Nanomaterials for Innovative Energy Systems and Devices (Materials Horizons: From Nature to Nanomaterials) 9811905525, 9789811905520

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Materials Horizons: From Nature to Nanomaterials

Zishan H. Khan   Editor

Nanomaterials for Innovative Energy Systems and Devices

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research.

More information about this series at https://link.springer.com/bookseries/16122

Zishan H. Khan Editor

Nanomaterials for Innovative Energy Systems and Devices

Editor Zishan H. Khan Department of Applied Sciences & Humanities Jamia Millia Islamia New Delhi, India

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-19-0552-0 ISBN 978-981-19-0553-7 (eBook) https://doi.org/10.1007/978-981-19-0553-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

The progress of mankind is defined in terms of advances in materials. Materials science is an interdisciplinary field that deals with the study of matter and its properties as well as the invention and design of new materials. “There’s plenty of room at the bottom”, the 1959 dream statement of the legendary Richard Feynman has been realized in less than 60 years by constant efforts and significant contributions from the scientific community across the globe. The world of materials science is thus witnessing a revolution in the exploration of matter at the nanoscale. New and improved properties of materials whose constituent units are nanosized objects make one explore these classes of materials called nanostructured materials. Nanostructured materials also referred to as nanophase materials, nanocrystalline materials, or nanomaterials can perform functions better than known conventional materials and make many inconceivable objectives in the past achievable. Molecular nanotechnology has the potential to revolutionize our livelihood in the future. Materials with at least one dimension below one micrometre but greater than 1 nm can be considered as nanoscale materials. Ultrafine microstructures having an average phase or particle size on the order of nanometres are known as nanostructured materials. These materials act as a bridge between isolated atoms and bulk macroscopic materials. As a result of the small size of their constituent particles or molecules, such materials can be designed to show novel and considerably improved physical, chemical, and biological properties, phenomena, and processes. Nanostructured materials exhibit some remarkable properties distinctively different from that of bulk. When the characteristic dimension is sufficiently small, bulk semiconductors become insulators. Nanostructured materials include nanoparticles, nanorods and nanowires, thin films, and bulk materials made of nanoscale building blocks or consisting of nanoscale structures. For the fabrication of nanomaterials, various techniques have been used. The structural features of nanomaterial lie in between those of atoms and the bulk materials. Materials in the micrometre scale mostly show the same physical properties as that of bulk form, but the materials in the nanometre scale may show distinctively different physical properties from that of bulk. Due to their small dimensions, nanomaterials have an extremely high surface-to-volume ratio, which makes v

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the surface or interfacial atoms to be large, resulting in more ‘surface’-dependent material properties. The quantum effects are due to the nanometre sizes of nanomaterials also having a spatial confinement effect on the materials. The energy band structure and charge carrier density in the materials can be modified differently from their bulk and hence will change the electronic and optical properties of the materials. Energy supply has a strong positive correlation with the development of the economy and society. Energy often becomes a rate-limiting factor in this process emphasizing the requirement of unrestricted energy supply. But the quest to attain this is driving the human race away from the concept of sustainable development and towards the vicious cycle of exploitation and damage of the ecosystem. To attenuate this detrimental process leading to negative environmental consequences, without compromising the development aspect, worldwide research has started to focus on energy saving, generation, harvest, conversion, and storage. Nanomaterials owing to their unique mechanical, electrical, and optical properties have been a pivotal player in recent advances in the field of energy systems and devices. The development of novel energy harvesting devices like solar cells and thermoelectric devices has been aided by the complementary advancement in nanoscience. The effect brought about by the quantum confinement of materials aids the electron transport, and band engineering in nanomaterials enhances the performances of devices. Research in this field have still miles to cover till it becomes a complete alternative for fossil fuels. Along with energy harvesting, innovation in energy storage devices like advanced batteries (Li-ion batteries), fuel cells, and ultra-capacitors is also enhancing the transition towards the utilization of renewable sources of energy. Portable electronics, electric vehicles, grid-scale energy storage, etc. thus rely on this technology. Nanomaterials provide a higher energy and power density to energy storage devices. Also, in the case of supercapacitors, usage of nanopowders with high surface area provides higher electrode compaction calling for increased usage of nanomaterials in supercapacitor-based storage sources. This book presents the latest advancements in nanomaterials for innovative energy systems and devices, thereby providing an overview of the current status of this rapidly developing field. This book includes 14 chapters authored by experts. Chapter 1 presents a review of nanomaterials used for Perovskite Solar Cells (PSC). This chapter provides a brief description of the different generations of solar cells, and an exclusive study on the role of nanomaterials as charge transport layers, absorber layers, and electrodes in the fabrication of PSCs has been presented. It also explores the merits and demerits of PSCs and provides possible solutions to overcome their limitations. Chapter 2 provides a detailed description of the synthesis, properties, and applications of graphitic carbon nitrides in perovskite solar cells (PSCs). It also includes the role of Graphitic Carbon Nitrides in Perovskite Solar Cells. Chapter 3 provides a detailed description of transition metal dichalcogenides (TMDs) nanocomposites-based supercapacitors. This chapter introduces the need for renewable energy and the importance of supercapacitors as an energy storage device in comparison to batteries. Furthermore, this chapter presents the various

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synthesis methods of TMDs and their nanocomposites with their applications in supercapacitors. Chapter 4 highlights the major challenges towards the development of efficient thermoelectric materials from high figure-of-merit (zT) materials to devices. This chapter gives an in-depth knowledge of the state-of-the-art techniques to optimize the properties of thermoelectric materials. The authors discussed various strategies for enhancing the power factor and their effects in some of the classic thermoelectric materials—Bi2 Te3 , PbTe, SnSe, SnTe, Mg2 Si, and MnSi2 . Chapter 5 discusses the role of Bi2 Se3 topological insulator thin films for various device applications. In this chapter, the authors report the deposition of Bi2 Se3 thin films on various substrates by the R.F magnetron sputtering system and their optical, electrical, and thermoelectric properties. It also includes the analysis of high-resolution X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray, and Raman Spectroscopy. Chapter 6 describes the photovoltaic application of Zinc Oxide. This chapter provides a review on synthesis methods of ZnO nanostructures and the structural, optical, and electrical properties of the ZnO nanostructures and thin films which are important for PV applications. Chapter 7 discusses the role of electrode materials for the improved performance of the batteries and the application of nanomaterials for attaining better capacity and long life cycle of rechargeable batteries. Chapter 8 presents the extraction and experimentation of biodiesel produced from leachate oils of landfills coupled with nano-additives aluminium oxide and copper oxide on diesel engines. The authors aimed at extraction and utilization of leachate oils obtained from food landfills and further combine them with nanoparticles to evaluate their performance and emission characteristics in engines. Chapter 9 describes the thin film deposition process, characterization, and photovoltaic application of cadmium selenide. Various methods of synthesis for thin films have been discussed in detail. The characterization of thin film by XRD, FESEM, elemental analysis using EDS, and UV-visible spectroscopy has also been discussed in this chapter. Chapter 10 highlights the recent developments in electrolyte materials for rechargeable batteries. This chapter discusses the role of different electrolytes and nanomaterials as additives for improved performance of the batteries. Chapter 11 describes the recent progress in separators for rechargeable batteries. This chapter discusses several types of separators in accordance with their use in different batteries and their physical and electrochemical properties, performance, fabrication, and production techniques. The role of nanomaterials in separators is also presented in this chapter. Chapter 12 focuses on the opportunities and challenges of organic photovoltaic cells. This chapter gives an overview of the understanding of Organic Photovoltaics’ (OPVs) working mechanism and device structures (conventional and inverted) comprising different types of layers. It also gives insight into the chemical and physical degradation mechanisms leading to instability and elaborates the different encapsulation techniques to improve the stability of the device. Chapter 13 presents a review on Polyaniline-based composites with and without binder as advanced supercapacitor electrode materials. This chapter focuses on achieving high performance of the newly cultivated PANI-based supercapacitors with and without binder addition into the PANI network. Moreover, many new interesting advanced PANI

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composites/nanocomposites supercapacitors have also been included in this chapter. Chapter 14 includes the aspect of green nanocomposites in green technology and sustainable development. This chapter portrays the evolution of the metals from the ancient stage to the latest green nanotechnology. The key challenges in developing green composites and the importance of green nanotechnology have also been included in this chapter. New Delhi, India

Zishan H. Khan

Acknowledgement

I take this opportunity to thank people who have been helpful in the successful completion of this book. First and foremost, I bend my head in humble gratitude before the Omnipotent God Almighty for showering me with his blessings throughout this venture. He has given me the power to believe in my passion and pursue my dreams. Without his grace, this book could not have become a reality. I am extremely grateful to all the authors who have significantly contributed to this book. Their timely inputs and response to every query had aided me to undertake and accomplish this challenging task. All the authors have justified their contribution to this book by presenting their work on the latest areas of nanomaterials and their applications in energy systems and devices, for which I am indebted. I would like to thank Prof. Najma Akhtar, Vice-Chancellor, Jamia Millia Islamia, New Delhi (India), for her constant support, and encouragement in creating an academic environment in the pursuit of higher education. Her pearls of wisdom, coupled with motivation, have supplemented largely to the completion of this book. I thank the people who had given valuable comments and suggestions for the improvement of this book. I also appreciate the efforts of those who assisted in the editing, proofreading, and design of the book. I am thankful to all my Ph.D. students, especially Mohd. Bilal Khan, Hasan Abbas, and Sultan Ahmad, who helped me a lot in the accomplishment of this book. I am thankful to the editorial team of Springer (India) for bringing out the book in its present form on time. I would never be able to complete this book without the unconditional love and support from my family. They have been a constant source of inspiration in all struggles of my life, and I dedicate this book to them. Words cannot express my gratefulness towards my mother for her encouragement and support for letting me live my life exactly the way I want to. I wish to express my heartfelt gratitude to my wife Rubina Mirza, for all the support and care that she has provided for me to excel in my career. I heartily thank my amazing children Ayanab, Alina, and Ali, for always making me smile in such a stressful life and provided stability and constant entertainment at my home. ix

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Finally, I thank everyone who helped me directly and indirectly in the successful completion of this book. As an editor, suggestions and feedback for the book are cordially invited at [email protected].

Contents

1

Nanomaterials for Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . Rasha Sultan, Hasan Abbas, Mohd. Bilal Khan, and Zishan H. Khan

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Graphitic Carbon Nitrides: Synthesis, Properties, and Applications in Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . Fareed Ahmad, Zishan H. Khan, and Sundar Singh

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Transition Metal Dichalcogenides (TMDs) Nanocomposites-Based Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . Shrestha Tyagi, Kavita Sharma, Ashwani Kumar, Yogendra K. Gautam, Anil Kumar Malik, and Beer Pal Singh

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Major Challenges Toward the Development of Efficient Thermoelectric Materials: From High Figure-of-Merit (zT) Materials to Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 S. Neeleshwar, Anjali Saini, Mukesh Kumar Bairwa, Neeta Bisht, Ankita Katre, and G. Narsinga Rao

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Bi2 Se3 Topological Insulator Thin Films for Various Device Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Sudhanshu Gautam and Sunil S. Kushvaha

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Zinc Oxide: A Fascinating Material for Photovoltaic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Premshila Kumari, Avritti Srivastava, Ruchi K. Sharma, Deepak Sharma, and Sanjay K. Srivastava

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Advances in Electrode Materials for Rechargeable Batteries . . . . . . 243 Nadeem Ahmad Arif, Mohammad Mudassir Hashmi, Syed Mehfooz Ali, Mohd Bilal Khan, and Zishan H. Khan

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Extraction and Experimentation of Biodiesel Produced from Leachate Oils of Landfills Coupled with Nano-Additives Aluminium Oxide and Copper Oxide on Diesel Engine . . . . . . . . . . . 319 Osama Khan, M. Emran Khan, Mohd. Parvez, Khan Adnan Ahmed Rizwan Ahmed, and Inzamam Ahmad

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Cadmium Selenide Thin Film Deposition and Characterization for Photovoltaic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Suresh Kumar and K. P. Tiwary

10 Recent Developments in Electrolyte Materials for Rechargeable Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 Syed Mehfooz Ali, Nadeem Ahmad Arif, Mohammad Mudassir Hashmi, Mohd Bilal Khan, and Zishan H. Khan 11 Recent Progress in Separators for Rechargeable Batteries . . . . . . . . . 417 Mohammad Mudassir Hashmi, Nadeem Ahmad Arif, Syed Mehfooz Ali, Mohd Bilal Khan, Mukesh P. Singh, and Zishan H. Khan 12 Organic Photovoltaic Cells: Opportunities and Challenges . . . . . . . . 499 Mukesh P. Singh and Mohd Amir 13 Review on Polyaniline-Based Composites With and Without Binder as Advanced Supercapacitor Electrode Materials . . . . . . . . . . 551 Gyan Singh and Samina Husain 14 The Aspect of Green Nanocomposites in Green Technology and Sustainable Development: State of the Art and New Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583 M. S. Kiran Sankar, Mohd. Parvez, Moti Lal Rinawa, Vijay Chaudhary, Sumit Gupta, and Pallav Gupta

About the Editor

Zishan H. Khan is currently Professor and Head at the Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi. He obtained his Ph.D. degree from Jamia Millia Islamia, New Delhi. He has almost 25 years of research experience in semiconductor physics and nanotechnology. He has published more than 140 research papers in various international reputed journals and guided a number of Ph.D. students. He has presented many research papers in various national and international conferences. He has completed several research projects on various topics in nanotechnology. He has worked at several positions in the universities abroad. He was a post-doctoral fellow at Department of Materials Science & Engineering & Centre of Nanoscience and Nanotechnology National Tsing Hua University, Hsinchu, Taiwan during 2001 to 2005. During the post-doctoral research, his work on the fabrication of FET (field effect transistor) using individual (single) carbon nanotube was highly appreciated by the scientific community. With this significant experience in nanotechnology, he was selected to establish a Centre of Nanotechnology at King Abdul Aziz University, Jeddah, Saudi Arabia in 2007. During his stay there, he established the world class facilities in nanotechnology with a clean room of level 100 at King Abdul Aziz University, Jeddah, Saudi Arabia. He is also actively involved in designing various courses in nanotechnology and energy sciences for graduate and research students. He

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is also the regular reviewer for many international journals of high repute. In addition, he has edited several special issues for reputed international journals. Dr. Khan has edited many books for reputed publishers including Springer Nature and published many book chapters with reputed publishers. His present research interests include hybrid solar cells, OLEDs, transition metals di-chalcogenides, carbonaceous nanomaterials, nano-sensors and nano-biosensors. He has started M.Tech. (Energy Science and Technology) program at the Department of Applied Sciences and Humanities and currently managing this program as the Program Coordinator. This program has been widely appreciated by the academic community as well as the industry experts.

Chapter 1

Nanomaterials for Perovskite Solar Cells Rasha Sultan, Hasan Abbas, Mohd. Bilal Khan, and Zishan H. Khan

1 Introduction 1.1 Nanomaterials: Privilege Over Bulk Materials Nanotechnology entails the creation and application of materials with dimensions of a billionth of a metre or less—nearly 100,000 times tinier than the diameter of a human hair. This places the nanomaterials/nanoparticles in the ultrafine particle category. We can control materials and gadgets down to the level of individual atoms and molecules using nanotechnology [1, 2]. Nanoparticle size has a significant impact on their qualities. When a particle’s bulk size is compared to its microscale size, there isn’t much of a change in its attributes. However, the characteristics of the particle change considerably when it reaches to a nanometre scale, relative to its bulk condition. For example, at the nanoscale, gold particles have a purple colour, which contrasts with the yellow colour of the bulk. Due to the confinement effect, their band type changed from continuous to discrete, resulting in a colour change [3, 4]. The fundamental causes for the ‘tunability’ of attributes are quantum processes at the nanoscale. We can vary the material attribute of our material by just changing the particle size. Nanomaterials exist in nature and can be created from a variety of products, such as carbon or minerals like silver, however nanomaterials must have at least one dimension in order of 100 nm (nm) [4–6]. Nanomaterials can be classified into three categories, namely, natural, incidental and engineered nanomaterials/ENMs. ENMs have been developed specifically for a variety of commercial product and procedures. Such materials have shown unique optical, magnetic, electrical and other properties. ENMs are ideal for a variety of applications including electronics, medicine, cosmetics, etc. One of the R. Sultan (B) · H. Abbas · Mohd. B. Khan · Z. H. Khan Department of Applied Sciences & Humanities, Jamia Millia Islamia, New Delhi 110025, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_1

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Fig. 1 Nanometer scale. Reprinted with permission from [6]

most important applications of nanomaterials is in the fabrication of efficient and stable solar cells.

1.2 Solar Energy (Cells): Overview As solar energy has the potential to accomplish global energy consumption demands, therefore, solar photovoltaic technology has become the most promising renewable energy source. The global energy consumption is constantly rising and the Energy Information Administration has predicted that the current consumption will rise by 40% till 2040 (Fig. 2) [7]. At present, the total energy produced by conventional (non-renewable) energy sources is more than two-third of the total energy consumed which is causing high level of pollution and global climatic changes. Therefore, scientist and researchers are seeking for better alternatives, i.e. renewable energy sources. The use of clean energy sources such as wind, solar, hydrogen and biomass has increased to a great extent over the last few years. Among all renewable energy sources, solar energy is known to be the source of nearly all energy on earth (Fig. 3). It is the most abundant and clean form of energy. Earth receives 1.5 × 1018 KW/h energy per year by the sun [10]. The energy received by sun in 1.5 h is ample to satisfy yearly global energy demands. Solar cells are one way to harvest the solar energy [11]. Solar cells work on photovoltaic effect which means the conversion of solar radiations into usable energy (electricity) and thus have the capability to meet humans’ accelerating energy demands.

1 Nanomaterials for Perovskite Solar Cells

Fig. 2 Global energy consumption [8] Source U.S. Energy Information Administration

Fig. 3 U.S. Primary energy consumption, 2020 [9]. Source U.S. EIA (April 2021)

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2 Evolution of Solar Cells 2.1 History and Background Solar cell, also known as photovoltaic cell, is an electronic device that converts light into electricity using the concept of photovoltaic effect. The photovoltaic effect is the process that produces voltage and current when the solar cell is exposed to sunlight or photon energy (Fig. 4). The growth of solar cell technology commenced in 1839 when the French experimental physicist Edmond Becquerel discovered the photovoltaic (PV) effect. While investigating a solid electrode in an electrolyte, he observed that when the electrode was exposed to light rays, a potential developed across it. Later in 1883, Charles Fritts fabricated first solar cell having junctions made by covering selenium (semiconductor) by a very fine layer of gold [12]. The efficiency of the solar cell was nearly 1% [13]. Though solar cell technology could not gain enough pace for development. This technology got some pace after early 1940s, when Russell Ohl invented solar cells with efficiency around 1% [14]. Some great work of solar cells was done by Daryl Chapin, Calvin Fuller and Gerald Pearson at the Bell Telephone Labs in 1954 [15]. Initial efficiency of the first crystalline silicon solar cell fabricated in Bell Labs was around 1% but soon they got efficiency of 6%, after few years it was increased to 10% [16]. Today the crystalline silicon-based photovoltaic technology is well developed and widely commercialized. The data shows that higher growth rate is

Fig. 4 Basic solar cell structure (Author)

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Fig. 5 Photovoltaic installations from 2010 to 2020 [13]. Attribution 4.0 International (CC BY 4.0). Copyright (c) MDPI, 2020

expected for the photovolaic capacity in 2020. It is predicted that new installations will range from 120 to 154 GW. The Renewable 2019 report of the International Energy Agency/IEA predicts an addition of new photovoltaic power between 586 and 765 GW by 2024 [17]. The development of solar cell technology has undergone three phases, which are known as three generations (Fig. 5).

2.2 First-Generation Solar Cells Today, silicon-based solar cells, also called first-generation solar cells, are the most prevalent solar cells available in the market. It’s a well-established PV technology and dominates the photovoltaic market by 80% [10]. The power conversion efficiency (PCE) of the first-generation solar cells has improved from 12 to 26% in the last 40 years [19]. Silicon solar cells have longer lifetime of approximately 25 years and the cost of manufacturing the cells reduces with mass production by large industries [20]. But the disadvantage of these cells is that the absorption is low, cost of fabrication is high and the manufacturing process is tedious and toxic due to use of materials like CIS, CdTe, silicon tetrachloride and cadmium indium gallium di-selenide. A lot

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Fig. 6 Silicon solar cell. Reprinted with permission from [21]

of heat energy is required for producing crystalline silicon solar cells. The firstgeneration solar cells included are further classified as monocrystalline silicon solar cells, polycrystalline silicon solar cell and mono-like multi silicon (MLM) solar cells (Fig. 6). The most prevailing bulk material for solar devices is crystalline silicon/c-Si also known as ‘solar-grade silicon’ bulk silicon is parted into various categories based on crystallinity and crystal size in the produced ingot, wafer or ribbon. These devices are totally based on the concept of a p–n junction. Photovoltaic devices made of c-Si have wafers between 160 and 240 μm thick [22]. The crystalline solar cells can be monocrystalline (m-Si) or polycrystalline (p-Si) depending upon the crystal structure of silicon. The m-Si is the oldest type of photovoltaic technology. The efficiency of m-Si is greater than 25% under laboratory conditions and for commercial applications it is between 15 and 22% [23–25]. The efficiency varies with the fabrication process. Generally, the monocrystalline silicon cells are manufactured by using Czochralski process, designed by the Polish chemist Jan Czochralski in 1916 [26]. The manufacturing method of these solar cells desires a material purity of almost 99.9999% [27, 28]. Solar cells made from monocrystalline silicon have higher efficiency and are more costly than other types of cells. As the wafer material is cut from cylindrical ingots, the corners of these cells are trimmed, like an octagon [29, 30]. The technology of polycrystalline silicon modules is also derived from crystalline silicon but they have non-aligned crystal pattern [31]. The efficiency of these modules recorded for the first time in 1984 was found to be less than 15%. Currently, in laboratory environment, it is possible to obtain efficiency of 22.3% [23, 24] and in case of large-scale production efficiency is between 14 and 20% [32]. Polycrystalline silicon solar cells have a lower production cost and the silicon residues produced are less as well but these cells have lower efficiency than m-Si solar cells. The fabrication cost of p-Si is low because of the block smelting method. Ingot formation step is not required in block smelting method. The silicon is liquefied

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and then transferred to a square crucible coated with SiO/SiN graphite. The material then goes through a controlled cooling process that yields a solid block of p-Si [33]. This block is sliced into square wafers to produce the solar cells. The drawback of this method is that the greater p-Si contact with the crucible permits more impurity transfer from the crucible to the p-Si. Therefore, the efficiency of polycrystalline cells is lower than monocrystalline cells. The square-shaped wafers have an ideal design for the better utilization of space in solar modules so there is no need to cut off the edges of the solar cells. Hence, regardless of the less efficiency than the m-Si cells, the low fabrication cost, less quantity of residues and the proper utilization of space by the solar modules have made p-Si cells popular [27]. Another type of crystalline silicon solar cells is the mono-like-multi silicon (MLM) solar cells. These cells were developed in the 2000s and are also known as cast-mono. This technology makes use of p-Si manufacturing chambers with tiny ‘seeds’ of monocrystalline material. The product is similar to monocrystalline material but the edges are polycrystalline. After being cut the inner portions have shown high efficiency like monocrystalline material (but square and not clipped), whereas the edges are sold as poly. This fabrication process gives mono-like cells at the cost of poly [34, 35].

2.3 Second-Generation Solar Cells Wolf and Lofersky developed the first thinner silicon wafers for solar cells [36–38]. Thin film solar modules require less material and manufacturing processes [39]. The solar devices of this technology are very thin, between 35 and 260 nm [40]. The solar cells produced are cheaper as glass substrate is generally used to place the PV material compromising the efficiency. Thin film solar cells can be made in any dimension but the only limitation is the base area for fabricating the module. As a result, in modules using thin-film technology, the contrast between the cell and module doesn’t exist [27]. The main thin-film technologies are, namely, amorphous silicon (a-Si), CdTe (Cadmium Telluride) and CIGS (Copper Indium Gallium Selenide) (Fig. 7). Among all the thin-film technologies, the a-Si technology is a well-developed technology. The a-Si solar cells are considered as a dependable energy source in electronic equipment’s like calculators, watches, etc. This technology requires the minimum quantity of material and is less harmful as compared to the other thinfilm technologies [41, 42]. Amorphous silicon has a disordered structure unlike monocrystalline silicon but its rate of absorption is 40 times higher than m-Si [43]. The disordered configuration results in high absorption due to high bandgap of 1.7 eV [44]. However, small amounts of a-Si and pendant bonds lead to short diffusion rate of minority carriers and anomalous electrical behaviour [41]. Amorphous silicon has achieved the lowest efficiency among the three main technologies. These cells have attained an efficiency of 14% while the modules have achieved efficiency of 9.8% [23]. Whereas the CdTe technology is considered one of the most promising technologies in the second-generation solar cells. It absorbs nearly all the light in the visible range with a bandgap of 1.45 eV and reveals good thermodynamic stability [45, 46].

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Fig. 7 CIGS thin film solar cell. Reprinted with permission from NREL Photovoltaic Research

These features allow it to attain better performance in comparison with the crystalline silicon technologies under high temperature and low light conditions [47, 48]. CdTe is the only thin-film technology so far to strive c-Si in cost per watt. However, Cd is extremely toxic and Te supplies are limited. The cadmium content present in the solar cells would be dangerous to the environment if released. However, release of cadmium is impossible during normal working of the devices and is doubtful during fires in residential roofs. The amount of cadmium present in a square metre of CdTe is nearly the same as present in a single-cell nickel–cadmium battery. However, the Cd present in CdTe solar cells has more stability and low solubility. The fabrication procedure of CdTe cells is easy and less costly as compared to c-Si and other thin-film technologies. Nearly 40% of the existing devices of the thin-film technology are from CdTe [48, 49]. PV devices using this technology have achieved an efficiency of 22.1% and their modules have obtained efficiency of 18.6% [23]. The efficiency reported commercially is generally 2–4% lower [50]. Another material used in the thin-film technology is copper indium gallium selenide/CIGS. It has a direct bandgap. These cells have achieved notable efficiency (~26.4%) among all the thin-film materials [51]. The fabrication procedure of the CIGS cells is carried out in vacuum conditions. On a glass/plastic holder with electrodes on the front and on the back, a thin layer of 2–3 m of copper, indium, gallium and selenium is deposited [27]. This deposition technique requires rigid substrates or flexible substrates that allow for new application fields and lower production costs. Recent advances at IBM and nano-solar used non-vacuum solution processes in order to lower the cost. The CIGS bandgap is proportional to the content of gallium, varying from 1.04 eV (pure CuInSe2 ) to 1.68 eV (pure CuGaSe2 ). The cells having bandgap of about 1.1 to 1.24 eV generally have higher efficiency [52]. CIGS showed the highest potential among the thin-film technologies, attaining efficiencies comparable to p-Si cells [53]. The reported efficiency of CIGS cells is 22.9% and their modules have attained an efficiency of 19.2% [23]. Gallium arsenide (GaAs) is another semiconductor material that is used for single crystalline thin-film PV cells. Even though GaAs cells are

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expensive, their single-junction photovoltaic cells with efficiency of 28.8% hold the world’s record [54]. GaAs is generally used in multi-junction solar cells for concentrated PVs (CPV, HCPV) and for solar panels on spacecraft because efficiency is preferred over cost by the industry for space-based solar power. Numerous explanations why Gallium Arsenide has such high PCE have been discussed based on the previous works and theoretical analysis. First, bandgap of GaAs is 1.43 eV which is nearly ideal for PV cells [55]. Secondly, as Ga is a by-product of the smelting of other metals, solar cells of GaAs are very resistant to heat and can maintain high efficiency even at elevated temperatures. Third, GaAs give various sorts of design options. Engineers use GaAs material as an active layer in PV devices and have various choices of other layers that can generate better charge carriers in GaAs.

2.4 Third-Generation Solar Cells The third generation of solar cells is also known as nanomaterials/NMs-based solar cells. This generation consists of Organic Photovoltaics (OPV), quantum dot PV cells, dye-sensitized solar cells/DSSCs and perovskite solar cells/PSCs. Different types of nanomaterials, polymers and organic dyes are used to make the third generation of solar panels. These cells are lightweight, easy fabrication process and cost-efficient (Fig. 8). DSSCs also known as Grätzel cells are thin-film solar cells that are under immense study for more than two decades because of their ease of production, simple fabrication methods, low toxicity and low cost. DSSCs were invented in 1991 with 7% efficiency [56]. These cells were co-invented by Brian O’Regan and Michael Grätzel in 1988 at UC Berkeley. These cells were further advanced till 1991 by the above-mentioned scientists at Ecole Polytechnique Fédèrale de Lausanne/EPFL. Still, because of their lower abundance and stability, there is a lot of possibility of replacing current DSSCs. Currently, the DSSCs have attained an efficiency of 12%, using Ru(II) dyes by optimizing material and structural properties. However, the Fig. 8 Perovskite solar cells. Source OENRL, JMI (Author)

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efficiency is still lower than the efficiency achieved by thin-film PVs and Si-based PV cells which offer nearly 20–30% efficiency [57]. The OPV or plastic solar cell, which is a type of PV technology based on organic electronics, is another category of solar cell in the third generation. Organic electronics deal with conductive organic polymers/small organic molecules for the absorption of light and the transport of charges for the generation of electricity by means of a PV effect. Most of the organic PV cells are polymer cells [58, 59]. The OPV materials are solution processed at high performance and are low cost making their mass production cheaper. Moreover, the flexibility of organic molecules makes OPVs cost-effective for PV applications [60, 61]. The bandgap can be modified by molecular engineering (e.g. altering the length and functional group of polymers), enabling electronic tenability. OPVs have attained efficiencies close to 11%, but efficiency limitations and long-term stability are still the major concern [61, 62]. The absorption coefficient of organic molecules is higher, so a small quantity of materials (order of hundreds of nanometre) can absorb large amount of light. The main drawbacks of organic PV cells are poor stability, lower efficiency and low strength compared to inorganic PV cells such as silicon solar cells. Typical organic photovoltaics (OPVs) include three layers, an active layer, a cathode and an anode. Active layer is sandwiched between the anode and cathode layers. However, one of the electrodes must be transparent for the light to penetrate [63]. Research has been done to develop suitable device architectures for improving the charge transfer across the OPV devices. Single layer organic solar cell was the first device structure proposed. It had a simple configuration made of a photoactive organic layer kept between cathode and anode [64]. The device architecture is as shown in Fig. 9. The maximum PCE of the solar device with this architecture was very low (0.1%), due to inefficient charge separation and therefore recombination of hole and electron is more [65]. Double-layer organic solar cell was developed, consisting of two layers (n-type and p-type semiconductors, also termed as donor (D) and acceptor (A)) sandwiched between the two electrodes. The maximum PCE achieved with this architecture was 1% [64]. Due to limited diffusion length of exciton and the short lifetime, the active layer must be thin enough to ensure the electron and hole can be separately collected before recombination. The bulk heterojunction (BHJ) structure is made from mixing

Fig. 9 Single layer organic solar cell. Reprinted with permission from [18]. Attribution 4.0 International (CC BY 4.0) copyright (c) 2017 PEN

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Fig. 10 Double layer and bulk heterojunction organic solar cell. Reprinted with permission from [18]. Attribution 4.0 International (CC BY 4.0) copyright (c) 2017 PEN

from a p-type and n-type material was invented in 1995 by Friend group and then by Heeger [66–68]. A BHJ organic solar cell basically comprises a multilayer structure in which different manufacturing techniques can be used to deposit each layer in the device architecture. The absorber layer is composed of two components: a donor material is generally a conjugated polymer, conjugated pigments or oligomers and for an acceptor material fullerene derivatives are often used [69]. The absorber layer is kept between the anode and cathode [70, 71] (Fig. 10). A quantum dot solar cell (QDSC) uses quantum dots as the absorbing PV material. It tries to replace bulk materials, viz. silicon, CIGS or cadmium telluride [72]. By changing their size, QDs have tunable bandgaps across a wide range of energy levels [73]. The bandgap of bulk materials is fixed by the material(s) selection. This property makes QDs attractive for multi-junction PV cells, where a range of materials are used to enhance efficiency by capturing multiple sections of the solar spectrum. Using CsPbI3 perovskite QDs (coated with formamidinium iodide (FAI) to increase the carrier mobility) as the absorbing material, a solar cell with a certified efficiency of 13.4% has been reached, and a high open-circuit voltage (Voc ) of 1.2 V has been attained [74].

3 Perovskite Solar Cells (PSCs) Since the first use of PSCs in 2009, the efficiencies of PSCs have improved from 3.81% in 2009 to ~23% for recent devices as of June 2018 [75–77]. The key features of the perovskites include high solar absorption, low production cost, facile fabrication, low non-radiative carrier recombination rates and the potential to capitalize on over 20 years of development of related DSSCs and OPVs [78, 79]. A fairly high carrier mobility along with proper device structures has great attribution in the fabrication of highly efficient and stable PSCs [79]. The PSCs have gained considerable admiration in recent years because of their advantageous features and the increasing PCEs. Toxicity and long-term stability are two major concerns of perovskite solar cells [80]. Lead being a vital constituent of almost all high-performing perovskite devices

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raises toxicity issues during device production, use and disposal. PSCs generally undergo degradation when exposed to humidity and UV radiation [81].

3.1 Perovskite: History and Background The name perovskite comes from titanium oxide (CaTiO3 ) mineral which was discovered by L.A. Perovski in 1839 in the Ural Mountains of Russia [82, 83]. Perovskite structure was first described by Victor Gold Schmidt in 1926 [84, 85] and the perovskite materials were used in organic photovoltaics field for the first time by German scientist David Mitzi in 1991 [86]. Perovskite materials have three main crystal structures, namely, cubic, tetragonal and orthorhombic [87, 88] as shown in Fig. 11. ABX3 describes the perovskite crystal structure, where ‘A’ represents larger cation, ‘B’ represents smaller cation cations and X symbolizes an anion that bonds with both the ‘A’ and ‘B’. The ideal cubic structure of ABX3 perovskite material comprises octahedral corner-sharing [BX6 ] wherein ‘A’ cation is surrounded by 12 ‘X’ anions [89]. The structure is identical to a cube at the corners with ‘A’ cations, ‘X’ anions at the face centres and ‘B’ cations at the body centre, where ‘A’ may be potassium, strontium, sodium, calcium, lead, or any other rare earth metal. ‘X’ represents an anion like oxide or halide and ‘B’ is a metal cation having coordination number six. Organic–inorganic halide perovskite consists of an organic part (amine derivative) and an inorganic part (lead halogen) [90]. Perovskites cubic structure stability can be deduced with a tolerance factor [91] t, √  t = (RA + RX )/ 2(RB + RX )

Fig. 11 Perovskite crystal structure (Author)

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where RA , RB and RX are the respective ionic radii of the ions A, B and X [93]. The formula is valid for empirical ionic radii at normal temperature [79]. The possible structure of perovskites can be evaluated using an octahedral factor µ, known as RB /RX ratio. Typically, 0.81 < t < 1.11 and 0.44 < µ < 0.90 for halide perovskite materials. The perovskite has cubic structure if the value of tolerance factor is in the range 0.89–1.0 and lower t values gives less symmetrical orthorhombic or tetragonal structures [79, 91]. Regardless of these restrictions, transitions between these structures on heating are same for any given perovskite, with the elevated temperature phase mostly being cubic [79]. The perovskite materials were discovered years earlier, but the first PSC was developed in 2009 by Miyasaka et al. The architecture of the device was almost identical to dye-sensitized solar cell (DSSC) [93]. They achieved only 3.8% PCE with the device structure: TiCl4 -treated FTO (anode)/mesoporous TiO2 /perovskitesensitizer/Pt-coated FTO (cathode) [76]. The full historical evolution of perovskite solar cells has been shown in Fig. 12.

Fig. 12 Historical evolution of the PSCs. Reproduced with permission from [94]. Copyright (2013) American Chemical Society

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3.2 Perovskite Solar Cells: Composition and Architecture Perovskite solar cell has different components that has specific functions. The functions of different components of a perovskite solar cell are discussed as shown in Fig. 13. Glass layer acts as a barrier which safeguards the solar cell against damages that may because due to external factors, viz. dirt, water and vapour. The glass also provides high transmissivity, strength and low reflection [95, 96]. Bottom electrode is a transparent electrode that allows the incident light to reach the absorber layer of the solar cell. Indium tin oxide/ITO and fluorine-doped tin oxide/FTO are most appropriate bottom electrode materials because of their transparency and conductivity. FTO performs effectively in solar cells as it is stable at high temperatures as well as under atmospheric conditions. It is also less expensive than ITO. Moreover, metaloxide-free electrodes are being considered for flexible solar cells. These comprise graphene and metal- or graphene-based electrodes, viz. silver nanowire or graphene oxide (GO) flake composite electrode [96, 97]. Electron transport layer (ETL) is a semiconducting layer that functions as electron absorber and charge carrier. ETL is used to remunerate and equalize the difference of hole and electron diffusion length. It effectively prevents the holes from travelling to the counterelectrode. ETL enhances the carrier’s separation effect and reduces the recombination, thereby improving the device performance. The main attribute of ETL is that it must match the band alignment of perovskite layer [78, 98]. The following should be taken into consideration while selecting an ETL: 1. 2. 3. 4.

An n-type semiconductor with high electron mobility is recommended. Material should have good optical transmittance in the visible range, and therefore should have a wide bandgap. The fabricating conditions should be mild and at low temperatures so as to obtain high-quality electron transport material (ETM) film. The energy levels of the ETMs should match with the energy levels of perovskite material. In fact, the ETL fabricated by different systems and structure can obtain

Fig. 13 Architecture of a PSC (Author)

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high PCE, showing that the selection of frequently used ETM is not the main factor that limits the PCE of photovoltaic cells. Perovskite layer is the main layer of PV cell which plays a key role in light absorption and photoelectric conversion [98]. In some designs of PV cells it also acts as a charge carrier. Optimizing of the materials and structures is one of the ways to improve the photoelectric conversion efficiency of the device [96]. Hole transporting layer (HTL) plays an essential role in migrating holes and blocking electrons, thereby restricting electrons from reaching the opposite electrode. Key role of HTL is to collect and transport holes from the photoactive layer, which helps in reducing electron–hole pair recombination in perovskite materials. In hole transporting materials (HTM), the HOMO must be aligned with the valance band of absorber material for hole transport [98]. On the basis of chemical composition, hole transporting material in PSCs can be categorized into inorganic and organic hole transporting materials. Spiro-OMeTAD is the most frequently used organic HTM. Inorganic p-type semiconductors display the potential to substitute organic HTMs because of some advantages like high charge carrier mobility, wide bandgap and easy solvent treatment process. Many inorganic HTMs have been reported, viz. CuI, CsSnI3 , NiO and CuSCN. The conductivity of CuI is superior to that of SpiroOMeTAD, and therefore CuI efficiently increases the fill factor of the photovoltaic device and, therefore, is a potent competitor to Spiro-OMeTAD [99]. Top electrode is an extremely conductive layer for the extraction and transportation of electrons. The frequently used metal electrodes like Au, Ag and Al can react with the perovskite absorber material. However, it was studied that devices with copper electrodes revealed stable performance (98% of initial efficiency) after more than 800 h of storage without encapsulation in the atmosphere. Carbon electrodes for perovskite solar cells have also been suggested besides the metal electrodes. They are more stable, cheaper and easily processable by low-cost methods. However, in most cases, efficiencies of solar devices with carbon electrodes are less than 10% with some exceptional cases [101, 102]. Various device architectures for PSCs have been developed till date achieving considerable PCEs. The device design architecture for PSCs can be mesoporous architecture, meso-superstructured, planar architecture or bulk heterojunction architecture. The mesoporous architecture is based on the traditional DSSC structure and it is the leading device architecture of PSCs. In this design, the TiO2 layer is made porous by annealing nanoparticles on it and then the perovskite layer is selfassembled in the space of mesoporous TiO2 layer. This mesoporous TiO2 network is essential to promote the transfer of electrons within the active layer and the FTO electrode [102, 103]. A further interesting improvement in the architecture of perovskite solar cell has been done by Lee et al. when they substituted n-type TiO2 mesoporous scaffold with an inert mesoporous Al2 O3 network. Since Al2 O3 was not directly involved in charge transporting mechanism, this architecture has been referred to as ‘meso-superstructured’ thin-film PSCs [104, 105]. The addition of Al2 O3 scaffold provided the perovskite with outstanding optoelectronic properties, encouraging researchers to further improve PSCs planar architecture. Another merit of replacing

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TiO2 with Al2 O3 is that the chemical instability concern of TiO2 when it is exposed to UV light can be resolved [106, 107]. The meso-superstructured PSC developed by Ballet al. employed mixed halide perovskite CH3 NH3 PbI3-x Clx as an absorber layer, with processing temperature of the material falling from 500 °C to less than 150 °C and PCE reaching up to12.3% [105]. The planar heterojunction architecture (PHJ) has an easy construction because it doesn’t have a mesoporous scaffold/electronic layer in it, enabling facile fabrication. Liu et al. reported that complex nanostructuring of the perovskite is not essential to attain higher efficiencies with the PSCs. They verified that simple PHJ architectures having vapour-deposited perovskite layer can attain PCEs greater than 15%. This architecture has closest resemblance with the silicon-based solar cells [108, 109]. There are basically two common device structures: conventional structure/n-i-p and inverted structure/p-i-n. The main difference between the two structures is that the charge transport layer is placed at different positions. In p-i-n structure, the first layer through which light penetrates is a p-type hole transport layer (HTL), whereas in ni-p, the first layer is an n-type electron transport layer (ETL) as shown in Fig. 14. A disadvantage of the perovskite structure is that the diffusion length of electrons and holes is different. Bulk Heterojunction (BHJ): to achieve the higher PCE of PSCs Kai et.al manufactured the bulk heterojunction perovskite hybrid PV cells and achieved a 22% increase in Fill Factor (FF) and the performance of the solar cell. The final PCE of the device reached approximately 14% [110].

Fig. 14 P-i-n and n-i-p structure of perovskite solar cells (Author)

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Fig. 15 Schematic diagram of planar heterojunction structure (PHJ) and bulk heterojunction structure (BHJ) (Author)

3.3 Application of Nanomaterials in Perovskite Solar Cells Nanotechnology has opened up new opportunities for improving solar cell quality, stability and performance. Nanomaterials can help with solar cell design in a variety of ways, all of which have an impact on their performance. The nanomaterial-based solar cells are cost-effective, stable and display better device performance. Nanotechnology has the potential to overcome present performance barriers and considerably increase the collection and conversion of solar energy. Nanoparticles and nanostructures have improved light absorption, higher light-to-electricity conversion and improved thermal storage and transmission.

3.3.1

Nanomaterials Used as Electron Transport Layer (ETL)

The electron transport layer (ETL) is a semiconducting layer that serves as a charge carrier and electron absorber. ETL compensates for and equalizes the disparity in hole and electron diffusion lengths. The holes are efficiently prevented from reaching the counterelectrode. ETL improves device performance by increasing the carrier’s separation effect and reducing recombination. Here, we have studied the different nanomaterials used as ETL in perovskite solar cells. Elseman et al. [111] studied TiO2 as an advanced ETM to boost the efficiency and stability of PSCs. TiO2 nanotube (TD-NT) was synthesized using economical hydrothermal method. The device they fabricated had a conventional architecture (FTO/TD-NT/MAFAPbI3 /SpiroMeOTAD/Gold).They reported JSC of 23.85 mA/cm2 , VOC of 1.14 V, FF of 68.32% and PCE of 19.14%. They concluded that TD-NT led to a better film morphology with larger grain size and few pinholes. The TD-NT layer enhanced charge separation and reduced charge recombination, in addition to better energy alignment. Furthermore, Bi and co-workers [112] stated the earliest use of perovskite absorber

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layer together with ZnO nanorods for photovoltaic application. The stability of spiroMeOTAD/CH3 NH3 PbI3 /zinc oxide nanorod devices was observed by storing them at room temperature in air without further encapsulation. They fabricated a solar cell (ZnO NRs/MAPbI3 /spiro-MeOTAD/Ag) with efficiency of 5% under 1000 W/m2 AM 1.5 G illumination. Moreover, the device showed a fine long-term stability. ZnO showed a lower performance in comparison to CH3 NH3 PbI3 /TiO2 solar cells because of more recombination losses. Moreover, You et al. [113] reported perovskite solar cell using solution process method that had NiOx as HTL and ZnO nanoparticles as ETL. This device showed improved stability against oxygen and water degradation in comparison to devices having organic charge transport layer. The device they fabricated had a p-i-n architecture (ITO/NiOx/MAPbI3 /ZnO/Al). Degradation was prevented as zinc oxide layer isolated the perovskite and aluminium layers. The all-metal-oxide devices kept nearly 90% of the initial efficiency regardless of being stored in air at room temperature for about 60 days. Whereas the control devices having organic transport layers completely degraded within 5 days. The initial PCE of the fabricated devices was 14.6 ± 1.5%, with an unaccredited maximum value of 16.1%. The metal-oxide devices fabricated showed a remarkable advancement in stability in air in comparison with the devices having organic transport layers. Maxim et al. [114] deposited carbon QDs embedded in thin layers of PMMA on the illumination side of perovskite devices. A bilayer structure made up of SnO2 QDs and SnO2 NPs that were deposited one after another was used as ETL. The structure followed was (FTO/SnO2 /Cs0.05 (MA0.17 FA0.83 )0.95 Pb(I0.84 Br0.16 )3 /spiroMeOTAD/Gold). They stated the VOC , FF, JSC and PCE of 1.13V, 68.19%, 23.21 mA/cm2 and 17.89%, respectively. Moreover, Ifa Laila and co-workers [115] explored the result that ZnO nanorods had on the electrical characteristics of MAPbI3 /ZnO nanorod PSCs. They used hydrothermal method for synthesizing ZnO nanorods on ITO with the help of hexamethylenetetramine and zinc nitrate as precursors in molar ratio 1:1 for about 6 h. The temperature was maintained at 90 and 100 °C, even the concentration of Zn(NO3 )2 was kept at 25 and 50 mM. Two-step deposition method was used to prepare the perovskite using PbI2 and CH3 NH3 I as precursors. The SEM revealed that the concentration and temperature of zinc nitrate [Zn(NO3 )2 ] depends directly on the size of the diameter and length of the rods. The best results (morphology) were obtained by the ZnO nanorods synthesized by using Zn(NO3 )2 with 50mM concentration at 90 °C. The XRD results revealed orientation at (101) for the zinc oxide nanorods. They reported that the zinc oxide NRs had an excellent crystal quality. The reported maximum current value of nearly 235 μA and high dielectric constant due to the existence of lead iodide that causes recombination. They concluded that the structure and synthesis condition of CH3 NH3 PbI3 is essential for device efficiency. Fibriyanti et al. [116] investigated the growth time for zinc oxide nanorods. They also studied the effect it has on the morphology of zinc oxide NRs as well as on the performance of the device and photoresponse. Zinc oxide NRs were developed on the zinc oxide seed layer with solvo-thermal method at growth time of 3 and 4 h. CH3 NH3 PbI3 perovskite material was fabricated using one-step spin coating. They reported the best power efficiency cell of ZnO NRs/CH3 NH3 PbI3 to be 0.4%. The photoresponse of ZnO NRs/CH3 NH3 PbI3 with the longer rod was found

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to be stable. It is seen that the performance of solar cell as well as the photoresponse is improved by zinc oxide nanorods with longer rods. Meanwhile, Khalid Mahmood and co-workers [117] studied the blade coating method for synthesizing ZnO nanorods in comparison to the ZnO NRs produced using spin-coating technique. They reported that the zinc oxide film grown using blade coating was found to be denser and the film was evenly distributed over the complete substrate. The best PCE of blade-coated zinc oxide NRs was found to be nearly 16.56%, whereas the PCE of spin-coated zinc oxide films was about 13.78%. They successfully synthesized highly efficient, scalable and hysteresis-free PSCs. Furthermore, Bu [118] has fabricated zinc oxide nanobranches by using solution processed method and used it as an ETL in PSCs. He reported that the zinc oxide nanobranches performed 85% better than the vertically aligned zinc oxide nanowire because of lesser structural defects and improved charge extraction. He revealed that the defects were reduced and carrier lifetime was enhanced because of the annealing process that was done for secondary growth. Dehghan and co-workers [119] worked to produce ZnO nanoparticles using precipitation method and used the same in PSCs with the structure FTO/ZnO/MAPbI3 /Au. They used various deposition methods to synthesize ZnO film (ETL), namely, spin coating, SILAR and spraying. The best device performance was reported with the use of spin-coating method. They achieved an average efficiency of nearly 7% for HTL free PSCs. However, an efficiency of nearly 10.25% was reported with the use of spiro-OMeTAD as HTL. SILAR and spray coating methods also revealed observable device performance. They concluded that the SILAR and spraying methods provided certain advantages like efficiency, inexpensive and ease of fabrication. Moreover, Yun et al. [120] have fabricated ZnO NRs of different lengths on FTO using water bath at low temperature. They revealed that orderly aligned vertical ZnO NRs with suitable length provided a better electron transportation and hence the PCE also increased. The crystalline quality of the film was also improved. The best PCE was reported to be 14.22% with NR length of nearly 400 nm. They concluded that ZnO NRs prove to be a good ETM for fabricating efficient PSCs with good reproducibility. Zhang et al. [168] focused on optimizing the length of ZnO NRs that are used as ETM in PSCs. To enhance the performance of the device, they have done sulfidation treatment on the zinc oxide NRs that helped to lower the degradation of perovskites, passivated surface defects and eased charge carrier transportation. All these factors led to the increase in PCE of the PSCs. They reported that the PCE of the cell was improved from 10% to 11.72% after sulfidation treatment. Liu and co-workers [171] synthesized an ETL composite made up of graphene oxide (GO) and SnO2 NPs. They fabricated PSCs on ZnO:Al (AZO) substrate. The revealed that the devices showed high efficiency and stability. SnO2 -GO with a modest incorporation ratio of 3 wt. percent GO greatly improves the device performance for planar PSCs on AZO substrates. They reported a PCE of 16.87% with reduced hysteresis and JSC increased from 21.45 to 22.57 mA/cm2 and FF increased from 57% to 73%. Furthermore, Song et al. [172] synthesized ETL composed of ZnO-SnO2 nanocomposite (NC) having various ratios of Zn/Sn at low temperatures. They reported that the device with ZnOSnO2 -NC with the ideal ZnO content of ~89 mol% has a higher PCE than ZnO. PCE of 14% was attained using ZnO-SnO2 -NC (2:1 wt. ratio in solution) as the ECL. Guo

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et al. [173] followed a straightforward, one-step, solution processed to synthesize a TiO2 /SnO2 -NC ETL. They achieved a PCE of 16.8%, JSC of 19.9 mA/cm2 , VOC of 0.83 V and reported that the use of the TiO2 /SnO2 ETL increases the stability of PSCs substantially. Moreover, G.S. Han and co-workers [174] synthesized PSC having a NC containing reduced graphene oxide (rGO) and mesoporous (mp)-TiO2 . This device showed better electron extraction property owing to lower interfacial resistance. They reported that the devices displayed a rise in JSC , VOC and FF in comparison to devices having mp-TiO2 . In the device with 0.4 vol.% rGO/mp-TiO2 NC, they reported that JSC , VOC and FF increased from 9.6 mA/cm2 to 21.0 mA/cm2 , 0.86 to 0.91 V and from 66.8% to 70.8%, respectively. Xiaorni Jia et al. [175] demonstrated the use of ZnO:PFN nanocomposite as the cathode buffer layer (CBL). On the perovskite/PC61 BM surface, this NC could form a compact and defect-free CBL film. In addition, the high conductivity of the NC film allows it to perform well with 150 nm film thickness. The device with ZnO:PFN CBL gave the best PCE of 12.76%. Whereas the device without CBL gave a PCE of 9%. Their study showed that using a ZnO:PFN composite buffer layer to enhance the stability and efficiency of PSCs is a viable option. Batmunkh and co-workers [176] reported the use of single-walled carbon nanotube/SWCNTs in the mesoporous photo-electrode which resulted in a substantial improvement in the device performance and stability. They revealed that the use of SWCNTs provided quick electron transfer and improved the overall performance of the PSCs by favourably shifting the conduction band minimum of the TiO2 photo-electrode. The device they fabricated displayed a PCE of about 16.11%. Furthermore, Chandrasekhar and Komarala [177] revealed the use of graphene/ZnO nanocomposite (G/ZnO NC) as an ETL in PSCs. They used a spray deposition method to deposit pristine ZnO and G/ZnO NC layers. They reported that a graphene concentration of 0.75 wt.% in the G/ZnO NC film gave an optimum device performance. They reported an increase in JSC and PCE from 15.54 mA/cm2 to 19.97 mA/cm2 and 7.01% to 10.34%, respectively, with the use of G/ZnO nanocomposite. Iqbal et al. [178] reported the use of reduced graphene oxide/tin oxide (RGO/SnO2 ) composite as transparent nanoelectrodes for PSCs. They used modified Hummer’s method for the synthesis of GO and sol-gel method to prepare SnO2 solution. The addition of GO increased the absorbance of visible light. They concluded that the results obtained a different way to study the GO/TSnO2 nanostructures. Moreover, Garcia and co-workers [179] synthesized ZnO-TiO2 nanocomposite ETL using a low-temperature-processed method. They revealed that the PSCs fabricated using ZnO-TiO2 nanocomposite gave a JSC of 13.5 mA/cm2 , VOC of 0.2V, FF of 34.4, PCEavg of 0.73% and PCEmax of 0.93%.

3.3.2

Nanomaterials Used as Photo-Active Layer

The main layer of a solar cell is the photoactive layer, which is responsible for light absorption and photoelectric conversion. It also serves as a charge carrier in some photovoltaic cell types. Here, we have studied the different use of nanomaterials in photoactive layer. Liu et al. [166] fabricated a bulk heterojunction inverted

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planar perovskite solar cell by adding ZnO nanoparticles in perovskite absorber layer. The growth of perovskite grain size is induced by zinc oxide NPs that act as catalytic centres. This improved the film quality of perovskite material and resulted in better electron mobility as well as charge extraction. As the result of this hysteresis-free PCE of 17.26% is reported while specifically controlling the concentration of zinc oxide NPs in CH3 NH3 PbIx Cl3-x :ZnO BHJ absorber layer. High FF of 77.3% and JSC of 22.71 mA/cm were also reported. They concluded by saying that the BHJ structure synthesized by intermixing perovskite and metal oxides is very significant to the growth and commercialization of upcoming PSCs. Their devices with perovskite:zinc oxide composite (1.5 mg/mL) film maintained a PCE of 16.86% in spite of 960-hour degradation (greater than 97% of the original PCE value). While the PCE of the solar cells without ZnO nanoparticles maintained only 86% of the initial one. Increase in current density was responsible for a significant advancement in the performance of PSCs with zinc oxide NPs. The device using perovskite:zinc oxide (1.5 mg/mL) showed greater IPCE intensity as well as continuous improvement in almost entire visible range. The grain size of the crystals of perovskite material progressively increases with the increase in zinc oxide concentration. Larger perovskite grains help in evading the energy loss in the PV process, resulting in a high VOC . Furthermore, Yang and co-workers [167] incorporated nanoparticles/nanotubes (TNNs) of TiO2 into the absorber layer of perovskite solar cells. The TNN-containing cells displayed a significant enhancement in JSC , from 23.9 to 25.5 mA/cm2 (with nanotubes). They reported a PCE of 15.32% and concluded that the TiO2 NTs boosted the charge transfer and collection. Du et al. [180] presented an organic bulk heterojunction {poly(3-hexylthiophene2,5-diyl):[6, 6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM)} absorber layer to increase the charge extraction and to broaden the spectral absorption (650 nm). They optimized the thickness of P3HT:PSBM absorber layer to achieve a higher PCE of 8.94% that is much greater than 6.28% for the pristine CsPbBr3 PV device. They reported that the unencapsulated device maintained outstanding long-term stability for over 75 days even when kept under persistent attack by 70% humidity in air. Moreover, Huang and co-workers [181] developed a novel technique to reduce the water absorbance of the perovskite material. They combined the CH3 NH3 PbI3 solution and exfoliated montmorillonite (exMMT) to form a protective covering of exMMT on CH3 NH3 PbI3 crystals. They revealed that this technique helped to improve the long-term stability of PSCs and reduced the hysteresis if fullerene is used as the top ETL. They reported that the perovskite layer integrating exMMT (0.01 wt.%) remarkably protected the perovskite device against light and humidity. They concluded that this novel method would effectively increase the lifetime of PV devices based on organo-metal halide perovskites and as this device modification method is simple and inexpensive, it could be further explored for large-scale production. Ahmed et al. [182] fabricated PSCs based on MAPbI3 :ZnO bulk heterojunction absorber layer. They used two different concentrations of ZnO nanoparticles. They used the following structure: ITO/NiOx /MALI:ZnO/ZnO/Al. They revealed that the best performance was shown by MAPbI3 :ZnO (1 mg/ml) solar cell with PCE of 8.97%. This device retained 83.82% of the initial value even

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after 30 days under ambient air. They concluded that the bulk heterojunction layer enhanced the stability of the PSCs in ambient atmosphere. Furthermore, Bi et al. [190] fabricated PSC using aliphatic fluorinated amphiphilic (FEAI) as an additive to CH3 NH3 PbI3 absorber layer. They revealed that this boosted the stability without affecting the device performance. This is accomplished by carefully screening the perovskite precursor solution with a molar ratio of FEAI/PbI2 ranging from 1 to 8 mol.%. The best performance was shown by device with 3 mol.% FEAI as additive. They reported a VOC of 1.06 V, JSC of 21.2 mA/cm2 , FF of 0.79 and a PCE of 18.0%. Hadadian and co-workers [191] synthesized a composite of heteroatom-doped graphene (N-RGO) and organic–inorganic lead halide perovskite in the absorber layer of the solar devices. The device structure used is FTO/TiO2 /FA0.85 MA0.15 Pb(I0.85 Br0.15 )3 -N-RGO/spiro-OMeTAD/Gold. They integrated N-RGO with the perovskite material by dispersing N-RGO in the perovskite precursor solution. They reported a PCE of 18.7%. They also revealed that the FF and JSC also increased due to the large grain size and denser perovskite film. Whereas surface passivation of the perovskite film led to rise in VOC. They concluded that their approach is a novel and effective tool for the advancement of the PSCs. Moreover, Wang et al. [192] synthesized MAPbI3-x Clx -NiO composite layer by introducing Al2 O3 /NiO at the TiO2 /perovskite interface. They followed the following structure: FTO/c-TiO2 /mp-TiO2 /Al2 O3 /NiO/CH3 NH3 PbI3-x Clx -NiO/spiro-OMeTAD/Gold. This approach displayed a PCE of 18.14% and outstanding reproducibility. They revealed that the devices even without encapsulation showed a substantial improvement in long-term stability and maintained nearly 100% of the initial VOC , about 94% of the initial JSC , nearly 91% of the initial FF and about 86% of the original PCE for over 210 days in an ambient atmosphere. Chen et al. [193] fabricated effective plasmonic Gold nanoparticles: QD-CsPbBr3 /PEDOT:PSS/CH3 NH3 PbI3 PSCs by incorporating Gold NPs in perovskite QD-CsPbBr3 that manages efficient light harvesting to improve PSCs. When compared to the original PSCs, the PCE of the plasmonic PSCs increased by 27.8%. They reported a VOC , JSC , FF and PCE of 0.9 V, 22.5 mA/cm2 54.0% 10.9%, respectively. Moreover, Bag and co-workers [194] incorporated MWCNTs in the bulk of the absorber film of PSCs. This reduced charge recombination and enhanced the VOC . They revealed an approximately 87% decrease in recombination rate that was reached by the introduction of MWCNTs in the PSC containing mixed counterions. The VOC of perovskite/MWCNTs solar cells was improved by 70 mV, while the JSC and FF remained unchanged. They concluded by saying that their method of CNT integration into the perovskite was well suited to both conventional and inverted device structures. Balis et al. [195] incorporated rGO nanoflakes into the ITO layer, the photoactive layer and the HTL of the solar device. When added in CH3 NH3 PbI3 , rGO improved the grain size of perovskite and created a more homogenous film morphology. This led to greater crystallinity thus enhancing the overall performance of the PSC. Whereas the presence of rGO in Spiro-MeOTAD is disadvantageous for the device performance. Incorporating rGO as an additive in both the ETL and the photoactive layer improved the performance of a MAPbI3 -based PSC for the first time in the literature. As a result, they were able to generate devices with optimal electrical characteristics, which resulted in a

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stable 13.6% PCE, surpassing the reference (rGO-free) devices by 20%. Furthermore, the inclusion of rGO provided extra stability to the devices, which kept 40% of the original PCE even after 50 days in a somewhat damp, dark environment. They reported JSC , VOC and FF of 22.9 mA/cm2 , 1.00 V, 72%, respectively. Moreover, Liang et al. [196] revealed that TiO2 nanotube arrays with optimum tube diameter and length could ease perovskite precursor solution penetration, resulting in the creation of a dense, smooth and large grain-size perovskite layer. The produced PSCs displayed a remarkable PCE of nearly 14.5% attributed to the superior perovskite layer, strong interfacial contact at the perovskite/TiO2 tube interface, better light harvesting, and quick electron collection and transport generated by one-dimensional TiO2 NTs. Zhang and co-workers [197] integrated sulfonate CNTs in CH3 NH3 PbI3 perovskite precursor solution during the spin-coating process. They revealed that this led to improved perovskite grain size and filled grain boundaries with the s-CNTs. When compared to pristine CNT integrated PSCs, the performance of s-CNT integrated PSCs improved dramatically from 10.3% to 15.1%. Furthermore, Xu et al. [198] synthesized carbon quantum dots (CQDs) from two separate carbon sources, namely, A-CQDs and CA-CQDs. They used them as additives in a one-step MAPbI3 precursor solution. The integrated perovskite layer of A-CQDs is discovered to have a larger grain size, superior crystallinity, boosted carrier extraction ability and a highly hydrophobic surface. With a maximum PCE of 13.28%, carbon-based PSCs with A-CQDs additive exhibited improved device performance. They stated that the PCE of PSCs modified by CA-CQDs additive is 7.85%, whereas the PCE of PSCs without additive is 10.50%. They discovered that the A-CQDs additive enhanced device performance, showed excellent long-term stability and sustained more than 90% of the original PCE after 200 h.

3.3.3

Nanomaterials Used as Hole Transport Layer (HTL)

The HTL is vital for migrating holes and stopping electrons from reaching the opposite electrode. HTL’s main function is to collect and transport holes from the photoactive layer, minimizing electron–hole pair recombination in perovskite materials. Here, we have studied the different nanomaterials used as HTL in perovskite solar cells. Chi et al. [168] prepared NiO2 NPs using chemical precipitation method for enhanced hole extraction in carbon-based PSCs. By optimizing the concentration of NiO-NPs, PCE of carbon-based perovskite solar devices reached 13.6%. The structure they followed is FTO/TiO2 /CH3 NH3 PbI3 /NiO-NPs/C. The best performance was with 25 mg/mL NiO. They reported maximum PCE of 13. 6% with 22.49 mA/cm2 JSC , 0/97 V VOC and 62% FF. They concluded that their work provided a facile and inexpensive process for preparing organic–inorganic hybrid PSCs with high PCE and stability. Furthermore, Lee and co-workers [169] enhanced carrier mobility and conductivity of PSC by incorporating multi-walled carbon nanotubes (MWNTs) in Spiro-OMeTAD. The hierarchical structure of pure SpiroOMeTAD and Spiro-OMeTAD/MWNTs was designed to prevent back-electron flow

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and fully use Spiro-OMeTAD/MWNTs’ enhanced charge transport. The concentration of MWNT was kept to be 2 wt.%. The structure followed is FTO/biTiO2 /CH3 NH3 PbI3 /Spiro-OMeTAD/MWNTs/Gold. The PCE, FF, and JSC were reported to be 15.1%, 69% and 21.6 mA/cm2 , respectively. Moreover, Gil et al. [170] incorporated CuCrO2 nanoparticles into PTAA as HTL in PSCs. CuCrO2 nanoparticles were manufactured hydrothermally. They used a facile spin-coating method and consecutively coated CuCrO2 NPs and PTAA on perovskite. This resulted in uniform HTL with outstanding coverage. Integration of stable CuCrO2 to the device enhanced its stability in heat and light. After nearly 900 h of storage in 85 °C/85% relative humidity and constant solar illumination at MPPT, encapsulated PSCs with CuCrO2 /PTAA HTL preserved over 90% of the initial efficiency. They have reported a VOC of 1.02V, JSC of 22.8 mA/cm2 , FF of 75% and PCE of 17.4%. The device structure they followed was ITO/SnO2 /Cs0.05 (FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 /CuCrO2 NPs/PTAA. Hu and co-workers [183] synthesized a HTM composite using P3HT polymer with copper (II) phthalocyanine nanostructure for PSCs. They revealed that the composite material displayed more appropriate energy levels and improved hole extraction capability. These composite HTM devices showed better PV performance and achieved efficiency of nearly 16.61%. Moreover, the stability of solar devices was also enhanced; the devices maintained 90% PCE in 800 h. They concluded that the inexpensive and facile production methods for P3HT and phthalocyaninebased nanocomposite materials would be important hole transporting materials in the development of perovskite devices with outstanding device performances, as well as enhanced long-term stability. Wang et al. [184] substituted the hygroscopic spiroMeOTAD with hydrophobic P3HT and used Au-NPs as hole transporting material. The best device performance was shown with a doping concentration of 20% and the values reported for FF, JSC , VOC and PCE are 64.79%, 22.05 mA/cm2 , 0.75 V and 10.71%, respectively. They concluded that this approach could also be used for small molecules or other conjugated polymers for further development and advancement of organic optoelectronic devices. Furthermore, Giuri et al. [185] doped graphene oxide (GO) and C6 H12 O6 in PEDOT:PSS to create a new nanocomposite. They revealed that the synergistic effect of these dopants altered the surface wettability of PEDOT:PSS and enhanced the composite’s charge transferring characteristics. The best results were displayed by GGO-PEDOT device with JSC , VOC , FF and PCE of 17.6 mA/cm2 , 1.05 V, 69% and 12.8%, respectively. Cogal and co-workers [186] synthesized nanocomposites using graphene (GR) with polythiophene (PTh) or PEDOT:PSS by plasma method. PTh–GR and PEDOT/GR composites demonstrated a variety of features, such as amorphous and high cross-linking structure, when compared to standard methods-assisted composites, depending on the type of plasma polymerization. The manufactured devices yielded PCE of 8.79% (for PEDOT/GR) and 4.95% (for PTh/GR). They concluded that by further optimizing the doping ratio, concentration and other parameters, the use of these materials in the manufacturing of PSCs can be made more economical and stable HTMs. Furthermore, Giuri et al. [187] studied the impact of graphene oxide (GO)-doped PEDOT:PSS thin nanocomposite on ITO anode, as HTL in PSCs. They concluded that their research is significant for the PV application, since the reducing GO could enhance the composite

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layer’s stability. Moreover, the use of GO in PEDOT:PSS promises better substrate coverage. Chen and co-workers [188] were the first one to report a HTL combining Cu/Cu2 O and a spiro-OMeTAD in conventional heterojunction PSCs. They revealed that their research increased the stability, hole collection ability and performance of the device. A high mobility of about 60.5 cm2 /V-s was observed for Cu/Cu2 O layer (20 nm thick) with an oxygen flow ratio of 60%, resulting in a higher PCE of 17.11% for the perovskite device when compared to the standard solar cell having spiroOMeTAD as the HTL (PCE of 13.97%). The Cu/Cu2 O layer proved to be more durable in ambient circumstances, according to the stability test. Moreover, Giuri et al. [189] studied a nanocomposite buffer layer for PSCs, based on polyelectrolyte PEDOT:PSS and GO. The nanocomposite exhibited a good thermal stability during annealing (140 °C) or at exercise temperature (RT) of the solar device. Additionally, the perovskite precursors’ high wettability on the PEDOT:PSS+GO UV-reduced nanocomposite confirmed that it is acceptable for use in a hybrid PSC. PVs having PEDOT:PSS+GO nanocomposites were evaluated with and without UV treatment to determine the effect of UV-reduced GO on the performances of the devices.

3.4 Promises of PSCs Possibilities prevail for technologies that assure either considerably high PCEs or remarkably lower fabrication cost [78, 79]. The novel generation of organic–inorganic halide perovskites offers fascinating outlook on both fronts. Some unique features of the perovskite materials are high absorbance, tunable bandgap, long charge carrier diffusion length, and high PCEs using cost-effective materials and facile device production methods, making them ideal for photovoltaics and other optoelectronic applications. The absorption coefficient is the most significant attribute for an active layer. The greater the absorption coefficient, the thinner the photoactive layer is required to effectively absorb light. Perovskite material (CH3 NH3 PbI3 ) has absorption coefficient of 1.5 × 104 per cm (at 550 nm) and 4.3 × 105 per cm (at 360 nm) that allows sufficient absorption even with a small film thickness [121]. That implies only about 100 nm−1 μm of perovskite layer is sufficient to effectively absorb most of the incident photons. The property of high absorption coefficient permits thin films at about 500 nanometre to completely absorb the visible spectrum [122] (Fig. 16). Bandgap is a crucial factor for a PV material, as according to Shockley–Queisser limit the maximum theoretical efficiency that can be attained depends on the bandgap of the material. The bandgap of material can be determined using Tauc plot (graph between α2 vs hυ) [124]. The perovskite materials have shown a direct bandgap of 1.58 eV [125]. According to the Shockley–Queisser limit, the maximum efficiency that can be attained from a single-junction PSC is 31%. The maximum efficiency attained from a double-junction tandem PV cell as perovskite as high bandgap material and either crystalline silicon or CIGS as low bandgap material is approximately 44% [126]. Thus, concluding that perovskite can give enormous enhancement in

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Fig. 16 Comparison of absorption coefficient at distinct wavelengths among various PV materials. Reprinted with permission from [123] Attribution 4.0 International (CC BY 4.0) copyright (c) 2014 OSA

PCE. Hoke et al. revealed a remarkable feature of perovskite in 2015. They revealed that by changing the relative composition of Iodine and Bromine for a mixed halide perovskite (CH3 NH3 PbI3-x Brx ), the bandgap can be changed from about 1.55 eV to nearly 2.20 eV [127]. Thus, showing that the bandgap of PSC is tuneable. The carrier diffusion lengths of perovskite materials are very high. Numerous groups have stated that the diffusion length is of the order of few micro-metres [128– 130]. With the help of time-resolved terahertz spectroscopy, La-o-vorakiat et al. have calculated the diffusion length of carriers at varied temperatures. They reported that the diffusion length could surpass 1 μm. The diffusion length measured at normal temperature could be different from that calculated at low temperature. This reveals that the recombination process changes with temperature changes [130]. Some research groups have revealed that perovskite has very high carrier mobility and lifetime of the carrier, resulting in longer diffusion lengths and greater collection efficiency [90, 128–130].

3.5 Current Status: Challenges and Possible Solutions PSCs have a lot of advantages as discussed above. However, perovskite materials have some major issues, viz. long-term stability and toxicity [80]. Lead being an important constituent of almost all high-performing perovskite devices raises toxicity issues during device fabrication, deployment and disposal [81]. The key barrier to the commercialization of PSCs is the stability concern. PSCs degrade in the ambient environment due to humidity and oxygen [131, 132]. They also degrade at high temperature or the exposure of high-intensity sunlight [133]. Preliminary investigations have revealed that the material is unstable when exposure to moisture because of its ionic nature and the hygroscopic amine component present in the material. It is seen that water could convert the perovskite material back to its precursors. The perovskite material gets decomposed into methyl ammonium

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iodide (CH3 NH3 I) and lead iodide (PbI2 ), followed by decomposition into methyl ammine (CH3 NH2 ) and hydrogen iodide (HI) as represented with the help of a series of chemical reactions under different conditions [134]. CH3 NH3 PbI3 (s) → PbI2 (s) + CH3 NH3 I(aq)(in presence of moisture) CH3 NH3 I(aq) → CH3 NH2 (aq) + HI(aq) 4HI(aq) + O2 (g) → 2I2 (s) + 2H2 O(1)(in presence of oxygen) 2HI(aq) → H2 (g) + I2 (s)(exposure to UV light) This effect is even accelerated in presence of an applied electric field. It is seen that if the perovskite is exposed to light for a longer duration it self-decomposes. This can be simply observed as the colour of the perovskite film changes from black to yellow as shown in Fig. 17. Therefore, these devices are usually produced in a glove box filled with inert gas to attain high efficiency [134]. Moreover, because of the influence of humidity, storing the solar cell under atmospheric condition also causes degradation of perovskite materials. It has been revealed that a humidity of 55% or higher would cause notable decrease in the device performance with CH3 NH3 PbI3 because of the degradation of the light-absorbing materials [135]. Leguy et al. have studied the hydration mechanism of perovskite active layer, single crystals and devices. CH3 NH3 PbI3 breakdown was characterized by reversible hydration and irreversible decomposition [136]. They concluded that the key reason for the variation in the reversible and irreversible deterioration phase was the existence of condensed water. In the manufacturing procedure of PSCs, low-temperature sintering is generally used. The sintering temperature has been reported to have a crucial influence on the morphology of perovskite film, solar cell stability and PV efficiency [137, 138]. The Fig. 17 Degraded perovskite film. Source OENRL, JMI (Author)

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study by UV–visible absorption spectroscopy, Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) of the perovskite layer synthesized at various temperatures have shown that a temperature of 80 °C is crucial for complete conversion of MAI and PbI2 to CH3 NH3 PbI3 . Further rise in the sintering temperature, however, could cause changes in the perovskite film morphology and bigger crystalline particles will be produced on the surface of the TiO2 mesoporous layer substrate. Tan et al. studied the thermally induced structural evolution of MAPbI3-x Clx by in situ grazing incident broad angle X-ray scattering and scanning electron microscopy/SEM studies [139]. Based on this study, the perovskite materials can go through the following stages in thermal annealing: 1. 2. 3.

At normal temperature, a crystalline perovskite precursor structure. Structure of perovskite at medium temperatures (below 120 °C). A number of compounds including PbI2 formed as a result of degradation at elevated temperature.

Annealing temperature, film thickness and atmosphere of the environment are responsible for the transition between the above-discussed structures. Furthermore, Tan et al. reported that the crystalline MAPbX 3 precursor went through a solid–solid phase transformation forming perovskite structure when annealed at 80 °C for 10 min. The complete transition was achieved at 90 °C [139]. Whereas the phase transformation was reduced to 3–10 min in the inert atmosphere at 100 °C. Thereafter, at about 20–30 min of annealing, degradation of the film into PbI2 began. The presence of moisture or damage from X-rays was the cause of the deterioration. PSCs work in the presence of light, whereas perovskite absorber and charge transport materials are usually degraded in the presence of UV radiation [103]. Schoonman studied the photolysis of the binary lead halides [140]. He reported that PbX 2 under UV irradiation formed metallic lead within short irradiation time. If MAPbX 3 showed the same sort of decomposition mechanism like the binary lead halide, then it would also contribute to the instability of perovskite devices. Therefore, it is essential to study the characteristics of the charge carriers generated in CH3 NH3 PbX 3 and trapping of the electrons at Pb ions. Moreover, Wei et al. also revealed that the chemical structure of organic and inorganic parts influences the degradation of organic–inorganic lead halide perovskite under UV light [141]. The stability of crystals is also influenced by their spatial arrangement. They recommend using fluorinated perovskite to decrease the oxidation of perovskite material caused by ultraviolet light. In order to enhance the stability of perovskite solar cells, different techniques can be used such as solvent engineering, optimization of perovskite layer, optimization of charge transport layer (ETL/HTL), encapsulation and so on. Continuous efforts have been made by scientists to increase the stability of the perovskite absorber materials. This has led to the discovery of some suitable methods that help in enhancing the stability of these materials. Any modification that improves the robustness of the material would be beneficial. Habisreutinger et al. developed a smart methodology for attaining long-term stability of PSCs [142]. They reduced thermal degradation of

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Fig. 18 Visible degradation of the perovskite layer. Reprinted with permission from [142]. Copyright (2014) American Chemical Society

PSCs by substituting the organic HTM with polymer-functionalized single-walled (Fig. 18). This colour and structural change might begin due to the formation of weak hydrogen bonds between H2 O and moisture-absorbing methylammonium cations that leads to a bond dissociation of the crystal constituents. Thus, the unbound methylammonium iodide (MAI) would escape the compound structure and a residual layer of lead iodide would be left behind. Interfacial engineering is also used in order to enhance the stability of perovskite devices. Here, ultrathin Al2 O3 layer is used under or above the spiro-OMeTAD hole transport material which helps to boost the stability of perovskite solar cells [143]. The atomic layer deposition technique was used for depositing aluminium oxide layer in order to fabricate continuous, precise and pinhole-free oxide layers. The aluminium oxide layer functioned as an effective oxygen and moisture barrier. The cells with the FTO/TiO2 /CH3 NH3 PbI3 /Al2 O3 /Spiro/Ag structure, on the other hand, exhibited CH3 NH3 PbI3 breakdown due to the employment of the heavy oxidant O3 for Al2 O3 deposition. Therefore, devices with improved structure of FTO/TiO2 /CH3 NH3 PbI3 /spiro/Al2 O3 /Ag were produced, which showed no presence of PbI2 , signifying better stability. It must be stated that the thickness of Al2 O3 layer plays a significant role in the device performance. A very thick layer of Al2 O3 will decrease the quantum tunnelling of the holes and thus decrease the charge collection. Niu et al. have investigated the effect of Al2 O3 on the degradation of PSCs due to the presence of moisture [144]. They stated two key roles for Al2 O3 :

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(i)Shielding perovskite active materials and sensitive films against degradation due to humidity and UV radiation. (ii)Delays the recombination process by performing the role of a barrier between hole transporting material and TiO2 . Moreover, Leijtens stated that by substituting the TiO2 mesoporous layer with Al2 O3 , the photo-stability of the device could also be improved [104, 107]. Under constant light exposure (1000 h) of AM 1.5 solar spectrum at 40 °C, the TiO2 free cells revealed stable performance. Light-induced degradation could reduce the performance of organic metal halide PSCs in the presence of UV radiations during long-term operation because of generation of several possible fading processes. Electron recombination thus generates localized trapping sites and reduces the depletion layer and TiO2 band excitation. This would cause oxidation of halogen atoms in the organic metal halide perovskite or hole transporting material arising because of UV radiation. There are several other factors that lead to the degradation of PSCs such as oxidation, heat and ordinary visible light. The main reason for degradation in most cases is UV irradiation and therefore is of great concern. Therefore, it is essential to use stable and broadly absorbing ultraviolet materials to guard equally the light absorber and the substrate by either starting a decomposition that proceeds by other mechanism like oxidation or by directly absorbing ultraviolet light [145]. Fortunately, the evolution of complex perovskite structures having temperature-stable formamidinium (FA) cation, the mixture of various organic and inorganic cations, the use of both iodine and bromine and the incorporation of 2D materials into the perovskite absorber film have all lead to enhanced stability of PSCs. The most stable perovskite PV devices have revealed no performance decrease for over 10,000 h in solar radiation [146]. Some reviews of key techniques used for the optimization of perovskite active layer to obtain high efficiency devices have been discussed, namely, the composition of the perovskite, encapsulation, as well as the solvent engineering of the perovskite solution and post-deposition operations done on the perovskite film. The selection of solvents taken to dissolve the perovskite could alter the film’s morphology in solution processing. Dimethylformamide/DMF, γ-butyrolactone/GBL and dimethyl sulfoxide/DMSO are widely used solvents to spin-coat perovskite products. The dissolving of perovskite precursors in many highly efficient perovskite devices is done with DMSO. The anti-solvent removes any unreacted organic material as the DMSO that stops the formation of perovskite crystal structure as it forms MAIDMSO-PbI2 complexes. DMSO is released from the film upon heating at 100 °C, and crystallization of perovskite occurs. This leads to formation of large crystal grains and improved film morphology [147–149]. The deposition of CH3 NH3 PbI3 active layers can be done using a volatile and clean solvent method based on a methylamine-bubbled acetonitrile perovskite precursor [150]. Post-deposition Engineering here deals with the annealing temperature required for perovskite layer. The sintering temperature selected to eliminate solvent and crystallize the perovskite absorber layer could influence the solar cell performance. Excess organic component is removed from perovskite through annealing which

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Fig. 19 Diagram of the solvent annealing process and SEM images of perovskite layer before and after solvent annealing. Reprinted with permission from [153]. Copyright (2015) American Chemical Society

results in crystallization of the perovskite. Crystallization of methyl ammoniumbased PV devices must take place at temperatures lower than 110 °C, as enhancement in halide deficiencies occur in films at higher sintering temperatures [151]. The application of vapour or solvent annealing has witnessed to enhance the device performance. The existence of extra solvent gives rise to substantially bigger crystal grain sizes, causing improvement of device JSC [152]. The perovskite material is exposed to a solvent vapour at a higher temperature in this process (Fig. 19). A quasi-stable liquid-phase atmosphere is set between a polar solvent used to dissolve CH3 NH3 PbI3 surfaces and grain boundaries during solvent annealing [153]. This process results in growth of grains until the conditions are no longer favourable [154]. Compositional engineering has been used for enhancing stability of PSCs. There are several effective perovskite precursors that facilitate the production of highperformance photovoltaic devices. MAI:PbX2 (3:1) with X = Chlorine (Cl) or Acetate (Ac) perovskite precursors have been used repeatedly as an alternative for MAI:PbI2 (1:1) perovskite precursor. In a study linking various lead sources revealed that the average power conversion efficiencies of devices employing PbAc2 , PbI2 and PbCl2 were found to be 14.0%, 9.3% and 12%, respectively. This indicates the benefit of using a different lead source [151]. MAAc is the by-product released when lead acetate (PbAc2 ) is used in MAI:PbAc2 perovskite layers. It decomposes at 97.4 °C, which makes it energetically favourable in comparison to other possible by-products such as MACl and MAI. The estimated release temperature for organic by-products is 226.7 °C for MACl and 245 °C for MAI. At temperatures below 100 °C, these by-products are known to sublime from a forming

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perovskite film, with the perovskite film only resembling a CH3 NH3 PbI3 structure when the organic components have totally sublimed. The MAI:PbAc2 approach produces ultra-smooth perovskite films that are comparable in quality to perovskites fabricated through vacuum-aided evaporation [151, 155]. PSCs made with the lead acetate approach, on the other hand, are extremely unstable. The performance of non-stoichiometric precursor solutions of perovskites when utilized to manufacture photovoltaic devices has been studied. Excess MAI perovskite solutions are typically linked with perovskite layer with better grain alignment and bigger grain sizes, while excess PbI2 has been revealed to persist in the perovskite film. It decreases the density of charge trap states by acting as a humidity barrier at grain boundaries and interfaces [156, 157]. The addition of cesium to multi-cation PSCs with a triple-cation active layer structure (CsI0.05 ((FAPbI3 )0.85–0.83 (MAPbBr3 )0.15–0.17 )0.95 ) has revealed manufacturing of devices with PCEs above 21% [158]. Quad-cation perovskites were created by adding potassium as a molar proportion (0–10%) of the total monovalent cations (MA, FA, Cs). This permitted for the development of solar cells with restrained ion movement and lower non-radiative losses [159]. These progressive perovskite compositions are stated to attain larger grain dimensions, passivated defect, delayed halide migration and phase segregation, boosted grain luminesce and decreased the amount of non-luminescent grains. Encapsulation is also done to enhance the stability of PSCs. Encapsulation must be transparent, cheaper, lightweight, and should be able to block moisture and oxygen entrance. Encapsulation can be mounted on a photovoltaic module’s back or front. By having good clarity, any front facing encapsulation can aim to increase the collection of photons, and probably have an anti-reflecting coating. To maximize photon collection, a coating of reflective materials can also be applied to the back of the encapsulated surface. Many of the degradation effects that destabilize perovskite solar cells will be avoided by encapsulation. The ability of ultraviolet-triggered epoxies to completely encapsulate organic photovoltaics has been demonstrated, with T80 lifetimes exceeding 10,000 h, thus working effectively under real-world conditions [160, 161]. However, perovskite solar cells encapsulated with ultraviolet curable epoxies don’t usually reveal long-term stable device performance metrics [162–164].

4 Future Scope In recent years, the solar cell industry has seen a rise in the production of extremely efficient solar panels and solar cells. However, present technology is insufficiently efficient, and manufacturing solar cells for large-scale electricity generation is too expensive. On the contrary, there have been possible breakthroughs in technology that could improve the ability to make more cost-effective and efficient solar cells. Furthermore, nanotechnology has the potential to improve the efficiency of solar cells, the most attractive application of nanotechnology in the solar cell sector is

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the significant reduction in production costs. Perovskite solar cells using nanomaterials are being developed in order to lower the cost and increase the amount of energy generated. As a result, using nanotechnology to design and fabricate lowcost perovskite solar cells significantly aids in environmental preservation. Nanophotonic structures, as opposed to traditional methods such as surface texturing, are a superior alternative for solar cells since they do not affect the surface topography of thin films, which has a significant impact on device performance. The manufacturing cost of these structures must be greatly lowered in future development, especially for big area substrates. They are unlikely to be employed in commercial manufacturing otherwise.

5 Conclusion Nanomaterials have shown unique optical, magnetic, electrical and other properties. These materials are ideal for a variety of applications including electronics, medicine, cosmetics, etc. One of the most important applications of nanomaterials is in the fabrication of efficient and stable solar cells. Solar energy is considered to be the most plentiful, reliable and best alternative for conventional sources of energy. The potential of solar energy is enormous, the energy received by sun in 1.5 h is ample to satisfy yearly global energy demands. Silicon solar cells are the most commonly used solar cells and dominate the photovoltaic market by 80%. The reason for silicon solar cells being so famous is because of its abundance, non-toxicity, high efficiency and longer lifetime. The problem with silicon solar cells is its low absorption and high cost, so we moved further to the next generation of solar cells as discussed earlier. In this search, perovskite solar cells (PSCs) have gained considerable attention in recent years because of their advantageous features such as high absorbance, low production cost and ease of fabrication. Herein, we have focused on the different applications of nanomaterials in perovskite solar cells. Furthermore, we have discussed about the main barrier to the commercialization of PSCs and the different techniques that can be used to overcome these problems such as solvent engineering, optimization of perovskite layer, optimization of charge transport layer (ETL/HTL), encapsulation, etc. Nanomaterials used as ETLs, HTLs and absorber layer have been studied. The use of nanomaterials in perovskite solar cells has led to a significant enhancement in the device performance. It is seen that the long-term stability also increased. Moreover, key techniques used for the optimization of perovskite active layer to obtain high efficiency devices have been discussed in this search, namely, the composition of the perovskite, encapsulation, as well as the solvent engineering of the perovskite solution and post-deposition operations done on the perovskite. Continuous enhancements have been made to enhance the stability and efficiency of PSCs resulting in better performance of these devices. Further development of new technologies and methods is also essential for the enhancement of the stability of perovskite PVs and to ease their commercialization.

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Chapter 2

Graphitic Carbon Nitrides: Synthesis, Properties, and Applications in Perovskite Solar Cells Fareed Ahmad, Zishan H. Khan, and Sundar Singh

1 Introduction Graphitic carbon nitride (g-C3 N4 ) is a metal-free, polymeric, organic semiconducting material that serves as an efficient, low-cost, and stable catalyst [1–3]. It is the most stable allotrope of carbon nitride. The structure of polymeric graphitic carbon nitride, especially nanosheets (NSs), is very analogous to two-dimensional (2D) sp2 hybridized sheets of graphene where there is a presence of the nitrogen atom among carbon atoms [4, 5]. In the nanosheets morphology, g-C3 N4 has a high surface-tovolume (S/V) ratio which is very useful for catalytic applications. The carbon-tonitrogen (i.e., C/N) ratio in graphitic carbon nitride is ¾, so these are nitrogen-rich compounds. Graphitic carbon nitride has two polymeric structural isomers composed of chiral molecules [6]. Chiral molecules basically superimpose over their mirror image [7, 8]. They are symmetric about any number of rotational or transformational lines of symmetry. The isomers are s-triazine composed of units (ring of C3 N3 ) and tri-s-triazine composed of units (tri-ring of C6 N7 ) also known as heptazine [9] (Fig. 1). They also exist in amorphous and crystalline forms depending upon their synthesis methods. Graphitic carbon nitride nanomaterials have tunable structure and morphology that make them suitable for a wide variety of nanocatalytic applications [11, 12]. This polymeric semiconducting material is frequently utilized as visible lightinduced heterogenous photocatalyst. It is also suitable for utilization in ion-transport membranes, photoelectrochemistry, and also in organic photovoltaics. Graphitic carbon nitrides find applications in light-induced catalysis, batteries, biosensors, hydrogen evolution, etc. Incorporation of g-C3 N4 into polymers is usually done to F. Ahmad · Z. H. Khan Department of Applied Sciences and Humanities, Jamia Millia Islamia, New Delhi-110025, India S. Singh (B) Department of Physics, Bareilly College Bareilly, Bareilly -243005, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_2

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Fig. 1 a Triazine, b tri-s-triazine (heptazine) structures of g-C3 N4. Reprinted (adapted) with permission from [10], Copyright 2016, American Chemical Society

tailor the mechanical properties of the polymer. Due to its role as a photoinitiator, g-C3 N4 can be used for polymer synthesis. In fact, g-C3 N4 /polymer conjugates are used to introduce new properties in polymeric materials or to enhance the existing ones. Graphitic carbon nitrides possess high thermal and chemical stability. Some of the drawbacks associated with g-C3 N4 are the existence of a large number of lattice defects causing frequent recombination of photo-generated carriers, structural disorder, poor dispersibility, low processability, and low conductivity.

2 Synthesis There are several methods for the synthesis of graphitic carbon nitrides. The precursors for the development of graphitic carbon nitride are nitrogen-based compounds such as cyanamide (CH2 N2 ), melamine (C3 H6 N6 ), urea (CH4 N2 O), thiourea (CH4 N2 S), dicyanamide (C2 H4 N4 ), and cyanuric acid (C3 H3 N3 O3 ) [13]. All these precursors are metal-free, oxygen-free, abundant, and nitrogen-rich compounds. The most used synthesis method is the thermal condensation for the fabrication of bulk g-C3 N4 under an inert atmosphere in the temperature range of 400–600 °C. Other synthesis methods include chemical vapor deposition (CVD), electrochemical deposition for fabricating the film or membrane g-C3 N4 , and microwave method for the fluorescent g-C3 N4 . Different precursors undergo thermal condensation at different temperatures to produce bulk g-C3 N4 as shown in Fig. 2. Various nitrogenous precursors like melamine, cyanamide, dicyanamide, urea, and thiourea are subjected to thermal polymerization to give the finished product— graphitic carbon nitride [14–16]. The stereo chemistry and bondings of different precursors lead to their thermal polymerization occurring at different temperatures. One important reaction pathway is the development of graphitic carbon nitride using cyanamide as a precursor [17, 18]. This is illustrated in Fig. 3.

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Fig. 2 Synthesis of g-C3 N4 through the thermal polymerization of different precursors. Reprinted (adapted) with permission from [10], Copyright 2016, American Chemical Society

Fig. 3 Stepwise reaction in the formation of g-C3 N4 with cyanamide as the precursor [10]

In this reaction pathway, cyanamide is heated and changed to dicyanamide [19, 20], which is again heated and gets morphed to melamine which is again heated and get transformed into melem (C6 H6 N10 ) which finally is heated to give the final product—polymeric graphitic carbon nitride.

2.1 Ordered Mesoporous Graphitic Carbon Nitride Mesoporous graphitic carbon nitride is basically homogeneously organized nanosheets having pores of diameter ranging from 2 to 50 nm. Templates used are chiefly ordered mesoporous silica templates to initiate the development of graphitic carbon nitride [21, 22, 23]. The most commonly used silica template, the SBA15 (Santa Barbara Amorphous Type Material), is a mesoporous silica colander

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Fig. 4 Template method for synthesis of mesoporous materials; a soft-templating method and b hard-templating method (nanocasting) method. Reproduced from [28] with permission of The Royal Society of Chemistry

consisting of uniform hexagonal pores having a small pore size distribution and a tunable pore diameter in the range of 5–15 nm [24, 25]. Mesoporous g-C3 N4 is better than bulk g-C3 N4 as it has a high specific surface area and porosity [26]. Due to the availability of a large number of active sites, the performance is enhanced in catalytic applications [27]. There are various templating methods to develop mesoporous graphitic carbon nitride templates of which the two most common methods are soft-templating or self-assembly and hard-templating or nanocasting. The schematics are illustrated in Fig. 4. Using the SBA-15 silica template as mentioned before, there are two ways to develop ordered mesoporous graphitic carbon nitride [29]. Firstly, we treat the SBA15 silica template with cyanamide and water mixture. We stir the mixture for 4 h at room temperature. Finally, we get an SBA-15 silica cyanamide composite. It is heated to 550 °C for 4 h and then treated with NH4 HF2 for 24 h to finally obtain ordered mesoporous graphitic carbon nitride as illustrated in Fig. 5a. Another method is by using acidified HCl and SBA-15 silica template with liquid cyanamide. It is then vacuum sonicated for 4 h at 55 °C and then treated again with NH4 HF2 for about 24 h to obtain ordered mesoporous graphitic carbon nitride [30, 31]. This second approach is illustrated in Fig. 5b.

2.2 Graphitic Carbon Nitride Nanorods Graphitic carbon nitride nanorods (NRs) are one of the possible morphologies of nanoscale graphitic carbon nitride with every dimension lying in the range of 1 to

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Fig. 5 Ordered mesoporous g-C3 N4 synthesis techniques: a simple method and b a novel method combining acidification and vacuum-sonication processes. Typical c SEM and d TEM images of ompg-CN-2. CA and ompg-CN denote cyanamide and ordered mesoporous g-C3 N4 , respectively. Reprinted with permission from [32]. Copyright 2013, John Wiley & Sons, Inc

100 nm and aspect ratios 3–5. The g-C3 N4 NRs may be synthesized using metals or semiconducting materials. Nanorods can be produced through direct chemical synthesis [33]. One such method is using anodic aluminum oxide (AAO) templates which are ordered porous templates on which the semiconducting graphitic carbon nitride nanorods can be electrodeposited. They are treated with cyanamide and then heated under a nitrogen-rich environment [34]. The photons react with the rods releasing electrons and holes which oxidize and reduce hydrogen cation and water, respectively. At last, the AAO template is removed to yield graphitic carbon nitride nanorods [35] (Fig. 6). Another method of developing mesoporous graphitic carbon nitride NRs is by using other nanorods templates like SBA-15 silica nanorod template. The template is infiltrated with cyanamide [37, 38]. It further goes through thermal condensation, and once the graphitic carbon NRs are condensed, the SBA-15 silica/cyanamide template is removed through etching by NH4 HF2 and we finally obtain mesoporous graphitic carbon nitride [39]. Figure 7a shows the synthesis of mesoporous graphitic carbon nitride NRs using SBA-15 silica nanorod template.

2.3 Ordered Porous Graphitic Carbon Nitride One of the most important synthesis procedures of ordered porous graphitic carbon nitride is by using silica nanospheres (SNSs) having spherical diameters of 20, 30,

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Fig. 6 a Steps involved in the synthesis of g-C3 N4 NRs, (1) Filling the AAO template (gray) with cyanamide (white), (2) heating the cyanamide/AAO under an N2 environment, and (3) removal of the AAO template to yield g-C3 N4 NRs (yellow). b, c SEM images of g-C3 N4 NRs. The inset of c showing the HR SEM image of g-C3 N4 NRs. Reprinted with permission from [36]. Copyright 2011, American Chemical Society

Fig. 7 a Synthesis process of mesoporous g-C3 N4 NRs (denoted by m-CNR) (yellow) using SBA15 NRs template (gray). b TEM and c HRTEM images of mesoporous g-C3 N4 NRs. The insets of b and c show the diameter distribution and pore size distribution, respectively. Reprinted with permission from [40]. Copyright 2012, Royal Society of Chemistry

50, and 80 nm in size. They are treated with cyanamide (CNNH2 ) at 823 K to form a graphitic carbon nitride–silicon dioxide composite (g-C3 N4 /SiO2 ). Then finally the SiO2 is removed through etching by HF acid and we obtain ordered porous graphitic carbon nitride [41]. This synthesis approach is illustrated in Fig. 8a.

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Fig. 8 a Synthesis protocol of ordered porous g-C3 N4 . Field emission SEM (FESEM) images of porous g-C3 N4 synthesized using various silica spheres with diameters of b 20, c 30, d 50, and e 80 nm. Reprinted with permission from [42]. Copyright 2011, John Wiley & Sons, Inc

Another method to produce ordered porous graphitic carbon nitride is using the facile-Sulfur template-mediated approach. Melamine and Sulfur are grinded and heated together and a mixture is formed in which Sulfur is melted [43, 44]. Molten Sulfur is treated with ammonia (NH3 ) in which bubbles are generated which on further treatment with ammonia generate ordered porous graphitic carbon nitride [45]. The schematic along with the TEM image of Sulfur-mediated synthesis of ordered porous graphitic carbon nitride is shown in Fig. 9. A different method to produce highly porous graphitic carbon nitride is by the thermal condensation of sucrose and melamine mixture [45, 46]. The mixture is heated which results in the thermal condensation of melamine. It is followed by increasing the temperature and further heating to lose one NH3 unit [47]. We get graphitic carbon nitride by this phase. Further heating removes an additional NH3

Fig. 9 a Schematics for the synthesis of porous g-C3 N4 via a facile-sulfur bubble templatemediated approach. b TEM image of sulfur-mediated g-C3 N4 . Reprinted with permission from [45]. Copyright 2015, Royal Society of Chemistry

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Fig. 10 a Schematics for the synthesis of mesoporous sucrose-mediated g-C3 N4 . b UV– absorption spectra and c EIS Nyquist plots of unmodified g-C3 N4 and sucrose-mediated g-C3 N4 with various contents of sucrose. Reprinted with permission from [43]. Copyright 2015, Royal Society of Chemistry

unit. There is oxidation and decomposition of sucrose that produces carbonaceous gases resulting in the formation of highly porous graphitic carbon nitride [48, 49] (Fig. 10). Before discussing about the applications of graphitic carbon nitride in PSCs, let us first briefly review PSCs for developing a better understanding of applications of g-C3 N4 .

3 Perovskite Solar Cells: Brief Review Perovskite Solar Cells are basically named after the mineral Perovskite discovered by Lev Perovskite. Perovskites have the same crystal structure as that of the mineral calcium titanium tri-oxide (CaTiO3 ). It has a ABX3 structure in which A is basically an organic cation like methyl ammonium ion and B is a big inorganic cation like lead II cation. X3 is basically a smaller halogen anion usually chloride or iodide. Silicon, the primary semiconductor material used in solar cells, has semiconducting properties, which, for the most part, are perfectly matched with the spectrum of solar radiation. Moreover, this material is found in somewhat abundance and is fairly stable [50]. The drawback of using large silicon crystals in traditional solar panels arises due to multi-level manufacturing methods involving huge capital and large energy consumption. In the quest to get a better material as a replacement for silicon, the scientific community has strived hard to harness the tunability of perovskite materials to develop semiconducting materials possessing silicon-like properties [51]. Perovskite solar cells can be fabricated at the lowest cost and energy requirements

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Fig. 11 a Schematics of the basic crystal structure of perovskites, b charge separation and transportation in a PSC, and c, d planar and mesoporous device architectures, respectively. Reproduced with permission from [54], Copyright 2019, Elsevier

by the use of facile additive-deposition methods such as printing. The suitability of perovskites in solar cells arises because of possessing a flexible composition, these materials could be perfectly aligned with the solar spectrum for harvesting solar energy. The golden journey of PSCs started in the year 2012 when scientists could produce a stable, thin-film perovskites-based solar cell device giving more than 10% conversion efficiencies. This device used a lead-halide perovskites layer for the absorption of light. Since then, power conversion efficiency (PCE) of PSCs has increased tremendously with the laboratory efficiency reaching around 25.2% [52]. Tandem solar cells combining perovskite solar cells with normal silicon solar cells have recorded still higher efficiency, with the maximum efficiency reaching to 29.1% [53]. Figure 11 shows the basic crystal structure, charge separation and transportation mechanism, and the two device architectures—planar and mesoporous PSC.

3.1 PSC Operating Principle PSCs use materials having a structure similar to those of perovskites for absorbing light to be used in light-to-energy conversion dynamics. In a PSC, the perovskite material absorbs incident sunlight, and excitons are generated. The excitons then dissociate into electrons and holes (see Fig. 11b). The dissociation of excitons takes place at the contact layer of the perovskite absorber layer and the layer used for the transport of charge [55]. The separation of the electron from the hole and subsequently

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upon being pumped into the electron-transporting layer (ETL), it drifts to the positive electrode which is in general made of fluorine-doped tin oxide (FTO) glass. At the same time, the other charge carrier in the pair is created from exciton dissociation, i.e., the hole is pumped into the hole-transporting layer (HTL) and thereafter drifts to the (metallic) negative electrode. Thus, electrons are collected by the working (or photoanode) and the holes by the counter electrode (i.e., cathode) and are transferred to produce current in the associated circuit [12]. In this manner, perovskites play a very important role in carrier generation and energy harvesting in PSCs.

3.2 Perovskite Solar Cell: Device Architecture A PSC has a structure that comprises of a metal electrode, an ETL, a light-absorbing layer of perovskite material along with a mesoporous metal oxide, a HTL, and a FTO substrate [7]. Metal electrodes are made up of silver, gold, or platinum. They help in the collection of electrons and holes. They also prohibit the recombination of photogenerated charge carriers [2]. The ETL is basically made up of titanium dioxide (TiO2 ) which is a mesoporous structural framework. It transports the electrons to the electrode, block holes, and prevents the recombination at FTO substrate to increase the overall working efficiency of the solar cell [56]. The light-absorbing active layer of perovskite is embedded with porous and mesoporous metal oxide like zinc dioxide (ZnO), di-aluminum tri-oxide (Al2 O3 ), and zirconium dioxide (ZrO2 ). The photocatalytic generation of electron–hole pairs (EHPs) occur here. The HTL is made up of Spiro-O Me-TAD or PEDOT: PSS which transports the generated holes and blocks electrons (see Fig. 12). The FTO substrate is the electrode that collects holes and electrons and prevents their recombination.

Fig. 12 a Schematics of the ETL-free planar mixed halide PSC configuration and b energy level diagram of the planar PSC showing collection and separation of photo-generated charge carriers without an ETL [60]

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Sunlight falls on the solar cell and the perovskite layer absorbs it releasing EHPs [1, 57]. Electrons migrate to the anode or FTO substrate via the ETL and holes migrate to the cathode or metallic electrode via the HTL [58, 59].

4 Graphitic Carbon Nitride-Modified PSCs Nitrogenous precursors showed enhanced power conversion efficiency for PSCs as confirmed through several recent studies. By infusing the nitrogen functionalism in the organic–inorganic halide perovskite materials effectively passivated their surfaces against different kinds of defects [61, 62]. Lee and his group of researchers in 2017 documented that the defects present in the methylammonium lead tribromide (MAPbBr3 ) could be passivated via coordinate bonding among the nitrogen atoms on the one side and the insufficiently coordinated lead ions on the other side. This passivation resulted because of using the amine-based treatment over the film of perovskites [63]. Hsieh and his coworkers, in another work, made use of urea along with thiourea which were dissolved in the organic solvent dimethyl formamide (DMF) at a concentration of 25% by weight and agitated it for a night. Then such urea and thiourea dissolved in DMF were used as supplements in MAPbI3-x Clx and MAPbI3 perovskite solar cell devices [64]. In both scenarios, the power conversion efficiency was enhanced. The perovskite solar cell using MAPbI3 showcased a power conversion efficiency of 18.8% against 14.6% that observed for the standard cell. The characterization of the supplements was done by the authors, containing perovskite absorbers by using the techniques such as ultraviolet–visible (UV-vis) spectroscopy, scanning electron microscopy (SEM), and grazing incidence wide angle XRD measurements, and the performance enhancement was observed to reduced grain boundaries of MAPbI3 stopping charge recombination and making the effective charge carriers (electrons and holes) transport. In a similar way, by infusing 4 mol% of urea (a bi-functional nonvolatile Lewis base) in MAPbI3 , Lee et al. [65] got significantly improved photoluminescence lifetime along with suppression of the trap-related non-radiative recombination. Analysis done by XRD and FTIR spectroscopy was applied to observe and record the perovskite growth and intra-molecular interactions. Alongside, time-resolved photoluminescence (TRPL) measurements allowed the determination of charge carrier lifetime and trap densities. Thus, the interaction of perovskite precursors with urea in solution was confirmed. The urea additive slows down the growth and improves the perovskite crystallinity, and then passivates the defects in the absorber layer by precipitating at the boundaries of the grains.

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4.1 Graphitic Carbon Nitride Integrated into Perovskite Absorber Layers During the previous years, a lot of studies have tried to focus on the incorporation of g-C3 N4 into the perovskite layer as a means of passivizing the surface to enhance the crystalline character of the material, its conductivity, and to enhance the all-inclusive working of the devices. A noteworthy example utilizing this approach is the study of Liao et al. [66] which dictates toward the integration of a little quantity of g-C3 N4 obtained by the technique of pyrolysis of dicyandiamide into the perovskite ink. This modification resulted in PCEs up to 21.1%. The g-C3 N4 supplements resulted in close-packed perovskite films which become highly crystalline in nature having defects that have been passivized, have bigger grains, and become more conducting (Fig. 13a, b, c, d and e). These improved characteristics were confirmed through characterization techniques such as SEM, FTIR, XPS, and conductive-AFM (i.e., c-AFM) measurements. Further, the resulting devices had a considerably low charge recombination rate. The g-C3 N4 altered PSCs exhibit smaller values of hysteresis and become more stable. The quoted study established that after 500 h of continuous exposure to the sunlight, a drop of only about 10% occurred in the PCE of the devices (Fig. 13f). Also, in an earlier study, Jiang et al. [62] documented about the incorporation of 0.6% by weight of pristine nanosheets of g-C3 N4 developed by basic pyrolysis of urea

Fig. 13 Top-view SEM images of the perovskite films based on a control MAPbI3 and b 0.1% by weight g-C3 N4 -modified MAPbI3 film, c-AFM images of c control MAPbI3 , and d 0.1% by weight g-C3 N4 -modified MAPbI3 films having a scanning area of 100 μm2 , e XRD patterns of control and 0.1 wt% g-C3 N4 -modified MAPbI3 films, and f stability response of the encapsulated PSCs fabricated with or without g-C3 N4 under the exposure of constant 1 sun. Reproduced with permission from [66], Copyright 2019, The Royal Society of Chemistry

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Fig. 14 a Architecture of MAPbI3 :g-C3 N4 -based n-i-p PSC and b corresponding crosssectional SEM image, c Hall mobility of MAPbI3 and MAPbI3 :g-C3 N4 in various solvents films, d current density–voltage (J-V) curves of hole dominated devices with a structure of FTO/PEDOT:PSS/MAPbI3 (with and without g-C3 N4 )/MoO3 /Ag, e J-V curves of MAPbI3 and MAPbI3 :g-C3 N4 -based devices by forward and reverse scan, and f photoluminescence (PL) spectra of MAPbI3 and MAPbI3 :g-C3 N4 (DMF) films deposited on TiO2 /FTO. Reproduced with permission from [62], Copyright 2017, Wiley-VCH GmbH

into MAPbI3 layer (Fig. 14a, b) enhanced the crystallinity and made bigger grains in the perovskite film. These modifications in the film were confirmed by SEM characterization and also by mobility values of carriers (Fig. 14c). Thus, c-AFM measurements proved the enhancement of the respective perovskite absorber/spiro HTM interface. Along with this, the defects density was lowered (Fig. 14d) which subsequently reduced hysteresis (Fig. 14e) and increased the PCE to 19.49%. Herein the authors observed keenly the effect of various solvents used for dissolving g-C3 N4 on the perovskite’s crystal growth and they were able to command the crystallization besides lowering down the recombination of charge carriers (Fig. 14f). Out of the solvents used (DMF, DMSO, and ethanol), DMF could not considerably affect the parameters of the solar cell. Wei et al. developed ultra-fine exfoliated graphitic carbon nitride (E-g-C3 N4 ) nanoparticles, employing melamine precursor and annealing at 550z C which was followed by the intercalation in H2 SO4 and fast stripping in ammonia aqueous solution [67]. The developed nanomaterials, self-located at the MAPbI3 grain boundaries through hydrogen-bonding interaction (Fig. 15a), worked as prohibitory of electron–hole recombination. Thus, they were successfully infused into the PSC devices enhancing the device performance by 35%. Integrated graphitic carbon nitride is displayed by FESEM top-view images (Fig. 15b, c) of the tweaked MAPbI3 films showcasing that most of the ultra-fine E- g-C3 N4 nanoparticles are present on the grain boundaries. During the crystallization process usually a big number of electron

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Fig. 15 a Schematics showing the self-location process of E-g-C3 N4 at MAPbI3 grain boundaries through hydrogen-bonding interaction. b, cTop-view FESEM images of MAPbI3 film and E-gC3 N4 -modified MAPbI3 film, respectively. Reproduced with permission from [67], Copyright 2019, Elsevier

trap sites are produced, resulting in electron loss and charge recombination. The ultra-fine E-g-C3 N4 nanoparticles are decorated with N-H or O-H groups and, thus, they are easily adsorbed by forming hydrogen bonds with N-H bonds present on the perovskite grain boundaries. These nanoparticles were put to application as intermediate materials in between the perovskite absorber and the HTM and an increase in PCE by the passivation of the defects at grain boundaries was observed. In 2019, Yang and his coworkers incorporated g-C3 N4 into carbon-based PSCs [68]. This group observed that the incorporation of g-C3 N4 into the precursors to the perovskite may result in (i) better surface morphology, (ii) grains of larger size, and (iii) better crystal quality of perovskite films. According to their observation

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adding 0.5% g-C3 N4 by weight has two major effects: (i) it lowered the root-meansquare (RMS) roughness from 15.3 nm for the normal perovskite to 11.5 nm, and (ii) an increase in the average grain size from 150 to 270 nm. These observations were verified through SEM and AFM characterizations. The better crystal quality led to the enhanced power conversion efficiency of carbon-based PSCs from 10.5% to 12.8%. To further improve the power conversion efficiency of the device through lowering down the charge carrier recombination, an insulating layer of Al2 O3 was spin-coated on the surface of ETL. This modification between the ETL and perovskite interface brought down the recombination of charge carriers which increased the PCE of the device up to 14.3%. In another study, Li and his coworkers used sulfonic, amino, nitrato, and hydroxy organic groups infused in g-C3 N4 as perovskite supplements for the purpose of passivation. These perovskites were developed by the precursor melamine and then subjected to heat treatment and finally cured in sulfuric acid [69]. Top-view SEM was employed to study the morphology of materials. Their opto-electronic properties were characterized with steady-state time-resolved photoluminescence (TRPL) and ultraviolet–visible absorption spectra. The PV performance of the corresponding perovskite solar cells was characterized by J-V curves. Enhanced nucleation and crystallinity were obtained following successful g-C3 N4 passivization of the perovskite. This was established through (i) an increase in grain size, (ii) a decrease in energy disorder, and (iii) absorber’s effective passivation. The PSCs passivized by graphitic carbon nitrides show PCEs as high as 20.08% and obviously are better than devices that do not undergo passivation. It got support from higher charge mobility and by Nyquist plots (Fig. 16a) reinforcing larger charge recombination resistance (Rct ). In addition, doping of iodine in g-C3 N4 (i.e., g-CNI) and its integration into triple cation perovskite film were studied by Cao along with his coworkers [70]. This group developed g-CNI by directly fusing C2 H4 N4 with NH4 I and following the heating at 5500 C. The resulting perovskite films had (i) high crystallinity and (ii) low density of trap states, besides other beneficial effects. The corresponding PSCs

Fig. 16 a Nyquist plot of control and devices passivated by functionalized C3 N4 . Reproduced with permission from [69], Copyright 2019, Wiley–VCH GmbH. b Schematics showing the mechanism of g-CNI-modified PSCs. Reproduced with permission from [70], Copyright 2019, The Royal Society of Chemistry. c Schematic illustration of charge transfer between CsPb(Brx I1-x )3 nanocrystals and g-C3 N4 . Reproduced with permission from [71], Copyright 2020, Wiley–VCH GmbH

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showed improved PV performance because of the interaction of the iodide of the g-CNI group (i) with the under-coordinated Pb ions on the surface and (ii) at the boundaries of grains of the perovskite absorber layer (Fig. 16b). After the optimization of the g-CNI concentration, the modified devices showcased PCE as high as 18.28%, quite higher than that of the reference cell. Based on XRD measurements showcasing that the lattice diffraction peaks (in shape and position) of the two perovskites (modified and reference) are quite similar, the scientists suggested that graphitic carbon nitride was not incorporated in the perovskite lattice but is positioned either on the surface of the material or at the grain boundaries. To further investigate the role of g-C3 N4 in energy or charge transfer, Sheng along with his coworkers [71] developed heterostructures of CsPb(Brx I1-x )3 and gC3 N4 NSs using the normal procedure from C3 H6 N6 . They observed an improvement of charge separation and its transport resulting from unusual band positioning (Fig. 16c). These effects were noticed through steady-state photoluminescence spectra, time-resolved photoluminescence spectra, and photodegradation and temperature-dependent photoluminescence graphs. In comparison to other heterostructures, the heterostructures developed by perovskite nanocrystals (PNCs) and g-C3 N4 nanosheets (CN) have two important advantages: they have (i) wider absorption spectrum and (ii) improved absorption potential. The efficient charge transfer enhances the separation between the electron and hole, which ultimately results in the charge transport efficiency as high as 98.16%. So it was established through the above-mentioned studies that the use of g-C3 N4 either as a supplement or in combination with functionalized groups for passivation purposes enhances appreciably the comprehensive PV performance through a reduction in the recombination of charge carriers.

4.2 Graphitic Carbon Nitride-Modified Transport of Charge Carriers Literature survey indicates that notably important research studies have concentrated on the incorporation of g-C3 N4 into the absorber layer of perovskites. Some recent studies have been carried out by using g-C3 N4 as ETL or/and HTL. With the increasing number of future investigations dealing with the application of g-C3 N4 in the ETL of PSCs, it is expected that we would get more promising results out of this fast-developing, innovative, and highly encouraging technological approach. In 2019, to the author’s knowledge, for the very first time, Chen et al. produced a hybrid ETL nanocomposite comprising of SnO2 with g-C3 N4 quantum dots (QDs) [72]. The synthesis included treatment with H2 SO4 and HNO3 acids as well as hydrothermal cure performed in autoclave with NH3 ·H2 O in order to develop porous graphitic carbon nitride quantum dots (g-CNQDs). Quantum dots are basically a monolayer of nanosheets having a diametric range of 2–10 nm . In this way, several benefits were achieved. Firstly, the intrinsic interface crystal defects of SnO2 were

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Fig. 17 a The side view for the charge density difference of SnO2 (up) and G-SnO2 (down) with oxygen vacancy, reproduced with permission from [72], Copyright 2020, Elsevier. b Schematic illustration of the PSCs with g-CNQD-modified SnO2 layers, and top-view SEM images of perovskite films based on c SnO2 and d SnO2 /g-CNQD ETL. Reproduced with permission from [73], Copyright 2020, The Royal Society of Chemistry

lowered down. In fact, the lone-pair electrons of nitrogen can interact with the undercoordinated Sn to properly passivate the defects of SnO2 related to oxygen vacancies (Fig. 17a). This resulted in a PCE crossing 22%, which was due to the ability of gC3 N4 for trap healing, its high resistance to corrosion, and its high chemical stability. In addition, devices based upon the G-SnO2 ETL showcased exceptional long-term stability, withholding 90% of their initial performance when they are stored for 1500 h in a humid environment (with about 60% humidity). In conclusion, the use of g-C3 N4 as ETL in perovskite solar cells improves significantly both the performance and the long-term stability. Liu et al. [73], in the following study also utilized g-CNQDs as modifying reagent at the SnO2 /perovskite interface (Fig. 17b). The g-CNQDs were synthesized with the help of heat treatment of urea and sodium citrate (C6 H5 O7 Na3 ). This caused the decrease of the SnO2 surface roughness, less grain boundaries, and the facilitation of the crystal growth of the perovskite absorber (Fig. 17c, d) as captured by SEM measurements. The best PSC displayed a PCE of 21.23% with negligible hysteresis. Furthermore, the long-term stability of the device was judged after being stored in ambient air and was exposed to natural light in a room for a period of one month in presence of low humidity of about 30%. The best part of their study was that the power conversion efficiency of the perovskite solar cell modified with g-CNQDs was lowered only by a factor of 10% over a period of one month. Liu

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and his coworkers [74] through their innovating approach by tweaking the contacts of ETL/perovskite and perovskite/HTL by incorporating g-C3 N4 obtained from urea observed that the g-C3 N4 modification does not showcase needed band orientations among ETL, HTL, and absorbing layer of MAPbI3 perovskite at the two contacts. They recorded a tremendous PCE enhancement to 19.69% for the dual embodied perovskite solar cell as compared to 18.03% for that of the standard device. The improvement in the performance occurred due to the lowering down of the trap density of states as confirmed through the TEM and SEM measurements at both ETL/perovskite and perovskite/HTL contact layers. Thus, the incorporation of gC3 N4 either as a supplement in the perovskite-absorbing layer and/or to modify the ETL/HTM transport layer provides higher efficiencies (Table 1) and greater stability of the PSCs.

5 Some Recent Applications of Graphitic Carbon Nitride 5.1 Graphitic Carbon Nitride for Optical Devices Graphitic carbon nitrides belong to the category of materials with a high value of the refractive index. Materials having high refractive indices have very useful optical applications. Therefore, due to the feature of high refractive index (n), g-C3 N4 finds applications in optical light trapping as well as in light-harvesting systems possessing a nanostructure. These can also be used to develop optical nanodevices and highly efficient sensors. The major drawback associated with high-n materials is that these materials are mostly of inorganic origin. This nature leads to disadvantages which include among others (i) mechanical rigidity of the materials and (ii) costly development methods. These disadvantages are the major force behind limiting their applications. Lately, Giusto and his coworkers documented the CVD-grown g-C3 N4 film with great homogeneity having a high refractive index of 2.43 which showed a highly transparent behavior [82]. The g-C3 N4 film having a layered structure is tremendously conjugated aligned disposition, showcasing deep blue luminescence characteristics (Fig. 18a, b, c and d). According to characterization performed through variableangle spectroscopic ellipsometry, it was observed that the absorption of light by g-C3 N4 film mostly lies in the ultraviolet region. Further, it was seen to have an extinction coefficient as high as 1.97, which is one of the highest among all catenated polymers (Fig. 18e). If g-C3 N4 is exposed to ultraviolet light of wavelength 375 nm, then it displays highly intense photoluminescence emission of blue color consisting of a highest single wavelength of 466 nm (Fig. 18f). Semiconducting polymer materials possess high extinction coefficients because of (i) great alignment, (ii) significantly high DOS, (ii) large values of transition dipole moments, and (iv) great stiffness existing in the polymer chains. Based upon prediction, this g-C3 N4 film with good optical performance may find lots of applications in light management devices with great efficiency.

24.31 23.00 23.20 23.80 24.00 22.63 22.97 22.84 22.47 22.56 22.37 24.03 21.45 23.39

FTO/compact TiO2 /g-C3 N4 modified MAPbI3 /spiro-OMeTAD/MoO3 /Ag

FTO/compact TiO2 /g-C3 N4 modified MAPbI3 /spiro-OMeTAD/Au

FTO/compact TiO2 /exfoliated g-C3 N4 modified MAPbI3 /spiro-OMeTAD/Au

FTO/c-TiO2 /m-TiO2 /Al2 O3 /MAPbI3 + 0.5 wt% g-C3 N4 /carbon

FTO/c-TiO2 /m-TiO2 /MAPbI3 + 0.5 wt% g-C3 N4 /carbon

FTO/TiO2 /g-C3 N4 modified CsFAMAPbI3−x Brx /spiro-OMeTAD/Au

FTO/TiO2 /iodine doped g-C3 N4 modified CsFAMAPbI3−x Brx /spiro-OMeTAD/Au

ITO/PTAA/CsFAMAPbI3−x Brx + NO3 functionalized g-C3 N4 /PCBM/BCP/Ag

ITO/PTAA/CsFAMAPbI3−x Brx + SO3 functionalized g-C3 N4 /PCBM/BCP/Ag

ITO/PTAA/CsFAMAPbI3−x Brx + NH3 functionalized g-C3 N4 /PCBM/BCP/Ag

ITO/PTAA/CsFAMAPbI3−x Brx + OH functionalized g-C3 N4 /PCBM/BCP/Ag

ITO/g-C3 N4 QDs modified SnO2 /CsFAMAPbI3−x Brx /Spiro-MeOTAD/Au

FTO/SnO2 /g-C3 N4 /MAPbI3 /g-C3 N4 /Spiro-OMeTAD/Au FTO/SnO2 /g-C3 N4

QDs/(FA/MA/Cs)PbI(3−(x+y)) Br(x) Cl(y )/Spiro-OMeTAD/Au

Jsc (mA.cm+2 )

Structure

Table 1 Progressive Development in the photovoltaic performance of g-C3 N4 -modified PSCs [48]

1.14

1.14

1.18

1.08

1.07

1.06

1.11

1.07

1.06

0.92

1.00

1.10

1.16

1.07

Voc (V)

79.6

80.7

78.3

75.68

76.96

75.96

79.20

74.0

73.0

58.2

60.1

62.0

79.0

74.0

FF (%)

21.23

19.67

22.13

18.28

18.58

18.09

20.08

18.28

17.53

12.85

14.34

15.80

21.10

19.49

PCE (%)

[81]

[80]

[51]

[79]

[79]

[79]

[78]

[78]

[78]

[77]

[77]

[76]

[75]

[12]

Ref

2020

2020

2020

2019

2019

2019

2019

2019

2019

2019

2019

2019

2019

2018

Year

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Fig. 18 Thin films of g-C3 N4 deposited on a substrate of a, b fused silica, and c, d quartz substrate with the shape of flower. e Extinction coefficient k (red line) and refractive index n (black line) of a gC3 N4 thin film plotted for different values of energy and wavelength, respectively f Photoexcitation spectrum contour plot on the y-axis with 466 nm emission versus Photoluminescence spectrum contour plot onthe x-axis with 375 nm excitation for thin films of g-C3 N4 (inset showing the g-C3 N4 thin-film emission color). Images taken from [82]. Licensed under CC BY-ND 2.0

5.2 Graphitic Carbon Nitride for Actuation and Sensing Functions Lately, g-C3 N4 has gathered a great deal of recognition to be used for developing not only gas and ion sensors but also bio- and humidity sensors [83]. It was observed that a sample of carbon nitride may undergo changeable deformity through triggering with H2 O. This fact was used by Zhang and his coworkers to develop a humidity sensor through the coating of dispersion of carbon nitride nanoribbon and carbon nanotubes on interdigitated electrodes. Humidity induces anisotropic deformity in the nanoribbons. This effect forms the basis for the development of humidity sensors [84]. The g-C3 N4 nanosheets were also used for developing a sensor [85]. This nanosheets-based sensor was used to capture polycyclic aromatic hydrocarbons by utilizing the distinctive photoluminescent properties of g-C3 N4 . The homogeneous distribution of the nanosheets was the result of using a polymer matrix. The further addition of β-cyclodextrin converted the above sensor capable of recognizing the molecules and ensures a continuous sensing function. The actuator has an important role to play in the case of (i) wearable devices, (ii) rehabilitation robotics, (iii) prosthetics, and (iv) artificial intelligence besides some other applications. Lately, a number of materials have been used for working as actuators. The g-C3 N4 materials and their membrane derivatives due to possessing great electronic, optical, and bio-compatible properties have shown promises for developing stimulus-responsive actuators. Arazoe et al. produced a humidity-driven

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Fig. 19 Humidity actuators. a Free-standing g-C3 N4 membrane bent under different relative humidity (RH) conditions at room temperature. b Bending and straightening motions of a g-C3 N4 membrane in response to ultraviolet (365 nm) irradiation with turn-on and turn-off. c Proposed mechanism for the efficient conversion of water adsorption and desorption. Reproduced with permission from [86]. Copyright 2016 Springer Nature

autonomous actuator by a free-standing g-C3 N4 membrane, which was developed by the VDP method utilizing guanidinium carbonate [86]. Upon fluctuations in ambient humidity, the membrane automatically reacts to the absorption and desorption of a particular amount of water. Figure 19a shows that the membrane can undergo gradual mutation starting from a bent state to a straightened state in response to a 36% reduction in the humidity of surroundings, specifically from 54 to 18%. Such a graphitic carbon nitride membrane-based actuator shows a fast response for humidity reduction-driven bending. Figure 19b showcases the bending and straightening alterations of a g-C3 N4 membrane when UV radiation falls on it. The unblemished membrane, the one in the straight condition, shows a quick curling-up response in just 50 ms with exposure of ultraviolet light of the intensity of 200 mWcm−2 . But the reverse process is slow, i.e., the curled membrane slowly gets back to straightened state when ultraviolet light exposure is removed. Due to being super lightweight and possessing ultrafast actuating response, the g-C3 N4 membrane can be made to show a large transverse response upon being exposed to light. The jumping effect shown by the g-C3 N4 membrane can be expositioned through its wide and highly intense UV light absorption. The ultraviolet light exposure to the membrane makes it heated up to reduce its water content. Figure 19c depicts the transformation of the membrane arising as a result of increasing and decreasing its water content. Due to the varying proportions of unreacted sp3 nitrogen to sp2 nitrogen at varying depths, the membrane produced in the quoted research study is not symmetric along the transverse dimension. This asymmetry makes it possible for it to be actuated through asymmetric increase and decrease of water by the membrane. The graphitic carbon nitride-based membrane, therefore, has a good actuating response.

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5.3 Graphitic Carbon Nitride for Transport and Separation Applications Inspired by biological ion channels, ion transport across porous membranes and nanochannels holds great importance due to their potential applications ranging from sensing to energy conversion [87–89]. To manifest a similar ion-transport property with biological nanopore, multifarious solid-state nanopore/nanochannel/nanotube with unique structure and surface property has been developed by different methods [90–92]. Similar to the graphene analogue, the g-C3 N4 membrane showcased potential ion transport due to its laminate and molecular porous structure [93]. Xiao et al. devised a facile van der Pauw technique for developing extensive and unsupported g-C3 N4 membrane by using melamine [94]. They observed the suitability of such produced g-C3 N4 membrane for use in ion-transport applications. Further, these kinds of membranes can also be used for harvesting energy from the salt gradient because of being porous and exhibiting a structure having several layers (Fig. 20a). In the quoted study, three membranes lying in the thickness range of 250 to 880 nm were developed. These membranes were utilized for intensive study on the transport of ions. Observations established that resistance against ion current is proportional to the thickness of the membrane. The ion current through the membranes, therefore, decreases with an increase in the thickness of the membranes. Not only the thickness of the membrane enhances the resistance against the flow of ion current but also the pores due to defects and homogeneity in structure increase it. The incomplete polymerization leads to the membrane being negatively charged ultimately resulting in an NH functional group which have a great number of electrons. Because of being

Fig. 20 Ion transport. a Schematic illustration of ion transport through a free-standing g-C3 N4 membrane. Optical image and cross-section of the g-C3 N4 membrane. b Electro-osmotic power generation from the salt concentration gradient across the free-standing g-C3 N4 membrane and ion selectivity transport. Reproduced with permission from [94]. Copyright 2018 John Wiley and Sons. c Carbon nitride nanotube membrane used for photo-induced ion pumping and energy harvesting. d Generation of photo-driven ionic current. Images adapted from [95]. Licensed under CC BY-ND 2.0

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negatively charged, the membrane becomes selectively permeable for cations. In a manner similar to this, we can garner the ionic current and voltage resulting from the salinity gradient occurring because of the resultant cation flow along the concentration gradient. Figure 20b shows the efficiency of energy conversion obtained in the quoted work when there exists a concentration gradient of three orders of magnitude across the membrane. The JSC and VOC values could be noted down directly from the two axes. These values were then utilized for calculating the output power density for which a value of 0.21 W/m2 was obtained for the membrane with the smallest thickness. Based on the above observations, the researchers proposed that a decrease in the membrane thickness will result in still higher power density. This research group went still further in producing a membrane (CNNM) based upon carbon nitride nanotubes through the vapor deposition method. This kind of membrane ensures the realization of the function of the ion pump which is driven through light (Fig. 20c). When the membrane is exposed on one side with the light, ions are made to transport thermodynamically “uphill” and against an up to 5000-fold concentration gradient with a light density of 380 mW/cm2 . The significant efficiency is comparable to biological ion pumps, which have not been documented before in artificial ion pumps. The further experiment proved that the CNNM-based ion pump showcased repeatable instant response and was highly stable, which can be attributed to the fast separation and recombination of charge carriers under light irradiation (Fig. 20d). It is expected that the performance may be further improved by reducing the length of the nanotubes to several nanometers, just as protein nanopore.

6 Conclusions 6.1 Role of Graphitic Carbon Nitrides in Perovskite Solar Cells Graphitic Carbon Nitride has a tunable bandgap of 1.8–2.7 eV that allows the harvesting of visible light in the spectrum of 460–698 nm and allows 13–49% of solar energy conversion efficiency [10, 40]. Graphitic carbon nitride acts as a photocatalyst for PSCs. It is infused with the light absorber perovskite layer using methods such as thin-film deposition and spin coating [28, 96]. Basically, in the perovskite layer of a solar cell, photoreaction occurs where the interaction of photons with the layer generates electron–hole pairs. These pairs are generated at a particular rate [97]. Thus, the role of a photocatalyst is to speed up or increase the rate of a photoreaction under its presence. This is what graphitic carbon nitride does when infuse with perovskites [75]. The synthesis is quite feasible and it gives a unique structure to the PSC with excellent properties as well (Fig. 21). Another use of graphitic carbon nitride in planar PSCs is the application in charge transport layer like the ETL. As energy level alignment and carrier mobility are crucially important for electron extraction and transport [32]. Nanocomposites like

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Fig. 21 a Progressive development in the power conversion efficiency of PSCs during the period 2009–2020 and b the structure and morphology (SEM image) of g-C3 N4 . Reproduced with permission from [48]. Copyright 2014, Elsevier

SnO2 -doped g-CNQDs were created in the laboratories that showed peaked performances. For a typical hybrid SnO2 -based ETL, a maximum recombination efficiency of 22.13% was obtained with VOC of 1.176 V and JSC of 24.03 mA/cm2 . A fill factor of 0.783 was obtained with negligible hysteresis and long-term stability. Only 10% power conversion efficiency decay was observed after 1500 h under observation when the relative humidity was around 60% in thin-film solar cells [36]. TiO2 is an excellent photocatalyst but it has a wide bandgap. The bandgap lies in the range of 3.0 to 3.2 eV . Thus, it is, in general, photoactive for the most part under UV irradiation which limits its promising application in photocatalysis. So, the solution is the synthesis of g-CNQDs-TiO2 composites. Hence, visible lightdriven photocatalysis is possible. The crystal defects of quantum dots infused planar hetero-junction PSCs are reduced and the power conversion efficiency is improved.

6.2 Preparation of Graphitic Carbon Nitride Quantum Dots Using the hydrothermal method, bulk graphitic carbon nitride is treated with acid for etching purposes. The porous g-C3 N4 obtained is then treated with hydrothermal solvents to obtain the desired graphitic carbon nitride quantum dots (g-CNQDs). One other method is by heating melamine for 2 h under 6000 C and then the bulk graphitic carbon nitride which is obtained is treated with a mixture of KOH and ethanol [28, 32]. The suspension is transferred to an autoclave and then heated at 1800 C for 16 h. This is followed by exfoliation and oxidation, and the final product g-CNQDs are obtained which are basically the monolayers of graphitic carbon nitride sheets having an average diameter of about 3.3 nm [40] (Fig. 22). Hence, we can conclude by saying that not only do graphitic carbon nitride nanomaterials when infused with the perovskite layer improve the photocatalysis but also improve the overall power conversion and recombination efficiency of the PSCs. In the form of ordered porous and mesoporous nanorods and quantum dots, their

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Fig. 22 a Molecule structure of g-C3 N4 . b TEM image of g-C3 N4 particles. c XPS spectrum of g-C3 N4 film. d XRD pattern of g-C3 N4 film. (e) Device structure of CH3 NH3 PbI3 :g-C3 N4- based n-i-p structure perovskite solar cell. f Cross-sectional SEM image of CH3 NH3 PbI3 :g-C3 N4 -based n-i-p structure perovskite solar cell [48]

promising applications in the charge transportation layer and in driving visible light photocatalysis make them a very pertinent nanomaterial for PSCs [46, 48]. PSCs have gathered great attention in scientific circles because of their (i) exceptional opto-electronic characteristics, and (ii) PCE has seen rapid enhancement. This rate of increase in PCE has surpassed other third-generation solar PV mechanisms. The g-C3 N4 promises extraordinary opto-electronic characteristics because of which it has found great application in perovskite solar cells [36]. When g-C3 N4 was integrated either in the light-absorbing layer of perovskites or in the ETL or in both the layers, it resulted in a power conversion efficiency of more than 22%. This improvement in PCE is mainly attributed to (i) passivation of defects, (ii) enhancement in conductivity, (iii) better crystallinity, and (iv) reduced rate of recombination of charge carriers. The integration of g-C3 N4 in the interfaces of a perovskite solar cell resulted in remarkable performance enhancement as well as an improvement in the stability of the device. This improvement in the device performance and stability could be observed because of the hydrophobic/hydrophilic nature of g-C3 N4 and also due to the fine-tune characteristics associated with it for the energetics of respective interfaces [40, 45]. During the previous decade, the amount of research done with cost-effective PSCs has increased exponentially. The efficiencies of such devices have crossed 25% in a short period. However, there are, still, challenges linked with charge carrier recombination both in the perovskite material and at the interfaces within the photovoltaic and with long-term PSC device stability. In this way, researchers infused graphitic carbon nitride in PSCs because of its suitable properties. Graphitic carbon nitride (g-C3 N4 ) is a very promising nanomaterial for application in devices operating under direct sunlight, as the material has an energy gap (Eg ) in the visible spectrum [43]. Also,

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the existing nitrogen sites seem to improve crystallinity, diminish grain boundaries, and result in defects passivation and charge carrier transport facilitation. Besides, g-C3 N4 is quite chemically and thermally stable as a material, a fact that makes this material favorable for PSCs’ applications [42]. This approach came up only a few years before and the most remarkable works deal with the application of g-C3 N4 as an additive or modification at the perovskite materials and layers, respectively. Lately, very limited studies have been carried out on the effect of g-C3 N4 as a modification of the ETL and the HTL [47]. Despite having impressive enhancement on efficiency and stability, considerable scientific problems and technological challenges still exist, requiring wholesome answers and proper solutions, such as controlling the thin-film growth and deposition and scaling the fabrication process and the reproducibility of results, toxicity of lead, and the current–voltage hysteresis along with the deterioration under sunlight. The incorporation of g-C3 N4 in PSCs raised significantly the PCE of the devices in every case and this is due to the main following facts. Charge recombination was controlled, the crystallization and the grain size of the perovskites grew, and the conductivity at the interfaces was improved. Thus, g-C3 N4 could be labeled as a universal material for all-round optimization of the PSCs [58]. Although g-C3 N4 has been the best and most extensively studied in the field of photocatalysis, optoelectronic devices based on g-C3 N4 are still in their childhood stage. We advise that further research on properly controlling the g-C3 N4 nanostructures’ fabrication with an importance to the enhancement of opto-electronic properties and the deposition method at the interfaces concerned with the wetting behavior could result in the development of novel PSC devices with great power conversion efficiency and increased stability [44]. Thus, the multifunctional nature of graphitic carbon nitride and its application as a universal material in PSCs are thought to overcome barriers and address challenges usually faced with proper and precise approaches incorporating carbon nanostructures, dyes, transition metals, and/or solution additives.

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Chapter 3

Transition Metal Dichalcogenides (TMDs) Nanocomposites-Based Supercapacitors Shrestha Tyagi, Kavita Sharma, Ashwani Kumar, Yogendra K. Gautam, Anil Kumar Malik, and Beer Pal Singh

1 Introduction The extensive energy consumption and the ever-increasing demand for energy have raised great concerns about environmental pollution and the limited availability of non-renewable fossil fuel resources. The excessive use of conventional energy sources leads emission of greenhouse gases and various pollutants, which in turn deteriorate the environment in the form of global warming, acid rain, depletion of ozone, and eutrophication [1]. Presently, it is necessary to replace the traditional energy sources with renewable energy such as solar energy, hydrothermal energy, geothermal energy, wind energy, and biomass energy. Therefore, the development of renewable energy storage devices such as batteries, supercapacitors, and fuel cells is essential for the replacement of conventional fossil fuels. Energy storage technologies are a growing asset along with the trends of miniaturization of electronic devices and renewable energy generation. Supercapacitors or ultracapacitors are considered important elements of modern energy storage technologies owing to their emerging applications in wind power generation, photovoltaics, aerospace, railways, hybrid electric vehicles, portable, and wearable electronics [2]. However, Li-ion batteries have been also at the forefront of devices used for energy storage but several aforementioned challenges such as slow charging/discharging, limited cycle life, high cost of Li metal, flammability, undesirable side reactions along with serious health risks associated with them limit their widespread applications [3]. These drawbacks have encouraged the ongoing research on energy storage systems that are environment-friendly, faster, mechanically robust, and offer wide operational temperatures. S. Tyagi · K. Sharma · Y. K. Gautam · A. K. Malik · B. P. Singh (B) Department of Physics, Chaudhary Charan Singh University, Meerut 250004, India A. Kumar Nanoscience Laboratory, Institute Instrumentation Centre, IIT Roorkee, Roorkee 247667, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_3

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Presently, supercapacitors are used as a complementary aid to batteries owing to their intrinsic problems of low energy density and low operating voltage [4]. The high energy density as compared to conventional capacitors power, superior power density in comparison to batteries, high cyclability, excellent response time, wide operating temperature range, high safety, and environmentally friendly nature make supercapacitors suitable candidates to combatting growing global energy challenges by storing as well as delivering clean energies [5]. The most significant feature that efficiently controls the performance of supercapacitors is the kind of electrode material used, which relies on the surface area, wetting behavior of electrode electrical conductivity, and permeability of electrolyte ions [6]. Developing and designing such kinds of electrode materials that exhibit all these expected features and provide efficient energy storage has become a key challenge in this field. Recent advancements in the field of supercapacitors suggest that transition metal dichalcogenides (TMDs) including MoS2 , MoSe2 , WS2 , TiS2 , NbS2 , and VS2 are promising candidates to bridge the gap between the present performance and modern necessities for energy storage devices [7]. This class of materials possesses unique properties such as two-dimensional (2D) layered morphology, large surface area, highly exposed active edge sites, enhanced electrochemical kinetics, and large interlayer spacing. Moreover, their exceptional superiorities including reactive surfaces and rich coordination sites have gained tremendous research attention over traditional electrode materials and graphene [8]. TMDs are found in nature in two phases, i.e., 2H (hexagonal) and 1T (trigonal), which are semiconducting and metallic phases, respectively. The numbers displayed in various phases indicate the stacking sequence of TMDs. The bulk, as well as the 2D form of TMDs, can be used as electrode material but the bulk form has several limitations in the field of supercapacitor owing to the low surface area. However, the 2D sheets of TMDs offer high surface area and abundant oxidation states which enable charge storage through faradaic and non-faradaic mechanisms [9]. TMDs-based supercapacitors demonstrate high performance which is attributed to the anisotropic crystal structures [10]. Moreover, the diversified morphologies and surface engineering strengthen TMDs as a potential candidate for supercapacitor electrodes [11]. In the next parts of this chapter, first various synthesis routes of TMDs and their composite have been summarized. Thereafter the factors that determine the high performance of supercapacitors are discussed. Furthermore, the utilization of TMDs and their nanocomposites as electrode materials and recent advancements in this field are also demonstrated. Finally, the future challenges and prospects in TMDs-based supercapacitors are discussed.

2 Synthesis Methods of TMDs The surface morphology of the electrode material influences the electrochemical performance of the supercapacitor. Moreover, the various surface morphologies such as nanoparticles, nanoflower, nanosheets, nanosphere, nanoflakes, etc. demonstrated

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to be a high surface area. The high surface area with uniform porosity, allocation of pore size, pore network, and pore length of the electrode material are highly desired. These electrodes materials provide additional electroactive area and a more faradaic reaction. In consequence, it resulted in high specific capacitance, high energy density, and long cycle life of supercapacitor. The proper morphology provides a channel for faster transportation of electrons; as a result, the active materials completely contact the electrolyte and supply more electroactive sites for superior electrical properties of the device [7]. Due to the easy fabrication of complex structures, transition metal dichalcogenide (TMD) has been found of great scientific interest for applications in versatile energy storage applications. Many recent scientific reports reveal the successful synthesis of various 1D, 2D, and 3D TMDs. There are many methodologies for the synthesis of nanostructured TMDs. This includes hydrothermal method [12], the vapor-phase growth method [13], microwave-assisted method [14], lithiumbased intercalation [8, 15], scotch-tape method [16, 17], chemical vapor deposition (CVD) [18], atomic layer deposition (ALD) [19], pulsed laser deposition (PLD) [20], etc.

2.1 Hydrothermal Method The hydrothermal method is a highly used synthesis method for the synthesis of hierarchical, porous nanostructured TMDs. The hydrothermal method is simple, low cost, and easy to scale up. The nanostructured TMDs, ranging from quantum dots to 3D are easily synthesized using the hydrothermal method. Many recent reports demonstrated the synthesis of high surface area and complex nanostructured TMDs using the hydrothermal method. Masikhwa et al. synthesized VS2 nanosheets using ammonium metavanadate, thioacetamide, and ammonia as precursors. The precursors were enclosed in a pressure control stainless-steel autoclave at a temperature of 180 °C for 20 h [21]. The carnation flower-like SnS2 was successfully prepared by a onestep solvothermal synthesis approach without using a surfactant [22]. The isopropyl alcohol and C2 H5 NS were used as a precursor in Teflon-lined stainless-steel autoclave at moderate temperature (180 °C) for 24 h to synthesize SnCl4 ·5H2 O. Further, as-prepared SnS2 was used as electrode materials in supercapacitor applications [22].

2.2 Microwave-Assisted Method The microwave-assisted method is one of the simple and low-cost, eco-friendly, and highly employed synthesis methods used for the synthesis of TMDs. In the microwave-assisted process, the radiation (microwave) heats the precursor to a high temperature in a tiny time by giving close contact among different chemical compositions in the precursor. The microwave-assisted method avoids agglomeration prepared of the product, which further retains the high surface area and

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uniform porosity in synthesized material [23, 24]. The films of layer-structured TMDs NbS2 , VSe2 , NbSe2 , and VTe2 were grown on Si wafers by Kwak et al., using the microwave-assisted technique. This study proves the robust way for the synthesis of various groups [25]. Fu et al. prepared MoS2 /graphene composite via microwave-assisted synthesis method for supercapacitor application. The prepared MoS2 /graphene composites exhibited high specific capacitance and high energy density of 401.1 Fg−1 at 1 Ag−1 and 26.4 Wh kg−1 . Also, the prepared composite demonstrated excellent capacity retention of 95.0% over 10,000 charge/discharge cycles at 5 Ag−1 [24]. Vattikuti et al. synthesized dry leaf-like mesoporous MoSe2 nanostructure via microwave irradiation process for hydrogen evolution and supercapacitor applications. The prepared MoSe2 nanostructure demonstrated a high specific capacitance of 257.38 Fg−1 at 1 Ag−1 and high cyclic stability for 5000 cycles [14].

2.3 Chemical Vapor Deposition (CVD) The chemical vapor transport/deposition (CVD) method is a green and simple synthesis route for the synthesis of TMDs. For the large-scale fabrication and high purity synthesis of TMDs, the CVD synthesis method is highly preferred over the other synthesis routes. In CVD, the vapor phase of the desired material is produced; further, these vapor atoms get condensed and deposited on an appropriate substrate. The large scale with high purity production in a very short time is the main advantage of this technique. CVD provides an easy route to achieve material having the required morphology for high electrochemical performances [26, 27]. TaSe2 threedimensional conductive quasi-array based on 2H-TaSe2 nanobelts was synthesized by Wang et al. Also, the conductive quasi-arrays are used as a substrate to form cylinder-like composite nanostructures by in situ electrodeposition of polypyrrole. The high areal capacitance of 835 mF cm−2 at a scan rate of 2 m Vs−1 the wide potential window of 1.2 V, remarkable cycling strength with 98.7% capacitance retention after 10,000 cycles, and outstanding electrochemical performance are found for this developed composite [18]. VSe2 nanosheets were vertically grown on the surface of the CNT cluster by the CVD method [28]. In this study, the conductive CNT cluster provides a large specific surface area to VSe2 nanosheets for storing the charges. VSe2 /CNT composites showed high specific area capacitance of 1854 µF/cm2 and stable cycling stability, and ∼93% capacity retention after 10,000 charge/discharge cycles [28].

3 Types of Supercapacitors The supercapacitors can be categorized based on their charge storage mechanism, the electrode materials, and also the type of electrolytes, as illustrated in Fig. 1. The

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Fig. 1 Schematic diagram of classification of various kinds of supercapacitors [35]

widely recognized categories of supercapacitors are: (i) the electrochemical doublelayer supercapacitors (EDLCs), which utilize permeable carbon electrodes amid highly accessible surface area. These electrodes act like an ‘electric sponge’, which get charged by adsorbing ions from the electrolytes and discharged by releasing them into it [29–33], (ii) the pseudocapacitors, which are operated through redox reactions that occur mostly on oxide-based material surfaces [34]; and (iii) hybrid supercapacitors, a combination of EDLCs and pseudocapacitors, yield more pronounced features than rest of the two. The storage of energy in EDLCs is accomplished by the intrinsic shell area and the partition length of atomic charge [35]. On the other hand, in pseudocapacitors, the storage of energy is furnished by the rapid recurring redox reactions between the electroactive units laid on active electrode material and electrolyte solution [36]. The amalgamation of storage mechanisms of EDLCs and pseudocapacitors collectively comprises the energy storage mechanism of hybrid supercapacitors. Consequently, the hybrid supercapacitors possess higher energy densities as compared to the two. The choice of electrode materials leads to further three subtypes of supercapacitors, namely metal oxides, carbon-based, and conducting polymers supercapacitors. In addition, with the application of different electrolytes, the supercapacitors can also be expressed as aqueous electrolytes and organic electrolyte supercapacitors.

3.1 Electrochemical Double-Layer Supercapacitors Electrochemical double-layer supercapacitors (EDLCs) can be fabricated with two carbon-based electrodes, an electrolyte, and a separator. The schematic depiction of

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Fig. 2 Schematic illustration of electrochemical double-layer supercapacitor [36]

the EDLCs structure is shown in Fig. 2. EDLCs accumulate charges electrostatically, or non-faradaically just like the conventional capacitors, as well as charges will not be transferred between electrode and electrolyte. EDLCs use an electrochemical dual layer of charge for storing the energy. When a voltage is applied, charges start to accrue on the surface of the electrodes. Because of the natural attraction of distinct charges, ions in the electrolyte solution start to diffuse across the separator into the pores of the electrode of opposite charge. However, the electrodes have fabricated a way to avert the recombination of the ions. Consequently, a dual-layer of charges is deposited on each electrode. These dual layers, enabled with an increase in surface area and a decrease in the distance between electrodes, permit EDLCs to attain elevated energy densities than the conventional capacitors [36]. Because no charge is transported between electrolyte and electrode and no chemical or composition changes are associated with non-faradaic processes, the charge storage capacity of EDLCs is highly reversible. This makes EDLCs acquire superior cycling serenities. EDLCs are usually functional with steady performance characteristics such as high charge/discharge cycles, around 100 times greater than the conventional batteries. Because of their cycling steadiness, EDLCs are best suitable for applications that engross unmanned serviceable sites, for example, deep-sea or mountain environments [35].

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3.2 Pseudocapacitors The charge storage mechanism of pseudocapacitors (PCs) is different from EDLCs. In the pseudocapacitors, the charges are stored faradaically via the transport of charge between electrode and electrolyte. This is accomplished through electrosorption, reduction/oxidation reactions, and intercalation processes [37, 38]. These faradaic processes may allocate PCs to attain greater capacitances and energy densities than EDLCs. Three types of faradaic processes are mainly found in PCs electrodes: reversible adsorption, redox reactions of transition metal oxides, and reversible electrochemical processes in conductive polymer-based electrodes [39]. These faradaic electrochemical processes not only extend the working voltage but also increase the specific capacitance (SC) of the supercapacitors [40].

3.3 Hybrid Supercapacitors Hybrid supercapacitors utilize the amalgamation of faradic and non-faradic mechanisms to store charges. The energy and power densities of these supercapacitors are greater than the EDLCs exclusive of giving up the cycling stability and affordability that have narrowed down the capacity of PCs. These supercapacitors consist of an asymmetric electrode arrangement in which one electrode is made up of electrostatic carbon material, and the other with faradaic capacitance material. Because of this reason, these supercapacitors are named hybrid supercapacitors. In hybrid supercapacitors, both electrical dual-layer capacitance and faradic capacitance mechanisms occur concurrently, out of which one plays a dominant role. In order to attain high capacitance, a large surface area, high conductivity, and appropriate pore size distribution are vital criteria for choosing the appropriate electrode materials. In topical times, numerous varieties of hybrid supercapacitors with an asymmetrical configuration have captivated noteworthy attention [41, 42].

4 Main Factors Responsible for Soaring Capacity of Supercapacitors Supercapacitors are novel energy storage devices that possess some distinctive characteristics such as soaring power density, high capacitance, and an extended charge/discharge cycle. Some requisite requirements like higher stability, durability, catalytic activity, and in addition lower cost are crucial for the implementation of new materials in supercapacitors. The key factors for designing a high-performance supercapacitor are novel types of electrode and electrolyte materials. The capacitance and working voltage of a supercapacitor can be enhanced by increasing the specific capacitance, the effective

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precise surface area, optimizing the pore size and volume, and infusing surface functional groups to elevate the pseudocapacitance input. In recent times, major supercapacitor researches are focused on the optimization of its energy output by designing the intrigue electrode materials [43, 44]. For this streak, the search for new electrolytes and the escalation of modern configurations are also being carried out. The n-doped carbon materials are the better choice for the electrode material since the addition of nitrogen functionalities into the carbon network boost some key properties of the supercapacitors like electrical conductivity, wettability, and the stability of the material. Regarding another important aspect for enhancing the energy storage of supercapacitors is the electrolytes. The ionic liquids are being investigated in recent times because of their large electrochemically stable voltage. In addition, aqueous electrolytes have also come into the spotlight as these possess better capacitance of the electrode materials as compared to the organic electrolytes [45, 46]. Based on recent advancements in the area of supercapacitors, it is to be anticipated that the prospective trend might be aimed at the improvement of trivial and supple electrode materials so that these devices can be implemented by inconvenient applications like portable electronics or hybrid electric vehicles. Graphene and CNTs seem to be the key alternatives for fabricating electrodes. Nonetheless, the large-scale production of supercapacitors using these carbon materials is still a big challenge because of their great cost of production.

5 TMDs for the Supercapacitor Electrode TMDs are 2D materials in which transition metal atoms are covalently bonded between two chalcogen atoms and thus constitute a layered structure having weak van der Waals forces. These materials due to their large surface area, excellent processability, active absorption sites, and cost-effectiveness have triggered wide scope of research for their application in energy storage, energy conversion, sensors, and nanoelectronics [11]. The 2D layered structure of atomically thin TMDs offers maximum contact with the active material of electrodes for faradaic and non-faradaic interactions. The first report on TMDs supercapacitor is the CVD-grown MoS2 films which exhibited high specific capacitance of 70 mF cm−2 at a scan rate of 1 m Vs−1 in the 0.5 M H2 SO4 electrolyte [47]. MoS2 micro-supercapacitor fabricated by spray painting and subsequent laser patterning demonstrated remarkable electrochemical performance having a specific capacitance of 8 mF cm−2 [48]. Flower-like MoS2 nanospheres having a mean diameter of 300 nm were prepared by hydrothermal method and they exhibited the specific capacitance of 122 Fg−1 at 1 Ag−1 [12]. In addition to MoS2 , WS2 has also been considered an appropriate electrode material for supercapacitor application. Presently used WS2 -based electrodes suffer from poor conductivity and fast capacitance fading. WS2 synthesized using a hydrothermal method with improved crystallinity demonstrated excellent areal capacitance of 0.93 F cm−2 at a discharge density of 4 mA cm−2 [49]. Metallic 1 T-WS2 nanoribbons prepared via hydrothermal method showed a significant enhancement in the value

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of specific capacitance 2813 µF/cm2 which is 12 times higher in comparison to the semiconducting 2H-WS2 [50]. The most significant factors for an ideal supercapacitor electrode are large surface area with a high density of active sites. The TMDs synthesized as quantum dots will be a better approach. WS2 quantum dots having a size of ~2.2 nm were prepared by the hot injection method to expose more edge atoms for enhancement in the electrochemical activity [51]. Selenide-based TMDs possess more electrical conductivity than sulfide-based TMDs. MoSe2 nanosheets are also promising electrode material for supercapacitors. Few layered MoSe2 synthesized via facile hydrothermal route displayed maximum specific capacitance of 198.9 F g−1 . Moreover, the capacitance retention was observed about 75% after 10,000 cycles at a high charge/discharge current density of about 5Ag−1 [52]. 2H-MoSe2 prepared by the chemical route and their charge storage performance in the organic electrolyte was studied by fabrication of symmetric supercapacitor. The electrochemical analysis displayed that the MoSe2 supercapacitor offers a high capacitance of 16.25 F g−1 , an energy density of 20.31 Wh kg−1 along outstanding cyclic stability having capacitance retention of 87% over 10,000 cycles [53]. Nickel diselenide (NiSe2 ) also falls under the category of emergent and potential TMD electrode material having various oxidation states along with tunable electronic configuration. The very first report of using NiSe2 as an active material for the electrode is the synthesis of ~30 nm hexapod-like NiSe2 nanoparticles that showed a maximum capacitance of 75 F g−1 at a scan rate of 2 mVs−1 . These particles also demonstrated remarkable cyclic stability retaining 94% capacitance after 5000 cycles as shown in Fig. 3 [54]. Monolayer sub-nanopore TaS2 -based microsupercapacitors synthesized via acid-assisted exfoliation showed capacitance of 508 F/cm3 at a scan rate of 10 mV/s along with a high energy density of 58.5 Wh/L [55].

6 TMDs-Based Hybrids for the Supercapacitor Electrode Despite several fascinating features, there are several drawbacks including poor electrical conductivity, low surface area, poor cycling stability, and others. To overcome all these disadvantages researchers fabricated hybrid electrodes which are synthesized by combining several electrochemically active inorganic and organic materials along with pristine TMDs. The novel morphologies such as nanowires, nanosheets, nanorods, nanoflowers, and nanobelts along with active edge sites, large area, subatomic thickness allow TMDs to easily combine and deposit on other active materials for the construction of hybrid electrodes with improved physical and chemical properties [8]. Another active material used in hybrid electrode material enhances the surface area which could not happen in the pristine TMDs owing to the restacking of layers. Carbon-based materials are the most widely employed electrode materials owing to their interesting features, including large surface area, superior electrical conductivity, and high electrochemical stability [56, 57]. They are suitable candidates for being used as conductive additives for the preparation of the hybrid electrodes in

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Fig. 3 a Cyclic voltammetric diagrams at different scan rates. b Variation of specific capacitance with scan rate. c GCD curves at various current densities. d Cycling stability curve while (inset displays the comparison of first and last cycles) e Nyquist plot of hexapod-like NiSe2 electrode [54] (Reprinted with permission from Elsevier)

combination with TMDs. In addition to carbonaceous material, several other additives such as conducting polymers, metal oxides, and other 2D compounds potentially enhance the electrochemical performance of TMDs due to their redox-active capacitance. Solution exfoliated MoS2 -graphene coin cell supercapacitor electrodes exhibited high specific capacitance of 11 mF/cm2 at 5 mV/s [19]. A scalable solution-based approach was used for the growth of polypyrrole thin films on MoS2 monolayers for supercapacitor application. Due to strong interface interaction between polypyrrole and MoS2 monolayers, high specific capacitance of 695 F g−1 at a discharge current of 0.5 A g−1 , excellent rate capability, and enhanced cycling stability are achieved [58].

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Furthermore, reduced graphene oxide (rGO) has its structural resemblance with TMDs which provides improved stability owing to 2D/2D atomic hetero-interfaces. Ternary heterostructures based on reduced graphene oxide, molybdenum disulfide, and tungsten disulfide (rGO-MoS2 -WS2 ) having a large surface area of 109 m2 g−1 with hierarchical pore structure were synthesized via a chemical method. The rGOMoS2 -WS2 heterostructures demonstrate high specific capacitance (Cs) of 365 F g−1 at 1 A g−1 and outstanding cyclic stability of approximately 70% of the initial value after 3000 cycles [59]. Transition metal oxides were also considered as effective additives which improve the performance of TMDs. Cauliflower-like ZnO/VS2 nanocomposites are synthesized via a wet chemical method. ZnO nanospheres formed on VS2 prevent restacking and exhibited specific capacitance of 2695.7 F/g at the current density of 1 Ag−1 [60]. The TEM images of Cauliflower-like ZnO/VS2 are shown in Fig. 4.

Fig. 4 a TEM images of bulk VS2 . b ZnO/VS2 nanocomposite. c HR-TEM images of ZnO/VS2 nanocomposite. d EIS spectra of two comparative materials where the inset is the enlarged view of Nyquist plots of the ZnO/VS2 nanocomposite [60] (Reprinted with permission from Elsevier)

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7 Recent Advancements in TMDs Supercapacitors The recent development in energy conservation and storage is required to fulfill our daily energy needs. Different nanostructured materials are encouraging the scientific community to develop electrochemical supercapacitors to fulfill energy needs. Traditionally used supercapacitor’s electrodes are made up with the help of binders which decrease the effective surface area of electrode materials and incorporate the useless mass of the devices. The transition metal dichalcogenides (TMDs) also showed potential applications in quantum computing, DNA sequencing, electronics, water splitting, flexible electronics, spintronics, electrocatalysis, optoelectronics, energy harvesting, renewable energy technology, and high energy supercapacitor. The monolayered TMDs were first described in 1986 [61]. The monolayer TMDs have a direct bandgap which was predicted in 2000 and confirmed in 2010 when ~1.8 eV bandgap is found in MoS2 monolayers [62, 63]. In band structure study, it is found that TMDs are metals (NbS2 and TaS2 ), semimetals (VS2 and WTe2 ), and semiconductors (MoSe2 , MoS2 , and WS2 ) [64]. TMDs give high physicochemical properties due to the layered composition of chalcogens (X: Se, S, and Te) and transition metal (M). These compositions of inorganic materials have MX2 chemical configuration in an XM-X manner [65]. Recently, two-dimensional (2D) TMDs are used as electrode material in supercapacitors, i.e., molybdenum diselenide (MoSe2 ), molybdenum disulfide (MoS2 ), tungsten diselenide (WSe2 ), tungsten disulfide (WS2 ), tantalum diselenide (TaSe2 ), tungsten ditelluride (WTe2 ), titanium disulfide (TiS2 ), tantalum disulfide (TaS2 ), niobium disulfide (NbS2 ), zirconium disulfide (ZrS2 ), vanadium diselenide (VSe2 ), vanadium disulfide (VS2 ), etc. [66]. The most studied TMD is MoS2 but MoSe2 is a potential alternative because it is a good electrical conductor. Transition metal sulfide has lower electrical conductivity than their selenide. Therefore, transition metal selenides become promising electrode material in supercapacitor devices [67]. The structure of TMDs is dependent on the sequence of the chalcone–metal– chalcogen atomic layer and these atomic layers are perpendicular and top of each other. TMDs exhibit in two phases metallic phase (1 T) and the semiconducting phase (2H) [68]. Figure 5a, b, and c shows the trigonal prismatic phase (hexagonal, 2H), octahedral (tetragonal 1 T), and their distorted phase 1 T, respectively [11]. In the 2H-phase of TMDs, each metal atom puts six branches + z and -z directions while the hexagonal symmetry can be seen in the top view. In the 1 T-phase of TMDs, the trigonal chalcogen layer is on the top and 180° rotated structure at the bottom in a single layer. Metal atoms are distorted further 1 T (dimerized in one direction), called T-phase resulting in the alteration of chalcogen atoms displacement by the side of the z-direction. The nature of the phase is decided by the electron distribution in the d-subshell of transition metal (TM), and filled d-orbitals give semiconducting behavior while partially filled decide the metallic nature of the TMDs. A phase change in TMDs from semiconductor to metallic encourages the scientist to develop a new category of metallic TMDs which helps to attain high energy density [69]. All TMDs materials have high electrochemical properties due to their abundant active edge sites, large surface area, sheet-like morphology, and anisotropic crystal

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Fig. 5 Different phases of the 2D layer structure of the transition metal dichalcogenides (TMDs): a 2H phase b 1 T phase, and c 1 T phase [11] (Reprinted with permission from Elsevier)

structures [70, 71]. The technology that includes the development of nanostructured (0D, 1D, 2D) material has been provided a route to get rid of the limitation of the materials. TMDs play an important role as electrode material in supercapacitors due to the variable oxidation state and large size that allows collecting their energy values. But the major drawback of the bulk state of TMDs is its lower surface area. However, in the case of atomically thin sheets, they have a huge surface area as well as multiple oxidation states. In two dimensions (2D), TMDs store charge through faradaic, as well as non-faradaic, mechanism that leads to high energy density and specific capacitance of the device [70]. In spite of this, TMDs have many challenges for large-scale utilization and fabrication. TMDs exist in three atomic layers and they are arranged as a transition metal layer in between the two chalcogen layers. Furthermore, the chalcogen atoms are not highly reactive. The theoretical and experimental study concludes that TMDS cannot be used directly in electrochemical hydrogen evolution reaction (HER) because the basal phase of 2D 2H-MoS2 is catalytically inert [72]. To activate the basal plane of 2D TMDs, doping of noble metal is needed or create some defect. Edge-oriented TMDs have higher absorbing power of ions than the basal plane because they exist in open van der Waals gaps for big inter-layer and large density of electrochemically active sites. Soon and Loh, first reported on chemical vapor deposited monolithic edge-oriented MoS2 film, and observed an excellent electrochemical performance with a very high capacitance of 100 F/g at a scan rate of 1 mV/s in the 0.5 M H2 SO4 electrolyte [47]. An edge-oriented MoS2 electrode was designed by electrochemical anodization of Mo in sulfur vapor ambient. The electrode exhibited a capacitance of 12.5 mF/cm at a scan rate of 50 mV/s [73].

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MoS2 is found to have poor electrical conductivity due to a larger bandgap and it suffers from volume change during the charge/discharge cycles. Sun et al. synthesized oxygen-incorporated MoS2 (O-MoS2 ) microspheres by hydrothermal method to find the solution of the problem (poor electrical conductivity). The inclusion of oxygen improved the intrinsic conductivity by decreasing the bandgap from 1.8 to 1.3 eV and increased electrolyte diffusion after expansion of interlayer spacing up to 9.8 Å. It delivers a specific capacitance of 744.2 F/g at a current density of 1 A/g with capacitance retention of 77% after 10,000 cycles [74]. WS2 is also an attractive supercapacitor electrode material (layered structure) with high capacitance. To overcome the drawback of the short life cycle and poor conductivity of WS2 , it is grown on carbon cloth by different methods. Hydrothermally grown WS2 nanosheets on carbon fiber exhibit a specific capacitance of 211 F/g at a current density of 4 mA/cm2 with superior cycling stability of 46.4–85.6% after 10,000 cycles [49]. WS2 nanoplates were well dispersed on carbon fiber cloth (CFC) by the solvothermal method. A 3D framework of the CFC prevents aggregation of nanoplates which provides high Cs of 399 F/g at a current density of 1 A/g with retaining 99% capacitance after 500 cycles [75]. The growth of TMDs on such a substrate also allows designing wearable supercapacitors, which is demand for wearable electronics. The main factors for an ideal supercapacitor electrode are large surface area and high density of active sites which can be achieved by synthesizing of TMDs as quantum dots (QDs) [51, 76]. Restacking issues of WS2 can be resolved by capping them with organic groups using ethanedithiol. The specific capacitance of capped QDs showed a high value of 457 F/g compared to only 151 F/g obtained for uncapped QDs. The capping agent has increased the cycle life of the WS2 QDs up to 81% after 8000 GCD cycles as shown in Fig. 6 [51]. 2D TiS2 nanocrystal electrode-based supercapacitors exhibit a Cs of 320 F/g [77]. Ultra-thin VS2 nanoplates were synthesized by colloidal chemical method with inplane and out-of-plane defect which create a more active site and enhance the surface area for redox reaction. This VS2 nanoplate delivers ultrahigh-specific capacitance of 2200 F/g at a current density of 1 A/g [78]. Hydrothermally synthesized MoSe2 nanosheet-based electrodes exhibited Cs of 198.9 F/g at a scan rate of 2 mV/s with cyclic stability of 70% after 10,000 cycles [52]. A unique distributed NiSe2 @MoSe2 was hydrothermally synthesized with fine crystalline nature and delivers a high capacitance of 1000 F/g (138.8 mAh g−1 ) achieved at a current density of 1 A/g with cycle stability of 77.67% for 5000 continuous cycles [79]. Another important approach is to synthesize MoSe2 nanosheets on the Ni-foam substrate using the hydrothermal method which shows outstanding electrochemical performance with a Cs of 1114.3 F/g with an extended long cycle life of 104.7% after 1500 cycles [80]. This large value of capacitance is due to the sieve-like feature of MoSe2 developed on the Ni foam creating pores for smooth transport of ions to the entire surface. Some other 2D TMDs electrode-based supercapacitor performance is shown in Tables 1 and 2.

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Fig. 6 a CV curves of supercapacitor cells at a scan rate of 50 mV/s. b CV curves of electrodes based on EWS at various scan rates. c GCD curves at the current densities of 1 A/g for the supercapacitor electrodes with OWS and EWS. d GCD curves based on EWS at different current densities. e CF of two supercapacitor electrodes at different current densities. f Cycling stability of the electrodes during 8000 charges/discharge cycles [51] (Reprinted with permission from Elsevier)

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Table 1 Comparison of electrochemical properties of sulfides and selenides-based electrodes Material

Specific capacitances (F/g)

Energy density (Wh/kg)

Power density (kW/kg)

Capacitances retention (%)

Ref

MoS2 nanosheets

199

8.1

-

89.36% over 2500 cycles

[81]

MoS2 /Ketjen black

429

25.7

16

93.2% after 5000 cycles

[82]

MoS2 nanoflowers

430

48.9

5

77% up to 5000 cycles

[83]

MoS2 nanosheets

2236.6

21.4

0.9

86.1% after 2000 cycles

[84]

MoS2 nanorods

766

12.2

-

92% after 9000 cycles

[85]

MoSe2

257.38

-

-

95% after 5000 cycles

[86]

MoSe2

775.3

63.4

5.3

98% after 1500 cycles

[87]

WSe2

244

-

-

-

[88]

8 Future Challenges and Prospects in TMDs Based Supercapacitors The development of an energy storage system is equally important as energy production. With the ever-increasing energy demands in portable electronics, computers, laptops, mobile phones, etc., the demand for high energy density, high capacitive, and low-cost energy storage devices has increased. In addition to the high capacity of the energy storage systems, the high efficiency, constant operation, environmentalfriendly, and low-cost goals provide a capable approach to resolve the global energy crisis. Recently, an alternative to the lithium-ion battery the supercapacitor is the best choice for the consumers. Out of the different existed electrode materials used in a supercapacitor, the TMDs are the important and promising class of material used in supercapacitors. The TMDs-based supercapacitors demonstrated high energy density, high power density, and long cycle life which are highly popular in adaptable portable applications. TMDs are the high potential supercapacitive candidate. The molybdenum diselenide (MoSe2 ), molybdenum disulfide (MoS2 ), tungsten diselenide (WSe2 ), tungsten disulfide (WS2 ), tungsten ditelluride (WTe2 ), tantalum diselenide (TaSe2 ), tantalum disulfide (TaS2 ), niobium disulfide (NbS2 ), titanium disulfide (TiS2 ), zirconium disulfide (ZrS2 ), vanadium diselenide (VSe2 ), vanadium disulfide (VS2 ), etc. are important types of TMDs which are highly used in supercapacitors [66]. Recently, due to smart physicochemical properties of these materials such as high surface area, electrochemical activity, mechanical permanency, excellent processability, and low cost have triggered wide research for utilization

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Table 2 Comparison of electrochemical properties of sulfides and selenides composite-based electrodes Material

Specific capacitances (F/g)

Energy density (Wh/kg)

Power density (kW/kg)

Capacitances retention (%)

Ref

Core-shell NiMoO4 @MoS2 nanorods

2246.7

47.5

0.44

88.4% after 5000 cycles

[89]

g-C3 N4 and flower-shaped MoS2

45.5

–

–

∼98% after 1000 cycles

[90]

MoS2 -rGO

329

165.66

1.992

91.90% after 10,000 cycles

[91]

MoS2 /Ni3S2

133

35.93

1.1

-

[92]

HR-GOs/Ni-doped MoS2

544

–

–

91.2% after 2500 cycles

[93]

MoS2 /CNTs-MnO2

366

124

916

6.5% at 8 A/g

[94]

NiS/MoS2 @N-rGO

1028

35.69

0.6

94.5% after 50,000 loops

[95]

1 T-MoS2 @TiO2 /Ti

428.1

48.2

2.48

97% after 10,000 cycles

[96]

Nickel sulfides/MoS2

108

40

0.4

100% after 10,000 cycles

[97]

4MoS2 /rGO

365

89

16.7

–

[98]

CoS@MoS2

374.75

95.7

0.7

95.5% after 10,000 cycles

[99]

MoS2 nanowires/NiCo2 O4

51.7

18.4

1.2

98.2% after 8000 cycles

[100]

412.7

47.3

0.5

92% cyclic stability for 2000 cycles

[101]

WS2 /graphene

383.6

–

–

79.9%

[102]

WS2 /PEDOT:PSS

86

–

–

86% after 5000 cycles

[103]

WS2 /N,S-rGO

1563

31.19

0.59

95% after 500 cycles

[104]

WS2 /CFC

399

–

–

99% after 500 cycles

[75]

NRGO-PANI/WS2

715

19

–

–

[105]

435

22

0.3

92% over 5000 [106] cycles

MoS2

WS2 Co3 S4 /WS2

MoSe2 VACNTF@MoSe2 /NF)

(continued)

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Table 2 (continued) 2D MoSe2 -Ni(OH)2

1175

43

8.1

91.6% after 5000 cycles

[107]

Carbon aerogel nanospheres embedded in ultrathin MoSe2

775.3

63.4

5.3

98% after 1500 cycles

[108]

MoSe2 /MWCNTs

232

7.41

0.68

93% after 1000 cycles

[109]

WSe2 /rGO

389

34.5

0.4

98.7% after 3000 cycles

[110]

WSe2 /reduced graphene oxide

419

42.8

14.4

–

[111]

WSe2

in supercapacitors. The phase engineering of chemically exfoliated sheets of metallicTMDs shows enhanced electrochemical behavior by allowing high intercalation of electrolyte ions. However, still there is limited utilization of TMDs-based supercapacitors in practical applications. This is due to the low energy density, low specific capacitance, and high cost of TMDs. Therefore, the fabrications of composites of TMDs with different conducting polymers, conductive carbons such as SWCNTs, MWCNTs, and graphene, etc. are highly encouraged. In addition, the utilization composites of hierarchical 2D, 3D nanostructured TMDs in supercapacitors can enhance energy density and specific capacitance. Hence, there is a need for advanced synthesis methodologies for the synthesis of hierarchical 2D, 3D nanostructured TMDs. This demands special attention and efforts toward the synthesis of different composites of TMDs for applications in supercapacitors. This is because the existing synthesis strategies are not enough to synthesize high surface area and 2D and 3D nanostructured TMDs. This needs advanced synthesis technologies to form hierarchical 2D and 3D nanostructures TMDs. Moreover, to enhance the energy density and specific capacitance of the existed TMDs-based supercapacitors, one needs to consider the theoretical aspects of TMDs materials. In addition to this, the electrochemical parameters of electrode material used in supercapacitor are equally dependent on the electrolyte used in it. Therefore, for enhancement energy density and specific capacitance of TMDs-based supercapacitors, one needs to find a compatible electrolyte for TMDs. In the view of above, researchers are required to more emphasis on their scientific approach to obtain a stable and compatible electrolyte for different TMDs.

9 Conclusions TMDs have been gaining global attention in achieving excellent electrode materials for their applications in high-performance supercapacitors. The unique layered

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structure, high mechanical stability, ultra-thin geometry, large surface area, and flexibility make TMDs a promising candidate for supercapacitor application. Despite various advances, TMDs and their nanocomposites offer low specific capacitance due to their low electrical conductivity than other active electrode materials which can be further improved by phase manipulation and hybridization. TMDs hybrid heterostructures prepared by wrapping or combining them with carbon-based materials (graphene, CNT, MWCNT, etc.), conductive polymers, and metal oxides show high synergistic effects and enable highly active electrodes/electrolyte interfaces. Thus, taking account of the aforementioned challenges it is expected that surface engineering and making hybrid structures open a new pathway for the improvement in the features of electrode materials for future research endeavors in energy storage technology.

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Table 5 Crystallographic information and thermoelectric transport parameters of Mg2 Si material [123] S. No.

Details

Description

1

Chemical formula

Mg2 Si

2

Molar mass

76.695 g/mol

3

Appearance

Dark blue

4

Theoretical density

1.99 g/cm3

5

Melting point

1102 °C, 1358 K

6

Bandgap

0.3–0.6 eV

7

Crystal structure

Cubic

8

Space group

Fm3m

9

Unit cell dimensions

a = 6.3900 Å, b = 6.3900 Å, and c = 6.3900 Å

10

Carrier concentration

0.69–2.83 × 1020 cm−3

11

Seebeck coefficient

−185.06 to −269.26 μVK−1

12

Electrical conductivity

469.92–112.72 Scm−1

13

Power factor

15.67–53.93 μWcm−1 K−2

14

Thermal conductivity

0.98–2.79 W m−1 K−1

15

Figure of merit

0.7–1.2

(HMS) possesses p-type conductivity without any doping with low mobility (∼10 cm2 V−1 s−1 at 300 K ) and high carrier concentration (∼1021 cm−3 ) [161]. As for thermal expansion and mechanical characteristics, both p-type and n-type materials with comparable compositions are required for effective thermoelectric generators. Both p-type and n-type HMS materials were fruitfully produced, although the measured zT value for n-type samples shows low value than for p-type samples. As a result, in order to build viable thermoelectric generators, we must also create good n-type HMS materials. The undoped HMS exhibits anisotropic thermoelectric characteristics and offers the capability to work under wide temperature ranges [161]. The generic formula used to represent HMS is Mnn Si2n-m [162]. All the phases of HMS have very similar unit cells; one of the phases (Mn4 Si7 ) is shown in Fig. 9. Mnn Si2n-m shows high electrical conductivity with high power factor low thermal conductivity. The elements containing HMS show a cheap, earth abundance and non-toxic nature. As can be seen from Table 6, these compounds exhibit a wide bandgap ~0.8 eV. HMS exhibits complex tetragonal incommensurate structure (Novotny chimney-ladder structure) [163], somewhere boost up phonon scattering which leads to reduction of lattice thermal conductivity (when substituting W and Re on Mn sites, due to mass difference between lighter (Mn) and heavier (W or Re) atoms), on comparison with other good silicides [164]. Besides that, HMS overcome the main hindrance of Mg2 Si and based material, is oxidation. HMS has better oxidation stability as compared to magnesium silicide [165].

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Fig. 9 The Nowotny chimney-ladder phases possess a tetragonal crystal structure with almost equal a-axes and unusually long c-axes of higher manganese silicide (HMS) composed of Mn sub-lattice and superimposed Si sub-lattice (Mn4 Si7 )

The best approach to generate p-type nanoparticles for HMS reported ball milling [166], Bridgman [167], chemical vapor transport [168], melting [169], melt-spinning technique [170] and, spark plasma sintering [171]. At 800 K, Sadia et al. fabricated MnSi1.75 and reported 0.62 zT [172]. The stoichiometric elemental powder of HMS was also prepared via mechanical alloying. Truong et al. demonstrated zT 0.55 at 850 K [173]. Unfortunately, this zT value is not equivalent to conventional thermoelectric material. The necessity to increase the performance of HMS via optimizing transport properties and a new fabrication strategy that may boost the figure of merit. The performance of HMS by substituting Cr [174], Fe [175], Co [176], Fe [176], Re [177], and W [164], at the Mn site or Al [178] at the Si site. Ghodke et al. reported zT maximum zT = 1.15 at 873 K [165] by the substitution heavy-element. Challenges [180]: • The secondary phase (MnSi) was observed during the synthesis of HMS. • The secondary phase exhibits high thermal conductivity and low Seebeck coefficient. • Inappropriate thermoelectric material for thermoelectric applications (Fig. 10).

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Table 6 Crystallographic information and thermoelectric transport parameters of MnSi2 material [179] S. No.

Details

Description

1

Chemical formula

Mnn Si2n-m

2

Molar mass

111.109 g/mol

3

Appearance

Gray black

4

Theoretical density

5.19 g/cm3

5

Melting point

1280 °C, 1550 K

6

Bandgap

0.4–0.8 eV

7

Crystal structure

Tetragonal

8

Space group

I-42d

9

Unit cell dimensions

a = 5.5250 Å, b = 5.5250 Å, and c = 65.550 Å

10

Carrier concentration

0.22–2.50 × 1021 cm−3

11

Seebeck coefficient

~181.11–250 μVK−1

12

Electrical conductivity

~171–708 Scm−1

13

Power factor

9.72–18.20 μWcm−1 K−1

14

Thermal conductivity

~1.5–2.6 Wm−1 K−2

15

Figure of merit

0.4–1.1

Fig. 10 3D plots indicating the variation of electronic thermoelectric figure of merit (zT) for the TE materials (Bi2 Te3 [181–189], SnTe [105, 190–197], PbTe [70–76, 198–201], SnSe [89, 90, 202–207], Mg2 Si [135, 140, 146, 160, 208–213]), HMS [165, 169, 172, 177, 214–219], plotted against the temperature with their corresponding years of publication

4 TE Device and Challenges To improve the TEG performance is a continuous process for future expectations. According to theoretical expectations, the efficiency of the TEG significantly improved in nanostructure engineering [220]. To develop efficient material, the challenges are vital. Solving these challenges is a topic of research, which leads to

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growing TEGs and opening new opportunities in various applications. For example, the major challenge of TEG is acquiring higher zT, which is interrelated between thermal conductivity and electrical conductivity of most materials. The TE device construction is still challenging, the utilization of thermal-toelectric conversion has been realized only in niche applications. The important challenge behind the TE device module is the interface between thermoelectric elements and electrodes. The proposed important materials for the TE device module are (i) TE materials, (ii) TE interface materials, (iii) TE package materials, and (iv) TE housing materials. Among these materials, the TE materials attracted more attention from thermoelectric researchers. Moreover, the TE interfaces with TE materials are still open challenges for TEG device assembly. General problems are the high cost and low efficiency of TEG as well as high output resistance. Furthermore, Fig. 11 shows the various factors other than materials used for efficient TEG. The essential factors are a high figure of merit, durability, low maintenance, mass flow rate, and gradient temperature, which influence the design and application. These factors affect the consistency, which are interrelated. Over the years, TE materials have gained a lot of attention due to their exceptional growth from both industrial and research perspectives. The choice of a TEG mainly depends upon the type of TE material used and the credibility with which it can be used in different application areas. The TE compounds can be classified depending upon their operating or working temperature range. Table 7 shows the various applications of the TE compounds that were discussed in Sect. 3. Nowadays, TE materials are needed in many industrial applications where it fills a great need in the military [27], electronic sensors, and cars [222]. Additionally, TE materials have acquired significant importance, especially after being integrated into the low-dimensional structure (nanotechnology). The useability of the TE materials has also been increased in terms of flexibility in carrying the TEG modules as running personal electronics through body heat, such as IoT devices in Fig. 11 The various factors that regulate the performance of TEG [221]

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Table 7 Summary of the investigated applications of the reviewed compounds in different areas and classified in the low- to mid- to high-temperature ranges [27, 81, 222–225] Range

Materials

Applications

Low temperature (800 K)

Automobile Airborne Mobile applications Optoelectronic silicon technology

the form of mobile phones and wearable sensor systems [222]. A low-power thermoelectric generator (ranges from 5 μW to 1 W) would be a wearable thermoelectric device (wristwatches), which uses thin bulk TEDs. Altogether the simplicity, durability, eco-friendliness, and noise-free nature serve as an advantage for TE material and add value to the TE-based device in various applications [221]. It is a known fact that the traditional TE materials like Bi2 Te3 and PbTe are known for their longevity and continuity in the TE field, although their usability can be found more in aerospace and military applications. The thermoelectric properties of PbTe were extensively studied in both the USA and the Soviet Union for military and space applications in the 1950s and early 1960s [27]. Recently, TEG module is used in the NASA space mission 2020, for providing power to Mars perseverance; this source is called a Multi-Mission Radioisotope Thermoelectric Generator [223]. Bi2 Te3 and PbTe are considered as best TE materials at and above room temperature, but it contains expensive and toxic elements (Bi and Pb). One of the best materials found to date in thermoelectrics with a high zT and the desired temperature range (300–800 K) is SnSe. The low-temperature phase (Pnma) SnSe has been widely studied for either application in energy storage (Li-ion batteries), electronic devices due to its high hole mobility, or applications in solid-state devices such as optoelectronic silicon technology and solar cells [224]. Similar to SnSe, there are other tin compounds like SnTe. Among all the above-mentioned TE materials, the family of Silicides (Mg2 Si and so forth) materials is superior of relative abundance, non-poisonousness, thermal stability, cost-effectiveness, and environment friendliness. Mg2 Si has been considered as the best TE material in the 500–1000 K temperature range [122, 148] and is notable for its application in the automobiles and aerospace industry because of its high explicit strength. Considerably, these TE materials have been widely used in

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both commercial and industrial applications. Starting from spacecraft, automobiles, and ending in fast and efficient wearable electronics (IoTs), these materials have also been employed in other energy application purposes such as energy storage, supercapacitors and PV cells. Further advancement in the experimental methods for the chemical and physical synthesis of these compounds with achieving high theoretical efficiency assures enormous progress in green energy projects. To overcome all these challenges, the growth of TE materials requires continuous research in all the fields like material scientists, physicists, and chemists.

5 Conclusion Extensive research is being carried on for the past few decades to find highperformance thermoelectric materials. To further accelerate this process, we require a clear understanding of the associated challenges at each step toward designing an efficient thermoelectric device. In this book chapter, we have summarized these challenges originating at different length scales—sub-nano scale (atomic and electronic level), nanoscale (nanostructures and nanoparticles), and micro-scale and beyond (device and module level). At all these levels, there are several critical aspects to be considered, which otherwise could collectively deteriorate the final performance of a thermoelectric device. One such aspect is selecting an appropriate device architecture that maintains proper heat-sink and heat-source temperatures, has thermally and electrically conducting contacts, ulfils thermal expansion matching criteria at interfaces, considers efficient thermal management, and most importantly, maintains device reliability. Moreover, manifesting the ideas of further technological improvements, such as device miniaturization and flexible thermoelectrics, imposes additional bottlenecks on device integration and performance. We have discussed these aspects in this book chapter. We have also shed some light on the device packaging elucidating the air sensitivity of thermoelectric materials. Device fabrication is crucial only when we have a constituting material that has a high thermoelectric figure of merit zT. As a high zT depends on a high power factor and a low thermal conductivity, simultaneously optimizing these properties in a material is also important. This throws a major challenge because the thermal and electrical properties are not decoupled. Thus, many efforts in optimizing one distort the other. Moreover, the electrical conductivity and Seebeck coefficient— that collectively contribute to the power factor—have competing trends too. All these factors make it an even more challenging problem to search for a high zT material. Thus, in this book chapter, we have also discussed the state-of-the-art techniques to optimize the properties of thermoelectric materials. For achieving low thermal transport, firstly the materials with a high intrinsic phonon–phonon scattering and consequently low intrinsic thermal conductivity are interesting. Additionally, including point defects, line defects, grain boundaries, and nanostructuring further restricts the thermal transport in the material. To enhance the power factor, several techniques such as band structure engineering, introducing multi-valley degenerate

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bands by doping, resonant scattering, and modulation doping are employed. We have discussed these strategies and their resulting effects in some of the classic thermoelectric materials—Bi2 Te3 , PbTe, SnSe, SnTe, Mg2 Si, and MnSi2 . We have compared their properties and have also demonstrated the challenges in synthesizing these materials. All the listed challenges in the current status of thermoelectric technology provide a roadmap for the future. In recent years, thermoelectrics research has come a long way. On the one hand, several cutting-edge computational and theoretical findings have enhanced our conceptual understanding of optimizing zT, which undoubtedly shows a promising path ahead. On the other hand, advancements in experimental techniques have also improved significantly and have enabled us to push the limits of device fabrication. Such a prior groundwork will certainly help to overcome the challenges highlighted in this work. Finally, it is also important to get rid of the toxic elemental constituents in a thermoelectric material. This leads to an important concern of finding non-toxic, environmentally friendly thermoelectric materials for our ultimate goal of clean energy harvesting in the future. Acknowledgements This work is supported by GGSIPU under the FRGS scheme and grant no. GSIPU/DRC/FRGS/2021/594/13. Ankita Katre and Neeta Bisht acknowledge the support from DST-INSPIRE Faculty Scheme (Grant No. IFA17-MS122). Author’s Contribution All the authors have contributed equally. Conflict of Interest The authors declare no conflicts of interest.

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Chapter 5

Bi2 Se3 Topological Insulator Thin Films for Various Device Applications Sudhanshu Gautam and Sunil S. Kushvaha

1 Introduction In general, the quantum states of matter are categorized by the type of symmetry breaking associated but the discovery in the 1980s of the integer and fractional quantum Hall effects have given birth to a new organizational principle of quantum matter [1, 2]. In quantum Hall phenomena, the “bulk” of the electron gas considered as an insulator whereas the circulating edge states are extremely robust as they pertinacious even in the presence of impurities. In past few years, “robust” conducting edge states observed in the quantum Hall state are also seen on the boundary of two-dimensional (2D) band insulators with large spin–orbit coupling, known now as topological insulators (TIs) [3–6]. TIs are an exotic state of quantum matter in which insulating bulk states and gapless conducting surface states or metallic edge present due to an inverted ordering of their bulk electronic bands [6]. The schematic of TI materials and their different band gap diagrams are presented in Fig. 1 [6]. The highly conductive states on the surface of TIs are immune to localization and topologically protected by the time reversal symmetry. It implies that surface states of TIs are extremely robust against any crystalline defects or impurities, scattering by nonmagnetic impurities and surface distortion [6, 7]. The population of electrons in TI surface states has only one spin state per momentum state (spin-momentum locking) in comparison to conventional materials. The TI materials may serve as a platform to study the various fundamental quantum behaviors as well as technological applications in the field of spintronics, quantum anomalous Hall effect, thermoelectric dissipationless electronics, and quantum information processing [8–10]. S. Gautam CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India S. S. Kushvaha (B) Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_5

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Fig. 1 (a) Schematic representation of topological insulator material: bulk is insulating where surface is conducting or metallic nature. Energy band diagram of: (b) insulator with large band gap (>10 eV) and (c) metallic or conducting with no gap between conduction band and valance band. Energy band diagram of 2D (d) and 3D (e) topological insulators showing 1D and 2D Dirac cone, respectively

The TI materials are generally having two categories on the basis of dimensionality: 2D and 3D TIs in which 2D TIs are also related to integer quantum Hall state where the spin–orbit interaction act as an effective magnetic field and this induced magnetic field act in opposite directions for opposite spins [5]. The existence of a 2D topologically insulating state was predicted by Kane et al. in 2005 [5] and the first 2D TI material (HgTe/CdTe quantum wells) was predicted by Andrei-Bernevig et al. in which TI state was characterized by 1D conducting channels on the edge of materials [11]. In the most widely investigated TIs, the chalchogenides Bi2 Se3 , Bi2 Te3 and Sb2 Te3 are model examples of 3D TIs as these materials possess the single Dirac-like band dispersion in topological surface state due to large spin–orbit effect [4, 12–14]. These materials have realistically large (~few 100 meV) and simple electronic structures, which made them as ideal templates to realize for various exotic quantum phenomena and device applications. Among various binary chalchogenides materials, bismuth-based compounds such as Bi2 Se3 and Bi2 Te3 are intrinsically narrow-band semiconductors, they possess the extrinsic in nature due to the unintentional doping induced by native defects [4, 14, 15]. For instance, Bi2 Se3 often show n-type conductivity whereas strong tendency of p-type conductivity appears in Sb2 Te3 . The Bi2 Se3 has been studied extensively due to larger band gap (~0.3 eV) compared to Bi2 Te3 (~0.1 eV), offering more control at

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elevated temperature [4, 16]. Apart, high melting point of Bi2 Se3 (706 °C) compared to Bi2 Te3 (585 °C) material give advantage to use in relatively high-temperature applications. The crystal structure of Bi2 Se3 has a rhombohedral tetradymite-type with the space group R-3 m, which is described in terms of quintuple layer made of five covalently bonded monoatomic sheets along c-axis, Se-Bi-Se-Bi-Se. The detailed crystal structure of Bi2 Se3 is shown in Fig. 2a with all details such as quintuple layers, van-der Walls gap, and ABC stacking. The quintuple layers (one quintuple thickness ~0.955 nm) are bonded with weak van der Waals forces forming a unique crystal structure with lattice parameters: a = b = 0.414 nm and c = 2.864 nm [17]. The stacking in x–y direction of Bi, Se (1) and Se (2) structures is presented in Fig. 2b.

Fig. 2 (a) Rhombohedral unit cell of Bi2 Se3 with all the details such as quintuple layer, stacking and van der Waals gap (b) Top view of the crystal unit cell structure of Bi2 Se3 with coordinate transformation at the sample rotation. The Se-1 and Se-2 refer to different Se-atom lattice positions in Bi2 Se3 crystal structure

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Table 1 Important physical properties of important chalcogenides-based topological and thermoelectric materials [17–22] Property Lattice constants

Bi2 Se3

Bi2 Te3

Sb2 Te3

a = b (Å)

4.14

4.38

4.25

c (Å)

28.64

30.49

30.48

Hexagonal

Hexagonal

Hexagonal

Stable crystal phase Band gap energy Eg (eV) at 300 K

0.3

0.1–0.14

0.21

Exciton binding energy (meV)

80

–

36

Thermal expansion

α perp. (K−1 ) α para.

Thermal conductivity (W

m−1

(K−1 )

K−1 )

Acoustic phonon potential (eV)

1.1 × 10–5

1.4 × 10–5

1.8 × 10–5

1.9 ×

2.1 ×

3.2 × 10–5

10–5

10–5

1.70

1.37

2.2

11

22

25

Density ρ (g cm−3 )

7.51

7.85

6.5

Melting point (K)

979

858

893

Mobility (cm2 V−1 s−1 )

1407

481

675

The physical and crystalline properties of the well-known chalcogenides-based Bi2 Se3 , Bi2 Te3 and Sb2 Te3 TI materials are presented in Table 1 [17–22]. Even before the discovery of TIs behavior in Bi2 Se3 material, this material is well known for various applications (Fig. 3) in the field of smart windows, infra-red photodetectors, electrically regulated optical filters, optical recording and laser photonics [15–27]. The near infrared photo detectors have tremendous applications in many Fig. 3 Application of Bi2 Se3 materials in various strategic sectors and devices

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strategic and health sectors such as medical imaging, telecommunication, and hazard gas sensing. For photo-detector applications, materials band gap and photocatalytic action are key parameters for the detectivity and responsivity of the photo-detectors [21]. It will be beneficial to select earth-abundant and cheaper materials, which can be readily synthesized/ processed, and Bi2 Se3 is one among these categories of materials. Optical and electrical property characterizations of Bi2 Se3 thin films are necessary for better understanding of reflection, photocurrent sensitivity and light absorption for the optoelectronic devices. The chemical deposition of Bi2 Se3 thin films showed the optical band gap in the range of 1.41–1.7 eV whereas it decreased to 1.06–1.57 eV after annealing at 200 °C under nitrogen ambient [23]. In other study by Pejova et al., they found that as-chemical deposited Bi2 Se3 films are characterized with optical band gap energy 2.3 eV, and it is almost same upon annealing process [24]. Adam et al. reported the optical properties of the Bi2 Se3 thin films deposited by vacuum thermal evaporation at different annealing temperatures, and they found that the absorption coefficient decreases whereas optical band gap increases from 1.51 to 1.83 eV with annealing temperature [15]. The systematic further studies of optical and electrical properties of Bi2 Se3 thin film deposited on various substrates are required for better understanding of the variation in optical band gap of large-area sputtered Bi2 Se3 thin films. Along with topological surface states, the Bi2 Se3 material is also an excellent thermoelectric material as it possesses narrow band gap and consists of heavy Bi element. Capability of direct energy conversions between heat and electricity in thermoelectric materials, these materials have tremendous application in the field of refrigeration and power generation. Compared with conventional materials used for refrigeration, the Bi2 Se3 -based materials have several advantages such as lightweight, solid-state devices without moving parts, eco-friendly, no noise or vibration generation, precise temperature control and long/reliable working life with relatively low cost, among others [10]. The thermoelectric figure of merit (ZT) for thermoelectric materials is defined as ZT = S2 σT/κ where S is thermopower or Seebeck coefficient, T is absolute temperature, σ and κ are electrical and thermal conductivities, respectively. The power factor of thermoelectric materials is quantitively defined by S2 σ and increase in power factor enhanced the ZT value. Kadel et al. have studied the thermoelectric properties of Bi2 Se3 nanostructures synthesized by solvothermal process, and they obtained the dimensionless ZT value of 0.096 at 523 K. They have reported the Seebeck coefficient value for Bi2 Se3 nanoparticles of about −115 μV/K at room temperature [25], which is nearly two times as much as for bulk Bi2 Se3 value of − 59 μV/K at room temperature [26]. Due to the presence of topological surface states on Bi2 Se3 , it has been predicted the improved thermoelectric figure of merit and Seebeck coefficient in thin layers of bismuth chalcogenides [12]. Guo et al. reported the tuning of the thermoelectricity in a molecular beam epitaxy (MBE) grown Bi2 Se3 TI film on SrTiO3 (111) with varied thickness with Seebeck coefficient value of − 104.3 μV/K at room temperature [27]. Further work on the thermoelectric properties of Bi2 Se3 thin films on Si-based substrates on large-scale area will lead to fabrication of low-cost thermoelectric devices.

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The Bi2 Se3 single crystals and thin films have been obtained by several growth and deposition techniques as per the requirements of the applications [12–17, 22– 27]. Xia et al. have grown Bi2 Se3 single crystals by melting stoichiometric mixtures of Bi and Se in a quartz tube where the sample was cooled from 850 to 650 °C slowly for 2 days followed by annealing at 650° C for a week time [12]. Irfan et al. used modified Bridgman technique for the growth of Bi2 Se3 single crystals in which high purity ingot of bismuth and selenium in a stoichiometric ratio was kept in a tapered ampoule at a pressure of about 10–6 Torr and heated at 850 °C for 36 hours followed by gradually cooling over a period of 5–6 days to 650 °C and finally ampoules were quenched in liquid nitrogen [28]. However, the bulk defects and natural doping in Bi2 Se3 single crystals due to the presence of Se-vacancies lead to a contribution of the bulk carrier density to the conductivity. Therefore, for investigations of the topological surface states, thin layers of Bi2 Se3 are preferred over single crystals. In case of exfoliated flakes, there may be several issues such as irregular shape, unreproducible size and presence of intrinsic defects due to the mechanical breaking of the crystal during exfoliation [29]. In this respect, fabrication of thin films and heterostructures of Bi2 Se3 have several advantages such as large surface to volume ratios, tuning of the thickness, compatible substrates for device fabrication, control over the doping and interface engineering, among others [27, 30]. Several techniques have effectively prepared Bi2 Se3 thin films and heterostructures, including pulsed laser deposition (PLD) and MBE and studied their crystalline and other properties for various application perspectives [27, 30, 31]. For example, Bigi et al. recently reported the growth of c-axis-oriented Bi2 Se3 thin films by PLD on SrTiO3 (001) substrates [31]. The epitaxial Bi2 Se3 thin films have been also grown on Al2 O3 (0001) substrates at relatively low temperature with low laser repetition rate by PLD via domain matching epitaxy paradigm [32]. Lai et al. also used PLD technique to grow Bi2 Se3 thin films on c-plane sapphire to measure the nanomechanical and nanoindentation properties of Bi2 Se3 films [33]. In addition, it has been reported that epitaxial and polycrystalline Bi2 Se3 thin films have been grown by PLD [34, 35], and for example, Bi2 Se3 films on InP (111) have triangular pyramids with growth along the c-axis [29]. Epitaxial Bi2 Se3 thin films and heterostructures were grown using MBE system on various single crystalline substrates with control of mono-layer thick films [27, 30, 36–40]. Chen et al. reported the large tunablity in carrier density in MBE-grown epitaxial Bi2 Se3 films on SrTiO3 substrates using back gate [39]. In some cases, the Bi2 Se3 epitaxial thin films were grown by MBE on epitaxial graphene/SiC (0001) without forming any interfacial dislocations, and it follows the spiral Bi2 Se3 thin film growth [36]. Levy et al. introduced the pre-growth optimization process in MBE growth for Bi2 Se3 layers on sapphire (0001) substrates and obtained low twinning and very smooth surface of Bi2 Se3 thin films [40]. Even ordered growth of Bi2 Se3 thin films along the (001) direction was obtained on amorphous SiO2 using Se-passivation in MBE growth process [41]. However, these PLD and MBE techniques are quite expensive, and there is limitation for growth of largearea thin films for the industrial applications. Compared with these growth techniques, magnetron sputtering deposition has been widely used for the production of

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large-area thin films owing to its low cost, high deposition rate, good film-forming uniformity and relatively simple process. Recently, several studies showed the deposition of large-area Bi2 Se3 thin films using magnetron sputtering and evaporation techniques on various substrates [42– 47]. The Bi2 Se3 thin films were deposited using magnetron sputtering system on Si (100), and surface exhibited terrace-like quintuple layers with good crystalline quality [42]. Wei et al. have reported the deposition of Bi2 Se3 thin films on Si (111) substrates using magnetron sputtering system, and they achieved highly caxis-oriented Bi2 Se3 thin films after post-annealing under Se-rich environment [43]. Recently, Mahendra et al. reported the room-temperature high spin–orbit torque in the magnetron-sputtered Bix Se1-x thin films on thermally oxidized silicon substrates, and they found the polycrystalline nature of sputtered Bix Se1-x films for high spin–orbit torque due to the quantum confinement effect [44]. The Bi2 Se3 thin film deposited by thermal evaporator technique showed the good crystalline quality after postannealing, and it also shows the TI behavior in the film [45]. Wang et al. reported the growth of Bi2 Se3 TI thin films on SrTiO3 (111) substrates by r.f. magnetron sputtering system, and they have found the 2D weak anti-localization effect at low temperature [46]. Recently, Kumar et al. reported the deposition of single crystalline Bi2 Se3 thin films along the [001] direction on SrTiO3 (111) substrate by DC magnetron sputtering after post-annealing in Se-environment [47]. These reported works showed the importance of magnetron sputtering system for deposition of high-quality Bi2 Se3 thin films for various applications by relatively simpler and easily scalable deposition technique. The formation of good crystalline quality of the sputtered Bi2 Se3 thin films has been studied previously, and mostly, topological surface states studies have been reported on selected substrates [42–44]. Here, we report the r.f. magnetron sputtering depositions of Bi2 Se3 thin films on various substrates such as Si(111), Si(100), sapphire (0001), and quartz under various deposition parameters such as deposition temperature, Ar-gas flow, and r.f. power. All the sputtered films were post-annealed in the Se-environment to maintain the stoichiometry in Bi2 Se3 thin film. The structural, crystalline, optical, thermoelectric, and electrical characteristics of Bi2 Se3 thin films deposited on various substrates were analyzed using different characterization techniques. In case of Bi2 Se3 on Si(100), the effect of r.f. power on the electrical properties of Bi2 Se3 thin films has been studied, and it showed the structural and morphological dependent characteristics. The optical properties of Bi2 Se3 thin films on quartz and sapphire substrates have been studied in detail for better understanding of optoelectronic properties such as transmission, optical band gap and light absorption. The thermoelectric properties of Bi2 Se3 thin films deposited on n-type Si(100) have been studied, and it showed high Seebeck coefficient of ~−190 μV/K at room temperature.

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2 Experimental Section The deposition of Bi2 Se3 films on various single crystalline substrates was carried out in a high vacuum r.f. magnetron sputtering system. A magnetron sputtering system is equipped with substrate heater, various confocal sputtering targets, and Ar gas mass flow controller as shown in Fig. 4a. The magnetron sputtering system has two chambers separated by a manual gate valve: main deposition chamber and load lock chamber having transfer arm for transferring the samples without breaking the vacuum in main chamber. The base pressure in the main deposition chamber was better than 2 × 10–7 mbar. We have used various substrates such as Si(111), p-Si(100), n-Si(100), quartz, and sapphire (0001) for the deposition of Bi2 Se3 thin films. All substrates were cleaned by organic solvents followed by de-ionized water. The substrates were heated inside the growth chamber using a resistive heater. High purity commercial Bi2 Se3 (99.99%) sputtering target was used as sputtering source. The sputtering process was performed in a high pure Ar (99.9999%) atmosphere with a gas flow rate of 10–20 sccm and working pressure of ~3–5 × 10–3 mbar. We have deposited Bi2 Se3 thin films on these substrates at deposition temperature of 360–400 °C with sputtering powers of 10–20 W, depending on the requirement of the deposition rate and nature of the substrates. The plasma generated during sputtering of Bi2 Se3 target in the presence of Ar-gas and thin film deposition process on Si-substrate is shown in Fig. 4b, c. The selenium deficiency can be expected in sputtered Bi2 Se3 samples due to difference in the momentum transfer of Bi and Se during the sputtering process. To maintain the stoichiometry, we have performed the post-selenization process in tubular furnace in Se-rich environment at temperature range of 300–400 °C for 1–2 hours, and the schematic of the selenization process is

Fig. 4 (a) Magnetron sputtering system equipped with confocal sputtering guns, main deposition chamber separated by load lock chamber; plasma during sputtering process (b) and during deposition on Si-based substrate (c)

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Fig. 5 Schematic representation of post-selenization process of sputtered Bi2 Se3 thin films at temperature range of 300–400 °C in the presence of continuous Ar-gas flow

shown in Fig. 5. The samples and Se powder were kept inside the graphitic box, and the temperature was kept constant during selenization process. The structural properties of the Bi2 Se3 thin films were characterized using highresolution X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. An XRD system was employed to characterize the crystalline nature of sputtered Bi2 Se3 thin films on various substrates using CuKα1 radiation. The plan view SEM image and energy dispersive X-ray analysis (EDAX) of Bi2 Se3 thin films deposited by magnetron sputtering system followed by post-selenization process were carried at voltage range 5–15 kV without any Au coatings on the film surface. The thickness of Bi2 Se3 films on various substrates was measured using a stylus profilometer by proper masking on the film. Raman spectra were measured at room temperature using an excitation laser source with wavelength of 514.5 nm. The p-n diode characteristics of Bi2 Se3 thin films deposited on p-Si(100) were carried out by depositing Al metal contacts, and I–V measurement was performed using twoprobe electrodes at room temperature under dark conditions. The room temperature carrier concentration and mobility of the Bi2 Se3 thin films were measured using Hall effect measurement system. The optical properties of sputtered Bi2 Se3 thin films on quartz and sapphire (0001) were characterized using broadband optical spectrometer in the wavelength range of 300–2500 nm (Agilent Technologies, Cary Series UV– Vis-NIR spectrometer). The Bi2 Se3 thin films deposited on n-Si(100) were cut into a rectangular bar shape for the measurement of thermoelectric parameters such as temperature-dependent electrical resistivity, power factor, and Seebeck coefficient using the four-probe method.

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3 Result and Discussion We have used different substrates to deposit Bi2 Se3 thin films using magnetron sputtering system for various applications. The detailed structural, electrical, optical, and thermoelectric properties of the magnetron sputtered Bi2 Se3 thin films are discussed below.

3.1 Structural and Crystalline Properties oF Bi2 Se3 Film on Si(111) The Bi2 Se3 thin film was deposited on Si (111) substrates at 360 °C using magnetron sputtering system at 20 W r.f. power. Figure 6a represents SEM image of the assputtered Bi2 Se3 thin film on Si(111), and it clearly shows the growth of truncated triangle or hexagonal grain with lateral sizes in the range of 300–400 nm. As-deposited Bi2 Se3 thin films show the Se-deficiency in the deposited film as

Fig. 6 Plan-view SEM image (a) and EDAX (b) of as sputtered-Bi2 Se3 thin film on Si(111) at 360 °C. SEM image (c) and EDAX (d) of post-selenized Bi2 Se3 thin film deposited on Si(111) at substrate temperature of 360 °C

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atomic percentages for Bi and Se are 57% and 43%, respectively, as characterized by EDAX analysis as shown in Fig. 6b. Further, these sputtered Bi2 Se3 thin films were post-selenized in the tubular furnace at temperature of 360 °C under Ar atmosphere for 90 min in the presence of Se. The surface morphology Fig. 6c of the post-selenized Bi2 Se3 thin film on Si(111) surface shows also similar triangle or hexagonal morphology. The EDAX measurement discloses the Bi2 Se3 thin film with stoichiometric ratio close to 2:3 (Bi: 38%, Se: 62%) as shown in Fig. 6d. The surface morphology of the Bi2 Se3 thin films grown on different substrates shows the triangular and hexagonal shapes due to the hexagonal phase of Bi2 Se3 and preferred growth along c-axis on Si(111) and other substrates [38, 48–51]. Zhang et al. also reported the chemical vapor deposition of Bi2 Se3 on graphene/SiO2 /Si, and they have observed coalesced plates of triangular and hexagonal shapes as these are well aligned along graphene layer [48]. Wang et al. reported the MBE growth of Bi2 Se3 on various substrates such as Si(111), GaN and In2 Se3 and mostly triangular spiral type single-crystalline Bi2 Se3 thin film was obtained on substrates those have small lattice misfits with Bi2 Se3 and chemically inert for Bi and/or Se precursors [38]. In other report, it was observed the nanometer-sized quasi-equilateral Bi2 Se3 triangular terraces growth on SrTiO3 (111) substrates using MBE technique [50]. These observations clearly revealed that triangular or hexagonal Bi2 Se3 shapes obtained on Si(111) by sputtering process are related with the inherent nature of hexagonal Bi2 Se3 phase, and this morphology is qualitatively similar to previous work [51]. The quintuple layer of Bi2 Se3 as shown in Fig. 2a possesses five atomic layers in which Bi and Se atoms are arranged alternatively like -Se1-Bi-Se2-Bi-Se2- ways separated by van der Waals gap. According to group theory, the Bi2 Se3 material has gerade (Raman active modes) and ungerade (infra-red active modes), and it can be represented as zone center phonon as follow:  = 2Eg + 2A1g + 2Eu + 2A1u , where g and u denote the gerade and ungerade modes, respectively. The three Raman active modes for Bi2 Se3 thin films are generally obtained: E2 g in in-plane direction, A1 1g (out-of-plane), and A2 1g (out-of-plane) [28, 51–53]. Figure 7a shows the Raman spectra of as-deposited and post-selenized Bi2 Se3 thin film on Si (111) recorded at room temperature. As-sputtered film shows four Raman peaks in range of 50– 200 cm−1 , and only one peak at 68.2 cm−1 seems to close to A1 1g Raman mode of Bi2 Se3 . However, other Raman peaks correspond to non-stoichiometric Bi2 Se3 thin films obtained after sputtered process only and further post-selenization treatment is required. After post-selenization process, Raman spectroscopy shows three pronounced characteristic peaks in the low wavenumber region at 71.9, 131.2, and 173.9 cm−1 , and they belong to; namely, A1 1g (out-of-plane), E2 g (in-plane), and A2 1g (out-of-plane) modes in Bi2 Se3 thin films, respectively. The E1 g Raman active mode in lowest wave number range is not observed, similar to other reports [51– 53]. The A1 1g peak observed in this spectrum is narrow and has full width at half maximum (FWHM) of ~4.4 cm−1 whereas A2 1g mode has broader peak with FWHM value of 9.5 cm−1 , reflecting the high structural quality of Bi2 Se3 thin film on Si(111) substrate. The high-resolution XRD 2θ-omega scan of post-selenized Bi2 Se3 thin film on Si(111) has been presented in Fig. 7b. The diffraction peaks at ~9.4, 18.7, 28.1, 37.9,

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Fig. 7 a Raman spectra of as sputtered and post-selenized Bi2 Se3 thin film on Si(111). b High-resolution XRD 2theta-omega scan of post-selenized Bi2 Se3 thin film deposited on Si(111) at substrate temperature of 360 °C

47.8, 58.4, and 69.1° corresponding to (0003), (0006), (0009), (00,012), (00,015), (00,018), and (00,021) planes of hexagonal Bi2 Se3 , respectively. These observations clearly show that the Bi2 Se3 has a high degree of texturing along c-axis normal to the Si (111) with the expected rhombohedral structure and showed only {0003n} family of planes of Bi2 Se3 , similar to previous work on epitaxial films on sapphire (0001) and Si(111) substrates [51, 54–56]. One additional XRD peak is observed for Bi2 Se3 thin film on Si(111), and it is likely related to excess Se present on surface during post-selenization process. Le et al. also reported that the deposition temperature in PLD growth of Bi2 Se3 on SrTiO3 (111) at 180 °C and above the deposited films showed polycrystalline with a highly c-axis-preferred orientation where the crystallinity of films deduced by estimating FWHM of (0003), (0006), (0009), and (00,015) diffraction peaks [57]. Throughout in this chapter, we have adopted similar post-selenization process for the sputtered Bi2 Se3 thin films on various substrates for their electrical, optical, and thermoelectric properties.

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3.2 Electrical Properties of Bi2 Se3 /p-Si(100) P–N Junction To study the electrical properties of the Bi2 Se3 thin films, we have used Si(100) substrates as Si(100) is more preferred over other Si planes due to easy device fabrication processes. In addition, it has been also proposed to electrically measure the spin-momentum coupling in the Bi2 Se3 -based TI by injection of spin-polarized electrons from silicon [58]. Figure 8a represents the SEM image of Bi2Se3 thin film on Si(100) deposited at 360 °C and hexagonal or truncated hexagonal large variation in grain sizes ranging from 300 to 700 nm. The EDAX measurements revealed the atomic percentages for Bi and Se are 38 ± 2 and 62 ± 2%, respectively. With further increase in deposition temperature to 400 °C, the surface morphology studied reveals the uncovered Si(100) surface as shown in Fig. 8b. Interestingly, we observed the formation of multiple layer types of structures, and it is believed to be the one-layer thick equivalent to one quintuple layer for Bi2 Se3 . Apart from the hexagonal grains, the spiral or layer-by-layer type of Bi2 Se3 grains was also seen, and the EDAX Fig. 8 Plan-view SEM images of Bi2 Se3 thin films deposited on p-Si(100) at: 360 (a) and 400 °C (b)

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measurement discloses the Bi2 Se3 thin film with stoichiometric ratio nearly close to 2:3 (Bi: 38%, Se: 62%). Meng et al. also reported the growth of Bi2 Se3 thin films on Si(100) by PLD techniques from varying the growth temperature from room temperature to 400 °C, and they found the hexagonal Bi2 Se3 structure with increased surface roughness with substrate temperature [59]. These observations clearly indicate that the surface morphology of the Bi2 Se3 thin film on Si(100) obtained in our study is similar to the previous work [59], and surface morphology/coverage critically depends on the substrate temperature. The structural quality of the Bi2 Se3 thin films deposited on p-Si(100) at different substrate temperatures is measured using Raman spectroscopy in the backscattering mode at room temperature. Figure 9a represents the different Raman modes present for stoichiometric Bi2 Se3 compound in which relative motion of Bi and Se atoms

Fig. 9 (a) Schematic representation of Raman active modes (E1 1g , E2 g , A1 1g and A2 1g ) for rhombohedral Bi2 Se3 compound (b) Raman spectra of Bi2 Se3 thin films deposited on Si(100) at 360 and 400 °C, keeping all deposition parameters similar

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for various phonon modes, i.e. E1 1g , E2 g , A1 1g , and A2 1g [28, 60–62]. Raman spectra of Bi2 Se3 thin films deposited on p-Si(100) at substrate temperature of 360 °C and 400 °C are presented in Fig. 9b. The three pronounced Raman peaks corresponding to A1 1g , E2 g , and A2 1g modes have been observed for both samples, and the peak positions at 360 °C (400 °C) are obtained as in the low wavenumber region at 72.6 (71.9), 132.1 (132.6), and 175 (174.5) cm−1 , respectively. It has been reported that single crystal of Bi2 Se3 possesses these three active Raman modes at 72, 130, and 174 cm−1 [28, 60]. Compared with the reported literatures, the peak position of E2 g mode is slightly blue-shifted compared to the bulk Bi2 Se3 single crystal, and it is likely due to the presence of stress in the films as thickness of these films is relatively lower than bulk single crystals or flakes [28, 60]. The electrical properties of Bi2 Se3 thin film deposited on the p-Si(100) substrates at 360 and 400 °C were also measured, and schematic diagram of I–V measurements is shown in Fig. 10a. The aluminum (Al) metal contacts on the heterostructure were patterned with the help of a metallic shadow mask, which have circular open dots of 300 μm diameter with spacing of 700 μm. The mask is well aligned to ensure contacts are made on the Bi2 Se3 and p-Si(100) of the junction. In order to understand Fig. 10 (a) Schematic diagram of I–V measurement on Bi2 Se3 /Si(100) using Ag metal contacts; (b) I–V curves across p–n junction formed on Bi2 Se3 /p-Si(100) for films deposited at 360 and 400 °C

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the effect of deposition temperature on the electrical properties of the Bi2 Se3 /Si(100) p–n junction, the I–V curves are measured on the Bi2 Se3 , p-Si(100) and across Bi2 Se3 and Si(100). The Hall measurement was used to identify the type of charge carriers in the Bi2 Se3 film using the Van der Pauw method, and it showed the n-type carrier. The I–V curves measured in forward and reverse bias condition in the range of + 5 to −5 V on the p–n junction formed by n- Bi2 Se3 on p-Si(100) are presented in Fig. 10b. The I–V curves clearly show that the forward bias current is obtained to be 9.37 × 10–6 and 1.56 × 10–5 A at 5 V for samples deposited at 360 and 400 °C, respectively. In both samples, the reverse bias leakage current at −5 V was obtained to be very low, i.e. 3.57 × 10–9 and 3.38 × 10–9 A for samples deposited at 360 and 400 °C, respectively. The rectification ratios (forward to reverse current ratio) for Bi2 Se3 /p-Si(100) junction formed on Bi2 Se3 deposited at 360 and 400 °C are 2.62 × 103 and 4.62 × 103 , respectively. Compared with the recent report on the SnTe/Bi2 Se3 p–n junction, the rectification ratio obtained for both films om p-Si(100) is very high compared to SnTe/Bi2 Se3 rectification ratio of 700 [63]. Generally, the improved crystalline quality of the film improves the p–n junction characteristics and rectification ratio [64]. It is understood that the improved crystalline quality of Bi2 Se3 thin films on flat p-Si(100) substrate results in the reduced leakage current compared to Bi2 Se3 flakes on Si-nanowires [65]. Due to the presence of conducting surface states and insulating bulk states in Bi2 Se3 TI as well as locking the orientations of spin and momentum of the propagating electrons in opposite directions, it is likely shown the difference diode characteristics behavior at forward and reverse bias [65]. From these observations, it is clear that the p–n junction formation on Bi2 Se3 films on pSi(100) substrates possesses excellent p-n diode properties and easy band alignment with Si is suitable for futuristic infra-red photodetectors and opto-electronics devices.

3.3 Optical Properties of Bi2 Se3 Thin Films on Various Substrates The optical energy bandgap of the Bi2 Se3 single crystals, thin films, and nanostructures were measured using different techniques, and these reports revealed the band gap in the range of 0.1–2.6 eV [16, 66–68]. The large differences in the band gap values for Bi2 Se3 were explained on the basis of thickness-dependent tuning of the band gap, quantum confinement effects, surface-interface, structural-dependent, and quantum size effects, among others [16, 69–71]. Yang et al. showed the importance of structure on the optical constants of the Bi2 Se3 by tuning the shape from nanocrystals to nanoflakes, and it has huge advantage for various technological applications [70]. The optical band gap properties of ultra-high vacuum magnetron sputtered few-layer Bi2 Se3 thin films on Si/SiO2 and quartz substrates were investigated, and blue shift in band gap arises due to the approaches to two-dimensional limit [71]. Bari et al. reported the change in optical band gap for chemical bath deposited Bi2 Se3 thin films by varying the Bi to Se ratio [72]. All these reports showed the importance of the

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deposition parameters, Bi/Se ratio, shape, and size on the optical properties of the Bi2 Se3 thin films, and further studies are required for better understanding of the optical properties of Bi2 Se3 thin films. Figure 11 shows the high-resolution XRD 2thea-omega spectra for magnetron sputtered Bi2 Se3 thin films on quartz and sapphire (0001) substrates at substrate temperature of 400 °C. In case of quartz substrate, we have obtained the pure hexagonal phase of Bi2 Se3 , and these prominent planes are (0003), (0006), (0009), (015), (00,012), (00,015), (0210), (00,018), and (00,021). Except one peak obtained for the Se, no other compounds such as Bi-Se or Bi were observed as shown in Fig. 11a. For film deposited on sapphire (0001), only {0003n} family related to hexagonal Bi2 Se3 and sapphire substrate peaks were obtained as shown in Fig. 11b, and it revealed the highly c-axis-oriented Bi2 Se3 thin films on sapphire (0001) substrate [73]. The FWHM value for Bi2 Se3 (0006) peaks for film deposited on quartz and sapphire Fig. 11 High-resolution XRD 2theta-omega scan of magnetron sputtered Bi2 Se3 thin film at substrate temperature of 400 °C on: (a) quartz and (b) sapphire (0001) substrates

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(0001) was estimated to be 0.88 and 0.31, respectively. These observations disclosed the better crystalline quality of the Bi2 Se3 thin film on sapphire (0001) substrate. The surface morphology and elemental analysis of the Bi2 Se3 thin film on quartz and sapphire substrates were characterized by SEM and EDAX analysis, respectively. Figure 12a represents the SEM image of post-selenized Bi2 Se3 thin film on quartz substrate. Mostly truncated hexagonal granular film with grain size in the range of 300–600 nm was seen. The composition analysis by EDAX revealed the stoichiometric Bi2 Se3 thin film with Bi atomic percentage of 41 and Se atomic percentage of 59 (Bi/Se ratio: ~2/3). Figure 12b represents the field emission SEM image of Bi2 Se3 thin film on sapphire (0001) substrate and mostly connected hexagonal islands were observed. The size of the grains falls in the range of 400–900 nm. The fully surfacecovered Bi2 Se3 thin film on sapphire is likely related to the high sticking coefficient of sapphire compared to quartz [74]. The EDAX measurement on Bi2 Se3 thin film on sapphire (0001) substrate showed the stoichiometric ratio of 2:3 (Bi: 40.6%, Se: 59.4%). The structural quality of the Bi2 Se3 thin films was also characterized by Fig. 12 SEM images of magnetron sputtered Bi2 Se3 thin films followed by post-selenization on: (a) quartz and (b) sapphire (0001) substrates

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Raman spectroscopy, and spectra are shown in Fig. 13. Raman spectra of Bi2 Se3 thin films deposited on quartz and sapphire substrates showed three active Raman peaks, i.e. A1 1g , E2 g , and A2 1g . The peak position (FWHM) values for Bi2 Se3 thin film on quartz substrates are at low wavenumbers at 73.1 (4.6), 132.7 (7.6), and 175.2 (10.1) cm−1 , respectively. For Bi2 Se3 thin film on sapphire (0001), three active Raman modes (FWHM) were found at 72.8 (4.4), 132.5 (8.2), and 175.2 (9.9) cm−1 . These observations revealed that the Bi2 Se3 thin film deposited on sapphire (0001) has better structural and crystalline quality compared to quartz substrates likely due to the single crystalline nature of sapphire (0001). The transmittance vs. wavelength spectra of the Bi2 Se3 thin films deposited on quartz and sapphire substrates in the wavelength range of 300–2500 nm are presented in Fig. 14a. The Bi2 Se3 thin film on quartz showed low transmittance compared to film on sapphire at same magnetron sputtering deposition parameters. Adam et al. reported Fig. 13 Raman spectra taken at room temperature of Bi2 Se3 thin films on: (a) quartz and (b) sapphire (0001) substrates

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Fig. 14 (a) Coefficient of transmittance versus wavelength of magnetron sputtered Bi2 Se3 thin films on quartz and sapphire substrates. Variation of (αhν)2 vs. (hν) for Bi2 Se3 thin film on: (b) quartz and (c) sapphire (0001) substrates. The noise/abrupt change in the optical data visible range is related to the change in the lamp during data acquisition from 300 to 2500 nm

that the low transmittance obtained for those Bi2 Se3 thin films has small grains, relatively rough surface and higher dislocation density, which caused the strong photon scattering [15]. The optical band gap of Bi2 Se3 thin films was calculated on the basis of the following equation [15, 23, 24]: n  (αhν) = B hν − Eg

(1)

Here, α is the optical absorption coefficient, hν is the photon energy, Eg is the optical band gap, h is Planck’s constant, B is a constant, n is the characteristic coefficient of materials (½ for direct-band gap transition and 3/2 for indirect-band gap materials). Here, we have considered n = 1/2 for Bi2 Se3 , being a direct band gap semiconductor material. The plot of (αhν)2 with (hν) for Bi2 Se3 thin films on

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quartz and sapphire substrates is presented in Fig. 14b, c, respectively. The estimated band gap for Bi2 Se3 films on quartz and sapphire surfaces by extrapolating the linear part of the (hν) axis is obtained to be 1.42 and 1.38 eV, respectively. Kannan et al. reported the optical band gap in the range of 1.22–1.03 eV for Bi2 Se3 thin films prepared by spray pyrolysis at different temperatures [75]. It has been reported that band gap of chemically deposited Bi2 Se3 thin films falls in the range of 1.7–2.3 eV [24, 76]. The Bi2 Se3 nanostructures synthesized through a hydrothermal process showed the large optical band gap: for example, Bi2 Se3 nanoplatelets have band gap of 2.95 eV [77] and Bi2 Se3 nanorods with band gap value of 2.25 eV [78]. Our optical band gap of Bi2 Se3 thin films on quartz and sapphire (0001) substrates is well matched with the film deposited by vacuum thermal evaporation technique [15]. We have also seen that the optical band gap of the Bi2 Se3 thin films deposited under similar condition on various substrates is same. These observations revealed that large-area magnetron sputtered Bi2 Se3 thin films are suitable for further infra-red photodetectors and opto-electronics devices.

3.4 Thermoelectric Measurements on Bi2 Se3 Thin Films Several strategies have been employed to increase the efficiency of thermoelectric materials, and these include the surface/interface tailoring for efficient phonon scattering to reduce the thermal conductivity, low-dimensional structures with quantum confinement effect, organic/inorganic hybrid structures for enhanced Seebeck coefficient, among others [27, 78–84]. Particularly for Bi2 Se3 , quantum confinement effect has been observed for nanoscale size, which eventually suppress the thermal conductivity and enhance the thermopower [27, 84, 85]. In addition, Bi2 Se3 is one of the thermoelectric materials of great interest as it does not possess any rare or toxic elements (Te or Pb). Other approach to improve efficiency of Bi2 Se3 material is achieved by doping of Sb, Sn, and other elements as it introduces resonant impurity levels, controls the carrier type and Fermi level position [85–89]. However, large-area deposition of Bi2 Se3 thin films by magnetron sputtering system on technologically important Si(100) has several advantages such as simple process, cost-effective, and easy to integrate on microelectronics chips/devices for cooling purposes. We have deposited Bi2 Se3 thin films on Si(100) using magnetron sputtering system at deposition temperature of 400 °C, and Fig. 15a shows the XRD pattern of Bi2 Se3 thin film. All XRD peaks are indexed to the pure hexagonal Bi2 Se3 phase with estimated lattice parameters of a = 0.42 nm and c = 2.87 nm [61, 78]. Out of these several peaks, the high intense XRD peaks for (0003), (0006), and (00,015) planes showed the dominant orientation of the Bi2 Se3 along c-axis with other directions. The structural quality of the Bi2 Se3 thin film was also analyzed by Raman spectroscopy as shown in Fig. 15b. The peak position (FWHM) values for active Raman A1 1g , E2 g , and A2 1g modes for Bi2 Se3 thin film obtained at 73.1 (4.5), 132.7 (7.2), and 175.5 (9.9) cm−1 , respectively. The SEM analysis of this sample shows the different shapes of the island along with truncated hexagonal features due to the random

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Fig. 15 (a) XRD 2theta-scan and (b) Raman spectrum of post-selenized Bi2 Se3 thick film deposited on Si(100) at substrate temperature of 400 °C

orientation of the film, and it is presented in Fig. 16a. Some locations, we have seen the nucleation of small islands on the top of flat structures. This type of spiral or multi-layer features is usually seen in the Bi2 Se3 thin film grown on other substrates [57]. The chemical composition is one of the key parameters, which can affect the thermoelectric properties, the EDAX measurement on Bi2 Se3 thin film on Si(100) was performed Fig. 16b, and we obtained the stoichiometric ratio of 2:3 (Bi: 41.6%, Se: 58.4%). Figure 17a shows the temperature dependence of electrical conductivity for the Bi2 Se3 thin films on Si(100). It shows that electrical conductivity of Bi2 Se3 increases monotonically with temperature, and it confirms the semiconducting nature of the film. The electrical conductivity nearly at room temperature is found to be 6.5 × 103 S/m. The contribution from n-type Si(100) is nearly negligible as the measurement

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Fig. 16 (a) Plan-view SEM image and (b) EDAX of Bi2 Se3 thin film on Si(100) after post-selenization process

of electrical conductivity of the bare oxidized Si substrate was not possible due to high resistivity in the range of 10–20 -cm. The thermoelectric power factor of the Bi2 Se3 thin film on Si(100) with a function of temperature is presented in Fig. 17b. The power factor value is found to be 1.9 × 10–4 Wm−1 K−2 at 304 K. The power factor value obtained here is comparable to the Bi2 Se3 thin film deposited by vapor–solid technique on graphene [85]. The temperature dependence of Seebeck coefficient value for Bi2 Se3 thin film on Si(100) substrate is shown in Fig. 17c. The negative sign of Seebeck coefficient value obtained for the Bi2 Se3 thin film discloses the n-type conduction. The Hall measurement also supports the domination of ntype conductivity. The value of Seebeck coefficient for sputtered Bi2 Se3 thin film is obtained to be −190 μV/K at nearly room temperature. Andzane et al. recently studied the thermoelectric properties of the Bi2 Se3 thin films deposited by vapor– solid technique on quartz and mica substrates and their Seebeck coefficient value falls in the range of −74 to −227 μV/K [85]. Hong et al. reported the Seebeck coefficient value of −155 μV/K for pellet composed of single-layered Bi2 Se3 nanosheets [90]. The metal organic chemical vapor deposited Bi2 Se3 thin films show the Seebeck coefficient value in the range of −130 to −159.2 μV/K for Se/Bi ratio of 3 to 15 [91].

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Fig. 17 Thermoelectric properties variation with temperature of Bi2 Se3 film on Si(100) substrate: (a) electrical conductivity, (b) power factor, and (c) Seebeck coefficient

Seebeck coefficient value of −90 μV/K was reported at 300 K for five-atom thick Bi2 Se3 free-standing single layers [92]. The reported value of Seebeck coefficient for undoped n-type Bi2 Se3 single crystal was −190 μV/K at room temperature [93]. Compared with the value of Seebeck coefficient with bulk and thin films, our obtained Seebeck coefficient value is comparable [85, 90–93]. These observations revealed that the quality of sputtered Bi2 Se3 thin films is also comparable with film grown by other advanced techniques and it opens the possibility to deposit large-area uniform film for futuristic cost-effective opto-electronics, photodetectors, and thermoelectric devices (Fig. 17).

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4 Conclusion and Future Remarks In summary, we have deposited Bi2 Se3 thin films on various substrates such as Si(111), sapphire (0001), Si(100), and quartz using r.f. magnetron sputtering deposition technique. Being a multi-application capability of Bi2 Se3 compound, we have explored to study the various physical properties such as optical, electrical, and thermoelectric on suitable substrates using different characterization techniques. The triangular and/or truncated hexagonal island-like structures of Bi2 Se3 were obtained on Si(111) substrates at substrate temperature of 360 °C. The high-resolution XRD and Raman spectroscopy analysis revealed the highly c-axis-oriented film with rhombohedral Bi2 Se3 phase after post-selenization process. Electrical property measurements on Bi2 Se3 /p-Si(100) showed excellent p-n diode characteristics with high rectification ratio. The variation of optical band gap of Bi2 Se3 thin film and nanostructures is one of the key parameters to utilize Bi2 Se3 in photo-detector application as it will change the light sensitivity and selectivity. We have obtained the optical band gap of ~1.4 eV for magnetron sputtered Bi2 Se3 thin films on various substrates, and it showed the potential applications in infra-red photodetectors. The thermoelectric properties of the Bi2 Se3 thin film deposited on Si-based substrates revealed the high Seebeck coefficient value of −190 μV/K, which is higher than the Seebeck coefficient value of bulk Bi2 Se3 materials. From these observations, we conclude that the Bi2 Se3 material can be used in different applications such as p-n diodes, infra-red photodetectors, TI-based quantum devices, and toxic-free thermoelectric devices. The magnetron sputtered Bi2 Se3 thin films on large area on various substrates with good crystalline, structural, optical, and thermoelectric quality pave the way for futuristic integrated opto-electronics and thermoelectric devices. In the future, few challenges in developing infra-red photodetectors based on Bi2 Se3 thin films and nanostructures as the band gap of this material is very sensitive to the Bi/Se ratio as well the deposition techniques used for the fabrication of thin films. Recently, Wang et al. also clearly pointed out that that type of defect or impurities is a critical parameter than optical band gap in determining photo-detection sensitivity. They have found that doped Bi2 Se3 with In and Sn has advantages over undoped Bi2 Se3 for application in ultra-violet and red-light photodetectors [94]. The selection of doping elements in Bi2 Se3 is one of the key parameters for making p-type Bi2 Se3 as intrinsic conductivity of the Bi2 Se3 is dominated by n-type. For example, light doping of Ca in Bi2 Se3 material can change to p-type so it is necessary to develop Bi2 Se3 materials in a way to protect any unintentional doping [93]. Along with doping, the hybrid nanostructures based on the various 2D materials (MoS2 , WS2 ) with Bi2 Se3 will be one of the promising materials for the futuristics optoelectronics and photoelectrochemical energy conversion due to tuning of the band structures and increased light absorption energy. Acknowledgements The authors thank the Director, NPL for the constant encouragement and support. The authors are grateful to Dr. H. K. Singh, Dr. K. K. Maurya, Dr. B. Gahtori, Dr. S. P. Singh, Dr. P. K. Siwach, Dr. V. N. Singh, Ms. S. Sharma of CSIR-NPL, and Dr. P. Kumar from IIIT, Allahabad, Dr. B. S. Yadav from SSPL, Delhi for their help in different sample characterizations.

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This work was funded by Science and Engineering Research Board under Early Career Research Award Scheme (ECR/2017/001852).

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Chapter 6

Zinc Oxide: A Fascinating Material for Photovoltaic Applications Premshila Kumari, Avritti Srivastava, Ruchi K. Sharma, Deepak Sharma, and Sanjay K. Srivastava

1 Background It is well known that world’s energy consumption is increasing rapidly day by day. The conventional energy sources like coal, oil, etc. are limited and depleting continuously. Further, the environmental issues are also becoming serious concern caused by the huge consumption of these energy sources. Keeping in mind these issues, researchers worldwide have been exploring new sources of energy like wind, solar, ocean, biomass, etc. which are largely green energy sources. Among these sources, solar is an abundant source of energy in which 60% of the solar power is generated through solar photovoltaic devices as one of the best alternates toward energy requirements. The phenomenal journey of solar photovoltaic was firstly started in 1941 when a silicon solar cell was fabricated in the form of the melt grown p–n junction having conversion efficiency of less than 1% by Ohl et al. [1]. In 1950s Pearson, Fuller, and Chapin at the famous Bell laboratories, USA, fabricated silicon solar cell with efficiency up to 4.5% with a diffused p–n junction [2]. Since the very first practical demonstration of p–n junction silicon solar cells in 1954, this technology has come up a long way. Post 1954, many researchers around the world have put a lot of efforts with a common objective to make this solar photovoltaic technology efficient and cost competitive with the conventional power sources. Several silicon solar cell technologies have been developed for improving the power output/efficiency of the solar cells, like aluminum-alloyed back junction (also commonly known as screen printed solar cells), passivated emitter and rear contact (PERC), metal-wrap through P. Kumari · A. Srivastava · R. K. Sharma · D. Sharma · S. K. Srivastava (B) Photovoltaic Metrology Section, Advanced Materials and Device Metrology Division, CSIR-National Physical Laboratory (NPL), New Delhi-12, India e-mail: [email protected]; [email protected] Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi-12, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_6

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(MWT), MWT-PERC, emitter wrap through (EWT) for p-type silicon homojunction solar cells and different methods like, n-type cells with boron (B) diffused front emitter, inter-digited back contact (IBC) solar cells with B-diffused back junction for n-type silicon homojunction solar cells and heterojunction silicon solar cells with thin intrinsic layer (commonly known as ‘HIT’) for n-type silicon [3–5]. Even a world record efficiency of 26.1% has been achieved with p-type crystalline silicon solar cell [6]. Not only in solar cells, but tremendous progress has been made in the entire value chain of the solar photovoltaic technology, be it raw silicon materials, solar cell fabrication, testing, solar panels technology as well their efficient deployment in the field for practical applications. However, despite all the advantages of the high efficiency homojunction silicon solar cell, a serious drawback is its relatively higher fabrication cost. Since last few decades, researchers are moving toward the heterojunction solar cell concept combining other inorganic semiconductors with silicon, third-generation solar cells concepts such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), organic semiconductor/silicon-based heterojunction solar cells, and quantum dot solar cells (QDSCs) and therefore, new generation solar cells have come into the picture. Among several inorganic wide band gap semiconductors like SiC, GaN, GaAs, GaP, PbSe, PbS, TiO2 , etc., used for different solar cell applications, zinc oxide (ZnO) has drawn a great attention in researchers due to its fascinating structural and optoelectronic properties. ZnO material has a wurtzite structure which provides it stability toward radiation damage and prevents photo-degradation and hence ensures longer device lifetime [7]. It is also a desirable property toward the radiation stability of photovoltaic devices. Besides, ZnO plays different roles in photovoltaic devices like active layer, buffer layer, window layer, antireflection and passivation layer as well in different types of photovoltaic devices like conventional silicon p–n junction photovoltaic, organic photovoltaics, perovskite photovoltaics, and dye-sensitized photovoltaics to enhance the performance of the devices. In addition, in organic and hybrid photovoltaics, ZnO is an excellent candidate as an efficient cathodic layer, buffer layer, and window layer. It is relatively easy to synthesize with cost-effective techniques, having wide band gap and good optical transmittance to fulfill the requirements of such applications. Thus, use of ZnO material is increasing day by day in various optoelectronic devices as well as in the various photovoltaic applications either in its nanostructure form or in its thin film architecture. Also, there are lots of easy and cost-effective techniques to synthesize ZnO nanostructures and ZnO thin films which make it more effective in various applications. Here, different fabrication techniques of ZnO nanostructures/thin films are getting attention including some chemical methods like precipitation method, colloidal method, chemical vapor deposition, spray pyrolysis technique, sol–gel method, solvothermal and hydrothermal methods, and that of physical methods like high energy ball milling technique, physical vapor deposition, pulsed laser technique, thermal evaporation, ultrasonic irradiation, and laser ablation technique which will be described further briefly in this chapter. Researches also made it more environmental friendly by green synthesis of ZnO nanostructures via plant extraction, microbes mediated, and biochemistry method. The different structural, optical, electrical, and

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electronic properties of the ZnO material dependent on its various shapes and size are briefly elaborated in this chapter. It is to be noted that undoped ZnO nanostructures have usually low resistivity due to its native defects related to oxygen vacancies and zinc interstitials. The lower resistivity can be controlled by doing defects engineering technique which may be possible either by doping with group III elements like aluminum (Al) and group VII elements like fluorine (F) [8] or by doing annealing treatment at different temperatures in different annealing environment (air, H2 , Ar, etc.) [9]. Furthermore, the chemical, conductive, and electrical properties of the ZnO nanostructures can also be tailored by doping with suitable materials like Al, Ga, F, Cu, Ag, Sn, and rare earth metals. After brief discussion of the structural, optical, electrical properties and popularly known methods/approaches of synthesis of ZnO nanostructures/thin films, a comprehensive review of applications of ZnO nanostructures and thin films in various photovoltaic applications are discussed. Finally, the chapter is concluded with future prospects for further improvements and applications of performances of different ZnO-based solar cell concepts.

2 Properties of ZnO 2.1 Structural Properties The ZnO has received significant attention due to its three different phases known as wurtzite, zinc blend, and rock salt shown in the schematics in Fig. 1 [10]. In ordinary condition, the wurtzite phase (hexagonal crystal system) is more stable and resistant toward radiation hazardous. The ZnO wurtzite structure is represented as two

Fig. 1 Schematic representation of zinc oxide (ZnO) in different phases a Rocksalt, b Zinc blend, and c Wurtzite. Reproduced after permission from Ref. [10], © 2009 WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

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intertwined hexagonal close packed structure with phase group P6 mc which indicates Zn and O lattices, arranged in such a way that each Zn2+ ion is coordinated by four O2− ions in a tetrahedral arrangement and each O2− ion is coordinated by four Zn2+ ions in the same way. The bonds are sp3 hybridized bonds showing almost equally ionic and covalent character. Its wurtzite crystalline structure has lattice parameters ‘a’ and ‘b’ equal to 3.2495 Å and ‘c’ equal to 5.2062 Å. The structural properties of the wurtzite ZnO material have been confirmed by X-ray diffraction (XRD) pattern in which 11 crystal planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) correspond to 2θ values of 31.83°, 34.46°, 36.29°, 47.57°, 56.62°, 62.88°, 66.39°, 67.93°, 69.12°, 72.71°, and 76.96°, respectively [11] (JCPDS data 36–1451). Lot of parameters like lattice constants, crystallite size, lattice strain, c/a ratio are dependent on the crystal plane orientation which directly affects θ values. The lattice parameters are found as a = b =√

λ 3 sin θ100

c=

λ sin θ002

Inter-planar spacing between two planes calculated by 1 = d2

4 3

 2  h + hk + k2 l2 + 2 2 a c

The crystallite size of the ZnO material (with the help of XRD pattern) can be calculated by using Debye-Sherrer formula: D=

kλ β cos θ

where λ k β θ D

X-ray wavelength (1.5406 A°) 0.89 Full width at half maxima (FWHM) Bragg angle (degree) crystallite size (nm).

The crystallite sizes play a very important part in the ZnO properties as by reducing crystallite size, surface to volume ratio can be enhanced which leads the nanoparticles to become more reactive. Such a variation in crystallite sizes can be easily modified by controlling the factors like synthesis route, precursor concentration, solution pH, and annealing effect depending on the synthesis method used. Further, in case of annealing treatment, the structural properties of the ZnO material can be improved by annealing at a certain temperature level and after that phase transformation comes into the consideration. It may be possible due to lattice strain and stress developed into the material caused by the annealing treatment or by tensile loading [12].

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The zinc blend ZnO structure on the other hand can be stabilized only by growth on cubic substrates such as ZnS, and SiO2 . The symmetry of the zinc blend structure has space group F-43 m and is composed of two interpenetrating fcc sublattice shifted along the body diagonal by one-quarter of the length of the body diagonal. There are four atoms per unit cell and every atom of one type (group II) is tetrahedrally coordinated with four atoms of other types (group VI), and vice versa. Zinc blend phase of ZnO material can be identified by XRD spectra having highly oriented crystal plane (004) at 2θ value of 44.6°, which is the characteristic peak of cubic zinc blend crystalline phase of ZnO [13]. Rocksalt (RS) phase of ZnO is actually the transformed structure of wurtzite one at relatively modest external hydrostatic pressure. This is due to the reduction of the lattice dimensions which causes the interionic Coulomb interaction to favor the ionicity more over the covalent nature. The space group symmetry of the RS structure is Fm-3m and the structure is sixfold coordinated [10]. The stability of RS phase of ZnO can be increased by doping the material with metals like Mg, Mn, and Co as reported in Ref. [14].

2.2 Morphology of ZnO The ZnO, whatever has the phase, is a very funny and diverse material and exists in different shapes and sizes depending on the synthesis techniques and environmental conditions. It exists in the form of nanospheres [15], nanoplates [16], nanorods [17], nanotubes [18], nanoneedles [19], nanoribbons [20], nanobelts [21], nanosheets [22], nanotrees [23], nanodendrites [24], nanoflowers [25], nanoshells [26], nanocorals [27], nanovolcanoes [28], nanopyramids [29], nanocolumns [30], nanotowers [31], nanocombs [32], nanorings [33], nanosprings [34], nanowires [35], nanocages [36], nanopencils, nano-pin-cushion cactus [37], and so on. The morphology of the samples can be recorded by scanning electron microscope (SEM). It is also to be noted that the morphology of the samples is condition dependent, i.e., it can be modified by varying pH of the solution, precursor concentration, annealing temperature, and time of the samples. For example, Amin and others have done a detailed study in this regard and reported the effect of different parameters on the morphology of the ZnO nanostructures [38]. The different morphology with varying parameters is shown in Fig. 2. Further, the size, shape, composition and defects, and structural properties of the different nanostructures can also be examined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) techniques [39].

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Fig. 2 SEM images of different ZnO nanostructures with aqueous solution of different pH values a 1.8, b 4.6, c 6.6, d 9.1, e 10.8, and f 11.2. The insets show the enlarged SEM images of ZnO NSs (scale bar = 100 nm). Adopted from Ref. [38]; Open Access

2.3 Optical Properties 2.3.1

Absorbance

It is known that photovoltaic applications fundamentally depend on the absorbance and transmittance properties of the material used in the device. So, if ZnO is to be used in the photovoltaic application it is to make sure that its absorbance should be optimum in case of ZnO as active layer and transmittance should be high when ZnO is used as antireflection layer. UV–Vis spectroscopy is a widely used technique to examine the optical properties of the nanostructures and thin films. In case of bulk ZnO material, a strong absorption peak is observed at ~368 nm which corresponds to the band gap energy value of ~3.37 eV. The absorption peak of the material depends on various factors like annealing time, temperature, dimension, morphology, etc. Band gap energy can be estimated from the absorbance spectra by using following the formula: n  αhν = B Eg − hν where α (=2.303A/t) is absorption coefficient, t is the thickness of cuvette, ν is frequency of incident radiation, B is constant, h is plank’s constant, Eg is band gap of the material. Here, ‘n’ is a number which depends on the type of materials. The

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value of ‘n’ is considered as 2 for indirect band gap material while it is ½ for direct allowed transitions as also in the case of ZnO nanostructures.

2.3.2

Transmittance

Good transparency of the ZnO layer in visible and IR region is very important for solar cell point of view. It has been reported that maximum of 60–70% transmittance was observed for undoped ZnO films which improved after doping with cobalt (Co). More than 80% of transparency observed in case of 10% Co doping concentration due to d-d* interionic transition in the region from 530 to 692 nm, whereas more than 90% transmittance in the visible region is obtained with Fe and Ga doping in ZnO thin films [40] (heavily doping will obviously decrease the transmittance). Howsoever, transmittance spectra of the ZnO thin films show variations with respect to different fabrication techniques, concentration of solvent or stabilizer used in the solution, annealing temperature and time as well. Xue et al. reported that there was 20% increment in transmittance with increasing annealing temperature from 600 to 950 °C [41]. The optical transmittance (T ) of the ZnO thin film can be calculated by using the formula below: T = exp[−α(λ)d] where T is the optical transmittance, d is the thickness of ZnO thin film and α(λ) is absorbance coefficient. It is known that complex refractive index of the material depends on extinction coefficient k(λ) and that extinction coefficient is related by absorption coefficient in the following relation: k(λ) =

α(λ)λ 4π

The complex refractive index of the material n* can be expressed as n∗ (λ) = n (λ) + jk(λ) where n is the refractive index. It can be predicted that the refractive index of the material decreases with wavelength in the visible region while increases in the UV region. Finally using this data, the dielectric constant of the ZnO material can be determined by the relation: εr (λ) = n2 (λ) −k2 (λ)

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Photoluminescence Properties

The ZnO nanostructures/thin films owing to the presence of various intrinsic and extrinsic defects in the material exhibit excellent photoluminescence (PL) properties. Here, a brief description on the optical and PL properties of the ZnO nanostructures/thin films are presented and emphasized that the optical and PL properties of bulk ZnO can be tailored by nanostructuring and defects engineering in the samples. Nanostructuring of the samples results in the increase in surface to volume ratio which thereby produces stronger surface defects related emissions. These defects are generally related to zinc and oxygen in ZnO material which acts as donor or acceptor defects depending on the position of their energy levels. In principle, there are numerous defect states within the band gap of the ZnO. The donor defects are Zni + , Zni++ , Zn*i (neutral), VO + , VO ++ , VO * (neutral), and the acceptor defects are VZn + , VZn ++ . Electron irradiation study has suggested that Zn interstitials are the dominant native donor in ZnO while oxygen vacancies are the shallow donors [42]. Actually, these defects are extrinsic defects which get induced during the synthesis of the material itself and depend on various external factors like structure, composition, doping concentration, nanostructures dimensions, morphology, post processing temperature (annealing) under different environments, etc. Furthermore, these defects play an important role in doping of the material, compensation, minority carrier lifetime, and luminescence property as well [43]. Different coloured spectra (like violet, blue, green, yellow, orange, and red) have been obtained by exciting the sample with a certain energy source. For example: if the ZnO sample is excited with 325 nm laser source, a UV band causing due to band-to-band excitation is obtained along with several colored spectra such as violet emission in the range of 400–445 nm, blue emission in the range of 445–495 nm, green emission in the range of 495– 565 nm, yellow emission (565–590 nm) and orange-red emission in the range of 590–650 nm. The emission in the different wavelength range is due to the transition of electrons from conduction band to different defect levels. The transition explanation is somehow complicated to deeply understand; howsoever some literatures are available to discuss these aspects in detail [44, 45]. It is to be noted that the intensity of defect states can vary via doping the material with other elements like Al, Ga, Co, Cu, Fe, etc. For example, in case of Al doping, intensity of defect states continuously increases with Al concentration. In a thermal decomposition method, Al3+ ions in ZnO are incorporated either by creating oxygen vacancies or by incorporating them as interstitials. In case of low doping concentration, the most of the Al3+ ions are substituted while for high concentration, the excess Al3+ ions incorporated interstitially, creating large amount of lattice defects [46]. Similarly, the defect densities are also affected by heat treatment (annealing) of the material at different temperatures as annealing provides enough formation energy for different energy levels related defects. It is simple to observe that when ZnO is heated, it losses a minute amount of oxygen which turns into Frenkel defect in the material which is generally governed by the following reaction:

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1 ZnO (s) ↔ Zn2+ + O2 + 2e− 2 It is to be noted that oxygen vacancies (VO ) require lower formation energy than the Zn interstitials (Zni ) and hence should be more in numbers in a Zn-rich conditions. This oxygen vacancy leads to the formation of Zn interstitials as its migration energy barrier is as low as 0.57 eV which promotes to create Zn vacancies (VZn ). In oxygen (O)-rich conditions, the VZn should be abundant. The VZn is expected to have 2 negative charges (−2) in n-type ZnO where its formation is more favorable. The formation of VZn can be understood by the following equation [47]. ZnZn ↔ Zn∗i + V∗Zn − Zn∗i ↔ Zn+ i + e ++ Zn+ + e− i ↔ Zni Further, it is also noted that excess oxygen atoms in the ZnO lattice are accommodated in the form of oxygen interstitials (Oi ). At higher annealing temperature, reduction in the oxygen atoms takes place and thus intensity of oxygen interstitials decreases [43]. Zinc antisite (ZnO ) and oxygen antisites (OZn ) have comparatively high formation energy than zinc and oxygen interstitials, so they show a higher thermal stability and their migration is quite low. Keeping in mind the applications of ZnO material in optoelectronic and photovoltaic devices, it is quite important to understand the significance of these defects discussed above. The as-synthesized ZnO generally behaves as n-type material which is attributed to these native defects. The oxygen vacancies and Zn interstitials have been found to be main source of n-type conductivity in the ZnO. ZnO nanostructuring controls the defects densities by increasing surface to volume ratio, increasing grain boundaries, and by adjusting the energy levels.

2.4 Electrical Properties As mentioned earlier, the ZnO being a direct and wide-band-gap material has been paid a lot of attention for various electronic and optoelectronic applications as it can provide higher breakdown voltages, lower noise generation, high temperature, high-power operation and can sustain large electric fields. It is known that undoped ZnO with a wurtzite structure behaves as naturally n-type semiconductor due to the presence of intrinsic or extrinsic defects, which were generally attributed to native defects such as VZn , VO , and Zni , The highest room temperature electron mobility of 205 cm2 V−1 s−1 with a carrier concentration of 6.0 × 1016 cm−3 has been reported for a bulk ZnO single crystal grown by vapor-phase transport method [48]. The observation is well matched with the theoretical value of mobility for a bulk ZnO as shown in Fig. 3. A peak value mobility of about 2000 cm2 V−1 s−1 at 50 K was

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Fig. 3 Temperature-dependent Hall mobility of bulk ZnO is shown by experimental data (circle) and theoretical data (solid line). Reprinted from Ref. [48] Copyright (1998), with permission from Elsevier

obtained. On the other hand, Al-doped ZnO (AZO) films grown by reactive pulsed magnetron sputtering have been reported to have best electron mobility value of 46 cm2 V−1 s−1 which is approaching the practical limit of 55 cm2 V−1 s−1 [49]. Surface resistivity, also popularly known as ‘sheet resistance’, is an important property of materials, especially for thin films, which indicate their electrical charge transport capabilities through the films. It is a very important parameter for the solar cells, where low sheet resistance materials are needed to extract the charge carriers efficiently. Also, the resistivity (or conductivity) can be determined from the sheet resistance measurements. The sheet resistance/resistivity can be measured using the four-probe measurements in which a fixed current (I) is injected into ZnO thin film of thickness (t) through the two outer probes and a voltage (V) is measured between the two inner probes. The electrical resistivity (ρ) then can be determined as [50]   V t = Rs t ρ = 4.532 I where Rs is the sheet resistance of the film. Cosmas and others reported ρ of 5.39 × 103 cm for ZnO nanoparticle films fabricated by chemical spray pyrolysis (CSP) technique [51]. Further, the electrical resistivity was found to be 6.03 × 101 cm

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for undoped microstructured ZnO thin film fabricated via the same method as earlier [50]. It can be noted that electrical resistivity of ZnO thin film is dependent on the annealing temperature of the film. Nasir and others reported that the ZnO films annealed at 500 °C have highest electrical conductivity value of 1.87 × 10–5 Scm−1 among the four samples annealed at 400, 450, 500, and 550 °C [52]. The electrical properties of ZnO nanostructure/thin film depend on doping materials as well as doping parameters. It has been observed that with increasing doping concentration, conductivity increases which happen may be due to the improvement in the crystallinity of the synthesized material. With increasing doping concentration, charge carrier scattering at the grain boundary decreases due to increment in grain size which also enhances the mobility of electrons.

2.5 Band Gap Engineering Band gap energy is a very important physical property which affects the electrical behavior of the material. The band gap engineering technique is the most effective approach in material science to control the properties of the semiconductor materials, in particular, for the optoelectronic devices. Band gap engineering can be realized in different ways such as via (i) doping, (ii) annealing, or (iii) alloying.

2.5.1

Doping

ZnO doped with different elements has attracted great attention because of its various properties in gas sensors, liquid crystal display, solar cells, solar UV-radiation monitoring, etc. The ZnO has been doped with different elements like Al, Mg, Cu, Co, Mn, Ni, etc. in order to tune the band gap property. The ions related to doping material produces color centers (defects) due to unpaired electron which affects decrease in the band gap energy level. In a report it is clearly shown that for undoped ZnO band gap was found as 3.33 eV, which decreased to the value 3.32 eV in case of 1% doping (Zn0.99 Mn0.01 O) of Mn element and decreased to the value 3.29 eV in case of 1% doping (Zn0.99 Cu0.01 O) of Cu element [53]. Similarly, band gap modification has been reported for Ni- doped ZnO [54]. It is known that bulk ZnO has a band gap value ~3.37 eV and that of NiO in the range of (3.7–4.0) eV. The band gap modification upon doping is achieved due to the interaction of Ni states and ZnO states. A decrease in the band gap of approximately 12% was reported for 15% Ni-doped ZnO and the band gap was found to decrease with increasing Ni concentration [54]. Similarly, the effect of Al doping on the band gap tuning of ZnO film (AZO film) is reported by Mondal et al. in which Al-doped ZnO thin film was deposited by successive ion layer adsorption and reaction (SILAR) method with average particle size of ~23 nm [55]. The value of band gap of undoped ZnO was observed as 3.229 eV which increased to 3.29 eV for 1% doping of AZO and was further decreased for 2% doped AZO. Such modification was attributed to the stress relaxation in the AZO sample due to

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doping [55]. Similarly, there are various examples of tailoring the band gap of ZnO nanostructures/thin film via doping of various elements.

2.5.2

Annealing

The band gap tuning in ZnO can also be possible by both annealing temperature and annealing time variation. Annealing treatment affects the variation of the crystallite size, grain boundaries, morphology, and local lattice defects which indirectly affects the band gap properties of the material. In a report, ZnO thin film was fabricated by spray pyrolysis technique and was analyzed that for the film annealed at 450 °C for 1 h showed the band gap value of 3.29 eV which decreased to 3.20 eV for 6 h annealed ZnO thin film [56]. Moreover, variation in annealing time would also change its structural, morphological, and optical properties. Lim W. C. et al. reported that the ZnO thin films deposited by RF magnetron sputtering with no annealing treatment had band gap energy 3.19 eV. Band gap energy increased with annealing temperature and found 3.24 eV for 800 °C annealing treatment [57]. The increase in band gap energy implies the reduction in defects and vacancies in ZnO thin film. On the other hand, ZnO nanospheres show decreasing band gap energy with increasing annealing temperature. The value was found to be 3.35 eV for ZnO nanomaterial annealed at 300 °C which was decreased to 3.25 eV for annealing at 700 °C [58].

2.5.3 (a)

Alloying Zn1-x Mgx O

The band gap engineering of ZnO can also be performed by alloying. ZnO can be alloyed with MgO (Eg = 7.7 eV) to form a ternary Mgx Zn1-x O compound. Since undoped ZnO prefers the wurtzite hexagonal structure and MgO favors the cubic rock salt, so in case of high ZnO concentration Mgx Zn1-x O exhibits wurtzite hexagonal structure while in case of high MgO concentration, cubic rock salt structure is preferred. The Mgx Zn1-x O alloy has been considered as a suitable material for barrier layer in ZnO/(Zn, Mg)O superlattice because it widened the band gap of ZnO. J. Liriano and others deposited Zn1-x Mgx O film on Al2 O3 substrate with different Mg concentrations and found that band gap of ZnO increased from 3.37 eV to 3.65 eV with increment of Mg concentration upto 25% while the UV photoluminescence characteristic deteriorated with increasing Mg concentration [59]. Although crystallite size increased and crystallinity decreased with increasing Mg concentration in Zn1-x Mgx O alloys. (b)

Zn1-y Cdy O alloys

As CdO has small a band gap (2.3 eV) compared to ZnO (3.37 eV), alloying of these two materials results in the lower band gap than that of ZnO, so it can be considered as band gap narrowing agent. It has been found that 7% Cd concentration could provide

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band gap of 2.99 eV with respect to 3.28 eV in case of 0% Cd concentration. It was also noticed that lattice constant parameters ‘a’ and ‘c’ increased with increasing Cd concentration [60]. J. Jiang et al. deposited Zn1-y Cdy O thin films by pulsed layer deposition method and found increasing grain size with increasing Cd concentration. Also increase in lattice parameter is subject to the size of Cd+2 (0.97 Å) with respect to Zn+2 (0.74 Å). At room temperature, without Cd doping ZnO exhibited the band gap energy level of 3.26 eV while 9.6% Cd doping in Zn1-y Cdy O thin films resulted in the band gap narrowing up to 2.88 eV [61].

3 Synthesis of ZnO Nanostructures, Thin Films, and Their Techniques Based on the properties of ZnO nanomaterials/thin films mentioned above, lots of efforts have been made to synthesize the ZnO nanomaterials/thin films by using different approaches like chemical, physical, and green synthesis techniques. A lot of fascinating shapes/nanostructures of ZnO such as, nanospheres, nanorods, nanotubes, and tetrapods have been successfully synthesized via different routes. Some of the popular methods are briefly discussed in the chapter. These are precipitation method, sol–gel method, spray pyrolysis technique categorized under chemical methods and electron beam physical vapor deposition (EBPVD), pulsed laser deposition (PLD), laser ablation technique, and other methods under physical methods. Green synthesis techniques have also been developed to avoid the high use of either lot of chemicals or use of high energetic physical methods. Some of the popular methods are described briefly in the following section.

3.1 Chemical Routes 3.1.1

Precipitation Method

In an aqueous solution, precipitation is a method of transforming a dissolved into an insoluble solid which is known as precipitates, i.e., it is the process of converting a solution into solid via making the substance into insoluble form or making the solution a super-saturated one. The process is influenced by several factors, such as (i) type and concentration of ionic metals in the solution, (ii) the precipitant used, (iii) the reaction conditions (like pH of the solution), and (iv) other constituents in the solution that may inhibit the precipitation reaction. Precipitation method is a low cost and facile approach for large-scale production which do not need for costly raw material and sophisticated equipments. To synthesize ZnO nanostructures, a certain amount of desired Zn salt is dissolved into a solvent (IPA, ethanol, DI water, etc.) and well mixed by ultrasonication/stirrer. Alongwith that, different hydrolysis agent

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(like NaOH, KOH, LiOH, etc.) is also dissolved in the same solvent and performs proper mixing by ultrasonication/stirrer. After proper mixing, the hydroxide solution is poured into the zinc solution slowly and kept it for precipitation at room temperature. After a certain period of time, washing and centrifugation of the precipitated solution followed by drying are done to get the desired ZnO nanostructures which are further characterized to establish its structural, optical, electrical, and electronic properties. The complete process steps of synthesis of ZnO nanostructures via precipitation method are presented schematically in Fig. 4. Raoufi et al. reported ZnO nanoparticles synthesized via precipitation method using precursors zinc nitrate and ammonium carbonate. The as-prepared ZnO nanoparticles were also annealed at different temperatures. It was demonstrated that crystallite size of ZnO nanoparticles was increased with increasing annealing temperature from 8.34 nm at 250 °C to 27.58 nm at 550 °C [62]. Belkhaoui et al. studied Mn doped ZnO nanostructure using precursor zinc chloride and sodium hydroxide in ethanol solvent with different Mn concentrations and found the nanostructure in hexagonal wurtzite

Fig. 4 Schematic presentation of process steps for synthesis of ZnO nanoparticles by precipitation method (Source Author)

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phase. The enhancement in average crystallite size and decrease in lattice strain and dislocation density due to doping suggested improvement in crystalline quality. It was concluded that 3% of Mn content exhibited the best crystalline quality and the highest optical transmittance in the visible region [63]. Zinc sulfate has also been used as a precursor to synthesize ZnO nanostructures using sodium hydroxide (NaOH) as hydrolysis agent. The synthesized ZnO nanopowder was annealed at 600 °C for 4 h in a muffle furnace. The ZnO nanostructures having nanospheres and elongated rod-like structures with average crystallite size of 38 nm were achieved [64]. The dissociation procedures in case of precursors zinc acetate dihydrate and lithium hydroxide, which govern the reaction, are as followings: 

→ Zn(OH)2 + 2CH3 COOC2 H5 Zn(CH3 COO)2 · 2H2 O + C2 H5 OH −

(1)

Zn(OH)2 + Li+ OH−  (Zn[OH]4 )2− + Li+

(2)

Zn(OH)2 + H+ OH−  (Zn[OH]4 )2− + H+

(3)

ZnO + OH− → ZnOOH−

(4)

ZnOOH− + Li+ → ZnOOH − Li

(5)

(Zn[OH]4 )2−  ZnO + H2 O + 2OH−

(6)

The ZnO nanostructures are obtained according to Eq. (6). The synthesized samples are then subjected to different characterization for their structural, optical, electronic properties. Almost similar protocols as discussed above are followed for all precursors used in the synthesis of ZnO nanostructures via precipitation method.

3.1.2

Sol–Gel Method

It is a wet-chemical method often used for the synthesis of both glassy and ceramic materials. Here, the solution (known as sol) gradually converts into a gel-like network (called as gel) where in both liquid and solid phases co-exist. In this method, mostly metal alkoxides and metal chlorides are used as precursors. A colloid is formed after the hydrolysis and polycondensation reactions of the precursors. In general, the following steps are required for sol–gel deposition technique: • • • •

Selection of suitable precursors; After various reaction steps converting the precursors into sol; Thin film deposition by various coating methods such as spin and spray. Conversion of sol to polymeric gel to synthesize powders;

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Fig. 5 Schematic of sol–gel method for the fabrication of ZnO thin films (Source Author)

• Annealing is done for the synthesis of material oxide films; A stabilizing agent is essential in a sol–gel process to avoid the premature precipitations and controlling the rapid sol to gel conversion [65]. The schematic diagram of a sol–gel method for the synthesis of ZnO thin films is represented in Fig. 5. Firstly, the Zn source material is dissolved in a suitable solvent. Stabilizing agents are needed for the stabilization of Zn solution in sol–gel process. Monoethanolamine, acetylacetone, diethanolamine, tetramethylammonium hydroxide, ethylenediamine tetraacetates, etc. are commonly used as stabilizers [66]. It is also noted that the precursor is subjected to the process of hydrolysis and condensation in which the following three reaction steps are involved M − X + H2 O → M − OH + HX

(i)

M = metal or Si; X = reactive ligand like halogen, OR, NR2 , acetate M − OH + X − M → M − O − M + HX

(ii)

M − OH + HO − M → M − O − M + H2 O

(iii)

The hydrolysis rate, in general, is precursors dependent and thus produces reactive monomers at different rates. In contrast, the condensation rate is controlled by monomers production rate [57]. Since sol–gel method is cost effective, reliable, repeatable, and easy process, synthesis of ZnO nanostructures through this approach has been investigated extensively. For example, Vafaee et al. reported spherical-shaped ZnO nanoparticles of size 3–4 nm synthesized using precursor zinc acetate dihydrate and triethanolamine

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(TEA). Nanoparticles in such sols are recognized having hexagonal wurtzite structures. In this type of formation, it was also noticed that the band gap decreases with the increase of particles size [67]. Modified sol–gel method has also been used to synthesize ZnO nanostructures. Fe doped ZnO nanospheres were synthesized using modified sol-gel route employing water as a solvent [68]. Change in the lattice parameters and inter-planar distance of the ZnO nanostructures were found as a result of Fe doping. For example, a 9 mol% of Fe doping changed the inter-planar distance from 25 to 11 nm causing a distortion in crystal lattice and hence greater tension eventually affecting the crystal growth. The band gap of the ZnO also decreased with increasing Fe doping until 10% concentration [68]. Many applications of the sol–gel derived ZnO nanoparticles have been analyzed time to time. Recently, enhanced photocatalytic degradation of acid blue 1 has been studied by A. Lida et al. in which Ni decorated ZnO NPs were synthesized by sol–gel method. The spherical-shaped ZnO NPs with uniform hexagonal wurtzite phase were observed [69]. The major advantages of the sol-gel method for ZnO thin film deposition are summarized below: • Major advantages of this process are the high degree of uniformity and control on the thickness of the sample. • Another advantage is the size of the substrate to be coated. The substrate size can be varied as required. • Easy to control the kinetics of various chemical reactions by low processing temperatures and other mild conditions. • Ability to produce multi-layer coating allows the fabrication of layers with wide range of optical characteristics, i.e., transmission and reflectance. • Ability to produce highly pure products via simple purification processes of the precursor materials (using distillation, crystallization, or electrolysis). 3.1.3

Spray Pyrolysis Technique

In this approach, a suitable precursor solution is atomized in a droplet generating set up, followed by evaporation in a heated reactor to decompose into particles and thin films. It is a cost effective and efficient method utilizing simple equipments. The major steps involved in the process of depositing thin films by this process are summarized below in a sequential manner. • • • • •

Generation of micro-sized droplets of the precursor solution, Evaporation of the solvent, Condensation of the solute, Decomposition and reaction of solute and, Sintering of solid particles.

The schematic diagram of a typical spray pyrolysis setup is shown in Fig. 6. It is a well-established technique for the formation of films of noble metals, metal oxide, chalcogenide, and high temperature superconducting compounds. Other advantages of using this method are the easy formation of doped film, no requirement of vacuum

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Fig. 6 Schematic presentation of spray pyrolysis setup for the deposition of ZnO nanostructures/thin films (Source Author)

apparatus, good productivity on large scale, and moderate operating temperature (50– 500 °C). The ZnO thin films have been prepared by using analytical grade of precursor zinc acetate dehydrate in a mixture of deionized water and isopropyl alcohol onto heated glass substrates at 397 °C. The films with high optical transmittance were deposited for a different span of time 20–480 s. The spraying time of 30 s led to continuous crystalline film consisting of well-shaped grains with mean grain size of ~35 nm [70]. Similarly, zinc acetate was used to deposit ZnO thin films of different thicknesses on glass substrate and found that refractive index increased with film thickness. Such films consisted of grains of ~20 nm and the films exhibited direct band gap energy in the range 3.21–3.31 eV [71]. Different other zinc precursors like zinc chloride [72], zinc nitrate hexahydrate [73], zinc acetyl acetone [74] have also been used to synthesize ZnO nanostructures by spray pyrolysis technique. Further, pulsed spray pyrolysis technique has also been used for ZnO thin films [75]. Similarly, low resistive p-type ZnO thin films have been deposited using ultrasonic spray pyrolysis technique [76, 77]. Recently Kurtaran et al. [78] used spray pyrolysis technique to deposit AZO thin film and investigated annealing time effect on the structure and properties. An island like ZnO nanostructures with hexagonal wurtzite phase was obtained with preferred (101) orientation and 3 h annealing treatment resulted the minimum electrical resistivity of 1.39 × 101 cm with a film thickness of 94 nm [78]. In similar fashion, several other chemical methods have been successfully utilized to produce the ZnO nanostructures and thin films [64, 79–85]. Some of the popular ones are presented schematically along with representative SEM/TEM images of the nanostructures in Fig. 7.

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[64]

[79] [85]

[80]

[84] [81]

[83]

[82]

Fig. 7 Schematic presentation of various ZnO nanostructures synthesized via different chemical methods. The representative SEM images of the nanostructures are also shown for each method with corresponding references from which the images are used (Source References for images mentioned with permission of re-use)

3.2 Physical Methods 3.2.1

Physical Vapor Deposition (PVD)

Electron-beam physical vapor deposition or EBPVD is a highly energetic process of physical vapor deposition in which a target anode is bombarded with a high energy electron beam having high voltages of 10–20 keV generated by a thermionic emitting metal tungsten filament under high vacuum pressure of ≤1 × 10–5 Torr [86]. The high energy electron beam causes atoms from the target to transform into the vapor phase. As a result, the evaporated material gets coated with a thin layer on the substrate of interest or on everything in the vacuum chamber. Resistive thermal evaporation technique is also based on the same principle except heating the target materials

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Fig. 8 Schematic of a typical electron beam physical vapor technique for the synthesis of ZnO nanostructures (Source Author)

by using resistive heating of the boat/crucible (by passing high current through it) containing the target material. In case of ZnO deposition, ZnO target impinges with high energetic electron beam which after transferring energy heats up the material (ZnO) causing its evaporation [87]. Figure 8 represents the schematic of a typical EBPVD setup. The EBPVD is a well established, widely used technique to fabricate ZnO thin films for various types of optoelectronic devices. This method provides highly transparent, conducting, single phase, and highly oriented ZnO thin films either by evaporation of ZnO at different substrate temperatures or by evaporation of ZnO at room temperature and then annealed it at different temperatures in oxygen ambient. Agarwal et al. [88] reported ZnO thin film via EBPVD technique with a band gap energy value of 1.51 eV with resistivity 14.65 × 10–2 cm for the films on the substrate at room temperature. However, the band gap and resistivity increased to 3.26 eV and 86 cm, respectively, for annealing temperature of 800 °C [88]. It was observed that the ZnO thin films annealed at 400 °C exhibited high electrical conductivity and carrier concentration confirming the presence of Zn interstitials. High temperature annealing caused reduction in both of the above parameters which suggested the elimination of Zn interstitials at higher annealing temperature. These parameters can tailor the properties of ZnO thin films to opt in various optoelectronic

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devices [89]. Electric field assisted PVD has been used for rapid synthesis of hexagonal structured and strongly (101) oriented ZnO nanobars of width of 40–80 nm and average length of 300–500 nm by Jouya et al. [90].

3.2.2

Pulsed Laser Deposition (PLD)

PLD is a technique wherein a high pulsed laser power is focused inside a vacuum chamber to hit a target material which is desired to be deposited on the substrate. After striking with pulsed laser beam the target material turns in the form of plasma plume which later deposited as a thin film [91]. PLD is a very complex physical phenomenon. When the laser pulse is absorbed by the target, plasma of the target material, which is created by electronic excitation energy, moves toward the substrate in the vacuum (dynamic of plasma) and deposition on the substrate occurs after ablation of plasma. At last, nucleation and growth process of the film starts on the surface of the substrate. The schematic diagram of a typical PLD setup is shown in Fig. 9. In this approach, usually UV exciter lasers (ArF: =193 nm and KrF: = 248 nm) [92] and Nd: yttrium aluminum garnet (YAG) pulsed lasers (λ = 355 nm) are used for ablation of the ZnO target in an ambient of oxygen environment [93]. Tablets of ZnO powder can be used as target material and single crystal ZnO may be used for high quality ZnO thin films. For example, a high quality nanostructured ZnO thin film was fabricated via nanoparticle assisted PLD in oxygen ambient by Fig. 9 Schematic of a typical PLD setup used for the synthesis of ZnO nanostructures/thin films (Source Author)

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Kawakami et al. [94]. Crystallized and highly oriented (002) ZnO nanorods having average diameter and length of 300 nm and 6 μm, respectively, were obtained at an optimized substrate temperature. Ultrafast PLD method has also been opted for the synthesis of ZnO nanomaterial due to its complex light-material interactions under the extreme conditions of short time scale and high-power density. ZnO photodetectors are also fabricated by PLD technique. The Mg alloyed ZnO in the context of Zn1-x Mgx O photodetectors fabricated by PLD technique possess several advantages for UV photodetectors due to its low deposition temperature, high absorption coefficient, high radiation hardness, and tunable band gap energy (3.3–7.8 eV) [95]. In similar fashion, several other physical methods are successfully employed to produce ZnO nanostructures and thin films [96–101]. Some of the popular methods used are presented schematically along with representative SEM images of the nanostructures in Fig. 10.

3.3 Green Synthesis of ZnO As we know, ZnO is a very versatile material as used in different areas of optoelectronic devices and has well-established chemical and physical synthesis techniques since last few decades. Where one side chemical synthesis routes promote the use of lots of raw chemicals which is hazardous to environment, on the other side physical synthesis methods lead to promote use of heavy equipments which overall increase the cost of material. So since last few years researchers have been developing green synthesis technique of ZnO nanostructures for application in different areas. Among the various nanoparticles, ZnO has been studied using different plant extracts such as Hibiscus sabdariffa [102], Agathosma betulina [103], Tecoma castanifolia leaf extract [104], Zingiber officinale [105], Stevia rebaudiana [106], Aristolochia indica, potato [107], and green tea leaves [108]. ZnO nanostructures synthesized via Salvadora persica plant extract were reported by Verma et al. [109]. The chemical constituents which are present in Salvadora persica plant play a vital role in the reduction of zinc acetate dihydrate precursor to ZnO nanoparticles. XRD pattern confirmed the single-phase wurtzite structures with crystallite size ~25 nm. FTIR analysis confirmed the presence of biological active compounds which gives stabilization to nanoparticles by adsorption of phytochemicals on the synthesized ZnO NPs [109]. The different green synthesis approaches for ZnO nanostructures are summarized schematically in Fig. 11.

4 Photovoltaic Applications In general, a solar cell or photovoltaic device converts the sunlight energy into electricity by photovoltaic (PV) effect. Absorption of solar radiation in the light harvester/absorber layer generates charge carriers (exciton/electron–hole pairs).

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[96] [97]

[98]

[99]

[100] [101]

Fig. 10 Schematic presentation of various physical methods used for the synthesis of ZnO nanostructures. Representing SEM image of the ZnO nanostructures are also shown for respective methods with corresponding references from where the images are used (Source References for images mentioned with permission of re-use)

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Fig. 11 Schematic presentation of some of green synthesis routes for ZnO nanostructures (Source Author)

These photo-generated charge carriers must be separated and transported to and collected efficiently at the respective electrodes in order to produce solar power from the device. A photovoltaic cell performance is characterized and evaluated by the current (I)-voltage (V) characteristics measured under the standard test conditions (STC). The STC are very important in performance evaluation of any photovoltaic cell as its performance depends upon several parameters. In order to have a uniform and globally accepted performance of different solar cells for terrestrial applications, the STC have been designed. These test conditions are (i) Radiation intensity of 100 mW/cm2 (or 1000 W/m2 ), (ii) Air Mass 1.5 Global (AM1.5G, IEC 60904-3) spectrum, and (iii) Measuring temperature of 25 °C. Typical current (I)-voltage (V) characteristics of a PV cell, under dark and illuminated conditions are schematically presented in Fig. 12. The performance parameters, which can be determined from the characteristics, are also shown in the curve. Brief descriptions of these parameters are also presented in the following section [4, 5].

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Fig. 12 Typical I-V characteristics of solar cell under dark and illumination conditions along with performance parameters (Source Author)

4.1 Basic Performance Parameters of a Photovoltaic Cell/Solar Cell 4.1.1

Open-Circuit Voltage (Voc )

The Voc is defined as the maximum voltage generated from the device under open circuit condition (i.e., no current in the circuit). In devices where minority carrier lifetime is low (caused by low-quality material used, poor device design, high density of defects, etc.) or have high recombination losses, the Voc will be low and vice versa.

4.1.2

Short-Circuit Current (Isc )

The Isc of a solar cell is the maximum current generated in its external circuit under the short-circuit condition. It depends on the intensity and the solar spectrum incident on the solar cell as well as on the area of the cell. Therefore, short-circuit current density (Jsc ) (Isc /area of the cell) is considered for practical application and scientific comparison of the quality of the devices.

4.1.3

Fill Factor (F.F.)

The F.F. is the ratio of the maximum output power (V m × I m ) to the product V oc × I sc , where V m and I m represent the voltage and current on the illuminated I-V characteristics where the power delivered by the device is maximum. Ideally, the F.F. should be unity, however, its value is always less than unity due to parasitic resistances and recombination losses. Lower the parasitic losses (shunt resistance, Rsh and series resistance, Rs ) higher will be the F.F.

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F.F. = Vm Im /Voc Isc

4.1.4

Power Conversion Efficiency (PCE)

The PCE or efficiency (η) is the most important parameter of a solar cell which is used to evaluate its performance. Quantitatively, it is the ratio of the maximum output electrical power to the total incident solar power density (Pin ). The PCE of the solar cell reflects the quantitative capability of the solar cell to convert the incident light into electrical power. PCE (η) = Pmax /Pin = Vm Im /Pin = F.F.Voc Isc /Pin In a photovoltaic cell, irrespective of solar cell technology (conventional silicon wafer based, or emerging solar cells), generally, the prime aim is to produce maximum power by (i) increasing Isc , (ii) increasing Voc, and (iii) minimizing the parasitic power loss (particularly Rs and Rsh ) and hence, maximizing the F.F. [4, 5]. The ZnO nanostructures and thin films, owing to various fascinating and tunable structural, morphological, outstanding physical properties, along with various routes of easy and cost-effective synthesis, have made this material a key and highly researched component/material for various photovoltaic cell applications starting from the first-generation silicon wafer-based photovoltaic cell to emerging photovoltaic devices like dye-sensitized solar cells (DSSCs), organic PVs (OPVs), quantum dots solar cells (QDSCs), perovskite solar cells (PSCs), and various heterojunction-based photovoltaic device concepts such as inorganic heterojunction and organic/silicon hybrid solar cells. A pictorial presentation illustrating the various photovoltaic applications of ZnO nanostructures/thin films and their role in such applications are summarized in Fig. 13.

4.2 ZnO/Silicon Heterojunction Solar Cell As mentioned in the introduction section of the chapter, although, single crystal silicon-based solar cell is a well-established technology and it is commercialized [110], its relatively higher cost and complicated fabrication technique is a major concern. So crystalline silicon (c-Si) based heterostructure solar cells have attracted a great deal of interest among researchers to enhance efficiency at the same cost. It has been observed that Si-based homojunction solar cells show high efficiency at a particular wavelength around 600 nm and have high losses in blue and red region. To overcome this problem, c-Si-based heterojunction solar cells have been explored by joining hands with wide band gap semiconductor material. The heterojunction solar cell concept based on a wide bandgap transparent conducting oxide (TCO) coated on lower band gap c-Si, gained lots of attention among the researchers due to

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Fig. 13 Schematic illustration of ZnO nanostructures/thin film for photovoltaic applications (Source Author)

its several advantages including the simple and low temperature processing. Previously indium tin oxide (ITO) films were used as a wide band gap semiconductor on Si substrate. Due to limited indium availability, an alternative material abundant in nature is required for utilization in such applications. The ZnO can be used as an effective alternative to the ITO. As discussed earlier, the ZnO has excellent optoelectronic properties as well as low-cost, non-toxic, natural abundance, and relatively low temperature processing [111]. Moreover, it offers more resistant to radiation damage as compared to other popular semiconductors (Si, GaAs, or GaN) and hence can provide stability against photo-induced degradation and the longer lifetime [112]. In addition, n-type ZnO can easily be realized by excess of Zn or by doping of Al, Ga, or In [113]. Recently, applications of n-ZnO layer as emitter layer on p-Si substrate and AZO as TCO layer were explored in a heterojunction device structure AZO/n-ZnO/p-Si/p+ -Si/Al by TCAD simulation to enhance the heterojunction solar cell efficiency [114]. Heterojunction device structure AZO/n-ZnO/p-Si/p+ -Si/Al with different emitter thicknesses (ranging from 1 to 80 nm) was considered in the simulation. A maximum theoretical PCE of 27.88% with Voc of ~0.628 V and Jsc of 54 mA/cm2 was obtained for 80 nm thickness of the n-ZnO emitter. The Voc and Jsc

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and hence the PCE increased with the thickness of absorber layer (p-Si) [114]. The SCAPS-1D (a Solar Cell Capacitance Simulator-1D) simulation has also been used to simulate the n-ZnO/p-Si heterojunction solar cells and PCE >17% is predicted by regulating the interface defects in Si surface [115]. Such simulation studies have motivated to explore and investigate the potential of ZnO/Si heterojunction solar cells and several experimental efforts have been made to realize such solar cells with promising performances. The schematic of a typical and simple n-ZnO/p-Si heterojunction solar cell is presented in Fig. 14a. As shown in Fig. 14b, values of electron affinity for Si and ZnO are 4.05 and 4.35 eV, respectively. Band gap energy for Si and ZnO in this case are 1.12 and 3.37 eV, respectively. Also, conduction band (CB) and valance band (VB) offsets are 0.3 eV and 2.55 eV, respectively, [116]. When a photon gets absorbed in p-Si after being transmitted through a wide bandgap in n-ZnO, it generates electron– hole pair. The electron and hole are separated by the band offsets at the junction. The electrons are then transported to cathode through the n-ZnO and holes (left behind in the p-Si) to the anode (back electrode). Therefore, under illumination, a minority carrier in conduction band of n-ZnO is generated. That is how an n-ZnO/p-Si heterojunction works as a solar cell. For photovoltaic applications, the ZnO nanostructures/thin films via different processing techniques have been used for n-ZnO/p-Si heterojunction design. For example, the ZnO nanoparticles (NPs), synthesized via colloidal method, were deposited as thin film on the p-Si substrate (500 um thickness, , 1–3 cm) by spray techniques for solar cells fabrication. Deposition of the ZnO NPs films was followed by oxidation in the temperature range 300–500 °C [117]. The synthesized ZnO films were found to be n-type having mobility values in the range of 7–24 cm2 V−1 s−1 . With this approach, the best solar cell of PCE 6.79% was made having Jsc : 25 mA/cm2 , Voc : 0.375 V and F.F.: 0.72 [117]. Very recently, Pietruszka et al. achieved PCE of 14% for n-ZnO/p-Si heterojunction solar cells after employing the ZnO nanorods (ZnONR ) as an Al/pSi/ZnONR /Zn1-x Mgx O/AZO/Al cell design as shown in Fig. 15a as compared to only 10.5% obtained with and Al/p-Si/Zn1-x Mgx O/AZO/Al cell design without ZnONR (i.e., with cell planar geometry) as shown in Fig. 15b. Here, AZO and Zn1-x Mgx O thin layers were deposited using the ALD, and ZnO nanorods were grown using hydrothermal method. However, ZnO seed layer required for the growth of ZnO nanorods was prepared by ALD on 180 μm thick p-Si wafers [118]. Several efforts have been made to demonstrate the potential of n-ZnO/p-Si heterojunction solar cell by introducing various ZnO nanostructures and thin films via different routes. Some of the prominent works are summarized in Table 1. However, the PCE achieved is far below than their potential for more than 20% [7, 126]. Efforts are being made to improve the quality of the ZnO nanostructures/thin films and p-Si interface, the electrical conductivity of the ZnO layer, surface passivation schemes to address the electrical and electronic losses which may definitely lead to achieve the simulated performances.

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(a)

(b)

Fig. 14 a Schematic representation of ZnO/Si heterojunction solar cell, b band energy diagram for p-Si/n-ZnO heterojunction [116]

4.3 ZnO Thin Film as Antireflection and Passivation Layer/Conventional Si Solar Cells Anti-reflection coatings (ARC) and surface passivation are the two most investigated subjects in Si wafer solar cell technology to address the reflection losses (>35% in 400–1100 nm spectral range) and the surface recombination losses for improving the performance [4, 5]. The surface recombination losses become significant and

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(a)

(b)

Fig. 15 A schematic drawing of the photovoltaic cells; a with ZnO nanorods, b planar ZnO. Reprinted from Ref. [118] Copyright (2017), with permission from Elsevier

decisive for the thinner wafers [4, 5, 127–129]. The most common ARC used for Si solar cell is silicon nitride (SiNx ) layer deposited by plasma enhanced chemical vapor deposition (PECVD) as it not only reduces reflection losses but also provides an effective passivation to n+ region (the front/emitter) of n+ -p-p+ structured Si solar cell. Though it is not effective for p+ surface. The PECVD-SiNx thin film of ~70 nm (with refractive index ~2.0) in conjunction with surface micro-texturing reduces the reflection loss to 10% [164–166, 171, 172, 196, 197, 199–205]. However, the state-of-the-art PCEs in OPVs which are based on fullerene-based active layers (acceptor, PCBM) are very limited. Very recently, however, it has been established that the performances (both PCE as well as stability) of the OPVs based on the fullerene-based active layers are being compromised due to the limited light absorption and morphological instability of the active layers [206–208]. Therefore, a lot of investigations are being put in to replace the PCBM and the other fullerene-based acceptor with some non-fullerene ones. Zhao et al. used a non-fullerene n-type acceptor, ITIC, and PBDB-T as active layer and ZnO as CBL in the inverted structure. A PCE of 10.71%, with a stability of 83% in PCE after >4000 h was demonstrated with such cell design [209]. Li et al. introduced ethylene diamine tetra-acetic acid (EDTA) in the ZnO precursor and could achieve ZnO layer at low annealing temperature along with passivating the defects in ZnO originated by its chelation function. The lower conductivity of the EDTA

Device structure and role of ZnO

ITO/ZnO NPs/P3HT:PCBM/PEDOT:PSS/Ag (ZnO as ESL)

Flexible ITO/ZnO/P3HT:PCBM/MoO3 /Ag (ZnO as cathodic layer in inverted solar cells)

ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag (ZnO as ETL)

ITO/GZO/PffBT4T-2OD:PC70 BM/MoOx /Al (Ga-ZnO as ETL)

ITO/ZnO (or ZnO/Ba(OH)2 or ZnO: (BaOH)2 nanocomposite/PTB7-Th:PCBM/MoOx /Ag

ITO/AZO/PCDTBT:PCBM/PEDOT:PSS/Au (AZO as ETL)

Year

2008

2013

2015

2017

2018

2020

Table 2 ZnO/doped ZnO as electron transport/buffer layer in OPVs Voc : 0.623 V Jsc : 10.69 mA/cm2 FF: 54.2 η: 3.61%

PV parameters*

Spin coating of AZO layer on ITO

ZnO (or ZnO:Ba(OH)2 or ZnO and ZnO:Ba(OH)2 nanocomposite) ITO by spin coating

GZO thin films by spin

ZnO sol–gel thin films by spin-coating method

Voc : 0.81 V Jsc : 8.82 mA/cm2 FF: 0.46 η: 2.9%

Voc: 0.818 V Jsc: 15.69 mA/cm2 FF: 0.67 η: 8.66%

Voc : 0.771 V Jsc : 18.65 mA/cm2 FF: 0.67 η: 9.74%

Voc : 0.60 V Jsc : 9.39 mA/cm2 FF: 0.41 η: 2.31%

ZnO films on PEN/ITO substrate by RF magnetron Voc : 0.59 V sputtering Jsc : 9.1 mA/cm2 FF: 0.47 η: 2.5%

ZnO NPs by sol–gel and spin-coating method

Method of ZnO synthesis/deposition

(continued)

[198]

[197]

[196]

[195]

[194]

[193]

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Source Author *All the PCE are under the simulated STC of AM 1.5G (100 mW/cm2 )

ITO/ZnO:Au NPs/PTB7-Th:PC71 BM/MoO3 /Ag (ZnO as Incorporation of Au NPs of size 10–20 nm into ETL) solution-processed ZnO ETLs

2021

Method of ZnO synthesis/deposition

Device structure and role of ZnO

Year

Table 2 (continued) Voc : 0.80 V Jsc : 21.75 mA/cm2 FF: 0.68 η: 11.8%

PV parameters* [199]

Ref

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was balanced by the higher conductivity of the ZnO film. Combining these two and PBDBT:IT-M as active layer, the inverted device exhibited a PCE of 11.67% [210]. In this series, PDBD-T-2F:Y6, has also been used as active layer and ZnO as the CBL. By using the ligand (tetrahydroxy-perylene bismide, HO-PBI ligand) embedded onto a ZnO thin film, the device PCE could be improved to 15.95% [211]. The enhanced PCE was accounted for the improved solubility and molecular dispersion of ZnO:HO-PBI, leading to increased mobility and improved transport for the electrons. In a very latest work, PCE of >16% has been achieved with high long-term stability. For example, Han et al. [212] have shown that surface pH conditions affect the performance (PCE and stability) of the non-fullerene acceptor-based OPVs as the interface contacts, between the CBL (ZnO) and the active layer, play key role in the device performance. This is due to the fact that the solution-processed ZnO surface always contains some base or Zn precursor related contaminants, like large number of hydroxyl groups on the surface, etc. which are responsible for oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindering the efficient charge transfer/collection, and hence the lower PCE and relatively rapid degradation of the non-fullerene acceptor at the ZnO/active layer interface. Treatment by acid solutions of 2-phenylethylmercaptan (PET) was found the most effective and in optimized treatment conditions of ZnO films with PET, a state-of-the-art PCE of 16.46% was demonstrated for the PM6:Y6 based OPV cells employing ZnO-CBL and a non-fullerene active layer. The long-term stability under continuous illumination conditions was also enhanced more than 400 folds (T80 lifetime >4400 h) compared with untreated ZnO-based OPVs (T80 lifetime of 14.3%) has been obtained with Co(III/II) complex redox electrolyte solution through collaborative sensitization by the dyes using silyl-anchor dye ADEKA-1 and carboxy-anchor dye LEG4 [235]. Photoanodes (wide band gap semiconductor) play a critical role in the DSSCs and TiO2 (anatase) has been the material of choice for this application since the discovery of the DSSC concept, owing to its stability, lower cost, catalytic activity, and easy synthesis [219, 220, 229, 231]. Several alternative wide band gap oxides such as ZnO [221, 223, 227], and Nb2 O5 [222, 237], SnO2 [238, 239], Fe2 O3 [240], Ag2 O-ZnO [224, 241], and Au-TiO2 [224, 242] have also been investigated. Among those, the ZnO has been found as the best alternative to TiO2 since the very beginning of TiO2 -based DSSCs. This is because of facts that ZnO possess quite similar or even better optoelectronic properties than TiO2 . For example, it has band gap (~3.3 eV) which is quite close to that of TiO2 (~3.2 eV), same electron affinity, higher electron diffusivity (or life time) than TiO2 [243], and high electron mobility (>200 cm2 V−1 s−1 ) [244], thus can have efficient electron transport and reduced recombination rates. In fact, ZnO was the first one to be used as the photoanode of a DSSC [245] and was also cited in the pioneering work by Grätzel and O’Regan on TiO2 based DSSC [219]. The first evidence of irreversible electron injection from organic molecules into the CB of a wide gap semiconductor was experimentally demonstrated in ZnO only [246, 247]. As mentioned earlier, the ZnO has a large excitation binding energy (60 meV), low cost and easy processing, and it is relatively stable against the photo-corrosion process. A very important requirement of the photoanode is its mesoporous/nanostructured (high surface area) morphology which facilitates the anchoring of more and more dye molecules and hence more light absorption and photocurrent. The ZnO, thanks to its diverse morphologies, easy and cost-effective synthesis, is considered to be richer than those of other counterparts known so far. Therefore, various designs of photoanodes can be explored in the ZnO-based DSSCs. The DSSCs with photoanodes of nanoparticles, nanorods, nanowires, nanotubes, nanosheets, nanoflowers, and many more diverse morphologies are possible with ZnO, which is not easy with TiO2 [248]. A comprehensive review on all aspects of ZnO photoanode-based DSSCs have been compiled by Vittal and Ho [223]. For example, Baxter et al. (2006) used ZnO nanowires-based photo-electrode in the DSSCs [248]. A schematic diagram of the ZnO nanowire-based DSSC is shown in

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Fig. 24 ZnO nanowires (red color) used as photo-electrode in DSSC. Reference [248]. © IOP Publishing. Reproduced by permission of IOP Publishing. All rights reserved

Fig. 24. The DSSCs assembled with 8 μm long ZnO nanowires exhibited Jsc value of 1.3 mA/cm2 and PCE of 0.3% [248]. In the recent years, Shashanka et al. reported DSSCs with green synthesized ZnO nanoparticles calcined at 400 °C for 15 min. Here, they could achieve a maximum PCE of 1.97% with Voc of 650 mV, Jsc of 6.26 mA/cm2 and F.F. of 48.5% [249]. Similarly, there have been lot of investigations on the nanostructured ZnO-based DSSCs for the improved performance. Some of the examples based on different ZnO morphologies are summarized in Table 3. Since very first pure ZnO photoanode-based DSSCs of efficiency 0.4% in 1994 [259], ZnO-DSSCs have made a significant progress. The highest PCE for a pure ZnO-DSSC using the liquid electrolyte was achieved to be 7.5% in 2011 [254] and further improved to 8.03% in 2015 [255]. Though DSSC with the highest PCE of 8.2% has been reported in 2019 but in this work, TiO2 is used as photoanode and the Pt decorated ZnO NWs as an efficient counter electrode [260]. It is no doubt that highest PCE for ZnO-DSSCs is inferior to that of TiO2 -DSSCs (12.3–14.3%) [233–235]. The poor PCEs of ZnO-DSSCs are mostly attributed to the instability of ZnO in presence of acidic dye and liquid electrolyte. The ZnO gets dissolved to form Zn2+ by the adsorbed acidic dye and I− /I3 − electrolyte followed by the formation of agglomerates (insulating layer) of Zn2+ and dye molecules (such as N719 or black dye). Subsequently, the injected electrons (from the dye molecules to the semiconductor) are prevented by the insulating layer formed [261, 262]. It is to be noted that the isoelectric point of ZnO is higher (~9) as compared to that of TiO2 (~6) and implying the more basic nature of the ZnO and thus more prone to attack by the acidic dye [263]. The core–shell structures of ZnO have been proposed in which a buffer layer is coated on the ZnO surface to avoid the formation of Zn2+ /dye agglomerates or complexes. In this direction, SiO2 has been demonstrated to be a very effective shell material on ZnO because of a strong interaction between Si4+ and O2− ions [264]. The TiO2 has also been proposed to be quite effective for such application [265]. Several core–shell structures of ZnO nanocrystals and nanowires

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Table 3 Summary of DSSCs based on ZnO nanostructures as photoanode Year Device structure

Morphology of nanostructure and method of preparation

PV parameters*

Ref

2007 ZnO nanorods/dye/electrolyte/electrode

ZnO nanorods and ZnO-nanoflowers by hydrothermal method

Voc : 0.65 V Jsc : 5.5 mA/cm2 FF: 0.53 η: 1.9%

[250]

2008 F:SnO2 /branched ZnO NWs/dye/electrolyte/F:SnO2

Branched ZnO nanowires by solvothermal method

Voc : 0.675 V Jsc : 4.27 mA/cm2 FF: 0.522 η: 1.51%

[251]

2010 FTO(F:SnO2 )/ZnO/Electrolyte dye/Pt-electrode/FTO(F:SnO2 )

Al-ZnO nanorod arrays by hydrothermal method; ZnO seed layer by RF magnetron sputtering

Voc : 0.477 V Jsc : 8.86 mA/cm2 FF: 0.32 η: 1.34%

[252]

2011 FTO/ZnO NWs forest/electrolyte/FTO

Hierarchical ZnO NWs by hydrothermal method

Voc : 0.680 V Jsc : 8.78 mA/cm2 FF: 0.53 η: 2.63%

[253]

2011 FTO/compact ZnO (buffer layer)ZnO Spray pyrolysis nanocrystallites followed by aggregates/dye(N719)/electrolyte(I3 − /I− redox post-deposition annealing couple)/platinized FTO

Voc : 0. 64 V Jsc : 19.8 mA/cm2 FF: 0.59 η: 7.5%

[254]

2012 FTO/ZnO/electrolyte//Pt foil/FTO

Nanospikes decorated ZnO sheets by hydrothermal method

Voc : 0.680 V Jsc : 6.07 mA/cm2 FF: 0.60 η: 2.51%

[255]

2015 FTO/ZnO (air plasma treated)/dye/electrolyte/platinized FTO

Solid-state synthesis of Zn(OH)2 nanostructures followed by low temperature air plasma treatment for reducing hydrogen related defects in ZnO

Voc : 0.60 V [256] Jsc : 22.18 mA/cm2 FF: 0.60 η: 8.03%

2019 FTO/ZnO NWs (or nanoparticles)/dye/electrolyte/Pt electrode/FTO

ZnO nanowires by PVP assisted solvothermal process

Voc : 0.58 V Jsc : 6.64 mA/cm2 FF: 0.47 η: 1.81%

[257]

(continued)

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Table 3 (continued) Year Device structure

Morphology of nanostructure and method of preparation

PV parameters*

Ref

2021 TCO/ZnO nanostructures/dye/electrolyte/Pt/TCO

ZnO nanostructures by precipitation method

Voc : 0.62 V [258] Jsc : 12.32 mA/cm2 FF: 0.54 η: 4.1%

Source Author *All the PCE are under the simulated STC of AM 1.5G (100 mW/cm2 )

have been developed by coating a buffer layer of different oxides like SiO2 [264], Al2 O3 [266], TiO2 [266], or even ZnO [267] on the surfaces of the ZnO nanostructures. Greene et al. made a core–shell structure of ZnO@TiO2 -NWs by ALD and the PCE was improved to 2.25% as compared to 0.45% for the corresponding DSSC with bare ZnO-NWs [268]. Similarly, layered double hydroxide is also investigated as photoanode materials to overcome the limitations of ZnO with corrosive redox I− /I3 − electrolyte [269]. Therefore, the current focus in research on ZnO photoanodebased DSSCs is to improve the PCE to make it at par with a TiO2 photoanode-based DSSC (>14%) and still there is a lot of scope for research. Some of the key strategies identified for improving the PCE of ZnO-based DSSCs are (i) exploring new architectures of ZnO photoanodes; (ii) effective mechanism to prevent the Zn2+ /dye aggregation, (iii) mechanism to minimize recombination reactions between injected electrons with redox species, and (iv) developing novel dyes as per ZnO photoanode requirements [223].

4.6 Perovskite Photovoltaics In recent times, perovskite solar cells (PSCs), which are based on inorganic–organic metal halides have been given a tremendous attention as one of the most attractive solar PV technologies. In this device, the perovskite crystal structure acts as efficient solar radiation absorber. The general formula for the perovskite material is ABX3 , where A, B represent large and small cations, respectively, and X is anion. In detail, A is aliphatic/aromatic or ammonium group, B is divalent material and X is halogen. The inorganic–organic lead (Pb) halides possess large absorption coefficient in visible spectral range and long exciton diffusion length. The PSCs, therefore, achieve PCE comparable to the well-established Si wafer-based solar cells at lab scale and also PSCs are promised to be quite cost effective as compared to the conventional Si photovoltaic devices [270]. The PCEs of PSCs have increased at an incredible rate reaching the world record of 25.6% (certified eff. 25.2%) through significant advancements in the perovskite film growth control, properties, interface,

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and device engineering [271–274]. The high efficiency of the PSCs is attributed to the excellent optoelectrical properties of the perovskite, such as high light absorption, low exciton binding energy, long carrier diffusion length, and carrier lifetime [275–277]. However, long-term stability of the devices is still a challenging task limiting the large-scale application of the PSCs technology [278, 279]. The photo-generated exciton in the absorber layer of the device must be separated and the charge carriers (electrons and holes) must be transported to and collected efficiently at the respective electrodes in order to generate solar power in a PV device. The schematic of typical perovskite solar cell structure is presented in Fig. 25. It has been reported that the holes have longer diffusion length than the electrons in a lead (Pb) based inorganic–organic metal halide perovskite like methyl ammonium lead iodide (MAPbI3 ) [280]. An ETL is therefore essential to facilitate electron collection by the cathode (like ITO). Besides, the ETL should also act as a hole Fig. 25 a The simplest PSC design in planar (n-i-p) structure. b Corresponding energy band diagram depicting the operation principle of the PSCs in simple planar structure as shown in a (Source Author)

(a)

(b)

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blocking layer (HBL) simultaneously. The ETL therefore must be engineered in way that its energy level is well-matched to that of the light absorber perovskite to act as both ETL and HBL simultaneously. Though a lot of works on PSCs have been done utilizing the TiO2 , and other metal oxides (MOs) as ETL [281, 282], the ZnO has been found an excellent and promising candidate as an ETL [283]. As also mentioned in other sections, the ZnO has a band gap of 3.37 eV, minimum of CB (CBM) at 4.2 eV and exciton binding energy of ~60 meV [284]. The exciton binding energy for MAPbI3 is 45 meV and its HOMO and LUMO are at 5.2 eV and 3.6 eV, respectively. Therefore, the ZnO can efficiently extract electrons and block holes and hence dissociate the excitons from the perovskite (light absorber layer) [285]. Moreover, the bulk ZnO has electron mobility of >200 cm2 V−1 s−1 which can be further enhanced to 1000 cm2 V−1 s−1 in ZnO nanostructures (nanorods, nanowires) [286]. On the other hand, the TiO2 has electron mobility of only 0.1–4 cm2 V−1 s−1 , much lower than that of ZnO nanostructures [286]. It is to be noted that higher the electron mobility lower may be the recombination loss and thus improved PV performance of the solar cell. Besides, as mentioned earlier, the ZnO has a very good structural flexibility at nanoscale to form diverse morphology, such as nanoparticles, nanorods, nanowires, nanoflowers and therefore like in DSSCs, it has also been extensively investigated as the ETL in planar PSCs. For HTLs, Lior Co-doped 2,2 ,7,7 -tetrakis(N, N-di-pmethoxyphenylamine)-9,9 spirobifluorene (Spiro-OMeTAD) and Li-spiked poly triarylamine (PTAA) are the most successful candidates due to their easy solution-processing and favorable hole mobility (1–2 × 10–3 cm2 V−1 s−1 ) [287, 288]. For PSCs, ZnO nanoparticles/thin films were firstly introduced in 2014 by Liu and Kelly [289] wherein the ZnO NPs were synthesized via a solution method using KOH and Zn(CH3 COO)2 .2H2 O. Subsequently, compact ETL based on crystalline ZnO NP film was also developed by a room temperature process and applied in the planar PSCs. They could achieve a high PCE of 15.7% PSCs in a conventional device structure via optimizing the ZnO film properties and use of large crystallite sized perovskite films. Thereafter, different deposition methods have been used for the ZnO as efficient ETL in the PSCs. For example, Lee et al. [290] used ALDbased compact ZnO films as the ETL and carried out a detailed investigation on the influence of ZnO film thickness on the performance of the PSCs. On a similar approach, Dong et al. achieved a PCE of 13.1% by using ALD-ZnO [291]. Kumar et al. employed both a compact ZnO film (by electro-deposition method) and a ZnO nanorod film (by chemical bath deposition method) to demonstrate flexible PSCs with reasonable performances [292]. Tseng et al. [293] used RF sputtering-based ZnO film and achieved a PCE up to 15.9% in regular planar PSC under the optimized deposition parameters of the film. It was also found that the deposition parameters like ratio of working gases (Ar and O2 ) have large influence on the electric and electronic properties of ZnO film due to the oxygen vacancies in the film. The performance was further improved by using AZO-ETL [294], primarily due to increased conductivity, better band matching with MAPbI3 and higher acid resistance of AZO than ZnO. Therefore, PSC based on this AZO ETL efficiently promoted the device performance. Very recently, Mazumdar et al. reported the efficient PSCs employing PLD-ZnO as

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ETL [98]. The perovskite layer was deposited with a stoichiometric composition of FA0.75 MA0.25 PbI2.5 Br0.5 by anti-solvent method followed by spin coating of spiroOMeTAD layer as the HTL. The best PSCs exhibited Jsc of 19.2 mA/cm2 , Voc of 1.09 V and F.F. of 72%, hence a PSC of 15.1% for an optimized layer of ZnO-ETL [98]. Recently, low temperature solution-processed ZnO nanoparticles treated with carbon nanotubes (CNTs) was explored [295]. It was found that CNT modified ZnO surface prevents the potential reaction between the ZnO and perovskite film resulting in significant increase in the PCE from 15.05 to 18.79% [295]. Similarly, some of the other key developments in PSCs using ZnO-ETL by different deposition methods are summarized in Table 4. Despite the tremendous and rapid progress in the ZnO-based PSCs, the thermal instability at ZnO/perovskite interface caused by the hydroxyl groups contaminants and/or acetate ligands of the ZnO surface as well as the oxygen vacancies, lead to a faster degradation of ZnO-ETLs based PSCs and hampers its applications [299, 304–306]. Recently, modification of ZnO surface by introducing organic/inorganic materials to avoid the direct contact between perovskite and ZnO was investigated [294, 297, 298, 307]. However, the PCEs of all ZnO-based PSCs were below 18%, with poor long-time stability [308]. Some other efforts have also been made to overcome such issues of ZnO. For example, Chen et al. [309] introduced a method to convert ZnO surface into ZnS at the ZnO/perovskite interface by sulfidation. The sulfide on ZnO-ZnS surface binds strongly with Pb2+ and creates a novel pathway of electron transport to accelerate electron transfer and reduce interfacial charge recombination, yielding a highest PCE of 20.7% with improved stability and no appreciable hysteresis. The long-term stability for 1000 h at 88% of initial performance under storage condition was observed. On the other hand, device maintained its performance at 87% (of initial one) for 500 h under UV radiation. Here, the ZnS worked as both ETL and a passivating layer [309]. Song et al. [310] used commercial ZnO NPs based ETL for PSCs by spin-coating and employed HC(NH2 )2 + instead of CH3 NH3 + to synthesize the perovskite via sequential deposition method. PCE of 18.9% and excellent environment durability and photo-stability was achieved with ZnO NPs-ETL with a triple cation perovskite, a stable light absorber [311]. In the same line, recently an ultrasonic-assisted method was reported for more transparent ZnO NP solution facilitating the formation of denser and uniform ZnO film. An aging step of ZnO film in air at room temperature was also introduced to improve the thermal stability between ZnO and MAPbI3 . Consequently, the degradation of PSC was effectively minimized exhibiting a high stability over 45 days in the air [312]. Similarly, a lot of exciting work in this rapidly progressing PV technology are being reported every now and then via improvement in interfacial engineering on the ZnO/perovskite interface to enhance the PCE as well as stability of the device and ZnO has certainly played key role in the fast development of the PSCs and however the best is yet to be reported.

Method of ZnO deposition

RF sputtering

Spin coating

Hydrothermal

ALD

RF sputtering

ALD

Year

2015

2015

2015

2016

2016

2017

FTO/bl-ZnO/mp-ZnO/Al2 O3 /MAPbI3 /spiro-OMeTAD/Au

ITO/AZO/MAPbI3 /spiro-OMeTAD

FTO/ZnO/mp-Al2 O3 /MAPbI3 /spiro-OMeTAD

FTO/bl-ZnO/N doped ZnO/PEI/MAPbI3 /spiro-OMeTAD/Au

ITO/ZnO/C3-SAM/MAPbI3 /spiro-OMeTAD/MoO3 /Ag

ITO/ZnO/MAPbI3 /spiro-OMeTAD/Ag

Structure of perovskite solar cells

Table 4 Progress in perovskite solar cells using ZnO as electron transport material

Voc : 1.0 V Jsc : 22.42 mA/cm2 FF: 0.71 η: 16.08%

Voc : 1.1 V Jsc : 21.7 mA/cm2 FF: 0.756 η: 17.6%

Voc : 1.0 V Jsc : 18.9 mA/cm2 FF: 0.62 η: 15.55%

Voc : 1.0 V Jsc : 21.5 mA/cm2 FF: 0.70 η: 16.12%

Voc : 1.1 V Jsc : 22.5 mA/cm2 FF: 0.65 η: 15.7%

Voc : 1.1 V Jsc : 19.9 mA/cm2 FF: 0.65 η: 13.9%

PV parameters*

(continued)

[299]

[294]

[298]

[297]

[284]

[296]

Ref

224 P. Kumari et al.

Hydrothermal

ZnO/Graphene ETL Precipitation method

Spin coating method

RF magnetron sputtering

2017

2019

2020

2021

FTO/ZnO/SnO2 /MAPbI3 /spiro-OMeTAD/Ag

FTO/ZnONRs/CH3 NH3 PbI3 /spiro-OMeTAD/Ag

FTO/MLG-ZnO/PbI2 /FAI-MABr-MACl/spiro-OMeTAD/Au MLG/ZnO

FTO/bl-ZnO/mp-ZnO/Al2 O3 /MAPbI3 /spiro-OMeTAD/Au

Structure of perovskite solar cells

Source Author *All the PCE are under the simulated STC of AM 1.5G (100 mW/cm2 )

Method of ZnO deposition

Year

Table 4 (continued)

Voc : 1.20 V Jsc : 20.20 mA/cm2 FF: 0.65 η: 15.82%

Voc : 0.98 V Jsc : 22.70 mA/cm2 FF: 0.63 η: 14.23%

Voc : 1.15 V Jsc : 23.43 mA/cm2 FF: 0.78 η: 21.03%

Voc : 1.0 V Jsc : 20.9 mA/cm2 FF: 0.78 η: 17.3%

PV parameters*

[303]

[302]

[301]

[300]

Ref

6 Zinc Oxide: A Fascinating Material for Photovoltaic Applications 225

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4.7 ZnO in Miscellaneous Other Photovoltaic Devices In addition to the above major PV applications, the ZnO nanostructures and thin films have found applications in varieties of other emergent solar cell concepts; for examples (i) different inorganic heterojunction solar cells, like QDSCs, excitonic solar cells as buffer layer, cathode layer or ETLs or as the n-type semiconductor in the active layer; and (ii) organic/inorganic hybrid solar cells as active layer as well as ETL/cathode layer, etc.

4.7.1

Inorganic Heterojunction Solar Cells

The most common one is fully heterojunction solar cells based on n-type ZnO thin films/nanostructures and nanocrystals (NCs)/quantum dots (QDs)/thin films based on lead (Pb) based p-type semiconductors (e.g., PbS, PbSe, PbTe) or cadmium (Cd) based p-type (CdS, CdSe or CdTe) semiconductors [313–320]. The performances of such PV devices are relatively low as compared to other ZnO-based PV concepts and the PCE as high as ~7% have been achieved. For example, Ren et al. [321] reported efficient PbS-QDSCs with applying both Mg-doped ZnO (MZO) as window layer and ZnO nanocrystals (ZnO NCs) as the interface passivation layer to minimize recombination losses at the interface for inverted PbS-QDSCs with a device configuration: (ITO)/MZO/ZnO-NC(w/o)/PbS/Au as shown schematically in Fig. 27. The PbS colloidal quantum dots (CQDs) solar cells were fabricated by inserting 8 layers of tetrabutylammonium iodide (TBAI) treated PbS CQDs and 2 layers of 1,2-ethanedithiol (EDT) treated PbS CQDs on ZnO NCs layer followed by 100 nm thick Au layer for front contact. The thickness and annealing treatment temperature of the MZO layer was found to be the key for the performance of the PbSCQDSCs. Without ZnO NCs thin layer, the best PCE of 5.52% (Jsc : 23.37 mA/cm2 , Voc : 0.54 V, FF: 43.47%) was achieved for an optimum MZO layer thickness of 50 nm annealed at 300 °C [321]. However, the PCE of the best cell enhanced significantly to 7.06% after introducing a thin layer of ZnO NCs between MZO and PbS-QDs layer, primarily due to the reduced interface recombination. Therefore, the combination of the MZO buffer layer with the ZnO NC interface passivation approach might lead to further advancement of QDSCs (Fig. 26). Similarly, Cu-based p-type semiconductors are also often used with n-ZnO, such as Cu2 O, Cu2 ZnSnS4 (CZTS), and Cu(In,Ga)Se2 (CIGS) [322–324]. The Cu-based solar cells have advantage of being low cost and non-toxic, when compared with the Pb- or Cd-based cells. However, the best PCE reported for solar cells based on Cu2 O and ZnO thin film combination is only 3–4%, far lower than the predicted PCE of ~20% [325–327]. A lot of efforts are being made to improve the PCE by improving the interaction between ZnO and p-type semiconductors/NCs to increase interfacial area, modifying the ZnO nanostructures’ morphology, to have efficient electron pathways, and minimizing electron recombinant losses. Recently, ZnO NPs were employed in a PbS-CQDs system. Here, the ZnO NPs were given additional oxygen annealing

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Fig. 26 Schematic representation of ZnO-PbS-based quantum dots solar cells with Mg-doped ZnO (MZO) as window layer [321]

Fig. 27 a Schematic of a typical PEDOT:PSS/Si hybrid solar cell and b operation principle of PEDOT:PSS/Si HSC via energy band diagram (Source Author)

treatment to passivate their defects [328] As a result, QDSCs of efficiency of 9.05% were achieved compared to 7.98% and 6.90% of the cells employing ZnO treatment in ambient air and nitrogen ambient, respectively. Therefore, it was demonstrated that surface defects caused from the oxygen vacancies in the ZnO NPs could be reduced via additional oxygen annealing and hence the greatly improved PCE.

4.7.2

Hybrid Organic/inorganic Heterojunction Solar Cells

Organic/inorganic hybrid solar cells (HSCs) have received a great attention in the recent past owing to their potential for low cost and high efficiency [329]. Application of ZnO in HSCs can be categorized in two classes (i) ZnO as active layer in HSCs, and (ii) ZnO in supporting role like as ETL/CBL.

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ZnO as Active Layer in Hybrid Solar Cells These HSCs, similar to a fully OPVs, are based on two components to convert light energy into electrical power (i) a conjugated organic semiconductor (like P3HT, poly[2-methoxy-5-(3 , 7 -dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), etc.), which acts as light harvester and electron donor, and (ii) an inorganic semiconductor (rather than an organic one) acting as the electron acceptor [330]. ZnO has been widely used as an electron acceptor (in place of the organic electron acceptor of a fully OPVs), owing to the higher electron mobility of the ZnO compared to most of the known n-type organic semiconductors. Moreover, the ZnO offers higher physical and chemical stability as well [331]. The overall operation principle of such devices is very similar to that of OPVs (as explained in the section of OPVs). Various ZnO nanostructures (nanoparticles, nanowires or nanorods, etc.) have been investigated for such ZnO-organic HSCs applications [332–335]. ZnO in such device concepts have been used mainly in three solar cell designs (i) bulk heterojunction HSCs with randomly dispersed NCs of ZnO [332, 334, 336–340], (ii) Hybrid cells with vertically aligned ZnO nanostructures [341–344], and (iii) the hybrid cells with organic/inorganic bi-layer structure [342, 345–349]. However, PCEs of such devices are quite low ranging 20% PCE with quite promising long-term stability and expected to have even better performances. The ZnO-based DSSCs, although has little lower performance to its counterpart TiO2 DSSCs. However, ZnO-DSSCs is also one of the highest researched subjects after TiO2 -DSSCs and expected to achieve a competitive PCE via development of new architecture ZnO photoanodes; efficient mechanism to avoid ZnO and dye reaction, minimize recombination reactions between electrons and redox species, and development of novel dyes as per the need of the ZnO photoanodes. Similarly, the inorganic semiconductor/ZnO, organic semiconductor/ZnO-based HSCs and PEDOT:PSS/Si HSCs have shown quite interesting performances and engineering of ZnO microstructures, their dimensions, and interface properties will definitely result in even better power conversion efficiencies. However, despite tremendous progress achieved so far in ZnO and ZnO-based PV devices, some critical issues still need to be addressed for achieving even better performances of ZnO-based various PV devices. The interface in various solar cell designs using ZnO as an active layer or as supporting layer should be made more effective via further interface engineering. Such developments may lead to an improved PCE and device stability. Further, the progress in achieving a cost-effective method of producing high quality ZnO with good reproducibility and the further development of a sustainable and environment friendly green method in this regard will definitely lead to socio-economic, efficient, and reliable ZnO-based PV devices in the near future. Acknowledgements Authors are grateful to Director, CSIR-National Physical Laboratory, New Delhi India for kind support. Authors P.K., A.S., R.K.S., and D.S. acknowledge NREF-MNRE, Govt. of India (grant code: 342-12/5/2019-HRD); Department of Science and Technology (DST), Govt. of India (Inspire fellowship, grant code: DST/INSPIRE/Fellowship/2018/IF 180040); University Grants Commission (UGC) (grant code: 16-6(DEC.2018)/2019(NET/CSIR); and CSIR (grant code: 31/001(0623)/2019-EMR-I), respectively, for the research fellowships.

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Chapter 7

Advances in Electrode Materials for Rechargeable Batteries Nadeem Ahmad Arif, Mohammad Mudassir Hashmi, Syed Mehfooz Ali, Mohd Bilal Khan, and Zishan H. Khan

1 Introduction The use of fossil fuel and environmental degradation are critical issues worldwide as of today. Most of the world’s energy supply is fuelled by fossil fuels, which in turn are linked to increasing greenhouse gas emissions and global warming. It is one of the most significant problems being faced by the scientific and social communities to achieve a safe and sustainable energy future. Wind, solar, and other carbon–neutral energy supplies are progressively replacing fossil fuels, with a higher priority on green energy. Availability of renewable energy sources, such as wind and sunlight, is unlimited but is varying and therefore energy storage is the need for using them efficiently. For more sustainable and environmentally friendly, energy storage devices including rechargeable batteries, fuel cells, and super-capacitors are playing a very important role [1]. In these electrochemical energy storage devices, electron and ion transport occur at the electrolyte–electrode interface, and the energy is then stored or released by means of the charge/discharge process. In addition, technological advancement requires energy storage systems that should be of higher energy density, long lasting, cost efficient and environmentally friendly. Rechargeable batteries are playing an important role as one of the prominent energy storage systems. During last few years, a lot of progress in the development of rechargeable batteries has been made. The electrode material of rechargeable battery plays an important role in the charge transport. Researchers are trying to develop advanced electrode materials so that the charge transport might be efficient resulting in better energy storage. Improvements in electrode materials and cell designs have enabled rechargeable batteries to

N. A. Arif (B) · M. M. Hashmi · S. M. Ali · M. B. Khan · Z. H. Khan Organic Electronics and Nanotechnology Research Laboratory, Department of Applied Science and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_7

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provide greater specific energy, higher specific power, and a longer lifespan. These batteries have also shown enhanced safety and reduced cost [2]. Many types of batteries have been developed, and their energy storage capacities have been studied. In this chapter, a discussion on some of the popular batteries has been included. This discussion starts with the lead acid batteries. These have many attractive features, including low cost, ease of manufacture, and stability. Their popularity is mostly attributable to their low cost. It converts chemical energy into electrical power using lead and lead oxide. Lead acid batteries should have less charging time, high capacity, and slow discharge for better performance. Carbon is being used as negative/positive electrode active material in lead acid battery. The use of carbon results in the extended battery life and improved charge/discharge cycles. Extensive study has been conducted on the operation of lead acid batteries using different carbon materials, and it has been discovered that carbon primarily used in the negative electrodes may reduce sulphation in the partially charged condition [3, 4]. Recently a lot of work is focused on the use of nanostructured materials as the electrodes. The application of lead nanowires as advanced electrodes has been reported [5]. Research has shown the use of CNTs and graphene as electrode materials, resulted in longer cycle life [6]. Two-dimensional layered molybdenum disulfide (MoS2 ) nanosheets have tremendous promise for lithium-ion batteries, as their role in lead-acid batteries (LAB) remains unknown. The use of Pb nanoparticles on mesoporous carbon (MPC) showed encouraging results. Pb-O hierarchical porous carbon composites (rice husk based) are also used for cathodes of lead acid battery [4]. Further, due to relatively low energy density of lead acid battery, researchers have turned their focus towards lithium batteries. Batteries powered by lithium are the most popular energy storage systems throughout the globe nowadays. Lithium batteries may be subdivided into various categories namely, lithium-ion batteries (LIBs), lithium oxygen batteries (LOBs), lithium air batteries (LiABs) and lithium sulphur batteries. Due to high gravimetric energy density and high-performance, lithium-ion batteries are regarded to be one of the most optimistic options for wide scale energy storage system [7–10]. The main features of LIBs are their high operating voltage, lack of memory effect, and the range of available electrode materials and electrolytes. While their primary issues are in terms of safety, energy density, and cost, they are also having trouble with cyclability, structural stability, and energy efficiency. Various electrode materials with complex structural designs have been developed to address these issues. Recently discovered hollow structures with bowl-like or yolk-shell geometry have shown improvements to the volumetric energy density owing to the decreased porosity of the material [11]. However, the production of delicate structures is time taken and often requires complex processes. Fabrication of vertically aligned nanotube and nanofiber arrays is another option to improve the volumetric energy density due to high surface to volume ratio. Microspheres have also been shown to be extremely effective in enhancing electrode material’s electrochemical properties when they are arranged hierarchically. These micron-sized hierarchical spheres have shown remarkable properties, including excellent mechanical strength and decreased charge transport distance,

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while maintaining high tap density. A lot of work on the use of carbonaceous materials as the electrodes in LIBs have been reported [12]. Some of these reports will be discussed in this chapter. Rechargeable LIBs may be classified based on electrochemical reaction mechanism of their respective materials. In insertion/de-insertion applications, several materials based on carbon such as single and multiwall carbon nanotubes, graphene, graphite, porous carbon, and hard carbon have been tested. Metal oxides (including nickel oxide, copper oxide, manganese oxide, molybdenum oxide and tin oxide and titanium oxide), metal phosphides, sulphides, and nitrides and alloys/de-alloys including silicon, aluminium, germanium, bismuth, tin, and tin oxide have been used as conversion materials [13, 14]. The theoretical capacity of 3580 mAhg−1 of Si-based materials makes it a primary contender in LIBs for the replacement of graphite as anode [15]. However, Si exhibits mechanical failure that causes poor cycling performance. It suffers the cracking and pulverisation due to significant volume changes of about 280% [16]. To overcome these problems, researchers suggested the application of nanoengineering, carbon inclusion, and composite materials with active Si and inactive materials. There have been several studies on SiSiOx nanostructured materials for LIB anodes. The use of nanostructured electrode materials offers a high surface-to-volume ratio that enhances the charge/discharge processes in an electrochemical cell since the reactions and charge transfer occurs at the electrode/electrolyte interface. The use of citric acid as a binder resulted in better electrochemical performance similar to that of silicon anodes produced with poly (acrylic acid) (PAA) that is one of the best silicon anode binders known so far [17]. It has been reported [18] that 2D structures offer certain benefits over 1D and 0D nanostructures, including larger surface area and superior stress relaxation mechanisms. 2D silicon nanosheets (SiNS) seen promising for the electrode materials [19]. Sb-based materials with their theoretical capacity of 660 mAhg−1 have been considered as high-capacity positive electrode material for Li-ion batteries [20, 21]. A group six element, Te, may be fused with Li to produce Li2 Te (theoretical capacity 422 mAhg−1 ). While some group six elements such as S and Se have lower theoretical gravimetric capacity, Te has large theoretical volumetric capacity (around 2625 mAhcm−3 ) because of its higher density (6.26 gcm−3 ). Two nanocomposite materials, one of them being a nanostructured Te/C nanocomposite and the other being a nanostructured ZnTe/C nanocomposite, were reported [22]. Over the years, LiMeO2 , a classic family of anode materials for LIBs, has also been investigated extensively. Out of the various kinds of cathode materials of lithium-ion batteries, LiMnO2 , LiCoO2 , LiNiO2 , LiNi1/2 Mn1/2 O2 , and LiNi1/3 Co1/3 Mn1/3 O2 have attracted significant interest [23, 24]. The other material being studied nowadays is a mixed lithium salt with a suitable solvent, such as glyme and CH3e Oe (CH2e CH2e O)n -CH3 , which is recognised as one kind of ionic liquid. It is known as a “solvate ionic liquid” [25]. Mn3 O4 is very desirable since it is cost effective and ecologically safe and thus attracted a lot of attention [26]. Researchers have explored the use of carbonbased compounds to improve the performance of Mn3 O4 [27, 28]. Furthermore, LiTiPO4 F cathode materials are particularly well known because of their safety, ecofriendliness, satisfactory discharge capacity, and long cycle stability [29]. A novel

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and appealing electrode material is proposed for extended cycle life using a multielectron transfer process in LiTiPO4 F [30]. Layered materials with higher lithium content may improve the electrochemical performance of a battery by improving structural stability [31]. A Li-rich Li[Li0.2 Co0.3 Mn0.5 ]O2 which has many layers is an excellent material. It exhibits superior electrochemical characteristic in both aqueous and non-aqueous rechargeable battery [32]. Fluorophosphate polyanion material for electrodes provides a higher energy density, extended cycle life, and environmental friendliness for use in modern Li-ion battery applications [33]. Meanwhile, due to its high initial charge capacity, lithium nickel manganese cobalt oxide (NMC) is an attractive positive electrode material for LIBs [34]. Due to improved cycle performance of NMC811 [35], diphenyl carbonate was used as additive in electrode materials [36]. Moreover, the host structure of Li4 MoO5 with hexavalent molybdenum ions has been used for creating novel high-capacity electrode materials [37]. Furthermore, creating a hybrid composite by mixing electrically conductive compounds such as one-dimensional MWCNT and two-dimensional graphene is a well-known approach for developing the effective electrode materials [38]. Meanwhile, highest possible specific capacity can be achieved by pairing lithium and oxygen in an electrochemical cell, thus researchers focused on lithium-oxygen battery. The lithium-oxygen battery (LOB) has garnered considerable interest owing to its theoretically ultrahigh energy density/Capacity (11.68 kWhkg−1 ) [39]. The production of Li2 CO3 and LiOH causes poor energy efficiency [40]. Organic electrolyte LOBs are the most popular kind of batteries because of their excellent capacity and long cycle life. Unfortunately, discharge by-products (li2 O and li2 O2 ) in organic electrolytes which get deposited at the cathode, where the oxygen-reduction process occurs [41]. The critical strategy for resolving these problems is to develop a suitable porous air electrode structure that allows the production of reaction products, as well as a bifunctional electrocatalyst with high activity for both the oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) [42]. Co-based catalysts may significantly fasten the kinetics of oxygen catalysis and facilitate the complicated redox reaction process in alkaline electrolytes. Co3 O4 is one of the catalytic oxygen electrode materials [43]. It has the best discharge capacity and cycle stability. Recently, in lithium-oxygen batteries, ternary ACo2 O4 (A = Ni, Cu, Fe, and Zn) have been utilised and showed remarkable results [44]. Due to their exceptional electrical conductivity, catalytic activity, and thermal stability, ternary metal sulphides with spinel structure, such as CuCo2 S4 , CoNi2 S4 , NiCo2 S4 and MnCo2 S4 , have been identified as the most promising electrode materials [45]. Researchers have also studied and used perovskite nanoparticles and Mn oxide-based catalysts as the electrode for Li-oxygen battery [46]. Manganese dioxide (MnO2 ) is a powerful catalyst for oxygen reduction and evolution processes. However, the poor specific surface area, electrical conductivity, and unstable structure of MnO2 catalyst for Li-O batteries had restricted its use. So MnO2 as a catalyst material for Li-O batteries must be enhanced structurally and chemically [47]. The band gap of ZnIn2 S4 (2.4 eV) and its chemical stability make it a promising catalyst candidate. Using pristine ZnIn2 S2 as electrocatalyst showed limited results. Adding Ru to a ZnIn2 S4 -based heterostructure can enhance electronic transmission of pure ZnIn2 S4 [48]. Platinum (Pt) is the

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most suitable catalyst for the ORR in aqueous electrolytes. However, there are little resources [49]. The catalyst’s Pt content can be minimised by alloying with different metals such as iridium, vanadium, chromium, nickel, cobalt, iron, and manganese [50]. Meso-cellular carbon foams, carbon nanotubes, and super P have all been suggested recently and showed promising results, when used as electro-catalysts in LOBs. These have inherent advantages, such as a large specific surface area, high electrical conductivity, and low cost [51]. Metal phosphides, such as MoP, Ni2 P, and Co2 P, have attracted recent interest as an efficient oxygen catalyst because of their metalloid properties, which facilitate enhanced surface creation. Many researchers had successfully utilised La0.6 Sr0.4 Co0.2 Fe0.8 O3-d (LSCF) in SOFC stacks. Electrode performance of LSCF has degraded due to Sr surface segregation impeding the oxygen surface exchange mechanism [52]. A significant void area in LOBs was reported because of the spherical shape of carbon bubbles, which also helped lithium and oxygen to travel between electrodes. Layered Li-rich oxides are now a leading option for positive electrode materials with high capacity, reaching 200 mAhg−1 [53]. Moreover, it is easy to use air instead of pure oxygen, thus lithium air batteries have also been investigated. With a theoretical energy density of 3458 Whkg−1 or 12,300 Whkg−1 using Li2 O2 as a discharge product, Li-air battery (LAB) is optimistic metal-air battery under study. Lithium-air batteries, which use oxygen from the air, have extra difficulties because of moisture, CO2 , and other by-products that corrode the lithium anode, damaging the battery’s functionality. As a result, a barrier against water penetration from the air should be suggested for use in natural environment. Highly hydrophobic materials are good for oxygen transfer but not for water penetration, which may extend the life of Li/air batteries in the environment. The insoluble discharge products are Li2 O and Li2 O2 . They deposited on electrode surface progressively and block oxygen flow route in the electrode thus limiting cell capacity. A lot of research has been reported on porous electrode structures [54]. Carbon materials with a hierarchical porous structure have been studied recently. In this arrangement, while discharging, the mesopores operate as reservoirs for Li2 O2 products, and the macropores act as a road to the interior regions of the air electrode. When the carbon interacts with Li2 O2 during charging at high voltage, the Li2 CO3 is formed, encouraging electrolyte decomposition [55]. The side reaction is substantially suppressed by stable polymer layers, which is created on carbon surface, and it restrict contact with the electrolyte or Li2 O2 . Another promising positive electrode material for lithium-based battery is sulphur. It has very high theoretical specific capacity of 1676 mAh g−1 and density of 2610 Whkg−1 . This is 5–7 times greater than the traditional Li-ion batteries [56]. The benefit of sulphur is that it is safe, cost effective, and readily available in nature and is environmentally friendly. However, it has various critical problems, such as the active species insulation, discharge products of Li2 S/Li2 S2 at cathode, the polysulfides dissolution in the electrolyte, and the electrolyte’s large volumetric expansion [57]. Recently, a lot of efforts are focused on encapsulating sulphur active materials in porous nanostructures to collect lithium polysulfides during charge–discharge processes [58]. Many materials, based on carbon such as carbon spheres, nanotubes,

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nano-fibres, and 3D interconnected porous structures, were employed as transporters for sulphur. The cathode has been found to include trappers and catalysts such as VO2 -VN, TiO2 -TiN, TiC-graphene, MoS2 -graphene, and BN-graphene. The sulphur adsorption of sulphur immobilisers through the strong Ti-O bond was shown to have a significant impact on the performance of lithium-sulphur batteries. TiC is a standard titanium metal complex. Its chemical stability and conductivity are exceptional [59]. Bimetallic sulphides containing both Ni and Co ions have electrochemical contributions that may offer richer redox active sites and higher electrical conductivity. It results in improved electrocatalytic activity and rapid electronic transmission during catalytic conversion [60]. It is reported [61] that designing and synthesising appropriate MOFs as host materials is successful in trapping polysulfides and advancing the performance of Li-S batteries. Polymer synthesis with S-ion in covalent bond can enhance the effectiveness of S-based cathodes by accelerating production time and cycle life. In situ chemically produced polysulfide species lead to liquid-based cathodes labelled “catholytes”, outperform the traditional Li-S cell design [62]. After carbonization, it is found that bacterial cellulose carbon materials have high electrical conductivity. This may improve the electrical characteristics, rate capability, and cycle performance of electrode materials [63]. Li et al. [64] in their recent research showed that electrochemical performance of LiSBs was enhanced by coating or modifying the sulphur cathode with electropolymerized conductive polyaniline (PANI) and poly-pyrrole (PPY). The annealing environment may significantly affect the permeability and chemical characteristics of nitrogen-doped graphene. It resulted in remarkable battery performance as sulphur cathode [65]. As a result of rising concern about the availability of lithium and the high expenses of lithium-ion batteries, rechargeable sodium ion batteries are a strong option for wide scale energy storage applications. Additionally, Li and Na, being members of the same group, it has similar properties. Similar to lithium-ion batteries, sodium-ion batteries use transition metal elements on the cathode side, coupled with a carbonbased negative electrode. Notably, hard carbon polyanion compounds, layered oxide compounds and Prussian blue are utilised as the cathode in sodium ion batteries (NIBs). There is a huge cycle loss in first charge/discharge of NIB. It is due to the development of an electrically insulating solid electrolyte interphase at the anode surface, which increases internal resistance and causes a subsequent decrease in cell performance. Also, hard carbon and alloying materials have extremely similar voltage plateaus as the sodium plating voltage that might create safety hazards. The NaVPO4 F, which is created on sodium-vanadium phosphate and uses F to replace O, is a NIB electrode material that has two voltage plateaus. The use of NaVPO4 F nanofibers as a NIB cathode resulted in a high working potential, extended cycle life, and excellent safety [66]. Developing a nano/microporous graphene (self-standing foam) with uniform doping of sulphur is an effective method to enhance the performance of S-graphene [67]. Molybdenum disulfide (MoS2 ) has a layered structure comparable to graphite and have wider interlayer gap; thus, it is an ideal intercalation host material for sodium ions in NIB [68]. Fe2 F.5H2 O pyrochlore is the sole fluoride with a comparable structure to both neutral and anionic clusters. Its strong electron affinities make it a super halogen, which is helpful for Na+ transport and storage,

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and having a large capacity for discharge. Recently, the Na2 VTi(PO4 )3 electrode material for aqueous rechargeable sodium batteries was introduced. It is structurally identical to Na3 V2 (PO4 )3 and NaTi2 (PO4 )3 . Due to the presence of mixed metals in the crystal structure, two distinct types of redox couples coexist. These are the high-potential couple of V3+ /V4+ and the low-potential couple of Ti3+ /Ti4+ [69]. The conduction band of silicon carbide (SiC) as anode electrode has interstitial gaps owing to floating electronic states. The Si vacancy is shown to be more efficient than the SiC carbon vacancy [70]. Moreover, nanotubes having a tunnel structure, such as Na2 Ti7 O15 (NTO), have recently been utilised as anode materials in sodium ion batteries (NIBs) [71]. Due to their easy synthesis and good electrochemical properties, NaTMO2 (TM-transition metal) layered oxides have received considerable interest for their use as a promising cathode material in sodium-ion batteries [72]. Tin sulphide (SnS) and tin sulphide oxide (SnS2 ) are two important members of the metal sulphide family that have various desirable properties. They are abundant, are good for environment and have high theoretical capacities (SnS-1022 mAh/g, SnS2 -1136 mAh/g) [73]. The octahedral BiFeO3 crystal provides an ion conductive interior that make it easier to inject Li or Na ions. This property facilitates long lifetime and great theoretical capacity. Also, 2D structures may be used for higher energy densities, controlled expansion and faster ion movement because of their huge surface area and flat surface. Recently, atomically thin two-dimensional structures, such as SnO2 , α-Fe2 O3 , and TiO2 sheets, have drawn attention [74]. Titanium carbide is reported to be the best suited active materials in electrodes of NIB. It has been shown that core/shell hybrid nanowires can not only transport electrons quickly but also buffers large volume changes and slow capacity degradation during long-term discharge–charge processes, particularly at high rates [75]. Particularly in application of long-term energy storage system, metal-sulphur batteries have shown remarkable results. Since lithium-sulphur batteries are expensive, sodium-sulphur batteries (NSBs) would be a better option [76]. Despite the significant challenges that impede the development and use of metal-sulphur batteries, such as large volume expansion, poor electrical conductivity of sulphur, and the shuttle effect, where metal polysulfides dissolve into the electrolyte and are transported from the sulphur cathode to the metal anode, they may offer a costeffective solution to energy storage for large scale applications. For metal-sulphur batteries, the shuttle effect should be actively countered. Room temperature NSBs are still restricted by the weak electrochemical activities. The slow movement of big sodium ions, delayed reaction kinetics and sodium polysulfide parasitic conversion to sodium sulphide are bigger problem for NSBs. A few combined experimental and theoretical investigations have shown that sodium polysulfides (NaPSs) with graphite, S-terminated Ti3 C2 Tx -MXene, and bilayer graphene all have strong anchoring properties [77]. By employing a short chain S2-4 cathode, it may shorten the reaction route and produce just soluble Na2 S3-4 or insoluble Na2 S2 intermediates. This allows faster reaction kinetics and less polysulfide dissolution [78]. Due to the existence of an internal electric field, ferroelectric materials are helpful for trapping polysulfide. The C/S composite gained greater strength when BaTiO3 nanoparticles (ferroelectric material) were added to it at a concentration of approx. 3 wt.%

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[79]. Nitrogen-doped porous carbons, derived from nitrogen-containing MOFs, are synthesised by thermal carbonization. They will significantly improve the characteristics of porous carbons for battery storage. ZIF-8, the sodalite-type structure of a MOF, has an imidazolate framework with regular mesoporous/microporous pore diameters [65]. In the succeeding sections, a detailed discussion of the use of electrode materials for improved performance of batteries is provided.

2 Advances in Rechargeable Battery’s Electrode Materials To develop next-generation rechargeable batteries, advanced electrode materials should be developed. Batteries have differing performances, depending on the kinds of materials and technology used. The performance of rechargeable batteries is only safe if their materials have been carefully chosen. Various types of electrode materials were developed and tested to meet the current demand of high-density battery energy storage system. For rechargeable battery electrode materials, different nanomaterials gained attention. Metal organic frameworks have recently been used as progenitors or catastrophic layouts to produce porous carbon, metal oxides, other metal compounds and their composites among various nanostructured materials. Here, some of the advanced materials, applicable for different rechargeable batteries, are discussed. Organic rechargeable batteries have a lot of potential as a next-generation energy storage technology. They do, however, suffer from a low organic active material loading in the electrode and a poor practical energy density consequently. Graphite substrate is used as the current collector instead of aluminium foil to create a novel flexible carbon conductor and binder-free organic electrode. The weight of a graphite current collector was just a third of that of an aluminium current collector. Furthermore, this modification enabled the creation of an electrode with a 100% organic active material. The electrochemical characteristics of the electrode were evaluated by Kim et al. [80], with polyimide based GPE produced by electrospinning method. With discharge capacities of 221, 184.8, 147.3, 108.0, and 72.3 mAhg−1 at 1/10, 1/2, 1, 5, and 10 C rates, respectively, the Li/GPE/PTMA-GS cell demonstrated excellent capacity and rate capabilities. The carbon/PTMA-GS full-battery had a measured energy density of 470 Whkg−1 , which is almost double that of the commercial graphite LiNi1/3 Co1/3 Mn1/3 O2 (NCM) battery. As a result, the commercialization of this novel rechargeable organic battery with adjustable geometries seems potential. Moreover, organo-sulphides are a new kind of sulphur-based cathode material that is gaining popularity. Bhargav and Manthiram [81] produced a new kind of active substance called xanthogen polysulfides. By using different materials characterization methods, di-isopropyl xanthogen polysulfide (DIXPS) was utilized as a specimen in a lithium battery to study the chemical changes and unique electrochemical behaviour of this class of materials. DIXPS had a high electrochemical utilization (up to 93.5%) as a cathode material during lower rate cycling. At a high rate of 4 C, it maintained a long cycling stability (thousand cycles). It had a good power

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density (1313 W h kg−1 and 1694 W h L−1 ) on a material level. DIXPS also demonstrated exceptional resistance to performance downgrades when evaluated in a prototype pouch cell under realistic high-loading and lean-electrolyte circumstances. This showed its practical feasibility. In addition, there is a possibility of utilizing DIXPS with other metal anodes, such as sodium. The capacity to manufacture xanthogenbased battery materials using sustainable, naturally accessible resources such as sugar and alcohol as row materials has also been verified. The results showed that xanthogen polysulfides may be used in a variety of high-energy-density batteries. In an alkaline electrolyte, the electrochemical capacity, cycle life of bifunctional electrodes (La3/5 Ca2/5 CoO3 or Sr1/5 Sm1/5 CoO3-δ ), and the carbon contents of the perovskite catalyst were investigated by Velraj and Zhu [82]. The electrochemical efficiency of the new electrodes was essentially like conventional electrodes, but the carbon had a direct effect on cycle life. The graphitized vulcan-based electrodes had more than two times the cycle life of the common vulcan-based electrodes because of their enhanced corrosion resistance to graphited carbon. The cycle life of the carbon-type electrodes was limited to 100–110 cycles in the testing parameters of this research and carbon was the limiting factor instead of the catalyst, i.e., anodic carbon degradation/oxidizing. In addition, catalyst powder size/morphology had a significant effect on the degradation in electrode functionality during cycling. Furthermore, multi-shelled (Co2/3 Mn1/3 ) (Co5/6 Mn1/6 )2 O4 hollowed microcapsules with adjustable shell counts to septuple shells were created using a newly found consecutive modelling method by Zhao et al. [83]. The septuple shelled sophisticated metal oxide hollow microcapsule was produced by injecting Mn into Co3 O4 , which resulted in a change in the precursor’s crystalline rate. It has a noteworthy amendable capacity (237 mAh/g with a current density of 1 A/g with 3 electrode system and 107 mAh/g at 0.5 A/g in batteries) and good cycle life performance as electrode materials for alkaline batteries because of its advanced structure. Low-cost method and design strategy for preparing an efficient material for bifunctional O2 electrocatalysis and describing its further integrating into a gas diffusion electrode (GDE) layout was evaluated by Marini et al. [84] under appropriate load circumstances for rechargeable zinc-air battery application. The active material is made up of -MnO2 produced via a simple synthesis method, commercially available carbon black, and Ni/NiO nanoparticles in a simple preparation. The design of a bifunctional electrocatalyst was shaped by a structured improvement of the surface concentration of the active catalytic merged effects. This improved reduction of oxygen and oxygen evolution processes. With stable overpotentials (approx. 0.35 V for every reaction, 55% efficiency) over 400 h at 20 mAhcm−2 load cycles (both the charge and discharge), GDEs outperform most of the previous concepts and established a link among promising electrocatalyst material and realistic functional electrodes. The presence of polymorphism in more than one form or crystal structure has a significant impact on electrochemical characteristics. Bi-based oxides have a wide range of coordination geometries, making them excellent options for electrode materials. BiO6 polyhedral, PbO6 polyhedral, and VO4 tetrahedra make up both intended polymorphs and -PbBiVO5 . BiO6 and PbO6 polyhedral create zigzag [Bi2 Pb2 O12 ] chains spanned by VO4 tetrahedra in -PbBiVO5 , whereas BiO6 and PbO6

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polyhedral form [BiO4 ] and [PbO4 ] chains bridged by VO4 tetrahedra independently in -PbBiVO5 . It has a specific capacitance of 85 mAh/g with a current density of 0.5 A/g, that is about 21% greater than -PbBiVO5 . Moreover, theoretical simulations by Yang et al. [85] showed that at the exposed surface (011), -PbBiVO5 almost becomes a conductor with a bandgap as low as 0.02 eV, explaining the improved electrode performance. The [BiO4 ] chains allow greater charge transfer, according to a bader charge analysis. At J (current density) = 0.5 A/g, the electrode made from the optimized materials of -PbBiVO5 at 5 wt.% hydrogenated graphene flakes (HG) have a high specific capacity of 96 mAh/g and capacity retention of approximately 87.6% after 1000 cycles. A 3 V white LED is powered by the assembled batteries. Also, for sophisticated energy storage systems, rational structural assembly is a remarkable approach. Further, we have included the classification of different electrode materials for secondary batteries based on battery types. Recent research work on lead acid, Liion, Li-O, Li-Air, Li-S, Na-ion, and Na-S batteries electrode materials has also been discussed in this chapter. Here we started from the traditional lead acid battery, which is widely used commercially all over the world, and discussed the application of improved electrode materials for this battery type. Moving further, we have discussed about other batteries.

2.1 Advances in Lead Acid Batteries A lot of work has been reported during last few years on improvement of the performance of lead acid battery (LAB). The primary aim is to replace traditional plates with novel electrodes that are stable, have a large capacity, and have a large surface area. Improved rates of electrochemical conversion processes at the electrode-solution interface, as well as electrical continuity during multiple charge/discharge cycles, are also required. Nanostructured electrodes have become the focus of research to accomplish these objectives. One of the most popular nanostructures used for these batteries is nanowires (NWs). Insinga et al. [5] produced lead nanowires by electrodeposition in a polycarbonate membrane that served as a template. They were tested as anode in electrochemical cells that simulated a lead-acid battery on a lab scale. The results were promising by following a proper initial charge and a curing time of approximately 100 cycles, the battery operated at a C-rate of up to 1 C, with a coulombic efficiency of around 90% and a lengthy time frame of constant potential. The findings were considerably more useful when considering the extreme straining conditions used to cycle the battery. For example, researchers set cut off V of 1.2 V, which is not recommended for commercial lead-acid batteries. The excellent performance of this type of electrode had been ascribed to a unique morphology that could only be produced from NWs (Fig. 1). Furthermore, lead nanoparticles with mesoporous carbon (MPC) were produced by Sadhasivam et al. [86] for advance LAB applications to improve electrochemical

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Fig. 1 SEM image of Pb nanowire and nanostructured Pb [5]

energy performance. The main issue with them was the substantial crystallisation of PbSO4 in the anode material during unit cell operations. The lead nanoparticles were coated on MPC to reduce this problem. The Pb on the MPC was made via a simple chemical reduction technique. X-ray diffraction study confirmed the architectural analyses of Pb, MPC. Brunauer–Emmett–Teller measurements were used to determine the specific surface area and pore size distribution of MPC and Pb on MPC. For MPC and Pb on MPC, the SSA achieved was 245.38 and 32.42 m2 g−1 , respectively. The micro-structural examination of higher resolution TEM showed the presence of Pb on MPC. The Pb nanoparticles had a particle size of around 5 nm. It was proposed that lead on MPC may be utilised as an effective active material for cathode in advanced lead acid battery systems based on cyclic voltammetry, structural, and microstructural studies. Graphene nanosheets, such as chemically converted graphene, graphene oxide and pristine graphene, increase the utilization rate of the lead acid battery’s positive active material. According to Dada [87] study of graphene improvements in the interphase of the positive electrode of a lead-acid battery, the greatest performance was achieved by GO-PAM (Graphene oxide Positive active material), which had the maximum utilisation of 41.8%, followed by CCG-PAM (chemically converted graphene) (37.7%) at 0.2 C rate. The discharge capacity and cycle performance of GO and CCG optimised samples were both improved. All samples except the unreactive but conductive PbO core, exhibited improved capacity even after 50 cycles. This was owing to the graphene additions as chemical interaction with the lead salt contacts. More hydrated crystals resulted in less passivation and the development of electroactive crystals in the interfacial zone. Electrochemical activity and interfacial contact of graphene (GO and CCG) aided ion mobility, which was balanced by enhanced charge uptake. Higher capacity utilisation is attributed to nano-recrystallization (of the PbO2 and PbSO4 phases), increased conductivity, improved inter-particle and structural integrity of the graphene tailored active masses, and higher electrochemical

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Fig. 2 SEM image of PAM, cycle performance. Reproduced with permission [87]. Copyright 2019, Elsevier

conversion and reversibility. Peukert dependencies were reduced in GO enhanced PAM, indicating improved current stability and degradation of electrode materials of LABs (Fig. 2). For the development of an improved and carbon-based hybrid Pb battery system, a homogenous integration of lead particles having nano size on the surface and pore of carbon has been reported by Thangarasu et al. [88]. The lead precursor (30%) in carbon showed homogeneous doping of lead particles (5 nm in size) in carbon with no agglomeration. AC-Pb was a Pb nanoparticle-impregnated carbon electrode active material that inhibits hydrogen more effectively than carbon alone. The single cell achieved a greater discharge capacity of 0.96 Ah at J = 1 A/cm2 with an ACPb30 coating on the cathode surface, which was higher than the solely electrode with coating of carbon (0.89 Ah). Surprisingly, the coated ACPb cathode lead battery (30,000 cycle life) had a consistent lifetime that was ten times of a standard Pbacid battery (3 thousand cycle life). Interfacial interactions provide many benefits when active material ACPb used to cover the lead cathode surface. The carbon introduction to lead and the ACPb coating on the cathode, restrained the formation of larger-sized PbSO4 , enhanced the Faradaic redox reaction of battery and capacitive functioning, and constrained evolution of H2 gas during continuous operation for long time (Fig. 3). Yin et al. [89] propose a PbO hierarchical (based on rice husk) carbon with porous structure (RHHPC@PbO1-n ) compound, an efficient negative electrode additive in a Pb-carbon battery. Simple annealing technique used to make the RHHPC@PbO1-n composite. Physicochemical techniques such as SEM, TEM, X-ray diffraction, as well as numerous methods, were used to analyse the RHHPC@PbO1-n compound and the electrode of Pb-carbon with RHHPC@PbO1-n add-on. The RHHPC@PbO1-n composite improves the electrode dynamics and cycle firmness of the Pb-carbon negative electrode by enhancing the active surface area and strengthening the structural steadiness of the lead–carbon electrode. Consequently, 2 V and 4 Ah VRLcarbon battery containing RHHPC@PbO1-n composite additives had a cycling life of

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Fig. 3 a, b Discharge capacity and cycle performance of AC and AC-Pb based electrode. c, d Traditional and advanced electrode. Reproduced with permission [88]. Copyright 2020, ACS

9 hundred cycles. Findings provided information on how to tune the Pb-carbon additives interphase and porosity in Pb-carbon batteries. On current established lead-acid battery manufacturing facilities, this RHHPC@PbO1-n composite has the potential to be utilised in Pb-carbon batteries for sustainable storage of energy. In the study of Arun et al. [90] a full-scale tubular positive flooded lead-acid battery with MoS2 in the negative electrode was compared to a control battery with just carbon black for stationary/float applications. According to the Indian standard for testing stationary lead acid battery (with tubular positive plate) in monobloc container, discharge capacity and an endurance test for 2000 h under continuous current charging regime were studied (IS 13369:1992, reaffirmed in 2017). The addition of MoS2 had no detrimental effect on discharge capacity or endurance; rather, it increased those metrics by 2–5% over the control battery. During 2000 h of endurance testing, the battery containing MoS2 showed a substantial decrease in charging voltage. At 15.2 V, a battery containing MoS2 carries a current of 15 A, which was comparable to the current passed by a control battery at 15.8 V. As a result, batteries containing MoS2 may be recharged at lower voltages to achieve the same Ah as control batteries. This delayed the beginning of a typical failure mechanism i.e., grid corrosion caused by high charging voltages. Even after lots of research on lead acid battery, it is still not fulfilling the current need of high-density storage system and lithium-ion battery is dominating LAB because of its high density, capacity, cycle life etc. and becoming the choice of current generation energy storage system. Now in upcoming sections, recent work by different researchers on LIBs for improved electrode materials shall be discussed.

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2.2 Advances in Lithium-Ion Batteries The basic working principle of lithium-ion batteries is that Li-ions are exchanged from negative and positive electrodes during charging and discharging. The electrodes (positive and negative), a separator, and electrolyte are the three main components of a lithium-ion battery. The cell of a Li-ion battery is composed of the electrolyte, which facilitates ionic mobility, separator for insulating electron transmission between electrodes but allowing ion transmission, and the two electrodes responsible for generating electric charge by ion and electron exchange. The first generation of manufacturers used LiCoO2 as positive and graphite as its negative electrodes. These materials were synthesised from millimetre-sized particles. Despite having a high energy density, the lithium-ion battery has limited operations (slow charge/discharge, safety issues etc.). Lithium-ion batteries created at an early stage are not useful for the present need of power or energy applications. Regardless of how creative individuals may be in creating novel lithium intercalation hosts at greater rates, the inherent diffusion of the solid lithium ion ultimately limits the intercalation/deintercalation rate. The advancement of LIB technology has led to the use of nanotechnology to fulfil the needs of future batteries and eliminate those drawbacks.

2.2.1

Silicon Based Lithium-Ion Batteries

Silicon based materials are considered one of the best suited anode materials for advance LIBs because of its very high specific capacity. Despite this, it is facing lots of issues like large volume change (causing large anisotropic stresses), reactivity of the charge materials, unstable solid electrolyte interphase layer formation etc. Recently, a lot of work on nanostructured composite of silicon has been focussed as possible solution to these issues. Kambara et al. [91] prepared a core–shell SiOx nanocomposite powders by using a single continuous plasma spray procedure. They reported that the injection of CH4 at the appropriate quantities during SiOx plasma spraying was extremely effective at enhancing SiOx reduction and, therefore, increasing the quantity of crystalline Si remaining after the disproportionation process. The negative electrode of a half-coin cell constructed using these powders has shown a steady capacity of even more of over 1000 mAh/g and a coulombic efficient of about 99.30% are both much better than that of a cell made entirely of raw SiO2 . Based on electrochemical study, the resistance at SiOx particle surface was shown to possibly decrease with Li2 O generation beginning at the start of the 1st lithiation. The addition of CH4 further enhanced the reduction in resistance, but a greater volume change is anticipated because of the higher crystalline Si phase concentration in the solution. As a result, the core–shell silicon oxide nanocomposite produced by plasma splattering with CH4 proved to be more advantageous as it displayed high capacity and retention efficiency. Also, Wada et al. [92] utilised a 3D de-alloy Mg2 Si precursor, and Bi melted the interconnected nanoporous Si material with adjusted diameter pore and ligament. The magnesium atoms in the precursor preferably dissolve into

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Fig. 4 3D nano-porous Si. Reproduced with Permission [92]. Copyright 2015, Elsevier

Bismuth, while the other Si atoms auto organize into a nano-porous architecture with typical lengths of several decades to hundreds of nanometres. When compared to commercial Si nanoparticles, Li-ion battery electrodes produced from nanoporous silicon have larger capacity, longer cycle lifetimes, and better rate performances. The better performance of nanoporous Si electrodes is attributed to the following factors, according to measurements of Change in electrode resistivity and thickness induced by lithiation: (1) (2)

The impedance of nano-porous Si is much lower due to the obvious n-type dopant introduced during deallocating than of nanoparticle Si. Because of the existence of intra-particle pores, the nanoporous Si-based electrode has a greater porosity and can tolerate Si expansion up to higher degrees of lithiation (Fig. 4).

Kim et al. [93] produced Si-SiOx -Al2 O3 carbon-coated composite materials via two-step methods of high-energy mechanical friction and pyrolysis, and then their electrochemical properties were studied as Li-ion anode material for application. The best materials for the nano-crystalline Si integrated SiOx composite were chosen for amorphous SiO2 and metallic Al. To partly reduce amorphous SiO2 using Al metal, a simple high energy mechanical milling method was used first. The spontaneous oxidation of Al to Al2 O3 resulted in the formation of a nanocrystalline Si scattered amorphous SiOx -Al2 O3 phase. Second, naphthalene as a source of carbon, the composite was heat-treated. An amorphous carbon coating layer was produced via the pyrolysis reaction of naphthalene; this layer stabilised the crystal structure of SiOx -Al2 O3 . Various analytical techniques were used to examine the materials properties of the composites. During the cycling tests, a reversible capacity (ca.) of approx. 500 mAh/g maintained at good rate capabilities for up to 300 cycles. The impact of the carbon coating, which both promoted rapid charge and Li+ transport and buffered significant volume change of Si-based anode materials, may be ascribed to this performance. Furthermore, Wang et al. [94] proposed a concept for “silica strengthening,” in which silica-enhanced carbon nanofibers with SB nanoparticles (SIO2 /Sb@CNFs) were made utilising an electrospinning method, to resolve changes in volume of electrode pulverisation problems. In addition, it had a lithium

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storage capacity, strengthened by silica fillers (isolating), which was enhanced by the embedding of Sb nanoparticles in the C-silica matrix. This varied the buffer volume during LiSb alloying and deployment. The porous carbon nanofiber frame ensured that lithium was stored more effectively. The SiO2 /Sb@CNF electromagnetic discharge capacity at 200 mAg−1 , 500 mAg−1 and 1000 mAg−1 was 700 mAhg−1 , 572 mAhg−1 and 468 mAhg−1 , respectively after 400 cycles. The coherence of the Sb@CNF (Silica Reinforced) electrodes withstands the stress (mechanical) caused by volume changes, based on TEM ex situ and on-site investigations. A good reversible ca. was provided for SiO2 /Sb@CNF/LiCoO2 (after 800 cycles at 500 mAg−1 the ca. was 400 mAhg−1 ; after 500 cycles at 1000 mAg−1 , the ca. was 336 mA hg−1 ). The high cost, complex manufacturing technique, and poor electrochemical performance had long hampered the practical use of Si-based materials. In the study of Xu et al. [95], new Si/C granules were developed and manufactured using a simple and scalable manufacturing method. The 3D conducting network and high tap density produced Si/C anodes that have good electrochemical performance at high mass loading (8.5 mgcm−2 ). Furthermore, even when the density is raised to that of commercial graphite anodes, the cycling performance of Si/C anodes remains constant (1.6 gcm−3 ). Furthermore, the flexible manufacturing method may be used to produce Si/C anodes with different reversible capacities (450–750 mAh/g). The pouch cell with Si/C anode and LiNi0.5 Co0.3 Mn0.2 O2 cathode effectively powers electronic devices, demonstrating the viability of Si/C anodes. The improved characteristics of the complete cell suggest that spherical Si/C granules are a promising option for Li-ion battery applications. In order to achieve more capacity on high-rate charging/discharging, Ando et al. [96] studied using a 2:1 stoichiometry combination based on monoglyme solvents and Li bis-(trifluoromethanesulfonyl) amide (LiTFSA), for the electrochemical behaviour of a silicon electrode in a half-cell. 1,2diethoxyethane (DEE), Monoglyme (G1 ), 1,2-dimethoxypropane (P-G1 ) and 1,2dibutoxyethane (DBE) were the solvents used in their work. The cell maintained about 20% capacity at a rate of 3 C when Li(P-G1 )2 /Li (DEE)2 employed, while the other samples exhibited practically negligible capacity under the same circumstances. Li+ may be found in two forms in the mixture, according to Raman spectroscopy: a solvent shared ion pair (SSIP) [Li+ solvent TFSA− ], a contacted ions pair [Li+ TFSA− ]. SSIP was the main species for Li (DEE)2 , Li(P-G1 )2 , whereas the contact ion pair was the leading species for Li (G1 )2 and Li (DBE)2 . Li(P-G1 )2 > Li (DEE)2 > Li (DBE)2 > Li (G1 )2 was the rate property order, which was largely compatible with the persistence of free-energy level of Li+ in the different monoglymes against Contact ion pair Li+ TFSA− as shown by Raman spectra. High-energy ball milling was used by Ding et al. [97] to expose magnesium and silicon monoxide to an in-situ oxidation and reduction process, resulting in a composite of silicon and magnesia, which was then mixed with graphite. To manufacture a composite structure of SMG@C by carbon coating for anode material of li-ion batteries, the Si/MgO/G (SMG) composite powder was coated by carbon through carbonization at high temperature. The phase, structure, morphology, and electrochemical characteristics of silicon-based anode composite materials were studied

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Fig. 5 (1), a SMG@C synthesis and SEM image. b, c After 75 cycle and 20 cycle. (2), (3) cycle and rate performance. Reproduced with permission [97]. Copyright 2020, Elsevier

using XRD, SEM, cyclic voltammetry, and impedance spectroscopy. The developed SMG@C anode composite material was reported to have a cored shell structure made up of silicon, Mg oxide, graphite, and amorphous carbon. SMG@C showed outstanding electrochemical performance, including a low impedance as well as good cycle stability and rate performance, according to electrochemical tests. SMG@C had a capacity of 1124 mAh/g after 75th cycles at a current density of 100 mA/g, and the cycle performance was reasonably constant. When evaluated at 100 mA/g current density, SMG@C capacity was back to its original condition after cycling at various current densities. The SMG@C anode material, which is made using a simple HEBM method and a calcination at high-temperature of a carbon coating, has the potential to replace traditional graphite anodes in high density li-ion batteries (Fig. 5). Park et al. [98] used a simple and economical reduction through magnesiothermic method to produce mesoporous silicon microparticles (mpSi-Y). The electrochemical properties of the carbon-coated mpSi-Y/C anodes were examined in a full cell and half-cell configuration in a lithium-ion battery. They compared this anode with those of SiNP/C and Gr based anodes. MpSi-Y was noted for its mesoporous structure, and its high-capacity half-cell performance (1200 mAh/g at 0.05 C). The half-cell performance was improved in comparison to SiNP/C composite because of lesser impedance development and rise in electrode thickness. This graphite anode is more difficult to charge/discharge than the new material. To make a complete cell, a pretreatment mpSi-Y/C was coupled with an NCM cathode (LiNi0.6 Co0.2 Mn0.2 O2 ). The mpSi@NCM cell provided the same cycling performance as the Gr@NCM cell, with 200 cycles performed at 0.5 C. Even more intriguingly, the size of the mpSi@NCM cell is found to be much lower than that of the Gr@NCM by more than 50%. The mpSi@NCM’s specific energy density increased by approximately 33% compared

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to Gr@NCM. Because of this, the mpSi-Y/C may be useful for high energy Li batteries who provide steady discharge. Furthermore, in wearable electronics sector, the development of foldable lithium-ion batteries with improved electrochemical performance has raised attention. Flexible carbon current collectors with excellent capacity retention are important components of their design to accomplish this. Park et al. [99] presented a material for foldable lithium-ion batteries made of silicon nanosheets (SiNS) on flexible carbon textile (CT), which combines the benefits of SiNS (e.g., high capacity, wide surface area, and stress relaxation) with the benefits of CT (e.g., foldability). Chemical vapour deposition and electrospray deposition were both used to manufacture the SiNS-CT anode. Both methods were determined to be manufacturing-ready. The SiNS-CT anode demonstrated excellent foldability and sustained potential stability after 100 bending cycles under ideal circumstances. Furthermore, when compared to silicon nanoparticles-carbon textile electrodes, this flexible electrode had excellent specific capacity (approximately 2500 mAh/g) and a 100% of coulombic efficiency across 200 lifecycles, yielding better results.

2.2.2

Antimony Based Lithium-Ion Batteries

Antimony (Sb)-based composite is regarded appropriate for use in lithium battery anodes due to their high capacity. Their cycling ability is restricted by the larger volume change seen during Li+ exchange cycling. Sb-active materials may be used with intercalation-based active materials to circumvent these issues. Seo et al. [100] used a simple solvothermal technique to fabricate Sb core/Nb2 O5 spherical shell shaped composite materials, and a carbon coating was added during heat treatment using a naphthalene precursor. The resulting double-shelled materials were analysed using XRD, Raman spectroscopy, XPS, and electron microscopy. Electrochemical testing showed a reversible ca. of more than 450 mAh/g after 100 cycles. The improved performance is due to the double-shelled structure. The doubleshelled structure, composed of crystallized orthorhombic Nb2 O5 and amorphous carbon, reduced the nano-sized Sb core material’s substantial volume shift. The shell materials aided in very high charge transmission (Fig. 6). Sb2 Te3 was developed by Nam et al. [101] as Li-ion battery electrode material. They studied the reaction mechanism of the electrode. Then after Sb2 Te3 /C (nanocomposite) was produced and used for advanced LIB electrode based on the reaction mechanism of Sb2 Te3 . The Sb2 Te3 /C electrode performance was outstanding (coulombic efficiency of 80.4%, charge ca. 607 mAh/g or 1243 mAh/cm−3 , long cycling behaviour over two hundred cycles, and 1230 mAh/cm−3 (rapid rate cap. at 3 C rate). The development of five to eight nm Sb2 Te3 nano crystallites inside the carbon matrix (amorphous) and its reassembling while Li reactions are credited to excellent electrochemical performances. The Sb2 Te3 /C nanocomposite seems a promising material for electrode of LIB. Moreover, Agostini et al. [102] created a self-standing anode made entirely of highly conductive 3D sponged nano-fibres, with no current collectors, binders, or other conductive materials. The tiny diameter of the fibres, coupled with an interior sponge-like porosity, results in short distances

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Fig. 6 (1) The Sb/Nb2 O5 -C double-shell synthesis. (2) and (3) shows cycle and rate performance [100]

for lithium-ion diffusion and 3D routes, making electrical conduction easier. Furthermore, functional groups at the fibre surfaces cause a stable solid-electrolyte interphase to develop. They revealed that this anode allowed Li-ion cells to operate at specific currents as high as 20 A/g (about 50 C) with good cycle stability and an energy density that is 50% greater than a commercial graphite anode.

2.2.3

Vanadium Based Li-Ion Batteries

Vanadium oxides have been the focus of research as cathode materials for LIBs. If it is being lithiated, it holds high-energy storage capabilities. The study of Xi et al. [103] presented the microwave irradiation process for the rapid synthesis of Li3 V2 (PO4 )3 /C as positive electrode materials for LIBs using an automatic microwave reaction reactor and LiH2 PO4 , V2 O5 and sucrose as raw components. Sucrose was used as a source of carbon and a reducing agent. To describe its structure and morphology, researchers utilised TG analyses, XRD, FE-SEM, TEM CV and charge–discharge cycling. The electrochemical characteristics of the Li3 V2 (PO4 )3 /C materials were also studied. The results indicated that the diffraction peaks of the sample correlate to a single phase, and the structure was monoclinic with a space group of P21 /n. Li3 V2 (PO4 )3 /C had a high electrochemical capacity of 138 mAh/g at 0.2 C rate and 124.1 mAh/g at 5 C rate, with a steady cycling ability. Furthermore, Liang et al. [104] utilised hydrothermal techniques to create precursor NH4 V4 O10

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Fig. 7 1 H-POM and Li-POM synthesis. 2 Crystal structure. Reproduce with permission [105]. Copyright 2020, Elsevier

nanowires on a conductive substrate. To produce nanowire of vanadium pentoxide, the NH4 V4 O10 precursor was calcined at 360 °C, and the nanowires demonstrated a 135.0 mAhg−1 initial capacity at current density of 50 mA/g in 5 M aqueous solution of LiNO3 . However, the specific capacitance rapidly decreased after 50 cycles. After the interface of V2 O5 based nanowires was covered with a non-soluble polypyrrole, the resultant nanocomposite electrode had a discharge ca. of 90.0 mAhg−1 at 50 mA/g (after 100 cycles). Rechargeable lithium batteries containing V2 O5 @PPy and LiMn2 O4 have a discharge capacity of 95.2 mAhg−1 at the start of the first cycle and this enhanced to 81.5 mAhg−1 after the first 100 cycles. Priyadarshini et al. [105] developed Keggin-type polyoxometalates as the materials for electrode of LIBs. As a result, they showed that [PMo10 V2 O40 ]5− POM with hydrogen and lithium as counter cations can be synthesised by a simple and costeffective process for anode and cathode, respectively, with power densities of 230 Wh/g and 329.4 Wh/g at an average. At 0.1 C, the lithium cation material had a higher initial specific capacity than the hydrogen cation material. It showed the capacity of 1414 mAh/g for the negative electrode and 332 mAh/g for the positive electrode. Among them, cathode (positive electrode) seemed to be the most promising, with a long and steady cycle life (Fig. 7).

2.2.4

Molybdenum Based Li-Ion Batteries

Molybdenum-based materials have a very high specific capacitance, due to their numerous oxidation states that favour rapid charge storage. It is regarded as potential electrode material for aqueous batteries (Li-ion, Li-S, Na-ion etc.) because of its unique layered structure and low cost. In lithium-ion batteries, the electrochemical characteristics of MoS2 covered MoO3 nanobelts were investigated by Villevieille et al. [106]. A film of MoS2 was coated effectively on the nanobelts of MoO3 using a simple sulphur transfer technique, and the crystallographic characteristics of this composite material were disclosed by X-ray synchrotron diffraction. When evaluated

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in the 0.05–3.5 V versus Li/Li range, the coated sample had a specific charge of more than 1100 Ah/kg (at 1 C) and 1400 Ah/kg (at C/10). The Coulombic efficiency was found to be greater than 98% with coating, indicating a highly reversible process. In situ X-ray synchrotron diffraction (1st and 50th cycle) and post-mortem SEM evaluation were also used to investigate the electrochemical reaction and degradation process. By means of a thick amorphous layer on the active material’s surface, both showed a structural as well as a surface deterioration of the electrode. Furthermore, for high-capacity secondary lithium anode materials, a binary system of n Li4/3 Ni1/3 Mo1/3 O2 -(1-n) LiNi1/2 Mn1/2 O2 was investigated by Zhao et al. [107]. In this binary system, the structural and electrochemical characteristics of oxides with various compositions were investigated. With a monoclinic symmetry, molybdenum ordering is preserved for 1/3 ≤ n ≤ 1 and vanished for n = 1/6 with a rhombohedral symmetry. Partial replacement of Mn for Mo improved reversible capacity and reduced polarisation when compared to Li4/3 Ni1/3 Mo1/3 O2 . High reversible capacities of approximately 200 mAhg−1 were achieved for Li6/5 Ni2/5 Mn1/5 Mo1/5 O2 (n = 1/3) and Li9/8 Ni7/16 Mn5/16 Mo1/8 O2 (n = 1/6). The optimisation of voltage ranges led to the improved cycle performance. In recent years, layered cathode materials rich in Li, which include extra Li inserted to the layer of metal, have attracted attention because of their high capacities (250 mAh g−1 ). Babu et al. [108] described Li4 FeMoO6 , a Li-excess positive electrode containing blended valent cations that have a stoichiometric composition of Li4 Fe20.45 Fe30.55 Mo50.55 Mo60.45 O6 . It contained blended valent cations having a stoichiometry of Li4 Fe20.45 Fe30.55 Mo50.55 Mo in accordance with XRD results. Also, Li4 FeMoO6 crystallises in a structure of monoclinic, which was comparable with Li2 MnO3 . Therefore, according to the structure concept with layer, it may be represented as Li[Li0.33 Fe0.33 Mo0.33 ]O2 . Lithium and ferrous were irregular across sites (4d and 2a), while the sites of 2a were exclusively occupied with the molybdenum atoms, according to the results of the XRD study. According to the results of HRTEM and electron diffraction investigations, there was irregularity in structure and faults in the planar lattice, for example twinning/stacking faults, present. It was found that Li4 FeMoO6 has an initial charge capacity of 313 mAh g−1 when subjected to electrochemical cycling in the range of 1.5–4.6 V. It had a reversible capacity of 90% during the 1st cycle, and after 100th cycles, capacity retention of about 76%. After reaching 4.60 V, molybdenum dissolution got detected, resulting in capacity retention dropping to 44% after 100 cycles of operation. Li4 FeMoO6 exhibits excellent rate functioning that may be ascribed to the high conductivity due to available blended Mo5 /Mo6 valent and mixed valent Fe2 /Fe3 ions (Fig. 8).

2.2.5

NMC Based Li-Ion Batteries

Despite all the achievements, cathode material for Li-ion batteries based on nickelmanganese-cobalt has shown most successful and promising. Here in this context, some of the advancement in NMC based electrode materials are discussed.

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Fig. 8 Capacity retention for Li/Li4 Fe-MoO6 . Reprinted with Permission [108], Copyrights 2019, Elsevier

The two most often used negative electrodes at the present, L4 T5 O12 and the graphite, had safety problem related to plating of lithium (working potential is less than 0.1 V compared to Li-ion) and a poor ca. (170–5 mAh/g). In this work, Wang et al. [109] described LiNi1/3 Co1/3 Mn1/3 O2 as an alternate anode material. It had a voltage of 01.10 V and a high ca. of 335–40 mAhg−1 when operated at a rate of 15 mA/cm2 . Each electrode of cells symmetrically had LiNi1/3 Co1/3 Mn1/3 O2 and an average discharge voltage of 2.2 V were delivering a discharge voltage of 2.2 V on average. First-principles calculations, XRD and HR-TEM on a LiNi1 /3 Co1/3 Mn1/3 O2 anode show that the reaction mechanism is primarily a conversion reaction, rather than a reduction process. Due to the advancements in both basic knowledge and practical demonstrations, lithium nickel cobalt manganese oxide (LiNi1/3 Co1/3 Mn1/3 O2 ) proved to be a potential cathode material for LIBs (Fig. 9). A study by Karayaylali et al. [110] explored the impact of diphenyl carbonate addition onto the surface reaction of LiNi1/3 Mn1/3 Co1/3 O2 , LiNi3/5 Mn1/5 Co1/5 O2 , and LiNi4/5 Mn1/10 Co1/10 O2 (NMC111, NMC622 and NMC811). DRIFT spectroscopy

Fig. 9 SEM image of LiNi1/3 Co1/3 Mn1/3 O2 and capacity at diff rate. Reproduced with permission [109]. Copyright 2017, Elsevier

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showed that LiNi1/3 Mn1/3 Ni1/3 O2 was converted to LiNi4/5 Mn1/10 Ni1/10 O2 with the addition of DPC to the electrolyte (NMC111 to NMC811). Additionally, the electrolyte including DPC created a lowered collapse of PF6 salt anions, resulting in a smaller number of fluorine-coordinated species, such as lithium nickel oxyfluorides or PF3 O-like species, according to merged IR spectroscopy and XPS for NMCs rich in Ni. The lower voltage needed for the electrochemical oxidation of DPC to produce surface reaction outcome may be attributed to the reduced reaction between NMC, like NMC811, and the electrolyte containing DPC. Analysis of in situ IR spectroscopy demonstrated that electrochemical oxidation of diphenyl carbonate occurs at 3.90 V Li, while infrared analysis identified a feature near 1824/cm, which is assigned to organic oxidative dissolved on the oxide surface and a steady electrode surface on NMC811 at greater voltages.

2.2.6

Titanium Based Li-Ion Batteries

Titanium based materials as anode of LIBs considered potential material because of its outstanding Li ion insertion/de-insertion with very less structural change. Recently, a lot of work are focussed to use these materials as anode of LIBs. TiS3 active materials were synthesized by Matsuyama et al. [111] in both crystalline and amorphous forms. Amorphous TiS3 was used in the all-solid-state lithium secondary batt. The initial discharge capacity was about 560 mAhg−1 , and capacity was approximately 550 mAhg−1 after cycle of 10 at the current density of 0.0642 mA/cm2 , which corresponded to the theoretical capacity of TiS3 . The crystalline TiS3 battery, on the other hand, demonstrated irreversible ca. after just one cycle. After being subjected to cycle of 10 at J = 0.064 mAcm−2 and testing the battery’s reversible ca., it demonstrated an approximate ca. of 400 mAh/g from the second to tenth cycles. For the battery containing amorphous TiS3 irreversible capacities were not detected after 10 cycles of operation. XRD and HR-TEM for structural changes identification that occurred at both crystalline and amorphous TiS3 during cycling. During the cycling process, the crystalline TiS3 became partly amorphous. When compared to crystalline TiS3 , amorphous TiS3 retained its amorphous state for a longer period and therefore had superior cyclability (Fig. 10). Mechanical milling of a-TiS3 and S/KB based electrodes by Matsuyama et al. [112], resulted in the formation of high-capacity a-TiS3 /S/carbon (Ketjen black; KB) composite electrodes. The composites were formed into coin-type liquid cells and all-solid-state cells and utilised as rechargeable batteries at room temperature. Due to the dissolution of polysulfides formed during redox reactions of a-TiS3 /S in the liquid electrolyte, the reversible capacity of coin type liquid cells decreased from 484 to 33 mAh/g after 50 charge–discharge cycles. On the other hand, all-solid-state cells had a higher reversible ca. and cyclability than coin type liquid cells. To increase the cycle life of the composite electrodes, solid electrolyte (SE) powders were added to serve as lithium-ion conduction routes to the active (negative and positive) materials. At the first cycle, the cell containing a-TiS3 /S/KB composite with 30% SE exhibited the highest reversible capacity of about 850 mAhg−1 and retained a reversible capacity of

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Fig. 10 a, b (top) SEM image of Li/TiS3 . a, b (bottom) charging/discharging and cycle performance [111]

approximately 650 mAh/g after 50 cycles. Due to its relatively high sulphur content, the composite anodes of a-TiS3 /S/KB proved to be attractive anodes with a high capacity for all-solid-state lithium rechargeable batteries. Using tavorite LiTiPO4 F as anode and Li[Li1/5 Co3/10 Mn1/2 ]O2 as cathode in aqueous electrolyte with 2 M Li2 SO4 , Rangaswamy et al. [113] showed a safe, cheap, and stable cycle-life aqueous LIB system. These materials were made using a simple and efficient process known as reaction under autogenic pressure at elevated temperature. They investigated the electrochemical characteristics of LiTiPO4 F in an aqueous electrolyte for the first time. XRD and SEM techniques used for characterising morphological and the structural features, while cyclic voltammetry, galvanostatic charge/discharge studies, potentiostatic intermittent titration techniques, electrochemical impedance spectroscopic and galvanostatic intermittent titration techniques were used to investigate electrochemical studies. The cycle life, columbic efficiency, and capacity of LiTiPO4 F in conjunction with Li [Li1/5 Co3/10 Mn1/2 ]2 cathode have been evaluated in galvanostatic charge/discharge experiments. LiTiPO4 F has a ca. of 82 mAh/g, capacity retaining of 90% above the 45th cycle too. Furthermore, Rangaswamy et al. [114] used lonothermal technique to synthesise LiTiPO4 F/C electrode material by 1,2-dimethyl-3-(3-hydroxypropyl) imidazolium dicyanamide

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ionic liquid as a reacting medium, followed by a detailed investigation of electrochemical performance in non-aqueous electrolytes. Long-term cycle performance, rate capability, and high reversible capacity have been improved significantly using lonothermally prepared LiTiPO4 F/C. Cyclic voltammetry, galvanostatic intermittent titration techniques, galvanostatic charging-discharging investigations, electrochemical impedance spectroscopy, and potentiostatic intermittent titration method were used to evaluate the electrochemical characteristics. After 200 cycles, the 1/2 cell LiTiPO4 F/C versus Li/Li+ generated an output of 153 milliamp hours per cycle for the very first cycle and 131 milliamp hours per cycle for the 200th cycle when operated at the C/15 rate. It was necessary to utilise more than 85% columbic efficiency to guarantee a steady cycle life. The entire cell design with graphite as anode has an efficiency of more than 83% and offers a ca. of 149 mAhg−1 for the very 1st cycle and 129 mAh/g for 200th cycle at the C/15 rate, for a total efficiency of more than 83%. It was extensively studied the influence of temperature, ex situ X-ray analysis for analysing the ca. fading process, and the impact of coating of carbon on the performance of LiTiPO4 F versus Li/Li+ batteries. Based on the findings of this study, it was suggested that LiTiPO4 F/C may be used as anode for high-performance LIBs that were suitable for use as huge scale power devices. Copper sulphide has been extensively studied as a positive electrode material for Li-ion batteries. The in-situ production of copper sulphide by reaction of sulphur coated copper foil current collector was studied by Mohanty et al. [115]. Sulphur was mixed with either acetylene black or TiO2 nanotubes as a porous and non-conducting addition, and electrodes were made on Cu foil. Electrodes were also made without any additives. XRD investigations have proven that copper interacted with sulphur to produce copper sulphide. In all the coatings, the formation of sulphides with plateletlike shape has been observed. The S-TiO2 electrode-based cell had a 282 mAh/g specific discharge capacity, compared to 317 mAhg−1 for the S-C electrode-based cell. The cells of the sulphur electrode have shown discharge ca. of 230 mAh/g without any addition. This is due to copper sulphide’s greater conductivity than sulphur, as well as the production of conducting copper particles after discharge. After exposing the cells to 100 discharge–charge cycles, the S-TiO2 electrode-based cell showed almost steady performance. Furthermore, the discharge capacity of the S-TiO2 based cell is higher than that of the S-C based cell, and it had grown with repeated cycling (Fig. 11). Iriyama et al. [116], using aerosol deposition (AD) on platinum sheets were able to fabricate composite electrodes (9 micro m thick), comprised of LiNi0.5Mn1.5O4 (LNM) (a 5 V class electrode) and a good Li+ conductive Li1.4 Al0.4 Ti1.6 (PO4 )3 (LATP). These were produced at room temperature. The resulting composite electrodes of LNM-LATP were merged well with Li and the LiPON, and then secondary lithium batteries (all solid state) of the 5 V class were created using the resulting electrode materials. By annealing the composite electrode (LNM into the LNM-LATP), the crystallinity of the LNM was enhanced significantly. The thermogravimetry-mass spectroscopy and the XRD analysis confirmed that the LNM and the LATP got the side reactions above 500 °C with oxygen as by-product. Due to these findings, the composite electrode’s (LNM-LATP) annealing temperature was determined to be

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Fig. 11 a, b SEM image of CuS-C, CuS-TiO2 . (1, 2) Cycle and rate performance. Reproduced with Permission [115]. Copyright 2017, Elsevier

500 °C. It was chosen because of the increased crystallinity of the LNM and the absence of side reactions. A total of 100 mAh/g was supplied by the LNM-LATP SSBs (9 micro m thick, 40 vol%). It was discovered that the decrease of discharge capacity with the re-iteration of the insertion-de-insertion reactions attributed to the significant change in volume of the LNM (approx. 06.49%). As organic electrodes in rechargeable batteries, a novel redox-active family of aromatic dicyanides was discovered by Deng et al. [117]. The electrical characteristics of two main di-cyano-benzene (DCB) and 9,10-di-cyano-anthracene (DCA) representatives were investigated in detail. When compared to DCB, the substantial aromatic conjugation in DCA resulted in greater electron conductivity and more robust reduced states of DCA− and DCA2− . The functioning of Li-ion batteries with DCA electrodes proved to be extremely reversible (Fig. 12).

2.2.7

Carbonaceous Based Li-Ion Batteries

Carbonaceous anode materials are extensively utilised in the present electrochemical energy storage sector for a range of purposes. Carbon nanospheres, on the other hand, are considerably more difficult to build into self-supporting arrays. The hollow

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Fig. 12 (i) DCB and DCA in Li-ion battery, (ii) modified rout for DCA and performance. Reproduced with permission [117]. Copyright 2017, Elsevier

nickel microtube/carbon nanosphere core–shell arrays (Ni/CNS) were successfully synthesised by Qi et al. [118] with a simple and regulated method. As a binderfree electrode, the unique core–shell arrays structure is advantageous to electron conduction and structural integrity of the whole composite material. By effectively improving electrical conductivity and building open channels for Li ion diffusion, the hollow nickel microtube/carbon nanospheres core–shell arrays showed excellent reversible capacity, rate capability, and a specific capacity of 148 mAh/g next to hundred cycles (Fig. 13). A one-step polyol-assisted pyro-synthesis by Alfaruqi et al. [119] yielded a nanostructured Mn3 O4 /C electrode without any post-heat treatments. As shown by electron microscopy, the prepared Mn3 O4 /C exhibited a nanostructured morphology composed of secondary aggregates formed from C-coated initial particles with an average size of 20–40 nm. The N2 adsorption experiments had shown that the nanostructured electrode has a hierarchical porosity characteristic. The current fast combustion method seems to be linked to the nanostructured morphology. When used as a negative electrode material for li-ion batteries, the nanostructured porous Mn3 O4 /C electrode demonstrated impressive electrode properties, including reversible ca. of 666 mAh/g at a current density of 33 mA/g, excellent capacity retention (1141 mAh/g to 100% Coulombic efficiency at the 100th cycle), and rate capabilities of 307 and 202 mAh/g at 528 and 1056 mA/g, respectively. CoO/Co3 O4 microspheres with porous particles that have been inherited from their shape may be manufactured efficiently by using a simple solvothermal method. The intermixed compound preferred monoxide conversion. Its capacity retention (after 250 cycles, 905 mA/g) was 1.60 times greater than that of a CoOx -type anode, which is very useful for use in secondary batteries. Kim et al. [120] indicated that the novel microspheres made of the CoO/Co3 O4 composite were extremely effective at containing both the deleterious volume exchanges that occurs during charging discharging as well as the numerous reactive sites that preserve the reversible Li-ion -LiOx reaction. These materials had proved a positive effect on the performance of the electrode of LIBs (Fig. 14).

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Fig. 13 a–f Metal Ni/CNS fabrication. (1), (2) cycle and rate performance. Reproduced with permission [118]. Copyrights 2018, Elsevier

Fig. 14 Rate capability of simple (black) and CoO/Co3 O4 (yellow) electrode. TEM image of CoO/Co3O4 electrode. Reprinted with Permission [120]. Copyrights 2018, Elsevier

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Fig. 15 Structure of AFePO4. Reproduced with permission [121]. Copyright 2016, Elsevier

Nakayama et al. [121] synthesised LiFePO4 (olivine), a positive electrode material for secondary LIBs that has a good energy density. However, using this as a material for electrode of NIBs, the olivine-type NaFePO4 had a poor power density compared to the other Na alternatives. First-principles density functional theory (DFT) was used to analyse the ion and electron transport characteristics of LiFePO4 and NaFePO4 to explain the significant difference in power density between the two materials. According to the results of the current DFT investigations, there was no statistically significant difference between the electronic migration energies of bulk LiFePO4 and NaFePO4 . As a result of this, the sodium ion’s migration energy in NaFePO4 was about 0.05 times greater than that of the lithium ion in LiFePO4 , which may explain for the sluggish kinetics seen in the NaFePO4 electrode. According to the results of further investigations into the phase stability and alkaline ion movement at the interfaces of the two phases of (Li/Na)-FePO4 and FePO4 , the disparity in rate performance of LiFePO4 and NaFePO4 is linked to the development of this interface (Fig. 15).

2.2.8

Zinc Based Li-Ion Batteries

Zinc oxide (ZnO) has been regarded a potential anode material option for Liion batteries due to its high theoretical specific capacity, low cost, and environment friendliness. ZnMn2 O4 /C nanoparticles are produced by Alfaruqi et al. [122] using a one-step polyol aided pyro-synthesis method and were suitable for usage as the cathode in LIBs. The ZnMn2 O4 /C as prepared was tetragonal, with spherical particle sizes ranging from 10 to 30 nm. The powders were used for the active material in li-ion cells and electrochemical measurements taken. After 50 cycles, the electrode (ZnMn2 O4 /C) had a starting charge ca. of approx. 666.2 mAh/g and a capacity it retained was of about 81% (~539.4 mAh/g). A new spherical ZnO@carbon with nanocomposite (porous) was produced by Fu et al. [123] using a zeolitic imidazolate frameworks (ZIFs-8)-directed approach for LIBs. The preparation of ZnO@ZIF-8 began with the production of sphere ZnO nanoparticles. Then, under

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Fig. 16 (1) Synthesis of ZnO@C, (2) Zno@ZIF-8, (3) After 100th Cycle, (4) Cycle life. Reprinted with permission [123]. Copyright 2017, Springer

ultrasonography, the addition of Zn2+ and the 2-methylimidazolate was alternated. Finally, the new spherical ZnO@C (porous) nanocomposites was created by incinerating the ZnO@ZIF-8 that was used in the experiment. The unique porous spherical ZnO@C nanocomposite was studied using a variety of analytical techniques, including SEM, TEM, and X-ray diffraction. In addition, after 100 cycles, the resultant spherical ZnO@C nanocomposites had a very high reversible capacity of 932 mAh/g of carbon, which was much greater than standard ZnO nanoparticles. Along with the porous structure and high BET surface area generated by ZIFs, as well as the exceptional electrical conductivity of the carbon materials (amorphous) generated by ZIFs, all these characteristics lead to the excellent performance of the resulting ZnO@C nanocomposites for LIBs (Fig. 16).

2.3 Advances in Lithium Oxygen Batteries Non-aqueous Li-O2 batteries typically consist of a metallic lithium positive plate, a porous negative plate (often carbon or titanium to act as catalysts), and a separator saturated in non-aqueous electrolyte (containing lithium-ion solution) in between. With a thermodynamic potential of 2.96 V, the main discharge/charge process occurs at the oxygen containing electrode [124]. The formation of insulating side products because of irreversible parasitic processes is a significant impediment to practical usage of rechargeable lithium oxygen batteries. Thus, multifunctional oxygen

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electrodes that can be used for oxygen reduction, oxygen evolution, and decomposition of side reaction products are essential. For resolving the energy crisis and decreasing pollution, developing a low-cost, stable, and efficient oxygen electrode catalyst for use in lithium-oxygen batteries with extended cycle stability and high discharge/charge capacity is critical. A major step toward cleaner and more sustainable energy sources is the development of excellent performance and economic electrode for oxygen evolution reaction. For solid oxide cells to be usable, it is essential to have electrodes that help facilitate rapid O2 reduction and O2 evolution processes at lower temperatures (less than 700 °C) and can maintain durability for required 40,000-h lifetimes. There have been many different electrode materials studied; however, it is often observed that electrode performance tends to decrease with time. Some of the advanced electrode materials by different researchers are discussed in this context who have potential to bring the Li-O2 battery to practical use. Shu et al. [125] presented the production of a mesoporous Nitrogen-doped onionlike carbon electrocatalyst, which was used to aid in the Li-O2 battery. The catalysts involved have several, concentric, quasi-spherical sp2 graphitic shells. The mesoporous N-doped onion-like carbon material, besides facilitating Li ion and O2 diffusion, provided lot of electrocatalytic positions. As a result, it was able to improve the performance of the Li-O2 battery, which had excellent round-trip efficiency, outstanding rate performance, higher specific capacity and stability of about 200-cycle. Based on morphology and structural analysis, a potential oxygen electrode reaction mechanism while charge discharge operations was suggested. This work presented intriguing prospects for the creation of heteroatom-doped carbon materials that may potentially enhance the performance of Li-O2 batteries. Pham et al. [126] synthesised nanosheets of perovskite oxide [(LaNi9/10 M1/10 O3 ) (M = Co, Cu)] with transition metal doping to work as a C and binder-free threedimensional porous air electrode for LOBs. The porous air electrode’s design allows for quick gas and electrolyte diffusion while also creating a continuous electrical conducting web. LaNi0.9 Cu0.1 O3 demonstrated a strong ORR and OER, which improved gradually across both aqueous and non-aqueous frameworks, because of a combination of massive lattice deformation and an oxygen deprivation effect produced by the replacement of a component with a distinct valence state in Ni sites. This LaNi9/10 Cu1/10 O3 nano-sheet of perovskite oxide catalyst produced on threedimensional micro-porous Ni foam obtaining high bi-functional catalytic performance in the lithium-oxygen battery, with the same highly active catalytic kinetics and larger round-trip effectiveness as reported earlier precious metal catalysts, especially when the oxidation/reduction facilitator tetrathiafulvalene is added. The overpotential stays at 0.72 V under circumstances of J = 0.1 mA/cm2 and a ca. limit of 1000 mAh/g, and the round-trip effectiveness may reach 80%. Furthermore, Lin et al. [127] developed a method for producing large-scale usable electrodes with efficient conductive yarns and RuO2 -coated nitrogen-doped carbon nanotubes with active material ink dyes. This lithium-oxygen battery design had a discharge cap. of 1980 mAh/gcarbon at J = 320 mA/gcarbon and could operate without showing a significant decline in ca. after 100 cycles (more than 600 h) when exposed to flexure.

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The novel freestanding type electrode was a crucial milestone on the way of making and using flexible Li-O2 batteries. Mohazabrad et al. [128] investigated the impact of the opening ratio of cathode on the performance of lithium-oxygen batteries at various discharge current densities using experimental and numerical simulations. The maximum discharge ca. is gained at a 25% open ratio, which was the highest of the open ratios evaluated at J = 0.1 mA/cm2 (0–100%). When the open ratio was elevated from 25 to 100%, the sp. discharge ca. decreases from 995.0 to 397.0 mAh/gcarbon . Open ratio values with open ratios of 1, 3, and 6% are noted to correspond to a discharge capacity of 1, 2, and 4 mA/cm2 , respectively. The model which claimed the electrode was often saturated by the electrolyte did not match experimental findings, while the design that incorporates electrolyte loss by evaporation and solid volume alter may be more accurate. Oxygen availability, electrolyte evaporation, and contact resistance were all impacted by the open ratio. Higher open ratios resulted in quicker electrolyte evaporation, which may be a significant factor in reducing discharge capacity (above 25%). As the open ratio rose from 3 to 95%, the battery’s contact resistance increased from 3.97 to 7.02 when measured using EIS. While the resistive over-potential potential was very less (about 1 mV), it was negligible due to the low discharge/charge currents (0.1 mA) (Fig. 17). In non-aqueous Li-O2 cells, nanoparticles of a Pt3 Ni alloy dispersed over graphene sheets were investigated by Kumar et al. [129] as a catalyst for the oxygen reduction process. In an ethylene glycol medium, graphene oxide, Pt4+ , and Ni2+ ions were all reduced by hydrazine at the same time. Additionally, to Pt3 Ni-RGO, Pt nanoparticles are produced independently on reduced graphene oxide sheets (Pt-RGO) for comparative investigations. Physicochemical methods were used to characterise samples. Using cyclic voltammetry and rotating disc electrode (RDE) methods, the oxygen reduction process in a non-aqueous electrolyte was investigated. When compared to Pt-RGO, Pt3 Ni-RGO had higher catalytic activity for the ORR. In a non-aqueous

Fig. 17 Oxygen diffuser and 1st charge discharge cycle. Reproduced with Permission [128]. Copyright 2017, ACS

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electrolyte, O2 reduction occurred through the one-e reaction. The catalytic performance of the samples was determined in non-aqueous Li-O2 cells. With J = 0.10 mAcm−2 just on Pt3 Ni-RGO catalyst, the discharging peak happened at 2.86 V, that is close to the expected value of 2.96 V. In contrast to Pt-RGO, the overpotential of charging was smaller with Pt3 Ni-RGO. Saito et al. [130] developed air-electrode catalysts composed of manganese nanosheets (Mn-NSs) and Ketjen Black, referred to as a KB-composites. Mn-NS catalyst was used to improve the charge and discharge characteristics of a rechargeable Li-Oxygen battery. The catalyst (composite) was prepared by electrostatically stacked nanomaterials in a colloid solution that contains Li+ , and its electrocatalytic activity was evaluated as an air electrode for a LAB test cell. The KB-composites and Mn-NS catalyst worked very well, especially in the oxidation of Li2 O2 accumulated on the air-electrode interface during charging, resulting in increased reversible characteristic of the Li2 O2 deposition/disintegration process during discharging/charge cycles. The cyclability was increased to 28 cycles, and the first discharge capacity was enhanced to 5458 mAh g(carbon) −1 with excellent capacity reversibility. XRD analysis and SEM, X-ray spectroscopy (Energy dispersive) were used to examine and validate these processes. At the air-electrode, Li2 O2 was precipitated and deteriorated more evenly with the KB-comp. Mn-NS catalyst than with just KB. Resulting, even at 0.40 mA, the composite catalyst substantially decreased overpotential and enhanced cyclability to 27th cycle. Kou et al. [131] were able to make an integrated electrode using an in situ grown Cl-doped Co (OH)2 grown on carbon cloth via face electrodeposition. To remove chlorine atoms (electro-oxidation (EO)/Cl-doped Co (OH)2 ), anodic potential was given to the Cl-doped Co (OH)2 in an alkaline solution. This allowed the electrocatalytic activity to be improved without needing any heat treatment. Electrochemical oxidation of EO/Cl-doped Co (OH)2 produced a structure with a greater electrochemically active surface area that yields superior performance for both ORR and OER due to the defect development. A novel methodology for creating active electrocatalysts while introducing flaws had been discovered. Moreover, the activation and Fe-loading of carbon (CMPACs and CMPACs-Fe) was done by pyrolysis in the N2 environment at 900 °C, using CMP peel as a precursor in the work of Li et al. [132]. They used KOH as an activator. According to electrochemical tests, the CMPACbased Li-O2 battery had a specific ca. of 7800 mAhg−1 , 466 cycles of stable cycling performance having a matching coulombic efficiency of 92.5%, an outstanding rate capability and reversibility. In addition, O2 electrodes based on CMPACs provided significantly lower overpotential for both loading and unloading operations. They found that the porous structures, large specific surface areas (900 and 768 m2 /g), and plentiful active sites in the CMPACs and CMPACs-Fe-based O2 electrode enhanced electrochemical performance. They believed that the cellular carbons produced from biomass materials could be an exciting option for realising efficient lithium oxygen batteries (Fig. 18). Lim et al. [133] was able to manufacture a hollow 3D graphene sphere by using vacuum residue (VR) in a green manner, and it had been demonstrated to provide as an oxygen electrode material for LOB. Asphalt-paved roads are known to contribute

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Fig. 18 SEM image of CMPAC and CMPACs-Fe. Charge discharge performance (70 cycle). Reproduced with Permission [132]. Copyright 2018, American Chemical Society

to pollution, and VR is often employed on these surfaces. VR, on the other hand, may be an advantageous predecessor for carbon-based electrodes in LOBs. The lowvalued VR was converted into graphene hollow spherical carbon by using Fe3 O4 as both a catalytic and a template in m-xylene at very critical situations. Due to its hollow spherical structure, it may include a multi-layer graphene feature, which would provide space for the discharge product of Li2 O2 and prevent the formation of Li2 CO3 . This resulted in a capacity of 16 805 mAhgcarbon −1 , which was higher than the 5493 mAhgcarbon −1 of commercial KB600J carbon. Additionally, the cycle performed better. Zhang et al. [134] selected Sr(Ti3/10 Fe7/10-n Con )O3-δ material, which offers stellar conjunction of long-term stability and robust oxygen electrode functionality. Adding only a little bit of Co, e.g., n = 0.07, to Sr(Ti0.3 Fe0.7 )O3-δ lowered the electrode polarisation resistance by around two times. In all fuel cell and electrolysis modes, the STFC electrode provided consistent performance at 1 A/cm2 . The coefficients of the primal diffusion of oxygen and STFC’s surface exchange were calculated and provided to be much superior to La3/5 Sr2/5 Co1/5 Fe4/5 O3-β , the most utilized electrode of oxygen (SOC). STFC had shown superior oxygen transfer coefficients than other electrode materials, yet its stability still outpaced them. Furthermore, to show the benefits of STFC Sr(Ti3/10 Fe7/10-n Con )O3-δ material, Zhang et al. [134], provided an O2 electrode demonstration. An addition of Co, e.g., n = 0.07 of a very modest quantity to Sr(Ti3/10 Fe7/10 )O3-δ results in >2 times the reduction in the electrode polarisation resistance. In all fuel cell and electrolysis modes, the STFC electrode provides consistent performance at 1 Acm−2 . The coefficients for STFC’s basic diffusion of oxygen and surface exchange were calculated and given to be much superior of the SOC oxygen electrode material La3/5 Sr2/5 Co1/5 Fe4/5 O3-β . While STFC

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Fig. 19 MoP QD@HCF O2 electrode mechanism. Graph shows profile of charge/discharge. Reproduced with permission [135]. Copyright 2019, Elsevier

does offer an advantage in oxygen transfer efficiency, other electrode materials cannot match its stability. A new 3D stand-alone film O2 electrode composed of N, P-codoped hollow carbon fibre packed with MoP quantum dots (MoP QD@HCF) was reported by Wei et al. [135]. The MoP QD@HCF O2 electrode had a good ca. of 6.75 mAh/cm2 , a reduced charge voltage 500 cycles), and 100% coulombic efficiency. Furthermore, Sun et al. [186] reported the first use of pectin-enriched BiFeO3 nanoflakes in rechargeable Na-ion batteries, and it was done via enhanced hydrothermal processing. They discovered that the BiFeO3 /pectin nanoflakes work well as an anode, retaining charge and discharge ca. of 450 mAhg−1 and long-term stability, as well as retaining a full 100% capacity after 100 cycles. BiFeO3 crystal lattices may have a viscoelastic pectin that contributed to the remarkable performance of Na-ion batteries. Organic pectin acted as a buffer in the expansion of BiFeO3 crystal lattice because of Na ions being introduced, which showed that the BiFeO3 /pectin nanoflakes will not degrade readily, if used for negative electrode in Na-ion batteries (Fig. 35). Bag et al. [187] developed a two-dimensional nanostructured hybrid anode material, Sb2 S3 with nitrogen and sulphur double doped reduced graphene oxide, which is useful for Na-ion batteries. Solvothermal synthesis led to the creation of hybrid material. Two-dimensional layered material was successfully synthesised through x-ray diffraction pattern, Raman and FTIR spectrum, and microscopic investigations. This new hybrid material is adept at holding onto sodium ions. The capacity of 507 mAhg−1 was likewise acquired after 150 cycles of continuous charging and discharging. Hybrids have shown a better electrochemical capacity and performance than free-Sb2 S3 materials. The reason for the electrochemical characteristics of the hybrid material being so good is that ions may easily diffuse across the layered structure, which had two dimensions. NaMn(x+z)1-y TMy-z TMz O2 (x < 0.33, TM = transition metal/s) were the most effective NIB positive electrodes. While doped with electrochemically inert elements like as Mg or Ti, their stability has increased, but their reason has not been grasped up to this point. Zarrabeitia et al. [188], has demonstrated that the presence of a TiIV doping (z = 0.1) in the sodium manganese rich coated oxides by soaking up

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the electrochemical-induced strain is vital in stabilising the crystal structure of these materials, this was a significant advancement in the search for the optimum NIB positive electrode. Above a z = 0.1-point Mn-Ti substitution, Jahn–Teller active MnIII increases, and strains were not well absorbed. This leads to instability of the crystal structure having low electrochemical performance. Furthermore, Na2 Ti7 O15 Nano Wires were effectively synthesised by Li-Ying et al. [189] utilising phase separation technique and low reaction pressure liquid phase growth. Their morphology, dispersion, and aspect ratio were all excellent. In addition, the nanowire’s structure had Na ion channels along the growth direction (020), which would significantly aid Na ion diffusion for high-performance SIBs. These findings indicated that Na2 Ti7 O15 NWs exhibit a substantial elasticity strain of above 02.56% and a variable Young’s modulus of 26.5 GPa, that might favour the creation of SIBs with a large reversible ca. and stability. Additionally, NTO NWs showed exceptional gas sensitivity. This study suggested a potential negative electrode material for highperformance performance SIBs, as well as a suitable gas sensing material for flexible wearable devices, due to the linked tube structure and good mechanical stability. Additionally, Chen et al. [190] worked on the production of carbon coated NaVPO4 F (NaVPO4 F/C) including their usage in complete batteries of symmetric sodiumion as bi-polar electrodes. The anode and cathode execution of NaVPO4 F@C electrodes have been extensively studied. The symmetric system required NaVPO4 F@C electrodes with little structural modification and adjustment of constant valence. Anode and cathode electrodes provided 136 and 134 mAh/g reversible capacity high diffusion of ions coefficients of 03.10 and 02.56 × 10−11 cm2 /s. The SIFBs and NaVPO4 F@C electrodes as both positive and negative have an elevated reversible capacity, excellent rate functioning, and extended cycle life [90% ca. retention above 400 cycles]. This study showed symmetric SIFBs with NaVPO4 F/C bipolar electrodes as a commercial possibility. Ren et al. [191] used lightweight (rGF) reduced graphene fibre textiles as leading platform to decrease wt. % and improve the functionality of non-active components. Ultrathin SnS2 nanosheet/rGF hybrid electrodes were created as binder-free sodiumion battery electrodes (SIBs). A conductive network is formed with the graphene fibres entwined with the electrode. By stopping the phase transition (in situ) of SnS2 by SnO2 , the powerful inter-interface contacts in graphene and SnS2 may be preserved. Advantages include a ~500 mAhg−1 specific capacity above the cycle count of 500 on 0.5 Ag−1 (current rate) with nearly hundred percent charge efficiency. Moreover, proportion of SnS2 in the entire electrode could approach to 68%, considerably better by traditional electrode configuration with Copper, Aluminium foil, or carbon cloth, and a considerable contrast to the usual binder-free electrode with the rGF textiles (Fig. 36). Shen et al. [192] used chemical vapour deposition (CVD) to create a core/shell nanowire array of TiC@C, then used it as the negative plate in SIBs. The carbon-shellencrusted conductive TiC core was intricately fashioned. The TiC@C nanowires acquired from the growth medium have a core/shell structure and porous architecture with electronic conductivity and stability that had been strengthened. The TiC@C electrode had an impressive rate and cycle performance with 135.3 mAh/g capacity at

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Fig. 36 SnS2 @rGF characteristics [191]

0.1 A/g and retains 91% ca. after thousand cycling on 2 A/g. The proposed approach would allow for the creation of binder-free electrode arrays for the storage of sodium ions. Furthermore, Ledwoch et al. [193] examined the impact of electrode formulations by partially replacing the electronic conductive addition carbon black with the ionic conductivity additive zeolite. The ionically conducting zeolite was partially replaced with the electrically conductive carbon black in a negative electrode of (115) GSM weight and 35% porosity. The additive to active ingredient and binder concentration ratios were maintained (90: 5: 5, active: binder: additive). EIS and GITT were utilised to evaluate the composite electrodes’ ionic conductivity, resistance, rate performance, and capacity retention. The variations in porosity during cycling were imaged using an electron microscope. The zeolite: carbon electrode composites (4:1) have shown better rate performance (90%) and cycle stability (reduced sodium plating). These coefficients increased from 1.2 × 10−13 cm2 s−1 (0:5) during sodiation to 1.5 × 10−12 cm2 s−1 (4:1) at the lower voltage plateaux (15% SOC). In the slope voltage area, de-sodiation at higher states of charge (70% SOC) seemed to reduce effective diffusion (0.4 × 10−10 cm2 s−1 (4:1) 1 × 10−10 cm2 s−1 (0:5)). This demonstrated the importance of ionic and electronic conductivity variations during cycling when developing electrode microstructures. Sodium-ion batteries benefit from NaTi2 (PO4 )3 ’s open 3D crystal channels that permit ion diffusion. A novel technique was used by Liu et al. [194], to develop the NaTi2 (PO4 )3 electrode, including the use of acceptable crystal design for Zr doping and surface modification by carbon coating. Sodium-ion batteries were created using the NaTi2-x Zrx (PO4 )3 /C composites (x = 0.05, 0.1, 0.15) via a simple sol–gel method.

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The result revealed that all the sample crystals were very pure. The dispersions of the Zr-doped samples were improved over the blank sample, however. While all the samples had their strengths and weaknesses, NTP-Zr-0.1 (NaTi1.9 Zr0.1 (PO4 )3 /C) excelled in terms of charge/discharge performance. The capacities delivered were 206.3 mAhg−1 at 40 mAg−1 and 79.2 mAh/g at 2000 mA/g. In comparison, the NTP discharge ca. were 116.2 and 26.9 mAh/g, respectively. NTP-superior Zr-0.1 ’s electrochemical performance was largely due to the carbon coating structure, which was believed to improve the dispersion and increase the lattice volume. Additionally, this improved the transport of both electron and sodium-ions. A new approach for boosting the performance of NaTi2 [PO4 ]3 /C composite was being proposed in this research, and it involved carbon coating and Zr doping (Fig. 37). Park et al. [195] investigated the impact of surface characteristics and microstructures on carbon-based electrode materials for sodium metal anodes. Oxygen and nitrogen functional groups reduced the sodium metal nucleation over-potential from 18 mV for P-CNTs to 9 mV for O-CNTs and N-CNTs, respectively. In the existence of nitrogen and O2 dual function groups elements, synergistic impact of nitrogen and oxygen functional groups for sodium metal nucleation was observed. The heteroatom-rich CNTs also had better CE values and lower cell-to-cell variability than plain Al foil and P-CNTs. On NO-CNTs and NO-S-CNTs, average CE values of 99.9 were obtained with just 0.6% cell-to-cell variance in each current range. Despite their comparable surface characteristics, NO-CNTs and NO-S-CNTs exhibited significant differences in cycling performance. Several ex-situ characterisation findings showed that the carbon microstructure is important for long-term cycle

Fig. 37 Sample preparation illustration, a rate performance of sample, b Coulombic efficiency of NTP-KB and NTP-Zr-0.1 . Reproduced with permission [194]. Copyright 2020, Elsevier

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lifetimes. The electrochemical performances of various SMBs with varied excess sodium metal loadings were investigated to obtain the optimum SMBs. This resulted in a practical electrochemical performance of 214.7 Wh kg−1 , 4219.2 W kg−1 and 75.2% capacity retention after 1000 cycles for the excessive anode designed SMBs. The layered oxide Na1/2 Ni1/4 Mn3/4 O2 (P3 type) as a cathode material of Na ion battery has a voltage of ~4.2 to 2.5 V versus Na+ @Na and a ca. of above 130 mAh/g when cycling at 10 mA/g. Despite it, the significance of the 4 V elevated voltage phase was never fully understood, limiting attempts to intelligently alter and enhance its performance. Liu et al. [196] utilised neutron diffraction (in situ) to demonstrate that the phase over 4 V was a modified, non-sodium transitional O3 phase with a considerably shorter interlayer spacing. P3 -O3 -O3 s are the development stage with increased voltage. With its known structure, they used first-principles electronic structure simulations to show that the reversible oxygen redox had a major role in electrochemical activity above 4 V in this O3 phase. In the case of O/transition metal vacancies, the O3 phase was shown to be rather stable. The findings might be derived for future study into the O3 s phase to enhance performance and cycle stability in sodium ion batteries. Furthermore, O3 -NaNi0.5 Mn0.5 O2 is a Potential sodium-ion battery material because of its cheap prices and ecological effect. Although, complicated phase transitions cause poor cycle stability and rate performance. Ren et al. [197] successfully synthesised O3 -Na49/50 X1/50 Ni1/2 Mn1/2 O2 at 5% Na-Mn-O (X 14 Li, Zr) by constructing and modifying structure of cored shell, with elemental doping. First, a high-nickel core and high-manganese shell enhanced cycle stability. Ex situ X-ray diffraction (XRD) measurements showed that doping Li and Zr into Na sites suppresses phase transition. Capacity retention rates of Na49/50 Li1/50 Ni1/2 Mn1/2 O2 at 5% NaMn-O and Na49/50 Zr1/50 Ni1/2 Mn1/2 O2 at 5% Na-Mn-O samples were 61% and 67%, respectively, while the pristine (NaNi1/2 Mn1/2 O2 ) sample was 52% cycling at 3 C. Excellent electrochemical performance of cathode materials was achieved via twofold modification. Moreover, in study of Ma et al. [198], detailed group of Na(Ni1/3 Mn1/3 Fe1/3)1-x Alx O2 (x = 0, 0.03, 0.05, 0.07) oxides prepared via spray pyrolysis as sodium-ion battery positive electrode materials. XRD, SEM, and CV, among other methods, have been used to better understand the morphology, structure, and electrochemical performance of Na(Ni1/3 Mn1/3 Fe1/3 )1-x Alx O2 (x = 0, 0.03, 0.05, 0.07). Na(Ni1/3 Mn1/3 Fe1/3 )0.95 Al0.05 O2 offers a 145.4 mAh/g initial ca. 0.10 C, retaining 128.4 mAh/g of capacity above 80 cycles at 0.2 C with an overall ca. retention of 77.5%. Analysis using XPS showed that the Jahn–Teller effect is produced by Mn3+ and may be counteracted by Al-doping, which enhanced the structural stability of layered oxides. Our findings showed that a little amount of Al-doping (5 wt.%) resulted in improved structural stability, which therefore led to good electrochemical performance. Furthermore, for use in Solid Injection Moulding in SIBs, a novel quinary O3 -type MgCo-NaMnNiAlO cathode material was developed by Zhang et al. [199] using the two methods of hydro-thermal reaction and solid-state reaction. This unit could deliver a reversible ca. of 118, 109, 88, and 73 mAhg−1 at a rate of 1 to 10 C. At 1 C, the prepared cathode maintained its original rate capacity for 1000 cycles, with a 0.018% capacity loss rate in every cycle. To support low-voltage

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and high-voltage capacities, respectively, Mn and Ni were employed during sodiation/desodiation. Additionally, Co ions caused charge compensation. In addition, Al and Mg ions are known to promote charge transfer, which may be a major contributor to the structure’s stability when Na+ intercalation/intercalation taken place. Moreover, Al ion and Mg doping, when used in combination, may help increase the rate of Na ion migration during circulation at ambient temperature. In situ XRD characterisation of advanced O3 materials provided insight into structural reforms in Na-ion batteries, which was important in the understanding of their characteristics. The inexpensive mechanochemical technique by Dogrusoz et al. [200], that was used to produce orthorhombic tin (II) sulphide involves using a high-energy ball mill over various period. The ideal milling duration was discovered to be 2 h, and this is determined to be most optimal for anode use in sodium-ion batteries. Polyvinylidene fluoride (PVdF) and sodium carboxymethyl cellulose (NaCMC) were shown to function better than Na alginate binder, which was more favourable in sodium-ion battery anodes (CMC). The presence of Na-involvement alginate’s in EIS measurements was first confirmed by the measurement of the Na ion diffusion coefficient (CMC binder: 7.4 × 10−17 cm2 /s, Na-alginate: 3.28 × 10−17 cm2 /s), with the resistance value of the Na-alginate being much lower. SEM analysis of post-mortem samples of the electrodes also indicated no cracks after performance because of the C-rate due to the rich and homogenous distribution of carboxylic groups and polar sites that was played by the Na-alginate binder’s self-healing properties, which can better withstand the volume expansion. Additionally, as the 2 h ball milled SnS composite without carbon addition in synthesis could not provide more than 300 mAh/g capacity, the conductive carbon and the coating methods were explored to produce SnS cathode with greater capacity. Moreover, greater specific volume and low discharge point of BiPO4 made it a popular NIB anode. PO43− has a stable structure that reduces structural changes during circulation, improving cyclic stability. However, BiPO4 is a weak electrical conductor with a low specific capacity at less 100 cycles. Xi et al. [201] used, reduced graphene oxide in a composite (BiPO4 @rGO) that improved its functionality. The inclusion of rGO enhanced the nanocomposite’s conductivity and cyclic stability. Its reversible specific capacity was 443.2 mAhg−1 on 100 AHG and 406 mAhg−1 on 200 AHG for over 200 cycles. That was very high than that of pure BiPO4 which is 10−4 Scm−1 ). Gel polymer electrolytes (GPEs) are more practical than solid polymer electrolytes [21], therefore receiving considerable attention due to their adaptability, excellent interfacial compatibility, low volatility, excellent thermal stability, safety [22] and superior electrochemical characteristics [23]. GPEs based polymers such as poly (methyl methacrylate) (PMMA), poly vinylidene fluoride (PVDF), poly (ethylene oxide)

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(PEO), copolymer, i.e. poly vinylidene fluoride co-hexa-fluoro-propylene (PVDFco-HFP) and poly-acrylonitrile PAN have been extensively researched during the last few decades [24, 25]. When organic solvents are used, it has been observed that GPEs have poor thermal and electrochemical characteristics. Organic solvent lowers heat stability of GPEs because of their volatile nature. Furthermore, their electrochemical stability range is limited by a very small electrochemical potential window. In order to overcome these issues, the organic solvents are replaced by ionic liquids (ILs) in GPEs [26]. Ionic liquids (ILs) are typically molten salts with melting point less than 100 °C. The majority of such salts are organic salts having a wide range of design possibilities with a number of benefits, including inherent ionic conductivity (10−3 –10−2 S cm−1 ), a broad electrochemical window, inertness, chemical and heat stabilities [27, 28]. Additionally, ILs acts as solvents, increasing the amount of charge carrier ions in a GP matrix. As a result, they may be used as advanced electrolytes in LIBs [29, 30]. These molten salts are often made up of an asymmetric organic cation pyridinium (PY), imidazolium (Im), pyrrolidinium (PYR), ammonium, sulfonium, etc. and a weak organic/inorganic anion (BF4 − , PF6 − σ, triflate (CF3 SO3 − ), bis (tri-fluoro-methane-sulfonyl imide) (TFSI) ((CF3 SO2 )2 N− ), etc.) combination [31, 32]. Imidazolium-based IL electrolytes attracted the researchers because of their low viscous characteristics and strong ionic conductivity [33, 34]. Plashnitsa et al. [35] used EMIBF4 (1-ethyl-3-methyl imidazolium tetra-fluoro-borate) in a symmetric Na-cell (both cathode and anode made up of Na3 V2 (PO4 )3 ) in early 2010. When compared to cells using a combustible carbonate-based electrolyte, this cell demonstrated much greater heat stability and cyclic performance even at extreme temperatures. Kumar et al. [36] described another use of the imidazolium-based IL in a polymer electrolyte composed of EMI-triflate (EMITf) immobilised with poly (vinylidene fluoride–hexa-fluoro-propylene) (PVDF–HFP). At room temperature, the electrolyte of EMITf/PVDF–HFP (4:1w/w) + 0.5 M NaTf exhibited an excellent charge conductivity of 5.7 mS/cm and exceptional mechanical strength. Even though, the ionic liquid electrolytes have a lot of advantages but its two physical characteristics exclude their use in lithium batteries. Firstly, cells constructed with IL electrolytes usually have a low percentage of electrical conductivity derived from mobile Li-ions (i.e. low lithium transference No. TLi + ). As a result, it tends to be more susceptible to polarisation. Lithium TLi + (ion transference No.) below 0.2 is common for pyrrolidinium-based TFSI/LiTFSI systems and imidazolium-based TFSI/LiTFSI systems [37, 38]. Second, the majority of ILs has comparatively moderate compressive and tensile strengths, which restricts the range of possible configurations of lithium batteries. IL-based electrolyte also failed to avoid the development of lithium dendrites in a lithium-metal battery. An innovative type of electrolyte, known as hybrid electrolyte, has been developed in order to address the scarcity of ILs and PEs in the market. The term “hybrid electrolyte” refers to a diverse group of materials. These may be categorised into two groups, i.e. hybrid all-solid-state electrolytes (HSEs) and hybrid quasi-solid electrolytes (QSEs). HSEs are a Combination of (at least) two well-defined lithium-ion conductive solid phases. These may be both inorganic and

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polymeric phases [39] or a mixture of both [40]. On the other hand, hybrid quasisolid electrolytes (QSEs) combine an inorganic conductor with an ionic liquid [41, 42] or organic liquid [43] phase. Hybrid electrolytes may be created by combining the properties of ILEs, GPEs and SLEs. For instance, various ceramic nano-particles may be combined with ILEs to create hybrid electrolytes. Moganty et al. [44] showed the synthesising method for adding SiO2 NPs to ILEs using imidazolium cations. In a similar manner, polymer electrolytes may be utilised in conjunction with ceramics to create composite structures [45]. All these types of electrolytes are discussed in detail in their respective sections.

2 Properties and Classification of Electrolytes In general, the electrolyte determines energy density, safety, cycle life, storage performance, operating circumstances, etc. of a battery system. The voltage of rechargeable batteries is determined by the potential difference between the electrodes, and its capacity is determined by the exchange of ions between the electrodes through the electrolyte solution. Hence, the performance of battery depends on the type of electrolyte used. Therefore, the synthesis of proper electrolyte should have the following properties (Fig. 1) [46]. Electrochemical window: Electrolytes must be stable across a broad voltage range in order to allow cell processes without electrolyte breakdown. Conductivity: It’s ideal to have a high ionic conductivity and a low electrical conductivity. The minimum ionic conductivity significant for battery electrolytes is a few mS/cm. Viscosity: Low viscous electrolytes have higher ion mobility. Thermal stability: Electrolytes that are thermally stable and liquid across a broad temperature range are desirable. High-flashpoint solvents are safer. Wettability: To ensure constant ion mobility, the electrolyte should thoroughly wet the electrodes and separator. Cost: Electrolytes that are low in cost and ecologically friendly should be chosen. Compatibility: Electrolytes that are noncorrosive and nonreactive guarantee that cell components last a long time [10]. The electrolytes can be classified according to their properties and can be classified as Organic, Aqueous, Non-Aqueous, Polymer, Ionic liquid and Hybrid Electrolytes.

3 Organic Electrolytes Organic electrolytes are the most widely used electrolytes. A typical organic electrolyte is made by dissolving a metal salt containing a large and weakly coordinating anion, such as BF4 − or PF6 − , with a concentration of 1 mol /dm3 [10]. Organic liquid

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Fig. 1 Properties of different types of electrolytes and their future work. Ref [47], copyright @ 2020, Elsevier

electrolytes have been researched extensively in recent decades due to their attractive properties, which include greater ionic conductivity than solid electrolytes and a stable contact with a variety of electrodes [47]. Ionic conductivities of organic solvents are typically about 10 mS/cm [48]. However, many physical criteria, particularly thermal stability, electrochemical stability and safety, remain unmet. Recent advancement in organic liquid electrolytes has attempted to resolve these issues by changing existing electrolyte systems (e.g. by adding different electrolyte additives) or designing new salts and organic solvents. More interestingly, the highly concentrated organic liquid electrolytes have been extensively investigated in recent years due to their superior oxidation resistance and wider electrochemical window than water [47]. In highly concentrated organic liquid electrolytes, there are only few freesolvent molecules, and have no direct coulombic interaction with ions. These characteristics help to reduce oxidation stability as well as to improve the thermal stability and rate capability [49]. Additionally, certain highly concentrated electrolytes aid in the development of SEI on the negative electrode and hence improve the current collector stability. Organic carbonates including ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and their combinations have traditionally been utilised in Li-ion batteries. These solvents, particularly EC, are necessary for the operation at 4 V

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of Li-ion batteries [8]. EC is an important component in carbonate solvent mixtures because it forms the passivation layer and has high flash point. Carbonates, on the other hand, cannot be used in high-voltage (>5 V) systems because it is more than its anodic stability limit [50]. Li batteries have also utilised a variety of esters, ethers, and sulfones [51, 52]. Flamme et al. [50] published a thorough research on the selection of organic solvents for Li-ion batteries, in which they evaluated several solvents based on their physicochemical characteristics, as well as their environmental effect and manufacturing routes. They found that the addition of nitrile and fluorine groups improved stability, whereas the addition of alkoxy groups lowered the melting point and increased conductivity. Desymmetrisation of solvent species in general improved solvation and lowered the melting point. The most significant disadvantage of these materials is their flammability. When it comes to commercialising a battery technology, safety is a top priority. As the advancement of battery technologies are more common, finding an electrolyte that is not intrinsically hazardous is utmost important. High-flash point solvents, such as Gamma-butyrolactone (95.5 °C) and sulfolane (165 °C), are safer alternatives of carbonates. Such safer solvents, which can be manufactured in a cost-effective manner and have a low environmental impact, are required. Based on the salt-concentrated electrolyte design, organic fire extinguishing electrolytes for rechargeable batteries are created. Wang et al. [53] demonstrated that they are not only a powerful fire extinguishing agents but also allow a stable charging cycle for more than one year with negligible degradation. In comparison to hard carbon or graphite anodes, using concentrated NaFSA/TMP (trimethyl phosphate) and LiFSA/TMP electrolytes are more suitable for sodium and lithium ion battery packs. The spontaneously generated inorganic salt derived SEI is superior to the standard, solvent-based organic inorganic hybrid SEI. Thus, high cycle stability is achieved. It is the LUMO modification, a distinctive solution structure specific to the concentrating electrolyte system that is responsible for this unique passivation. The design approach may be expanded to include different combinations of non-flammable or fire retardant solvents and alkaline springs with its exceptional simplicity and flexibility, giving a welcome boost for the creation of secure and high performance rechargeable batteries. In this regard, the aqueous electrolytes can play an important role.

4 Aqueous Electrolytes Aqueous electrolytes provide a number of benefits, including low cost, inherent safety and also environmental friendly. Due to these properties aqueous electrolytes have attracted more attention, even though they have significant stability limitations with electrodes as compared to other liquid electrolytes. Aqueous liquid electrolytes have a very high ionic conductivity (generally hundred times higher than organic liquidbased electrolytes) [11], as well as much cost effective since they are essentially water-based. The present focus of aqueous electrolyte research is on overcoming

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the stability issues that arise with H2 O breakdown (narrow the operational electrochemical window, around 1.23 V) [54]. Highly concentrated aqueous electrolytes, commonly known as “water-in-salt” electrolytes (WiSE), have recently attracted a lot of attention because of their potential to expand the electrochemical window while resulting in low solvent activity, low flammability and good chemical ability [55]. Aqueous electrolytes, when combined with excess sodium, make aqueous sodium ion batteries (ASIBs) more appealing for application in large-scale ESS than other battery systems [56]. The most often utilised electrolyte in ASIB experiments is Na2 SO4 at different concentrations in deionised water [57]. The potentials of Na intercalation and de-intercalation of certain materials may be regulated in the stable range of water by changing the pH of electrolyte. Wang et al. [58] reported a promising cathode material, Na0.66 [Mn0.66 Ti0.34 ]O2 , and used it in a complete cell with NaTi2 (PO4 )3 /C as the anode in an aqueous electrolyte of 1 molar Na2 SO4 having pH = 7. The cell demonstrated excellent performance, with a specific capacity of about 54 mAh/g at the rate 10C and cyclic stability of 89% after 300 cycles at the rate 2C. In another work, they synthesised Ti-substituted Na0.44 MnO2 (which is frequently used as a cathode) and utilised it in anode material. This material was tested in a three-electrode cell using Na2 SO4 electrolyte with a pH = 13.5 [57]. According to Kumar et al. [59] the presence of an electrolyte additive, as well as the presence of salt content influenced the pH of electrolytes as well as other characteristics such as viscosity. The pH of the aqueous electrolyte containing 10 molar NaClO4 and 2 vol% vinylene carbonate (VC) was 1.7, this ensured that the Na3 V2 O2x (PO4 )2 F3-2x /multiwall carbon nanotube half-cell cycled between 0 to 0.9 V when compared to a saturated calomel electrode (SCE). They concluded that the additives (CMC, Ag, or VC) lowered the pH value, with VC having the most noticeable impact [1]. Apart from number of benefits aqueous Na-ion batteries also have several serious short comings. Firstly, having a small electrochemical window, i.e. only 1.23 V. Also energy densities of ASIBs’ limited to less than 100 Whkg−1 . Secondly, the corrosive nature of aqueous electrolytes also restricted ASIBs’ performance [60]. In order to overcome these problems, different types of aqueous electrolytes are investigated. Few of them are discussed below.

4.1 Lithium Nitrate The non-flammable and high conductive characteristics of aqueous electrolyte such as Lithium Nitrate make it the first choice as electrolytes in aqueous rechargeable batteries (ARB) in the early 1990s [11]. However, owing to a narrower (1.23 V) stability window and poor cycle performance due to the presence of water in electrolytes, researchers are started to think on various electrode combinations for high performance ARBs. The ARB is produced as an electrode with a greater discharge capacity of 60 mAhg−1 even after 500 cycles without dissolved oxygen in 9 M lithium nitrates with LiV3 O8 and LiFePO4 /C [13]. It showed the ARB potential as

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more efficient energy storage battery. It is also worth noting that the LiNO3 has greatly improved oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) when used as an electrolyte in polyether solutions. Sharon et al. [61] suggested that NO3 − anions stabilise Li+ cations and hence made them less Lewis acidic during ORR. This reduction resulted from high binding between the Li cations and nitrate anions in the ethereal solutions. The lifespan of the radical anions (superoxide and peroxide moieties) produced by oxygen reduction is prolonged because the Li-ions (which promote precipitation of Li2 O2 as the ultimate product) immediately engaged with them. Under these circumstances, the first step produces less oxygen, but soluble products increased in the solution phase. The scenario helped Li2 O2 precipitation to occur in a top-down manner, which caused thick particles to accumulate on the surface of cathode and provided them, a higher specific capacity (per electrode weight and volume). The ORR process in LiTFSI was more efficient than the normal precipitation mechanism of Li2 O2 . When the homogeneous insulating product layer was produced and had a specified nanometric thickness, the process is stopped by the later blocks. The cyclic behaviour of Li–O2 with one molar LiNO3 and one molar LiTFSI is shown in Fig. 2 By adding 1,4,di-oxane as a co-solvent to a Li bis (fluoro-sulfonyl) imide/1,2,dimethoxy-ethane solution, a novel ether-based electrolyte suitable for Li-metal electrode, is formed. Even at relatively high current rates, this simple liquid electrolyte shows stable lithium cycling with dendrite-free lithium deposition, excellent coulombic efficiency (CE) of about 98 percent, and strong anodic stability up to 4.87 V versus Li RE due to the synergetic action of solvents and salt. Its outstanding performance had fulfilled the requirement of next generation of highenergy–density rechargeable Li-metal battery systems [62]. Meanwhile Giordan et al. [63] proved that the high concentration of LiNO3 enhances the stability of the cycling of Li/O2 cell. Cells with greater LiNO3 concentrations have better coulombic efficiency, lowered H2 and CO2 evolution, and reduced Li2 CO3 buildup in O2 electrode. These occurrences are explained using a quantum chemical model of autoxidation.

Fig. 2 Cyclic behavior of a Li-O2 cells with a 1 molar LiNO3 or b 1 molar LiTFSI solutions. Ref. [61]. Copyright @2015, ACS

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Water interacts with Li metal to produce LiOH, an anticipated by-product of autoxidation. When X-ray diffraction (XRD) signals for LiOH on the Li electrode in cyclic Li/O2 and symmetric Li/Li cells are compared, it suggested that (1) the O2 electrode promoted autoxidation and (2) high molarity LiNO3 inhibited autoxidation. These findings were in line with density functional theory (DFT) calculations that provided a molecular explanation for autoxidation in Li/O2 cells. The solid option to increase the specific energy density is the next-generation lithium-metal batteries (LMBs) [64]. Metallic lithium anode offers unique advantages over graphite anode (372 mAh g−1 ) in LIBs because of its high theoretical capacity (3860 mAh/g), minimum electrochemical voltage (3.04 V vs. SHE), and the source is pure lithium. The instability of Li-metal with all organic solvents, as well as the short circuit produced by lithium dendrite deposits, pose two significant technological obstacles to the commercialization of LMBs. The Li bis-(fluoro-sulfonyl) imide salt has shown to be effective in suppressing lithium dendrites and improving cycle efficiency at low current densities. Furthermore, the passivation of the Li-metal anodes, Jin et al. [65] utilised LiNO3 as a film-forming additive. The efficiency of cells with 2.5 molar LiFSI-0.75 molar LiNO3 /DOL as electrolyte at large currents improved noticeably on the addition of LiNO3 . The stripping/plating capacity of the symmetrical Li–metal cells was 2mAh/cm2 and for 1000 cycles with more than 1000 h at 5 mA/cm2 . Furthermore, the Li/Cu cell could be cycled 400 times with a steady and averagely high CE = 98.8% at 3 mA/cm2 with a capacity of 2 mAh/cm2 . Furthermore, the electrolyte adapted well to the LiFePO4 electrode and enhanced the Li- metal’s cycle stability. As a result, LiNO3 performed well as a layer forming addition in the LiFSI/DOL electrolyte. This allowed the Li-metal anodes to be further passivated.

4.2 Saturated LiCl Electrolyte In case of contact voltage less than −3.04 V, water does not disintegrate generally from electrolytes [66]. A saturated solution of lithium chloride may thus be a viable option in the presence of LiOH and H2 O. In this case, the pH value is below 10, which allows the NASICON type to conduct glass–ceramic electrodes of stability owing to the existence in the electrolytes of the lithium ions [67]. The complete usage of aqueous electrolytes may be used with a LISCON Film demonstrated as a means of reducing the exposure of lithium to water [68]. The cathode to the aquatic solutions here contacts the organic layer. This kind of design may contribute to the achievement of a specific energy density similar to that of a combustion engine (Fig. 3).

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Fig. 3 LISCON film. Ref. [68]. Copyright @2009, ACS

4.3 Aqueous Sodium Salt Sodium salts like NaOH, NaNO3 and Na2 SO4 are excellent options for the electrolytes of rechargeable batteries as they are economical and easily available in nature. With sodium sulphate 1M solution that delivered 40 mAh g−1 specific capacities at 20 °C temperature, Pang et al. [69] reported good performance and stability. Qin et al. [70] showed the advantage of 5 molar NaNO3 solution as an electrolytes polyimide anode. Minakshi et al. [71] have shown that maricite may durable up to 100 cycles with a capacity of 45 F/g discharge values in sodium hydroxide aqueous median as a cathode. However, the major drawback of sodium-based electrolytes is very low decomposition voltage which is theoretically 1.23 V. Additionally Xu et al. [72] demonstrated that ZnSO4 -based electrolyte, rechargeable aqueous ZIBs with Na2 SO4 additive removed zinc dendrites by altering the structure of Zn2+ ions. The Zn/NMO battery also showed excellent cycle-life capacity (367.5 mAh/g at 0.65 °C) and stability (only 0.007% capacity fading after 10,000 cycles at 6.5 °C). Additionally, the formation of by-products on the cathode surface during operando and ex situ electrochemical characteristic indicated an increased capacity and a decreased CE during the electrochemical activation process. DFT calculations indicated that the solvation energy of electrolyte ions is responsible for the insertion/extraction of H+ /Zn2+ ions in the cathode material.

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4.4 Alkaline Electrolytes Alkaline aqueous solutions including potassium hydroxide (KOH) [73], sodium hydroxide [74] and lithium hydroxide [75] have been the most often utilised electrolytes in Zinc–based batteries (e.g. RAM, NiZn, or ZAB). Nonetheless, owing to the rapid electrochemical kinetics and great dissolvability of Zn-salts, KOH has been the most often utilised among them [76] because of the higher ionic conductivity of K+ (73.50 −1 cm2 /equiv) as compared to sodium ions (50.11 −1 cm2 /equiv) or Li+ ion (50.11 −1 cm2/ equiv) [77]. An alkaline battery with a Zn anode has been preferred because it’s intrinsic electrochemical reversibility. The high dissolvability of Zn-salts, durability, non- toxicity and long life-span positive electrodes make them more attractive electrolytes (e.g. NiOOH and MnO2 ) [76]. Secondary alkaline zinc–air batteries have a fundamental requirement, i.e. the selection of proper electrolyte (concentration and additives). Ionic conductivity is strong at OH− concentrations over the critical level, however dissolving of Zn leads to change of shape and dendritic development. On the other side, lesser pH leads to Zn species (as shown in Fig. 4) which are not desirable. In order to improve the performance of the battery, additives should be included to address these issues. The mixing of the aqueous alkaline electrolyte has a strong relationship with the reversibility of zinc anode. Although a high concentration of OH− causes zinc to dissolve in the aqueous alkaline electrolyte, it is desirable to avoid the more severe restrictions that occur with a lower concentration of OH− . As a result, for the growth of rechargeable zinc–air technology, reducing zinc dissolution is critical. The use of proper additives may help to alleviate this restriction [73]. Zinc solubility (associated with intermediate species, Zn corrosion, change of shape, and dendritic growth), insoluble carbonate deposition, electrolyte evaporation or environmental moisture absorption, and H2 evolution are all drawbacks of using aqueous alkaline electrolytes [73]. Hence, in the next section, a neutral electrolyte is discussed.

Fig. 4 Scheme of alkaline electrolyte pH: issues shown by red and solutions by green

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4.5 Neutral Electrolytes Carbonization of the electrolyte is one of the most technological challenges faced by ZABs. A neutral electrolyte can resolve this problem, as it offers two significant advantages: (i) it prevents the electrolyte from carbonization and (ii) it minimises the dendrite formation. These advantages are due to the neutral pH value of electrolyte. Neutral electrolytes are eco-friendly and they are advantageous for biofuel and biosensor applications that need severe pH conditions [78]. With solutions like KCl, KNO3 , Na2 SO4 and K2 SO4 , the pH of the electrolyte may be lowered to about 7 or around 5 with ammonium salts. In contrast to H2 SO4 (acid) or KOH solutions, the latter is regarded as near-neutral electrolyte (alkaline) [79]. The conventional alkaline electrolyte interacts with CO2 and lowers the conductivity of ions. Precipitated carbonate particles obstruct the diffusion channels of air electrodes [74, 80, 81]. Carbon oxidation in an alkaline electrolyte leads to the generation of carboxylic acids such as mellitic acid and humic acid. As a result, it is desired to lower the pH of the electrolyte in order to increase the long-term stability of zinc–air batteries with a carbon-based air cathode. Another issue faced in ZABs is the production of insoluble K2 CO3 due to the reaction of airborne CO2 with KOH. Carbonate will ultimately precipitate out, resulting in increased electrolyte consumption and decreased conductivity of the electrolyte. Precipitates may also obstruct the pores inside the air cathode, gradually passivating it and impairing the batteries’ longterm efficiency [82]. The development of ZABs is limited by the absence of a strong bifunctional catalyst for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) at the air-electrode (ORR). The synthesis of NiFe2 O4 /FeNi2 S4 hetero-structured nano-sheets (HNSs) in controlled way that are extremely effective in catalysing OER and ORR, thereby allowing neutral secondary ZABs, was demonstrated by An et al. [83] The oxygen binding energy of the catalyst may be successfully modified to improve OER and ORR activities by generating abundant oxide/sulphide interfaces on the surfaces of NiFe2 O4 /FeNi2 S4 HNSs (Fig. 5). The optimised NiFe2 O4 /FeNi2 S4 HNSs had outstanding oxygen electrocatalytic activity and stability in a 0.2 molar PO4 3− buffer solution, has significantly lesser OER and ORR over potentials as compared to NiFe2 O4 or FeNi2 S4 monocomponents and non-significant performance loss in accelerated endurance testing. In an air-electrode configuration, the NiFe2 O4 /FeNi2 S4 HNS’s can confer a density of power about 44.4 mW/cm2 and have effective cycling stability (at 0.5 cm−2 only 0.6 percent degradation after 1,000 cycles), making the resulting zinc–air battery the high efficient and durable with a neutral aqueous electrolyte reported (Fig. 6). Goh et al. [84] developed a series of relatively close chloride baths using zinc chloride (ZnCl2 ) and ammonium chloride (NH4 Cl), together with polyethylene glycol and thiourea additions, for use in rechargeable ZABs. A chloride solution comprising 0.51 M ZnCl2 , 2.34 molar NH4 Cl, 1000 ppm polyethylene glycol (PEG), 1000 ppm thiourea and a pH of 6.0 demonstrated good electrochemical and ZAB’s efficiency. The application of PEG and thiourea additives prevented Zn accumulation from chloride baths. Impedance spectroscopy and exchange current densities revealed that the

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Fig. 5 a, b Showing optimized geometry of O2 adsorption over NiFe2 O4 /FeNi2 S4 . Ref. [83]. Copyright @ 2018, ACS

Fig. 6 Synthesis of NiFe2 O4 /FeNi2 S4 HNSs using wet chemical sulphidation method. Ref. [83]. Copyright @ 2018, ACS

air cathode’s reaction kinetics were nearly one order of magnitude lower than those of the Zn anode. For both the Zn anode and the air cathode in a three-electrode arrangement, the cathodic and anodic limbs of the quasi-steady-state polarisation curves were asymmetric. Tests on rechargeable ZABs indicated that this kind of chloride electrolyte system was capable of lasting over 1000 h and hundreds of charging/discharging cycles with charging/discharging capacities ranging from 20 to 120 mAh. No Zn dendrite growth was detected after a long zinc–air battery test.

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4.6 Advanced Aqueous Electrolytes Due to the limited lifespan of dendritic electro deposits of metallic Zn, their further growth was hindered. Huijun Yang et al. suggested the use of a metal–organic framework (MOF) as the front surface layer in order to maintain a supersaturated electrolyte layer on a zinc anode. The ion complexes with a high degree of coordination moving via MOF channels were distinct from the solvation structure in bulk electrolyte, according to Raman spectroscopy. Symmetric Zn cells survived up to 3000 h at 0.5 mA cm−2 , almost 55 times longer than plain Zn anodes. The Zn electro-deposit became round-edged and tightly packed, with reduced side-product accumulation. As an electrolyte for the battery, the created nano-composite material has shown commendable and exceptional results. FE-SEM analysis was used to investigate the structure of a synthetic nano-composite with intermediate synthesis of nano-fibres (poly N-methyl aniline) and spheres of lithium. After 100 charging and discharging cycles, the power density of the cell was measured to be 119.233 W /m3 . As an electrolyte, the new material has showed tremendous promise as a viable material for use in batteries [85]. At ambient temperature, the Ga-Sn liquid metal nano-particles (LMNPs) selfheal, preventing fracturing and delamination caused by volume expansion during cycling. Ex situ and operando XRD analysis revealed that the Ga-Sn electrode crystallises as Ga-Sn liquid metal—Li2 Ga7 —LiGa—Li2 Ga. A solid solution is produced when lithium is introduced into a LiGa crystal. Only at extremely low current or extremely high temperature Li2 Ga can be produced. Both operando XRD and CV testing showed that the 8 percent Sn did not engage in the electrochemical process to produce Lix Sn alloys reversibly. By employing a PEO-based solid electrolyte at a temp of 60 °C, the liquid metal anode demonstrated reversible Li insertion and extraction as well as increased cycling capacity to the control cells with Sn and MCMB anodes. The active material’s mass ratio is another element that impacted its cycling efficiency [86]. Furthermore, Qu et al. [87] investigated the electrochemical behaviour of V2 O5 . 0.6H2 O nano-ribbons in three neutral aqueous electrolytes (0.5 mol /L Li2 SO4 , Na2 SO4 , and K2 SO4 ), as well as the structure and composition changes of V2 O5 . 0.6H2 O electrodes in charging-discharging process. The findings reveal that alkaline metal ions are critical in the electrochemical response of the V2 O5 . 0.6H2 O electrode. Intercalation/de-intercalation of potassium ion into/from the inter layered region of V2 O5 . 0.6H2 O is the easiest of the three alkaline metal cations (Li-ion, Na ion and K ion), resulting in the greatest capacity in K2 SO4 solution. Because of its high contact with the inter layer H2 O of V2 O5 . 0.6H2 O, intercalation and de-intercalation of Li+ is challenging. These findings indicate that optimising electrolytes may help with the creation of excellent performance batteries, as well as understanding the electrochemical mechanisms of electrode materials (Fig. 7).

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Fig. 7 Different electrochemical behavior and structural changes of V2 O5 ·0.6H2 O electrodes [87]. Copyright 2013, Elsevier

5 Non-aqueous Electrolytes The non-aqueous electrolyte used in rechargeable batteries has a substantial impact on their capacity, rate capability, cycling performance and safety features. As a result, the development of improved electrolytes for SIBs is critical. Typically, electrolytes used in SIBs must first satisfy the following criteria: (a) (b) (c) (d) (e)

Broad electrochemical window, Excellent chemical/electrochemical compatibility with active electrode materials, Wide spectrum of liquids, High ionic conductivity and Minimal toxicity.

Non-aqueous electrolytes for SIBs are typically comprised of one or more sodium salts and additives dissolved in a solution of two or more organic solvents, as shown in Fig. 8 [88]. The electrolytes can classify into three broad categories: chloroaluminate salts, room temperature molten salts/ionic liquids and organic solvent-based systems [89, 90]. Mostly used non-aqueous electrolytes in rechargeable batteries are lithium hexa

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Fig. 8 Chemical compositions of the electrolytes for SIBs

fluoro phosphate (LiPF6 ), but LiPF6 must be mixed with organic carbonates. When ethylene carbonate organic material is mixed with LiPF6 , the physicochemical parameters get enhanced. This was first developed in the early 1990s, but as the need for rechargeable batteries with high density of power has grown, unique combination of non-aqueous salts and organic solvents have begun to emerge in recent years. As a result, a variety of organic solvents other than EC are used, including PC, EPE, fluorinated linear carbonate (F-EMC), fluorinated cyclic carbonate (F-AEC), FEPE, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC). As a result, Zhang et al. [18] proved that mixing 1.2 molar Li hexafluorophosphate in fluorinated ether, EMC and ethyl carbonate at a ratio of 2:6:2 can improve voltage limit and performance. Other salts such as LiCF3 SO3 (lithium trifluoro-methane-sulfonate), LiClO4 (lithium perchlorate), Li(CF3 SO2 )2 N (lithium bis-tri-fluoro-methyl-sulfonyl imide) may be dissolved in ethers that are cyclic or may be linear in nature, such as 1,3-di-oxolane (DOL), THF (tetra-hydro-furan), TEGDME (tetra(ethylene–glycol) dimethyl Ether), DME (di-methoxy-ethane) for the sake of the following generation’s performance of EC [91]. Since the chosen organic liquid solvents satisfy the following criteria, non-aqueous liquid electrolytes have been significantly optimised and extensively used in LIBs [92, 93]. (a) (b)

(c)

The salt has a sufficient dissolving capability. A solvent with a high dielectric constant ensures strong ionic conductivity. Over a wide temperature range, it has a low viscosity. This is necessary to ensure that the solvent exhibits low resistance for movements of ion and maintains its liquid state across a wide range of temperature. High chemical and electrochemical durability. The development of a high energy density cell with outstanding electrochemical performance requires a solvent with high electrochemical stability and strong compatibility between solvents and electrodes [46].

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NaCl-AlCl3 was developed in the 1970s [94]. Na+ may be replaced for K+ in these systems, and molar ratio of MCl:AlCl3 can be adjusted. The greater the aluminium salt concentration, the more acidic the solution, and the main charge carrier is therefore moved away from AlCl4 − anions and toward Al2 Cl− 7 anions. Furthermore, both complexes are capable of discharging on the cathode, allowing for aluminium deposition on the cathode. The dependency of the electrode exchange current density on the melt composition (mole percentage of AlCl3 ) was established for the aluminium ion charge transfer method, which is a critical characteristic. As a result, a mechanism involving several electrode reactions was suggested [95, 96]. Additionally, studies have shown that the aluminium chloride (AlCl3 ) concentration has a significant impact on the look of aluminium deposits obtained. Thus, the deposited layer’s dendritic development may be manipulated and overridden. To achieve this desired property, either the current density must be carefully regulated or an organic suppressor such as tetraethyl ammonium chloride or urea must be considered [97]. On the other hand, inorganic additives such as PbCl2 , SnCl2 and MnCl2 are of lower significance. Numerous efforts were attempted using different cathodes, including chlorine, sulphur and sulphides. Due to their widespread availability, FeS2 and FeS are mostly used cathode materials. For instance, at 175 °C, an Al/NaClAlCl3 /MeS2 battery system [98] had a high discharge capacity, but with a substantial capacity loss during cycling. The latter was ascribed to the metal sulphides’ solubility in the molten electrolyte. Further research [99, 100] shown that by optimising the electrolyte composition, the operating temperature of the cell could be reduced to 100 °C. Das et al. [101] provide a comprehensive review of the various chloride-based electrolytic systems suitable in secondary aluminium cells, as well as the juxtaposition of acceptable cathode materials [102]. Additionally, the authors identify the high operating temperature of the aforementioned systems as their primary disadvantage and propose that they be supplemented with room temperature ionic liquids (RTILs) as a partial solution to this issue. Additional temperature reductions may be accomplished by using just organic chlorides, such as n-butyl-pyridinium chloride, 1-methyl-3-ethylimidazolium chloride and 2-dimethyl-3-propylimidazolium chloride [103]. The aluminium chloride in these RTILs dissolves to form an electrolyte. As a result, it has been suggested to utilise a molten combination of 1,4, dimethyl-1,2,4-tri-azolium chloride and AlCl3 in the secondary battery. AlCl3 and 1-butyl-3 methyl imidazolium chloride are another IL that is often used as an aluminium secondary cell electrolyte [88]. It is shown that by adjusting the Al2 Cl− 7 concentration and therefore the molar ratio of the components, the electrochemical performance of these chlorides may be substantially influenced. Wang et al. [104] investigated the same cation-based ionic liquid– AlCl3 electrolytes that included Cl, Br and I counter ions as their organic component. The chlorides had the largest stability window (4.7 V), while the bromides had 3.9 V and the iodides had the smallest (only 2 V). The ionic conductivity of these additive electrolytes followed a similar trend and it was discovered that the stability is composition dependent. For example, for Al:BMI molar ratios ranging from 1:1 to 2:1, the anode limiting potential was determined to be 2.6 V against Al/Al3 + , whereas it was restricted to 1.75 V for a ratio of 0.8. In another research, Wang et al. [105]

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successfully tested a superior quality natural graphite cathode in a cell using the commercially available electrolyte AlCl3 /[EMIm]Cl. The molar ratio of AlCl3 to 1-ethyl-3-methyl imidazolium chloride ([EMIm]Cl) was set at about 1:3, and the electrolyte was synthesised by simple mixing. Zafar et al. [106] tested the same electrolyte against a CMK-3 mesoporous carbonbased cathode, resulting in a device dubbed “superlong-life”. The battery’s stability was found to be greater than 36,000 reversible cycles, with a CE = 97%. Additionally, organic solvents such as toluene, benzene, dichloromethane (DCM) and 1,2dichloroethane (DCE) were used to alter the performance of the ionic liquid-based aluminium electrolytes using AlCl3 /Et3 NHCl. Xia et al. [107] examined compositions with solvent: electrolyte molar ratios ranging from 0.1 to 1. DCM and DCE were shown to be superior than the two aromatic compounds, with a volume ratio of 0.2 being identified as the optimal value. It was discovered that by using these techniques, the performance of the aluminium secondary battery may be significantly enhanced. It is worth noting that the present efficiency of aluminium deposition and dissolution in quaternary ammonium chloroaluminate-based ionic liquids is between 85 and 100%. Unfortunately, haloaluminate-based ionic liquids have a number of disadvantages, as outlined by Das et al. in [95]. They are corrosive, viscous and hygroscopic in nature. As a result, they explored halogen-free versions of the RTIL electrolyte bases. On the other hand, there have been just a few reports of their usage in batteries too far. Additionally, Wang et al. [108], created a noncorrosive and water-stable electrolyte using aluminium tri-fluoro-methane sulfonate (Al[TfO]3 ) and 1-butyl-3-methyl imidazolium. In an Al-V2 O5 cell, the solution was evaluated. 1-butyl-1-methyl-pyrrolidinium bis(tri-fluoro-methane-sulfonyl) imide and 1ethyl-3-methyl-imidazolium bis(tri-fluoro-methane sulfonyl) imide were suggested as Al-cell electrolytes in the same research article [109] (Fig. 9). Additionally, Kitada et al. [110] demonstrated the reversible electrodeposition of Al from an AlCl3 solution in a diglyme (1:5 molar ratio). Their study was expanded

Fig. 9 Treated and untreated Al deposition/ dissolution on Al anode. Ref. [109]. Copyright 2016, ACS

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to include additional ether-based electrolytes, including triglymes, tetraglymes and butyldiglyme. According to the authors, glyme-based systems may also be thought of as ionic liquids, which sound like a rather strange assertion. Reed et al. [111] used a diglyme solution of Al[TfO]3 as the electrolyte. The second case findings are partly analogous to the former, since reversible Al deposition was verified here as well, albeit for just a few initial electrochemical cycles. Later, it was discovered that extreme passivation of the Al electrode was impeding the desired process. Furthermore, Zhang et al. [112] synthesised a novel Al(ClO4)3 -based electrolyte by dissolving a thoroughly dehydrated salt in a propylene carbonate (PC)-fluoroethylene carbonate combination. This resulted in a battery with high discharge voltage plateaus (about 1 V), a high rate capacity, and a long cycle life (more than 400 cycles). The authors assert that the battery is an unique “dual ion” design owing to the parallel plating of aluminium on the anode and the intercalation of the ClO− 4 anions into the specifically constructed 3D graphene-based cathode. Finally, it is necessary to emphasise that immobilisation of the RTIL electrolyte in a polymer gel results in an undesirable complexation of the chloroaluminate anions with the polymeric chains. This undesirable property occurs with PEO, PAN, PMMA and PFdF. On the other hand, Sun et al. [113] effectively utilised PAAM for this purpose in a [EMIM]Cl-AlCl3 system.

5.1 Advanced Non-Aqueous Electrolytes Non-aqueous electrolytes have been researched as an alternative to conventional aqueous electrolytes due to issues such as water evaporation or ambient moisture uptake, bicarbonate production, a narrow electrochemical window and very short battery life. Unfortunately, there has been very little research carried out in secondary ZABs. Secondary zinc anodes exhibit exceptionally remarkable outcomes in nonaqueous electrolytes. Guerfi et al. [114] employed an organic electrolyte including propylene carbonate and 0.3 molar fluoro-based salts to achieve over 1700 cycles at η = 99.8% charge/discharge rate of 1 °C. A noval ZAB system should, in general, have the following desired characteristics [75, 115]. • Non-aqueous electrolyte conducts Zn2+ ions with high conductivity. • Non-volatile solvents. • Low-toxic compounds. At the air cathode of aqueous ZABs, the well-known 3-phase reaction at boundary occurs. The oxygen reduction process is enabled by the interaction of two interpenetrating sub-systems: a hydro-phobic sub-system for O2 diffusion and hydrophilic μ-channels for metal ion transport and reaction site formation (ORR). Due to the fact that the most of electrolytes easily soak the entire pores electrode, filling the air channels and a major barrier to the functioning of air cathodes are non-aqueous electrolytes. As a result, in a two-phase border reaction zone only dissolved O2 take part in ORR. Dissolved oxygen has a mobility that is at least ten times lesser than

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gaseous oxygen. Increased oxygen pressure reduces this problem (enhancing specific capacity) by increasing liquidity and the concentration in electrolytic solution and de-wetting of certain pores of cathode, resulting in the development of favourable 3-phase reaction zones across the air electrode [116, 117]. The redox process and end products of the Li/FeS2 and Na/FeS2 batteries, first discharge have been thoroughly documented. The initial discharge comprises two equal capacity stages, first converting (S–S)2 into S2 and then converting Fe2+ into Fe, resulting in metallic Fe and Li2 S (or Na2 S for Na/FeS2 cells). Following recharging and cycling, two conversion processes occur: one between Fe and Fe2+ for the reduced voltage plateau and another between Li2 S and Li poly-sulphides or elemental sulphur for the higher voltage plateau. The majority of the debates surrounding Li/FeS2 cells have centred on the redox process and redox intermediate of the higher voltage plateau. Numerous experimental findings suggest that the meta-stable Li2 FeS2 transition is improbable to occur, and that the larger voltage plateau is caused by a breakage–recombination process of the (S–S)2 bond instead of an intercalation/deintercalation process of the lithium ion. Following the discharge in starting, the Li/FeS2 cells transform into a hybrid of Li/S and Li/FeS cells. The slow transition kinetics and development of iron particles or even dendrites in the Li/FeS cell, as well as the decomposition of long-chain Li poly-sulphides into the liquid electrolyte and the subsequent parasitic reactions in the Li/S cell, are responsible for the majority of the problems with the Li/FeS2 cells including inadequate rate capability, bad reproducibility, and rapid capacity fading [118]. Meanwhile, First Emission-SEM analysis was used to look into the networked structure of a synthetic nano-composite with intermediate morphology of nano-fibres (poly N-methyl aniline) and spheres (Lithium). After 100 charging and discharging cycles, the power density of the battery was measured to be 119.233 W/m3 . As an electrolyte, the new material has showed tremendous promise as a viable material for use in batteries [85]. The localisation of +ve charge in the molecules was verified in carbonatebased electrolytes, resulting in limited reversibility of the O2 radical. The nucleophilic O2 radical Li-air batteries must breakdown the electrolytes. The nitrile- and piperidinium-based electrolytes, on the other hand, demonstrated strong O2 radical reversibility since all of the atomic charges in molecules and cations were either negative or almost zero. The electrical distribution of electrolyte solvents was discovered to influence their electrochemical stability against O2 radical. Piperidinium-based ionic liquid was chosen as an electrolyte solvent due to its electrochemical and chemical durability against Li metal. In the discharge–charge profiles, the cell with piperidinium-based electrolyte obtained a much lower charging voltage of about 3.2 V and a lower voltage gap of around 0.75 V when compared to conventional cells with carbonate-based electrolyte. As a result, the charging capabilities had a significant impact on the durability of O2 radical of the electrolyte solvent [119]. The charging capability of LABs was enhanced by raising the test temperature and utilising a Li2 O2 -based cathode at a current density (J) of 100 mA g−1 . Electrochemical impedance spectroscopy shows that charge-transfer resistance reduces with increasing temperature, and X-ray diffraction (XRD) confirms oxidative breakdown of Li2 O2 during charging operations. Even at a current density of 500 mA g−1 ,

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the charging voltage plateau (3.6 V) is not excessively high for the Li2 O2 -based cathode in the absence of a catalyst [120]. By carefully accounting for the electrochemical processes during discharging (OER and ORR) and the resulting species or charge movement across different parts of a Li-air cell, a 3-D transient lithium-air model was created. A considerable focus was placed on the precision of predicted Li2 O2 development inside the air electrode, as well as the consequent potential losses due to activation, ohmic, and concentration polarisations. A comparison of predicted and actual voltage development curves, as well as detailed profiles of Li+ , oxygen, Li2 O2 , and the electrolyte percentage in the air electrode, clearly demonstrated that the air electrode surfaces near the membrane interface were not used for ORR throughout the discharge operation [121]. The primary cause of rechargeable Li-air battery performance degradation is the solvent molecules instability toward OR species. One requirement for the use of Li-air batteries is the identification of suitable electrolyte solvents. Meanwhile, Wang et al. [122] proposed N-methyl-2pyrrodione (NMP) as a non-aqueous electrolyte for Li-air batteries. Despite the fact that the presence of Li salt has a significant effect on the behaviour of ORRs and OERs, the fundamental electrochemical reaction is a one-electron shifting mechanism that produces O2 /O2 − pairs. Due to the high acidic nature of lithium, the initial product, i.e. LiO2 is disproportionate to the main discharge product Li2 O2 . When charging takes place, Li2 O2 may breakdown at a high over-potential and gold atoms would accelerate Li2 O2 oxidation, without passivation on the surface of gold electrode shown by many cycles. In NMP-based electrolyte, it has been observed that there exist nucleophilic interactions between O2 − and the solvent molecules. Because of NMP’s which makes it chemically stable against OR species, NMP-based cells function well. Furthermore, avoiding NMP breakdown on the surface of porous air electrodes is critical for the future use of NMP-based electrolytes in secondary LABs with excellent cycle efficiency. A novel family of electrolytes has been developed by combining aqueous and non-aqueous solvents, which inherit the non-flammability and non-toxicity of aqueous systems as well as improved electrochemical stability from non-aqueous systems. The secondary interfacial ingredient (alkyl-carbonate) introduced by the non-aqueous component contributes to the expansion of the hybridised electrolyte’s electrochemical window to 4.1 V, allowing a 3.2 V aqueous Li-ion battery based on Li4 Ti5 O12 and LiNi0.5 Mn1.5 O4 to provide a high energy density of 165 Wh/kg for more than 1,000 cycles [123]. Because of the less volatility of propylene carbonate (PC) and tris-(2,2,2-tri-fluoro-ethyl) phosphate (TFP), the PC/TFP electrolyte is an excellent option for long period of operation of LABs in dry atmospheric conditions. Despite the fact that the use of TFP raises the solvent’s viscosity and decreases the conductivity of ions of the electrolyte, the LAB with PC/TFP electrolyte exhibits higher specific capacity and better performance. Furthermore, the use of TFP benefits LABs that work in settings with low oxygen partial pressure. TFP’s improved performance is due to the enhanced dissolution kinetics and oxygen solubility in TFP-containing electrolytes. Furthermore, since PC/TFP electrolyte has an electrochemical window of 5.15 V which is appropriate to ustilise in secondary LABs [124]. Furthermore, Kolomoiets et al. [125] proposed Mg(ClO4 )2 in glyme-based solutions,

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for the next design of advanced magnesium (Mg) batteries. These electrolytes are very conductive and electrochemically stable. Furthermore, Mg(ClO4 )2 -based electrolytes are compatible with MnO2 -cathodes up to a high voltage of 3.5 V against Mg/Mg2 + . Organic electrolytes have the ability to overcome a variety of drawbacks, including hydrogen evolution [126], dendrite development, electrolyte dry-out owing to water evaporation and alkaline electrolyte carbonation. They have a wider temperature range [127] and an electrochemical window that is quite large. However, to prevent possible issues with non-aqueous MABs such as its volatile nature, high flammability, and toxicity, the proper composition must be used [128]. This is why the electrolyte in metal-air batteries has historically been a liquid solution (usually water-based), allowing ion transfer between the metal and air electrodes [129]. On the other hand, patented fluorinated compounds may be used in non-aqueous electrolytes as co-solvents for MABs to improve oxygen solubility [130].

6 Polymer Electrolytes Polymer electrolytes are ionically conducting solid phases formed by the dissolution of salts by ion-coordinating macromolecules that are free of low molecular weight additions or contaminants [131]. Fenton et al. [132] invented the polymer electrolyte in 1973, and its technological importance was recognised in the early 1980s. Numerous researchers have been motivated to develop new PEs during the past three decades by their prospective use in electrochemical/electrical power generation, storage, and conversion systems [133]. A polymer electrolyte (PE) membrane is made up of salts dissolved in a heavy molecular weight (wt) polymer matrix [134]. Because this solid solvent-free solution has ionic conduction properties, it is extensively used in electrochemical devices like solid-state batteries and rechargeable batteries, particularly in lithium ion batteries. Super-capacitors, dye-sensitised solar cells, fuel cells, secondary batteries, electrochemical sensors and analogue memory devices are just a few of the innovative electrochemical, electro-chromic, and electrical devices in which PEs may be used [135, 136]. Polymer electrolytes are significant because they are intrinsically safer than certain traditional electrolytes like LiPF6 /ethylene carbonate. These have the potential to be multifunctional and are suitable with open systems such as fuel cells. The electrolyte of today’s lithium ion batteries, which power mobile electronics and certain electric cars, is a salt dissolved in volatile organic solvents. The burning of the electrolyte is a common cause of catastrophic failure of lithium ion batteries [136]. PE may be classified into two types depending on its sources and origins: (i) natural and (ii) synthetic. Chitosan [137, 138], rice starch [139, 140] and maize starch [141, 142] are examples of natural PEs that have been studied extensively. PEs may also be classified into three categories depending on their physical condition and composition, as shown in the Fig. 10.

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Fig. 10 Classification of polymer electrolytes on the basis of sources and physical condition

6.1 Gel Polymer Electrolytes Feuillade and Perche developed gel polymer electrolyte (GPE), also known as plasticised PE, in 1975 [143]. GPE is a gelled polymer matrix that is swelled in a liquid electrolyte by the addition of plasticiser [144]. GPE is readily made by heating a combination that includes a polymer matrix, such as poly (ethylene oxide) (PEO), an alkali-metal salt, such as Li-salt, and a solvent. The viscous clear liquid combination is then cast in a hot condition and cooled to produce a thin film under the pressure of electrodes [133]. PVDF (Poly(vinylidene fluoride)) is a more common GPE because of its additional benefits, such as safety, stability, and low volatility. The PVDF-HFP GPEs have a porous structure illustrated in Fig. 11, may assist to achieve conductivity of up to 1.03 mS/cm with temperature stability up to 350 °C, which provides excellent safety characteristics [145]. The other two well-known GPEs are poly-acrylonitrile and poly (methyl-methacrylate) (PMMA) [146]. Yang et al. [147] demonstrated that adding poly (methyl-methacrylate) gel polymer electrolytes may increase conductivity up to 2.31 × 10−3 Scm−1 while also improving strength. Additionally, Balo et al. [148] described the process of synthesis and their characteristics of a gel polymer electrolyte (GPE) based on the polymer polyethylene oxide (PEO), the salt Li bis(tri-fluoro-methyl-sulfonyl) imide, and various amounts of 1-ethyl-3-methylimidazolium bis(tri-fluoro-methyl-sulfonyl) imide are added. Differential scanning calorimetry, thermo-gravimetric analysis, EIS, ion transfer number tests, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) studies, all show that manufactured GPEs have promising features for use in lithium polymer batteries (LPBs). The GPEs have a high thermal stability (i.e. no weight loss up to 310 °C) and a high ionic conductivity (2.08 × 10−4 S cm−1 at 30 °C). They also have a high lithium transference number (=0.39) and a excellent electrochemical stability window (~4.6 V). By sandwiching the top performing GPE between LiMn2 O4 cathode and a Li-metal anode, a minimum cost simple thermal lamination technique was used to enclose the entire LPB assembly. The constructed cell demonstrated good electrochemical performance

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Fig. 11 SEM PVDF-HFP GPEs (a, b) front side, (c, d) back side, (e, f) cross section views. Ref.[145]. Copyright 2014, Springer Nature

in galvanostatic charge/discharge cycles. Furthermore, Karuppasamy et al. [26] introduced the nonaflate anion-based ionic liquid and lithium salt into gel polymer electrolyte chemistry for the first time as an ionic conductive additive in the polymer matrix. The GPE enclosed in a lithium nonafluoro-1-butanesulfonate LiNfO/IL electrolyte combination showed good thermal and electrochemical stability. The selfstanding, flexible GPE film exhibited a sufficiently broad electrochemical window (5.4 V) and a strong ionic conductivity, reaching a maximum conductivity of 10−2 S cm−1 at 100 °C. The cathode has a greater discharge capacity of 164 mAhg−1 at room temperature and excellent capacity retention up to 45 cycles at the C/10 current rate. The ILGPE’s safe nature, high conductivity and broad electrochemical window make them a viable option for use in lithium ion batteries alongside lithium anode and LiCoO2 cathode.

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6.2 Solid Polymer Electrolytes (SPE) SPEs are made up of lithium or sodium salts spread in a polymer matrix, which is typically based on polyethylene oxide (PEO). Wright et al. [149] pioneered solid polymer electrolyte (SPE) research three decades ago. The technical use of SPEs in electrochemical devices has been confirmed by Vashishta et al. [150] the first SPE studied was a “dry solid” polymer electrolyte based on PEO. This is a solvent-free system that does not utilise any organic liquid. The ionic conductivity of this PEObased SPE was low at room temperature, resulting in poor performance [151]. To improve the ionic conductivity in EC, SPE like PEO (Poly(ethylene oxide)) may be combined with lithium salts such LiTFSI, LiTf, LiBETI, LiBOB and LiClO4 [152]. To achieve optimum ionic conductivity, nanoceramic material may be combined with SPEs such as PMA/PEG [153]. Sarangika et al. [154] developed solid polymer electrolytes (SPEs) based on polyethylene oxide (PEO) complexed with magnesium triflate Mg(Tf)2 or Mg(CF3 SO3 )2 ) and integrating the ionic liquid (IL) (1-butyl-1methylpyrrolidinium bis(trifluoromethan esulfonyl)imide (PYR14 TFSI). Electrical conductivity, cationic transport number tests and CV were used to optimise and characterise the electrolyte. At ambient temperature, the maximum conductivity of the PEO/Mg(Tf)2 , 15:1 (molar ratio), electrolyte was 1.19 × 10−4 Scm−1 , which was enhanced to 3.66 × 10−4 Scm−1 by adding 10 wt.% ionic liquid. With increasing ionic liquid concentration in the PEO-Mg(Tf)2 electrolyte, the Mg2+ ion transit number increased significantly. At the optimum electrolyte composition, the highest Mg2+ ion transit number was 0.40. The Mg/ and [(PEO)15 :Mg(Tf)2 + 10%IL]/TiO2 -C batteries were constructed and analysed. According to preliminary tests, the battery’s discharge capacity was 45 mAh/g.

6.3 Composite Polymer Electrolytes In order to address the drawbacks and limits of SPEs, a new kind of material, composite polymer electrolytes, was developed (CPEs). For the preparation and design of CPEs, researchers have used a variety of methods. Polymer blending [155, 156], cross-linking polymer matrices [157, 158], comb-branched copolymers, binary salt systems, additives such as plasticisers [159], doping of nonmaterials [160], impregnation with ionic liquids [161, 162] and reinforcement by inorganic fillers [163, 164] are some of the techniques used. Lithium batteries using traditional organic liquid electrolytes have been found to have a number of safety issues in recent years. As a result, solid polymer electrolytes are being studied as potential candidates to replace presently available organic liquid electrolytes in lithium batteries, because to their form variability, flexibility, low weight, and reduced processing costs. However, in these promising solid polymer electrolytes, poor ion transport and mechanical performance remain a problem. Polymers are being coordinated with other components, such as liquid electrolytes, polymers and inorganic fillers,

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to create polymer-based composite electrolytes, to address these issues and enhance overall comprehensive performance [165]. Electrolytes based on lithium hectorite and carbonate solvents were shown by Riley et al. [166] to be promising candidates for use in lithium-ion batteries. The performance of these electrolytes is dependent on hectorite dispersion and lithium ion solvation. Because the lithium ions get stuck between the clay platelets due to incomplete dispersion, they have limited mobility. In these systems, room-temperature conductivities as high as 2 × 10–4 S/cm have been reported with excellent hectorite dispersion. Patel et al. [167] studied on the development of a soft matter solid composite electrolyte by incorporating a polymer into a semisolid organic plastic lithium salt electrolyte. In comparison to lithium bis-tri-fluoro-methane-sulfonimide succinonitrile (LiTFSI-SN), the (100 − x)% − [LiTFSI-SN]: x% − P Polyacrylonitrile (PAN), polyethylene oxide (PEO) and polyethylene glycol dimethyl ether (PEG) composites exhibit increased ionic conductivity at ambient temperatures, increased mechanical strength, and a relatively broad electrochemical window. At 25 °C, the ionic conductivity of 95% −[0.4 M LiTFSI-SN]: 5% − PAN was 1.3 × 10−3 −1 cm−1 , more than double that of LiTFSI-SN. The Young’s modulus (Y) increased from Y → 0 for LiTFSI- SN to 1.0 MPa for samples containing (100 − x)% − [0.4 M LiTFSI-SN]: x% − PAN. For composites, the electrochemical voltage window was approximately 5 V (Li/Li+ ). Without the use of a separator, excellent galvanostatic charge/discharge cycling performance was obtained with composite electrolytes in Li/LiFePO4 cells. The effect of dispersion of micron-sized magnesium oxide particles on a magnesium-ion (Mg2+ ) conducting GPEs based on poly-(vinyl-idene-fluoride–cohexafluoropropylene) (PVDF–HFP) has been investigated utilising a variety of electrical and electrochemical methods. The composite gel films are self-supporting, flexible, and provide sufficient mechanical strength. At ambient temperature, the optimum composition with 10% MgO particles provides a maximum electrical conductivity of 6 × 10−3 Scm−1 (at 25 °C). Cyclic voltammetry, impedance spectroscopy and transport number analyses all validate the Mg2+ ion conduction in the gel film. The application of the composite gel electrolyte to a rechargeable battery system was investigated [168] by constructing a prototype cell with negative and positive electrodes of Mg (or Mg–MWCNT composite) and V2 O5 , respectively. When Mg metal was replaced with a Mg–MWCNT composite as the -ve electrode, the cell’s rechargeability increased (Fig. 12). The electrochemical results of a variety of composite electrolytes show that the insertion of a ceramic component in a polymer matrix leads to better conductivity, increased Li-ion transport number, and enhanced electrode–electrolyte interfacial durability. Conductivity increases with increasing ceramic phase weight percentage, processing conditions, type of polymer-ceramic combination, and temperature. Additionally, the inclusion of ceramics enhances the effective glass transition temperature, decoupling structural and electrical relaxation modes, thus increasing the Li transport number. Additionally, the ceramic additives provide a variety of free energy interactions with lithium [169]. Mechanical milling was used to make the 75Li2 S(25 − x)P2 S5 ·xP2 O5 oxysulfide glasses. The oxysulfide glasses included oxysulfide units

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Fig. 12 Variation of conductivity of CGPE at ambient temperature. Ref. [168]. Copyright @2011, Elsevier

such as PO2 S2 , in which a phosphorus atom is adjoining by both sulphur and O2 atoms. The conductivity of ions of oxysulphide glasses reduced as the P2 O5 concentration increased. The rate of H2 S production from air-exposed glasses was reduced when P2 O5 was partially replaced with P2 S5 . Furthermore, adding 10 mol% ZnO to the x = 10 glass reduced H2 S gas leakage into the atmosphere. As a lithium secondary battery, the all-solid-state In/LiCoO2 cells with Li2 S–P2 S5 –P2 O5 –ZnO composite electrolytes were used. The cell using a composite electrolyte made up of 90 mol % 75Li2 S·21P2 S5 ·4P2 O5 glass and 10 mol% ZnO had excellent performance and maintained a capacity of 75 mAh/g after 50 cycles [170]. Composite polyethylene oxide/garnet electrolytes with LiTFSI as the Li salt have a Li+ conductivity greater than 10–4 S cm−1 at 55 °C and a low plating/stripping impedance of a dendrite-free Li-metal anode designed for a solid-state Li-metal rechargeable battery. Hot-pressing is used to manufacture composites ranging from “ceramic-in-polymer” to “polymer-in-ceramic” that are versatile and mechanically robust. The fabrication of pouch cells that are safe with exceptional flexibility has been accomplished. At 0.2 and 55 °C, solid-state LiFePO4 |Li batteries with “ceramic-in-polymer” and “polymer-in-ceramic” electrolytes exhibit high cycling stability combined with high discharge capacities (139.1 mAh g−1 which retain 93.6% capacity after 100 cycles) and high capacity retention (103.6% with 100% CE after 50 cycles). Both electrolytes are compatible with solid-state lithium batteries [171]. Solid-state lithium-metal batteries seem to be a viable next-generation energy storage technology. However, the major bottlenecks are the solid electrolyte’s low conductivity and the high interfacial resistance. Although polymers have a lower interfacial resistance than ceramics, they often need the addition of volatile solvents (Fig. 13). Dai et al. [172] proposed a highly conductive composite polymer electrolyte (CPE) membrane composed of a PVDF, a lithium-conductive perovskite (i.e.,

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Fig. 13 Schematic diagram for PEO-LLZTO CSE: a “ceramic-in-polymer”; b “intermediate”; c “polymer-in-ceramic” Ref. [171]. Copyright @ 2018, Elsevier

Li0.38 Sr0.44 Ta0.70 Hf0.30 O2.95 F0.05 , a fire-retardant solvent and (TMP) tri-methyl phosphate. Conductivities of the CPE membrane containing 10 wt% LSTHF are as high as 0.53 mS/cm at ambient temperature and 0.36 mS cm−1 at zero degrees Celsius (0 °C). Additionally, model batteries, including those containing the CPE-10 electrolyte, demonstrate excellent discharge capacities initially, high rate capabilities, and durable cycle efficiency at room temperature or between 5 and 60 °C. This research demonstrated that incorporating a lithium-conductive perovskite and TMP into a PVDF-based polymer material may result in safe, high-performance quasi-solid-state Li-metal batteries with a reasonably broad temperature range of operation. A ceramic electrolyte of the NASICON type has been explored as a possible option for solid-state electrolytes with high conductivity at ambient temperature and good stability in the presence of oxygen. Shi et al. [173] showed NASICON-type Li1.3 Al0.3 Ti1.7 (PO4 )3 (LATP) dispersed in poly(vinylidene fluoride) (PVDF) to produce LATP/PVDF composite electrolyte membranes (CEM) through the casting technique. The effects of the LATP filler on the LATP/PVDF CEM structure, morphology, ionic conductivity and stability were investigated. The LATP/PVDF CEM demonstrated outstanding lithium ion conductivity and electrochemical stability, with an electrochemical stability window of up to 5.67 V against Li+ /Li. Lithium iron phosphate cells built with LATP/PVDF CEM had a discharge capacity of 163.5 mAh g−1 and maintained over 95.7% of their original capacity after 50 cycles. The findings indicate that when LATP filler is added to PVDF matrix, the amorphous phase of the polymer is enhanced by serving as a gel centre, and the concentration of Li+ is increased, which benefits synergism in Li+ migration and improves ionic conductivity.

6.4 Advanced Polymer Electrolytes Kataria et al. [174] proposed various methods to manufacture and analyse NPGEs (nano-composite polymer gel electrolytes) based on polymer PVDF-HFP,

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ionic liquid, 1-butyl-3-methyl-imidazolium bis (tri-fluoro-methane-sulfonyl) imide (BMIMTFSI), lithium-salt and SiO2 . The prepared NPGEs exhibit a high ambient temperature ionic conductivity (103 S/cm) and a broad electrochemical window (3.3–3.5 V ). By sandwiching the highest efficient NPGEs b/w a LiFePO4 cathode and a Li-metal anode, the galvanostatic charge/discharge profile was investigated. The specific discharge capacity of the battery (Li/NPGE/LiFePO4 ) at ambient temp is 138 mAh g−1 at the rate of 0.1 C. Meanwhile Guo et al. [175] proposed a freestanding IL-based PE film of ILGPE as the electrolyte for Li/LiFePO4 battery with operative fillers of LAGP. For 10% LAGP loading at 25 °C, the ILGPE has a good ionic conductivity (0.76 × 10−3 Scm−1 ) and a broad electrochemical window of around 4.8 V versus Li+ /Li. In comparison to inert fillers, LAGP particles not only efficiently decrease the crystalline structure of the polymer matrix, but the filler can also supply Li+ ions and serve as a Li+ ion conductor, resulting in a excellent conductivity of ions and Li+ ion transference No. Furthermore, ILGPE-10% LAGP has been shown to have excellent stability against heat and no combustibility, allowing it to be utilised in rechargeable batteries with better protective characteristics. The Li-metal batteries have a high capacity of discharge and cycle performance due to their excellent electrochemical durability and suitable with Li electrodes. As a consequence of these findings, ILGPE-x percent LAGP was identified as a viable and another electrolyte for the safer and significant efficiency of solid-state Li-metal batteries. An adhesive bonding solid polymer electrolyte was proposed by Dong et al. [176] for rechargeable zinc-ion batteries and showed strong anti-ageing effects as well as a broad electrochemical window. The ionic conductivities may be raised to 3.77 × 10−4 S cm−1 by increasing the quantity of plasticiser (PC). Hydrated hydro-gels are proposed by Tran et al. [177] for rechargeable ZABs on the basis of ion conduction, chemical durability, electrochemical windows and mechanical characteristics. Three different types of hydro-gel networks, i.e. poly PVA (vinyl alcohol), PAA (poly-(acrylic acid)) and PAM, were used due to their chemical mixture and the presence of various charged functional groups within the polymer network. The alkaline solutions with a concentration of 6 molar, PVA, PAA and PAM are stable chemically; after water removal, the GPEs exhibit a durable electrochemical window of 2 V, which is adequate for zinc-air batteries. Furthermore, PAA with 6 molar KOH exhibits the greatest mobility of ions which rises with temperature. The high hydrophilicity of PAA 6 M is most likely to blame for this behaviour. In general, ionic conductivity and mechanical stiffness have an inverse relationship. At 20 °C, the conductivities were 161, 204 and 21 mS/cm of PAA, PVA and PAM with 6 molar KOH, respectively, while their compacting moduli were 30.9, 5.4 and 2704.4 kPa. On testing of a fully charged cell it is confirmed that the battery using PAM of 6 molar as the electrolyte failed after many cycles, while the batteries with PVA 6 M or PAA, 6 M as the electrolyte sur-passed batteries with standard aqueous 6 M KOH in terms of cycling number. Furthermore, at 0.5 mA/cm, the battery with PAA 6 M electrolyte exhibited the greatest starting discharge/charge η = 79%. The performance may be improved by decreasing the thickness of gel polymer electrolyte; as a result, bulk resistance is reduced (Fig. 14).

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Fig. 14 a Discharge curves with different electrolytes. b Discharge–charge cycling tests at 0.5 mA cm–2 . Ref. [177]. Copyright@ 2019, Elsevier

The most plentiful biomass lignocellulose (LC)-based gel polymer electrolyte (GPE) to the Li–S battery was successfully developed and produced by Song et al. [178]. The GPE based on a membrane with a suitable LC length size (150– 300 μm) exhibits high and excellent overall performance in terms of liquid electrolyte uptake, mechanical properties, ionic conductivity (4.52 mS cm−1 ), Li-ion transference number of 0.79, electrochemical stability window of 5.3 V, compatibility with Li electrodes, and stability against heat. The aforementioned findings lead to a high initial discharge specific capacity and electrochemical reversibility when the GPE is tested using a sulphur cathode and Li anode. The reason for this is due to the stable passivation layer on the surface electrodes and the hydrogen bond between polysulfides and LC matrixes, for all of these remarkable characteristics. As a result, this new GPE is one of the most promising options in green energy storage due to its abundance, biodegradability and ease of production, as well as its outstanding

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overall performance. Lithium rich nickel manganese cobalt oxide cathode material (Li1.2 Ni0.6 Mn0.1 Co0.1 O2 ) and ionic liquid (IL)-based mix gel polymer electrolytes (BGPEs) were synthesised by Srivastava et al. [179]. Solution combustion and solution casting techniques are used to create Li1.2 Ni0.6 Mn0.1 Co0.1 O2 cathode material and BGPEs, respectively. The cathode material is obviously in pure phase, with a well-defined layered structure, as shown by X-ray diffraction (XRD). Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), impedance spectroscopy and linear sweep voltammetry are used to study the thermal, electrical and electrochemical characteristics of BGPEs (LSV). Maximum Li-ion conductivity (Li+ 1.1 mS cm−1 ), thermal (250 °C) and electrochemical stability (4.3 V vs. Li/Li+ ) are found when 70 wt.% IL containing BGPE is used. The cell (Li/70 wt.% BGPE/Li1.2 Ni0.6 Mn0.1 Co0.1 O2 ) has well-defined redox peaks corresponding to Ni2+ /Ni4+ and a specific discharge capacity of 166 mAh g−1 at 0.1 °C-rate with 97% efficiency up to 140 cycles, according to electrochemical studies.

7 Ionic Liquids Electrolytes (ILE) Ionic liquids (ILs) are often made up entirely of ions with melting points below 100 °C. Paul Walden reported the first IL (ethylammonium nitrate) in 1914, and he had no idea that ILs would become a significant research topic nearly a century later [180]. ILEs provide additional benefits such as reduced volatility and increased safety qualities such as non-flammable properties. However, it also has some drawbacks, such as high room temperature viscosity. This influences the movement of ions in electrolytes indirectly. Some of the most famous ionic electrolyte anions include BF4− , PF6− and bis(tri-fluoro-methane sulfonyl)imide TFSI− . On the other side, quaternary ammonium, piperidinium and pyrrolidinium are the anions of the ILEs. The novel tri alkyl imidazolium ILEs was proposed by Jin et al. [181]. Ionic fluids offer distinct and adjustable physiochemical characteristics, including a broad electrochemical window, excellent conductivity of ions, large ranges of liquids, low fumes pressure, highly chemically and thermally stable, non-flammable and non-toxic. ILs is suitable for many electrochemical applications, including batteries, petroleum cells, super capacitors and dye-sensitised solar cells [182]. In both fuel cells and proton-conduction batteries, the high proton conductivity of ILs has been demonstrated. Di-ethyl-methyl-ammonium tri-fluoro-methane-sulfonate ([DEMA][TfO]) has a conductivity of 10 mS cm−1 and is used to display an open circuit voltage of 1.03 V using fuel cells [183, 184]. In the nickel/metal hydride (Ni-/MH) battery, Meng et al. [182] utilised an IL/acid combination for replacing a 30 wt% KOH watery electrolyte, and confirmed the conducting proton nature of the mixture using electrochemical charges/discharges. Acetic acid dilution of ILs has been shown to enhance proton conductivity efficiently. Stable charging/discharging characteristics including low charging-discharging over potentials, an output voltage plateau at approx. 1.2 V, specific capacity of 161.9 mAh/g, and a steady cycling power for an AB5 metal-hydride cathode were achieved

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by using 2 molar acetic acid in 1-ethyl-3-methyllimidazolium acetate. In rechargeable aluminium batteries IL electrolytes are very sensitive to moisture and highly corrosive. To overcome these issues, a 4-ethylpyridine/AlCl3 IL for the Al/graphite RAB is proposed. The optimum composition for graphite capacity (95 mAhg−1 at 25 mAg−1 ) and rate capability was determined to be a molar ratio of 1.3 of AlCl3 to 4-ethylpyridine. In situ synchrotron X-ray diffraction, as well as X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) mapping, confirmed stage-3 of GIC formation at the end of charging and the de-intercalation of AlCl4 during discharging [185]. The 4-ethylpridine–AlCl3 IL demonstrated significantly less corrosive toward Al, copper and nickel electrodes than the conventional EMICl–AlCl3 IL due to the absence of Al2 Cl7 -and the presence of the neutral 4ethylpyridine ligand. Most significantly, the 4-ethyl-pyridine–AlCl3 IL is not susceptible to moisture, allowing charging/discharging of the aluminium/graphite open cell in a natural environment. This type of IL electrolyte could result in RABs with higher reliability, less corrosive damage, and having lesser safety issues [185]. Li et al. [186] developed a unique ternary AlCl3 -Urea-[EMIm]Cl electrolyte that increases the ionic conductivity and coulombic efficiency of Al- graphite battery. The battery showed voltage peaks during discharge vary from 1.9 to 1.5 V and 1.5 to 1.0 V (vs. Al/AlCl4 ) and a specific discharge capacity up to 60 mA h/g after 150 cycles with a excellent CE = 96% ± 1%. In situ XRD, X-ray photoelectron spectra, and other methods have been used to confirm the mechanisms of intercalation/de-intercalation behaviours of the AlCl4 anions. The findings, which uses a cheaper AlCl3 -Urea-[EMIm]Cl electrolyte, indicate a feasible technique to the development of ESS using low-cost Al/graphite batteries. The dissolving of tiny organic compounds into liquid electrolytes remains a difficult task. 1,4-benzoquinone was coupled with four heavy phthalimide groups to produce 2,3,5,6-tetra-phthalimido-1,4-benzoquinone (TPB) as the cathode materials and built into an AI/TPB cell to successfully solve the decomposition problem. As a result, an Al/TPB cell with a capacity of 175 mAh g−1 outperformed a cell with a capacity of 250 mAh/g [187]. Cobalt sulphide (CoS) spheres with 3-D hierarchical porosity, CoSHE , were synthesised by Pan et al. [188] using a solvothermal technique in a mix solvent including equal proportions of water and ethylene glycol for rechargeable magnesium batteries. The resulting CoSHE had substantial porus volume (0.227 cc/g) and specific surface areas (SSAs 27 m2 /g), additionally, flexible architectures are required that enable effective Mg2+ transportation routes and volume expansion during the discharge–charge process. On the other hand, their findings show that adding an IL to an electrolyte containing TBMPOMgCl [2tert-butyl-4-methylphenolate magnesium chloride] and AlCl3 at a mole ratio of 2:1 not only importantly charging-discharging activities, but also increases the specific capacity, possibly by changing the thermodynamics and kinetics of the CoS-Mg reaction. As a result, the CoSHE cathode and the ionic liquid electrolyte additive worked together to generate an excellent capacity (approx 370 mAh/g), strong cycling durability (approximately 340 mAh g−1 after 88 cycles), and significant rate efficiency (about 300 mAh g−1 at 50 mA g−1 ). Additionally, Chellappan et al. [189]

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presented a new non-nucleophilic IL-based electrolyte comprising magnesium bis(di-sopropyl) amide (Mg[(DIPA)2 ]) and 1-ethyl-3-methyl-imidazolium tetra-chloroaluminate ([C2 mim][AlCl4 ]) in a mole ratio of 1:2 for secondary MIBs. For the Mg deposition process, the electrolyte outperforms in terms of reversibility and coulombic efficiency (CE). It is observed that the electrolyte is electrochemically durable up to approx 4.5 V when used with molybdenum (Mo) as the current collector electrode, and there is no corrosion of Mo even after holding it at 4.5 V for 48 h. With a Mo6 S8 cathode, a full-cell test yielded a long-life of over 300 cycles and retains more than 80% capacity. In addition, the electrolyte has been shown to function with a high-voltage V2 O5 cathode as a proof of concept. The new electrolyte’s straight forward, simple and scalable production procedure, together with its good half-cell qualities and full-cell efficiency with model Mo6 S8 and V2 O5 cathodes make it an excellent option for secondary MIBs. With the objective of overcoming the drawbacks of organic-liquid and all-solid electrolytes and optimising their usage in the upcoming generation of safe, secure and high-energy rechargeable LIBs. According to Guo et al. [190], an efficient ionic liquid gel polymer electrolyte is produced easily by utilising core–shell structured SiO2 nano-particles as functional fillers, and its structure and purity are described. The shell layers of SiO2 -PAA@Li serve as lithium ion sources, increasing the electrolyte’s ionic conductivity and lithium transference number. Due to the inherent properties of the silica core structure, its thermal stability and compatibility between electrolyte and Li electrode enhanced. More significantly, morphological examination of the cycled lithium anode demonstrates that a steady solid electrolyte interface may prevent the development of Li dendrites during cycling. Furthermore, the solidstate Li cell LiFePO4 /Li with this type of electrolyte shows stable charge/discharge profiles and acceptable performance, demonstrating the critical significance of its unique structural design in influencing the overall electrochemical characteristics. These encouraging findings indicate that composite GPEs have enormous promise for safe, stable, and even high-power secondary Li-metal batteries. Meanwhile, Singh et al. [191] reported on the synthesis and characterization of free-standing, flexible IL-based gel polymer electrolyte (ILGPE) membranes comprising polymer PVDFHFP, imidazolium-based ionic liquid EMIMFSI and lithium salt LiTFSI. Thermal durability of manufactured membranes is shown up to 200 °C. The ionic conductivity of 40 wt.% IL containing GPE is determined to be 3.8 × 10−4 S cm−1 at 25 °C and 6.0 × 10−4 S cm−1 at 50 °C. At 25 °C, the Li transference number and conductivity of Li-ion of 40 wt.% IL containing GPE having highest values of 0.4 and 1.5 × 10−4 Scm−1 , respectively, with an electrochemical window of 4.7V against Li/Li+ . The GPE containing 40 wt.% IL is utilised in battery applications because it is more compatible with lithium electrodes than other produced ILGPEs. At 0.1C, the discharge capacity reached its maximum of 141.2 mAh/g at 25 °C and 160.3 mAh/g at 50 °C, respectively. Up to 100 cycles at 50 °C, about 99% Coulombic efficiency is achieved. These findings suggested that the Li/40wt.% IL having GPE/LiFePO4 battery exhibits a excellent CE, a high charge–discharge capacity, and cyclic durability up to 100 cycles (Figs. 15 and 16).

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Fig. 15 Schematic diagram of manufactured ILGPE membrane. Ref. [191]. Copyright@ 2018, Elsevier

Fig. 16 Conductivity of ions of the ILGPE membranes with different wt% of EMIMFSI at 25 °C and 50 °C. Ref. [191]. Copyright @2018, Elsevier

8 Hybrid Electrolytes Hybrid electrolytes are a new type of solid electrolyte that could be a viable replacement of liquid electrolytes. Finding appropriate electrolytes that are compatible with the Mg metal anode and allow high coulombic efficiency and extended cycle life is a significant roadblock in the development of rechargeable magnesium batteries (RMBs). Since ether oxygens may align to Mg2+ ions in the presence of different Mg

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salts to enable reversible Mg electrochemistry, ether-based solvents have been extensively investigated as Mg electrolytes [192]. Ionic liquids (ILs) have a very low vapour pressure and a very low flammability, which gives them a great stability against heat [193]. While many ILs are costly, their use as co-solvents in an organic medium may help decrease costs, enhance heat stability, expand the electrochemical window and enhance the electrolyte’s conductivity [194]. Furthermore, Ma et al. [195] studied the electrochemical reversibility of magnesium in hybrid electrolytes composed of IL and glyme-based organic solvents for use in RMBs (rechargeable magnesium batteries). The addition of magnesium di-[bis(tri-fluoro-methane-sulfonyl) imide] (Mg[TFSI]2 ) (0.3 M) to N-butyl-n-methyl-pyrrolidinium bis-(tri-fluoro-methanesulfonyl) imide [C4mpyr][TFSI]/tetra-glyme at a mole ratio of 1:2 demonstrated steady CV cycling for almost three hundred cycles, while observed by scanning electron microscopy (SEM) and X-ray. Further thermo-gravimetric analysis (TGA) revealed that this electrolyte retained 79% of its mass at 250 °C, indicating that the inclusion of the IL significantly improves heat stability, which makes these hybrid electrolytes appropriate for RMBs [195]. Organic and inorganic HSE (hybrid solid electrolytes) are anticipated to combine the advantages of both parts in order to overcome the difficulties associated with producing rapid ion movement and high stability in energy storage applications. Zheng et al. [196] developed Li10 GeP2 S12 (LGPS)—PEO (bis (tri-fluoro-methane) sulfonimide lithium (LiTFSI)) hybrid electrolytes with ionic conductivity up to 0.22 mS cm−1 and excellent for a long period of time cycling stability against Lithium metal. The local structural contexts of Li+ ions in LGPS-PEO hybrids are investigated using high-resolution solid-state Li NMR, which detects Li+ from PEO (LiTFSI), LGPS in bulk, and at LGPS-PEO interactions. Tracer-exchange the majority of Li+ ions are transferred via LGPS-PEO interactions, according to Li NMR. They looked at how the amount of LGPS & LiTFSI in LGPS-PEO hybrid electrolytes affected the interfacial chemistry. The ionic conductivity of LGPS-PEO hybrids are shown to be positively correlated with the amount of Li+ ions accessible at the LGPS-PEO interfaces. This research sheds light on how to design organic–inorganic hybrid interfaces in order to create excellent-performance electrolytes for solid-state secondary batteries (Fig. 17). Since the secondary ZIB system of Mgx V2 O5 ·nH2 O/ZnSO4 //zinc has a problem with capacity fading, MgSO4 is selected as an addition. Electrolytes with a range of ZnSO4 and MgSO4 concentrations are examined by Zhang et al. [197]. The 1 molar ZnSO4 −1 MgSO4 electrolytes are used by other competitors which provide a high

Fig. 17 Schematically shown the production of LGPSPEO hybrid films and the symmetrical battery cells. Ref [196] Copyright @ 2019, ACS

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Fig. 18 Scheme of the Mg2+ functional mechanism for hybrid electrolyte. Ref. [197] Copyright @ 2020, Springer Nature

specific capacity of 374 mAh g−1 at a present density (ρ) of 100 mAg−1 and display comparable rates of reversible performance of 175 mAh g−1 in the range of 5Ag−1 . This research has offered a novel way to enhance the effectiveness of vanadium-based cathodes in ZIBs that uses electrolyte enhancement and is affordable (Fig. 18). Pagot et al. [198] report the development of novel hybrid Al/Mg electrolytes based on IL, paving the way for further advancements in the area of rechargeable multivalent MABs (metal alloy batteries). In fact, these electrolytes are capable of co-depositing and stripping an Al/Mg alloy synergistically with a CE = 99.66% and an over-voltage of less than 50 mV. They have conductivity of ions about to 2.7 × 10−3 S cm−1 at normal temperature. It is observed that high quantities of Al2 Cl7 − dimers in their anion domains enhance their electrochemical performance. In detail, coexistence in anion domains of cation Mg-chloro-aluminate species has the benefit of (i) increasing the concentration and solubility of Mg2+ ions in electrolytes; (ii) providing Mg2+ ion and Al3+ ions in a proper structural state during the deposition process, which ease their reducing process and (iii) interchange the co-deposition process of Mg2+ ion and Al3+ ions. Taken together, these electrolytes enable continuous deposition/stripping of an Al/Mg alloy on a magnesium electrode for 253 h, resulting in a prototype rechargeable battery capable of cycling for more than 100 cycles.

9 Conclusion This chapter concludes that the design of improved electrolytes is influenced by a number of variables, including the stability window, the nature of the material, temperature stability, non-reactiveness, abundance, non-hazardousness and costeffectiveness. Different kinds of electrolytes may be used in rechargeable batteries applications, including aqueous electrolytes, non-aqueous electrolytes, organic electrolytes, ionic electrolytes, polymer electrolytes and hybrid electrolytes. Many additives (salts and solvents), including nanomaterials, have been used to develop better

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electrolytes with high energy density, oxidation durability, and non-flammability for the use in mobile devices and electric vehicles. Facile and simple design approaches should be adopted to develop safe and high performance rechargeable batteries.

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Chapter 11

Recent Progress in Separators for Rechargeable Batteries Mohammad Mudassir Hashmi, Nadeem Ahmad Arif, Syed Mehfooz Ali, Mohd Bilal Khan, Mukesh P. Singh, and Zishan H. Khan

1 Introduction Fossil fuels are exhaustible in nature but are still being used as the primary source of energy production. Excessive use of fossil fuels is causing a lot of environmental damage and thus has become a global concern. In order to prevent the exploitation of fossil fuel reserves and to protect the environment, the world is shifting towards renewable energy resources. On the basis of numerous studies, it is found that a large amount of produced energy is wasted. Therefore, to minimize this waste of energy, the storage of energy is very important. In the last few years, energy storage systems have been developed at full pace. Among all the energy storage system (ESS), the battery energy storage system (BESS) is the most feasible and efficient. A battery is an energy storage device which works on the principle of electrochemical reaction. It has three major components, namely, electrodes, electrolyte and a separator. Separator, which is one of the most important components of rechargeable batteries that is also perhaps one of the least discussed. The separator is a super thin, porous membrane that permits the positive and negative electrodes to be physically separated and thus prevented short circuit. As a result, it is a critical component for cell safety. A separator plays a crucial role in the operation of the cell because its porosity allows the movement of ions in the electrolyte to pass back and forth between electrodes thus preventing the passage of electrons. This means that it should be ionically conductive while being electrically isolating. It is important to remember the basic principles for separators. Separators must be safe and reliable, having a resistible and robust structure to prevent M. M. Hashmi (B) · N. A. Arif · S. M. Ali · M. B. Khan · M. P. Singh · Z. H. Khan Organic Electronics and Nanotechnology Research Laboratory, Department of Applied Science and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India e-mail: [email protected] Z. H. Khan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_11

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punctures for transporting particles. Therefore, these are used in the batteries in the sealed form [1]. Batteries that can be recharged are the secondary or rechargeable kind. These rechargeable batteries are widely used due to their outstanding performance and low cost compared to other battery types [2]. There are several battery technologies, each of which has its own unique set of parameters to be incorporated into each unit. However, there are certain common characteristics across all battery separators [3]. The separator should ensure that the anodic and cathodic components of the cell are mechanically separated. When submerged in the battery electrolyte, the separator should have good conductivity. Chemically, the separator should be stable and compatible with all of the cell’s components. The electrochemical processes must be able to pass through the separator. It should allow unrestricted flow of electrolytes while providing the least barrier to electrolyte diffusion. It should be robust enough to be handled readily during assembly and capable of being prepared in such a way that cell construction may be accomplished easily. When kept under normal circumstances prior to installation in a battery, it should not degrade. In both dry and wet states, it should be dimensionally stable. It should be low cost, made of non-critical materials and easily produced in order to allow widespread usage. Electrolyte retention separators should be extremely absorbent, quickly wetted and non-shedding [4]. The first known separator materials date back to A. Volta’s efforts towards the end of the eighteenth century. Volta used a cloth soaked in a salt solution as a separator to illustrate the production of energy from his voltaic pile. Other separator materials, such as cellulosic papers, cedar, shingles and sausage casings, wood, nonwoven textiles, foams, microporous polymeric membranes or cellophane became recognized later. The expanding chemical industry fuelled the development of specialized separators in the second half of the twentieth century. A separator can be classified as ion permeable or ion conductive (solid electrolytes). Physical qualities, crystallographic traits, shape and composition are all factors that go into this classification [5]. Industrially manufactured microporous films made from polymers such as polyethylene (PE) [6], polypropylene (PP) [7] or naturally abundant materials (e.g. rubber or cellulose), polytetrafluoroethylene (PTFE) [8] and nonwovens are prominent examples of the first group. Nonwovens are sheets, webs or mats made of fibres bound together by cohesion, adhesion, friction, heat treatment or chemical bonding. Woven and papers, tufted or stitched items, on the other hand, are not allowed. PE (polyethylene), polyethylene terephthalate (PET) [9], polyvinylidene difluoride (PVDF) [10] or PTFE are the most commonly used nonwovens, which are generally manufactured using a wet-laid method. Separators consisting of microporous polymeric sheets, loaded with Li salts, predominate in the commercial batteries [11]. Moreover, there have been many more significant findings reported by numerous research groups in the development of effective separators based on manufacturing methods. Among all, one option is to create new AGM with improved characteristics by changing the diameter of the glass fibre or combining it with other reinforcing material [12–14]. Burashnikova et al. [15], found that impregnation the AGM separator with various concentration of polymer solution increased the efficiency of the

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oxygen cycle and Drenchev et al. [16] found that a significant reduction in overcharge current was observed throughout the polymer composite coating on the AGM separator. AGM separator with polymer emulsion was also found to have a reduction in resistance and stratification property by Pavlov and colleagues [17], among other things. Furthermore, separators must have appropriate mechanical characteristics to withstand different forces within and outside the batteries throughout the lithium-ion batteries (LIBs) application process, particularly compressive stress. Over the whole life cycle of batteries, a dynamic pressure environment may be sensed. During each charge/discharge cycle of a lithium-ion batteries, intercalation and deintercalation of lithium are not only an electrochemical process, but also a mechanical evolution of battery components, resulting in significant volume expansion of electrode materials; [18, 19] 2% for lithium cobalt oxide and 10% for graphite electrodes [20], with up to 400% expansion for graphite electrodes [21]. As a result, the expanding electrodes unavoidably strain the battery’s limited interior space, compressing the soft separator and producing significant micropore deformation resulting in reduced ionic conductivity along the thickness direction. The local pressure on the separator is significantly higher than this level due to the coarse and rough electrode surface of two electrodes. As a result, it’s critical to look at the effects of compressive stress on the standard commercial separator. Polyolefin recently has been used for manufacturing three of the most widely used commercial separators [22]: the dry(parched) approach using uniaxial embroidering of strong elastic iPP [23, 24], the dry process using biaxial embroidering of β nucleated polypropylene (β-iPP) [23, 25] and the wet(soak) approach using ultra-high molecular weight polyethylene (UHMWPE) [26, 27]. A coated Li-ion battery separator using β-cyclodextrin as a binder is used to control water absorption and improve high C-rate stability. Organic or inorganic layers are deposited onto the membrane surface to increase wettability and dimensional stability, therefore improving the membrane’s safety and cycle performance. Separators are typically layered with particles of ceramic like as silica (SiO2 ) [28], alumina (Al2 O3 ) [29], titanium dioxide (TiO2 ) [7] and the binders such as poly(lithium 4-styrenesulfonate) (PLSS) and carboxymethyl cellulose (CMC) [30], polyvinyl alcohol (PVA) [31], polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) [32] are normally employed. A porous composite separator, poly(aryl ether ketone) (PAEK) [33] may be produced via electrospinning that yields crystallized Al2 O3 -enhanced separators for high-performance lithium-ion batteries. Commercial polyolefin separators suffer from many deficiencies, including inadequate interaction with electrolyte, lower heat stability and absorption capacity. When the temperature rises beyond 90 degrees Celsius, the porous polyolefin separators would experience significant thermal shrinkage. To enhance thermostability and liquid electrolyte adhesion, porous LIB separators are altered using a variety of nanoparticles which are inorganic in nature (e.g. Al2 O3 /SiO2 /ZrO2 /TiO2 ) [34, 35]. A surface-alkaline-etching was used to coat a titanium (TiO2 ) nanoshell onto a polyimide (PI) [36] nanofiber membrane, and then composite separators demonstrated increased thermostability, enhanced wettability and higher flame remonstrance. Simultaneously, a growing number of polymers, such as poly(ethylene oxide) [37], poly(methyl methacrylate) [38], poly(acrylonitrile)

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[39], polyimide, poly(vinylidene fluoride) and polyetherimide, are being developed as porous membranes for LIBs [40]. The development of cellulose and its derivatives (like as cellulose acetate, cellulose acetate butyrate, cellulose diacetate, methyl cellulose (MC) [41], ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose (CMC) [42] and hydroxypropyl methyl cellulose) [43] based composite separators has been widely studied to enhance the applicability of cellulose materials for lithium battery separators. CNFs (cellulose nanofibrils) [44] are kindly a new unidimensional cellulosic nanomaterial that has recently received a lot of attention. To prevent the shuttle effect and sulphur’s insulating properties as well as its discharge products in lithium–sulphur batteries result in a limited use of active sulphur material, porous carbon as a host material, such as graphene, reduced graphene oxide (RGO) [45], multiwall carbon nanotube (MWCNT) [46], porous carbon nanosphere [47], biomass carbon materials [48], meso-hollow and microporous carbon nanofibers (MhMpCFs) [49], heteroatom-doped carbon materials and carbon nanocomposite, are used to physically confine the polysulfide on the cathode side. Furthermore, it has been discovered that by using LiPSs absorbing materials on the separator, [50] the Li dendrite development may be suppressed at the same time. The combination of TiSx and carbon allows the separator to efficiently minimize polysulfide dissolution without affecting the cell’s conductivity [51]. For reducing polysulfide diffusion, Chung et al. [52] created a bifunctional separator using a lightweight CNTs layer. Kim et al. [53] showed that Al2 O3 can efficiently trap polysulfides, resulting in improved cycle stability. Cui et al [54] created a new SiO2 nanoparticles sandwiched layer to improve the separator’s mechanical properties and avoid lithium dendrite development. One significant advantage for the electrostatic layer by layer (LBL) self-assembly innovation is that it’s extremely efficient, cheap and easy to use. It also makes for a possible route to mass production because of its low environmental impact, cost-effectiveness and the simplicity of successive electrostatic interaction. A polypropylene (PP) membrane may be used to enhance the Coulombic efficacy and cycling results of Li metal batteries. A coating of Gr, BN or a mixture of the two may be applied to enhance the Li metal battery’s cyclability [55]. Easy fabrication of flexible electrodes for sodium-ion batteries using a polyvinylidene fluoride (PVDF)/Si3 N4 composite separators have been fabricated by electrospinning [56]. Taking advantage of the P/SN separator’s excellent porosity and ionic conductivity, the P/SN@PBNi combination shows high initial coulomb efficiency (96.8%), superior cycle stability and rate stability [57]. After introduction, the next part of this chapter covers the fundamentals of separators including its classification, manufacturing processes, properties and requirements.

2 Classification of Separators in Rechargeable Batteries A number of separating materials are now being used by manufacturers. The use of separators became common in the chemical manufacturing sector for

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Separator

Microporous membrane

Ion Exchange Membranes

Non wovens Membranes

Nano porous Membrane

Dry Process

Grafting of Hydrophilic Monomers

Web Formation

Top Down

Wet Process

Coating of different Polymers

Web Bonding

Bottom Up

Phase Inversion

Electrospinning

Fig. 1 Separator classification and their fabrication techniques

last few decades [11]. Most separators are made using polyolefin membranes, nonwoven membranes, microporous membranes, composite membranes, electrospun membranes and polymer blends. They must have high dimensional, mechanical stability, porosity, be easily wetted, uniform thickness by liquid electrolytes, thermal and chemical stability [58, 59]. As a consequence, battery separators are evaluated for material composition, types, thermal (high-temperature melt integrity and cooling), electrical (chemical stability and ionic conductivity) and mechanical properties (shrinkage and tensile strength). All variables, particularly charge and discharge characteristics, have an impact on battery safety and performance [60, 61] (Figs. 1 and 2).

2.1 Microporous Membranes The pores are usually large in size, with diameters in the range of 50–100 Å [62]. While low-temperature (less than 100 °C) separators for batteries which work at normal temperatures have utilized materials like Polymer sheets (Polyethylene(PE), Polypropylene(PP), nonwoven fibres (e.g. cotton, nylon, glass, polyesters), Poly(vinyl chloride) (PVC), Poly(tetrafluoroethylene) (PTFE) and naturally occurring substances (e.g. wood, rubber, asbestos) [63, 64]. Polyethylene separators, which are made using microporous polyolefins are used for storing electrolyte in lithium-based non-aqueous batteries and lead acid batteries, respectively [65].

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(a)

(b)

Fig. 2 SEM image of Microporous Polyolefin membrane produced by a dry method, b wet method. Readdressed via permission [69]. Copyright 2007, Elsevier

In addition, other polymers, such as isotactic polyoxymethylene, polymer blends and poly(4-methyl-1-pentene) that include poly(propylene) (PP)/polystyrene (PS), PP/poly(ethylene terephthalate) (PET) and P(VDF-co-HFP)/PAN were utilized to make microporous membranes by both the wet and dry methods, as shown in Figs. (3, 4), as well as through the phase-inversion method [66, 67]. Polyolefin microporous membranes (which are either polyethylene or polypropylene) have been broadly utilized as separators for commercial lithium-ion batteries during the last decades, since they are appealing, safe and less expensive [68–70]. In addition, the separators have been further categorized into single- and multiple-layer microporous polyolefin

Fig. 3 Block diagram of Dry process

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Fig. 4 Block diagram of Wet process

(PP/PE or PE/PP/PE/PP) separators depending on the number of layers, which helps to ensure thermal shutdown behaviour for safety concerns [71]. Even though it is water-repellent, this makes it less compatible with organic electrolytes, which causes decreased efficiency and cycle times. So, modified microporous polyolefin (PP, PE, PE/PP, PP/PE/PP) separators have been made by surface grafting of monomer units using plasma, electron beam (EB) and gamma irradiation [72–74]. The difficulties in obtaining optimum characteristics, such as mechanical stability, concurrently for microporous monolayer membranes, such as hydrophilicity and ion transmission potential, heat defiance and electrochemical process, may be shown by the example of high rate capability. To circumvent this limitation, they are made from different polymers [71]. Manufacturing of membrane separators must satisfy the following conditions: • • • • • • • •

minimum electric resistance (1500kg/cm2 ), spatial strength (shrinkage 1 represent a more complex network. More tortuous shapes are good for dendritic, but perhaps bad for separator performance. Size and Distribution of pores: Regardless of battery type, a separator must have consistent pore distribution to stop efficiency loss due to an uneven density of current. Due to the thin separators used in lithium-ion cells, sub-micrometre hole sizes are required to avoid internal shorts. Composition of polymer and stretching conditions such as frequency, draw ratio as well as temperature influence pore structure. The conventional method of describing separators by means of mercury porosimetry included the use of average pore sizes, percentage porosity and pore diameter dispersions. Non-mercury porosimetry (e.g. Aqua pore) is also used to assess hydrophobic separators (such as polyolefins). Polyolefin separators are described in great depth using this technique. Porosimetry gives information on the pore size distribution, pore medium diameter, pore surface area and pore volume. Porous Materials Inc. [127] has created a second method, capillary flow porometry, to characterize battery separators [128, 129] which measure the pore constriction of the widest gap, largest size of pore, permeability, distribution of pore and the coating surface area. SEM may also be used to analyse separator morphology. To describe the pores of synthetic membrane materials, image analysis has been used [130]. The pore size should be less than 1 micron so that molecules don’t get through. Puncture strength: Separators should be physically robust enough to resist assembly pressures and routine charging or discharging cycles. Structural power is always a requirement to counter fundamental manipulation, cell interference, physical stress, perforations, erosion and compressive pressures. The puncture solidity (PS) is the amount of load required to penetrate a separator fully with a needle. During battery construction and charging–discharging cycles, the separator may have holes created in it by the electrode’s rough surface, which in turn may cause short circuits. Separator strength is significantly affected by the materials and production process utilized. To ensure the electrodes’ substance doesn’t pierce the separator, the separators used in cell winding must have good puncture stability. The puncture stability of most separator applications should be a minimum of 400 g/mil. Mix Penetration strength: It is defined as an amount of force needed to produce a short through a separator because of mix (electrode material) penetration. The mixing intensity of penetration is used to indicate the propensity of separators to generate short circuits during construction. The assessment of mixed piercing resistance is closely linked to particle penetration resistance compared to the puncture resistance test. Increasing the sensitivity of separators to particles infiltration may be accomplished by choosing a higher mix penetration power from a drop-down

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menu, such as 100 kgf/mil for separator of lithium-ion batteries [131]. Increasing the sensitivity of separators to particles infiltration may be accomplished by choosing a higher mix penetration power from a drop-down menu, such as 100 kgf/mil for separator of lithium-ion batteries [131]. Tensile strength: These are determined by the fabrication process. A separator should be robust enough to bear the physical supports throughout cell construction and winding. Due to the narrow diameter, the electrodes will be able to make contact and form a short circuit. As a result, the tensile characteristic of the separator in the MD direction must be strong enough in contrast to the direction of TD. Under stress, the separators must not stretch too much. Occasionally, tensile stability is specified. Commonly, 2% offset output is acceptable while less than 2% is at 1,000 psi. Shrinkage: This test is run in both the directions (TD and MD). In this test, the separator dimensions are established and then kept for a specific amount of time at 90 °C. Shrinkage (%) =



  L i − L f /L i ×100

where li is the initial length and lf is the final length of separator after high-temperature storage. Shutdown: Significant and important method for temperature control and avoiding ventilation in cells with short circuit is the separator shutdown. The collapse of the pores converting a porous polymeric film which is ionically conductive into a non-porous isolating layer between the electrons normally occurred at polymers melting temperature. The cell impedance increases considerably at this temperature, and the current flow across a cell is regulated. It further prevents any electrochemical activity inside the cell, preventing an explosion. It is defined by its percentage of crystallinity, molecular mass and density and production process that the PEbased separator may shut down the battery. Material characteristics and processing techniques may need a sudden and full shutdown reaction. The optimization must be carried out without compromising the material’s physical property within the temperature domain. The shutdown attribute of separators is assessed by a linear rise in the impedance of the separator. A separator is safer when the physical integrity of the separator is greater beyond 130 °C. This avoids direct contact between the electrodes, resulting in heat loss. So, the separator’s heating and electrical resistance management are selected for shutdown [132]. Melt integrity: Lithium-ion separators must have superior heat tolerance to maintain high-temperature melting strength. The separator must not break down or come into touch with the electrodes to prevent short circuits after the device has been turned off. This keeps the cell away from becoming dangerously hot even when heated. To evaluate separators for melt integrity at large temperature, thermal mechanical analysis (TMA) is a valuable tool. For lithium-ion batteries, separators need to have a melt integrity of 150 °C or above. Polypropylene-covered trilayer separators may improve melt integrity at high temperature in comparison to a mono-layer PE separators. This is critical for larger Li-ion batteries used in hybrid and electric cars. Li-ion batteries

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are sensitive to water contamination. Therefore, materials used in manufacturing of cell is dried at 80 °C in a vacuum. Separators should therefore remain tight and unwrinkled. Each battery manufacturer employs a unique process of drying. TMA determines the separators’ high temperature strength. So, the separator is maintained between 200 and 300% elongation and continuous load versus defined temperature. Wettability and wetting speed: Electrolyte absorption and electrolyte retention are important features for the operational parameters of any battery. For best results, a separator must be capable to soaking up an electrolyte in appropriate amounts during cell operation, as well as retain the absorbed electrolyte. The wettability of the separator may reduce battery performance by raising separator and cell resistance. In actual cells, the wetting speed of the separator may be linked to the time it takes for the electrolyte to fill. These elements of the separators namely porosity, tortuosity, surface energy and pore size all influence the wetting speed. There is no universally recognized wettability test for separators. The contact angle may also be used to determine wettability. A separator’s ability to absorb and hold electrolyte is significant. Ion transfer requires electrolyte absorption. Generally, microporous membranes do not expand in response to electrolyte absorption. Thickness: The separators typically measure 20–50 μm in thickness in a rechargeable battery. A Li-ion battery separator’s thickness is often 2,500 m). The OH and Zn (OH)4 2– ions preferential migration of the PBE membrane might be a significant step towards rechargeable Zn–air batteries.

10 Separators Used in Alkaline Zn/MnO2 Batteries This chemistry has dominated the main battery industry since its discovery because to its cheap cost, excellent safety and high theoretical energy density (>400 Wh/L) due to the high gravimetric capacity of MnO2 (310 mAh/g per electron) [246]. A frequent cause of failure in rechargeable alkaline Zn/MnO2 batteries is zinc poisoning of the cathode, which leads to the formation of an electrochemically inactive zinc manganese spinel phase. Huang et al. [247] found that interlayering Ca (OH)2 sheets between separators and Zn anodes, which trapped zincate ions through a polyfunctional mechanism, led to an improved capacity retention of 90mAh/g-MnO2 at 100% DOD over 60 cycles, with no presence of unwanted spinel phase in the bulk electrolyte. For grid storage applications, alkaline zinc–manganese dioxide (Zn–MnO2 ) batteries are an attractive option because of its environmentally safe, aqueous electrolyte and the established materials distribution network. Recent advances were achieved in Cu/Bi-stabilized birnessite cathodes with the ability to handle MnO2 (617 mAh/g) with complete twoelectron capacity equivalence. With this new electrode arrangement, which blocks Zn (OH)4 – transit from the anode to the cathode, the need for more effective selective separators has arisen. Kolesnichenk et al. [248] described the development of polysulfide (NBI-PSU)-based separators that were capable of delivering hydroxide over zincate and synthesized using N-butylimidazolium. After this, they investigated the effect of their use in between the cathode and anode of high-discharge capacity Zn/(Cu/Bi–MnO2 ) batteries. Initially, they determined the selectivity of their membranes using zincate and hydroxide diffusion tests, which demonstrated a significant improvement in zincate-blocking (DZn (cm2 /min): 0.17 ± 0.04 × 10–6 for 50-PSU, their most selective separator vs 2.0 ± 0.8 × 10–6 for Cellophane 350P00 and 5.7 ± 0.8 × 10–6 for Celgard 3501), while maintaining comparable crossover rates for hydroxide (DOH (cm2 /min): 9.4 ± 0.1 × 10–6 for 50-PSU compared to 17 ± 0.5 × 10–6 for Cellophane 350P00 and 6.7 ± 0.6 × 10–6 for Celgard 3501). then they

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Fig. 29 SEM image of EMD electrodes [247]. Readdressed via permission Copyright 2017, Elsevier

incorporated their membranes into cells and found an increased cycle life compared to cells that solely contain commercial separators (cell lifetime extended from 21 to 79 cycles) (Figs. 29 and 30).

Fig. 30 Graphical presentation of Ionic conduction via Nyquist plot permeation schematic and concentration of zincate ion by using different separators [247]. Readdressed via permission Copyright 2017, Elsevier

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11 Separators used in Iron-Air Batteries A special kind of sophisticated high-temperature electrochemical energy storage (EES) device is the rechargeable molten salt batteries (MSB) [249]. They are very energy efficient and powerful. They also have an excellent durability, safety and may be used to satisfy the increasing worldwide need for sustainable energy [250]. High-temperature electrochemical energy storage (EES) with unparalleled reversible electrode kinetics and high ion-conductivities is enabled by molten salts, making them an excellent option for energy storage and power capabilities. Although they evaporate and flow rapidly at high temperatures, the EES devices that include them provide a design and fabrication challenge due to their high production cost and cycle durability. Furthermore, the lithium-based molten salts required as the electrolyte for these EES devices not only adds to the cost but also limits the already restricted supply of lithium. Zhang et al. [251] reported a novel electrolyte produced from a molten eutectic mixture of Na2 CO3 -K2 CO3 with yttrium-resistant zirconia (YSZ) nanoparticles as a 1:1 mass ratio, which was almost solid-state (QSS). Compared with the purely eutectic Na2 CO3 -K2 CO3 , the volatility of QSS electrolytes has been lowered, resulting in reduced evaporation of liquid salt via increased contact at the high-temperature interface between molten salt and YSZ nanoparticles. An iron-air battery that achieved high columbic and energy efficiency was constructed using the QSS electrolyte. Furthermore, they postulated and demonstrated a redox process in the negative electrode’s three-phase interlines. This research may lead to a more practical and efficient method to the creation of inexpensive and highly reliable molten salt metal-air batteries.

12 Conclusion This chapter indicates that there are many distinct requirements for separators utilized in the various battery technologies. After introduction, the fundamentals of separators have been discussed. Fusion of precise requirements of these technologies with appropriate engineering is essential to get the most advantageous separators. The important aspect is that the separator thickness reduces when the electrochemical electrolyte increases. Li-ion battery separators have extremely low membrane thickness. The membrane surface finishing through plasma treatment performs a significant role in the choice of cell performance in membrane separators. The key criteria for customers and producers will be decreased cost and self-discharge. It will thus consider future studies. In future, tailor-made battery separators will become much more important separators. Also, for future research particularly for HEV, long-term stability, safety, affordability and better performance are also important factors (Tables 1 and 2).

Method preparation

Wet-laid

Melt-blown

Melt-blown

Casting solution into nonwoven

In situ polymerization and cross-linking reaction

Soaked and hot-pressed

–

–

Materials

PEGDMA/ PET

PP/SiO2

PVDF/SiO2

PVDF- HFP/SiO2

PI

Bacterial cellulose/ Al2 O3

Alginate

PET

1 M LiPF6 in EC/EMC (3: 7 by vol)

–

1 M LiPF6 in EC/DMC/DEC (1/1/1, v/v/v)

1 M LiPF6 in EC/DMC/EMC 1:1:1 (wt.)

1 M LiPF6 in EC/DMC/EMC 1:1:1 (vol)

1 M LiPF6 in EC/DMC/EMC 1:1:1

1M LiPF6 in EC-DMC

1M LiPF6 in EC-DMC (1:1v/v)

Electrolyte solution

0.6; 1.75 mAh (4 C)

1.4; 98 (10 C)

4.91; 161 (0.2 C)

2.7; 108.4

3.45; 122 (10 C)

– ;170 (0.2)

4.33; 152 (0.2 C)

2.14; –

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

Graphite/ LiCoO2

Li metal/ LiNi0.5 Mn 1.5 O4

Li metal/LiFePO4

Graphite/ LiCoO2

Li metal/LiCoO2

Li metal/LiCoO2

Li metal/LiFePO4

–

Anode/cathode

Table 1 Nonwoven membranes with different polymers and their application [252]. Copyright 2019, Elsevier

(continued)

Higher electrolyte uptake and ionic conductivity value

Heat resistant and excellent cycling stability

Large porosity and improved electrochemical stability

Internal short circuit protection and stable Interfacial resistance

Excellent interface compatibility and Capacity retention

Better cycling performance

Higher capacity value

Higher porosity, good chemical stability

Main achievement

478 M. M. Hashmi et al.

Method preparation

Wet-laid

Dip-coating

Wet-laid

Coating

Materials

PVDF-HFP

PVDF- HFP/PET

PET/cellulose

Ester/Al2 O3 / PET

Table 1 (continued)

1M LiPF6 in EC/EMC (30:70, v/v)

EC/DEC

1M LiPF6 in DMC/EMC/EC (1:1:1, v/v/v)

1 M LiPF6 in EC/DEC/EMC (1/1/1, v/v/v)

Electrolyte solution

0.824; 100 (0.2 C)

160 (0.2 C)

1.20; 131.33 (5 C)

1.2-1.8; 130 (1 C)

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

Graphite/ Li (Ni1/3 Co1/3 Mn1/3 ) O2

Li metal/LiFePO4

mesocarbon microbeads/ LiNi1/3 Co1/2 Mn1/3 O2

Graphitized mesocarbon microbeads/LiCoO2

Anode/cathode

Stable up to 200 ºC

High mechanical strength and hydrophilicity

Superior thermal stability and higher electrolyte uptake

Better rate capability and long-term stability

Main achievement

11 Recent Progress in Separators for Rechargeable Batteries 479

Filler

TEOS: PSZ

Silica

TiO2

EMImTFSI

LLZO

SiO2

SiO2

LiTFSI and PYR13 FSI

TiO2

Materials

PAN

PAN

PEO

PEO

PEO

PEO

PEO

PEO

P(AN-VAc)PMMA

2.43; 107 (0.02 C)

154.9 (0.1 C)

2; 96 (5 C)

5–10−5 S/cm at 80 °C

10; –

0.0211; –

2.1; 162.1 (0.5 C)

1.04; 121 (0.2 C)

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

1M LiPF6 in EC: DMC: 4.5; 84 (20 C) DEC

–

1M LiPF6 in EC: DEC

1M LiPF6 in EC: DEC

–

–

–

1.15 M LiPF6 in EC EMC/ DEC (3:5:2 by vol) + 5.0 wt.% FEC

LiPF6 in EC: DMC: DEC

Electrolyte solution

Graphite/LiFePO4

Li metal/ LiNi1/3 Mn1/3 Co1/3 O2

Li metal/LiFePO4

Li metal/LiFePO4

–

–

–

Graphite/ LiNi0.6 Co0.6 Mn0.2 O2

Graphite/LiCoO2

Anode/Cathode

Table 2 Composite membranes with different polymers and their application [252]. Copyright 2019, Elsevier

(continued)

Suppresses anion decomposition to improve the stability and lifespan of the batteries

Electrochemicaly stable up to 4.5 V(vs. Li/Li+ )

Higher thermal and electrochemical stability

High-performance lithium-ionic conductor and reliable separator for lithium metal batteries

Decrease of the degree of crystallinity

Improved ionic conductivity

Improved thermal stability and mechanical integrity

Good electrolyte wettability and excellent thermal stability

Ceramic domains Leading increased amorphous regions within the fibre

Main achievement

480 M. M. Hashmi et al.

Filler

IL-TFSI

SiO2

SiO2

LLZO

Nanoclays

NCC

BMImNfO

Materials

PMMA

β-iPP

PP

PVDF-HFP

PVDF

PVDF

PVDF-HFP

Table 2 (continued)

–

0.512; 120 (C)

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

1.3; 148.8 (0.5 C)

–

26.1 at 100 °C; 126 (C/4)

1 M LiPF6 in EC/DMC 2.53; – (1:1)

1 M LiPF6 in EC/DMC – (1:1)

1.15 M LiPF6 in EC/DEC 3:7 by vol

1 M LiPF6 in EC/DMC 0.63; 100 (8 C) (1:1)

EC: DEC (1:1)

–

Electrolyte solution

Li metal/LiCoO2

–

–

Graphite/LiCoO2

Li metal/LiFePO4

–

Li4 Ti5 O12 /Li metal

Anode/Cathode

(continued)

Enhancement of the electrical behaviour

Mechanical reinforcement, associated with a lower strain at break

Enlarged pore size and reduced degree of crystallinity

Improved thermal stability and high ionic conductivity

Decreases the polarization of Li-ion batteries and leads to improved power performance and cycle stability

Lower thermal shrinkage and shrinkage rate and suppressed porosity

Excellent elasticity with large elongation-at-break (up to 1600%)

Main achievement

11 Recent Progress in Separators for Rechargeable Batteries 481

Filler

PDA

MMT, NaY, BaTiO3 , MWCNT

OIL-Br

Graphene oxide

Silica

ZrO2

AlO (OH)

Materials

PVDF-HFP

PVDF-TrFE

PVDF-HFP

PVDF-HFP

PVDF-HFP

PVDF-HFP

PVDF

Table 2 (continued) Conductivity (mS cm−1 ) and capacity (mAh g−1 )

2; 152 (0.1 C)

0.45; 103.1 (2 C)

1 M LiPF6 in EC/DEC (3/7 w/w)

1M LiPF6 in EC: EMC (1:3)

1M LiTFSI in EMITFSI/EC/PC

1.72; 80 (C)

2.06; 149.7 (0.1 C)

1.11; 108.6 (0.2 C)

1M LiPF6 in EC: DEC: 1.12; 118 (2 C) EMC

1M LiPF6 in EC: DEC

1 M LiTFSI in PC

1M LiPF6 in EC: DMC: 1.4; 105 (0.5 C) DEC

Electrolyte solution

Graphite/LiCoO2

Li metal/LiCoO2

–

mesocarbon microbead/LiCoO2

Li metal/LiFePO4

Li metal/LiFePO4

–

Anode/Cathode

(continued)

Superior wettability and thermal stability

High electrolyte uptake, high ionic conductivity and high electrochemical l stability potential

Good compatibility and dendrite-free electrodeposition of lithium metal at intermediate current densities

Improved electrochemical, mechanical properties and thermal stability

Improved safety and superior performance

Improvement of the overall electrochemical behaviour of the separator membranes

Excellent cyclic stability and good rate performance

Main achievement

482 M. M. Hashmi et al.

Filler

Si2-TiO2

ZrO2

SiO2

EMImNfO

ZIF-4

LATP

reduced graphene oxide

Materials

PVDF

PVDF-HFP

PVDF-HFP

PVDF-HFP

PVDF

PVDF-HFP

PVDF

Table 2 (continued)

18 at 100 °C; 164 (C/10)

– ; 127 (C)

1; 123 (0.1 C)

–

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

1 M LiTFSI and 0.1M LiNO3 in a DME/DOL (1:1 vol)

– ; 1070 (0.2 C)

1 M LiTFSI with 0.25 0.88; 1614 M LiNO3 in DME/DOL (1/1, v/v)

1 M LiPF6 in EC/DMC 3.4; 155 (0.2 C)

–

1M LiPF6 in EC/DMC (1: 2, v/v)

1M LiPF6 in EC: EMC (1:1)

LiPF6 or Li BOB

Electrolyte solution

Li metal/Sulphur

Li metal/Sulphur

Li metal/ Li [Ni1/3 Co1/3 Mn1/3 ] O2

Li metal/LiCoO2

Li metal/ LiNi0.5Mn1.5O4

Li metal/LiFePO4

Graphite/LiFePO4

Anode/Cathode

(continued)

Enhancement of the cycling stability and rate capability

Good ionic conductivity as well as the reduced polysulfide shuttle within the cell

Larger liquid electrolyte uptake, higher retention, higher ionic conductivity and lower interfacial resistance

Improved thermal stability and excellent electrochemical properties

Excellent thermal stability and safety towards fire

Improved interfacial and electrochemical properties

Increase in battery capacity

Main achievement

11 Recent Progress in Separators for Rechargeable Batteries 483

Filler

ZrO2

Graphene

Al2 O3

Al2 O3

glass microfiber

Al2 O3

Al2 O3

SiO2

Materials

PVDF-HFP

PVDF

PVDF

PVDF-HFP

Melamine formaldehyde

PVA-co-PE

PEG

Bacteria Cellulose

Table 2 (continued) Conductivity (mS cm−1 ) and capacity (mAh g−1 )

1M LiPF6 in EC/DEC (1:1, w/w)

1M LiPF6 in EC/DMC (1:1, w/w)

Li4 Ti5 O12 /LiFePO4

Li metal/LiCoO2

Li metal/ Li [Ni1/3 Co1/3 Mn1/3 ] O2

Anode/Cathode

0.52; 140 (0.2 C)

6.5; 153 (0.1 C)

0.93; 156 (0.1 C)

Li metal/LiFePO4

Graphite/LiNi1/3 Co1/3 Mn1/3 O2

Li metal/LiFePO4

Li metal/LiCoO2

1.3 at 80 ºC; 155 (0.5 Li metal/LiFePO4 C)

0.82.154 (0.2 C)

1 M LiPF6 in EC/DMC 0.92; 157 (0.2 C) (1/1, v/v)

1M LiPF6 in EC/DMC (1:1, v/v)

1 M LiPF6 in EC: DMC: EMC with a ratio of 30/15/35/20 + 2% VC

1M LiPF6 in EC: DEC

1M LiPF6 in EC: DEC: 3.61; 149 (C) EMC

1M LiPF6 in EC: DEC: 0.320; 165.7 (0.2 C) EMC

Electrolyte solution

(continued)

Good thermal stability up to 200 °C and high ionic conductivity

Excellent cycling stability and good rate performance

High porosity and superior electrolyte affinity

Enhanced tensile strength and a suitable porous structure

Excellent electrochemical performance and remarkable rate capacity

High-rate electrochemical l performance and enhanced thermal stability

Improved specific discharge capacity and discharge performance

Superior thermal stability and improved electrochemical performance

Main achievement

484 M. M. Hashmi et al.

Filler

graphene oxide

SiO2

Li6.75 La3 Zr1.75 Ta0.25 O12

HAP

Nano-ZrO2

SiO2

SiO2

Materials

HBPE

PPO

PPC

Cellulose

bacterial cellulose

Succinonitrile

PI

Table 2 (continued)

1M LiPF6 in EC/DEC (1/1, v/v)

2.27; 105 (0.5 C)

2; 151 (0.4 C)

2.14; 120 (2 C)

1 M LiPF6/EC + DEC (1/1, v/v) –

3.07; 138 (0.5 C)

0.52; 120 (0.3 C)

2.62; 142 (0.2 C)

1.7; 118 (5 C)

Conductivity (mS cm−1 ) and capacity (mAh g−1 )

1M LiPF6 in EC/DMC (v/v)

–

1M LiPF6 in EC/DMC

1M LiPF6 in EC/DEM: EMC (1:1:1, w/w/w)

Electrolyte solution

Li metal/LiMn2 O4

Li4 Ti5 O12 /LiCoO2

Li metal/LiFePO4

Li metal/LiFePO4

Li metal/LiFePO4

Li metal/LiFePO4

Li metal/LiFePO4

Anode/Cathode

Large electrolyte uptake and enhanced conductivity

Inhibition of lithium dendrite growth and improved electrochemical stability up to 5.2 V

High thermal resistance, electrolyte wettability and ionic conductivity

Excellent thermal stability, fire resistance and improved electrolyte wettability

Excellent rate capability and cycling stability

Improved discharge capacity and cycling stability

Improved electrochemical performance

Main achievement

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Chapter 12

Organic Photovoltaic Cells: Opportunities and Challenges Mukesh P. Singh and Mohd Amir

1 Introduction The global energy demand is rising day by day due to the continual increase in the world population and technological advancement of our society. Which is subsequently, leading to climate change and global warming [1]. At present, the maximum percentage of the world’s energy demand is delivered through the consumption of conventional energy resources such as fossil fuels. Therefore, these resources are continually depleting due to the high consumption rate. Moreover, the consumption of these resources leads to climate change and global warming [2]. Hence, it is high time to endeavour serious attempts for finding renewable and environment-friendly resources of energy to meet the global energy shortfall. Solar energy is the most prominent renewable and environment-friendly resource of energy [3]. Photovoltaic (PV) cells are electronic devices based on the photoelectric effect, using which solar energy can directly be converted into electrical energy. There are many photovoltaic technologies available in today’s world. The main difference between the different PV technologies relies on the composition of constituent materials and layer structure of the device [4]. Crystalline silicon (c-Si)-based PV cells are the star candidate in the PV market due to the abundance of Si, long-term stability, good conversion efficiency, and well-developed popular fabrication processing of Si-based electronic devices. The fabrication of Si-based PV cells is done by utilizing a 200–500 µm thick wafer of silicon [5, 6]. The efficiency of PV cells and the amount of constituent materials used in the fabrication of PV cells directly influence the cost of generated power. Therefore, in the total cost of the PV power plant, the cost of Si-based panels stands alone for more than half due to high material consumption [7]. Hence, as an M. P. Singh (B) · M. Amir Organic Electronics & Nanotechnology Research Laboratory, Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_12

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alternative to Si-based PV cells, industrialists and researchers are searching for more efficient materials and low-cost techniques to reduce the overall cost of the PV power plants. The thin-film PV cells such as organic photovoltaic cells (OPVs), consume less material comparative to Si-based cells and can be fabricated by using the low-cost solution processing techniques, consequently lowering the cost per unit watt power [8–10]. In today’s industry and academic research field, the OPVs have emerged as one of the most promising alternatives to inorganic material-based PV cells such as those based on Si, gallium arsenide (GaAs), copper indium gallium selenide (CIGS), etc. With many advantages like lightweight, flexibility, use of non-scarce materials, and the ease of fabrication techniques including solution-processing and roll-to-roll manufacturing techniques, the OPVs have gained significant attention during the last few years [11–13]. Also, in terms of efficiency and cost, the OPV is competing with their inorganic and metal halide perovskite counterparts at an amazingly fast rate. The rapid progress in OPVs is directly attributed to continuous advancement in the fabrication technologies and development of new organic semiconducting materials and metamaterials along with the applications of nano technology in the field of OPVs. Moreover, the detailed understanding of film morphology, molecular packing, and the physics of the device has opened new paths toward achieving highly efficient devices [14, 15]. The performance of the OPVs is strongly influenced by the active layer comprising donor and acceptor materials. These are the two key materials to be considered in the fabrication processes of OPV. The exploration of new donors, acceptors, and the electrically and optically advanced carriers (electrons and holes) transporting materials has further improved the performance of OPV. Recently the emergence of nanotechnologies, such as the incorporation of nano metals and metal oxides, use of nano plasmonic, nano structuring, carbon nanotubes, and graphene, has boost up the efficiency and stability of OPVs very significantly [16–20].

2 Organic PV Cells In every PV cell, the basic principle of energy conversion is the physical phenomenon known as the photoelectric effect [21]. The conversion of energy is possible by making use of the electrical and optical properties of semiconductors, the group of materials having conductivity between the insulator and conductor. In OPVs, the light-absorbing material generally is an organic semiconducting material, such as conjugated polymers or small organic molecules. The semiconducting behaviour of polymers was first discovered by Shirakawa et al. [23, 24]. This discovery made it possible to design and fabricate the PV cell comprising polymers and a new area of research was born [25]. The OPVs show numerous potential advantages which include low-cost processability, flexibility, no or minimal use of scarce resources, and use of low-cost materials. The processability of OPVs avoids the use of highly expensive vacuum-assisted techniques which are used in fabrication of Si-based cells. The layers of OPV cells can be casted directly from solutions comprising of constituent materials (in a solvent) for designated layers. This makes

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Fig. 1 Efficiencies of different photovoltaic technologies, printed from the Ref. [22]

up-scaling fabrication possible thus reducing the cost of OPVs [26]. The layers can be deposited using coating or printing techniques on large rolls of the substrate, known as the roll-to-roll (R2R) fabrication technique [27]. Due to the flexible nature of these cells, the solar panels comprised of OPVs can be rolled out onto any surface such as a roof. Initially, the efficiency of OPVs was very poor, when a single crystal anthracene was studied for OPV in early 1959 [28]. Since then, many researchers are working toward improving the efficiency of organic-based photovoltaic cells. During the past years, in a short period, as compared to other PV technologies, the efficiency of OPVs has improved significantly, according to the National Energy Laboratory (NERL) record of certified efficiencies for different PV technologies as shown in Fig. 1 [22].

3 Working Principle of OPVs Like any other PV cell, the OPVs convert light energy into electrical energy. The most basic structure of OPVs consists of two organic semiconducting materials, i.e., donor (D) and acceptor (A) layers sandwiched between the two electrodes across which electrical power is generated. However, the conversion is made more efficient by inserting several other layers in the basic layer stack of OPVs. The basic mechanism of energy conversion by OPVs involves the following steps [29–33] as shown in Fig. 2. Step 1: A photon impinging on a conjugated polymer, with an energy more than or at least equal to the band gap, results in the creation of excitons in the donor phase.

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Fig. 2 Schematic of energy conversion mechanism: (1) impinging of photon (2) creation of exciton (3) diffusion (4) capturing of free carriers. Source Author

Note: excitons are the quasiparticles, i.e., the electron and hole (e–h) pairs bounded by the electrostatic Coulombic force. Step 2: The generated excitons start to diffuse and may reach to D-A interface. Step 3: The exciton is subsequently separated over a built-in gradient provided by the donor–acceptor interface due to their different electron affinities, ionization potentials, and different positioning of LUMOs and HOMOs. Step 4: Finally, the electrons and holes are captured at opposite electrodes, thereby instigating a potential difference and hence the electricity.

3.1 Absorption of Light   A semiconductor material is characterized by certain band gap energy E g . Which signifies the energetic separation between the lowest of the conduction band and the highest of the valance band. However, in the case of organic semiconductors, this gap is defined as the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as illustrated in Fig. 3a. E g = E LU M O − E H O M O   The typical band gap energy E g of organic semiconducting material is around 1.7 to 2.1 eV [34]. Since the thermal energy available at room temperature is in order of meV (≈26 meV), the electrons in the HOMO cannot excite to LUMO in dark, rendering the material non-conductive. Wherein on the absorption of photon energy (E hν ) greater than the E g , the electrons are excited from the HOMO to the LUMO state. E hν =

ch 1.24 = eV ≥ E g λhν λhν

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Fig. 3 Energy band diagram of a typical organic semiconductor: a in the dark, b under illumination, c excess energy loss. Source Author

where c is the speed of light, h is the Planck’s constant and λhν is the wavelength of the incident photon. These excited electrons (with the absorption of the photon) leave behind an unoccupied valence state, termed as the hole in the HOMO state, the absorbed photon energy stored as the potential-energy difference between these excited e–h pairs, shown in Fig. 3b. However, the electrons that are excited much higher than the LUMO, quickly fall to the LUMO level after thermal relaxation, resulting in heat [35]. Therefore, all the excess photon energy exceeding the band gap energy will turn into heat, as shown in Fig. 3c. E ther mal = E hν − E g Therefore, for an optimized OPV cell, the prediction of balancing the energy (E hν ) (i.e., absorbed energy) with the total lost energy (E ther mal ) is required. In 1961, Shockley and Quisser did this excellent work, defining the ‘detailed balance theory’ [36]. This helps in determining the optimum band gap and the theoretical limit of the cell efficiency.

3.2 Charge Separation In OPVs the e–h pair, created through the absorption of the incident light, is bounded together by the Coulombic attractive forces, behaving like quasiparticles known as an exciton. For a photovoltaic cell to generate the electrical power, the electron and hole must be separated, and subsequently need to be collected at electrodes with

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opposite polarities. To achieve this, the exciton bond of the quasi-particle must be broken. This is accomplished by introducing another organic semiconductor, having an energetically lower LUMO level, in the active layer, to make electron transfer possible between the two semiconductors (i.e., D and A) as shown in Fig. 2. For this reason, the organic material with the highest LUMO is called the electron donor and the other is called the electron acceptor. The following condition must be satisfied to facilitate electron transfer acceptor

donor exciton E LU M O − E LU M O ≥ E bond

exciton where E bond is the exciton bond energy. The materials used as a donor are most often conjugated polymer, while the C60 fullerene-based small molecule is often used as an acceptor. The excitons are generated mostly in the donor region, as the highly symmetric C60 molecule, has incredibly low absorption compared to conjugated polymers. Even though the absorption in the acceptor region is quite low, it cannot be neglected. Therefore, for successive transfer of holes from acceptor to the donor, the following condition needs to hold true acceptor

exciton E donor H O M O − E H O M O ≥ E bond

Moreover, an exciton possesses a very short lifetime. Hence, for the successful dissociation of the electrons and holes from the excitons, the typical distance between the exciton creation site and a donor–acceptor interface needs to be in the order of 5–15 nm (nm) [37, 38].

3.3 Charge Collection After successful separation of electrons and holes of an exciton at the donor/acceptor (D-A) interface, the electrons and holes transfer to the acceptor and donor regions, respectively. To generate the electrical current these separated carriers need to be collected at the opposite electrodes, i.e., the electrons at the cathode and holes at the anode as shown in Fig. 2. The control of current flow is maintained by making use of the electrodes having significantly different work functions. The anode is made up of a high work function material and a low work function metal is chosen for the cathode. After dissociation from an exciton, the holes and electrons move to the high work function anode and low work function cathode, respectively. More details about the electrodes will be discussed later in this text.

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Fig. 4 Layer staking of a typical OPV. Source Author

4 Device Structure The fabrication of OPVs includes the dissolution of polymers in organic solvents, then transferred onto a substrate using printing or coating. The solution of different materials which is used for different purposes is stacked in a certain order, resulting in a photovoltaic cell. The layers in the stack comprise an active layer (which may be a mixture of D and A materials). Where the excitons are created and subsequently separated into electrons and holes at the D-A interface. For the selective collection of carriers, only electrons at the cathode and holes at the anode, carrier selective layers such as electron transport layer (ETL) and hole transport layer (HTL) are positioned in the device as shown in Fig. 4. The detailed functioning and material used to construct the different layers are discussed in the subsequent sections.

5 Active Layer The active layer consists of a donor which absorbs the light and an acceptor which extracts the electrons from the excitonic bound electron–hole pairs. The electrondonating unit in the OPVs is either a conjugated polymer or a small molecule organic compound. The most widely used conjugated polymers as a donor are based on PPVs [Poly(phenylene vinylene)s] and PTs (polythiophenes). The PPV-based materials such as MEH-PPV and MDMO-PPV exhibits drastically low current densities due to their wide band gaps and weaker absorptions [39, 40]. On the other hand, P3HT [poly(3-hexylthiophene)] a derivative of poly(3-alkyl-thiophene), has a lower band gap and hence exhibits better absorption and hole mobility. Therefore, the OPVs based on P3HT results in a more efficient device as compared to the MEHPPV and MD-PPV-based devices [14]. Although P3HT shows promising properties, its absorption spectrum (500–650 nm) restricts it to be an ideal choice for a donor. The two-dimensional polythiophenes such as P3HDTT exhibits a broader absorption spectrum and better hole mobilities. Therefore, enhanced performance of

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OPVs has been observed with the P3HDDT donor. There are several other conjugated polymers, such as polymers based on 2,1,3-Benzothioazole (BT), Pyrrole[3,4c]pyrrole-1,4-dione (DDP), and Benzo[1,2-b;4,5-b]bithiophene (BDT) have been investigated to be used as a donor for further improving the performance and stability of the device [41]. Moreover, small molecular materials such as Zinc phthalocyanine (ZnPc), subphthalocyanine (SubPc), Copper(II) phthalocyanine (CuPc), diketopyrrolopyrroles (DPP), and perylene diimides (PDI) has also been potentially used as a donor material [42, 43]. Fullerene (C60) exhibits good electron mobility due to its highly symmetric nature, therefore, fullerene and its derivatives are a good choice for acceptor materials. The ultrafast electron transferring has been observed between the D-A interface comprised of C60 as acceptor [30]. However, C60 has very limited solubility in most of the commonly used solvents in the process of ink formation for manufacturing the OPVs. To improve its solubility, the solubilized version of C60, such as [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-PhenylC71-butyric acid methyl ester (PC71BM) has proved to be a better acceptor [44, 45] (Fig. 5). Energy levels and electrochemical properties of fullerene-derived acceptors are of much importance for OPVs because the open circuit voltage (V OC ) depends on these factors. The V OC is determined by the energy difference between the HOMO of the donor and the LUMO of the acceptor [46]. Therefore, to tune the energy levels

Fig. 5 Molecular structure of commonly used materials in OPVs. From left to right, MEH-PPV: poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene vinylene]; MDMO-PPV: Poly[2-methoxy-5(3 ,7 -dimethyloctyloxy)-1,4-phenylenevinylene]; P3HT: Poly(3-hexylthiophene); C60 : fullerene; PCBM: [6,6]-phenyl-C61-butyric acid methyl ester; ZnPc: Zinc phthalocyanine. Reprinted with the permission from springer nature, Ref. [41]

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Fig. 6 OPV comprising single polymer layer: a schematic of the device, b energy band structure. Source Author

of the compound, other derivatives of the fullerene, such as indene-bis-C61-adduct (ICBA), have been derived which give an enhanced performance of the OPVs [47]. Some other fullerene derivatives that have been incorporated as an acceptor in OPVs include Indene-bis-C70-adduct (IC70BA), PC84BM, endohedral fullerene, etc. The acceptor based on small molecules such as the derivatives of perylene and pentacene has also been utilized [48, 49]. The interesting feature of OPVs is that the photovoltaic current can be obtained just by inserting an organic material (a molecular or polymer thin film layer) in between two metallic electrodes, resulting in a metal–insulator-metal (MIM) structure, as shown in Fig. 6a [50]. To get photocurrent through such structures, the anode material with excellent transparency and high work function (φ H igh ) is required. Indium tin oxide (ITO) is one of the best choices for anode material due to its outstanding conductivity, high work function, high chemical stability, and excellent transparency [4]. Whereas the cathode should be a low work function (φ Low ) material such as aluminium (Al). Figure 6b depicts the energy band structure of an OPV cell comprising only single organic material active layer. In such structures, the HOMO and LUMO act as electron donors and electron acceptors, respectively. The light impinging on the device gets absorbed in the organic active layer resulting in the exciton creations. The difference of work function between the two electrodes sets up an electric field within the active layer that breaks the excitons to produce free carriers (electrons and holes). Finally, the electrons and holes are pulled toward the anode and cathode respectively due to the presence of an electric field [50]. In the organic polymers, the exciton diffusion length is of the order of 5–10 nm, consequently, the light (photons) needs to travel a certain thickness (≈100 nm) of the active layer for efficient creation of excitons [51–54]. Therefore, due to this

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Fig. 7 Schematic presentation of different heterojunction OPVs: a bilayer, b bulk heterojunction, c Ordered junction. Source Author

mismatch between the thickness of the active layer and the exciton diffusion length, most of the generated excitons will collapse inside the active layer. The excitons are the pairs of electrons and holes bounded by the Coulombic force. Generally, the excitonic binding energy is of the order of 0.5 eV [55, 56], hence the dissociation of electrons and holes in these types of devices is very difficult [50]. These issues can be overcome using an active layer comprising of two components, an electron-donor (D) material, and an electron-acceptor (A) material. The resulting interface between D-A provides the energetic offset to break the excitons efficiently [57–59]. The mixture of two different materials in the active layer results in nanosized heterojunction interfaces. Therefore, depending upon the interface morphology between the D and A the heterojunctions are classified as bilayer, bulk heterojunction, and more advanced ordered heterojunction as shown in Fig. 7.

5.1 Bilayer Structures The simplest form of the heterojunction OPVs is the bilayer or planer heterojunction (PHJ) structure in which the separate films of donor and acceptor materials are sandwiched between the two opposite electrodes, as shown in Fig. 7a. The donor and acceptor materials have different electron affinities and ionization energies as shown in Fig. 8. The donor and acceptor materials are chosen in such a way that these differences are large enough to create a strong local electric field, resulting in enough potential energy difference, more than the exciton binding energy. Thus, results in the strong potential difference at the D-A interface, providing the driving forces to favour the dissociation of the excitons. The dissociated electron moves in the acceptor while the hole in a donor. These carriers are then finally collected at the respective electrodes and contribute to the photocurrent. Therefore, the bilayer structure results in more

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Fig. 8 Energy band diagram of an OPV consisting of donor and acceptor films in the active layer. Source Author

efficient dissociation of excitons as compared to organic/metal interface, resulting in comparatively more photocurrent, and hence more efficiency [28, 59–61]. However, in planer heterojunction or bilayer structures, the driving forces are confined within a few nanometres near the D-A interface. Therefore, for efficient charge separation and collection, light needs to create excitons within the small region, of the order of 10 nm on each side of the junction, which would contribute to the current generation. Moreover, the diffusion length of the exciton is much shorter than the absorption depth of organic films, only those excitons that are created within the diffusion distance from the D-A interface can reach the D-A interface to get dissociated [37, 38]. This phenomenon puts the limit on the photocurrent generation in the cell, compromising the overall performance of the device. An elegant solution to overcome this limitation is to increase the surface area between the D-A interface. To achieve this, the donor and acceptor materials can be blended resulting in threedimensional interconnecting nanoscale phases of D and A materials, called bulk heterojunction (BHJ).

5.2 Bulk Heterojunction The bulk heterojunction OPVs are very much similar to the bilayer OPVs except that the interfacial surface area between the donor and the acceptor has increased enormously as shown in Fig. 7b. This can be achieved, by simply mixing and dissolving the D and A materials in a common solvent, the solution is then cast as the active layer, upon drying, this active layer consists of distinct three-dimensional interconnected phase-separated regions of D and A with effective distribution of heterojunctions interfaces within the bulk of the active layer [62, 63]. The length scale of these distributed heterojunctions is of the order of a few nano-meters which is comparable to exciton diffusion length. Hence, for every generated exciton, there is an interface (D-A) in the proximity and the exciton dissociation takes place almost at every point

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within the bulk of the active layer. Therefore, in BHJ, the exciton loss is reduced dramatically as every exciton has a very high probability to dissociate and produce free electrons and holes. Consequently, these free carriers contribute toward generating high photocurrent, provided, that there exists a continuous path from interfaces of each material (donor and acceptor) to the electrodes. Therefore, BHJ results in much better efficiency as compared to bilayer heterojunctions [64, 65]. Although BHJ devices effectively resolve the problem of short diffusion length of excitons, but resulted electron and holes may still be geminated. An electric force is required to separate these geminated pairs for subsequent capturing at their respective electrodes. Moreover, the randomly oriented phases of donor and acceptor regions in BHJ lead to deteriorating the conductivity of the device and so its efficiency [66]. This problem may be resolved by making use of advanced heterojunctions, where the controlled growth of D and A materials is used at desired positions. Such structures are called ordered heterojunctions (OHJs) as shown in Fig. 7c. Usually, the OHJs are fabricated with ordered inorganic material embedded in an organic active layer. In these kinds of heterojunctions, carriers diffuse to contact through the polymer along the length of the pore. In an ideal OHJ, all the incident photons will be in the vicinity of the D-A interface within the diffusion length of excitons and separated charge carriers immediately find a path to the electrodes. Therefore, the OPVs based on OHJ exhibits excellent conversion efficiency [67–69].

5.3 Incorporation of Nanoparticles in the Active Layer In organic semiconductors, the excitonic diffusion length is of the order of a few nanometres due to the poor mobilities of charge carriers in these polymers [14]. Thus, to facilitate the charge transfer, and efficient extraction of free charge carriers, the active layer/medium in OPVs is kept quite thin. Wherein, for the sufficient absorption of light (photons), it is required that light should cover a minimum distance of around 100 nm within the active medium. Therefore, the low thickness of the active medium results in optical loss, i.e., a large fraction of the photons that could be harvested, is lost due to incomplete absorption, resulting in low power conversion efficiency (PCE) [51–54, 70]. Conversely, the thick layers can absorb sufficient photons but have increased recombination which limits the photocurrent and hence the PCE. Therefore, for an efficient OPV, minimum recombination and high absorption are the two desired conditions. This can be achieved by enhancing the optical absorption of the active medium, without increasing its thickness, so that the maximum light can be harvested, and there is no upswing in recombination as well. The enhancement of the optical absorption in the thin active layer (without altering its thickness) can be achieved by incorporating the plasmonic nanostructures such as nanoparticles, nanorods, nanotubes, nanowires, nanostars, grating, nanocomposites, etc., in this layer as shown in Fig. 9 [71–77]. Incorporation of plasmonic nanostructures such as noble metals like gold (Au) and silver (Ag) nanoparticles (NPs) within active medium/layer results in localized

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Fig. 9 Some different types of plasmonic nanostructures used in OPV: a Au nanoparticles, b Au nanorods, c Single-walled carbon nanotubes, d Ag nanowires, e Au nanostar, f Ag grating, g metal nanocomposite, h graphene film. Reprinted with permission from the Refs. [71–77]

Fig. 10 Schematic representation of a incorporation of nanoparticles in the active layer, b resulted plasmonic effect. Source Author

surface plasmon resonance (LSPR) effect. The collective oscillation of conductive electrons that are confined within the nanoparticles, leads to a highly intensified electric field around the surface of NPs see Fig. 10b. This intensified field enhances the light absorption (in the active layer) in the surrounding area of NPs. The intensity of this field around the NPs is frequency dependent. The field attains a maximum when the frequency of incident light matches with the frequency of oscillating electron cloud (within the NPs), a condition known as plasmonic resonance, and this frequency is called resonant frequency. Hence, the maximum absorption takes place at the resonate frequency of plasmonic oscillation. The resonant frequency depends on the particle geometry and the surrounding environment of the NPs within the bulk material. Therefore, the LSPR spectral peak position can be controlled by tailoring the dielectric properties, geometry, size, and poisoning of NPs [78]. Thus, by modifying the size and geometry of the NPs appropriately, a specific demand of design can be achieved. Furthermore, for plasmonic excitation, the size of the NPs needs to be comparatively less than the wavelength of the incident light [70]. The increase in optical absorption of P3HT:PCBM (active medium) due to incorporation of the Au and Ag NPs is depicted in Fig. 11a and b,

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Fig. 11 Optical absorption spectra of P3HT: PCBM without NPs and with different wt% concentrations of a Au NPs, b Ag NPs. Reprinted with permission from springer nature, Ref. [79]

respectively, as evidence of LSPR. The different percentage weight (wt%) concentrations of NPs results in different absorption spectra. The device shows initial efficiency of 2.11% without NPs. Whereas the after embedment of Au and Ag NPs in the active layer, device efficiency increased to 3.11% and 3.20% with the optimum concentration of 1.5wt% (Au NPs) and 0.5wt% (Ag NPs), respectively [79]. An upward shift in the absorption spectra of P3HT:PCBM can be noticed in Fig. 11a when the doping concentration of Au NPs changes with the ratio of 0.5, 1, and 1.5 wt% compared with the pristine P3HT:PCBM and in Fig. 11b for Ag NPs with doping ratio 1.5 and 2 wt%. The resonant peak of LSPR is at around 505 nm for all the doping ratios. The increase in current densities due to this increase in absorption is illustrated in Fig. 12 [79]. Where Fig. 12a and b demonstrate current–

Fig. 12 Influence of LSPR on the J-V characteristics of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al OPV with a Au NPs, b Ag NPs. Reprinted with permission from springer nature, Ref. [79]

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voltage (J-V ) characteristics of the OPV with and without incorporation of Au and Ag NPs respectively. It can be noticed from these figures that the current density of the pristine device is low as compared with Au and Ag doped devices. This increase in the current densities is due to the plasmonic effect of Au and Ag NPs embedded in the active layer (P3HT:PCBM). Since Au and Ag metals are very costly, the incorporation of these metals raises the device cost, however, the OPVs are more commonly known for their low cost. So, the replacement of these costly materials with some cheap metals is desirable. The metals such as copper (Cu) and aluminum (Al) provide cost-effective alternatives [80]. The incorporation of metal NPs exhibits narrow bandwidth of plasmonic absorption enhancement corresponding to LSPR spectra due to which the enhancement in the absorption is higher only in a particular frequency range over the entire solar spectrum. Therefore, to expand the absorption spectral range, the incorporation of multiple metals or composite metals NPs in OPVs is one of the best possible solutions [81–84]. Furthermore, the broadening of the spectral region may also be achieved by incorporating the nanometals with asymmetric and complex geometries. In these structures, there exists a coupling between the branches and the central part of the structure, resulting in a concentrated electric field with high intensity along with an extraordinary scattering of light. Hence, such structures have great potential toward enhancing the absorption of light in the active medium [85, 86]. The main drawback of incorporating metal NPs in the active medium is that it can create the recombination centers for electron and hole recombination and thus degrades the performance of the device. Wherein, encapsulation of these particles within some non-absorbing shells such as TiO2 and SiO2 can resolve this problem effectively [87]. Furthermore, nowadays two-dimensional (2D) monolayer materials, such as graphene and carbon nanotubes, have gained significant attention in the field of OPVs for enhancing their optical absorbance [19, 88, 89].

6 Transport Layers The OPVs consist of two carrier transport layers, placed between the electrode and active layer as shown in the layer stack of OPVs. Figure 4. The layer based on materials that favour only electron transportation is known as an electron transport/selective layer (ETL), while the other, based on a material that makes only hole migration possible is known as hole transport/selective layer (HTL). In an OPV, especially with the bulk heterojunction without ETL and HTL, both the donor and acceptor can reach the anode and cathode as can be seen from Fig. 7b. This provides a pathway to electrons as well as holes leading to the flow of these carriers toward both the electrodes simultaneously. This affects the device performance and hence reduces efficiency. Incorporation of ETL and HTL allows a selective flow of electrons and holes, respectively, i.e., only electron can pass through the ETL and reach the cathode, whereas only hole can pass through the HTL and can reach the anode. The migration of only electron/hole through these layers is due to the suitable positioning of their

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energy levels. Moreover, the transport layers play a very crucial role in OPVs in terms of enhancing their PCE and long-term stability, protecting the active layer from the negative impact of water and oxygen in the environment [90]. Excellent conductivity, high transparency, and chemical stability are the key properties for a material to be incorporated as a transport layer [13]. The materials used as ETL in OPVs mainly include polymer electrolytes, small molecules, fullerenes, metal fluorides, and n-type metal oxides. Among these ETL materials, organic materials have gained significant attention due to their adjustable molecular structure and compatibility with solution processing for large-scale manufacturing [14–18]. Wherein the materials used for HTL can either be inorganic or organic. The commonly used inorganic material as HTL includes molybdenum trioxide (MoO3 ), nickel oxide (NiO), tungsten trioxide (WO3 ), iron oxide (Fe3 O4 ), ruthenium oxide (RuO2 ), molybdenum sulfide (MoS2 ), etc., [19–24]. Even though OPVs incorporating inorganic HTL have gained significant PCEs, they are not compatible with large-scale roll-to-roll manufacturing, due to no use of high-temperature vacuum processing. Whereas, PEDOT:PSS, PCPDTT, PEDOT-S, DVTPD, etc., are some new emerging organic HTL materials, having compatibility with large scale manufacturing [91–94].

6.1 Effects of Nanotechnology in ETL and HTL Even though incorporation of plasmonic nanostructures in the active layer has several advantages, some negative effects, such as the creation of recombination centers can decrease the number of available free charge carriers, thus reducing the photogenerated current and hence the efficiency [95]. Moreover, the nanostructure does not form a continuous network, which can worsen the transfer of charge between the nanoparticles and the acceptor [96]. These disadvantages can be encountered by placing plasmonic nanostructured in the transport layers instead of the active medium of the device. The incorporation of metallic NPs in transporting layers ETL or HTL may result in forward or backward scattering, depending upon the position where they are placed in the device stack as shown in Fig. 13. The scattering increases the optical path of the incident photons and its interaction time with the active layer and thus enhancing the optical absorption in the active Fig. 13 Schematic diagram of NPs incorporated in a HTL, b ETL. Source Author

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medium. By tailoring the geometry and size of the nanoparticles, the direction of scattering can be tuned. For the particle diameters below 50 nm, the scattering is predominated in the forward direction, whereas if the particle size is larger than 50 nm, more radiation gets scattered backward [97]. If the metallic nanoparticles are placed in the front transporting layer, the forward scattering is utilized to increase the optical path length of the incident photons and to enhance the absorption Fig. 13a. However, if they are placed in the rear transporting layer, the backward scattering is utilized, which scatter back the incoming photons toward the active medium, improving the absorption as shown in Fig. 13b. The incorporation of Au NPs, in the front electron transport layer (ZnO) of an OPV (ITO/ZnO/PTB7-Th:PC71 BM/MoO3 /Ag), has resulted in an increase of 24% PCE with the optimized concentration of 0.05 wt%, as compared to the pristine efficiency of the device [98]. The J-V characteristics of the device with/without Au NPs are shown in Fig. 14a. It can be observed that the device with pristine ZnO layer exhibits lower short-circuit current density (J SC ) as compared to that of Au NPs doped ZnO layer. The increase in current is attributed to factors such as scattering and the plasmonic effect of Au NPs. The corresponding enhancement in the external quantum efficiency (EQE) can be seen in Fig. 14b. The current density and the EQE are maximum at the optimum concentration of 0.05 wt% of Au NPs doped in the ZnO. In addition to conventional NPs other plasmonic nanostructures such as hybrid NPs with CNT and nanoarrows, have also been explored for incorporation in the transport layers. Incorporation of Au nanoarrows in the ZnO ETL leads to lower resistance of ETL and higher J SC and EQE. The 27.3% increase in PCE has been obtained with an optimized concentration of 1.5 wt% of Au nanoarrows [99]. An ETL comprising of the hybrid of ZnO NPs and CNTs decorated Au NPs has boosted up the PCE by 7.9% [100]. The incorporation of metallic NPs in HTL such as PEDOT:PSS, MoO3 , or WO3 has also resulted in enhanced PCE. For instance, the incorporation of bimetallic Au–Ag NPs in the WO3 HTL extended the absorption spectra due to

Fig. 14 Influence of Au nanoparticles incorporation in ZnO ETL. a J-V characteristics for a varying concentration of Au NPs and b corresponding effect on external quantum efficiency. Reprinted with permission from Elsevier, Ref. [98]

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the LSPR effect and backward scattering which enhances the absorption spectra due to the LSPR effect and backward scattering [74].

7 Electrodes For the collection of charge carriers (obtained by breaking of excitons), OPVs require two electrodes, i.e., anode and cathode. The high conductivity, suitable positioning of the energy levels, and excellent transparency of at least one of the electrodes are the key properties to be considered. Generally, the top electrode (anode/cathode) is a transparent conductive oxide (TCO) such as zinc oxide (ZnO) in the form of thin-film or nanoparticles, indium tin oxide (ITO) and tin oxide (SnO2 ), etc. [101– 104]. Whereas the bottom electrode consists of a metal such as aluminum (Al), cupper (Cu), silver (Ag), or gold (Au), etc. However, during the past few years, various materials such as doped metal oxides, transparent conductive polymers, thin metal layers have been successfully incorporated as top transparent electrodes. The composite, transparent and flexible top electrode has also been used in OPV [105]. More recently the advancement of nanotechnology has gained significant attention toward the development of various nanoscale materials, such as carbon nanotubes (CNTs), carbon nanosheets (CNSs), metal nanoparticles and nanowires, graphene, etc. The excellent optical and electrical properties of these materials, along with their compatibility of solution-processing for large area manufacturing, indicates that these are the promising materials for the transparent electrodes (TEs) to be used in OPVs [106–109]. For instance, the incorporation of Ag nanoparticles on the top of the ITO surface improves the PCE from 3.05% to 3.69%, majorly due to the enhanced photocurrent, directly attributed to the high optical absorption in the active medium as a result of plasmonic effects. The J-V characteristics and the incident photon to current conversion efficiency (IPCE) of doped ITO with Ag NPs are compared with the pristine (reference) device as shown in Fig. 15a and b, respectively [110]. Moreover, other than the embedment of metal NPs in the TEs, the films of a randomly distributed network of metal nanowires (NWs) have also gained significant attention. The metallic nanostructures such as gold (Au), silver (Ag), copper (Cu), cupronickel (Cu–Ni), etc., have been demonstrated as promising candidates for TEs. The Ag nanowires (NWs) are the prime focus of the research community due to their excellent electrical and optical properties [109, 111, 112]. Further, graphene exhibits several properties such as ultrahigh carrier mobilities, excellent optical transmittance, good mechanical flexibility, and great stability, therefore, it demonstrates a high potential, to get incorporated as electrodes (cathode/anode) in OPVs. Although the sheet resistance (RS ) of intrinsic monolayer graphene transferred onto a glass substrate is much higher at ∼980 /sq, it exhibits an excellent transmittance of 97.6% [113]. The problem of high RS can be resolved by stacking the layers of graphene, as the RS of the graphene reduces with the increasing number of layers but consequently lowers the transmittance. Therefore, the optimization of

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Fig. 15 a J-V Characteristics and b IPCE spectra of ITO doped with Ag NPs OPV and reference OPV. Reprinted with permission from AIP Publishing, Ref. [110]

layer stacking is necessary so that sufficient transmittance and lower RS can be maintained for OPVs applications. The optimized condition for intrinsic graphene layers is found for the two layers staking (2L-G) with the reduced RS (∼320) /sq along with the average transmittance of 94 ± 0.5% at wavelength 550 nm [114]. Furthermore, the sheet resistance of 2L-G can be modified by employing doping techniques. For instance, the thin film of ZnO prepared on graphene and the incorporation of ZnO NPs on graphene surface increases the RS to ∼780 /sq and ∼380 /sq, respectively. Whereas the coating of PEDOT:PSS followed by the thin layer of ZnO NPs on the graphene results in a much lower RS of ∼230 /sq. All these three structures have been incorporated as a cathode in the OPV (Au/MoO3 /PTB7:PC71 BM/buffer layer/ graphene/glass) [115]. Here the buffer layer consists of any one material from ZnO film, ZnO NPs or PEDOT:PSS followed by a thin layer of ZnO NPs. The current– voltage (J-V ) characteristics and the corresponding external quantum efficiencies (EQEs) of the device are shown in Fig. 16a and b, respectively. The schematic of the energy bands for the device Au/MoO3 /PTB7:PC71 BM/ZnO-NPs/PEDOT:PSS graphene/glass is inserted in Fig. 16a. The resulted PCEs of the device are 4.64, 4.89, and 5.78% with the graphene cathode coated by the ZnO film, ZnO-NPs, and PEDOT:PSS/ZnO-NPs buffer layers, respectively. The graphene layer coated with a thin layer of PEDOT:PSS results in a much lower RS of around ∼170 /sq has been incorporated as anode resulting in a semi-transparent OPV having the device structure DMS/PMMA/graphene/PEDOT:PSS/PTB7:PC71BM/ZnO-NP/ITO/glass. The energy band structure of the device is inserted in Fig. 17a. This device can be illuminated from both sides. The J-V characteristics for the device are compared when the device is illuminated from graphene and ITO sides is shown in Fig. 17a and the corresponding EQEs are compared in Fig. 17b. The PCEs for the device have been reported as 3.84% and 4.23% when illuminated from ITO and graphene sides, respectively [115].

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Fig. 16 The characteristics of (Au/MoO3 /PTB7:PC71BM/buffer layer/ graphene/glass) OPV comprising graphene cathode with different buffer layers. a J-V characteristics. b EQE spectrums. Reprinted with permission from ACS publication, Ref. [115]

Fig. 17 For an OPV (PDMS/PMMA/graphene/PEDOT:PSS/PTB7:PC71BM/ZnO-NP/ITO/glass) with graphene anode and ITO cathode measurement of a J-V characteristics and b EQE spectra illuminated from either side. Reprinted with permission from ACS publication, Ref. [115]

8 Substrate In the fabrication process of OPVs, the substrate plays the role of support providing mechanical strength to the device. Two types of substrates are mainly used in the fabrication of OPVs, i.e., the plastic substrate and the glass substrate. The glass substrate with ITO as a transparent electrode is used in lab-scale production while the flexible poly(ethylene terephthalate) (PET) foil substrate is used in upscaled production where the transparent electrode is either ITO or a printed transparent electrode [116, 117]. Furthermore, the substrate has also played a remarkable role in improving the optical absorption within the active layer. For instance, texturing

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Fig. 18 The micro-structured PET film with line patterns, SEM images with periods a 1.8 µm and b 2.6 µm, c deposited OPV on the patterned side of PET film, d schematic illustration of patterned PET film. Reprinted with permission from AIP publishing, Ref. [120]

of substrate results in extraordinary scattering of incident photons, increasing the optical path within the active medium, in which more light can be absorbed. The increase in photon absorption provides a remarkable improvement in the efficiency of the device [118, 119]. The line pattern with the period ( ) 1.8 and 2.6 µm has been created on the PET substrate by using direct laser interference patterning (DLIP) as shown in Fig. 18a and b. An OPV fabricated on the patterned side of this substrate is depicted in Fig. 18c. The structured line pattern on the PET film diffracts the light in guided modes of the active layer, resulting in increased optical path length. The optical path increment depends on the angle of diffraction, where larger diffraction angles lead to a longer optical path inside the active layer. However, it has been observed that an OPV fabricated on the patterned side of PET substrate results in low PCE and poor fill-factor (FF) due to inappropriate coverage of polymer layers on the sharp edges of the structured line patterns as can be seen in Fig. 18c, while the OPV constructed on the planer side of the structured PET substrate exhibits outstanding performance. The J-V characteristics and the EQEs spectrum of the OPV are shown in Fig. 19a and b, respectively for performance comparison between the device fabricated on the untextured (planer) PET substrate and that of fabricated on the planer side of line-patterned PET substrate. The PCEs of the three OPVs were obtained as 6.62, 7.70, and 7.39% with planer PET substrate and with the line-patterned PET substrates having periods 1.8 µm and 2.6 µm, respectively. The later two PCEs are higher as compared to the first PCE, as expected because the later two PCEs correspond to the OPVs fabricated on the planer side of the line patterned PET substrate. Moreover, it can be seen that the maximum PCE (i.e., 7.70%) is obtained for the OPV fabricated on the planer side of line-patterned PET substrate with a period of 1.8 µm [120]. V

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Fig. 19 Comparison between the performance of the OPVs fabricated on unstructured and on planer side of line patterned (with period 1.8 µm and 2.6 µm) PET substrates, a J-V characteristics, b EQEs spectrum. Reprinted with permission from AIP publishing, Ref. [120]

9 Geometric Description In general, depending upon the layer stack geometry, OPVs can have two architectures conventional and inverted as shown in Fig. 20. In OPVs, the layer on which the sunlight impinges is treated as the top layer close to the substrate. In a typical conventional or normal structure, the electrode coated on the substrate behaves as an anode which is the top layer in this case. A low work function metal electrode on the top of the layer stack works as the cathode. The HTL is inserted between the anode and active layer and ETL is used between the cathode and the active layer, as shown in Fig. 20a. Conversely, in the inverted geometry, the transparent electrode at Fig. 20 Schematic diagram of an OPV device: a conventional architecture, b inverted architecture. Source Author

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the substrate act as a cathode, and a high work function metal on the top of the layer stack is an anode. The positions of the ETL and HTL are also interchanged. Therefore, the direction of the photogenerated current in inverted structure is reversed as compared to conventional structures as shown in Fig. 20b [121]. The inverted OPVs exhibit better performance than those of conventional OPVs. The desired condition, for the proper functioning of OPVs, requires gradual distribution of donor and acceptor materials toward the anode and the cathode, respectively. However, due to the lower surface energy of donor materials (for example that of P3HT) as compared to the acceptor counterpart (like PCBM), the donor materials have a strong tendency to move toward the surface, resulting in an unfavorable distribution of donor material in the conventional architecture, having cathode at the top, this is known as vertical phase separation [122] (Fig. 20). This problem can be overcome by making use of the inverted architecture of OPVs where the top electrode (anode) is formed by using a high work function metal and the bottom electrode (cathode) is a transparent conducting oxide such as ITO. The vertical phase separation is favorable in inverted geometry, as it has anode at the top instead of a cathode, resulting in efficient carrier transportation. Moreover, the use of high work function metal as a top electrode (anode) in inverted OPVs provides better stability as it does not degrade in the ambient [123, 124].

10 Stability and Degradation Aside from PCE, another two important aspects, namely the stability and the processability of OPVs are the issues of concern to make the OPVs viable. All these three areas: efficiency, processability, and stability are of equal importance and are defined as the unification challenge collectively for OPVs as shown in Fig. 21 [125, 126]. The focus on just one of these areas is very unlikely to provide the solutions for Fig. 21 Illustrating the unification challenge, as the unification of efficiency, process, and stability for the same material. Source Author

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the remaining two areas inherently. Unfortunately, the prime focus of industrialists and the research community long remained toward improving the PCEs of OPVs, to compete with highly matured Si-based PV technology. However, until the focus will not be taken collectively on all these areas together, the progress of the OPV technology applications will remain slow. In recent years, stability and processability have gained the significant attention of researchers. The processing of OPVs has become more mature with the advancement of new fabrication technologies and their compatibility with the roll-to-roll manufacturing for large area manufacturing of OPVs [127, 128]. Therefore, the major issue that remained with the use of OPV technology is their poor stability leading to low operational lifetimes. It has been observed that due to the poor stability, OPVs struggle to last even a year, whereas their inorganic counterpart Si-based PV can last up to order of 25 years. The poor stability of OPV arises due to the degradation of polymer materials under illumination as well as in the dark. It has been observed that organic polymers are more prone to chemical degradations as compared to their counterpart inorganic materials. The degradation process of OPVs can be categorized as chemical and physical degradations. Primarily the molecular diffusion of oxygen and water molecules into the organic layers accounts for the chemical degradation. The physical degradation generally indicated the degradations arise due to mechanical delamination and morphological instabilities [125, 129].

10.1 Chemical Degradation The chemical degradation in OPVs is primarily subjected to the molecular diffusion of oxygen and water molecules in the device. Water molecules and oxygen are considered as the extrinsic sources of chemical degradation. Oxygen and water sensitivity of polymer materials have a strong tendency toward photo-oxidizing the organic layers and the interfaces in the layered structure of OPVs [130, 131]. In the presence of oxygen, there is a high chance of electron transfer to an oxygen molecule, leading to a high concentration of free radicals in the device. This oxidation strongly affects the electrical and optical properties of the material as well as the interfacial energetics [131, 132]. Similarly, in metal electrodes, the low work function metals act as a reducing agent, especially in the presence of hydrogen donating reagents like water. Consequently, metal oxides are formed between the metal electrode and the rest of the device. This metal oxide layer creates a barrier and restrains the carrier transportation. The encapsulation of the device, as detailed in Sect. 11, provides a pathway to reduce the oxygen and water-induced degradations [133–137]. Moreover, the advancement of nanotechnology has provided excellent options toward protecting the OPVs degradations due to extrinsic sources. For instance, the use of multilayer graphene as a top anode provides excellent protection to the device against water and oxygen diffusion. The stacking of graphene sheets results in narrow spacing between the two successive sheets and the pores of the underneath sheet are sealed by a sheet on its top. Therefore, the oxygen and water molecules cannot diffuse through this

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multilayer structure of the graphene and the system exhibits high potential toward improving the device stability. The aging effect of the photovoltaic parameters such as J SC , FF, and PCE indicates that the devices with graphene anode provide better stability as compared to the device structures which use gold (Au) as anode instead of graphene as can be seen in Fig. 22 [114]. However, increasing the number of graphene layers reduce the transparency, thus the optimization of layers number is required to provide sufficient transparency as well as a blockage to oxygen and water molecules diffusion. The optimized thickness of graphene layers is obtained with a two-layer system, as a further increase in the graphene layers does not provide any significant change in the degradation speed of the devices comprising more than two graphene layers as can be from Fig. 22. The intrinsic sources of chemical degradation, such as ultraviolet (UV) light are particularly more dangerous, as they cannot be stopped from entering the system simply by encapsulation. UV light is the primary source of intrinsic degradation in OPVs, framing the photochemical reactions. The energy of UV light is sufficient to tear up the carbon–carbon bond of polymers [138]. To minimize this type of degradation, UV filters can be used [133, 139]. Moreover, in organic materials, excitons with

Fig. 22 Aging effect on photovoltaic parameters evaluated in air, including a short-circuit current (JSC), b Fill-factor (FF), and c Power conversion efficiency (PCE), for the OPVs consisting of 1 to 4 layers of graphene or Au as the top anode. Reprinted with permission from John Wiley and Sons, Ref. [114]

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longer life due to molecular fragmentation of polymers have also been reported [140]. Therefore, rapid quenching of these excitons has shown an outstanding pathway towards restricting the photooxidation in polymers [141]. Photo-oligomerization of fullerenes is also an intrinsic class of chemical degradation of OPVs, leading to burn-in and substantial losses [142–146].

10.2 Physical Degradation The OPVs contain multiple layers, composed of dissimilar materials. Under mechanical stress, these layers particularly the flexible ones, are subjected to mechanical delamination. This stress is unavoidable, which can occur in R2R fabrication, transportation, and during the installation of OPV modules. Moreover, environmental conditions such as wind and rain can also lead to mechanical stress to the OPVs. The delamination of layers reduces the contact area at the interfaces between the layers hindering the carrier transportation and extraction, thus degrading the overall performance of the device [147]. Delamination is a type of extrinsic source of physical degradation, whereas the intrinsic source of physical degradation refers to the morphological instabilities of the device. As we already discussed, D and A materials are blended resulting in bicontinuous nanoscale morphology in the active layer. An optimum morphological condition is required for efficient dissociation of excitons and suppression of the carrier recombination simultaneously. Although, at equilibrium, the optimum morphology is not a thermodynamically steady-state case. Therefore, once the optimum morphology is formed, it will slowly stabilize toward a steady-state causing morphological phase separation. In most cases, the high mobilities of D and A materials further contribute to this phase separation along with, the relatively high temperature of the device due to the continuous illumination under the sun, which provides additional thermodynamic energy, accelerating the morphological phase separation. The phase separation of materials within the active layer, at longer distances than the diffusion length of excitons, reduces free carrier generation in the device [148–150]. The temperature, at which the degradation in OPVs begins, is correlated with the glass transition temperature (T g ) of the constituent materials [151]. Moreover, the transport layers and the electrodes material exhibit a strong tendency to diffuse in the active layer and into each other due to their respective mobilities. The diffusion of these layers alters the energy levels of the layers and introduces the trap centers in the active layer, which enhances the non-radiative recombination [152]. One of the progressively explored approaches to improve the thermal stability of OPVs is the use of fullerene mixtures, which either prevents the crystallization of the fullerene acceptor or results in the controlled nucleation of fullerene crystals [153–157]. Moreover, Non-fullerene-acceptors (NFAs)-based OPV cells have been reported with enhanced thermal and photochemical stability [158, 159]. The lifetime of these devices is also longer when compared with those based on fullerene acceptors. Along with this, the burn-in losses can also be countered by making use of NFAs [160, 161].

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11 Encapsulation The different degradation challenges of OPVs can be controlled by encapsulation techniques. Encapsulation act as a barrier against the sources of degradations such as oxygen and moisture, it also provides mechanical stability to the device. Along with this, it can act as a UV filter, to stop the photochemical reactions filtering the most harmful part of the solar spectrum. Although, the exact requirements for encapsulation depend on the factor such as the materials used to design the layer stack of the device. High optical transmittance, light weight, low cost, and flexibility are the key feature of a material to be used for the encapsulation of OPVs [162, 163]. The most general example of encapsulation includes complete sealing of the device, within a glass case with an inert atmosphere. Further, the sealing of the device with a suitable adhesive between the top glass sheet and the bottom substrate has also been explored. The ability of moisture and oxygen to penetrate the barrier film is measured as the water transmission rate (WVTR) and oxygen transmission rate, respectively. For OPVs, it has been observed that a barrier film with WVTR of 10–3 g (m2 day)−1 along with 40% relative humidity (RH) at 25 °C has the potential to ensure 3–5 years life of device [164]. These two discussed methods of encapsulation ensure the zero-transmission rate of oxygen and moisture but compromise the flexibility of OPVs [133, 165]. The most common encapsulation techniques, compatible with the large-scale fabrication, are laminating OPVs module with thin-film barrier foils or thin-film encapsulation (TFE) is directly deposited onto the sub-cells of the module. In the former case, flexible barrier foil acts as the substrate for the device, and the top of the device is laminated by a second foil or the device is proceeded on a flexible substrate and consequently sandwiched between two barrier foils. The earliest TFE encapsulation techniques comprise of the poly(ethylene terephthalate) (PET) and foils as a barrier, but this does not provide sufficient protection due to their high WVTR, and OTR of 4 g (m2 day)−1 and 10 cm3 (m2 day bar)−1 respectively [162, 164, 166, 167]. More recently, the perhydropolysilazanes (PHPS), an inorganic polymer, have emerged as an excellent option for encapsulating the OPVs. It has been observed that the adhesive laminated barrier films, prepared by deep UV light irradiation on PHPS coated PET substrate, result in excellent barrier properties. However, the use of adhesives for laminating these films on the OPVs causes residual degradation, hence affecting the device performance [168]. This difficulty can be overcome by developing in-line processing of encapsulation in which the barrier material (PHPS) is directly applied on the device in the form of solution and subsequently irradiated by vacuum UV light [166]. The resulting barrier film shows WVTR, RH, and OTR of 10–2 g (m2 day)−1 , 85%, and 10–2 cm3 (m2 day bar)−1 , respectively. The lifetime test for several hours, investigated for a P3HT:PCBM OPVs with PET barrier foil, with a commercial available Mitsubishi barrier, and with a PHPS barrier foil, demonstrate that the OPV encapsulated by the former two did not show any significant degradation, while the device encapsulated with the plain PET foil dies within less than 50 h as shown in Fig. 23.

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Fig. 23 The lifetime test for several hours for an OPV based on P3HT:PCBM encapsulated by plain PET, commercial Mitsubishi barrier, and PHPS foils. Reprinted with permission from John Wiley and Sons, Ref. [166]

12 Loss Mechanism Following thermodynamic limits, there is always a certain amount of energy loss in the process of light to electrical energy conversion, in a photovoltaic cell, even if an ideal semiconductor is used to fabricate the device. Imperfect absorption, Carnot loss, and etendue expansion are the core factors contributing to this energy loss [36, 169, 170]. The first factor refers to the imperfect absorption of photons, while the remaining two are evidence of additive entropy losses. In a typical heterojunction OPVs, external quantum efficiencies (EQEs) have exceeded over 85% along with nearly 100% internal quantum efficiencies (IQEs) [171–173]. Wherein their open circuit voltage (V OC ) and fill-factor (FF) are still limited and hence are the major contributor to energy loss. Thus, the incurred energy loss in an OPV is defined as the energy difference between the optical band gap (E g ) of the material and eV OC . For a typical photovoltaic cell, the maximum theoretical limit for V OC can be estimated by Shockley–Quessier’s (SQ) theory [36]. Applying the assumptions of SQ theory, under suppressed nonradiative recombination, GaAs photovoltaic cell exhibits a small V OC loss of 0.3 V [174]. Whereas, in OPVs, the total voltage loss ranges from 0.7 to 1 eV [176]. Therefore, OPVs exhibits more energy losses, than their inorganic and metal halide perovskite counterparts, limiting their efficiencies below 20% [177, 178]. The energy losses in OPVs incurred due to three main reasons, i.e., the energy transfer loss in the successive separation of the electron, from bounded excitons in the D/A materials, into e–h pairs known as charge transfer (CT) states along with the radiative and nonradiative recombination as shown in Fig. 24 [175].

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Fig. 24 Schematic illustration of energy loss mechanism in a typical OPV. Reprinted with permission from AIP publishing, Ref. [175]

12.1 Charge Transfer Loss The reason behind the additional energy loss in a typical OPV cell is primarily due to the excitonic character of photogenerated e–h pairs. It has already been discussed that excitons are e–h pairs tied together by the Coulombic forces. The binding energy of an exciton is strongly influenced by the chemical nature of molecule and its surrounding environment in the bulk. In organic semiconductors, the excitonic binding energy is in the order of 0.5 eV or more [52, 53]. Therefore, efficient separation of charge carriers from the excitons requires a heterointerface (i.e., D/A interface) providing an  energetic offset E o f f set . This mechanism leads to the formation of charge-transfer (CT) states with energy (E CT 100,000 cycles) [96].

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5.2 Pseudocapacitor Pseudocapacitors are a special class of supercapacitors that store the electric charge through the faradaic mechanism. They have electrodes that are made up of redoxactive materials. In a report, the room temperature pseudocapacitive properties of PANI nanofibers delivered a high specific capacity of 901 Fg−1 at 10 mVs−1 . PANI samples synthesized via ultrasonic emulsion-assisted polymerization showed a high capacitance of 704 Fg−1 at 10 mVs−1 . The ultrasound-assisted emulsion polymerization has the advantage of improved polymerization rate, even particle size distribution, and high monomer conversion than the conventional emulsion polymerization process. The results showed that it is possible to obtain PANI with an adequate electrochemical performance which is extremely important for the industrial purpose [97]. Liu et al. [98] reported PANI and PPy as excellent materials for pseudocapacitor electrode with good cycling stability. They displayed remarkable capacitance with a good retention rate ~95% after 10,000 cycles. Another group reported PANI solid-state pseudocapacitor morphology-based performances with CTAB surfactant using PVA/H2 SO4 . The pseudocapacitor cell exhibited the highest specific capacitance 367 Fg−1 at 5 mV/s with 91.5% retention after 1000 GCD cycling [99]. Ebrahim et al. [100] reported CP-based pseudocapacitor electrode on a flexible substrate. It exhibited a high specific capacitance of 594.92 Fg−1 at 5 mV/s in a −0.8 to 0.8 V potential window. Results also displayed 82.63 Wh/kg energy density using 4M KOH aqueous electrolytes.

6 Supercapacitor 6.1 Flexible Supercapacitors Flexible CNTs/PANI supercapacitor electrodes display high capacitive behaviour of 660 Fg−1 at 1 Ag−1 and low charge transfer resistance of 0.5 . This flexible supercapacitor also shows an enhanced specific capacitance of 439 Fg−1 at 0.05 Ag−1 after 500 cycles and an excellent stability of 95.4% [101]. Yanilmaz et al. [102] fabricated flexible PANI-CNF composites and analyzed them for supercapacitor electrodes. They fabricated a symmetric flexible CNFs supercapacitive cell via sol–gel and electrospinning techniques to obtain enhanced electrode properties. It showed a high capacitance of 234 Fg−1 with good cyclic stability of 90% after 1000 cycles. Films of PANI nanofibres (PANI-NFs) and chemically converted graphene (CCG) composite films were prepared by vacuum filtration. It was envisaged that it was highly flexible and was more mechanically stable. The conductivity of CCG (5.5 × 102 Sm−1 ) was about 10 times that of a PANI-NF film and had a specific capacitance of ~210 Fg−1 at a discharge rate of 0.3 Ag−1 [65].

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6.2 High-Rate Supercapacitors PANI nanofibres prepared in PPD at a current density of 0.18 Ag−1 [33] had the specific capacitance value ~548 Fg−1 and a specific power value of 127 Wkg−1 . Mao et al. [103] HSAFCs obtained high specific capacitance in a 2M KOH aqueous solution. The values of specific capacitance obtained were 474 Fg−1 at 0.5 Ag−1 and 285 Fg−1 at 100 Ag−1 at high currents with potential window 1.8 V. The device gave an excellent performance after 10,000 cycles with 93% capacitance retention. In another study, Graphene/Pt films exhibited 120 Fg−1 at 50 Ag−1 . The device indicated a good potential of rGO in the field of energy storage [104].

6.3 Smart Supercapacitors The fabrication of a smart electrochromic supercapacitor using Au mesh and PANI electrodes was reported by Kiruthika et al. [105]. The device displayed a high specific capacitance of ≈15 mFcm−2 with a good charge/discharge cycling stability. Another metal-oxide W18 O49 /PANI-based smart supercapacitor having excellent electrochromic behaviour and operating at different potential ranges [106] was reported. Nano-morphology of PANI [107] has been one of the pioneer electroactive materials which has paved way for the commercialization of smart supercapacitors. This particular nano form of PANI has shown superior electrochemical properties for future generation smart electrochemical energy storage systems.

6.4 Electrolytic Supercapacitor Aluminium solid electrolytic capacitors with PANI doped with inorganic and organic acids were fabricated and their electrochemical properties were studied [108]. In one analysis, PANI/Mesoporous MnO2 obtained a specific capacitance of 1405 Fg−1 which was 10 times more enhanced than that of pure MnO2 . It was observed that PANI/Mesoporous MnO2 composite has a good discharge property (857 F/g) at high current density. It was also analyzed that the composite exhibited a good cycling stability after 500 cycles and also retained 90% of its original capacitance [109]. Ruo et al. [110] prepared PANI doped with LiPF and HCl, respectively, using a chemical method. They obtained an initial capacitance of ~115 F/g using two electrodes in a polymer electrolyte membrane.

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7 Physical and Electrochemical Characterization of PANI-Based Supercapacitors 7.1 SEM Analysis of Different Morphologies of PANI Figure 3a, b displays the SEM micrographs of PANI nanostructures at different urea to aniline ratios. It was analyzed that the PANI sample exhibited aligned nanowires (100–150 nm), which were all oriented perpendicular to the substrate. Figure 3c displays the SEM micrographs of P porous PANI-S2 and PANI nanofibers. The nanofibers morphology of PANI is due to a faster polymerization rate where 1D PANI is easily synthesized by homogeneous nucleation and suppression of the secondary growth. Yolk–shell structures of S-PANI were clearly observed in Fig. 3d by Zhou et al., where the PANI structure did not shrink during vulcanization which indicated the porous nature of PANI. They also suggested that due to the porous structure of PANI, it was able to retain its structure when diffusing out of sulphur at a specific temperature. This resulted in the strong mechanical strength of the PANI shell during the heat treatment. They suggested that this can be attributed to the crosslinking of sulphur bonds (sulphide/disulphide) in the PANI chains.

Fig. 3 SEM images of a) PANI nanotubes, b) PANI nanowires, c) PANI-S2 nanofibres, d) S-PANI core–shell composite Reproduced with permission from [30, 111–113]

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7.2 Cyclic Voltammetry (CV) Analysis CV is an important analysis to calculate the capacitive nature of the materials. The ideal rectangular-shaped voltammogram indicates symmetric cathodic and anodic reactions and a large current separation between the charge and discharge cycles. The fact that voltammograms do not show perfect box-type rectangular images implies that there is a contribution of significant pseudocapacitance to the overall calculated direct capacitance. It is being noted there that the capacitance values are calculated by measuring the ratio of the magnitude of the current separation with the scan rate. Figure 4a displays the CV measurement of PANI nanostructured electrodes. It is displayed that all PANI nanostructured samples have similar CV curves resulting in pseudocapacitance. PANI-3 in Fig. 4a has a larger CV area than other PANI nanostructured materials, which shows good electrochemical performances due to the surface to volume ratio in comparison to nanorods. On the other hand, there is a fast reaction kinetics in PANI-3 nanotubes because they carry a higher current than nanorods. It is considered that PANI nanowire arrays are good electrode material for supercapacitors. The CV curves of PANI nanowire within −0.2 to 0.85 V versus

(a)

(b)

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Fig. 4 CV curves of a) PANI nanotube at scan rate 5 mV/s, b) PANI nanowire array at different scan rate, c) S-PANI yolk–shell at 0.05 mV/s and 0.02 mV/s. Reproduced with permissions from [111–113]

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SCE at different scan rate in 1M HClO4 solution is shown in Fig. 4b. The typical voltammogram shows two redox peaks. The first peak relates to the conversion from the leucoemeraldine base to emeraldine salt and the second redox peak describes the conversion between emeraldine salts and pernigraniline base character of PANI. Therefore, it was analyzed that the peak current increases with increasing the scan rate, which showed good performance of PANI nanowires arrays. Here, it is indicated that the redox peaks generally come from the conversion of the salts: leucoemeraldine to emeraldine and from hydroquinone to benzoquinone, respectively. This is the diffusion process of the redox reaction in PANI peaks which is influenced by the capacitance of counter anions. CV curves of heat-treated S-PANI yolk–shell cathode with Li–S were analyzed at 0.05 and 0.02 mV/s and are shown in Fig. 4c. The two reduction peaks 2.35 and 2.08 V were centred at 1.9 V at a lower scan rate. Also, it was observed that the peak at 1.9 V could be seen in every reduction peak of the sample. Hence, this 1.9 V peak was assigned to the reduction of disulfide bonds with PANI during vulcanization.

7.3 Galvanostatic Charge–Discharge (GCD) Analysis GCD is another critical analysis parameter for the prediction of the performance of the supercapacitor cell. A constant current I is applied to the working electrodes and voltage Vs time is recorded between initial and final potential window values. PANI nanotubes were examined for electrode properties and the obtained charge– discharge curves (Fig. 5a) were analyzed from 0.2 to 1 Ag−1 . It was observed that the specific capacitances decreased to 313 Fg−1 and 295 Fg−1 at current densities 0.5 Ag−1 and 0.7 Ag−1 respectively. This was due to a faster diffusion rate that could not catch up with the redox reaction at a large value of current density. The capacitance was still 263 Fg−1 at even 1 Ag−1 which was 64.9% of the capacitance 405 Fg−1 at 0.2 Ag−1 . This indicated a good rate performance of the PANI nanotubes. The specific capacitance is calculated using the formula Csp =

(I ∗ t) , (m ∗ 4V )

(1)

where I and t are charge–discharge current and time, 4 V is 0.7 V in the measurement, and m is the mass of PANI film. They calculated the mean value of five consecutive charge–discharge cycles. It was concluded that specific capacitance decreased with an increase in the current density. The calculated specific capacitance of PANI nanowire arrays was about 950 Fg−1 at 1 Ag−1 . They reported that the achieved specific capacitance of 780 Fg−1 was ~ 82% higher than previously obtained specific capacitance. During the faradaic process, there can be an initial IR drop in voltage that may be generated by the internal resistance of the electrolyte and the PANI electrodes.

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(b)

)

(a)

4 1

Pote

2 3

Time

(c)

(d)

Fig. 5 GCD curves a) PANI-3 nanotubes at various current densities, b) PANI nanowire at several current densities, c) S-PANI yolk–shell at 0.2 C and, 0.5 C d) Basic curve of the galvanostatic charge–discharge cell, Reproduced with permission from [111–113]

Figure 5c shows the different discharge–charge profiles of electrode coin cells of heattreated S-PANI yolk–shell composites. There were two clear discharge plateaus of the S-PANI core–shell composite electrode observed which were different from the two less-well-defined discharge plateaus centred at 2.35 and 2.08 V for heat-treated S-PANI yolk–shell composites. Figure 5d shows the Cell behaviour during charging (1), during discharging (2), initial process (3), IR drop (4).

7.4 Nyquist Plot Analysis The Nyquist plot calculates the indirect specific capacitance of the cell and determines the complex capacitance and complex power through EIS data. It produces impedance

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Fig. 6 Nyquist plot of a) PANI nanotube, b) PANI nanowires, c) PANI and PT1-PT5, Reproduced with permission from [111, 112, 150]

of the cell in the terms of real impedance versus imaginary impedance. This technique also determines the electrical parameters of the materials such as relaxation time constant, response frequency, phase angle, and charge transfer resistance. These parameters are very significant for the characterization of a supercapacitor. Since interaction between the electrolyte and the electrode is an essential parameter, hence EIS (Fig. 6a) was done to further analyze the kinetics of the electrode process. The results of Nyquist plots were analyzed and displayed for different PANI nanostructure samples. Figure 6b shows the Nyquist plot of PANI nanowires in the 1M HClO4 electrolyte in the frequency range of 20 kHz to 1 Hz with 5 mV amplitude in the open circuit voltage. The junction of the plots at the X-axis represents resistance offered by the solution (Rs ) or equivalent series resistance (ESR). The plot shows that the strong electrolyte (1M HClO4 ) resulted in only a small solution resistance (Rs ) ~ 0.6 . In the high-frequency region, the semicircle displayed a charge transfer resistance (Rct ) ~ 0.12  in the electrochemical system, which was calculated from the slope of the curve at the low-frequency region. This low value of Rct of PANI nanowires signifies that the small diameters in PANI spheres help the electrolyte ions to penetrate inside the polymer matrix and were efficiently able to reach the inner layer of the polymer. Similar Rb , Rct resistance patterns were observed for PANI-NR (Fig. 6c). The only important difference noted in the Nyquist plots of

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Fig. 7 Bode plot of a) A-SC with the APPH-2 electrodes under 200% stretching, 50% compression, and 180° bending, b) MoS2 -NH2 /PANI-150. Reproduced with permission from [114, 115]

PANI-N and PANI-NR was that the slope at low frequency in PANI-NRs was less vertical compared to that of PANI-NS. This indicated a higher diffusion resistance because of large ion penetration across the electrode/electrolyte barrier.

7.5 Bode Plot Bode plot is another vital analysis in supercapacitors in describing the response frequency, response time, phase angle from resistive to capacitive behaviour. This plot calculates the relaxation time constant which can be calculated from capacitor response frequency. Figure 7a shows the gravimetric capacitances of A-SC with the APPH-2 measured from the GCD curves. The curves revealed that the gravimetric capacitance achieved was more than 85% when it was stretched to 200% and at 50% compression. It showed that A-SC could maintain 100% of its original capacitance. The highest phase angle of the MoS2 -NH2 /PANI-150 was observed close at 90° and characteristics response frequency were shown in Fig. 7b.

7.6 Cyclic Stability Analysis The cyclic stability analysis of the device is calculated by measuring the specific capacitance value at the 1st cycle and the nth cycle. This gives us the specific capacitance retention (%) and is a crucial factor in determining the long-term stability of the supercapacitor device. The Coulombic efficiency (ï) is expressed by the formula η (%) =

td × 100 tc

(2)

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(b)

Fig. 8 Cyclic stability of a) PANI, MWNT/PANI, and P-MWNT-PANI composite at current density 2 A/g, b) D-CNT and PANI/VACNTs at current density 4 A/g. Reproduced with permission from [116, 117]

where td is the discharge time and tc is the charging time of the device. Figure 8a generalizes the charge–discharge cycling capability of PANI, MWNT/PANI, and P-MWNT-PANI composite. There was 85% retention from the initial specific capacitance indicating both high-rate performance and excellent long-term stability of P-MWNT-PANI composites. But the capacitance of pure PANI decreased continuously through the cycling process. This is a feature of a poor stable material. PANI value decreased to 80 Fg−1 from 225 Fg−1 , dropped to 35.6% of its initial value. The reason was attributed to the covalent interaction between the active material and the electrolyte. The cycling stability of D-CNTs and PANI/VACNTs (Fig. 8b) were compared between the voltage 0–3 V at a current density of 4 Ag−1 during a galvanostatic charge/discharge cycle. D-CNTs displayed excellent reversibility and capacitance retention of 98.3% after 3000 cycles. However, PANI/VACNTs were also able to preserve 90.2% of the capacitive retention after 3000 cycles. This indicated a good interaction and a strong network between the PANI and CNTs in the PANI/VACNTs composites.

8 Beyond PANI-Based Supercapacitors It is already known that CPs are promising candidates for smart future supercapacitor applications. Some of the CPs have already established themselves and have gained immense popularity in the branch of electrochemistry. Although the main achiever is PANI but other CPs such as PPy [118–124], PEDOT [125–131], Polythiophene [132] are also gaining importance in this category. Normally, the specific capacitance of any CP composite/nanocomposite is not only dependent on the interaction between the constituent materials, but also more deeply dependent on the polymer structure (as can be seen from significantly different values reported for PANI supercapacitor

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in Table 1). It is important to understand here is that the polymer matrix being used for the preparation of flexible supercapacitor [133–146] has its usefulness in it. This is the current focus of the search being carried out for the supercapacitor application. Nevertheless, since all the materials used for making nanocomposites/composites interact with each other chemically, (i.e., metal oxides and carbon nanomaterials), the selection of the CP becomes crucial for designing the supercapacitors. All these CPs have been engaged for similar energy applications in the literature, but it will be useful to contrast the pseudocapacitive behaviour of different CPs for supercapacitors. Generally, these CPs suffer from limitations like low specific capacitance, swelling/deswelling, low cyclability, low stability, and low rate capability [147–149]. To avoid these limitations, most of them are combined with two/three materials such as metal oxides, carbon materials/nanomaterials that can overcome their limitation and deliver enhanced electrochemical properties. PPy is like PANI which delivers high specific capacitance (although less than that of PANI [150, 151]) because of similar architecture. However, in some cases, the electrochemical performance of PPy is closer to a perfect pseudocapacitive behaviour. It shows distinct redox peaks in the CV curves [122, 152]. The charge carriers in CP are stored in the polymer chain which causes them to self-discharge easily which is unlike the inorganic electroactive materials where the counter ions are stored within the lattice. This is a severe limitation for a few CPs such as Polyacetylene but moderately limits the electrochemical properties of PPy and Polythiophene [125].

9 Conclusions CPs have essential advantages over inorganic electrode materials in supercapacitor applications because of the polymer matrix. PANI is the first most researched electroactive material which paved the way and feasibility of development of pseudocapacitors. It is the most favourable candidate in terms of supercapacitors. Recently, PANI has gained considerable attention because of its unlimited advantages with electroactive electrode materials. The future scope of PANI-based supercapacitor builds upon the suitable network and well interaction with the advanced smart composites. The constituent materials making the composite or nanocomposites must be placed well in the PANI network or vice versa. Different architectural morphologies can attain flexibility in PANI electrode substrates. For example, flexible carbon substrate like carbon cloth for pasting inorganic electroactive materials can be used to achieve superior electrochemical performances of PANI. In some cases, PANI chain can combine with metal oxides to produce a good interconnected network within the nanocomposite. Carbon nanomaterials furnish a spacious scaffold for the PANI to develop on, but their capacitive offering is not as much as pseudocapacitive materials like metal-oxide nanoparticles. Moreover, PANI should be synthesized in such a way that it acts like an additive or a binder in supercapacitors to enhance its performance. Most of the current works are based on a feasible approach to evaluate the supercapacitor performances in numerous PANI- based composite or nanocomposites. It

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is, therefore, the need of the hour to have keen and focussed contemplation of this energy storage system with and without binder for future applications. Acknowledgements We would like to thank all the Authors of the Refs. [30, 111–117] and [150] manuscripts mentioned in our review paper to give us valuable data in the form of their figures.

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82. Simotwo SK, Kalra V (2018) Polyaniline-carbon based binder-free asymmetric supercapacitor in neutral aqueous electrolyte. Electrochim Acta 268:131–138 83. Nam MS, Patil U, Park B, Sim HB, Jun SC (2016) A binder free synthesis of 1D PANI and 2D MoS2 nanostructured hybrid composite electrodes by the electrophoretic deposition (EPD) method for supercapacitor application. RSC Adv 6(103):101592–101601 84. Wang L, Lv S, Zhang J, Zhang C, Dong S, Kong Q, Pang S (2014) Low-cost, flexible graphene/polyaniline nanocomposite paper as binder-free high-performance supercapacitor electrode. Funct Mater Lett 7(06):1440010 85. Babu RS, de Barros ALF, de Almeida Maier M, da Motta Sampaio D, Balamurugan J, Lee JH (2018) Novel polyaniline/manganese hexacyanoferrate nanoparticles on carbon fiber as binder-free electrode for flexible supercapacitors. Compos B Eng 143:141–147 86. Du J, Li Y, Zhong Q, Yang J, Xiao J, Chen D, Li W (2020) Boosting the utilization and electrochemical performances of polyaniline by forming a binder-free nanoscale coaxially coated polyaniline/carbon nanotube/carbon fiber paper hierarchical 3d microstructure composite as a supercapacitor electrode. ACS Omega 5(35):22119–22130 87. Asif M, Tan Y, Pan L, Li J, Rashad M, Fu X, Usman M (2016) Improved performance of a MnO2 @ PANI nanocomposite synthesized on 3D graphene as a binder free electrode for supercapacitors. RSC Adv 6(52):46100–46107 88. Sha R, Badhulika S (2017) Binder free platinum nanoparticles decorated graphene-polyaniline composite film for high performance supercapacitor application. Electrochim Acta 251:505– 512 89. Hong X, Zhang B, Murphy E, Zou J, Kim F (2017) Three-dimensional reduced graphene oxide/polyaniline nanocomposite film prepared by diffusion driven layer-by-layer assembly for high-performance supercapacitors. J Power Sources 343:60–66 90. Zhang LL, Zhao XS (2009) Carbon-based materials as supercapacitor electrodes. Chem Soc Rev 38(9):2520–2531 91. Mostazo-López MJ, Ruiz-Rosas R, Morallón E, Cazorla-Amorós D (2016) Nitrogen doped superporous carbon prepared by a mild method. Enhancement of supercapacitor performance. Int J Hydrogen Energy 41(43):19691–19701 92. Wang T, Le Q, Zhang G, Zhu S, Guan B, Zhang J, Zhang Y (2016) Facile preparation and sulfidation analysis for activated multiporous carbon@ NiCo2 S4 nanostructure with enhanced supercapacitive properties. Electrochim Acta 211:627–635 93. Tang Y, Zheng S, Xu Y, Xiao X, Xue H, Pang H (2018) Advanced batteries based on manganese dioxide and its composites. Energy Storage Mater 12:284–309 94. Hsia B, Marschewski J, Wang S, In JB, Carraro C, Poulikakos D, Maboudian R (2014). Highly flexible, all solid-state micro-supercapacitors from vertically aligned carbon nanotubes. Nanotechnology 25(5):055401 95. Ahmed S, Bhat MY, Rafat M, Hashmi SA (2017) Low-temperature thermal exfoliation of graphene oxide for high performance supercapacitor. J Mater Sci Surf Eng 5:571–576 96. Miller JR, Burke A (2008) Electrochemical capacitors: challenges and opportunities for realworld applications. Electrochem Soc Interface 17(1):53 97. Mahdavi H, Shahalizade T (2019) Investigation of the pseudocapacitive properties of polyaniline nanostructures obtained from scalable chemical oxidative synthesis routes. Ionics 25(3):1331–1340 98. Liu T, Finn L, Yu M, Wang H, Zhai T, Lu X, Li Y (2014) Polyaniline and polypyrrole pseudocapacitor electrodes with excellent cycling stability. Nano Lett 14(5):2522–2527 99. Grover S, Goel S, Marichi RB, Sahu V, Singh G, Sharma RK (2016) Polyaniline all solidstate pseudocapacitor: role of morphological variations in performance evolution. Electrochim Acta 196:131–139 100. Ebrahim SA, Harb ME, Soliman MM, Tayel MB (2016) Preparation and characterization of a pseudocapacitor electrode by spraying a conducting polymer onto a flexible substrate. J Taibah Univ Sci 10(2):281–285 101. Luo H, Lu H, Qiu J (2018) Carbon fibers surface-grown with helical carbon nanotubes and polyaniline for high-performance electrode materials and flexible supercapacitors. J Electroanal Chem 828:24–32

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102. Yanilmaz M, Dirican M, Asiri AM, Zhang X (2019) Flexible polyaniline-carbon nanofiber supercapacitor electrodes. J Energy Storage 24:100766 103. Mao N, Wang H, Sui Y, Cui Y, Pokrzywinski J, Shi J, Mitlin D (2017) Extremely high-rate aqueous supercapacitor fabricated using doped carbon nanoflakes with large surface area and mesopores at near-commercial mass loading. Nano Res 10(5):1767–1783 104. Zhang D, Zhang X, Chen Y, Wang C, Ma Y, Dong H, Hu W (2012) Supercapacitor electrodes with especially high rate capability and cyclability based on a novel Pt nanosphere and cysteine-generated graphene. Phys Chem Chem Phys 14(31):10899–10903 105. Kiruthika S, Kulkarni GU (2020) Smart electrochromic supercapacitors made of metal mesh electrodes with polyaniline as charge storage indicator. Energy Technol 8(5):1901364 106. Tian Y, Cong S, Su W, Chen H, Li Q, Geng F, Zhao Z (2014) Synergy of W18O49 and polyaniline for smart supercapacitor electrode integrated with energy level indicating functionality. Nano Lett 14(4):2150–2156 107. Majumdar D (2019) Polyaniline as proficient electrode material for supercapacitor applications: PANI nanocomposites for supercapacitor applications. In: Polymer nanocomposites for advanced engineering and military applications. IGI Global, pp 190–219 108. Eftekhari A, Li L, Yang Y (2017) Polyaniline supercapacitors. J Power Sources 347:86–107 109. Hu Z, Zu L, Jiang Y, Lian H, Liu Y, Li Z, Cui X (2015) High specific capacitance of polyaniline/mesoporous manganese dioxide composite using KI-H2 SO4 electrolyte. Polymers 7(10):1939–1953 110. Ryu KS, Wu X, Lee YG, Chang SH (2003) Electrochemical capacitor composed of doped polyaniline and polymer electrolyte membrane. J Appl Polym Sci 89(5):1300–1304 111. Pang S, Chen W, Yang Z, Liu Z, Fan X, Fang D (2017) Facile synthesis of polyaniline nanotubes with square capillary using urea as template. Polymers 9(10):510 112. Wang K, Huang J, Wei Z (2010) Conducting polyaniline nanowire arrays for high performance supercapacitors. J Phys Chem C 114(17):8062–8067 113. Zhou W, Yu Y, Chen H, DiSalvo FJ, Abruña HD (2013) Yolk–shell structure of polyanilinecoated sulfur for lithium–sulfur batteries. J Am Chem Soc 135(44):16736–16743 114. Li L, Zhang Y, Lu H, Wang Y, Xu J, Zhu J, Liu T (2020) Cryopolymerization enables anisotropic polyaniline hybrid hydrogels with superelasticity and highly deformation-tolerant electrochemical energy storage. Nat Commun 11(1):1–12 115. Zeng R, Li Z, Li L, Li Y, Huang J, Xiao Y, Chen Y (2019) Covalent connection of polyaniline with MoS2 nanosheets toward ultrahigh rate capability supercapacitors. ACS Sustain Chem Eng 7(13):11540–11549 116. Wu G, Tan P, Wang D, Li Z, Peng L, Hu Y, Chen W (2017) High-performance supercapacitors based on electrochemical-induced vertical-aligned carbon nanotubes and polyaniline nanocomposite electrodes. Sci Rep 7(1):1–8 117. Jin L, Jiang Y, Zhang M, Li H, Xiao L, Li M, Ao Y (2018) Oriented polyaniline nanowire arrays grown on dendrimer (PAMAM) functionalized multiwalled carbon nanotubes as supercapacitor electrode materials. Sci Rep 8(1):1–10 118. Pattananuwat P, Aht-ong D (2017) Controllable morphology of polypyrrole wrapped graphene hydrogel framework composites via cyclic voltammetry with aiding of poly (sodium 4-styrene sulfonate) for the flexible supercapacitor electrode. Electrochim Acta 224:149–160 119. Devi M, Kumar A (2016) In-situ reduced graphene oxide nanosheets–polypyrrole nanotubes nanocomposites for supercapacitor applications. Synth Met 222:318–329 120. Peng S, Xu Q, Fan L, Wei C, Bao H, Xu W, Xu J (2016) Flexible polypyrrole/cobalt sulfide/bacterial cellulose composite membranes for supercapacitor application. Synth Met 222:285–292 121. Zhang X, Wang J, Liu J, Wu J, Chen H, Bi H (2017) Design and preparation of a ternary composite of graphene oxide/carbon dots/polypyrrole for supercapacitor application: importance and unique role of carbon dots. Carbon 115:134–146 122. Wang N, Zhao P, Liang K, Yao M, Yang Y, Hu W (2017) CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO2 for high performance asymmetric supercapacitors. Chem Eng J 307:105–112

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142. Wang R, Wu Q, Zhang X, Yang Z, Gao L, Ni J, Tsui OK (2016) Flexible supercapacitors based on a polyaniline nanowire-infilled 10 nm-diameter carbon nanotube porous membrane by in situ electrochemical polymerization. J Mater Chem A 4(32):12602–12608 143. Sankar KV, Selvan RK (2016) Fabrication of flexible fiber supercapacitor using covalently grafted CoFe2 O4 /reduced graphene oxide/polyaniline and its electrochemical performances. Electrochim Acta 213:469–481 144. Li H, Song J, Wang L, Feng X, Liu R, Zeng W, Wang L (2017) Flexible all-solid-state supercapacitors based on polyaniline orderly nanotubes array. Nanoscale 9(1):193–200 145. Chang CM, Hu ZH, Lee TY, Huang YA, Ji WF, Liu WR, Wei Y (2016) Biotemplated hierarchical polyaniline composite electrodes with high performance for flexible supercapacitors. J Mater Chem A 4(23):9133–9145 146. Zhu M, Huang Y, Deng Q, Zhou J, Pei Z, Xue Q, Zhi C (2016) Highly flexible, freestanding supercapacitor electrode with enhanced performance obtained by hybridizing polypyrrole chains with MXene. Adv Energy Mater 6(21):1600969 147. Yang C, Zhang L, Hu N, Yang Z, Wei H, Zhang Y (2016) Reduced graphene oxide/polypyrrole nanotube papers for flexible all-solid-state supercapacitors with excellent rate capability and high energy density. J Power Sources 302:39–45 148. Sen P, De A (2010) Electrochemical performances of poly (3, 4-ethylenedioxythiophene)– NiFe2 O4 nanocomposite as electrode for supercapacitor. Electrochim Acta 55(16):4677–4684 149. Simon P, Gogotsi Y (2010) Materials for electrochemical capacitors. Nanoscience and technology: a collection of reviews from Nature journals, pp 320–329 150. Singh G, Kumar Y, Husain S (2020) High charge retention and optimization of polyaniline– titanium dioxide nanoparticles composite nanostructures for dominantly stable pseudocapacitive nature. J Energy Storage 31:101660 151. Singh G, Kumar Y, Husain S. Improved electrochemical performance of symmetric polyaniline/activated carbon hybrid for high supercapacitance: comparison with indirect capacitance. Polym Adv Technol 1–12 152. Snook GA, Peng C, Fray DJ, Chen GZ (2007) Achieving high electrode specific capacitance with materials of low mass specific capacitance: potentiostatically grown thick micro-nanoporous PEDOT films. Electrochem Commun 9(1):83–88

Chapter 14

The Aspect of Green Nanocomposites in Green Technology and Sustainable Development: State of the Art and New Challenges M. S. Kiran Sankar, Mohd. Parvez, Moti Lal Rinawa, Vijay Chaudhary, Sumit Gupta, and Pallav Gupta

1 Introduction Composites are the promising material for future economic growth in all sectors, mainly aerospace, defense, and locomotive sectors. The composite materials were developed at the existence of human beings. Woods and animal bones are the natural primitive composites during an early age. The drastic development of high fatigue and corrosion resistance composites began after the Second World War (1939–1945) [1]. The high strength–weight ratio, lightweight, and easy processability of the composite made it superior to the traditional materials in all applications. The main constituents of composites are (1) matrix and (2) reinforcement. The matrix transfers stress, binds the reinforcement, and ultimately provides a unique shape to the composites. Based on the types of matrix and reinforcing material composites are categorized. The common matrices used are metal, polymer, and ceramic. The typical reinforcements in the metal matrix composites are particle, short fiber or whisker, and continuous fiber. Among the reinforcements particle, short fiberbased composites are very important because they are inexpensive w.r.t. continuousbased composites [2]. The important matrices used in MMC are alloys of aluminum, M. S. K. Sankar NSH Corporation, Manama, Bahrain Mohd. Parvez Department of Mechanical Engineering, Al-Falah University, Faridabad, India M. L. Rinawa Department of Mechanical Engineering, Government Engineering College Jhalawar, Jhalawar, Rajasthan, India M. S. K. Sankar · V. Chaudhary · S. Gupta · P. Gupta (B) Department of Mechanical Engineering, A.S.E.T., Amity University Uttar Pradesh, Noida, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Z. H. Khan (ed.), Nanomaterials for Innovative Energy Systems and Devices, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-0553-7_14

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magnesium, copper, and lithium. The lightweight magnesium matrices are used in aircraft gearbox housings, chain saw housings, and electronic equipment. The high strength/weight, high melting point, good oxidation, and corrosion resistance of titanium alloys make it an ideal material for aerospace applications. Some intermetallic compounds like nickel aluminide and molybdenum disilicide are also used in specific applications. Copper-based composites are mainly used in niobium-based superconductors. The long-range order in the intermetallic compounds restricts the movement of dislocations which will lead to retention of the strength at elevated temperatures [2]. The ceramic matrix composites were developed to overcome the drawbacks of the monolithic ceramics (aluminum oxide, silicon carbide, aluminum nitride, etc.): brittle and poor electrical conductor. Among the various reinforcing fibers, the carbon fibers are the highest performance one investigated [3]. The emergence of carbon nanotube which is considered as the second stage evolution of the carbon fiber is nowadays the promising material in the CMC. The unique properties of the single-walled CNT and multi-walled CNT have replaced the conventional materials in high-temperature applications. Some of the drawbacks associated with the formation of CNT–ceramic composites are (1) uniform dispersion of the carbon nanotube in the matrix to prevent the agglomeration which will lead to the area of high-stress concentration and (2) availability of the carbon nanotube with desired properties at affordable cost. The CNT properties mainly depend upon the manufacturing methods, and the chemical vapor deposition (CVD) method is the reliable one to produce CNT for the composite reinforcement. The polymer matrix composites (PMC) mainly constitute thermoset/thermoplastic polymer and reinforcement. The polymer derived its word from poly and mer which means “many” and “units”. The polymer is formed by the repeated combination of macromolecules called monomers. The choice of the thermoset or thermoplastic polymer depends upon the specific application. The linear molecular structure of the thermoplastic polymer enhances the movement of the molecules compared to the cross-linked structure of the thermoset polymer. Hence, the energy absorption capacity of the thermoplastic (toughness) is very large compared to the thermoset polymer. The main disadvantage of the thermoplastic polymer is the high processing temperature compared to the thermoset polymer with a low processing temperature. Epoxy (Thermoset) is the commonly used matrix for carbon fiber-based composites. The epoxy possesses an excellent combination of mechanical property and corrosion resistance, greater dimensional stability, is relatively inexpensive, and exhibits good adhesion with the reinforcement fiber. Other than epoxy the polymers used are phenol–formaldehyde resins, urea–formaldehyde bismaleimide resins, polyether, and polyester polyols [4]. The examples of thermoplastic polymers include polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polyamide (PA), polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polycarbonate (PC), polyetheretherketone (PEEK), polyethersulfone (PES), and polyetherimide (PEI). The most effective method for the fabrication of thermoplastic composites is the injection molding/screw extrusion process. The other methods rarely used are pultrusion, filament winding, spray

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layup process, diaphragm forming process, etc. [5]. Due to superior mechanical properties (high modulus, high electrical and thermal electrical conductivities, and low coefficient of thermal expansion), carbon fiber-based composites play a vital role in industrial applications [6]. The usage of the PMC will be 100-fold times in defense applications for the next 30 years. The PMC provides a significant reduction in automobile weight by 25% in replacing the conventional steel structures [7]. The aerospace industry has progressively shifted from conventional metal structures to composites mainly polymer matrix composites. Nowadays, aggressive research is going on in these areas to promote the development of a PMC to suit the current challenges in this sector. The major concern in PMC manufacturing is the environmental impact of the entire life cycle of the resins used. The environmental concerns in the manufacture of PMC are the hazardous air pollutants (HAP) and hazardous wastes (HW). The HAP is produced during the curing of the composite and HW during the cleaning and manufacturing of resins. The composite wastes are normally disposed of by incineration which causes the evolution of hazardous chemicals like nitrogenous aromatics and phenolics. Another drawback is the hurdles faced in the recycling of polymer resins due to their chemical bonding. The recycling of carbon fiber composites coated with hexavalent chromium in the aerospace industry is a very serious concern because disposal of these compounds will lead to the leaching of chromium into the ground [8]. Several studies revealed the use of polymer additives, unreacted monomers, and catalysts in the food packaging industry causing severe health issues. Polyvinyl-based composites will release dioxin and furans on combustion. Polymer matrix composite (PMC) with natural fibers are nowadays dominating in the universal market due to its environmental friendliness. In this scenario sustainable development is the key concern that can be achieved through the evolution of green composites. Green composites are the combination of natural resins with natural fibers which results in the formation of an eco-friendly product. The green composites provide a complete solution for today’s environmental concerns of depletion of oil reservoirs and the waste disposal of petroleum-based polymers. This chapter exploits the crucial role of green composites in the development of eco-friendly innovation in the engineering sector. Figure 1 shows the MSW generated per year globally and Fig. 2 discusses the forecast of MSW generation.

2 Initiatives Toward Sustainability The major concerns in using synthetic polymer and resins are enlisted below: 1.

The depletion of fossil fuel in the coming years is the origination of matrices and reinforcements.

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Fig. 1 MSW generated per year globally [https://datacatalog.worldbank.org]

Fig. 2 Forecast of MSW generation [https://datacatalog.worldbank.org]

2.

3.

The high environmental impact and serious prolonged health issues like respiratory, etching of skin due to the dispensable usage of the PMC in various applications. The processing of synthetic polymer resin is an energy-intensive process.

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The above-mentioned issues gave birth to a new era of composites called green or biodegradable composites. These are rectified to some extent by (1) landfilling, (2) incineration, and (3) recycling. The composites are abandoned at the end-of-life majority from the automobile and aerospace industry. Due to the overall depletion of fossil fuel and the environmental impact of the resins, researchers focused on the development of polymers filled with natural fibers. The first attempt was to use recycled polymer resins with natural fibers. This invokes a reduction in the consumption of petroleum-based inorganic fillers. In the beginning, some of them are reused by mechanical grinding. But this process has limitations as it cannot be applicable for all polymer resins. A significant increase in the overall property of cement blended with crushed waste glass is an alternative method for reducing glass waste [9]. The polymeric material can be recycled (1) directly without separating the constituents and (2) treating each constituent separately into a useful product [9]. The elements in the green composites are completely biodegradable which plays a significant role in balancing the ecosystem. The methodology adopted for attaining the overall sustainable development is (1) reduce, (2) reuse, (3) recycle, and (4) recover. The reuse of the waste is limited to some extent as it depends upon knowledge in various technical aspects. However, the reduction of waste can be achieved by replacing conventional polymers with biodegradable resins. Recycling is one of the major components in the spectrum which has to be determined properly for maintaining ecosystem balance. The composite recycling methods range from (1) chemical recycling, (2) pyrolysis, (3) microwave pyrolysis, to (4) mechanical recycling. In these methods the highest energy demand is for chemical recycling and the least is for mechanical recycling. The recycling depends upon the (1) waste quantity, (2) nature of the polymer, (3) polymer purity, and (4) net polymeric content in the waste [10]. To adopt suitable recycling methods, a vast and in-depth understanding of the waste must be done. Mechanical recycling is one of the methods used to produce the recyclate as filler or secondary element in the new product. Mechanical recycling consists of serious mechanical processes like grinding, washing, separating, drying, regranulating, and compounding. Slow cutter or shredding mills are used to reduce the size of the particles to 50–100 mm. The key parameters in the recycling are the purity of the recyclate, the degree of interfacial bonding between the recyclate and the virgin polymer, and the fiber length of the recyclate. The fiber length determines the mechanical property and is governed by the type of process used in recycling. The technology readiness level (TRL) is the framework used to measure technological maturity. For the mechanical grinding of carbon and glass fiber, the TRK approached nearly 9. The pyrolysis of carbon fibers has a median TRL of 8 [12]. To some extent the decision tree will be helpful in making the appropriate method [10]. Figure 3 discusses the impact comparison of (1) landfill, (2) incineration, and (3) recycling. Figure 4 shows the decision tree for the recycling process. In thermal recycling the fibers are separated from the resin by the heat application. The processing temperature depends upon the type of resin used. In heating the insignificant volatile materials moved out leaving behind the fiber only. But the two major drawbacks are (1) undercooking of fiber and (2) diameter reduction in

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Fig. 3 Impact comparison of (1) landfill, (2) incineration, and (3) recycling [11]

Is the composite waste polymeric only Yes

Reuse

Yes

No

Any scope for Reuse

Can it be sorted

Yes No

Mechanical recycling process

Fig. 4 Decision tree for recycling process [10]

No

Chemical Energy Recovery process

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fibers due to overcooking. The main methods falling under this category are (1) pyrolysis, (2) fluidized bed pyrolysis, and (3) microwave pyrolysis. Pyrolysis is the thermal decomposition of the composite scrap in the absence of oxygen. The processing temperature varies from 450 to 700 °C depending upon the resin type in the composite. The oil and gas extracted are stored in the pyrolysis reactor for secondary usage. It is one of the efficient methods for the recycling of carbon and glass fiber-based composites. The charring on the fiber is the major drawback of this process which results in a significant reduction of the mechanical properties. These can be rectified to some extent by post-heating of fiber and using carbon dioxide and water vapor. The recent advancement in the field is using a controlled atmosphere in the pyrolysis process: (1) Vacuum atmosphere, (2) nitrogen atmosphere, and (3) superheated steam atmosphere [13]. The vacuum process is most suitable for glass fiber-reinforced composite. The nitrogen and superheated steam atmosphere are very efficient in recycling both the carbon and glass fiber-based composite. The microwave-assisted pyrolysis replaces the conventional heat source with microwave addition. This serves less energy consumption with more thermal energy transfer [13]. The fluidized bed pyrolysis is used in the conversion of many resins like LDPE, HDPE, PP, etc. for converting them into petrochemical feedstock. Chemical recycling is the process of disintegrating the polymer matrix by adding it to solvents like acid and base. After disintegration, the fiber is washed and then it proceeds further. The solvent is selected depending upon the volume of polymer substrate. The resin degradation is obtained either with solvents by solvolysis or by water with hydrolysis. Solvolysis is one of the efficient methods for obtaining high and good quality fibers [12]. But the main disadvantage is the use of some toxic solvents which are harmful to the environment. Nitric acid is one of the better solvents for recycling the carbon and glass fiber reinforced polymer [13].

3 Green Composites The green composites can be partly renewable or fully renewable. In the fully renewable both the matrix and fillers are from a renewable source. But in the partly renewable either the matrix or constituent will be from a renewable source. The green composites technology already existed in ancient Egypt where they used the straw as a reinforcing material for the mud-based wall structures for the house.

3.1 Introduction to Green Composites These are polymer composites consisting of natural fibers and biopolymers. The green composites can be polymer or cement-based. The bio fibers used are from animal fiber, mineral, and plant fiber. In recent years the plant fibers like jute, hemp, coconut, and wood fibers replaced conventional fibers like glass, carbon, aramid,

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etc. The commonly used biodegradable polymer matrix are polylactic acid, polyhydroxybutyrate, starch, etc. The natural fibers used in the green composites are classified into protein-based, cellulose-based, and mineral-based. The protein-based fibers are from animal fibers (silk and wool), cellulose-based are from vegetables, and the mineral-based are from asbestos (rock). The vegetable-based are further classified according to their origin, i.e., leaf, bast, seed, stalk, and grass. Wood fiber is a promising candidate as a natural fiber in many industrial applications. The commonly used wood fibers are aspen, birch, spines, and spruces. Natural fibers generally consist of cellulose, hemicellulose, and lignin. Cellulose provides the strength and stability of the fiber and hemicellulose contributes to the structural nature of the fiber. The overall property of the natural fiber is determined by the structure, microfibrillar angle, cell dimension, defects, and chemical composition of the fiber [14]. The structure of the fiber also depends upon the origin of location, climatic conditions, and the method of cultivation. Some of the extensively used natural fibers are bamboo, coir, flax, hemp, jute, bagasse, kenaf, etc.

3.1.1

Flax Fiber

Flax fiber is extracted from the bast beneath the surface of the stem of the flax plant. The flax fiber is stronger than cotton fiber. The flax fiber has great tensile strength/elongation, good vibration absorbing properties, low cost/kg, and is ecofriendly. The major drawback is the degradation of the fibers above 200 °C and bad adhesion to all substances [15]. The flax fiber-based composite is manufactured by hand lay-up method and the obtained composite exhibits excellent mechanical and impact strength (21–38 kJ/m2 ) [15].

3.1.2

Hemp Fiber

Hemp fiber is extracted from the Hemp by the process called decortication. Hemp fibers are extensively used in many industrial applications in the last many years. The hardness and tearing strength of the hemp fiber-reinforced rubber composite varies directly with the proportion of the fiber [16]. The hemp/natural composite is manufactured by the ecological method of cross-linking, namely electron beam irradiation. Hemp is classified under the green category of building design due to its positive effect. It has the capacity of absorbing large amounts of carbon dioxide thereby purifying the local environment. The other significant properties include better insulator, low embodied energy, non-toxic, etc.

3.1.3

Jute Fiber

Jute fiber belongs to the bast fiber family. It is cultivated in the tropical areas of India, China, and Bangladesh. It has similar properties compared to glass fiber and

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is extensively used in the textile industry. The jute fiber composite is manufactured commonly by hand lay-up method, pultrusion as well as extrusion and resin transfer molding method. The tensile strength of the jute/thermoplastic starch composite is found to enhance by the various proportions of the composite [17]. The thermal stability of the jute/TPS is excellent due to the hydrogen bonding between matrix and fibers. The composites with jute fiber exists higher degradation resistance than pure TPS [17].

3.1.4

Bamboo Fiber

Bamboo fiber is a cellulosic fiber from bamboo through alkaline hydrolysis and multiphase bleaching. It has an excellent moisture absorbing capacity due to large number of microholes in the structure. In a recent study of green composites comprising polylactic (PLA), bamboo fiber, and ammonium polyphosphate (APP), the stiffness and moduli of the composite are found to enhance with the addition of bamboo fiber [18]. The fracture toughness and Young’s modulus have been increased significantly in the Soya protein concentrate (SPC) with the addition of bamboo microfibrils [19]. The bamboo fiber biodegradable composites have the potential to replace conventional polymer composites.

3.1.5

Biodegradable Polymers

Starch-based green composites are becoming popular due to their prodigious abundance and degree of biodegradability. Starch is a polymeric carbohydrate produced in every plant for energy storage. It consists of numerous glucose units linked by glycosidic bonds. The commercially refined starches nowadays are cornstarch, tapioca, arrowroot, wheat, rice, and potato starches. The starch reinforced with cellulose fibers shows excellent mechanical properties comparable to the synthetic polymer [20]. Starch-based structures are manufactured by the extrusion process. In the extrusion the starch undergoes multiphase transition changes, hence the product completely depends upon the processing technique and conditions. The reinforced starch wheat composite shows a remarkable increase in mechanical properties. Polylactic acid (PLA) is thermoplastic polyester formed by the condensation of lactic acid. The basic mechanical property lies between polystyrene and PET. The brittleness and poor thermal stability of PLA limit its application to a wide range. The flame retardation property of the PLA can be enhanced by the addition of the ammonium polyphosphate (APP). The polybutylene adipate terephthalate (PBAT) is a biodegradable aliphatic–aromatic copolyester that exhibits superior toughness compared to the other resins. The PLA/PBAT blends with natural fibers and compatibilizers are utilized in the automotive fields for their outstanding properties. Figure 5 discusses the strength to weight ratio of natural resins.

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Fig. 5 Strength to weight ratio of natural resins [Source Shekar et al. Green composites: a review, 2018. (Online). Available: www.sciencedirect.comwww.materialstoday.com/proceedings2 214-7853]

3.2 Key Challenges in Developing Green Composites Green composites are the ruling material in the coming era due to the severe awareness of the environmental impact, sustainable development, and total ecological balance of the universe. But the development of fully efficient biodegradable composite faces a lot of critical challenges. This is due to the incompatibility of the composites with the existing divergent conditions. The main concerns are listed below. 1. 2.

3.

4.

5. 6.

The thermal stability of the natural fiber in disparate circumstances which restricts its application. The hydrophilic and hygroscopic nature of the renewable fiber leads to a shaky interfacial bond between the matrix and filler which results in poor mechanical properties. The high moisture absorption capacity of the fiber engendered swelling of the fiber, yielding dimensional stability and finally affecting the mechanical properties. The fiber properties depend upon its length, aspect ratio, and chemical composition. Hence, the composites’ overall property will vary depending upon the fiber property. The type and content of both resin and fiber are affected by the geographical location, unpredicted climate variations, and finally the way of cultivation. Availability of deposition lands for the polymer biodegradation with effective processing technique otherwise will lead to contamination.

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8. 9.

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The natural fibers are modified by the expensive process of chemical treatment or surface modification to compete with performance. Also, the selection of the process is extremely critical. The PLA is a high performer natural polymer but comparatively highly expensive. The commercialization of biodegradable resin PHA (polyhydroxyalkanoates) is awfully expensive due to the complexity and high cost of production [21]. Similar cases are associated with other natural resins and fibers also. The overall property of the composite depends upon the interfacial adhesion between the fiber and resin. The accomplishment of these is a major task in the development of green composites.

4 Role of Green Composites in Sustainable Development According to World Commission on Environment and Development (WCED), sustainable development is defined as “Development that meets the needs of the present without compromising the ability of future generations in order to meet their own needs”. Synthetic polymers dominated in each and every corner of the industrial sector due to their lightweight, excellent mechanical, and thermal properties compared to the traditional metal-based components in the eighteenth century and thereafter. The extensive usage of plastics leads to unpredicted environmental impact and jeopardizes the entire ecosystem. The annual production of plastics increased drastically from 1.5 million tons in 1950 to 400 million tons in 2018 [21]. It is projected that the global plastics annual production will reach up to 2000 million tons by 2020. Of the overall plastics used, nearly 18% of it is recycled. The remaining is threatening to the environment. Around 8 million tons of plastics are entering the marine environment, thereby smashing the entire water life. The microplastic contamination is adversely affecting the ocean life and ultimately leading to the total imbalance of the ecosystem. Microplastics are invisible particles that are lesser than 5 mm in size. Microplastics are the carriers of heavy dangerous metals (Cu, Hg, Cr, etc.). This happened in two ways: (1) Materials added in the process to improve the mechanical properties. (2) Adsorption of materials on the surface. Sustainable growth is hindered by the extensive use of plastics which can be summarized below: 1. 2.

3.

A very few percentages of the polymer are recyclable. The remaining portion is abandoned on the earth invoking environmental hazards. The entrapment of heavy metals in the microplastic prompts interruption in the food chain. Its presence will cause drastic effects such as a high mortality rate and severe reproductive issues. Ultimately the food chain gets interrupted. Among the aquatic life, the most affected organisms by microplastics are fish (38%) and mollusks (18%) [22]. Global warming is caused by the endless emission of carbon dioxide. The carbon dioxide concentration increases drastically from 280 ppm in the late eighteenth

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century to the twenty-first century [22]. The global temperature is increased by 0.9 °C. The processing of polymers results in the emission of toxic gases which are harmful to human nature. The exposure will cause reproductive abnormalities, digestive problems, respiratory issues, etc. Depletion of fossil fuel by the enormous production of the polymers.

For attaining sustainable development, the aforesaid factors should be eliminated completely. Green composites are the only alternative solution for this. By the evolution of green composites, the conventional crop varieties will be stimulated which catalyzes the balance of the ecosystem. An efficient fully developed green composites must be developed to meet the overall development through sustainability.

5 Sustainability Through Green Nanotechnology 5.1 Nanotechnology Nanotechnology is the art of science that encompasses the processing and controlled manufacturing of particles with a size of less than 100 nm in length. Table 1 illustrates the applications of nanotechnology in different areas. Table 1 Applications of nanotechnology in different areas Areas of application

Description and benefits

Animal science

Synthetic drug delivery systems for efficient drug usage; nanotubes within the skin for the proper mating; silver nanoparticles for the cleanliness in poultry farms; nanocoatings for nutritional materials

Agricultural science

Nanoscale sensors for detecting contamination; nanoscale additives and coatings in fertilizer for nutrition supply; nanopoisons for detection of weeds, insect, and agricultural pests

Water treatment

Nano-TiO2 and fullerene derivatives in photocatalytic reactors; carbon nanotubes and nanoscale metal oxides in adsorption media filters; nanozeolites and nano-Ag in high permeability membranes; dye-doped silica nanoparticles in optical detection. The aforementioned technologies are used for water purification and treatment process

Pharmaceutical and medical sciences Smart nanomaterials in multiple therapeutic functions for metastasis process; nanotherapeutics in the treatment of primary tumors; nanosized drugs for enhanced solubility; rate of dissolution; oral bioavailability; rapid therapeutic action; and less patient-to-patient variability

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Nanotechnology provides functionality to the product, improves the existing process, and exploits the benefit of unique quantum and surface phenomena at the nanolevel. A renowned physicist Richard Feynman disseminate his ideas in a presentation “Their’s plenty of room at the bottom”, which laid the foundation for the development of nanotechnology. The applications of nanotechnology range from biomedical sensors in the medical field to the food packaging industry, catalyst, lubricants and fuel additives, nanoelectronics and sensor devices, cosmetics, conductive link and printing materials, etc. Apart from the extensive use of the nanomaterials for a high output product, its environmental impact on the entire life cycle stages of the product must be validated. The hurdles in these are lack of inventory data for the material and new manufacturing process with confidential credibility [23]. The carbon fiber and nanofibers processing require 50 times more energy than aluminum production for equal mass [23]. Before finalizing the environmental benefits of nano-enabled products, a life cycle analysis with risk management should be done in order to analyze the environmental consequences.

5.1.1

Deleterious Effects of Nanotechnology

Despite the fact that it leads to the development of a high-performance product, the harmful effects of nanotechnology are very perilous. The lack of knowledge in the reactivity of nanoparticles under different circumstances will prompt severe health issues and environmental problems. Nanotoxicology is a subpart dealing with the toxicity of the different nanoparticles. More studies should be done to analyze the serious effects of the transportation of nanoparticles to different stages. The harmful effects of nanotechnology in various stages are enlisted below [24]. Table 2 discusses the potential risks of various nanoparticles. • Severe health issues while nanoparticles enter the human body through nose, skin, respiratory system, and digestion. • Inhaling of airborne nanoparticles leads to fibers. • More energy consumption due to the synthesis of nanoparticles. • Lower rates of recovery and recycling. • Unpredictable impact on the different life cycle stages.

5.2 Green Nanotechnology The harmful effects of nanotechnology have been elucidated in the previous paragraph. In order to facilitate sustainability, a new evolution emerged in nanotechnology strictly adhering to eliminate the harmful effects and environmental pollution using the so-called green nanotechnology. Green nanotechnology embraces the principle of green chemistry and green engineering. It engulfs on reducing waste and pollution, minimum use of materials, use of renewable inputs wherever applicable,

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Table 2 Potential risks of various nanoparticles [25] Nanoparticle type

Source

Potential risks

Silver nanoparticles

Mining activities, incineration of biomedical wastes

Oxidative stress, altered cell signaling

Titanium dioxide nanoparticles

Pigments in paints, coatings, plastics, foods, etc.

Oxidative stress and retarded cell growth

Carbon-based nanoparticles

Byproducts of hydrocarbons, celestial activities, etc.

Retarded cell growth and apoptosis

Molybdenum, iron oxide, aluminum oxide, and silicon-based nanoparticles

Automobiles fuels, metallurgical operations, research activities, etc.

Cytotoxicity, hemolysis, and cell death

Polymeric nanoparticles

Biomedical applications

Inflammation, oxidative stress, and alteration in cell morphology

Quantum dots and nanodevices, etc.

Sensing materials, probes, and advanced detecting methods

Free radical formation, arrest of cell growth, and cell death

and development of healthy environment. Green nanotechnology aims at developing eco-friendly nanoproducts and providing solutions for existing environmental and health issues by developing nano-based products. It succeeded in the development of green nanocomposites as versatile materials for diverse application needs. Green nanotechnology is linked with green chemistry and green engineering and manufacturing. Green chemistry deals with the more efficient and eco-friendly process for nanomaterials production. The use of carbon nanotubes, nanosilica replacing the conventional materials carbon black in the tire has enhanced the tensile strength and hardness [26]. Green chemistry emphasizes economic and social benefits while incorporating every aspect of nanoparticles. The adverse and unpredictable effect of nanotechnology is eliminated by the principles of green chemistry and green engineering [27–32]. The 12 principles of green chemistry imply the reduction of waste and producing environmental-friendly products. Nowadays, plant-based nano-additives are used in the synthesis of nanomaterials, thereby eliminating the production of toxic gases. Figure 6 illustrates the applications of green nanotechnology. Figure 7 illustrates the sustainable development by adapting green nanotechnology.

6 Conclusion For sustainability development, green nanotechnology is to be incorporated throughout the life cycle. The chapter portrayed the evolution of the metals from the ancient stage to the latest green nanotechnology. In the past decades, only

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Biotechnology and Agriculture

Energy Storage

Automobile, Texle and clothing Food Preservaon

Fig. 6 Applications of green nanotechnology [10]

Fig. 7 Sustainable development by adapting green nanotechnology (Source [10])

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performance-oriented development was the target, neglecting the environmental impacts, health hazards, and renewable resource depletion. In spite of the huge potentials of the PMC nanocomposites, the unpredictable and adverse impact of the polymers on the environment and human health was countless. The generation of municipal solid waste and industrial waste drastically increased exponentially in the global market. That was a serious predicament in the world and leads to the evolution of the sustainability concept. Hence, green nanotechnology is imperative for overall sustainable development.

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