Advanced PEDOT Thermoelectric Materials 012821550X, 9780128215500

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Woodhead Publishing Series in Electronic and Optical Materials

ADVANCED PEDOT THERMOELECTRIC MATERIALS

Edited by

FENGXING JIANG CONGCONG LIU JINGKUN XU

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2022 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-821550-0 For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

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Contributors Chunmei Gao Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, PR China; Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, PR China Yanhua Jia Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Fengxing Jiang Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China; Flexible Electronics Innovation Institute, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Qinglin Jiang Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong, PR China Congcong Liu Flexible Electronics Innovation Institute, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Peipei Liu Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Shouli Ming College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong, PR China Hui Shi Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China; National-Local Joint Engineering Research Center of Heavy Metals Pollutants Control and Resource Utilization, Nanchang Hangkong University, Nanchang, PR China Haijun Song College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing, Zhejiang Province, PR China

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Contributors

Xiaodong Wang School of Materials Science and Engineering, Institute of Materials Genome & Big Data, Harbin Institute of Technology, Shenzhen, PR China Lei Wang Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, PR China; Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, PR China Jingkun Xu Flexible Electronics Innovation Institute, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China; Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Ge Zhang School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China Shijie Zhen Guangxi Key Laboratory of Electrochemical and Magneto-Chemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, Guangxi, PR China Zhengyou Zhu Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, PR China; Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, PR China

Foreword Conducting polymers stand out for an unique combination of their mechanical, optical, electrical, and thermal properties. With the benefit of advanced synthesis and its related processing techniques, tremendous breakthroughs have been made in the development of high-performance organic thermoelectric materials; these materials have potential uses in wearable heating and cooling devices as well as near-room-temperature energy generation. Among polymer thermoelectric materials, Poly(3,4ethylenedioxythiophene) (PEDOT)-based materials are known as some of the best organic thermoelectric materials and have been extensively investigated because of their high electrical conductivity and thermal stability. With rational design, some PEDOT-based materials have achieved high figure-ofmerit values comparable to those of conventional inorganic thermoelectric materials at room temperature. This progress has triggered renewed interest from the scientific community, with over 600 publications including the keywords “PEDOT” and “thermoelectric.” These scientific works cover the design of molecular structural design, morphology, advanced polymer film preparation strategy, theoretical mechanisms, and thermoelectric devices. Despite a wealth of reported methods, a systematic summary and a clear in-depth understanding of the thermoelectric conversion processes associated with PEDOT-based polymers are still needed. This book aims to describe the development process of PEDOT-based thermoelectric materials and point out future challenges and opportunities. To comprehensively develop functional knowledge of this field, this work encompasses all relevant aspects of PEDOT thermoelectric materials, beginning with a historical overview of conductive PEDOT and thermoelectric principles. Chapter topics include various optimization pathways to improve the thermoelectric performance of PEDOTs, as well as different measurement techniques to explore organic thermoelectric theory. In addition, various derivatives, heteroanalogs, copolymers, and nanocomposites/hybrids of PEDOT-based thermoelectric materials are also introduced. This book is primarily intended for graduate students beginning their research on organic thermoelectric materials, researchers working on PEDOT electron devices, and readers with any level of familiarity with PEDOT materials. The details of the theoretical basis of PEDOT are

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addressed, including preparation, characterization, discussion, and applications as well as its development as a high-performance thermoelectric material. Balancing sufficient details and references for further study, this book is a powerful and timely tool for anyone working in the field of thermoelectricconjugated polymers. I wish the book great success and foresee it having a substantial impact on the field and all of the areas with which it connects. Zhi-Gang Chen Professor of Energy Materials, University of Southern Queensland, Australia 2021-06-09

Preface Organic electronic materials have become one of the main topics of discussion within materials science, physics, and chemistry. Thermoelectricity is a typical representative direction for these frontier interdisciplinary subjects. At present, research on organic thermoelectric materials involves a wide variety of materials, including p-type and n-type, that consider light weight, good flexibility, and abundant resources. Nevertheless, few books on organic thermoelectric materials have been published to date, especially not a monograph such as this, referring specifically to PEDOT-based thermoelectric materials. Poly(3,4-ethylenedioxythiophene) (PEDOT), with excellent air/environmental stability, is currently the most widely used polymer material in research and development for organic electron devices. Moreover, its hybrid poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has become a commercially available product and has been widely used in organic photovoltaics, capacitors, transistors, and antistatic layers. Many features are due further exploration and research, just like thermoelectric performance. With numerous research efforts, PEDOT/PEDOT:PSS is regarded as a potential organic thermoelectric material and has gained a breakthrough process in thermoelectric figure-of-merit (ZTw10 1) at room temperature. Compared with other conducting polymers, it has significantly higher electrical conductivity and better mechanical properties. However, some problems have already arisen with current PEDOT thermoelectric materials. The challenge for PEDOT to be considered as a promising thermoelectric material is how to improve its Seebeck coefficient in response to high electrical conductivity to achieve a higher thermoelectric power factor. For the current key scientific issues and technologies, researchers still need continuous efforts and input. Therefore, it is necessary to summarize the current progress and challenges of PEDOT thermoelectric materials and clarify the direction of future development. We hope this monograph can help researchers working on PEDOT materials to prepare, characterize, analyze, test, and apply their findings while contributing to the future development and application of organic thermoelectric materials. Our group has persisted in the development of PEDOT/PEDOT:PSS thermoelectric materials for more than 10 years. In 2008, our group

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systematically reported the thermoelectric performance of PEDOT:PSS pellets (ZTw10 3) for the first time. Until 2010, we reported the ZT value of free-standing PEDOT:PSS film (ZTw10 2). Since 2010, PEDOT/ PEDOT:PSS has become a focus of researchers around the world. In 2015, we developed a highly thermoelectric performance of PEDOT:PSS film (ZTw10 1) with submicron thickness. Based on our work in, and understanding of, the field over many years, we were encouraged to start writing a monograph on thermoelectric PEDOT. Nevertheless, editing such a book has been a huge challenge for us because (1) its success is bounded by the limits of our knowledge and ability; (2) the thermoelectric performance of PEDOT/PEDOT:PSS involves a wide range of knowledge, covering physics, chemistry, and materials science; and (3) PEDOT has developed rapidly in recent years, making its current status, which is in constant flux, very difficult to grasp and elaborate on. This book is composed of 10 chapters with topics from basic to potential applications. Chapter 1 reviews the development of thermoelectric properties of conducting polymers (CPs), focusing on the developmental history and basic situation of PEDOT and PEDOT:PSS. Chapter 2 systematically introduces PEDOT, including derivative synthesis, morphology, structural analysis, and thin-film preparation technologies. Chapters 3 and 4 describe the basic knowledge related to PEDOT/PEDOT:PSS thermoelectric transport properties. Chapter 5 summarizes the advanced methods and principles for optimizing and improving the thermoelectric performance of PEDOT-based materials. Chapter 6 introduces the developmental status of PEDOT derivatives, analogs, and copolymers in thermoelectric performance from the perspective of molecular structure. Chapter 7 is about composite materials based on PEDOT/PEDOT:PSS, which is also one of the most popular research methods currently. Chapter 8 focuses on testing techniques and methods for measuring PEDOT thermoelectric performance such as electrical conductivity, Seebeck coefficient, and thermal conductivity, as well as including the testing of carrier-related information. Chapter 9 describes the state-of-the-art progress of PEDOT/PEDOT:PSS as flexible and wearable thermoelectric devices. Finally, Chapter 10 summarizes the challenges that PEDOT/PEDOT:PSS may face in the future as a thermoelectric material and opportunities for its development.

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It is hoped that this book will become a chosen reference and guidance for those engaged in or who want to become engaged in PEDOT thermoelectric performance research. Because the study of organic thermoelectric materials covers a broad range of knowledge, this book may not provide a comprehensive overview of every aspect of current PEDOT/ PEDOT:PSS thermoelectric-related research. At the same time, due to our relatively limited levels of knowledge and experience, mistakes and improprieties will inevitably appear in the book. We sincerely ask readers to provide criticism and corrections to jointly promote the development of PEDOT thermoelectric materials. Jingkun Xu 2021-06-10

Abbreviations AF AFM AS B BBL BDT BN BP BTFMSI 2Cz-D CB CNFs CNTs CPs CSA CTAB Cz D-A-D DBSA DC DEG DES DFT DMF DMSO DN DOS DWCNT e Ea ECAs EDOT EF Eg EG EMIM-BF4 ESR Et F F4TCNQ FIT FTS GE

Ammonium formate Atomic force microscopy Cross-sectional area Magnetic induction intensity Benzimidazo-benzophenanthroline Benzodithiophene Boron nitride Black phosphorus Bis(trifluoromethylsulfonyl)imide 1,12-Bis(carbazolyl)dodecane Conduction band Carbon nanofibers Carbon nanotubes Conjugated (or conducting) polymers Camphor sulfonic acid Hexadecyl trimethyl ammonium bromide Carbazole Donor-acceptor-donor Dodecyl benzenesulfonic acid Direct current Diethylene glycol Deep eutectic solvents Density functional theory N,N-Dimethylformamide Dimethyl sulfoxide Double network structure Density of states Double-wall carbon nanotube Electronic charge Activation energy Electrically conductive adhesives 3,4-Ethylenedioxythiophene Fermi level Energy bandgap Ethylene glycol 1-Ethyl-3-methylimidazolium tetrafluoroborate Electron spin resonance Energy at transport edge Force Tetrafluorotetracyanoquinodimethane Fluctuation-induced tunneling Tridecafluoro-1,1,2,2-tetrahydrooctyl-trichlorosilane Graphene

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Abbreviations

GF GIWAXS GNPs GQDs h HI HOMO HRTEM HZ I ICPs IDT ILs IPA IPN ITO K k, kB l L LbL LCA Lnc LUMO MGI MW MWCNTs n NMP NP NSs NWs oCVD OECTs OFETs OLEDs OMIEC OPV OSC OTE P P2Cz-D P3HT PAc PANi PBTTT PCB

Gauge factor Grazing incidence wide angle X-ray scattering Graphene nanoplates Graphene quantum dots Planck constant Hydroiodic acid Highest occupied molecular orbital High resolution transmission electron microscope Hydrazine Current flow Intrinsic conducting polymers Indacenodithiophene Ionic liquids Isopropyl alcohol Interpenetrating network Indium tin oxide Absolute temperature Boltzmann constant Length of samples Lorenz number Layer-by-layer assembly Life-cycle assessment Average distance between adjacent nanocrystals Lowest unoccupied molecular orbital Materials Genome Initiative Molecular weight Multiwall carbon nanotubes Charge carrier concentration N-Methyl-2-pyrrolidone Nanoparticle Nanosheets Nanowires Oxidative chemical vapor deposition Organic electrochemical transistors Organic field-effect transistors Organic light emitting diodes Organic mixed ionic-electronic conductor Organic photovoltaics Organic solar cells Organic thermoelectric Power Poly(1,12-bis(carbazolyl)dodecane) Poly(3-hexylthiophene) Polyacetylene Polyaniline Poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) Printed circuit board

Abbreviations

PCz PDADMAC PDMS PEDOS PEDOT PEDOT:PSS PEG PEI PEIE PEO PET PF ph PI PIDT PMMA PPP PProDOT PPV PPy ProDOT PS PSe PSS PTE PTh PU PVA PVC q, Q R R2R rGO RH Rs RT s S SCP SEM SPS SWCNTs T Tc TCR TDAE TDTR

Polycarbazole Poly(diallyldimethylammonium chloride) Polydimethylsiloxane Poly(3,4-ethylenedioxyselenophene) Poly(3,4-ethylenedioxythiophene) Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) Polyethylene glycol Polyethylenemine Polyethylenimine ethoxylated Poly(ethylene oxide) Polyethylene terephthalate Thermoelectric power factor Phonons Polyimide Polyindacenodithiophene Polymethylmethacrylate Polypropylene glycol-polyethylene glycol Poly(3,4-propylenedioxythiophene) Poly(p-phenylene vinylene) Polypyrrole 3,4-Propylenedioxythiophene Polystyrene Polyselenophene Polystyrenesulfonate Photo-thermo-electric Polythiophene Polyurethane Polyvinyl alcohol Polyvinyl chloride Charge quantity Resistance Roll-to-roll Graphene oxide Hall coefficient Square resistance Room temperature Transport parameter Seebeck coefficient or thermopower Solution casting polymerization Scanning electron microscopy Spark plasma sintering Single-walled carbon nanotubes Temperature Temperatures at cold side Temperature coefficient of resistivity Tetrakis(dimethylamino)ethylene Time-domain thermal reflectance

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TE TEG TEM Th THF Tos UPS UV-Vis-NIR V VA VB VPP VRH W WFL WPU wt.% Xc XPS ZT

Thermoelectric Thermoelectric generator Transmission electron microscope Temperatures at hot side Tetrahydrofurane p-toluenesulfonate Ultraviolet photoelectron spectra Ultraviolet near infrared spectra Voltage Vapor annealing Valence band Vapor phase polymerization Variable-range hopping Activation energy or width of samples Wiedemann-Franz law Waterborne polyurethane Mass percent Degree of crystallinity X-ray photoelectron spectroscopy Figure-of-merit

Symbol

a s k m h F r u ε hc ke kl sS2 DT DV kǁ kt

Thermopower or Seebeck coefficient Electrical conductivity Thermal conductivity Carrier mobility Thermoelectric conversion efficiency Work functions Resistivity Current frequency Resistance coefficient Carnot efficiency Electron thermal conductivity Lattice or phonon thermal conductivity Thermoelectric power factor Temperature difference Thermopower voltage In-plane thermal conductivity Out-of-plane thermal conductivity

Acknowledgments The contents of this book are mainly based on the thermoelectric research on PEDOTs published in recent decades. In the run-up to the completion of the manuscript, we felt deeply honored to receive much support and help from many teachers and friends. We hope that readers will think of the fruit of our editing journey as a compelling read. The smooth compilation of this book is also inseparable from the hard work of the staff at Elsevier and their encouragement and assistance. Finally, our deepest gratitude goes to the contributors who dedicated their precious time to write and revise the corresponding chapters. Fengxing Jiang Congcong Liu Jingkun Xu

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Biographies Chunmei Gao is an Associate Professor at the College of Chemistry and Chemical Engineering, Shenzhen University. Gao received her PhD degree in Polymer Chemistry and Physics from Guangzhou Institute of Chemistry, Chinese Academy of Sciences, in 2007. Gao’s recent research interests are focused on the design and preparation of organic/carbon nanotube-based composites and new conducting polymers as thermoelectric materials. Jia Yanhua received her MSc degree from Jiangxi Science and Technology Normal University (2019). Since 2020, she has been working at the State Key Laboratory of Luminescent Materials and Devices at South China University of Technology for a PhD degree. Her current research interests include organic thermoelectric materials and devices. Fengxing Jiang received his PhD degree in Physical Chemistry in 2013 from Soochow University. He is a Professor in the Department of Physics at Jiangxi Science and Technology Normal University. Dr. Jiang’s research interests are centered on the design and synthesis of new PEDOTbased conducting polymers and their applications in thermoelectric conversion, supercapacitors, biochemical sensors, and fuel cells. His primary goal is to develop new high-quality, freestanding conducting polymer films and their composites for flexible organic electronic devices. Qinglin Jiang is a Postdoctoral Scholar at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, China. He obtained his MSc degree from Jiangxi Science and Technology Normal University (2015). In 2017, he carried out collaborative research at Linköping University, Sweden. He obtained his PhD from South China University of Technology in 2019. His research interests are focused on organic thermoelectric materials and wearable thermoelectric devices. Congcong Liu received his PhD degree from the School of Materials Science and Engineering from the Tongji University in 2019. His main contribution is a report on freestanding PEDOT:PSS film as a promising thermoelectric polymer with a stable thermoelectric figure-of-merit (ZTw10 2) at room temperature (Synth. Met. 2010, 160, 2481e2485). This is the highest value to date. His current research interests center on the design and synthesis of conducting polymers/2-D inorganic composites and their applications in energy conversion and energy storage.

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Peipei Liu received her PhD degree in Green Energy Chemistry and Technology in 2018 from the South China University of Technology. She is currently a Lecturer in the Department of Physics at Jiangxi Science and Technology Normal University. Dr. Liu’s research interests center on the preparation and design of metal oxides and conducting polymers-based materials, and their applications in thermoelectric conversion and supercapacitor use. Her current research work aims to prepare and design metal oxides and conducting polymers-based materials with high performance for thermoelectric conversion and supercapacitors. Shouli Ming received his PhD degree in Polymer Chemistry and Physics in 2019 from Beijing Normal University. He is a Lecturer at the College of Chemistry and Chemical Engineering, Liaocheng University. Dr. Ming’s research interests are centered on the design and synthesis of new conducting polymers and their applications in electrochromism, supercapacitor, and photoelectric/thermoelectric conversion. His primary goal is to develop new high-performance conducting polymers for organic electronic devices. Hui Shi received her PhD degree in Materials Physics and Chemistry in 2018 from South China University of Technology. She is currently a Lecturer at the School of Enviromental and Chemical Engineering, Nanchang Hangkong University. Her research interests are conjugated polymers for thermoelectric properties, photovoltaic devices, and wastewater treatment. Haijun Song received his PhD degree in Materials Science and Engineering in 2017 from Tongji University under the supervision of Prof. Kefeng Cai. He is currently a Lecturer at the College of Mechanical and Electrical Engineering, Jiaxing University. His research is focused on improving the thermoelectric properties of PEDOT-based materials and their applications in flexible thermoelectric generators. Lei Wang is a Professor at the Shenzhen Key Laboratory of Polymer Science and Technology and College of Materials Science and Engineering in Shenzhen University. Wang received his PhD degree in Polymer Chemistry and Physics from Guangzhou Institute of Chemistry, Chinese Academy of Sciences, in 2006. Dr. Wang’s research interests are focused on the design, preparation, and properties of new conducting polymers as thermoelectric materials and the proton exchange membrane in fuel cells. His primary goal is to determine the relationship between polymer structure and properties.

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Xiaodong Wang received his bachelor’s degrees from Northeastern University (China) and his PhD degree (2018) from Jilin University in China. Now, he works as a Postdoctoral Scholar in Prof. Qian Zhang’s group at the School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen, China. His current research interest is in the study of novel high-performance organic/inorganic composite films and their application in wearable thermoelectric devices. Jingkun Xu is a Professor at the Jiangxi Key Laboratory of Organic Chemistry at Jiangxi Science and Technology Normal University. Dr. Xu received his PhD degree in Polymer Chemistry and Physics from Tsinghua University in 2003. Dr. Xu’s research interests are centered on the preparation of new conducting polymers with high performance and their applications in thermoelectric conversion, electrochromic, supercapacitor, biochemical sensor, and fuel cells. His primary goal is to design new highquality conducting polymer-based materials for various energy storage and conversion devices. Ge Zhang received her PhD degree from the Shandong University in 2017. She is now a Lecturer at the Jiangxi Science and Technology Normal University. Dr. Zhang’s research interests center on the design and synthesis of new conjugated polymers and their applications in thermoelectric conversion, supercapacitors, and chemical sensors. Her primary goal is to develop novel conjugated polymers with good photoelectric properties. Shijie Zhen received his PhD degree from the South China University of Technology in 2018 and conducted his postdoctoral work under the supervision of Prof. Ben Zhong Tang at the State Key Laboratory of Luminescent Materials and Devices from 2018 to 2021. He is currently an Associate Professor in the Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, Guilin University of Technology. His research focuses on developing novel functional organic materials and exploring their optoelectronic and biological applications, such as singlemolecule wires and fluorescent diagnosis and treatment. Zhengyou Zhu received his PhD degree in 2019 from the College of Electronic Information and Optical Engineering in Nankai University. He is currently a Postdoctoral Fellow at Shenzhen University. His research interests are focused on the thermoelectrics of PEDOT-based films, design and synthesis of metal oxide semiconductors (MOS)-based nanomaterials, and their applications in gas sensors. Now he is focusing on the development of high-performance, low-power, and flexible sensing electronics.

CHAPTER 1

Short history of thermoelectric conjugated PEDOT development Fengxing Jiang Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China

1.1 Introduction The discovery of conjugated (conducting) polymers has completely changed people’s understanding of plastics that are not conductive. At the same time, they have quickly attracted the attention of researchers due to their unique physical and chemical properties as well as easy processing. Since the discovery of conjugated polymers, in the nearly half a century of development, it has benefited from the great efforts of countless scientific researchers in this field, whether it is in the synthesis and modification of conjugated polymers, or the principles and applications. On the one hand, only then has the important position of today’s conjugated polymers been developed in the development of science and technology. In the future development, conjugated polymers will lead human life, especially in the field of flexible microelectronics, and will leave an indelible mark on the development of humankind. Conjugated polymers are typical semiconductor materials and have shown potential applications in the fields of semiconductor light, electricity, heat, and magnetism. The thermoelectric effect is one of the important properties of semiconductor materials and plays an important role in energy conversion. Research on the thermoelectric properties of conjugated polymers has become an important part of this field. Compared with inorganic thermoelectric materials, the research history of conjugated polymers as thermoelectric materials is relatively short. At the earliest stages of research on conjugated polymers at the end of the 20th century, thermopower (also called Seebeck coefficient), the most important parameter in thermoelectric materials, was mainly used to study the problem of charge transport in organic semiconductors. Because of its stability and other issues, it has not been studied as a thermoelectric material. Of course, this does not

Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00008-1

© 2022 Elsevier Ltd. All rights reserved.

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mean that the thermoelectric effect of conjugated polymers has not aroused the interest of researchers, and more attention is focused on the study of charge-transport mechanisms. With the development of synthesis and preparation technologies and processes, conjugated polymers have been rapidly developed in many research fields, including thermoelectrics. Among a large number of conjugated polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is predominant in the field of organic semiconductor materials with its outstanding photo/electric properties. This is in line with countless researchers in its synthesis, microstructure control, preparation process, charge-transport mechanism, and characterization. It is inseparable from the investment and effort in testing technology and other aspects. In the field of organic thermoelectric (OTE) materials, PEDOT has also become a leader with its excellent electrical conductivity (s), film-forming properties and processability. Although PEDOT still has many problems in the research of thermoelectric materials, the rapid development of its thermoelectric properties in decades has made it one of the most promising organic thermoelectric materials. In this chapter, we will make a brief review of conjugated polymer thermoelectric materials, focusing on the development history of conductive PEDOT as a promising thermoelectric material. We hope that the knowledge in this part can give readers a clearer and deeper understanding of thermoelectric conjugated polymers.

1.2 Evolution of thermoelectric conjugated polymers The thermoelectric effects can realize the direct conversion of heat into electric voltage, and vice versa. This mainly depends on the directional movement of carriers inside the material under thermal excitation. The thermoelectric effect is composed of three separately determined effects: Seebeck effect, Peltier effect, and Thomson effect. Here, we will not give the introduction to the three effects one by one, please refer to the relevant reference books. As we know, the dimensionless figure-of-merit is used to evaluate thermoelectric performance based on inorganic thermoelectric materials, defined as ZT ¼

sS2 T k

(1.1)

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where, s, S (also as a), k, and T are the electrical conductivity, Seebeck coefficient (thermopower), thermal conductivity, and absolute temperature, respectively. The higher the thermoelectric ZT value, the better the thermoelectric performance of materials or devices. Observed from the above equation, a high-performance thermoelectric material should have the high electrical conductivity and Seebeck coefficient as well as the low thermal conductivity. Conjugated polymers have a unique inherent advantage that is lower thermal conductivity, which is one to two orders of magnitude lower than inorganic thermoelectric materials,1,2 which implies that conjugated polymer as thermoelectric materials in the middle and low-temperature regions are expected to be comparable to inorganic thermoelectric materials even better. Therefore, many efforts on conjugated polymers have been devoted to achieving a high thermoelectric power factor (sS2) often used instead of ZT to evaluate their thermoelectric performance. Compared with electrical conductivity, Seebeck coefficient contributes more to the power factor, but this does not mean that it can sacrifice one of them. Generally, it is not easy to obtain a high power factor, because electrical conductivity and Seebeck coefficient have an opposite correlation with the carrier concentration in polymers. Currently, most of this work is to maximize the power factor by seeking a compromise between them, which has to sacrifice one party to meet the improvement of the other party. Therefore, finding a suitable material, using an effective control strategy, and adopting a feasible preparation process is the direction for current researchers to pursue and explore high-performance thermoelectric polymers. As early as 1969, Barnes et al. discovered the thermoelectric currents in nonconjugated polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), and polystyrene (PS) by establishing a temperature gradient after irradiation.3 They found that the charge carriers induced in the presence of a temperature gradient were observed to be positive due to added impurities. More importantly, their investigation showed that a temperature gradient can be used effectively to generate thermoelectric currents for the study of charge trapping in certain organic polymers, although the induced current is small. This is very obvious in the early studies on the electrical properties of polymers. Because thermoelectric materials require good electrical conductivity, the early studies of polymers as thermoelectric materials did not receive enough attention until conjugated conducting polymers were developed to a greater extent.

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Conjugated polymer is consistent with sp2 hybridization between two neighboring carbon atoms resulting in the p electrons in chain skeleton therefore available to delocalize into a band which would give rise to metallic behavior. The Nobel Prize in Chemistry 2000 was awarded jointly to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa “for the discovery and development of conductive polymers.” After that, the investigation on the thermoelectric properties of conjugated polymers began to enter the field of vision of researchers, such as doped and undoped polyacetylene (PAc), polypyrrole (PPy), polyaniline (PANi), polythiophene (PTh), and their derivatives. MacDiarmid and Heeger et al.4 devoted a lot of efforts into the conductivity and thermopower of conjugated polymers. The thermopower is used as a zero-current transport coefficient, allowing evaluation of the intrinsic properties, and as a measure of entropy per carrier. Later, the conductivity and thermopower of have been studied in many new conjugated polymers, and these works have made outstanding contributions to the subsequent development of thermoelectric polymers. For example, Ueno and Yoshino performed detailed electrical conductivity and thermopower measurements in the graphitized poly(p-phenylene vinylene) (PPV) films for the various heat-treatment temperatures.5 Kaiser analyzed the thermopower of conducting polymers in heterogeneous media.6 For some highly conducting polymers, the thermopower shows typical metallic temperature dependence similar to the diffusion thermopower of metals where a knee is produced at low temperatures by the electron-phonon interaction, which was considered as providing a good basis for subsequent research on the thermoelectric properties of conjugated polymers.

1.3 Typical thermoelectric conjugated polymers As we know, a key property of a conjugated polymer is doping by analogy with the doping of semiconductors resulting in a semiconductor-metal transition. More importantly, conjugated system in polymers allows a dedoping process enabling an easily controlled by chemical, electrochemical or other means to optimize conductive behavior. In this regard, inorganic semiconductor materials are incomparable. Moreover, the types of dopants are so abundant that it has great selectivity and operability in the regulation space of charge-transport property. In addition, conjugated system in polymers has a desired molecular structure through functional group modification and synthesis techniques, thereby realizing good electron transport properties. Therefore, conjugated polymers have considerable potential regarded as a promising thermoelectric material. The following briefly introduces several common thermoelectric conjugated polymers.

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5

1.3.1 Polyacetylene In 1974, polyacetylene (PAc) as a silvery crystalline film was prepared from acetylene by Shirakawa and coworkers, already known as black powder. For doped polyacetylene film, its electrical conductivity is allowed to systematically and continuously increase over 11 orders of magnitude from pristine to doped state, forming a new class of conducting polymers. The polyacetylene served as the simplest conjugated polymer is more nearly analogous to the traditional inorganic semiconductor and is therefore of special fundamental interest in organic electron transport. Also, its thermoelectric performance has gained great attention. Kwak et al.7 first investigated the resistance and thermopower between 10 and 300 K on both cis- and trans-rich polyacetylene films doped with 10% AsF5 in Fig. 1.1. The change in resistance with temperature shows three obvious stages including a positive (metallic) temperature coefficient above 250 K, a negative (nonmetallic) one below 250 K, and a flat one becoming essentially temperature independent below 30 K. The electrical conductivity can achieve a large value sRT w1000 S cm1. For heavy doping, the metallic polymer can be regarded as composed of discontinuous metallic strands with dc transport owing to barrier penetration and/or phonon-assisted hopping. Note that the disorder may play a key role in doped polyacetylene crystalline films. Generally, the plots of lns versus 1/T do not show a straight-line behavior, but a more nearly straight-line trend for lns versus T1/4 (or T1/2) at a low dopant concentration range, indicating the typical of transport in disordered and amorphous systems.8 The Mott variable range-hopping (VRH) between localized states has been found to be consistent with the law: h i sðT Þfexp  ðT0 =T Þ1=ðdþ1Þ (1.2)

Figure 1.1 (A) Dc resistance versus the log of temperature and (B) thermopower versus temperature for polyacetylene doped with 10% AsF5.7 (Copyright 1985, Elsevier Ltd.)

6

Advanced PEDOT Thermoelectric Materials

Here d is the dimensionality of the transport. For polyacetylene, its polymer structure shows strong p overlap along the chain with weaker interchain coupling. The carrier mobility of polyacetylene doped 14% AsF5 was estimated to be 1 cm2 V1 s1 from s ¼ nem

(1.3)

where n is the carrier density, e is the electronic charge, and m is the mobility. It is worth mentioning that it is still a challenge to date to retain the mobility based on the Hall measurements. Because a low-doped conjugated polymer has a high contact resistance due to the hopping nature of system causing too excessive noise to successfully measure a small Hall voltage. In contrast, a higher carrier density in heavily doped polymers probably leads to a smaller Hall voltage due to their inverse dependence.9 It is expected toward the development of a new technology suitable for the measurement of mobility in conjugated polymers in the future. In subsequent chapters, we will further discuss the test methods of mobility focused on PEDOT:PSS, such as organic field effect transistor (OFET) and electrochemical transistor (OECT). Thermopower usually serves as a measure of the entropy per carrier (a function of dopant concentration) to study the semiconductor-metal transition. In general, a large thermopower is desired for semiconductors (few carriers with many possible states per carrier) whereas the entropy of degenerate electron gas is small in the metallic state (of order kB (kBT/EF) per carrier).10 As shown in Fig. 1.1B, both cis and trans films showed a fairly small magnitude of thermopower SRTw10 mV K1 (metal-like), and the positive value indicated polyacetylene doped AsF5 belonged to hole conduction (p-type doping). The linear S versus T indicates a general result for a degenerate Fermi gas, independent of the dimensionality. Park et al.10,11 also observed the similar linear temperature dependence and the highly anisotropic dc conductivity consistent with quasi-one-dimensional behavior in polyacetylene with heavy iodine or ferric chloride doping. A nonlinear S versus T observed in heavily doped polyacetylene is similar to the characteristic of the electron-phonon effect in metallic diffusion thermopower. Note that the thermopower of the undoped polyacetylene for different films varies in the range from 800 to 1000 mV K1 owing to variations in purity or oxygen introduced in samples. In addition, the thermopower under pure, low concentration doping, or the additional sp3 defects is temperature independent suggestive of localized charge carriers with hopping transport. Unfortunately, due to the poor environmental stability in air, polyacetylene is not an ideal organic thermoelectric material, so it gradually faded out of the field of investigation.

Short history of thermoelectric conjugated PEDOT development

7

1.3.2 Polythiophenes Unlike polyacetylene, polythiophene (PTh) and its derivatives can be synthesized directly in the doped form and have excellent environmental stability in (un)doped states, and ease of structural modification as well as solution processability. Despite a low electrical conductivity (6000 S cm1), served as a typical semimetal with only one order of magnitude lower than silver and copper, has been discovered for PEDOT films prepared with vapor phase polymerization (VPP) or oxidative chemical vapor deposition (oCVD) by many research groups (introduced in Chapter 2).40,41 In addition, the resultant high carrier mobility (>10 cm2 V1 s1) will enable PEDOT to become a promise organic electronic material with low energy consumption and better charge carrier transport. At the same time, PEDOT shows significant advantages over inorganic semiconductor materials in the field of flexible electron devices due to its good film-forming, stretchable, and bendable properties (Fig. 1.7). PEDOT also has another advantage that other conducting polymers are incomparable, that is its commercial product poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) aqueous solution (polyelectrolyte complex). Its appearance as a commercial product has taken an extremely important step for the development of organic

16

Advanced PEDOT Thermoelectric Materials

Figure 1.7 Chemical structure of EDOT, PEDOT (doped PEDOTþ and undoped PEDOT0), and PEDOT:PSS.

electron devices. The main function of water-soluble PSS (HPSS/NaPSS) is dual: (1) to solve the problem of PEDOT solubility in water and (2) to balance the charge of PEDOT during polymerization. It has been found that the sulfonic acid group as counteranion is the most conducive to the stability of PEDOTþ.42 Factually, the PEDOT positive charges (PEDOTþ) requires only a very small amount of PSS negative charges to balance through the coulombic interactions, and most of PSS acts as surfactant to enhance the dispersion of PEDOT in water. Moreover, the PEDOT/PSS ratio can be varied between 1/2.5 and 1/20 and the work function between 4.8 and 5.8 eV.43 This combination resulted in a homogeneous and stable PEDOT:PSS aqueous dispersion with good film-forming properties for safe handling and easy to use by coating and printing. The required PEDOT:PSS films can be prepared by many application methods such as screen printing, inkjet printing, gravure printing, slot-die coating, spray coating, dip and spin coating. The resultant films exhibit some key characteristics including tunable conductivity over wide range, excellent mechanical flexibility, high light transmissivity (transparency >90% at l ¼ 550 nm77), high thermal and chemical stability. The solid PEDOT:PSS is a typical organic mixed ionic-electronic conductor (OMIEC) composed of the electronic PEDOT and ionic PSS, leading to the wide interest in next-generation bioelectronic, optoelectronic, and energy storage devices. As a matter of fact, PSS also produces another side effect at the same time, that is, its excessive existence leads to poor charge transfer properties

Short history of thermoelectric conjugated PEDOT development

17

in PEDOT:PSS solid (w1 S cm1). Because the PSS chains adopt a coil conformation that the PEDOT follows due to the shorter chain length of repeating PEDOT unit. This results in the structure distortion and charge localization leading to the poor conduction pathways in PEDOT:PSS solid. However, the charge transfer properties can be optimized by introducing additives (also namely secondary doping), or easy pre- and posttreatment to achieve the phase separation between PEDOT-rich and PSS-rich domains. Note that the additives (salts, high boiling point organic solvents, and ionic liquids) induce charge screening and conformation changes from coil to linear structure in PEDOT:PSS, which allows two orders of magnitude improvement in electrical conductivity. The pre- and posttreatment is mainly based on organic solvents or acids for the secondary treatment of PEDOT:PSS solid, which enables the electrical conductivity to increase by three orders of magnitude up to 103 S cm1 that is not inferior to pure PEDOT. This could be attributed to two reasons: (1) The solvents using for posttreatment remove a large amount of excess insulating PSS, so as to gain more conductive PEDOT-rich phase in PEDOT:PSS, (2) the used solvent also induces the conformation changes and close packing of PEDOT molecules. For more about the optimization of thermoelectric properties, we will discuss and introduce in detail in Chapter 5.

1.5 Thermoelectric PEDOT/PEDOT:PSS Thanks to the above advantages that PEDOT/PEDOT:PSS has become the benchmark conducting polymer for organic thermoelectric materials at present. As an organic thermoelectric material, PEDOT has inherently low thermal conductivity ( 1) due to its large power factor which could be achieved by adjustable electrical conductivity and thermopower. Since its attention, PEDOT/PEDOT:PSS as a p-type organic thermoelectric material has been studied for less than 20 years, and its ZT value has increased from 103 to 101 at room temperature. Its mechanical properties and applications in the field of microelectronics have unmatched characteristics of inorganic semiconductor materials. Nevertheless, thermoelectric PEDOT leaves a relatively large gap when compared with practical applications. Due to the complex correlation of electrical conductivity, thermopower, and thermal conductivity with carriers and microstructures in semiconductor thermoelectric materials (Chapter 4), researchers need to continue in-depth studies, and then obtain the PEDOT

18

Advanced PEDOT Thermoelectric Materials

thermoelectric transport mechanism and the controllable methods in thermoelectric performance. Next, we divide the development of thermoelectric PEDOT into three stages, and make a brief introduction and summary. It needs to be emphasized that the rapid development of thermoelectric PEDOT is inseparable from the previous research results on the thermoelectric properties of numerous conjugated polymers. 1.5.1 Discovery at an early stage PEDOT is penetrated into the field of organic thermoelectric material owing to its highly conductive and stable properties as well as easy chemical oxidation synthesis, which have been confirmed based on its broad application prospects including solid electrolyte capacitors, printed wiring boards, packaging films, touch screens and as hole transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaics (OPV). Early (Before 2008), a large number of researches on PEDOT are mainly in respect of the preparation method, charge-transport mechanism, processability and its application in the field of antistatic coatings and organic optoelectronic devices. These common conjugated polymers, such as PANi, PTh, PPy, and PEDOT etc., are composed of conjugated chains with various lengths and uneven defects during preparation process, and their combination leads to the inherently disordered structure. Due to the weak van der Waals forces (p-p interactions) between chains, conjugated polymers are constituted with crystalline domains (tens of nm) and amorphous domains due to stacking of chains. Therefore, PEDOT also shows different charge transport from that in metals and classic semiconductors, depending on the degree of disorder. This is still the huge obstacle to the theoretical charge transport in PEDOT. In general, the temperature dependences of the electrical conductivity s(T ) is often used to study the heavily doped conjugated polymers with metallic transport properties and charge-carrier transport. Aleshid et al.45 studied the charge transport of metallic PEDOT films doped with PF 6,  BF 4 , and CF3SO3 based on s(T ) from room temperature down to 1.2 K. The electrical conductivity for the best PEDOT films is typically 200e300 S cm1 with PF6 doping at room temperature. They found that the transition in s(T ) dramatically changed from negative to positive temperature coefficient of resistivity (TCR) at about 10 K. This lowtemperature ( Et Þ sE ðE; T Þ ¼ sE0 ðT Þ  kB T (1.8) ¼ 0 ðE < Et Þ

Short history of thermoelectric conjugated PEDOT development

33

States below the transport edge Et are truly localized and do not contribute to conduction, while states above Et conduct electricity, but this involves thermally activated conduction. It can be found that the highperformance PEDOT-based samples are fundamentally different giving a better fit to the S-s curve by s ¼ 1 than s ¼ 3 of other polymers such as P3HT and PBTTT doped with F4TCNQ. These different transports in polymers may be owing to the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions. Moreover, the curves are qualitatively different, and also that the sE0 is orders of magnitude higher in the case of the samples shown. However, the as-described charge-transport model is not very successful for PEDOTbased polymers including the their thermopower-conductivity relation. Therefore, further investigation is greatly expected to provide advanced insight regarding microstructure, disorder and percolation on a complex polymer system. On the other hand, density functional theory (DFT) investigations have been conducted for PEDOT/PEDOT:PSS to unravel the impacts of geometric structure and doping at molecular level on the thermoelectric transport properties.72 PEDOT exhibits a distinct transition from the aromatic to quinoid-like structure of backbone, and semiconductor-to-metal transition with an increase in the level of doping. When the Tos doping degree is around 12.5%, a high electrical conductivity and thermopower can be gained for PEDOT at the same time. The doping effects have been elucidated by explicitly including the scattering of charge carriers with dopants on the thermoelectric properties of PEDOT. The predicted mobility and power factors are in very good agreement with the stateof-the-art experimental data. Moreover, the thermoelectric transport is highly anisotropic in ordered crystals. It has been suggested that a large power factor can be utilized in the direction of polymer backbone, while a low lattice thermal conductivity can be realized in the stacking and lamellar directions, which are viable in chain-oriented amorphous nanofibers. According to the theoretical investigation, the doping effects on thermoelectric performance of “ideal” crystalline lightly doped PEDOT:Tos unravel that by tuning the doping level a power factor of 3150 mW m1 K2 along the polymer chain and 209 mW m1 K2 in the p-p stacking direction can be realized.72 The crystalline PEDOT is predicted an excellent heat conductor in the direction of polymer backbone, thereby the thermoelectric performance is very poor in the chain direction despite of its electron-crystal nature. However, it exhibits the phonon-glass nature in the interchain

34

Advanced PEDOT Thermoelectric Materials

directions due to its anisotropic chemical bonding nature, covalent versus van der Waals bonding.75 It has been proposed that the rational design and tailoring the chain length and crystallinity of chain-oriented PEDOT fibers allow for a significant decrease of the lattice thermal conductivity and an impressive increase of ZT. Furthermore, a heterogeneous conduction has been created and found that the thermoelectric transport can be governed by only quantum mechanical tunneling of charge carriers through nanoscale Coulomb barriers at order-disorder boundaries in conducting polymer networks.76 Also, it can be engineered without doping or intrinsic trades-off. Despite many theoretical models have been created for conjugated polymers, more elaborate and accurate computational methods are required toward understanding the thermoelectric performance better to gain more experimental outcomes in the future.

1.6 Concluding remarks In the last two decades, the thermoelectric ZT of PEDOT/PEDOT:PSS has been improved from 103 to 101 at room temperature, which is now regarded as a benchmark organic thermoelectric polymer. During the development and innovation of PEDOT-based thermoelectric materials, new theories, technologies, and experiments have established the strong foundation for the further development of highly efficient organic thermoelectric materials, which is inseparable from the long-term cultivation and hard work of countless researchers. From theory to experiment, and then from basic to application, PEDOT-based systems have achieved the significant improvement on the thermoelectric transport properties. Although many challenges still exist, more improvements and innovations are expected and required in the established but also burgeoning field of thermoelectric PEDOT.

References 1. Jiang, F.; Xu, J.; Lu, B.; Xie, Y.; Huang, R.; Li, L. Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate). Chin. Phys. Lett. 2008, 25, 2202. 2. Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic Thermoelectric Materials for Energy Harvesting and Temperature Control. Nat. Rev. Mater. 2016, 1, 16050. 3. Barnes, J. E.; Hoecker, F. E.; Kevan, L. Radiation-induced Trapped Charge Carriers in Organic Polymers. A Model of Thermoelectric Current Generation. Radiat. Res. 1969, 40, 235.

Short history of thermoelectric conjugated PEDOT development

35

4. Wnek, G.; Chien, J.; Karasz, F.; Druy, M.; Park, Y.; MacDiarmid, A.; Heeger, A. Variable-density Conducting Polymers e Conductivity and Thermopower Studies of a New Form of Polyacetylene e (CH)x. J. Polym. Sci. Polym. Lett. Ed. 1979, 17, 779. 5. Ueno, H.; Yoshino, K. Electrical Conductivity and Thermoelectric Power of Highly Graphitizable Poly(p-Phenylene Vinylene) Films. Phys. Rev. B 1986, 34, 7158. 6. Kaiser, A. B. Thermoelectric Power and Conductivity of Heterogeneous Conducting Polymers. Phys. Rev. B 1989, 40, 2806. 7. Kwak, J. F.; Clarke, T. C.; Greene, R. L.; Street, G. B. Transport Properties of Heavily AsF5 Doped Polyacetylene. Solid State Commun. 1979, 31, 355. 8. Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Electrical Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098. 9. Wei, Q.; Mukaida, M.; Naitoh, Y.; Ishida, T. Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents. Adv. Mater. 2013, 25, 2831. 10. Park, Y. W.; Denenstein, A.; Chiang, C. K.; Heeger, A. J.; MacDiarmid, A. G. Semiconductor-metal Transition in Doped (CH)x: Thermoelectric Power. Solid State Commun. 1979, 29, 747. 11. Park, Y. W.; Han, W. K.; Choi, C. H.; Shirakawa, H. Metallic Nature of Heavily Doped Polyacetylene Derivatives: Thermopower. Phys. Rev. B 1984, 30, 5847. 12. Kaloni, T. P.; Giesbrecht, P. K.; Schreckenbach, G.; Freund, M. S. Polythiophene: From Fundamental Perspectives to Applications. Chem. Mater. 2017, 29, 10248. 13. Kaneto, K.; Hayashi, S.; Ura, S.; Yoshino, K. ESR and Transport Studies in Electrochemically Doped Polythiophene Film. J. Phys. Soc. Jpn. 1985, 54, 1146. 14. Masubuchi, S.-i.; Kazama, S. A Possible Phase Transition in Metallic Polythiophene and Poly(3-Methylthiophene) at Low Temperature. Synth. Met. 1995, 69, 315. 15. Hiraishi, K.; Masuhara, A.; Nakanishi, H.; Oikawa, H.; Shinohara, Y. Evaluation of Thermoelectric Properties of Polythiophene Films Synthesized by Electrolytic Polymerization. Jpn. J. Appl. Phys. 2009, 48, 071501. 16. Wang, Z. H.; Li, C.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Three Dimensionality of "metallic" States in Conducting Polymers: Polyaniline. Phys. Rev. Lett. 1991, 66, 1745. 17. Kroon, R.; Kiefer, D.; Stegerer, D.; Yu, L.; Sommer, M.; Muller, C. Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly P-Doped Polythiophene. Adv. Mater. 2017, 29, 1700930. 18. Glaudell, A. M.; Cochran, J. E.; Patel, S. N.; Chabinyc, M. L. Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes. Adv. Energy Mater. 2015, 5, 1401072. 19. Lim, E.; Peterson, K. A.; Su, G. M.; Chabinyc, M. L. Thermoelectric Properties of Poly(3-Hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8Tetracyanoquinodimethane (F4TCNQ) by Vapor-phase Infiltration. Chem. Mater. 2018, 30, 998. 20. Zuo, F.; Angelopoulos, M.; MacDiarmid, A. G.; Epstein, A. J. Transport Studies of Protonated Emeraldine Polymer: A Granular Polymeric Metal System. Phys. Rev. B Condens. Matter. 1987, 36, 3475. 21. Wang, Z. H.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Transport and EPR Studies of Polyaniline: A Quasi-One-Dimensional Conductor with Three-Dimensional "metallic" States. Phys. Rev. B Condens. Matter. 1992, 45, 4190. 22. Reghu, M.; Cao, Y.; Moses, D.; Heeger, A. J. Counterion-induced Processibility of Polyaniline: Transport at the Metal-Insulator Boundary. Phys. Rev. B Condens. Matter. 1993, 47, 1758.

36

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23. Yoon, C. O.; Reghu, M.; Moses, D.; Heeger, A. J.; Cao, Y. Counterion-induced Processibility of Polyaniline: Thermoelectric Power. Phys. Rev. B Condens. Matter. 1993, 48, 14080. 24. Yan, H.; Ohta, T.; Toshima, N. Stretched Polyaniline Films Doped by (L)10-Camphorsulfonic Acid: Anisotropy and Improvementof Thermoelectric Properties. Macromol. Mater. Eng. 2001, 286, 139. 25. Cao, Y.; Smith, P.; Heeger, A. J. Counter-ion Induced Processibility of Conducting Polyaniline and of Conducting Polyblends of Polyaniline in Bulk Polymers. Synth. Met. 1992, 48, 91. 26. Li, J.; Tang, X.; Li, H.; Yan, Y.; Zhang, Q. Synthesis and Thermoelectric Properties of Hydrochloric Acid-Doped Polyaniline. Synth. Met. 2010, 160, 1153. 27. Yao, Q.; Chen, L.; Xu, X.; Wang, C. The High Thermoelectric Properties of Conducting Polyaniline with Special Submicron-Fibre Structure. Chem. Lett. 2005, 34, 522. 28. Liang, L.; Chen, G.; Guo, C.-Y. Polypyrrole Nanostructures and Their Thermoelectric Performance. Mater. Chem. Front. 2017, 1, 380. 29. Li, M.; Luo, C.; Zhang, J.; et al. Electrochemical Doping Tuning of Flexible Polypyrrole Film with Enhanced Thermoelectric Performance. Surf. Interfaces 2020, 21, 100759. 30. Maddison, D. S.; Tansley, T. L. Variable Range Hopping in Polypyrrole Films of a Range of Conductivities and Preparation Methods. J. Appl. Phys. 1992, 72, 4677. 31. Maddison, D. S.; Unsworth, J.; Roberts, R. B. Electrical Conductivity and Thermoelectric Power of Polypyrrole with Different Doping Levels. Synth. Met. 1988, 26, 99. 32. Bekkar, F.; Bettahar, F.; Moreno, I.; Meghabar, R.; Hamadouche, M.; Hernaez, E.; Vilas-Vilela, J. L.; Ruiz-Rubio, L. Polycarbazole and its Derivatives: Synthesis and Applications. A Review of the Last 10 Years. Polymers 2020, 12, 2227. 33. Wellinchoff, S. T.; Deng, Z.; Kedrowski, T. J.; Dick, S. A.; Jenekhe, S. A.; Ishida, H. Electronic Conduction Mechanism in Polycarbazole Iodine Complexes. Mol. Crysf. Liq. Crysr. 1984, 106, 289. 34. Wellinghoff, S. T.; Deng, Z.; Reed, J. F.; Jenekhe, S. A. The Role of Polymer Cations in the Polymerization and Electrical Conductivity of Polycarbazole. Mol. Crysf. Liq. Crysr. 1985, 118, 403. 35. Lévesque, I.; Bertrand, P.-O.; Blouin, N.; Leclerc, M.; Zecchin, S.; Zotti, G.; Ratcliffe, C. I.; Klug, D. D.; Gao, X.; Gao, F.; Tse, J. S. Synthesis and Thermoelectric Properties of Polycarbazole, Polyindolocarbazole, and Polydiindolocarbazole Derivatives. Chem. Mater. 2007, 19, 2128. 36. Aïch, R. B.; Blouin, N.; Bouchard, A.; Leclerc, M. Electrical and Thermoelectric Properties of Poly(2,7-Carbazole) Derivatives. Chem. Mater. 2009, 21, 751. 37. Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4ethylenedioxythiophene) and its Derivatives: Past, Present, and Future. Adv. Mater. 2000, 12, 481. 38. Goel, M.; Thelakkat, M. Polymer Thermoelectrics: Opportunities and Challenges. Macromolecules 2020, 53, 3632. 39. Groenendaal, L.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R. Electrochemistry of Poly(3,4-Alkylenedioxythiophene) Derivatives. Adv. Mater. 2003, 15, 855. 40. Cho, B.; Park, K. S.; Baek, J.; Oh, H. S.; Koo Lee, Y. E.; Sung, M. M. Single-crystal Poly(3,4-Ethylenedioxythiophene) Nanowires with Ultrahigh Conductivity. Nano Lett. 2014, 14, 3321. 41. Wang, X.; Zhang, X.; Sun, L.; Lee, D.; Lee, S.; Wang, M.; Zhao, J.; Shao-Horn, Y.; Dinca, M.; Palacios, T.; Gleason, K. K. High Electrical Conductivity and Carrier Mobility in oCVD PEDOT Thin Films by Engineered Crystallization and Acid Treatment. Sci. Adv. 2018, 4, eaat5780.

Short history of thermoelectric conjugated PEDOT development

37

42. Kirchmeyer, S.; Reuter, K. Scientific Importance, Properties and Growing Applications of Poly(3,4-Ethylenedioxythiophene). J. Mater. Chem. C 2005, 15, 2077. 43. Lövenich, W. PEDOT-Properties and Applications. Polym. Sci. Ser. C 2014, 56, 135e143. 44. Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 45. Aleshin, A. N.; Kiebooms, R.; Heeger, A. J. Metallic Conductivity of Highly Doped Poly(3,4-Ethylenedioxythiophene). Synth. Met. 1999, 101, 369. 46. Menon, R.; Yoon, C. O.; Moses, D.; Heeger, A. J. Handbook of Conducting Polymers; Marcel Dekker: NY, 1996. 47. Aleshin, A. N.; Williams, S. R.; Heeger, A. J. Transport Properties of Poly(3,4Ethylenedioxythiophene)/poly(styrenesulfonate). Synth. Met. 1998, 94, 173. 48. Kaiser, A. B. Systematic Conductivity Behavior in Conducting Polymers: Effects of Heterogeneous Disorder. Adv. Mater. 2001, 13, 927. 49. Kim, J. Y.; Jung, J. H.; Lee, D. E. Enhancement of Electrical Conductivity of Poly(3,4Ethylenedioxythiophene)/poly(4-Styrenesulfonate) by a Change of Solvents. Synth. Met. 2002, 126, 311. 50. Gueye, M. N.; Carella, A.; Faure-Vincent, J.; Demadrille, R.; Simonato, J.-P. Progress in Understanding Structure and Transport Properties of PEDOT-Based Materials: A Critical Review. Prog. Mater. Sci. 2020, 108, 100616. 51. Bubnova, O.; Berggren, M.; Crispin, X. Tuning the Thermoelectric Properties of Conducting Polymers in an Electrochemical Transistor. J. Am. Chem. Soc. 2012, 134, 16456. 52. Liu, C.; Lu, B.; Yan, J.; Xu, J.; Yue, R.; Zhu, Z.; Zhou, S.; Hu, X.; Zhang, Z.; Chen, P. Highly Conducting Free-Standing Poly(3,4-Ethylenedioxythiophene)/poly(styrenesulfonate) Films with Improved Thermoelectric Performances. Synth. Met. 2010, 160, 2481. 53. Scholdt, M.; Do, H.; Lang, J.; Gall, A.; Colsmann, A.; Lemmer, U.; Koenig, J. D.; Winkler, M.; Boettner, H. Organic Semiconductors for Thermoelectric Applications. J. Electron. Mater. 2010, 39, 1589. 54. Gao, X.; Uehara, K.; Klug, D. D.; Patchkovskii, S.; Tse, J. S.; Tritt, T. M. Theoretical Studies on the Thermopower of Semiconductors and Low-Band-Gap Crystalline Polymers. Phys. Rev. B 2005, 72, 125202. 55. Gao, X.; Uehara, K.; Klug, D. D.; Tse, J. S. Rational Design of High-Efficiency Thermoelectric Materials with Low Band Gap Conductive Polymers. Comput. Mater. Sci. 2006, 36, 49. 56. Pernstich, K. P.; Rossner, B.; Batlogg, B. Field-effect-modulated Seebeck Coefficient in Organic Semiconductors. Nat. Mater. 2008, 7, 321. 57. Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12, 719. 58. Zykwinska, A.; Domagala, W.; Czardybon, A.; Pilawa, B.; Lapkowski, M. In Situ EPR Spectroelectrochemical Studies of Paramagnetic Centres in Poly(3,4Ethylenedioxythiophene) (PEDOT) and Poly(3,4-Butylenedioxythiophene) (PBuDOT) Films. Chem. Phys. 2003, 292, 31. 59. Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J. B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190.

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60. Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043. 61. Jin, H.; Li, J.; Iocozzia, J.; Zeng, X.; Wei, P. C.; Yang, C.; Li, N.; Liu, Z.; He, J. H.; Zhu, T.; Wang, J.; Lin, Z.; Wang, S. Hybrid Organic-Inorganic Thermoelectric Materials and Devices. Angew. Chem. Int. Ed. 2019, 58, 2. 62. Yee, S. K.; Coates, N. E.; Majumdar, A.; Urban, J. J.; Segalman, R. A. Thermoelectric Power Factor Optimization in PEDOT:PSS Tellurium Nanowire Hybrid Composites. Phys. Chem. Chem. Phys. 2013, 15, 4024. 63. Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038. 64. Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching Disorder-free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384. 65. Kee, S.; Kim, N.; Kim, B. S.; Park, S.; Jang, Y. H.; Lee, S. H.; Kim, J.; Kim, J.; Kwon, S.; Lee, K. Controlling Molecular Ordering in Aqueous Conducting Polymers Using Ionic Liquids. Adv. Mater. 2016, 28, 8625. 66. Mazaheripour, A.; Majumdar, S.; Hanemann-Rawlings, D.; Thomas, E. M.; McGuiness, C.; Alencon, L.; Chabinyc, M. L.; Segalman, R. A. Tailoring the Seebeck Coefficient of PEDOT:PSS by Controlling Ion Stoichiometry in Ionic Liquid Additives. Chem. Mater. 2018, 30, 4816. 67. Petsagkourakis, I.; Pavlopoulou, E.; Cloutet, E.; Chen, Y. F.; Liu, X.; Fahlman, M.; Berggren, M.; Crispin, X.; Dilhaire, S.; Fleury, G.; Hadziioannou, G. Correlating the Seebeck Coefficient of Thermoelectric Polymer Thin Films to Their Charge Transport Mechanism. Org. Electron. 2018, 52, 335. 68. Tanaka, H.; Kanahashi, K.; Takekoshi, N.; Mada, H.; Ito, H.; Shimoi, Y.; Ohta, H.; Takenobu, T. Thermoelectric Properties of a Semicrystalline Polymer Doped beyond the Insulator-To-Metal Transition by Electrolyte Gating. Sci. Adv. 2020, 6, eaay8065. 69. Dongmin Kang, S.; Jeffrey Snyder, G. Charge-transport Model for Conducting Polymers. Nat. Mater. 2017, 16, 252. 70. Patel, S. N.; Glaudell, A. M.; Peterson, K. A.; Thomas, E. M.; O’Hara, K. A.; Lim, E.; Chabinyc, M. L. Morphology Controls the Thermoelectric Power Factor of a Doped Semiconducting Polymer. Sci. Adv. 2017, 3, e1700434. 71. Kang, K.; Watanabe, S.; Broch, K.; et al. 2D Coherent Charge Transport in Highly Ordered Conducting Polymers Doped by Solid State Diffusion. Nat. Mater. 2016, 15, 896. 72. Shi, W.; Zhao, T.; Xi, J.; Wang, D.; Shuai, Z. Unravelling Doping Effects on PEDOT at the Molecular Level: From Geometry to Thermoelectric Transport Properties. J. Am. Chem. Soc. 2015, 137, 12929. 73. Zhang, B.; Wang, K.; Li, D.; Cui, X. Doping Effects on the Thermoelectric Properties of Pristine Poly(3,4-Ethylenedioxythiophene). RSC Adv. 2015, 5, 33885. 74. Shi, W.; Wang, D.; Shuai, Z. High-Performance Organic Thermoelectric Materials: Theoretical Insights and Computational Design. Adv. Electron. Mater. 2019, 5, 1800882. 75. Shi, W.; Shuai, Z.; Wang, D. Tuning Thermal Transport in Chain-Oriented Conducting Polymers for Enhanced Thermoelectric Efficiency: A Computational Study. Adv. Funct. Mater. 2017, 27, 1702847. 76. Qiu, M.; Baxendale, M. Quantum-tunneling Controlled Thermoelectricity in Polymers. Org. Electron. 2020, 78, 105553. 77. Worfolk, B. J.; Andrews, S. C.; Park, S.; Reinspach, J.; Liu, N.; Toney, M. F.; Mannsfeld, S. C. B.; Bao, Z. Ultrahigh Electrical Conductivity in Solution-Sheared Polymeric Transparent Films. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 14138.

CHAPTER 2

PEDOT preparation, morphology, and electronic structure Peipei Liu and Yanhua Jia Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China

2.1 Introduction With the introduction of the concept of “Internet of things,” devices based on organic electronics have flourished, which widely used in flexible displays, touch panels, and soft sensors.1e3 As one of components of the aforementioned systems, a solution-processed, flexible, and highly conductive polymer film is a crucial element. Poly(3,4-ethylenedioxythiophene):poly(4styrenesulfonate) (PEDOT:PSS), developed by Bayer AG in 1990 and commercially available as a water dispersion of colloidal particles, is one of the most popular conductive polymers.4 Owing to its high electrical conductivity (up to 103 S cm
1), transparency, and thermal stability, PEDOT:PSS has been used in a wide field of applications including antistatic coatings and transparent electrodes (as an alternative to indium tin oxide) for various electronic devices.5,6 However, PEDOT:PSS has some technical issues stem from the use of PSS as external ions to compensate the positive charges on PEDOT, impeding its further practical application. The hydrophobic PEDOT core is surrounded by a shell of excess hydrophilic PSS7 for water dispersion as a colloidal particle,5 so the PEDOT:PSS colloids are tend to aggregate and precipitate when stored for an extended period. Besides, additives are indispensable to improve electrical conductivity of the pristine PEDOT:PSS, such as H2SO4, ethylene glycol (EG), and dimethyl sulfoxide (DMSO),8e13 which can change the conformation of the PEDOT core and remove the insulating PSS shell to enhance the transport of charge carriers between the PEDOT nanocrystals by a hopping mechanism.14e17 Furthermore, prescient and intensive studies on fully soluble self-doped PEDOT (S-PEDOT) have been reported in the literature.18e22 For example, Zotti and coworkers19 synthesized an EDOT derivative bearing a sulfonate group, and then polymerized to obtain water-soluble S-PEDOT a maximum electrical conductivity of 10 S cm
1. In addition, Konradsson Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00002-0

© 2022 Elsevier Ltd. All rights reserved.

39

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Advanced PEDOT Thermoelectric Materials

et al.20 applied chemical polymerization route to obtain the S-PEDOT and found that the electrical conductivity (s) was up to 12 S cm
1. Nevertheless, these acquired conductivity values of the S-PEDOT were much lower than that of PEDOT:PSS (up to 1000 S cm
1). For the past few years, surprisingly, Yano et al. first synthesized a novel EDOT derivative (S-EDOT) bearing a sodium alkylsulfonate side chain, sodium 4-[(2,3dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy] butane-2-sulfonate. From the viewpoints of safety, low environmental load, and mass production for commercialization, the S-EDOT monomer was designed because of the lower toxicity of 2,4-buthanesultone used in the synthetic process. It was found that the electrical conductivity achieved as high as 1089 S cm
1, which is two orders of magnitude higher than those of previously reported S-PEDOT,18e22 and exceeded the conductivity of PEDOT:PSS (up to 1000 S cm
1). Given their point of view, the molecular weight of S-PEDOT is the critical parameter for increasing the number of nanocrystals, which can reduce both the average distance between adjacent nanocrystals and the activation energy for the hopping of charge carriers, achieving desired bulk conductivity. These results confirm that the prepared self-doped PEDOT can be an excellent candidate for developing ideal electronic devices.

2.2 Precursor synthesis 2.2.1 HMEDOT As shown in Fig. 2.1, there were two routes for the synthesis of hydroxymethyl functional EDOT (HMEDOT): (1) HMEDOT was synthesized by the reaction of 3,4-dihydroxythiophene-2,5-dicarboxylic acid diethyl ester and 2,3-dibromo-1-propanol, with subsequent hydrolysis and decarboxylation18; (2) 3,4-dimethoxythiophene as the starting material for the synthesis of 2-chloromethyl-3,4-ethylenedioxythiophene (EDOT-MeCl), and then EDOT-MeCl underwent substitution reaction with sodium succinate in dry acetonitrile to obtain HMEDOT.23 2.2.1.1 Alkoxysulfonate The S-EDOT monomer, sodium 4-[(2,3-dihydrothieno[3,4-b][1,4] dioxin-2-yl) methoxy]butane-2-sulfonate (b), was synthesized by the sulfonation of HMEDOT with 2,4-buthanesultone. By changing the structure or configuration of sultone, different PEDOT structures can be obtained in Fig. 2.2.24

PEDOT preparation, morphology, and electronic structure

41

Figure 2.1 Two routes for the synthesis of Hydroxymethyl EDOT (HMEDOT) precursor including (i)18 and (ii).23 (Copyright 1998, Elsevier and Copyright 2013, Elsevier Ltd.)

2.2.1.2 Alkylcarboxylic C4-EDOT-COOH (d) was obtained by mixing succinic anhydride, triethylamine, 4-dimethylaminopyridine, and HMEDOT (c) anhydrous dichloromethane and stirring at room-temperature overnight. By changing the structure or configuration of anhydride, different PEDOT structures can be obtained in Fig. 2.2.25

2.3 Polymerization methods This part focuses on introducing various polymerization methods to prepare PEDOT-based materials, including oxidation polymerization in solution, electrodeposition, and vapor phase polymerization (VPP, also called oxidative chemical vapor deposition (oCVD)).

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Advanced PEDOT Thermoelectric Materials

Figure 2.2 The chemical structure of (a) sodium 4-[(2,3-dihydrothieno[3,4-b][1,4] dioxin-2-yl)methoxy]butane-1-sulfonate, (b) sodium 4-[(2,3-dihydrothieno[3,4-b][1,4] dioxin-2-yl)methoxy]butane-2-sulfonate, (c) HMEDOT and (d) C4-EDOT-COOH, respectively. (e) Synthetic route of PEDOTs based on HMEDOT.

2.3.1 Oxidation polymerization in solution The choice of oxidants plays a key rule during the polymerization process of EDOT (Fig. 2.3a). The common oxidants are iron-III and manganeseIV, or other metal ions in a suitable higher oxidation state, and iron-III is the most preferred candidate in reported literature. The solubility requirements for iron-III oxidants are determined by the limited solubility of EDOT in water, which is in contrast to its miscibility with alcohols like ethanol or n-butanol in any ratio. Therefore, the well alcohol-soluble iron salts of sulfonic acids are favorable oxidants. Especially, p-toluenesulfonate (Tos) has been established as a very suitable anion with respect to solubility and reactivity of the corresponding iron-III salt. Recently, Iron-III toluenesulfonate has become the most widely used oxidant for EDOTs to prepare PEDOT-based materials.

PEDOT preparation, morphology, and electronic structure

43

Figure 2.3 The polymerization scheme (a) and kinetic process (b) for PEDOT and PEDOT:PSS.

The whole oxidative polymerization reaction can be separated into two steps (Fig. 2.3b). Firstly, EDOTs lost electron and is oxidized to the neutral, leading to the occurrence of the undoped polythiophene. Meanwhile, the byproducts of p-toluenesulfonic acid and iron-II p-toluenesulfonate are formed stoichiometrically. Secondly, the neutral PEDOT is doped by the action of “excess” Fe-III tosylate. In fact, more detailed investigations reveal that not only the two-step character of this reaction, but also an even more complex reaction mechanism. The neutral PEDOT can be isolated in traces to moderate amounts. Using iron-III chloride instead of the Fe tosylate, which is not suitable for technical applications due to the formation of corrosive acids and of deeply colored tetrachloroferrate counterions, neutral PEDOT can be synthesized in moderate yields.27 Beyond that, instead of metal salts, peroxides and other more sophisticated oxidants were suggested. Rather closely related to the preferred method for the manufacture of PEDOT:PSSdthe oxidative polymerization with peroxodisulfatesdin situ polymerization by peroxidic compounds.28,29 In addition, several interesting properties may be a motivation for the use of hypervalent iodine compounds. For example, EDOT derivatives with thioether function like 3,4-ethyleneoxythiathiophene cannot be oxidatively polymerized with peroxides due to the oxidation of the thioether function to a sulfone group. Use of periodate excludes this reaction and facilitates oxidative polymerization. Nevertheless, achievable conductivities are only very moderate, and so the practical value is limited.

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Advanced PEDOT Thermoelectric Materials

2.3.2 Electrodeposition Electrochemical deposition displays the following advantages compared to conventional chemical reaction processes (Fig. 2.4a): (1) It can provide high electron transfer in electrodeposition, which can make it achieve the kind of oxidation-reduction ability that ordinary chemical reagents do not have. (2) The synthesis reaction system and its products will not be contaminated by reducing agents (or oxidants) and their corresponding oxidation products (or reduction products). (3) Due to the particularity of the electric redox process, many substances and aggregates that cannot be prepared by other methods can be prepared. Electrochemical polymerization is mainly potentiostatic, galvanostatic, and repetitive multisweep, which results in different polymer films with various properties.30 In addition, counterion is also a key factor in the quality of the film, the choice of the counterion may

Figure 2.4 The polymerization scheme of EDOT by electrodeposition (a) and vapor phase polymerization (VPP) (b).26 (Copyright 2017, Elsevier Ltd.)

PEDOT preparation, morphology, and electronic structure

45

also have an effect on the oxidation level (doping level) (Fig. 2.3b). In 1995, Granström and Inganäs modeled the X-ray diffraction pattern for PEDOT with perchlorate as the counterion as obtained electrochemically in welldefined pores.31 The best fit was obtained for a crystal with one perchlorate counterion for every four thiophene repeat units in an orthorhombic cell. Niu et al.32 polymerized 3,4-ethylenedioxythiophene (EDOT) in the presence of tetra-n-butyl ammonium hexafluorophosphate on platinum foils and were able to obtain the resulting polymer as a powder. The X-ray diffraction pattern revealed an orthorhombic unit cell with four thiophene units and one hexafluorophosphate ion, in which stacks of PEDOT are separated by layers of counterions. A different doping level using perchlorate was obtained by Zotti et al.,33 who used electrochemical quartz crystal microbalance analysis to relate the charge stored in the polymer to the dried mass. Using tosylate and perchlorate they obtained a ratio of one counterion for every three thiophene units. Using the same technique, Zotti examined the electropolymerization in the presence of PSS. In this case the resulting film contained more sulfonate moieties than required for the charge balance simply due to the polymeric nature of the counterion. Interestingly, it was found that the polymeric composition in the film was independent from the PSS concentration in the solution. Furthermore, Zotti33 found that at low concentrations of EDOT the presence of tosylate hinders the polymerization. No polymerization is observed at EDOT concentrations below 0.1 M in the presence of tosylate, whereas the polymerization of EDOT in the presence of perchlorate proceeds readily. This can be explained by the chemical nature of the counterion. The tosylate ion is a base strong enough to deprotonate the EDOT radical cation, and the neutral radical then undergoes further reactions to nonconductive products. This pathway plays a negligible role at higher concentrations but limits the electrochemical polymerization at low concentrations.33 On the other hand, the tosylate ion is better at forming ion pairs and shielding the radical cation and thereby speeds up the polymerization processes. Perchlorate on the other hand is very poor in forming ion pairs.34 2.3.3 Vapor phase polymerization The advantage of VPP is the simplicity of the procedure combined with the variety of both substrate and monomer that can be employed. During VPP, polymerization occurs at the liquid-vapor interface created by casting of an oxidant solution on the desired substrate which is subsequently

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Advanced PEDOT Thermoelectric Materials

exposed to the monomer vapor (Fig. 2.4b). VPP has the ability to synthesize soluble and insoluble polymers on various substrates, provided the substrate itself can be initially overcoated by an oxidant solution, and the precursor monomer can be vaporized. The polymerization kinetics of VPP was basically consistent with the above two methods (Fig. 2.3b). The ease with which both monomer and oxidant can be enhanced to manipulate polymer properties is a major advantage over other polymerization methods. For example, additives can be incorporated into the oxidant solution to manipulate the polymer that forms.35 As discussed above, the VPP process is a practical implementation of the oxidative polymerization method, where the oxidant (in solution) and monomer (as a vapor) come into intimate contact at the liquid-vapor interface. The rate of polymerization at this interface will be governed by many factors, including the concentration of the oxidant and monomer at the interface, their replenishment rate, and physical constraints regarding the physical mixing of the two chemical species. In a practical sense these can in one way or another be controlled by the conditions (temperature, pressure, etc.) and parameters (chamber, oxidant, etc.) used in VPP, directly impacting on the resulting intrinsic conducting polymer (ICP) thin-film properties (conductivity, optical properties, etc.).36e38 Arguably the most important parameter is the choice of oxidant, where oxidant refers to both the oxidizing agent and the doping anion which reside together in the form of a salt. Oxidants with varying oxidation strengths have been incorporated into VPP with interesting results. The strength of the oxidizer relates to the standard electrode potential of the cation. Fe3þ is the most commonly used cation with a standard electrode potential of 0.77 V to convert to Fe2þ. This is typically paired with anions such as Cl
and Tos
,39 and a variety of sulfonates,36,40,41 which are used to dope ICPs. These anions are essential for stabilizing the polaron and bipolaron states along the conjugated backbone of ICPs.42 This is an important consideration as different dopants that are known to alter the properties of the resulting polymer.43e45 FeCl3 has been shown to polymerize many monomers,46e48 while Fe(Tos)3, due to its lower effective oxidation strength, is limited to selected monomers.36,41,49 Herein, the effective oxidation strength refers to the fact that despite the standard electrode potential of the cation reduction (Fe3þ to Fe2þ) is constant, the change in anion leads to a change in the polymerization rate. Owing to its lower effective oxidation strength and hence slower polymerization rate, Fe(Tos)3 however is the oxidant of choice for many applications, albeit with certain additives included.50e54

PEDOT preparation, morphology, and electronic structure

47

Furthermore, after ion-exchange was performed on the synthesized ICP films, no significant change in interchain spacing or conductivity was observed. This result indicated that while the specificity of anion was important during polymerization, once ICP was formed the mere presence of any anion (irrespective of type) was sufficient to render ICP the enhanced conductivity. Observations such as these lead to the conclusion that electrostatic interactions between the anion and ICP govern chargetransport, without implication of the anion chemical nature.55 For example, the synthesized PEDOT-Tos remained highly conductive after ion exchange to create PEDOT-Cl, while synthesized PEDOT-Cl retained a lower conductivity after ion exchange with PEDOT-Tos. All these studies suggest that the kinetics of the polymerization is a critical factor in defining ICP properties. Generally, a higher oxidant concentration leads to faster rates of polymerization; thicker ICP films that are rougher and less conductive. Based on this, it stands to reason that slowing the polymerization kinetics by some means would lead to thin, smooth, highly conductive films. Appreciation of the monomer vapor pressure and the temperature at which the polymerization takes place is paramount to designing a VPP process to yield slow kinetics.

2.4 Fabrication techniques for nano-/micro-PEDOTbased thin-film materials The main fabrication techniques to prepare PEDOT-based materials can be classified into four types, including coating, printing, filtration, and gel methods. 2.4.1 Coating Dip-coating is a simple, old, waste-free, low cost, and low energy consumption technique. For such reasons, it has been used in laboratories for the basic and the applied research even in industry for the development and production. The main procedures of the dip-coating are as follows: (1) the substrate is immersed in the solution of a soluble/dispersible material at a constant speed (preferably judder free). (2) The substrate keeps fully immersed and motionless for some time, and then the thin layer covered on the substrate while it is pulled up. (3) The substrate is withdrawn again at a constant speed to prevent any judders, excess liquid drains from the surface, and the solvent evaporates from the liquid, forming the thin layer. Dipcoating is easy to scale up and provides a good control for the thickness

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Advanced PEDOT Thermoelectric Materials

of films through coating viscosity and the speed of withdrawal from the soluble/dispersible material, but dip-coating also has some drawbacks, for instance, it encounters the difficulty to homogeneously wet the substrate, especially when high surface tension solvents like water are selected. The thickness of film is difficultly controlled when the ultrathin film (1000 nm) needs to be obtained from highly viscous/ dilute liquids. The solubilized precursors may alter their initial properties when the film was deposited onto the porous substrates/layers or multilayer systems. The drop-coating is one of the most extensively used methods due to its advantages such as the facile process, no waste of material, and no requirement of specific equipment. However, the drop-coating also deals with limitations in large-area coverage, thickness hard to control, nonuniform coverage. Besides, various combinations of solvents and solvents evaporation time can influence the evaporation process and/or film morphology (heating of the substrate can speed up the evaporation process and/or improve film morphology). The spin-coating is one of the most common coating techniques with the uniform thickness of thin-film on a substrate using spin coater. The spin-coating is mainly separated into three stages: (1) a soluble/dispersible material is dropped onto the center of the substrate by the microsyringe or pipette, and the substrate is stationary or is rotated at a very slow speed. (2) The substrate is accelerated quickly after dispensing the soluble/dispersible material, and a thin-film is produced by rotating the substrate at a fast speed, the soluble/dispersible material flows radially owing to centrifugal force, and covers the substrate completely. (3) The film of the soluble/dispersible material thins thanks to convective stable outflow driven by centrifugal force, and the excess soluble/dispersible material is ejected from the edge of the substrate in the form of droplets, leaving a thin layer. The advantage of spin-coating includes commercially available equipment, low cost, no timeconsuming delays, fast operation (only a few seconds per coating) and good uniformity/reproducibility of the coating thickness over the whole substrate, and coating thickness is well controlled. While the main obstacle of the spin-coating is that the waste of excess soluble/dispersible material needed to ensure full coverage, less than 10% soluble/dispersible material applied remains after centrifugation. Large substrate is not able to be spun at a high rate to allow the film to thin and dry in a timely manner leading to the decreased throughput.

PEDOT preparation, morphology, and electronic structure

49

The spray-coating is a handy and cost-effective coating technique, it has been considered to be suitable for the production of large-area thin-film onto the substrate with any shape or size because of adjustable thickness, large-area coverage, independence on substrate topology, which has high potential for industrial production. In this technique, a soluble/dispersible material is sprayed onto a substrate directly. It can produce very thin dense and stable film onto the surface of the substrate. The surface morphology of the film is controlled by air pressure, the viscosity of the soluble/dispersible material, gun tip geometry, solvent properties, distance between nozzle and substrate. 2.4.2 Printing Screen printing is a traditional printing technique for making patterns onto diversified substrates, including glass, plastics, fabrics, metals, paperboard, papers, and so on, which is a commonly used industrial technique for fast, inexpensive deposition of soluble/dispersible materials over large areas due to advantages like versatility, low cost, microminiaturization, particularly the possibility of mass production, Screen printing is parsimonious and essentially no loss of soluble/dispersible materials during printing, and its main difference from all other coating and printing techniques is a large wet film thickness and a requirement for a relatively high viscosity and a low volatility of the printing medium. Inkjet printing is another popular printing technique with direct patterning, low wastage, low cost, and high-resolution patterning. The formation of the droplet is critical part for the inkjet processing. Surface tension of the ink plays important role in generating a stream of the droplet. Typically, different solvents and additives are added into the solution to get suitable formulations for high-quality printed films. Gravure printing and flexography printing are well established techniques with a resolution of down to 10 mm, high printing speeds (up to 15 m s
1), and low ink wastage. The advantage of gravure is that low viscosity inks can work very well. The morphologies of the rotogravure printed films are affected by the fluid properties of the inks like the surface tension. 2.4.3 Filtration Filtration is another simple method to obtain the PEDOT:PSS thin-film. However, it is difficult to prepare a thin-film by the direct filtration of a

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Advanced PEDOT Thermoelectric Materials

pristine PEDOT:PSS aqueous solution, owing to its narrow particle size distribution of 20e30 nm. The buildup of large nanoparticles is a necessary requirement to achieve the filtration separation of PEDOT:PSS from solution. The common organic solvents were chosen to dilute the pristine PEDOT:PSS. This process led to the agglomeration of PEDOT:PSS nanoparticles over 250 nm due to the partial dissolution of PSS in the diluent. Xiong et al.56 successfully separated PEDOT:PSS nanoparticles from the solvent via the operation of dilution filtration in Fig. 2.5. The asprepared PEDOT:PSS film has an average thickness of about 233 nm. 2.4.4 Gel The gel method (Fig. 2.6) is a facile method that can be fabricated into different shapes such as films, fibers, and columns with arbitrary sizes for practical applications. 2.4.4.1 In situ polymerization The in situ polymerization method is divided into the following steps. First, a nonconductive hydrogel skeleton material is obtained by a certain polymerization method and dehydrated. Then, the nonconductive hydrogel skeleton is immersed in a conductive polymer monomer solution. Finally, the conductive polymer hydrogel is prepared by chemical oxidation polymerization (adding oxidant) or electrochemical polymerization (applying voltage). Conductive polymer hydrogels prepared by in situ polymerization generally have an interpenetrating network structure (IPN) or double network structure (DN).

Figure 2.5 Schematic of normal filtration for PEDOT:PSS thin films.56 (Copyright 2017, Royal Society of Chemistry.)

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Figure 2.6 Schematic of gelation based on PEDOT:PSS.57 (Copyright 2017, The Authors.)

2.4.4.2 Supramolecular self-assembly Supramolecular self-assembly can be initiated by multivalent cations or positively charged polymer chains, interacting with negatively charged polymer chains via electrostatic interactions. Currently, the preparation of PEDOT-based hydrogels is mainly based on the first type. Aqueous dispersions of PEDOT:PSS consist of dilute PEDOT:PSS microgel particles which are able to flow due to electrostatic repulsions between the negatively charged sulfonates at the particles surfaces. Increasing the ionic strength of these dispersions, for instance by adding ions or lowering pH, screens the electrostatic repulsions between microgel particles, enabling the particles to aggregate and form gels stabilized by physical crosslinks like p-p stacking interactions.58 Thus, hydrogels can be formed directly from PEDOT:PSS, simply by the addition of ions. Different types of additives have been shown to induce gelation of PEDOT:PSS including Ca2þ, Fe2þ/3þ, Ru2þ/3þ, ionic liquids, and sulfuric acid.

2.5 Morphology structure 2.5.1 SEM The morphology of the PEDOT-based materials obtained are particles, which seriously limited studies on one-dimensional (1D) transport in these systems. Zhang et al.55 describing for the first time: a one-step, roomtemperature method to chemically synthesize bulk quantities of microns long, 100w180 nm diameter nanofibers of electrically conductive PEDOT in the form of powders, or as optically transparent, substrate supported films

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using a V2O5 seeding approach. The V2O5 nanofibers are not polymerization templates in the conventional sense, but are seeds that help orchestrate a change in morphology. As shown in Fig. 2.7a and b, the unseeded control reaction yielded only PEDOT powders having granular morphology. Subsequently, Zhang et al. again reported a rapid, roomtemperature, reverse emulsion polymerization method to chemically synthesize bulk quantities of micrometers long nanotubes of electrically conducting PEDOT having tube diameters in the range 50w100 nm (Fig. 2.7c).59 S. Patra et al. systematically studied scanning electron microscopy of PEDOT prepared by various electrochemical routes.60 As shown in Fig. 2.8, The PEDOT films prepared at low current densities of galvanostatic route, low potentials of potentiostatic route and low potential ranges of potentiodynamic route essentially possess globular surface

Figure 2.7 SEM images of PEDOT powder synthesized: (a) without added V2O5 seeds (control); (b) with added V2O5 seeds. Scale: 500 nm.55 (c) Microscopy images of PEDOT nanotubes. Scale: 3 mm.59 (a and b) Copyright 2005, Royal Society of Chemistry and (c) Copyright 2006, American Chemical Society.)

Figure 2.8 Scanning electron micrographs of PEDOT/stainless steel (SS) electrode prepared by (a) galvanostatic method at a fixed current of 0.5 mA cm
2; (b) deposited potentiostatically at 0.90 V versus SCE; (c) prepared by cyclic voltammetry with sweep rate of 10 mV s
1 in the potential range of 0w1.10 V versus SCE. The charge of the electrode is 1.4 C cm
2.60 (Copyright 2008, Elsevier Ltd.)

PEDOT preparation, morphology, and electronic structure

53

morphology. The morphology becomes porous at higher current densities and higher potentials of preparation. In the case of potentiodynamic preparation, however, the morphology turns out to be rod-like and fibrous. Moreover, Figs. 2.9 and 2.10 shows the SEM and TEM image of PEDOT doped with Tos and PSS, respectively.61,62 It can be seen the granular morphology of PEDOT:Tos particles and the smooth surface morphology of the PEDOT:PSS film. 2.5.2 TEM Recently, many efforts have been directed toward fabricating nanometerscale conducting polymer materials because of the beneficial characteristics derived from their small dimensions and high surface-to-volume ratio.64,65 Hyeonseok and coworkers systematically investigate the formation of PEDOT nanomaterials via chemical oxidation polymerization in reverse (water-in-oil) microemulsions.63 Fig. 2.11 shows the TEM images of the

Figure 2.9 SEM images of the PEDOT:Tos61 and PEDOT/PSS film.62 (Copyright 2017, The Authors and Copyright 2010, Elsevier Ltd.)

Figure 2.10 TEM images of PEDOT nanomaterials fabricated with different amounts of aqueous FeCl3 solution: (a) ellipsoidal nanoparticles, (b) nanorods, and (c) nanorods and nanotubes.63 (Copyright 2007, Wiley.)

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Figure 2.11 Microstructure of PEDOT/PSS nanofilms. (a, c, and e) BF-TEM images of PEDOT/PSS nanofilms at RT, LT and HT collected at a dose of 50 e
Å
2. Insets are TEM diffraction patterns. (b, d and f) HAADF-STEM images of PEDOT:PSS nanofilms at RT, LT and HT collected at a dose of 187 e
Å
2. Insets are corresponding profiles of PEDOT:PSS grains measured in the marked areas.66 (g) HRTEM image of PEDOT:OTfNMP. Inset image is a magnification of the outlined square.67 (awf) Copyright 2005, Royal Society of Chemistry and (g) Copyright 2016, American Chemical Society.)

1D PEDOT nanomaterials fabricated in each H2O/FeCl3/AOT/hexane system, including ellipsoidal nanoparticles, nanorods, and nanotubes. Zhou et al. reported the temperature-dependent microstructure of PEDOT/PSS nanofilms.66 TEM imaging of PEDOT:PSS nanofilms was carried out at room (RT, 21  C), low (LT,
177  C) and high (HT, 100  C) temperatures. Fig. 2.11a shows the BF-TEM image of the PEDOT:PSS nanofilm at room temperature (RT). The granular structure of PEDOT:PSS could be observed with an average diameter of 30  4 nm, though the contrast was very weak and the granular structure was somehow blurred. However, in the HAADF-STEM image of PEDOT:PSS at RT display enhanced contrast and individual grains could be clearly observed. The high magnification HAADF-STEM image confirmed the core-shell structures of PEDOT:PSS grains (Fig. 2.11b). The inset image of Fig. 2.11b shows the intensity distribution of the signal across one PEDOT:PSS grain at RT. The intensity at the border is higher than that at the center, which can help us to measure an average grain diameter of about 63  12 nm and an average shell thickness of about 8 nm. When it comes to LT, we found that the contrast of the BF-TEM image improved (Fig. 2.11c), which is due to the

PEDOT preparation, morphology, and electronic structure

55

reduced thermal diffuse scattering of samples at low temperature and this effect greatly reduces the background intensity. However, Fig. 2.11d shows that the HAADF-STEM images at LT indicate no obvious coreeshell structure of PEDOT:PSS grains. The intensity distribution of the marked area in Fig. 2.11d indicates that the intensity at the border is lower than that at the center, which differs from the RT situation for which the coreeshell structure was observed. These results indicate that the LT might induce a structure change in PEDOT:PSS grains, as the LT largely reduces the movement of the polymer chains. Also, when the specimen was cooled, it attracted water vapor, which condenses as ice on the surface of the sample. As far as the HT was concerned, we perceived that there was no water on the PEDOT:PSS film due to the heating. Fig. 2.11e shows that only a few of the PEDOT:PSS grains are observed, which is quite different from the BF-TEM images at RT and LT. Moreover, Fig. 2.11f shows that in the HAADF-TEM image, most of the PEDOT:PSS grains are connected to each other with bright boundaries. The average diameter of the grains is about 52  10 nm. The intensity distribution of the marked area in Fig. 2.11f shows that the intensity in the connected area is higher than that in the nonconnected area, indicating that the PEDOT:PSS grains overlap at the boundaries at HT. Fig. 2.11g shows the HRTEM image of PEDOT:OTf-NMP.67 The HRTEM images provide clear information about the length of PEDOT chains forming the crystallites, that is, around 6 nm for the crystallite, which is the same order of magnitude as the crystallite size along the b axis, that is, around 10 nm. 2.5.3 AFM Crispin et al. present tapping mode atomic force microscopy (AFM) performed on thin PEDOT:PSS films to understand the electrical conductivity increase following the addition of diethylene glycol (DEG) in the conducting polymer emulsion.68 The secondary dopant effect of DEG is associated with the phase separation observed with. AFM (Fig. 2.12). Small islands with a very distinct contrast in the noncontact AFM phase signal are attributed to phase-separated PSS. This phenomenon induces a better connection between PEDOT:PSS grains and hence more efficient pathways for charge-transport. Using AFM and transport measurements, Nardes et al. confirmed that the increase in conductivity happens under thermal annealing due to the presence of high boiling solvent and that the aggregation of PEDOT-rich grains is accompanied with their broadening.12,64

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Advanced PEDOT Thermoelectric Materials

Figure 2.12 AFM topography images (a, c) and phase images (b, d) of PEDOT: PSS without solvent addition. In the phase images, darker areas correspond to softer zones. The sharp contrast suggests two phases, one PEDOT-rich (the brighter zones) and one PSS-rich (the darker zones). Therefore, PEDOT:PSS particles are surrounded by excess PSS. AFM topography images (e, g) and phase images (f, h) of PEDOT: PSS after DEG addition. PSS-rich regions are swollen after the solvent addition so that PEDOT-rich regions are more interconnected.68 (Copyright 2006, American Chemical Society.)

2.6 Electronic states 2.6.1 X-ray photoelectron spectroscopy (XPS) XPS is an ideal tool to follow the reduction of PEDOT and estimate the oxidation level. Take PEDOT-Tos for example, the S (2p) signals from the sulfonate group of Tos (166w170 eV) and the thiophene units in PEDOT (163w166 eV) are clearly distinguishable because of the different chemical environment of their sulfur atoms. The S (2p) electrons in the sulfonate have a high binding energy owing to the presence of three electronegative oxygen atoms.69 In addition, the oxidation level of PEDOT films can be estimated thanks to the ratio of thiophene units to other sulfur-based components. Simonato et al. reported the S 2p spectra of PEDOT:OTfNMP and PEDOT:Sulf-NMP.67 As shown in Fig. 2.13, the S 2p doublet from the thiophene units of PEDOT is distinct from that of the counterions. Both thiophenes and sulfonates are detected in the S 2p spectrum of PEDOT:OTf-NMP. An oxidation level of 25.6% was measured, which is in good agreement with the oxidation level of PEDOT:OTf, which is

PEDOT preparation, morphology, and electronic structure

57

Figure 2.13 Chemical analysis of the PEDOT materials using XPS.67 (Copyright 2016, American Chemical Society.)

about 27.8%. The co-solvent addition does not alter the oxidation of PEDOT, and the conductivity enhancement is more likely due to structural and morphological changes rather than a doping process. Regarding the acid-treated films, not only were thiophenes and sulfonates detected, but sulfates were detected as well. This, along with the reduced amount of fluorine found in the films, can be explained by the replacement of triflate by hydrogenosulfates as counteranions. The degree of oxidation of PEDOT:Sulf-NMP is about 39.1%, which means not only that some CF3SO3 are replaced by HSO4
but also that the film undergoes further oxidation. The use of a less steric counteranion introduced after acid treatment, combined with a higher oxidation level, allows a substantial conductivity enhancement from 3600 to 5400 S cm
1. 2.6.2 UV-Vis-NIR absorbance spectroscopy UV-Vis-NIR spectroscopy is frequently used to analyze electron transitions and the bandgap structure in PEDOT. The transition emerges at 600 nm that originates from the absorption of neutral PEDOT segments of the polymer chains. The absorption band around 900 nm attributed to polaron. The bipolaronic band displays broad infrared absorption with a shoulder at 1000 nm.69 As shown in Fig. 2.14, upon reduction with moderate reducing agents (Na2SO3 and Na2S2O3), the absorption band around 900 nm attributed to polaron increased while we observed a concomitant decrease of the bipolaronic contribution (above 1250 nm). Further reduction, obtained with strong reducing agents like TDAE and NaBH4, led to the onset of a new band around 660 nm, corresponding to polymeric neutral chains.70

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Advanced PEDOT Thermoelectric Materials

Figure 2.14 Top: chemical structures of a PEDOT:PSS (left) neutral chain, (center) a radical cation charge carrier, (right) a dication charge carrier. Bottom: absorbance spectra of (a) pristine PEDOT:PSS and thin films treated with (b) Na2S2O3, (c) Na2SO3, (d) NaBH4, and (e) TDAE.70 (Copyright 2005, Royal Society of Chemistry.)

2.6.3 Raman spectroscopy Raman spectroscopy is also a useful tool to confirm the oxidation (doping) level. In Table 2.1 are listed the principal Raman lines, the comparison with parent compounds.71 It has previously been reported that the C]C symmetric stretching vibration can be decomposed into two separate peaks: one for the neutral structure of PEDOT (centered at 1413.5 cm
1) and one for the oxidized structure of the PEDOT segments (centered at 1444.5 cm
1). The relative intensities of these two independent bands change with the oxidant level or the degree of doping in the PEDOT films. Therefore, both the frequency and line shape of their combined band vary according to the oxidant level in the PEDOT. When the oxidant (doping) level increases, this combined band moves toward higher wave number (blue shift) and is more dominated by the oxidized structure of PEDOT. Hence, the observed slight blue shift after posttreatment indicates an increased oxidant (doping) level (Fig. 2.15).72 The band observed at approximately 2800 cm
1 depends on the force constants of the CeH bond.

Table 2.1 Observed and calculated frequencies of neutral poly(3,4-ethylenedioxythiophene) and parent compounds, with assignment of the principal bandsa.71 poly(3,30 dibutoxy- 2,20 bithiophene)

Polythiophene Raman

Infrared

Raman

poly(ethylenedioxythiophene) Raman

Infrared

exptl

calcd

exptl

calcd

exptl

calcd

exptl

calcd

1497 1455

1498 1460

1488 1441

1504 1429

2931 1509

2934 1488

2933 1468

1359

e

1338

1512 1456 1401 1300

1520

1365

1522 1451 1415 1375

1408 1350

1210 1197 1039

1224

1225

1190

1065

1055

1201 1170 1004 1108

1444 1366 1267 1228

1427 1366

1220 e 1045

1431 1369 1270 1226 1163 1111

1110 1061 964

1051

1070

991

988 806

892

692 571

691 577 486 440

740 700 e

746 696

737 590 522

747 587

754 699

757 676 523

a Key: asym, asymmetric; bend, bending; def, deformation; str., stretching; sym., symmetric. Copyright 1999, American Chemical Society.

911 961 866 802

565 525

CH2 str asym C]C str sym Ca]Cb (eH) str sym Ca]Cb (eO) str CbeCb str CaeCa0 (inter-ring) str CaeCa0 (inter-ring) str þ Cb-H bend CbeH bend CeOeC def CeOeC def oxyethylene ring def sym CeSeC def oxyethylene ring def oxyethylene ring def asym CeSeC def sym CeSeC def oxyethylene ring def

59

calcd

PEDOT preparation, morphology, and electronic structure

exptl

1112

Approximative description of vibrations

60

Advanced PEDOT Thermoelectric Materials

Figure 2.15 Raman spectra of PEDOT-Tos-IL films posttreated with different solvents.72 (Copyright 2017, Wiley.)

2.6.4 GIWAXS Conjugated organic molecules arrange in complex multiphase systems that are so sensitive to processing, chemistry, and local environment that detailed characterization has proven elusive, often requiring simplified and qualitative morphological descriptions. It has become clear that the use of quantitative characterization techniques is the key to obtaining such precise description of the molecular structure and microstructure of the materials of interest. From conductive polymers (CPs) in the form of thin-film hardly any scattering signal can be gained simply due to the lack of scattering volume. By changing from transmission to reflection geometry and hence to grazing incidence conditions, this problem is solved by the footprint effect and thin films can be probed with sufficient statistics.73 Grazing incidence wide angle X-ray scattering (GIWAXS) is sensitive to the crystalline parts and allows for the determination of the crystal structure and the orientation of the crystalline regions, providing unique insights into the molecular arrangement.74 As seen in Fig. 2.16a, a common choice of the coordinate system in GIWAXS is with the x-axis along the X-ray beam direction, the Y-axis parallel to the sample surface and the Z-axis along the surface normal. All angles are probed with respect to the surface, which is located in the (x, y)-plane.

PEDOT preparation, morphology, and electronic structure

61

Figure 2.16 (a) Schematic picture of the experimental set-up used in GIWAXS; Sketch of film crystallinity and corresponding 2D GIWAXS data in case of (b) oriented domains and (c) full rotational disorder of crystallites.73 (Copyright 2017, Wiley.)

Fig. 2.16b and c summarize two frequently observed scenarios of GIWAXS pattern for CPs. Having a more textured film with domains oriented with an angular distribution around the horizontal alignment, the Bragg peaks along the vertical direction will broaden (Fig. 2.16b). In case of powder like films with a large degree of orientational disorder of the crystallites, these Bragg peaks smear out into Debye-Scherrer-like rings (Fig. 2.16c). However, care has to be taken for conjugated polymers. Rivnay and coworkers have shown that the simple analysis, using the Scherrer equation, does not properly account for deviations of the lattice constant caused by, e.g., the soluble alkyl side chains of conjugated

62

Advanced PEDOT Thermoelectric Materials

polymers. The para-crystalline nature of the conjugated polymers gives rise to g-values close to that of glass, hence broadening Bragg peaks. Thus, for additional analysis, higher order Bragg peaks need to be used to overcome such problems of the analysis. In order to assess which aspects of the structure dominates chargetransport, researchers attempt to correlate structural information to conductivity. GIWAXS is often employed to assess structural order, allowing for the determination of the ordering within various aspects of the structure. In particular, for PEDOT and composites thereof, GIWAXS has been used to measure p-stacking distances. PEDOT forms a well-characterized, well-ordered system. The interlayer and p-stacking distances have been measured,75 are 14 and 3.4 Å respectively, and it was additionally demonstrated that depending on which oxidant is used as a precursor to the polymer,76 stacking distances can be adjusted. GIWAXS measurements of VPP PEDOT during the different stages of the manufacturing process were performed and are presented in Fig. 2.17. In Fig. 2.17a, one can see scattering profile of the amorphous deposited oxidant. Fig. 2.17b shows the initially polymerized EDOT, in the uncollapsed state (i.e., including the now-reduced form of the oxidant). It can be seen that significant crystallization occurs, and this behavior is commonly associated with the crystallization of the reduced oxidant under ambient conditions and is thus, to a large extent, due to the duration of the GIWAXS measurement. Finally, Fig. 2.17c shows the highly anisotropic collapsed, washed and dried PEDOT, which consistent with literature-reported scattering profiles of PEDOT.41,75,77,78 Charge transport studies of cast PEDOT samples demonstrate that interchain interaction is the limiting factor for charge transport for samples where the fraction of polymerized PEDOT approaches the percolation threshold.79 The solution processability of PEDOT was achieved by introducing insulating PSS chain, which induce solubility by decreasing interchain interactions in the solution state, impedes interchain coupling in the solid state and thus limits charge-transport through p-orbital overlapping.80,81 Regarding the origin of conductivity enhancement, numerous studies have suggested that dipoles or ions in these solvents/solutions reduce the electrostatic interaction between PEDOT and PSS, resulting in their phase separation and the restructuring of the PEDOT:PSS complex toward more ordered structures. The correlation between the degree of counterion exchange and the structural transformation for PEDOT:PSS can be investigated by GIWAXS.

PEDOT preparation, morphology, and electronic structure

63

Figure 2.17 (a) The GIWAXS scattering pattern for Fe(III) PTS. (b) The GIWAXS pattern for unwashed PEDOT, (c) the GIWAXS pattern of washed PEDOT and (d) the sector averages, in-plane and out-of-the-plane for the data shown in (a), (b), and (c), with a vertical shift as an offset for visual clarity.79 (Copyright 2016, Royal Society of Chemistry.)

Figs. 2.18a and 2.19 presents 2D GIWAXS patterns that are converted from the raw detector images into q space, taking account of the Ewald sphere,73 and 1D scattering qz profiles in Fig. 2.18b were obtained from the two-dimensional (2D) GIWAXS images which were converted from raw detector image to qz space. The decreases in the (100) spacing indicate that more dense lamellar structures of PEDOT:PSS were formed, and this result is probably caused by the reduced PSS. The enhancement of interchain interactions with the shortened pep stacking distance between PEDOT

64

Advanced PEDOT Thermoelectric Materials

Figure 2.18 (a) 2D GIWAXS patterns of the PEDOT:PSS films with and without various ionic liquids; (b) 1D scattering profiles in the qz direction; The relationship between the electrical properties and structure of PEDOT:PSS: (c) conductivities, (d) charge carrier mobilities, and (e) pep stacking distances of the PEDOT:PSS films.82 (Copyright 2016, Wiley.)

molecules induced by the counterion exchange improves the crystallinity, as revealed by the decrease in the full width at half-maximum of the (010) peak.82 The relationship between the structure and electrical properties of PEDOT:PSS system can be seen in Fig. 2.18cee. The enhancement of conductivity (s) mainly arises because of an increase in mobility (m), as shown in Fig. 2.18d. The m values were calculated from the relationship s ¼ nem, where the charge carrier density (n) was estimated from the plasma frequency. The spacing between conjugated molecules greatly affects the intermolecular overlap of p-orbitals and thus the charge transfer integral.83

PEDOT preparation, morphology, and electronic structure

65

Figure 2.19 GIWAXS of distinct CP films of PEDOT:Tos treated with different pH solutions: (a) 0.01 M HCl treated film and (b) 10 M HCl treated film, (c) domain size at various acid/base treatments indicating a decrease in the domain size at acidic and basic treatments confirming the fact that bulky tosylate ions have been replaced with small Cl-1 or OH-1.84 (Copyright 2015, Royal Society of Chemistry.)

Zia et al.84 proposed that pristine PEDOT:Tos film and the films treated with weak acid or base show the typical texture of a spin coated film of a semicrystalline conjugated polymer, with “edge-on” orientation, i.e., with the aromatic planes of the backbone oriented perpendicular to the substrate surface and the p-stacking along the surface, as shown in Fig. 2.19. At higher acid or base concentration, the diffracted intensity at the scattering vector corresponding to the p-stacking interchain distance is more evenly distributed over azimuthal angles, whereas the azimuthal distribution of intensity in the lamellar peak is unaffected. The latter observation indicates that the effect of acid or base treatment is not an overall change in the domain texture, but rather a disruption of the p-stacking alone, with ordering along the surface normal left more or less unaffected. The domain size along the surface normal is diminished in films treated with strong acid or base. The acido-basic treatment leads to an ion-exchange that actually disrupts the overlap between the p-orbitals of adjacent PEDOT chains; which is thus expected to deteriorate the charge-transport. But systematically, the interstack distance decreases when Tos is replaced by smaller anions; which could favor interstack transport. The charge-transport property of PEDOT-based materials are governed by the degree of molecular ordering associated with chain expansion, interchain stacking, and percent crystallinity. In the analysis of GIWAXS data, the crystalline information from PEDOT under investigation is commonly extracted. The complexity of this analysis depends on the degree of order of the PEDOT-based materials. GIWAXS analyses and conductivity measurements allowed us to unravel the organization, doping, and transport mechanism of these highly conductive PEDOT materials, then bring about high conductivity enhancements.

66

Advanced PEDOT Thermoelectric Materials

2.7 Concluding remarks The microstructure of organic materials has a great influence on the electronic transmission performance. Accurately obtaining the electronic structure information is very important for the preparation and research of high-performance organic electronic materials. High-performance PEDOT is due to the continuous improvement and development of the current preparation technology, which provides reliable technical support for numerous researchers engaged in organic electronics. In the hope of obtaining more PEDOT microstructure information, more advanced technologies are eager to emerge, providing theoretical and technical support for further analyzing the relationship between its structure and charge transfer performance.

References 1. Xiong, L.; Jin, H.; Lu, Y.; Li, X.; Ai, X.; Yang, H.; Huang, L. A Solvent Molecule Driven Pure PEDOT:PSS Actuator. Macromol. Mater. Eng. 2020, 305, 2000327. 2. Zhang, D.; Tang, Y.; Zhang, Y.; Yang, F.; Liu, Y.; Wang, X.; Yang, J.; Gong, X.; Zheng, J. Highly Stretchable, Self-Adhesive, Biocompatible, Conductive Hydrogels as Fully Polymeric Strain Sensors. J. Mater. Chem. A 2020, 8, 20474. 3. Lee, A.-H.; Lee, J.; Laiwalla, F.; Leung, V.; Huang, J.; Nurmikko, A.; Song, Y.-K. A Scalable and Low Stress Post-CMOS Processing Technique for Implantable Microsensors. Micromachines 2020, 11, 925. 4. Jonas, F. Poly(3,4-Ethylenedioxythiophene): Conductive Coatings, Technical Applications and Properties. Macromol. Symp. 1995, 100, 169. 5. Kirchmeyer, S.; Reuter, K. Scientific Importance, Properties and Growing Applications of Poly(3,4-Ethylenedioxythiophene). J. Mater. Chem. 2005, 15, 2077. 6. Hohnholz, D.; Okuzaki, H.; MacDiarmid, A. G. Plastic Electronic Devices through Line Patterning of Conducting Polymers. Adv. Funct. Mater. 2005, 15, 51. 7. Takano, T.; Masunaga, H.; Fujiwara, A.; Okuzaki, H.; Sasaki, T. PEDOT Nanocrystal in Highly Conductive PEDOT:PSS Polymer Films. Macromolecules 2012, 45, 3859. 8. Shi, H.; Liu, C.; Jiang, Q.; Xu, J. Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review. Adv. Electron. Mater. 2015, 1, 1500017. 9. Ashizawa, S.; Horikawa, R.; Okuzaki, H. Effects of Solvent on Carrier Transport in Poly(3,4-Ethylenedioxythiophene)/Poly(4-Styrenesulfonate). Synth. Met. 2005, 153, 5. 10. Ouyang, J.; Xu, Q.; Chu, C.-W.; Yang, Y.; Li, G.; Shinar, J. On the Mechanism of Conductivity Enhancement in Poly(3,4-Ethylenedioxythiophene):Poly(styrene Sulfonate) Film through Solvent Treatment. Polymer 2004, 45, 8443. 11. Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of Electrical Conductivity of Poly(3,4-Ethylenedioxythiophene)/poly(4-Styrenesulfonate). Synth. Met. 2002, 126, 311. 12. Nardes, A. M.; Janssen, R. A. J.; Kemerink, M. A Morphological Model for the Solvent-Enhanced Conductivity of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2008, 18, 865.

PEDOT preparation, morphology, and electronic structure

67

13. Jönsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; Denier van der Gon, A. W.; Salaneck, W. R.; Fahlman, M. The Effects of Solvents on the Morphology and Sheet Resistance in Poly(3,4-Ethylenedioxythiophene)e Polystyrenesulfonic Acid (PEDOTePSS) Films. Synth. Met. 2003, 139, 1. 14. Horii, T.; Li, Y.; Mori, Y.; Okuzaki, H. Correlation between the Hierarchical Structure and Electrical Conductivity of PEDOT/PSS. Polym. J. 2015, 47, 695. 15. Okuzaki, H.; Harashina, Y.; Yan, H. Highly Conductive PEDOT/PSS Microfibers Fabricated by Wet-Spinning and Dip-Treatment in Ethylene Glycol. Eur. Polym. J. 2009, 45, 256. 16. Yamashita, M.; Otani, C.; Shimizu, M.; Okuzaki, H. Effect of Solvent on Carrier Transport in Poly(3,4-Ethylenedioxythiophene)/poly(4-Styrenesulfonate) Studied by Terahertz and Infrared-Ultraviolet Spectroscopy. Appl. Phys. Lett. 2011, 99, 143307. 17. Horii, T.; Hikawa, H.; Katsunuma, M.; Okuzaki, H. Synthesis of Highly Conductive PEDOT:PSS and Correlation with Hierarchical Structure. Polymer 2018, 140, 33. 18. Stephan, O.; Schottland, P.; Gall, P.-Y. L.; Chevrot, C.; Mariet, C.; Carrier, M. Electrochemical Behaviour of 3,4-Ethylenedioxythiophene Functionalized by a Sulphonate Group. Application to the Preparation of Poly(3,4Ethylenedioxythiophene) Having Permanent Cation-Exchange Properties. J. Electroanl. Chem. 1998, 443, 217. 19. Zotti, G.; Zecchin, S.; Schiavon, G.; Groenendaal, L. B. Electrochemical and Chemical Synthesis and Characterization of Sulfonated Poly(3,4-Ethylenedioxythiophene): a Novel Water-Soluble and Highly Conductive Conjugated Oligomer. Macromol. Chem. Phys. 2002, 203, 1958. 20. Karlsson, R. H.; Herland, A.; Hamedi, M.; Wigenius, J. A.; Åslund, A.; Liu, X.; Fahlman, M.; Ingana€s, O.; Konradsson, P. Iron-catalyzed Polymerization of Alkoxysulfonate-Functionalized 3,4-ethylenedioxythiophene Gives Water-Soluble Poly(3,4-Ethylenedioxythiophene) of High Conductivity. Chem. Mater. 2009, 21, 1815. 21. Persson, K. M.; Karlsson, R.; Svennersten, K.; Loffler, S.; Jager, E. W.; RichterDahlfors, A.; Konradsson, P.; Berggren, M. Electronic Control of Cell Detachment Using a Self-Doped Conducting Polymer. Adv. Mater. 2011, 23, 4403. 22. Persson, K. M.; Gabrielsson, R.; Sawatdee, A.; Nilsson, D.; Konradsson, P.; Berggren, M. Electronic Control over Detachment of a Self-Doped Water-Soluble Conjugated Polyelectrolyte. Langmuir 2014, 30, 6257. 23. Zhang, L.; Wen, Y.; Yao, Y.; Xu, J.; Duan, X.; Zhang, G. Synthesis and Characterization of PEDOT Derivative with Carboxyl Group and its Chemo/bio Sensing Application as Nanocomposite, Immobilized Biological and Enhanced Optical Materials. Electrochim. Acta 2014, 116, 343. 24. Yano, H.; Kudo, K.; Marumo, K.; Okuzaki, H. Fully Soluble Self-Doped Poly(3,4Ethylenedioxythiophene) with an Electrical Conductivity Greater Than 1000 S cm
1. Sci. Adv. 2019, 5, eaav9492. 25. Ayalew, H.; Wang, T. L.; Yu, H. H. Deprotonation-induced Conductivity Shift of Polyethylenedioxythiophenes in Aqueous Solutions: The Effects of Side-Chain Length and Polymer Composition. Polymers 2019, 11, 659. 26. Brooke, R.; Cottis, P.; Talemi, P.; Fabretto, M.; Murphy, P.; Evans, D. Recent Advances in the Synthesis of Conducting Polymers from the Vapour Phase. Prog. Mater. Sci. 2017, 86, 127. 27. Jiang, Y.; Liu, T.; Zhou, Y. Recent Advances of Synthesis, Properties, Film Fabrication Methods, Modifications of Poly(3,4-Ethylenedioxythiophene), and Applications in Solution-processed Photovoltaics. Adv. Funct. Mater. 2020, 30, 2006213.

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28. Li, J.; Zhang, M.; Liu, J.; Ma, Y. Effect of Attached Peroxyacid on Liquid Phase Depositional Polymerization of EDOT over PI Film with Adsorbed Ferric Chloride. Synth. Met. 2014, 198, 161. 29. Tewari, A.; Kokil, A.; Ravichandran, S.; Nagarajan, S.; Bouldin, R.; Samuelson, L. A.; Nagarajan, R.; Kumar, J. Soybean Peroxidase Catalyzed Enzymatic Synthesis of Pyrrole/EDOT Copolymers. Macromol. Chem. Phys. 2010, 211, 1610. 30. Groenendaal, B.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R. Electrochemistry of Poly(3,4-Alkylenedioxythiophene) Derivatives. Adv. Mater. 2003, 15, 855. 31. GranstrGm, M.; Inganas, O. Electrically Conductive Polymer Fibres with Mesoscopic Diameters: Studies of Structure and Electrical Properties. Polymer 1995, 36, 2768. 32. Niu, L.; Kvarnström, C.; Fröberg, K.; Ivaska, A. Electrochemically Controlled Surface Morphology and Crystallinity in Poly(3,4-Ethylenedioxythiophene) Films. Synth. Met. 2001, 122, 425. 33. Zotti, G.; Zecchin, S.; Schiavon, G. Electrochemical and XPS Studies toward the Role of Monomeric and Polymeric Sulfonate Counterions in the Synthesis, Composition, and Properties of Poly(3,4-Ethylenedioxythiophene). Macromolecules 2003, 36, 3337. 34. Erlich, R. H.; Popov, A. I. Spectroscopic Studies of Ionic Solvation. X. A Study of the Solvation of Sodium Ions in Nonaqueous Solvents by Na Nuclear Magnetic Resonance. J. Am. Chem. Soc. 1971, 93, 5620. 35. Fabretto, M.; Muller, M.; Zuber, K.; Murphy, P. Influence of PEG-Ran-PPG Surfactant on Vapour Phase Polymerised PEDOT Thin Films. Macromol. Rapid Commun. 2009, 30, 1846. 36. Winther-Jensen, B.; Chen, J.; West, K.; Wallace, G. Vapor Phase Polymerization of Pyrrole and Thiophene Using Iron(III) Sulfonates as Oxidizing Agents. Macromolecules 2004, 37, 5930. 37. Wu, D.; Zhang, J.; Dong, W.; Chen, H.; Huang, X.; Sun, B.; Chen, L. Temperature Dependent Conductivity of Vapor-phase Polymerized PEDOT Films. Synth. Met. 2013, 176, 86. 38. Ali, M. A.; Kim, H.; Jeong, K.; Soh, H.; Lee, J. Effects of Solvents on Poly(3,4Ethylenedioxythiophene) (PEDOT) Thin Films Deposited on a (3-Aminopropyl) Trimethoxysilane (APS) Monolayer by Vapor Phase Polymerization. Electron. Mater. Lett. 2010, 6, 17. 39. Ali, M. A.; Kim, H. H.; Lee, C. Y.; Soh, H. S.; Lee, J. G. Effects of the FeCl3 Concentration on the Polymerization of Conductive Poly(3,4-Ethylenedioxythiophene) Thin Films on (3-Aminopropyl) Trimethoxysilane Monolayer-Coated SiO2 Surfaces. Met. Mater. Int. 2009, 15, 977. 40. Subramanian, P.; Clark, N.; Winther-Jensen, B.; MacFarlane, D.; Spiccia, L. Vapourphase Polymerization of Pyrrole and 3,4-ethylenedioxythiophene Using Iron(III 2,4,6-trimethylbenzenesulfonate. Aust. J. Chem. 2009, 62, 133. 41. Subramanian, P.; Clark, N. B.; Spiccia, L.; MacFarlane, D. R.; Winther-Jensen, B.; Forsyth, C. Vapour Phase Polymerisation of Pyrrole Induced by Iron(III) Alkylbenzenesulfonate Salt Oxidising Agents. Synth. Met. 2008, 158, 704. 42. Khan, M. A.; Armes, S. P. Surface Characterization of Poly(3,4Ethylenedioxythiophene)-Coated Latexes by X-Ray Photoelectron Spectroscopy. Langmuir 2000, 16, 4171. 43. Kumar, D.; Sharma, R. C. Advances in Conductive Polymers. Eur. Polym. J. 1998, 34, 1053. 44. Glenis, S.; Tourillon, G.; Garnier, F. Photoelectrochemical Prooperties of Thin Films of Polythiophene and Derivatives: Doping Level and Structure Effects. Thin Solid Films 1984, 122, 9.

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45. Mitchell, G. R.; Davis, F. J.; Leegge, C. H. The Effect of Dopant Molecules on the Molecular Order of Electrically Conducting Films of Polypyrrole. Synth. Met. 1988, 26, 247. 46. Jang, K. S.; Eom, Y. S.; Lee, T. W.; Kim, D. O.; Oh, Y. S.; Jung, H. C.; Nam, J. D. Fabrication of Poly(3-Hexylthiophene) Thin Films by Vapor-phase Polymerization for Optoelectronic Device Applications. ACS Appl. Mater. Interfaces 2009, 1, 1567. 47. Xu, C.; Wand, P.; Bi, X. Continuous Vapor Phase Polymerization of Pyrrole. I. Electrically Conductive Composite Fiber of Polypyrrole with Poly(p-Phenylene Terephthalamide). J. Appl. Polym. Sci. 1995, 58, 2155. 48. Kim, J.; Kim, E.; Won, Y.; Lee, H.; Suh, K. The Preparation and Characteristics of Conductive Poly(3,4-Ethylenedioxythiophene) Thin Film by Vapor-phase Polymerization. Synth. Met. 2003, 139, 485. 49. Fabretto, M. V.; Evans, D. R.; Mueller, M.; Zuber, K.; Hojati-Talemi, P.; Short, R. D.; Wallace, G. G.; Murphy, P. J. Polymeric Material with Metal-like Conductivity for Next Generation Organic Electronic Devices. Chem. Mater. 2012, 24, 3998. 50. Fabretto, M.; Zuber, K.; Hall, C.; Murphy, P. High Conductivity PEDOT Using Humidity Facilitated Vacuum Vapour Phase Polymerisation. Macromol. Rapid Commun. 2008, 29, 1403. 51. Xia, J.; Chen, L.; Yanagida, S. Application of Polypyrrole as a Counter Electrode for a Dye-Sensitized Solar Cell. J. Mater. Chem. 2011, 21, 4644. 52. Winther-Jensen, B.; Krebs, F. C. High-conductivity Large-Area Semi-transparent Electrodes for Polymer Photovoltaics by Silk Screen Printing and Vapour-phase Deposition. Sol. Energy Mat. Sol. C. 2006, 90, 123. 53. Thuy Le, T.; Nguyen, D. L.; Nam, J.-D. Poly(3,4-Ethylenedioxythiophene) Vaporphase Polymerization on Glass Substrate for Enhanced Surface Smoothness and Electrical Conductivity. Macromol. Res. 2007, 15, 465. 54. Ho, T. A.; Jun, T.-S.; Kim, Y. S. Material and NH3-sensing Properties of PolypyrroleCoated Tungsten Oxide Nanofibers. Sensor. Actuator. B Chem. 2013, 185, 523. 55. Zhang, X.; MacDiarmid, A. G.; Manohar, S. K. Chemical Synthesis of PEDOT Nanofibers. Chem. Commum. 2005, 5328. 56. Xiong, J.; Jiang, F.; Zhou, W.; Liu, C.; Xu, J. Highly Electrical and Thermoelectric Properties of a PEDOT:PSS Thin-Film via Direct Dilution-Filtration. RSC Adv. 2015, 5, 60708. 57. Wu, Q.; Wei, J.; Xu, B.; Liu, X.; Wang, H.; Wang, W.; Wang, Q.; Liu, W. A Robust, Highly Stretchable Supramolecular Polymer Conductive Hydrogel with SelfHealability and Thermo-Processability. Sci. Rep. 2017, 7, 41566. 58. Feig, V. R.; Tran, H.; Lee, M.; Liu, K.; Huang, Z.; Beker, L.; Mackanic, D. G.; Bao, Z. An Electrochemical Gelation Method for Patterning Conductive PEDOT:PSS Hydrogels. Adv. Mater. 2019, 31, e1902869. 59. Zhang, X.; Lee, J.-S.; Lee, G. S.; Cha, D.-K.; Kim, M. J.; Yang, D. J.; Manohar, S. K. Chemical Synthesis of PEDOT Nanotubes. Macromolecules 2006, 39, 470. 60. Patra, S.; Barai, K.; Munichandraiah, N. Scanning Electron Microscopy Studies of PEDOT Prepared by Various Electrochemical Routes. Synth. Met. 2008, 158, 430. 61. González, F. J.; Montesinos, A.; Araujo-Morera, J.; Verdejo, R.; Hoyos, M. ‘In-situ’ Preparation of Carbonaceous Conductive Composite Materials Based on PEDOT and Biowaste for Flexible Pseudocapacitor Application. J. Compos. Sci. 2020, 4, 87. 62. Liu, C.; Lu, B.; Yan, J.; Xu, J.; Yue, R.; Zhu, Z.; Zhou, S.; Hu, X.; Zhang, Z.; Chen, P. Highly Conducting Free-Standing Poly(3,4-Ethylenedioxythiophene)/Poly(Styrenesulfonate) Films with Improved Thermoelectric Performances. Synth. Met. 2010, 160, 2481.

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63. Yoon, H.; Chang, M.; Jang, J. Formation of 1D Poly(3,4-Ethylenedioxythiophene) Nanomaterials in Reverse Microemulsions and Their Application to Chemical Sensors. Adv. Funct. Mater. 2007, 17, 431. 64. Meng, L.; Turner, A. P. F.; Mak, W. C. Tunable 3D Nanofibrous and BioFunctionalised PEDOT Network Explored as a Conducting Polymer-Based Biosensor. Biosens. Bioelectron. 2020, 159, 112181. 65. Cai, Y.; Wang, R.; Xu, L.; Jiang, F.; Liang, A.; Duan, X.; Xu, J.; Zhou, W.; Xu, Q.; Lu, X. Self-assembly of Reverse Micelles to Engineer PEDOT Nanoribbons, Nanotubes, Nanorods and Their High Capacitance Performances. J. Electrochem. Soc. 2020, 167, 080538. 66. Zhou, J.; Anjum, D. H.; Chen, L.; Xu, X.; Ventura, I. A.; Jiang, L.; Lubineau, G. Temperature-dependent Microstructure of PEDOT/PSS Films: Insights from Morphological, Mechanical and Electrical Analyses. J. Mater. Chem. C 2014, 2, 9903. 67. Gueye, M. N.; Carella, A.; Massonnet, N.; Yvenou, E.; Brenet, S.; Faure-Vincent, J.; Pouget, S.; Rieutord, F.; Okuno, H.; Benayad, A.; Demadrille, R.; Simonato, J.-P. Structure and Dopant Engineering in PEDOT Thin Films: Practical Tools for a Dramatic Conductivity Enhancement. Chem. Mater. 2016, 28, 3462. 68. Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; Haesendonck, C.v.; Auweraer, M. V.d.; Salaneck, W. R.; Berggren, M. The Origin of the High Conductivity of Poly(3,4-Ethylenedioxythiophene)-Poly(Styrenesulfonate) (PEDOT-PSS) Plastic Electrodes. Chem. Mater. 2006, 18, 4354. 69. Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 70. Massonnet, N.; Carella, A.; Jaudouin, O.; Rannou, P.; Laval, G.; Cellea, C.; Simonato, J.-P. Improvement of the Seebeck Coefficient of PEDOT:PSS by Chemical Reduction Combined with a Novel Method for its Transfer Using Free-Standing Thin Films. J. Mater. Chem. C 2014, 2, 1278. 71. Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. In Situ Spectroelectrochemical Raman Studies of Poly(3,4-Ethylenedioxythiophene) (PEDT). Macromolecules 1999, 32, 6807. 72. Jia, Y.; Li, X.; Jiang, F.; Li, C.; Wang, T.; Jiang, Q.; Hou, J.; Xu, J. Effects of Additives and Post-treatment on the Thermoelectric Performance of Vapor-phase Polymerized PEDOT Films. J. Polym. Sci. B Polym. Phys. 2017, 55, 1738. 73. Mueller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692. 74. Rivnay, J.; Mannsfeld, S. C.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale. Chem. Rev. 2012, 112, 5488. 75. Aasmundtveit, K.; Samuelsen, E.; Pettersson, L.; Inganäs, O.; Johansson, T.; Feidenhans, R. Structure of Thin Films of Poly (3, 4-Ethylenedioxythiophene). Synth. Met. 1999, 101, 561. 76. Winther-Jensen, B.; Forsyth, M.; West, K.; Andreasen, J. W.; Bayley, P.; Pas, S.; MacFarlane, D. R. Orderedisorder Transitions in Poly (3, 4-Ethylenedioxythiophene). Polymer 2008, 49, 481. 77. Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J.-B.; Geerts, Y. H. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190. 78. Palumbiny, C. M.; Liu, F.; Russell, T. P.; Hexemer, A.; Wang, C.; MüllerBuschbaum, P. The Crystallization of PEDOT: PSS Polymeric Electrodes Probed In Situ during Printing. Adv. Mater. 2015, 27, 3391.

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79. Mayevsky, D.; Gann, E.; Garvey, C. J.; McNeill, C. R.; Winther-Jensen, B. Decoupling Order and Conductivity in Doped Conducting Polymers. Phys. Chem. Chem. Phys. 2016, 18, 19397. 80. Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038. 81. Heeger, A. The Fourth Generation of Polymeric Materials Semiconducting and Metallic Polymers. Rev. Mod. Phys. 2001, 73, 681. 82. Kee, S.; Kim, N.; Kim, B. S.; Park, S.; Jang, Y. H.; Lee, S. H.; Kim, J.; Kim, J.; Kwon, S.; Lee, K. Controlling Molecular Ordering in Aqueous Conducting Polymers Using Ionic Liquids. Adv. Mater. 2016, 28, 8625. 83. Giri, G.; Verploegen, E.; Mannsfeld, S. C.; Atahan-Evrenk, S.; Kim, D. H.; Lee, S. Y.; Becerril, H. A.; Aspuru-Guzik, A.; Toney, M. F.; Bao, Z. Tuning Charge Transport in Solution-Sheared Organic Semiconductors Using Lattice Strain. Nature 2011, 480, 504. 84. Khan, Z. U.; Bubnova, O.; Jafari, M. J.; Brooke, R.; Liu, X.; Gabrielsson, R.; Ederth, T.; Evans, D. R.; Andreasen, J. W.; Fahlman, M.; Crispin, X. Acido-basic Control of the Thermoelectric Properties of Poly(3,4-Ethylenedioxythiophene)tosylate (PEDOT-Tos) Thin Films. J. Mater. Chem. C 2015, 3, 10616.

CHAPTER 3

Thermoelectric properties of PEDOTs Zhengyou Zhu1, 2, Lei Wang1, 2 and Chunmei Gao1, 2 1

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, PR China; 2Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, PR China

3.1 Introduction TE materials can directly convert heat into electricity without any moving parts in solid state, showing great research significance in the context of energy crisis facing all mankind. TE efficiency (figure of merit) of a TE material is described as ZT ¼ sa2 T =k

(3.1)

where s is the electrical conductivity, a represents the Seebeck coefficient or thermopower, T means the absolute temperature and k stands for the thermal conductivity. Accordingly, one can conclude that the TE efficiency (expressed as ZT value) is positively proportional to s and a2 but negatively change with k at a given temperature. For most TE materials, these parameters influence each other. Generally, s increases with the increase of carrier concentration which is beneficial for improving ZT. However, the a and k simultaneously decrease and increase, respectively, which indicates that it is very challenging to effectively optimize the ZT value owing to this conflicting relationship (Fig. 3.1).2 At present, the widely investigated TE materials are focusing on some inorganic materials such as Bi2Te3, PbTe etc. which exhibit large S and high ZT value. Although some progress has been made in the enhancement of TE performance, the development and application of inorganic TE materials are limited by their high cost, toxicity, difficulty in large-scale synthesis, high brittleness, high working temperature and high thermal conductivity. Compared with inorganic materials, p-type conducting polymers such as polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh), and poly (3,4-ethylene-dioxythiophene) (PEDOT) and their derivatives etc., show great potential as TE materials because of their rich Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00011-1

© 2022 Elsevier Ltd. All rights reserved.

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Figure 3.1 Dependence of s, a, k, sa2 (power factor), and ZT value on carrier concentration, which were modeled from Bi2Te3.2 (Copyright 2008, Springer Nature.)

sources, low cost, easy synthesis by large-scale, tunable conductivity, and much lower (one order of magnitude ranging from 0.01 to 0.6 W m1 K1) thermal conductivity than inorganic materials at room temperature. Among them, PEDOT is one derivative of polythiophene families, possessing similar properties with other conducting polymers like wide-range high conductivity, excellent chemical stability in air and low thermal conductivity, which has been currently regarded as the most promising and most studied TE material among the polymer families. PEDOT was first synthesized in 1988 by Jonas et al.1 via the chemical oxidation of EDOT monomer with FeCl3 acting as the oxidizing agent, and then electrochemical polymerization was developed to obtain PEDOT, which broadened the synthetic strategies of PEDOT and promoted its wide applications. Afterward, via the oxidation polymerization of EDOT, PEDOT was successively synthesized with chloride (Cl), perchlorate (ClO 4 ), and p-toluenesulfonate (Tos) as the counterions. These counterions can be considered as the dopants, playing a role for charge balancing (PEDOT is charged with positive charges). Taking PEDOT:Tos as an example, in 2004, Winther-Jensen et al.3 obtained PEDOT:Tos by VPP method and found that the electrical conductivity could reach 1000 S cm1, which suggested that PEDOT:Tos was a good candidate for TE application. In 2011, Bubnova et al.4 prepared PEDOT:Tos

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nanofilm via spin-coating and achieved an optimal TE performance by precisely controlling the oxidation level of PEDOT chains by tetrakis(dimethylamino)ethylene (TDAE). At an optimized doping level of 22%, the thermoelectric power factor (PF) and ZT value were 324 mW m1 K2 and 0.25, respectively, which was a breakthrough in organic TE field. However, similar with most other conducting polymers, PEDOT is insoluble which makes it difficult for large-scale processing and limited its widespread application. To address this problem, Bayer AG first synthesized polystyrolsulfon acid (PSS) doped PEDOT, namely the famous PEDOT:PSS, the chemical structure is shown in Fig. 3.2. PEDOT:PSS is composed of positively charged PEDOT chains and negatively charged PSS chains, where the PSS chains act as both the dopant and the counterion to balance PEDOT via Columbic force. PSS chains are hydrophilic, making the PEDOT also dispersible in water. For better stabilizing PEDOT:PSS in water, PSS chains are usually excessive. Thus the PEDOT:PSS is solution-processible and possesses excellent film-forming properties, and becomes the only stable PEDOT dispersion. Since the advent of PEDOT:PSS, it has been viewed as the most successful conducting polymer and applied in various fields. Due to a variety of

Figure 3.2 Chemical structure of PEDOT:PSS.

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advantages, initially in early stage, PEDOT:PSS was mainly employed in solar cells,5,6 OLEDs,7,8 supercapacitors,9 and electrochromic devices10 etc. The first report of both Seebeck coefficient and electrical conductivity of PEDOT:PSS was performed by Kim et al. in 2002.11 Until 2008, Xu’s group for the first time systematically investigated the TE performance of PEDOT:PSS pellets.12 The electrical conductivity was improved by addition of polar solvents (DMSO and EG) and the ZT value was 1.75  103. However, the ZT was still very low probably owing to the noncompact, discontinuous and disordered structure in the pellets. Therefore, preparation of film structure became a better choice for TE PEDOT:PSS. Following that, they did lot of efforts on PEDOT:PSS films and had further improved the ZT value from 103 to 102. These works have drawn a new research hotspot in the organic TE field. In the year of 2013, Pipe et al.13 prepared PEDOT:PSS nanofilm by spin-coating, and achieved a recorded ZT value of 0.42 in organic TE field by simultaneously optimizing electrical conductivity and Seebeck coefficient via DMSO dedoping. All these works have convincingly proved that PEDOTs, especially the representative PEDOT:Tos and PEDOT:PSS, are outstanding potential candidates as TE materials. In this chapter, the main goal is to introduce and discuss the basis of thermoelectric PEDOTs (PEDOT/PEDOT:PSS).

3.2 From insulator to semimetal Because of the wide-range tunable electrical conductivity via doping, the electrical properties of PEDOTs can be engineered to display insulating, semiconducting or semimetallic behavior. Undoped PEDOT is nonconductive due to the lack of available charge carriers. Polarons or bipolarons acting as the charge carriers can be generated during doping. With the increase in doping level (oxidation level), PEDOT structure is turned from benzoid structure to quinoid structure, which is a structure much more beneficial for carrier transport (Fig. 3.3). In the lightly doped PEDOT, polarons are dominated species, which display semiconductor property. While in the heavily doped PEDOT, bipolarons are the main available charge carriers which makes PEDOT a semimetal. For example, PEDOT films via doping by vapor-phase polymerization was found to possess an electronic structure similar to semimetal with an electrical conductivity as high as 2000 S cm1, which is only one

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Figure 3.3 Transition of PEDOT structures between neutral form (A), polaron (B), and bipolaron (C).

order of magnitude lower than that of titanium or stainless steel. Fabretto et al.14 also found that PEDOT film doped with Tos exhibited a metal-like electrical conductivity of over 3400 S cm1. In fact, besides PEDOT doped small molecules (Tos), electrical conductivity of PEDOT:PSS has also been significantly enhanced to semimetal or metal-like values of 103 S cm1 level. Pristine PEDOT:PSS film is an amorphous phase with very low electrical conductivity because of the presence of PSS insulators and the poor crystallinity (molecular chain orientation), although the polarons or bipolarons are presented in PEDOT:PSS, which indicates that the crystallinity of PEDOT phase significantly affects the electrical properties. Treatments to PEDOT:PSS with acids or organic solvents etc. can effectively improve the crystallinity,

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which makes it a semimetal or even a metal. Generally, in an amorphous phase, the polymer (PEDOT) chains arrange in a disorder fashion, where the polaron or bipolaron are localized on a segment of the chains, and the carrier transport can only be achieved by phonon-assisted hopping. The Fermi level lies among localized states in the middle of the polaron band, which is considered as a Fermi glass. While in a well-crystalline phase, short distances between interchain will lead to an overlap of the p electronic density of adjacent packed chains, which therefore promotes the delocalization of the electronic wavefunction.15,16 This change enables the polymer (PEDOT) chain undergo a transition from Fermi glass to semimetal/metal.

3.3 Thermoelectric power factor In the formula ZT ¼ sa2T/k, sa2 is defined as the thermoelectric power factor usually expressed using PF, which determines the ZT value together with the thermal conductivity k. When the k values of TE materials are close each other, PF can be used to directly evaluate the ZT performance. As indicated, PF is proportional to electrical conductivity and the square of Seebeck coefficient, which means for optimization of PF either electrical conductivity or Seebeck coefficient need to be enhanced without obvious loss of one parameter. In order to obtain a higher PF value, an ideal strategy is to simultaneously improve the both parameters. However, considering the conflicting relationship between electrical conductivity or Seebeck coefficient, researchers usually tried to optimize the PF value by carefully controlling the changes of the two parameters. For PEDOTs, the understanding of conductive mechanism and the influence factors on the two parameters should be made clear first. 3.3.1 Electrical conductivity of PEDOTs Electrical conductivity is positively proportional to carrier concentration and carrier mobility, expressed as: s ¼ qnm

(3.2) 3

where q is the carrier charge (C), n is the carrier concentration (cm ) and m is the carrier mobility(cm2 V1 s1). One should also make clear that q is a constant, n and m can be modified via various treatments. Electrical conductivity is one of the key parameters that determines the ZT value of TE materials. According to the ZT formula mentioned above, obtaining a

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high electrical conductivity is of significance for enhancing ZT, and the improvement of electrical conductivity by optimizing n and mis the most used strategy. Below, from various aspects, PEDOT is basically understood in term of its electrical conductivity. 3.3.1.1 Origin of (s) To understand the origin of electrical conductivity of PEDOT, one must first consider the unique features of conjugated polymers. For conducting polymers, the backbone structure usually presents the characteristics of complete planarization, and the p-electron orbits of adjacent atoms constituting the backbone overlap each other, and finally form a delocalized p-channel to afford a conductive path enabling the transfer of charge carriers. To obtain electrical conductivity, carriers ((bi)polarons) must be introduced by doping method, namely the oxidation of the polymer chains. Similarly, undoped PEDOT is nonconductive because of the absence of available charge carriers. PEDOT can be highly conductive only after the  doping process, such as by chloride (Cl), perchlorate (ClO 4 ) and Tos , etc. The p-type doping in PEDOT is an oxidation reaction, therefore the dopants are oxidants, like Fe3þ which can be removed after the formation of products, and the PEDOT will be positively charged as polarons or bipolarons. For example, in PEDOT:Tos, the Tos doping not only changes the charge distribution in PEDOT lattice but also the inner structure of PEDOT chains. Studies show that pristine-state PEDOT shows an aromatic structure with CeC longer than C]C. After Tos doping, bond length of C]C increases while that CeC decreases, resulting that the C]C becomes equal or longer than that of CeC.17 This observed variation is related to the Tos doping concentration. The heavier the doping is, more significant the lattice structure changes. As a result, the doped PEDOT converts into a quinoid structure, which enables the charge transfer along the polymer chains. In a commonly seen deposited film structure of TE PEDOT, molecular packing enables good intermolecular electronic coupling, therefore the charges can move between chains. 3.3.1.2 Influencing factors on (s) (1) Microstructure. Mcrostructure exhibits an effect on the measured electrical conductivity of PEDOT samples. Generally, a film structure possesses a high conductivity because of the formation a compact and continuous structure. PEDOT films via doping show electrical conductivity values up to 102e103 S cm1, however PEDOT pellets or hydrogels with similar

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treatments only have conductivity values of w101 S cm112,18 (2) Doping level. Electrical conductivity is significantly dependent on the doping level of PEDOT chains, due to the largely changed carrier concentration. With the increasing of doping level, more polarons or biopolarons would be generated, accompanied by the molecular structure transition from benzoid structure to quinoid structure (a structure beneficial for carrier transport). A previous work indicated that highly doped (oxidized) PEDOT possesses up to one charge carrier per three monomer units.19 From quantum chemical calculations, a bipolaron in PEDOT spreads over six monomer units or more.20 The mass density of PEDOT:PSS is about 1.5 g cm3, considering the mass ratio of PEDOT to PSS is about 1:2.5, the calculated density of monomer (EDOT: Mw¼142.18 g mol1) should be saturate around 1.9  1021 cm3 (n¼1.5 g cm3  28.6%  6.0231023 mol1/142.18 g mol1) with the maximum oxidation level. From electrochemical measurement, the level of oxidation per monomer unit is known to be approximately 1 charge per 3 EDOT units. Consequently, the density of holes in PEDOT can be estimated to be n ¼ 61020 cme3. (3) Crystallinity. In an amorphous phase, polarons or bipolarons levels cannot form a continuous network by overlap, but can only be localized on a segment of the chains. As a comparison, in a phase with good crystallinity, the molecular chains are stacked in a dense and order manner. The short interchain distances lead to an overlap of the p-electronic density of adjacent polymer chains, promoting the delocalization of the electronic wavefunction hence the hopping and transportation of carriers. Treatments by acids or organic solvents (EG, etc.) enable the restacking of PEDOT chains thus improving the crystallinity, which can result in a significantly enhanced electrical conductivity. Vapor-phasepolymerization (VPP) method tends to form well-crystalline PEDOT structures. Kim et al.21 achieved self-assembly and crystalline growth of PEDOT nanofilms with a polycrystalline structure by VPP method, observing a high carrier mobility. Sung et al.22 developed PEDOT nanowires that have closely packed single-crystalline structures with orthorhombic lattice units. The electrical conductivity of the single-crystal PEDOT nanowires show an average value of 7619 S cm1 and the highest can reach 8797 S cm1, which remarkably exceeds values of PEDOT samples in other reports. 3.3.1.3 Methods for improving (s) As PEDOT:PSS with quite low pristine electrical conductivity of 0.2e1 S cm1, one form of PEDOT, has been the most studied as TE

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material, much effort has been paid on PEDOT:PSS to enhance its electrical conductivity hence its TE properties. As shown in Fig. 3.4, various methods have been developed that have effectively enhance the electrical conductivity of PEDOT:PSS such as thermal treatment,26 light treatment,27 organic solvent treatment,11,24,25,28e30 ionic liquids treatment,31 surfactant treatment,32 salt solution treatment,33,34 zwitterion treatment35 and acids treatment36,37 etc. Among these, organic solvent and acid treatments have been found more effective and they are the most frequently employed. In

Figure 3.4 Mechanism for conductivity enhancement of PEDOT:PSS. (A) Schematic illustration of the conformational change of PEDOT chains with PEG treatment.23 (B) Transition of PEDOT molecule from benzoid structure to quinoid structure.24 (C) Morphological change in the PEDOT:PSS films caused by the addition of EG.25 (A) Copyright 2013, Royal Society of Chemistry. (B) Copyright 2004, Elsevier Ltd. (C) Copyright 2013, Wiley.)

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2002, Kim et al.11 for the first time found that the electrical conductivity of PEDOT:PSS film could be enhanced by DMSO addition. Since then, EG, ethanol, methanol etc. have been successively found effective, especially when they were used as posttreatment regents. In fact, posttreatment route on PEDOT:PSS films is more effective than direct additive method because the molecular chain rearrangement on surface can significantly promote the carrier transport hence the in-plane electrical conductivity. For acids treatment, posttreatment is also preferred due to its higher efficiency. For example, CSA/L-CSA solution addition improved the electrical conductivity of PEDOT:PSS film to 206.2 S cm1, while up to 644.7 S cm1 by CSA/L-CSA posttreatment.38 A recent work reported that the electrical conductivity of PEDOT:PSS film could reach up to 4380 S cm1 by H2SO4 posttreatment, which was among the highest value for this system.36 3.3.1.4 Mechanism and characterizations for enhancing (s) In spite that large number methods have been adopted to improve the electrical conductivity of PEDOT:PSS, the mechanism has been a subject of debate. In pristine PEDOT:PSS, PSS insulator is excess consisting of large amount of free PSS and complexed PSS binding to PEDOT segments. The quite low electrical conductivity of pristine PEDOT:PSS mainly come from two aspects: (1) large amount of free PSS insulators hinder the transport of charge carriers. (2) PSS chain is longer than PEDOT chain and adopts a coiled structure, which makes PEDOT also a coiled structure. In view of this, the main efforts are aiming at removing free PSS and changing the conformation of PEDOT. Currently, the most widely accepted mechanism is the phase segregation between PEDOT and PSS induced by secondary doping, which results in partial PSS loss from PEDOT:PSS and conformational change of the PEDOT chains from a coil structure to an coil-prolonged or linear structure (Fig. 3.4B). In fact, doping such as by organic solvents, or acids etc. can induce the transition of PEDOT molecule from benzoid structure to quinoid structure and polarons to biopolarons accompanied by the conformational change, which finally leads to the charge more delocalized on PEDOT chains. Proper characterizations/testings can be employed to reflect the boosted electrical conductivity of PEDOT:PSS after treatment such as atomic force microscopy (AFM) image, X-ray photoelectron spectroscopy (XPS), Raman spectrum, electron spin resonance (ESR), and Hall measurement, etc. AFM images can testify the conductivity enhancement of PEDOT:PSS film by the change of surface roughness increase that indicates the

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enrichment of conductive PEDOT grains. XPS spectra can confirm the PSS loss/increased PEDOT to PSS ratio via S2p change. Raman spectra can indicate the change of PEDOT chains from a benzoid with a preferred coiled structure to a quinoid with a preferred linear or expanded-coil structure. ESR spectra can suggest the transition of PEDOT from polarons to bipolarons and Hall measurement can provide the direct evidence of conductivity enhancement by measuring the carrier concentration and carrier mobility. 3.3.2 Thermopower In 1821, Thomas Seebeck first observed that in a circuit formed by two kinds of conductors or semiconductors via contacting each other, a potential difference can be generated when a temperature difference is given at the two contacting sites, which is the well-known Seebeck effect, and the Seebeck coefficient (or thermopower) is calculated as a ¼ DV/DT. When the current and thermal transport are in the same direction, a negative Seebeck coefficient is obtained, indicating the n-type feature; Otherwise, the Seebeck coefficient is positive (p-type). The Seebeck coefficient characterizes the fundamental carrier transport property of materials, which measures the entropy of a carrier with unit charge. Assuming a simple system without strong interactions between charge carriers, the entropy of a carrier can be statistically determined by:
 1r qS ¼ kB ln (3.3) r In which r is the charge density of each state, q is the carrier charge and kB is the Boltzmann constant. Accordingly, an increase in carrier concentration results in a decreased entropy thereby a reduced Seebeck coefficient. For metals or degenerate semiconductors, the relationship between carrier concentration and Seebeck coefficient can be expressed as 8p2 k2B *  p 2=3 a¼ mT (3.4) 3n 3qh2 where h is the Planck constant, m* is the effective mass of carrier and n is the carrier concentration, respectively. Obviously, lower carrier concentration leads to larger Seebeck coefficient, however, this doesn’t mean the lower carrier concentration, the better, because the electrical conductivity also decreases with the increased carrier concentration.

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Similarly, the Seebeck coefficient of PEDOT can be improved by adjusting the carrier concentration. Generation/reduction of charge carriers of PEDOT can be achieved by doping/dedoping method, and this method is based on the redox reactions between PEDOT chains and oxidation/ reduction reagents. In PEDOT, three redox states might present, namely the bipolaron (PEDOT2þ), polaron (PEDOTþ), and neutral state (PEDOT0). In an oxidized state, bipolarons are dominated, while the polarons or neutral states are the main species in a reduced/dedoped state. For a doped PEDOT, the Seebeck coefficient can be largely improved by chemical or electrochemical dedoping, where the bipolarons would be converted into polarons or neutral states, accompanied by the decrease of carrier concentration, therefore the increase in Seebeck coefficient. The commonly utilized reducing reagents are hydrazine (HZ), ammonium formate (AF), sodium sulfite (Na2SO3), tetrakis(dimethylamino)ethylene (TDAE), sodium borohydride (NaBH4), hydroiodic acid (HI), and 1-ethyl3-methylimidazolium tetrafluoroborate (EMIM-BF4), etc. For examples, Bubnova et al.4 found that the largest Seebeck coefficient of PEDOT:Tos film can reach 780 mV K1 by decreasing the oxidation level (carrier concentration) with TDAE reduction. Massonnet et al.39 also obtained a significantly improved Seebeck coefficient of 160 mV K1 by chemically treating PEDOT:PSS film using TDAE. Park et al.40 observed that the Seebeck coefficient of PEDOT film sharply increased from 79.8 to 190 mV K1 when the applied voltage decreased from 0.5 to 2 V by electrochemical dedoping. Unlike PEDOT doping with small molecules, pristine commercial PEDOT:PSS film shows both low electrical conductivity (0.2e1 S cm1) and Seebeck coefficient (15e18 mV K1). The secondary doping can significantly enhance the electrical conductivity without much sacrifice in Seebeck coefficient, while a dedoping process will lead to a remarkably increased Seebeck coefficient with a severely falloff in electrical conductivity. Therefore, the optimization of Seebeck coefficient by dedoping is usually performed before a booming to the electrical conductivity via secondary doping, which is a feasible strategy to improving TE performance. Apart from the reduction reactions, the Seebeck coefficient of PEDOTs can also be enhanced by acidity tuning. Protonic doping effect is highly dependent on the pH value. Increasing the pH value will lead to the removal of these protons, which is known as deprotonation, similar to the chemical dedoping effect. Stepien et al.42 added a strong base, KOH, into PEDOT:PSS solution, and the pH value increased from 1.8 to 13. The

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Figure 3.5 Carrier concentration and Seebeck coefficient as a function of pH value.41 (Copyright 2014, Elsevier Ltd.)

as-obtained film exhibited an increased Seebeck coefficient from 15 to 90 mV K1. Tsai et al.41 observed that the carrier concentration of NaOHtreated PEDOT:PSS film decreased from 3.0  1020 cm3 to 1.7  1020 cm3 with the pH value increase from 1.5 to 13.42 (Fig. 3.5). As a result, the Seebeck coefficient showed an increased tendency with carrier concentration. An enhancement in Seebeck coefficient of PEDOT can be indirectly confirmed by characterizing its reduction level using XPS S2p spectra,4 Raman spectra43,44 and UV-Vis adsorption spectra39,40 etc. For instance, Massonnet et al.39 found that deeper the dedoping is, higher the Seebeck coefficient is, as shown in Fig. 3.6. UV-Vis spectra indicated that deeper dedoped PEDOT showed adsorption mainly around 600 nm, assigning to neutral states, which suggested the significantly decreased carrier concentration, hence a larger Seebeck coefficient. In addition, with the dedoping level goes deeper, the color of PEDOT film also changes from light blue to dark blue. 3.3.3 Power factor When the thermal conductivity values are close to each other, the power factor (PF ¼ sa2) can be employed to determine the TE performance. As a matter of fact, the thermal conductivity of conducting polymers including PEDOT is low and changes in a small range even via various treatments, therefore the effective optimization of PF always leads to a high ZT value. Based on that PF is positively proportional to the square of Seebeck coefficient, careful adjustment of tradeoff relationship between electrical conductivity and Seebeck coefficient through redox level can be employed

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Figure 3.6 (A) UV-Vis-NIR spectra at 0.5 V to 1.2 (with 0.1 V intervals), 1.5, and 2 V compared to pristine PP-PEDOT (dashed line). (B) The absorbance change at 600, 900 and 1600 nm under different applied potentials. (B) Photo images showing film color change under different electrochemical dedoping level.40 (Copyright 2013, Royal Society of Chemistry.)

to optimize PF value. Bubnova et al.4 observed that freshly polymerized PEDOT:Tos film (at a 36% oxidation level) had a high electrical conductivity but a mediocre Seebeck coefficient (40 mV K1), which resulted in a low PF of 38 mW m1 K2. However, after careful control of its oxidation level to 22%, the Seebeck coefficient exhibited a substantial increase to above 200 mV K1, accompanied by a relatively small attenuation in electrical conductivity, resulting in a significantly enhanced PF of 324 mW m1 K2. For PEDOT:PSS having a poor initial electrical conductivity, the PF optimization can be achieved from two strategies. One straightforward route is secondary doping by chemical agents such as organic polar solvents, acids, or small molecules. This process can effectively enhance the electrical conductivity with small changes in the Seebeck coefficient. The other method is combining doping and dedoping, which can simultaneously enhance the electrical conductivity and Seebeck coefficient compared to pristine PEDOT:PSS. The second strategy has been mostly proved more effective than the first one to obtain a higher PF. Jiang et al.45 reported that a nontoxic polyethylenimine ethoxylated (PEIE) served as a reductant to

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successfully realize an enhanced S for PEDOT:PSS, followed as another significant anion-blocking role in enabling efficient modulation of oxidation level by sulfuric acid (H2SO4) with a longer operating time (Fig. 3.7). Eventually, a good PEDOT-rich nanocrystal was achieved by sequential dipping process in PEIE/ethylene glycol (PEIE/EG) and H2SO4 solutions. A large PF of 133 mW m1 K2 can be ascribed to the good formation of PEDOT-rich nanocrystal and an effective compromise between s and a of PEDOT:PSS films. Ouyang et al.46 posttreated PEDOT:PSS film with 1 M H2SO4 for multiple times, finding that the electrical conductivity increased from 0.2 S cm1 to 3088 S cm1 with negligible variation in Seebeck coefficient (15 mV K1 to 17.3 mV K1) and the PF was enhanced to be 92.4 mW m1 K2 mainly originating from the contribution of electrical conductivity. After a subsequent

Figure 3.7 The TE performance of PEDOT:PSS nanofilms depending on the dipping time in (A) PEIE/EG solution and (B) H2SO4.45 (Copyright 2019, American Chemical Society.)

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treatment with 1 M NaOH at room temperature, although the electrical conductivity decreased to 2170 S cm1, the Seebeck coefficient increased from to 39.2 mV K1. Therefore, the optimal PF was further enhanced to 334 mW m1 K2, which is 3.6 times higher than that of single H2SO4 treated PEDOT:PSS film, and this value is among the highest for PEDOT:PSS. Earlier actually, the highest PF of PEDOT:PSS was obtained by Pipe et al. in 2013 by combining doping and dedoping method using DMSO and EG, where the PF could reach 469 mW m1 K2 that was nearly half of the calculated maximum possible value for PEDOT:PSS (1100 mW m1 K2).13 The above mentioned reports indicate that PEDOT:PSS shows high promise for TE investigation and there is still much room for its TE development. Later in the same year, Park et al.40 achieved an ultrahigh PF of 1270 mW m1 K2 in PEDOT:Tos film via tuning of electrochemical dedoping level. This value is far more beyond the highest value in PEDOT:PSS and even higher than its calculated maximum possible value, which once again suggest the huge potential for PEDOT in TE field.

3.4 Thermal conductivity Thermal conductivity is also one of the three key parameters significantly affecting the ZT performance. As the only one parameter in the denominator of the ZT formula, ZT varies a lot even if the thermal conductivity changes a little. Compared with inorganic TE materials, conducting polymers possess intrinsic low thermal conductivity (0.1e0.7 W m1 K1), which makes it challenging and meaningful to further reduce the thermal conductivity. Depending on the different types and sources, thermal conductivity of conducting polymers like PEDOT includes electronic thermal conductivity (ke) and lattice (or phonon) thermal conductivity (kl), and the former is calculated as the sum of the latter two, expressed as k ¼ ke þ kl

(3.5)

where ke is that is related to electrical conductivity and kl is depending on the phonon transport and independent of the electrical conductivity. For conducting polymers, the electronic contribution is always marginally small compared with the phonon contribution, given the low electrical conductivity of most conducting polymers. Even the electrical conductivity of conducting polymers tune over several orders of magnitude, the thermal conductivity always shows minor variation, which suggests that the thermal

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conductivity of conducting polymers including PEDOT is dominated by lattice thermal conductivity. In addition, similar to the electronic that is anisotropic, the thermal transport shows in-plane (k||) and out-of-plane (kt) thermal conductivity due to the anisotropy. All these will be detailly expounded below. 3.4.1 Electronic thermal conductivity Electronic thermal conductivity is generated via electronic transport/ diffusion, which means that there is no electronic thermal conductivity in an insulator. Due to the inherent low electrical conductivity, the electronic thermal conductivity for most conducting polymers only contributes little compared to lattice thermal conductivity. For inorganic TE counterparts, the electronic thermal conductivity relating to electrical conductivity can be described by WiedemanneFranz law: ke ¼ LT s

(3.6)

where L is the Lorentz factor (L ¼ 2.4  108 J K2 C2) and T is the absolute temperature. However, WiedemanneFranz law is often invalid for TE conducting polymers due to their stronger their stronger chargelattice coupling. For the sake of estimating an upper limit of electronic thermal conductivity contribution, one can assume the applicability of ideal electron gas model (efficient heat transport) and the electrical conductivity (w300 S cm1) for typical conducting polymers, e.g., PEDOT. The calculated electronic thermal conductivity is only 0.002 W m1 K1, which is negligible as compared to the ever reported or measured thermal conductivity values of PEDOT usually falling in range of 0.2e0.5 W m1 K1. For example, electrical conductivity of pristine PEDOT:PSS film (s ¼ 0.1e0.2 S cm1) can be tuned over four orders of magnitude to 103 S cm1 after various treatments, while the measured thermal conductivity shows relatively minor variation of less than one order of magnitude. This again confirms that the total thermal conductivity of conducting polymers is dominated by phonons. 3.4.2 Lattice thermal conductivity Lattice thermal conductivity also known as phonon thermal conductivity is generated via lattice vibration or phonon transport, which is independent on electronic conduction but related to the crystallinity. For PEDOT as well as other conducting polymers, they can be either amorphous or semicrystalline. The lattice thermal conductivity in semicrystalline

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conducting polymer is usually higher that in amorphous ones because the phonon transport is more promoted. Even in a conducting polymer with crystalline structure, the crystallinity is still much lower than that of inorganic TE materials, and crystalline regions and amorphous regions coexist in the solid system, which produce large number of boundaries and defects. Therefore, the phonon scattering is more serious, which well accounts for the inherent quite low thermal conductivity in conducting polymers. Total thermal conductivity can be decreased by reducing its lattice thermal conductivity via enhancing phonon scattering (phonon-phonon scattering, phonon-boundary scattering, and phonon-defect scattering). Therefore, good TE candidates are crystalline materials that manage to scatter phonons without obviously disrupting the electrical conductivity. 3.4.3 In-plane and out-of-plane thermal conductivity Because of the anisotropic transport in film samples, the thermal conductivity is predicted in the directions parallel and perpendicular to the film surfaces, which is defined or called as in-plane and out-of-plane (crossplane) thermal conductivity, respectively. Similar with heat transport, the charge transport also shows anisotropy, consisting of the in-plane and outof-plane electrical conductivity. For thermoelectric film, the measured electrical conductivity is from the in-plane (lateral) direction. By rationality, to get a ZT value, the in-plane thermal conductivity should be given. However, the in-plane thermal conductivity is still facing difficulty to be accurately measured currently due to heat conduction through the substrate, radiation loss, and thermal contact resistance. Recent years, both inplane and out-of-plane thermal conductivities of PEDOT films have been reported. Generally, the in-plane thermal conductivity values are higher than those obtained from out-of-plane directions, which suggested the strong anisotropy in thermal conduction property of PEDOT films. The ratio between in-plane and out-of-plane thermal conductivity (k||/kt) is defined as anisotropy ratio, which was reported to be about 1.1 for PEDOT:Tos and 1.4e1.6 for PEDOT:PSS nanofilm. Up to now, special researches on thermal conductivity are relatively less than other TE parameters because of the lack of theoretical understanding of the thermal transport and the challenges in thermal conductivity measurements. Three methods, namely the laser flash, time domain thermal reflectance (TDTR) and differential three-omega (3u), are most widely employed to measure thermal conductivity. Only out-of-plane thermal

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conductivity measurement is available with laser flash and TDTR methods, and both thermal conductivities can be measured using 3u method. The details on thermal conductivity measurements are given in Chapter 8.

3.5 Thermoelectric figure of merit As mentioned before, at a given temperature, ZT increases with increase in electrical conductivity and Seebeck coefficient, but decrease in thermal conductivity. The thermal conductivity is dominated by phonons (lattice thermal conductivity), and it is still challenging for researchers to improve ZT by specially decreasing lattice thermal conductivity. Therefore, enhancing electrical conductivity or coordination optimization of electrical conductivity and Seebeck coefficient are the main strategies to improve ZT value. The past 10 years have witnessed significant progress on organic TE materials especially the PEDOT based TE candidates, from ZT ¼ 103, 102 to either ZT ¼ 0.25 for PEDOT:Tos or ZT ¼ 0.42 for PEDOT:PSS, which have been acknowledged among the highest values in organic TE field. In addition, Fan et al. achieved an estimated ZT value of 0.75 in IL/PEDOT:PSS heterostructures film, which is comparable to that of the conventional bismuth telluride (Bi2Te3) at room temperature.47 Furthermore, Park et al. reported a much higher estimated ZT value as high as 1.02 in electrochemical dedoped PEDOT film, revealing great ZT potential in PEDOT.40 Despite these remarkable achievements, the development of organic TE materials, even for the most promising PEDOTs, is still in the initial stage and far inferior to their inorganic counterparts. Generally, for TE materials, the higher ZT value is, the higher conversion efficiency is, which can be expressed as: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT  1 h ¼ hc pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (3.7) 1 þ ZT þ Tc =Th Where h is the TE conversion efficiency from heat to electricity, Tc and Th are the temperatures at cold and hot sides of TE device, respectively, and hc represents the Carnot efficiency. The dependence of conversion efficiency on ZT values suggests that only a ZT value at least above three can ensure the TE device is competitive with that of traditional power generators, corresponding to the Carnot efficiency of 40%.48 However, the ZT values

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reported to date for PEDOT based TE materials are still too far below to meet the requirements for practical applications, which means further efforts are still highly expected.

3.6 Concluding remarks PEDOTs have been deemed as the most promising organic TE potentials among the conducting polymer families. In this chapter, we have provided a broad and basic understanding of PEDOTs in terms of electrical conductivity, Seebeck coefficient (thermopower), PF, thermal conductivity (lattice thermal conductivity and electronic thermal conductivity), and the ZT value. In addition, how each parameter is positively or negatively affected and the relationship between them were also detailed. Some issues remain unsolved, especially regarding thermal conductivity and the precise control for an optimal TE performance. Therefore, further efforts are still highly needed to understand the basis on thermoelectric PEDOTs.

References 1. Jonas, F.; Heywang, G.; Werner, S. Novel Polythiophenes, Process for Their Preparation, and Their Use, 1988; p 22. Bayer AG German Patent DE 3813589A1. 2. Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2011, 7, 101. 3. Winther-Jensen, B.; West, K. Vapor-phase Polymerization of 3,4ethylenedioxythiophene: a Route to Highly Conducting Polymer Surface Layers. Macromolecules 2004, 37, 4538. 4. Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 5. Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.; Hummelen, J. C. Temperature Dependent Characteristics of Poly(3 Hexylthiophene)-Fullerene Based Heterojunction Organic Solar Cells. J. Appl. Phys. 2003, 93, 3376. 6. Wan, L.; Wang, B.; Wang, S.; Wang, X.; Guo, Z.; Dong, B.; Zhao, L.; Li, J.; Zhang, Q.; Luo, T. Well-dispersed PEDOT: PSS/graphene Nanocomposites Synthesized by In Situ Polymerization as Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Sci. 2015, 50, 2148. 7. Craats, A. M. V. D.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Friend, R. H. Mesoepitaxial Solution-growth of Self-organizing Discotic Liquid-crystalline Semiconductors. Adv. Mater. 2003, 15, 495. 8. Ha, Y. G.; You, E. A.; Kim, B. J.; Choi, J. H. Fabrication and Characterization of OLEDs Using MEH-PPV and SWCNT Nanocomposites. Synth. Met. 2005, 153, 205. 9. Ryu, K. S.; Lee, Y. G.; Hong, Y. S.; Yong, J. P.; Wu, X.; Kim, K. M.; Kang, M. G.; Park, N. G.; Chang, S. H. Poly(ethylenedioxythiophene) (PEDOT) as Polymer Electrode in Redox Supercapacitor. Electrochim. Acta 2004, 50, 843. 10. Shimelis, A.; Olle, I. Electrochromism in Diffractive Conducting Polymer Gratings. J. Electrochem. Soc. 2004, 151, H153.

Thermoelectric properties of PEDOTs

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11. Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of Electrical Conductivity of Poly(3,4-Ethylenedioxythiophene)/poly(4-Styrenesulfonate) by a Change of Solvents. Synth. Met. 2002, 126, 311. 12. Jiang, F.; Xu, J.; Lu, B.; Xie, Y.; Huang, R. L. L. Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate). Chin. Phys. Lett. 2008, 25, 2202. 13. Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12, 719. 14. Fabretto, M. V.; Evans, D. R.; Mueller, M.; Zuber, K.; Hojati-Talemi, P.; Short, R. D.; Wallace, G. G.; Murphy, P. J. Polymeric Material with Metal-like Conductivity for Next Generation Organic Electronic Devices. Chem. Mater. 2012, 24, 3998. 15. Koller, G.; Berkebile, S.; Oehzelt, M.; Puschnig, P.; Ambrosch-Draxl, C.; Netzer, F. P.; Ramsey, M. G. Intra- and Intermolecular Band Dispersion in an Organic Crystal. Science 2007, 317, 351. 16. Bubnova, O.; Khan, Z. U.; Wang, H.; et al. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190. 17. Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. In Situ spectroelectrochemical Raman Studies of Poly(3,4-Ethylenedioxythiophene) (PEDT). Macromolecules 1999, 32, 6807. 18. Wang, X.; Liu, P.; Jiang, Q.; Zhou, W.; Xu, J.; Liu, J.; Jiang, F. Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT: PSS-Based Aerogel. ACS Appl. Mater. Interfaces 2018, 11, 2408. 19. Zotti, G.; Zecchin, S.; Schiavon, G.; Louwet, F.; Groenendaal, L.; Crispin, X.; Osikowicz, W.; Salaneck, W.; Fahlman, M. Electrochemical and XPS Studies toward the Role of Monomeric and Polymeric Sulfonate Counterions in the Synthesis, Composition, and Properties of Poly(3,4-Ethylenedioxythiophene). Macromolecules 2003, 36, 3337. 20. Geskin, V. M.; Brédas, J. L. Polaron Pair versus Bipolaron on Oligothiophene Chains: a Theoretical Study of the Singlet and Triplet States. ChemPhysChem 2003, 4, 498. 21. Kim, J.-Y.; Kwon, M.-H.; Min, Y.-K.; Kwon, S.; Ihm, D.-W. Self-assembly and Crystalline Growth of Poly(3,4-Ethylenedioxythiophene) Nanofilms. Adv. Mater. 2007, 19, 3501. 22. Cho, B.; Park, K. S.; Baek, J.; Oh, H. S.; Koo Lee, Y. E.; Sung, M. M. Single-crystal Poly(3,4-Ethylenedioxythiophene) Nanowires with Ultrahigh Conductivity. Nano Lett. 2014, 14, 3321. 23. Chu, C. W.; Mengistie, D. A.; Wang, P.-C. Effect of Molecular Weight of Additives on the Conductivity of PEDOT:PSS and Efficiency for Ito-free Organic Solar Cells. J. Mater. Chem. 2013, 1, 9907. 24. Ouyang, J.; Xu, Q.; Chu, C. W.; Yang, Y.; Li, G.; Shinar, J. On the Mechanism of Conductivity Enhancement in Poly(3,4-Ethylenedioxythiophene):poly(styrene Sulfonate) Film through Solvent Treatment. Polymer 2004, 45, 8443. 25. Wei, Q.; Mukaida, M.; Naitoh, Y.; Ishida, T. Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents. Adv. Mater. 2013, 25, 2831. 26. Huang, J.; Miller, P. F.; Mello, J. C. D.; Mello, A. J. D.; Bradley, D. D. C. Influence of Thermal Treatment on the Conductivity and Morphology of PEDOT/PSS Films. Synth. Met. 2003, 139, 569. 27. Lin, Y.-J.; Yang, F.-M.; Huang, C.-Y.; Chou, W.-Y.; Chang, J.; Lien, Y.-C. Increasing the Work Function of Poly(3,4-Ethylenedioxythiophene) Doped with Poly(4Styrenesulfonate) by Ultraviolet Irradiation. Appl. Phys. Lett. 2007, 91, 092127. 28. Thomas, J. P.; Zhao, L.; McGillivray, D.; Leung, K. T. High-efficiency Hybrid Solar Cells by Nanostructural Modification in PEDOT:PSS with Co-solvent Addition. J. Mater. Chem. 2014, 2, 2383.

94

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29. Xia, Y.; Ouyang, J. Highly Conductive PEDOT:PSS Films Prepared through a Treatment with Geminal Diols or Amphiphilic Fluoro Compounds. Org. Electron. 2012, 13, 1785. 30. Xia, Y.; Sun, K.; Ouyang, J. Highly Conductive Poly(3,4-ethylenedioxythiophene): Poly(styrene Sulfonate) Films Treated with an Amphiphilic Fluoro Compound as the Transparent Electrode of Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5325. 31. Dbbelin, M.; Marcilla, R.; Salsamendi, M.; Pozo-Gonzalo, C.; Mecerreyes, D. Influence of Ionic Liquids on the Electrical Conductivity and Morphology of PEDOT:PSS Films. Chem. Mater. 2007, 19, 2147. 32. Fan, B.; Mei, X.; Ouyang, J. Significant Conductivity Enhancement of Conductive Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) Films by Adding Anionic Surfactants into Polymer Solution. Macromolecules 2008, 41, 5971. 33. Xia, Y.; Ouyang, J. Anion Effect on Salt-Induced Conductivity Enhancement of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) Films. Org. Electron. 2010, 11, 1129. 34. Xia, Y.; Ouyang, J. Salt-induced Charge Screening and Significant Conductivity Enhancement of Conducting Poly(3,4-Ethylenedioxythiophene): poly(styrenesulfonate). Macromolecules 2009, 42, 4141. 35. Xia, Y.; Zhang, H.; Ouyang, J. Highly Conductive PEDOT:PSS Films Prepared through a Treatment with Zwitterions and Their Application in Polymer Photovoltaic Cells. J. Mater. Chem. 2010, 20, 9740. 36. Kim, N.; Kee, S.; Lee, S. H.; Lee, B. H.; Kahng, Y. H.; Jo, Y.-R.; Kim, B.-J.; Lee, K. Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization. Adv. Mater. 2014, 26, 2268. 37. Xia, Y.; Sun, K.; Ouyang, J. Solution-processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Adv. Mater. 2012, 24, 2436. 38. Song, H.; Kong, F.; Liu, C.; Xu, J.; Hui, S. Improved Thermoelectric Performance of PEDOT:PSS Film Treated with Camphorsulfonic Acid. J. Polym. Res. 2013, 20, 1. 39. Massonnet, N.; Carella, A.; Jaudouin, O.; Rannou, P.; Simonato, J. P. Improvement of the Seebeck Coefficient of PEDOT:PSS by Chemical Reduction Combined with a Novel Method for its Transfer Using Free-Standing Thin Films. J. Mater. Chem. C 2014, 2, 1278. 40. Park, T.; Park, C.; Kim, B.; Shin, H.; Kim, E. Flexible PEDOT Electrodes with Large Thermoelectric Power Factors to Generate Electricity by the Touch of Fingertips. Energy Environ. Sci. 2013, 6, 788. 41. Tsai, T. C.; Chang, H. C.; Chen, C. H.; Huang, Y. C.; Whang, W. T. A Facile Dedoping Approach for Effectively Tuning Thermoelectricity and Acidity of PEDOT:PSS Films. Org. Electron. 2014, 15, 641. 42. Stepien, L.; Roch, A.; Schlaier, S.; Dani, I.; Kiriy, A.; Simon, F.; Lukowicz, M. V.; Leyens, C. Investigation of the Thermoelectric Power Factor of KOH-Treated PEDOT:PSS Dispersions for Printing Applications. Energy Harvest. Systems 2016, 3, 101. 43. Luo, J.; Billep, D.; Waechtler, T.; Otto, T.; Toader, M.; Gordan, O.; Sheremet, E.; Martin, J.; Hietschold, M.; Zahn, D. R. T.; Gessner, T. Enhancement of the Thermoelectric Properties of PEDOT:PSS Thin Films by Post-treatment. J. Mater. Chem. 2013, 1, 7576. 44. Park, H.; Lee, S. H.; Kim, F. S.; Choi, H. H.; Cheong, I. W.; Kim, J. H. Enhanced Thermoelectric Properties of PEDOT:PSS Nanofilms by a Chemical Dedoping Process. J. Mater. Chem. 2014, 2, 6532. 45. Li, X.; Liu, C.; Zhou, W.; Duan, X.; Du, Y.; Xu, J.; Li, C.; Liu, J.; Jia, Y.; Liu, P.; Jiang, Q.; Luo, C.; Liu, C.; Jiang, F. Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene)Rich Nanocrystal Films. ACS Appl. Mater. Interfaces 2019, 11, 8138.

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46. Fan, Z.; Li, P.; Du, D.; Ouyang, J. Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films through Sequential Post-treatments with Common Acids and Bases. Adv. Energy Mater. 2017, 7, 1602116. 47. Fan, Z.; Du, D.; Guan, X.; Ouyang, J. Polymer Films with Ultrahigh Thermoelectric Properties Arising from Significant Seebeck Coefficient Enhancement by Ion Accumulation on Surface. Nanomater. Energy 2018, 51, 481. 48. Han, C.; Li, Z.; Dou, S. Recent Progress in Thermoelectric Materials. Chin. Sci. Bull. 2014, 59, 2073.

CHAPTER 4

Thermoelectric transport and PEDOT dependence Congcong Liu Flexible Electronics Innovation Institute, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China

4.1 Introduction Now that polymer semiconductors have been demonstrated to display the high electrical conductivity (s), high Seebeck coefficient (also called thermopower, S), and low thermal conductivity (k) necessary for good thermoelectric performance. The fundamental understanding of their charge transport is extremely important for designing advanced polymer thermoelectric materials. In crystalline semiconductors, the free-electron approximation for the band structure enables the free-carrier concentration (n) and mobility (m) to describe the electrical conductivity (s ¼ nme). In disordered polymers, there are many localized charge carriers which charge transport is often described by hopping from site to site. For a wide range of semiconducting polymers over the entire measurable range of conductivity, two essential features of transport edge (Et), and transport parameter(s) will help identify the transport function (Eq. 4.1) as following:1
s E  Et sE ðE; T Þ ¼ sE0 ðT Þ  ðE > Et Þ (4.1) kB T where E/kBT is the reduced energy of charge carriers (electrons or holes), and sE0(T) is the transport coefficient which is a temperature-dependent but energy-independent parameter. For the semiconductors that the electron chemical potential (EF) close to Et, they are characterized by the energy dependence above the edge, using s as an exponent, where s can be understood as the different “type” of charge transport. Temperature gradient can cause the charge carriers, which can be characterized by the Seebeck coefficient (thermopower). When each charge carrier has an e charge, it also has an “excess” energy (E  EF).

Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00010-X

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Transport function sE(E) simply describes the relationship between Seebeck coefficient and the energy of each carrier:1
Z
 
1 kB E  Et vf S¼ dE (4.2) sE  s e vE kB T For insulators or hopping conductors, their Seebeck coefficient can be approximated as S¼(EtEF)/eT, showing that S is sensitive to EtEF. The measurement of Seebeck coefficient and electrical conductivity for the same samples can confirm the transport mechanism, specifically once EF reaches near Et. sE(E) usually depends on the density of states (DOS), charge carrier velocity, and their relaxation time or hopping frequency, which have the power-law relationship to the energy of carrier above the transport edge, thus resulting the different dependencies between transmission mechanisms and energy, and resulting sE(E) shows a power law with s. This form can be used to calculate the relations of S and s, identifying the parameter s, and connecting to transport mechanisms. While s spans eight orders of magnitude, the relationship between S and s can be fitted by Eq. (4.1) with s ¼ 3, indicating that the typical conducting polymers have a same transport exponent. In three-dimensional (3D) crystalline solids, s equals to 1. However, when the charge carriers are scattered by ionized impurities, s be expected as 3, which may be attributed to the charge-transfer nature for the doped polymer semiconductors. Recently, poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials have attracted extensive attention in some fields such as thermoelectric because of their high conductivity. Interestingly, compared with other polymers that follow s ¼ 3, these high-performance PEDOT-based materials can be better fitted to the S  s curve by s ¼ 1. Their sE0 extracted from fitting data show several orders of magnitude higher than that of most polymers, resulting in the higher electrical conductivity in same thermopower.1 The transport edge described in the aforementioned model probably describes an effective global transport edge of the most polymers. However, because of the presence of poorly conducting disordered domains acting as an energy barrier, the subpercolation limit as a cause for the temperature dependency of conductivity should also be considered. Furthermore, the geometric structure, electronic structure, band structure, and density of states are expected to have significant impact on the thermoelectric performance of conducting polymers, which will be discussed in the next sections.

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4.2 Thermoelectric transport theory 4.2.1 Stable geometric structure The micro (molecular) and macro structure order deeply affects the charge transport of conducting polymers (Fig. 4.1).2,3 Because of the differences of the electronic coupling between molecules, charge transport in the crystalline domains is highly anisotropic.4e7 It is necessary to require close intermolecular contacts for coupling the p electronic levels of molecules, resulting the effective charge transfer between molecules.6 Conjugated molecular units usually result the planar structures that stack against each other (namely, p stacking), making the intermolecular electronic coupling. The conjugated repeat units of semiconducting polymers are strongly electronically coupled along the backbone, but the electronic coupling between stacked chains is weaker. Because of the disordered structure, carriers must traverse many domains during transport, resulting in the markedly lower charge mobility than that of expected single crystals. It could be the promising strategy to enhance the charge transport of conducing polymers by controlling the crystallinity and constructing stable geometry structure with the crystal alignment from macroscale to microscale.8 On the macroscale, the size of polycrystalline domains in solution-deposited conducing polymers can reach tens of nanometers to micrometers. These ordered domains orientation which is relative to the direction of transport can observably affect the macroscale electrical conductivity. On the microscale, highly anisotropic charge transport in individual crystalline regions is indicated in the changes of electronic

Figure 4.1 Effect of structural order of conducting polymers on charge transport properties. (Copyright 2014, American Chemical Society. Palumbiny, C. M.; Heller, C.; Schaffer, C. J.; Korstgens, V.; Santoro, G.; Roth, S. V.; Mueller-Buschbaum, P. Molecular Reorientation and Structural Changes in Cosolvent-Treated Highly Conductive PEDOT:PSS Electrodes for Flexible Indium Tin Oxide-free Organic Electronics. J. Phys. Chem. C 2014, 118, 13598.)

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coupling between molecules and polymeric chains. The strategies making chain straighter, such as, mechanical stretching and cosolvents treating, can effectively improve chain alignment and electronic coupling of conducing polymers. In addition, the stack against each planar functionality by the conjugation of molecular units (pep stacking) can lead to the stronger coupling, including the coplanar molecule design, the functionalities, and the molecules doping. Disparate crystallographic and morphological structures of PEDOT-based films can be caused by the different methods, especially the structures of small counterions (PEDOT:X) and PEDOT:PSS. Generally, PEDOT:X reveals more crystalline and oriented structure than that of PEDOT:PSS. For example, PEDOT:tosylate (PEDOT:Tos) films show advantageous lamellar stacking in the out-of-plane direction, and have the obvious pep stacking in the in-plane direction, indicating PEDOT chains are preferentially “edge-on” oriented in these materials system. Moreover, in PEDOT:Tos films, PEDOT chains are aggregated in crystallites consisting of 3e6 pep stacked chains. The crystallites are embedded in an amorphous matrix of PEDOT chains, and are linked by interpenetrating pep stacked chains. Comparatively, PEDOT:PSS films usually show amorphous and entangled structures, mainly due to the uncontrolled packing of polymer chains during solution deposition, and the excessive PSS counterions which damage the dense packing and alignment of PEDOT chains.9e11 4.2.2 Electronic structure The pz electrons in the backbone carbon atoms of the polymer chains are the main source of semiconducting polymers electrical properties, calling as “conjugated system.”12 In this system, each carbon atom has sp2 pz-hybridized orbitals, resulting in the overlapping orbitals. Besides, carbon atoms share sp2 electrons to each other, forming the s-bonds for polymer backbone. The pz electrons and s-bond plane are orthogonal, tending to delocalize along the polymer chains. Notably, conjugated polymers are not metals, but the semiconductors, because the Peierls distortion of one-dimensional (1D) crystal results in a dimerized chain structure with alternating bond lengths. As one electron is removed from the PEDOT chain, its spin degeneracy will rise, and an empty state corresponded to one of the spin species will appear in the gap, which state is called as polaron (Fig. 4.2). The polaron is usually located in several monomer units, resulting the character of the bond alternation of PEDOT chains partly transform from the quinoid to aromatic. The modern density functional theory (DFT) approach has calculated the spin state of the chain while the polaron is

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Figure 4.2 Electronic structure of doped PEDOT chains. Positively charged polaron and bipolaron states in a PEDOT chain N ¼ 12 in the ground state configuration for a different number of positive charges q ¼ þ1 ,., þ6. (The spin S of the ground state is indicated.) (A, B) Bond length alternation (red curves) and the electron density distribution in a PEDOT chain with a polaron þ e (corresponding to the band diagram (F)). (C, D) the same as (A, B), but for a bipolaron þ2e in (F). The black lines in (A) and (C) correspond to the bond length alternation for a neutral chain. (EeK) The band diagram for the polaron and bipolaron states as the PEDOT chain is consequently charged from 0 to þ6e. All band diagrams are aligned by setting EHOMO ¼ 0. Occupied electronic levels (in the valence band) are shown in blue, where electrons filling the levels are represented by arrows. The empty electronic levels (in the conduction band) are shown in red. Dashed red lines are unoccupied polaronic/bipolaronic levels. (L) and (M) are the electron density distribution for the polaronic (þ5e) and bipolaronic (þ6e) states (J) and (K), along with the corresponding bond length alternation. Arrows in the conduction band describe the occupied spin-up and spin-down electron levels. (Copyright 2019, American Chemical Society. Zozoulenko, I.; Singh, A.; Singh, S. K.; Crispin, X.; Berggren, M.; Polarons, Bipolarons, and Absorption Spectroscopy of PEDOT, ACS Appl. Polym. Mater. 2019, 1, 83.)

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doublet (S ¼ 1/2).13e18 While two removed electrons correspond to two empty states in the gap, this state is called as bipolarons. DFT approach has forecasted the spin state of the chain while a bipolaron is triplet (S ¼ 1). Along with the increased oxidation level of chain, more empty states will appear in the gap. For even number of electrons (bipolaronic states þ4e, þ6e), the spin state of the chain is singlet (S ¼ 0); and for odd number (polaronic states þ3e, þ5e), the spin state is doublet (S ¼ 1/2). Notably, the major electronic structure characteristics of bipolaronic and polaronic states are extremely similar, showing the analogous width of (empty) polaronic/bipolaronic states band which is separated from the conduction and valence band. Similar charge localization and the character of the wave function forcefully indicates the character and the electron transport features for polarons and bipolarons are parallel. Both the DOS for valence and polaronic/bipolaronic bands reveal the Gaussian tails toward the gap, as shown in Fig. 4.3. In addition, the DOS of polymer thin-film with the deeply reduced gap between the bipolaron and the valence bands shows typical semimetal characteristic.19,20

Figure 4.3 (A) Calculated energy diagram of a representative crystallite composed of several pep stacked PEDOT chains. (B) A schematic diagram of the DOS of a PEDOT thin film (polycrystallic PEDOT thin film composed of crystallites embedded in an amorphous matrix). Occupied electronic states (in the valence band) and the corresponding DOS are depicted in blue, unoccupied electronic states are in red (solid lines for the bipolaronic band and dashed lines for the conduction band). The tail of the DOS in the valence band can be approximated by the Gaussian with a typical broadening gives s z 0.1 eV. (Copyright 2018, Wiley. Munoz, W. A.; Crispin, X.; Fahlman, M.; Zozoulenko, I. V.; Understanding the Impact of Film Disorder and Local Surface Potential in Ultraviolet Photoelectron Spectroscopy of PEDOT, Macromol. Rapid Commun. 2018, 39, 1700533.)

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4.2.3 Transport property Electrical conductivity of conducting polymers show a typical temperaturedependent behavior with the thermally assisted hopping transport. Under the recognized hopping transport mechanism, the quasiparticles (polarons/ bipolarons) can obtain the energy from the lattice vibrations (phonons), transmitting to the higher energy states (i.e., the valence band states of p-doped systems), and then traveling past the polymer film by the phononassisted hopping. The carrier mobility of PEDOT strongly depends on the morphology and structure.21 As shown in Fig. 4.4, the charge carrier mobility of PEDOT with different chain lengths (from N ¼ 3 monomers per chain to N ¼ 18) and different water content are calculated by a multiscale computational model, finding that the mobility exponentially increases with the chain length, and reaches a plateau while the chain length N > 15. With the increased the chain length, the morphology of PEDOT changes from orderly disconnected crystallites to a network with less orderly pep stacking, extending throughout the material. This result indicates that the pep connections between polymeric chains can form percolative network, ensuring good carrier mobility of PEDOT, and also explaining the reason that carrier mobility of PEDOT quickly increases with the increased chain length but saturating for sufficiently long chains.4

Figure 4.4 Multiscale transport calculation for PEDOT:Tos. (A) Morphology obtained from MD simulation and corresponding resistive network connecting polymer chains in the thin film. (B) Dependence of the mobility on the chain length N of the PEDOT oligomers. W is the water content (%w/w). (Copyright 2018, American Physical Society. Rolland, N.; Franco-Gonzalez, J. F.; Volpi, R.; Linares, M.; Zozoulenko, I. V.; Understanding Morphology-Mobility Dependence in PEDOT:Tos, Phys. Rev. Mater. 2018, 2, 045605.)

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Seebeck coefficient and electrical conductivity of conducting polymers reveal different coupling behavior compared with that of many inorganic materials, due to the complicated dependence of carrier concentration and microstructure. In the most basic Seebeck coefficient model of homogeneous materials, the electrical conductivity and Seebeck coefficient are determined by DOS for the system and the location of Fermi level (EF). Only the energy of carriers closes to the chemical potential (EF), they can participate in transport. Seebeck coefficient can be defined as the average entropy of each charge carrier, which is weighted by the carrier’s contribution to conductivity, and is therefore directly related to conductivity DOS, s(E)22: Z kB E  EF sðEÞ a¼  dE (4.3) s e kB T where a shows diverse temperature-dependent forms depending on the transport type of present system. To the lightly doped semiconductor polymers, Seebeck coefficient shows the consistent hopping behavior with the homogeneous system, and is irrelevant with temperature.23 However, there is not the same behavior to be found in heavily doped polymers, showing the increased Seebeck coefficient with the enhanced temperature.24e27 Experimental reports reveal the electronic transport processes are not simply follow a single transport mechanism, when the electrical conductivity is thermally activated but the Seebeck coefficient shows metallic dependence with temperature.28 In most instances, the transport models of polymers are assumed as that of homogeneous mediums, which carriers uniformly distribute in the whole medium. However, in some cases, doping of polymers may be uneven, resulting in the changed structural order at the nanoscale. It can be dividually treated the transport mechanisms of electrical conductivity for doped polymers by the measurements of electrical conductivity and Seebeck coefficient, which is similar to the competing transport mechanisms of homogeneous and disordered materials. 4.2.4 Model setup An important direction for the development of organic thermoelectric applications is the precise regulation of electronic doping. Around room temperature, conjugated polymers are considered to have similar thermopower (the efficiency of converting temperature differences into electric

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potential) with inorganic materials (e. g. Bi2Te3).29 Typical conjugated polymer is PEDOT,30,31 which is the most popular in recent years, with high thermopower and stability.3,32,33 The unique properties of conjugated polymers are attributed to the special structure. A semiconductor polymer comprises a conjugated electron backbone with an unsaturated carbon-carbon bond. While applied an electric field, charge can be transported along the backbone in a conjugated delocalization, showing a high mobility. Electric charge transfer is realized in conductive polymer films through electron coupling between molecules (Fig. 4.5). To improve the electrical conductivity for thermoelectric applications, extra carriers must be introduced into the polymer chain by means of oxidation or reduction (namely, doping). In organic polymers, carriers with a single charge are called polarons and, and carriers with two charges are called bipolarons. For example, the polythiophene in the high oxidation state has bipolarons as the dominant charge carrier.3,32,33

Figure 4.5 Schematic representation of (A) the variable-range hopping model versus (B) the tunneling model. (Copyright 2020, Elsevier Ltd. Gueye, M. N.; Carella, A.; FaureVincent, J.; Demadrille, R.; Simonato, J. P.; Progress in Understanding Structure and Transport Properties of PEDOT-Based Materials: A Critical Review. Prog. Mater. Sci. 2020, 108, 100616.)

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There is a large gap in semiconductors between the valence states (the highest energy state) and the conduction states (the lowest unfilled states). A band of states in a metal is partially occupied by carriers, resulting in the lack of band gap around the Fermi energy level. PEDOT:PSS as one of the typical polymer thermoelectric materials, polarons and bipolarons coexist in a certain ratio (depending on the number of PEDOT and PSS), which is the same as many doped semiconductor polymers. In contrast, in vapordeposited PEDOT samples with tosylate counterions (Tose), dipolarons are considered to be the dominant species. For the electronic structure model of PEDOT:Tos, an empty band of bipolaron states is formed and is close (or touching) to the valence band (VB) in energy. The semimetals are defined as a material with no gap between the valence state and the conduction state. The high-energy distribution of states near the Fermi level, indicating a sharp rise in semimetals, is in connection with the Seebeck coefficient: the steeper the distribution, the higher the Seebeck coefficient. The high S value and remarkable metal conductivity of PEDOT:Tos films confirm the semimetal-like behavior of these organic materials.34

4.3 Band structure In conjugated systems of the conductive polymers, the formation of pbonding and p* -antibonding states are also predicted by quantum physics after creating a molecular orbital from the P atomic orbitals. These formed states correspond to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which is similar to the valence band and conduction band of inorganic semiconductors. Within these states, the much longer chain will create more states; and the energy band gap between the HOMO and the LUMO will be smaller. A sufficiently long chain and tight molecular orbitals will cause the stop of discretization and the formation of energy bands. The highest energy of a completely filled VB is HOMO, while the lowest energy of a completely holed conduction band (CB) is LUMO. The conjugated polymers exhibit semiconductor properties attributed to the relatively wide energy band gap between HOMO and LUMO (generally, 1w4 eV).35 Crispin et al. employed the ultraviolet photoelectron spectroscopy (UPS) and electron paramagnetic resonance spectroscopy (EPR) to analyze the band structure of PEDOT: PSS and PEDOT:Tos. The UPS results indicate that the amorphous PEDOT:PSS is a Fermi glass. Compared with PEDOT:PSS (5.1 eV), the valence band of PEDOT:Tos with lower work

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function (4.3 eV) is closer to Fermi level (EF). For the EPR testing, PEDOT:PSS contains 7% polarons and 93% dipolarons based on the monomers with a carrier density of about 33%. The ratio can be determined by DEG, which may promote the conversion of paired polarons into bipolarons.36e38 The EPR signal (remaining) is intrinsically associtated with the molecular disorder which impedes the remnant polaron from coupling to other charged defects. Surprisingly, there is no ESR signal at all in the polycrystalline PEDOT:Tos, which indicates that the only type of carriers is attributed to polaron pairs or bipolarons.39,40

4.4 Density of states The polymer chains in the solid state are arranged in disorder or selforganized in crystalline domains.41 The level of the polaron or bipolaron is located in a part of the chains in the amorphous phase. The wave functions of charged defects at high oxidation levels locate on the same chain overlap and form a one-dimensional “intrachain” band.42 However, the disorder and the lack of electron coupling between the chains impede this band from extending through the three-dimensional space of the solid, which results in the localization in space and spreads in energy distribution.43 For the solids of disordered polarized polymer, the Fermi level is located among the localized states in the middle of the polarized band; and for the solids of disordered bipolarized polymer, it locates between the valence band and the bipolarized band.44 Both of these solids can be regarded as Fermi glasses.12,45 In polymer crystal domains and molecular crystals, the short distance between chains causes the electron density of adjacent filled chains to overlap, thereby promoting the delocalization of the electron wavefunction,46 such as a polaron distributed on multiple chains.47 The polyaniline highly oxidized can be a metal with the characteristics of a half-filled polarized band, which is derived from the formation of a polarized network.48,49 The slope of the DOS at EF is related to the Seebeck coefficient (S) in the first approximation.29 The low thermopower (S < 10 mV K1) of metals and polyaniline highly oxidized are attributed to the Fermi level being in the middle of a band.50 A lower degree of disorder will result in the transition of polarized polymers, such as polyaniline, from Fermi glass to metal.51 For bipolarons, it is unclear. Compared with polyaniline, defects (e.g., dipolarons) exist in polythiophene (e.g., PEDOT).52 In highly oxidized PEDOT, there are not more than one carrier for every three monomer units.36 According to quantum chemistry

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calculations, it can be concluded that the bipolarons in PEDOT are distributed on six or more monomer units.39 The Mott formula shows that S is proportional to d(lns(E))/dE at EF and is valid for both mechanisms of hopping and band motion transport.53 S is proportional to [d(lnN(E))/dE]E ¼ EF, which is primarily ascribed to the energy dependence of conductivity determined by DOS N(E). In the bipolaron system of amorphous, EF depends on the local states which fades out from the valence band and bipolaron bands. According to UPS detection, the DOS of EF not only has little change, but is close to the minimum value, which makes the lower S of PEDOT:PSS explained. For PEDOT:Tos, [d(lnN(E))/dE]E ¼ EF and S are larger, which is attributed to the fact that EF is in a greatly varying DOS area. It should be noted that the asymmetry of DOS will be magnified as the number of structural levels increases because the local level will lead to smooth DOS, which makes the larger S of PEDOT:Tos with high s. Obviously, a further improvement in the structural order (higher s) shouldlead to a larger S in principle, which in turn results in an enhanced thermoelectric power factor (sS2). In disordered or metallic polyaniline, where polarons dominate the doping species, EF is located in a region of the DOS varying slowly. Thus, compared with the polaron network (metallic), the thermoelectric properties of the conductive polymers composed of a bipolaron network (semimetallic) are expected to be better. In the semimetallic PEDOT:Tos, the absence of (residual) polarons is a unique feature which can be used in spintronics, because in solids without unpaired electrons, spin scattering and spin lifetime extension are hindered.40

4.5 Thermoelectric performance dependence 4.5.1 Electrical conductivity and thermopower The electrical conductivity and Seebeck coefficient are interrelated with the carrier concentration. Due to s ¼ emn, electrical conductivity is increased with the increased carrier concentration. The Seebeck coefficient is related to the density of state, which represents average transport entropy of each charge carrier and thus decreases with increasing carrier concentration. Decoupling these parameters closely related to carrier concentration is critical for optimizing the thermoelectric performance. The S is defined as open circuit voltage obtained when two ends of the material are at different temperatures.54 S represents the thermal diffusion of charge carriers in the material. According to Mott’s formula,

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Sz

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vlnDOS vE

, where DOS and EF refer to densities of states and E¼EF

Fermi level, respectively. Hence S is proportional to slope of the DOS at the EF; and can be determined by filling and shape of the DOS of the material. This gives explanation to S in semiconducting polymers will decrease as the doping level increases. High crystallinity materials are accompanied by intense DOS.40 The conjugated backbone of PEDOT-based polymers can form linear chains; and can be easily separated by networks of some organic materials, such as PSS and Tos. Previous studies reported that S only minor changes for all PEDOT:PSS samples despite strong s variation. Samples with different doping levels are generally accompanied by opposite changes in S and s. PEDOT:PSS thermally activated transport, which refers to the behavior of semiconductors with a positive temperature coefficient; PEDOT:Tos shows metal behavior with negative temperature coefficient at room temperature. As a result, the Seebeck coefficient has an unexpected relationship with the charge transport mechanism. The different of the metal s versus T for the PEDOT:Tos indicates that the two transport modes with different weights are active meanwhile: hopping conduction and metallic conduction. Notably, s and S all increase with the increase of metallic contribution. PEDOT:Tos with larger S belong to semimetals rather than metals compared to polyaniline and other metals. According to the analysis of electronic structure, PEDOT:Tos is resemble a bipolaron network with a hollow delocalized bipolaron band integrating with delocalized valence bands.40 4.5.2 Electrical conductivity and thermal conductivity The thermal conductivity consists of lattice contribution kl and electron contributions ke, with k ¼ klþ ke; the latter is positively correlated with the carrier concentration because charge carriers can also transmit heat. The predicted conductivity in WiedemannFranz law (WFL) ke ¼ LsT (L is the Lorenz number, s is the electrical conductivity, and T is the absolute temperature) is proportional to the electronic contribution in the thermal conductivity for metals and highly doped semiconductors. The Lorenz number L is typically observed to be within 20% of the Sommerfeld value L ¼ 2.45  108 W U K2. Lorenz number can be significantly reduced as bandwidth of the electron or hole dispersion is small by theoretical calculations.55

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Figure 4.6 The WiedemannFranz law and Sommerfeld value of Lorenz number L,  plotted as a dashed line kk ¼ kph þLsT . (Copyright 2015, American Chemical Society. Liu, J.; Wang, X.; Li, D.; Coates, N. E.; Segalman, R. A.; Cahill, D. G.; Thermal Conductivity and Elastic Constants of PEDOT:PSS with High Electrical Conductivity, Macromolecules 2015, 48, 585.)

Liu et al. reported that the in-plane thermal conductivity (kk) of dropcast PEDOT films can be used as a function of the electrical conductivity to estimate the electronic contribution of the thermal conductivity using the WFL and the Sommerfeld value of the Lorenz number, kk ¼ kph þ LsT ,   where the contribution of the phonons to the thermal conductivity kph can be regarded as constants. This relationship is plotted as a dotted line in Fig. 4.6. The conductivity in the thickness direction is much smaller than the in-plane conductivity when assumed Lorenz number also applies in the through-thickness direction. The phonon contribution to the thermal conductivity does not change significantly with the increased electrical conductivity of PEDOT films. 4.5.3 Thermal conductivity and semicrystalline The ideal thermoelectric material should allow the diffusion of electron charge carriers in the material and prevent the transport of thermal carriers, which indicates that the material should have high conductivity and low thermal conductivity.29 The thermal conductivity k of semiconductor systems derives from the contribution of electrons ke and lattice kl. The ke suggests that electron charge transport also contributes to heat conduction, and ke is related to conductivity by WeidemanneFranz lawke ¼ ðkB =eÞ2 LT s, where L is the Lorenz number. Compared to the

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thermal conductivity of inorganic semiconductors (approximately 100 W m1 K1), polymers have relatively low thermal conductivity (approximately 0.01w2 W m1 K1).57 This low thermal conductivity can be attributed to the semicrystalline and paracrystalline of polymers, while the electron contribution can be neglected. However, doped semiconductors including multiple PEDOT formulations do not belong to the latter.3 Weathers et al. proposed that doping the self-supporting PEDOT:Tos film can increase its electrical conductivity by 3 times and thermal conductivity approximately to 2 W m1 K1.58 They believe that the contribution of electrons to thermal transport leads to this behavior, revealing a close association between conjugated polymers and doping levels. Liu et al. also reported similar values for PEDOTPSS. The in-plane thermal conductivity of PEDOTPSS films is reported by combining the measurement of elastic constants with the time-domain thermoreflectance.56 Wei et al. emphasized the importance of structure to the thermal conductivity of PEDOT systems.59e62 Crystallinity of the PEDOT film is positively correlated with the thermal conductivity, while the orientation of the morphology can make the directional dependence for thermal conductivity. 4.5.4 Temperature Carrier movements of thermoelectric materials respond to changes in temperature gradients, which is Seebeck effect. Due to the existence of temperature gradient, carrier diffusion can form charge accumulation, thus producing a potential difference, DV. The conduction which occurs in slightly doped organic materials mainly depends on the thermal assisted process and the Arrhenius-type relationship with respect to temperature which means s is proportional to expð  EA =kB T Þ, where EA, kB, and T refer to activation energy, Boltzmann constant and absolute temperature, respectively. This can also be used to characterize the charge hopping occurred at nearest domains.63,64 Obstacles to the neighborhood boundary of polycrystalline materials can also lead to the occurrence of this temperature gradient. Owing to s ¼ s0 exp½  ðT0 =T Þg , where g represents the dimension of the system (g ¼ 0.25 for 3D variable-range hopping (VRH) and 0.5 for 1D VRH) and T0 is the characteristic temperature, temperature dependence indicating variable-range hopping may occur at high carrier concentrations.65e69 Typical hopping mechanism such as VRH model, where s disappears as the temperature approaches 0 K, is not suitable for charge transport in highly conductive states. By combining the heterogeneous transport model which combines the quasi-1D metallic

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transport and the fluctuation-induced tunneling (FIT) model, the typical morphology of the mixture of metal and nonmetallic properties is consisted with crystallites which arrange in chains, and are surrounded by the chainentangled amorphous regions. Notably, the real metal behavior with negative temperature coefficients over the whole temperature range has been reported for ordered, vapor-phase polymerized PEDOT:Tos film.40 4.5.5 Carrier concentration and mobility Doping is the key method to optimize the thermoelectric power factor (S2s) which can affect the carrier mobility and determine the free-carrier concentration.30 Dopants have greater effect on organic semiconductors with lower thermoelectric power factor, which can be attributed to dopants in Van der Waals bonded solids can not only change the conformation of conductive host molecules and hence change their carrier transport properties, but also increase the tunneling distance between these molecules and thus greatly reducing the rate of thermal activation hopping. Kim et al. reported that the conductivity and Seebeck coefficient of PEDOT:PSS (treated with DMSO or EG), can be optimized simultaneously due to the removal of excess insulating PSS and the enhancement of crystallinity of PEDOT, which improves the charge transport of the system.70 When PEDOT:PSS film was treated with oxidative reductants, the Seebeck coefficient increased due to the adjustment of oxidation level and the structural change of PEDOT chains.71 Thermoelectric behavior of PEDOT systems can also be influenced by macromolecular structures. Longer PEDOT chains facilitate the thermal diffusion of carriers, which acts as highly conductive bridges between the PEDOT crystallites.4 As a result, the high molecular weight grade (PH1000) has high conductivity and Seebeck coefficient.72 Seebeck coefficient as a function of carrier concentration is calculated by Monte Carlo calculation and semianalytical method in thermal activation jump model. For the PEDOT, Ihnatsenka et al. reported that hopping transport of polaron and bipolaron quasiparticles extended to more than 2e3 EDOT monomer units,74 while the typical jumps distance is more than 3e4 monomer units. The energy disorder is estimated at 0.1 eV. Highly efficient thermoelectric polymers should exhibit semimetallic behavior. The Seebeck coefficient of the PEDOT:Tos is proportional to the charge carrier mobility of the system at constant oxidation levels (Fig. 4.7), with S z m0:2 . The increased crystallinity of PEDOT film is closely related to higher carrier mobility. The increase in crystallinity is accompanied by sharper DOS slopes and higher Seebeck coefficients.73

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Figure 4.7 The extracted relationship between Seebeck coefficient and charge carrier mobility with constant oxidation levels for PEDOT:Tos thin films. (Copyright 2018, Elsevier Ltd. Petsagkourakis, I.; Pavlopoulou, E.; Cloutet, E.; Chen, Y. F.; Liu, X.; Fahlman, M.; Berggren, M.; Crispin, X.; Dilhaire, S.; Fleury, G.; Hadziioannou, G.; Correlating the Seebeck Coefficient of Thermoelectric Polymer Thin Films to Their Charge Transport Mechanism, Org. Electron. 2018, 52, 335.)

4.5.6 Order and disorder Owing to the different transport processes in the heterogeneous phase, the Seebeck coefficient and electrical conductivity can be weighed by the ordered and disordered regions. Different degrees of heterogeneity and ordering in the microstructure cause the mixed transport behavior. Improved predictive power in thermoelectric material design can be achieved by developing many models of high-performance polymer heterogeneity. The three illustrated material scenarios are: a highly disordered microstructure, a highly ordered microstructure, and an intermediate microstructure in which both highly ordered and disordered domains exist. Carrier conduction occurs in highly ordered and highly disordered domains through electron disordered band conduction and thermally assisted hopping, respectively. The change from disorder to order for microstructure is accompanied by the increase of average mobility, which leads to the increase of conductivity. However, even if there are metal bands in the ordered region, the total conductivity may not be a temperature function of the metal due to the existence of the disordered region.7

4.6 Concluding remarks PEDOT appears as a breakthrough in thermoelectric performance, representing a unique opportunity of conducting polymers in the organic

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thermoelectric field, and has now brought more attention. Because of the structural disorder for conducting polymers, along with the polaronic nature of the charge carriers, various transport mechanisms have been proposed. In this section, we review and discuss recent progress on the thermoelectric transport model of conducting polymers. Particular emphasis is given to the PEDOT (PEDOT/PEDOT:PSS) systems, with special attention on the influence factors of geometric structure, electronic structure, band structure, and density of states for the thermoelectric performance. An in-depth fundamental understanding of the charge transport will further facilitate the design of advanced PEDOT Thermoelectric Materials.

References 1. Kang, S. D.; Snyder, G. J. Charge-transport Model for Conducting Polymers. Nat. Mater. 2017, 16, 252. 2. Palumbiny, C. M.; Heller, C.; Schaffer, C. J.; Korstgens, V.; Santoro, G.; Roth, S. V.; Mueller-Buschbaum, P. Molecular Reorientation and Structural Changes in CosolventTreated Highly Conductive PEDOT:PSS Electrodes for Flexible Indium Tin Oxidefree Organic Electronics. J. Phys. Chem. C 2014, 118, 13598. 3. Chabinyc, M. Thermoelectric Polymers: Behind Organics’ Thermopower. Nat. Mater. 2014, 13, 119. 4. Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship Between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038. 5. Rivnay, J.; Jimison, L. H.; Northrup, J. E.; Toney, M. F.; Noriega, R.; Lu, S.; Marks, T. J.; Facchetti, A.; Salleo, A. Large Modulation of Carrier Transport by GrainBoundary Molecular Packing and Microstructure in Organic Thin Films. Nat. Mater. 2009, 8, 952. 6. Coropceanu, V.; Cornil, J.; Filho, D. A. D. S.; Olivier, Y.; Silbey, R.; Brédas, J. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926. 7. Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic Thermoelectric Materials for Energy Harvesting and Temperature Control. Nat. Rev. Mater. 2016, 1, 1. 8. Mei, J. G.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. A. Siloxane-terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors. J. Am. Chem. Soc. 2011, 133, 20130. 9. Franco-Gonzalez, J. F.; Zozoulenko, I. V. Molecular Dynamics Study of Morphology of Doped PEDOT: from Solution to Dry Phase. J. Phys. Chem. B 2017, 121, 4299. 10. Modarresi, M.; Franco-Gonzalez, J. F.; Zozoulenko, I. Morphology and Ion Diffusion in PEDOT: Tos. A Coarse Grained Molecular Dynamics Simulation. Phys. Chem. Chem. Phys. 2018, 20, 17188. 11. Franco-Gonzalez, J. F.; Rolland, N.; Zozoulenko, I. V. Substrate-dependent Morphology and its Effect on Electrical Mobility of Doped Poly(3, 4ethylenedioxythiophene)(PEDOT) Thin Films. ACS Appl. Mater. Interfaces 2018, 10, 29115. 12. Heeger, A. J. Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials (Nobel Lecture). Angew. Chem. Int. Ed. 2001, 40, 2591.

Thermoelectric transport and PEDOT dependence

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13. Zozoulenko, I.; Singh, A.; Singh, S. K.; Crispin, X.; Berggren, M. Polarons, Bipolarons, and Absorption Spectroscopy of PEDOT. ACS Appl. Polym. Mater. 2019, 1, 83. 14. Brédas, J. L.; Wudl, F.; Heeger, A. J. Polarons and Bipolarons in Doped Polythiophene: A Theoretical Investigation. Solid State Commun. 1987, 63, 577. 15. Cornil, J.; Beljonne, D.; Brédas, J. L. Nature of Optical Transitions in Conjugated Oligomers. II. Theoretical Characterization of Neutral and Doped Oligothiophenes. J. Chem. Phys. 1995, 103, 842. 16. Salzner, U. Investigation of Charge Carriers in Doped Thiophene Oligomers through Theoretical Modeling of Their UV/Vis Spectra. J. Phys. Chem. 2008, 112, 5458. 17. Heimel, G. The Optical Signature of Charges in Conjugated Polymers. ACS Cent. Sci. 2016, 2, 309. 18. Volkov, A. V.; Singh, S. K.; Stavrinidou, E.; Gabrielsson, R.; Franco-Gonzalez, J. F.; Cruce, A.; Chen, W. M.; Simon, D. T.; Berggren, M.; Zozoulenko, I. V. Spectroelectrochemistry and Nature of Charge Carriers in Self-Doped Conducting Polymer. Adv. Electron. Mater. 2017, 3, 1700096. 19. Munoz, W. A.; Crispin, X.; Fahlman, M.; Zozoulenko, I. V. Understanding the Impact of Film Disorder and Local Surface Potential in Ultraviolet Photoelectron Spectroscopy of PEDOT. Macromol. Rapid Commun. 2018, 39, 1700533. 20. Muñoz, W. A.; Singh, S. K.; Franco-Gonzalez, J. F.; Linares, M.; Crispin, X.; Zozoulenko, I. V. Insulator to Semimetallic Transition in Conducting Polymers. Phys. Rev. B 2016, 94, 205202. 21. Rolland, N.; Franco-Gonzalez, J. F.; Volpi, R.; Linares, M.; Zozoulenko, I. V. Understanding Morphology-Mobility Dependence in PEDOT:Tos. Phys. Rev. Mater. 2018, 2, 045605. 22. Fritzsche, H. A General Expression for the Thermoelectric Power. Solid State Commun. 1971, 9, 1813. 23. Park, Y. W.; Denenstein, A.; Chiang, C. K.; Heeger, A. J.; Macdiarmid, A. G. Semiconductor-metal Transition in Doped (CH)x: Thermoelectric Power. Solid State Commun. 1979, 29, 747. 24. Zhang, Q.; Sun, Y. M.; Xu, W.; Zhu, D. B. What to Expect from Conducting Polymers on the Playground of Thermoelectricity: Lessons Learned from Four High Mobility Polymeric Semiconductors. Macromolecules 2014, 47, 609. 25. Reghu, M.; Cao, Y.; Moses, D.; Heeger, A. J. Counterion-induced Processibility of Polyaniline: Transport at the Metal-Insulator Boundary. Phys. Rev. B 1993, 47, 1758. 26. Yoon, C. O.; Reghu, M.; Moses, D.; Heeger, A. J. Transport Near the Metal-InsulatorTransition: Polypyrrole Doped with PF6. Phys. Rev. B 1994, 49, 10851. 27. Nogami, Y.; Kaneko, H.; Ishiguro, T.; Takahashi, A.; Tsukamoto, J.; Hosoito, N. On the Metallic States in Highly Conducting Iodine-Doped Polyacetylene. Solid State Commun. 1990, 76, 583. 28. Kaiser, A. B. Electronic Transport Properties of Conducting Polymers and Carbon Nanotubes. Rep. Prog. Phys. 2001, 64, 1. 29. Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105. 30. Bubnova, O.; Ullah Khan, Z.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 31. Kim, G.-H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12, 719. 32. Heeger, A. J. Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials. J. Phys. Chem. B 2001, 105, 8475.

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Advanced PEDOT Thermoelectric Materials

33. Gueye, M. N.; Carella, A.; Faure-Vincent, J.; Demadrille, R.; Simonato, J. P. Progress in Understanding Structure and Transport Properties of PEDOT-Based Materials: A Critical Review. Prog. Mater. Sci. 2020, 108, 100616. 34. Conwell, E. M.; Mizes, H. A. Surface-phonon Dispersion of MoS2. Phys. Rev. B 1991, 44, 937. 35. Patil, A. O.; Heeger, A. J. F.; Wudl. Optical Properties of Conducting Polymers. Chem. Rev. 1988, 88, 183. 36. Zotti, G.; Zecchin, S.; Schiavon, G.; Louwet, F.; Groenendaal, L.; Crispin, X.; Osikowicz, W.; Salaneck, W.; Fahlman, M. Electrochemical and XPS Studies toward the Role of Monomeric and Polymeric Sulfonate Counterions in the Synthesis, Composition, and Properties of Poly(3,4-Ethylenedioxythiophene). Macromolecules 2003, 36, 3337. 37. Domagala, W.; Pilawa, B.; Lapkowski, M. Quantitative In-Situ EPR Spectroelectrochemical Studies of Doping Processes in Poly(3,4-Alkylenedioxythiophene)s: Part 1: PEDOT. Electrochim. Acta 2008, 53, 4580. 38. Crispin, X.; Marciniak, S.; Osikowicz, W.; Zotti, G.; Denier van der Gon, A. W.; Louwet, F.; Fahlman, M.; Groenendaal, L.; Schryver, F. C.; Salaneck, W. R. Conductivity, Morphology, Interfacial Chemistry, and Stability of Poly(3,4-Ethylene Dioxythiophene)-Poly(styrene Sulfonate): A Photoelectron Spectroscopy Study. J. Poly. Sci. B Polym. Phys. 2003, 41, 2561. 39. Geskin, V. M.; Brédas, J.-L. Polaron Pair versus Bipolaron on Oligothiophene Chains: A Theoretical Study of the Singlet and Triplet States. ChemPhysChem 2003, 4, 498. 40. Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J.-B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190. 41. Martin, D. C.; Wu, J.; Shaw, C. M.; King, Z. A.; Spanninga, S. A.; RichardsonBurns, S. M.; Hendricks, J. L.; Yang, J. The Morphology of Poly(3,4Ethylenedioxythiophene). Pol. Rev. 2010, 50, 340. 42. Stafström, S.; Brédas, J. L. Evolution of the Electronic Structure of Polyacetylene and Polythiophene as a Function of Doping Level and Lattice Conformation. Phys. Rev. B 1988, 38, 4180. 43. Prigodin, V. N.; Efetov, K. B. Localization Transition in a Random Network of Metallic Wires: A Model for Highly Conducting Polymers. Phys. Rev. Lett. 1993, 70, 2932. 44. Lavarda, F. C.; Santos, M. C.; Galvão, D. S.; Laks, B. Insulator-to-metal Transition in Polythiophene. Phys. Rev. B 1994, 49, 979. 45. Sariciftci, N. S.; Heeger, A. J.; Cao, Y. Paramagnetic Susceptibility of Highly Conducting Polyaniline: Disordered Metal with Weak Electron-Electron Interactions (Fermi Glass). Phys. Rev. B 1994, 49, 5988. 46. Koller1, G.; Berkebile1, S.; Oehzelt1, M.; Puschnig, P.; Ambrosch-Draxl, C.; Netzer, F. P.; Ramsey, M. G. Intra- and Intermolecular Band Dispersion in an Organic Crystal. Science 2007, 317, 351. 47. Beljonne, D.; Cornil, J.; Sirringhaus, H.; Brown, P. J.; Shkunov, M.; Friend, R. H.; Brédas, J. L. Optical Signature of Delocalized Polarons in Conjugated Polymers. Adv. Funct. Mater. 2001, 11, 229. 48. Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C. W.; Lee, S. H. Metallic Transport in Polyaniline. Nature 2006, 441, 65. 49. Stafström, S.; Brédas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Polaron Lattice in Highly Conducting Polyaniline: Theoretical and Optical Studies. Phys. Rev. Lett. 1987, 59, 1464.

Thermoelectric transport and PEDOT dependence

117

50. Mateeva, N.; Niculescu, H.; Schlenoff, J.; Testardi, L. R. Correlation of Seebeck Coefficient and Electric Conductivity in Polyaniline and Polypyrrole. J. Appl. Phys. 1998, 83, 3111. 51. Yoon, C. O.; Reghu, M.; Moses, D.; Heeger, A. J.; Cao, Y.; Chen, T.-A.; Wu, X.; Rieke, R. D. Hopping Transport in Doped Conducting Polymers in the Insulating Regime Near the Metal-Insulator Boundary: Polypyrrole, Polyaniline and Polyalkylthiophenes. Synth. Met. 1995, 75, 229. 52. Zykwinska, A.; Domagala, W.; Czardybon, A.; Pilawa, B.; Lapkowski, M. In Situ EPR Spectroelectrochemical Studies of Paramagnetic Centres in Poly(3,4Ethylenedioxythiophene) (PEDOT) and Poly(3,4-Butylenedioxythiophene) (PBuDOT) Films. Chem. Phys. 2003, 292, 31. 53. Cutler, M.; Mott, N. F. Observation of Anderson Localization in an Electron Gas. Phys. Rev. 1969, 181, 1336. 54. Bubnova, O.; Crispin, X. Towards Polymer-Based Organic Thermoelectric Generators. Energy Environ. Sci. 2012, 5, 9345. 55. Jeong, C.; Kim, R.; Lundstrom, M. On the Best Band Structure for Thermoelectric Performance: A Landauer Perspective. J. Appl. Phys. 2012, 111, 113707. 56. Liu, J.; Wang, X.; Li, D.; Coates, N. E.; Segalman, R. A.; Cahill, D. G. Thermal Conductivity and Elastic Constants of PEDOT:PSS with High Electrical Conductivity. Macromolecules 2015, 48, 585. 57. Petsagkourakis, I.; Tybrandt, K.; Crispin, X.; Ohkubo, I.; Satoh, N.; Mori, T. Thermoelectric Materials and Applications for Energy Harvesting Power Generation. Technol. Adv. Mater. 2018, 19, 836. 58. Weathers, A.; Khan, Z. U.; Brooke, R.; Evans, D.; Pettes, M. T.; Andreasen, J. W.; Crispin, X.; Shi, L. Significant Electronic Thermal Transport in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Adv. Mater. 2015, 27, 2101. 59. Wei, Q.; Mukaida, M.; Kirihara, K.; Ishida, T. Experimental Studies on the Anisotropic Thermoelectric Properties of Conducting Polymer Films. ACS Macro Lett. 2014, 3, 948. 60. Wei, Q.; Uehara, C.; Mukaida, M.; Kirihara, K.; Ishida, T. Measurement of In-Plane Thermal Conductivity in Polymer Films. AIP Adv. 2016, 6, 045315. 61. Shi, W.; Shuai, Z.; Wang, D. Tuning Thermal Transport in Chain-oriented Conducting Polymers for Enhanced Thermoelectric Efficiency: A Computational Study. Adv. Funct. Mater. 2017, 27, 1702847. 62. Genovese, C.; Antidormi, A.; Dettori, R.; Caddeo, C.; Mattoni, A.; Colombo, L.; Melis, C. Linking Morphology to Thermal Conductivity in PEDOT: An Atomistic Investigation. J. Phys. D 2017, 50, 494002. 63. Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Highly Efficient Organic Devices Based on Electrically Doped Transport Layers. Chem. Rev. 2007, 107, 1233. 64. Pingel, P.; Neher, D. Comprehensive Picture of P-type Doping of P3HT with the Molecular Acceptor F4TCNQ. Phys. Rev. B 2013, 87, 115209. 65. Heeger, A. J.; Kivelson, S. A.; Schrieffer, J. R.; Su, W. P. Solitons in Conducting Polymers. Rev. Mod. Phys. 1988, 60, 781. 66. Wang, Z. H.; Li, C.; Scherr, E. M.; Macdiarmid, A. G.; Epstein, A. J. Three Dimensionality of “metallic” States in Conducting Polymers: Polyaniline. Phys. Rev. Lett. 1991, 66, 1745. 67. Paloheimo, J.; Laakso, K.; Isotalo, H.; Stubb, H. Conductivity, Thermoelectric Power and Field-Effect Mobility in Self-Assembled Films of Polyanilines and Oligoanilines. Synth. Met. 1995, 68, 249. 68. Ruit, K.; Cohen, R. I.; Bollen, D.; Mol, T.; Yerushalmi-Rozen, R.; Janssen, R. A. J.; Kemerink, M. Quasi-one Dimensional In-Plane Conductivity in Filamentary Films of PEDOT:PSS. Adv. Funct. Mater. 2013, 23, 5778.

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69. Wang, S.; Ha, M.; Manno, M.; Frisbie, C. D.; Leighton, C. Hopping Transport and the Hall Effect Near the InsulatoreMetal Transition in Electrochemically Gated Poly(3Hexylthiophene) Transistors. Nat. Commun. 2012, 3, 1210. 70. Palumbiny, C. M.; Liu, F.; Russell, T. P.; Hexemer, A.; Wang, C.; MuellerBuschbaum, P. The Crystallization of PEDOT:PSS Polymeric Electrodes Probed In Situ During Printing. Adv. Mater. 2015, 27, 3391. 71. Fan, Z.; Li, P.; Du, D.; Ouyang, J. Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films Through Sequential Post-treatments with Common Acids and Bases. Adv. Energy Mater. 2017, 7, 1602116. 72. Fan, Z.; Du, D.; Yao, H.; Ouyang, J. Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric Property of PEDOT:PSS: A Comparative Study of Clevios P and Clevios PH1000. ACS Appl. Mater. Interfaces 2017, 9, 11732. 73. Petsagkourakis, I.; Pavlopoulou, E.; Cloutet, E.; Chen, Y. F.; Liu, X.; Fahlman, M.; Berggren, M.; Crispin, X.; Dilhaire, S.; Fleury, G.; Hadziioannou, G. Correlating the Seebeck Coefficient of Thermoelectric Polymer Thin Films to Their Charge Transport Mechanism. Org. Electron. 2018, 52, 335. 74. Ihnatsenka, S.; Crispin, X.; Zozoulenko, I. V. Understanding Hopping Transport and Thermoelectric Properties of Conducting Polymers. Phys. Rev. B 2015, 92, 035201.

CHAPTER 5

Optimizing the thermoelectric performance of PEDOTs Hui Shi1, 2 1

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, PR China; 2National-Local Joint Engineering Research Center of Heavy Metals Pollutants Control and Resource Utilization, Nanchang Hangkong University, Nanchang, PR China

5.1 Introduction PEDOTs (PEDOT/PEDOT:PSS) have recently emerged as a promising class of materials for thermoelectric (TE) energy harvesting, attributing to the prominent merits of high electrical conductivity, low thermal conductivity, good stability as well as commercial availability.1e3 Generally, an efficient TE material with a high figure of merit (ZT) requires a high electrical conductivity (s), high Seebeck coefficient (S), and low thermal conductivity (k), which defined as ZT ¼ sS2T/k. However, simultaneous optimization of the electrical conductivity, Seebeck coefficient and thermal conductivity is inherently challenging as these three performance parameters depend upon one another. Thus, it is of fundamental and practical significance to develop novel strategies for the regulation of TE performance for waste heat utilization and conversion. Posttreatment of PEDOTs by secondary dopants including polar organic solvents, inorganic salts, nonionic surfactant polymers, acids, or alkalis, reducing reagents was considered one of the most commonly adoptive approaches.4 The enhancement of the crystallinity of the ordered domains in PEDOTs contributed to the compromise between electrical conductivity and Seebeck coefficient.5 Numerous efforts have been taken in the controllable doping of PEDOTs to enhance the electrical conductivity and Seebeck coefficient simultaneously, achieving the stateof-the-art TE performance of PEDOTs (ZTw0.25) upon doping in the presence of tosylate counterions following an in situ chemical or vaporphase polymerization.6 Additionally, low dimensionality, phonon scattering and molecular conformation are essential for excellent TE materials.7e9 To the best of our knowledge, PEDOT:PSS exhibited a record ZT value of 0.75 at room temperature via ion accumulation of an Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00015-9

© 2022 Elsevier Ltd. All rights reserved.

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ionic liquid on the polymer surface,10 which is comparable to that of conventional inorganic TE materials such as bismuth telluride. This chapter systematically introduces effective approaches including doping/ dedoping, low dimensionality, crystal structure, phonon scattering, molecular conformation, and posttreatment to optimize TE performance of PEDOTs, with the main results summarized in Table 5.1.

5.2 Doping and dedoping PEDOTs possess easy doping tenability, and the regulation of oxidation level via chemical or electrochemical methods is of great significance for the optimization of TE performance of conducting polymers. 5.2.1 Chemical doping and dedoping Crispin research group synthesized poly(3,4-ethylenedioxythiophene)tosylate (PEDOT-Tos) via chemical oxidative polymerization, and optimized the power factor by controlling the oxidation level with varying tetrakis(dimethylamino) ethylene (TDAE) exposure time. The electrical conductivity of PEDOT-Tos films experienced a dramatic decline with the decrease of the oxidation level, while the Seebeck coefficient gradually enhanced, achieving the power factor of 324 mW m1 K2 at the optimum oxidation level of 22%. With the low intrinsic thermal conductivity, a record figure of merit of 0.25 was yielded at room temperature, and the PEDOT-Tos based all-organic thermoelectric generator (TEG) provided a maximum power output of 0.128 mW at DT ¼ 10 C.6 Additionally, Cai et al. prepared PEDOT-Tos-polyethylene glycolepolypropylene glycole polyethylene glycol (PPP) by vapor-phase polymerization (VPP), and the films were treated with NaBH4/DMSO to adjust the redox level. Despite the decrease in the electrical conductivity upon chemical dedoping, the maximum power factor of PEDOT-Tos-PPP reached 98.1 mW m1 K2 due to a great increase in the Seebeck coefficient, leading to an enhanced ZT value of 0.064.11 Calzolari et al. presented a first principles study of the electronic and thermal transport of highly doped PEDOT:Tos (doping ratio 1:4) system, specifically the effect of Tos on the TE properties of crystalline PEDOT assemblies (Fig. 5.1). Density functional theory (DFT) calculations indicated that the inclusion of dopants impacts the bulk configuration by inflating the packing structure and worsening the intrinsic transport properties of the PEDOT host.59 Similarly, Cui et al. carefully studied the influence of

Table 5.1 The TE performance of PEDOTs. Material Strategy

s (S cm1)

S (mV K1)

P (mW m1 K2)

k (W m1 K1)

ZT

Refs.

PEDOT-Tos PEDOT-Tos-PPP PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS

6  104e300 5.7e1550 65 298 249 1399

40e780 14.9e143.5 13 14.2 18.81 18.6

324 98.1 1.1 6 8.81 48.3

0.37 0.451 0.17 0.17 0.108 0.18

0.25 0.064 1.75  103 1.02  102 0.024 0.1

[6] [11] [12] [13] [14] [15]

1067

22.5

53.3

0.3

0.05

[16]

0.036

436.3

0.69

e

e

[17]

295

27.47

22.28

0.17e0.52

[18]

224 1000 708e2074

12.5 27 14e42

3.5 75 147

e e 0.19

0.013 e0.039 e e 0.22

[19] [20] [21]

80e766

9.6e37.0

16.3

0.35  0.02

1.4  102

[22]

0.2e150 7.9 540

19e123 122 25.7

7.9 12 35.8

0.186 e e

0.013 e e

[23] [7] [24]

PEDOT:PSS

PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT PEDOT PEDOT PEDOT PEDOT

TDAE treatment NaBH4/DMSO dedoping DMSO or EG doping DMSO doping DMSO-urea doping Chemical doping DMSO Liquid exfoliated graphene doping- hydrazine treatment Chemical doping Ammonium formate D-sorbitol doping-TDAE vapor treatment SDBS doping LiTFSI doping Electrochemical doping of ClO4, PF6 and BTFMSI The voltage of 0.2e1.0 V S-PHE as electrolyte Nanowire Nanowire

Continued

Table 5.1 The TE performance of PEDOTs.dcont’d Material

Strategy

s (S cm1)

S (mV K1)

P (mW m1 K2)

k (W m1 K1)

ZT

Refs.

PEDOT:PSS/ PPy/SWCNT PEDOT:Tos

1D PPy as template

e

e

45.3  1.4

e

e

[25]

High boiling point additives Nanofibers, nanotubes and nanocubes H2SO4 treatment Wet-spinning and PEI dedoping PEIE/EG reductionH2SO4 oxidation Ga-ZnO nanoparticles Electric field Electric field Bi0.5Sb1.5Te3 composite PEI as reducing agent Te/SWCNT composite Acido-basic treatment Graphene nanocomposite Thickness-dependent H2SO4 treatment EMImTCM treatment HNO3 treatment-N2 pressure DMSO-vapor annealing

640

34  5

78.5

e

e

[26]

560

4

0.9

0.009

1.75  102

[27]

2500 830

20.6 19

107 30

0.64 e

0.05 e

[5] [28]

2770.7

21.9

133

e

e

[29]

1015 930 1373 1285 100 900.3  20.5 290e970 875.2 18.1 2170  201 1100 1175  23

19.5 16 16.5 49 59 43.4  0.6 15e23 167.2 24.1 37.1  2.9 31.9 32  1

38.4 23.8 38.6 308 34.8 169.8  7.8 26 24.3 1.05 334 117 121  10

e e e 0.19 e e 0.14 0.23 e e e

e e e 0.484 e e e 0.0506 1.37  103 e e e

[30] [31] [32] [33] [34] [9] [35] [36] [37] [38] [39] [40]

496

14.5

9.47

0.17

1.67  102

[41]

PEDOT FS-PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT PEDOT PEDOT-Tos PEDOT-Tos PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS

PEDOT:PSS PEDOT:PSS PEDOT:PSS/ Bi2Te3 PEDOT-TosPPP PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT PEDOT:PSS

PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS

EG-vapor annealing DMSO-vapor annealing EG-vapor annealing

520 126 1026

16.9 24.1 47

14.9 11.3 226

e 0.188 e

e 2.0  102 e

[42] [43] [44]

H2SO4 treatment

1750

14.6

25.7

0.474

0.024

[45]

Oxalic acid treatment Camphorsulfonic acid treatment Oxalic acid treatment H2SO4-vapor treatment Trifluoromethanesulfonic acid treatment Trifluoroacetic acid treatment EG/[BMIm][BF4] treatment Methylammonium iodide in 80 vol.% DMF-20 vol.% water PEG/H2O treatment Formamide two-step treatment DMSO and HI vapor two-step treatment CH3NO, H2SO4, and NaBH4 triple treatment Green DES mixture treatment

8 644.7

40.7 18.1

1.35 21.1

0.11 0.37

3.68  103 0.017

[46] [47]

823 1167 2980

9.2 12.1 21.9

6.96 17.0 142

e e 0.224

e e 0.19

[48] [49] [50]

3748

16.0  1.1

97.1  5.4

0.415

0.1

[51]

869

22.9

45.3

0.37

0.036

[52]

1831

28

144

0.29

0.11

[53]

1415.7  21.2 2929

20.8  1.5 17.4

61.4 88.7

e 0.19

e 0.04

[54] [55]

298

33.02

45.02

e

e

[56]

1786

28.1

141

e

e

[57]

424.2

24.4

25.26

e

e

[58]

124

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Figure 5.1 Phononic (left) bandstructures and (right) total transmission function of the (A) PEDOTsc, (B) PEDOTbulk, and (C) PEDOT:Tos.59 (Copyright 2018, Royal Society of Chemistry.)

the Tos/Tos doping concentration on the TE properties of the pristine PEDOT in virtue of DFT method, and found that the Tos doping simultaneously improved the electrical conductivity and Seebeck coefficient upon an appropriate concentration.60 These theoretical researches might offer deeper understanding of the doping/dedoping processes to optimize the TE conversion capability of organic systems in near future. Xu research group systematically investigated the effective doping of DMSO, EG, and DMSO-urea on the thermoelectric properties of poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS),12e14 and common organic solvents (DMSO, EG, NMP, DMF, IPA, etc.) doped PEDOT:PSS was filtrated to fabricate highly conductive thicknesscontrollable films.15 Meanwhile, they fabricated liquid exfoliated graphene (GE)-doped PEDOT:PSS nanofilm via vacuum filtration with enhanced TE performance, which was attributed to the removal of PSS and the good interaction between PEDOT and GE, and a further enhanced power factor of 53.3 mW m1 K2 was obtained with hydrazine treatment.16 Subsequently, ammonium formate (AF), sorbitol, anionic surfactants, ionic liquids (ILs) were also adopted as dopants in PEDOT:PSS system.17e20 In order to simultaneously increase the electrical conductivity and Seebeck coefficient of PEDOT:PSS, Gong et al. introduced two secondary dopants of DMSO and poly(ethylene oxide) (PEO), the molar ratios of PEDOT to PSS are tuned by PEO, resulting in increased proportions of PEDOT in the bipolaron states,61 which correlates well with the conclusions drawn by Stöcker et al.62 Chemical dedoping of PEDOT:PSS nanofilms by over-coating a mixture of DMSO and

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hydrazine (HZ) was reported, and an optimized power factor of 112 mW m1 K2 by controlling HZ/DMSO ratio, with the corresponding electrical conductivity and Seebeck coefficient of 578 S cm1 and 67 mV K1.63 Encouragingly, a chemically stable dopant dimethylsulfone (DMSO2) was proposed by Yang and coworkers to remarkably raise the electrical conductivity (1080 S cm1) and long-term humidity stability of the PEDOT:PSS film, which was promising for an eco-friendly alternative to the widely employed dopants of DMSO, EG and various acids (Fig. 5.2A). As further verify by XRD in Fig. 5.2B, the enhancement in electrical conductivity was due to polymer alignment induced by crystallization of DMSO2 in the PEDOT:PSS system.64 More recently, Hu and colleagues adopted the triple treatment of ethylene glycol ((CH2OH)2), ethanol (C2H5OH) doping and (CH2OH)2 dedoping to tune the micro stacking structure of PEDOT:PSS, yielding the highest power factor up to 330.597 mW m1, and TEG with p-legs of PEDOT:PSS and n-legs of Ni were successfully fabricated to improve the utilization of solar energy photothermal conversion.65 5.2.2 Electrochemical doping and dedoping Doping of counterions and controlling of potential were effective approaches for electrochemical doping and dedoping. Culebras et al. syn thesized PEDOT doped with ClO 4 , PF6 and bis(trifluoromethylsulfonyl) imide (BTFMSI) by electropolymerization, the electrical conductivity of PEDOT increased by a factor of three due to the stretching of the polymer chains by changing the counterions, while the Seebeck coefficient

Figure 5.2 (A) The electrical conductivity, Seebeck coefficient and power factor at ambient temperature for DMSO2 doped PEDOT:PSS at different loading. (B) XRD patterns of pristine, DMSO2-doped PEDOT:PSS and DMSO2 crystals.64 (Copyright 2019, The Authors.)

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remained at the same order of magnitude, resulting in an optimal ZT value of 0.22 for PEDOT:BTFMSI at room temperature.21 Kim et al. fabricated PEDOT films with efficient TE properties by solution casting polymerization and used as electrodes, the maximum power factor of 1270 mW m1 K2 was obtained by precisely controlling the oxidation level of PEDOT electrochemically, which could be processed as flexible and cuttable TE films to generate electricity by fingertips.66 Additionally, highly conductive PEDOT free-standing films were prepared by electrochemical polymerization at different oxidation levels regarded as bipolaron, polaron and neutral states by varying the voltage of 0.2e1.0 V. Changing the doping states of the PEDOT chains from bipolarons/polarons (0.0 < V < 1.0 V) to neutral states results in changes in the optical properties of the polymer (Fig. 5.3A). As observed in Fig. 5.3B, the electrical conductivity increases from 80 to 766 S cm1, while the Seebeck coefficient decreases from 37.0 to 9.6 mV K1, achieving the maximum TE efficiency of 1.4  102 at 0.01 V.22 Besides, Harima et al. prepared freestanding PEDOT:S-PHE films by galvanostatic polymerization and investigated the TE performances by changing the oxidation level by controlling the applied potential.23 These results provided very simple strategies to optimize the TE performance of PEDOT.

5.3 Low dimensionality One-dimensional (1D) conducting polymers such as PEDOT nanowires (NWs) are very promising as organic TE materials due to the highly

Figure 5.3 (A) UV-Vis spectra and (B) electrical conductivity, Seebeck coefficient and power factor of PEDOT as a function of voltage referred to the Ag/AgCl electrode.22 (Copyright 2015, The Owner Societies.)

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ordered structure induced fast carrier transport. Penner et al. reported the electrical conductivity and Seebeck coefficient of electrodeposited PEDOT NWs, which consistently produced Seebeck coefficient (up to 122 mV K1 at 310 K) higher than those for PEDOT films (57 mV K1). The variation in Seebeck coefficient for PEDOT NWs and films with a wide range of critical dimensions was fully explained by variations in the carrier concentrations in accordance with the Mott equation. In addition to the higher Seebeck coefficient, PEDOT NWs also had higher average electrical conductivity than films, for electron mobilities were greater in nanowires by a factor of 3.7 Zhang et al. systematically investigated the TE transport in ultrathin PEDOT NWs-based films, showing high electrical conductivity of 540 S cm1, and enhanced Seebeck coefficient of 50.55 mV K1 upon hydrazine treatment (2.6 times of that of PEDOT:PSS), achieving a state-of-art power factor of 35.8 mW m1 K2. Furthermore, the origin of s and S enhancements in PEDOT NWs-based films was carefully interpreted. The large S in PEDOT NWs based films stems from the sharp feature of the density of states at Fermi level. The high s mainly results from the significant increment of carrier mobility, which is mainly due to preferred edge-on orientation and high crystallinity of PEDOT chains. As qualitatively verified by grazing incidence wide-angle X-ray scattering (GIWAXS), compared to PEDOT:PSS (Fig. 5.4A), the T-PEDOT NWs (Fig. 5.4B) and C-PEDOT NWs (Fig. 5.4C) exhibited several sharp diffraction peaks, and the peak at qw1.82 Å1 held higher intensity at qxy axis (Fig. 5.4E) than in qz axis (Fig. 5.4D) for the C-PEDOT NWs film, indicating a preferential edge-on orientation instead of face-on orientation.24 More recently, PEDOT:PSS/PPy/SWCNT composites were designed in virtue of 1D nanostrutured PPy as template, revealing dramatically-enhanced TE performance with an optimal power factor of 45.3  1.4 mW m1 K2.25

5.4 Crystal structure Transport properties of conducting polymers are driven by the polymer structure, and higher polymer crystallinity results in higher carrier mobility.67 Fleury research group observed that processing PEDOT:Tos with high boiling point additives such as DMSO, EG, DMF, etc. can result in more crystalline films, leading to an increase of the power factor from 25 to 78.5 mW m1 K2 with electrical conductivity three-fold maximum improved and Seebeck coefficient remained unaffected. It was demonstrated

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Figure 5.4 2D GIWAXS patterns of PEDOT:PSS and PEDOT NWs-based thin films. GIWAXS patterns obtained for (A) PEDOT:PSS, (B) T-PEDOT NWs, (C) C-PEDOT NWs films; 1D scattering profiles in the near out-of-plane qz (D) and in-plane qxy (E) direction with three samples.24 (Copyright 2018, Elsevier Ltd.)

that electrical conductivity was driven solely by carrier mobility changes, and the behavior of mobility was dictated by the structural properties of the PEDOT:Tos films, and specifically by the thin film crystallinity combined to the preferential edge-on orientation of the PEDOT crystallites.26 Khoso et al. synthesized PEDOT nanofibers, nanotubes, and nanocubes with various monomer to oxidant (PEDOT:FeCl3) ratio of 1:3, 1:6, and 1:9, among which 3D nanocubes showed noticeably higher TE performance due to regular and controlled shapes. The resultant TE nanogenerator showed power factor of 0.9 mW m1 K2 at room temperature, which was promising for harvesting energy from the human body heat.27 These studies underlined the substantial importance of structural fine-tuning through processing methods. Free-standing conducting polymer PEDOT:PSS films with high power factor of 107 mW K2 m1 was demonstrated with an output power density of 99  18.7 mW cm2 in a corresponding free-standing TE device under a temperature gradient of 29 K, attributing to the high conductivity

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of the thick PEDOT:PSS films derived from the ordered stacking structure of the PEDOT chains and the absence of a substrate. In addition to the high power factor, the high thermal stability in chemical as well as crystalline structure of the PEDOT:PSS films up to 250 C was confirmed by in situ temperature-dependent X-ray diffraction and GIWAXS, making PEDOT:PSS films promising candidates for thermoelectric applications.5 Recently, Yoon and coworkers prepared robust conducting polymer fibers by wet-spinning of PEDOT:PSS into sulfuric acid, achieving an electrical conductivity of 830 S cm1 and a thermoelectric power factor of 30 mV m1 K2. In conjunction with excellent stability upon water exposure, semicrystalline PEDOT:PSS fibers featured a Young’s modulus of up to 1.9 GPa and a high degree of mechanical robustness. On this basis, an embroidered TE module was constructed, which illustrated their usefulness for the construction of energy-harvesting textiles with promising TE performance.28 Xu et al. reported a good PEDOT-rich nanocrystal achieved by sequential dipping process in polyethylenimine ethoxylated (PEIE)/EG and H2SO4 solutions. As illustrated in Fig. 5.5, the ethanol-dilution induced the efficient phase separation between PEDOT and PSS as well as the aggregation of PEDOT crystals, and the PEIE/EG dipping resulted in a slight increase in the p-p stacking direction of PEDOT backbone but no obvious effect on the lamellar distance, and the further H2SO4 dipping significantly decrease in both p-p stacking and lamellar spacing distances leading to a highly ordered PEDOT-rich nanocrystal film. A large TE power factor of 133 mW m1 K2 can be ascribed to the good formation of PEDOT-rich nanocrystal and an effective compromise between s and S of PEDOT:PSS films.29 Shiraishi et al. reported a PEDOT-PSS film including Ga-ZnO

Figure 5.5 The molecular packing arrangement and the crystalline domain in PEDOT:PSS films after dipping in PEIE/EG and H2SO4.29 (Copyright 2019, American Chemical Society.)

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(GZO) nanoparticles, and the PEDOT-PSS film containing GZO nanoparticle of one atom% Ga exhibited a higher in-plane TE power factor (38.4 mW m1 K2) than the pure PEDOT:PSS (27.4 mW m1 K2). It suggested that the Zn and Ga components were an efficient carrier promoter for improving the crystal growth and conformation of PEDOT.30 Yamaguchi research group synthesized PEDOT:PSS films by applying an electric fields Epr up to 4 kV cm1 perpendicular to the substrate during film formation, and investigated their TE properties. With increase of Epr, the electrical conductivity increased from 1.5 S cm1 for Epr ¼ 0 kV cm1 to 8.0 S cm1 for Epr ¼ 1 kV cm1, then saturated for Epr above 1 kV cm1, while the Seebeck coefficient was almost constant at 16 mV K1 for all Epr values. Thus, the power factor improved from 0.04 mW m1 K2 for Epr ¼ 0 kV cm1 to 0.20 mW m1 K2 for Epr ¼ 1 kV cm1. XPS analysis showed that the molecular ratio of PEDOT to PSS molecules was significantly increased by applying an electric field during film formation. These results suggested that the electric field promoted PEDOT:PSS crystal growth near the film surface, resulting in improved TE properties due to enhanced carrier mobility.31 Similar conclusion was drawn by Chonan et al.32

5.5 Phonon scattering Kim and his coworkers fabricated Bi0.5Sb1.5Te3 (BST)-PEDOT:PSS nanocomposites and optimized the TE performance. The interfacial phonon scattering at the low-dimensionality filler surface dramatically reduced the thermal conductivity, while the pathway for charge carriers is maintained at the interfaces not to decrease the electrical conductivity. The BST-PEDOT:PSS nanocomposite achieved an electrical conductivity 1285 S cm1 and a Seebeck coefficient of 49 mV K1. The corresponding power factor was 308 mW m1 K2 to yield a recording ZT value of 0.484 at room temperature among the organicinorganic TE materials, which was promising fulfilling the requirement for efficient waste energy harvesting at low temperatures.33 Shuai et al. presented that the inclusion of the counterions in the host matrix also altered the phonon scattering effect, mainly because different packing structures between pristine PEDOT and doped PEDOT:Tos, with intercalation of the counterions in the latter, lead to different elastic constants.8

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5.6 Molecular conformation Xu and coworkers combined vapor-phase polymerization and H2SO4 post-treatment to effectively tune the TE performance of PEDOT.34 Similarly, Yin et al. combined dynamic three-phase interfacial electropolymerization of EDOT with physical mixing of single-walled carbon nanotubes (SWCNTs) and tellurium (Te) nanowires to prepare PEDOT/ Te/SWCNT composites, affording the optimal power factor of 169.8  7.8 mW m1 K2. The electrochemical polymerized PEDOT with ordered arrangement integrating the addition of Te and SWCNT benefited the carrier transportation, producing significant effect on the improvement of TE performance.38 Crispin research group investigated the variation in the TE performance of PEDOT-Tos by the acido-basic treatment, and identified that changing the pH tuned the oxidation level of the polymer. The electrical conductivity decreased from acidic to basic treatments, which was attributed to structural transformation from quinoid to benzoid and ring opening along the PEDOT chains.35 In addition, PEDOT-Tos-graphene nanocomposites were synthesized, and degree of structural order increased with gradual incorporation of graphene, which increased the hopping rate within the polymer matrix and reduced the pep conjugation defects in the backbone of the polymer chains, increasing the charge carrier mobility of the synthesized samples, therefore enhancing the electrical conductivity.36 Choi and coworkers investigated the thickness-dependent change in the TE properties of PEDOT:PSS nanofilms, and both the conductivity and Seebeck coefficient improved with increasing thickness of the nanofilms, due to the conformation change of PEDOT that exposed the PEDOT on the surface of the PEDOT:PSS phase, and a maximum ZT of 1.37  103 was achieved.37 Similar conclusion was drawn on single and multiple layered PEDOT:PSS films.68 Ouyang’s group indicated that the treated CleviosTM PH1000 films showed both superior electrical conductivity and Seebeck coefficient over the treated CleviosTM P films, which was attributed to longer PEDOT chains and large PEDOT domains of the treated CleviosTM PH1000.38 In the case of solvent-mixed flexible PEDOT:PSS films, the selective eviction of PSS from typical PEDOT:PSS core-shell structure caused conformational change in PEDOT chains from benzoid to quinoid structure, and re-arranged PSS in more stretched form.69 The structural ordering tuning

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and oxidation level adjustment of PEDOT:PSS in molecular scale synergistically improved the TE and mechanical properties also discussed in the most recently research.39 The influence of N2 pressure on the structure and TE properties of acidtreated PEDOT:PSS was studied, resulting in enhanced electrical conductivity and superior TE properties. The enhancement in electrical conductivity was attributed to the removal of insulating PSS and the conformational change of the PEDOT chain from benzoid to quinoid structure, and the N2 pressure was responsible for the additional conformation of PEDOT chain favoring the linear orientation of the structure.40 Furthermore, Vuong et al. demonstrated the static magnetic-field-induced alignment of PEDOT:PSS particles, and the thin films cast under modest mT-level magnetic fields exhibited doubled electrical conductivity and a four-fold increase in the Seebeck coefficient. High-resolution scanning electron microscopy identified a consistent structural coil-to-rod transition, as shown in Fig. 5.6.70

Figure 5.6 Experimental setup with deposited PEDOT:PSS (blue) showing control samples heated on a Peltier module and substrate-perpendicular and substrateparallel alignments of the magnetic field. In three-dimensional (3D) TOF-SIMS images (300  300  4 mm3), the fragments of PEDOT [red, molecular weight (MW) ¼ 41] are distinguished from the PSS and indium tin oxide (ITO) coating (green, MW ¼ 115). SEM micrographs depict morphology of the top surface.70 (Copyright 2018, American Chemical Society.)

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5.7 Posttreatment Posttreatment by secondary dopants including polar organic solvents, acids or alkalis, inorganic salts, nonionic surfactant polymers, and reducing reagents as well as thermal annealing displayed satisfying effects to enhance the TE performance of PEDOTs.71e79 5.7.1 Polar organic solvents Xu research group introduced solvent vapor (DMSO, EG, DMF, NMP, and H2O) annealing (PSVA) method into the preparation of PEDOT:PSS films, which exhibited significantly enhanced electrical conductivity up to 496 S cm1, thereby a maximum power factor of 9.47 mW m1 K2 without distinct change of Seebeck coefficient. The enhancement was attributed to the smoother morphologies and simultaneous increase of carrier mobility and carrier concentration of PSVA-treated PEDOT:PSS films.41 Similar treatment of PEDPT:PSS films by DMSO, EG, NMP, DMF to improve the TE performance was also reported.42e44 Additionally, Urban et al. firstly fabricated PEDOT:PSS aerogels via freeze-drying technique and the TE properties were investigated upon treatment with EG, achieving the optimal power factors 6.8 mW m1 K2 (Fig. 5.7).80 5.7.2 Acids or alkalis PEDOT-Tos-polypropylene glycol-polyethylene glycol (PEDOT-TosPPP) films were vapor-phase polymerized, and the TE performance (ZTw0.016) was significantly improved by treating with H2SO4 at different concentrations, attributing to the combined effects of the removal of the insulating PPP from the PEDOT-Tos-PPP film, and the increase of the doping level and conformational change of the PEDOT chains resulted from the H2SO4 treatment.45 Xu et al. systematically investigated dependence of the TE performance of free-standing PEDOT:PSS films on the pH value treated by acids (oxalic acid and HCl) and alkalis (NaOH, EDA and NH4OH). The TE performance was enhanced by decreasing the pH value, and it decreased rapidly with the increase in pH value, achieving the maximum power factor of 1.35 mW m1 K2.46 Enhancement of TE performance of PEDOT:PSS films by posttreatment with camphorsulfonic acid, oxalic acid, sulfuric acid, trifluoromethanesulfonic acid, and trifluoroacetic acid have also been reported.47e51 Furthermore, Ouyang et al. proposed sequential posttreatments of PEDOT:PSS with H2SO4 and

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Figure 5.7 Thermoelectric measurements of both PEDOT:PSS aerogels and thick films. (A) Conductivity of both the aerogels and thick films show similar trends of increased conductivity based upon solvent treatment. After correcting for density differences between the thick films and aerogels, the conductivities are close in magnitude. (B) Seebeck coefficient remains relatively constant regardless of solvent treatment. (C) Power factors for thick films are two to four times greater than those for uncorrected aerogel samples.80 (Copyright 2016, Wiley.)

NaOH, and the TE performance was remarkably enhanced after the treatments, yielding the optimal power factor of 334 mW m1 K2, with the corresponding electrical conductivity and Seebeck coeffcient of 2170 S cm1 and 39.2 mV K1. It suggested that the enhancement was

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Figure 5.8 (A) UV-vis-NIR spectra vary at different NaOH concentrations. (B) Analysis of resistance-temperature relationship of the untreated, H2SO4-treated, and H2SO4e NaOH treated PEDOT:PSS films with the VRH model.81 (Copyright 2017, Wiley.)

attributed to the synergetic effect of the optimal oxidation level tuned by the base treatment (Fig. 5.8A) and the high charge mobility by the acid treatment (Fig. 5.8B). 5.7.3 Humidity conditions Wei et al. studied the effect of water content on the TE properties of PEDOT:PSS, and found an increase in the power factor from 23  5 to 225  130 mW m1 K2 in high-humidity conditions. The enhancement was mainly due to the apparent Seebeck coefficient improvement related to morphological change after water absorption or electrochemical reaction of PEDOT in air. It indicated the necessity for well-controlled measurement conditions particularly humidity during evaluating the TE performance.82 Then, humidity-dependent TE properties of PEDOT:PSS were investigated by Pipe and coworkers.83 5.7.4 Mixture treatments Xu research group post-treated the vapor-phase polymerized PEDOT film with mixed EG/[BMIm][BF4] solvent, which displayed the optimal power factor of 45.3 mW m1 K2 along with enhanced electrical conductivity of 869 S cm1 and Seebeck coefficient of 22.9 mV K1.52 Ouyang and coworkers proposed a facile approach to enhance the TE properties of PEDOT:PSS through a treatment with cosolvents of organic solvent and water or solutions consisted of cosolvent and organic salt. The electrical conductivity and Seebeck coefficient of PEDOT:PSS reached 1831 S cm1 and 28 mV K1 when treated with a solution of 0.1 M methylammonium

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iodide in 80 vol.% DMF-20 vol.% water, yielding an optimal power factor of 144 mW m1 K2. It indicated that the treatments increased the charge mobility of the PEDOT:PSS films by partially depleting PSS from PEDOT:PSS and causing the conformational change of PEDOT chains.53 Similar conclusion was obtained by Whang and colleagues.84 Most recently, a novel binary treatment method by using polyethylene glycol (PEG) and water was reported to significantly enhance the conductivity of the PEDOT:PSS thick films. The soaked PEDOT:PSSfilm with 4%PEG200 showed a power factor of 61.4 mW m1 K2, with the corresponding electrical conductivity and Seebeck coefficient of 1415.7  21.2 S cm1 and 20.8  1.5 mV K1. The enhancement was resulted from the synergetic effect of PEG and water induced the conformation change of PEDOT chains, the phase segregation between PEDOT and PSS and the removal of PSS with PEG.54 5.7.5 Multistep processing The sequential treatment of drop followed by dip or dip followed by drop using a polar solvent formamide was reported to improve the TE performance of PEDOT:PSS. The electrical conductivity significantly increased from 0.33 S cm1 for the pristine PEDOT:PSS film to 2929 S cm1 for the treated film, and the Seebeck coefficient maintained as 17.4 mV K1, resulting in a power factor of 88.7 mW m1 K2.55 Two-step treatment of PEDOT:PSS films was also investigated by Fu et al.78 and Jang et al.85 Furthermore, triple post-treatments with CH3NO, H2SO4, and NaBH4 in sequence was adopted to engineer flexible PEDOT:PSS TE films, showing a high power factor of 141 mW m1 K2, which was stemmed from the high electrical conductivity of 1786 S cm1 and Seebeck coefficient of 28.1 mV K1 (Fig. 5.9A). Both the CH3NO and H2SO4 treatments contribute to the selective removal of excess PSS acting as insulating domains within the films, while the H2SO4 treatment also causes a conformation change in the PEDOT chains from benzoid to quinoid character due to an oxidative dehydrogenation process (Fig. 5.9B).57 5.7.6 Environment-friendly posttreatment Xu et al. demonstrated a green surface treatment reagent of DES (ChCl/ urea fluid) to improve the TE performance of PEDOT:PSS films. Simultaneous increases were found in the electrical conductivity and Seebeck coefficient, achieving the values of 85.6 S cm1 and 30.1 mV K1 by raising

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Figure 5.9 (A) TE performance of PEDOT:PSS treated with CH3NO for 15 min two times and with H2SO4 for 10 h, and with NaBH4 with various concentrations sequentially. (B) XPS results for PEDOT:PSS films with different treating conditions with inset showing an enlarged view of XPS near 164 eV.57 (Copyright 2019, American Chemical Society.)

the treating temperature to 120 C.58 Aqueous vitamin C solution as an environment-friendly reducing agent was also employed for tuning the TE properties of p-toluenesulfonatedoped PEDOT-Tos films. A 42% increase in ZT value was determined for the 5% aqueous vitamin C solution-treated PEDOT-Tos films with respect to that of the untreated films, mainly attributing to the enhancement of Seebeck coefficient.86

5.8 Concluding remarks Effective approaches to optimize TE performance of PEDOTs including doping/dedoping, low dimensionality, crystal structure, phonon scattering, molecular conformation, and posttreatment were systematically summarized. From the theoretical point of view, the main feasible strategies to further enhance the TE performance of PEDOTs include the following three points: (1) Breaking the “universal” antagonistic correlation between the electrical conductivity and Seebeck coefficient in virtue of precisely controlling of the microscopic morphology of PEDOT films, achieving simultaneous increase in both parameters. (2) Exploring facile strategies to go from thin films to thick films while maintaining good TE properties, creating vertical legs in TE modules for electricity generation or cooling. (3) Gaining insight into the mechanism for the TE performance optimization of PEDOTs, providing the guidance and future research focus on further TE performance enhancement and flexible TE devices construction.

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References 1. Petsagkourakis, I.; Kim, N.; Tybrandt, K.; Zozoulenko, I.; Crispin, X. Poly(3,4-ethylenedioxythiophene): Chemical Synthesis, Transport Properties, and Thermoelectric Devices. Adv. Electron. Mater. 2019, 5, 1800918. 2. Song, H.; Meng, Q.; Lu, Y.; Cai, K. Progress on PEDOT:PSS/nanocrystal Thermoelectric Composites. Adv. Electron. Mater. 2019, 5, 1800822. 3. Xu, Y.; Jia, Y.; Liu, P.; Jiang, Q.; Hu, D.; Ma, Y. Poly(3,4-ethylenedioxythiophene) (PEDOT) as Promising Thermoelectric Materials and Devices. Chem. Eng. J. 2021, 404, 126552. 4. Zhu, Z.; Liu, C.; Jiang, F.; Xu, J.; Liu, E. Effective Treatment Methods on PEDOT:PSS to Enhance its Thermoelectric Performance. Synth. Met. 2017, 225, 31. 5. Li, Z.; Sun, H.; Hsiao, C.-L.; Yao, Y.; Xiao, Y.; Shahi, M.; Jin, Y.; Cruce, A.; Liu, X.; Jiang, Y.; Meng, W.; Qin, F.; Ederth, T.; Fabiano, S.; Chen, W. M.; Lu, X.; Birch, J.; Brill, J. W.; Zhou, Y.; Crispin, X.; Zhang, F. A Free-Standing High-Output Power Density Thermoelectric Device Based on Structure-Ordered PEDOT:PSS. Adv. Electron. Mater. 2018, 4, 1700496. 6. Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 7. Taggart, D. K.; Yang, Y.; Kung, S.-C.; McIntire, T. M.; Penner, R. M. Enhanced Thermoelectric Metrics in Ultra-long Electrodeposited PEDOT Nanowires. Nano Lett. 2011, 11, 2192. 8. Shi, W.; Zhao, T.; Xi, J.; Wang, D.; Shuai, Z. Unravelling Doping Effects on PEDOT at the Molecular Level: from Geometry to Thermoelectric Transport Properties. J. Am. Chem. Soc. 2015, 137, 12929. 9. Yin, S.; Lu, W.; Wu, R.; Fan, W.; Guo, C.-Y.; Chen, G. Poly(3,4ethylenedioxythiophene)/Te/single-walled Carbon Nanotube Composites with High Thermoelectric Performance Promoted by Electropolymerization. ACS Appl. Mater. Interfaces 2020, 12, 3547. 10. Fan, Z.; Du, D.; Guan, X.; Ouyang, J. Polymer Films with Ultrahigh Thermoelectric Properties Arising from Significant Seebeck Coefficient Enhancement by Ion Accumulation on Surface. Nanomater. Energy 2018, 51, 481. 11. Wang, J.; Cai, K.; Shen, S. A Facile Chemical Reduction Approach for Effectively Tuning Thermoelectric Properties of PEDOT Films. Org. Electron. 2015, 17, 151. 12. Jiang, F.; Xu, J.; Lu, B.; Xie, Y.; Huang, R.; Li, L. Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate). Chin. Phys. Lett. 2008, 25, 2202. 13. Liu, C.; Lu, B.; Yan, J.; Xu, J.; Yue, R.; Zhu, Z.; Zhou, S.; Hu, X.; Zhang, Z.; Chen, P. Highly Conducting Free-Standing Poly(3,4-Ethylenedioxythiophene)/poly(styrenesulfonate) Films with Improved Thermoelectric Performances. Synth. Met. 2010, 160, 2481. 14. Kong, F.; Liu, C.; Xu, J.; Huang, Y.; Wang, J.; Sun, Z. Thermoelectric Performance Enhancement of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) Composite Films by Addition of Dimethyl Sulfoxide and Urea. J. Electron. Mater. 2012, 41, 2431. 15. Xiong, J.; Jiang, F.; Zhou, W.; Liu, C.; Xu, J. Highly Electrical and Thermoelectric Properties of a PEDOT:PSS Thin-Film via Direct Dilution-Filtration. RSC Adv. 2015, 5, 60708. 16. Xiong, J.; Jiang, F.; Shi, H.; Xu, J.; Liu, C.; Zhou, W.; Jiang, Q.; Zhu, Z.; Hu, Y. Liquid Exfoliated Graphene as Dopant for Improving the Thermoelectric Power Factor of Conductive PEDOT:PSS Nanofilm with Hydrazine Treatment. ACS Appl. Mater. Interfaces 2015, 7, 14917.

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17. Tsai, T.-C.; Chang, H.-C.; Chen, C.-H.; Whang, W.-T. Widely Variable Seebeck Coefficient and Enhanced Thermoelectric Power of PEDOT:PSS Films by Blending Thermal Decomposable Ammonium Formate. Org. Electron. 2011, 12, 2159. 18. Yang, E.; Kim, J.; Jung, B. J.; Kwak, J. Enhanced Thermoelectric Properties of SorbitolMixed PEDOT:PSS Thin Films by Chemical Reduction. J. Mater. Sci. Mater. Electron. 2015, 26, 2838. 19. Kishi, N.; Kondo, Y.; Kunieda, H.; Hibi, S.; Sawada, Y. Enhancement of Thermoelectric Properties of PEDOT:PSS Thin Films by Addition of Anionic Surfactants. J. Mater. Sci. Mater. Electron. 2018, 29, 4030. 20. Li, Q.; Deng, M.; Zhang, S.; Zhao, D.; Jiang, Q.; Guo, C.; Zhou, Q.; Liu, W. Synergistic Enhancement of Thermoelectric and Mechanical Performances of Ionic Liquid LiTFSI Modulated PEDOT Flexible Films. J. Mater. Chem. C 2019, 7, 4374. 21. Culebras, M.; Gómez, C. M.; Cantarero, A. Enhanced Thermoelectric Performance of PEDOT with Different Counter-ions Optimized by Chemical Reduction. J. Mater. Chem. 2014, 2, 10109. 22. Culebras, M.; Uriol, B.; Gomez, C. M.; Cantarero, A. Controlling the Thermoelectric Properties of Polymers: Application to PEDOT and Polypyrrole. Phys. Chem. Chem. Phys. 2015, 17, 15140. 23. Zhang, L.; Goto, T.; Imae, I.; Sakurai, Y.; Harima, Y. Thermoelectric Properties of PEDOT Films Prepared by Electrochemical Polymerization. J. Polym. Sci., Polym. Phys. Ed. 2017, 55, 524. 24. Zhang, J.; Zhang, K.; Xu, F.; Wang, S.; Qiu, Y. Thermoelectric Transport in Ultrathin Poly(3,4-Ethylenedioxythiophene) Nanowire Assembly. Compos. B Eng. 2018, 136, 234. 25. Liang, L.; Fan, J.; Wang, M.; Chen, G.; Sun, G. Ternary Thermoelectric Composites of polypyrrole/PEDOT:PSS/carbon Nanotube with Unique Layered Structure Prepared by One-Dimensional Polymer Nanostructure as Template. Compos. Sci. Technol. 2020, 187, 107948. 26. Petsagkourakis, I.; Pavlopoulou, E.; Portale, G.; Kuropatwa, B. A.; Dilhaire, S.; Fleury, G.; Hadziioannou, G. Structurally-driven Enhancement of Thermoelectric Properties within Poly(3,4-Ethylenedioxythiophene) Thin Films. Sci. Rep. 2016, 6, 30501. 27. Khoso, N. A.; Ahmed, A.; Deb, H.; Tian, S.; Jiao, X.; Gong, X. Y.; Wang, J. Controlled Template-free In-Situ Polymerization of PEDOT for Enhanced Thermoelectric Performance on Textile Substrate. Org. Electron. 2019, 75, 105368. 28. Kim, Y.; Lund, A.; Noh, H.; Hofmann, A. I.; Craighero, M.; Darabi, S.; Zokaei, S.; Park, J. I.; Yoon, M. H.; Müller, C. Robust PEDOT:PSS Wet-spun Fibers for Thermoelectric Textiles. Macromol. Mater. Eng. 2020, 305, 1900749. 29. Li, X.; Liu, C.; Zhou, W.; Duan, X.; Du, Y.; Xu, J.; Li, C.; Liu, J.; Jia, Y.; Liu, P.; Jiang, Q.; Luo, C.; Liu, C.; Jiang, F. Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene)Rich Nanocrystal Films. ACS Appl. Mater. Interfaces 2019, 11, 8138. 30. Hata, S.; Taguchi, K.; Oshima, K.; Du, Y.; Shiraishi, Y.; Toshima, N. Preparation of Ga-ZnO Nanoparticles Using Microwave and Ultrasonic Irradiation, and the Application of Poly(3,4-Ethylenedioxythiophene)-poly(styrenesulfonate) Hybrid Thermoelectric Films. Chemistry 2019, 4, 6800. 31. Aizawa, K.; Chonan, Y.; Komiyama, T.; Yamaguchi, H.; Aoyama, T. Enhancement of Thermoelectric Properties of PEDOT:PSS Films Prepared under an Electric Field. J. Electron. Mater. 2017, 46, 5145. 32. Chonan, Y.; Sato, N.; Komiyama, T.; Kotani, K.; Yamaguchi, H. Enhancement of Thermoelectric Properties of PEDOT:PSS Films by Applying an Alternating Electric Field during Preparation. J. Electron. Mater. 2019, 48, 3854.

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33. Lim, J. Y.; Cho, S.; Kim, H.; Seo, Y. Optimum Thermoelectric Performance of Bismuth-Antimony-Telluride alloy/PEDOT:PSS Nanocomposites Prepared by an Innovative Redox Process. ACS Appl. Energy Mater. 2019, 2, 8219. 34. Jia, Y.; Liu, C.; Liu, J.; Liu, C.; Xu, J.; Li, X.; Shen, L.; Jiang, Q.; Wang, X.; Yang, J.; Jiang, F. Efficient Enhancement of the Thermoelectric Performance of Vapor Phase Polymerized Poly(3,4-Ethylenedioxythiophene) Films with Poly(ethyleneimine). J. Polym. Sci., Polym. Phys. Ed. 2019, 57, 257. 35. Khan, Z. U.; Bubnova, O.; Jafari, M. J.; Brooke, R.; Liu, X.; Gabrielsson, R.; Ederth, T.; Evans, D. R.; Andreasen, J. W.; Fahlman, M.; Crispin, X. Acido-basic Control of the Thermoelectric Properties of Poly(3,4-Ethylenedioxythiophene)tosylate (PEDOT-Tos) Thin Films. J. Mater. Chem. C 2015, 3, 10616. 36. Maity, S.; Kulsi, C.; Banerjee, S.; Das, S.; Chatterjee, K. Dependence of Thermoelectric Power and Electrical Conductivity on Structural Order of PEDOT-Tos-Graphene Nanocomposite via Charge Carrier Mobility. Mater. Res. Express 2019, 6, 105095. 37. Yoo, D.; Son, W.; Kim, S.; Lee, J. J.; Lee, S. H.; Choi, H. H.; Kim, J. H. Gradual Thickness-dependent Enhancement of the Thermoelectric Properties of PEDOT:PSS Nanofilms. RSC Adv. 2014, 4, 58924. 38. Fan, Z.; Du, D.; Yao, H.; Ouyang, J. Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric Property of PEDOT:PSS: a Comparative Study of Clevios P and Clevios PH1000. ACS Appl. Mater. Interfaces 2017, 9, 11732. 39. Li, Q.; Zhou, Q.; Wen, L.; Liu, W. Enhanced Thermoelectric Performances of Flexible PEDOT:PSS Film by Synergistically Tuning the Ordering Structure and Oxidation State. J. Materiomics 2020, 6, 119. 40. Myint, M. T. Z.; Inoue, H.; Ichimura, S.; Nishikawa, T.; Nishina, Y.; Kyaw, A. K. K.; Hayashi, Y. Influence of Pressure of Nitrogen Gas on Structure and Thermoelectric Properties of Acid-Treated PEDOT:PSS Films. J. Mater. Sci. Mater. Electron. 2019, 30, 13534. 41. Jiang, Q.; Liu, C.; Song, H.; Shi, H.; Yao, Y.; Xu, J.; Zhang, G.; Lu, B. Improved Thermoelectric Performance of PEDOT:PSS Films Prepared by Polar-Solvent Vapor Annealing Method. J. Mater. Sci. Mater. Electron. 2013, 24, 4240. 42. Liu, E.; Liu, C.; Zhu, Z.; Shi, H.; Jiang, Q.; Jiang, F.; Xu, J.; Xiong, J.; Hu, Y. Enhanced Thermoelectric Performance of PEDOT:PSS Films by Solvent Thermal Treatment. J. Polym. Res. 2015, 22, 240. 43. Wang, X.; Liu, P.; Jiang, Q.; Zhou, W.; Xu, J.; Liu, J.; Jia, Y.; Duan, X.; Liu, Y.; Du, Y.; Jiang, F. Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT:PSS-based Aerogel. ACS Appl. Mater. Interfaces 2018, 11, 2408. 44. Kim, W. S.; Anoop, G.; Jeong, I.-S.; Lee, H. J.; Kim, H. B.; Kim, S. H.; Goo, G. W.; Lee, H.; Lee, H. J.; Kim, C.; Lee, J.-H.; Mun, B. S.; Park, J.-W.; Lee, E.; Jo, J. Y. Feasible Tuning of Barrier Energy in PEDOT:PSS/Bi2Te3 Nanowires-Based Thermoelectric Nanocomposite Thin Films through Polar Solvent Vapor Annealing. Nanomater. Energy 2020, 67, 104207. 45. Wang, J.; Cai, K.; Shen, S. Enhanced Thermoelectric Properties of Poly(3,4Ethylenedioxythiophene) Thin Films Treated with H2SO4. Org. Electron. 2014, 15, 3087. 46. Kong, F.; Liu, C.; Song, H.; Xu, J.; Huang, Y.; Zhu, H.; Wang, J. Effect of Solution pH Value on Thermoelectric Performance of Free-Standing PEDOT:PSS Films. Synth. Met. 2013, 185e186, 31. 47. Song, H.; Kong, F.; Liu, C.; Xu, J.; Jiang, Q.; Shi, H. Improved Thermoelectric Performance of PEDOT:PSS Film Treated with Camphorsulfonic Acid. J. Polym. Res. 2013, 20, 316. 48. Liu, C.; Shi, H.; Xu, J.; Jiang, Q.; Song, H.; Zhu, Z. Improved Thermoelectric Properties of PEDOT:PSS Nanofilms Treated with Oxalic Acid. J. Electron. Mater. 2014, 44, 1791.

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49. Kim, J.; Jang, J. G.; Hong, J.-I.; Kim, S. H.; Kwak, J. Sulfuric Acid Vapor Treatment for Enhancing the Thermoelectric Properties of PEDOT:PSS Thin-Films. J. Mater. Sci. Mater. Electron. 2016, 27, 6122. 50. Wang, X.; Kyaw, A. K. K.; Yin, C.; Wang, F.; Zhu, Q.; Tang, T.; Yee, P. I.; Xu, J. Enhancement of Thermoelectric Performance of PEDOT:PSS Films by Post-treatment with a Superacid. RSC Adv. 2018, 8, 18334. 51. Yemata, T. A.; Kyaw, A. K. K.; Zheng, Y.; Wang, X.; Zhu, Q.; Chin, W. S.; Xu, J. Enhanced Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene):poly(4styrenesulfonate) (PEDOT:PSS) with Long-term Humidity Stability via Sequential Treatment with Trifluoroacetic Acid. Polym. Int. 2019, 69, 84. 52. Jia, Y.; Li, X.; Jiang, F.; Li, C.; Wang, T.; Jiang, Q.; Hou, J.; Xu, J. Effects of Additives and Post-treatment on the Thermoelectric Performance of Vapor-phase Polymerized PEDOT Films. J. Polym. Sci., Polym. Phys. Ed. 2017, 55, 1738. 53. Zhang, S.; Fan, Z.; Wang, X.; Zhang, Z.; Ouyang, J. Enhancement in the Thermoelectric Properties of PEDOTPSS via One-step Treatment with Cosolvents or Their Solutions of Organic Salts. J. Mater. Chem. 2018, 6, 7080. 54. Sun, Z.; Shu, M.; Li, W.; Li, P.; Zhang, Y.; Yao, H.; Guan, S. Enhanced Thermoelectric Performance of PEDOT:PSS Self-Supporting Thick Films through a Binary Treatment with Polyethylene Glycol and Water. Polymer 2020, 192, 122328. 55. Kyaw, A. K. K.; Yemata, T. A.; Wang, X.; Lim, S. L.; Chin, W. S.; Hippalgaonkar, K.; Xu, J. Enhanced Thermoelectric Performance of PEDOT:PSS Films by Sequential Post-treatment with Formamide. Macromol. Mater. Eng. 2018, 303, 1700429. 56. Zhang, L.; Deng, H.; Liu, S.; Zhang, Q.; Chen, F.; Fu, Q. Enhanced Thermoelectric Properties of PEDOTPSS Films via a Novel Two-step Treatment. RSC Adv. 2015, 5, 105592. 57. Xu, S.; Hong, M.; Shi, X.-L.; Wang, Y.; Ge, L.; Bai, Y.; Wang, L.; Dargusch, M.; Zou, J.; Chen, Z.-G. High-performance PEDOT:PSS Flexible Thermoelectric Materials and Their Devices by Triple Post-treatments. Chem. Mater. 2019, 31, 5238. 58. Zhu, Z.; Liu, C.; Jiang, Q.; Shi, H.; Xu, J.; Jiang, F.; Xiong, J.; Liu, E. Green DES Mixture as a Surface Treatment Recipe for Improving the Thermoelectric Properties of PEDOT:PSS Films. Synth. Met. 2015, 209, 313. 59. Cigarini, L.; Ruini, A.; Catellani, A.; Calzolari, A. Conflicting Effect of Chemical Doping on the Thermoelectric Response of Ordered PEDOT Aggregates. Phys. Chem. Chem. Phys. 2018, 20, 5021. 60. Zhang, B.; Wang, K.; Li, D.; Cui, X. Doping Effects on the Thermoelectric Properties of Pristine Poly(3,4-Ethylenedioxythiophene). RSC Adv. 2015, 5, 33885. 61. Yi, C.; Wilhite, A.; Zhang, L.; Hu, R.; Chuang, S. S.; Zheng, J.; Gong, X. Enhanced Thermoelectric Properties of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) by Binary Secondary Dopants. ACS Appl. Mater. Interfaces 2015, 7, 8984. 62. Stöcker, T.; Köhler, A.; Moos, R. Why Does the Electrical Conductivity in PEDOT:PSS Decrease with PSS Content? A Study Combining Thermoelectric Measurements with Impedance Spectroscopy. J. Polym. Sci., Polym. Phys. Ed. 2012, 50, 976. 63. Park, H.; Lee, S. H.; Kim, F. S.; Choi, H. H.; Cheong, I. W.; Kim, J. H. Enhanced Thermoelectric Properties of PEDOTPSS Nanofilms by a Chemical Dedoping Process. J. Mater. Chem. 2014, 2, 6532. 64. Zhu, Q.; Yildirim, E.; Wang, X.; Soo, X. Y. D.; Zheng, Y.; Tan, T. L.; Wu, G.; Yang, S. W.; Xu, J. Improved Alignment of PEDOT:PSS Induced by In-Situ Crystallization of “green” Dimethylsulfone Molecules to Enhance the Polymer Thermoelectric Performance. Front. Chem. 2019, 7, 783. 65. Feng, K.; Xu, L.; Xiong, Y.; Sun, L.; Yu, H.; Wu, M.; Thant, A. A.; Hu, B. PEDOT:PSS and Ni-Based Thermoelectric Generator for Solar Thermal Energy Conversion. J. Mater. Chem. C 2020, 8, 3914.

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66. Park, T.; Park, C.; Kim, B.; Shin, H.; Kim, E. Flexible PEDOT Electrodes with Large Thermoelectric Power Factors to Generate Electricity by the Touch of Fingertips. Energy Environ. Sci. 2013, 6, 788. 67. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685. 68. Andrei, V.; Bethke, K.; Madzharova, F.; Beeg, D.; Knop-Gericke, A.; Kneipp, J.; Rademann, K. Size Dependence of Electrical Conductivity and Thermoelectric Enhancements in Spin-Coated PEDOTPSS Single and Multiple Layers. Adv. Electron. Mater. 2017, 3, 1600473. 69. Bharti, M.; Singh, A.; Samanta, S.; Debnath, A. K.; Marumoto, K.; Aswal, D. K.; Muthe, K. P.; Gadkari, S. C. Elucidating the Mechanisms behind Thermoelectric Power Factor Enhancement of Poly(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) Flexible Films. Vacuum 2018, 153, 238. 70. Zarubin, V. A.; Li, T.-D.; Humagain, S.; Ji, H.; Yager, K. G.; Greenbaum, S. G.; Vuong, L. T. Improved Anisotropic Thermoelectric Behavior of Poly(3,4Ethylenedioxythiophene):poly(styrenesulfonate) via Magnetophoresis. ACS Omega 2018, 3, 12554. 71. Mengistie, D. A.; Chen, C. H.; Boopathi, K. M.; Pranoto, F. W.; Li, L. J.; Chu, C. W. Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants. ACS Appl. Mater. Interfaces 2015, 7, 94. 72. Jeong, M. H.; Sanger, A.; Kang, S. B.; Jung, Y. S.; Oh, I. S.; Yoo, J. W.; Kim, G. H.; Choi, K. J. Increasing the Thermoelectric Power Factor of Solvent-Treated PEDOT:PSS Thin Films on PDMS by Stretching. J. Mater. Chem. 2018, 6, 15621. 73. Zhu, Z.; Liu, C.; Shi, H.; Jiang, Q.; Xu, J.; Jiang, F.; Xiong, J.; Liu, E. An Effective Approach to Enhanced Thermoelectric Properties of PEDOT:PSS Films by a DES Post-treatment. J. Polym. Sci., Polym. Phys. Ed. 2015, 53, 885. 74. Seki, Y.; Takahashi, M.; Takashiri, M. Enhanced Thermoelectric Properties of Electropolymerized Poly (3,4-ethylenedioxythiophene) Thin Films by Optimizing Electrolyte Temperature and Thermal Annealing Temperature. Org. Electron. 2018, 55, 112. 75. Sarabia-Riquelme, R.; Ramos-Fernández, G.; Martin-Gullon, I.; Weisenberger, M. C. Synergistic Effect of Graphene Oxide and Wet-Chemical Hydrazine/deionized Water Solution Treatment on the Thermoelectric Properties of PEDOT:PSS Sprayed Films. Synth. Met. 2016, 222, 330. 76. Fan, Z.; Du, D.; Yu, Z.; Li, P.; Xia, Y.; Ouyang, J. Significant Enhancement in the Thermoelectric Properties of PEDOT:PSS Films through a Treatment with Organic Solutions of Inorganic Salts. ACS Appl. Mater. Interfaces 2016, 8, 23204. 77. Du, Y.; Shi, Y.; Meng, Q.; Shen, S. Z. Preparation and Thermoelectric Properties of Flexible SWCNT/PEDOT:PSS Composite Film. Synth. Met. 2020, 261, 116318. 78. Liu, S.; Deng, H.; Zhao, Y.; Ren, S.; Fu, Q. The Optimization of Thermoelectric Properties in a PEDOT:PSS Thin Film through Post-treatment. RSC Adv. 2015, 5, 1910. 79. Luo, J.; Billep, D.; Waechtler, T.; Otto, T.; Toader, M.; Gordan, O.; Sheremet, E.; Martin, J.; Hietschold, M.; Zahn, D. R. T.; Gessner, T. Enhancement of the Thermoelectric Properties of PEDOT:PSS Thin Films by Post-treatment. J. Mater. Chem. 2013, 1, 7576. 80. Gordon, M. P.; Zaia, E. W.; Zhou, P.; Russ, B.; Coates, N. E.; Sahu, A.; Urban, J. J. Soft PEDOT-PSS Aerogel Architectures for Thermoelectric Applications. J. Appl. Polym. Sci. 2017, 134, 44070. 81. Fan, Z.; Li, P.; Du, D.; Ouyang, J. Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films through Sequential Post-treatments with Common Acids and Bases. Adv. Energy Mater. 2017, 7, 1602116.

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82. Wei, Q.; Mukaida, M.; Kirihara, K.; Naitoh, Y.; Ishida, T. Thermoelectric Power Enhancement of PEDOT:PSS in High-Humidity Conditions. APEX 2014, 7, 031601. 83. Kim, G.-H.; Kim, J.; Pipe, K. P. Humidity-dependent Thermoelectric Properties of Poly(3,4-Ethylenedioxythiophene):poly(styrene Sulfonate). Appl. Phys. Lett. 2016, 108, 093301. 84. Tsai, T.-C.; Chen, T.-H.; Chang, H.-C.; Chen, C.-H.; Huang, Y.-C.; Whang, W.-T. A Facile Surface Treatment Utilizing Binary Mixtures of Ammonium Salts and Polar Solvents for Multiply Enhancing Thermoelectric PEDOT:PSS Films. J. Polym. Sci. Polym. Chem. 2014, 52, 3303. 85. Kim, S.-K.; Mo, J.-H.; Kim, J.-Y.; Jang, K.-S. Improving the Thermoelectric Power Factor of PEDOT:PSS Films by a Simple Two-step Post-treatment Method. E-Polymers 2017, 17, 501. 86. Khan, E. H.; Thota, S.; Wang, Y.; Li, L.; Wilusz, E.; Osgood, R.; Kumar, J. Environment-friendly Post-treatment of PEDOT-Tos Films by Aqueous Vitamin C Solutions for Tuning of Thermoelectric Properties. J. Electron. Mater. 2018, 47, 3963.

CHAPTER 6

Thermoelectric PEDOTs: Derivatives, analogs, and copolymers Shouli Ming1, Shijie Zhen2 and Ge Zhang3, * 1

College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong, PR China; 2Guangxi Key Laboratory of Electrochemical and Magneto-Chemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, Guangxi, PR China; 3School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, PR China *Corresponding author

6.1 Introduction Inorganic semiconductor materials have been commercialized for use in large-area thermogenerators, while organic materials are far less studied even though they are abundant and easy to manufacture. However, conducting polymers have theoretically and experimentally been regarded as promising thermoelectric materials, whose potential remains to be further explored. As one of the optional green energy materials in future, it is of great significance to design novel organic molecules to develop advanced thermoelectric materials for the production of flexible electron devices at low temperatures. Lately, thermoelectric performance based on conducting polymers has improved greatly as a wide variety of strategies, such as processing technology (Chapter 2), doping or energy band engineer (Chapter 5), composite, or hybrid (Chapter 7), as well as molecular engineer. In terms of thermoelectric performance of optimization, conducting polymer has more abundant characteristics on structure design compared with inorganic materials. The rational molecular design and synthesis allows thermoelectric performance to be tuned readily. Poly 3,4ethylenedioxythiophene (PEDOT) is one of the most successful organic semiconductors based on the principles of electronic structure, which has led to significant advancements in thermoelectric devices. However, a few issues, such as the low thermopower with high electrical conductivity, the pervasive conformational and energetic disorder, the hygroscopic property and counterion effects of PEDOT, increase the necessity of finding Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00013-5

© 2022 Elsevier Ltd. All rights reserved.

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alternative organic TE molecules. Intensive research into the modification of 3,4-ethylenedioxythiophene (EDOT) polymers is lacking so far, the advanced design rules of molecular structure are just now being uncovered for highly thermoelectric performance. Therefore, this chapter briefly describe PEDOT-based thermoelectric materials including its derivatives, analogues, and copolymers to explore the structure-performance relationship of PEDOT-based polymers. Here, the detailed types and functions of side chains and the synthesis of monomers are not systematically discussed in this section.

6.2 Derivatives As we know, EDOT monomer can be easily functionalized by modification of various flexible side chains onto EDOT backbones such as hydroxyl, alkyl, alkoxy, and oligo (ethylene oxide) groups. Initially, PEDOT derivatives are subject to considerable current interest motivated due to their stability, moderate bandgap and high visible transparency considering technological application such as antistatic coatings, transparent electrodes, energy storage, biomaterials in organic electronic devices.1e5 Generally, the more extensive research on PEDOT derivatives typically involves two main categories (I and II) in the field of organic optoelectronics, as shown in Fig. 6.1. Significant effort has focused on tunning PEDOT’s physical properties (e.g., absorption, emission, energy level, molecular packing, and charge transport) and inducing solubility to make PEDOT derivatives compatible with high-throughput printing and coating methods.6 For organic semiconductors, main work established that manipulation of polymer backbone affects solid-state charge transport properties such as mobility and electrical conductivity, which are significantly affected by molecular conformation, degree of ordering, and orientation of conjugated backbones.7

Figure 6.1 EDOT derivatives.

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The most common alkyl chains involve the linear hexyl, octyl, and dodecyl chains. Very recently, the longer tetradecyl and octadecyl side chains were introduced in donor-acceptor conjugated polymers.8 Statistically, linear alkyl chains with even numbers of carbon atoms are more popular than those with odd-numbered carbon atoms, presumably due to their commercial availability.6 Compared with polythiophene derivatives, the thermoelectric properties for side-chain functionalized PEDOT are very limited. Jiang and his coworkers prepared a free-standing polymer film (P(EDT-E5-EDT)) based on oligo(oxyethylene)-functionalized polythiophene with two EDOT units as building blocks (P1 in Fig. 6.2) by electrochemical polymerization. The as-prepared P1 achieved a higher Seebeck coefficient (35.9 mV K 1) than PEDOT/PEDOT:PSS films.9 Duan et al.10 synthesized a flexible conductive poly(1,6-bis((2,3dihydrothieno[3,4-b] [1,4]dioxin-2-yl)methoxy)hexane) (PBEDTH as P2) by cross-linking two EDOT units based on electrodeposition (Fig. 6.3). Although this PBEDTH showed an enhanced Seebeck coefficient (37.2 mV K 1), a low electrical conductivity (0.11 S cm 1) led to a small thermoelectric power factor (0.015 mW K 2 m 1) at 300 K. From the perspective of practical application, this cross-linking method with linear

Figure 6.2 Chemical structure of PEDOT derivatives.

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Figure 6.3 The digital images of flexible PBEDTH (P2) films by electrodepostion.10 (Copyright 2017, Elsevier Ltd.)

side chain can obtain high-quality elastic thin films. However, their thermoelectric performance have suffered a huge discount, which is probably ascribed to low carrier mobility from the formation of amorphous structure during electrodeposition. To enhance solubility of PEDOT, different modification of basic EDOT unit-bearing alkyl, oligooxyethylene, alkylsulfonate, or perfluoroalkyl groups is only possible by substituents at the ethylenedioxy bridge by covalent attachment of functional groups at the conjugated backbone.11 Akoudad and Roncali demonstrated that the introduction of hydroxymethyl group significantly improves the ability of EDOT monomer to electropolymerize in water and the electroactivity of the resultant polymer (P3) in aqueous media.1 For the introduction of a polyether chain (P4), a negative shift of the redox potential led to an enhanced sensitivity toward molecular oxygen, suggesting possible spontaneous doping by molecular oxygen. In addition, a significant enhancement of effective conjugated chain length with a 0.10 eV reduction of bandgap was revealed by optical properties of oligo(oxyethylene)-substituted polymers. On the other hand, the hydroxymethyl (-MeOH) modified PEDOT has been demonstrated to be an excellent hole transport layer used to tune the work function in solar cells.3 The similar result was also observed on the poly(4(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butanesulfonic acid) (PEDOT-S, P5) to match the hole transport level in organic solar cells (OSC)12 and organic light-emitting diode (OLED).13 Moreover, P5 gains a higher in-plane electrical conductivity above 12 S cm 1 without any posttreatment or additive than the pristine PEDOT:PSS (1 S cm 1).11 Compared to linear alkyl chains, branched alkyl chains generally preclude interchain interdigitation because their bulkiness usually hinders

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interchain interactions and impart better solubility when both chains have identical molecular formulas.6 Very recently, Yano et al.15 further developed a novel fully soluble PEDOT (P6) with a branched side chain. The resulting electrical conductivity was as high as 1089 S cm 1 without additives and posttreatment by the commonly oxidative polymerization. This amazing value is about three orders of magnitude higher than previous reported results of P511,16 and is comparable to PEDOTs (PEDOT/ PEDOT:PSS) with additives or posttreatment. This study has attempted to correlate the polymer crystallites and the transport of charge carriers. The experimental results indicated that P6 showed less entanglement in water because its polymer chains possibly had an extended coil configuration owing to rigid p-conjugated backbone and electrostatic repulsion between the alkylsulfonic acid side chains, conducive to crystallization by p-stacking in the solid state. Noted that, the degree of crystallinity (cc) was as high as 72%, which is even higher than PEDOT:PSS, which is attributed to the larger number of crystallites with the average particle sizes of D100 z 6 nm and D020 z 3 nm corresponding to (100) and (020) planes, respectively. Moreover, an average distance between adjacent nanocrystals (Lnc) was as small as 3.95 nm with the activation energy (Ea) of 4.4 meV, which is lower than the thermal energy at room temperature (26 mV). This suggests the charge carriers in P6 are allowed hopping with enough energy, which explains the highly electrical conductivity at room temperature. An et al.14 further developed an electroactive shape memory composite film with P6 and polyurethane-based polymer (P6:SMP). The as-prepared P6 film exhibited a high electrical conductivity of 576 S cm 1 in Fig. 6.4A. XRD results indicated a significant increase in the diffraction peaks from crystalline P6 (Fig. 6.4B). These results suggest that introduction of a suitable side chain is conducive to improving the electronic transport property of PEDOT. Another important derivative of EDOT is 3,4-propylenedioxythiophene (ProDOT) with or without covalent side chains served as the second category (II) in Fig. 6.1. Most studies of ProDOT polymers (PProDOT) focus on electroactive materials, such as electrochemical and electrochromic properties. PProDOT and its derivatives have the advantages of good optical transmittance and narrow bandgap (w1.6 eV) similar to PEDOT.17,18 For conjugated polymers, an effective dope is allowed to manipulate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels and to tune the electronic transport properties.

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Figure 6.4 (A) Relation between electrical conductivity (s) and volume fraction of P6:SMP composite films. Inset: relation between s and WS-PEDOT. (B) XRD patterns of P6:SMP composite films with different WS-PEDOT. Inset: relation between crystallinity (cc) of P6:SMP composite films and WS-PEDOT. fS-PEDOT is the volume fraction of the conductive filler P6, and WS-PEDOT is the weight ratios of P6 in DMSO solution.14 (Copyright 2020, The Japan Society of Applied Physics.)

Generally, the electronic bandgap depends on the degree of p-overlap along the backbone via steric interactions and the change of electron-donating or accepting substituents.19 For the former, the bridge side chains in PProDOT produce two primary steric effects including (1) the increase in the distance between side chain and polymer backbone and (2) the position for side chains out of plane relative to backbone direction.7 For the latter, the energy gap of the family of PProDOT is slightly higher than PEDOT due to substituents and corepeat units. Bargigia et al.20 found in doped P7 that the formation of polaron led to a more planar chain structure with enhanced torsional order and relaxed intraring bonds conductive to charge transport. Note that, the bipolaron formation at high doping levels is accompanied by a stiffening of both inter- and intraring double bonds and a subsequent decrease in electrochemical conductance. Pittelli et al.7 revealed that the oligoether functionalized PProDOT polymers (P8 and P9) exhibited no crystalline ordering by GIWAXS. Moreover, the incorporation of linear side chain for P9 showed the better solid-state charge transport compared to branched P8. This conclusion seems to be contrary to P5 and P6. Nevertheless, the fact is that the substituents in PProDOT plays an important role in intermolecular order and ability to charge transport. The changes in PEDOT derivatives allow us to

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understand the relationship between structure and property better. The abundant PEDOT derivatives also can provide a good platform for optimizing and studying the organic thermoelectric performance.

6.3 Analogs Previous studies suggest that thermoelectric polymers with no or low energy bandgap and low density of states (DOS) at the Fermi level (EF) typically have a higher Seebeck coefficient and low thermal conductivities, thus being suitable for thermoelectric application.21 Studies indicate that polyselenophenes (PSes) have many unique properties stemming from their unique properties of Se atom and selenophene including22: (1) the intermolecular Se/Se interactions are conductive to enforcing inter-China charge transfer. (2) PSes can be obtained by a lower oxidation based on corresponding monomers. (3) PSes are much easier to polarize compared to polythiophenes due to the Se atom. (4) PSes are allowed to accommodate more charge upon doping than polythiphenes (PThs) owing to the larger size of Se atom. (5) PSes have a lower bandgap than PThs. Gao et al.23 suggests appropriate and specially engineered doped low-band-gap polymers can also be promising candidate materials. Among the PSe family, poly(3,4-ethylenedioxyselenophene) (PEDOS; P10 in Fig. 6.5) is the most prominent due to its low bandgap (1.4 eV, Table 6.1) and high stability in the oxidized state in despite of a low electrical conductivity.24 Given the similar molecular structure to PEDOT and its many advantages, PEDOS has become a promising member for organic electronic devices. PEDOS has a relatively narrow band gap (Table 6.1), its designing demands the identification of more rigid conjugated systems capable of bearing various substituents on their backbone while retaining their planarity.25 Thermoelectric performance of conducting polymer can be controlled by tunning doping level based on chemical or electrochemical method. Conducting polymers with intrinsic electrical conductivity can absorb light from the

Figure 6.5 Chemical structure of PEDOT analogues.

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Table 6.1 Experimental and calculated data for polymers.a,25 Polymer onset [V]

PEDOT PEDOS (P10) PEDTT (P12) PEDTS (P13)

0.44 0.64 0.43 d 0.20

Polymer lmax [nm]

Optical Eg [eV]

635 673

1.6 1.4

311, 426 d CA. 610

2.0 d 1.4

LUMOc [eV]

Eg calcd [eV]

3.52 3.44

1.68 1.79

1.84 1.66

4.10 4.91 4.33

2.05 1.78 2.50

2.06 3.13 1.83

HOMOb [eV]

a

Calculated at the PBS/B3LYP/6-31G(d) level. Highest occupied crystal orbital from PBC calculations. c Lowest unoccupied crystal orbital. Copyright 2009, Wiley. b

UV/V is range to the near-infrared (NIR) range. Importantly, the light driven heat could be converted into electricity when coupling the photothermal conversion with the thermoelectric effect, allowing photo-thermoelectric (PTE) conversion in conducting polymer. Kim et al.26 synthesized a conductive poly(hexyl-3,4-ethylenedioxyselenophene) (PEDOS-C6, P11) film by electrochemical deposition and studied its PTE property by controlling its doping state via an external potential, as shown in Figs. 6.6 and 6.7. The P11 film at doped state exhibited visible to NIR electrochromism with the almost colorless in the visible region but the strong absorption (polaron and bipolaron band) in the IR region (Fig. 6.6A). By the electrochemically controlled doping level at 0.1 V or higher, the electrical conductivity (s) of P11 film increased up to 355 S cm 1 with a low Seebeck coefficient (Sw30 mV K 1) in Fig. 6.6B. The Seebeck coefficient can increase with a negative applied potential and reach to 770 mV K 1 at 0.5 V. Apparently, the electrical conductivity and Seebeck coefficient of P11 film exhibited two opposite trends with the change of applied potential. This is attributed to the change of charge carrier concentration by controlling electrochemical potential. However, a maximized thermoelectric power factor (sS2) can be observed to be 354.7 mW m 1 K 2 at 0.1 V for P11 film in Fig. 6.6C, which is comparable to PEDOT films. Moreover, the P11 film was demonstrated to have reproducible PTE effect and the stable light driven voltage over 1000 cycles during NIR switching (Fig. 6.6D). There are many analogues of PEDOT (P12eP15), which have unique electrical and optical properties. However, few efforts have been devoted to

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Figure 6.6 (A) In situ spectroelectrochemistry of PEDOS-C6 (P11) films as a function of the applied potential between 0.9 and 0.5 V at intervals of 0.1 V in a 0.1 M LiBF4/PC supporting electrolyte using a platinum wire as the counter electrode. (B) Electrical conductivity and Seebeck coefficient. (C) The power factor of the P11 film at various doping state precisely controlled by applied potentials. Seebeck voltage generation of P11 at 0.1 V with the body heat and NIR laser as a hot source. (D) The cyclability of Seebeck voltage generation with a NIR laser intensity of 2.33 W cm 2 at a switching time of 12 s over 1000 cycles.26 (Copyright 2013, Wiley.)

the research of thermoelectric performance. No matter in theory or experiment, it is expected there are more works to better guide the research on thermoelectric conducting polymers.

6.4 Copolymers The development of conducting polymers with high electrical conductivity and Seebeck coefficient has continued to pose a massive challenge in organic thermoelectric materials. New structural insights into the chargecarrier transport are necessitated for the realization of high-performance

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Figure 6.7 Chemical structure of PEDOT copolymers.

thermoelectric polymers. Very recently, many efforts have demonstrated that tuning solid-state packing by molecular design for both p-type and ntype polymers should be a promising strategy, which can significantly affect backbone orientation on thermoelectric performance.27e30 Most of researches for thermoelectric polymers are focused on improvement of crystalline morphology, dopants related to impact on polymer microstructure, DOS, tailoring side chains and so on to finely achieve the optimal combination of high electrical conductivity and Seebeck coefficient to

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maximize the thermoelectric power factor. Therefore, semicrystalline thermoelectric conducting copolymers are typically constructed to connect various organic monomer molecules. At present, many research groups make efforts to develop highperformance thermoelectric p-type conducting copolymers. For instance, Chabinyc and his coworkers recently reported a remarkably high thermoelectric power factor over 100 mW m 1 K 2 based on the classic poly(2,5bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14).31 The as-prepared PBTTT-C14 films doped with (tridecafluoro-1,1,2,2tetrahydrooctyl)-trichlorosilane (FTS) vapor or immersion in 4ethylbenzene-sulfonic acid solution gained the high electrical conductivity of w1000 S cm 1 and Seebeck coefficient of w33 mV K 1, resulting in power factor of w110 mW m 1 K 2. In addition, the doped PBTTT-C14 films with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) by vapor phase showed the electrical conductivity of 670 S cm 1 and Seebeck coefficient of w42 mV K 1, and the resultant power factor of 120 mW m 1 K 2.32 They find that alignment of ordered domains is the critical factor leading to higher electrical conductivity without lowering Seebeck coefficient, thereby leading to improvement in the thermoelectric power factor. To date, many works have revealed that the advances in molecular design are allowed to achieve a high charge carrier mobility (m > 1 cm2 V 1 s 1) strongly depending on the degree of electronic and structural disorder of conducting polymers. EDOT as a popular precursor is often used to combined with other organic molecules by repeating unit structure to obtain a copolymer and affect its electronic charge transport properties. The side chains have an influence on the properties of conducting polymer, such as charge transport, molecular aggregation, film morphology, etc. Xu’s group studied that the different side chains substituted thiophene and EDOT as copolymerization units are used to construct conducting polymers as thermoelectric materials (P16).33 An alkyl group bearing electronic ability at the thiophene ring effectively achieved a large increase in the electrical conductivity with nearly invariable Seebeck coefficient, resulting in an enhancement by one order of magnitude for the thermoelectric power factor (Fig. 6.8A). Compared to chain structure polymers, the thermoelectric performance for network structure polymers have also been investigated. Furthermore, they proposed a spin-spray coating polymerization strategy by spraying of solid-state trithiophenes as precursors in a common low-boiling point organic solvent for a homogeneous conjugated polymer thin-film.34

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Figure 6.8 The thermoelectric performance of EDOT-based copolymers: (A) P1633, (B) P1734, (C) P2035, and (D) P2136. (A) Copyright 2017, American Chemical Society. (B) Copyright 2020, American Chemical Society. (C) Copyright 2017, Wiley. (D) Copyright 2017, American Chemical Society.

The as-fabricated conducting polymer (P17) films exhibited the uniform morphology with the fast and easy polymerization and acceptable thermoelectric performance compared to the traditional chemical and electrochemical oxidation in solution (Fig. 6.8B). The planar conjugated backbone can strengthen intermolecular interaction and facilitate charge carrier mobility, which is helpful to improve the thermoelectric performance of conjugated polymer. Therefore, designing planar molecular structure is the effective strategy to prepare highperformance conjugated polymer. The copolymers containing EDOT and thiophene derivatives exhibit planar conjugated backbone due to the intramolecular noncovalent interaction between oxygen in EDOT and sulfur in thiophene derivatives, which triggers the research of EDOT-(thiophene derivatives) conjugated copolymers on thermoelectric performance. Imae et al.37 reported that two types of copolymers (P18) composing of EDOT

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and 3-hexylthiophene with different composition ratios were synthesized by the electrolytic polymerization. Considering the excellent performance of EDOT unit, many novel polymers based EDOT have also been explored. A low-bandgap D-A-D type polymer (P19) was designed and showed the low electrical conductivity between 10 1w101 S cm 1 and the low Seebeck coefficient of 14.0 mV K 1 at room temperature.38 Compared to the copolymers containing two kind of conjugated units, a series of polymers containing alternate EDOT and methine were prepared by chemical oxidation method. Wang’s group reported that the introduction of methine in polymeric block results into the target polymers (P20) possessing high Seebeck coefficient.35 The results confirmed that lower-bandgap polymers (1.72w1.79 eV) tend to exhibit higher Seebeck coefficients, resulting in a ZT w6.75  10 3 at 120  C (Fig. 6.8C). Also, they revealed the significant impact of structural modification of polymers on the improvement in the understanding of thermoelectric performance. Li et al.36 synthesized a series of copolymers (P21 and P22) based on EDOT and thiophene with or without a sulfur atom inserted between thiophene rings and dodecyl chains. P21 doped with F4TCNQ exhibits a high electrical conductivity of 140 S cm 1 and a Seebeck coefficient of w25 mV K 1, resulting in a power factor of 11 mW m 1 K 2 (Fig. 6.8D). In contrast, the P22 doped with NOBF4 shows a low electrical conductivity although at high doping levels. They reveal that the high electrical conductivity for P21 mainly depends on the bulk molecular packing and charge transport in the doped state, and the mobility of polymer without doping is not a dominant factor for gaining a high electrical conductivity due to the impact of doping on packing of bulk film. Due to the good planarity and high carrier mobility, fused ring units, such as carbazole (Cz), benzodithiophene (BDT), indacenodithiophene (IDT), and their derivatives, have been used intensively to construct conjugated molecules applied as photovoltaic materials. Carbazole and their derivatives (P24-27), owning their unique properties such as good packing, high charge mobility, and high thermal stability resulted from the rigid backbone, have attracted much attrition and been used to construct thermoelectric materials. Leclerc’s group designed and synthesized thermoelectric polymeric materials based on polycarbazole derivatives by introducing different alkyl side chains, EDOT unit and bi-3,4(ethylenedioxy)thiophene (Bi-EDOT) unit (P24-26).39 The electrical conductivity of P25 is 4  10 3 S cm 1, and the low value is probably due to the steric conformation by the hexyl substituents. By introducing only

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one EDOT comonomer and no substituents on carbazole, P24 revealed the increase in electrical conductivity (2  10 2 S cm 1).40 The dispersive band of P26 was higher than that with thiophene rings, indicating a possibly higher Seebeck coefficient. Xu et al.41 electrochemical synthesized a copolymer containing 1,12-bis(carbazolyl)dodecane (2Cz-D) and EDOT by direct anodic oxidation (P27).13 The highest electrical conductivity of P27 film was 0.6 S cm 1, four orders of magnitude higher than that of P2Cz-D (2.7  105 S cm 1), which was due to the conducting copolymer combines the advantages of the homopolymers (P2Cz-D and PEDOT). Moreover, the copolymers containing EDOT and solubilizing 3,4propylenedioxythiophene (ProDOT) units have been developed for thermoelectric materials. Ponder et al.42 found that high mole fraction of EDOT (fEDOT) in a repeat unit of the polymer was preferable from the viewpoint of electrical conductivity, while low fEDOT was preferable from the viewpoint of Seebeck coefficient. It is noteworthy that the two copolymers show inferior ZT relative to pure PEDOT. As increase with fEDOT, the electrical conductivity of the polymers is tuned from 1  10 3 to 3 S cm 1. However, the electrical conductivity and Seebeck coefficient are not inversely correlated in this system. P29 exhibits the highest electrical conductivity and acceptable Seebeck coefficient values. After optimization, P29 achieves a high electrical conductivity of over 250 S cm 1 and a thermoelectric power factor of 7 mW m 1 K 2. The Seebeck coefficient for these copolymers (P23, P28-30) in their oxidatively doped states are relatively low and positive as expected for a good conductor, similar to PEDOT, with P28 having the highest value of 35 mV K 1 at room temperature. Interestingly, a significant increase can be observed in electrical conductivity with 33% EDOT to the repeating unit of PProDOT to obtain P28, and further increase with the 67% EDOT (P29) is more gradual. Recently, studies focused on polymeric thermoelectric materials based on EDOT-(fused ring units) are increasingly reported. Wang’s group reported that two IDT-based conjugated polymers, PIDT-EDOT and PIDTT-EDOT, are designed and synthesized as organic thermoelectric materials.44 PIDTT-EDOT based on larger fused ring showed better thermoelectric properties than PIDT-EDOT, and the highest power factor for PIDTT-EDOT was 0.867 mW m 1 K 2, which is approximately 25 times higher than that of PIDT-EDOT. The better thermoelectric properties of PIDTT-EDOT mainly result from that the increased planarity and longer effective conjugation enhance the carrier concentration and mobility. However, PIDTT-EDOT exhibited a relatively lower Seebeck

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coefficient relative to PIDT-EDOT due to its high carrier concentration. The enhancement of DOS is regarded as the promising strategy to improve Seebeck coefficient of conducting polymers. Furthermore, they introduced alkylthienyl substitutes onto BDT units to construct two dimensional conjugated polymer P32 as thermoelectric material, which show the larger Seebeck coefficient compared with one dimensional P31 films (Fig. 6.9).43 The work functions (F) of neutral P31 and P32 were estimated to be 3.59 and 3.68 eV, respectively, which were determined from the binding energy of the secondary electron cutoff in the ultraviolet photoemission spectroscopy (UPS) spectrum (Fig. 6.9A). During the doped state, their EF both shifted toward a lower binding energy (Fig. 6.9B), implying a downward movement of EF toward the HOMO level, indicating the generation of hole carriers and effective p-doping that was consistent with the positive Seebeck coefficients. The high Seebeck coefficient value of P32 can be attributed to the enhancement of the DOS around the Fermi level, which has been proved by the ultraviolet photoemission spectroscopy measurement and Mott parameters calculation. Besides the molecular structure, the molecular weight of conducting polymer is crucial to determine the thermoelectric performance.

Figure 6.9 Chemical structure of P31 and P32. (A) UPS spectra showing the secondary electron cutoff region for P31 and P32 in pristine condition and doped condition. (B) Schematic diagrams of electronic states derived from the UPS results.43 (Copyright 2019, Royal Society of Chemistry.)

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6.5 Concluding remarks Molecular modifications as incorporating side chains, atom substitution, or EDOT units into organic unit rings or backbone enable us to tune the thermoelectric performance of conducting polymers. Importantly, such modifications could play an important role in controlling the charge transport and thermoelectric properties based on molecular backbone orientation, doping level, and technology, DOS engineer. Many efforts have been devoted in PEDOT-based polymers including derivatives, analogues, and copolymers, which are constantly revealing the relationship between structure and thermoelectric performance. At present, despite the slowness of progress, it is believed that the structure that high-performance thermoelectric polymers should have will become increasingly clear under constant attention and research.

References 1. Akoudad, S.; Roncali, J. Modification of the Electrochemical and Electronic Properties of Electrogenerated Poly(3,4-Ethylenedioxythiophene) by Hydroxymethyl and Oligo(oxyethylene) Substituents. Electrochem. Commun. 2000, 2, 72. 2. Emanuelsson, R.; Sterby, M.; Stromme, M.; Sjodin, M. An All-Organic Proton Battery. J. Am. Chem. Soc. 2017, 139, 4828. 3. Dong, H.; Zheng, E.; Niu, Z.; Zhang, X.; Lin, Y. Y.; Jain, P.; Yu, Q. Hydroxymethylfunctionalized PEDOT-MeOH:PSS for Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 17571. 4. Stavrinidou, E.; Gabrielsson, R.; Gomez, E.; Crispin, X.; Nilsson, O.; Simon, D. T.; Berggren, M. Electronic Plants. Sci. Adv. 2015, 1, e1501136. 5. Åkerlund, L.; Emanuelsson, R.; Renault, S.; Huang, H.; Brandell, D.; Strømme, M.; Sjödin, M. The Proton Trap Technology-Toward High Potential Quinone-Based Organic Energy Storage. Adv. Energy Mater. 2017, 7, 1700259. 6. Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2013, 26, 604. 7. Pittelli, S. L.; De Keersmaecker, M.; Ponder, J. F., Jr.; Österholm, A. M.; Ochieng, M. A.; Reynolds, J. R. Structural Effects on the Charge Transport Properties of Chemically and Electrochemically Doped Dioxythiophene Polymers. J. Mater. Chem. C 2020, 8, 683. 8. Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching Disorder-free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384. 9. Hu, Y.; Jiang, F.; Lu, B.; Liu, C.; Hou, J.; Xu, J. Free-standing Oligo(oxyethylene)Functionalized Polythiophene with the 3,4-ethylenedioxythiophene Building Block: Electrosynthesis, Electrochromic and Thermoelectric Properties. Electrochim. Acta 2017, 228, 361.

Thermoelectric PEDOTs: Derivatives, analogs, and copolymers

161

10. Ye, G.; Xu, J.; Ma, X.; Zhou, Q.; Li, D.; Zuo, Y.; Lv, L.; Zhou, W.; Duan, X. Onestep Electrodeposition of Free-Standing Flexible Conducting PEDOT Derivative Film and its Electrochemical Capacitive and Thermoelectric Performance. Electrochim. Acta 2017, 224, 125. 11. Karlsson, R. H.; Herland, A.; Hamedi, M.; Wigenius, J. A.; Åslund, A.; Liu, X.; Fahlman, M.; Inganäs, O.; Konradsson, P. Iron-catalyzed Polymerization of Alkoxysulfonate-Functionalized 3,4-ethylenedioxythiophene Gives Water-Soluble Poly(3,4-Ethylenedioxythiophene) of High Conductivity. Chem. Mater. 2009, 21, 1815. 12. Cai, W.; Musumeci, C.; Ajjan, F. N.; Bao, Q.; Ma, Z.; Tang, Z.; Inganäs, O. Selfdoped Conjugated Polyelectrolyte with Tuneable Work Function for Effective Hole Transport in Polymer Solar Cells. J. Mater. Chem. 2016, 4, 15670. 13. Cutler, C. A.; Bouguettaya, M.; Kang, T.-S.; Reynolds, J. R. Alkoxysulfonatefunctionalized PEDOT Polyelectrolyte Multilayer Films: Electrochromic and Hole Transport Materials. Macromolecules 2005, 38, 3068. 14. An, Y.; Okuzaki, H. Novel Electro-Active Shape Memory Polymers for Soft Actuators. Jap. J. Appl. Phys. 2020, 59, 061002. 15. Yano, H.; Kudo, K.; Marumo, K.; Okuzaki, H. Fully Soluble Self-Doped Poly(3,4Ethylenedioxythiophene) with an Electrical Conductivity Greater Than 1000 S Cm 1. Sci. Adv. 2019, 5, eaav9492. 16. Zotti, G.; Zecchin, S.; Schiavon, G.; Groenendaal, L. B. Electrochemical and Chemical Synthesis and Characterization of Sulfonated Poly(3,4-Ethylenedioxythiophene): A Novel Water-Soluble and Highly Conductive Conjugated Oligomer. Macromol. Chem. Phys. 2002, 203, 1958. 17. Groenendaal, L.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R. Electrochemistry of Poly(3,4-Alkylenedioxythiophene) Derivatives. Adv. Mater. 2003, 15, 855. 18. Gaupp, C. L.; Dean, M.; Welsh, a.; Reynolds, J. R. Poly(ProDOT-Et2 ): A HighContrast, High-Coloration Efficiency Electrochromic Polymer. Macromol. Rapid Commun. 2002, 23, 885. 19. Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4ethylenedioxythiophene) and its Derivatives: Past, Present, and Future. Adv. Mater. 2000, 12, 481. 20. Bargigia, I.; Savagian, L. R.; Osterholm, A. M.; Reynolds, J. R.; Silva, C. Chargetransfer Intermediates in the Electrochemical Doping Mechanism of Conjugated Polymers. J. Am. Chem. Soc. 2021, 143, 294. 21. Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J. B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190. 22. Patra, A.; Bendikov, M. Polyselenophenes. J. Mater. Chem. 2010, 20, 422. 23. Gao, X.; Uehara, K.; Klug, D. D.; Patchkovskii, S.; Tse, J. S.; Tritt, T. M. Theoretical Studies on the Thermopower of Semiconductors and Low-Band-Gap Crystalline Polymers. Phys. Rev. B 2005, 72, 125202. 24. Patra, A.; Wijsboom, Y. H.; Zade, S. S.; Li, M.; Sheynin, Y.; Leitus, G.; Bendikov, M. Poly(3,4-ethylenedioxyselenophene). J. Am. Chem. Soc. 2008, 130, 6734. 25. Wijsboom, Y. H.; Patra, A.; Zade, S. S.; Sheynin, Y.; Li, M.; Shimon, L. J.; Bendikov, M. Controlling Rigidity and Planarity in Conjugated Polymers: Poly(3,4Ethylenedithioselenophene). Angew. Chem. Int. Ed. 2009, 48, 5443.

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26. Kim, B.; Shin, H.; Park, T.; Lim, H.; Kim, E. NIR-sensitive Poly(3,4Ethylenedioxyselenophene) Derivatives for Transparent Photo-Thermo-Electric Converters. Adv. Mater. 2013, 25, 5483. 27. Wang, Y.; Takimiya, K. Naphthodithiophenediimide-bithiopheneimide Copolymers for High-Performance N-type Organic Thermoelectrics: Significant Impact of Backbone Orientation on Conductivity and Thermoelectric Performance. Adv. Mater. 2020, 32, e2002060. 28. Lee, J.; Kim, J.; Nguyen, T. L.; Kim, M.; Park, J.; Lee, Y.; Hwang, S.; Kwon, Y.-W.; Kwak, J.; Woo, H. Y. A Planar CyclopentadithiopheneeBenzothiadiazole-Based Copolymer with Sp2-Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices. Macromolecules 2018, 51, 3360. 29. Liu, J.; Ye, G.; Zee, B. V.; Dong, J.; Qiu, X.; Liu, Y.; Portale, G.; Chiechi, R. C.; Koster, L. J. A. N-type Organic Thermoelectrics of Donor-Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States. Adv. Mater. 2018, 30, e1804290. 30. Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X.; Portale, G.; Dong, J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; Loi, M. A.; Chiechi, R. C.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Enhancing Molecular N-type Doping of DonorAcceptor Copolymers by Tailoring Side Chains. Adv. Mater. 2018, 30, 1704630. 31. Patel, S. N.; Glaudell, A. M.; Kiefer, D.; Chabinyc, M. L. Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase. ACS Macro Lett. 2016, 5, 268. 32. Patel, S. N.; Glaudell, A. M.; Peterson, K. A.; Thomas, E. M.; O’Hara, K. A.; Lim, E.; Chabinyc, M. L. Morphology Controls the Thermoelectric Power Factor of a Doped Semiconducting Polymer. Sci. Adv. 2017, 3, e1700434. 33. Hu, Y.; Liu, X.; Jiang, F.; Zhou, W.; Liu, C.; Duan, X.; Xu, J. Functionalized Poly(3,4Ethylenedioxy Bithiophene) Films for Tuning Electrochromic and Thermoelectric Properties. J. Phys. Chem. B 2017, 121, 9281. 34. Shen, L.; Liu, P.; Liu, C.; Jiang, Q.; Xu, J.; Duan, X.; Du, Y.; Jiang, F. Advances in Efficient Polymerization of Solid-State Trithiophenes for Organic Thermoelectric Thin-Film. ACS Appl. Polym. Mater. 2020, 2, 376. 35. Lai, C.; Li, J.; Xiang, X.; Wang, L.; Liu, D. Effects of Side Groups on the Thermoelectric Properties of Composites Based on Conjugated Poly(3,4Ethylenedioxythiophene Methine)s with Low Bandgaps. Polym. Comps. 2018, 39, 126. 36. Li, H.; DeCoster, M. E.; Ireland, R. M.; Song, J.; Hopkins, P. E.; Katz, H. E. Modification of the Poly(bisdodecylquaterthiophene) Structure for High and Predominantly Nonionic Conductivity with Matched Dopants. J. Am. Chem. Soc. 2017, 139, 11149. 37. Imae, I.; Koumoto, T.; Harima, Y. Thermoelectric Properties of Polythiophenes Partially Substituted by Ethylenedioxy Groups. Polymer 2018, 144, 43. 38. Ming, S.; Zhen, S.; Lin, K.; Zhao, L.; Xu, J.; Lu, B.; Wang, L.; Xiong, J.; Zhu, Z. Thermoelectric Performance of DonoreAcceptoreDonor Conjugated Polymers Based on Benzothiadiazole Derivatives. J. Electron. Mater. 2015, 44, 1606. 39. Lévesque, I.; Bertrand, P.-O.; Blouin, N.; Leclerc, M.; Zecchin, S.; Zotti, G.; Ratcliffe, C. I.; Klug, D. D.; Gao, X.; Gao, F.; Tse, J. S. Synthesis and Thermoelectric Properties of Polycarbazole, Polyindolocarbazole, and Polydiindolocarbazole Derivatives. Chem. Mater. 2007, 19, 2128. 40. Zotti, G.; Schiavon, G.; Zecchin, S.; Morin, J.-F.; Leclerc, M. Electrochemical, Conductive, and Magnetic Properties of 2,7-Carbazole-Based Conjugated Polymers. Macromolecules 2002, 35, 2122.

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41. Yue, R.; Lu, B.; Xu, J.; Chen, S.; Liu, C. Electrochemistry, Morphology, Thermoelectric and Thermal Degradation Behaviors of Free-Standing Copolymer Films Made from 1,12-bis(carbazolyl)dodecane and 3,4-ethylenedioxythiophene. Polym. J. 2011, 43, 531. 42. Ponder, J. F.; Menon, A. K.; Dasari, R. R.; Pittelli, S. L.; Thorley, K. J.; Yee, S. K.; Marder, S. R.; Reynolds, J. R. Conductive, Solution-processed Dioxythiophene Copolymers for Thermoelectric and Transparent Electrode Applications. Adv. Energy Mater. 2019, 9, 1900395. 43. Zhou, X.; Pan, C.; Gao, C.; Shinohara, A.; Yin, X.; Wang, L.; Li, Y.; Jiang, Q.; Yang, C.; Wang, L. Thermoelectrics of Two-Dimensional Conjugated Benzodithiophene-Based Polymers: Density-Of-States Enhancement and Semi-metallic Behavior. J. Mater. Chem. 2019, 7, 10422. 44. Wei, C.; Wang, L.; Pan, C.; Chen, Z.; Zhao, H.; Wang, L. Effect of Backbone Structure on the Thermoelectric Performance of Indacenodithiophene-Based Conjugated Polymers. React. Funct. Polym. 2019, 142, 1.

CHAPTER 7

PEDOT-based thermoelectric nanocomposites/hybrids Haijun Song College of Mechanical and Electrical Engineering, Jiaxing University, Jiaxing, Zhejiang Province, PR China

7.1 Introduction Recently, conducting polymers have drawn more and more attention as a new kind of thermoelectric (TE) materials because of their abundance, intrinsic low thermal conductivity, relatively simple synthesis, easy processing into versatile forms, flexibility, and light weight. The most widely investigated conducting polymer TE materials include poly(3,4ethylenedioxythiophene) (PEDOT) and its water-processable dispersion poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh), and poly(3hexylthiophene) (P3HT), as well as their composites with inorganic materials or carbon nanostructures.1e8 Among these conducting polymers, PEDOT and PEDOT:PSS have emerged as significant candidates for TE applications because of their high electrical conductivity (as high as 103 S cm
1 through appropriate doping treatment),9e11 low thermal conductivity (w0.2 W m
1 K
1, which is 1w2 orders of magnitude lower than that of inorganic TE materials),12,13 solution-processability and flexibility, which make them promising candidates to construct flexible TE generator and replace traditional batteries to power wearable devices.14,15 All these characteristics are undeniable merits as TE materials, which is usually evaluated via the dimensionless figure of merit, ZT ¼ S2sT/k, where S is the Seebeck coefficient, s is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature, respectively. For PEDOT:PSS, posttreatment is one of the most convenient and effective methods to optimize its TE property. For commercialized PEDOT:PSS, i.e., Clevios PH1000, the weight ratio of PSS/PEDOT is 2.5:1. And the excess PSS content helps for a better water solubility, whereas it also reduce the chain alignment of conductive PEDOT and result in a low electrical conductivity. When PEDOT:PSS is treated by some organic solvents or acids, such as dimethylsulfoxide (DMSO), Advanced PEDOT Thermoelectric Materials ISBN 978-0-12-821550-0 https://doi.org/10.1016/B978-0-12-821550-0.00006-8

© 2022 Elsevier Ltd. All rights reserved.

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ethylene glycol (EG), or sulfuric acid, the electrical conductivity can obtain 2w3 orders of magnitude improvement due to the reduction of PSS and the reorientation of PEDOT chains.9,16e19 However, the intrinsic low Seebeck coefficient of pristine PEDOT or PEDOT:PSS is only around 12e18 mV K
1, which is one to two orders of magnitude lower than that of inorganic TE materials and becomes the main bottleneck for further improvement of its power factor (PF ¼ S2s).20e22 Another strategy to increase the TE properties of PEDOTs (PEDOT/PEDOT:PSS) is constructing PEDOT-based composites and hybrid TE materials. These composites can combine the individual strengths of PEDOT (high electrical conductivity) and inorganic components or carbon nanostructures (high Seebeck coefficient or power factor). Through the synergistic effect and energy-filtering effect of the composites, taking advantage of the low thermal conductivity of these composites and optimizing composite method, some reported TE performances are comparable to that of traditional inorganic TE materials.23,24 Therefore, in this chapter, we mainly focused on the recent progress of PEDOT-based composites for TE applications. Inorganic nanocrystals and carbon nanomaterials fillers with different structures and properties are discussed and the effect of different fillers and prepared methods on the TE properties of PEDOT has been analyzed.

7.2 Thermoelectric properties of PEDOT/inorganic nanocrystals and composites 7.2.1 TE properties of PEDOT/metal nanoparticle composites Metal nanomaterials, such as Au, Ag, Cu, etc., show extremely high electrical conductivity w105 S cm
1, and can be easily prepared through wetchemical process, making them used as fillers to enhance the electrical conductivity of PEDOT-based TE composites. Toshima et al. have systematically studied the TE properties of PEDOT:PSS/metal nanoparticle (NP) composites.25e28 Early in 2012, PEDOT:PSS/Au-NPs (protected by two kinds of ligands, terthiophenethiol, and dodecanethiol) composite was prepared by a physical mixing and drop-casting method. In the composite, the dodecanethiol protected Au NPs showed better effect to enhance the electrical conductivity of PEDOT:PSS than that of the terthiophenethiol protected Au NPs (AuDT), which could be attributed to the enhanced carrier hoping caused by the well dispersion of Au-DT in the PEDOT:PSS solution.25 Besides,

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they also prepared Au NPs with different shapes (spherical or rod) to composite with PEDOT:PSS, and the rod-shaped Au NPs showed a great enhancement in electrical conductivity (w2000 S cm
1), revealing that one-dimensional (1D) particles with larger aspect ratio (rods and wires) are favorable nanocomponents for development of highly conductive hybrid materials.27 In 2016, they investigated length of Ag nanowires (NWs) on the TE properties of PEDOT:PSS/Ag NWs composite. The composite showed an increase in electrical conductivity with increase of Ag NWs concentration and the maximum value of electrical conductivity could be as high as 10,000 S cm
1. Due to percolation effect, the as-prepared composite films containing long Ag NWs showed higher electrical conductivity relative to short Ag NWs at given concentration. The Seebeck coefficient of the composite showed opposite tendency because of increase in carrier number due to the Ag NWs, and decreased with the increased Ag NWs concentration, leading to undesirable reduction of TE performance.28 Cai’s group has also made some research on the TE properties of PEDOT/metal composites. Previously, they synthesized PEDOT/Cu and PEDOT/Ag nanocomposites through an interfacial polymerization method, and measured the TE properties of the pellets cold pressed from composite powders with different Ag or Cu contents.29 When the oxidant/ EDOT molar ratio is 2:1, both PEDOT/Ag and PEDOT/Cu (spherical structure) nanocomposites reach their largest power factors of 1.49 and 7.07 mW m
1 K
2. And compared to zero-dimensional (0D) spherical Cu NP, 1D needle-like Cu NPs showed a better effect to enhance the TE properties of the composite. When the oxidant/EDOT molar ratio is 1, the PEDOT/Cu (needle-like structure) showed the largest power factor of 12.47 mW m
1 K
2 with a ZT value of 0.01 at room temperature. The research confirmed that 1D inorganic nanostructures as a filler is better than 0D inorganic nanostructures for enhancing the TE properties of polymerbased composites. Besides, Liu et al. prepared PEDOT:PSS/Ag NWs bulk TE hybrids by cryogenic grinding and a spark plasma sintering processes.30 The composite showed much higher electrical conductivity than the pure bulk PEDOT:PSS, meanwhile, the Seebeck coefficient and thermal conductivity of the composite remained with the same level as pure PEDOT:PSS, and consequently the maximum ZT value was found to achieve a four-time enhancement. Son et al. fabricated PEDOT:PSS/GeNP composite using a mechanically mixing method and measured its TE properties.31 As the Ge NPs content of the composite film was increased, the electrical conductivity decreased from 593.4 to 28.48 S cm
1 and the

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Seebeck coefficient increased from 18.98 to 46.77 mV K
1. They attributed this trade-off relationship to the energy-dependent scattering of charge carriers at the interface between PEDOT:PSS and the Ge NPs. The maximum power factor was 31.20 mW m
1 K
2, and due to the interfacial phonon scattering effect, the thermal conductivity was 0.417 W m
1 K
1, the optimized ZT value is 0.0223 at room temperature. Recently, Pasha et al. prepared PEDOT:PSS/Ag-NP nanocomposites treated with polyethylene glycol (PEG) by drop-cast technique.32 Due to the removal of non-complexed PSS and synergetic interaction between PEDOT:PSS and Ag NPs segments via PEG, the electrical conductivity of the composite was enhanced remarkably, and the optimal power factor of 85 mW m
1 K
2 was obtained with 10 wt.% Ag NPs. As a whole, the metallic NPs mainly play a positive role for the enhanced electrical conductivity of PEDOT materials due to the intrinsic high electrical conductivity of NPs and the enhancement of carrier concentration, and show insignificant effect on Seebeck coefficient and thermal conductivity of PEDOT:PSS. Besides, 1D NPs show better effect at a given concentration due to being easier to form a percolation structure. 7.2.2 TE properties of PEDOT/inorganic semiconductor composites The typical inorganic semiconductors include Bi2Te3, PbTe, Te, SnSe, etc., which possess large Seebeck coefficient and/or high power factors around room temperature. Therefore, a series of PEDOT-based inorganic semiconductor composites have been prepared with various methods, and their TE properties have been studied. Low-dimensional tellurium (Te) shows a large Seebeck coefficient at room temperature33 and can be easily synthesized using wet-chemical methods, therefore, it can serve as an effective inorganic filler to improve the TE properties of PEDOT-based composite materials. Segalman’s group did some pioneer and excellent works on exploring the TE properties of PEDOT:PSS/Te composites.23,34,35 In 2010, they synthesized a watersoluble PEDOT:PSS passivated Te nanorods composite film through a thermal reduction method of Na2TeO3 in the presence of PEDOT:PSS (Fig. 7.1).23 PEDOT:PSS serves as a conformal coat on Te nanowires and the composite film shows both high electrical conductivity (w19.3 S cm
1, which is one order of magnitude higher than that of pristine PEDOT:PSS and two orders of magnitude higher than Te nanowires) and high Seebeck coefficient (163 mV K
1, which is nine times larger than that of pristine

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Figure 7.1 (A) Synthesis of PEDOT:PSS passivated Te nanorods, followed by formation of smooth nanocomposite films during solution casting; (B) picture of a typical dropcast composite film on a 1 cm2 quartz substrate. (All panels reproduced with permission See, K. C.; Feser, J. P.; Chen, C. E.; Majumdar, A.; Urban, J. J.; Segalman, R. A. Waterprocessable Polymer-Nanocrystal Hybrids for Thermoelectrics. Nano Lett. 2010, 10, 4664. Copyright 2010, American Chemical Society.)

PEDOT:PSS). A high power factor of 70 mW m
1 K
2 was obtained with a low cross-plane thermal conductivity of 0.22w0.30 W m
1 K
1, and the ZT value was 0.1 at room temperature. In order to gain a better understanding of the TE transport in the PEDOT:PSS/Te composite material, they varied the relative ratio of organic and inorganic components and measured the TE transport properties of these composite films and demonstrated a highly conductive interfacial layer between the coated Te nanowires and PEDOT:PSS.34 This work indicated the importance of the interface control between PEDOT:PSS and inorganic fillers to further optimize the TE performance of composite films. Also the TE properties of PEDOT:PSS/Te composite was further improved by (1) varying the nanowire morphology, (2) by addition of polar solvents (e.g., EG or DMSO), (3) adjusting the relative ratio between Te nanowires and PEDOT:PSS matrix.35 It was showed that the composite with longer Te nanowires together with 16 wt.% PEDOT:PSS and 5 vol.% DMSO contributed to the highest power factor of 100 mW m
1 K
2. Furthermore, they demonstrated a way to adjust the TE performance of PEDOT:PSS/Te composite by controlling the growth of heterostructures in the films (as shown in Fig. 7.2).36 Through introducing Cu2þ into the PEDOT:PSS/Te composite, an alloy subphase of CuxTe can be formed. An energy-dependent carrier scattering mechanism could be introduced by this alloy subphase structure, thus, the carrier scattering and TE properties of the composite could be tuned. After optimization the PEDOT:PSS loading, up to 22% enhancement of the power factor is obtained.

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Figure 7.2 (A) Cartoon depiction of Cu incorporation and nucleation of alloy phases within PEDOT:PSS-Te NWs; (B) the original PEDOT:PSS-Te NWs are straight and rigid; and (C) PEDOT:PSS-Te(Cux) NWs exhibit morphological transition to bent wires.36 (Copyright 2016, American Chemical Society.)

To further enhance the TE properties of PEDOT:PSS/Te composite, some posttreatment method were used to increase the electrical conductivity of composites.37e39 In 2016, Song et al.37 used Hexadecyl trimethyl ammonium bromide (CTAB) as template to synthesis PEDOT:PSS/Te nanorods composite films followed with H2SO4 posttreatment. After the treatment, the electrical conductivity of the composite films increased from 0.22 to 1613 S cm
1 due to the removal of insulating PSS and the structural rearrangement of PEDOT. An optimum power factor of 42.1 mW m
1 K
2 was obtained at room temperature from a PEDOT:PSS/Te (80 wt.%) composite film, which was about 10 times greater than that before treatment. Besides, a flexible PEDOT:PSS/PF-Te (PEDOT:PSS functionalized Te) composite films was prepared using a vacuum-assisted filtration process (as shown in Fig. 7.3).38 With increase content of PF-Te, the Seebeck coefficient of the composites increased from 15.6 to 51.6 mV K
1, and the electrical conductivity decreased from 1262 to 122.4 S cm
1. The sample contained 70 wt.% PF-Te showed a maximum power factor of 51.4 mW m
1 K
2. And a flexible TE generator module was fabricated

PEDOT-based thermoelectric nanocomposites/hybrids

171

Figure 7.3 Schematic illustration of the fabrication of flexible PEDOT:PSS/PF-Te composite film on PVDF via a filtration process.38 (Copyright 2017, Elsevier.)

with an output voltage of 2.5 mV at a 13.4 K temperature difference between a forearm and the ambient. The H2SO4 treatment was also employed to enhance the TE performance of the PEDOT:PSS/PF-Te composite film prepared by a drop-casting method.39 After the treatment, the power factor increased to141.9 mW m
1 K
2 for the film containing 90 wt.% PF-Te, which was 2.75 times greater than that of the untreated composite film. Though much progress have been made on the commercial PEDOT:PSS products, especially PH1000, the price is high. Therefore, high conductive PEDOT materials prepared from some novel methods were also chosen to composite with Te NWs. Hu et al. fabricated highmobility PEDOT-NWs/Te-NWs composite by a physical mix and filter method.40 The TE properties of PEDOT-NWs/Te-NWs composite film were investigated by altering the Te NWs content and found that the enhancement of the TE properties in the composites should be assigned to the energy filtering effect. Ni et al. presented a novel method to prepare PEDOT/Te composites.41 They used free-standing and highly conductive PEDOT nanowire film as a working electrode to electrodeposit Te onto PEDOT films (as shown in Fig. 7.4). By adjusting the electrodeposition conditions, the TE performance of the composite film was optimized, and the maximum power factor can be as high as 240.0 mW m
1 K
2, which was about 8 times higher than that of the pristine PEDOT-NW film. Shi et al. fabricated high quality small-sized anions codoped PEDOT:dodecylbenzenesulfonate/Cltellurium (PEDOT:DBSA/CleTe) composite films using a series of novel Te(IV)-based oxidants.42 Due to the

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Advanced PEDOT Thermoelectric Materials

Figure 7.4 Schematic diagram of Te electrodeposition on the free-standing PEDOT film.41 (Copyright 2020, Elsevier.)

synchronized production of PEDOT and Te, the Te quantum dots were well protected and distributed evenly in the composite films. A high doping levels was caused by the codoping of Cl
/DBSA
, resulting in an enhanced electrical conductivity. The controlled synthesis under certain Te(IV)/Fe(III) oxidant ratios results in Te quantum dots with size 10 nm), the thermal conductivity is close to the bulk value, because phonon scattering at the film-substrate and filmatmosphere interfaces is limited owing to the short intrinsic phonon mean free paths.22 The in-plane thermal conductivity (kǁ) for PEDOT-Tos films was measured to be 0.37  0.07 W m
1 K
1 by 3u-technique, while the out-of-plane thermal conductivity (kt) was about
1
1 23 0.37  0.07 W m K . For pristine PEDOT:PSS thick films, the kǁ values were found to be 0.42  0.07 W m
1 K
1 and
1
1 0.52  0.11 W m K with additives of DMSO and EG, which are 1.4e1.6 times (kǁ/kt) than kt, respectively.24 The difference between kǁ and kt is related to the preparation method of PEDOT/PEDOT:PSS films, and the lamellar and orienting (face-on or edge-on to the substrate) of molecular conjugate plane. Once this impact is ignored, the thermoelectric ZT may have a large error even by order of magnitude. Overall, considerable progress in thermoelectric PEDOT/PEDOT:PSS has been achieved with the help of state-of-the-art theoretical and experimental research. There is no doubt that numerous efforts have made great contributions to the development of advanced thermoelectric PEDOT. This is not only conducive to in-depth understanding the relationship between structure and property, but also promoting the development of high-performance organic thermoelectric materials. PEDOT, a champion in thermoelectric polymers with excellent features, provide an avenue for exploring energy harvest from waste heat to electrical power, which is attracting the growing concern. In addition, it should give explicit recognition to the enormity of high-performance PEDOT thermoelectric material ahead in the future.

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References 1. Kwak, J. F.; Clarke, T. C.; Greene, R. L.; Street, G. B. Transport Properties of Heavily AsF5 Doped Polyacetylene. Solid State Commun. 1979, 31, 355. 2. Park, Y. W.; Han, W. K.; Choi, C. H.; Shirakawa, H. Metallic Nature of Heavily Doped Polyacetylene Derivatives: Thermopower. Phys. Rev. B 1984, 30, 5847. 3. Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic Thermoelectric Materials for Energy Harvesting and Temperature Control. Nat. Rev. Mater. 2016, 1, 16050. 4. Kang, S. D.; Jeffrey Snyder, G. Charge-transport Model for Conducting Polymers. Nat. Mater. 2017, 16, 252. 5. Jacobs, I. E.; Moule, A. J. Controlling Molecular Doping in Organic Semiconductors. Adv. Mater. 2017, 29, 1703063. 6. Poehler, T. O.; Katz, H. E. Prospects for Polymer-Based Thermoelectrics: State of the Art and Theoretical Analysis. Energy Environ. Sci. 2012, 5, 8110. 7. Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J. B.; Geerts, Y. H.; Desbief, S.; Breiby, D. W.; Andreasen, J. W.; Lazzaroni, R.; Chen, W. M.; Zozoulenko, I.; Fahlman, M.; Murphy, P. J.; Berggren, M.; Crispin, X. Semi-metallic Polymers. Nat. Mater. 2014, 13, 190. 8. Liu, J.; Wang, X.; Li, D.; Coates, N. E.; Segalman, R. A.; Cahill, D. G. Thermal Conductivity and Elastic Constants of PEDOT:PSS with High Electrical Conductivity. Macromolecules 2015, 48, 585. 9. Chabinyc, M. Thermoelectric Polymers: Behind Organics’ Thermopower. Nat. Mater. 2014, 13, 119. 10. Heeger, A. J. Semiconducting and Metallic Polymers: the Fourth Generation of Polymeric Materials. J. Phys. Chem. B 2001, 105, 8475. 11. Goel, M.; Siegert, M.; Krauss, G.; Mohanraj, J.; Hochgesang, A.; Heinrich, D. C.; Fried, M.; Pflaum, J.; Thelakkat, M. HOMO-HOMO Electron Transfer: An Elegant Strategy for P-type Doping of Polymer Semiconductors toward Thermoelectric Applications. Adv. Mater. 2020, 32, 2003596. 12. Kaiser, A. B. Electronic Transport Properties of Conducting Polymers and Carbon Nanotubes. Rep. Prog. Phys. 2001, 64, 1. 13. Wang, X.; Zhang, X.; Sun, L.; Lee, D.; Lee, S.; Wang, M.; Zhao, J.; Shao-Horn, Y.; Dinca, M.; Palacios, T.; Gleason, K. K. High Electrical Conductivity and Carrier Mobility in oCVD PEDOT Thin Films by Engineered Crystallization and Acid Treatment. Sci. Adv. 2018, 4, eaat5780. 14. Wei, Q.; Mukaida, M.; Naitoh, Y.; Ishida, T. Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents. Adv. Mater. 2013, 25, 2831. 15. Honma, Y.; Itoh, K.; Masunaga, H.; Fujiwara, A.; Nishizaki, T.; Iguchi, S.; Sasaki, T. Mesoscopic 2D Charge Transport in Commonplace PEDOT:PSS Films. Adv. Electron. Mater. 2018, 4, 1700490. 16. Li, X.; Liu, C.; Zhou, W.; Duan, X.; Du, Y.; Xu, J.; Li, C.; Liu, J.; Jia, Y.; Liu, P.; Jiang, Q.; Luo, C.; Liu, C.; Jiang, F. Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the Thermoelectric Performance of Poly(3,4-Ethylenedioxythiophene)Rich Nanocrystal Films. ACS Appl. Mater. Interfaces 2019, 11, 8138. 17. Bubnova, O.; Berggren, M.; Crispin, X. Tuning the Thermoelectric Properties of Conducting Polymers in an Electrochemical Transistor. J. Am. Chem. Soc. 2012, 134, 16456. 18. Inal, S.; Malliaras, G. G.; Rivnay, J. Benchmarking Organic Mixed Conductors for Transistors. Nat. Commun. 2017, 8, 1767.

264

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19. Rivnay, J.; Inal, S.; Salleo, A.; Owens, R. M.; Berggren, M.; Malliaras, G. G. Organic Electrochemical Transistors. Nat. Rev. Mater. 2018, 3, 17086. 20. Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching Disorder-free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384. 21. Goel, M.; Thelakkat, M. Polymer Thermoelectrics: Opportunities and Challenges. Macromolecules 2020, 53, 3632. 22. Losego, M. D.; Moh, L.; Arpin, K. A.; Cahill, D. G.; Braun, P. V. Interfacial Thermal Conductance in Spun-Cast Polymer Films and Polymer Brushes. Appl. Phys. Lett. 2010, 97, 011908. 23. Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythiophene). Nat. Mater. 2011, 10, 429. 24. Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat. Mater. 2013, 12, 719.

Index Note: ‘Page numbers followed by “f ” indicate figures and “t” indicate tables.’

A

D

Acids, 133e135, 135f AFM. See Atomic force microscopy (AFM) Alkalis, 133e135, 135f Alkoxysulfonate, 40, 42f Alkylcarboxylic, 41, 42f Alkyl chains, 147e148 Anisotropic thermal conductivity, 209e210 Atomic force microscopy (AFM), 55, 56f

Density functional theory (DFT), 33e34, 100e102 Density of states (DOS), 108 Dimethylsulfoxide (DMSO), 165e166 Dip-coating, 47e48 Doping, 4, 7, 14, 21 Drop-coating, 48

B Band structure, 32 Bipolarons, 259e260

C Camphor sulfonic acid (CSA), 11 Carbon nanotube (CNT), 181 Carrier density and mobility field effect transistor method, 210e211 Hall effect method, 211e214, 211f, 213f Charge transport, 1e2, 9, 14, 97, 259e260 in conjugated polymer films, 260e262 Conjugated (conducting) polymers, 145, 165 discovery, 1 photo-thermoelectric conversion, 151e152 physical conduction processes, 260 synthesis and preparation technologies, 2 thermoelectric effect, 1e4 transport properties of, 127e128 Copolymers, 153e159, 156f, 159f Crystalline PEDOT, 33e34

E Electrical conductivity, 2e3, 200e202, 200fe201f commercial PEDOT:PSS, 21 of PEDOT, 15 influencing factors, 79e80 mechanism and characterizations, 82e83 methods, 80e82, 81f origin, 79 polyacetylene, 5 polyaniline, 9e11 polycarbazole, 13e14 polypyrrole, 12 polythiophene, 7 of pristine PEDOT:PSS, 39 temperature dependences, 18e19 and thermal conductivity, 109e110, 110f thermoelectric power factor, 78e83 and thermopower, 108e109 Electrochemical polymerization, 44e45 Electrodeposition, 44e45, 44f Electronic thermal conductivity, 89 3,4-Ethylenedioxythiophene (EDOT), 146, 155e156 Ethylene glycol (EG), 165e166

F Fabrication techniques coating, 47e49

265

266

Index

Fabrication techniques (Continued) filtration, 49e50, 50f gel method, 50e51, 51f printing, 49 Field effect transistor (FET) method, 210e211 Figure of merit, 2e3, 91e92 Filtration, 49e50, 50f Flexible and wearable devices, 220e221 thermoelectric fiber, 231e237, 232te234t, 235f thermoelectric film, 222e231, 223te225t, 226fe228f, 230f thermoelectric module, 238e251, 239te242t inkjet printing, 246e248, 247f photolithography, 250e251, 251f roll-to-roll, 248e250, 249fe250f screen printing, 238e245, 243f, 245f Flexography printing, 49 Free-standing conducting polymer, 128e129 Fused ring units, 157e158

G Gel method, 50e51, 51f GIWAXS. See Grazing incidence wide angle X-ray scattering (GIWAXS) Graphene, 185e188 Graphene quantum dots (GQDs), 185e188 Gravure printing, 49 Grazing incidence wide-angle X-ray scattering (GIWAXS), 60e65, 61f, 63fe64f vs. PEDOT:PSS, 126e127, 128f

H Hall effect method, 211e214, 211f, 213f Highest occupied molecular orbital (HOMO), 149e151 Hopping conductors, 98 Humidity conditions, 135 Hydrophobic PEDOT core, 39 Hydroxymethyl EDOT (HMEDOT), 40e41, 41f

I Inhomogeneous disorder model, 32 Inkjet printing, 49, 246e248, 247f Inorganic nanocrystals, 166 Inorganic semiconductor materials, 145 Inorganic thermoelectric materials, 2e3 In-plane thermal conductivity, 90e91 In situ polymerization method, 50 Internet of things, 39

L Lattice thermal conductivity, 89e90 Linear alkyl chains, 147e148 Log-log scale, 19e21 Low dimensionality, 126e127, 128f Lowest unoccupied molecular orbital (LUMO), 149e151

M Metal-semiconductor transition, 260 Molecular conformation, 131e132, 132f Mott formula, 108

O Octadecyl side chain, 147e148 One-dimensional (1D) conducting polymers, 126e127 Optimizing thermoelectric performance crystal structure, 127e130, 129f doping and undoping chemical, 120e125, 124fe125f electrochemical, 125e126, 126f low dimensionality, 126e127, 128f molecular conformation, 131e132, 132f phonon scattering, 130 posttreatment, 133e137 acids or alkalis, 133e135, 135f environment-friendly, 136e137 humidity conditions, 135 mixture treatments, 135e136 multistep processing, 136, 137f polar organic solvents, 133, 134f Organic electrochemical transistors (OECTs), 257e258 Organic field-effect transistors (OFETs), 24e26, 257e258

Index

Organic light-emitting diode (OLED), 148 Organic solar cells (OSC), 148 Organic thermoelectric polymers, 257 Out-of-plane thermal conductivity, 90e91 Oxidation polymerization, 42e43, 43f Oxidative chemical vapor deposition (oCVD). See Vapor phase polymerization (VPP) Oxide semiconductor materials, 177e178

P Peltier effect, 199 Performance dependence carrier concentration and mobility, 112, 113f electrical conductivity, 108e110 order and disorder, 113 semicrystalline, 110e111 temperature, 111e112 thermal conductivity, 109e111, 110f thermopower, 108e109 Phonon scattering, 130 Photolithography, 250e251, 251f Polarons, 259e260 Polar organic solvents, 133, 134f Poly(3,4-ethylenedioxythiophene) (PEDOT) advantage of, 15e17, 16f electronic states GIWAXS, 60e65, 61f, 63fe64f Raman spectroscopy, 58, 59t, 60f UV-Vis-NIR spectroscopy, 57, 58f X-ray photoelectron spectroscopy, 56e57, 57f fabrication techniques coating, 47e49 filtration, 49e50, 50f gel method, 50e51, 51f printing, 49 insulator to semimetal, 76e78, 77f morphology structure AFM, 55, 56f SEM, 51e53, 52fe53f TEM, 53e55, 54f

267

organic thermoelectric polymers, 257 p-conjugation, 257 polymerization methods, 41e47 electrodeposition, 44e45, 44f oxidation polymerization, 42e43, 43f vapor phase polymerization, 45e47 synthesis, 40e41 Polyacetylene (PAc), 5e6, 5f Polyaniline (PANi), 9e11, 12f Polycarbazole (PCz), 13e14 Polymer semiconductors, 97 Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS), 39 chemical structure, 75e76, 75f Polypyrrole (PPy), 12e13, 13f Polyselenophenes (PSes), 151 Polythiophene (PTh), 7e9, 7fe8f, 10f derivatives, 147e148 Polythiphenes (PThs), 151 3,4-Propylenedioxythiophene (ProDOT), 149e151

R Raman spectroscopy, 58, 59t, 60f Reduced graphene oxide (rGO), 185e188 Roll-to-roll (R2R), 248e250, 249fe250f

S Scanning electron micrography (SEM), 51e53, 52fe53f Screen printing, 49, 238e245, 243f, 245f Seebeck coefficient, 97e98, 259 of carrier concentration, 112, 113f Seebeck effect, 199, 202e206 analysis of the error, 204e206, 205f quasi-static method, 203e204, 204f static method, 203, 203f SEM. See Scanning electron micrography (SEM) Soluble self-doped PEDOT (S-PEDOT), 39e40 Solution casting polymerization (SCP), 226e228

268

Index

Spin-coating, 48 Spray-coating, 49 Sulfuric acid, 165e166 Supramolecular self-assembly, 51

T Tellurium (Te) nanostructure, 168e169 TEM, 53e55, 54f Temperature coefficient of resistivity (TCR), 18e19 Temperature gradient, 97e98 Tetradecyl side chain, 147e148 Tetrakis(dimethylamino)ethylene (TDAE) vapor, 24 Thermal conductivity, 3, 17e18, 21e22, 28, 33e34, 88e91 anisotropic, 209e210 characterization, 206e207 electrical conductivity, 109e110, 110f electronic, 89 in-plane and out-of-plane, 90e91 lattice, 89e90 quantitatively calculate, 209 and semicrystalline, 110e111 thermal power, 207e208 triple frequency method, 207, 207f Thermoelectric (TE) analogues, 151e153, 151f, 152t, 153f band structure, 106e107 copolymers, 153e159, 156f, 159f density of states, 107e108 dependence carrier concentration and mobility, 112, 113f electrical conductivity, 108e110 order and disorder, 113 semicrystalline, 110e111 temperature, 111e112 thermal conductivity, 109e111, 110f thermopower, 108e109 derivatives, 146e151, 146fe148f, 150f figure of merit, 91e92 material, 73 performance, 119e120 of optimization, 145e146 power factor, 78e88 electrical conductivity, 78e83

power factor, 85e88, 87f thermopower, 83e85, 85fe86f properties of PEDOT-based ternary composites, 188e190, 189fe190f of PEDOT/carbon nanomaterial composites, 180e188, 182fe183f, 185fe187f of PEDOT/inorganic semiconductor composites, 168e180, 169fe174f, 176fe177f, 179f of PEDOT/metal nanoparticle composites, 166e168 transport theory electronic structure, 100e102, 101fe102f model setup, 104e106, 105f stable geometric structure, 99e100, 99f transport property, 103e104, 103f Thermoelectric conjugated polymers evolution, 2e4 typical, 4e14 polyacetylene, 5e6, 5f polyaniline, 9e11, 12f polycarbazole, 13e14 polypyrrole, 12e13, 13f polythiophene, 7e9, 7fe8f, 10f Thermoelectric fiber, 231e237, 232te234t, 235f Thermoelectric film, 222e231, 223te225t, 226fe228f, 230f Thermoelectric generators (TEGs), 219, 221f Thermoelectric module, 238e251, 239te242t inkjet printing, 246e248, 247f photolithography, 250e251, 251f roll-to-roll, 248e250, 249fe250f screen printing, 238e245, 243f, 245f Thermoelectric PEDOT/PEDOT:PSS, 17e34 awaited stage, 30e34 early stage, 18e22, 20f exploratory stage, 22e30, 23f, 25f, 29f Thermoelectric performance, 2e3, 5, 17e18, 22, 24

Index

Thermoelectric power factor, 3, 12e14 Thermopower, 6 electrical conductivity, 108e109 estimation, 26 polyaniline, 9e10 polypyrrole, 12 semiconductors, 6 thermoelectric power factor, 83e85, 85fe86f zero-current transport coefficient, 4 Thiophenederivatives, 156e157 Three-dimensional (3D) crystalline solids, 98 Time-domain thermal reflectance (TDTR) method, 23e24 Tosylate ion, 44e45 Transport edge, 98 Transport theory electronic structure, 100e102, 101fe102f model setup, 104e106, 105f stable geometric structure, 99e100, 99f transport property, 103e104, 103f Triple frequency method, 207, 207f

269

2D layered materials, 178e180 Typical thermoelectric conjugated polymers, 4e14 polyacetylene, 5e6, 5f polyaniline, 9e11, 12f polycarbazole, 13e14 polypyrrole, 12e13, 13f polythiophene, 7e9, 7fe8f, 10f

U UV-Vis-NIR spectroscopy, 57, 58f

V Vacuum-filtration method, 178e180 Vapor phase polymerization (VPP), 15, 26e27, 45e47

W Weak dependence, 31e32 White graphene, 178e180

X X-ray photoelectron spectroscopy (XPS), 56e57, 57f