Dye-sensitized Solar Cells 9783110344363, 9783110344202

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Songyuan Dai (Ed.) Dye-sensitized Solar Cells GREEN Alternative Energy Resources

GREEN Alternative Energy Resources

Volume 7

Dye-sensitized Solar Cells Edited by Songyuan Dai

Editor Songyuan Dai No. 2 Beinong Road, Zhuxinzhuang Changping District Beijing People’s Republic of China [email protected]

ISBN 978-3-11-034420-2 e-ISBN (PDF) 978-3-11-034436-3 e-ISBN (EPUB) 978-3-11-038373-7 Library of Congress Control Number: 2022938226 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2022 Walter de Gruyter GmbH, Berlin/Boston and China Science Publishing & Media Ltd. Cover image: vencavolrab/iStock Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface The demand for energy runs through the history of human activities. From the earliest simple utilization of natural energy (such as biomass materials and water power) to the development of fossil energy (such as coal, oil, and natural gas) in the industrial era, various types of energy are always an indispensable part of human activities. Especially in the modern society, with the highly intensive human society and the increasing population, the human demand for energy also presents explosive growth. However, with the large-scale development of fossil energy, a series of accompanying problems are increasingly becoming prominent contradictions in today’s society, such as the international situation turbulence caused by energy shortage, environmental pollution, and greenhouse effect caused by energy use, and geological structure change caused by energy mining. Therefore, it is an urgent problem to find one or several new conventional energy sources to replace the existing energy. Solar energy will undoubtedly become one of the main ways of new energy utilization in the future because of its characteristics of being free from regional restrictions, clean, and pollution free. In the application of solar energy, solar photovoltaic power generation directly converts solar energy into electrical energy, which is a more convenient form of direct use of solar energy. The carrier of solar photovoltaic power generation is solar cell. Many types of solar cells and their classifications were introduced in this book. In various types of solar cells, most of them are p–n junction structures, which generate electric current through the separation of electrons and holes. However, dye-sensitized solar cell is a type of special solar cell. The process of charge generation and transport is different from most other solar cells. In the actual working state, the cell involves many physical transport processes and chemical reaction processes. Therefore, the working principle, composition system, and solar materials of the whole solar cell are also complex. The understanding of dye-sensitized solar cells requires not only certain physical thinking but also certain chemical skills. Therefore, the editor believes that it is of far-reaching significance to write a book describing dye-sensitized solar cells in an all-round way. Readers can understand the whole dye-sensitized solar cells and the parts they are interested in deeply by reading the book; beginners can start through the book, and researchers can inspire new ideas and new research directions through the book. The editor hopes that the publication of this book can give a good introduction and play a positive role in the research of dye-sensitized solar cells. Dye-sensitized solar cells are composed of four main parts: nanoporous film, dye, electrolyte, and counter electrode. After the introduction of Chapter 1, this book makes a comprehensive discussion on the above four parts through four chapters. In addition, the generation, transfer, transport, and recombination of charge in dye-sensitized solar cells are complex, especially a series of photoelectrochemical reactions at the interface are the key factors affecting the performance of solar cells. Therefore, Chapter 6 of this book focuses on the detailed description of the https://doi.org/10.1515/9783110344363-202

VI

Preface

interfacial reaction and its principle in combination with the commonly used electrochemical and spectroelectrochemical characterization methods. After more than 20 years’ development, dye-sensitized solar cells have been put into pilot-scale industrialization. Therefore, in addition to the above-mentioned basic materials and theories, the practical research has been made based on the developmental experience in the past 20 years. From the perspective of industrialization, we have discussed the photoelectric conversion simulation and structural design of dyesensitized solar cells in Chapter 7, and made a preliminary discussion on the test standards of the dye-sensitized solar cells in Chapter 8. The editor hopes that through the introduction of the above chapters, the dye-sensitized solar cells can be described clearly and comprehensively. In 1993, academician Huo Yuping (the director of the Institute of Plasma Physics of the Chinese Academy of Sciences) and academician Zhou Guangzhao (the president of Chinese Academy of Sciences), as members of the world laboratory, met professor M. Grätzel of EPFL in Switzerland (inventor of dye-sensitized solar cells). When they attended the annual meeting of the World Laboratory in Lausanne, Switzerland, as scientists who devoted their whole lives to the research of nuclear fusion physics, they are keenly aware of the importance of this new solar cell technology, so they are determined to introduce this technology into China. At that time, the patent of the technology had been licensed by STA company of Australia (now changed to Dyesol company) and INAP company of Germany. After many negotiations and exchanges, the company finally signed a long-term free cooperation agreement with STA company of Australia at the end of 1996, laying a good foundation for the introduction of the technology into China and future industrialization. In April 1997, the two sides officially sent personnel for technical exchanges. In this process, the World Laboratory successively used the renewable energy project to support the international exchange of Chinese researchers, and the special fund of the president of the Chinese Academy of Sciences supported the project twice. In June 2000, the research of dye-sensitized solar cells was supported by the key direction project of knowledge innovation project of Chinese Academy of Sciences. In September 2000, dye-sensitized solar cells and other thin-film solar cells were supported by the national key basic research and development project (973 program) “Basic research of low-cost and long-life new-type photovoltaic cells.” With the support of these projects, it is strengthened that the advantages of dye-sensitized solar cell research in China and the exchange between international peers and a 500 W demonstration system have been established. In 2006, the basic research of dye-sensitized solar cells was supported by the “973” project “Basic research on key scientific and technological issues of large-area low-cost long-life solar cells.” In 2008, the research on related equipment and key technologies was also supported by the key direction project of knowledge innovation project of Chinese Academy of Sciences entitled “Research on dye-sensitized solar cell pilot test technology” and national high technology research and development program

Preface

VII

(“863” program) project entitled “Research and development of key technologies for dye-sensitized solar cell assembly” in 2009. At the end of 2011, the design, installation, and commissioning of 0.5 MW dye-sensitized solar cell pilot line were completed, and a 5 kW demonstration system was established. Significantly, in order to improve the lifetime and related technology level of dye-sensitized solar cells, a “973” project entitled “Basic research on high-efficiency and low-cost new thin-film photovoltaic materials and devices” has given greater support to the research of dye-sensitized solar cells. The materials of this book come from the research results of the author who has been engaged in the basic research and technical research of dye-sensitized solar cells in the past 20 years. The book strives to be illustrated with detailed data and clear images. In the process of writing, the editor also refers to the latest progress and research results in relevant fields at home and abroad, and quotes some viewpoints, contents, charts, and data in literatures and books. We would like to express our sincere thanks to these authors. In the process of writing this book, the researchers, doctoral students, and master students in his laboratory have made different contributions to the formation of the content of this book. The purpose of this book is to give a clear and detailed description of the basic principles, materials, and practical technology of dye-sensitized solar cells. The author hoped that this book will be conducive to the further development of dye-sensitized solar cells and the improvement of the research level of relevant researchers. Due to the cultivation of graduate and undergraduate students and the deepening of the understanding of dye-sensitized solar cells by other industry personnel, and due to the rapid development of dye-sensitized solar cells, the research covers a wide range of subjects. Although the author uses the latest literature as much as possible, when the reader reads this book, it is inevitable that there will be many new research results published. At the same time, in view of the limited level of the author, there are inevitably omissions and deficiencies in the book, so the reader is requested to criticize and correct. Songyuan Dai

Contents Preface

V

List of contributors

XI

Songyuan Dai, Linhua Hu Chapter 1 Introduction 1 Linhua Hu, Songyuan Dai Chapter 2 Nano-semiconductor materials

41

Fantai Kong, Songyuan Dai Chapter 3 The dyes used in dye-sensitized solar cells Xu Pan, Songyuan Dai Chapter 4 Electrolyte used in dye-sensitized solar cells Xu Pan Chapter 5 Counter electrode for dye-sensitized solar cells

139

205

269

Weiqing Liu, Songyuan Dai Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells 299 Yang Huang, Songyuan Dai Chapter 7 Structure design and performance simulation of dye-sensitized solar cells 441 Shuanghong Chen, Songyuan Dai Chapter 8 Performance test and module application of dye-sensitized solar cells Index

515

483

List of contributors Chapter 1 Songyuan Dai, North China Electric Power University

Chapter 5 Xu Pan, Hefei Institutes of Physical Science, Chinese Academy of Sciences

Linhua Hu, Hefei Institutes of Physical Science, Chinese Academy of Sciences

Chapter 6 Weiqing Liu, Nanchang Hangkong University

Chapter 2 Linhua Hu, Hefei Institutes of Physical Science, Chinese Academy of Sciences

Songyuan Dai, North China Electric Power University

Songyuan Dai, North China Electric Power University Chapter 3 Fantai Kong, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University Chapter 4 Xu Pan, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University

https://doi.org/10.1515/9783110344363-204

Chapter 7 Yang Huang, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University Chapter 8 Shuanghong Chen, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University

Songyuan Dai, Linhua Hu

Chapter 1 Introduction Energy is an important requirement for survival and development of mankind and is the foundation of the national economy and social development. As a result of fossil energy depletion and global awareness on the “green-house effect,” developing and using clean and renewable energy becomes the theme of the present and future world energy technology development. The peak mining of fossil fuel will happen before the middle of this century, and human energy use will change radically in the first half of the century. Fossil fuel extraction peak is only 20 years from now, and conventional energy (coal or hydro) power generation costs have been rising year on year. Humanity is facing a very urgent need for alternative energy. Renewable energy saves resources, shows clear development prospects, and is a less controversial technology. Development and utilization of renewable energy involves saving and substituting part of fossil fuels, promoting the adjustment of energy structure, thus, putting the environmental pressure down; It is not only an important measure to solve the current energy supply and demand, but also a strategic choice for the future energy, economic, social, and environmental sustainable development, ensuring the safety of national energy and the environment as well as the development of low-carbon economy. By the end of 2009, the concepts of “energy conservation and emissions reduction” and “low carbon” had gained importance. In the Copenhagen conference, “low carbon” became a development direction and a new navigation mark in global economy. As a kind of clean renewable energy, photovoltaic (PV) power generation is one of the ideal energies, and has gained extensive attention all over the world. As an important application field of renewable energy, solar photovoltaic power generation has seen fast development in the past 10 years. Solar photovoltaic (PV) will be developed as the main source of the world energy in the future. In order to tackle climate change, Chinese government has put forward the goal that by 2020, non-fossil energy should be able to meet 15% of primary energy consumption targets, as also that carbon intensity will be reduced by 40%–50% of the binding forces. Thus, renewable energy and low-carbon economy development path, is the surest way to achieve this ambitious goal. The abundant solar energy is a huge plus for developing photovoltaic power generation, in most places.

Songyuan Dai, North China Electric Power University Linhua Hu, Hefei Institutes of Physical Science, Chinese Academy of Sciences https://doi.org/10.1515/9783110344363-001

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Songyuan Dai, Linhua Hu

1.1 History and development of solar cell Solar cell is a kind of electricity generation device that can convert sunlight directly into electricity. As long as there is light on the solar cell, the cell can output voltage and current, and this is known as the solar photovoltaic effect in physics (Photovoltaic – combining the two words of photo and voltaic, PV for short).

1.1.1 Brief history of development of solar cell Solar cell development history can be traced back to 1839, during the times of the French physicist, Alexander – Edmond Becquerel, who discovered the Photovoltaic effect. From then on, people began study in this field. – In 1883, the American, Fritts, made the first large area (30 cm2) solar cells on a metal substrate. – At the end of nineteenth century, Moster first reported the photoelectric effect of dye sensitization in the University of Vienna. – In 1930, Schottky first proposed the Cu2O barrier theory of photovoltaic effect. In the same year, Longer first proposed manufacturing solar cell using the photovoltaic effect and then converting the solar energy into the electrical energy. – In 1954, the first practical monocrystalline silicon p–n junction solar cells were developed successfully. After a few months, the efficiency of this kind of solar cell increased up to 6%. Shortly thereafter, silicon cells were commercially used in the aerospace field. – In 1959, Hoffman Company in the United States launched commercial silicon cells with 10% efficiency. – In 1960, silicon solar cells were connected to the grid. – In the 1860s, CdTe thin-film cells acquired 6% photoelectric conversion efficiency. – In 1967, the first piece of GaAs cell with efficiency of up to 9% was developed successfully.。 – In 1974, amorphous silicon solar cells were developed successfully. – In 1980, the first CuInSe cell, whose efficiency is more than 10%, was made in the United States; Carlson of RCA developed the amorphous silicon solar cells, whose efficiency was up to 8%. – In 1981, the 350 KW of concentrator cell matrix was established in Saudi Arabia. – In 1982, the first 1 MW photovoltaic power stations were installed in California, United States. – In 1990, the Germans put forward the “2000 photovoltaic roof plan”; it marked that the coming of age of PV grid-connected power generation technology.

Chapter 1 Introduction

3

– In 1991, Lausanne, Switzerland, Grätzel proposed a mesoporous thin-film dyesensitized solar cell (DSC), the efficiency of which reached 7.1%; it was a breakthrough in DSC research. – In 1993, the efficiency of mesoporous thin-film DSCs reached 10%. – In 1997, the US President Clinton’s “Million photovoltaic roofs” initiative, Japan “new sunshine plan”, and the “Millions of Photovoltaic roofs” plan of the Dutch government began, one after another. Then, the capacity of photovoltaic cells reached 100 MW in the same year. – In 1999, a number of photovoltaic power stations reached 1,000 MW in the world. – In 2002, the STA Company in Australia set up the world’s first 200 m2 DSC PV roof, fully embodying the photovoltaic power generation industrialization prospects of future of DSCs. – In 2002, the world’s total installed PV capacity reached 2 GW. – At the end of 2012, the global total installed capacity reached 95 GW. It took 25 years from 1954, the year in which the photovoltaic cell appeared first, for the total PV installed capacity to reach 1 GW. However, it took only 3 years for total PV installed capacity to increase from 1 GW to 2 GW. In 2013, the new and total installed capacity touched 37 and 137 GW, respectively. In 2013, China’s PV installed capacity of 11 GW, ranked it the first in the world, and its total installed capacity was 18 GW, taking it to the second position in the world. Germany’s solar photovoltaic power generation capacity had reached 36 GW, playing an important role in the global photovoltaic development. Many international agencies believe that solar energy will be the subject of future energy supply, and they expect solar energy will account for over 50% of the total energy consumption by the end of this century.

1.1.2 The current situation in the development of solar cells Since the year that the first practical solar cell was developed in 1954, great progress has been made in increasing the efficiency of different types of solar cells and modules. At the same time, a few types of solar cells have practical application, or have been researched. Table 1.1 lists the various solar cell efficiency certified by third parties till July 2013.

1.1.3 Solar cell applications Solar photovoltaic power generation has obvious advantages. Firstly, it is rich in resources as a renewable clean energy. Photovoltaic power generation, especially, can work without water, so the construction of photovoltaic power station is not affected

. (ap) . (ap) . (da)

. ± . . ± . . ± . . ± . . ± .

Amorphous silicon Nanocrystalline silicon

Dye-sensitized cell Dye-sensitized modules

Organic thin film . ± .

. (ap) . (ap)

. ± .

CdTe (cell)

Organic cells (submodule)

. (ap)

. ± . . ± .

CIGS (cell) CIGS (submodule)

. (da)

. (ap) . (ap)

. (ap) . (t) . (t)

. ± . . ± . . ± .

III–V cells GaAs (thin film) GaAs (multicrystal) InP (crystal)

. (da) . (ap) . (ap) . (ap)

Area (cm)

 ± . . ± . . ± . . ± .

Efficiency (%)

Silicon Crystalline Multicrystalline Thin-film transfer Thin-film submodules

Classification

Table 1.1: Certified solar cell and module efficiency in 2013 [1].

.

.

. .

. .

.

. .

. . .

. . . .

Voc (V)

.

.

. .

. .

.

. .

. . .

. . . .

Jsc (mA·cm−)

.

.

. .

. .

.

. .

. . .

. . . .

FF (%)

AIST (/)

AIST (/)

AIST (/) AIST (/)

NREL (/) FhG-ISE (/)

Newport (/)

NREL (/) FhG-ISE (/)

NREL (/) NREL (/) NREL (/)

Sandia (/) NREL (/) NREL (/) FhG-ISE (/)

Testing center and date

Toshiba, four series

Mitsubishi Chemical

Sharp Sony, eight parallel

Oerlikon Solar EPFL, Neuchatel

GE Global Research

NREL glass substrate Solibro, four Series

Alta Devices RTI, Ge substrate

UNSW PERL FhG-ISE Solexel ( μm) CSG Solar

Description

4 Songyuan Dai, Linhua Hu

. (ap) . (ap)

. ± . . ± . . ± .

a-Si/nc-Si/nc-Si (thin film)

a-Si/nc-Si (thin film)

a-Si/nc-Si (thin-film module)

.

.

.

.

.

.

.

.

.

.

.

.

AIST (/)

AIST (/)

NREL (/)

AIST (/)

Kaneka

Kaneka

LG Electronics

Sharp

(ap), aperture area; (t), total area; (da), designated illumination area. FhG-ISE, Fraunhofer-Insitut für Solare Energiesysteme; JQA, Japan Quality Assurance; AIST, Japanese National Institute of Advanced Industrial Science and Technology.

. (ap)

. (ap)

. ± .

GaInP/GaAs/InGaAs

Chapter 1 Introduction

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Songyuan Dai, Linhua Hu

by water resource. It can work in free land or even in the desert. Secondly, the photovoltaic cell installation is simple, flexible, and convenient to change the installed scale. It can be combined with building construction (so-called BIPV, buildingintegrated photovoltaics), it almost does not need operational maintenance, and has a great potential for development and utilization. Thirdly, the output characteristics of photovoltaic power generation are consistent with power load characteristic, which can effectively reduce the pressure of pitching electric load peak. This is beneficial in optimizing the power system operation and saving nonrenewable fossil energy resources, so as to achieve energy conservation and emissions reduction targets. In 2010, the performance of the global PV industry increased dramatically. A large number of new capacities greatly expanded the supply of photovoltaic market. In 2011, the global newly installed PV capacity reached 29.6 GW, a year-on-year growth of 76.2%, compared to 16.8 GW in 2010. By the end of 2010, the top five manufacturers of the global photovoltaic cells were Hebei JA company, First Solar company in the United States, Suntech, Baoding Yingli, and Trina Solar companies, with production of 1,500, 1,300, 1,200, 1,000 and 930 MW, respectively. It can be seen that the world solar photovoltaic enterprise competition has been upgraded to GW formal level. Figure 1.1 shows the photovoltaic solar power installed capacity in China.

Figure 1.1: Solar photovoltaic-installed capacity in China.

In recent years, many governments in the world have a photovoltaic industry development plan in place. The photovoltaic cell industry has grown at a high speed around the world, and diversified demand situation has emerged. Grid power still accounts for the market demand; however, PV and building integration will become the future trend. From 2010 to 2020, the global annual growth rate of the PV industry has reached 34%, with the installed capacity reaching 11.34 GW. According to the relevant forecast,

Chapter 1 Introduction

7

in 2030, solar photovoltaic power generation will account for more than 10% of the world’s total electricity supply rate, while, in the end of the twenty-first century, the proportion of solar photovoltaic power generation will be as high as 60%. “China’s new energy industries development plan” has put forward a target of solar power installed capacity of 20 GW till 2020. But in 2012, the international solar energy market changed dramatically, and then the National Energy Administration of China changed the solar energy development plan. The National Energy Administration of China announced the “solar power development twelfth five-year plan.” By the end of 2015, China’s solar power capacity has reached 35 GW. These numbers can depict the solar photovoltaic industrywide development prospects in the future. At the same time, drawing a roadmap for national renewable energy development clearly, building the new energy economy policy system, and strengthening the supervision of the new energy industry are important for the future healthy development of the PV industry [2].

1.2 Solar cell classification and application 1.2.1 Solar cell classification The core of photovoltaic power generation systems is the solar cell. From the development technology angle, solar cells can be classified into the following several stages. The first-generation solar cell: crystalline silicon solar cells; second-generation solar cell: amorphous silicon thin-film cells (a-Si), CdTe solar cells (CdTe), CIGS solar cell (CIGS), GaAs solar cells (GaAs), DSC; third-generation solar cell: all types of tandem solar cells, thermophotovoltaic cell (TPV), quantum dot super lattice and quantum well solar cell, intermediate band solar cells, the upconversion solar cell, the downconversion solar cell, hot carrier solar cell, collision ionization solar cell, and other new concept solar cells. According to the cell structure, solar cells can be classified as the crystalline silicon solar cell and thin-film solar cells. According to the materials, solar cells can be classified as the silicon solar cell, compound solar cells, dye-sensitized cells, and organic thin-film cells. (1) Silicon solar cell Silicon solar cell is based on the silicon crystal morphology and can be classified as monocrystalline silicon solar cell, polycrystalline silicon solar cells, and amorphous silicon solar cells. Monocrystalline silicon solar cells have higher conversion efficiency, compared to the other two silicon solar cells. Compared to monocrystalline silicon, polycrystalline silicon solar cell has lower cost and higher conversion efficiency than amorphous silicon solar cells. Amorphous silicon thin-film solar cell has low cost, light weight, is convenient for mass production, and has other advantages, lending it great potential.

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Songyuan Dai, Linhua Hu

(2) GaAs solar cell The conversion efficiency of single-junction GaAs III–V group cell can reach 28% with strong irradiation resistance, but the price is very high. Thus, it is usually used in space and for other special use. (3) CdTe and other II–VI group compound solar cells Cadmium telluride (CdTe) thin-film cells have higher efficiency than amorphous silicon thin-film solar cell, the cost is lower than that of monocrystalline silicon cell, and can be mass-produced easily, so it needs to focus on developing environmentfriendly technology in the process of industrialization. (4) CIGS thin-film solar cell Copper–indium–gallium–selenium (CIGS) thin-film solar cell is suitable for photoelectric conversion with no light recession. The photoelectric conversion efficiency of CIGS solar cell is comparable with polycrystalline silicon cell, while it has the advantages of low cost, good performance, and simple process. (5) DSC Based on nano-TiO2 porous films, a DSC is made up of the nanoporous anode, an electrolyte with redox couple, and the catalytic counter electrode. The key materials and preparation technology offer a low-cost and good application prospect. (6) Polymer solar cell Polymer solar cells have rich sources, require a simple process, and are low cost, but it is also easy to implement “roll-to-roll” production. (7) Quantum dot solar cell Quantum dot solar cells are set to achieve a new generation of one of the important structures of the cell because of the quantum effect and high photoelectric conversion efficiency. Hence, it has attracted much attention in recent years. (8) The new-structure solar cell In order to improve the efficiency of the cell, researchers have been trying to find new materials or structures, looking at improving the conversion efficiency of semiconductor thin-film cells and new efficient and low-cost solar cells. The new-structure solar cells mainly include nano-silicon, black silicon, multijunction tandem, light trapping, and quantum well and super lattice. The related study is still preliminary.

1.2.2 Solar cell module classification and application Solar cell modules can be classified into the following types: normal DC output solar cell modules, building solar cell modules, lighting type solar cell modules, new type of solar cell modules, etc.

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(1) Normal DC output solar cell modules For normal DC output solar cell modules, the module size is different for different manufacturers. The output voltage of solar cell modules varies as cell component size changes. At present, the commercial individual modules have close to 300 W; the output voltage is commonly 17–40 V, with the output power of 100–300 W. (2) Building-integrated solar cell modules Building-integrated solar cells of new solar cell module can be of three types, namely, roof integrated modules, wall body integrated modules, and soft modules. Among them, roof integrated modules are mainly used for personal home solar photovoltaic system; wall body integrated modules are mainly used for public building walls. Soft modules are mainly used in window glass and building surfaces. (3) Daylighting-type solar cells Daylighting-type solar cells’ modules are designed for enterprise office building, factory, and public facilities, such as for building glass or curtain with aesthetic considerations. Daylighting-type solar cell modules can generate power for the use of the building in coordination with environment. Daylighting-type solar cell modules, can be classified into various forms in accordance with solar cell types used, namely by the combination of crystalline solar cell with glass structure, composite glass daylighting type solar cell modules, the thin-film solar cell combinations, and composite glass daylight-type solar cell modules. (4) New type of solar cell modules There are many different kinds of new type of solar cell modules, including the AC output solar cell modules, electric energy storage functional solar cell modules, snow-melting functional-type solar cell modules, and double-side power-generating amorphous silicon germanium hybrid heterojunction solar cell modules.

1.2.3 Silicon solar cells Silicon-based cells include polycrystalline silicon solar cell, monocrystalline silicon solar cell, and amorphous silicon solar cell. Of several silicon series cells, monocrystalline silicon solar cell has the highest photoelectric conversion efficiency; the technology is also the most mature and is still dominant in the large-scale application and industrialization. But the cost of single-crystal silicon cell is relatively high. In order to reduce cost and the dosage of silicon materials, polycrystalline silicon and amorphous silicon solar cells were developed as an alternative to traditional monocrystalline cells. The cost of amorphous silicon cell is relatively low and

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the weight is light, which is suitable for mass production, but it is limited by material-related light-induced degradation, and stability needs to be further improved. The cost of polysilicon cells is less than monocrystalline cells, while the efficiency is higher than amorphous silicon cells that also have very good application prospects. The efficiency of commercial crystalline silicon cells can reach 14%–20% (16%–20% for single-crystal silicon, 14%–18% for polycrystalline silicon). Among commercial solar cells, polycrystalline silicon and monocrystalline silicon solar cell account for nearly 90%. Silicon cells are widely used in grid generation, off-grid power generation, and commercial applications.

1.2.3.1 Single-crystalline silicon solar cell In 1954, Pearson, Fuller, and Chapin invented the first modern monocrystalline silicon solar cell at Bell Lab [3]. The main characteristics of the cell are diffusional p–n junction on monocrystalline silicon slice with double electrode structure in the substrate. However, the crystal silicon cell developed rapidly until the 1980s and 1990s. Now, great progress has been made not only in the basic research of highly efficient cells, but also in the practical research and the practical applications. At present, the photoelectric conversion efficiency of monocrystalline silicon solar cell has reached 25% [3]. Figure 1.2 shows the efficiency evolution of monocrystalline silicon cell.

Figure 1.2: The efficiency evolution of monocrystalline silicon solar cell.

High-performance monocrystalline solar cells are based on high quality single-crystal silicon materials and the related mature processing technology. Monocrystalline silicon cell technology has been applied in cell production, the general method of surface texture, emitter passivation, partitional doping technology, etc. The electrode includes a flat monocrystalline cell grid electrode and grooving buried grid electrode

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for monocrystalline cells. Improving the conversion efficiency mainly involves processing with monocrystalline silicon surface microstructure and partition of doping process improvement. Though the monocrystalline silicon solar cells have the highest conversion efficiency, the complex process and the high price of key materials lead to the high cost of monocrystalline silicon and complex production line process. In order to save high quality materials consumption, polycrystalline silicon and amorphous thin-film solar cell developed gradually, as an alternative to the monocrystalline silicon solar cell.

1.2.3.2 Polycrystalline silicon solar cell Polycrystalline silicon solar cell is a solar cell that uses polycrystalline silicon as substrate materials. According to the thickness of the silicon material, it can be classified as polycrystalline silicon cell or polycrystalline silicon thin-film cell. Generally, polycrystalline silicon solar cells are called bulk polysilicon solar cell. The performance of polycrystalline silicon solar cell is almost the same as that of the monocrystalline silicon solar cell. The crystalline silicon solar cells are usually prepared on 200–300 μm thick high-quality silicon wafer, which are sawed from the silicon ingot by lifting or casting. So, it actually consumes many silicon materials. In order to save silicon material, from the 1970s, researchers began to deposit polycrystalline silicon thin film on cheap substrate. However, the development of polycrystalline silicon thin-film solar cells is slow due to the small grain size of the film. The polycrystalline silicon thin film is made up of many small grains of different crystal orientations and sizes. In order to obtain large grain-size film, tireless researches were conducted by scientific research personnel who put forward many methods. At present, most polycrystalline silicon thin-film cells are prepared by using chemical vapor deposition (CVD), including low-pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) techniques. In addition, the methods of liquid-phase epitaxy (LPPE) and sputtering deposition could also be used in the preparation of polycrystalline silicon thin-film cells. Reaction gases such as SiH2Cl2, SiHCl3, SiCl4, or SiH4 react to obtain silicon atoms under a protective atmosphere and deposit on the hot Si, SiO2 or Si3N4 substrate with LPCVD method. But it is very difficult to form a larger grain-size on the silicon substrate, while it is easy to form in intergranular space. The solution to this problem is to use LPCVD for first depositing a thin layer of amorphous silicon layer on the substrate, then annealing the amorphous silicon layer to get larger grain size, and lastly, depositing polycrystalline silicon thin film on the layer of seed crystal. As a result, the recrystallization technology is undoubtedly a very important process. Current technology mainly includes solid phase crystallization method and the central melting recrystallization method.

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Polycrystalline silicon thin-film cells also adopted almost all other preparation technologies of monocrystalline silicon solar cells, in addition to the recrystallization process. Then, the conversion efficiency of polycrystalline silicon solar cell improves obviously. Figure 1.3 shows the efficiency evolution of polycrystalline silicon solar cell.

Figure 1.3: The efficiency evolution of polycrystalline silicon solar cell.

Due to the relatively less single-crystal silicon use in polycrystalline silicon thin-film solar cell, the stable performance, and ease in preparation on the cheap substrate, the cost of polycrystalline thin-film solar cells is far lower than the monocrystalline silicon cell. Moreover, the efficiency is higher than that of amorphous silicon thinfilm cells. Therefore, polycrystalline silicon thin-film cells will dominate solar electricity in the near future.

1.2.3.3 Thin-film amorphous silicon solar cells There are two key issues in the development of solar cell: improving conversion efficiency and reducing cost. Amorphous silicon thin-film solar cells have got great attention and seen rapid development for advantages such as low cost and ease in mass production. Actually, in the early 1970s, the development of the amorphous silicon cell had already started and have developed rapidly, in recent years. Many companies in the world are engaged in the production of this kind of cell . Amorphous silicon, which is also called a-Si, is a direct absorption semiconductor material with high optical absorption coefficient. Only a few microns of amorphous silicon film can fully absorb sunlight. So, the amorphous silicon solar cell can be very thin; its material and production costs are low. The effect of temperature increase on the efficiency decline is smaller for amorphous silicon solar cells than that for crystalline silicon solar cells. However, amorphous silicon solar cell

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can produce 10%–30% of the electrical energy attenuation after illumination. This is called light attenuation effect of amorphous silicon solar cells. This effect limits the potential of amorphous silicon solar cell as a power generating device for largescale application. As a solar energy material, amorphous silicon has the optical band gap of 1.7 eV. Amorphous silicon material is not sensitive to long-wavelength solar radiation, thereby limiting the amorphous silicon solar cell conversion efficiency. In addition, the photoelectric efficiency will decrease with the continuation of illumination time; this is known as the recession S-W effect, leading to the instability of the amorphous silicon solar cells. These problems can be solved through the preparation of the tandem solar cell. Tandem solar cells are made by deposition of one or more p–i–n subcell on single-junction solar cell. The key issues of tandem solar cell with improved conversion efficiency and stability are as follows: (1) it combines materials with different band gaps and improves the spectral response range; (2) the thin i layer leads to easy extraction of photoinduced carriers due to less change of the electric field intensity; (3) carriers at the bottom cell are about half as much as that of the single cell, leading to the decreased light recession effect; and (4) each subcell of tandem solar cells are connected in series.

Figure 1.4: The efficiency evolution of amorphous silicon solar cell.

At present, there are many methods to prepare amorphous silicon thin-film solar cell, including reaction sputtering, PECVD, or LPCVD technique. Using reaction raw material gas of H2 diluted SiH4 combining glass stainless steel slice as substrate, single–junction, or tandem amorphous silicon thin film can be obtained with different depositional procedure. Great progress has been made on amorphous silicon solar cells in two ways: firstly, the three-junction amorphous silicon solar cell conversion efficiency set a new record, reaching 13% [4]; secondly, the annual production capacity of the three-junction amorphous silicon solar cell is up to 5 MW. United Solar Companies (VSSC) has the highest single-junction solar cell conversion efficiency of

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9.3%; the maximum conversion efficiency of three band gap three-junction cell reached 13%, with a small active area of 0.25 cm2 [4]. Domestic research on amorphous silicon thin-film cells, especially the tandem solar cells, is not much. Xin-hua Geng and Ying Zhao from Nankai University prepared a-Si/a-Si tandem solar cells with the area of 20 cm × 20 cm and the conversion efficiency of 8.28%, by the combination of industrial materials with aluminum back electrode. Amorphous silicon solar cells are known for the advantages of relatively high conversion efficiency, lower cost, and light weight, which have a great developmental potential. But at the same time, the instability directly impacts its practical application. If the stability and the conversion efficiency are improved further, amorphous silicon solar cell will be one of the main members of the commercial solar cell family.

1.2.4 CdTe solar cells 1.2.4.1 A brief introduction on CdTe solar cells CdTe is one of II–VI compound semiconductors with a band gap of 1.5 eV, which is a kind of good photovoltaic material and one of the most suitable for photoelectric conversion. The CdTe solar cell can match well with the solar spectrum in spectral response, and its theoretical efficiency can reach 28%. The CdTe solar cell is stable and is one of the rapidly developing thin-film solar cells, in recent years. In 1993, the University of South Florida in the United States used the sublimation method to make the conversion efficiency of 15.8% solar cells [5] with 1 cm2 area. In 1996, Xuan Wu, at the National renewable energy laboratory (NREL) of the United States, achieved 15.8% efficiency by the combination of composite transparent conductive film, heterojunction, and magnesium fluoride antireflective film. In March 2013, The First Solar Company in the United States announced that its CdTe solar cell had obtained conversion efficiency up to 18.7%. In June, the same year, the CdTe cell efficiency was improved up to 19.6% by GE Global Research Center. Figure 1.5 shows the evolution of conversion efficiency for CdTe solar cells, in recent years. CdTe is easy to deposit into the large-area thin film with a high deposition rate. CdTe thin-film solar cells are usually based on CdS/CdTe heterojunction. Although the lattice constant of CdS and CdTe are different with 10%, the heterojunctions composed show excellent electrical performance. In the early 1990s, CdTe solar cell has achieved large-scale production, but its market developed slowly, and the market share has been hovering at nearly 1%. CdTe thin-film solar cells are photovoltaic devices that are composed of the multilayer thin films deposited in glass or other flexible substrate, with the average commercial conversion efficiency of 8–10%. General standard GdTe thin-film solar cells are composed of five structures, as illustrated in Figure 1.6.

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Figure 1.5: The efficiency evolution of CdTe solar cell.

Back electrode Back contact layer

CdTe CdS

TCO Glass substrate

Light Figure 1.6: Typical structure schematic of CdTe solar cells.

1.2.4.2 CdTe solar cells’ preparation methods At present, the preparation of CdTe polycrystalline thin film and a variety of technologies have been developed, such as near space sublimation, electrodeposition, PVD, CVD, CBD, screen printing, vacuum sputtering, and evaporation. The screen printing sintering process is as follows: CdTe and CdS film are sintered in 600–700 °C under the controlled atmosphere for 1 h to form big-grain film after screen printing with CdTe and CdS paste. Near space sublimation method: using glass as substrate, the substrate temperature of 500–600 °C, the deposition rate of 10 μm/min. Vacuum evaporation method: The CdTe sublimate from about 700 °C heating crucible with typical deposition rate of 1 nm/s before condensation on 300–400 °C substrate. The typical structure of semiconductor heterojunction CdTe solar cell with CdTe as absorption layer and CdS as window layer is antireflective film/glass/(SnO2: F)/CdS/P-CdTe/back electrode.

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1.2.4.3 CdTe solar cells’ development situation at home and abroad (1) Foreign CdTe solar cell industry development status and trends CdTe solar cell is a photovoltaic device that has shown rapid development in the thin-film solar cells. Generally, CdTe solar cells are the easiest to produce, so they show the fastest progress in commercialization. At present, the highest efficiency of small-area device is 19.6% in the laboratory, which has become the main object in research and development of solar cells in the United States, Germany, Japan, Italy, and other countries. It is necessary to increase the efficiency by optimization of the cell structure and the layers of material technology. Reducing the thickness of the thin CdS window layer can reduce the loss of incident light, thus increasing shortwavelength response and the short circuit current density. This strategy has been adopted for high efficiency record of CdTe solar cells. To reduce costs, CdTe deposition temperature must be dropped below 550 °C for cheap glass as substrate. Developing from laboratory to industry involves going through the modules and the design of production mode, research, and optimization process. In recent years, many teams all over the world have been able to produce CdTe solar cells at low substrate temperatures with conversion efficiency more than 12% and large modules. Many companies are making pilot or factory constructions for the CdTe thinfilm solar cell production. On the basis of extensive application research, CdTe thin-film solar cells have moved from the laboratory research stage to the scale industrial production in many countries. In 1998, the CdTe solar cells output of the United States was 0.2 MW. BP Solar Company plans to produce the CdTe thin-film solar cells in Fairfield. (2) The status and trends of the CdTe solar cells in China CdTe cells’ research began in the early 1980s in China. Inner Mongolia University adopted evaporation technology while Beijing solar energy Institute adopted the electrodeposition (ED) technology research. The research work was in the pause state during the mid-1980s to mid-1990s. In the late 1990s of the ninth five-year period, Lianghuan Feng’s group in Sichuan University carried out the study on CdTe solar cells. They achieved good results by using the near-space sublimation technology to prepare the CdTe solar cells. Lately, the cells’ efficiency has been above 16%. The tireless efforts of several generations of scientists for many years on CdTe solar cell technology has progressed from the basic laboratory research to the rapid development of industrialization application. During the period of “11th five-year plan” in Sichuan University and Wuxi Suntech Power Co., Ltd. set up Sichuan Suntech Power Co., Ltd., They have successfully established a 5 MW CdTe solar cell production line in May 2012. In our country, the industrialization of CdTe solar cells will see rapid development, and will be a world leader.

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1.2.4.4 The future development of CdTe solar cells CdTe solar cells are easy to manufacture than other thin-film cells and show fastest progress to the commercialization. The related research has moved from the laboratory to the large-scale industrial production. The next step of research and development is to further reduce costs, improve efficiency, and perfect the production process. CdTe solar cells exhibit many factors that are conducive to competition, but its global market share was only 0.42% in 2002. Currently, CdTe cell efficiency of commercial products is more than 12%. The following points explain why it cannot become the market mainstream: A) The cost of module and substrate material are too high; the overall CdTe solar cells materials accounted for 53% of the total cost, despite semiconductor materials accounting for only about 5.5%. B) The natural capacity of tellurium is limited, the total of which is unable to cater to photovoltaic power generation. C) The toxicity of cadmium makes it difficult for people to accept this kind of solar cell. CdTe solar cells are one of the large-scale productions and applications of photovoltaic devices; however, the most notable disadvantage is the problem of environment pollution. The toxic element Cd causes pollution to the environment and the health of the operator cannot to be ignored. The technology for effective disposal of broken CdTe modules is very simple; Cd is a heavy metal that is not good for the environment. As regards broken glass modules, the Cd and Te element should be recycled, and the damaged modules should be properly handled. Moreover, the waste water in the production and processing waste should comply with environmental protection standards. It is believed that the problem will be solved and the CdTe solar cells will be one of the ingredients of the future society as a new type of energy. As the representative of the second-generation thin-film solar cell material, CdTe/CdS will become the most potential future thin-film solar cell materials. In 1996, renewable energy laboratory of United States and First Solar Inc., made a series of solar cells from laboratory to production scale. Domestic research and development team of Xuanzhi Wu have realized the alignment of the production line successfully, obtaining 10% of the cell modules in September 2011. After a year, the module efficiency was close to 12%, and the average efficiency was 11.4%.

1.2.5 CIGS solar cells CIGS is the abbreviation for CuInxGa1–xSe2. CIGS solar cell is a chalcopyrite thin-film solar cell constituted by Cu (copper), In (indium), Ga (gallium), and Se (Se) in the best proportion. CIGS solar cell can absorb the visible absorption spectrum, which is similar

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to crystalline silicon and amorphous silicon solar cell. Moreover, the absorption wavelength of CIGS can be extended to the infrared region between 700 and 1,200 nm. It has good stability, good radiation resistance, low cost, high efficiency, and other advantages; its conversion efficiency evolution in recent years is shown in Figure 1.7. The conversion efficiency and capacity of large-area modules are in the range of 10%–15%, depending on the preparation process. The CIGS thin-film technology in China is still in the laboratory stage. The Institute of Optoelectronics of Nankai University is leading the CIGS research and development at the domestic level. They have developed aCIGS solar cell with the conversion efficiency touching 14%. Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences (SEAT) has successfully developed a CIGS solar cell with conversion efficiency of up to 19%, with the cooperation of the Chinese university of Hong Kong.

Figure 1.7: The efficiency evolution of CIGS solar cell.

1.2.5.1 Characteristics and preparation methods of CIGS solar cells The high efficiency of CIGS solar cells is mainly due to the following features: (1) introducing Se and Ga not only increases the band gap of the absorption layer material, but also controls the band gaps formed in cell absorption layer gradient distribution, adjusting the match between the absorption layer interface and other material, and optimizing the whole band structure; (2) CBD method is used to deposit the CdS layer and double layer ZnO film instead of the thick CdS window material formed by evaporation deposition. This is beneficial in improving the performance of the heterojunction performance and the short-wave spectral response range; (3) while using the alternative Na-free glass as opposed to the common Na glass, Na diffuses into the CIGS thin-film material through Mo boundary, which improves the structural properties and electrical properties of CIGS thin-film material, thus improving the open circuit voltage and fill factor.

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Preparation methods of CIGS thin-film material can be roughly classified into two categories: vacuum deposition and the non vacuum deposition. The multielement direct evaporation method and prefabricated metal layer followed by selenization method are the most commonly used methods. Direct evaporation method using multiple elements make the preparation of high photoelectric conversion efficiency of CIGS cells successful. However, metal prefabricated layer followed by selenization method is the preferred process of industrialization for its easy precision control of the stoichiometric ratio of each element in the film, film thickness, and composition characteristics of uniform distribution. Other methods include electrochemical deposition, spray pyrolysis method, laser-induced synthesis, screen printing method, mixing process, and liquid phase deposition method.

1.2.5.2 The structure of CIGS solar cell Figure 1.8 shows the typical structure of CIGS solar cells, including substrate that is coated with Mo layer of soda lime glass, CIGS absorption layer, CdS buffer layer (or Cdfree materials), and i–ZnO and Al–ZnO window layer, MgF2 antireflective film, and the top electrode Ni–Al thin-film materials. The detailed material preparation methods and specific performance of different layers can be referred in the literature [6]. Ni-Al electrode

Mg2F ~100 nm Al-ZnO ~ 400 nm i-ZnO ~ 50 nm

CdS ~ 50 nm Cu(In, Ga) Micro scale Mo ~ 100 nm Glass substrate

Figure 1.8: The typical structure of CIGS.

1.2.5.3 State of the art of CIGS solar cells at home and abroad The efficiency of CIGS solar cell is close to polycrystalline silicon solar cell, with the advantages of low cost and high stability, and the breakthrough achieved in the bottleneck of industrialization. To meet the rising price and shortage background of raw materials for crystalline silicon solar cell, many companies invest heavily, so the CIGS industry shows a tendency of vigorous development. More than 30 companies focus on the industrialization of CIGS in the world, such as Jonanna Solar, Wurth Solar, Surlfulcell, and AVANCIS in Germany, Global Solar Energy and Nanasolar lnc in the United States, and Honda and Showa Shell in Japan. In 2010, the modules production capacity of CIGS solar cell in the world was 325 MW.

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The CIGS industry in China lags far behind Europe, the United States, Japan, and other countries. During the “tenth five-year” period, Nankai University successfully constructed a 0.3 MW pilot line and prepared integration modules with 30 cm × 30 cm size and 7% efficiency. In February 2008, Shandong Sunvim Solar Technology Co., Ltd. introduced the first copper indium gallium sulfur selenium compounds (CIGSSe) commercial line from the German photovoltaic technology supplier, Johanna Solar Technology GmbH (JST). Shenzhen institute of advanced technology center independently researched and developed a set of photovoltaic solar energy using CIGS total evaporation- magnetron sputtering growth system, combining the two mature technologies of the total evaporation and sputtering selenide in the CIGS solar cell preparation, which can achieve the CIGS module of 100 mm × 100 mm with 14% efficiency.

1.2.5.4 Development and problems of CIGS solar cell In summary, the efficiency of CIS/CIGS solar cell can be compared to the traditional PV, in the near future. Although it has made some progress, there are still differences in conversion efficiency between the thin-film technology and traditional PV. Then, it must compete with traditional PV, over cost. The advantages such as flexibility, uniform color, and high stability of CIGS solar modules make it more suited to the application of building integration. In summary, The CIGS cells have the advantages of high efficiency and low cost of materials. However, it is also facing three major problems: (1) complex production process, high cost of investment; (2) the key raw materials are in short supply; and (3) buffer-layer CdS has potential toxicity.

1.2.6 Dye-sensitized solar cell Nanomaterials have shown a rapid development since the 1990s, due to the rising interests of nanostructure semiconductor materials. The group led by professor Grätzel from Swiss Federal Institute of Technology in Lausanne (École Polytechnique Fédérale de Lausanne, EPFL) made the so-called dye-sensitized solar cell (DSC) by replacing the plate electrode with nanoporous electrode in 1991, and achieved breakthrough conversion efficiency (PCE) of 7.1% [7] from about 1% for plate electrode. At present, the PCE of DSC is more than 12% [4]. DSC is mainly constituted of five components: conductive glass coated with transparent conductive film (F doped SnO2), nano-TiO2 porous films, dye sensitizer, electrolyte, and counter electrode (as shown in Figure 1.10). The working mechanisms of DSC: dye sensitizer absorbed on nano-TiO2 porous film transits to the excited state after absorbing sunlight, due to instability of excited dye; electrons transfer to the conduction band of TiO2 driven by the interaction of dye molecules and TiO2 surface; the electrons

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Figure 1.9: The PCE evolution of DSC certified by the third part.

in TiO2 conduction band have a macro orientation transport process by diffusion or drift and transfer to the counter electrode through the external circuit; at the same time, I3– in electrolyte solution is reduced to I– on the electrode, and the oxidized state of dye after electron injection is reduced to ground state by I–, then I– is oxidized into I3–, thus completing the entire cycle as also shown in Figure 1.11. Nanocrystalline Semiconductor Film

Conducting Substrate

Dye Sensitizer

Counter Electrode

Electrolyte

I3I

External Circuit

Figure 1.10: Structure diagram of DSC.

-

Conducting Substrate

Figure 1.11: The working principle of DSC.

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(1) Conductive glass coated with transparent conductive film (F doped SnO2) (TCO): This kind of glass has good light transmittance (> 85%), square resistance of 10–20 Ω/□, and can collect or transfer electrons. (2) Nano-porous TiO2 film: Nanoporous TiO2 thin film is prepared by screen printing technology with pre-prepared TiO2 paste. The spongy porous structure enables the film to adsorb more dye molecules, absorbing more sunlight, at the same time. The film also has the function of electronic collection and transmission. (3) Dye sensitizer: It absorbs photons to emit electrons. Moreover, it modifies TiO2 thin film, reducing leaking current. (4) Electrolyte: It reduces the oxidized dye molecules and gains electrons to complete the cycle. (5) Counter electrode: Covers a thin layer of Pt on TCO substrate. Pt not only can improve the light absorption rate of nano-TiO2 thin film, but can also help export electrons, improving the electron collection efficiency. DSC attracts the attention of scientists and companies from all over the world for its advantages such as cheap raw materials, simple production technology, stable performance, etc. Sustainable Technology International (STA, Australia), Institute of Photovoltaic (Germany), Léclanche S. A. company (Switzerland), Solaronix company (Switzerland), Energy Research Centre of the Netherlands (ECN), Institute of Plasma Physics, Technical Institute of Physics and Chemistry and other institutes of Chinese academy of sciences, Peking University, and many big companies and universities have carried out research on DSC. After more than a decade of research and development, DSC has made great progress. The current research focus is to further improve the PCE and practical application.

1.2.7 Polymer solar cells Polymer solar cells are constructed by placing the blend membranes of conjugated polymers donor and soluble acceptor of fullerene derivatives between transparent conductive glass substrate and metal electrodes. It has attracted much attention in recent years, due to its simple structure, low cost, light weight, and flexibility.

1.2.7.1 The structure and working principle of polymer solar cells In 1986, Dr. Ching W. Tang from Kodak prepared organic photovoltaic device with double-layer structure, using Copper phthalocyanine as donor and perylene as acceptor [8]. Under simulated sunlight, the PCE was close to 1%, which has increased research interest in organic solar cells. The structure of ITO/ organic acceptor/organic

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donor/metal grid electrode, is shown in Figure 1.12. The metal electrode is usually obtained by vacuum evaporation of metal such as gold, silver, copper, or aluminum.

Figure 1.12: Structure diagram of polymer solar cells [9].

At present, polymer solar cells are usually constructed by placing the blend membranes of conjugated polymers (electron donor) and PCBM (soluble derivatives of C60, electron acceptor) between transparent conductive glass substrate ITO (indium-tin oxide) and Al metal cathode (Figure 1.12). Generally, ITO electrode needs a transparent conductive polymer PEDOT: PSS layer with a thickness of 30–60 nm and photosensitive active layers with a thickness of 100–200 nm by spin-coating method. When the active layer is exposed to light, the conjugated polymer donors in active layer absorb photons and generate excitons (electron-hole pair). Then, excitons migrate to the interface of polymer donor/acceptor, and electrons at the interfaces transfer to the LUMO energy levels of electron acceptor PCBM, while holes remain in the HOMO energy levels of polymer donors, so as to realize the charge separation. Then, under the effect of internal potential field (its magnitude is related to the difference of work function between the positive and negative electrodes, as well as the thickness of active layer), the isolated holes transmit to the anode along the channel constructed by conjugated polymer donors, while electrons transmit to the cathode through the channel formed by acceptors. Holes and electrons are collected by anode and cathode, respectively. Thus, the current and voltage are generated, which is the so-called photovoltaic effect.

1.2.7.2 Recent progress in polymer solar cells After Dr. Tang obtained the efficiency of 1% of organic small-molecule solar cells for the first time in 1986, Professor Heeger realized ultrafast optoelectronics charge transfer, using conjugated polymer/C60 in 1992 [10]. In the twenty-first century, the development of polymer solar cells has accelerated significantly. Firstly, in 2002, Brabec succeeded in reaching an efficiency of 3.85% with the introduction of LiF

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modification layer, blending soluble C60 derivatives PCBM, and using dichlorobenzene as solvent [11]. In 2005, Yang Yang increased the conversion efficiency of polymer solar cells to 4.38%, by controlling the solvent evaporation rate and heat treatment of spincoated photosensitive active layer [12]. Heeger made tandem polymer solar cells in 2007, and the efficiency of the device was more than 6.0% [13]. Heliatek GmbH in Germany recently announced a PCE of 12% for organic solar cell. This world record has been certified by the examination organization, SGS Company. The efficiency evolution of polymer solar cells in recent years is shown in Figure 1.13.

Figure 1.13: The efficiency curve of polymer solar cells.

1.2.7.3 Future development of polymer solar cells Although the PCE of polymer solar cells has been more than 10%, and the application prospect has been revealed, compared to the existing mature silicon-based solar cells, the commonly used conjugated polymers have low utilization rate of sunlight (absorption spectra do not match the solar spectrum and has narrow absorption spectrum) and low charge carrier mobility (Usually, the charge carrier mobility conjugated polymer is 10–5–10−3 cm2 V−1 s−1). Also, the problems of charge transfer, collection efficiency, and fill factor also exist. Design and synthesis of conjugated polymer donor materials, high absorption coefficient in the visible and infrared region, and high hole mobility will be the emphasis in the future research of polymer solar cells.

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1.2.8 Quantum dot solar cells In recent years, quantum dot solar cells mainly include quantum dot-sensitized solar cells, quantum dot polymer hybrid solar cell, quantum dot Schottky junction and quantum dot depletion heterojunction solar cells. Though the concept of quantum dot-sensitized solar cell has been proposed since the late 1980s, it has got rapid development recently. The PCE of quantum dot solar cells based on liquid electrolyte has reached 6%. The current efficiency of quantum dot polymer hybrid solar cells is close to 5%. The efficiency of quantum dot depletion heterojunction solar cells is about 7%. In 2002, Nozik proposed the application of quantum dot structure in a variety of new concepts, such as tandem, multi exciton generation, hot carrier injection, and intermediate band. On the one hand, the process of quantum dot optical absorption and generation of electron hole pairs need not meet the momentum conservation principle; and quantum dots doped semiconductor thin film can produce multiphoton absorption and multiple exciton generation effect, so as to improve the PCE. On the other hand, semiconductor quantum dots have good optical absorption and photoluminescence properties, which can be easily controlled by changing the size of the quantum dots, and can easily realize full spectrum absorption. Therefore, quantum dot solar cells have the potential to improve the efficiency and to break through the limit of Shockley-Queisser. Nozik not only pointed out the potential advantages of the solar cells using quantum dot structure, but also put forward some specific structures of quantum dot solar cells. A high rate of multi exciton generation in the structure of PbSe and PbS was found and related theoretical models research were done after that. More importantly, the researchers have made a breakthrough, recently, on the multi exciton effect in solar cells. In 2010, people first observed multiple exciton collection effect in PbS colloidal quantum dot-sensitized TiO2 single-crystal system [14]. Subsequently, Nozik confirmed the 130% internal quantum efficiency PbSe colloidal quantum dot solar cells (ITO/ZnO/PbSe/Au), [15] and, thus, demonstrated for the first time, the use of multiple exciton effect in photoelectric devices. Also, the existence of hot carrier injection phenomenon by adsorbing PbSe quantum dots in single-crystal rutile TiO2 (110) surface [16] was proved. The above important research results fully prove the superiority of the application of quantum dots in new solar cells and attract extensive interest of many researchers, which will vigorously promote the rapid development of quantum dot solar cells. As a distinctive nanotechnology of the twenty-first century, quantum dot materials bring us the hope of achieving high efficiency, green, and low-cost solar cells, which has a profound scientific and social significance.

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1.2.9 Other types of solar cells The main problems of solar cells are conversion efficiency and cost. The first generation of crystalline silicon cells, including monocrystalline silicon and polycrystalline silicon solar cell, has realized large-scale production. However, the price, production process, and requirements are not ideal. The second-generation semiconductor thinfilm photovoltaic cells, including amorphous silicon, gallium arsenide, cadmium tellurium, copper indium gallium selenium, are also facing problems of low theoretical efficiency. Therefore, in order to further improve the efficiency and reduce the cost, people are trying to find new materials or new structures. At present, people have developed new materials and put forward many conceptual cell structure models, including new DSCs, full spectral absorption solar cells, black silicon solar cells, quantum dots/nanostructured solar cells, organic/inorganic hybrid solar cells, and so on. The “third-generation solar cells” have the following characteristics: thin-film, high-efficiency (higher than the efficiency of single-junction cell), abundance in raw materials, nontoxic, etc. New concept solar cells, such as the full spectrum solar cells, usually have very high PCE, and the theoretical PCE is 80%. As a result, all types of new concept solar cells have been focused on, the demand and the market are attractive, especially the development of new type of photovoltaic materials. Those new concept solar cells whose core material preparation technology is not yet mature are in the laboratory research stage, and some of the mechanisms are not completely clear. Countries from all over the world have given great attention to the research of new concept solar cells, and the development is very fast with new results reported continuously. The third-generation solar cells, which are expected to achieve high conversion efficiency, mainly include: (1) Multi threshold devices, such as tandem cells, impurity photovoltaic cells (2) Quantum multiplication devices, such as the impact excitation, the photon up/ down conversion (3) Hot carrier cells (4) Thermal methods, such as thermal ionization, thermophotovoltaics (TPV) (5) Others In summary, compared to DSCs and organic solar cells, the third-generation solar cells are in the theoretical research stage, and many of them even have no prototype cells production. It is worth noting that the third-generation solar cells generally have high efficiency and require cheap materials, theoretically. There will be big changes once the third-generation solar cells make a great breakthrough.

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1.3 Dye-sensitized solar cells 1.3.1 The development of dye-sensitized solar cells Since Daguerre invented photography in 1837 and Fox Talbot invented silver halide plate in 1839, photoelectric response has aroused people’s attention and has been a hotspot in the field of scientific research. In 1839, Becquerel found that metal electrodes coated with copper oxide or silver halide generated photoelectric phenomena [17], which further confirmed the possibility of photoelectric conversion. In 1887, Moser [18] coated erythrosine dye on silver halide electrodes, further confirming the photoelectric phenomena. Thus, the concept of “dye sensitization” was introduced from photography to the photoelectric effect. In 1949, Putzeiko and Trenin firstly reported sensitization of organic photosensitive dyes to wide band gap semiconductor. Moreover, they observed photoelectric effect when rhodamine B, eosin y, erythrosine, and cyanine were adsorbed on compressed ZnO powder. From then on, dye-sensitized semiconductors became a research hotspot in the field. The synchronous development of the sensitization effect on the photograph and photoelectricity has puzzled several generations of chemists. In fact, the essence of sensitization is a photoelectric-induced charge transfer process. In the solid photosensitive International Conference held in Chicago in 1964, the viewpoint presented by Namba and Hishiki [19] that the sensitization mechanism of organic dye in photography and photoelectric conversion system are the same obtained peer recognition. Then, there was a consensus that the dye must be adsorbed onto the semiconductor surface in a closely spaced single molecular layer, in order to get the best sensitization efficiency [20]. But it was not very clear whether the mechanism of sensitization is electron transfer process or the energy coupling process. After this conference, the work of dye- sensitized ZnO semiconductor by Tributsh made it clear that the mechanism is electron transfer. In the 1960s, Gerischer, Tributsch, Meier, and Memming systematically studied the charge transfer reaction between photoinduced organic dye and semiconductor and pointed out the mechanism of dye adsorption on the semiconductor producing electric current, which has become an important basis for the photoelectron chemical cell. In the beginning of the 1970s, Honda and Fujishima successfully produced hydrogen by water photolysis using TiO2, and converted the light energy into chemical energy [21]. This experiment has become a milestone in the history of photoelectric chemicals, and made people realize that TiO2 is an important semiconductor material in the field of photoelectric chemistry. As the limitation is the cost, the strength, and efficiency of hydrogen production using single-crystalline TiO2 semiconductor material, the method did not show great prospect, let alone practicality. Since the 1980s, the focus of photoelectric conversion research has transferred to artificial photosynthesis, including natural photosynthesis simulation experiments, light energy, chemical energy (photocatalytic water splitting, light nitrogen

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fixation, and so on), and the photoelectric conversion application. The group led by Gust and Moore from Arizona State University successfully simulated photosynthetic electron transfer process in ternary compounds C-P-Q (porphyrin, carotene, quinone) [22], for the first time. After that, they began to research quaternary [23] multivariate compounds and achieved certain results. Making photovoltaic diode using the photoelectric characteristics of organic molecules is a great achievement in the field of optoelectronics since the 1980s. Fujihira assembled photo diodes with organic molecules by Langmuir–Blodgett film (LB film), achieving a short circuit current of 0.28 A/cm−2, [24] which was a pioneering work in the field. During the two decades from the 1970s to the 1990s, the research of organic dye-sensitized wide-band-gap semiconductors has been very active. Memming, Gerischer, Hauffe, Bard, and Tributsch studied various organic dye sensitizers and thin-film semiconductor photosensitization. The dyes include rose red, porphyrin, coumarin, and so on, while semiconductor films like TiO2, CdS, WO3, Fe2O3, Nb2O5, ZnO, Ta2O5, and SnO2 are more emphasized. Early research in this field mainly concentrated on the plate electrode, whose main drawback is that only monolayer dye molecules can be absorbed on the surface. In order to overcome the disadvantage of low absorption of monolayer dye molecules, people tried to use multilayer dye to increase the absorption. However, the inner dye molecules hinder the charge transfer and separation, and photoelectric conversion efficiency is always below 1%. D. Chapin and G. Peanon introduced p–n junction into monocrystalline silicon in 1954 in RCA, and found the photoelectric phenomena. And then, they developed the silicon solar cell of today. The PCE was much higher than that of the electrochemical cell at that time, reaching the practical level. When it came to 1980s, the research on the conversion of solar energy by photochemical method was active, again. Early studies mainly focused on two methods: the first is that using solar energy to promote the conversion of chemical energy. In the 1980s, using green plants and bacteria to perform optical processing and optical synthesis and using photolysis water to obtain hydrogen were popular. In the 1990s, it was used to obtain the industrial and ecological reaction and the research on the degradation of toxic substances by the suspension of oxide semiconductor. The second method is to directly transform solar energy into electricity, which is more attractive. Such photovoltaic cells can be classified as two categories: one is to disperse dye in solution and the oxidation reduction reaction will take place; the other is charge separation at the interface of semiconductor and dielectric. In terms of the mechanism, the former has fast electronic exchange but low efficiency, and hence, was nearly abandoned in the 1990s. The latter includes single crystal from III to V (such as InP, MoS2, or WSe2), and the low-band-gap semiconductors usually show good photoelectric conversion efficiency between 15% and 20%. However, expensive cost and instability (light corrosion) of these narrow band-gap single crystals limits its development. Oxide semiconductors have good stability, but their wide band-gap limits its light absorption.

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In the past 30 years, with the important development of sol gel technique in the field of chemistry and micron and nanomaterials, people use the network structure from chalcogen and chalcogenide to prepare nanocrystalline semiconductor thin films, such as TiO2, ZnO, Nb2O5, WO3, Ta2O5, CdS, or CdSe, whose interconnected network structure promotes electron transport in semiconductors. Nanostructured materials provide many new opportunities for people to study the artificially controlled process, and also provide other uses for people. Its unique photoelectric properties can be used to fabricate photo electrodes for photovoltaic cells, photo-induced display, optical switch, chemical sensors, embedded battery, heat reflective and ultraviolet absorption layer, and improve chemical and mechanical stability of glass. It has taken over 30 years of research lead to deeply understand the basic process of dye-sensitized wide band-gap semiconductor. Fujihara studied the carboxyl of rhodamine B and found that it can form ester bonds with hydroxyl on the surface of semiconductor through dehydration. The resulting photocurrent is higher than that via amide linkage [25] by two orders of magnitude. After that, Goodenough extended the surface chemical reaction to bipyridine ruthenium complexes, in order to perform photo oxidation of water, effectively. Although the oxidation yield was very low, their work illustrated the role of bipyridine complex containing carboxyl and semiconductor oxide, which was adopted by the research group from EPFL led by Grätzel. Grätzel’s group was committed to study DSCs through the study the TiO2 particles [26] and photoelectrode [27]. They prepared DSCs with nanoporous TiO2 semiconductor film as electrode, transition metal Ru and Os complexes as dye, and appropriate redox electrolyte as the main material. In 1991, they achieved a major breakthrough with the photoelectric conversion efficiency of 7.1% (AM1.5). It is comforting that its cheap cost, simple technique, and stable properties provide a more effective method to use solar energy; its production cost is only one-tenth to one-fifth of the silicon solar cells, and the efficiency has exceeded 12%, in recent years [28]. Monolayer of dye can only absorb less than 1% of the light, while multilayers of dyes hinder the transfer of electron. Thus, the PCE of the electrochemical cells has been less than 1%. The introduction of nanoporous TiO2 film by Prof. Grätzel made the entire semiconductor film like a sponge with large internal surface area, which enables the film electrode to adsorb more dyes and overcome the disadvantage of low light absorption. Moreover, multiple reflection of light in the film increases the possibility of being absorbed by dye, which can help generate larger photocurrent, thus greatly improving the efficiency.

1.3.2 State of the art of dye-sensitized solar cells DSCs have attracted international concern and attention since the breakthrough made by Grätzel in EPFL 1991. Its low production cost, easy technology for industrial production, and broad application prospects attract many scientists and enterprises in Europe,

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America, and Australia. With the efforts of global research institutes, companies, and universities, it has made great progress in research and application.

1.3.2.1 State of the art of large-area dye-sensitized solar cells in China In China, the research level of DSCs both in the scientific fundamental research and the industrial research is close to the world level. At present, many research institutions in China have carried out research on DSC and have achieved many fruitful results in the basic research. As for the industrial research of DSC, it has reached a high level, although there are only a few institutions involved in this field. Over the past decade, researchers have carried out various studies of electrolyte materials and cell structures, and proposed different kinds of innovative structures and ideas. Dr. Mingdeng Wei from Fuzhou University proposed the combination of DSCs and energy storage. Dr. Qingbo Meng from Institute of Physics, CAS, proposed the environmentally friendly composite electrolyte. Dr. Hong Lin from Tsinghua University put forward new efficient low cost tandem flexible thin-film solar cells. Changchun Institute of Applied Chemistry, CAS, made a breakthrough in new dyes and ionic liquid electrolyte, developing dye C101 independently, with efficiency of 11%, and the efficiency of ionic liquid electrolyte cells reached 8.2%. On the basis of the systematic study of the small area DSC, the team of Tingli Ma from Dalian University of Technology carried out research on the large-area DSC. Our group in Institute of Plasma Physics, CAS, has done a lot of work on largearea DSC research [29–32]. We successfully prepared 15 cm × 20 cm and 40 cm × 60 cm cell modules with PCE close to 6%, in 2003 [29]. We have reported an efficiency of 7.4% of large strips cells; parallel large area cells (18 × 0.7 cm × 13 cm) obtained PCE of 5.9%. We also successfully assembled 45 cm × 80 cm cell panel with high efficiency, as shown in Figure 1.14. In 2004, Institute of Plasma Physics built a 500 W DSC demonstration system, which has had stable operation in the past 7 years, breaking the bottleneck of DSC in electrode and sealing. In 2012, 0.5 MW DSC pilot lines were successfully constructed, which laid a solid foundation for the industrial application. Study on the industrialization of the DSCs has successfully realized the modules pilot line construction and commissioning, which laid a solid foundation for further popularization and application of DSCs.

1.3.2.2 Foreign research results of large-area DSC After several years of development, DSCs have become a very active research field, with laboratory PCE close to the level of amorphous silicon solar cells. In addition to low cost, high efficiency, and a huge potential future market, relatively low threshold

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Figure 1.14: Dye-sensitized solar cell demonstration system established by Institute of Plasma Physics.

of the industry made companies willing to be engaged, compared to silicon-based solar cells. In the range of application of solar cells, DSCs have certain advantages. Japan, Germany, Australia, and other countries have also joined the ranks of the DSC industry research and made great progress. Japan has been in the leading position in the world in the basic research and application of DSC, and the research of largearea DSC has made some breakthroughs. In Japan, more than 100 companies are involved, with patents over 1,600. Han et al., in 2009, studied W and Z type DSC and reported the PCE of 8.2%, with W type DSC modules whose area is 50 mm × 53 mm with 85% active area of total area [33]. Afterward, they reported 8.4% PCE [34]. Fujikura Company from Japan has also made a few achievements in DSC. In 2003, they adopted gold, silver, platinum, titanium, nickel, aluminum, and other metals for gate electrode and compared the device life and production cost. Finally, they used Ni as the gate electrode, with the entire component showing a PCE of 4.3% and 5.1% (effective area of 68.9 cm2) [35]. In addition, Fujikura Company also studied the application of ionic liquid electrolytes on the large-area DSC and showed longterm thermal stabilitys in 2003 [36, 37]. Under optimized conditions, the photoelectric conversion efficiency of 0.9 cm × 0.5 cm small- area DSC is 4.5%; 10 cm × 10 cm largearea DSC is 2.7%, efficiency is 2.4% in ion – gel system (active area for 69 cm2). In 2009, Fujikura studied the thermal stability of DSC with Ag wire as the gate electrode, using ionic liquid electrolyte and found no obvious leakage at 85 °C. They also separate the different parts of DSC with outer humidity with double sealing method [38]. The related results show that the electrolyte in large-area DSC has no obvious leakage at the atmosphere of the temperature of 85 °C and the humidity of 85% and maintains the stability of more than 1,000 h. They also confirmed that the DSC can be

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stable for 200 cycles at − 40–90 °C, while the performance of the solar cell was not significantly decreased. In 2009, Kato from Japan developed a kind of monolithic series DSC (S type). They use carbon electrodes instead of conductive glass as substrate, greatly reducing the manufacturing cost. Kato also produced the DSC assembly that consists of nine unit-cells (10 cm × 11 cm) [39], and studied the stability of DSC by Raman spectroscopy and electrochemical impedance spectrum. The results show that the photocurrent of the cell was relatively stable, and voltage and fill factor decreased slightly in the outdoor place, after 2½ years. Researchers from Japan [40] produced DSC modules with area of 120 mm2 and 255 mm2 and studied the stability of 120 mm2, under high temperature. They found that the stability of this kind of DSC is maintained at 1,000 h, and PCE still maintains more than 95%, even at high temperature of 85 °C. Researchers in South Korea [41, 42] prepared photo anode on transparent conductive glass by screen printing method, with Ag as gate electrode, making a largearea DSC, with 80 μm thick Surlyn film. Under standard test conditions, the PCE of the large-area module was 5.52% [43]. Researchers also invented large-area DSC with nanocarbon powder as the counter electrode materials, and PCE was 4.23% [44]. Researchers from South Korea [45] studied the method to improve the photoelectric conversion efficiency of W type of DSC. They regulated the thickness of Pt, making Pt electrode with high reflectivity, and reduced the thickness of the electrolyte layer, with a thin metal film reflecting the transmitted light, increasing photon number absorbed by dye in the DSC modules. Yong Seok Jun [46] studied the influence of the size of TiO2 film on the performance of DSC and concluded that the best width of TiO2 film in a DSC is 8–9 mm. They produced DSC modules with the optimum size; the PCE of 10 cm × 10 cm DSC modules reached to 6.3% and 6.6%, after adding a scattering layer on TiO2 film [6]. Scientists from Israel studied other electrode materials with a resistance to corrosive electrolytes instead of Ag line [47]. There was no need for the protection of a gate electrode, which not only increases the active area of the large area DSC but also reduces the difficulty in packaging, thus improving the stability of the large-area DSC. Their DSC modules can be irradiated at 85 °C for 3,300 h under continuous irradiation of 1 sun, and the conversion efficiency was not significantly decreased. Paoli [48] from Germany assembled large-area DSC with polymer films as solid electrolyte. They use low-cost epoxy resin adhesive, which is easy to solidify, and to pack large-area cells. Although the effect of epoxy resin adhesive is not as good as that of Surlyn film, it is a good choice because the solid electrolyte does not have the problem of electrolyte leakage. R. Sastrawan [49, 50] from Germany used silver to collect current with glass as protection and packaging material. Glass materials not only have low cost, but also have good thermal stability, chemical stability, and mechanical stability.

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In recent years, research on the large-area DSC has attracted wide attention, and great progress has been made for the industrial application. The STA Company in Australia built the first DSC pilot factory on May 1, 2001. They used tungsten powder as the connection and sealing material and established 200 m2 DSC roof [51], laying a solid foundation for the architectural integration of DSC. Dyesol from Australia obtained the outdoor stability data of DSC through accelerated aging experiment: 20,000 h aging data (0.8 sun, 55–60 °C), which means solar cells can run for 32 years in central Europe, or run for 18 years in Sydney. For many years, the European Union headed by the Energy Research Centre of the Netherlands (ECN) has been working on large-area DSC and gained an efficiency of 8.2% in 2001 [52]. In 2003, ECN established production line of 10 cm × 10 cm cell module and got an efficiency of 5.9%, with good stability [52]. Researchers from Germany have produced large-area DSC modules with area of 30 cm × 30 cm, and the PCE was 4.2% [53]. At present, the stability of smallarea DSC at room temperature has been demonstrated, and the long-term stability of large-area DSC under high temperature and humidity conditions has been tested. Aisin, TOYOTA Central Research Institute, and Fujikura Company have also developed integrated solar cells and tested their outdoor durability [35, 54]. Japan has also achieved fruitful results in the study of large-area DSC, and the representative research groups are Arakawa and Yanagida. SHARP company [55] and Arakawa [56, 57] reported efficiency of 6.3% (26.5 cm2) and 8.4% (10 cm × 10 cm), respectively. Miyasaka [58, 59] from Toin University of Yokohama developed flexible large-area DSC, based on low temperature preparation technology. Compared to silicon solar cells, the price could be reduced to about one-tenth of silicon solar cells. They also produced large-area flexible DSC with area of 30 cm × 30 cm, including 10 unit-cells with output voltage of 7.2 V and current of 0.25–0.3 A. In 2005, Peccell Company, Fujimori Kogyo Co., Ltd, and Showa Denko Group in Japan jointly developed large-area plastic DSC production line. They use screen printing method, realizing low cost continuous production [60]. The unit cell size has a length of 2.1 m, width of 0.8 m, thickness of 0.5 mm, and weight of 800 g/m2, which is the largest with the relatively lightest DSC in the world. The DSC modules can also output high voltage of more than 100 V, even indoors.

1.3.3 Characteristics of dye-sensitized solar cells 1.3.3.1 Technological characteristics of DSC The main semiconductor material of DSC is TiO2. DSC has great advantages in terms of abundant raw materials, low cost, and stable performance in large-scale industrial production. All raw materials and production processes are nontoxic and nonpolluting. And conductive glass can be fully recycled, which offers good environmental protection.

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1.3.3.2 Technical advantages of DSC industrialization In recent years, the domestic PV industry, especially the development of solar cells industry, has made great progress. Taking crystal silicon solar cells as an example, constant technology improvement and breakthrough emerge in each process from the beginning of cleaning, to finally sintering, and each technology brought a decline in cost and a rise in efficiency. With such industry background, the PCE of this kind of new thin-film solar cells has exceeded 12% in the laboratory, and pilot scale has reached 0.5 MW. Compared to crystalline silicon and other solar cells, industrialization of DSC shows obvious technical advantages as follows: 1) Relatively simple technology The production cost of solar cells mainly comprises the following parts: ① The cost of raw materials: most of the raw materials needed are elements of the earth that have rich reserves, such as titanium and iodine, and the preparation methods are simple with low cost. ② Preparation process: the main technology of DSC is screen printing technology, which is relatively less and easy to be integrated, and favorable for the design of the assembly line. ③ Price of preparation equipment: the requirements for the accuracy of the preparation are also an important factor to measure the cost. In preparation of solar cells, the higher the accuracy requirements, the higher the value of the equipment and higher the difficulty of the preparation. From the test line building of 0.5 MW DSCs, we can see that there is no need for high temperature and high vacuum. The main steps are screen printing of paste, cell laminating, electrolyte perfusion, and component assembly. This relatively cheap equipment and low requirements of processing precision are prominent advantages of the DSC industry. 2) Abundant raw materials All raw materials used in solar cells and synthesis process, except the conductive glass, realized localization and independent intellectual property rights. Moreover, the raw materials do not require high purity as in crystalline silicon materials, and the preparation process does not require precise control of composition and crystal phase structure as do multivariate compound semiconductor cells. 3) Relatively simple industrial equipment The key of industrialization is equipment and the DSC equipment for the industrialization has nonstandard characteristics. Therefore, compared to other types of cells, DSC manufacturing equipment has characteristics of relatively simplicity and low cost. In summary, as new thin-film solar cells and due to the simple process, good practical prospect, and low cost, DSCs have gradually progressed due to their own special

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characteristics. The industrialization of DSC will have a greater breakthrough through continuous research, fully showing the advantages of the new energy.

1.3.4 Application prospects of dye sensitized solar cells Great changes will take place in the world energy in the twenty-first century. The present energy structure based on fossil fuel with limited resources and serious pollution will be gradually transformed into one based on clean renewable energy with unlimited and diverse resources. The development and utilization of solar energy and solar cells will be an important part in the development of new energy. Compared to other solar cells, DSCs have the very obvious advantages of simple production process, low equipment cost, small energy consumption, nontoxic raw materials, and so on; their industrialization cost is only one-fourth to one-third of silicon solar cells. These characteristics of DSCs will make their application prospects very extensive, and they will show huge social benefits. 1) Rural electrification: By the end of 2012, at least 10 million households and 60 million farming and animal husbandry population were still unable to use electricity. In these populations, many live in the northwestern five provinces, autonomous regions, Inner Mongolia, Tibet, Yunnan, Hainan, and Aba Town in Sichuan, where they have abundant solar energy resources. The most important requirement of rural power is the low price and reliable performance, and DSCs can meet these requirements. 2) Communication: Communication industry is the basic industry of the national economy. With the rapid development of national economy, the development of communication industry is bound to increase very fast. We estimate that photovoltaic cell modules applied to communications industry will increase with an annual rate of 10% in the first 7 years and 5% in the following 5 years; they are mainly used in fiber optic communication, microwave communication, rural communication, satellite receiving station, and so on. It will greatly reduce the investment in communications and save resources, if the DSC modules are used to provide electricity. 3) Civilian goods and others: Civilian goods mainly include solar hat, solar charger, solar calculator, solar watch, solar clock, solar lights, solar toys, solar advertising light boxes, solar car, solar powered yacht, and solar energy semiconductor refrigeration. In recent years, adapting PV power generation and construction together to constitute a photovoltaic roof power generation system has shown attractive prospects and a broad market, with rapid development in foreign countries.

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Green M A, Emery K, Hishikawa Y, et al., Solar cell efficiency tables (version 42). Prog Photovoltaics, 2013, 21(5): 827–837. Yan J D, Thoughts and suggestions on the development of new energy in China. China Basic Sci, 2010, 12(3): 10–12. Chapin D M, Fuller C S, Pearson G L, A new silicon P-N junction photocell for converting solar radiation into electrical power. J Appl Phys, 1954, 25(5): 676–677. Green M A, Emery K, Hishikawa Y, et al, Solar cell efficiency tables (version 39). Prog Photovoltaics, 2012, 20(1): 12–20. Britt J, Ferekides C, Thin-Film CdS/CdTe Solar-Cell with 15.8-Percent Efficiency. Appl Phys Lett, 1993, 62(22): 2851–2852. Xiong S Z, Zhu M F, Foundations and Applications of Solar Cells. Science Press: Beijing, 2009. O’regan B, Grätzel M, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature, 1991, 353(6346): 737–740. Tang C W, 2-layer organic photovoltaic cell. Appl Phys Lett, 1986, 48(2): 183–185. Li Y F, Molecular design of photovoltaic materials for polymer solar cells: Toward suitable electronic energy levels and broad absorption. Acc Chem Res, 2012, 45(5): 723–733. Sariciftci N S, Smilowitz L, Heeger A J, et al., Photoinduced electron-transfer from a conducting polymer to buckminsterfullerene. Science, 1992, 258(5087): 1474–1476. Brabec C J, Organic photovoltaics: Technology and market. Sol Energy Mater Sol Cells, 2004, 83: 273–292. Li G, Shrotriya V, Huang J S, et al., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat Mater, 2005, 4(11): 864–868. Kim J Y, Lee K, Coates N E, et al., Efficient tandem polymer solar cells fabricated by allsolution processing. Science, 2007, 317(5835): 222–225. Sambur J B, Novet T, Parkinson B A, Multiple Exciton collection in a sensitized photovoltaic system. Science, 2010, 330(6000): 63–66. Semonin O E, Luther J M, Choi S, et al., Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science, 2011, 334(6062): 1530–1533. Tisdale W A, Williams K J, Timp B A, et al., Hot-electron transfer from semiconductor nanocrystals. Science, 2010, 328(5985): 1543–1547. Becquerel A-E, Mémoire sur les effets électriques produits sous l’influence des rayons solaires. Comptes Rendus, 1839, 9: 561–567. Moser J, Notizüber Verstärkung photo-elektrischer Ströme durch optischer Sensibilierung. Monatsh Chem, 1887, 8: 373. Namba S, Hishiki Y, Color sensitization of zinc oxide with cyanine dyes. J Phys Chem-US, 1965, 69(3): 774. Nelson R C, Minority Carrier trapping and dye sensitization. J Phys Chem-US, 1965, 69(3): 714-&. Fujishima A, Honda K, Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238(5358): 37–38. Moore T A, Gust D, Mathis P, et al., Photodriven charge separation in a carotenoporphyrin quinone triad. Nature, 1984, 307(5952): 630–632. Gust D, Moore T A, Moore A L, et al., A Carotenoid-diporphyrin-quinone model for photosynthetic multistep electron and energy-transfer. J Am Chem Soc, 1988, 110(22): 7567–7569.

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[24] Fujihira M, Nishiyama K, Yamada H, Photoelectrochemical responses of optically transparent electrodes modified with Langmuir-Blodgett-films consisting of surfactant derivatives of electron-donor, acceptor and sensitizer molecules. Thin Solid Films, 1985, 132(1–4): 77–82. [25] Fujihira M, Ohishi N, Osa T, Photocell using covalently-bound dyes on semiconductor surfaces. Nature, 1977, 268(5617): 226–228. [26] Dung D H, Serpone N, Grätzel M, Integrated systems for water cleavage by visible-light – Sensitization of TiO2 particles by surface derivatization with ruthenium complexes. Helv Chim Acta, 1984, 67(4): 1012–1018. [27] Desilvestro J, Gratzel M, Kavan L, et al., Highly efficient sensitization of titanium-dioxide. J Am Chem Soc, 1985, 107(10): 2988–2990. [28] Yella A, Lee H W, Tsao H N, et al., Porphyrin-sensitized solar cells with Cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science, 2011, 334(6056): 629–634. [29] Dai S Y, Wang K J, Weng J, et al., Design of DSC panel with efficiency more than 6%. Sol Energy Mater Sol Cells, 2005, 85(3): 447–455. [30] Wang M, Pan X, Fang X Q, et al., A new type of electrolyte with a light-trapping scheme for high-efficiency quasi-solid-state dye-sensitized solar cells. Adv Mater, 2010, 22(48): 5526–5530. [31] Tian H J, Hu L H, Zhang C N, et al., Superior energy band structure and retarded charge recombination for anatase N, B codoped nano-crystalline TiO2 anodes in dye-sensitized solar cells. J Mater Chem, 2012, 22(18): 9123–9130. [32] Tang Y T, Pan X, Zhang C N, et al., Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells. Electrochim Acta, 2011, 56(9): 3395–3400. [33] Han L T, Fukui A, Chiba Y, et al., Integrated dye-sensitized solar cell module with conversion efficiency of 8.2%. Appl Phys Lett, 2009, 94(1): 013305. [34] Fukui A, Fuke N, Komiya R, et al., Dye-sensitized photovoltaic module with conversion efficiency of 8.4%. Appl Phys Express, 2009, 2(8): 082202. [35] Okada K, Matsui H, Kawashima T, et al., 100 mm x 100 mm large-sized dye sensitized solar cells. J Photochem Photobio A, 2004, 164(1–3): 193–198. [36] Papageorgiou N, Athanassov Y, Armand M, et al., The performance and stability of ambient temperature molten salts for solar cell applications. J Electrochem Soc, 1996, 143(10): 3099–3108. [37] Kubo W, Kitamura T, Hanabusa K, et al., Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator. Chem Commun, 2002, 38(4): 374–375. [38] Matsui H, Okada K, Kitamura T, et al., Thermal stability of dye-sensitized solar cells with current collecting grid. Sol Energy Mater Sol Cells, 2009, 93(6–7): 1110–1115. [39] Kato N, Takeda Y, Higuchi K, et al., Degradation analysis of dye-sensitized solar cell module after long-term stability test under outdoor working condition. Sol Energy Mater Sol Cells, 2009, 93(6–7): 893–897. [40] Noda S, Nagano K, Inoue E, et al., Development of large size dye-sensitized solar cell modules with high temperature durability. Synth Met, 2009, 159(21–22): 2355–2357. [41] Lee W J, Ramasamy E, Lee D Y, et al., Glass frit overcoated silver grid lines for nanocrystalline dye sensitized solar cells. J Photochem Photobio A, 2006, 183(1–2): 133–137. [42] Lee W J, Ramasamy E, Lee D Y, et al., Efficient dye-sensitized cells with catalytic multiwall carbon nanotube counter electrodes. ACS Appl Mater Interfaces, 2009, 1(6): 1145–1149. [43] Lee W J, Ramasamy E, Lee D Y, et al., Dye-sensitized solar cells: Scale up and current-voltage characterization. Sol Energy Mater Sol Cells, 2007, 91(18): 1676–1680.

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[44] Lee W J, Ramasamy E, Lee D Y, et al., Grid type dye-sensitized solar cell module with carbon counter electrode. J Photochem Photobio A, 2008, 194(1): 27–30. [45] Kang M G, Park N G, Ryu K S, et al., A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate. Sol Energy Mater Sol Cells, 2006, 90(5): 574–581. [46] Jun Y, Son J H, Sohn D, et al., A module of a TiO2 nanocrystalline dye-sensitized solar cell with effective dimensions. J Photochem Photobio A, 2008, 200(2–3): 314–317. [47] Goldstein J, Yakupov I, Breen B, Development of large area photovoltaic dye cells at 3G Solar. Sol Energy Mater Sol Cells, 2010, 94(4): 638–641. [48] de Freitas J N, Longo C, Nogueira A F, et al., Solar module using dye-sensitized solar cells with a polymer electrolyte. Sol Energy Mater Sol Cells, 2008, 92(9): 1110–1114. [49] Sastrawan R, Beier J, Belledin U, et al., New interdigital design for large area dye solar modules using a lead-free glass frit sealing. Prog Photovoltaics, 2006, 14(8): 697–709. [50] Sastrawan R, Beier J, Belledin U, et al., A glass frit-sealed dye solar cell module with integrated series connections. Sol Energy Mater Sol Cells, 2006, 90(11): 1680–1691. [51] Kroon J M, Bakker N J, Smit H J P, et al., Nanocrystalline dye-sensitized solar cells having maximum performance. Prog Photovoltaics, 2007, 15(1): 1–18. [52] Spath M, Sommeling P M, van Roosmalen J A M, et al., Reproducible manufacturing of dyesensitized solar cells on a semi-automated baseline. Prog Photovoltaics, 2003, 11(3): 207–220. [53] Hinsch A, Brandt H, Veurman W, et al., Dye solar modules for facade applications: Recent results from project ColorSol. Sol Energy Mater Sol Cells, 2009, 93(6–7): 820–824. [54] Toyoda T, Sano T, Nakajima J, et al., Outdoor performance of large scale DSC modules. J Photochem Photobio A, 2004, 164(1–3): 203–207. [55] Han L Y, Fukui A, Fuke N, et al., High efficiency of dye-sensitized solar cell and module. Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, 2006, 1–2: 179–182. [56] Yamaguchi T, Uchida Y, Agatsuma S, et al., Series-connected tandem dye-sensitized solar cell for improving efficiency to more than 10%. Sol Energy Mater Sol Cells, 2009, 93(6–7): 733–736. [57] Arakawa H, Yamaguchi T, Takeuchi A, et al., Efficiency improvement of dye-sensitized solar cell by light confined effect. Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, 2006, 1–2: 36–39. [58] Miyasaka T, Kijitori Y, Ikegami M, Plastic dye-sensitized photovoltaic cells and modules based on low-temperature preparation of mesoscopic titania electrodes. Electrochemistry, 2007, 75(1): 2–12. [59] Ikegami M, Suzuki J, Teshima K, et al., Improvement in durability of flexible plastic dyesensitized solar cell modules. Sol Energy Mater Sol Cells, 2009, 93(6–7): 836–839. [60] Miyasaka M, Flexible and solid-state dye-sensitized solar cells. Clean Energy, 2008, 17(9): 8–15.

Linhua Hu, Songyuan Dai

Chapter 2 Nano-semiconductor materials Nano-semiconductor materials usually refer to semiconductor materials transformed by nanotechnology, whose size is on the nanoscale level. Nanotechnology appeared in the 1980s and has developed quickly. A very important branch of nanomaterials is nano-semiconductor materials. A semiconductor material remolded by nanotechnology usually has some special properties such as high surface area. In recent years, a nano-semiconductor material has developed rapidly and has been applied in many fields, such as new type of solar cells, nanoscale electronic devices, light-emitting devices, biological sensors, and catalysts, showing attractive application prospects. In 1991, a group led by professor Grätzel applied nano-semiconductor TiO2 to dye-sensitized solar cells (DSCs) and achieved a great breakthrough. This chapter briefly focuses on the effect of nano-semiconductor materials in DSCs, especially the common nano-semiconductor materials and the new nano-semiconductor materials. In addition, we aim at the move and bend of energy band in DSCs, and electron transport and recombination in porous film. At the same time, this chapter also focuses on the physical and chemical modification of nano-semiconductor electrode and thinfilm electrode optimization design in DSCs in detail. What is more, nano-semiconductor electrode using the cathode-sensitized solar cells is introduced.

2.1 The application of nano-semiconductor materials in DSCs 2.1.1 Effect of semiconductor porous film Nano-semiconductor porous film is one of the key components of DSCs. In DSCs, there are some related processes: (a) the dye molecules in the ground state absorb energy to transfer to the excited state; (b) dye molecules in the excited state inject electrons to the semiconductor conduction band; and (c) electron transport and recombination in the film. Therefore, the performance of nano-semiconductor thin film directly affects the cells in the adsorption of dye sensitizer, the incident light transmission, and photoinduced electronic collection and transport in the film, thus influencing on the performance of the cells in photoelectric conversion.

Linhua Hu, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University https://doi.org/10.1515/9783110344363-002

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The size and morphology of nanoparticles directly affect the microstructure characteristics of porous film, further affecting the performance of light refraction, scattering, and transmission in the film, as well as the electronic transmission of the nano-semiconductor thin-film electrode. The role of DSC nano-semiconductor porous film is mainly embodied in the following three aspects: (1) For the structure and principle of DSCs, a semiconductor porous film plays a role in the adsorption of dye, and the amount of dye adsorption is determined by the structure and performance. As mentioned in the first chapter, the film in the early photoelectrochemical solar cells was basically smooth and dense, so only the surface of dense film can adsorb the monolayer dye molecule whose conversion efficiency reached 1%. Low utilization ratio of light leads to a lower photoelectric conversion efficiency. Although they can only adsorb monolayer dye molecules on the particle surface, the introduction of nanoporous film in DSCs, the spongy structure of the porous film can adsorb more dye molecules, and each dye molecule directly contacts nanoparticles, in which the electron stimulated by the dye can quickly and effectively transport to the collecting electrode. At the same time, the incident light is absorbed repeatedly by dye molecules on the surface of the nano-semiconductor porous film, which greatly improves the light absorbing efficiency of the dye molecules. Dye molecules can effectively excite electrons and can transmit to the semiconductor only if they contact directly with nanoparticles. Therefore, the increased contact rate and contact area between dye molecules and nanoparticles in a porous structure, making electrons produced by dye excitation inject into the semiconductor conduction band in time and collected by the conductive film on the conductive glass, resulting in current, and the photoelectric conversion efficiency of DSCs getting greatly improved. It is adopting the porous structure as an electrode by Grätzel’s team in 1991, which leads to the breakthrough of 7.1% photoelectric conversion efficiency of DSC. (2) The amount of dye adsorption in nano-semiconductor porous film mainly depends on the surface state of semiconductor, film thickness, specific surface area, porosity, and other factors. Specific surface area refers to the sum of external surface area and pore structure of internal surface area in unit mass powder particles, whose unit is m2 g–1. Porosity refers to the proportion of unit volume pores accounted for the pore volume in a porous film material. Internal specific surface area of the nanoporous film mainly depends on the size of the nanoparticles: the smaller the nanoparticles, the bigger the specific surface area, and the greater the amount of dye adsorption. However, with the decrease in nanoparticles, the pore diameter is reduced. If the pore diameter is too small to let dye molecules and electrolytes in the photoelectric conversion, the performance declines. Therefore, nanoparticles need to be large enough to let molecules of dye molecules and electrolytes in [2]. When film thickness increases, the specific surface area increases and the dye adsorption quantity increases; correspondingly, the light absorption rate increases

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significantly. At the same time, recombination of charge increased, which lead to loss of electrons [3–5]. Thus, there is an optimal value in nanoporous film thickness and the size of the nanoparticles, and only in the most optimal value, the photoelectric conversion efficiency can truly reach the highest. Nanoparticle size not only affects the dye adsorption but also has an effective light scattering and reflection. It was found that when the average particle size of nanoparticles is small, the film printed is usually transparent, and the diffuse reflection of thin film is weak. On the contrary, when the average particle size of nanoparticles is bigger, printed film is usually white, and its light diffuse is stronger [2]. Increasing the diameter of the nanoparticles, the corresponding specific surface area is inevitably lower which reduces the amount of dye adsorption. Therefore, size and composition of nanoporous semiconductor particle film need to be optimized. (3) Nanoporous semiconductor thin film plays a very important role in electronic transmission of internal cells [6–8]. Photogenerated electron transmission injecting into the semiconductor conduction band and the conductive base is achieved in the porous film. At the same time, not all dye molecules in the excited state make electrons effectively injected into the semiconductor conduction band, and then converted into current. There are many factors influencing the output current. Basically, there are three aspects that result in the generation of dark current: A) Dye molecules in excited states failed to make electrons effectively injected into the semiconductor conduction band but through the internal conversion directly returns to the ground state. B) Dye molecules are not reduced by I– in electrolyte but directly recombined by electrons in the semiconductor conduction band, which consumes electrons. C) I3− in the electrolyte is not reduced into I− by electrons in semiconductor conduction band instead of the electron from counter electrode. The electrons of the porous film spend most of the time in the trapping–detrapping, making its transport and diffusion at a slower rate. In general, the number of generated charges is related to some factors such as the amorphous layer of oxide, oxygen defects, and grain boundaries. Obviously, small particles, big pore, chaotic arrangement, amorphous state, film rupture, and thick film will slow down the speed of electronic transmission speed in the film, increase electronic composite chance, and ultimately affect the photoelectric conversion efficiency of the cells.

2.1.2 The preparation method of nano-semiconductor porous film As the key factor of DSC light absorption, electronic collection, and transmission, the preparation technology is directly related to the quality and efficiency of DSCs. The commonly used preparation methods are as follows: sol–gel method, hydrothermal

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synthesis method, magnetron sputtering, chemical vapor deposition (CVD) method, and electrodeposition method. (1) The sol–gel method Sol–gel synthesis (sol–gel) is recently developed as an alternative to high-temperature solid-phase synthesis preparation of ceramics, glass, and many new solid materials. From the hydrolysis and polymerization of metallic organic or inorganic compounds in aqueous solution, the sol solution containing metal oxide particles was formed. After further gelation reaction and heat up, eventually amorphous glass and polycrystalline ceramics were gained. At present, this method is one of the most important ways in the preparation of nanofilm. According to the types of raw materials, the sol–gel process is usually divided into two categories: organic way and inorganic way. In an organic way, hydrolysis and polycondensation reaction of a raw material, usually organic metal alkoxide, obtains sol solution. Then gel is gained by further polycondensation of sol solution. Metal alkoxide hydrolysis and polycondensation reaction can be represented as follows: Hydrolysis reaction: MðORÞn + xH2 O ! ðORÞn − x − M − ðOHÞx + xROH

(2:1)

where M is metal elements and R is various alkyl groups. The alkyl metal alcoholate monomer is obtained through the hydrolysis reaction. The polycondensation reaction includes dehydration polycondensation and dealcoholizing polycondensation. Dehydration polycondensation: ðORÞn − 1 − M − OH + HO − M − ðORÞn − 1 ! ðORÞn − 1 − M − O − M − ðORÞn − 1 + H2 O (2:2) Dealcoholizing polycondensation: ðORÞn − 1 − M − OH + RO − M − ðORÞn − 1 ! ðORÞn − 1 − M − O − M − ðORÞn − 1 + ROH (2:3) Monomer is prepared by the hydrolysis reaction which undergoes dehydration polycondensation and dealcoholizing polycondensation reactions, forming -M-O-M- oxygen bridge bonds, With the ongoing polycondensation reaction, two-dimensional or three-dimensional inorganic network is gradually formed in the solution. While in an inorganic way, cheap inorganic salts usually act as a raw material and sol can be made by the hydrolysis reaction of inorganic salt: Mn + + nH2 O ! MðOHÞn + nH +

(2:4)

By adding alkali solution (such as ammonia), the hydrolysis reaction can go to the positive direction, and M(OH)n precipitation is gradually formed. Sediment is dispersed in strong acid solution after fully washing, followed by filtration to get the stable sol. By special handling (such as heating dehydration), sol can become gel,

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oxide powder is formed after drying and calcination. Then by adding surface-active agent and solvent to the powder, a paste is prepared by grinding. Then, using suitable methods such as screen printing, coating, and spin coating methods to prepare film directly on the conductive substrate, through high-temperature heat sintering, a nano-semiconductor porous film electrode was obtained. Compared with the gas method, sol–gel is easy to be popularized for its simple equipment, less materials, and lower costs. What is more, it is easy to obtain relatively uniform composition system, meeting the requirements of quantitative doping, and to effectively control film composition and structure. In addition, the relatively low preparation temperature is good for the preparation of those containing volatile components or polynary system of phase separation under high temperature. Large area film is relatively easy to be prepared in different shapes and different substrates, and even achieve a coating layer on the powder material surface, which is difficult to achieve by other traditional methods. Therefore, it is widely used in the preparation of TiO2, ZnO, and SnO2 photoanode thin film. The following examples describe the specific process of this type of method in preparation of nano-TiO2. To the solution of tetraisopropyl titanate in isopropyl alcohol, the deionized water is slowly dripped. After 0.5 h strong stirring, a certain amount of nitric acid is added, and the solution is heated up to 80 °C. The transparent sol appears after a long time of mixing. Then, it becomes milky after 1.5 × 106–3.3 × 106 Pa pressure processing in the autoclave. In order to prevent the TiO2 film cracking in the sintering process and obtain high specific surface area, a certain amount of polymer (such as poly(ethylene glycol)) is added. After dehydration by vacuum at constant temperature or by other methods (sintering), nanocrystalline TiO2 (anatase) powder can be obtained. Using screen printing technology, the TiO2 gel is coated on the substrate directly and the TiO2 nanocrystalline thin film is gained. Figure 2.1 shows the nanoporous TiO2 (anatase) thin-film preparation flowchart. (2) The hydrothermal synthesis method In the preparation of TiO2 thin film, for example, hydrothermal synthesis method refers to that amorphous precipitate by the hydrolysis of titanium alkoxide or chloride precursor, and peptizes in acidic or alkaline solution to obtain sol solution. Then after the hydrothermal Ostwald ripening in the autoclave, the sol is coated in the conductive glass substrate, and sintering at a high temperature of 500 °C, a TiO2 thin film appears. The paste by mixing TiO2 alcohol solution with commercial TiO2 can also take the place of the above sol. This method is the improvement of traditional sol–gel method by introducing a hydrothermal aging process to control the crystallization of the product, which is beneficial to control the particle size and distribution of semiconductor oxides as well as the porosity of film. The oxide crystal is determined by the reaction conditions (e.g., the calcination temperature). At the same time, the temperature of the hydrothermal treatment has a decisive influence on the particle size [18]. The method is also used in the preparation of ZnO and SnO2 nanomaterials.

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Figure 2.1: Mesoporous TiO2 (anatase) thin-film preparation flowchart.

(3) Sputtering Sputtering mainly includes the direct current (dc) sputtering and plasma sputtering, radio-frequency sputtering, and magnetron sputtering method. The basic principle is that the plasma-generating cation, which is accelerated by the auxiliary facilities, moves to the cathode target material and bombards the target material so that some atoms diffuse deposition on the anode substrate. In the preparation process, a different material and thickness of the film can be achieved by changing the different material of the target and controlling different sputtering times. The film has high density, good uniformity, and good connection between substrate and film, which is suitable for ultra-thin optical coating. However, the equipment is expensive, and the target material also has certain requirements. (4) CVD method CVD makes one or more kinds of the gas to experience thermal decomposition, reduction, or other reactions to form a thin film by depositing nanoparticles on the surface of the substrate in conditions such as heat, electricity, magnetic, and chemical. Prof. Aydil prepared dendritic array ZnO nanowires by metal organic complexes in CVD [19]. (5) The electrodeposition method Electrodeposition is a kind of electrochemical process coating by the electricity solution method, and a kind of redox process is passing into dc in the solution containing metallized ions, making the positive ions in the substrate surface

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discharge to get the required film. Although this method is simple, the factor is quite complex, and it is difficult for preparation of complex film materials and cannot control the speed of the growth of the crystal nucleus. The semiconductor oxide thin film is mainly crystalline or amorphous, and has low performance. Nogami first used a conventional three-electrode system to prepare TiO2 porous film by the electrodeposition technology. Yamamoto uses cathodic electrodeposition technology to prepare TiO2 photoanode and to get the photoelectric conversion efficiency of 4.13% DSCs [20]. Youshida uses electrodeposition self-assembly methods to obtain ZnO/dye composite film [19] (6) Cold compression method Cold compression method is a highly regarded technology in the preparation of TiO2 thin film in low temperature for DSCs, by dispersion of nano-TiO2 powder into an organic solvent to make suspension and then coat to the conductive substrate. After the volatilization of organic solvent, the substrate is pressed between two pieces of steel plate and made films. Studies have found that [18] with the increase of pressure, the film gradually becomes compaction and porosity decreases. In addition, the large-sized particles are crushed, and the distribution of particle size becomes narrow. Therefore, changing the pressure is able to control the porosity of the film and adjusts its size. At the same time, this method can also be used for the preparation of ZnO and other films. (7) Spray pyrolysis deposition Spray pyrolysis deposition technology is that the solution is sprayed to preheat the substrate and to deposit as a film. In other words, when the solution is sprayed into fog, small droplets spread out on the substrate and evaporate, leaving the sediment thermal decomposition as a film. Using this method in preparation of thin film, only a simple device is needed. It has the advantages of forming film thickness and controllable surface morphology without vacuum equipment, which is often used in the preparation of nano-TiO2 thin film.

2.2 Commonly used nano-semiconductor materials in DSCs A nano-semiconductor porous film applied to DSCs should meet at least the following three conditions: (1) there is enough large specific surface area adsorbing a large amount of dye molecules; (2) thin-film dye adsorption means must ensure electrons into the conduction band effectively; (3) electrons have faster transmission speed in the thin film, and an electronic composite is relatively slow in the thin film. At present, the application is in DSCs of nano-semiconductor porous thin-film materials such as TiO2, ZnO, and SnO2. From the band diagram (Figure 2.2), in the

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electrolyte whose pH is 0, the conduction band of TiO2 and ZnO matches with LUMO of commonly used dyes (such as relative to the vacuum level, the LUMO energy levels of the N719 and N3 are about 3.85 and 4.10 eV, respectively, and the LUMO level of other dyes is similar or lower than N719), which means the LUMO energy levels of semiconductor conduction band is lower than the dye molecule.

Figure 2.2: Energy band gap of semiconductor materials in the electrolyte (pH = 0).

2.2.1 Titanium dioxide (TiO2) TiO2, a kind of white powder, commonly known as “titanium white,” is a kind of cheap, wide application, nontoxic, stable important inorganic materials with good corrosion resistance, as well as high refractive index and good chemical stability. Its preparation is simple, but the energy band gap is bigger with the absorption in the ultraviolet band. Then it needs dye sensitization to utilize the visible light energy. In low temperature, TiO2 has good stability in many inorganic and organic media and does not dissolve in water and other solvents. Moreover, rutile phase of TiO2 is also difficult to dissolve in sulfuric acid. Due to small particle diameter and large specific surface area, nano-TiO2 is completely different from TiO2 with micro-sized or larger sized particle structure in the reaction to light, mechanical stress, and electricity. Besides having the performance of common TiO2, nano-TiO2 also has many differences from generally excellent properties of normal TiO2, such as large specific surface area, humidity sensitive, oxygen sensitive, photocatalysis, and other features. Therefore, it is widely used in catalysts, sensitive elements. In addition, the nano-TiO2 is a kind of semiconductor material with good optical properties of permeability, refractivity, and chemical stability, especially strong reflection ability for the visible light between 400

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and 800 nm. Due to its very small particle size, the ability of nano-TiO2 to absorb ultraviolet light is much stronger than normal TiO2. The strength and hardness of ceramic made of nano-TiO2 is 2–3 times larger than that of normal ceramic [21]. Nano-TiO2 and aluminum pigment or mica pearlescent pigment for paints or plastic system with the special optical performance of color varying with different angles leads to the wide application prospect in special coatings, for example, luxury car paint. The effect of its special nanomaterial size and surface makes the nano-TiO2 to be widely used as the catalyst carrier, UV absorber, functional materials, functional ceramics, and gas sensors.

2.2.1.1 The physical properties of TiO2 Titanium element belongs to the transition metal IVB group. Its atomic number is 22, the nucleus contains 22 protons and 20–32 neutrons, and the extranuclear electric substructure alignment is 1s22s22p63s23d24s2, generally showing the highest +4 valence state. Atomic number of oxygen (O) is 8; its nucleus contains 8 protons, and the extranuclear electric substructure alignment is 1s22s22p4. Then TiO2 is a relatively strong ionic oxide. In common, there are three types of crystal structure, namely anatase, rutile, and brookite. TiO2 parameters with different crystal structures are shown in Table 2.1. Table 2.1: TiO2 crystal parameters with different titanium ore phases. Parameters

Rutile

Anatase

Brookite

Crystal system

Tetragonal

tetragonal

Orthorhombic

Space groups

P/mnm

I/amd

Pbca

Lattice constant (Ǻ)

a = . c = .

a = . c = .

a = . b = . c = .

Density (g · cm−)

.

.

.

Hardness

–.

.–

–

Ti–O bond length (nm)

.

.

.–.

O–Ti–O bond angle

.° .°

.° .°

.°–°

Atomic radius

.

Valence state radius

(+) . (+) . (+) .

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Anatase-type TiO2 crystal type belongs to the tetragonal crystal system, which is only stable at relatively low temperature. When the temperature reaches 610 °C, it begins to transform into rutile phase gradually. The TiO2 rutile phase is a very stable phase, which also belongs to the tetragonal crystal system and can decompose only at very high temperatures. Brookite TiO2 crystal type belongs to the orthogonal crystal system and is an unstable compound. In normal conditions, the prepared TiO2 is mainly in anatase and rutile phases, both of which are wide band-gap semiconductor (rutile band gap is 3 eV and anatase band gap is 3.2 eV). Figure 2.3 shows the crystal structure of anatase and rutile phases. Combining with the data in Table 2.1, in the rutile phase, a Ti atom coordinates with six O atoms, forming an octahedron. In the octahedral anatase phase, octahedron is distorted because the four O atoms, except the two O atoms’ octahedral vertex, are not on a plane (the angle between central Ti atom and the four O atoms is 92.6°). Anatase- and rutile-phase TiO2 crystal structure is often seen as composed of such an octahedral arrangement.

Figure 2.3: Anatase- and rutile-phase crystal cell diagram.

Crystal structure differences lead to two different density and electronic structure between anatase and rutile phases. Studies have shown that when these two types of crystal structures of TiO2 are used in DSCs, the open-circuit voltage is basically the same but short-circuit current of rutile phase is 30% smaller than that of anatase phase. The reason is that the specific surface area of rutile-phase TiO2 thin film is smaller resulting in less dye adsorption. Dynamics research showed that the connection number of rutile-phase particles per unit volume is low and the electronic transmission time in the rutile-phase film is slower [22]. Therefore, in the DSCs, TiO2 with anatase phase is used more commonly.

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From the aspect of thermodynamics, rutile is a thermodynamically stable phase, while anatase belongs to the metastable phase. After a certain temperature of heat treatment, its phase transitions into the rutile phase, which is the so-called structure of anatase and rutile phase change (hereinafter referred to as “A→R phase transformation”). Many properties differ between two types of crystal phases, for example, in photocatalytic and photoelectric conversion performance, anatase phase is obviously better than the rutile phase. Research has shown that decreasing the size of the TiO2 particles of powder can obviously decrease the beginning temperature of A→R phase transformation [23]. For example, TiO2 nano-powder prepared by the sol–gel method began A→R phase transformation at 550 °C, whose phase transition temperature range is 550–800 °C [23]. Other physical properties of rutile and anatase are shown in Table 2.2. Table 2.2: The physical properties of rutile and anatase. Property

Anatase

Rutile

Melting point Mohs hardness Refractive index UV absorption/% ( nm) Reflectivity/% ( nm) Dielectric constant

Transform into rutile .–. .  – 

, K .–. .  – 

2.2.1.2 The chemical property of TiO2 TiO2 has a relatively stable chemical property and is biologically nontoxic. Nano-TiO2 particles also have good chemical stability and biological nontoxicity. Because the particle size is greatly reduced to nanoscale, as a result, it leads to sharply increase the specific surface area. Compared with the ordinary TiO2, chemical properties of nanometer TiO2 are more active, with very strong adsorption and photocatalytic performance. (1) Adsorption The bond polarity of Ti–O is higher for nano-TiO2, and hydroxyl groups form easily on the surface, thereby improving the function of TiO2 as an adsorbent and a carrier. Alcohol and other organic molecules form strong adsorption by hydrogen bonding in the TiO2 surface. Chemical adsorption is not only influenced by the particle surface properties but also affected by the adsorption phase and solvent. Nano-TiO2 in aqueous solution with different pH values can be positive, negative, or neutral. When the pH value varied from small, middle to high, the surface forms different bonds by varying from Ti-OH2 bond, Ti-OH bond, to Ti-O bond, charging differently from positive, neutral to negative, accordingly [24].

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(2) Dispersion and aggregation Due to the activity on the surface of nanoparticles and the characteristics of nanoparticles, they easily aggregate with a weak connection interface and larger size. Under normal condition, nanoparticles are dispersed in the solution to form a suspension due to the factors such as Brownian movement (whether physical method or chemical method is used in the preparation). However, even in this case, due to the coulomb force or van der Waals’ force between particles, the aggregation phenomenon will still happen. In short, the aggregation phenomenon between nanoparticles is common, and it is still hard to disperse nanoparticles in good uniform. (3) Photocatalytic performance For most organic compounds or inorganic compounds, electron or hole transfer occurs on the semiconductor particle surface, leading to photocatalytic oxidation or reduction. A unique strong oxidation ability of photon-generated holes of nano-TiO2 makes it possible to biodegrade organic pollutants such as aromatic species and surfactants with complete oxidation. Nano-TiO2 with anatase phase has been widely used in the catalysis field.

2.2.1.3 Nano-TiO2 in the application of DSCs Since 1991, professor Grätzel and coworkers used nano-TiO2 porous films as the anode to gain 7.1% of the photoelectric conversion efficiency [25]. After that, smallarea DSCs based on TiO2 light of anode has made great progress in the photoelectric conversion efficiency. Now the photoelectric conversion efficiency was more than 13%. In the exploration of nano-TiO2 photoanode, the work by Grätzel’s team was the most prominent around the world. At the same time, many countries such as Switzerland, the United States, Australia, and Japan invested a lot of resources for research and development of DSCs based on TiO2. There were also many domestic scientific research institutes and universities carrying out a large amount of works, such as Institute of Plasma Physics, CAS, Technical Institute of Physics and Chemistry, CAS, and Peking University. Their research team made a lot of achievements in this aspect, drawing the attention from the international counterparts. Although nano-TiO2 is the most used DSC thin-film material, limitations exist. Large Brown–Emmett–Taylor (BET) surface area results in lots of surface state. The surface state in the forbidden band level is restricted as a trap, which hinders the movement of electrons in the film and prolongs the electronic transmission time. At the same time, some of the electrons can recombine with positively charged dye sensitizer cation, and some electrons can react with the electrolyte, and the reverse dark current electron transfer due to the absence of transition layer between oxide and electrolyte interface, thereby reducing the efficiency of DSCs. At present, so many researchers focus on the structure optimization and modification on nano-

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TiO2 porous film photoanode to improve TiO2 photoanode performance and the conversion efficiency of DSCs.

2.2.2 ZnO Zinc oxide (ZnO) is a kind of white powder, and it is nontoxic, nonpolluting, and environmentally friendly with the density of 5.6 g · cm−3. It is hardly soluble in water but soluble in acid and alkali, and is widely used in coatings, photocatalysis, and pressure sensor. There are three types of ZnO crystal structure: wurtzite structure, cubic sphalerite structure, and sodium chloride octahedral structure. The most commonly used wurtzite structure has the highest stability among them. Cubic sphalerite structure can be generated by gradually growing on the surface of zinc oxide. In two kinds of crystals, each zinc or oxygen atom can form a regular tetrahedron structure with adjacent atoms. The octahedral structure is observed only in 100 GPa high-pressure condition. Wurtzite structure and sphalerite structure have central symmetry without the axial symmetry. Crystal symmetry properties make the wurtzite structure, and the sphalerite structure embodies the piezoelectric effect. The wurtzite structure has point group of 6 mm (international symbol), and its space group is about P63mc. The lattice constants a = 3.25 Å and c = 5.2 Å; c/a ratio is about 1.60, which is close to the 1.633 hexagon ideal proportion. In the semiconductor material, zinc and oxygen connect with the ionic bond, which is one of the reasons for its high piezoelectric effect. Singlecrystal ZnO is a hexagonal system wurtzite structure, as shown in Figure 2.4.

Zn

O

Z y x

Figure 2.4: ZnO microstructure.

ZnO is a II–VI semiconductor and has the same band-gap width and similar conduction band potentials with TiO2, but it has larger electron mobility and shorter electronic transmission time. The first ZnO used as photoanode was reported by Gerischer in 1969. But the study of nanostructure ZnO thin-film solar cells increases gradually when Redmond succeeded in obtaining 13% monochromatic photon-current conversion

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efficiency at a wavelength of 520 nm and 0.4% photoelectric conversion efficiency for ZnO porous film DSCs in 1994 [26]. In 1997, Rensmo reported the nano-ZnO solar cells, whose monochromatic light conversion efficiency reaches 58% and the total photoelectric conversion efficiency is up to 2% [27], making people to find the potential of ZnO as an efficient DSC photoanode material. Gifu University used organic indoline as dyes, nano-ZnO thin film as an electrode prepared by electrodeposition, to prepare successfully colorful plastic DSCs, and the photoelectric conversion efficiency is up to 5.6%. In 2008, the photoelectric conversion efficiency of 7.2% was prepared by applying porous single-crystal ZnO in DSCs [28]. In addition, controlling ZnO nanostructures suggests that the smaller diameter, the better conductive performance. However, the conversion efficiency of DSCs with ZnO photoanode is still lower than that based on the TiO2 photoanode. The reasons boil down to: (1) the large grain size and small specific surface area of ZnO films leads to the small dye adsorption numbers; (2) different from the electronic coupling phenomenon in TiO2 films, dye/Zn2+ complex formation does not favor the injection of excited electrons into the conduction band from ZnO.

2.2.3 SnO2 Tin oxide (SnO2) is a powder with white, pale yellow, or pale gray color and with tetragonal, hexagonal, or orthorhombic system in crystal phase. It is a kind of welltransparent conductive material. In order to improve the conductivity and stability, it is often doped with other elements, such as F and Sb. SnO2 is a tetragonal rutile structure, as shown in Figure 2.5. SnO2 contains two Sn and four O atoms, whose lattice constant is a = b = 0.4737 nm, c = 0.3186 nm, and c/a = 0.637.

O Sn Figure 2.5: Rutile-phase SnO2 crystal original cell diagram.

SnO2 is a wide band-gap semiconductor, whose forbidden bandwidth is 3.5–4.0 eV, and the transmission rate of infrared and visible light was 80%. The plasma edge is located at 3.2 µm, refractive index is >2, and extinction coefficient tends to zero. The ability of SnO2 is high, and the adhesion strength between glass and ceramics is 20 MPa. What is more, its Mohs hardness is up to 7–8, has good chemical stability, and can withstand chemical etching. As a conductive film, the main carrier of SnO2 is from crystal defect, which means O space and provides electron doped with impurities. SnO2 has good transmittance for visible light, has excellent chemical stability in aqueous solution, and has specific conductivity and reflection characteristics to

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the infrared radiation. Thus, it has been widely used in lithium cells, solar cells, liquid-crystal display, photoelectric devices, transparent conductive electrode, and protection against infrared detection. The nano-SnO2 material has a significant change due to its advantages such as small size effect, quantum size effect, surface effect, and macroscopic quantum tunneling effect in light and heat, and physical properties such as electric, acoustic, and magnetic, and other macroscopic properties compared with the traditional SnO2. Therefore, you can use nanomaterials to improve the performance of the sensor material SnO2. For the dye sensitization system, the driving force of dyes on the semiconductor electron transfer from energy-level difference between the oxidation state dyes and semiconductor conduction band can be expressed as follows:
 (2:5) ΔE = − e E* ox − EFB where E*ox is the excited dye oxidation potential energy, EFB is the semiconductor conduction band potential energy, and e is the number of charge. The SnO2 conduction band position is passive by 0.5 V compared with the TiO2 conduction band; in theory, this is beneficial to the electron transition from the excited state dyes to the conduction band. At the same time, due to the wider band gap, SnO2 shows low sensitive to UV light. However, the photoelectric conversion efficiency of DSCs based on SnO2 photoanode is still very low (about 1–2%), which need further research and optimization.

2.2.4 Other semiconductor materials In addition, metal oxides such as Nb2O5, Fe2O3, CeO2, and Sb6O13 are also studied as DSC photoanode materials. Compared with TiO2, they have wider band and the similar conduction band edge position. Nano-Nb2O5 may become another suitable material preparation of high-performance DSCs instead of TiO2. Researchers took these oxide nanoparticles, nanobelt, Nb2O5/TiO2 composite layer, or blocking layer to prevent the reverse electron transfer in DSCs. The greater the specific surface area of Nb2O5 photoanode is, the smaller the DSC impedance is. Comparison of performance of TiO2, Nb2O5, and ZnO as anode DSCs suggests that the conversion efficiency based on Nb2O5 is second only to that of TiO2 and the high open-circuit voltage is obtained, which mainly contributes to the different electronic structure. When using Ru complex sensitizing Nb2O5 DSCs (0.2 cm2), the short-circuit current density of 1.7 mA · cm−2, fill factor (FF) of 55%, and photoelectric conversion efficiency of 5.0% were achieved under low light intensity [29]. Nb2O5 as a blocking layer between conductive glass and nano-TiO2 used in DSCs inhibits electronic composite, which greatly improved the cell’s open-circuit voltage and FF [30]. Some research works on Fe2O3 have been done. It is found that the carrier diffusion length Ld is very small, the electron recombination is large for Fe2O3, and the

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efficiency is only 1.7% [31]. Zr4+ and La3+ doping nano-CeO2 has photoelectric response characteristics, and DSCs with La3+ doping CeO2 as photoanode gets the open-circuit voltage of 0.9 V [32]. The conduction band of these materials can match with the LUMO of dye. However, there are many reasons leading to unsatisfactory efficiency of DSCs, and more studies need to be done to improve the efficiency. In addition, some simple ternary compounds (such as SrTiO3 and Zn2SnO4) were reported in the application of DSCs.

2.3 The application of new nanostructure materials in DSCs The material’s microstructure determines the specific surface area and the size of the hole. The size of the specific surface area determines the number of dye molecules load, and the size of the holes affects the diffusion of dye molecules in the film material. However, there is contradiction between the specific surface area and the size of holes for a certain material. The bigger the hole is, the smaller the specific surface area is. The too small the hole to allow the dye molecules to spread inside is useless for dye adsorption. At the same time, the surface defect and holes ratio of the material affects the electron transport in thin film. In these cases, the microstructure of materials determines the electron injection quantity and electron collection efficiency of DSCs, which determines the photoelectric conversion efficiency of DSCs.

2.3.1 One-dimensional materials A one-dimensional nanomaterial is named as a material whose two dimensions are in nanoscale materials in space, including nanotube, nanorod, and nanowire. Since carbon nanotubes were discovered by the Japanese scientists in 1991, carbon nanotubes have aroused an extensively international research interest due to their special physical, chemical, electrical, and mechanical properties, which also promote the development of other nanomaterials. During the process of carbon nanotube research, scientists gradually realized one-dimensional nanostructures of other materials that also have special properties.

2.3.1.1 Carbon nanotubes In 2002, researchers have firstly introduced the single-walled carbon nanotubes into TiO2 nanoparticle-based mesoporous film, which increases the electrical conductivity and light scattering properties. A significantly increased photocurrent of DSCs and a

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slightly reduced photovoltage contribute to a markedly increased power conversion efficiency. Afterward, the researchers further analyzed the performance of composite films composed of single-walled carbon nanotubes and TiO2 nanoparticles. The composite film containing 0.01 wt% of single-walled carbon nanotubes (length of 0.5– 3.5 µm) increases the adsorption amount of the dye molecules and increases the photocurrent by 25%, and photovoltage by 0.1 V, resulting in a significantly improved power conversion efficiency. However, the walls of single-walled carbon nanotube are thin with poor thermal stability, which are unable to bear calcination treatment at a high temperature. Thus, the multiwalled carbon nanotubes are introduced into DSCs. The growth of TiO2 nanoparticles onto the surface of carbon nanotubes by combination with the TiCl4 treatment leads to higher thermal stability, retarded electron recombination, and increased photocurrent by 35%. The multiwalled carbon nanotubes treated with concentrated nitric acid are mixed with TiO2 nanoparticles to prepare the mixed paste through the sol–gel method. After sintering at 450 °C for 30 min, the composite films containing 0.3 wt% of multiwalled carbon nanotubes possess the high roughness and small pore size and increase the amount of dye loading. Additionally, the multiwalled carbon nanotubes improve the conductivity of composite films, but the dark current increased significantly after exceeding 0.3 wt% of carbon nanotubes. Therefore, introducing 0.3 wt% of carbon nanotube into TiO2 nanoparticle-based film increases by 61% of the power conversion efficiency, compared with the pure TiO2 nanoparticle-based film. It has been discovered that the interface impedance of dyedTiO2/electrolyte (Rct) increases with increasing the content of carbon nanotube (0.1– 0.5 wt%) in the light condition after sintering the composite film at low temperature (150 °C), resulting from the reduced specific surface area of composite film and the corresponding dye loading amount. The highest Rct is found in the composite film containing 0.1 wt% of carbon nanotube under the dark condition, which could effectively inhibit the electron recombination and improve the electron lifetime, and thereby obtain the highest power conversion efficiency. Single-walled carbon nanotubes were synthesized by using a virus as a template to self-assemble and mixed with TiO2 nanoparticles to prepare the composite film. When 0.2 wt% of carbon nanotubes are added, the composite film achieves the highest power conversion efficiency of 10.6%. Considering the strong catalytic ability, carbon nanotubes have also been applied into the counter electrode to substitute the Pt-based counter electrode. Although the catalytic ability of carbon nanotubes is inferior to the Pt-based counter electrode, great attention has been paid on the development of the cheap carbon nanotubes.

2.3.1.2 One-dimensional ZnO nanomaterials ZnO nanomaterials have various morphologies, including nanowires, nanorods, nanotubes, nanoribbons, nanorings, and nanocombs. ZnO was one of the first semiconductors applied into DSCs. The band-gap and conduction band edge of ZnO similar to

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that of anatase TiO2 are beneficial to inject electrons from the excited state of dye molecules. Besides, one-dimensional ZnO nanostructures have a high electron mobility, which could accelerate the electron transport. In recent years, the synthesis method and properties of one-dimensional ZnO nanomaterials have been realized. Numerous routes have been explored to the synthesis of one-dimensional ZnO nanostructures. According to the growth mechanism, the representative approaches include gas–solid–liquid growth, gas–solid growth, solution–liquid–solid growth, and dislocation-induced growth. Based on the state of reactants, the routes are divided into gas-phase method and liquid-phase method. The following describes the main synthesis method and related formation mechanism of gas-phase method and liquid-phase method. Gas-phase method refers to that the precursor is a gas phase or through a certain process into a gas phase during the synthesis process and forms into desired one-dimensional nanomaterials under a certain mechanism. The mechanism of gas-phase method is the growth of tiny crystals after condensation of the supersaturation vapor through the process of distillation, evaporation, and decomposition into gas phase under appropriate conditions. This method is widely used for manufacturing highpurity and integrity good nanomaterials but needs the suitable heat treatment to eliminate the thermal stress and some defects. Therefore, according to different routes to convert the precursor into vapor phase, the vapor-phase method includes vapor-phase transport (thermal evaporation), molecular-beam epitaxy (MBE), CVD, metal organic CVD (MOCVD), plasma-enhanced CVD (PECVD), metal organic vaporphase epitaxy (MOVPE), radio frequency magnetron sputtering (RFMS), arc discharge, and thermal pyrolysis. (1) Vapor-phase transport, also known as thermal evaporation method, refers to heating the reactants into a vapor phase, and transporting or depositing on the substrate through the condensation and nucleation growth into one-dimensional nanostructures. This is one of the most effective ways for preparing the one-dimensional ZnO nanomaterials. The precursor (or mixed catalyst) is directly heated to high temperature on the stove, with the help of carrier gas which could blow the vapor to the cold end, which make it easy to nucleate and grow. There are many influencing factors in this method, including raw material, evaporation temperature, collecting temperature, presence or absence of catalysts and the type of catalysts, pressure, and type of carrier gas. The formation mechanism to prepare the one-dimensional nanomaterials with gas-phase method is divided into vapor–liquid–solid (VLS) (with metal catalysts) mechanism and vapor–solid (VS) mechanism (without metal catalysts). In the process of a catalyst-free thermal evaporation for preparation of the one-dimensional ZnO nanomaterials, the gaseous source forming at high temperature directly condenses at low temperature. After reaching the critical size, the nucleation and growth occur without the catalysts and the droplet from reactants participated, which is the typical VS mechanism. In 2001, the VS method has been used to fabricate the ZnO nanobelts without the presence of catalyst

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under the following parameters: evaporation time of 2 h; chamber pressure of 300 torr; and Ar flowing rate of 50 cm3 · min−1. The typical widths of 30–300 nm, width to thickness of 5–10 nm, and lengths of up to a few millimeters of single-crystal ZnO nanobelts were deposited onto the alumina substrate in the horizontal tube furnace, as shown in Figure 2.6 [39]. It is easy to synthesize the belt-like nanostructures based on the VS mechanism. The oxide vapor, evaporating from the starting oxide at a higher temperature zone, directly deposits onto a substrate at a low-temperature region and grows into belt-like nanostructures.

Figure 2.6: TEM image of ZnO nanobelts [39].

Researchers have used the modified VS method (catalytic gas-phase epitaxial growth method) to grow ZnO nanowires. About 20–150 nm thick and 2–10 mm long ZnO nanowires were grown on 1–3.5 nm thick Au-precoated sapphire substrates, and the morphology is depicted in Figure 2.7 [40]. Au deposited on the surface of the substrate may catalyze the growth of ZnO nanowires via VLS catalytic growth mechanism. The process involves the reduction of ZnO powder by graphite/hydrogen to form Zn and CO/ H2O vapor at a high temperature. The Zn vapor is transported into, and reacted with, the Au catalyst on silicon substrate located downstream at a lower temperature to form Au/Zn alloy droplets. As the droplets become supersaturated, crystalline ZnO nanowires are formed. The presence of a small amount of CO/H2O is not expected to significantly change the Au–Zn phase diagram, and then they serve as the oxygen source during the ZnO nanowire growth. The size control of the nanowire diameters was

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achieved by varying the thickness of the thin-film Au catalyst in the nanowire growth process. After the system was cooled, the alloy droplets were solidified at the top of the nanowires. Currently, the growth of one-dimensional ZnO nanomaterials is mainly prepared through the VLS mechanism in the thermal evaporation process. The precursor is usually a pure metal zinc, zinc oxide, or zinc sulfide. The evaporation temperature is slightly higher than the eutectic of the catalyst and precursor; when the zinc metal is used as a precursor, it needs a low evaporation temperature.

Figure 2.7: SEM image of ZnO nanowires [40].

(2) CVD refers to a process in which vaporized precursor materials (metal hydride, metal halide, or organic metal) transport, decompose, or react on the particular substrate surface at low temperature to form solid materials. CVD is usually used for the preparation of nano-semiconductor films. The growth process involves decomposition (decomposition of zinc sources), transport (transport by the inert gas to the substrate), nucleation and growth (zinc source nucleation and growth on the substrate), and final growth into a one-dimensional ZnO nanostructure at the vertical direction of the silicon substrate. CVD includes thermal pyrolysis reaction deposition, chemical reaction deposition, and metal organic vapor deposition. The deposition method may be divided into CVD and MOCVD whether the source used is organic metal or not. ZnO nanorods with diameters of 40–300 nm and lengths up to tens of microns were fabricated through a simple chemical reaction deposition method [41]. ZnO nanorods with diameters of 25–70 nm and lengths of 800 nm were obtained on Al2O3 (001) substrates using a low-pressure MOVPE system. For ZnO growth, diethylzinc and oxygen were employed as reactants and argon was used as a carrier gas. A typical growth temperature was in the range of 400 °C [42]. The obtained crystals synthesized from chemical vapor method have many advantages such as high purity, better compactness, and crystallographic orientation. The main factors on the CVD method are similar to the vapor-phase method, such as: catalyst types, reaction temperature, carrier gas, gas sources, and gas flow rate. Besides, the CVD technology is particularly interesting not only because of the low reaction temperature, mild conditions, simple equipment, large-scale production, continuous operation, and convenient collection but also it eases to form arrays. Then, the CVD method becomes the main method to prepare the one-dimensional

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ZnO nanomaterials. By choosing the appropriate catalyst and substrate, and proper flow rate and pressure, growth of nanorods can be vertically well-aligned and exhibit uniform thickness and length distributions. (3) Other gas-phase methods, altering the growth conditions in some gas-phase process, such as catalysts, carrier gas flow, and the growth rate are suitable to prepare thin films. MBE, PECVD, RFMS, and so on can also fabricate the one-dimensional nanomaterials. Solution-based growth method refers to the chemical reactions that occur in an appropriate solvent to control the crystal nucleation and growth by using chemical solution as the media to transport energy. Since the growth of each crystal plane is substantial in the same environment, the anisotropic crystal structure results in the anisotropy of the growth rate. The obtained crystal is usually the slow growth rate of lower index plane. The prepared crystals from this method are low stress, uniform, and complete with the polyhedron shape. According to the differences of energy transfer process or carrier, solution-based growth for preparation of one-dimensional ZnO nanomaterials is divided into hydrothermal/solvothermal method, microemulsion method, template method, solvothermal method, self-assembly method, electrochemical method, and sol–gel method. (1) Hydrothermal/solvothermal method: this method is widely used for manufacturing micro/nanoparticles in the ceramics industry with the reaction in aqueous or nonaqueous solutions, respectively. High temperature and high pressure increase the solubility and reaction rate of the solute to grow the crystalline which is insoluble crystalline at normal pressure and temperature. The synthesis is typically conducted in autoclaves at elevated pressure and temperature. Under such conditions, ion reaction and hydrolysis reaction are accelerated, and some slow and hard reactions allowed by the law of thermodynamics at normal pressure and temperature became fast reactions. This method to produce one-dimensional nanostructures include: (1) using crystal structure properties of the materials in the fast-growing direction to prepare one-dimensional nanomaterials; ZnO has a stable hexagonal phase and grows fast along the c-axis direction, which is easy to obtain the one-dimensional nanostructures. The ZnO nanorods and nanowire arrays are obtained in the Si substrate and thin films composed of ZnO nanoparticles, respectively, using zinc nitrate and hexamethylene tetramine as the starting materials at 95 °C for homogeneous precipitation. (2) Using a small number of organics as additives for preparing one-dimensional nanomaterials. There are currently many surfactants used, including cetyl trimethyl ammonium bromide (CTAB), hexamethylenetetramine, sodium dodecyl sulfate (SDS), complexing agent (thioglycolic acid (TGA)), and polymers. ZnO nanorods are synthesized by the CTAB favored hydrothermal oxidization of zinc metal at 180 °C [43]. The presence of CTAB is a cationic surfactant, which could be considered to influence the erosion process of zinc and the growth process of ZnO by the electrostatic and stereochemical effects. In addition, ZnO nanorods with the diameter of 70 nm

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and the length of 1 μm are prepared by CTAB favored hydrothermal oxidization of zinc metal at 180 °C under hydrothermal conditions. The CTAB favors to form a film in which molecules tend to be perpendicular to the adsorbed surface, and the growth units would tend to face-land onto the growing interface. This kind of landing and dehydration will result in three Zn–O–Zn bonds, which make this landing mode predominant in competition with other ones such as vertex and edge landing. The ZnO crystal should grow preferentially along the c-axis ([0001] direction). (3) Using the water phase and oil phase forming the reaction beam to prepare nanomaterials under the low temperature, mainly containing CTAB–water–alcohol–alkanes or amine system. ZnO nanowires were synthesized by using a quaternary microemulsion consisting of the CTAB, deionized water, cyclohexanol, and heptane at 140 °C for 20 h. The controllable precipitation of Zn(OH)2 inside the droplet microreactor is very beneficial to the growth of nanowires along preferred orientations. Besides, the reaction temperature seems to have certain effects on the growth of nanowires due to the relatively poor stability of reverse micelle at high temperature. Generally, employing the reverse micelle method to prepare the one-dimensional ZnO nanomaterials chooses a low reaction temperature (about below 160 °C) [44]. Effects on the morphology, crystal size, and structure of the one-dimensional nanomaterials are reaction temperature, reaction time, the type of raw materials, concentration, pH value, and organic additives. (2) Microemulsion methods: When two immiscible solvents are mixed together with the help of surfactant through mechanical agitation, liquid droplets of one phase can be dispersed into the other continuous phase, forming an emulsion. Solid phase precipitated from the emulsion, through the nucleation, growth, coalescence, and agglomeration process in the tiny spherical droplets, further avoiding aggregation. The main principle of the microemulsion method is to use the surfactant with two different end groups, namely the hydrophilic hydrogen bonding to connect the ionic or nonionic end group and the hydrophobic alkyl chain dominated by the van der Waals force. In micelles, the hydrophobic hydrocarbon chains of the surfactant are oriented to the interior of the micelle, and the hydrophilic groups of the surfactants are oriented toward the surrounding aqueous medium. Reverse micelles are formed in nonaqueous media, and the hydrophilic headgroups are directed toward the core of the micelles while the hydrophobic groups are directed outward toward the nonaqueous media. Such reaction takes place in the hollow part, so as to limit the growth of materials in certain directions. Because most of the hollow part formed are spherical, it was mainly used for the preparation of smoothly spherical nanoparticles. The shape of the micelle is a function of the molecular geometry of its surfactant molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic strength. Currently, it can be used for the synthesis of the columnar nanomaterials by selecting an appropriate surfactant. The main surfactants are CTAB and SDS. In addition to the hollow tunnel, microemulsion method also needs the chemical potential. Therefore, choosing surfactants is highly required to satisfy the low ion product. Because the application scope of this method is relatively narrow, this

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method is generally used in combination with other methods. To prepare the microemulsion, cyclohexane was used as the oil phase and a mixture of poly(oxyethylene)-5-nonyl phenol ether (NPS) and poly(oxyethylene)-9-nonyl phenol ether (P9), as well as (polysorbate 80) (TW80) was used as a nonionic surfactant. The precipitation was obtained after adding potassium chloride as a solvent salt into the reaction mixture of zinc chloride and sodium carbonate. Then the precipitates were repeatedly washed using anhydrous acetone followed by centrifuge [45]. ZnO nanorods with the length of tens of micrometers and diameter of 30–80 nm were dried in an oven at 80 °C before they were calcined at 770 °C for 3 h to form ZnO nanorods. (3) Sol–gel methods: These are widely used in synthesizing inorganic or organic–inorganic hybrid materials. In a sol–gel process, the sol is a colloidal suspension that is typically prepared by mixing the metal alkoxides with alcohol or other organic solvents. When stirring the sol for hydrolysis and polymerization reactions, loss of solvent converts the liquid sol into a gel. The gel has a continuous solid skeleton surrounded by a continuous liquid phase and is further converted into nanoparticles after heat treatment under proper conditions. This method exhibits a series of advantages, including a low preparation temperature, good uniformity of films, and strong adhesion of substrate. In addition, it is easy for atomic doping and to obtain homogeneous multicomponent system in a simple process for preparation of nanomaterials without vacuum equipment. There are three types: traditional colloid type, inorganic polymer type, and complex type. (4) Template methods: These are a very versatile synthesis technique that forms a nanostructure with a morphology that follows the featured pores or network structures in nanoscale. The templates being chosen can be orderly hole arrays of alumina template, disorderly hole of polymer template, nanohole of glass template, porous zeolite, porous silicon template, and metal template. The nanospace of templates provides nucleation sites and limits the growth direction of nanomaterials, which could be used to grow into the desired structural nanomaterials. The template method can generally be divided into hard template and soft template. The hard template method is the use of a template material that has a hollow channel to control the growth of one-dimensional nanomaterials. While the soft template method is using the chain curly or expansion force of organic molecular to control the growth of one-dimensional nanomaterials. The soft template method is a relatively broad concept. Basically, all methods of using organic molecules to control the growth of one-dimensional nanomaterials are classified into the soft template method. The most commonly employed templates include porous alumina template, porous polymer membrane, porous materials, and one-dimensional nanomaterials such as carbon nanotubes and DNA molecular template. There are many methods to fill the needed materials into the pores of templates, such as electrochemical method, chemical solution method, CVD method, and thermal evaporation method. About 15–90 nm diameter ZnO nanowires were synthesized on

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the alumina template (AAM) via the electrochemical method [46]. While using the sol– gel method, the ZnO nanofibers can be deposited on the alumina template [47]. (5) Self-assembly methods: These methods are usually under the condition of specific solvent and appropriate solution to assemble the determined components of radicals, supramolecular, molecular assembly, nanoparticles, and other sized particles, composed of atoms or molecules, into mesoporous nanostructural materials and devices. Generally, a self-assembly system contains artificial nanostructure assembly system, nanostructure self-assembly system, and molecular self-assembly system. Now, the nanostructure self-assembly system and molecular self-assembly system are relatively mature. The nanostructure self-assembly system is formed with the aid of weak and small directional noncovalent bond, such as hydrogen bonding, van der Waals bonding, and weak synergetic ionic bond, to assemble the atoms, ions, or molecules into a nanostructure or nanostructural pattern. Molecular devices and molecular regulation have an application value in the information and material science. The related design and research of the molecular self-assembly system have aroused great attention. The molecular self-assembly system is that the intermolecular noncovalent interactions or spontaneous assembly into molecular aggregates or supramolecular structures which have a specific function and properties. The main principle is the intermolecular synergistic effect and spatial relationship. The key to design the self-assembly system is to regulate the intermolecular noncovalent bond correctly and to overcome the negative factors of thermodynamics in the self-assembly process. The formation of high-quality single-crystalline ZnO nanorods is based on the oriented attachment of preformed quasi-spherical ZnO nanoparticles that were prepared from zinc acetate dehydrate in alcoholic solution under basic conditions through the sol–gel methods [48]. Besides, there is also the use of organic chemicals to control the growth of a material along the one-dimensional direction. The highly oriented growth of ZnO nanowhiskers has been obtained by using both the self-assembly technique to modify the surface and the VLS vapor conversion method. Adopting the drip or dip-coating method prepares a single micellar membrane with the coating of Au nanoparticles on the sapphire substrate. After the 0.1 mbar oxygen plasma treatment for 30 min to remove all organic constituents in the micelle, the Au micellar membrane was obtained on the sapphire substrate.

2.3.1.3 One-dimensional TiO2 mesoporous films One-dimensional TiO2 nanomaterials have aroused great attention due to superior physical and chemical properties. The one-dimensional TiO2 nanomaterials have faster electron transport rate and long electron lifetime, which is beneficial to collect photoinduced electron in DSCs. Besides, the one-dimensional nanomaterials have a high longitudinal light scattering effect and improve the light harvesting efficiency. The preparation of one-dimensional TiO2 nanomaterial-based films has

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many synthesis methods, such as hydrothermal method, template method, and anodic oxidation method. 2.3.1.3.1 Hydrothermal method The hydrothermal method is normally conducted in autoclaves under controlled temperature and/or pressure with the reaction in aqueous solutions. The temperature can be heated above the boiling point of water, reaching the pressure of vapor saturation to nucleate, grow, and form a certain size and crystal morphology of nanomaterials. In 1998, researchers have synthesized TiO2 nanotubes by employing the hydrothermal method. As shown in Figure 2.8, the TiO2 nanotubes have diameters of 8 nm, lengths of 100 nm, aspect ratio of more than 12.50, and specific surface area over 400 m2 · g−1 [49]. At the same time, by adjusting the temperature from 180 to 240 °C and reaction time to 24 h, TiO2 nanobelts with widths of 30–200 nm and lengths of up to a few microns were obtained [50].

Figure 2.8: TEM images of TiO2 nanotubes [49] (a) and nanobelts [50] (b).

In 2002, TiO2 nanotubes prepared by the hydrothermal method were introduced into DSCs. TiO2 nanotube-based mesoporous film had a high specific surface area than the TiO2 nanoparticle-based film and obtained a power conversion efficiency of 2.9% [51]. Using P25 nanoparticles as precursor, TiO2 nanotubes were prepared via the Kasuga hydrothermal method. By introducing the TiO2 nanotubes into DSCs, a 7.10% of power conversion efficiency was obtained. It was discovered that the TiO2 nanotubebased film has a long electron lifetime, resulting in the efficient electron collection [52]. TiO2 nanobelts were synthesized by the Kasuga hydrothermal method and introduced into DSCs. Due to the low specific surface area, TiO2 nanobelt-based mesoporous film could not adsorb enough dye molecular, leading to the low photocurrent and power

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conversion efficiency. The one-dimensional nanomaterials (such as nanotubes and nanobelts) prepared by the Kasuga hydrothermal method have uniform morphology and size. During the process, H+ and Na+ in the intermediate product composed of H2Ti3O7 and NaxH2–xTi3O7 are easy to be replaced by the other cations, which are easy to dope with other elements and improve the photovoltaic performance of DSCs [49, 53]. Although having longer electron lifetime, one-dimensional TiO2 nanomaterials in the films show almost horizontal distribution [52], which would extend the electron transport pathway, and thus prolong the electron transport time and reduce the electron collection efficiency [54]. TiO2 nanotubes were formed on a substrate that was seeded with TiO2 nanoparticles. During this process, a dilute TiO2 suspension was prepared by dispersing the P25 nanoparticles into deionized water with ultrasonic dispersion. The suspension was centrifuged to remove coarse particles and aggregates. TiO2 nanoparticles were deposited on titanium through dip coating from this TiO2 suspension. The titanium foil containing the predeposited TiO2 nanoparticles then reacted with an NaOH solution in a sealed Teflon reactor at 150 °C for 20 h. After the reaction, the TiO2 nanotubes were obtained, as shown in Figure 2.9(a) [55]. Because of low cost and simple synthesis, it is suitable for preparing TiO2 nanotubes and nanowires in large scale. In 2008, TiO2 nanoarrays were synthesized by hydrothermal oxidation of a titanium substrate in aqueous alkali solution and then post-annealed, as shown in Figure 2.9(b) [56]. The TiO2 nanoarraybased film with the thickness of 4.10 µm was used as a photoanode of DSCs, and a power conversion efficiency of 6.58% was obtained. TiO2 nanowires were grown on titanium foil through a three-step process. The titanium foil was transformed to Na2Ti2O4 (OH)2 nanotubes through hydrothermal oxidation in NaOH solution. Then the Na2Ti2O4 (OH)2 nanotubes are immersed in HCl solution to replace Na+ with H+ ions. Finally, the H2Ti2O4(OH)2 nanotubes were converted to nanowires through a calcination process at 500 °C. These mesoporous TiO2 nanowires were used as a photoanode in DSCs, and the overall power conversion efficiency was 1.8% [57]. Titanium tetrachloride or tetrabutyl titanate was used as a precursor, concentrated hydrochloric acid was used as hydrolysis inhibitors, toluene was used as a polar solvent, and the reaction solution is formed according to certain proportion. Clean FTO glass was put in reaction solution, and different lengths of rutile-type nanorod arrays were obtained through different times at 180 °C, as shown in Figure 2.10(a) [58]. When 2–3 µm long nanowire array was used as the photoanode combining with N719 dye, solar cells with 5.02% efficiency were achieved. While tetra-n-butyl titanate was used as a titanium source, concentrated hydrochloric acid as the hydrolysis inhibitors of butanol titanium, and deionized water as the solvent, put together of above resource materials according to suitable proportion. Under different heat treatment (80–220 °C) times, different thicknesses of a rutile nanorod array can be obtained in different times, as shown in Figure 2.10(b) [59]. By using 4 µm long nanowire array as the light anode and N719 dye, we can obtain 3% of efficiency.

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Figure 2.9: SEM images of TiO2 nanotube arrays [55] (a) and nanowire arrays [56] (b).

Preparing one-dimensional TiO2 nanomaterials by the hydrothermal method has simple process and high activity, but the degree of order for arrays is not high and close accumulation is insufficient, which has less dye loading amount and is easy to cause low photoelectric conversion efficiency of DSCs.

Figure 2.10: Cross-sectional SEM images of rutile TiO2 nanorod arrays (inset is top-view images) [58, 59]. (a) Annealing at 180 °C and (b) annealing at 150 °C.

2.3.1.3.2 Template method The template method is that the nucleation and growth of materials occur in the nanoscale aperture or the outside wall of template, and the size of the diameter or aperture and morphology determine the size and morphology of the product. This method can design a template according to the size of the composite materials and morphology in advance, and the size, morphology, structure, and layout of a material can be controlled based on the confined space function and regulation of templates. The template

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method can be divided into soft template and hard template. Both can provide the size of a limited reaction space. The difference is that the soft template provides a cavity in the dynamic balance. The substance can be spread in and out through the wall. While the hard template provides a static channel so that the material can go inside from the pore opening. The soft template is usually composed of surfactant molecules. Most of the soft templates include a variety of polymers formed by the orderly amphiphilic molecules, such as liquid crystals, vesicles, micelles, and microemulsions. This kind of template forms a certain spatial structure of cluster collective by using weak interaction among molecules, which makes the distribution of inorganic substances that present a specific trend so that the specific structure of nanomaterials is achieved. The hard template is a kind of rigid template by means of covalent bonds to maintain a certain shape such as a polymer with different spatial structures, anodic alumina film, porous silicon, metal template, natural polymer materials, molecular sieve, colloidal crystal, carbon nanotubes, and confined deposit on a quantum well. The hard template has high stability and good space confined effect, strictly controlling the size and morphology of nanomaterials, and the structure unitary, and high degree of order. The surfactant self-assembly method is to use multiple aggregation of surfactant molecules (micelles, capsules, and other forms) as a template to synthesize material need. Surfactant molecules are usually constituted by nonpolar, oleophylic (hydrophobic) part of hydrocarbon chain and polar, hydrophilic (lipophobic) groups. Surfactant molecules in aqueous solution system can self-assemble to form micelles. This method can be used to synthesize nanostructure materials with the advantage of orderly arranged pore, adjustable and uniform pore size, easily changed morphology, and controlled shapes such as pipe, rod, and sphere. In 2002, researchers used lauryl amine hydrochloride (LAHC) surface active agent dissolving in the deionized water to form micelles, and tetra-n-propyl ester titanate (TIPT) and acetyl acetone (ACA) mixture as the precursor to prepare the TiO2 nanotubes. TIPT and ACA are added to the aqueous solution containing LAHC at 40 °C under constant stirring for a few days until the solution becomes transparent, and then the sol solution is annealed to 80 °C for 3 days. Through centrifugate and washing, a diameter of 10 nm and length of 30–200 nm nanotubes were obtained. DSCs based on this nanotube as a thin-film electrode achieves 5% of the photoelectric conversion efficiency [60]. Then, the self-assembly method was used to get the single-crystal nanowires, whose mainly (101) crystal plane forming nanostructures on the outside of the material with the dye adsorption amount 4 times larger than that of P25 powder and the 9.33% of photoelectric conversion efficiency for solar cells [61]. After improvement and optimization, the nanotubes with the 9 nm diameter, up to hundreds of nanotubes achieved the efficiency of DSCs up to 8.43% [62]. Benzoic acid and ethylene glycol as surfactants, with tetraisopropyl titanate as Ti source, the nanorods with the diameter of 5 nm and length of 13–17 nm were prepared, and the porous film electrode made up of these nanorods obtained the DSCs with the efficiency of 7.50% [63].

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The hard template method is mainly using the orderly arranged pores and onedimensional nanoarray as a template, combined with the sol–gel deposition process, electrochemical deposition, and atomic deposition technology to prepare one-dimensional TiO2 nanomaterials. Using anodic-oxidized aluminum plate (AAM) as a template, TiCl4 solution dopes on the clean AAM template, making solution full of pores. After the hydrolysis for 2 h at 50 °C, the AAM template is removed by using 1 mol·L−1 NaOH solution. The TiO2 nanotubes and nanorods are prepared by varying the concentrations of TiCl4 solution, as shown in Figure 2.11 [64]. Electrons in thin-film electrode of nanorods have faster transmission rate, and effectively scatter light to improve the photocurrent of cells. Using anodic-oxidized aluminum plate as a template, tetra-isopropyl titanate ethanol solution (weight ratio 3:1) fills the pore by vacuum infiltration technology, by drying in the air for 12 h, and by sintering for 30 min at 500 °C to form highly ordered anatase TiO2 nanotube arrays before removing the AAM template with 3 mol · L− 1 NaOH solution. The TiO2 nanotube arrays adhesive on conductive glass substrate by tape, and sinter at high temperature to remove tape and TiO2 nanotube array is fixed on the substrate as film photoanode. The related DSCs get the photoelectric conversion efficiency of 3.50% [65] when natural fiber was used as the template and mixed solution of (NH4)2TiF6 and H3BO3 deposits a TiO2 layer on the fiber surface. In air atmosphere, the anatase-type hollow TiO2 nanofibers are prepared by sintering at 500 °C high temperature. The TiO2 nanofiber film electrode has longer electron lifetime and shorter electron collection time compared with nanoparticle film electrode, and the photoelectric conversion efficiencies of DSCs [66] were up to 7.20%.

Figure 2.11: SEM images of TiO2 nanotubes (a) and nanorods (b) [64].

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2.3.1.3.3 Anodic oxidation method Anodic oxidation method uses titanium or titanium alloy plate as anode in electrolyte and forms TiO2 thin film on the surface of the anode by electrolysis. According to the current types, anodic oxidation can be divided into dc anodic oxidation, ac anodic oxidation, and pulse current anodic oxidation. In recent years, researchers have systematically studied the preparation of thin-film electrode process by a layer of titanium coating on conductive glass by sputtering deposition as precursors; then, the TiO2 nanotube arrays were obtained in fluoride electrolyte by the anodic oxidation method, and then sintered at high temperature. Because the film sputtering on conductive glass has good adhesion, TiO2 nanotube arrays can firmly load on conductive glass, which solved the problem that a nanotube array was not easy to load on the conductive glass. In 2006, Paulose et al. sputtered 500 nm Ti thick film on conductive glass, and prepared nanotube arrays with 46 nm diameter, 17 nm thickness, and 360 nm length [67], and the photoelectric conversion efficiency was 2.90% [67]. In the same year, they changed the electrolyte with the electrolyte containing 0.10 mol · L−1 KF, 1.0 mol · L−1 NaHSO4, and 0.10 mol · L−1 sodium citrate, preparing 6 µm thick nanotubes array of thin-film electrode, and the efficiency of DSCs was up to 4.24% [68]. When comparing the efficiency of 6.20 μm long TiO2 nanotube array film electrode by irradiation from the front has higher open-circuit voltage and the higher photoelectric conversion efficiency in AM 1.5 light than that from the back [69]. By finite-difference time domain technique, the influence of the size of the nanotube arrays on the light absorption can be analyzed and the best morphology structure can be designed. It was showed that the larger in length and the smaller in the diameter of the nanotubes, the higher the surface roughness, absorption, and efficiency. They also pointed out that the charge on the barrier layer of light absorption has almost no influence [70]. In 2008, Paulose et al. used the electrolyte containing 0.14 mol · L−1 NH4F and 5% deionized water in formamide solution, and the optimized nanotube size and surface morphology (as shown in Figure 2.12), to improve the internal surface area and the dye molecules’ loaded amount and to reduce the loss of charge recombination. Then the thin film of TiO2 nanotube array with the length of 14.40 µm was used as an electrode, and 6.10% photoelectric conversion efficiency was obtained [71]. In 2009, they used HF and dimethyl sulfoxide mixed electrolyte, and obtained 0.30–33 µm long TiO2 nanotube array. Thus, the photoelectric conversion efficiency of DSCs reached 6.90% with 20 µm thick TiO2 nanotube array porous film as the photoanode [72]. In 2008, there was a breakthrough using the method of anodic oxidation on the flexible DSCs [73]. For TiO2 nanotube arrays with a length of 14 µm polyethylene naphthalate formic acid glycol ester (ITO/PEN) as a thin-film electrode, DSCs’ photoelectric conversion efficiency reached 3.60% with the ion electrolyte. When comparing nanotubes prepared by the hydrothermal method or template method, TiO2 nanotube arrays prepared by the anodic oxidation method has strong adhesion, high degree of order, long length, and good controllability.

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Figure 2.12: SEM image of TiO2 nanotubes: (a) top view and (b) cross section [71].

Because of the shortage of the existing nanotechnology which makes nanotube wall with lower roughness, larger diameter, and lower degree of array accumulation, the efficiency of DSCs based on one-dimensional nanomaterials such as TiO2 nanotube arrays as thin-film electrode has yet to catch up with traditional cells of nanoparticles as a porous film. Researchers also confirmed with the superiority of DSCs whose light anode is TiO2 nanotube array [74, 75]: under close thickness of the film and the same light source, compared with photoanode composed of nanoparticles, DSCs whose anode is TiO2 nanotube arrays can obtain larger current density, longer life, and higher photoelectric conversion efficiency. With similar thickness, the electronic transmission time on two types of light anode were similar, but on the nanotube film, the electronic recombination time is 10 times more than that on the nanoparticles’ thin films, which means that the nanotube film has better electronic collection efficiency. Nanotubes can have effectively scattering light at the same time, improving the utilization rate of light and increasing the current density of the cells. The result confirms that the electrons in one-dimensional TiO2 nanomaterials (nanotube, nanofiber, etc.) have longer life and greater diffusion coefficient electron collection efficiency than the traditional nanoparticle porous film, improving the cells’ current density. The onedimensional structure has excellent electron transport dynamics. Therefore, it is an effective and practical way to improve the photoelectric conversion efficiency by introducing the one-dimensional TiO2 nanomaterials efficiently into DSCs.

2.3.2 Three-dimensional TiO2 mesoporous films Although mesoporous TiO2 nanoparticle-based film can adsorb enough dye molecules, the efficient electron collecting efficiency is hard to be realized. Besides, the one-dimensional TiO2 mesoporous films are beneficial to transport the photogenerated electrons, and are difficult to adsorb enough dye molecular due to their low

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specific surface area. Therefore, researchers have explored the novel and highly promising TiO2 nanomaterials to satisfy the sufficient specific surface area, efficient light scattering effect, and frequent electron collection ability. Photonic crystal is a photonic band-gap material, which is a periodic dielectricstructure artificial crystal in optical scale. Its lattice size is comparable to the wavelength of light and is 1,000 times larger than that of the crystal lattice size of the crystal. Inverse opal structure refers that the low dielectric coefficient balls (always air balls) stacked with face-centered cubic are uniformly distributed in the medium with high dielectric coefficient. Such a structure forms the band structure and produces the photonic crystal with photonic band gap. Figure 2.13 shows the TiO2 inverse opal structure introduced into the DSCs as the photoanode to enhance the light scattering effect and the infrared absorption [76]. The photocurrent of the DSSC is enhanced by 26%, comparing with the traditional TiO2 nanoparticles. The main preparation process is as follows: polystyrene spheres (PS) and deionized water were mixed into the gel. After ultrasonic treatment, the PS spheres were uniformly distributed into the solution. A thin gel film was spin-coated onto the FTO glass by the spin coating method and dried to remove the water. This film adsorbed enough isopropyl titanate in the hole of the film and was immersed in the aqueous solution of ammonium hexafluorotitanate ((NH4)2TiF6) and boric acid (H3BO3) to produce TiO2 nanoparticles. A thin compact layer of TiO2 was calcined at 400 °C for 8 h to remove the PS spheres and created the inverse opal structure. Kwak et al. [77] have found that different sizes of PS spheres as the template on the morphology of TiO2-inverse opal structure and photovoltaic performance of DSCs. The 1,000-nmdiameter PS-templated TiO2 photoanode achieved the highest photocurrent. It was found that TiO2-reverse opal structure screen-printed on the top of nanocrystalline as a scattering layer achieved the optimum properties of DSCs with a 8.30% of power conversion efficiency obtained [78]. Figure 2.13 shows a TiO2/SiO2 periodic layered structure through a step-by-step deposition process [79–81]. The Bragg reflection peak can be tuned by changing the lattice parameter of the periodic structure, which is realized by varying the thickness of each type of layer in the structure. With the increase of the layer numbers, the reflection ratio will be increased. When the Bragg reflection peak was located in the 480–650 nm, such an inverse opal structure could improve the amount of the dye molecules to absorb incident light, thereby the photocurrent of the DSSC is enhanced by 25% compared with the TiO2 nanoparticle-based film. If the incident light vertically irradiates on the substrate, the Bragg reflection effect will be better. The hierarchical structure is aggregated by nanoparticles, one-dimensional nanomaterials (nanotubes, nanorods, nanowires, etc.), or two-dimensional nanomaterials (nanosheet, nanoflake, etc.) into large-sized three-dimensional structure (submicron or micron grade), which gave those materials to have multilevel, multidimensional, and multicomponent coupling effect. The hierarchical spherical TiO2 microspheres composed of nanoparticles have aroused great attention. Those

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Figure 2.13: SEM images of the template-inverse opal TiO2 films [76] (a) and the cross section of an eight-layer Bragg reflector made of silica and titania nanoparticles deposited alternately on the substrate (b) [79].

microspheres featuring high specific surface areas and variable particle sizes have recently achieved high performance in DSCs with improved dye loading, increased light scattering properties, and enhanced electron diffusion because of the well-interconnected networks with the microspheres. A range of synthesis routes have been applied to prepare porous TiO2 microspheres. Here the examples have been introduced to the DSCs. (1) Mesoporous spherical TiO2 structures have been fabricated using template methods. The cationic PS sphere and titanium tetra-isopropoxide (TTIP) were added into ethanol with little deionized water to hydrolyze the TTIP. The hierarchical TiO2 hollow microspheres with a diameter of 490 nm and a shell thickness of 30 nm were obtained after the calcination process at 480 °C to remove the PS spheres [82]. About 590-nm-sized TiO2 hollow microspheres were made into mesoporous photoanode, which exhibits the power conversion efficiency of 1.26%, for the TiO2 area density of the hollow microsphere is one-fifth lower than that of the P25 nanoparticles. If these coefficients are transformed into values per unit weight, the per unit of weight efficiency of the hollow microsphere is 2.5 times higher than those of the P25 nanoparticles, which provides possibilities to reduce the usage amount of TiO2. TTIP and a small amount of deionic water were added to ethanol. The mixture was stirred to form a clear solution for 30 min. Then the solution was transferred to the autoclave and thermal treatment is carried out for 6 h at 240 °C. The white hierarchical microsphere was obtained after natural cooling, centrifugation, and washing. The high efficiency of 10.34% has been obtained with DSCs based on the porous film electrode screen-printed with the paste of the hierarchical microsphere combining N719 sensitizer. (2) Sol–gel synthesis of spherical TiO2 colloids was conducted in a nonaqueous solvent containing small amounts of water or in the presence of diverse chelating agents to slow down the hydrolysis of alkoxides due to the high reactivity of titanium alkoxides. For example, researchers developed an acetonitrile and ethanol-modified sol–gel process with a small amount of water and methylamine

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to reduce the hydrolysis rate of the titanium alkoxides and thus fabricated the amorphous TiO2 microspheres. These microspheres were redispersed into an ethanol solvent and transformed into an autoclave for thermal treatment at 240 °C for 6 h. The obtained TiO2 microspheres with a diameter of 250 nm were composed of 13-nm-sized nanoparticles. A high power conversion efficiency of 10.52% was achieved on a TiO2 microsphere-based film after sensitization using N719, a Ru(II)-based dye. In addition, it was found that the electron diffusion coefficient of TiO2 microsphere-based film is smaller than that of the traditional TiO2 nanoparticle-based film. (3) The morphology and monodispersity in size and diameter of the mesoporous TiO2 beads can be fabricated by combining sol–gel chemistry with a solvothermal process in the presence of hexadecylamine (HDA) as a structure-directing agent [83–86]. KCl was used to control the monodispersity of the precursor beads by adjusting the ionic strength of the solution. The HDA was dissolved in the ethanol, followed by the addition of KCl solution. Then, titanium isopropoxide was added to this solution under vigorous stirring. The milky white precursor bead suspension was kept static for 18 h and then centrifuged to obtain the amorphous TiO2 beads. To prepare mesoporous TiO2 beads with a highly crystalline framework, a solvothermal treatment of the amorphous beads was performed. By adjusting the ammonia concentration in a solvothermal process or the temperature of solvothermal crystallization, crystallite size, specific surface area, and pore size distribution of the anatase TiO2 beads can be varied. Mesoporous TiO2 beadbased DSCs have demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 nanoparticle-based DSCs because of the well-interconnected, densely packed nanocrystalline TiO2 particles inside the beads, resulting in the enhanced electron collection efficiency. This has resulted in a power conversion efficiency of up to 10.6% using C106 dye without an additional scattering layer. (4) An electrostatic spray technique was used to synthesize hierarchically TiO2 spheres. The 10 wt% Degussa P25 nanoparticles were dispersed into ethanol by using an ultra-apex mill. The dispersed solution was loaded into a plastic springe. Then the dispersed P25 solution was electrosprayed directly onto the FTO substrates. To prepare the hierarchically structured TiO2 sphere with a diameter of about 640 nm, the electric field of 15 kV was applied between the metal orifice and the conducting substrate. The feed rate was controlled by a syringe pump at 35–30 μL min–1 [87]. Although three-dimensional nanomaterials have superior performance, some deficiencies are obvious, such as complicated synthesis process, low production, and high cost. Low specific surface area and the weak connection between the adjacent nanomaterials also existed in some three-dimensional nanomaterials, which will restrain the electron collection efficiency in DSCs.

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Figure 2.14: TEM image of TiO2 hollow microspheres [82] (a) and SEM image of TiO2 solid microspheres [83] (b).

2.4 Energy-level structure of TiO2 thin films 2.4.1 Flat band potential of the semiconductor electrode As one of the key components of DSC, the nano-TiO2 porous thin-film electrodes have a great influence on the photovoltaic performance. It not only affects the adsorption of dyes and the transmission of the incident light in the film, but also acts as the media of the electron transfer in the film. In the system of the semiconductor contact with the electrolyte solution, the Fermi level of the semiconductor is different from the redox couple of the electrolyte solution, then on the side of the semiconductor form the space charge layer, and the Helmholtz layer is formed on the side of the electrolyte so that the band of the semiconductor can be bent at the surface. If a potential is applied to polarize the semiconductor electrode and make the semiconductor energy band in the flat band condition, the potential is known as flat band potential (Vfb). The semiconductor/electrolyte interface capacitance is the basic method for determining the flat band potential and the position of the band edge of the solid film. Studying on the relationship between the capacitance of the semiconductor electrode and the electrode potential can determine the conducting type of solid, the space charge density, and the positions of the surface energy level Ecs and Evs. The commonly used method is the equivalent circuit analysis. Figure 2.15 shows a basic simple equivalent circuit. The potential change of the semiconductor electrode can be expressed as follows: ΔΦ = ΔΦsc + ΔΦH

(2:6)

where ΔΦsc and ΔΦH represent the space charge layer potential and Helmholtz layer potential, respectively.

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Csc

CH

Css

Rss

Figure 2.15: Equivalent circuit of a semiconductor electrode.

The electrode capacitance is defined as the ratio of the stored charge that changes with the applied potential. The neutral condition in the interface area is qsc + qss = qsoln

(2:7)

where qsc, qss, and qsoln are the space charge density, surface charge density, and solution charge density, respectively. So, the reciprocal of the capacitor (1/C) is 1 ∂ΔΦ ∂ΔΦsc ∂ΔΦH = + = C ∂qsoln ∂ðqsc + qss Þ ∂qsoln

(2:8)

Helmholtz layer capacitance CH, space charge layer capacitance Csc, and surface state capacitance Css are defined as follows: CH = ∂qsoln =∂ΔΦH Csc = ∂qsc =∂ΔΦsc Css = ∂qss =∂ΔΦsc then formula (2.8) can be written as 1 1 1 + = C Csc + Css CH

(2:9)

Equation (2.9) is the corresponding relationship between the three capacitors in the equivalent circuit. When the applied voltage decreases the band bending, the EF moves up. Then there will be more surface state energy level located under EF, and the charge is stored. On the contrary, if the applied voltage increases the band bending, then the EF moves down, and part of the electrons in the surface state energy level will return to the conduction band, and the charge is released. We can learn that Css has the function of “capacitor.” Charge transfer to surface states needs to overcome the activation barrier ΔΦsc, if the surface barrier is high, and the electron transfer rate

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will be very slow. Therefore, measuring the capacitance with high-frequency AC can reduce the contribution of Csc to the determination of C. The measurement of the semiconductor electrode capacitance is usually carried out in the depletion layer; thus, the relation between Csc and ΔΦsc is derived: 

1 Csc

2

    kT 2 kT = = ΔΦsc − φ − φfb − e0 εsc ND e0 e0 εsc ND e0 2

(2:10)

where ΔΦsc = φ–φfb, εsc is the capacitance rate of the semiconductor, that is, the product of the dielectric constant and dielectric constant in vacuum; ND is the doping concentration; q0 is the charge electric quantity; k is the Boltzmann constant; T is the thermodynamic temperature; φ is the electrode potential; and φfb is the flat band potential. Equation (2.10) is known as the Mott–Schottky formula. φfb is the flat band potential, that is, the position of the electrode potential where the band is not bent (position of solid Fermi level). Mapping Csc−2 to φ assume a straight line, from the slope we can obtain the doping concentration ND (for n-type semiconductor, the concentration of ionized donor) and the intercept on the potential axis is φfb. φ is a relative value, related to the reference electrode, for the sake of comparison, φfb is usually converted to the potential value using standard hydrogen electrode as the reference electrode. The known flat band potential φfb can determine the position of solid surface energy band edge and draw out the energy distribution diagram at the specified potential. Under the condition of flat band potential, ΔΦsc = 0, for n-type semiconductor: Ecs = φfb + μ

(2:11a)

Evs = e0 φfb − μ

(2:12a)

for p-type semiconductor:

where μ is the difference between the band edge and the energy of the semiconductor, which can be measured by the solid physical method. The doping concentration affects the value of μ; the higher the concentration is, the smaller the μ value is. The typical μ value of a semiconductor material is 0.1–0.2 eV. If the energy level is converted to electrode potential, for n-type semiconductor: Ecs ðNHEÞ = φfb − μ

(2:11b)

Evs ðNHEÞ = e0 φfb + μ

(2:12b)

for p-type semiconductor:

As long as the solution medium is the same, the Ecs and Evs of the same material will not be changed obviously due to the n- or p-type semiconductor. The pH value of the solution has a significant effect on the position of Ecs and Evs. When the pH value increases one unit, φfb negatively shifts about 60 mV, and

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this change is essentially due to an effect from pH value on ΔΦH. In the measurement of the semiconductor electrode capacitance, we cannot always get the ideal Mott–Schottky diagram. Mainly in three situations, the Csc−2–φ diagram is not a straight line, and the capacitance value and flat band potential measurement are changed with the frequency of the AC.

2.4.2 Measurement method of semiconductor electrode flat band potential The flat band potential is an important physical quantity in the semiconductor/ electrolyte system. By studying the relationship between the flat band potential of the semiconductor electrode and the corresponding DSC photovoltaic properties can help detect, characterize, and evaluate the properties of the nanoporous thin-film electrode. At present, there are three main methods to measure the flat band potential: Mott–Schottky mapping, spectral electrochemistry method, and electrochemical method. The following are the detailed introduction of the three methods and the corresponding advantages and disadvantages.

2.4.2.1 Mott–Schottky mapping Insert the semiconductor electrode into the electrolyte, and due to the adsorption of an electrolyte or solvent on the surface of the electrode, the charge transfers; so, the potential gradient is generated in the surface layer of the semiconductor, and the space charge layer is formed. Space charge layer is a kind of double layer. In the flat state, the potential gradient is small, and the double-layer capacitance is changed. So, we can obtain the flat band potential from the change of the double-layer capacitance. In the contact system of semiconductor and electrolyte, capacitance (C) is made by the space charge capacitance (Csc) and the Helmholtz-layer capacitance (CH) of the solution in series. Usually, CH and Csc in the electrolyte can be neglected, so C = Csc. Changing the electrode potential (V) of a semiconductor can change the capacitor of the space charge layer for the semiconductor layer. According to the Mott–Schottky equation, the relationship between the two is 

1 Csc

2 =

    kT 2 kT = ΔΦsc − φ − φfb − e0 εsc ND e0 e0 εsc ND e0 2

(2:13)

where ΔΦsc = V–Vfb is the potential drop of space charge layer. If (1/Csc)2 is the ordinate for the V plot, it will get a straight line, as shown in Figure 2.16. The intercept of straight line on the horizontal axis is Vfb + kT/q, and the slope of the line is

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2/(qεε0ND); thus, the flat band potential Vfb and the doping concentration N of the semiconductor can be calculated by the Mott–Schottky diagram. 2×1016

1 kHz 10 kHz 50 kHz

C-2 (cm4 F-2)

1.5×1016

1×1016

5×1015

0×100 -3

-2.5

-2

-1.5

-1

-0.5

0

1

1.5

Potential (VSCE) 2.5×1016

1 kHz 10 kHz 50 kHz

C-2 (cm4 F-2)

2×1016

1.5×1016 1×1016

5×1015 0×100

-1.5

-1

-0.5

0

0.5

Potential (VSCE) Figure 2.16: Mott–Schottky diagram of semiconductor: P-type semiconductor and (b) n-type semiconductor.

After measuring the electric potential, the Fermi level of the semiconductor can be obtained in the flat state (Vfb = EF). Then, using the relationship type between Fermi level and the position of conduction band (Ec) (n type as the conduction band and p type as the valence band), calculate the conduction band position of the n-type semiconductor: EF = Ec − kT ln ðNc =N Þ

(2:14)

where Nc is the effective density of the conduction band. The band gap (Eg) of the semiconductor can be obtained by its absorption threshold (λg), Eg = 1,240/λg, The equation Eg = Ec–Ev can determine the position of the valence band semiconductor (Ev).

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The case is different for the nano-sized undoped semiconductor. Since the self-built electric field of nanoparticles is very small, the surface energy band bending can be neglected, and the semiconductor nanocrystals can be considered in a flat state. Thus, the flat band potential of the semiconductor cannot be measured by measuring the change of the space charge layer capacitance. The main measurement of the flat band potential of semiconductor nanocrystals is by the spectral and electrochemical methods.

2.4.2.2 Spectral electrochemistry method Inserting a certain thickness and area of the nanocrystalline semiconductor films (working electrode), platinum wire (electrode) and saturated calomel electrode or Ag/AgCl electrode (reference electrode) into the appropriate electrolyte solution constitutes a three-electrode system. Under the three-electrode system, a different bias voltage is applied to the nanocrystalline semiconductor electrode to measure the change in absorbance at a fixed wavelength (e.g., TiO2 at 780 nm), as shown in Figure 2.17.

Figure 2.17: Absorbance–potential plot of TiO2 thin-film electrode at 780 nm.

When the electrode potential is positive, the absorbance is not changed; contrarily, the absorbance will increase sharply. When the absorbance begins to increase, the corresponding electric potential is the flat band potential of the semiconductor electrode.

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For monochromatic light with a wavelength of 780 nm, it is assumed that the optical absorption is due to the band transitions or free carrier absorption, so the intensity of the absorption is related to the electron concentration in the conduction band: ∞ ð

gcb ðEÞ

ncb = Ecb

1 dE eðE − EF Þ=kT + 1

(2:15)

Ecb is the energy of the conduction band edge, EF is the Fermi level of the semiconductor, and gcb (E) is the state density in the conduction band. Ignoring the hole concentration and assuming ionized donor concentration is a constant, the semiconductor potential meets the following equation: d 2 ϕ ð x Þ e0 = ½ncb ðϕðxÞÞ − n0  εε0 dx2

(2:16)

where n0 stands for conduction band electron concentration of the semiconductor. Because of the presence of electrolyte at the interface of the semiconductor, also taking the capacitances of the space charge layer and the Helmholtz layer into account, the electric potentials of the space charge layer and the interface can be obtained: Φ el − Φ b =

Csc ðΦ s − Φ b Þ + CH ðΦ s − Φ b Þ CH

(2:17)

In order to calculate the optical absorption of the semiconductor film, a surface super function is defined as follows:




∞ ð

G ϕs − ϕb =

½ncb ðϕðxÞÞ − n0 dx

(2:18)

0

The change in the electric potential of the space charge layer will result in the change of optical absorption, and the relationship between the amount of light absorption and the super function is ΔðabsorbanceÞ =

1 A σcb ΔG 2.303 a

(2:19)

where σcb is the optical cross sections of electrons in the conduction band at a given wavelength, A is the microarea of the semiconductor film at solid/liquid interface, and a is the macroscopic area of the semiconductor film. Briefly, this is because when add negative bias voltage to the TiO2 film electrode to reach its flat band potential, the Ti4+ will be combined with the conduction band − ! Ti3 + . And this process will absorb the electrons to produce Ti3+, that is Ti4 + + ecb monochromatic light of a specific wavelength so that its absorbance increased significantly.

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2.4.2.3 Electrochemical method Under the three-electrode system, the incident light is used to excite the semiconductor electrode to change the electric potential applied on the electrode. When the applied potential is more negative than that of the flat band potential, the electron cannot be transferred to the external circuit. In contrast, when the applied potential is more positive, the photocurrent can be produced. When the light current starts, the electric potential applied on the electrode is the flat band potential, as shown in Figure 2.18. 0.08

628 nm illuminated n-CdSe in Alkaline Ferro/Ferricyanide [KCN]

I2, mA2

0.06

0.04

0.001 M 0.002 M 0.005 M 0.01 M 0.02 M 0.05 M 0.1 M 0.2 M 0.5 M 1.0 M

0.02

Vfb 0.00 -1.2

-1.1

-1.0

-0.9

-0.8

E, volts vs Pt Figure 2.18: The relationship curve between the light current and the bias voltage of the semiconductor electrode in the light.

2.4.2.4 The advantages and disadvantages of several methods for measuring the flat band potential The spectral electrochemistry method is suitable for the polycrystal electrodes with more defects, and the result is more accurate. However, this method can only measure

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the semiconductor electrodes with good optical properties, and the extinction coefficient of local electron or free electron must be determined. The maximum advantage of the electrochemical method is that it is easy to operate. However, it is difficult to determine the initial voltage of dark current and the uncertainty of the results. The flat band potential measured by these two methods is related to the nature of the medium and the electrolyte. The maximum advantage of the spectral electrochemistry method is that it can simulate the medium conditions in the photoelectrochemistry, and thus it can reflect the level of the semiconductor electrode in the photoelectric conversion actually.

2.4.3 Effect of test condition on the flat band potential 2.4.3.1 Change result of TiO2 film electrode with different area

Absorbance at 780 nm/a.u.

We compared the influence of the size of the effective area of the porous thin-film electrode on the flat band potential test results using the spectrum electrochemistry method. The effective area of the design electrode was 5 mm × 5 mm and 25 mm × 8 mm, and the flat band potential of the nano-TiO2 thin-film electrode was measured with the above two different effective areas. The relationship between the absorbance of two different active area TiO2 thin-film electrodes and the applied bias voltage is shown in Figure 2.19.

bias voltage/V (vs Ag/AgCl)

Figure 2.19: The absorbance plot at 780 nm of TiO2 thin-film electrode with different active areas versus the bias voltage.

Figure 2.19 shows the test result that the two curves are basically identical, indicating that the flat band potential of the two different effective areas of the TiO2 thinfilm electrode is the same, which shows that the area of the TiO2 electrode has no effect on the measurement of the flat band potential of TiO2 electrodes.

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2.4.3.2 The relative geometric position of different working electrodes and counter electrodes In the measurement of the flat band potential of nano-TiO2 thin-film electrode by using the spectral electrochemistry method, the relative position between the working electrode and the electrode is changed and compared. One condition is that the working electrode and the counter electrode are relatively fixed, and the other is that the position of the working electrode is 1 cm higher than that of the counter electrode, as shown in Figure 2.20. Pt

TiO2

Matching

TiO2

Pt

Mismatching

Figure 2.20: Diagram of the relative position between different working electrodes and counter electrodes.

The relationship curve of the measured absorbance of the TiO2 film electrode and the applied scan voltage is shown in Figure 2.21. In these two different cases, the flat band potential of the TiO2 thin-film electrode is the same. This shows that the change of the relative space geometric position between working electrode and counter electrode has no effect on the result during the measurement of flat band potential of TiO2 thin-film electrode by using the spectral electrochemistry method.

2.4.3.3 Distance between different working electrodes and counter electrodes In the measurement of the flat band potential of nano-TiO2 thin-film electrode by using the spectrum electrochemistry method, the distance between the working

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Figure 2.21: The absorbance of the TiO2 thin-film electrode versus the bias voltage plot at 780 nm with different relative positions of the electrodes.

electrode and the counter electrode is changed and compared, and the distances between the electrodes are 4, 5, and 6 mm. In the three cases, the relationship between the absorbance of the TiO2 thin-film electrode and the applied scan voltage is shown in Figure 2.22.

Figure 2.22: The relationship curve of the absorbance of the TiO2 thin-film electrode and the bias voltage at 780 nm.

From Figure 2.22, we can see that the measured flat band potentials are basically consistent with the three different electrode distances. The result shows that the change of the distance between the working electrode and the counter electrode has no effect on the flat band potential of the nano-TiO2 thin-film electrode measured by the spectral electrochemistry method.

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2.4.3.4 Different bias voltage scan rate The relationship curves between the absorbance of the TiO2 thin-film electrode and the bias voltage at different bias voltage scan rates 0.00125, 0.0025, 0.005, 0.01, and 0.02 V s−1 are shown in Figure 2.23.

Figure 2.23: The absorbance of TiO2 thin-film electrode versus bias voltage plot at different bias voltage scanning rates.

As shown in Figure 2.23, for different bias voltage scan rates, the rise speed of the absorbance is not the same. When the scan rate is slow, the time to stay on a bias voltage is long. So the TiO2 film electrode can absorb the incident light more fully, and the rising speed of the absorbance is fast. In this case, although the absorbance curves are not the same, but it can be seen from the diagram that the starting voltages when the absorbance begin to rise are basically the same, that is, the flat band potentials are still the same. This shows that changing the bias voltage scan rate has no effect on the flat band potential of the TiO2 thin-film electrode measured by the spectral electrochemistry method.

2.4.3.5 Different incident light wavelength The relationship curves between the absorbance of the TiO2 thin-film electrode and the bias voltage at different incident light wavelengths of 356, 555, and 780 nm are shown in Figure 2.24. From Figure 2.24, we can see that, for three different wavelengths of incident light, when the wavelength is 356 nm, the obtained curve is very unstable with the severe fluctuations of data point, and the absorbance is reduced with the increase of negative bias voltage. When the wavelength is 555 nm, the obtained curve is relatively stable. However, when the negative bias exceeds the flat band potential by a short

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Figure 2.24: The absorbance of the TiO2 thin-film electrode versus the bias voltage plot under different incident light wavelengths.

distance, the increase of the absorbance is not so obvious. While when the wavelength is 780 nm, the obtained curve is very stable, and it is suitable for the measurement of the flat band potential. Therefore, 780 nm as the incident wavelength in the measurement of flat band potential was chosen.

2.4.3.6 Effect of temperature on the flat band potential The relationship curves between the absorbance of nano-TiO2 thin films and the bias voltage at 21, 25, and 29 °C are shown in Figure 2.25.

Figure 2.25: The absorbance of the TiO2 thin-film electrode versus the bias voltage plot at different temperatures.

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From the picture we can see that the flat band potential of the nano-TiO2 film electrode is moving in the positive direction with the increase of temperature. Due − = Ti3 + , and the Nernst equation to the system, the following reaction exists: Ti4 + + ecb can be expressed as follows: E = E0 +

RT aTi4 + ln aTi3 + F

(2:20)

The concentration of Ti4+ in the working electrode is much larger than that of Ti3+. It can be seen from the Nernst equation that with the increase of temperature, the Nernst potential is moving to the right direction.

2.4.3.7 Effect of the supporting electrolyte solution on the flat band potential

Figure 2.26: The absorbance of the TiO2 thin-film electrode versus the bias voltage plot at different pH values.

From Figure 2.26, we can see that the measured flat band potential differs 0.54 V at different pH values, which indicates that the flat band potential of TiO2 thin-film electrode is related to the pH value of the electrolyte solution. Because of the adsorption–desorption equilibrium of proton on the surface of the TiO2 electrode, that is, TiO22 − + H + = TiOH +, the potential difference between the electrode surface and the electrolyte solution can be approximately represented by the Helmholtz-layer potential drop ΔΦH (ignore the dispersion-layer effect): ΔΦH = constant − 0.06 × pH. And by the relationship between the flat band Vfb and the Helmholtz-layer potential drop ΔΦH: Vfb = − ðEF vac =q + 4.5 − ΔΦH Þ EF

vac

is the Fermi level of TiO2 thin-film electrode in vacuum.

(2:21)

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2.4.4 Energy-level structure of different microstructure thin-film electrodes Nano-TiO2 porous thin-film electrode is one of the important factors that affect the performance of solar cell, and has a great influence on the performance of DSC. It not only affects the adsorption of dyes and the transfer of the incident light in the porous film, but also acts as the media of the electron transfer in the porous film. How to retard the electron recombination and improve the film on the electron transporting ability is one of the key issues of nanocrystalline TiO2 porous film electrode. In recent years, many domestic and international studies have been carried out in this field: on the one hand to actively explore other wide band-gap semiconductor materials (such as ZnO and SnO2) in the application of DSC; on the other hand, introducing a new nanostructure of nanowire and nanoball into DSC; and modifying the nano-TiO2 porous thin-film electrode with physical and chemical methods to speed up the transmission of electrons in the film reduce the recombination between the electron and the I3− ion in the electrolyte. The structure of the corresponding energy level of different modified microstructure thin films can also be changed, and then influence electron injection, transmission, and recombination processes.

2.4.4.1 Effect of the thickness of nano-TiO2 thin film on the electrode level The thin-film electrodes with different thicknesses were prepared by changing the printing numbers of times of the screen-printing process. Figure 2.27 shows the relationship curves between the flat band potential evolution with the increase in the thickness of the thin-film electrode.

Figure 2.27: The absorbance of TiO2 thin-film electrode versus the bias voltage plot at 780 nm with different thicknesses.

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As shown in Figure 2.27, with the increase of film thickness, the flat band potential of TiO2 electrode is moving in the positive direction. I–V curve of the cells is shown in Figure 2.28 and the Jsc of the three groups is obviously different. The shortcircuit current density of the cell is proportional to the absorption coefficient; the greater the absorption coefficient is, the more the electron is. The open-circuit voltage decreases with the increase of film thickness, because of the film thickness increases, the contact area between the TiO2 film and the electrolyte solution increases, and the direct recombination rate of electron increases. Usually, the FF of DSC is between 0.6 and 0.8, and the lower FF reflects high interface resistance and the serious electron recombination in the cell, which lead to low open-circuit voltage.

Figure 2.28: I–V curves of the cells.

By the relation formula of DSC open voltage Voc = |Vfb − Vred|, we know that in the same electrolyte, the decrease in the open-circuit voltage of DSC indicates that the flat band potential of the TiO2 electrode is moving in a positive direction, which shows that the change trend of the flat band potential of TiO2 electrode in DSC can be reflected by the change trend of the flat band potential of independent TiO2 electrode. Because of the positive direction of flat band potential, the conduction band edge of the corresponding TiO2 electrode also shifts to the positive direction, and the energy difference between the conduction band level of TiO2 and the ground level of the dye is reduced, making more dye molecules in a low excited state injected electrons to the TiO2 conduction band. Moreover, due to the increase in the thickness of the film, the inner surface area of TiO2 thin film will be increased, and the number of surface defects on the TiO2 electrode is increased. The recombination of the electrons in the conduction band of TiO2 and the I3− ions in electrolyte becomes easy, and the open-circuit voltage of DSC is also decreased.

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2.4.4.2 Effect of TiCl4 surface modification on the level of TiO2 thin-film electrode The TiO2 thin-film electrodes were treated by TiCl4 in four different ways. The relationship curves between the absorbance of TiO2 thin-film electrode and the applied voltage are shown in Figure 2.29.

Figure 2.29: The absorbance of TiO2 thin-film electrode versus the bias voltage at 780 nm in different TiCl4 treatments. ① Treating the conductive glass with TiCl4, then screen printing a layer of TiO2 film; ② screen printing a layer of TiO2 film, then treating the TiO2 film with TiCl4; ③ treating the conductive glass with TiCl4, then screen printing a layer of TiO2 film, and finally, treating the TiO2 film with TiCl4; ④ only screen printing a layer of TiO2 film.

From Figure 2.29, we can see that the flat band potential of the four electrodes is basically coincident. This shows that the effect of TiCl4 treatment on the flat band potential of TiO2 thin-film electrode is not obvious. Four kinds of TiO2 films with different thicknesses were treated by TiCl4 solution, and the characteristics of electronic transmission and back reaction were measured by means of Intensity modulated photocurrent spectrum (IMPS)/Intensity modulated photovoltage spectrum (IMVS). Table 2.3 shows the values of no. A–H DSC parameters that are obtained by fitting the experimental data of the IMPS/IMVS theory. The device performance is shown in Figure 2.30. It can be seen that after the treatment, the Voc and FF changed a little, Jsc increased by about 10%, and the device efficiency increased by about 15%. The influence of TiCl4 on the microscopic parameters is shown in Table 2.3. After TiCl4 treatment, the τn value of the cell is significantly higher than the cell without TiCl4 treatment, which shows that TiO2 thin films, in a certain extent, can effectively inhibit the recombination of electrons after treatment. Furthermore, the τd value was significantly lower than that of untreated cell value, which illustrates that the nanoparticles produced by the hydrolysis of TiCl4 solution were partially filled in the pores, the electrical contact between the particles was improved, the electron transfer became more easily, and the transmission capability of the electron was greatly improved.

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Table 2.3: Parameters obtained by IMPS/IMVS fitting. Number of cells d (μm) τn (s) A B C D E F G H

. . . . . . . .

. . . . . . . .

α (cm−) Dn × − (cm s−)        

. . . . . . . .

τd × − (s)

Ln (μm)

Qoc (μC · cm−)

. . . . . . . .

. . . . . . . .

. . . . . . . .

IPCE .% .% .% .% .% .% .% .%

No. A, C, E, and G are standard cells, and no. B, D, F, and H are treated by TiCl4.

Figure 2.30: I–V curve of DSC. No. A, C, E, and G are standard cells, and no. B, D, F, and H are treated by TiCl4.

2.4.4.3 Effect of acid–base properties on the energy level of TiO2 thin-film electrode Nano-TiO2 thin-film electrodes were prepared by two kinds of acidic paste and two kinds of alkaline paste. The relationship curves between the absorbance of different TiO2 thin-film electrodes and the applied voltage are shown in Figure 2.31. It can be seen from the figure that the influence of the pH value of the paste on the flat band potential of the nano-TiO2 film electrode is small. The acidic paste was A2 and A1, respectively, and the alkaline paste was B2 and B1, respectively. The TiO2 thin-film electrode was prepared by the sequence of cell number.

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Figure 2.31: The absorbance of the TiO2 thin-film electrode versus the bias voltage plot at 780 nm.

2.4.4.4 Effect of TiO2 particle size on the energy level structure of TiO2 thin-film electrode TiO2 thin-film electrodes were prepared by five kinds of different particle sizes of TiO2 paste, which were labeled as A, B, C, D, and E, respectively. The flat band potential of the corresponding TiO2 thin-film electrode is shown in Table 2.4. Table 2.4: The flat band potential of TiO2 thin-film electrodes with different particle sizes. Number A B C D E

Particle size (nm)

Flat band Vfb (V)

    

−. −. −. −. −.

From Table 2.4, we can see that with the increase of the particle size of TiO2 film electrodes, the flat band potential is shifted to the negative direction. The cells made up nano-TiO2 particles with three different sizes were labeled by A, B, and C, with the particle size (Dhlk) of 14, 19, and 25 nm, respectively. Figure 2.32 is the DSC I–V curve, and with the increase in the particle size, the change of the short-circuit current density, the open-circuit voltage, and the efficiency is not obvious.

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Figure 2.32: DSC I–V curve.

Table 2.5: Parameters obtained by IMPS/IMVS fitting. No. d (μm) A B C

τn (s) α (cm−) Dn × − τd × − (s) Ln (μm) Qoc (μC cm–) IPCE Dhlk (nm) (cm · s−)

. . . . . .

  

. . .

. . .

. . .

. . . . . .

. . .

The influence of particle size on the microscopic parameters is shown in Table 2.5. With the increase of Dhlk value, Dn value increases and τn value decreases gradually. Dhlk value increased from 13.4 to 17.5 nm, the film surface area decreases, the boundary conditions between the particles and the particles are improved, and the electronic transmission is more easily, so the Dn value is gradually increased. The τn value decreased with the increase of Dhlk value because the recombination rate of electron and hole is related to the transition frequency of the defect. After the frequency of electrons transition between the defects is accelerated, the recombination rate of electron and the I3− ion in electrolyte becomes slow.

2.4.4.5 Effect of doping on the energy level of TiO2 thin film When the intrinsic semiconductor is mixed with moderate impurities, the charge carrier can be provided. And the different doping has different electric conduction mechanisms. n-Type semiconductor has excess electron, and p-type semiconductor has excess holes. In the TiO2 crystal, when many electrons with the same level in different isolated atoms form a crystal, due to quantum effects, there donnot have two electrons

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in the same energy level, their energy is staggered with each other, and each in a group of sublevels with slightly energy difference to form energy band. According to the semiconductor theory, in intrinsic condition, the number of electrons in the conduction band can be expressed as follows: ∞ ð

ncb =

NðEÞ × f ðE, TÞdE

(2:22)

Ec

where N(E) is the total density of states, f(E) is the electron occupation probability, Ec is the conduction band bottom level: 

2me * NðEÞ = 4π h2

32

3

×E2

(2:23)

where me* is the effective electron mass, different from the free electron mass, h is the Planck constant. In semiconductors, the occupation probability of electrons at different levels is strongly dependent on temperature (T) and energy (E), which can be given by the Fermi–Dirac distribution function: 1

f ðE, TÞ =

(2:24)

E − EF

1 + e kB T

EF is the semiconductor Fermi energy, which is the energy-level position when the electron occupation probability is 0.5. In intrinsic semiconductor, the position of the Fermi level is at the center of the band gap with several eV. For nondegenerate semiconductors, eq. (2.24) can be simplified as f ðE, TÞ = e

−

E − EF kB T

(2:25)

Substituting eqs. (2.23) and (2.25) into eq. (2.22): ∞    3 ð 3 2me * 2 3 E − EF 2 2 dE ncb = 4π × ðkB TÞ E × exp − h2 kB T

(2:26)

Ec

Make x = E/kBT, eq. (2.26) can be written as    3 3 2me * 2 Ec − EF 2 ncb = 4π × ðkB TÞ exp − h2 kB T

∞ ð

1

x2 e − x dx

Ec =kB T

The integral in formula (2.27) is standard, and the integral value is 

2πme * kB T ncb = 2 h2

32

(2:27)

pffiffiffi π=2, then

    Ec − EF Ec − EF = Nc exp − × exp − kB T kB T

(2:28)

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 3 2πme * kB T 2 Nc = 2 h2

(2:29)

where Nc is the defective density of states of conduction band. For DSC, a study shows that the effective mass of electrons in the TiO2 conduction band is 5.6 times of free electron, that is, me* = 5.6 me. The effective density of states is 3.317 × 1020 cm−3 through calculating. The electron density ncb,0 of the DSC conduction band in the dark state is obtained by eq. (2.28):   Ec − EF, redox (2:30) ncb, 0 = Nc exp − kB T It can be seen in eq. (2.30) that Ec–EF,redox value changes with the change in the band structure of TiO2 film electrode in the case of the DSC electrolyte system. Therefore, doping leads to the TiO2 conduction band edge shifts, which makes the DSC conduction band electron concentration ncb,0 change, and then affects the open-circuit voltage and short-circuit current of DSC. DSC based on the traditional single nano-TiO2 semiconductor thin-film electrode obtained is not a very ideal photovoltaic performance, and it needs other modification methods to improve the photovoltaic performance. Introducing impurities can change the distribution of the energy band structure and the surface states of the TiO2 light anode, which lead to the redshift of the absorption peak and the improvement of charge separation and transfer. Moreover, reduce the TiO2/dye/electrolyte interface electron recombination to improve the photoelectric conversion efficiency of DSC. Nano-TiO2 light anode doping mainly includes nonmetal elements, metal, metal ions, rare earth ions doping, and various metal oxide doping. The recombination of electrons and holes is affected by different atoms or ions doped in the lattice of the semiconductor. Doping transition metal ions [88] into the nano-TiO2 light anode was used, such as Zn2+ and Fe3+. However, the efficiency of DSC is not high. The reason is that the doped transition metal ions are easy to form the capture center of the electron, which is not conducive to the stability of DSC cells. Al, W [89] doped and (Al + W)-codoped TiO2, and the metal-ion-doped DSC with different characteristics were obtained. Among them, Al doping improved the Voc, but decreased the Isc because Al doping greatly reduced the recombination between the electrons and the I3− ions. But it made less electron transfer to the electrode, and the short-circuit current was reduced. W doping improved the short-circuit current of DSC but also reduced the open-circuit voltage. The reason was due to the direct recombination of electrons and I3−. Al + W codoping affected the surface electronic states, surface polarization, and defect charge balance, and improved the adsorption of unit area of TiO2 film, and ultimately improved the efficiency of DSC. W-doped TiO2 [90] was synthesized by the sol–gel method, and the doping amount of W was increased from 0.1% to 5%, and the TiO2 conduction band edge

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was shifted in the right direction, and the short-circuit current of DSC was improved. The efficiency of DSC reached 9.4%, an increase of 20%. Al3+-doped TiO2 [91] was realized by the electrochemical method. When Al3+ was doped into TiO2, the electron lifetime and electron transmission time were increased, and the electron recombination decreased. But the quantum efficiency of the electron injection is significantly decreased when the amount of Al3+ is high. The reason is in multiple trap models: Al3+ doping brought more traps, slowing down the dynamics of electrons. Cr-doped TiO2 [92] was synthesized by the hydrothermal method, and deposited it on undoped TiO2 thin films to form a double-layer structure, and the efficiency of prepared DSC was increased from 7.1% to 8.4%, increased by 18.3%. This is mainly due to the double-layer structure which formed a P–N homojunction energy barrier, and thus reduces the loss of electron recombination, improves the short-circuit current of the DSC, and ultimately improves the efficiency of DSC. Nb5+-doped TiO2 was prepared by the hydrothermal method. The collection efficiency of electron is greatly improved with low Nb5+ doping amount [93]. While the distribution of the surface defect states of TiO2 can be effectively controlled by varying different Nb doping amounts, which can improve the efficiency to 8.7%. For example, ytterbium is a valence alternating element with special f electron structure with stable full-filled 4f14 subshell. Ytterbium oxide was widely used in chemical synthesis, electrocatalysis, and bulb with special application, with many special chemocatalysis and electrocatalysis. Lanthanide-doped TiO2 can significantly inhibit the recombination of electron–hole pairs and prolong the carrier lifetime. When used in DSC, it will also inhibit the recombination of semiconductor/electrolyte interface, reduce the loss of electron, and improve the open-circuit voltage to a certain extent. TiO2 paste with different Yb doping contents were prepared by the sol–gel method, and the performances of the cells are shown in Table 2.6. The open-circuit voltage Voc increases by 8.7% from 0.69 V of pure TiO2 film to 0.75 V of doped TiO2 with the doping concentration of 6%. FF also increases. FF of DSCs is determined by many factors, and the charge recombination is one of them. Keeping other conditions the same, change the content of Yb only, and an FF increase means the charge recombination is inhibited effectively. The short-circuit current density Jsc and the photoelectric conversion efficiency of the cell gradually decreases with the increase in the doping concentration. Table 2.6: DSC current–voltage characteristics with different doping concentrations of Yb. Treatment concentration (wt%) Voc (V) Jsc (mA cm–) FF (%) η (%) Undoped .% .% % %

. . . . .

. . . . .

    

. . . . .

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When Yb is doped into the TiO2 paste, part of them come into the lattice, which leads to the lattice distortion and the dislocation defect. However, Yb exists in the film in another way, Yb2O3 is an insulating oxide, the band gap is very large, and electronic excitation requires a lot of energy. Coating it on the surface of TiO2 particles to form an insulating layer, the energy barrier formed by Yb2O3 hinders the recombination of the electrons from dye injected into TiO2 conduction band and the I3− in electrolyte. Charge recombination can be suppressed effectively.

Figure 2.33: Dark current of DSCs doped with different concentrations of Yb.

From Figure 2.33, it can be seen that the greater the doping concentration, the more the Yb2O3 is, the more powerful the ability to inhibit the charge recombination is, and the smaller the dark current is. The open-circuit voltage and the recombination reaction dynamics have the following relationship:     Iinj kT (2:31) ln Voc = ncb ket ½I3 −  e where Iinj is the intensity of the incident light, ncb is the charge concentration of TiO2 surface, ket is the I3− reaction rate constant, T is the thermodynamic temperature, and k is the Boltzmann constant. As it can be seen from the equation that, the reaction rate of I3− is higher, and that the open-circuit voltage Voc is lower. Thus, the charge recombination of the electrode surface is an important factor which restricts the photovoltage of a solar cell. The higher the doping concentration, the increase of the potential barrier, and the higher the filling level of the electrons in the TiO2, so the Femi level is higher and the open-circuit voltage is increased. However, the driving force of the electron from the excited state level to the Femi level of TiO2 is decreased, which leads to the decrease in the short-circuit current density Jsc. And due to the large particle size of the Yb2O3, the hole is blocked, the pore size decreases, the specific surface area decreases, the number of TiO2 particles in unit volume decreases, and the adsorption capacity of the dye is

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reduced. Yb2O3 particles also affect the diffusion and transfer of I− and I3− ions in the electrolyte, which may be the reason for the decrease of the optical current density. Study on nonmetal-doped TiO2 showed that nonmetal doping could effectively control the energy band structure of TiO2, partially substituted oxygen vacancy defects and extended the light response range to visible light region, and was not easy to form a recombination center. Doping nonmetals into anatase-phase TiO2 to control oxygen vacancy defects and to study the photovoltaic performance of the solar cells has attracted wide attention. In 2001, Taga et al. [94] published a study on Science about N-doped TiO2 with a very good visible light activity, which caused researchers to study nonmetaldoped TiO2. N doping can replace the oxygen vacancies in the TiO2 lattice. The reduction of oxygen vacancies in the crystal lattice can not only reduce the interfacial electron recombination and improve the open-circuit voltage, but also improve the stability of DSC. By the density of states model, the visible light activity of N-doped TiO2 is interpreted as the hybridization of the 2p orbit of N and the 2p orbit of O, and the band gap of TiO2 is reduced [95]. The doped state only meets the following requirements that can produce real visible light response: (1) doping can produce a state that can absorb visible light in the band gap of TiO2; (2) the lowest value of the conduction band energy level (including the TiO2 doped state) should be equal to the electrode potential of TiO2, or higher than that of H2/H2O; (3) the new band gap should be fully overlapped with the TiO2 band gap. Conditions (2) and (3) require anion doping, and the reason is that the d orbit of the cation is deep in the TiO2 band gap and can easily become the center of the carrier recombination. In Figure 2.34, when the mass ratio n (TiO2):n (urea) = 2:1, the flat band potential of the N-doped TiO2 thin-film electrode has not been significantly changed compared with the flat band potential of the undoped one with a slight change. When n (TiO2):n (urea) = 1:1, the flat band potential obviously shifts toward the negative direction, and this indicates that the surface properties of TiO2 film electrodes are changed by N doping, which leads to the negative shift of the TiO2 conduction band, and it is beneficial for the improvement of the open-circuit voltage of DSC. The photovoltaic performance comparison of N-doped cell with different content and the undoped is shown in Figure 2.35. In the J–V curve, the open-circuit voltage of N-doped DSC is increased, but the short-circuit current is decreased. With the increase of N content, the Voc increased from 724 to 745 mV and then increased to 752 mV. The filling factor is improved by 5.2% compared with the undoped DSC. The Jsc and the efficiency decreased. The reasons for the increase of open-circuit voltage are defined by the definition of Voc [96]: Voc = jVfb − Vred j

(2:32)

where Vfb is the flat band potential of TiO2 electrode; Vred is the redox potential of the electrolyte. Voc can be changed with the change of the Vfb of TiO2 electrode.

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Figure 2.34: Flat band potential of N-doped TiO2 thin film.

Therefore, because of the obvious negative shift of the flat band potential of the Ndoped TiO2 thin-film electrode, the Voc has an increasing trend. The driving force of the electron injected into the TiO2 conduction band decreases, resulting in the decrease of the DSC short-circuit current.

Figure 2.35: The photocurrent density and voltage curve of DSC based on undoped and N-doped TiO2 thin films.

The comparison of the dark current of DSC with undoped and series N-doped is shown in Figure 2.36. The test results show that introducing appropriate N can reduce the dark current. In other words, the recombination of electrons in the conduction band of TiO2 and I3− can be inhibited. How to improve the preparation process of the doping is the focus of the future research of DSC.

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Figure 2.36: Dark current curve of series urea-doped DSC.

Use the synergistic effect of codoping atoms or ions, on the one hand, to control the energy band structure of TiO2; on the other hand, it can inhibit the formation of electron recombination centers and improve the performance of TiO2 electrode [89]. The doping method also provides a theoretical guidance for the adjusting of the TiO2 energy band structure and the improvement of the open-circuit voltage and the short-circuit current of DSC. Doping ZrO2 into TiO2 thin-film electrodes, the obtained TiO2–ZrO2 mixed oxide powders had a larger specific surface area, and the band gap was expanded. This improved the short-circuit current, open-circuit voltage, and the photoelectric conversion efficiency of DSC.

2.4.4.6 Effect of surface coating on the level of TiO2 thin-film electrode As an effective method for particle modification, surface coating has been widely concerned. Applying surface coating to nano-TiO2 porous thin-film electrode can inhibit the recombination of electrons and improve the photoelectric conversion performance of the cell. However, the effect of surface coating is still controversial. A layer of barrier layer on the surface of nano-TiO2 thin-film electrode was coated to form the “core–shell” structure. The contact characteristics of TiO2 film/electrolyte and TCO substrate/electrolyte interface can be improved, which is conducive to the suppression of the electron recombination reaction, the decrease of dark current, and the increase of the cell property. When a thin Nb2O5 layer was covered on the surface of nano-TiO2 porous thin film, the electron recombination rate was slow, and the photoelectric conversion efficiency of the cell increased by 35% [97]. At present, the surface coating materials used in DSC are mainly divided into two categories: wide-band-gap semiconductors (such as ZnO and Nb2O5) and insulators (such as Al2O3, MgO, and ZrO2).

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There are three kinds of mechanisms that may be produced by the surface coating in DSC [98]: (1) the coating material itself forms the energy barrier, which allows the electron injection and the electron recombination reaction; (2) surface dipolar interaction, and the conduction band edge of the photo-anode material is moved; and (3) passivating the recombination center of the surface state, as shown in Figure 2.37.

Figure 2.37: Different mechanisms of surface coating on DSC. (a) The formation of an energy barrier, (b) the surface dipole effect, and (c) the passivation of the surface state.

The wide-band-gap semiconductor materials used in the surface coating of TiO2 mainly include Nb2O5 and ZnO. Usually they have a more negative conduction band edge position (Figure 2.37), so the anode/electrolyte interface can form an energy barrier, the injected electrons can be rapidly transferred to the conduction band of the semiconductor. and the energy barrier can inhibit the recombination between the electrons and the oxidation state dye or the redox couple in and electrolyte, to optimize the performance of the solar cell. Figure 2.38 indicates the core–shell structure of TiO2 coating by Nb2O5. The free electron concentration in the conduction band of TiO2 increased after coated with ZnO, the recombination of electron in the transmission process is reduced, the short-circuit current and open-circuit voltage of the cell increased, and the photoelectric conversion efficiency is increased by 27%. Coating with SrO made the sensitized TiO2 film enhance the absorption of visible light, especially in the short wavelength range, and the cell efficiency is increased from 7.3% to 9.3% [99]. A study on the preparation of SrTiO3/TiO2 photoanode in DSC suggests that the photoelectric conversion efficiency of the cell was improved, the conduction band edge of the TiO2 moved to the negative direction, they thought that the surface dipolar interaction was the primary cause of this phenomenon [100]. Using the insulator oxides (Al2O3, MgO, and ZrO2) as the shell material is an important aspect of the study of surface coating. However, when the TiO2 film electrode was coated with Al2O3, MgO, and Y2O3, although the open-circuit voltage and the FF of DSC increased, but the short-circuit current was greatly reduced, and the photoelectric conversion efficiency of the cell decreased [101].

Conductive substrate

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TiO2

103

Dye

Ox Nb2O5 layer

Figure 2.38: The core–shell structure of TiO2 coating by Nb2O5.

When the nano-TiO2 porous film is coated with insulator, the excited electrons passed through the insulating layer by means of quantum tunneling [102]. Quantum tunneling formula can be expressed as   16EðV0 − EÞ 2a pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp − − EÞ (2:33) 2mðV T= 0
h V0 2 where T indicates the probability of electron tunneling through a barrier layer; Vo and a are the barrier height and barrier width, respectively. The barrier height is related to the band gap of the semiconductor, and the barrier width is related to the thickness of the barrier layer; m and E are the mass and energy of the electron. In case of other parameters unchanged, the probability of the electron through the barrier layer is decreased with the increase of the thickness of the barrier layer. In DSC, if the nano-TiO2 is coated with insulating materials, the electron must pass through the barrier to the TiO2 conduction band. Therefore, it is necessary to consider the effect of the thickness of the insulation layer on the electron injection. Taking Y2O3 coating as an example, the effect of the surface coating on the energy structure and recombination process. The band gap of Y2O3 is about 5.6 eV, which is beneficial to the inhibition of electron recombination. The equivalent charge point (pzc = 9) is higher than that of TiO2 (pzc = 6.2), which is helpful for the deprotonation of the thin-film electrode on the surface and the enhancement of the dye adsorption amount; The large dielectric constant helps to store more charge and to improve the cell efficiency [101]. Dipped by the ethanol solution of Sm (CH3COO)3 with the concentration of 0.0005, 0.001, and 0.005 mol · L−1, the obtained coating thicknesses were approximately 0.6, 0.8, and 1.4 nm, respectively. The flat band potential Vfb of film at 780 nm monochromatic light is shown in Table 2.7. Compared with 0.6 nm film, the flat band potentials of 0.8 and 1.4 nm films were negative shifted 10 and 30 mV, respectively. This showed that the flat band potential of the film shifted in the negative direction after coating, and the thicker the coating layer, the more obvious the negative shift. The relationship between the thickness of the coating layer and the photovoltaic performance parameters of DSC is shown in Table 2.8. The open-circuit voltage Voc increases obviously with the thickness increase of the coating layer; the short-

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Table 2.7: Changes of the flat band potential of the films. Coating thickness

.

.

.

Vfb (V)

−.

−.

−.

circuit current density Jsc is significantly reduced, which leads to the decrease of the cell efficiency. The negative shift of the conduction band edge is small, so the voltage increases mainly due to the electrons in the TiO2 conduction band cannot cross the barrier of the Sm2O3 coating layer to react with I3− easily; thus, the dark current is effectively suppressed. the thicker TiO2 surface coating layer is the basic reason for the decrease of electron injection efficiency and the photoelectric conversion efficiency of the cell. Table 2.8: The effect of Sm2O3 coating thickness on the photovoltaic properties of DSC. Coating thickness (nm)  . . .

Voltage (V)

Jsc (mA · cm−)

FF (%)

η (%)

. . . .

. . . .

. . . .

. . . .

2.5 Nano-semiconductor electrode modification As the main component of DSC, oxide film is the key factor affecting the photovoltaic performance, mainly including a variety of photoanode materials, such as TiO2, ZnO, Nb2O5, and SnO2. Because of many good properties of the above materials, such as the forbidden band width matching the dye LUMO which make the excited dye electrons to effectively inject into the semiconductor conduction band. The porous film built by nanoparticles possess large specific surface area which could increase the dye adsorption, and thus increase the photogenerated electrons, making the as-prepared DSC has ideal photovoltaic performance. But a large number of grain boundaries and the existence of defects such as surface state can also lead to a decrease in the carrier diffusion length, an increase in the recombination chances between the grain boundaries, and finally restrict the efficiency improvement of the solar cells. Therefore, the researchers adopt a variety of methods to conduct physical and chemical modification for these oxide thin-film materials. At the same time, because the photovoltaic performance is not ideal, the DSC based on the traditional single nanoTiO2 semiconductor thin film need other chemical modifications to improve its photovoltaic performance. Doping can change the semiconductor energy level, the position of the conduction band, the band structure of the TiO2 photoanode, and can improve

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the charge separation and transfer, and consequently improve the photoelectric conversion efficiency of the DSC. Compared with the traditional surface modification, for example, surface coating, the purpose of doping is covering a wide-band-gap semiconductor layer on the surface of TiO2, forming the core–shell electrodes, and inhibits the recombination of electrons with oxidation state dyes and electrolyte, and eventually the formation of dark current. Doping modification will affect the performance of the nano-TiO2 thin-film electrode, such as the adjustment of the TiO2 band structure and inhibition of the recombination on the TiO2/dye/electrolyte interface in DSC. Presently, researches about physical and chemical modification of nano-TiO2 porous film electrode in DSC are mainly concentrated in the following aspects.

2.5.1 Surface physical and chemical modification Physical and chemical modification on nano-TiO2 porous film electrode surface can improve the state of the electrode interface, help enhance the dye adsorption activity and electronic injection and transport properties, inhibit the electron recombination, and eventually improve the photoelectric conversion performance of solar cells. Surface modification methods can be divided into three categories: One is the modification of nano-TiO2 thin-film surface by the physical method to improve the film surface state [103]. After treating a nanofilm with oxygen plasma and ion beam, oxygen vacancy number in TiO2 decreases, the recombination between electrons in TiO2 conduction band and I3– ion in electrolyte was efficiently restricted, the cell performance was improved, and the photoelectric conversion efficiency increased from 5.1% to 6.6% [104]. Another is the modification of nano-TiO2 porous film electrode by chemical materials, such as TiCl4, TiO2 sol, and acid (e.g., HCl and HNO3), to optimize the contact properties of TiO2 particle/particle interface and TCO/TiO2 film interface in the film interior. Adopting TiCl4 aqueous solution to treat nano-TiO2 photoanode can raise the electron injection efficiency and form a barrier layer in the semiconductor/electrolyte interface, which reduces the recombination between electrons and holes. At the same time, after TiCl4 treatment, despite the specific surface area of the nano-TiO2 thin film decreases, there are new nano-TiO2 particles formed in the TiO2 particles and particle interface inside the film, and the number of TiO2 per unit volume increase, which enhances the electrical contact between the particles and increases the short-circuit current density. Some researchers found that, after treating TiO2 film using TiCl4 and O2 plasma in turn, the cell photoelectric conversion efficiency increased from 3.9% to 8.4% [105]. TiO2 sol treatment is another commonly used approach, namely, before preparation of TiO2 porous film photoanode, pretreating conductive glass by the gel gained in the sol–gel process, and forming a uniform layer of TiO2 barrier layer through high heat sintering, obtaining the photoanode with TiO2 porous films/TiO2 compact layer/conductive glass structure. This method can efficiently improve the

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interface contact properties of TCO/TiO2 thin film, enhance the electrotransportation and collection efficiency, and at the same time, reduce the dark current. Using synthetized titanium organic sol to treat the nano-TiO2 photoanode, the PCE of 7.3% is obtained, which is 28% higher than that of untreated TiO2 photoanode. The titanium organosol treatment increases the two interfaces contact of nano-TiO2 particle/particle and TCO/TiO2 film, forming a good nano-TiO2 network microstructure, leading to the increase of short-circuit current. Similarly, anodic oxidation and acid treatment can also improve the interface characteristics and enhance the photoelectric conversion efficiency of DSC. In addition, the surface coating is also considered as an important method for photoanode surface modification in DSC. Surface coating methods include surface deposition, sol–gel, impregnation, magnetron sputtering, atomic-layer deposition, and metal thermal evaporation method. Among them, the surface deposition, sol–gel, and impregnation method have simple preparation technology, while the atomiclayer deposition, magnetron sputtering, and metal thermal evaporation can easily control the coating layer morphology. According to the difference of the treated object, the surface coating method can be divided into two categories: (1) first, conducting the coat treatment on the nano-TiO2 particles, and then making the core– shell structural nanoparticles to form thin-film photoanode; (2) directly treating the sintered nano-TiO2 thin film to obtain the photoanode with the coated structure. Besides, the first method may introduce the barrier layer in the grain boundary between TiO2 particles, leading to the extension of the electron transport time [108], affecting the coating effect. Tien et al. established the “core–shell” structure model by combining the XPS method to discuss the effect of coverage layer on the performance of the cell. They coated Al2O3 on the surface of TiO2 by the atomic-layer deposition and controlled the average thickness of Al2O3 layer by adjusting the deposition number. They found that the Al2O3 exhibited island-shaped growth obtained by the atomic-layer deposition method, and the cell showed the best photoelectric conversion efficiency at once deposition [109]. Guo et al. studied the effect of dielectric oxide coated layer on the electron injection dynamic in the DSC by ultrafast transient IR method. At the same time, they found that the coating layer existed at the incompletely covered and inhomogeneous coating thickness state [110]. When coating Al2O3 and MgO on the TiO2 electrode surface using metal thermal evaporation and ultraviolet ozone oxidation method and precisely control the coating thickness, the TiO2 conduction band edge moved to the negative direction after surface coating. The coating layer can effectively passivate the TiO2 surface state, resulting in the suppression of the electron recombination in the DSC interior and improved the cell performance. The thickness of oxide layer also has a crucial impact on the cell performance. Menzies et al. applied In2O3 and ZrO2 as shell materials to DSC and researched the influence of coating layer thickness on the cell performance. They found that the main reason for the decrease of the cell performance is the decrease of the short-circuit

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current density induced by the increased coating-layer thickness. The thinner coating layer is more beneficial to the photoelectric conversion efficiency. The introduction of the core–shell structure could effectively inhibit the electron recombination and the dark current generation as Zaban’s team introduced a core– shell structure to the DSC. Researches about a core–shell electrode composed of different materials have been extended, such as SnO2/TiO2, TiO2/Nb2O5, TiO2/Al2O3, SnO2/Al2O3, SnO2/MgO, SnO2/Y2O3, and SnO2/ZnO composite core–shell structure. The introduction of core–shell structure improved the photoelectric conversion efficiency of DSC, optimized the photoanode, and provided improved method for the transport of electrons and effective charge separation.

2.5.2 Element doping When mixed with the suitable amount of impurities, the intrinsic semiconductor can increase the carrier and make a lot of changes in the conductive ability. Doping different, the conductive mechanism is also different. For the impurities, as the free carrier, that is either electrons or holes increase, the semiconductor would become a doped semiconductor. If there are excess electrons, the semiconductor is so-called n-type semiconductor. While if there are excess holes, the semiconductor is socalled p-type semiconductor. Normally, the impurity providing conduction band electrons is called the donor, and the impurity providing the valence band holes is known as the acceptor. In doped semiconductor, the impurity atoms can dope to the crystal structure in two ways: one is to be located in the position between atoms and atoms, which is called interstitial impurity. Another is to replace atoms in the crystal with impurity atoms, keeping regularly arranged atoms in a crystal structure called substitutional impurity. As for TiO2, the typical lattice defects are oxygen vacancies. These oxygen vacancies will lead to the existence of low valence state titanium defects, such as Ti3+. Besides, the lattice distortion makes TiO2 crystallite lattice incomplete, and too much lattice defects also may become the carrier recombination center (electron–hole). In a TiO2 crystal, when the electrons possessing the same level in different isolated atoms form crystals, due to the quantum effect, namely the Pauli exclusion principle which do not allow two electrons in the same state, their energy is staggered to each other, and they formed the energy band each other on a set of sublevel whose energy is slightly different. In the process of preparing doped TiO2 thin-film materials, affected by doping, the material lattice constant deviated, and the local stress in the material also changed. This will greatly affect the concentration and type of the intrinsic defects

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of materials and foreign impurity, the resulting changes of band structure will affect the recombination rate of electrons and holes. In general, nano-TiO2 photoanode doping mainly include nonmetal elements doped, metal ions doped, rare earth ions doped, metal elements doped, and a variety of metal oxide doped.

2.5.2.1 Nonmetal-doped nano-TiO2 photoanode In 2001, Asahi et al. [94] reported the N-doped TiO2 which has excellent visible light activity on the journal of Science, booming the researches about nonmetallic element doped TiO2. In 2003, Lindquist et al. [111] obtained N-doped TiO2 thin-film electrode by magnetron sputtering method, and the photocurrent response in the visible light region is 200 times higher than the undoped film. In 2005, in the case of N2 atmosphere and a little carbon, after calcining 3 h under 500 °C, the commercial anatase powders became N-doped nano-TiO2 powder with deep yellow color, and then were further fabricated to DSC TiO2 photoanode [112]. Studies show that N doping broadens the light response range of the DSC TiO2 photoanode and extends the lifetime of DSC; The DSC efficiencies based on the N-doped TiO2 photoanode are 33% and 14% higher than that based on P25 and SL-D, respectively. From the point of the dye adsorption quantity of the three TiO2, the dye adsorption quantity of N-doped TiO2 photoanode was 1.6 and 1.2 times higher than that of P25 and SL-D, respectively. The aging experiment results show that N-doped TiO2 photoanode did not exhibit apparently visible light degradation phenomenon, and the DSC based on N-doped TiO2 photoanode has good stability. In 2007, the researchers fabricated nano-TiO2 porous film electrode with different content C doping. The doping TiO2 thin film has high specific surface area and high porosity. When the content of C was 1 wt%, the DSC shows good performance, the short-circuit current achieves 12.69 mA · cm−2, open-circuit voltage was 0.72 V, the FF was 62%, and its efficiency reaches 5.6% [113]. In 2008, some researchers introduced N doping to CdSe quantum dot-sensitized solar cell, and obtained good results [114]. Kusama et al. [115] adsorbed N heterocyclic on the anatase TiO2, such as pyrazole, imidazole, and 1,2,4-triazole; adopted the DFT theory by means of the fully optimized geometric model; and calculated the adsorption energy, Ti–N bond length, and Fermi energy levels of the N heterocyclic adsorbing on TiO2 (101), (100), and (001) surface. The results suggest that the Fermi energy level of the anatase TiO2 containing N heterocyclic shift to negative direction, resulting in larger open circuit voltage in DSC and smaller short-circuit current. In 2011, Hou et al. [116] prepared iodine (I2)-doped TiO2 thin-film electrode with visible light response. Using sol–gel synthesis method and combining hydrothermal process, the prepared iodine doped TiO2 has strong visible light absorption range in

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400–550 nm, and the absorption edge has a redshift. The efficiency of DSC based on iodine-doped TiO2 thin-film electrode increased by 42.9% than the undoped DSC. Moreover, the introduction of impurities of iodine can effectively improve the electron lifetime of DSC. For a long time, the study of oxygen vacancy is one of the important areas of semiconductor research, regulating the internal oxygen vacancy can effectively improve the performance of the semiconductor, whereas, the theory study to the oxygen vacancy is controversial. In the TiO2 photocatalytic field, the N doping TiO2 with visible light activity is assigned to N atoms replacing O atoms, accompanying generating oxygen vacancy, the doped N plays a role in hindering the oxidation of oxygen vacancy with oxygen vacancy theory. Batzill et al. [117] studied the formation of N doping TiO2 defect and the surface characteristics and found that N doping TiO2 can improve the catalytic thermal stability of TiO2. In addition, by N-doping, B-doping, or N, B codoping, all the cell performances were greatly improved. Among them, after N and B codoped, the DSC gained the PCE of 8.4%, and the cells keep good stability [118]. Nonmetal doping and codoping of TiO2 photoanode with N, B, or other nonmetals for DSC can effectively inhibit the DSC interface electron recombination and improve the stability of the DSC interface. Its energy-level structure was shown in Figure 2.39. Potential/V vs. NHE Conducting glass -2

Cathode

Dye

-1

eC.B.

0

LUMO

Recombination

e1 TiO2

2

0.4 V

HOMO N719

I- I3 Electrolyte

Figure 2.39: Energy level of the undoped and N, B codoped DSC. Dye: N719; electrolyte: I−/I3− redox.

Through the analysis of the grain size of N-doped TiO2, it is found that the visible light activity can be implemented on polycrystalline particles [119]. Oxygen vacancies could easily form on the interface of the polycrystalline particles, which shows that the formed oxygen vacancy on the interface of the polycrystalline particles is the main reason for N-doping TiO2 possessing visible light activity, and the effect of doping N is to prevent oxygen vacancies reoxidation. Through the theoretical calculation, N-doping in TiO2 is likely to accompany the formation of oxygen vacancies [120]. By plasma heat treatment of nano-TiO2 method, the oxygen vacancy-type nano-TiO2 has obvious visible light activity. By calculating its electron density

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function, it was proved that oxygen vacancies could form a narrow band below the TiO2 conduction band, so as to improve the photocatalytic activity of TiO2 under visible light. Introducing oxygen vacancy defects in the process of preparation of Ndoped TiO2, the visible light activity of the N-doped TiO2 is not only related to N but also related to the introduction of oxygen vacancy [121].

2.5.2.2 Metal, metal-ion-doped nano-TiO2 photoanode Doping with Al, W, or (Al + W) in TiO2 photoanode would result in different DSC photovoltaic performances [89]. Al doping could improve the Voc of DSC, but reduce the Isc. The reason lies that Al doping in TiO2 photoanode greatly reduces the recombination between the photoelectrons and I3−, but Al doping makes the relatively less electrons transport to the counter electrode, which reduces the short-circuit current. The Al + W codoping affected the TiO2 surface electron state, polarization and surface defect charge balance, increased the per unit area dye adsorption amount of the TiO2 thin film, and eventually improved the DSC efficiency. Besides, among the studies about ZnO thin-film electrode-based DSC, researches using Al-doped ZnO thin-film electrode received a wide attention. For instance, Hirahara et al. [122] prepared Aldoped ZnO thin-film substrate using the magnetron sputtering technology and improved the photoelectric conversion efficiency. Yun et al. [123] also successfully applied Al-doped ZnO nanowire thin-film electrode to DSC and achieved much higher photoelectric conversion efficiency. Wang et al. [92] synthesized tungsten (W)-doped TiO2 photoanode with sol–gel method. Adjusting the amount of doping W from 0.1% to 5% lead to the TiO2 conduction band edge shift to the positive direction and enhanced the short-circuit current of the DSC. Within the doping range, the doping can effectively improve the electron lifetime of the DSC, and the efficiency reached 9.4%, increasing by 20%. Wang et al. [124] synthesized transition metal ions, such as Zn2+,Cd2+, Fe3+, Co2+, 2+ Ni , Cr3+, and V5+-doped TiO2 thin-film electrode by the hydrothermal method. Under different incident lights, the light current change of the DSC based on the different doped thin-film electrodes had two different trends. For the Zn2+- and Cd2+-doped n-type semiconductor film, when the film thickness was 0.5 μm and doping amount is less than 0.5%, the IPCE of the doped DSC is higher than that of undoped DSC. The efficiency of Zn2+-doped DSC (1.01%) is higher than that of undoped (0.82%). The TiO2 thin-film electrode doped with Fe3+, Co2+, Ni2+, Cr3+, and V5+ ions showed p–n transition properties. Therefore, the photoelectric performance and photocurrent value of the cell depends on the way and the concentration of doping. To improve the voltage, Iwamoto et al. in Japan [125] obtained 1 V photovoltage under AM1.5 illumination using the Mg doped TiO2 photoelectrode, adopting NKY003 (2-cyano-5-(4-N,N-diphenyl-amino-phenyl)-trans,trans-penta-2,4-dienoic acid)

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as the photosensitizer, and by raising the lowest dye molecules possess orbital and semiconductor conduction band energy level. Figure 2.40(a) is the level distribution when using N719 dye. Figure 2.40(b) shows the level distribution when using NKY003 dye. The results primly proved the viewpoint of the photovoltage of the DSC originated from the level difference between the semiconductor TiO2 Fermi level and electrolyte redox.

a

b HOOC

COOTBA

NC HOOC

Potential/V vs. NHE Conducting glass -2.0

N N

NCS

N

Ru

N

NCS N

HOOC

eCOOTBA

LUMO -1.2 V

e-

-1.0 C.B.

C.B.

LUMO -0.6 V

-0.5 V

Maximum voltage

Maximum voltage

0 e 1.0

-

0.4 V

I -/ I3 -

e

0.4 V

-

I -/I3 -

HOMO 1.0 V HOMO 1.4 V N719

2.0

TiO2

Electrolyte Mg(x-)TiO2

NKY-003

Electrolyte

Figure 2.40: Scheme of the energy level of N719 and NKY-003 dyes.

2.5.2.3 Rare earth elements doped with nano-TiO2 light of anode Yang et al. [126] studied the effect of 13 kinds of rare earth ions, for example, Yb3+ on the TiO2 thin-film electrode. The DSC based on the Yb3+ ion-doped TiO2 thin-film electrodes of photoelectric conversion efficiency increases by 15% in the white light irradiation with light intensity of 73.1 mV · cm−2. There is an energy barrier formed on the surface of TiO2 electrode after rare earth ions doping, which effectively inhibits the electrons recombination on the surface of the electrode, reduces the dark current, and improves the DSC efficiency. Zalas et al. [127] studied the performance

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of the DSC based on gadolinium-doped TiO2 thin-film electrodes. They found that the efficiency of gadolinium-doped DSC increased by 0.22% compared with that of undoped one.

2.5.3 Other doping modification In the ZrO2-doped TiO2 thin-film electrode, the TiO2–ZrO2 mixed oxide powder has a larger specific surface area and expanded the band gap which can improve the short-circuit current, open-circuit voltage, and photoelectric conversion efficiency [128]. Coating the nanoporous CaCO3 on the TiO2 thin-film electrode using local pyrolytic deposition method improved the Isc, Voc, and FF of the DSC, and efficiency increased from 7.8% to 9.7% [129]. Therefore, as one of the most promising alternatives of the traditional silicon solar cells, DSC has achieved very good results and the DSC efficiency reached up to 14%. As the key component in the DSC, nanoporous TiO2 photoanode plays the role of the electron transport channels, which is directly related to the performance of the DSC. Doping TiO2 photoanode is one of the effective means to improve the DSC performance. Selective doping can effectively inhibit the recombination of the interface electrons in DSC, and suppress the dark current. Doping with different elements can affect the TiO2 band structure, and proper doping can extend the spectral response range of the DSC. Part elements doping can improve the stability of DSC, which laid the foundation for the practical application of DSC. Thus, how to improve the preparation of doping process as well as the nonmetal doped TiO2 photoanode is the focus of the DSC in the future.

2.6 Optimization design of the electrode structure 2.6.1 Introduction of small particles compact layer Generally, to improve electrical contact between the nano-TiO2 porous films with Fdoped SnO2 conductive glass (FTO) and speed up the rate of electron transfer and collection, a thin TiO2 compact layer was added between the FTO and porous film. This compact layer may be obtained through the following methods: Spin-coating a TiO2 layer within 1 μm thickness, TiCl4 hydrolysis, anodic oxidation electrodeposition method, and magnetron sputtering a TiO2 compact layer with the thickness below 100 nm. The photoanode can be obtained according to the following processes: (1) coating a compact layer on the FTO by spin coating, TiCl4 hydrolysis and anodic oxidation electrodeposition method; (2) screen printing the nanoporous film with an 8–15 μm

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113

Electrode position

FTO TiO2

1 µm

100 nm

(a)

(b)

Figure 2.41: The surface morphology of the compact layer.

thickness; and (3) screen printing a scattering layer composed with large TiO2 particles. A detailed process for spin coating, TiCl4 hydrolysis, and anodic oxidation electrodeposition can be found in the literature [130]. Figure 2.41 shows the FE-SEM images of the film surface before and after the dense-layer deposition. From Figure 2.41(a), it can be seen that the particle size on the FTO is 200–400 nm, the particles are easily distinguished, the boundary between particles is clear, and the thin film shows a rough surface. Whereas after the compact layer depositing, from Figure 2.41(b), it can be clearly seen that the TiO2 thin film obtained by electrodeposition is very dense and is composed with small particles compared to SnO2 layer. In addition, from the leak in the dense layer, we can see the substrate SnO2 particles and the morphology of dense layer. Table 2.9: Effect of the TiO2 compact layer on the photovoltaic performance of the DSC. Cell A B C D

Voc (V)

Jsc (mA · cm−)

FF (%)

η (%)

Method

. . . .

. . . .

   

. . . .

No Spin coating TiCl hydrolysis Electrodeposition

Table 2.9 shows the test results of the DSC performance after introduction of the compact layer with several different methods. It is found that, after introduction of compact layer, the Jsc increased from 14.34 to 14.43 mA · cm−2, the Jsc after electrodeposition increases to 14.71 mA · cm−2, the photoelectric conversion efficiency also shows different improvement. This is mainly because that the compact layer by deposition plays a role of bridge between the nanoporous film and conductive glass, accelerating the export rate of generated electrons. At the same time, the compact layer reduces the direct contact area between the FTO and electrolyte solution, inhibiting the charge recombination and reducing the dark current. Because the hole occupying rate in nano-TiO2 porous film is as high as 50%–60%, if the nanoporous film directly contacts with the FTO, the contact area between the nano-TiO2 particles

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and the conductive glass will be very low, greatly reducing the effective electrical contact, and the transport efficiency of the electrons in the porous film. Moreover, the direct contact between the nanoporous film and FTO also increased the recombination rate of electrons with I3− in electrolyte, resulting in the increase of dark current.

Figure 2.42: LSV curves of the DSC with different film electrode in dark.

Figure 2.42 shows LSV curves of effect of the compact layer introduced by the above several different ways on the dark current. The results show that the dark current of the DSC is the largest without compact layer. After coating a compact TiO2 layer by spin coating or electrodeposition method on conductive glass, the dark current of the DSC was effectively suppressed, and the current was improved. The compact layer with hundreds nm thicknesses not only improved the electrical contact between the porous film and conductive glass, but also conform to the structure design principle that the particle size of the thin-film electrode increases from the beginning of the conductive glass edge [131]. To study the effect of TiO2 thin film with much smaller particles and more compact layer on the performance of the DSC, by using the magnetron sputtering method, the researchers deposited a compact transparent TiO2 film with a thickness of 10–60 nm on the transparent conductive glass. Afterward, researchers screenprinted nano-TiO2 porous films with the thickness of about 12 μm on the transparent film (no big particle scattering layer). The cell test results were shown in Table 2.10. Compared with the thin film layer by anodic oxidation electrodeposition and TiCl4 hydrolysis, the TiO2 thin film by magnetron sputtering is more compact and has better thin-film surface smoothness. The deposition layer thickness testing by elliptic polarization meter with different sputtering time is only 12.5, 28.1, and 52.3 nm, so we can see that the grain is small and thin film is dense. In Table 2.10, the cell test results show that, with the increase of compact layer thickness, the DSC performance shows a

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Table 2.10: Effect of the TiO2 compact layer by magnetron sputtering method on the photovoltaic performance of the DSC. d (nm)  . . .

Voc (V)

Jsc (mA · cm−)

FF (%)

η (%)

. . . .

. . . .

   

. . . .

Figure 2.43: (α · hV)1/2 versus hV curve of the TiO2 compact layer by magnetron sputtering method.

trend of decline. It is mainly because the TiO2 thin film by magnetron sputtering is very dense, the dense layer beyond a certain thickness could significantly inhibit the electron transport and transfer. By fitting and transformation of the semiconductor optical absorption band edge formula, the compact layer thickness is 12.5 nm, forbidden band width is 3.35 eV. Figure 2.43 shows the Eg and hυ relationship curve obtained from the elliptic polarization instrument testing and fitting. Based on the above study, we found that the contact between the nanoporous film and FTO is very important for electron collection efficiency and the output of the whole cell current. If the porous film has poor electrical contact with FTO, the photogenerated electron cannot timely export and it will prolong the electron collection time, increasing the charge recombination chances and dark current output. Furthermore, the increased direct contact area of FTO with electrolyte solution can also lead to the increase of dark current.

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2.6.2 Effect of nano-TiO2 porous film layer The size of the nanoparticles has a great influence on the BET surface area of the porous film; therefore, characterization of nanoparticle size is commonly used to determine the BET surface area of the porous film. Except the porous film BET surface area, the nano-TiO2 grain size has a more direct relationship with the dye adsorption amount of the porous thin film. The DSC results provide an experimental basis for the implementation of porous film microstructure control. Figure 2.44 shows the UV–vis absorption spectrum of the nanoporous film(5 mm × 5 mm) composed with different TiO2 grain sizes before and after adsorption of the N719 dye with a concentration of 3 × 10−4 mol · L−1.

Figure 2.44: The absorption spectra before and after dye adsorption.

With the film thickness normalized to 15 μm, increasing the grain size leads to a gradual increase of the dye adsorption amount. Whereas when the grain size reaches 15 nm, the dye adsorption quantity presents the downward trend, and the result is shown in Figure 2.45. The result is reverse to the theoretical analysis. Theoretically, with the particle size increased, the specific surface area and the dye adsorption amount of the porous film should be reduced, not increased. The possible reasons for this result are: with the grain size (corresponding particle size) increased, the pore sizes also increases, which might lead to multilayer adsorption of dye molecules on the porous film. The results also showed that the dye adsorption is not the more the better. From Figure 2.46, it can be seen that the dye adsorption amount of four pieces of nanoporous thin film with the same sizes increased from 5.02 to 6.02 mg, while the corresponding cell short-circuit current density did not increase all the time. The results also showed that not all the adsorbed dyes have the effect of light absorption, part of which is only the physical adsorption. Therefore, controlling the

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Figure 2.45: The relationship between the grain size and dye adsorption.

Figure 2.46: The relationship between the dye adsorption and the short-circuit current of the DSC.

dye adsorption amount is not only by controlling the BET surface area of the porous film and film thickness but also by considering other parameters, such as particle size and pore size distribution, namely, the microstructure of the porous film.

2.6.3 Light scattering effect of big TiO2 particle thin-film layer 2.6.3.1 Effect of TiO2 large particles addition on the performance of the DSC The light absorption efficiency of the DSC can be improved by increasing of the multiple light scattering and refraction in nano-TiO2 thin film, so as to improve the

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photocurrent. Effective light scattering requires a certain number of large particles, whereas the addition of larger particles will reduce the BET surface area of the film and decrease the amount of dye adsorption. To improve the dye adsorption amount, the light absorption and the light utilization efficiency, optimization of TiO2 particle size is particularly important. Furthermore, the addition of nano-TiO2 with the particle size in 200–500 nm range in porous film could help increase visible light absorption of the TiO2 electrode in the wavelength range of 600–800 nm and make up the weak absorption of dye within this band. From the optical point of view, nanoTiO2 thin film with a thicker layer can have a better light absorption performance. However, in the thicker film electrode, electrons, iodine and iodine ions will have a longer transport path in the DSC and larger internal resistance, resulting in the decrease of photoelectric conversion efficiency of the DSC. When the TiO2 nanoparticle size is smaller, although the porous film can adsorb more dye molecules, the transport rate of the electrolyte in the film will decrease, leading to the transport not smoothly enough. If the nanoparticle diameter is larger, the BET surface area of porous film will be reduced accordingly, as a result, reducing the numbers of adsorption dye molecules. Furthermore, the light absorption of dye in 600–800 nm wavelength range is weak. Despite of using the porous film with higher BET surface area can adsorb more dye molecules, the visible light utilization efficiency of dye in the wavelength range of 600–800 nm is very low. Therefore, optimization design of the nanoparticle size in view of the application on nanoporous film electrode in the DSC is very important. By adding the big particle with size about 300 nm to the porous film, the shortcircuit current of the DSC achieves the largest value, leading to the highest efficiency. Because the incorporation proportion of large particles possesses 15% of the total amount of TiO2 particles, and the TiO2 particles size is mostly in 15–30 nm range, the doped large particles will have a big impact on the performance of the TiO2 film and DSC. After doping with 300 nm TiO2, the short-circuit current and efficiency of the DSC were all better than the other two, while the open-circuit voltage showed no obvious difference. The influence can be illustrated from the following two aspects: firstly, if the doped particles are too large (particle size is 400 nm), it will lead to the decrease in the specific surface area and pore volume of the TiO2 porous film, decreasing the film dye adsorption, and reducing the efficiency of the DSC. Whereas if the particles are too small, it will not play the role of enhancing the TiO2 film light scattering effect, showing no obvious effect on improving the photoelectric conversion efficiency of the DSC. Secondly, to improve the sunlight utilization of the porous film, large particles were doped in the film. If the doping particles are too large, it will enhance the light reflection effect, but the actual number of photons adsorption by dye reduced, affecting the light absorption utilization ratio of the DSC, and thus decreasing the photoelectric conversion efficiency. Therefore, the doping particle size has an optimum value, in this research, the optimum TiO2 size is 300 nm for the best performance of DSC (Figure 2.47).

Chapter 2 Nano-semiconductor materials

Figure 2.47: Effect of different large particle doping on the properties of the DSC.

Figure 2.48: Effect of the doping amount of large particles on the properties of DSC.

Figure 2.49: Effect of the doping amount of large particles on the properties of DSC.

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For multiple scattering, doping large TiO2 particles into the nano-TiO2 thin film will scatter light in different directions for making light transmission path increased in the nano-TiO2 thin films, enhancing light absorption and raising the light utilization efficiency. From the comparison results of different large particles doped cells, the DSC will have the best performance when the doping large particle size was around 300 nm. The short-circuit current and cell efficiency are all higher than the other two. If the doped particles are too large, it will easily increase large holes in the porous thin film, leading to less corresponding small holes in the film, and the greatly decreased the performance of the porous film. While the doped particles are too small, the light scattering effect of the film cannot reach the expected effect. Figures 2.48 and 2.49 show the comparison results of the DSC doped with 300 nm particles in different percentages. Apparently, the performance of the DSC with 25% doped proportion is better than the other proportion. From the experimental results, the specific surface area of the porous film isn’t the larger, the better, as long as it reaches a certain value, the adsorbed dye could just satisfy the inside cell electrons cycle. Although adding a certain percentage of large particles can reduce the specific surface area of the porous film, it will enhance the sunlight utilization of the nano-thin film, speeding up the electron transport, reducing the charge recombination, and improving the performance of the DSC.

2.6.3.2 Calculation of the adsorption of dye in the porous film In addition to the discussed experimental results in above chapters, through a simple simulating calculation, the dye adsorption of the porous films with different nanoparticle sizes can be analyzed and obtained. For the scattering assumption of the nano-TiO2 thin film to each wavelength, we can think that both single scattering and multiple scattering all exist on the surface of each TiO2 particle that composed the whole DSC. For the single scattering, the dye molecular size is about 1 nm, which is very small compared with the light wavelength, therefore, it is not necessary to view each dye molecules as a ball. It can be thought that the dye formed on the TiO2 nanospherical particle surface is a layer of thin film. Then, assuming the covered single layer dye molecules on the TiO2 sphere surface and the TiO2 spherical particles as a whole, we can calculate the scattering effect of the nano-TiO2 spherical particle covered with a dye layer. Not all the surface area of the nano-TiO2 spherical particle is effective for the dye adsorption. It is necessary to study how many adjacent particles the individual particles have. Because only knowing the distribution of nano-TiO2 particles, namely, the effective area of the nano-TiO2 particles, we can calculate that the amount of dye molecules adsorbed on every nano-TiO2 particles, more effectively analyze the dye adsorption of the entire cell, and the effect of dye on light, finally determine the role

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121

of the whole cell structure on light. Generally, there will be 4–6 adjacent particles around each TiO2 particle [132], therefore, it is assumed that only half of the TiO2 spherical particles surface area adsorbs dye, and the dye molecule connected with TiO2 surface by chemical adsorption. Taking the round nano-TiO2 thin film with radius of 15 mm as an example, hypothetic calculating the dye adsorption amount of the nano-TiO2 film. When the thickness of the thin film is about 8–9 μm, the dosage of the nano-TiO2 is about 0.005–0.008 g. Here, the average diameter of nano-TiO2 particles was analyzed by transmission electron microscope and micro densitometer test, the specific surface area and pore diameter were analyzed by BET specific surface area and porosity analyzer test. The hypothetic calculation results of the dye adsorption amount of different nano-TiO2 thin film were shown in Table 2.11. Table 2.11: The hypothetical calculation results of the dye adsorption amount of different TiO2 nano-thin films. TiO particle diameter (nm) ~ ~ ~ ~

BET (m · g−)

Dye adsorption (per cell · m−)

~ ~ ~ ~

~. ×  ~. ×  ~. ×  ~. × 

2.6.4 Microstructure design of the large-area porous film electrode Optimization design of the large area nanoporous film electrode is very important for the improvement of the DSC performance and the commercial application in the future. At present, the optimization design process and technology of the small area DSC (104 cm−1). Besides, porous CZTS film showed low reflectivity. Figure 2.55 shows the cell series connected n-type TiO2-based DSC and p-type CZTS-based DSC. After series connecting, the cell efficiency increased by 7% than n-type DSC, up to 1.23%, and the short-circuit current increased by 22%.

References [1] [2] [3] [4]

[5] [6] [7]

[8] [9] [10] [11]

[12] [13] [14] [15] [16] [17] [18]

O’Regan B, Grätzel M, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature, 1991, 353(6346): 737–740. Barbe C J, Arendse F, Comte P, et al., Nanocrystalline titanium oxide electrodes for photovoltaic applications. J Amer Ceram Soc, 1997, 80(12): 3157–3171. Kang M G, Ryu K S, Chang S H, et al., Dependence of TiO2 film thickness on photocurrentvoltage characteristics of dye-sensitized solar cells. Bull Kor Chem Soc, 2004, 25(5): 742–744. Gomez R, Salvador P, Photovoltage dependence on film thickness and type of illumination in nanoporous thin film electrodes according to a simple diffusion model. Sol Energy Mater Sol Cells, 2005, 88(4): 377–388. Ito S, Zakeeruddin S M, Humphry-Baker R, et al., High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv Mater, 2006, 18(9): 1202–1205. Liang L Y, Dai S Y, Hu L H, et al., Porosity effects on electron transport in TiO2 films and its application to dye-sensitized solar cells. J Phys Chem B, 2006, 110(25): 12404–12409. Oekermann T, Zhang D, Yoshida T, et al., Electron transport and back reaction in nanocrystalline TiO2 films prepared by hydrothermal crystallization. J Phys Chem B, 2004, 108(7): 2227–2235. Lindström H, Rensmo H, Sodergren S, et al., Electron transport properties in dye-sensitized nanoporous-nanocrystalline TiO2 films. J Phys Chem, 1996, 100(8): 3084–3088. Wang R B, Dai S Y, Wang K J, The influence of pH to prepare TiO2 by sol-gel and the characteristics, J Funct Mater, 2002, 33(003): 296–297. Hu L H, Dai S Y, Wang K J, Structural transformation of nanocrystalline titania grown by solgel technique and the growth kinetics of crystallites. Acta Phys Sin, 2003, 52(9): 2135–2139. Nazeeruddin M K, Kay A, Rodicio I, et al., Conversion of light to electricity by cis-x2bis (2,2ʹbipyridyl-4,4ʹ-dicarboxylate) ruthenium(ii) charge-transfer sensitizers (X = Cl−, Br−, I−, CN−, and SCN−) on nanocrystalline TiO2 electrodes. J Am Chem Soc, 1993, 115(14): 6382–6390. Grätzel M, Photoelectrochemical cells. Nature, 2001, 414(6861): 338–344. Chiba Y, Islam A, Watanabe Y, et al., Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn J Appl Phys Part 2-Lett Exp Lett, 2006, 45(24–28): 638–640. Kroon J M, Bakker N J, Smit H J P, et al., Nanocrystalline dye-sensitized solar cells having maximum performance. Prog Photovoltaics, 2007, 15(1): 1–18. Green M A, Emery K, King D L, et al., Solar cell efficiency tables (version 20). Prog Photovoltaics, 2002, 10(5): 355–360. Dai S, Weng J, Sui Y F, et al., Dye-sensitized solar cells, from cell to module. Sol Energy Mater Sol Cells, 2004, 84(1–4): 125–133. Yum J H, Kim S S, Kim D Y, et al., Electrophoretically deposited TiO2 photo-electrodes for use in flexible dye-sensitized solar cells. J Photochem Photobiol A Chem, 2005, 173(1): 1–6. Tang X, Qian J S, Huang J M, Photoelectrode Preparation Technology in Dye-Sensitized Solar Cells, Mater Rev, 2006, 20(3): 97–103.

Chapter 2 Nano-semiconductor materials

[19] [20]

[21] [22] [23] [24] [25] [26]

[27] [28] [29] [30]

[31] [32] [33] [34] [35] [36]

[37]

[38]

[39]

131

Baxter J B, Aydil E S, Nanowire-based dye-sensitized solar cells. Appl Phys Lett, 2005, 86(05): 053114. Yamamoto J, Tan A, Shiratsuchi R, et al., A 4% efficient dye-sensitized solar cell fabricated from cathodically electrosynthesized composite titania films. Adv Mater, 2003, 15(21): 1823–1825. Liu Y J, Wang A B, Claus R, Molecular self-assembly of TiO2/polymer nanocomposite films. J Phys Chem B, 1997, 101(8): 1385–1388. Park N G, van de Lagemaat J, Frank A J, Comparison of dye-sensitized rutile- and anatasebased TiO2 solar cells. J Phys Chem B, 2000, 104(38): 8989–8994. Ding X Z, Luo L, Cheng L F, Structural Phase Transition and Anatase Grain Growth Kinetics of Nano-TiO2, J Inorg Mater, 1993, 8(1): 114–118. Liu H Z, Kinetics and Application of Nano-TiO2 Reverse Micellar Hydrolysis Synthesis and Heat Treatment Process, Shanghai: Shanghai Jiao Tong University, 2000. O’regan B, Grätzel M, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature, 1991, 353(6346): 737–740. Redmond G, Fitzmaurice D, Grätzel M, Visible-light sensitization by cis-bis(thiocyanato)bis (2,2ʹ-bipyridyl-4,4ʹ-dicarboxylato) ruthenium(II) of a transparent nanocrystalline ZnO film prepared by sol-gel techniques. Chem Mater, 1994, 6(5): 686–691. Rensmo H, Keis K, Lindström H, et al., High light-to-energy conversion efficiencies for solar cells based on nanostructured ZnO electrodes. J Phys Chem B, 1997, 101(14): 2598–2601. Irene G V, Monica L C, Vertically-aligned nanostructures of ZnO for excitonic solar cells: A review. Energ Environ Sci, 2009, 2(1): 19–34. Guo P, Aegerter M A, Ru(II) sensitized Nb2O5 solar cell made by the sol-gel process. Thin Solid Films, 1999, 351(1–2): 290–294. Xia J B, Masaki N, Jiang K J, et al., Fabrication and characterization of thin Nb2O5 blocking layers for ionic liquid-based dye-sensitized solar cells. J Photochem Photobiol A: Chem, 2007, 188(1): 120–127. Bjorksten U, Moser J, Grätzel M, Photoelectrochemical studies on nanocrystalline hematite films. Chem Mater, 1994, 6(6): 858–863. Jose R, Thavasi V, Ramakrishna S, Metal oxides for dye-sensitized solar cells. J Amer Ceram Soc, 2009, 92(2): 289–301. Jung K H, Hong J S, Vittal R, et al., Enhanced photocurrent of dye-sensitized solar cells by modification of TiO2 with carbon nanotubes. Chem Letter, 2002, 8: 864–865. Jang S R, Vittal R, Kim K J, Incorporation of functionalized single-wall carbon nanotubes in dye-sensitized TiO2 solar cells. Langmuir, 2004, 20(22): 9807–9810. Kim S L, Jang S R, Vittal R, et al., Rutile TiO2-modified multi-wall carbon nanotubes in TiO2 film electrodes for dye-sensitized solar cells. J Appl Electrochem, 2006, 36(12): 1433–1439. Yen C Y, Lin Y F, Liao S H, et al., Preparation and properties of a carbon nanotube-based nanocomposite photoanode for dye-sensitized solar cells. Nanotechnology, 2008, 19(37): 375305. Kun-Mu L, Chih-Wei H, Hsin-Wei C, et al., Incorporating carbon nanotube in a lowtemperature fabrication process for dye-sensitized TiO2 solar cells. Sol Energy Mater Sol Cells, 2008, 92(12): 1628–1633. Dang X, Yi H, Ham M-H, et al., Virus-templated self-assembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nat Nanotechnology, 2011, 6(6): 377–384. Pan Z W, Dai Z R, Wang Z L, Nanobelts of semiconducting oxides. Science, 2001, 291(5510): 1947–1949.

132

Linhua Hu, Songyuan Dai

[40] Huang M H, Wu Y Y, Feick H, et al., Catalytic growth of zinc oxide nanowires by vapor transport. Adv Mater, 2001, 13(2): 113–116. [41] Li J Y, Chen X L, Li H, et al., Fabrication of zinc oxide nanorods. J Cryst Growth, 2001, 233(1–2): 5–7. [42] Park W I, Kim D H, Jung S W, et al., Metal organic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods. Appl Phys Lett, 2002, 80(22): 4232–4234. [43] Sun X M, Chen X, Deng Z X, et al., A CTAB-assisted hydrothermal orientation growth of ZnO nanorods. Mater Chem Phys, 2003, 78(1): 99–104. [44] Zhang J, Sun L D, Pan H Y, et al., ZnO nanowires fabricated by a convenient route. New J Chem, 2002, 26(1): 33–34. [45] Liu Y K, Liu Z H, Wang G H, Synthesis and characterization of ZnO nanorods. J Cryst Growth, 2003, 252(1–3): 213–218. [46] Li Y, Meng G W, Zhang L D, et al., Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties. Appl: Phys Lett, 2000, 76(15): 2011–2013. [47] Lakshmi B B, Dorhout P K, Martin C R, Sol-gel template synthesis of semiconductor nanostructures. Chem Mater, 1997, 9(3): 857–862. [48] Pacholski C, Kornowski A, Weller H, Self-assembly of ZnO: From nanodots to nanorods. Angew Chem Int Ed, 2002, 41(7): 1188–1191. [49] Kasuga T, Hiramatsu M, Hoson A, et al., Formation of titanium oxide nanotube. Langmuir, 1998, 14(12): 3160–3163. [50] Yuan Z Y, Colomer J F, Su B L, Titanium oxide nanoribbons. Chem Phys Lett, 2002, 363(3–4): 362–366. [51] Uchida S, Chiba R, Tomiha M, et al., Application of titania nanotubes to a dye-sensitized solar cell. Electrochemistry, 2002, 70(6): 418–420. [52] Ohsaki Y, Masaki N, Kitamura T, et al., Dye-sensitized TiO2 nanotube solar cells: Fabrication and electronic characterization. Phys Chem Chem Phys, 2005, 7(24): 4157–4163. [53] Kasuga T In 4th International symposium on transparent oxide thin films for electronics and optics; Elsevier Science Sa: Tokyo, Japan, 2005, p 141–145. [54] Kim G-S, Seo H-K, Godble V P, et al., Electrophoretic deposition of titanate nanotubes from commercial titania nanoparticles: Application to dye-sensitized solar cells. Electrochem Commun, 2006, 8(6): 961–966. [55] Tian Z R R, Voigt J A, Liu J, et al., Large oriented arrays and continuous films of TiO2-based nanotubes. J Am Chem Soc, 2003, 125(41): 12384–12385. [56] Wang W L, Lin H, Li J B, et al., Formation of titania nanoarrays by hydrothermal reaction and their application in photovoltaic cells. J Amer Ceram Soc, 2008, 91(2): 628–631. [57] Boercker J E, Enache-Pommer E, Aydil E S, Growth mechanism of titanium dioxide nanowires for dye-sensitized solar cells. Nanotechnology, 2008, 19(9): 10. [58] Feng X J, Shankar K, Varghese O K, et al., Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: Synthesis details and applications. Nano Lett, 2008, 8(11): 3781–3786. [59] Liu B, Aydil E, Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells. J Am Chem Soc, 2009, 131(11): 3985–3990. [60] Adachi M, Okada I, Ngamsinlapasathian S, et al., Dye-sensitized solar cells using semiconductor thin film composed of titania nanotubes. Electrochemistry, 2002, 70(6): 449–452. [61] Adachi M, Murata Y, Takao J, et al., Highly Efficient Dye-Sensitized Solar Cells with a Titania Thin-Film Electrode Composed of a Network Structure of Single-Crystal-like TiO2 Nanowires Made by the “Oriented Attachment” Mechanism. J Am Chem Soc, 2004, 126(45): 14943–14949.

Chapter 2 Nano-semiconductor materials

133

[62] Ngamsinlapasathian S, Sakulkhaemaruethai S, Pavasupree S, et al., Highly efficient dyesensitized solar cell using nanocrystalline titania containing nanotube structure. J Photochem Photobiol A: Chem, 2004, 164(1–3): 145–151. [63] Melcarne G, de Marco L, Carlino E, et al., Surfactant-free synthesis of pure anatase TiO2 nanorods suitable for dye-sensitized solar cells. J Mater Chem, 2010, 20(34): 7248–7254. [64] Yoon J H, Jang S R, Vittal R, et al., TiO2 nanorods as additive to TiO2 film for improvement in the performance of dye-sensitized solar cells. J Photochem Photobiol A: Chem, 2006, 180(1–2): 184–188. [65] Kang T S, Smith A P, Taylor B E, et al., Fabrication of highly-ordered TiO2 nanotube arrays and their use in dye-sensitized solar cells. Nano Lett, 2009, 9(2): 601–606. [66] Ghadiri E, Taghavinia N, Zakeeruddin S M, et al., Enhanced electron collection efficiency in dye-sensitized solar cells based on nanostructured TiO2 hollow fibers. Nano Lett, 2010, 10(5): 1632–1638. [67] Mor G K, Shankar K, Paulose M, et al., Use of highly-ordered TiO2 nanotube arrays in dyesensitized solar cells. Nano Lett, 2006, 6: 215–218. [68] Paulose M, Shankar K, Varghese O K, et al., Backside illuminated dye-sensitized solar cells based on titania nanotube array electrodes. Nanotechnology, 2006, 17(5): 1446–1448. [69] Paulose M, Shankar K, Varghese O K, et al., Application of highly-ordered TiO2 nanotubearrays in heterojunction dye-sensitized solar cells. J Phys D-Appl Phys, 2006, 39(12): 2498–2503. [70] Ong K G, Varghese O K, Mor G K, et al., Application of finite-difference time domain to dyesensitized solar cells: The effect of nanotube-array negative electrode dimensions on light absorption. Sol Energy Mater Sol Cells, 2007, 91(4): 250–257. [71] Shankar K, Bandara J, Paulose M, et al., Highly efficient solar cells using TiO2 nanotube arrays sensitized with a donor-antenna dye. Nano Lett, 2008, 8(6): 1654–1659. [72] Varghese O K, Paulose M, Grimes C A, Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells. Nat Nanotechnol, 2009, 4(9): 592–597. [73] Kuang D, Brillet J, Chen P, et al., Application of application of highly ordered TiO2 nanotube arrays in flexible dye-sensitized solar cells. ACS Nano, 2008, 2: 1113–1116. [74] Zhu K, Neale N R, Miedaner A, et al., Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett, 2006, 7(1): 69–74. [75] Zhu K, Vinzant T B, Neale N R, et al., Removing structural disorder from oriented TiO2 nanotube arrays: Reducing the dimensionality of transport and recombination in dyesensitized solar cells. Nano Lett, 2007, 7: 3739–3746. [76] Nishimura S, Abrams N, Lewis B A, et al., Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals. J Am Chem Soc, 2003, 125(20): 6306–6310. [77] Kwak E S, Lee W, Park N G, et al., Compact inverse-opal electrode using non-aggregated tio2 nanoparticles for dye-sensitized solar cells. Adv Funct Mater, 2009, 19(7): 1093–1099. [78] Lee S H A, Abrams N M, Hoertz P G, et al., Coupling of titania inverse opals to nanocrystalline titania layers in dye-sensitized solar cells. J Phys Chem B, 2008, 112(46): 14415–14421. [79] Colodrero S, Ocana M, Miguez H, Nanoparticle-based one-dimensional photonic crystals. Langmuir, 2008, 24(9): 4430–4434. [80] Colodrero S, Ocana M, Gonzalez-Elipe A R, et al., Response of nanoparticle-based onedimensional photonic crystals to ambient vapor pressure. Langmuir, 2008, 24(16): 9135–9139.

134

[81] [82]

[83]

[84]

[85]

[86]

[87]

[88] [89] [90] [91] [92]

[93]

[94]

[95]

[96] [97] [98]

[99]

Linhua Hu, Songyuan Dai

Silvia C, Agustin M, Leif H, et al., Porous one-dimensional photonic crystals improve the power-conversion efficiency of dye-sensitized solar cells. Adv Mater, 2009, 21: 764–770. Kondo Y, Yoshikawa H, Awaga K, et al., Preparation, Photocatalytic activities, and dyesensitized solar-cell performance of submicron-scale TiO2 hollow spheres. Langmuir, 2007, 24(2): 547–550. Chen D, Cao L, Huang F, et al., Synthesis of monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters (14~23 nm). J Am Chem Soc, 2010, 132(12): 4438–4444. Chen D, Huang F, Cheng Y-B, et al., Mesoporous anatase TiO2 beads with high surface areas and controllable pore sizes: A superior candidate for high-performance dye-sensitized solar cells. Adv Mater, 2009, 21(21): 2206–2210. Huang F, Chen D, Zhang X L, et al., Dual-function scattering layer of submicrometer-sized mesoporous TiO2 beads for high-efficiency dye-sensitized solar cells. Adv Funct Mater, 2010, 132(12): 4438–4444. Sauvage F D R, Chen D, Comte P, et al., Dye-sensitized solar cells employing a single film of mesoporous TiO2 beads achieve power conversion efficiencies over 10%. ACS Nano, 2010, 4(8): 4420–4425. Hwang D, Lee H, Jang S-Y, et al., Electrospray Preparation of hierarchically-structured mesoporous TiO2 spheres for use in highly efficient dye-sensitized solar cells. ACS Appl Mater Interf, 2011, 3(7): 2719–2725. Schmuki P, Bohni H, Bardwell J A, In situ characterization of anodic silicon oxide films by AC impedance measurements. J Electrochem Soc, 142(5): 1705–1712. Licht S, Peramunage D, Flat band variation of n-cadmium chalcogenides in aqueous cyanide. J Phy Chem, 1996, 100(21): 9082–9087. Mosaddeq-ur-rahman M, Krishna K M, Miki T, et al., Investigation of solid state Pb doped TiO2 solar cell. Sol Energy Mater Sol Cells, 1997, 48(1–4): 123–130. Ko K H, Lee Y C, Jung Y J, Enhanced efficiency of dye-sensitized TiO2 solar cells (DSSC) by doping of metal ions. J Colloid Interf Sci, 2005, 283(2): 482–487. Zhang X, Liu F, Huang Q L, et al., Dye-sensitized W-doped TiO2 solar cells with a tunable conduction band and suppressed charge recombination. J Phys Chem C, 2011, 115(25): 12665–12671. Alarcon H, Hedlund M, Johansson E M J, et al., Modification of nanostructured TiO2 electrodes by electrochemical Al3+ insertion: Effects on dye-sensitized solar cell performance. J Phys Chem C, 2007, 111(35): 13267–13274. Kim C, Kim K S, Kim H Y, et al., Modification of a TiO2 photoanode by using Cr-doped TiO2 with an influence on the photovoltaic efficiency of a dye-sensitized solar cell. J Mater Chem, 2008, 18(47): 5809–5814. Chandiran A K, Sauvage F, Casas-Cabanas M, et al., Doping a TiO2 photoanode with Nb5+ to enhance transparency and charge collection efficiency in dye-sensitized solar cells. J Phys Chem C, 2010, 114(37): 15849–15856. Asahi R, Morikawa T, Ohwaki T, et al., Visible-light photocatalysis in nitrogen-doped titanium oxides. Science, 2001, 293(5528): 269–271. Miyauchi M, Ikezawa A, Tobimatsu H, et al., Zeta potential and photocatalytic activity of nitrogen doped TiO2 thin films. Phys Chem Chem Phys, 2004, 6(4): 865–870. Kang T S, Chun K H, Hong J S, et al., Enhanced stability of photocurrent-voltage curves in Ru (II)-dye-sensitized nanocrystalline TiO2 electrodes with carboxylic acids. J Electrochem Soc, 2000, 147(8): 3049–3053. Zaban A, Chen S G, Chappel S, et al., Bilayer nanoporous electrodes for dye sensitized solar cells. Chem Commun, 2000, 22: 2231–2232.

Chapter 2 Nano-semiconductor materials

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[100] Wu X M, Wang L D, Luo F, et al., BaCO3 modification of TiO2 electrodes in quasi-solid-state dye-sensitized solar cells: Performance improvement and possible mechanism. J Phys Chem C, 2007, 111(22): 8075–8079. [101] Yang S M, Huang Y Y, Huang C H, et al., Enhanced energy conversion efficiency of the Sr2+-modified nanoporous TiO2 electrode sensitized with a ruthenium complex. Chem Mater, 2002, 14(4): 1500–1504. [102] Diamant Y, Chen S G, Melamed O, et al., Core-shell nanoporous electrode for dye sensitized solar cells: The effect of the SrTiO3 shell on the electronic properties of the TiO2 core. J Phys Chem B, 2003, 107(9): 1977–1981. [103] Kay A, Grätzel M, Dye-sensitized core-shell nanocrystals: Improved efficiency of mesoporous tin oxide electrodes coated with a thin layer of an insulating oxide. Chem Mater, 2002, 14(7): 2930–2935. [104] Palomares E, Clifford J N, Haque S A, et al., Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers. J Am Chem Soc, 2003, 125(2): 475–482. [105] Huang C M, Chen L C, Cheng K W, et al., Effect of nitrogen-plasma surface treatment to the enhancement of TiO2 photocatalytic activity under visible light irradiation. J Mol Catal AChem, 2007, 261(2): 218–224. [106] Parvez M K, Yoo G M, Kim J H, et al., Comparative study of plasma and ion-beam treatment to reduce the oxygen vacancies in TiO2 and recombination reactions in dye-sensitized solar cells. Chem Phys Lett, 2010, 495(1–3): 69–72. [107] Xin X K, Scheiner M, Ye M D, et al., Surface-Treated TiO2 Nanoparticles for dye-sensitized solar cells with remarkably enhanced performance. Langmuir, 2011, 27(23): 14594–14598. [108] Neo C Y, Ouyang J, Precise modification of the interface between titanium dioxide and electrolyte of dye-sensitized solar cells with oxides deposited by thermal evaporation of metals and subsequent oxidation. J Power Sources, 2011, 196(23): 10538–10542. [109] Tien T C, Pan F M, Wang L P, et al., Coverage Analysis for the Core/shell electrode of dyesensitized solar cells. J Phys Chem C, 2010, 114(21): 10048–10053. [110] Guo J C, She C X, Lian T Q, Effect of insulating oxide overlayers on electron injection dynamics in dye-sensitized nanocrystalline thin films. J Phys Chem C, 2007, 111(25): 8979–8987. [111] Lindgren T, Mwabora J M, Avendano E, et al., Photoelectrochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive DC magnetron sputtering. J Phys Chem B, 2003, 107(24): 5709–5716. [112] Ma T L, Akiyama M, Abe E, et al., High-efficiency dye-sensitized solar cell based on a nitrogen-doped nanostructured titania electrode. Nano Lett, 2005, 5(12): 2543–2547. [113] Kang S H, Kim J Y, Kim Y K, et al., Effects of the incorporation of carbon powder into nanostructured TiO2 film for dye-sensitized solar cell. J Photochem Photobiol A-Chem, 2007, 186(2–3): 234–241. [114] Lopez-Luke T, Wolcott A, Xu L P, et al., Nitrogen-doped and CdSe quantum-dot-sensitized nanocrystalline TiO2 films for solar energy conversion applications. J Phy Chem C, 2008, 112(4): 1282–1292. [115] Kusama H, Orita H, Sugihara H, TiO2 band shift by nitrogen-containing heterocycles in dyesensitized solar cells: A periodic density functional theory study. Langmuir, 2008, 24(8): 4411–4419. [116] Hou Q Q, Zheng Y Z, Chen J F, et al., Visible-light-response iodine-doped titanium dioxide nanocrystals for dye-sensitized solar cells. J Mater Chem, 2011, 21(11): 3877–3883. [117] Batzill M, Morales E H, Diebold U, Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase. Phys Rev Lett, 2006, 96(2): 026103.

136

Linhua Hu, Songyuan Dai

[118] Tian H J, Hu L H, Zhang C N, et al., Superior energy band structure and retarded charge recombination for Anatase N, B codoped nano-crystalline TiO2 anodes in dye-sensitized solar cells. J Mater Chem, 2012, 22(18): 9123–9130. [119] Ihara T, Miyoshi M, Iriyama Y, et al., Visible-light-active titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping. Appl Catal B-Environ, 2003, 42(4): 403–409. [120] Di Valentin C, Pacchioni G, Selloni A, et al., Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations. J Phys Chem B, 2005, 109(23): 11414–11419. [121] Orlov A, Tikhov M S, Lambert R M, Application of surface science techniques in the study of environmental photocatalysis: Nitrogen-doped TiO2. C R Chim, 2006, 9(5–6): 794–799. [122] Hirahara N, Onwona-Agyeman B, Nakao M, Preparation of Al-doped ZnO thin films as transparent conductive substrate in dye-sensitized solar cell. Thin Solid Films, 2012, 520(6): 2123–2127. [123] Yun S N, Lim S, Improved conversion efficiency in dye-sensitized solar cells based on electrospun Al-doped ZnO nanofiber electrodes prepared by seed layer treatment. J Solid State Chem, 2011, 184(2): 273–279. [124] Wang Y Q, Hao Y Z, Cheng H M, et al., The photoelectrochemistry of transition metal-iondoped TiO2 nanocrystalline electrodes and higher solar cell conversion efficiency based on Zn2+-doped TiO2 electrode. J Mater Sci, 1999, 34(12): 2773–2779. [125] Iwamoto S, Sazanami Y, Inoue M, et al., Fabrication of dye-sensitized solar cells with an open-circuit photovoltage of 1 V. ChemSusChem, 2008, 1(5): 401–403. [126] Yang S M, Li F Y, Huang C H, Photoelectrochemical Properties of Dye-Sensitized Rare Earth Ion Modified Titanium Dioxide Nanocrystal Electrodes, Sci China Ser B, 2003, 33(01): 59–65. [127] Zalas M, Walkowiak M, Schroeder G, Increase in efficiency of dye-sensitized solar cells by porous TiO2 layer modification with gadolinium-containing thin layer. J Rare Earth, 2011, 29(8): 783–786. [128] Kitiyanan A, Yoshikawa S, The use of ZrO2 mixed TiO2 nanostructures as efficient dye-sensitized solar cells’ electrodes. Mater Lett, 2005, 59(29–30): 4038–4040. [129] Lee S, Kim J Y, Youn S H, et al., Preparation of a nanoporous CaCO3-coated TiO2 electrode and its application to a dye-sensitized solar cell. Langmuir, 2007, 23(23): 11907–11910. [130] Xu W W, The optimization and design of nano crystalline TiO2 electrodes in dye-sensitized solar cells, Institute of Plasma Physics, Chinese Academy of Sciences, 2006. [131] Wang Z S, Kawauchi H, Kashima T, et al., Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coordination Chem Rev, 2004, 248(13–14): 1381–1389. [132] van de Lagemaat J, Benkstein K D, Frank A J, Relation between particle coordination number and porosity in nanoparticle films: Implications to dye-sensitized solar cells. J Phys Chem B, 2001, 105(50): 12433–12436. [133] Hu L H, Dai S Y, Weng J, et al., Microstructure design of nanoporous TiO2 photoelectrodes for dye-sensitized solar cell modules. J Phys Chem B, 2007, 111(2): 358–362. [134] Hu L H, Dai S Y, Wang K J, The influence of nanoporous TiO2 films microstructure to performance of dye-sensitized solar cells, Acta Phys Sinica, 2005, 54(4): 1914–1918. [135] He J J, Lindstrom H, Hagfeldt A, et al., Dye-sensitized nanostructured tandem cell-first demonstrated cell with a dye-sensitized photocathode. Sol Energy Mater Solar Cells, 2000, 62(3): 265–273. [136] Nattestad A, Mozer A J, Fischer M K R, et al., Highly efficient photocathodes for dyesensitized tandem solar cells. Nat Mater, 2010, 9(1): 31–35.

Chapter 2 Nano-semiconductor materials

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[137] Fujishima A, Zhang X T, Solid-state dye-sensitized solar cells. Proc Jpn Acad Ser B-Phys Bio Sci, 2005, 81(2): 33–42. [138] Awais M, Rahman M, MacElroy J M D, et al., Deposition and characterization of NiOx coatings by magnetron sputtering for application in dye-sensitized solar cells. Surf Coat Tech, 2010, 204(16–17): 2729–2736. [139] Awais M, Rahman M, MacElroy J M D, et al., Application of a novel microwave plasma treatment for the sintering of nickel oxide coatings for use in dye-sensitized solar cells. Surf Coat Tech, 2011, 205: S245–S249. [140] Vera F, Schrebler R, Munoz E, et al., Preparation and characterization of eosin B- and erythrosin J-sensitized nanostructured NiO thin film photocathodes. Thin Solid Films, 2005, 490(2): 182–188. [141] Durr M, Bamedi A, Yasuda A, et al., Tandem dye-sensitized solar cell for improved power conversion efficiencies. Appl Phys Lett, 2004, 84(17): 3397–3399. [142] Bandara J, Yasomanee J P, p-type oxide semiconductors as hole collectors in dye-sensitized solid-state solar cells. Semicond Sci and Tech, 2007, 22(2): 20–24. [143] Yu M Z, Natu G, Ji Z Q, et al., p-Type Dye-sensitized solar cells based on delafossite CuGaO2 nanoplates with saturation photovoltages exceeding 460 mV. J Phys Chem Lett, 2012, 3(9): 1074–1078. [144] Dai P C, Zhang G, Chen Y C, et al., Porous copper zinc tin sulfide thin film as photocathode for double junction photoelectrochemical solar cells. Chem Commun, 2012, 48(24): 3006–3008.

Fantai Kong, Songyuan Dai

Chapter 3 The dyes used in dye-sensitized solar cells 3.1 Introduction The wide band-gap semiconductor such as TiO2 responds only in UV region, which cannot absorb light in the visible region. The dye sensitizers adsorbed on nanocrystalline semiconductor with wide band gap (mainly TiO2) can absorb visible and near-infrared (IR) region of the sunlight, so as to broaden the wide band-gap semiconductors’ photoanode photoelectric response in visible light, which is described in Section 1.3. These dye sensitizers include inorganic complex dyes, organic dyes, and inorganic quantum dots (QDs).

3.1.1 The role of dye sensitizers Dye sensitizer is a key component of DSC. The dye sensitizer acts as a “light absorber” in DSC, similar to the chlorophyll in photosynthesis of green plants. It absorbs solar energy and generates light excitation, then the photons of excited state of the dye sensitizers inject into the conduction band of TiO2. So, the performance of the dye sensitizers directly determines the optical absorption efficiency and the photoelectric conversion efficiency of the solar cells. Figures 3.1 and 3.2 are sketch maps of role of the dye in DSC and the function of the green plant photosynthesis vector. As the mechanism of DSC is similar to the photosynthesis of green plants, it is called “artificial photosynthesis.”

3.1.2 Classification of dyes The classification of dyes applied in DSC is not unified. According to the molecular structure, the source, and the sensitized electrode of dyes, the preliminary classification is as follows: According to whether dyes contain metal of its molecular structure, it can be divided into inorganic dyes and organic dyes. The inorganic dyes mainly contain ruthenium and osmium metal polypyridyl complexes, metal porphyrin and phthalocyanine of the metal complex dyes, and inorganic QDs. The inorganic complex

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Figure 3.1: The “artificial photosynthesis” of DSC.

Figure 3.2: Schematic diagram of photosynthesis of green plant.

dyes include: ruthenium complex dyes, osmium complex dyes, rhenium complex dyes, platinum complex dyes, zinc complex dyes, copper complex dyes, iron complex dyes, and titanium complexes according to the central atom. The complex dyes include: bipyridine complexes, phenanthroline complexes, porphyrin complexes, and phthalocyanine complexes according to the different ligands. In addition, it contains mononuclear complex and polynuclear complex dyes according to the different number of central metal atoms. Inorganic QD dyes include: PbS, PbSe, CdS, and CdSe. Through the research of dyes of DSC in the past 20 years,it is found

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that Os and Ru of VIII family can meet the requirements for high-performance DSC. Especially, the photosensitive properties of the ruthenium complexes are the best. Organic dyes include synthetic organic dyes and natural organic dyes, the former are widely applied. These synthetic organic dyes include indoline dyes, coumarin dyes, triphenylamine dyes, cyanine dyes, squarylium dyes, dialkylaniline dyes, carbazole dyes, fluorene dye, perylene dyes, tetrahydroquinoline dyes, porphyrin dyes, and phthalocyanine dyes. Natural organic dyes are mainly extracted from leaves or fruits of green plants. The sensitization of natural dyes is related to the content of active ingredients. The pigment extracted from natural dyes is a mixture; the content of the active ingredient may be very low. To improve the sensitization ability of the natural pigment, the extraction of natural pigment and the effective ingredients to enrich the active ingredients are needed. The low efficiency of natural dyes must be further studied. The extraction of natural pigment can improve the concentration of active components, which will improve the sensitization ability of natural dyes. In the study of DSC, the stability, the price, and the long-term maneuverability are the main considerations. Therefore, it is a promising way to prepare DSC by easy access of the naturally dyes to form nanocrystalline semiconductor film. According to the sensitized electrode, dyes can be divided into anode dyes and cathode dyes.

3.1.3 Structure and molecular design of dyes In order to obtain the highly efficient DSC, the following principles should be followed in the design of dyes [1]: (1) Dyes should possess a wide spectral range, the absorption range should be in the entire visible region and the near-infrared region, that is to say dyes should be matched with the emission spectrum of the sun. And the molar absorption coefficient of dye should be as high as possible, so that the dyes can effectively collect the sun light on a thinner TiO2 film. (2) To obtain the best photoelectric conversion efficiency, dyes should be firmly combined with the semiconductor oxide surface with a kind of compact single molecule layer, and the electron is injected into the conduction band of the semiconductor oxide with high quantum efficiency. (3) The LUMO level of dyes should be confined to adsorbed groups (usually carboxylic acid or phosphonic acid group) and the excited state level of dyes should be higher than the conduction band level of the semiconductor electrode (usually TiO2), that is to make the electrons of excited state dyes effectively be transferred into the TiO2 conduction band and it can reduce the energy loss in the process of electron transfer.

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(4) To ensure dyes regeneration through the electron donor in electrolyte or electron of hole material, the HOMO level of dyes should be lower than redox potential energy levels in electrolyte. (5) In order to reduce the charge recombination between the injected electron and the oxidation state of dyes, the positive charge should be confined to the donor site of the dye after electron injection, which should be further away from the TiO2 surface. (6) In order to reduce the direct contact between electrolyte and anode and prevent the water induction desorption of dyes on the surface of the TiO2, and to enhance the long-term stability of the dye, the dye periphery should be hydrophobic. (7) Dyes should not be aggregated in the surface to avoid the nonradiative decay (dye excited state to the ground state), and this process tends to occur in a thicker film. (8) Dyes should be high stability, it can experience more than 108 cycles of oxidation reduction cycle, which is equivalent to 20 years or longer in the sun. In recent years, researchers have designed and synthesized a variety of organic dye, which is not only for the replacement of the ruthenium complex dyes, but also to broaden the spectral response range of dyes, and extending the photochemical properties of the material.

3.1.4 Quantitative calculation of dyes The relevant theoretical calculation is mainly involved in the calculation of HOMO and LUMO levels, the absorption spectra, the dipole moment, and the vibrational spectra of the dyes. At present, quantitative software Gaussian is widely applied in calculation of dyes. The software is a quantum chemistry software package, which is currently one of the most widely used computational chemistry software. The code was originally written by theoretical chemist, 1998 Nobel Prize for chemistry winner, John Pope. And the name is from the Gauss-type group that Pope used in the software. Using the Gauss type group is to simplify the calculation process and short computation time by an important approximation. The emergence of Gaussian software reduces the threshold of quantum chemical calculation, which makes ab initio calculation to be widely used. Thus, it greatly promotes the progress of the method in the methodology. Initially, Gaussian’s copyright belonged to Pope John at Carnegie Mellon University; the current copyright holder is Gaussian, Inc. The latest version of the software is Gaussian 09. Gaussian calculation can simulate the system in the gas and solution, and can simulate the ground state and excited state. It is a powerful tool to study the substitution effect, the reaction mechanism, the potential energy surface, and the excited state energy. The density functional theory (DFT) B3LYP is the most widely used in quantitative calculation of dyes in DSC.

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Figure 3.3 is the calculation result of the dye 1 (N621) using B3LYP algorithm. It is very consistent with the experiment results. The results (the parentheses brackets correspond to the theoretical calculation results) of absorption spectrum position are as follows: 2.41 eV (2.49 eV), 3.32 eV (3.46 eV), and 4.00–4.19 eV (4.09–4.37 eV) [2]. COOH

HOOC N N

N

Ru

SCN

N NCS

1

Intensity/a.u.

Theor. Exp.

HOMO 4.5

4.0

3.5

3.0 Energy/eV

LUMO 2.5

2.0

1.5

Figure 3.3: DFT calculations based on dye 1 absorption spectra and the experimental results.

3.2 Anode dyes in DSC The dyes adsorbed on nanocrystalline semiconductors (usually nanocrystalline TiO2) are from the ground state to the excited state by absorbing light energy (mainly visible light). Because of the instability of the excited state, the electron is injected into the conduction band of nanocrystalline semiconductors at extremely high speeds (τ < 7 ps), then transmitted to the conduction substrates. The electron goes back to the electrode via external circuit to generate photocurrent. And the dyes in oxidation state return to the ground state by electron donor in electrolyte (usually I–) reduction. At the same time, the I3– ion in the electrolyte is reduced at the interface of the electrode, thus forming a complete loop. According to above operational principle, we can clearly know that anode dyes must meet two basic requirements: (1) LUMO level (dyes excited level) of dyes should be higher than the conduction band of nanocrystalline semiconductor (usually TiO2);

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(2) the ground-state energy level of dyes is lower than the redox stated level of the electrolyte, so that the dyes can be rapidly regenerated from electron donor in electrolyte. We classified dyes into inorganic dyes and organic dyes and discuss those dyes in detail in the following sections.

3.2.1 Inorganic dyes Inorganic metal complex dyes usually contain adsorption ligands and auxiliary ligands, which own high chemical stability and thermal stability. Figure 3.4 shows the molecular structures of several typical polypyridine ruthenium complex dyes. The adsorption ligand of the dyes makes it adsorbed on TiO2 surface and acts as chromophore. The auxiliary ligand is not directly adsorbed on the surface of the nano-semiconductor, and the main function of which is to adjust the overall performance of the complex dyes. The ruthenium dyes have high chemical stability, good redox properties, and outstanding visible spectral response characteristics, and hence are widely used in DSC. This kind of dyes can be adsorbed on the surface of nano-TiO2 thin films by carboxyl or phosphonic acid, which makes the excited dyes be effectively injected into the conduction band of nano-TiO2. Table 3.1 lists the absorption peak of these dyes and the photovoltaic performance data of DSCs.

Figure 3.4: Molecular structures of several representative inorganic dyes.

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Table 3.1: Absorption spectra and optical properties of the complexes of ruthenium(II) complexes. Dye         

Abs (nm) (ε/ L · mol− · cm−)

IPCE (%)

Jsc (mA · cm–)

Voc (mV)

 (.)  (.)  (.)  (.)  (.)  (.)  (.)  (.)  (.)

        

. . . . . . . . .

        

FF

η (%)

. . . . . . . . .

. . . . . . . . .

3.2.1.1 Polypyridine ruthenium dyes Polypyridine ruthenium dyes are classified into carboxylic acid polypyridine ruthenium dyes, phosphonic acid polypyridine ruthenium dyes, and multinuclear ruthenium dyes according to the structures. The difference between the first two types lies in the adsorbing groups: the adsorbing group of the former is carboxyl and that of the latter is phosphonic acid group. The difference between them and the multinuclear bipyridine ruthenium is that they have only one metal center (MC). The adsorbing group of the carboxyl acid polypyridine ruthenium dyes is a planar structure, which makes electron be rapidly injected into the conduction band of TiO2. These dyes are the most widely used in DSC. In those dyes, the highest photoelectric conversion efficiency is obtained with N3, N719, and black dye represented carboxyl polypyridyl ruthenium dye. In recent years, the amphiphilic dyes represented by Z907 and K19, C101, C106 own the high molar extinction coefficient is a hot topic in the research of polypyridyl ruthenium dyes [3]. Figure 3.5 lists several representative dyes. Figure 3.6 is the IPCE curve of several dye-sensitized nanocrystalline TiO2 films. (1) Compound 11: It is applied in the homogeneous phase and multiphase water photolysis. It is a good photosensitizer due to its own good photochemical property [4–6]. (2) Compound 12: The absorption spectrum of compound 12 is redshifted due to the introduction of carboxyl group on the ligand. Wolfgang group from Australia first introduced carboxyl group to the pyridine ligand of compound 11 to obtain a more effective sensitizer in the water photolysis system [7]. The carboxyl group of compound 12 forms ester bonds with hydroxyl groups on the surface of the semiconductor by dehydration, which can be very effectively adsorbed on the surface of the semiconductor [8].

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Figure 3.5: Several representative polypyridine ruthenium dyes.

(3) Compounds 13, 14: Ru(H2-dcbpy)2X2 (13, X = –OH or 14, –Cl): A H2-dcbpy of compound 12 was replaced by –OH or –Cl with double negative charge, which make the absorption spectrum redshifting by 100 nm or more, and broaden the spectral response range greatly of the dyes [9]. (4) Compound 15, NC-Ru(bpy)2-NC-Ru(H2-dcbpy)2-CN-Ru(bpy)2-CN: It is a trinuclear complex using CN- as a ligand. CN- acts as both good σ-electron donor and good π-electron acceptor, and it forms a strong feedback bond with the MC. In 1991, O’Regan and Grätzel introduce the dye to solar cell, and obtain a PCE of 7.1% [10]. (5) Compound 16, Ru(H2-dcbpy)2(CN)2: It was studied with compound 15 at almost the same time, and the PCE of these two dyes are similar [11].

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Figure 3.6: The IPCE diagram of a nano-mesoporous TiO2 film with different dyes.

(6) Compound 2, Ru(H2-dcbpy)2(NCS)2: Known as the N3 dye, it is the most widely used dye in DSC. In 1993, Nazeeruddin et al. first report this dye [12], it has a wide range of visible light absorption, and its excited state lifetime is longer. It shows outstanding performance in terms of the absorber and charge transfer. The experiment demonstrated that compound 2 did not suffer any attenuation through 5 × 107 times of redox cycles, which is equivalent to 20 years in the outdoor operation. (7) Compound 3, TBARu(H2-dcbpy)2(NCS)2, also known as N719: It is formed by the removal of the four carboxyl groups of the N3 dye. Blueshift is shown in UV–vis absorption spectrum compared with N3, but the solubility, molar extinction coefficient of dyes, and PCE of cells are improved. Grätzel et al. reported the dye in 1999 [13]. Since the conduction band of the TiO2 is related to the pH value in a certain extent, in the process of the fully protonated N3 adsorbed on TiO2 film, the proton in carboxyl group will transfer to TiO2 surface, which makes PH higher. In this situation, the electric field of the surface dipole enhanced the photocurrent by the presence of the anionic of the ruthenium complexes on the TiO2 surface and this is helpful to electron of dyes inject into the conduction band of TiO2. While the dyes of complete removal protons can get a higher open-circuit voltage and lower shortcircuit current density. So, it is suitable for the N3 series dyes of appropriate degree of deproton to get a balance between the open-circuit voltage increasing and the short-circuit current decreasing, to achieve highest PCE of cells. At the same time,

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the appropriate protonation can increase the solubility of dyes and accelerate the rate of adsorption to nanocrystalline semiconductor film. (8) Compound 4, TBA3 [Ru(Htctpy)(NCS)3](Htctpy = 2,2ʹ:6ʹ,2″-terpyridyl-4,4ʹ,4″-tricarboxylic acid): It extends the response to sunlight to 920 nm of dyes, which covers the entire visible spectrum and part of near-IR spectrum, so-called black dye [14]. The dye and N719, N3 in the I3–/I– organic solvent electrolyte and nanocrystalline TiO2 electrode in the solar cell keeping the highest record. (9) Compound 5, Z907: For the shortcoming of N3 and N719, that is easy to be removed from the TiO2 surface in the presence of water. Wang et al. [15] reported the design and synthesis of amphiphilic dyes, combining with quasi-solid-state polymer gel electrolyte (from photochemical stability P(VDF-HFP) to solidify 3-methoxy propionitrile-based liquid electrolyte), and the PCE of it exceeding 6% (AM1.5, 100 mW · cm−2): And a PCE of 9.5% with Z907Na and acetonitrile-based electrolyte system was obtained, while the PCE of 7.4% was obtained based on Z907Na and ionic liquid electrolyte. (10) Compound 6, K19: K19 is mainly to adapt to the requirement of thin TiO2 film, the styrene group is added in the auxiliary ligand of dyes to expand the electron’s delocalization range, and introducing at the end of dye at the same time to disperse the positive charge on the dye cation after electron was injected. And long hydrophobic alkyl chain can improve the water resistance stability [16]. (11) Compound 7, K77: Because the K19 is difficult with purification for industrial application of DSC in the future, Grätzel group developed K77. The PCE of the dye achieved 9.47% with nonvolatile electrolyte and 6 μm porous TiO2 films together with 4 μm large particle scattering layer, which is closing to the PCE of cells with volatile organic solvents. The result is a big forward in the process of the DSC to the practical application [17]. (12) C series dyes: Wang Peng group developed a series of ruthenium dyes based on Z907 [18–22], the structure is shown in Figure 3.7. The spectral response range and the molar extinction coefficient of the dyes are improved by changing the structure of the auxiliary ligands. The PCE of 11.7–12.1% of the device was obtained by combining C106 of high absorption coefficient and acetonitrile-based electrolyte. 3.2.1.1.1 Carboxylic acid polypyridine ruthenium dyes Carboxylic acid polypyridine ruthenium dyes are most widely used dyes in DSC, which has been paid to great attention. Compound 2 (N3), 3 (N719), and 4 (black dye) showed excellent performance it the liquid I3–/I– electrolyte system. The high PCE of DSC with volatile liquid electrolyte system is based on the compounds 3 and 4. The absorption of N3, N719 dyes is weak in the red light, and utilization efficiency of these two dyes in the red and near-IR range in the solar spectrum is low.

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Figure 3.7: Dye structure of C series ruthenium of Wang Peng.

At the same time, hydrophilic dyes and the adsorption group of them are a carboxyl group, which is easy to be desorbed from the surface of nanocrystalline TiO2 particles. To solve this problem, people have modified the structure of N3 dyes to develop polypyridine ruthenium dyes with redshift absorption spectra by changing the position and type of substituents to increase the conjugated system. The spectral response range of dyes can be achieved from two aspects. On the one hand, introducing strong electron donating ligand to active the t2g orbitals of the ruthenium MC; on the other hand, introducing the ligand with low π* molecular orbital energy. However, introducing strong electron donating ligands only cannot meet the need of spectral response due to the HOMO and LUMO level of dyes shift to the same direction. Meyer et al. adjust the metal–ligand charge transfer (MLCT) of ruthenium complexes using this strategy, which achieved remarkable result. The dyes contain low π* energy bidentate ligand combine with ruthenium complexes with strong electron donating do own the property of so-called panchromatic dye [23]. However, the spectrum response in the near-IR region is achieved by adjusting the position of LUMO level the dye, and lower LUMO level limit the rate of electron injecting into the conduction band of TiO2 nanocrystals. The LUMO and HOMO levels must be maintained at a certain level, which can make the electron transfer to the conduction band of nanocrystalline TiO2 and can be reduced by I–. And two reactions should be carried out in the yield close to 100%. This limits greatly the possible selection of dyes. At present, it is considered that the MLCT transition of a good dye should be redshifted to 900 nm. Grätzel group developed the

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carboxylic acid polypyridine ruthenium dyes 4, so-called black dye, and obtained a PCE of 10.4% based on this dye [14]. Introducing phenyl group to the suitable position of pyridine ligands of ruthenium complexes can increase the molar extinction coefficient of the MLCT. This effect was observed in the ruthenium complexes with 4,4ʹ-di(p-carboxyphenyl)2,2ʹ-bipyridine ligand. Zhang et al. [24] introduced phenyl group to the middle of the pyridine ring and the carboxyl group to develop analogues of the “black dye” K[Ru(NCS)3(tpyphCOOH)] [tpyphRu dye, tpyphCOOH=2,2ʹ:6ʹ,2ʹ-terpyridyl-4ʹ-(4carboxyl)-phenyl)]. It was found that although the visible absorption spectrum of tpyphRu dye was wider than that of N3, the absorption spectra of dye adsorbed on TiO2 are blueshifted. Although the IPCE range of 400–600 nm is more than 80%, the PCE is only 2.9% while N3 is 6.8%. This may result from the central metal ion which is too far from the pull electron carboxyl, and the electron cloud of the dye is concentrated in the polypyridine ring and the electron cannot be efficiently injected into the conduction band of TiO2. Figure 3.4 shows the structures of N3, N719, and black dye, which are the most widely used dyes, that is, ruthenium dyes. Another hot spot in the research of the polypyridine ruthenium dyes in DSC is the development of the so-called amphiphilic dye. The initial work was published in Langumir in 2002 by Gräztel group [25]. In this article, the authors reported three amphiphilic dyes. Figure 3.8 shows the structures, and Table 3.2 lists the spectra and photovoltaic data of these amphiphilic dyes. The design idea of these dyes is changing two carboxyl groups of the N3 to long-chain alkyl groups, which obtains a dye with one end of the hydrophilic, and the other end of the hydrophobic. The long chain in amphiphilic dyes have two functions: (1) a fat network is formed on the TiO2 surface to prevent the I3– reaching to TiO2 surface, thereby reducing the dark current: (2) to prevent the desorption of dyes on the surface of TiO2 by water. The advantage of those dyes are enhancing its stability against water-induced desorption from the TiO2 surface. The disadvantage is reducing the π electronic delocalization range in bipyridine ligands, which reduce molar extinction coefficient of the dye in the visible light and affect the absorption efficiency of light and PCE. These kinds of dyes also contain Z907, which is reported by Wang Peng et al. [15]. In this paper, they use amphiphilic carboxylate polypyridine ruthenium Z907 as sensitizer and quasi-solid polymer–gel electrolyte (using P(VDF-HFP) gelated 3methoxypropionitrile based liquid electrolyte, and obtain a PCE of above 6% under AM 1.5, 100 mW · cm−2. Besides hydrophobic long-chain alkyl of amphiphilic polypyridine ruthenium, long-chain fatty alkyl amide group, and long-chain alkoxy group also achieved good results. Long-chain amide group of 4,4ʹ-dicarboxylic-2,2ʹ-bipridine acts as a hydrophobic ligand. The position of lowest energy absorption peak of MLCT is the same as and the position of the absorption peak of N3, and the stability against

Figure 3.8: The structures of three amphiphilic dyes in the original.

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Table 3.2: The absorption spectra, emission spectra, redox properties, and photovoltaic properties of three amphiphilic dyes. Dye

Absmax (nm)a (ε/ L · mol− · cm−) π–π L

π–π L

 (.)  (.)  (.) –

 (.) (.)(.)  (.) (.)(.)  (.) (.)(.)  (.) (.)(.)

*

   

*

*

dπ–π

ERuII/IIId Jsc embmax τc (nm) (ns) (mA · cm−)    

   

. . . .e

. . . .

Voc (V) FF

. . . .

. . . .

a

Measured in ethanol. b The emission spectra were tested in ethanol solution at room temperature, and the excitation wavelength was the lowest energy of the MLCT absorption band (the correlation concept was introduced in Section 2.3), the error was ±2 nm. c Tested in ethanol solution at room temperature, the error was ±1 ns. d V versus Ag/AgCl, tested in DMF solution, the value of Fc/Fc+ was 0.46 V under similar conditions (vs. Ag/AgCl).

water-induced desorption of dyes is enhanced at the same time. These amphiphilic dyes have the following advantages compared with N3 [26]: (1) The pKa value of the ground state of 4,4ʹ-bicarboxylic-2,2ʹ-bipyridine in the amphiphilic dyes is higher, which enhance the bonding of the complexes with TiO2 surface. (2) The reduction of charge on the dye decreases the electrostatic repulsion of dye and TiO2 surface, which improve the load amount of dyes. (3) The presence of a hydrophobic unit of the ligand enhances the stability of the dye against desorption from the surface of the TiO2 film. (4) The oxidation potential of these complexes is shifted to the negative direction compared with of N3 dye, which enhances the reversibility of the Ru(III/II) and the stability of dyes. Grätzel and coworkers developed as-called ion coordinating sensitizer [27] (K51). When K51 works as a dye, using fixed hydrocarbon ionic liquid and add Li+, the photocurrent density of the cells will increase sharply and the open-circuit voltage will decrease slightly. While the open-circuit voltage suffers sharply decreased Z907, which is without ion coordination group when Li+ is added. For solid-state DSC with organic hole-transport materials, the open-circuit voltage increases with the increase Li+ ion concentration. In recent years, to meet the requirement of improving the stability of dyes and using thin semiconductor film, Grätzel, Arakawa, and Yanagida et al. reported a lot of the polypyridine ruthenium dyes with good performance. Among these amphiphilic dyes, Z907 has high absorption coefficient, which has become a hot spot for the study of ruthenium dyes. Moreover, designing molecular structure of dyes to enhance light absorption efficiency of solid-state DSC has become a hot spot in this field.

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3.2.1.1.2 Phosphonic polypyridine ruthenium dyes Though phosphonic polypyridine ruthenium dyes own many advantages, it suffers desorption from nano-semiconductor surface in aqueous solution (pH > 5) [28]. The adsorption group of this type of dyes is phosphonic acid group, which is not easy to be desorbed from the surface of the TiO2 when the pH is high. As regards the combination ability on nano-semiconductor surfaces, the phosphonic polypyridine ruthenium dyes are superior to carboxylic polypyridine ruthenium dyes. However, the disadvantage of the phosphonic polypyridine ruthenium dyes is also obvious: because the sp3 hybridization of the central phosphate atom, which is not planar structure, and cannot conjugate well with the polypyridine. The life of the electronic excited state is short, which is a disadvantage to the electron injection. Péchy et al. [29] developed a polypyridine ruthenium dye 25, which have a strong MLCT absorption band at 498 nm in UV–vis absorption spectra in ethanol, and π–π* absorption of ligand are located in 280 and 320 nm. At room temperature and pH = 10, the fluorescence emission properties of 25 in ethanol solution are as follows: λe’max = 708 nm, ϕ = 0.0044, and τ = 15 ns. The emission quantum efficiency is smaller than compound 11. There is a quasi-reversible redox peak in 0.86 V by cyclic voltammetry measurement. The Langmuir adsorption constant of compound 25 on the TiO2 was about 8 × 106, which is approximately 80 times higher than N3. The adsorption ability of a phosphonic acid in nanocrystalline TiO2 surface is better than N3 dye which has four carboxyl groups [30]. The photocurrent spectrum shows that it can convert most light energy into electrical energy in the visible range. And its IPCE reached a maximum value of 70% at 510 nm. Zabri et al. [31] developed a series of phosphonic polypyridine ruthenium dyes like cis-Ru(L′)2X2 (L′ = 2,2ʹ-bipyridine-4,4ʹ-diphosphonic acid or 2,2ʹ-bipyridine-5,5ʹ-diphosphonic acid, X = Cl–, CN–, NCS–, Figure 3.9 shows the structures of compounds 26–29) and study their performance. The shows that the cis-Ru(L′)2(NCS)2 is best, but the overall efficiency is lower than that of N3. In 2004, Grätzel group reported Z955 as the analogues of the Z907 dye. In these dyes, carboxyl adsorption group of Z907 is replaced by phosphonic acid group of Z955, which improves the adsorption ability on the surface of TiO2 [32]. The maximum MLCT of spectra of Z955 is at 519 and 370 nm. And the π–π* absorption band of bipyridyl ligands with alkyl-substituted is at 295 nm, which has a shoulder peak at 305 nm. There is a structureless MLCT emission band at the emission spectra centered in 780 nm, and the emission quantum efficiency of it is 1(±5%) × 10−3(quinine sulfate as a reference). The DSC performance using Z955 as dye sensitizer, and electrolyte composition was respectively A: 0.6 mol · L−1 PMII, 30 mmol · L−1 I2, 0.1 mol · L−1 GuSCN, 0.5 mol · L−1 TBP/butylcyanide mix solution (volume ratio: 3:1); B: 0.6 mol · L−1 PMII, 0.1 mol · L−1 I2, 0.5 mol · L−1 NMBI in methoxypropionitrile solution; C: 0.2 mol · L−1 I2, 0.1 mol · L−1 GuSCN, 0.5 mol · L−1 NMBI, PMII, and EMINCS mix solution (volume ratio: 13:7) are listed in Table 3.3.

Figure 3.9: Structures of the phosphonic polypyridine ruthenium dyes.

154 Fantai Kong, Songyuan Dai

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Table 3.3: Photovoltaic performance of Z955 based on different electrolyte solutions. Device A B C

Jsc (mA · cm−)

Voc (V)

FF

. . .

. . .

. . .

η (%) . . .

3.2.1.1.3 Polynuclear ruthenium dyes Polynuclear ruthenium dyes are the complex containing several metal atoms which are connected to a different ruthenium MC by bridge bond. The advantage of it is that the ground state and excited state of the dye can be changed by different ligands, which can be matched with the solar emission spectrum to enhance the absorption efficiency of solar light. According to the theoretical study, the multinuclear ligands of this complex can transfer energy to other ligands, which have the function of “energy antenna.” Figure 3.10 shows the structures of a number of polynuclear ruthenium dyes. As is proved by Grätzel et al. [33], the antenna effect can increase the absorption coefficient of dyes. However, this effect cannot effectively increase the absorption efficiency of the dyes due to the decline of IPCE of bipyridyl ruthenium dyes comparing with single nuclear dyes. Moreover, the size of the polynuclear ruthenium dyes is larger than that of single nuclear dyes, which limits the absorption efficiency due to it is difficult to enter into the pores of nano-TiO2 for this type dyes. Although the absorption coefficient of the dye in solution increases, the dyes’ concentration on the TiO2 surface is decreased, which do not enhance light absorption of dyes on TiO2. In addition, the synthesis of these dyes is difficult than single nuclear dyes, which makes these dyes rarely applied in DSC. 3.2.1.1.4 Other ruthenium complex dyes In addition to bipyridine ruthenium complex dyes, it is also widely studied for ruthenium complex dye contain nitrogen-containing aromatic heterocyclic ligands, such as phenanthroline [34–36], pyridine quinoline [37], and dipyridyl benzimidazole [38]. Arakawa et al. synthesize and study ruthenium complexes with phenanthroline and pyridyl quinoline in DSC. Figure 3.11 and Table 3.4 show the device performance of several non-polypyridine ligand ruthenium complexes and the photoelectric conversion efficiency of the device. The influence of the location, the number, and the protonated degree of carboxyl group on the photovoltaic properties of the ruthenium complexes is studied. It was found that the photovoltaic performance of the complex with two deprotonations of four carboxyl groups in phenanthroline ligand (compound 37) was best with the highest PCE of 6.6% under one Sun irradiation.

Figure 3.10: The structures of several multinuclear ruthenium dyes.

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Figure 3.11: The structures of the ruthenium complexes with several non-polypyridine ligands. Table 3.4: The photovoltaic performance of the ruthenium complexes with some non-polypyridine ligands in DSC. Dye   

Jsc (mA · cm−)

Voc (V)

. . .

. . .

FF . . .

η (%) . . .

3.2.1.2 Os-polypyridyl complex dye Os-polypyridyl complexes are alternative high-efficiency sensitizers for Ru complexes. Figure 3.12 lists molecular structures of several Os-polypyridyl complex dyes. The properties of spectral response and photovoltage were summarized in Table 3.5 [39, 40]. Interestingly, the Os-complex [Os(H3tcbpy)(CN)3]– existed spin-forbidden singlet–triplet MLCT excitation. This can be attributed to strong spin–orbit coupling in heavy metal (heavy-atom effect), which combines with a certain singlet state characteristic absorption. This indicated high IPCE values at longer wavelength region, reaching maximum IPCE 50% at 510 nm.

3.2.1.3 Metal porphyrins and metal phthalocyanines The porphyrin is a conjugate electronic system, connecting four pyrrole rings with methylene to afford an 18-electron system (as shown in Figure 3.13). Its molecular coordination performance is outstanding; almost all the metal atoms in the periodic table can coordinate with the nitrogen atoms in the center of porphyrin. The porphyrin can connect with various substituted groups through chemical reaction at two substituted positions (meso- and β-). Porphyrin compounds with excellent

Figure 3.12: The molecular structures of Os-polypyridyl complex dye.

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Table 3.5: The properties of spectral response and photovoltage for these Os-complex dyes. Complex

Absorbance, λmax (nm) (εmax) (L · mol− · cm−)

Os(Hdcbpy)(CN) Os(Hdcbpy)(NCS) Os(Hdcbpy)Cl [Os(Hdcbpy)]+ [Os(Htcbpy)(CN)]–

 (.),  (.),  (.)  (.),  (.),  (.)  (.),  (.),  (.)  (.),  (.),  (.)  (.),  (.)  (.),  (.),  (.)

Jsc (mA · cm−)

Voc (V)

FF

. . . . .

. . . . –

. . . . –

light, thermal, and chemical stability, exhibit strong characteristic absorption spectra in the visible region. In recent years, the use of porphyrins and their metal complexes unique electronic structure and optical performance, functional design, and synthesis of optoelectronic materials and devices have become a very active research area in the world [41].

Figure 3.13: The main structure of metal porphyrins and metal phthalocyanines.

In terms of obtaining energy, the nature chose the metal porphyrin complexes. In natural photosynthesis, the metal porphyrin derivative of chlorophyll is the center of lightchemical energy converting reaction. The ability of long charge-separated excited state lifetime, which is an important prerequisite for the effective output of the charge, is the key for converting solar energy into chemical energy. Experiments show that the metal porphyrins performance is not inferior to Ru-polypyridyl compounds [42, 43] in solar cell, especially in the solid-state solar cell, whatever the electron injection efficiency or rate of TiO2. The process for electrons recombination of the conduction band with excited state of porphyrin complex requires a few microseconds, which is long enough for dye regeneration from the electrolyte [44]. These results suggest that metal porphyrins are expected to become good photosensitizers for solar cell. Figure 3.14 shows the structure of several metal porphyrins. The photovoltaic performance data was listed in Table 3.6. Currently, the meta-tetra(p-carboxyphenyl) porphyrin zinc complex (compound 45) is the most studied molecule, because of the longer excited state lifetime (>1 ns) and the appropriate HOMO and LUMO levels.

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Figure 3.14: The structures of Zn porphyrins.

Grätzel and Fox reported that the compound 45 sensitized solar cells exhibit 42% IPCE at B absorption band [45]. Boschloo and Goossens reported its photovoltage conversion efficiency of 1.1% (IPCE 40% at B band) [46]. Meta-tetratosylate zinc porphyrin (compound 46)-sensitized solar cell demonstrated IPCE up to 99.4%, significantly higher than the value of IPCE based on Ru-polypyridyl dyes [47]. Porphyrin-based liquid electrolyte DSC’s performance has been closed to Ru-polypyridyl dye-based DSCs. The liquid electrolyte DSC based on compound 48 exhibit IPCE up to 90% at B band, higher than Ru-polypyridyl compounds. Compound 47 gains relatively high photoelectric conversion efficiency with IPCE (B band) = 85%, η = 5.6% [48]. The use of porphyrins in dye-sensitized solar cells as a photosensitizer

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has great potential because of excellent light, heat, and chemically stable and higher molar extinction coefficient than Ru-polypyridyl compounds. Table 3.6: The photovoltage performances of Zn-porphyrin-based DSCs. Dye

Jsc (mA · cm−)

Voc (V)

FF

η (%)

 * *  

. . . . .

. . . . .

. . . . .

. . . . .

* Based on OMeTAD solid-state electrolyte.

Recently, Diao, Ye, and coworkers with Grätzel research group reported cosensitizer using the compounds 51 and 52 (structure shown in Figure 3.15), on 2.4 μm of TiO2 gained 6.9% efficiency. When using 11 μm porous membranes, compound 51 gave 11% of the photoelectric conversion efficiency in optimal conditions. An organic dye compound 53 and cosensitizer 54, with new cobalt electrolyte, won 12.3% of the photoelectric conversion efficiency. Phthalocyanine compound is a kind of high-performance dye, which has good light, has thermal stability, and has a strong absorption in the near-IR region. Phthalocyanine compound has two absorption bands: (i) at about 550 nm, medium absorption intensity with the molar extinction coefficient of about 104 L mol–1 · cm–1, known as Q band; (ii) at about 370 nm, the molar extinction coefficient of about 105 L mol–1 · cm–1, known as the B band or Soret band. Phthalocyanine and metal atom can generate a variety of metal complexes. The metal phthalocyanine molecule only has 16 p electrons, due to the effect of the conjugated molecules, covalent bond and coordination bond linked with metal atoms is equivalent in essence. This configuration demonstrated a very special stability and good corrosion resisting property in most mediums of acid, alkali, erosion, heat, light, and various organic solvents, and also high absorption efficiency. Phthalocyanine compounds as the photoactive substance adsorbed on nanocrystalline TiO2 electrode can broad the spectral response and improve the photoelectric conversion efficiency [49, 50]. A series of phthalocyanine compounds with carboxyl group, sulfonic acid group as anchor groups with Ti4+ were synthesized. Grätzel group reported that the compound 55 (structure shown in Figure 3.16) which absorbed on the surface of TiO2 nanocrystalline by its axial pyridine-3,4ʹ-dicarboxylic acid ligands was an efficient near-IR-sensitizer for solar cells [51]. And phthalocyanine dye 56 was also reported as sensitizer, obtained IPCE a maximum of 75% and the efficiency of 3.0% using a liquid electrolyte, at AM 1.5 standard sunlight, which is the best results for phthalocyanine dye.

Figure 3.15: Highly efficient Zn–Zn-porphyrin sensitizers.

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Figure 3.16: The molecular structure of two metal phthalocyanine dyes.

3.2.1.4 Other complex sensitizers Recently, in addition to the ruthenium complex dye, some other metal complexes as DSC dye sensitizer have been synthesized. These metals are mainly Fe (II) [52–54], Pt(II) [55, 56], Re(I) [57], Cu(0) [58, 59], and so on. Ligands contain bipyridine, o-phenanthroline, and quinoxaline disulfide. Iron bipyridine dye was developed mainly based on the higher price of the precious metal ruthenium and limited resource. Platinum complexes were synthesized because of planar structure of the platinum complexes which can improve the electron injection efficiency. At present, such work is still immature, the device efficiency based on these sensitizers are 80%) was performed, and the photoelectric conversion efficiency of compound 102 was 6.6%, the structures of compound 101 and 102 are shown in Figure 3.37.

Chapter 3 The dyes used in dye-sensitized solar cells

N

N

189

N

CN

CN CN

COOH

COOH COOH NC

N

HOOC N

99

100

N

101

102

N

Figure 3.37: The molecular structure of polyene dyes.

3.2.6.6 Carbazole dyes This kind of dyes is assembled with the carbazole unit as electron donor, substituted polythiophene as conjugated bridge, and cyanoacetic acid as electron acceptor. The typical structures are shown in Figure 3.38. The long alkyl chain not only inhibits the agglomeration of the dye molecules and the contact between the electron acceptor and TiO2 surface, but also increases the electron lifetime. Koumura et al. [121] synthesized three carbazole compounds 103, 104, and 105. Among them, compound 104 obtains the photoelectric conversion efficiency of 7.7%. It was found that the electron lifetimes of compound 103 and 104 dyes are significantly increased with the length of the alkyl chain. After that, Wang et al. [122] found that the conversion efficiency of dye-sensitized solar cell based on the ionic-liquid electrolyte and the 104 is 7.6%, which was comparable to the cell efficiency based on the liquid electrolyte. Zhang et al. [123] synthesized 106 and 107 on the basis of compound 103. The IPCE value of compound 106 reaches 83%, and the photoelectric conversion efficiency is 7.3%. The carbazole in these dyes works as good electron donor. The conjugated system prevents the charge recombination which results in the enlarged electron lifetime and enhanced the photoelectric conversion efficiency of the dye. C 8 H 17 R

N

NC

C 6H13 COOH

N

S

n

103: n=3, R=C 6H 13 104: n=4, R=C 6H 13 105: n=3, R=H

N H 17 C 8

Figure 3.38: The molecular structure of carbazole dyes.

S

106: n=2 107: n=3

n NC

COOH

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3.2.6.7 Fluorene dye Fluorene dye has a structure that fluorene units works as electron donor and cyanoacetic acid as electron acceptor. The typical structures are shown in Figure 3.39. Ko et al. [74, 124–127] prepared a series of efficient fluorene dye, and compound 108 got the efficiency of 8.6% which was the highest efficiency of such dyes. In order to enhance the stability of the dye, Xu et al. [128] introduced EDOT as the conjugated bridge and synthesized compounds 109 and 110. The conversion efficiency of 7.6% is obtained by compound 110. The plane structure reduces dye aggregation and thermal instability.

NC S

N S

CN 3

C6H13

N

n

COOH

108

COOH

S

O

O

109 n=1 110 n=2

Figure 3.39: The molecular structure of fluorene dyes.

3.2.6.8 Perylene dyes Recently, the researchers [72, 129, 130] synthesized a number of perylene dyes. As shown in Figure 3.40, the compounds 111–113 have been the representative of this series dye. Compound 113 obtained a conversion efficiency of 6.8%, which is the highest efficiency of such dyes. However, the IPCE of these dyes is very narrow. How to broaden the spectral response by group modification will be the key of the dyes.

3.2.6.9 Tetrahydroquinoline dyes Sun et al. [131–133] used the quinoline as electron donor and cyanoacetic acid as electron acceptor to synthesize the compounds 114–120. The structures are shown in Figure 3.41. The new organic dye (compound 119), which has the highest efficiency of 7%, was obtained by the modification of the conjugated bridge and the electron donor.

N

111

O O

O

Figure 3.40: The molecular structure of perylene dyes.

O

O O

N

112

O O

S

O

O

113

N

O

S

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NC

NC R

COOH S N

COOH S

n

114: n=1 115: n=2

R

N

116: R=methyl 117: R=hextyl NC COOH

R

C6H13 COOH

R

S

CN

N

118: R=methyl 119: R=hextyl

C6H13

N

120

Figure 3.41: The molecular structure of tetrahydroquinoline dyes.

3.2.6.10 Porphyrin and phthalocyanine dyes Porphyrin is the derivative of porphine. The phthalocyanine compounds condense porphyrin with four phenyl rings. The typical structures are shown in Figure 3.42. In order to get the effective electron injection, the metal porphyrin compounds [134–138] were designed by the introduction of metal. At present, the metal porphyrin dyes reached the conversion efficiency of 11% [139]. However, there is inferior absorption of porphyrin dyes near IR. Although the phthalocyanine dyes have strong absorption peak in the visible region, the dye aggregation and narrow absorption spectra cannot be prevented. The researchers also explored the cosensitization using porphyrin dyes and phthalocyanine dyes [140–142].

3.3 Cosensitization Single dye always has narrow absorption spectrum, and it is difficult to match with the emission spectrum of the sun. So, cosensitization achieved good results. A series of squaraine dyes, the structure shown in Figure 3.43, were synthesized by Zhang et al. [143, 144], and their absorption spectra have a very good complementary to that of ruthenium complexes. In 600–700 nm range, these dyes show a very strong absorption band and the extinction coefficients are obviously higher than that of N3. The maximum absorption peak redshifted 100 nm compared to that of N3. Table 3.10 shows the performance of the cosensitized cell. The maximum IPCE value of the TiO2 nanoparticles sensitized with N3 and these dyes was more than 85%, and the total photoelectric conversion efficiency of the cell was increased by

Figure 3.42: The molecular structure of porphyrin and phthalocyanine dyes.

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O 2 N R

O

-

N R

125: R = CH3 126: R = CH2CH2OH 127: R = CH2CH2CH2SO3- N

Figure 3.43: The molecular structure of squaraine dyes.

H

13% compared with the single N3. Lu et al. studied the cosensitization of TiO2 electrode system with Zn(II) tetracarboxy phthalocyanine, CdS, and zinc porphyrin [145–147]. It was found that the cosensitization could only broadened the single spectral response range, but also increased the quantum efficiency of the optical conversion. Table 3.10: The photovoltaic properties of device with N3 dye cosensitized by squaraine dyes. Dye     : (concentration ratio) : (concentration ratio)

Voc (V)

Jsc (mA · cm−)

FF (%)

η (%)

. . . . . .

. . . . . .

. . . . . .

. . . . . .

As a way to improve the performance of DSC, the variety of cosensitization is still fragmented and needs to be further studied.

3.4 P-type dyes 3.4.1 Structure characteristics of p-type dye The p-type dye sensitizer molecules have different structure to the p-type dye. The specific requirements are as follows: the HOMO of dye should be lower than the valence band of p-type semiconductor; the LUMO of the dye is higher than that of I3–/I–. By photoexcitation, the dye molecules inject electrons into the electrolyte, and then dye molecules are regenerated from the p-type semiconductor. Figure 3.44 shows the structure of some p-type dyes.

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3.4.2 Research progress of p-type dye In 1999, Lindquist and his coworkers [148] reported the first use of P-type dye in dye sensitized solar cells, the efficiency of the device reached 0.008%. Odobel et al. [149] designed and synthesized a series of ruthenium dye, which obtained compared efficiency to the standard P-type dye 65 (i.e., C343). Lin and his collaborators [150] synthesized a series of aromatic amine P-type dyes. The dye-sensitized solar cells obtained cell efficiency of 0.1%. Nattestad et al. [151] used this dye and N719 type to make a cosensitized solar cell, the photoelectric conversion efficiency of 1.91% was obtained. Sun and Hagfeldt [152–154] synthesized a series of dyes (132–135) which uses triphenylamine-COOH as electron donor, dicyanoethylene as electron acceptor dye. As shown in Figure 3.45, Dye 135 was applied in DSC as the sensitizer, and the photoelectric COOH

COOH HOOC I

I

O

O I

N

O

O

O Ar

OH

128

N

Ar

I

65 O

P+

N

N Ar

N

Ar O

C 8H 17 N O

Ar= COOH

O

N

O

O O

N N

129

O O

CN CN

S O

O O

O HOOC

O

N

O

HOOC

HOOC

130

131

S

132

CN

NC O S

CN S

N O

N

CN CN

NC

S NC

HOOC

HOOC

N

S

NC

N

S

S

O

N

S

N

S

N

NC CN

O

133

134

Figure 3.44: The molecular structure of p-type dyes.

CN

NC

COOH

135

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conversion efficiency and IPCE was 0.05% and 18%. Under the similar conditions, the IPCE of dye 65 was 7% and that of N3 was almost negligible. Hagfeldt et al. [155, 156] studied the charge transfer dynamics of p-type dye-sensitized solar cells by transient absorption spectroscopy. The results showed that the main restricted factor for the development of high efficiency p-type NiO dye-sensitized solar cells was the rapid recombination between reduction stated dyes and the holes in the valence band. Hagfeldt and Odobel [153] used dye 130 and 131 to investigate the performance of cathode in the presence of I3–/I–. The study found that the charge recombination rate of 130 was faster than that of 131. The charge recombination rate of 131 can be attributed to the charge separation state of the electron acceptor, which can effectively improve the charge transfer and the hole collection efficiency. Suzuki and coworkers [157] used TiO2/N3 photoanode and cyanine sensitized photocathode in DSC, the short-circuit current and open-circuit voltage reaches 3.6 mA cm−2 and 0.92 V, respectively. In general, the study of p-type dye is just beginning. The design and synthesis of new dyes, fast charge recombination and other basic physical and chemical problems need further investigation. After the basic problems have been solved, it is possible to improve dramatically the efficiency of dye sensitized solar cells in the future.

Figure 3.45: IPCE spectra of P1, C343, and N3 as p-type sensitizer.

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References [1] [2]

Hagfeldt A, Grätzel M, Molecular photovoltaics. Acc Chem Res, 2000, 33(5): 269–277. Nazeeruddin M K, De Angelis F, Fantacci S, et al., Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell Ruthenium sensitizers. J Am Chem Soc, 2005, 127(48): 16835–16847. [3] Kong F, Dai S, Dye-sensitized solar cells, Prog Chem, 2006, 18(11): 1409–1424. [4] Abruna H D, Teng A Y, Samuels G J, et al., Reduction of water to hydrogen by reduced polypyridine complexes of ruthenium. J Am Chem Soc, 1979, 101(22): 6745–6746. [5] Castellano F N, Heimer T A, Tandhasetti M T, et al., Photophysical properties of Ruthenium polypyridyl photonic SiO2 Gels. Chem Mater, 1994, 6(7): 1041–1048. [6] Ford W E, Wessels J M, Rodgers M A J, Electron injection by photoexcited Ru(bpy)32+ into colloidal SnO2: Analyses of the recombination kinetics based on electrochemical and augercapture models. J Phys Chem B, 1997, 101(38): 7435–7442. [7] Lay P A, Sasse W H F, Proton transfer in the excited state of carboxylic acid derivatives of tris (2,2ʹ-bipyridine-N,N’)ruthenium(II). Inorg Chem, 1984, 23(25): 4123–4125. [8] Argazzi R, Bignozzi C A, Heimer T A, et al., Enhanced spectral sensitivity from Ruthenium(II) polypyridyl based photovoltaic devices. Inorg Chem, 1994, 33(25): 5741–5749. [9] Liska P, Vlachopoulos N, Nazeeruddin M K, et al., cis-diaquabis(2,2ʹ-bipyridyl-4,4ʹdicarboxylate)ruthenium(II) sensitizes wide band gap oxide semiconductors very efficiently over a broad spectral range in the visible. J Am Chem Soc, 1988, 110(11): 3686–3687. [10] O’Regan B, Grätzel M, A low-cost,high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature, 1991, 353: 737–740. [11] Bignozzi C A, Argazzi R, Schoonover J R, et al., Electronic coupling in cyano-bridged ruthenium polypyridine complexes and role of electronic effects on cyanide stretching frequencies. Inorg Chem, 1992, 31(25): 5260–5267. [12] Nazeeruddin M K, Kay A, Rodicio I, et al., Conversion of light to electricity by cis-X2bis (2,2ʹ-bipyridyl-4,4ʹ-dicarboxylate) Ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline TiO2 electrodes. J Am Chem Soc, 1993, 115(14): 6382–6390. [13] Nazeeruddin M K, Zakeeruddin S M, Humphry-Baker R, et al., Acid-base equilibria of (2,2ʹ-bipyridyl-4,4ʹ-dicarboxylic acid)ruthenium(II) complexes and the effect of protonation on charge-transfer sensitization of nanocrystalline titania. Inorg Chem, 1999, 38(26): 6298–6305. [14] Nazeeruddin M K, Pechy P, Renouard T, et al., Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J Am Chem Soc, 2001, 123(8): 1613–1624. [15] Wang P, Zakeeruddin S M, Moser J E, et al., A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat Mater, 2003, 2(6): 402–407. [16] Wang P, Klein C, Humphry-Baker R, et al., A high molar extinction coefficient sensitizer for stable dye-sensitized solar cells. J Am Chem Soc, 2005, 127(3): 808–809. [17] Kuang D, Klein C, Ito S, et al., High-efficiency and stable mesoscopic dye-sensitized solar cells based on a high molar extinction coefficient Ruthenium sensitizer and nonvolatile electrolyte. Adv Mater, 2007, 19(8): 1133–1137. [18] Cao Y, Bai Y, Yu Q, et al., Dye-sensitized solar cells with a high absorptivity Ruthenium sensitizer featuring a 2-(hexylthio)thiophene conjugated bipyridine. J Phys Chem C, 2009, 113(15): 6290–6297. [19] Cao Y, Zhang J, Bai Y, et al., Dye-sensitized solar cells with solvent-free ionic liquid electrolytes. J Phys Chem C, 2008, 112(35): 13775–13781.

198

Fantai Kong, Songyuan Dai

[20] Gao F, Wang Y, Shi D, et al., Enhance the optical absorptivity of nanocrystalline TiO2 film with high molar extinction coefficient Ruthenium sensitizers for high performance dye-sensitized solar cells. J Am Chem Soc, 2008, 130(32): 10720–10728. [21] Gao F F, Cheng Y M, Yu Q J, et al., Conjugation of selenophene with bipyridine for a high molar extinction coefficient sensitizer in dye-sensitized solar cells. Inorg Chem, 2009, 48(6): 2664–2669. [22] Gao F F, Wang Y, Zhang J, et al., A new heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film for a high efficiency dye-sensitized solar cell. Chem Commun, 2008, 23: 2635–2637. [23] Anderson P A, Strouse G F, Treadway J A, et al., Black MLCT Absorbers. Inorg Chem, 1994, 33(18): 3863–3864. [24] Wang Z S, Huang C H, Huang Y Y, et al., Photoelectric behavior of nanocrystalline TiO2 electrode with a novel terpyridyl ruthenium complex. Sol Energy Mater Sol Cells, 2002, 71(2): 261–271. [25] Zakeeruddin S M, Nazeeruddin M K, Humphry-Baker R, et al., Design, synthesis, and application of amphiphilic ruthenium polypyridyl photosensitizers in solar cells based on nanocrystalline TiO2 films. Langmuir, 2002, 18(3): 952–954. [26] Klein C, Nazeeruddin K, Di Censo D, et al., Amphiphilic ruthenium sensitizers and their applications in dye-sensitized solar cells. Inorg Chem, 2004, 43(14): 4216–4226. [27] Kuang D, Klein C, Snaith H J, et al., Ion Coordinating sensitizer for high efficiency mesoscopic dye-sensitized solar cells: Influence of lithium ions on the photovoltaic performance of liquid and solid-state cells. Nano Lett, 2006, 6(4): 769–773. [28] Hagfeldt A, Lindquist S-E, Grätzel M, Charge carrier separation and charge transport in nanocrystalline junctions. Sol Energy Mater Sol Cells, 1994, 32(3): 245–257. [29] Péchy P, Rotzinger F P, Nazeeruddin M K, et al., Preparation of phosphonated polypyridyl ligands to anchor transition-metal complexes on oxide surfaces: Application for theconversion of light to electricity with nanocrystalline TiO2 films. Chem Commun, 1995(1): 65–66. [30] Ito S, Liska P, Comte P, et al., Control of dark current in photoelectrochemical (TiO2/I–/I3–) and dye-sensitized solar cells. Chem Commun, 2005, 34: 4351–4353. [31] Zabri H, Gillaizeau I, Bignozzi C A, et al., Synthesis and comprehensive characterizations of new cis-RuL2X2 (X = Cl, CN, and NCS) sensitizers for nanocrystalline TiO2 solar cell using bis-phosphonated bipyridine ligands (L). Inorg Chem, 2003, 42(21): 6655–6666. [32] Wang P, Klein C, Moser J-E, et al., Amphiphilic Ruthenium sensitizer with 4,4ʹ-diphosphonic acid-2,2ʹ-bipyridine as anchoring ligand for nanocrystalline dye sensitized solar cells. J Phys Chem B, 2004, 108(45): 17553–17559. [33] Kohle O, Ruile S, Grätzel M, Ruthenium(II) charge-transfer sensitizers containing 4,4ʹdicarboxy-2,2ʹ-bipyridine. Synthesis, properties, and bonding mode of coordinated thio- and selenocyanates. Inorg Chem, 1996, 35(16): 4779–4787. [34] Hara K, Horiuchi H, Katoh R, et al., Effect of the ligand structure on the efficiency of electron injection from excited Ru-phenanthroline complexes to nanocrystalline TiO2 films. J Phys Chem B, 2002, 106(2): 374–379. [35] Hara K, Sugihara H, Singh L P, et al., New Ru(II) phenanthroline complex photo sensitizers having different number of carboxyl groups for dye-sensitized solar cells. J Photochem Photobiol: A, 2001, 145(1–2): 117–122. [36] Sakaguchi S, Ueki H, Kato T, et al., Quasi-solid dye sensitized solar cells solidified with chemically cross-linked gelators-control of TiO2/gel electrolytes and counter Pt/gel electrolytes interfaces. J Photochem Photobiol A-Chem, 2004, 164(1–3): 117–122. [37] Yanagida M, Islam A, Tachibana Y, et al., Dye-sensitized solar cells based on nanocrystalline TiO2 sensitized with a novel pyridylquinoline ruthenium(II) complex. New J Chem, 2002, 26(8): 963–965.

Chapter 3 The dyes used in dye-sensitized solar cells

199

[38] Ruile S, Kohle O, Pechy P, et al., Novel sensitisers for photovoltaic cells. Structural variations of Ru(II) complexes containing 2,6-bis(1-methylbenzimidazol-2-yl)pyridine. Inorg Chim Acta, 1997, 261(2): 129–140. [39] Sauve G, Cass M E, Coia G, et al., Dye sensitization of nanocrystalline titanium dioxide with osmium and Ruthenium polypyridyl complexes. J Phys Chem B, 2000, 104(29): 6821–6836. [40] Sauve G, Cass M E, Doig S J, et al., High quantum yield sensitization of nanocrystalline titanium dioxide photoelectrodes with cis-dicyanobis(4,4ʹ-dicarboxy-2,2ʹ-bipyridine)osmium (II) or tris(4,4ʹ-dicarboxy-2,2ʹ-bipyridine)osmium(II) complexes. J Phys Chem B, 2000, 104(15): 3488–3491. [41] Wu D, Shen Z. Xue Z L et al, Applications of porphyrin-based sensitizer in dye sensitized solar cell, Chin J Inorg Chem, 2007, 23(1): 1–14. [42] Hagen J, Schaffrath W, Otschik P, et al., Novel hybrid solar cells consisting of inorganic nanoparticles and an organic hole transport material. Synth Met, 1997, 89(3): 215–220. [43] Tachibana Y, Haque S A, Mercer I P, et al., Electron injection and recombination in dye sensitized nanocrystalline titanium dioxide films: A comparison of ruthenium bipyridyl and porphyrin sensitizer dyes. J Phys Chem B, 2000, 104(6): 1198–1205. [44] Odobel F, Zabri H, Preparations and characterizations of bichromophoric systems composed of a Ruthenium polypyridine complex connected to a difluoroborazaindacene or a Zinc phthalocyanine chromophore. Inorg Chem, 2005, 44(16): 5600–5611. [45] Dabestani R, Bard A J, Campion A, et al., Sensitization of titanium dioxide and strontium titanate electrodes by ruthenium(II) tris(2,2ʹ-bipyridine-4,4ʹ-dicarboxylic acid) and zinc tetrakis(4-carboxyphenyl)porphyrin: An evaluation of sensitization efficiency for component photoelectrodes in a multipanel device. J Phys Chem, 1988, 92(7): 1872–1878. [46] Boschloo G K, Goossens A, Electron trapping in porphyrin-sensitized porous nanocrystalline TiO2 electrodes. J Phys Chem, 1996, 100(50): 19489–19494. [47] Deng H H, Zhou Y M, Mao H F, et al., The mixed effect of phthalocyanine and porphyrin on the photoelectric conversion of a nanostructured TiO2 electrode. Synth Met, 1998, 92(3): 269–274. [48] Wang Q, Zakeeruddin S M, Cremer J, et al., Cross surface ambipolar charge percolation in molecular triads on mesoscopic oxide films. J Am Chem Soc, 2005, 127(15): 5706–5713. [49] Yanagisawa M, Korodi F, Bergquist J, et al., Synthesis of phthalocyanines with two carboxylic acid groups and their utilization in solar cells based on nanostructured TiO2. J Porphyrins Phthalocyanines, 2004, 8(10): 1228–1235. [50] Yanagisawa M, Korodi F, He J J, et al., Ruthenium phthalocyanines with axial carboxylate ligands. Synthesis and function in solar cells based on nanocrystalline TiO2. J Porphyrins Phthalocyanines, 2002, 6(3): 217–224. [51] He J J, Benko G, Korodi F, et al., Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO2 electrode. J Am Chem Soc, 2002, 124(17): 4922–4932. [52] Ferrere S, New photosensitizers based upon [Fe(L)2(CN)2] and [Fe(L)3](L = substituted 2,2ʹ-bipyridine): Yields for then photosensitization of TiO2 and effects on the band selectivity. Chem Mater, 2000, 12(4): 1083–1089. [53] Ferrere S, New photosensitizers based upon [FeII(L)2(CN)2] and [FeII L3], where L is substituted 2,2ʹ-bipyridine. Inorg Chim Acta, 2002, 329: 79–92. [54] Ferrere S, Gregg B A, Photosensitization of TiO2 by [fell(2,2ʹ-bipyridine-4,4ʹ-dicarboxylic acid)2(CN)2]: Band selective electron injection from ultra-short-lived excited states. J Am Chem Soc, 1998, 120(4): 843–844. [55] Geary E A M, Hirata N, Clifford J, et al., Synthesis, structure and properties of [Pt(2,2ʹbipyridyl-5,5ʹ-dicarboxylic acid)(3,4-toluenedithiolate)]: Tuning molecular properties for application in dye-sensitised solar cells. Dalton Trans, 2003, 19: 3757–3762.

200

Fantai Kong, Songyuan Dai

[56] Islam A, Sugihara H, Hara K, et al., New platinum(II) polypyridyl photosensitizers for TiO2 solar cells. New J Chem, 2000, 24: 343–345. [57] Man K Y K, Tse C W, Cheng K W, et al., Fabrication of photovoltaic cells using rhenium diimine complex containing polyelectrolytes by the layer-by-layer electrostatic self-assembly method. J Inorg Organomet Polym Mater, 2007, 17(1): 223–233. [58] Bessho T, Constable E C, Gräetzel M, et al., An element of surprise-efficient copperfunctionalized dye-sensitized solar cells. Chem Commun, 2008, 32: 3717–3719. [59] Sakaki S, Kuroki T, Hamada T, Synthesis of a new copper(I) complex, [Cu(tmdcbpy)(2)]+ (tmdcbpy=4,4ʹ,6,6ʹ-tetramethyl-2,2ʹ-bipyridine-5,5ʹ-dicarboxylic acid), and its application to solar cells. J Chem Soc, Dalton Trans, 2002, 6: 840–842. [60] Anderson P A, Deacon G B, Haarmann K H, et al., Designed synthesis of mononuclear tris (heteroleptic) Ruthenium complexes containing bidentate polypyridyl ligands. Inorg Chem, 1995, 34(24): 6145–6157. [61] Anderson S, Constable E C, Dare-Edwards M P, et al., Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser. Nature, 1979, 280: 571–573. [62] Strouse G F, Anderson P A, Schoonover J R, et al., Synthesis of polypyridyl complexes of ruthenium(II) containing three different bidentate ligands. Inorg Chem, 1992, 31(14): 3004–3006. [63] Alessio E, Mestroni G, Nardin G, et al., Cis- and trans-dihalotetrakis(dimethyl sulfoxide) ruthenium(II) complexes (RuX2(DMSO)4; X = Cl, Br): Synthesis, structure, and antitumor activity. Inorg Chem, 1988, 27(23): 4099–4106. [64] Zakeeruddin S M, Nazeeruddin M K, Humphry-Baker R, et al., Stepwise assembly of trisheteroleptic polypyridyl complexes of ruthenium(II). Inorg Chem, 1998, 37(20): 5251–5259. [65] Kong F, Design and synthesis of polypyridyl ruthenium dye sensitizers and their application in dye-sensitized solar cells, Hefei: Hefei Institutes of Physical Science, Chinese Academy of Sciences, 2007. [66] Cao F, Oskam G, Meyer G J, et al., Electron transport in porous nanocrystalline TiO2 photoelectrochemical cells. J Phys Chem, 1996, 100(42): 17021–17027. [67] Meyer T J, Meyer G J, Pfennig B W, et al., Molecular-level electron transfer and excited state assemblies on surfaces of metal oxides and glass. Inorg Chem, 1994, 33(18): 3952–3964. [68] Kay A, Grätzel M, Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder. Sol Energy Mater Sol Cells, 1996, 44(1): 99–117. [69] Fujihira M, Ohishi N, Osa T, Photocell using covalently-bound dyes on semiconductor surfaces. Nature, 1977, 268(5617): 226–228. [70] Murakoshi K, Kano G, Wada Y, et al., Importance of binding states between photosensitizing molecules and the TiO2 surface for efficiency in a dye-sensitized solar-cell. J Electroanal Chem, 1995, 396(1–2): 27–34. [71] Nazeeruddin M K, Amirnasr M, Comte P, et al., Adsorption studies of counterions carried by the sensitizer cis-dithiocyanato(2,2ʹ-bipyridyl-4,4ʹ-dicarboxylate) Ruthenium(II) on nanocrystalline TiO2 films. Langmuir, 2000, 16(22): 8525–8528. [72] Ferrere S, Zaban A, Gregg B A, Dye sensitization of nanocrystalline tin oxide by perylene derivatives. J Phys Chem B, 1997, 101(23): 4490–4493. [73] Hara K, Horiguchi T, Kinoshita T, et al., Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells. Sol Energy Mater Solar Cells, 2000, 64(2): 115–134. [74] Kim S, Lee J K, Kang S O, et al., Molecular engineering of organic sensitizers for solar cell applications. J Am Chem Soc, 2006, 128(51): 16701–16707.

Chapter 3 The dyes used in dye-sensitized solar cells

[75]

[76] [77] [78] [79] [80] [81] [82]

[83]

[84] [85] [86]

[87] [88] [89] [90] [91] [92] [93] [94]

[95]

201

Sayama K, Hara K, Sugihara H, et al., Photosensitization of a porous TiO2 electrode with merocyanine dyes containing a carboxyl group and a long alkyl chain. Chem Commun, 2000, 13: 1173–1174. Horiuchi T, Miura H, Uchida S, Highly-efficient metal-free organic dyes for dye-sensitized solar cells. Chem Commun, 2003, 24: 3036–3037. Horiuchi T, Miura H, Sumioka K, et al., High efficiency of dye-sensitized solar cells based on metal-free indoline dyes. J Am Chem Soc, 2004, 126(39): 12218–12219. Horiuchi T, Miura H, Uchida S, Highly efficient metal-free organic dyes for dye-sensitized solar cells. J Photochem Photobiol A-Chem, 2004, 164(1–3): 29–32. Schmidt-Mende L, Bach U, Humphry-Baker R, et al., Organic dye for highly efficient solidstate dye-sensitized solar cells. Adv Mater, 2005, 17(7): 813–815. Ito S, Zakeeruddin S M, Humphry-Baker R, et al., High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2electrode thickness. Adv Mater, 2006, 18(9): 1202–1205. Ito S, Miura H, Uchida S, et al., High-conversion-efficiency organic dye-sensitized solar cells with a novel indoline dye. Chem Commun, 2008, 41: 5194–5196. Kuang D, Uchida S, Humphry-Baker R, et al., Organic dye-sensitized ionic liquid based solar cells: Remarkable enhancement in performance through molecular design of indoline sensitizers. Angew Chem Int Ed, 2008, 47(10): 1923–1927. Howie W H, Claeyssens F, Miura H, et al., Characterization of solid-state dye-sensitized solar cells utilizing high absorption coefficient metal-free organic dyes. J Am Chem Soc, 2008, 130(4): 1367–1375. Plank N O V, Howard I, Rao A, et al., Efficient ZnO nanowire solid-state dye-sensitized solar cells using organic dyes and core-shell nanostructures. J Phys Chem C, 2009, 113(43): 18515–18522. Hara K, Sato T, Katoh R, et al., Molecular design of coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem B, 2002, 107(2): 597–606. Hara K, Sayama K, Ohga Y, et al., A coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-energy conversion efficiency up to 5.6%. Chem Commun, 2001, 6: 569–570. Hara K, Kurashige M, Dan-oh Y, et al., Design of new coumarin dyes having thiophene moieties for highly efficient organic-dye-sensitized solar cells. New J Chem, 2003, 27(5): 783–785. Hara K, Sato T, Katoh R, et al., Molecular design of coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem B, 2003, 107(2): 597–606. Hara K, Dan-Oh Y, Kasada C, et al., Effect of additives on the photovoltaic performance of coumarin-dye-sensitized nanocrystalline TiO2 solar cells. Langmuir, 2004, 20(10): 4205–4210. Wang Z S, Hara K, Dan-Oh Y, et al., Photophysical and (photo)electrochemical properties of a coutnarin dye. J Phys Chem B, 2005, 109(9): 3907–3914. Hara K, Miyamoto K, Abe Y, et al., Electron transport in coumarin-dye-sensitized nanocrystalline TiO2 electrodes. J Phys Chem B, 2005, 109(50): 23776–23778. Wang Z S, Cui Y, Hara K, et al., A high-light-harvesting-efficiency coumarin dye for stable dyesensitized solar cells. Adv Mater, 2007, 19(8): 1138–1141. Wang Z-S, Cui Y, Dan-oh Y, et al., Molecular design of coumarin dyes for stable and efficient organic dye-sensitized solar cells. J Phys Chem C, 2008, 112(43): 17011–17017. Koops S E, Barnes P R F, O’Regan B C, et al., Kinetic competition in a coumarin dye-sensitized solar cell: Injection and recombination limitations upon device performance. J Phys Chem C, 2010, 114(17): 8054–8061. Sayama K, Tsukagoshi S, Hara K, et al., Photoelectrochemical properties of J aggregates of benzothiazole merocyanine dyes on a nanostructured TiO2 film. J Phys Chem B, 2002, 106(6): 1363–1371.

202

Fantai Kong, Songyuan Dai

[96] Kitamura T, Ikeda M, Shigaki K, et al., Phenyl-conjugated oligoene sensitizers for TiO2 solar cells. Chem Mater, 2004, 16(9): 1806–1812. [97] Velusamy M, Justin Thomas K R, Lin J T, et al., Organic dyes incorporating low-band-gap chromophores for dye-sensitized solar cells. Org Lett, 2005, 7(10): 1899–1902. [98] Hagberg D P, Edvinsson T, Marinado T, et al., A novel organic chromophore for dye-sensitized nanostructured solar cells. Chem Commun, 2006, 21: 2245–2247. [99] Hwang S, Lee J H, Park C, et al., A highly efficient organic sensitizer for dye-sensitized solar cells. Chem Commun, 2007, 46: 4887–4889. [100] Moon S J, Yum J H, Humphry-Baker R, et al., Highly efficient organic sensitizers for solid-state dye-sensitized solar cells. J Phys Chem C, 2009, 113(38): 16816–16820. [101] Liang Y, Peng B, Liang J, et al., Triphenylamine-based dyes bearing functionalized 3,4propylenedioxythiophene linkers with enhanced performance for dye-sensitized solar cells. Org Lett, 2010, 12(6): 1204–1207. [102] Li G, Jiang K-J, Li Y-F, et al., Efficient structural modification of triphenylamine-based organic dyes for dye-sensitized solar cells. J Phys Chem C, 2008, 112(30): 11591–11599. [103] Xu W, Peng B, Chen J, et al., New triphenylamine-based dyes for dye-sensitized solar cells. J Phys Chem C, 2008, 112(3): 874–880. [104] Yen Y-S, Hsu Y-C, Lin J T, et al., Pyrrole-based organic dyes for dye-sensitized solar cells. J Phys Chem C, 2008, 112(32): 12557–12567. [105] Li R, Lv X, Shi D, et al., Dye-sensitized solar cells based on organic sensitizers with different conjugated linkers: Furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene. J Phys Chem C, 2009, 113(17): 7469–7479. [106] Shi D, Cao Y, Pootrakulchote N, et al., New organic sensitizer for stable dye-sensitized solar cells with solvent-free ionic liquid electrolytes. J Phys Chem C, 2008, 112(44): 17478–17485. [107] Xu M, Li R, Pootrakulchote N, et al., Energy-level and molecular engineering of organic D-π-A Sensitizers in dye-sensitized solar cells. J Phys Chem C, 2008, 112(49): 19770–19776. [108] Zeng W, Cao Y, Bai Y, et al., Efficient dye-sensitized solar cells with an organic photosensitizer featuring orderly conjugated ethylenedioxythiophene and dithienosilole blocks. Chem Mater, 2010, 22(5): 1915–1925. [109] Zhang G, Bala H, Cheng Y, et al., High efficiency and stable dye-sensitized solar cells with an organic chromophore featuring a binary π-conjugated spacer. Chem Commun, 2009, 16: 2198–2200. [110] Liang M, Xu W, Cai F S, et al., New Triphenylamine-Based Organic Dyes for Efficient DyeSensitized Solar Cells. J Phys Chem C, 2007, 111(11): 4465–4472. [111] Pei J, Peng S, Shi J, et al., Triphenylamine-based organic dye containing the diphenylvinyl and rhodanine-3-acetic acid moieties for efficient dye-sensitized solar cells. J Power Sources, 2009, 187(2): 620–626. [112] Shi J, Wang L, Liang Y, et al., All-Solid-State dye-sensitized solar cells with alkyloxyimidazolium iodide ionic polymer/SiO2 nanocomposite electrolyte and triphenylamine-based organic dyes. J Phys Chem C, 2010, 114(14): 6814–6821. [113] Tian H, Yang X, Chen R, et al., Effect of different dye baths and dye-structures on the performance of dye-sensitized solar cells based on triphenylamine dyes. J Phys Chem C, 2008, 112(29): 11023–11033. [114] Wiberg J, Marinado T, Hagberg D P, et al., Effect of anchoring group on electron injection and recombination dynamics in organic dye-sensitized solar cells. J Phys Chem C, 2009, 113(9): 3881–3886. [115] Mikroyannidis J A, Suresh P, Roy M S, et al., Triphenylamine- and benzothiadiazole-based dyes with multiple acceptors for application in dye-sensitized solar cells. J Power Sources, 2010, 195(9): 3002–3010.

Chapter 3 The dyes used in dye-sensitized solar cells

203

[116] Wang Z S, Huang C H, Huang Y Y, A highly efficient solar cell made from a dye-modified ZnO-covered TiO2 nanoporous electrode. Chem Mater, 2001, 13(2): 678–682. [117] Chen X, Guo J, Peng X, et al., Novel cyanine dyes with different methine chains as sensitizers for nanocrystalline solar cell. J Photochem Photobiol A-Chem, 2005, 171(3): 231–236. [118] Kim S, Mor G K, Paulose M, et al., Molecular Design of near-IR harvesting unsymmetrical squaraine dyes. Langmuir, 2010, 26(16): 13486–13492. [119] Yum J H, Jang S R, Walter P, et al., Efficient co-sensitization of nanocrystalline TiO2 films by organic sensitizers. Chem Commun, 2007, 44: 4680–4682. [120] Hara K, Kurashige M, Ito S, et al., Novel polyene dyes for highly efficient dye-sensitized solar cells. Chem Commun, 2003, 2: 252–253. [121] Koumura N, Wang Z S, Mori S, et al., Alkyl-functionalized organic dyes for efficient molecular photovoltaics. J Am Chem Soc, 2006, 128(44): 14256–14257. [122] Wang Z-S, Koumura N, Cui Y, et al., Exploitation of ionic liquid electrolyte for dye-sensitized solar cells by molecular modification of organic-dye sensitizers. Chem Mater, 2009, 21(13): 2810–2816. [123] Zhang X-H, Wang Z-S, Cui Y, et al., Organic sensitizers based on hexylthiophenefunctionalized indolo[3,2-b]carbazole for efficient dye-sensitized solar cells. J Phys Chem C, 2009, 113(30): 13409–13415. [124] Choi H, Baik C, Kang S O, et al., Highly efficient and thermally stable organic sensitizers for solvent-free dye-sensitized solar cells. Angew Chem Int Ed, 2008, 47(2): 327–330. [125] Choi H, Baik C, Kim H J, et al., Synthesis of novel organic dyes containing coumarin moiety for solar cell. Bull Kor Chem Soc, 2007, 28(11): 1973–1979. [126] Choi H, Lee J K, Song K H, et al., Synthesis of new julolidine dyes having bithiophene derivatives for solar cell. Tetrahedron, 2007, 63(7): 1553–1559. [127] Kim C, Choi H, Kim S, et al., Molecular engineering of organic sensitizers containing pphenylene vinylene unit for dye-sensitized solar cells. J Org Chem, 2008, 73(18): 7072–7079. [128] Xu M, Wenger S, Bala H, et al., Tuning the energy level of organic sensitizers for highperformance dye-sensitized solar cells. J Phys Chem C, 2009, 113(7): 2966–2973. [129] Edvinsson T, Li C, Pschirer N, et al., Intramolecular charge-transfer tuning of perylenes: Spectroscopic features and performance in dye-sensitized solar cells. J Phys Chem C, 2007, 111(42): 15137–15140. [130] Li C, Yum J H, Moon S J, et al., An improved perylene sensitizer for solar cell applications. ChemSusChem, 2008, 1(7): 615–618. [131] Chen R, Yang X, Tian H, et al., Tetrahydroquinoline dyes with different spacers for organic dye-sensitized solar cells. J Photochem Photobiol A-Chem, 2007, 189(2–3): 295–300. [132] Chen R K, Yang X C, Tian H N, et al., Effect of tetrahydroquinoline dyes structure on the performance of organic dye-sensitized solar cells. Chem Mater, 2007, 19(16): 4007–4015. [133] Hao Y, Yang X, Cong J, et al., Efficient near infrared D-π-A sensitizers with lateral anchoring group for dye-sensitized solar cells. Chem Commun, 2009, 27: 4031–4033. [134] Balanay M P, Dipaling C V P, Lee S H, et al., AM1 molecular screening of novel porphyrin analogues as dye-sensitized solar cells. Sol Energy Mater Sol Cells, 2007, 91(19): 1775–1781. [135] Cherian S, Wamser C C, Adsorption and photoactivity of tetra(4-carboxyphenyl)porphyrin (TCPP) on nanoparticulate TiO2. J Phys Chem B, 2000, 104(15): 3624–3629. [136] Ma T L, Inoue K, Yao K, et al., Photoelectrochemical properties of TiO2 electrodes sensitized by porphyrin derivatives with different numbers of carboxyl groups. J Electroanal Chem, 2002, 537(1–2): 31–38. [137] Campbell W M, Jolley K W, Wagner P, et al., Highly efficient porphyrin sensitizers for dye-sensitized solar cells. J Phys Chem C, 2007, 111(32): 11760–11762.

204

Fantai Kong, Songyuan Dai

[138] Wang X F, Kakitani Y, Xiang J F, et al., Generation of carotenoid radical cation in the vicinity of a chlorophyll derivative bound to titanium oxide, upon excitation of the chlorophyll derivative to the Q(y) state, as identified by time-resolved absorption spectroscopy. Chem Phys Lett, 2005, 416(4–6): 229–233. [139] Bessho T, Zakeeruddin S, Yeh C Y, et al., Highly efficient mesoscopic dye-sensitized solar cells based on donor–acceptor-substituted porphyrins. Angew Chem Int Ed, 2010, 49(37): 6646–6649. [140] Kong F, Dai S, Wang K, Dye sensitizers used in dye-sensitized solar cells. Chemistry, 2005, 68(5): 338–345. [141] Fang J, Zhang X, Wu J et al. Photovoltaic study of dye-cosensitized nanocrystalline TiO2 solar cell. Acta Energ Sol Sin, 1997, 18(02): 164–167. [142] Zhou D, She X, Song G, The application of metal organic photosensitizers in dye sensitized solar cell. Precious Met, Zhou D, She X, Song G, The application of metal organic photosensitizers in dye sensitized solar cell. Precious Met, 2010, 31(1): 37–42. [143] Zhao W, Hou Y J, Wang X S, et al., Study on squarylium cyanine dyes for photoelectric conversion. Sol Energy Mater Sol Cells, 1999, 58(2): 173–183. [144] Zhao W, Zhang B, Cao Y, et al. Photoelectric conversion performance study of nanocrystalline TiO2 film electrodes modified with squarylium cyanine functional materials. J Funct Mater, 1999, 30(3): 304–306. [145] Shen Y C, Deng H H, Fang J H, et al., Co-sensitization of microporous TiO2 electrodes with dye molecules and quantum-sized semiconductor particles. Colloids Surf A, 2000, 175(1–2): 135–140. [146] Deng H-H, Mao H-F, Shen Y-C, et al Cosensitization of a nanostructured TiO2 Electrode with tetrasulfonated porphyrins and phthalocyanine. Acta Chim Sin, 1999, 57: 1199–1205. [147] Mao H-F, Tian H-J, Zhou Q-F, et al. The effect of co-adsorption on photovoltaic properties of porphyrin, phthalocyanine/TiO2 electrode. Chem J Chin Universities, 1997, 18(2): 268–272. [148] He J J, Lindstrom H, Hagfeldt A, et al., Dye-sensitized nanostructured p-type nickel oxide film as a photocathode for a solar cell. J Phys Chem B, 1999, 103(42): 8940–8943. [149] Pellegrin Y, Le Pleux L, Blart E, et al., Ruthenium polypyridine complexes as sensitizers in nio based p-type dye-sensitized solar cells: Effects of the anchoring groups. J Photochem Photobiol A-Chem, 2011, 219(2–3): 235–242. [150] Yen Y-S, Chen W-T, Hsu C-Y, et al., Arylamine-based dyes for p-type dye-sensitized solar cells. Org Lett, 2011, 13(18): 4930–4933. [151] Nattestad A, Mozer A J, Fischer M K R, et al., Highly efficient photocathodes for dye-sensitized tandem solar cells. Nat Mater, 2010, 9(1): 31–35. [152] Qin P, Linder M, Brinck T, et al., High incident photon-to-current conversion efficiency of p-type dye-sensitized solar cells based on nio and organic chromophores. Adv Mater, 2009, 21(29): 2993–2996. [153] Qin P, Wiberg J, Gibson E A, et al., Synthesis and mechanistic studies of organic chromophores with different energy levels for p-type dye-sensitized solar cells. J Phys Chem C, 2010, 114(10): 4738–4748. [154] Qin P, Zhu H, Edvinsson T, et al., Design of an organic chromophore for p-type dye-sensitized solar cells. J Am Chem Soc, 2008, 130(27): 8570–8571. [155] Bavykin D V, Friedrich J M, Walsh F C, Protonated titanates and TiO2 nanostructured materials: Synthesis, properties, and applications. Adv Mater, 2006, 18(21): 2807–2824. [156] Morandeira A, Boschloo G, Hagfeldt A, et al., Photoinduced ultrafast dynamics of comnarin 343 sensitized p-type-nanostructured nio films. J Phys Chem B, 2005, 109(41): 19403–19410. [157] Nakasa A, Usami H, Sumikura S, et al., A high voltage dye-sensitized solar cell using a nanoporous nio photocathode. Chem Lett, 2005, 34(4): 500–501.

Xu Pan, Songyuan Dai

Chapter 4 Electrolyte used in dye-sensitized solar cells An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent. The electrolyte is one of crucial components in DSCs and responsible for the charge transport between electrodes. The role of electrolytes in DSCs is achieved by the following processes. (i) Electron exchange reaction at the electrode/electrolyte interface. And the reaction rate can directly affect the fill factor (FF) and short-circuit current density (Jsc) of DSCs [1]. (ii) The recombination between the electrons in the conduction band of TiO2 with I3– in electrolytes. The reaction rate significantly affects the open-circuit voltage (Voc) of DSCs. (iii) The charge transfer of redox couple in electrolytes, which the charge transfer of redox couple is the main factor for determining the photovoltaic performance of DSCs [2, 3] and directly affects the interfacial electron exchange reactions associated with electrolytes. The effect of each component on the reactions on the platinum (Pt)-electrode–electrolyte interface was investigated in details [4, 5]. It was found that the faster the charge transfer of redox couple in the electrolyte, the faster the electron exchange reaction at the electrode–electrolyte interface, the more conducive to improve the photovoltaic performance of DSCs. Huo et al. reported that in the electrolytes with I– and I3– as the redox couple, the fast I3– transfer could slow down the recombination at the dye-coated TiO2 electrode/electrolyte interface, which enhances the Voc and in turn improves the photovoltaic performance of DSCs [6, 7]. In summary, the charge transfer of redox couple in electrolytes makes a significant effect on the parallel resistance, further on the photovoltaic performance of DSCs. The charge transfer of redox couple in electrolytes is mainly actualized by concentration-driven diffusion. The diffusion coefficient is an important parameter to evaluate the diffusion rate. A small diffusion coefficient would reduce the diffusion flux, which makes a direct influence on the Jsc [3]. Meanwhile, the redox couple diffusion in electrolytes can also produce a diffusion impedance, being the series resistance as a partial internal resistance in DSCs. The diffusion impedance is dominated by the diffusion coefficient and the concentration of the diffuse species [8]. Han and his collaborators studied the diffusion impedance in electrolytes by electrochemical impedance spectroscopy [9]. It was found that the resistance of electrolytes could be reduced to 0.7 Ω cm2 theoretically and the diffusion resistance decreased gradually with the decrease of the distance between electrodes.

Xu Pan, Hefei Institutes of Physical Science, Chinese Academy of Sciences Songyuan Dai, North China Electric Power University https://doi.org/10.1515/9783110344363-004

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However, in fact, the actual diffusion impedance was much larger than that in theory. For example, when the distance between electrodes was 20 μm, the diffusion resistance was usually 2 Ω cm2. In summary, the charge transfer of redox couple in electrolytes is an important factor to determine the performance of DSCs. The faster the charge transfer of the redox couple in electrolytes, the smaller the series resistance and the larger the parallel resistance in DSCs, the better the photovoltaic performance of DSCs.

4.1 Classification of electrolytes An electrolyte typically consists of a reversible redox couple (such as the commonly used I3– couple), a solvent that is used to dissolve redox couples, and additives. Electrolytes can be categorized as liquid electrolytes, quasi-solid electrolytes and solid-state electrolytes according to their physical states.

4.1.1 Organic solvent-based electrolytes Several aspects are essential for the electrolytes used in DSCs: (i) The electrolytes must have long-term chemical stability. (ii) The electrolytes must possess low viscosity to guarantee fast transport of redox couples. (iii) The electrolytes must be able to dissolve redox pairs as well as additives and at the same time not cause desorption and degradation of the dye sensitizer. This puts forward a high requirement for the solvent in electrolyte. At present, the most commonly used organic solvents are nitriles with high polarity, such as acetonitrile (ACN), methoxypropionitrile (MPN) and esters such as vinyl carbonate (EC), propylene carbonate (PC) and γ-butyrolactone. Compared to water, these organic solvents could inflitrate the electrodes and do not participate in the interface reactions. They have a wide electrochemical window so that dye desorption and degradation would not occur. Their freezing point is low enough to be adaptable in a wide temperature range. In addition, these solvents have a comparatively high dielectric constant and low viscosity so that inorganic salts are fully dissolved and dissociated and that the solution has a high conductivity. Grätzel’s group made a breakthrough in organic solvent-based electrolytes in 1991. The team at École Polytechnique Fédérale de Lausanne in Switzerland and Sharp group in Japan reported DSCs with organic solvent-based electrolytes to achieve efficiencies up to 11.18% in 2005 and 11.1% in 2006. Table 4.1 lists some organic solvents commonly used as electrolytes in DSCs and their physical parameters. ACN is usually used as a solvent tends to achieve high photoelectric conversion efficiency of DSCs, while 3-MPN is used as a solvent for obtain a long-term stability of DSCs.

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In addition, different organic solvents usually have a significant effect on the photovoltaic performance of DSCs. Arakawa group studied the effects of various organic solvent electrolytes including DMSO, DMF, nitriles, alcohols, and other organic solvents using iodide/triiodide redox electrolyte on the photovoltaic performance of DSCs [10]. The results show that the organic solvents with a strong electron–donor ability would affect the conduction band energy level of TiO2 and enhance the Voc of DSCs, but usually reduce the Jsc. Other organic solvents containing N atom, such as N-methyl pyrrolidone, pyridine, and so on, would lead to an increased Voc and a reduced Jsc of DSCs which is similar to the behaviors of N-heterocyclic additives in electrolytes. Table 4.1: Properties of common organic solvent-based electrolytes. Solvent

Water Ethyl alcohol Acetonitrile Propionitrile Valeronitrile Glutaronitrile Methoxyacetonitrile -Methoxypropanitrile γ-Butyrolactone Propylene carbonate

mp/bp (°C)a / −/ −/ −/ −/ −/ / −/ −/ −/

εrb   .    –   

Viscosity (mPa⋅s)c

DI dð106 cm2 · s1 Þ

. . . . . . – . . .

. – . – – – . – . –

3

Remarks

For efficient experiments

For stability testing

a The melting point/boiling point at 1 atm; b the relative dipole constant; c the relative dipole constant of pure solvent at 25 °C; d the apparent diffusion coefficient of I3–; f 1-methyl-3propylimidazolium iodide.

Although organic solvent electrolytes show remarkable advantages, there are some drawbacks to limit their further application. Organic solvents usually own a relatively high vapor pressure, which causes evaporation and leakage, and in turn brings the difficulties in sealing and packaging of DSCs. In addition, the organic solvent usually has poor thermal and photostability.

4.1.2 Ionic liquid electrolytes 4.1.2.1 Advantages of ionic liquids Ionic liquid is a novel green solvent developed in recent years. Compared to the conventional organic solvents, ionic liquids have many advantages. The most important feature of ionic liquids is that their properties can be modified by designing

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the structure of anions and cations. So, ionic liquids are also known as “designer’s solvents” [11]. They are called functionalized ionic liquids to be designed to satisfy the specific requirements. These properties cover the physical properties (e.g., mobility, conductivity, and liquid range) and the chemical properties (polarity, acidity, chirality, coordination capacity, and solubility). Introducing various functional groups into ionic liquids to realize the functionalization of ionic liquids is the forefront of the research on ionic liquids. In addition to being designed easily, ionic liquids have a number of outstanding advantages over the conventional solvents. (i) At room temperature, the density of ionic liquid is generally in the range of 1.1–1.6 g cm–3 [3] and the viscosity is between 50 and 200 cP, which is several to dozens of times to that of water. It can flow and conduct heat. (ii) Ionic liquids are characteristic of nonvolatility with negligible vapor pressure, nonflammability, excellent thermal stability, and low toxicity so that they can be used in the high-vacuum system and are green solvent with no solvent loss or environmental pollution. (iii) Ionic liquids are highly soluble. They can dissolve various organics, inorganics, and metal complexes, except for polyethylene, polytetrafluoroethylene, glass, and so on. This makes ionic liquids ideal alternatives in reactions solvents, avoiding the use of a variety of solvents. Furthermore, the solubility to inorganics, water, organics, and polymer could be adjusted by the rational design of anions and cations in ionic liquids. In addition, ionic liquids are insoluble with some organic solvents, which provides a two-phase non-water system with adjustable polarity. Meanwhile, the hydrophobic ionic liquids could also be used as the noneutectic polar phases of water. (iv) Ionic liquids have a wide range of stable temperature. Even up to 200 °C, it still has a very good thermal stability. Most ionic liquids can remain liquid at 300 °C, which is conducive to dynamically control. (v) Ionic liquids have a broad electrochemical stability window (also called an electrochemical stability potential window) and a high ionic conductivity. An electrochemical stability window refers to the difference between the oxidation potential and reduction potential of a substance. The electrochemical stability window of most ionic liquids is about 4 V, which is much broader than that of conventional solvents. For example, the electrochemical stability window of water is 0.4 V under an alkaline condition and 1.3 V under an acidic condition. In addition, the ionic conductivity is usually high with the conductivity at the order of 10−2–10−1 S·m−1, which makes them have wide applications in electrochemistry. (vi) Ionic liquids can be acidic, basic and neutral and the pH of ionic liquid can be manipulated by adding Lewis acid or base. For example, when Lewis acid such as AlCl3 was added into 1-butyl-3-methylimidazolium tetrachloroferrate, three different cases would occurs: (i) the ionic liquid is basic if the mole fraction of AlCl3 is less than 0.5; (ii) it is neutral if the mole fraction of AlCl3 is

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equal to 0.5; and (iii) it exhibits as a strong acidity if the mole fraction of AlCl3 is greater than 0.5. Moreover, it has been reported that the ionic liquid exhibits a “latent acidity [12, 13]” and a “super acidity” [14, 15]. For example, if the weak base pyrrole or N,N′-dimethylaniline was added into 1-butyl-3-methylimidazolium tetrachloroaluminate ([BMI]AlCl4), the ionic liquid exhibits a strong “latent acidity.” The ionic liquid exhibits a strong “super acidity” if an inorganic acid is added into the above acidic ionic liquid. (vii) It is easy to be separated from other chemicals and can be recycled. (viii) The preparation process is relatively simple. For example, to synthesize ionic liquids [BMI]Cl–AlCl3, the intermediate product can be synthesized directly from commercial methylimidazole and halogenated alkanes and consequently reacts with inorganic salts containing the target anion. (ix) Ionic liquids exhibit a weak tendency to be coordinated to other ions [16].

4.1.2.2 Classification of ionic liquids A wide variety of ionic liquids can be designed to synthesize ionic liquids by changing the different combinations of cations and anions. Based on the cations, ionic liquids at room temperature can be divided into imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphorus salts, pyrrole salts, thiazole salts, triazole salts, benzotriazole salts, guanidiniums, matte salts, tetrahydrothiophene salts, heterocyclic aromatic compounds, and the derivatives of natural products. The most common cations are 1,3-dialkylimidazole cations (also called as N,N′-dialkylimidazole cations) [R1R+ + 2Im] . Other common cations include alkyl quaternary ammonium cations [NRxH4-x] , + + alkyl quaternary phosphorous cations [PRxH4-x] , N-alkyl pyridine cations [RPy] , and alkyl matte cations [R1R2S]+ that has been widely developed in recent years. Based on the anions, ionic liquids can be classified into two groups. One is aluminochloric acid [AlCl3(AlBr3)] system with adjustable compositions. For example, 1-butyl-3-methylimidazolium chloride [BMI]Cl–AlCl3 contains several different ions and their properties including melting point depend on their compositions. This group of ionic liquids has been studied early and the chemical reactions with them as solvents have also been investigated. Despite many advantages, these ionic liquids are extremely sensitive to water, easy to decompose, and unstable in air. Thus, they should be kept or treated in a vacuum or in an inert atmosphere. Protons and oxides’ impurities also have a significant effect on the chemical reactions in these ionic liquids. In addition, AlCl3 would release HCl when exposed to water, which could exert skin irritation. For the above reasons, these kinds of ionic liquids are seldom used in DSCs. The other group of ionic liquid owns a fixed composition of anions and the cations are mainly heterocyclic or substituted alkyl containing N, P, and S elements. They are stable in water and air and have currently become the mainstream research. The common anions are X– (Cl–, Br–, I–), BF4–, PF6–, CF3SO3–,

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C4F9SO3–, CF3COO–, C3F7COO–, (CF3SO2)2N–, (CF3SO2)3C–, (C2F5SO2)3C–, NO3–, SbF6–, AsF6–, Sac–, CB11H12–, and their substituents. In recent years, a number of novel structure anions have been explored such as the ionic liquids with double cationic structures synthesized by Shreeve’s group [17] and a series of novel anions based on cyanide structures developed by Yoshida’s research group [18].

4.1.2.3 Synthesis of ionic liquids The synthesis of ionic liquids includes conventional synthesis and new technology synthesis. 4.1.2.3.1 Conventional synthesis The conventional synthesis of ionic liquids employs common chemical preparation techniques such as heating, stirring, reflux, extraction, vacuum distillation, and so on. There are two basic methods for the conventional synthesis of ionic liquids: direct synthesis and two-step synthesis. For the direct synthesis, the ionic liquid is synthesized in one step via a nucleophilic addition reaction of a nucleophile of the target compound with a haloalkane or ester. As an example, halogenated imidazolium salts and pyridine salts of commercial value were synthesized by this method [19, 20]. The operation is economical and simple and there are few by-products. The product is easy to be purified. The solvents used in the reaction are usually 1,1,1-trichloroethane, ethyl acetate, toluene, and so on. Features these solvents all have in common are that the reactant and the solvent are mutually soluble but the product and the solvent are not mutually soluble and that the density of the product is higher than that of the solvent so the product can be separated by liquid separation. Generally, recrystallization is employed for the purification of the ionic liquid in solid at room temperature. For the ionic liquid in a liquid state at room temperature, repeated washing with solvents such as 1,1,1-trichloroethane and ethyl acetate is used to remove organic impurities from the product followed by vacuum drying to remove water and residual solvents. An acid– base neutralization reaction is also commonly used for one-step preparation of ionic liquids. For instance, the nitrate ionic liquid of monoalkylamine can be produced from the neutralization reaction of an aqueous solution of amine and nitric acid. The preparation is simple and halide-free, involving several steps such as (i) the acid– base neutralization reaction proceeding until the solution is neutral, (ii) vacuum drying to remove excess water or by-product in the system, (iii) the product then being dissolved in ACN or tetrahydrofuran and other organic solvents followed by being treated with activated carbon, (iv) vacuum drying to remove organic solvents to obtain ionic liquids. Hirao et al. [21] synthesized a series of tetrafluoroborate ionic

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liquids with different cations by this method. In addition, various ionic liquids, such as 1-butyl-3-methylimidazolium salt, can be prepared by a quaternization reaction in one step [22]. Similar ionic liquids can be obtained by neutralizing imidazole derivatives with different acids [23], and the corresponding reactions include the preparation of tetraalkyl ammonium sulfonate ionic liquids by an equimolar mixture of sulfonic acid and tetraalkyl ammonium hydroxide [24]. Ohno [25] used this method to prepare 20 aminoacid ionic liquids based on imidazole cations. In addition, a new technique was reported to synthesize ionic liquids by imidazole derivatives being directly methylated. The obtained ionic liquid is not contaminated by halogen ions and exhibits good stability (heat resistance up to 400°C) [26]. As for the two-step approach, a halogen salt containing a target cation is first prepared by a quaternization reaction (quaternary matte, quaternary phosphating, etc.) between a compound containing a target cation and a haloalkane. In the second step, the halogen ion is replaced with other anions or the anion exchange occurs by adding a Lewis acid to obtain a target ionic liquid. In general, the first step takes a comparatively long time (48–72 h), which needs to be carried out in organic solvents (e.g., acetone) under the protection of nitrogen or argon. The involved haloalkane usually being alkyl iodide and alkyl bromide. The product is normally washed by ethyl acetate or ether to remove the excess target cationic compound and haloalkane and then followed by vacuum distillation to obtain an alkyl imidazole halide. The main contaminants involved in this step include the unreacted raw materials, solvents, and ester or ether waste containing a small number of products after washing. Appropriate reaction conditions and synthesis methods would efficiently improve the product yield and reduce the amount of organic solvents used for the reaction. For example, Souza et al. [27] synthesized the ionic liquid mixture of MMIBF4, BBIBF4, and MIBF4 from n-butyl amine, formaldehyde, glyoxal, and tetrafluoroboric acid in one step. The molar ratio of the three ionic liquids in the product was 1:4:5, and they did not employ the commonly used raw material, methimidazole. This avoids the condensation of glyoxal and is not valuable for popularizing in the synthesis of other ionic liquids. Holbrey et al. [28] synthesized a series of ionic liquids containing functional hydroxyl groups from acid, methylimidazole, and propylene oxide. The synthesis was completed in one step although the reaction involved using highly explosive propylene oxide. And the reaction time was shortened significantly to create economic value. Shi et al. [29, 30] prepared high-purity 1-methyl-3-propylimidazolium iodide (MPII) and 1,2-dimethyl-3-propylimidazolium iodide (DMPII) with a high yield by autoclave synthesis without using any solvent, indicating a wide application prospect. In the second step, there are many ways to replace halogen ions with target anions, such as metathesis reaction, ion exchange reaction, and electrolysis.

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(1) Metathesis reaction Metathesis reaction is the most commonly used method in the synthesis of ionic liquids. It is important to guarantee the completion of the reaction through certain methods, such as the formation of new liquid phase, precipitation, gas or changing the reaction solvent to precipitate one of the metathesis products. The salt (MY) used in the metathesis reaction is usually AgY, NH4Y, and NaY, where Na can also be substituted by K or Li and Y is usually BF4– and PF6–. The metathesis reaction proceeds quickly when AgY or NH4Y is applied as the anion source of the ionic liquid, accompanied by the generation of AgX precipitation or gases like NH3 and HX that can be easily removed, thus leading to relatively easy purification of the ionic liquid. Bonhöte et al. [22] prepared 1,3-dimethylimidazole trifluoroacetic acid ionic liquids from silver salts. This reaction proceeds with a high purity and good yield, but silver salts are very expensive. Considering the synthesis cost and the purification of ionic liquids, the most suitable salts are NH4Y and NaY. Fuller et al. [31] employed an ammonium salt to prepare 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) by metathesis reaction in ACN solution. The metal salt (NaY) is relatively desirable for the metathesis reaction to synthesize hydrophobic ionic liquids, such as the preparation of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6). However, for the NaY-based metathesis reaction, it takes a long time to completely replace the halogen ions, and the residual halogen ions are difficult to be removed. Singer compared the residual halogen ion content in the synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) by ICP-MS [32]. The results showed that under the comparable experimental conditions, the residual halogen ion content was the lowest (0.007%) by the substitution reaction of BMI–Cl with AgBF4 and HBF4, while the content of residue halogen ions was 0.45% when substituted with NH4BF4. Seddon et al. [33] reported that halogen ions have an adverse effect on the physical properties and catalytic ability of ionic liquids. Therefore, it has been an inevitable trend to explore new techniques to synthesize ionic liquids. Roger’s group [34] reported that the ionic liquids containing alkyl sulfate anions were prepared by the reaction of 1-alkylimidazoles with dimethyl sulfate and diethyl sulfate. This method possesses several features such as instant reaction, without heating, ease of preparation, and low cost of chemicals. The obtained ionic liquids own a relatively low melting point and exhibit high thermal stability. In addition, they can also be used to react with other acids to prepare other ionic liquids. These ionic liquids are halide-free but dialkyl sulfuric acid is relatively toxic. Dialkyl carbonic acid is also an ideal alkylation reagent [35]. The salt formed from dialkyl carbonic acid and imidazole can react with other acids to prepare new ionic liquids with the by-product of alcohols and CO2. This is halide-free and environment friendly, providing an ideal technique to synthesize ionic liquids. If strong protonic acid HY is used as the anion source, the reaction needs to be conducted at a low temperature with stirring. After the reaction, repeated washing should be applied until the system becomes neutral, followed by extracting with an

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organic solvent, and then vacuum drying to remove organic solvents to obtain pure ionic liquids. (2) Ion exchange [36] The ion-exchange reaction occurs when a precursor aqueous solution containing the target cation passes through an ion-exchange resin containing the target anion to obtain the aqueous solution of the target products. The evaporation is then employed to remove water molecules to gain the final products. For the synthesis of high-purity binary ionic liquids, this process usually proceeds in ion exchangers. (3) Electrolysis Here, the chloride precursor aqueous solution containing the target cation is electrolyzed directly to produce chlorine gas and hydroxide containing the target cation. The latter is then neutralized with an acid containing the target anion to obtain an aqueous solution of the target product. The water is then evaporated to obtain the pure ionic liquid. This method is not widely applied currently [37]. Two-step synthesis is currently a dominant method of ionic liquid synthesis. Those have been synthesized successively by this method including sulfocyanic acids, tris(trifluoromethylsulfonate) methylates, trifluoroacetic acids, hexafluorobutyric acid, octyl sulfuric acids, melamine, carboborane, dicyandiamide, perfluoroalkyl boron trifluoride, p-toluene sulfonic acid, tetraalkyl boron, and imidazole-based ionic liquids. This not only lays a solid foundation for the development of ionic liquids, but also provides the essential conditions for the synthesis of functionalized ionic liquids. In addition to the above synthesis methods, some other methods have been reported. Zimmermann group [38] reported that functionalized ionic liquids can be prepared by Michael-type reaction. The first step is to synthesize protonic salts from a base and an acid. The second step is to react with unsaturated compounds by the catalysis of weak bases to prepare functionalized ionic liquids with carbonyl groups. Dickenson et al. [39] found that fructose, a natural product, can be used to synthesize ionic liquids with a special solubility. Its trifluoromethyl sulfonimide salt is insoluble in water and methanol, but dissolves better in ether compared with imidazole-based ionic liquids. 4.1.2.3.2 Microwave and ultrasound-assisted synthesis Microwave and ultrasound as new techniques have widely been used in chemical synthesis in recent years and have also been employed in the synthesis of ionic liquids. Seddon et al. [40] applied microwave technology to the synthesis of halogenated salts without solvents, where there was no reactant surplus and the reaction was completed in only 1 h. This method accelerated the reaction rate and conversion ratio, shortened the reaction time, and improved the purity of ionic liquids, so it is applicable to large-scale production. Varma et al. [41] investigated the synthesis of RMI–X from alkyl halide and methylimidazole in a home hold inverter microwave oven with a polytetrafluoroethylene autoclave as the reactor. The results

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showed that the obtained RMI–X accelerated microwave absorption and that the reaction in microwave field could be shortened to a few minutes. The synthesis yield was equal to or higher than that in an oil bath at 80 °C. Afterward, it was reported that RMI–BF4 synthesized from RMI–X and NH4BF4 in solvent-free condition could be completed within a few minutes and the yield was significantly improved (>90%) [42]. Khadilkar et al. [43] studied the reactions of an alkyl halide with methylimidazole, pyridine, and 2,6-dimethyl pyridine in a modified household microwave oven with a reflux unit and a commercial microwave reaction system. It was found that the reaction with imidazoles was completed within several minutes and the reaction with pyridine took approximately 1 h. The microwave-assisted method significantly shortens the reaction time, but side reactions occur easily. Leadbeater et al. [44] investigated the formation of ROH from RMI–X and found that RMI–X could be easily decomposed into RIm+ and MeX under a long-term microwave irradiation. The microwave-assisted synthesis of ionic liquids is solvent-free and with short time consumption (several hours and even several minutes), but several disadvantages accompany such as being difficult to control, side reactions, and expensive reactors. It is only limited to the smallscale laboratory synthesis and is difficult for the large-scale industrial production. In addition to the microwave-assisted synthesis, ultrasonic-assisted technology has also been used for the synthesis of ionic liquids. Leveque et al. [45] discovered that ultrasound-assisted technology could also shorten the reaction time in the synthesis of ionic liquids but not at any price of reduced yield. Currently, the reported synthesis of ionic liquids by microwave and ultrasonic technology focuses on the common ionic liquids containing alkylimidazole and alkyl pyridines. The synthesis of other types of ionic liquids or the reaction involving special raw materials (e.g., flammable and explosive epoxides) has not been reported yet because of safety concerns. At present, the research on the synthesis of ionic liquids mainly focuses on the design of different anion–cation combinations to improve the properties of ionic liquids. The modification of synthesis process has not been paid enough attention to. This is mainly due to the fact that the small-scale preparation of common ionic liquids has been well developed and many companies (such as Merck and Solvent Innovation) are able to provide a small amount of commercial ionic liquids with a reduced price. The scientific research has currently paid more attention to the application of ionic liquids. The green and large-scale preparation of ionic liquids is closely related to its application prospects. Exploring new and green techniques to prepare ionic liquids will become crucial for the breakthrough in the application field. Researches should consider the synthesis-related problems for ionic liquids in order to find green techniques.

4.1.2.4 Application of ionic liquids in DSCs In DSCs, ionic liquids used as electrolytes can effectively improve the stability of DSCs and prevent electrolyte leakage in the package and transport process, which is

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of great significance to the industrial application of DSCs. The common ionic liquids as electrolytes in DSCs are based on imidazolium, pyridylium, guanidiniums, and so on. The structure of common anions and cations is shown in Figure 4.1. Among these ionic liquids, imidazole-based ionic liquids are the most widely used in DSCs:

Figure 4.1: Structures of common ionic liquids in DSCs.

In order to reduce the negative effect of organic solvent electrolyte on DSCs, Grätzel introduced ionic liquid 1-hexyl-3-methylimidazolium iodide (HMII) into the electrolytes in DSCs for the first time in 1996. However, the photoelectric conversion efficiency (PCE) of DSCs was low at that time due to the influence of high viscosity of HMII. In 2003, Wang et al. [46] used 1-propyl-3-methylimidazolium iodine with relatively low viscosity as the solvent and introduced I2, LiI, and N-methyl benzimidazole (NMBI) as electrolytes to obtain a DSC with a PCE as high as 6%. However, ionic liquids with electrically active I– as anions usually own a large viscosity, the result of which is a low mass transfer rate. This is the main factor to restrict the photovoltaic performance of DSCs. Therefore, it is necessary to change the structure of anions and explore other types of ionic liquids with low viscosity. Available experimental results show that introducing anions with a relatively large volume, such as SCN–, N(CN)2–, C(CN)3–, B(CN)4–, TFSI–, and so on, can significantly reduce the viscosity of ionic liquids. Since I– is indispensable in DSCs, when the viscosity of ionic liquids is reduced by changing anions, ionic liquids based on iodized salts

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Xu Pan, Songyuan Dai

should be introduced to prepare binary or mixed ionic liquids for electrolytes in DSCs. Figure 4.2 and Table 4.2 lists the structure of ionic liquids and the corresponding diffusion coefficient of I3– investigated in available literature (Table 4.2).

N

I

+

N

+

N

R 1-methyl-3-alkylimidazolium iodide

N

N

mPa s

+

N

N

+

N

N

– N(CN)2

MPIC(CN)3

18 mPa s

18 mPa s

NCS +

N

–

N

+

N

N

C2H5

EMIDCN

EMINCS

EMIBF4

20 mPa s

21 mPa s

21 mPa s

43 mPa s

+

+

N

– B(CN)4

+

N

– PF6 N

C4H9

– BF4 C2H5

MPIB(CN)4

– NH(CF3SO3)2 N N C2H5

C(CN)3 C3H7

C2H5

C3H7

HN

N

MPIDCN

C3, C4, C5, C6, C7, C8, C9 865, 963, 1362, ..................mPa s – B(CN)4

N(CN)2 C3H7

R = C3-C9 alkyl

+

–

–

–

+

– NH(CF3SO3)2

C4H9

EMITFSI

EMIDCN

BMIPF4

39 mPa s

20 mPa s

352 mPa s

BPITFSI 72 mPa s

Figure 4.2: Structure and viscosity of some ionic liquids.

Table 4.2: Diffusion coefficients of I– and I3– in ionic liquid electrolytes. Electrolyte composition . mol · L− I, . mol · L− NMBI in PMII . mol · L− I, . mol · L− NMBI in PMII  mmol · L− I, HMII  mmol · L− I, % HMII, % EMITFSI HMII + I (:) BMII + I (:) PMII + I (:) DMII + EMII + AMII + I (:::) . mol · L− I, . mol · L− GNCS, . mol · L− TBP, PMII + EMINCS (: v/v) . mol · L− I, . mol · L− GNCS, . mol · L− NMBI, PMII + EMINCS (: v/v)

D(I–)/ D(I–)/ References –  − ( cm ⋅s ) (– cm⋅s−) . . . . . . . . .

. . . – – – – – –

[] [] [] [] [] [] [] [] []

.

–

[]

Chapter 4 Electrolyte used in dye-sensitized solar cells

217

Table 4.2 (continued) Electrolyte composition . mol · L− I, PMII + EMITFSI + EMITfo (::) . mol · L− I, . mol · L− NMBI, PMII + EMIDCN (: v/v) . mol · L− I, . mol · L− LiI, . mol · L− NMBI, PMII + EMIDCN (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) . mol · L− I, . mol · L− NMBI, . mol · L− GNCS, PMII + EMITCB (: v/v) PMII + I (:) DMII + EMII + AMII + I (:::) PMII + EMITCB + I (::.) DMII + EMII + EMITCB + I (:::.) . mol · L− I, . mol · L− DMHII in EMITFSI . mol · L− I, . mol · L− DMHII in EMIF.HF . mol · L− I, . mol · L− NMBI, PMII + EMITCM (: v/v) . mol · L− I, . mol · L− NMBI, PMII + EMITCM (: v/v) . mol · L− I, . mol · L− NMBI, PMII + EMITCM (: v/v) TI/TDCA/I (::) TI/TTCM/I (::) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIDCA/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII) . mol · L− I, EMIBF/PMII ( mol% of PMII)

D(I–)/ D(I–)/ References –  − ( cm ⋅s ) (– cm⋅s−) . .

. –

[] []

–

[]

.

.

[]

.

.

[]

.

.

[]

.

.

[]

.

.

[]

.

.

[]

. . . . .  

– – – – – – .

[] [] [] [] [] [] []

–

.

[]

–

.

[]

– – – – – – – – – – – – – –

[] [] [] [] [] [] [] [] [] [] [] [] [] []

.

. . . . . . . . . . . . . .

218

Xu Pan, Songyuan Dai

In 2003, Wang et al. [53] prepared a binary ionic liquid mixture with relatively low viscosity by mixing EMImN(CN)2 with MPII. When the obtained ionic liquids were applied to DSCs, the PCE reached 6.6%. Further results show that the photostability of N(CN)2– based ionic liquids is poor and not suitable for electrolytes in DSCs. In 2004, Wang et al. [50] introduced the I– donor, MPII, into the low-viscosity EMImSCN ionic liquid to prepare a binary ionic liquid mixture with low viscosity and applied it into DSCs combined with Z907 as a sensitizer. The obtained DSC exhibited a PCE of 7% under AM1.5 irradiation. The viscosity of binary ionic liquid mixture is 21 cP. The apparent diffusion coefficient of I3– is 2.95 × 10−7 cm2 s−1, which is obviously higher than that of I3– in pure MPII. In 2005, Wang developed a low-viscosity ionic liquid, 1-ethyl-3-methylimidazolium dicyanomethyl salt (EMImTCM) (18 cP, 22 °C). The diffusion coefficient of I3– reached 7.37 × 10−7 cm2 · s−1 when the content of MPII is 20% and the corresponding PCE reached 7.4% when EMImTCM was used as electrolytes in DSCs. However, the thermal stability of SCN– and TCM– was relatively poor. In 2006, Kuang et al. [55] developed 1-ethyl-3-methylimidazolium tetracyanoborate (EMImB(CN)4) (19.8 cP, 20 °C) and used it into DSCs combined with MPII, sensitizer Z-907Na and co-adsorbent PPA to gain a DSC with a PCE as high as 6.4% and with an ideal thermal stability. The apparent diffusion coefficients of I3– and I– in the EMImB(CN)4/MPII system are 3.42 × 10−7 cm2 · s−1 and 4.08 × 10−7 cm2 · s−1, respectively. In addition to the binary ionic liquids mentioned above, Fei et al. also explored a series of imidazole-based supercooled ionic liquids with different substituents and mixed them with MPII to prepare a binary ionic liquid [61]. Combined with the K60 dye, the obtained DSC presented a PCE of 6.8% under AM1.5 irradiation and the PCE reached 8% at 30 mW cm−2. In addition, the DSC showed good stability in the accelerated aging test at 60 °C. In addition to the binary ionic liquids, Bai et al. [56] recently reported a ternary imidazole-based ionic liquid system as electrolytes. They mixed three imidazolebased iodized salts that are solid at room temperature, DMII, AMII, and EMII, to obtain eutectic melts. Then, they added iodine, N-butyl benzimidazole (NBB) and guanidinium thiocyanate (GSCN) into the eutectic melts to obtain ionic liquidsbased electrolytes. With the Z907 dye, the PCE of the obtained DSC reached 7.1% and the PCE went up to 8.2% after replacing AMII by EMITCB with lower viscosity. Shi et al. [62] then employed the C103 dye instead of Z907, in the same electrolyte system based on ionic liquids (DMII/EMII/EMITCB/NBB/GNCS), to yield a DSC with a PCE of 8.5% (under AM 1.5 irradiation). This is the highest PCE record of DSCs based on ionic liquid as electrolytes at that time. Till now, a variety of ionic liquids have been developed to be used as electrolytes in DSCs. Researchers then have begun to investigate how the structure makes effects on the properties of ionic liquids and consequently on the photovoltaic performance of DSCs. Kubo et al. [63] introduced the ionic liquids, 1-methyl-3-alkylimidazolium iodized salts with different lengths of alkyl chains, into DSCs. It was found that ionic liquids with hexyl alkyl chains (HMII) were better for the performance of DSCs than those with propyl alkyl chains (MPII). Kawano et al. [64] compared the physical

Chapter 4 Electrolyte used in dye-sensitized solar cells

219

properties of EMI+-based ionic liquid electrolytes with different anions including TFSI–, BF4–, PF6–, and DCA– and investigated the effects on the performance of DSCs. It was found that DCA– played a special role in DSCs and significantly improved the Voc of DSCs. Dai et al. [65] made similar experiments and prepared DSCs with the ionic liquid EMITCM, EMIDCN, and EMISCN as electrolytes, respectively. It was found that the DSC with EMITCM that owns the lowest viscosity as electrolyte exhibited the highest Jsc and PCE. Mazille introduced allyl and cyanide groups into imidazole ring and changed anions (N(CN)2– and TFSI–) to investigate the effect of different groups and anions on the properties of ionic liquids [66]. The results show that the substituents on the imidazole ring would cause the difference in the viscosity of the corresponding ionic liquids with an order of –CH2–CH2–CN >–CH = CH2 >–CH2–CH2–CH3. Different anions also bring the difference in the viscosity as follows: TFSI– > N(CN)2–. However, when these ionic liquids were used as electrolytes in DSCs, there is no significant change in the PCE of DSCs. In addition to imidazole-based ionic liquids, other ionic liquid electrolytes have been developed one after another, such as the ionic liquids based on alkylsulfonium, alkyl pyridinium, quaternary ammonium/quaternary phosphonium, guanidinium, and so on. Paulsson et al. [67] synthesized a series of trialkylsulfonium-based ionic liquids for DSCs. The DSC with (Bu2MeS)I-based ionic liquid electrolyte showed the best photovoltaic performance compared to the DSCs based on other ionic liquids under investigation. The PCE of the resultant DSC was 3.7% under one Sun illumination. Kawano et al. [64] developed an alkylpyridinium-based ionic liquid BPTFSI as the electrolyte in DSCs and the DSC showed a PCE of 2%. A series of ionic liquids based on quaternary ammonium iodide were prepared by Santa-Nokki et al. [68]. It was found that the DSC based on the quaternary ammonium salt containing n-hexyl group exhibited a higher PCE than that containing n-hexyl group. Cai et al. [69] explored low-melting pyrrole-based ionic liquids in conjunction with different anions. It was found that the viscosity value of the products varied with different anions in the order of I– < NO3– < SCN– < DCA–. The DSC with the DCA-based pyrrole ionic liquid as an electrolyte showed the best performance with an efficiency of 5.58%, compared to other DSCs. Wang et al. [70] employed guanidinium ionic liquids for DSCs, but the PCE was not desirable. Li et al. [71] developed a cyclic guanidinium-based ionic liquid, and the efficiency of the corresponding DSC reached 5.41%. Kunugi et al. [72] prepared DSCs using the quaternary phosphonium ionic liquids as the electrolyte to yield an optimum efficiency of only 1.2% at the standard light intensity. Recently, Xi et al. [59] developed a binary ionic liquid system containing a tetrahydrothiophene structure as the electrolyte in DSCs and achieved a nearly 7% PCE under AM 1.5 irradiation, which is comparable to the efficiency of DSCs based on traditional imidazole-based ionic liquids, indicating its great potential applications. All the ionic liquid electrolytes mentioned above employ I–/I3– as the redox couple, mainly because of the low price of I–/I3– couple and its high efficiency of electron transfer. In recent years, some researchers have also started to explore

220

Xu Pan, Songyuan Dai

novel redox couples. Some new electron transfer materials have been developed in succession such as cobalt (II/III) complex [73–75], 2,2,6,6-tetramethyl-1-piperidinyloxy [76] and liquid hole-transport materials (HTMs) [77]. These redox couples have shown good performance under low light intensity, but the corresponding efficiency of DSCs is less than 4% under the standard light intensity (AM1.5). Gorlov et al. [78] synthesized 14 types of interhalogen ionic salts in the form of [K+] XY2–, where there were three cations (1,3-dialkylimidazole cations and 1,2,3-trialkylimidazolium cations and N-alkyl pyridine ions) and two anions (IBr2– and I2Br–). Due to the existence of an equilibrium between different anions in solution, I–/IBr2– or I–/I2Br– was more flexible as a redox couple than I–/I3–. Five halogenated compounds PrMeI-IBr2, HexMeI-IBr2–, HexMeI-I2Br–, Me2BuI-IBr2, and BuPy–IBr2 were selected as components of the electrolytes in DSCs in the experiments. They also analyzed the effects of the different compositions with these compounds as electrolytes on the performance of DSCs. The results showed that the DSC based on ionic liquid HexMeI-IBr2– had the highest conversion efficiency (2.4%) under 100 mW cm−2 illumination. The efficiency decreased by 9–14% after 1,000 h of illumination at 35 mW cm−2. Oskam et al. [79] prepared DSCs using SCN–/(SCN)2 and SeCN–/(SeCN)2 as redox couples together with the N3 dye. However, they did not yield a good performance of DSCs. The reason is mainly that SCN– and SeCN– cannot regenerate oxidized dyes effectively. Wang et al. [80] employed (SeCN)3–/SeCN– as a redox couple and EMI-SeCN-based ionic liquid as an electrolyte to obtain a DSC with a PCE as high as 7.5%, which is comparable to that of the DSC based on I3– couple. However, it is difficult for this couple to replace I3– due to the rare reserves and high cost of selenium. The drawback of ionic liquid electrolytes is their excessively high viscosity, which prevents them from adequately permeating the titanium dioxide (TiO2) film. As a result, DSCs based on ionic liquid electrolytes are inevitably inferior to DSCs based on low-viscosity organic-solvent electrolytes in terms of PCE. The work mentioned above all focuses on lowering the viscosity of ionic liquid electrolytes. Instead, Yamanaka et al. [81] increased the diffusion coefficient of ionic liquids by creating a new diffusion path for I−/I3–, thereby improving the cell performance. Due to the presence of interlocked alkyl chains, 1-dodecyl-3-methylimidazole groups were able to self-assemble into a bilayer structure, forming an ionic liquid crystal (ILC) system with a smectic-A (SA) phase. The ionic conductivity was high in the horizontal direction of the SA phase. Therefore, I3− and I−could be positioned in the horizontal direction of the SA bilayer to form two-dimensional paths for electron transport (Figure 4.3). DSCs containing an electrolyte composed of the above ILC system and diatomic iodine (I2) were found to outperform DSCs based on a noncrystalline ionic liquid system in terms of both open-circuit voltage and current density. Yamanaka’s work also opened up a new path for the research on ionic liquids for DSCs. However, the high viscosity limits the physical diffusion of the ionic liquid in the ILC system along with the diffusion of I−/I3– based on the exchange reaction and transport.

I-

I-

I-

I3I-

I-

Im -

Im -

I-

I3-

I3-

I3-

I3-

I3-

II-

I-

I-

I3-

Im -

I3-

Im -

II-

I-

I-

I3-

I3-

I3-

I3-

II-

I-

I-

Im -

I3-

Im -

I3-

II-

I-

I-

I3-

Im -

I3-

Im -

II-

I-

I-

I3-

I3-

I3-

I3-

I-

I-

I-

I-

Im -

Im -

Im -

Im -

I-

I-

I-

I-

I3-

I3-

I3-

I3-

Im Im -

I-

I-

I-

I-

Im -

I-

I-

I-

Im -

I-

I3-

I3-

I3-

I3-

I-

I-

Chapter 4 Electrolyte used in dye-sensitized solar cells

Figure 4.3: Schematic diagram of electron-transport paths in ionic liquid crystal system.

221

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Xu Pan, Songyuan Dai

Therefore, the application of ILC systems in DSCs remains limited. Currently, the research on ionic liquid electrolytes for DSCs focuses primarily on the development of iodine-free, low-viscosity and low-cost ionic liquids and increasing the diffusion coefficient of I3− in electrolytes by reducing the viscosity of the ionic liquid to improve the photovoltaic performance of DSCs.

4.1.3 Quasi-solid electrolytes In 1991, Grätzel’s Group in Switzerland prepared the first DSC in the world using bipyridyl ruthenium (Ⅱ) complex as the dye and porous nano-TiO2 thin film to obtain a PCE of 7.1% [82]. In the next dozen years, worldwide scientists made great efforts and the efficiency of DSCs has been improved to 10–11% [83, 84], which is comparable to the efficiency of amorphous silicon solar cells. Such high efficiency greatly inspired researchers to carry on further investigations. However, it was found that the liquid electrolytes used in solar cells had a series of problems on the stability and application of the DSCs: (1) the liquid electrolyte may cause the dye molecules to detach from the TiO2 surface, which affects the stability of DSC; (2) the solvent in the liquid electrolyte is very volatile; (3) seal imperfections exist and the sealant might react with the electrolyte to cause new leaking, which shortens the cell lifetime; (4) the liquid electrolyte itself is unstable and prone to chemical reaction, which makes the cell failed; (5) the redox couple in the electrolyte is unstable under high-intensity illumination [85]. Researchers have proposed that solid or quasi-solid electrolytes could replace liquid electrolytes in DSCs to overcome these problems of liquid electrolytes as above. At present, solid-state electrolytes mainly include p-type semiconductors, conductive polymers and organic hole transport materials [[86–89]]. Although solid-state electrolytes overcome the shortcomings of liquid electrolytes, the PCE of solar cells is far from up to the application requirements and to be further investigated due to the low conductivity, poor wettability of the electrolyte/electrode interface and so on. Moreover, quasi-solid electrolytes have a higher conductivity and PCE, effectively preventing volatilization and leakage and prolong the lifetime of cells. A lot of research work has focused on the quasi-solid electrolytes. The mechanical properties of quasi-solid electrolytes are between that of liquid and solid electrolytes, with gelatinous appearance. The important strategy to prepare quasi-solid electrolytes is to add some substances to liquid electrolytes, such as organic small molecule gels, polymers, nanoparticles, and so on. These substances can achieve a three-dimensional network structure through a physical or chemical crosslinking among molecules in electrolytes to present a macrosolid and microliquid structure and to obtain quasi-solid electrolytes. According to the different gelation

Chapter 4 Electrolyte used in dye-sensitized solar cells

223

methods, quasi-solid electrolytes can be categorized into polymer gel electrolytes, organic small-molecule gel electrolytes, and quasi-solid electrolytes with nanoparticles.

4.1.3.1 Polymer gel electrolytes Polymers are the most commonly used gelators for preparing quasi-solid electrolytes, including high-molecular-weight polymers and low-molecular-weight polymers, each of which has its advantages and disadvantages. The spatial network structure formed by high-molecular-weight polymers is relatively stable and the mechanical strength is relatively good compared to that formed by low-molecular-weight polymers. However, the network structure of gel has an obvious obstruction effect on the charge transfer with a low electrical conductivity. In addition, the contact between electrolyte and TiO2 film is not good enough, resulting in the increase of impedance between the electrolyte and TiO2 film. The quasi-solid electrolyte formed by polymers of low molecular weight has a relatively high conductivity to achieve a good PCE of the resultant DSCs, although they possess relatively poor mechanical property. At present, the commonly used polymers include polyoxyethylene oxide (PEO, Figure 4.4) [90, 91], polyethylpyridine [92], polyacrylonitrile [93], poly(methyl methacrylate), vinylidene fluoride and hexafluoropropyl copolymer P(VDF-HFP) [94, 95], and so on. The copolymerization method of two types or multiple types of monomers is usually adopted in order to improve the conductivity and mechanical property of polymers. Polymer itself is a long-chain structure and would form a three-dimensional network structure in the quasi-solid electrolyte via molecular covalent bond, which is more stable than the structure formed by organic small-molecule gelators. Furthermore, quasi-solid electrolytes are usually thermally irreversible. There are two general methods of utilizing polymers to prepare the quasi-solid electrolytes. One is to add polymers into liquid electrolytes and then by melted by heating, obtaining quasi-solid electrolytes with a cross-linking network structure. Another is to prepare polymer films followed by absorbing liquid electrolytes to obtain quasi-solid electrolytes.

Figure 4.4: Structure of PEO inner-plasticized side-chain.

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Figure 4.5: Structure of polysiloxane polymer containing PEO inner-plasticized side chain and quaternary-ammonium-salt side chain.

Wang et al. [94] prepared a gel electrolyte containing MPII solidified by PVDF– cohexafluoropropylene(HFP) and the obtained DSC presented a PCE of 5.3% at a light intensity of 100 mW cm−2. Kim et al. [96] prepared a gel polymer electrolyte based on methyl methacrylate (MMA)–acrylonitrile (AN) co-polymer P(MMA–AN) and yielded a PCE of 2.4% at an light intensity of 100 mW cm−2. In 2004, Komiya et al. [97] in Japan introduced an oligomer whose structure is shown in Figure 4.6 into a quasi-solid electrolyte. A polymer film layer was deposited on the dye-coated electrode and then the electrode and the film were immersed in a liquid electrolyte. A short-circuit photocurrent density of 14.8 mA/cm2, an open-circuit voltage of 0.78 V, a FF of 0.70 and a PCE of 8.1% at a light intensity of 100 mWcm−2 were yielded for the DSCs with the polymer electrolyte. Our group [98] prepared a P(VDFHFP)-based DSC prepared with the PCE reaching 6.61% at a light intensity of 100 mW · cm−2. Lin et al. [99] developed a novel gel electrolyte employing poly(ethylene oxide) (PEO) inner-plasticized side-chain polysiloxane polymer to react with cross linker in liquid electrolyte. The polymer structure is shown in Figure 4.5. They introduced quaternary ammonium iodide as a side chain into the polymer backbone and prepared a DSC based on polysiloxane polymer containing PEO innerplasticized side chain and quaternary ammonium salt side chain. The polymer structure is shown in Figure 4.6. They gained a short-circuit current density of 5.0 mA · cm−2, an open-circuit voltage of 680 mV, a filling factor of 0.6, and a PCE of 3.4% for the DSC. Lin et al. [100] optimized PEO gel electrolytes containing I–/I3– by adding two functional additives (inorganic nano-TiO2 and the 1-methyl-3-propylimidazolium iodide salt ionic liquid). The PCE of the assembled DSC reached 3.2% at a light intensity of 100 mW · cm−2, which is much higher than that of the DSC if no additives were added into the electrolytes. Lin et al. [91] solidified the liquid electrolyte by PEO halogenate reacting with the derivative of polyamide (PAMAM, dendrimers poly (amide amine)). The PCE of the prepared DSC was 7.72% at a light intensity of

Chapter 4 Electrolyte used in dye-sensitized solar cells

225

100 mW cm−2. They also used a novel comb-like molten-salt polymers with oligopoly ethylene oxide chains (MOEMImTFSI) solidified liquid electrolytes with different organic solvents (N-methoxazolidadione, 3-methoxypropanitrile (MPN), and a mixture of vinyl carbonate and PC) to obtain a gel electrolyte. The DSC based on the gel electrolyte containing the mixture of vinyl carbonate and PC exhibited the best photoelectric performance compared to others and the PCE reached 6.58% at a light intensity of 100 mW cm−2. After aging at room temperature for 50 days, the PCE of the DSC decreased to 4% [46].

Figure 4.6: Oligomer structure employed by Ryoichi Komiya [97].

4.1.3.2 Organic small-molecule gel electrolytes Some organic small-molecule compounds form a three-dimensional network structure via molecular self-assembly in a liquid electrolyte to obtain a quasi-solid electrolyte. Small-molecule gelators including carbohydrate derivatives, amino acid compounds, amide (urea) compounds, and biphenyl compounds have been reported currently [101]. The molecular weight of these compounds owns a relatively low molecular weight (generally less than 1,000) compared to that of polymer gelators. They generally contain polar groups, such as amide bonds, hydroxyl groups, and amino groups [102] or long fatty chains. In organic solvents, the gelator molecules gelatinize liquid electrolyte by forming a self-assembled three-dimensional network structure via hydrogen bonds, hydrophobic interactions, electrostatic interactions, and π–π interactions [103]. Kubo et al. in Japan used organic small-molecule gelators to prepare quasi-solid electrolytes in DSCs for the first time. In 1998, they used an amino acid to gelatinize the liquid electrolyte and the prepared quasi-solid DSC gave a PCE higher than 3% under light illumination at 100 mW·cm−2. In 2001, they solidified the liquid electrolyte with four different small-molecule gelators whose structures are shown in Figure 4.7.

Figure 4.7: Structure of four organic small-molecule gelators.

226 Xu Pan, Songyuan Dai

Chapter 4 Electrolyte used in dye-sensitized solar cells

227

The composition of the liquid electrolyte was 0.6 mol · L−1 1,2-dimethyl-3-propylimidazole iodine, 0.1 mol · L−1 I2, 0.1 mol · L−1 · LiI, and 1 mol · L−1 4-tert-butylpyridine (TBP) and the solvent was 3-MPN. The performance of the resultant DSCs is listed in Table 4.3. It can be seen that the electrolyte after gelation had an insignificant impact on the performance of the DSCs. The DSC optimized by the first gelator exhibited a short-circuit current density of 12.8 mA · cm−2, open-circuit voltage of 0.67 V, and a PCE of 5.91% under light illumination at 100 mW·cm−2[104]. Mohmeyer et al. [103] solidified the liquid electrolyte with bis(3,4-dimethyl-diphenylmethylene sorbitol) and the PCE of the prepared DSC reached 6.1% under the light illumination at 100 mW·cm−2. Huo et al. [6] used 12-hydroxystearic acid (Figure 4.8) as an organic small-molecular gelator to solidify 3-MPN electrolyte via hydrogen bonding between hydroxyl and methoxy, where the sol–gel transition temperature TSG reached 66 °C and the assembled DSC exhibited a PCE of 5.36%. They investigated the photovoltaic performance of DSCs with gel electrolytes and examined how I2 content in the gel electrolyte and the formation of polyiodide ions made effects on the conductivity. The study of the electrochemical properties (including the conductivity and the activation energy) of gel electrolytes indicated that the conductivity of the quasi-solid electrolyte increases with the increase of I2 concentration, that is to say, the increase of iodine concentration facilitates the formation of polyiodine compounds. According to Grotthuss charge-transfer mechanism, the formation of polyiodine compounds can significantly improve the charge transport in the network structure of gelators. After accelerated aging at 60°C for 1,000 h, the PCE of the DSC with a quasi-solid electrolyte remained 97% of the initial value, indicating a satisfying stability.

Figure 4.8: Molecular structure of dodecyl stearic acid.

The organic small-molecule gelator used for gelatinizing the liquid electrolyte would also solidify the liquid electrolyte by forming a gel-network structure in the organic liquid, which is via the formation of quaternary ammonium salt from the reaction of amine with halogenated hydrocarbon. Murai et al. [105] employed various polybrominated hydrocarbons to react with organic small molecules and polymers of aromatic rings containing heteroatom nitrogen (such as pyridine, imidazole, etc.) to form quaternary ammonium salt, which can gelatinize the liquid electrolyte containing 0.3 mol L–1 of 1-methyl-3-hexylimidazole iodine, 0.05 mol · L−1 I2, 0.5 mol · L−1 LiI, 0.58 mol · L−1 TBP and ACN to obtain DSCs with quasi-solid-state electrolytes.

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Table 4.3: Photovoltaic performance of several small-molecule gel cells. Electrolyte Liquid electrolyte Gel electrolyte  Gel electrolyte  Gel electrolyte  Gel electrolyte 

Jsc/(mA · cm−)

Voc/(V)

. . . . .

. . . . .

FF

η (%)

. . . . .

. . . . .

4.1.3.3 Quasi-solid nanoparticle electrolytes Due to the special size effect, nanoparticles are usually used as the fillers to improve the properties of polymers. Inorganic nanoparticles have also been added to the DSC electrolytes to improve the conductivity and mechanical performance. Inorganic nanoparticles are easy to disperse and to form porous structures in liquid electrolytes, which is better to the conductivity of gel electrolytes. The most commonly used inorganic nanoparticles are nano-TiO2, nano-SiO2, carbon black, carbon nanotubes, and so on. In recent years, scientists have focused on the topic of inorganic-nanoparticle gel electrolytes. Wang et al. [46] added nano-SiO2 (particle size 12 nm) to MPII ionic liquid electrolytes containing iodine (I2) for the first time to solidify the electrolyte and obtain quasi-solid-state gel electrolytes. The results showed that the properties of the solidified ionic liquid electrolyte almost had no difference with those of the liquid electrolyte. The ionic liquid electrolyte was prepared by adding 0.5 mol L−1 I2 and 0.45 mol L−1 NMBI into the mixture of MPII and MPN (volume ratio of 13:7). The PCE of the corresponding DSC was 7.0%. After that, researchers started to investigate the use of nano-SiO2 to solidify ionic liquid. Yanagida et al. [106] prepared gel electrolytes with different inorganic nanoparticles respectively. The PCE of the obtained DSCs lied in the range of 4.57–5.00%. It was found that the DSC based on the nanoTiO2 gel electrolyte gave the highest PCE compared to other DSCs. Meng’s group in the Institute of Physics of Chinese Academy of Sciences [107] prepared a gel electrolyte by adding nano-SiO2 to the LiI/ethanol-based liquid electrolyte with a certain mass ratio and the assembled cell exhibited a PCE of 6.1%. In 2005, Yang et al. [108] in Fudan University gelatinized liquid electrolyte with mesoporous SiO2 nanoparticles. The obtained DSC presented a PCE of 4.34% under a light intensity of 100 mW cm−2. Yang et al. [109] reported that ionic liquid BMIBF4 was solidified by SiO2 nanoparticles. The solidifying mechanism was considered as self-assembly through hydrogen bond (O—H ···F) networks between the anion BF4– and the hydroxyl group in the surface of SiO2 nanoparticles. When the quasi-solid electrolyte and the N3 dye were introduced for DSCs, the efficiency was 4.7% at room temperature and 5% at 60 °C at a light intensity of 75 mW cm−2. After accelerated aging at 60 °C for 1,000 h, no decrease of the efficiency was observed for the DSCs. Chen et al. [110]

229

Chapter 4 Electrolyte used in dye-sensitized solar cells

prepared a succinitrile/BMI · BF4/SiO2 gel electrolyte system by adding SiO2 particles and BMI · BF4 into succinonitrile. This gel system had a wide range of thermal stability and its ion mobility would decrease significantly without succinonitrile. DSCs with this gel electrolyte and Z907 dye gave a PCE of 4.98% under the light intensity of 75 mW cm−2 at 20 °C and a higher PCE of 5.3% at a higher temperature of 80 °C. After that, they replaced BMIBF4 with BMIPF6 and solidified the binary ionic liquid electrolyte, a mixture of BMIPF6 and MPII, with nano-SiO2 particles [111]. They introduced 3-MPN to reduce the electrolyte viscosity. MPII/BMIPF6/MPN gel electrolyte with volume ratio of 2:2:1 was finally prepared. The results showed that the addition of MPN improved the ionic conductivity of gel electrolytes significantly. At 20 °C, the DSCs with the gel electrolyte gave a PCE of 5.77% (75 mW cm−2), which was much higher than before. The cell can work well at the high temperatures and have excellent long-time stability, which makes it possible for practical outdoor application. In summary, quasi-solid electrolytes have solved the problems with liquid electrolytes and have made some achievements (Table 4.4). Compared to solid electrolytes, quasi-solid electrolytes have better wettability to photoanode and excelent conductivity. Quasi-solid electrolytes would not flow like liquid electrolytes and the three-dimensional network structure of the quasi-solid electrolyte can effectively inhibit the volatilization of the liquid electrolyte, which improves the longterm stability of the DSCs. Furthermore, exploring the charge transfer mechanism can improve the properties of quasi-solid-state electrolytes and further improves the photovoltaic performance of the corresponding DSCs. Therefore, the research on developing quasi-solid electrolyte is of great significance for the practical application of DSCs. Table 4.4: Photovoltaic properties of DSCs based on quasi-solid electrolytes with gelators. Electrolyte composition

Gelators

Dyes Efficiency

. mol · L− DMPII, . mol · L− I, . mol · L− LiI and  mol · L− TBP in MPN

Organic small-molecule amides

N .%

. mol · L− DMPII, . mol · L− I, . mol · L− LiI and  mol · L− TBP in MPN

Organic small-molecule amides

N .%

HMII, I et al.

Organic small-molecule amides

N .%

. mol · L− I, . mol · L− GuSCN, and . mol · L− NMBI in PMII/EMINCS mixture (:, v/v)

 wt% organic small-molecule urea

K DPA

DMPII, I, and NMBI in MPN

Bis(, -dimethyl-diphenylene Z .% sorbitol)

.%

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Table 4.4 (continued) Electrolyte composition

Gelators

Dyes Efficiency

KI, I, EC, PC

Polysiloxane

N

DMPII, I, LiI, TBP . mol · L− DMPII, . mol · L− I and . mol · L− NMBI in MPN

PVDF–HFP  wt% PVDF-HFP

N .% Z .%

NaI, I, EC,PC, ACN

PAN

N

–%

. mol · L DMPII, . mol · L LiI and . mol · L− I in the mixture of EC and γ-butyrolactone (:, v/v)

Poly(ethylene oxide-cooxypropylene) tris (methacrylate) oligomer

N

.%

I, MPII, NMBI

% PVDF-HFP

Z .%

 wt% PVDF-HFP or  wt% SiO nanoparticles

Z

. mol · L− I and . mol · L− NMBI in MPII

SiO nanoparticles

Z .%

. mol · L− EMII, . mol · L− LiI, . mol · L− I and . mol · L− TBP in EMITFSI

 wt% Different kinds of nanoparticles

N

−

−

. mol · L PMII, . mol · L . mol · L− NMBI in MPN

−

−

I and

.%

.%, .%

.–.%

4.1.4 All-solid-state electrolytes Although quasi-solid electrolytes can prevent leakage and reduce the volatilization of organic solvent to a certain extent, there are still problems with them in the longterm stability. All-solid-state electrolytes have emerged as a response to this problem [112, 113]. At present, the research of all-solid-state electrolytes mainly focuses on inorganic p-type semiconductor materials, organic/polymer HTMs and ionic conducting polymers and so on. In all-solid-state electrolytes, the electron transport is slightly different from that of the electrolytes mentioned earlier in this chapter. For all-solid-state electrolytes, the oxidized dye injects holes into the HTMs and the holes reach the counter electrode through the HTMs to recombine with the external-circuit electrons, thus forming an electron-transport circuit.

4.1.4.1 Inorganic p-type semiconductor materials Inorganic p-type semiconductor materials have intensively been studied in recent years. Generally, the HTMs for dye-sensitized solar cells should possess the following essential properties. (i) It is transparent to the visible light that is absorbed by the dye. (ii) The deposition of p-type semiconductors would not cause any dye

Chapter 4 Electrolyte used in dye-sensitized solar cells

231

degradation or dissolution. (iii) The ground-state energy level of the dye is lower than the valence band of the p-type semiconductor, whereas the excited-state energy level of the dye is higher than the conduction band of TiO2. Cu-based materials including CuI, CuSCN, and CuBr have been widely investigated in the field of inorganic p-type semiconductors. In 1995, Tennakone et al. used CuI that was deposited from solutions as the HTM for DSCs [114]. Although the efficiency of the resultant DSC was very low, the results confirmed that inorganic p-type semiconductor materials can be used as an electrolyte in DSCs. One reason for the low efficiency of the DSC is that CuI is easy to crystallize, so that it would form large particles while being filled into TiO2 films. This results in poor contact between CuI particulates and TiO2. Researchers then started to employ crystal growth inhibitors to prevent the formation and growth of crystals. Meng et al. employed an ionic liquid to inhibit CuI crystal growth combined with an organometallic dye to obtain an all-solid-state DSC with a PCE of 3.8% [115]. Currently, the PCE of DSCs based on CuI electrolyte have reached 3%, which has been patented in Europe by Toshiba Corporation of Japan. CuSCN has also been used as the p-type semiconductor material. In 1995, O’Regan et al. [116] found that CuSCN can be used as a HTM and investigated the effect of ultraviolet light irradiation on the photovoltaic cells. It was indicated that ultraviolet light irradiation had a beneficial effect on the interfacial contact between TiO2 and CuSCN and on the formation of (SCN)x−. In addition, (SCN)x– can improve the regeneration rate of dye cations [88]. They replaced TiO2 with ZnO to obtain ZnO/ dye/CuSCN, which obtained an efficiency of 1.5% under AM1.5 light illumination [117]. Other researchers reported that CuSCN accelerated the recombination reactions in DSCs [118], but CuSCN facilitated electron transport better than other common liquid electrolytes [119]. In order to reduce the fast recombination in p-type semiconductor-based DSCs, researchers introduced Al2O3-based potential barrier between TiO2 and dye. It was found that the Al2O3 thin layer as a tunneling barrier increased the Voc and FF of the cells but reduced the Jsc [120], which is indicated by the data in Table 4.5. In addition, Fujishima et al. [121] studied the performance of CuI as a HTM in DSCs with surface-modified electrodes. A barrier layer with Al2O3, MgO, or ZnO was formed on the surface of the TiO2 electrode by surface modification. They also introduced MEISCN as a crystal inhibitor to improve the performance and stability of DSCs. However, the improvement in the stability of inorganic p-type semiconductor materials used in DSCs is still on the way. Moreover, the theory on the inhibition of recombination reactions in DSC is far from completion. These are the significant problems with inorganic p-type semiconductor materials and should be solved for their further application in DSCs.

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Table 4.5: Photovoltaic performance parameters for devices. Voc/(mV) Jsc/(mA cm−) FF

Electrolytes ZnO/dye/CuSCN TiO/dye/CuSCN TiO(AlO)/dye/CuSCN TiO/dye/CuI n-TiO/Se/p-CuCNS TiO/dye/CuI(MEISCN) TiO/dye/CuI TiO(AlO)/dye/CuI TiO(MgO)/dye/CuI TiO/dye/CuI TiO/dye/CuI (MEISCN) TiO(ZnO)/dye/CuI (MEISCN)

           

. . .  . . . . . . . .

. . . . – – – . . . . .

η (%) Light intensity (mW⋅cm−) . . .  . . . . . . . .

 – –   – –     

References [] [] [] [] [] [] [] [] [] – – –

4.1.4.2 Organic hole-transport materials HTMs can be categorized into two types: small-molecule and polymer HTMs. Active research on small-molecule HTMs has been ongoing for a long time. In 1998, Grätzel’s group developed a solid-state dye-sensitized mesoporous titanium dioxide (TiO2) solar cell with N3 as a sensitizer and 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9ʹ-spirobifluorene (spiro-OMeTAD) as an HTM. The incident photon-to-current conversion efficiency (IPCE) of this solar cell reached 33%. They also examined the dynamic process of charge separation in dye-sensitized heterojunctions using nanosecond pulse laser photolysis combined with time-resolved absorption spectroscopy. They concluded that the optically excited sensitizer injects electrons into the TiO2 conduction bands. Subsequently, the dye molecules in the oxidized state are regenerated by injecting holes into the HTM (see eqs. (4.1) and (4.2)). Figure 4.9 shows a schematic of the electron transfer process in dye-sensitized heterojunctions an IPCE of 0.74% was achieved under the white-light illumination at 9.4 mW · cm−2, whereas a short-circuit photocurrent of 3.18 mA · cm−2 was generated under white-light illumination at 100 mW · cm−2. Dye-sensitized heterojunctions are advantageous because the light absorber and the HTM can be selected independently, which is conducive to the optimization of cell performance. Dye-sensitized heterojunctions provide a viable option for future research focused on lowcost solid-state solar cells: RuðNCSÞ2 ðdcbpyÞ*2 ! RuðNCSÞ2 ðdcbpyÞ+2 + e− ðTiO2 Þ

(4:1)

Chapter 4 Electrolyte used in dye-sensitized solar cells

OMeTAD + RuðNCSÞ2 ðdcbpyÞ+2 ! RuðNCSÞ2 ðdcbpyÞ2 + OMeTAD+

233

(4:2)

/V -1

cb

0

inj. ( D +/D )* rec.

1

h reg.

hopping

+

TiO 2

( D /D ) Dye

HTM

Au

Figure 4.9: Schematic diagram of the electron transfer process in dye-sensitized heterojunctions.

Grätzel’s group also investigated the effects of ruthenium dyes with different carbon-chain lengths (Figure 4.10) on the photovoltaic performance of solid-state solar cells [125]. They found that a relatively long carbon chain can improve the performance of the solar cell. This was attributed to the increased distance between the electrode and the hole-transport layer in dyes with long carbon chains. This large separation distance allowed the dye to act as a barrier layer to inhibit charge recombination. As shown by the data in Table 4.6, each device parameter was improved to some extent by using long-carbon-chain dyes. Liquid electrolytes based on lithium ions (Li+) and TBP are also frequently introduced into small-molecule HTMs to increase the electrolyte conductivity and inhibit recombination reactions in DSCs. In 2007, Snaith et al. [126] significantly inhibited the recombination reactions in an OMeTAD electrolyte-based DSC by introducing ion chelation sites into the dye molecule. As a result, they obtained a PCE of 5%, which is currently the highest PCE achieved with small-molecule HTMs. Furthermore, conventional conductive polymers such as polythiophene, polypyrrole, and polyaniline (PANI) can also be used as hole-conducting materials. Most of these polymers exhibit properties similar to those of p-type semiconductors [127]. Therefore, they are also referred to as organic p-type semiconductors. However, no remarkable progress has been made in solar cells based on this type of polymers and the PCE of the corresponding DSCs all remains low. In 1997, Yanagida’s group [128] prepared an all-solid-state DSC with a conductive polypyridine polymer mixed with lithium perchlorate as the electrolyte. However, the PCE reached only 0.1%. Saito et al. [129] fabricated an organic HTM, poly(3,4-ethylenedioxythiophene) (PEDOT), via chemical polymerization. They also successfully enhanced the photovoltaic performance of an all-solid-state DSC by introducing a sulfonimide ionic liquid between the electrodes. Moreover, they found that the PCE of the all-solid-state DSC could be further improved by combining in-situ polymerized PEDOT [130] with a hydrophobic dye [131] and some ion dopants (Li+, TFSI–, CF3SO3–) [132, 133]. Other commonly used

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R

R

COOH

N

N

N

R=

Ru

N N C S

N

COOH

C S

Figure 4.10: Molecular structure of ruthenium dyes with different chain lengths. Table 4.6: Performance parameters of ruthenium dyes with different carbon-chain lengths [125]. Different carbon chain C C C C C

Voc/(mV)

Jsc/(mA cm−)

    

. . . . .

FF . . . . .

η (%) . . . . .

polymer-based organic HTMs include PANI [134], poly(o-phenylenediamine) [135], poly(3-octylthiophene), and poly(3-hexylthiophene) (P3HT) [136]. Yanagida’s group achieved a PCE of 2.7% using an ionic liquid- and TBP-containing P3HT electrolyte combined with a thiophene-containing dye. It has been demonstrated that the hole-transport rate is not the primary factor limiting the photocurrent in organic HTM-based DSCs [137]. Instead, improving the contact between the organic HTM and the TiO2 film is crucial for the photovoltaic performance of DSCs, which has currently drawn considerable attention from researchers. At present, the approach to better the contact between the organic HTM and the TiO2 film is to introduce an ionic liquid into the organic HTM or reduce the thickness of the TiO2 film.

4.1.4.3 Ionic conductive polymer materials The p-type semiconductor materials and organic HTMs has not exhibited a promising prospect because the problems such as hole-transport rate, interface contact, and others seriously limit the performance of DSCs. Ionic conducting polymers have become ideal candidates to solve these problems with all-solid-state electrolytes and have gained much attention in recent years because of their relatively high ionic

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mobility and being easy to solidify. The related research has achieved significant progress up to now. In 2001, Paoli et al. [138] copolymerized propylene oxide and ethylene oxide into polymer Epichlomer-16 and mixed with NaI and I2 to prepare a solid electrolyte. The assembled DSC (Figure 4.11) gave an efficiency of 2.6%. This efficiency was relatively low compared to those DSCs with liquid electrolytes. The main reason is that solid electrolytes have a relatively low conductivity and the contact between the electrolyte and the dye is not good enough. To solve this problem, the concept of inorganic polymer composite solid electrolyte was proposed by introducing plasticizers such as inorganic nanopowders to reduce the degree of crystallization. The related research showed that the ionic mobility of the system was greatly improved by adding inorganic nanopowders, which can keep the polymer crystallization and improve the stability of the electrolyte/electrode interface.

Figure 4.11: Schematic diagram of the assembled DSC using a polymer electrolyte.

Stergiopoulos et al. used PEO and TiO2 nanopowder as gelators and the liquid electrolyte was composed of I2, LiI, and ACN. The DSC gave an efficiency of 4.2% at a light intensity of 65.6 mW cm−2 (AM1.5) [139]. In 2004, Kim et al. [140] employed the solid electrolyte poly(ethylene oxide dimethyl ether) modified by silica nanopowder to obtain a DSC with a conversion efficiency of 4.5%. Han et al. [141] introduced TiO2 nanopowder into a PEO/PVDF polymer blend system. This reduced the crystallinity of the polymer, improved the ionic conductivity, and effectively lowered the recombination rate at the TiO2/solid-state electrolyte interface. The DSC exhibited a PCE of 4.8% under the irradiation of white light (65.2 mW cm−2).

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In addition, some other investigations also deserve attention. By mixing LiI and 3-hydroxypropionitrile (HPN), Meng’s group [142] prepared LiI (HPN)x compounds. Based on the single crystal structure (Figure 4.12), they found these compounds can provide three-dimensional paths for the transport of iodine ions and was expected to replace the solvent in the electrolyte. They also introduced SiO2 nanoparticles to inhibited the crystallization of LiI (HPN)x compounds. An optimized PCE of 5.4% was achieved under the light intensity of 100 mW cm−2.

Figure 4.12: Single crystal structure LiI(HPN)x viewed along the a, b, and c axes.

4.2 Redox couples in electrolytes 4.2.1 I–3 redox couple In 1991, Grätzel employed the I–/I3– redox couple for the first publication on dye-sensitized solar cells in Nature. The I–/I3– couple exhibits excellent properties. First, the I3– energy level matches the conventional dye to facilitate the dye regeneration. Second, the recombination of I3– with the TiO2 conduction band electrons is slow while the regeneration rate of I– is very fast [143]. Finally, I3– is relatively stable in the electrolytes and the DSC with I3– has a high conversion efficiency. However, I3– suffer from two major disadvantages: I2 corrodes electrodes and absorbs visible light. This promotes scientists to find an alternative to the current redox couples. Currently, based on I3– redox couples, the highest DSC efficiency obtained so far has reached 11.1% [145] after an optimization. In addition, researchers have found several iodine-free redox couples [146], such as ferrocene, diphenol, SCN–/(SCN)3–, SeCN–/(SeCN)3–, Br–/Br3–, and so on. However, these couples have not replaced I–/I3– couples currently since the PCE of the cells based on them is not high enough yet.

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4.2.1.1 Transport mechanism of the I–/I3– redox couple in electrolytes Researches on the DSCs for over 10 years have achieved a relatively mature transport mechanism of the I–/I3– redox couple. The diffusion of I3– in electrolytes is mainly driven by the concentration gradient. In viscous media, due to the friction resistance, there exists a relationship between the force and the velocity of the diffuse species (equivalent to spherical species) [147] as follows: F = 6πrην

(4:3)

Here η is the medium viscosity, r is the radius of the species, and v is the moving rate of the species. In addition, there is the Einstein–Strokes equation: D=

kT 6πrη

(4:4)

where D is the diffusion coefficient of the species and k is the Boltzmann constant. Equation (4.2) can be used to gain the diffusion coefficient of diffuse species in the medium with different viscosities at different temperatures. A slight change to eq. (4.4) yields the following equation: Dη k = T 6πr

(4:5)

According to eq. (4.3), we can calculate the diffusion coefficient of a given species in a given medium at a given temperature. At present, the study of redox couple transport in organic-solvent electrolytes is mainly based on the Einstein–Strokes equation. In DSCs, when the concentration of I3– is low or the viscosity of electrolyte is high, the diffusion of I3– is the rate-determining step. I3– is reduced at the counter electrode and this lowers the output voltage of the cell. However, the increase in diffusion coefficient with a high concentration of I3– can be explained by the Grotthuss ion-exchange mechanism that is expressed as follows [147]: I3− + I − ! I −    I2    I − ! I − + I3−

(4:6)

Charge transfer is achieved by the formation or cleavage of chemical bonds. At present, this theory has been widely accepted, especially in ionic liquid electrolytes with a high electric field (a high ionic strength). The diffusion coefficient (Dex) can be quantified by Dahms–Ruff equation as follows: Dex =

kex δ2 c′ 6

(4:7)

where kex is the rate constant of the exchange reaction, δ is the distance between I– and I3– while the exchange reaction occurs, and c′ is the concentration of I3– and I–

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in electrolytes. The diffusion coefficients of I– and I3– in electrolytes can then be expressed as the sum of two parts as follows: Dapp = Dphys + Dex =

kT kex δ2 c′ + 6 6πrη

(4:8)

It is known from eq. (4.8) that the exchange reaction occurs only when I- and I3– possess enough kinetic energy to overcome the potential energy produced when they approach each other so that I– and I3– can collide for the exchange reaction. Ionic liquids are room-temperature molten salts consisting of only anions and cations. The ionic strength of ionic liquids is very high and I3– is in a high electric field. Due to the kinetic salt effect [148], the collision between I– and I3– becomes easier (a significantly increased kex) and the contribution of Dex to Dapp increases significantly in ionic liquids. For example, I– and I3– move freely in MPN after solvation. However, I– and I3– are both negatively charged and they would repel each other when approaching, which makes it difficult for I– and I3– to collide and leads to a small kex. Therefore, Dex of I3– in liquid electrolytes based on Grotthuss-like exchange reaction can be approximately considered as zero.

4.2.1.2 Reactions associated with redox couples in electrolytes The operating principle of DSCs has been described earlier in this chapter (Figure 4.13). What is related to electrolytes is that the dye molecule S+ in the oxidation state is reduced to ground state by I– (from I3– redox couple) to realize the dye regeneration. Meanwhile, I– itself is oxidized to I3–, and then I3– diffuses to the photocathode and gain electrons to form I–. Therefore, redox couples play an important role in the elec-

0.05 150 ps ~1 ps

~1 ps Ec

20 ns ~1 ms 100 ns1 ms

h ~10 s ~0.5 s

TiO2

Dye(D)

Redox mediator

Figure 4.13: Schematic diagram revealing the operating principle of DSCs and the typical time constants for the reactions.

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tron transport in DSCs and their diffusion rate in electrolytes makes a significant effect on the photoelectric performance of DSCs. Iodine combines with I– in electrolytes to produce I3–. When the iodine content is high, the polyiodide anion, such as I5–, I7–, and I9–, can be formed easily [149]. However, the content of I3– in DSCs affects directly the DSC performance, so it is important to control the content of I– and I3–. In theory, the conductivity of an electrolyte would decrease with the increase of iodine concentration and the formation of polyiodide ions. However, in fact, the conductivity of an electrolyte increases with the formation of polyiodide ions. This indicates that the ion migration produced by the exchange reaction in electrolytes also makes an important effect on the conductivity of electrolytes. Moreover, the formation of polyiodide ions has a positive impact on the ion exchange reaction. Generally, the exchange reaction occurs between I– and I3– in electrolytes in DSCs and this plays an important role in the transport of I/I3– in electrolytes. When I5– is formed in electrolytes, I5– can also participate in the exchange reaction with I– and I3–, which is easier due to the larger ionic radius of I5–. Therefore, electrolytes are affected by the exchange reaction and Dex increases with the formation of polyiodide ions [150]. However, it is difficult to quantify the contribution of the formation of polyiodide to Dex: I2 + I − = I3 −

(4:9)

I3− + 2e − = 3I −

(4:10)

In electrochemical cells, the reaction as eq. (4.9) occurs on a counter electrode with sufficient catalysis and low overpotential. For the I3– redox couple, platinum (Pt) is commonly used as a catalyst to adsorb rapidly I–, I3–, and I2 undergoing one-electron reduction [151]. The charge transfer resistance RCT would be produced when the charge transfer occurs at the counter electrode. Therefore, at a certain value of current density, an overpotential (η) is the driving force for the reaction as eq. (4.9). When the η is small, there is a linear relationship between η and the current density J: RCT = η/J = RT/nFJ0. For eq. (4.9), n equals to 2 and J0 is the exchange current density. In the ideal case, RCT should be less than 1 Ω cm2 to avoid the current density drop too much. Equation (4.9) has little to do with the dye regeneration, but the equation 2I– = I2– + e– correlates with the reduction of the oxidized dye. Compared with the standard hydrogen electrode, the redox potential of the I3– couple is 0.35 V and the standard oxidation potential of N3 or N719 is 1.1 V. Therefore, the potential to drive the reduction of the oxidized dye is up to 0.75 V. The process is also the most energy-consuming process [151]. When photogenerated electrons are injected into TiO2, the dye is in the oxidation state and an electron donor is necessary to reduce the dye and achieve the dye regeneration. The lower limit of dye regeneration can be obtained according to the diffusion control kinetics. The diffusion coefficient Kdiff is not lower than 109–1010 L mol−1 s−1 in nonviscous medium and the concentration of electron donor is not less

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than 0.1 mol L−1 in electrolytes. Thus, the time of dye regeneration can be expressed as the product of Kdiff and the concentration of electron donor. The dye regeneration efficiency φreg is defined here as the possibility that the dye regeneration in the way of being reduced by electron donors in electrolytes or directly reduced by electrons in the conduction band of TiO2: φreg =

kreg kreg + krec

(4:11)

Here kreg is the regeneration rate constant and krec is the quasi-first-order rate constant of the recombination of oxidized dyes with conduction band electrons. In DSCs, iodide is a common electron donor (reductant) and shows a high φreg for different dyes. In the process of dye reduction, iodide is partially oxidized to diiodide I2 – [152], but I2– is chemically unstable and would form triiodide I3– and iodide I– by disproportion reaction. The reduction mechanism of oxidized dye (S+) is expressed as follows: S + + I − ! ðS    IÞ

(4:12)

ðS    IÞ + I − ! ðS    I2 −  Þ

(4:13)

ðS    I2 −  Þ ! S + I2 − 

(4:14)

I2 −  ! I3 − + I −

(4:15)

The first step is the single electron transfer reaction between S+ and I–. However, the I•/I– redox potential U0(I•/I–) in the aqueous solution is 1.33 V referring to the standard hydrogen electrode NHE [151] and that in ACN solution is 1.32 V [153]. This value is much more positive than the U0(S+/S) of dyes. Therefore, iodine will not be oxidized to iodine free radical I•. The redox potential of (S . . . I) is only slightly more positive than the U0(S+/S) of dyes, so the first step of dye regeneration might be the formation of (S . . . I). I– can continue to react with (S . . . I) to form (S∗∗∗I2–•) complex. (S∗∗∗I2–•) complexes would be dissociated into S and I2–•. Finally, I2–• undergoes disproportion into I3– and I–, whose second-order rate constant in ACN is 2.3 × 1010 L mol−1s−1. The (S∗∗∗I) intermediate has been confirmed by laser spectroscopy. Clifford et al. confirmed that the formation of (S∗∗∗I) from cis-Ru(dcbpy)2(CN)2 dye in the oxidation state [154]. Fitzmaurice et al. observed the formation of (S∗∗∗I2–) from Ru (dcbpy)32+ sensitizer and iodide [155]. By quantum chemical calculation, cis-Ru (dcbpy)2(CN)2 sensitizer is easy to form intermediate with iodide. For the standard sensitizer cis-Ru (dcbpy)2(CN)2, the half time for regeneration was 100 ns ~10 μs when 0.5 mol L−1 iodide was added to the electrolyte [156]. The regeneration kinetics is derived from the composition of electrolytes. Pelet et al. found that the properties of cations in iodized salt are closely related to regeneration [157]. Cations adsorbed on the surface of TiO2, such as Li+ and Mg2+, can promote the rapid dye

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regeneration, while TBA+ cations can reduce the reaction rate. The reason is that there is a locally high concentration of iodide ion on the surface of TiO2, when cations are adsorbed on the surface TiO2, thus improving the regeneration efficiency. Most sensitizers can be effectively reduced by iodides to achieve the dye regeneration. The oxidation potential of the dye is comparable to or slightly more positive than that of the standard sensitizer cis-Ru(dcbpy)2(CN)2(U0 = + 1.10 V vs. NHE). The redox potential of I3–/I– in organic solvents is +0.35 V versus NHE and for the standard sensitizer Ru(dcbpy)2(CN)2, the driving force of regeneration, ΔG0, is 0.75 eV, thus the required driving force can be estimated from these available data. Kuciauskas et al. [156] studied the dye regeneration dynamics for a series of Ru-based and Os-based dyes. They found that the sensitizer could not be reduced by the iodide when the driving force of the sensitizer ΔG0 was 0.52 eV and that the sensitizer Os (dcbpy)2(CN)2 can be reduced by iodide when ΔG0 = 0.82 eV. Clifford et al. [158] showed that the regeneration efficiency was very low when the driving force ΔG0 of Ru-dye Ru (dcbpy)2Cl2 is 0.46 eV. The results show that the driving force of 0.5–0.6 eV is required to reduce Ru-based dyes in I3–/I– system. Such a large driving force may be derived from the I–/I2–• redox couple with a more positive potential than I3–/I– in the initial regeneration reaction.

4.2.2 Iodine-free redox couples Although DSCs with iodine-based liquid electrolytes give a very respectable efficiency, currently a drawback of the iodine-couple system is that a large driving force is required for the dye regeneration [144] and that the iodine redox couple would corrode metal electrodes and absorb visible light to affect the efficiency and stability of DSCs [159]. Arakawa et al. [160] employed the LiBr/Br2 redox couple for DSCs, but obtained a low PCE. In 2005, Wang et al. [161] prepared a DSC with the LiBr/Br2 redox couple combined with eosin dye and gained an efficiency of 2.61%. This achieved an increase in efficiency by 56% compared to DSCs with I–/I3– redox couple under the comparable conditions. Yanagida et al. [162] synthesized a series of redox systems based on Cu(I)/(II) complexes and obtained an optimal PCE of 1.4%. In 1995, Tennakone et al. [114] prepared all-solid DSCs with p-CuI as the HTM and the current density of cells was 1.5–2.0 mA · cm−2 at 800 W · m−2 sunlight. In 2002, Kumara et al. [124] investigated solid-state DSCs with p-CuI as the electrolyte. It was found that the CuI crystal growth led to a rapid decay in the Voc and Jsc of deceives, so they added a small quantity of 1-methyl-3-ethylimidazolium thiocyanate (MEISCN) into electrolytes to inhibit the CuI crystal growth. The obtained DSC gave a PCE of 3.0% under AM 1.5 sunlight illumination. In 2011, Peng et al. [163] in Wuhan University developed a novel CuI-based gel electrolyte for DSCs using PEO as a plasticizer and lithium perchlorate (LiClO4) as a salt additive. It effectively

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avoided the problems caused by liquid electrolytes such as volatilization, leakage, and so on and owned a relative high conductivity and stability. This indicated that the efficiency of solid-state DSCs can be dramatically improved by adding PEO and LiClO4. There are Li+–O coordination interactions between PEO and LiClO4, and these coordination interactions have an important influence on the structure, morphology, and ionic conductivity of the CuI-based electrolyte. The optimal efficiency (2.81%) was obtained for the fabricated DSC, with a 116.2% improvement in the efficiency compared with the DSC without addition of LiClO4. In 2000, Tennakone et al. [164] prepared a highly stable DSC with CuBr3S(C4H9)2 as the hole collector. The cell delivered a Voc of 0.4 V and a Jsc of ~4.3 mA · cm−2 at the light intensity of 1,000 W · m−2. In 2005, Bandara adopted the p-type semiconductor NiO as the hole collector for DSCs and the cell gave an Isc and a Voc of 0.15 mA and 480 mV, respectively. In 1995, O’Regan et al. [116] prepared a solid-state DSC with CuSCN as the electrolyte with a low efficiency for the first time. In 2002, O’Regan [165] optimized the experiments and obtained a PCE of 2.0% for DSCs with a CuSCN-based electrolyte. In addition, researchers have tried to employ organic HTMs instead of inorganic p-type semiconductor materials as the solid electrolyte in DSCs. In 1998, Grätzel et al. [86] employed a hole-transport material spiro-OMeTAD (Figure 4.14) as the electrolyte in DSCs for the first time, the PCE was 0.7% under the 9.4 mW · cm−2 white light. In 2006, Grätzel et al. [166] prepared a DSC with another hole transport material tris-(4-(2-methoxy-ethoxy)-phenyl)-amine as the electrolyte combined with amphiphilic K51 dye. The corresponding PCE reached 2.4% under AM 1.5 sunlight illumination.

Figure 4.14: Structure of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine) − 9,9′-spirobifluorene (spiro-OMeTAD).

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243

In 2001, Oskam [79] reported the DSCs with SCN–/(SCN)2 or SeCN–/(SeCN)2 as the redox couple. The optimal IPCE was only 20% and much lower than that of DSCs with the I–/I3– couple. The reason might be that SCN–/(SCN)2 or SeCN–/(SeCN)2 cannot reduce the dye effectively. In 2004, Grätzel et al. [80] synthesized the low-viscosity MEISCN ionic liquid with a conductivity of 14.1 mS · cm−1 at 21 °C. The DSC with this ionic liquid as electrolytes in conjunction with Z907 dye exhibited a PCE of 7.5% under AM1.5 sunlight illumination. This performance is comparable to those DSCs with ionic liquid containing the I3–/I– redox couple. Grätzel [167] presented a new electrolyte based on disulfide/thiolate redox couple in 2010. This electrolyte has negligible absorption in the visible spectral range, which is a very attractive feature for flexible DSCs that use transparent conductors as current collectors. Using this novel, iodide-free redox electrolyte, they achieved a PCE of 6.4% under standard illumination and an IPCE higher than 81% at 520 nm. This offers us a new direction for improving the efficiency of DSCs. In 2011, Spiccia et al. [168] in Monash University reported a DSC combining the ferrocene/ferrocenium (Fc/Fc+) as redox couple in the electrolyte combined with an organic donor-acceptor sensitizer (Carbz-PAHTDTT). The results indicated a favorable matching of the redox potential of the Fc/Fc+ couple with that of the CarbzPAHTDTT sensitizer. The efficiency of these Fc/Fc+-based devices exceeds that of the devices based on I–/I3– electrolytes under comparable conditions, revealing their great potential of ferrocene-based electrolytes in future applications in DSCs. Mirkin et al. [169] used Ni(III)/Ni(IV) bis(dicarbollide) as the redox couple for DSCs. Although the potential of this redox couple lies ~140 mV negative of Fc/Fc+, a Voc of 580 mV which was much higher than that of Fc/Fc+ (200 mV) was achieved. There was an increase in Voc from 580 mV to 640 mV with one cycle of Al2O3 passivation, yielding a Jsc of 3.76 mAcm−2 and a PCE of 1.5%. In 2001, Nusbaumer et al. [74] found that the cobalt-based redox couple had a dyeregeneration kinetic behavior that was comparable to that of the I3–/I– couple. In 2011, Grätzel et al. [170] reported DSCs with the Co(II/III) tris(bipyridyl)-based redox electrolyte in conjunction with zinc porphyrin dye YD2-o-C8 (Figure 4.15) as a sensitizer. The specific molecular design of YD2-o-C8 greatly retarded the rate of interfacial back electron transfer from the surface of the nanocrystalline TiO2 film to the oxidized cobalt mediator, which enabled attainment of (Voc) approaching 1 V. In addition, the YD2-oC8 porphyrin harvests sunlight across the visible spectrum, so large photocurrents were generated. A high PCE of 11.9% was achieved under AM 1.5 sunlight. Cosensitization of YD2-o-C8 with dye Y123 (Figure 4.16) that possesses a complementary absorption spectrum to YD2-o-C8 improved the PCE to 12.3% under AM 1.5 sunlight.

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Figure 4.15: Molecular structure of YD2-o-C8 porphyrin dye.

Figure 4.16: Molecular structure of Y123.

4.3 Additives in electrolytes Both liquid electrolytes and quasi-solid electrolytes in DSCs contain redox couples, solvents, and organic or inorganic additives. The electrolytes are concurrently in contact with the photoanode and the counter electrode, forming five interfaces (TiO2/transparent conductive oxide, dye/TiO2, dye/electrolyte, TiO2/electrolyte, and Pt/electrolyte) as shown with the blue line in Figure 4.17) and two layers (the electrolyte layer and the dye-sensitized porous film layer). The recombination between TiO2 electrons and oxidizing species in the electrolyte are prone to occur at the TiO2/electrolyte interface, causing electrical leakage of the cell. This recombination is caused by the defect of unabsorbed dye molecules on the TiO2 surface. At the

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245

dye/TiO2 interface, only about one tenth of the dye is adsorbed on the TiO2 surface. In addition, hydrogen bonding-induced aggregation of dye molecules are also very prevalent. Interfacial electron recombination and dye aggregation are main factors to reduce the photovoltaic performance of devices. The electrolyte layer is closely related with the TiO2/electrolyte and dye/TiO2 interfaces. Hence, researchers have made an effective improvement of DSC performance by adding additives in electrolytes. Additives would be adsorbed on the TiO2 surface to inhibit electron recombination between injected electrons and I3– ions on TiO2 photoanode, thus reducing the electrical leakage on the photoanode and increase the Voc and Jsc of devices. However, the additive adsorption on the TiO2 surface can trigger the movement of the conduction band, which influences electron injection at the dye/TiO2 interface indirectly. As a result, additives make a two-side effect on the Voc and Jsc of devices. In summary, as an important component of electrolytes, additives can improve the efficiency and stability of devices and it is generally applicable to a wide range of DSC systems. Moreover, exploring the effect of additives helps to further investigate the interfacial dynamics of DSCs. TCO

TCO Pt

electrolyte dye e

TiO2 dye

I3-

I-

dye

e TiO2

TiO2 dye

Figure 4.17: Schematic structure of DSCs.

4.3.1 The principle of additives Overall, an additive fulfills the following roles at the TiO2 photoanode: it forms a barrier layer that inhibits dark current and regulates the interaction between the conduction band of TiO2 and dye. For most additives, their adsorption on the counter electrode exerts a relatively insignificant impact on the catalytic performance. The effect of electrolyte additives on ionic conduction and the electrochemical window has not been investigated systematically as yet. In DSCs, the Voc is determined by the difference between the Fermi voltage of TiO2 (Vf) and the voltage of the redox couple (Vred) (Figure 4.18). Therefore, increasing the Vf or reducing the Vred can increase the difference between the Voc and Vf

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and thereby increase the Voc. The Vf varies with the conduction-band energy of TiO2. The adsorption of different additives exerts different impacts on the conduction band of TiO2 due to the difference in electric charge and electronegativity of additives. However, the adsorption of additives has almost no impact on the energy level of the redox couple in electrolytes. As a result of the interaction between the additive and the hydrogen bonds in the dye, the change in the Jsc may lead to a reduction in dye aggregation and affect the ground-state energy level of the dye and, therefore, may further affect the excited-state energy level of the dye and alter the driving force of electrons injected from the excited state of the dye into the conduction band of TiO2. ENERGY excited state conduction band

h Fermi level

V =|V -V |

I /I ground state valence band TiO

Dye

Redox electrolyte

Figure 4.18: The energy level diagram of photoanode in DSCs.

The Voc of DSCs can be calculated by the diode as follows [171]:     nRT isc ln Voc = −1 io F

(4:16)

where n is the ideal factor in the range of 1–2 in DSCs and i0 is the reverse saturation current, namely the dark current of DSCs. When TiO2 film comes in contact with an electrolyte solution, a Helmholtz-layer capacitance will be formed on the TiO2 surface [172]. When a sufficient quantity of electric charge has accumulated on the TiO2 surface, there will be a change in the potential drop in the Helmholtz layer, which in turn causes the conduction-band edge to shift. When positive charge has accumulated on the surface, the conduction-band edge will shift towards the positive direction. When negative charge has accumulated, the conduction-band edge will shift towards the negative direction. The electron concentration in the conduction band (Qcb) dictates the difference between the conduction-band-edge energy (Ecb) and the Fermi energy (Ef), i.e., EðQcb Þ = Ecb − Ef [173]. When the Qcb remains unchanged, the Ef increases with the increase of the Ecb, resulting in an increase in the Voc. Similarly, the Ef decreases

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247

with the decrease of the Ecb, resulting in a decrease in the Voc. Research has found that when adding lithium ions (Li+) [158] or guanidiniums ions into an electrolyte solution or acidifying an electrolyte solution to promote the adsorption of hydrogen ions (H+) on the TiO2 surface [174], the TiO2 surface adsorbs positive charges, resulting in a shift of the conduction band towards the positive direction as well as a decrease in the Voc. Additives, such as TBP, NMBI, and benzimidazole (BP), can induce a significant negative shift of the conduction band of TiO2, resulting in an increase in the Voc. The effect of adding certain additives in practical applications remains unclear. For example, TBP may alter the recombination rate or shift the conduction band or both. There are four cases for the change in the interfacial recombination rate and the conduction band: (i) an increase in the recombination rate and a positive shift of the conduction band; (ii) an increase in the recombination rate and a negative shift of the conduction band; (iii) a decrease in the recombination rate and a positive shift of the conduction band; (iv) a decrease in the recombination rate and a negative shift of the conduction band. These changes can be demonstrated by experiments. In addition, it is also necessary to take into account two processes, namely, electron transfer and recombination. For example, a positive shift of the conduction band may increase the Jsc. The reduced dark current would result in reductions in the Voc if there is an insignificant impact on the current. The dark current would be lowered by reducing the electron recombination on the TiO2 surface. The electron exchange at each interface of cells in the dark state is usually characterized by dark-state impedance testing (Figure 4.19). The second semicircle in Nyquist diagram indicates how difficult the electron recombination of TiO2 at the interface, corresponding to the Rct and Cu in the right-side circuit diagram. Rct is the impedance value of electron recombination at TiO2/electrolyte interface. A great Rct indicates a small-rate electron recombination at the interface, which corresponds to an improved Voc and Jsc of the cells. Cμ is the chemical capacitance at TiO2/electrolyte interface, which is related to the adsorption at interfaces and reflects the density and distribution of TiO2 surface states. Adsorption of organic compound-type additives on the counter electrode is typically very weak and thus exerts a relatively insignificant impact on the reactions at the counter electrode. However, such additives may react with I3– and affect the I3– diffusion. An ionic liquid additive may be adsorbed on the counter electrode. In Figure 4.19, the first semicircle shows the properties of the Pt counter electrode/electrolyte interface. Polarization resistance (Rp) reflects how difficult the reduction reaction of I3– at the counter electrode is. The lower the Rp is, the more easily the reduction reaction of I3– occurs. The constant phase angle element (CPE) reflects the electrical double-layer capacitance (Cp) at the Pt/electrolyte interface. The admittance (YQ) of the CPE is expressed as YQ = Y0(jω)n. There are two variables for YQ, namely, Y0 and n. The value of Y0 reflects the Cp at the Pt/electrolyte solution interface and n (0 ≤ n ≤ 1) is a dimensionless exponent that reflects the

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extent of surface coarseness of the Pt electrode, i.e., the extent of the deviation from the plate capacitance. Therefore, Cp reflects the property of the electrical double layer at the Pt/electrolyte interface and is mainly affected by material adsorption on the counter electrode [175]. -40

N719 N719+TBP

Z''/( ·cm2)

-30

-20

-10

0

Rs

0

20

40

60

RCE

Rct

Cp

C

Rw

80

2

Z'/( ·cm ) Figure 4.19: Nyquist diagram without TBP and with TBP (left) and impedance-fitting circuit diagram of electrolytes (right).

4.3.2 The classification of additives In the available literature, additives mainly can be classified into neutral molecules containing nitrogen atom, positively charged inorganic compounds, and metal iodides such as sodium iodides and guanidiniums. In addition, there are also some ionic liquids containing heterocyclic cations used as additives. Nitrogen-containing heterocyclic molecules mainly include TBP, NMBI, BI, and so on. The N atom with a lone pair electron originating from the heterocycle is adsorbed to (101) surface of TiO2 film and this achieves the formation of a covalent bond Ti←N (Figure 4.20) [176]. TBP is the most widely used additive in DSC electrolytes. TBP molecules can be chemisorbed on nano-TiO2 surfaces via Ti←N coordinate covalent bonds. The totally symmetrical “breathing” vibration of the pyridine ring of free or physically adsorbed TBP occurs at 996 cm−1. When TBP is chemisorbed, TBP with Ti←N-coordinate covalent bonds is present mainly between TiO2 and TBP, and the totally symmetric “breathing” vibration shifts to 1,007 cm−1 [177, 178]. Thus, the nitrogen atom, as an electron donor,

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exhibits electronegativity and causes a negative shift of the conduction band of TiO2 after chemisorbing the additive. Consequently, there is a decrease in the probability of electrons being injected from the excited state of the dye into the conduction band of TiO2, which in turn results in a decrease in the Jsc of the DSC. In addition, there is an increase in the difference in the energy level between the conduction band of TiO2 and the redox iodine pair, which in turn leads to an increase in the Voc. Moreover, the adsorption of N-containing heterocyclic molecules results in the formation of a barrier layer that obstructs the recombination reactions, extends the electron-transport lifetime in the TiO2 film and improves the Voc of the DSC. The reduction in dark current loss can also improve the Jsc. TBP can inhibit the reduction of I3– on the TiO2 electrode or dye-sensitized TiO2 electrodes. When there are no dyes adsorbed on TiO2, TBP can be chemisorbed on the TiO2 surface to suppress the oxidation of I–. When a dye is adsorbed on TiO2, TBP have little impact on the oxidation of I–. This also explains that the sulfur atom in the dye molecule (NCS) plays a very important role in the reduction of I3– and the oxidation of I– [157, 179]. In recent years, a small amount of literature has reported that NMBI can reduce the aggregation of pyridine-based ruthenium dye. Nitrogen atom of heterocyclic compounds and I3– can form a ligand (eq. (4.17)), which reduces the concentration of I3– and raises the concentration of I–. An equilibrium reached between I3– and I– would cause an improved hole-capture ability of I–, reduced the electron recombination of I3– and improved the Voc [180, 181]. The TiO2 surface of DSC in service is negatively charged and metal cations adsorb on the TiO2 surface due to electrostatic interaction. The positively charged cations may cause the conduction band of TiO2 to move toward the positive direction, thus increasing the probability of electrons being injected to the conduction band of TiO2 from the excited energy level of dye. This would cause an increase in Jsc but a decrease in Voc [182]. Li+ is the common metal-cation additive in the electrolyte solution of DSCs. If with small ion radius is Li+ is added into the electrolyte solution, Li+ mainly adsorbs on the surface of TiO2 film. With Li+ concentration increasing, Li+ may both adsorb on the surface of TiO2 film and be embed into the TiO2 film. TiO2 anatase undergoes spontaneous phase separation into lithium-poor (Li 0.01TiO2) and lithium-rich (Li0.6TiO2) when inserted into Li+. This enhances the surface states of TiO2 film, accelerates electron recombination and shortens the lifetime of electrons [183]. Li+ adsorption would increase the electron diffusion coefficient in TiO2 [184]. Moreover, Li+ would form dipoles with electrons in the conduction band. The dipoles would migrate both on the TiO2 film surface and off from the TiO2 surface. Such a migration of dipoles reduces the resistance when the transport of electrons in the conduction band occurs between two adjacent or nonadjacent Ti atoms and also shortens the transport distance of electrons [183]. Hence, adding Li+ into electrolyte solution facilitates the electron transport in TiO2 film significantly, which in turn improves the shortcircuit current of solar cell. Meanwhile, the recombination rate of the formed dipoles

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c

a

Ti5c

b

Ti6c

O2c

c

c

c

O3c

A

A

A

Figure 4.20: Structures of (a) TiO2 anatase (101) surface, (b) TBP adsorbed on a TiO2 anatase (101) surface, and (c) imidazole adsorbed on a TiO2 anatase (101) surface.

and I3– in the solution is high, which reduces the FF of the DSC. Cation adsorption on the TiO2 surface is affected by the cation size. Li+ with a very small size is easy to be adsorbed on the TiO2 surface, followed by imidazolium cations. TBA+ with a big size would exert an insignificant adsorption on the TiO2 surface. This difference will accordingly make an effect on the electron lifetime (Figure 4.21) [185]. Metal ions undergo thermal motion although they won’t be adsorbed on the counter electrode. The Larger the diameter of metal ions, the stronger the thermal motion brings an obstruction to the reaction at the counter electrode. The strong obstruction is reflected by small values of Rp and Cp. Currently, alkylimidazole acts as a common iodine source in the electrolyte solution of DSCs. One example of alkylimidazolium iodide is DMPII with a common concentration of 0.60 mol · L−1. However, there are no systematic investigations so far about the influences of DMPII, MPII and 1-methyl-3-heximidazolium iodide on the redox behavior of I3– and I– in the electrolyte solution, on the mass transfer control of I3– and the photovoltaic performance of DSCs. Apparent diffusion coefficients of I– and I3– in MPN solution of the three alkylimidazolium iodides mentioned above at 25 °C were calculated, finding that if the concentration of alkylimidazolium iodide is fixed, the diffusion coefficient of I3– remains constant as I2 concentration changes, while the diffusion coefficient of I– decreases slightly. In addition, the interfacial transport resistance of electrode reactions can be reduced by increasing I– and I2 concentrations in the

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electrolyte solution. Multilayered alkylimidazolium cations adsorbed on the surface of TiO2 film would accelerate electron transport in the nanoporous film and inhibit the recombination between I3– and the electrons in TiO2 conduction band. All these can improve the PCE of DSCs. Imidazolium cations would be adsorbed on the counter electrode, which can make an effect on the double electric layer of the counter electrode and increase the adsorption capacity of the I–/I3– redox couple. However, molecules containing N heterocyclic ring would not be adsorbed on the counter electrode, but would form a complex with I3– (eq. 4.17). This reduces the adsorption concentration of I3– on the counter electrode and in turn the catalytic reaction of the counter electrode is restricted by the I3– diffusion: − Heterocycles + I3− ! Heterocycles I2 + I

(4:17)

The adsorption of guanidiniums causes a positive shift of the TiO2 conduction band and the formation of a barrier layer that inhibits the electron recombination. As a result, the current increases, whereas there is no notable change in Voc [186]. The formation of a complex from an alkali metal iodide and a crown ether or cryptand can nearly expose the I−. Therefore, in an organic solvent, the formed complex owns a significantly improved solubility and I− reduction activity compared to the original alkali metal iodide. Macrocyclic polyethers are a major class of neutral compounds that emerged after 1967. Single-macrocyclic polyethers are referred to as crown ethers, while double-macrocyclic polyethers containing bridgehead nitrogen (N) atoms are referred to as cryptands. The structural formulas of common crown ethers and cryptands are shown in Figure 4.22. Generally, saturated crown ethers are colorless, viscous liquids or low-melting-point solids. Cryptands are oily liquids or low-melting-point solids. Since it is found that crown ethers and cryptands play a special role in coordinating with metal ions, particularly alkali metal ion, a lot of research work have been made to investigate the relationship between the synthesis/structure and the coordination properties of crown ethers and cryptands and their applications. Crown ethers and cryptands selectively coordinate with cations to form complexes and thus can be used to capture and separate metals. The solubility of an alkali metal salt in an aprotic solvent would increase after being coordinated with a crown ether or cryptand. In addition, exposed anions have very high reactivity. Therefore, crown ethers and cryptands are widely used in the nuclear power industry, electronic industry, electrochemical industry, photographic materials industry, military industry, organic synthesis, chemistry and other fields. Crown ethers and cryptands can be used as additives to prepare highpower cells with nonaqueous electrolytes. These cells own a long discharge time, a large discharge capacity, a high discharge efficiency, exceptional low-temperaturedischarge (−20 °C), and high-load properties. Research has shown that the apparent diffusion coefficient of I3− in the complexes formed from an alkali metal iodide and a crown ether or cryptand is lower than that in DMPII. In contrast, the apparent diffusion coefficient of I− in the complexes of an alkali metal iodide and a crown ether

0

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-

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TBA+

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Distance from the surface of TiO2/ nm

l-/ l3

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Double layer formed by Li

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Figure 4.21: Radius of different cations (left) and electron lifetime (right).

0.0

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252 Xu Pan, Songyuan Dai

Electron Lifetime / s

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or cryptand is higher than that in DMPII. Additionally, the variation trend in the apparent diffusion coefficients of I− and I3– are consistent with those of the Jsc and FF of DSCs. Specifically, the Jsc of a DSC containing a complex of an alkali metal iodide and a crown ether or cryptand is higher than that of a conventional DMPII ionic-liquid-based DSC and the FF of a DSC containing a coordination complex of an alkali metal iodide and a crown ether or cryptand is lower than that of a conventional DMPII liquid-based DSC. O

O O

O

O

O

O

O

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O

O

O

O

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O

12-C-4

15-C-5

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O O

2.1.1 Cryptand

O

O

O

O

N

2.2.1 Cryptand

N

O

O

O

O

O

O

N

2.2.2 Cryptand

Figure 4.22: Structural formulas of common crown ethers and cryptands.

4.3.3 Research progress on additives Recent researches have developed further the mechanism of conventional additives. Wang et al. investigated several salts with different cations in electrolytes and found that as the strength of the Lewis acid became weaker in the order of H+ > TBA+ > guanidinium > Li+ > Na+ > K+, the maximum absorption peak of the dye gradually shifted to shorter wavelength in the same order [187]. Gao et al. found that Mg (OOCCH3)2 would form hydrogen bonds with the dye when added into the electrolyte, which led to a blue shift of the absorption peak but weakened the aggregation of dye molecules [188]. Some high-temperature molten salts with high stability and nonvolatility have been used as alternative electrolytes for DSCs. These molten salts usually owns relatively long alkyl chain and can be adsorbed on the TiO2 surface to form a blocking layer, thus achieving a high PCE [189]. Zhang et al. reported when both additives containing different cations and TBP modified the TiO2 surface, the difference in the cation would influence the TBP adsorption and result in a difference effect on the Fermi levels of TiO2 (Figure 4.23) [190].

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Various additives with new structures and functions have been emerging in the recent reports. Thomas et al. investigated in detail the action of TBP and NMBI additives in ionic liquid-based redox electrolytes with varying iodine concentrations to extract the optimum additive/I2 ratio for each system. NMBI remarkably suppressed the dark current in ionic liquid electrolytes with a high iodine concentration and improved the Voc and Jsc. Guanidinium thiocyanate (GSCN) inhibited the electron recombination at the electrolyte/TiO2/dye interface only in the ionic liquid electrolyte with NMBI [191]. Raja et al. synthesized triazole-based additives with dendritic structures (Figure 4.24). The dendritic structures were co-adsorbed with the dye on the TiO2 surface to enhance the absorption and electrochemical behaviors of DSCs. Furthermore, as the number of triazole units increased in dendrimers, the DSC performance was improved [192]. Marszale et al. synthesized a series of tridcyanomethyl salts containing imidazole, pyridine and pyrrole with a low saturated vapor pressure as the additives in ionic liquid electrolytes and they obtained a high photovoltaic performance [193]. Kisserwan et al. investigated the effect of cuprous iodide (CuI) as an electrolyte additive on the performance of DSCs based on ionic liquid electrolytes. Cuprous iodide can accelerate the dye regeneration and improve the Jsc. Cuprous iodide would not make any effect on the interface electron recombination at the electrolyte/TiO2/dye surface, which in turn resulted in no changes in the Voc of DSCs [194]. Current researches have gradually improved the theoretical calculation about the interfacial interaction of additives. It was reported that adding a common N-heterocyclic additive into electrolytes had little effect on the potential of the redox couple (Vred) in electrolytes. A shift in the conduction band and a decrease in electron recombination can ultimately help to improve the Voc and reduce the Jsc. The extent to which the N-heterocyclic compound affected the Voc and Jsc was also related to its inherent heterocyclic structure and substituents. Kusama analyzed a series of substituents and summarized the relationship between the photoelectric parameters of N-heterocyclic compounds with different structures and partial charges and electronegativity of nitrogen atoms [195], which is shown in Figure 4.25. As the partial charges of nitrogen atoms on the heterocycle increased, the dark current of the cell decreased. A larger increase in the Voc corresponded to a more significant reduction in the Jsc. Meanwhile, the higher the ionization energy of a compound, the higher the Voc. It indicated that the nucleophilicity of a heterocyclic compound would influence the modifying effects of additives. Additives optimize the performance of the TiO2/electrolyte interface primarily by adjusting the conduction band of TiO2 and forming a recombination inhibition layer. However, the available additives usually exhibit a conflict to improve both Voc and Jsc. Figure 4.26 includes four possible cases of the effects of additives on the TiO2 conduction band shift and the interface charge recombination. In the fourth case, with additives the Voc is improved and simultaneously there is little effect on the Jsc. Furthermore, when selecting an additive, it is necessary to consider its inherent stability

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Figure 4.23: Proposed scheme for relationship between adsorption of Li+ (TBA+) and TBP on the TiO2 film at short circuit and open circuit.

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Figure 4.24: Molecular structures of triazole-based dendritic compounds.

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1,3,5-Triazine Pyrazine Pyrimindine Pyridazine Pyridine Tetrazole 1,2,4-Triazole 1,2,3-Triazole Imidazole Pyrazole None 0.0 0.5

0.6

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N N H

Tetrazole

N

N

N N N H

N

N N H

1,2,4-Triazole 1,2,3-Triazole

N H

N H

Imidazole

Pyrazole

Figure 4.25: Photovoltaic performance of different N-heterocyclic additive.

along with its potential to react with other components in the DSCs and thereby affect the performance of DSCs. For example, TBP is prone to forming complexes with I3−, thereby affecting the interfacial stability. Additives may affect both the adsorption and energy level of ruthenium (Ru)-based dyes. The additives on organic dyes other than ruthenium pyridyl dye requires further investigation.

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Figure 4.26: Four cases for the change in the TiO2 conduction-band edge and the interfacial electron recombination rate with an additive (dotted line) and without additives (solid line). (A) A positive shift of the conduction-band edge and an increase in the recombination rate; (B) a negative shift of the conduction band edge and an increase in the recombination rate; (C) a negative shift of the conduction band edge and an increase in the recombination rate; and (D) a positive shift of the conduction band edge and a decrease in the recombination rate.

Whenever possible, it is advisable to use an additive that exerts no effect on the redox couple and the counter electrode and that can be adsorbed on the TiO2 surface to achieve an increase in Voc and an insignificant effect on Jsc as shown in Figure 4.26 (D). Moreover, an additive should be considered as an excellent auxiliary adsorbent that enables broad-spectrum dyes with low adsorption or high aggregation to be adsorbed to the TiO2 surface to form a sensitizing layer, which may effectively improve the photovoltaic performance of the dye-sensitized solar cells.

References [1] [2] [3] [4]

[5] [6]

Koide N, Islam A, Chiba Y, et al., Improvement of efficiency of dye-sensitized solar cells based on analysis of equivalent circuit. J Photochem Photobio A: Chem, 2006, 182(3): 296–305. Kang T E, Cho H H, Cho C H, et al., Photoinduced charge transfer in donor-acceptor (DA) copolymer: Fullerene bis-adduct polymer solar cells. ACS Appl Mater Inter, 2013, 5(3): 861–868. Lin J F, Tu G Y, Ho C C, et al., Molecular structure effect of pyridine-based surface ligand on the performance of P3HT:TiO2 hybrid solar cell. ACS Appl Mater Inter, 2013, 5(3): 1009–1016. Tang Y T, Pan X, Zhang C N, et al, Influence of different electrolytes on the reaction mechanism of a triiodide/iodide redox couple on the platinized FTO glass electrode in dye sensitized solar cells. J Phys Chem C, 2010, 114: 4160–4167. Tang Y T, Pan X, Dai S Y, et al., Research progress of the counter electrode in dye-sensitized solar cells. Key Eng Mater, 2010, 451: 63–78. Huo Z P, Dai S Y, Zhang C G, et al., Low Molecular mass organogelator based gel electrolyte with effective charge transport property for long-term stable quasi-solid-state dye-sensitized solar cells. J Phys Chem B, 2008, 112(41): 12927–12933.

Chapter 4 Electrolyte used in dye-sensitized solar cells

[7]

[8] [9] [10]

[11] [12] [13] [14]

[15] [16] [17] [18]

[19] [20] [21] [22] [23] [24]

[25] [26] [27] [28]

259

Huo Z P, Dai S Y, Wang K J, et al., Nanocomposite gel electrolyte with large enhanced charge transport properties of an I3–/I– redox couple for quasi-solid-state dye-sensitized solar cells. Sol Energy Mater Sol Cells, 2007, 91(20): 1959–1965. Hauch A, Georg A, Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochim Acta, 2001, 46(22), 3457–3466. Han L Y, Koide N, Chiba Y, et al., Improvement of efficiency of dye-sensitized solar cells by reduction of internal resistance. Appl Phys Lett, 2005, 86(21): 213501. Fukui A, Komiya R, Yamanaka R, et al., Effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized solar cell. Sol Energy Mater Sol Cells, 2006, 90(5): 649–658. Sheldon R, Catalytic reactions in ionic liquids. Chem Commun, 2001, 23: 2399–2407. Perelygin V P, Stetsenko S G, On the determination of bismuth concentration in specimens by an alpha-activation technique. Nucl Tracks Rad Meas, 1993, 22(1–4), 823–826. Quarmby I C, Mantz R A, Goldenberg L M, et al., Stoichiometry of latent acidity in buffered chloroaluminate ionic liquids. Anal Chem, 1994, 66(21): 3558–3561. Ma M H, Johnson K E, Carbocation Formation by selected hydrocarbons in trimethylsulfonium bromide-alcl3/albr3-hbr ambient-temperature molten-salts. J Am Chem Soc, 1995, 117(5), 1508–1513. Quarmby I C, Osteryoung R A, Latent acidity in buffered chloroaluminate ionic liquids. J Am Chem Soc, 1994, 116(6), 2649–2650. Gordon C M, New developments in catalysis using ionic liquids. Appl Catal A Gen, 2001, 222(1–2), 101–117. Singh R P, Winter R W, Gard G L, et al., Quaternary salts containing the pentafluorosulfanyl (SF5) group. Inorg Chem, 2003, 42(19): 6142–6146. Yoshida Y, Muroi K, Otsuka A, et al., 1-ethyl-3-methylimidazolium based ionic liquids containing cyano groups: Synthesis, characterization, and crystal structure. Inorg Chem, 2004, 43(4): 1458–1462. Hurley F H, Wier T P, Electrodeposition of metals from fused quaternary ammonium salts. J Electrochem Soc, 1951, 98(5), 203–206. Chan B K M, Chang N H, Grimmett M R, Synthesis and thermolysis of imidazole quaternarysalts. Aust J Chem, 1977, 30(9), 2005–2013. Hirao M, Sugimoto H, Ohno H, Preparation of novel room-temperature molten salts by neutralization of amines. J Electrochem Soc, 2000, 147(11), 4168–4172. Bonhote P, Dias A P, Papageorgiou N, et al., Hydrophobic, highly conductive ambienttemperature molten salts. Inorg Chem, 1996, 35(5): 1168–1178. Ohno H, Yoshizawa M, Ion conductive characteristics of ionic liquids prepared by neutralization of alkylimidazoles. Solid State Ionics, 2002, 154: 303–309. Poole S K, Shetty P H, Poole C F, Chromatographic and spectroscopic studies of the solvent properites of a new series of room temperature liquid tetraalkylammonium sulfonates. Anal Chim Acta, 1989, 218(2), 241–264. Fukumoto K, Yoshizawa M, Ohno H, Room temperature ionic liquids from 20 natural amino acids. J Am Chem Soc, 2005, 127(8), 2398–2399. Zhang J, Martin G R, DesMarteau D D, Direct methylation and trifluoroethylation of imidazole and pyridine derivatives. Chem Commun, 2003, (18): 2334–2335. de Souza R F, Padilha J C, Goncalves R S, et al., Room temperature dialkylimidazolium ionic liquid-based fuel cells. Electrochem Commun, 2003, 5(8): 728–731. Holbrey J D, Seddon K R, The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J Chem Soc Dalton, 1999, (13): 2133–2139.

260

Xu Pan, Songyuan Dai

[29] Shi C W, Ge Q, Han S K, et al., An improved preparation of 1-methyl-3-propylimidazolium iodide and its application in dye-sensitized solar cells. Sol Energy, 2008, 82(5): 385–388. [30] Shi C W, Dai S Y, Wang K J, et al., Application of 3-hexyl-1-methylimidazolium iodide to dyesensitized solar cells. Acta Chim Sin, 2005, 63(13): 1205–1209. [31] Fuller J, Breda A C, Carlin R T, Ionic liquid-polymer gel electrolytes from hydrophilic and hydrophobic ionic liquids. J Electroanal Chem, 1998, 459(1), 29–34. [32] McCamley K, Warner N A, Lamoureux M M, et al., Quantification of chloride ion impurities in ionic liquids using ICP-MS analysis. Green Chem, 2004, 6(7): 341–344. [33] Seddon K R, Stark A, Torres M J, Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl Chem, 2000, 72(12), 2275–2287. [34] Holbrey J D, Reichert W M, Swatloski R P, et al., Efficient, halide free synthesis of new, low cost ionic liquids: 1,3-dialkylimidazolium salts containing methyl- and ethyl-sulfate anions. Green Chem, 2002, 4(5): 407–413. [35] Picquet M, Poinsot D, Stutzmann S, et al., Ionic liquids: Media for better molecular catalysis. Top Catal, 2004, 29(3–4): 139–143. [36] Lall S I, Mancheno D, Castro S, et al., Polycations. Part X. LIPs, a new category of room temperature ionic liquid based on polyammonium salts. Chem Commun, 2000, (24): 2413–2414. [37] Moulton R Electrochemical process for producing ionic liquids. US, 2003/0094380A1. [38] Wasserscheid P, Driessen-Holscher B, van Hal R, et al., New, functionalised ionic liquids from Michael-type reactions-a chance for combinatorial ionic liquid development. Chem Commun, 2003(16): 2038–2039. [39] Handy S T, Okello M, Dickenson G, Solvents from biorenewable sources: Ionic liquids based on fructose. Org Lett, 2003, 5(14), 2513–2515. [40] Deetlefs M, Seddon K R, Improved preparations of ionic liquids using microwave irradiation. Green Chem, 2003, 5(2), 181–186. [41] Varma R S, Namboodiri V V, An expeditious solvent-free route to ionic liquids using microwaves. Chem Commun, 2001, (7): 643–644. [42] Namboodiri V V, Varma R S, Microwave-assisted preparation of dialkylimidazolium tetrachloroaluminates and their use as catalysts in the solvent-free tetrahydropyranylation of alcohols and phenols. Chem Commun, 2002, (4): 342–343. [43] Khadilkar B M, Rebeiro G L, Microwave-assisted synthesis of room-temperature ionic liquid precursor in closed vessel. Org Process Res Dev, 2002, 6(6), 826–828. [44] Leadbeater N E, Torenius H M, Tye H, Ionic liquids as reagents and solvents in conjunction with microwave heating: Rapid synthesis of alkyl halides from alcohols and nitriles from aryl halides. Tetrahedron, 2003, 59(13), 2253–2258. [45] Leveque J M, Luche J L, Petrier C, et al., An improved preparation of ionic liquids by ultrasound. Green Chem, 2002, 4(4): 357–360. [46] Wang P, Zakeeruddin S M, Comte P, et al., Gelation of ionic liquid-based electrolytes with silica nanoparticles for quasi-solid-state dye-sensitized solar cells. J Am Chem Soc, 2003, 125(5): 1166–1167. [47] Wang M, Chen P, Humphry-Baker R, et al., The influence of charge transport and recombination on the performance of dye-sensitized solar cells. ChemPhysChem, 2009, 10(1): 290–299. [48] Papageorgiou N, Athanassov Y, Armand M, et al., The performance and stability of ambient temperature molten salts for solar cell applications. J Electrochem Soc, 1996, 143(10): 3099–3108. [49] Cao Y M, Zhang J, Bai Y, et al., Dye-sensitized solar cells with solvent-free ionic liquid electrolytes. J Phys Chem C, 2008, 112(35): 13775–13781.

Chapter 4 Electrolyte used in dye-sensitized solar cells

261

[50] Wang P, Zakeeruddin S M, Humphry-Baker R, et al., A binary ionic liquid electrolyte to achieve ≥ 7% power conversion efficiencies in dye-sensitized solar cells. Chem Mater, 2004, 16(14): 2694–2696. [51] Mohmeyer N, Kuang D B, Wang P, et al., An efficient organogelator for ionic liquids to prepare stable quasi-solid-state dye-sensitized solar cells. J Mater Chem, 2006, 16(29): 2978–2983. [52] Ito S, Zakeeruddin S M, Humphry-Baker R, et al., High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv Mater, 2006, 18(9): 1202-+. [53] Wang P, Zakeeruddin S M, Moser J E, et al., A new ionic liquid electrolyte enhances the conversion efficiency of dye-sensitized solar cells. J Phys Chem B, 2003, 107(48): 13280–13285. [54] Kuang D B, Klein C, Zhang Z P, et al., Stable, high-efficiency ionic-liquid-based mesoscopic dye-sensitized solar cells. Small, 2007, 3(12): 2094–2102. [55] Kuang D B, Wang P, Ito S, et al., Stable mesoscopic dye-sensitized solar cells based on tetracyanoborate ionic liquid electrolyte. J Am Chem Soc, 2006, 128(24): 7732–7733. [56] Bai Y, Cao Y M, Zhang J, et al., High-performance dye-sensitized solar cells based on solventfree electrolytes produced from eutectic melts. Nat Mater, 2008, 7(8): 626–630. [57] Matsumoto H, Matsuda T, Tsuda T, et al., The application of room temperature molten salt with low viscosity to the electrolyte for dye-sensitized solar cell. Chem Lett, 2001(1): 26–27. [58] Wang P, Wenger B, Humphry-Baker R, et al., Charge separation and efficient light energy conversion in sensitized mesoscopic solar cells based on binary ionic liquids. J Am Chem Soc, 2005, 127(18): 6850–6856. [59] Xi C C, Cao Y M, Cheng Y M, et al., Tetrahydrothiophenium-based ionic liquids for high efficiency dye-sensitized solar cells. J Phys Chem C, 2008, 112(29): 11063–11067. [60] Wachter P, Schreiner C, Zistler M, et al., A microelectrode study of triiodide diffusion coefficients in mixtures of room temperature ionic liquids, useful for dye-sensitised solar cells. Microchim Acta, 2008, 160(1–2): 125–133. [61] Fei Z, Kuang D, Zhao D, et al., A Supercooled Imidazolium Iodide Ionic Liquid as a LowViscosity Electrolyte for Dye-Sensitized Solar Cells. Inorg Chem, 2006, 45(26): 10407–10409. [62] Shi D, Pootrakulchote N, Li R Z, et al., New Efficiency Records for Stable Dye-Sensitized Solar Cells with Low-Volatility and Ionic Liquid Electrolytes. J Phys Chem C, 2008, 112(44): 17046–17050. [63] Kubo W, Kitamura T, Hanabusa K, et al., Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator. Chem Commun, 2002(4): 374–375. [64] Kawano R, Matsui H, Matsuyama C, et al., High performance dye-sensitized solar cells using ionic liquids as their electrolytes. J Photochem Photobio A-Chem, 2004, 164(1–3): 87–92. [65] Dai Q, Menzies D B, MacFarlane D R, et al., Dye-sensitized nanocrystalline solar cells incorporating ethylmethylimidazolium-based ionic liquid electrolytes. C R Chim, 2006, 9(5–6): 617–621. [66] Mazille F, Fei Z F, Kuang D B, et al., Influence of ionic liquids bearing functional groups in dye-sensitized solar cells. Inorg Chem, 2006, 45(4): 1585–1590. [67] Paulsson H, Berggrund M, Svantesson E, et al., Molten and solid metal-iodide-doped trialkylsulphonium iodides and polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells. Sol Energ Mat Sol C, 2004, 82(3): 345–360. [68] Santa-Nokki H, Busi S, Kallioinen J, et al., Quaternary ammonium polyiodides as ionic liquid/ soft solid electrolytes in dye-sensitized solar cells. J Photochem Photobio A: Chem, 2007, 186(1): 29–33. [69] Cai N, Zhang J, Zhou D F, et al., N-Methyl-N-Allylpyrrolidinium Based Ionic Liquids for Solvent-Free Dye-Sensitized Solar Cells. J Phys Chem C, 2009, 113(10): 4215–4221.

262

Xu Pan, Songyuan Dai

[70] Wang P, Zakeeruddin S M, Gratzel M, et al., Novel room temperature ionic liquids of hexaalkyl substituted guanidinium salts for dye-sensitized solar cells. Appl Phys A-Mater, 2004, 79(1): 73–77. [71] Li D M, Wang M Y, Wu J F, et al., Application of a New Cyclic Guanidinium Ionic Liquid on DyeSensitized Solar Cells (DSCs). Langmuir, 2009, 25(8): 4808–4814. [72] Kunugi Y, Hayakawa H, Tsunashima K, et al., Dye-sensitized solar cells based on quaternary phosphonium ionic liquids as electrolytes. Bull Chem Soc Jpn, 2007, 80(12): 2473–2475. [73] Cameron P J, Peter L M, Zakeeruddin S M, et al., Electrochemical studies of the CoIII/CoII (dbbip)2 redox couple as a mediator for dye-sensitized nanocrystalline solar cells. Coord Chem Rev, 2004, 248(13–14): 1447–1453. [74] Nusbaumer H, Moser J E, Zakeeruddin S M, et al., Co-II(dbbiP)(2)(2+) complex rivals triiodide/iodide redox mediator in dye-sensitized photovoltaic cells. J Phys Chem B, 2001, 105 (43): 10461–10464. [75] Nusbaumer H, Zakeeruddin S M, Moser J E, et al., An alternative efficient redox couple for the dye-sensitized solar cell system. Chem-Eur J, 2003, 9(16): 3756–3763. [76] Zhang Z, Chen P, Murakami T N, et al., The 2,2,6,6-tetramethyl-1-piperidinyloxy radical: An efficient, iodine-free redox mediator for dye-sensitized solar cells. Adv Funct Mater, 2008, 18(2): 341–346. [77] Snaith H J, Zakeeruddin S M, Wang Q, et al., Dye-sensitized solar cells incorporating a “liquid” hole-transporting material. Nano Lett, 2006, 6(9): 2000–2003. [78] Gorlov M, Pettersson H, Hagfeldt A, et al., Electrolytes for dye-sensitized solar cells based on interhalogen ionic salts and liquids. Inorg Chem, 2007, 46(9): 3566–3575. [79] Oskam G, Bergeron B V, Meyer G J, et al., Pseudohalogens for dye-sensitized TiO2 photoelectrochemical cells. J Phys Chem B, 2001, 105(29): 6867–6873. [80] Wang P, Zakeeruddin S M, Moser J E, et al., A solvent-free, SeCN-/(SeCN)3- based ionic liquid electrolyte for high-efficiency dye-sensitized nanocrystalline solar cells. J Am Chem Soc, 2004, 126(23): 7164–7165. [81] Yamanaka N, Kawano R, Kubo W, et al., Dye-sensitized TiO2 solar cells using imidazoliumtype ionic liquid crystal systems as effective electrolytes. J Phys Chem B, 2007, 111(18): 4763–4769. [82] Oregan B, Grätzel M, A low-cost, high efficiency solar-cell based on dye-sensitized collodial TiO2 films. Nature, 1991, 353(6346), 737–740. [83] Nazeeruddin M K, Kay A, Rodicio I, et al., Conversion Of Light To Electricity by Cis-X2bis(2,2ʹBipyridyl-4,4ʹ-Dicarboxylate) Ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, And SCN-) on Nanocrystalline Tio2 Electrodes. J Am Chem Soc, 1993, 115(14): 6382–6390. [84] Nazeeruddin M K, Pechy P, Renouard T, et al., Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J Am Chem Soc, 2001, 123(8): 1613–1624. [85] Tennakone K, Perera V P S, Kottegoda I R M, et al., Dye-sensitized solid state photovoltaic cell based on composite zinc oxide tin (IV) oxide films. J Phys D-Appl Phys, 1999, 32(4): 374–379. [86] Bach U, Lupo D, Comte P, et al., Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature, 1998, 395(6702): 583–585. [87] Huynh W U, Dittmer J J, Alivisatos A P, Hybrid nanorod-polymer solar cells. Science, 2002, 295(5564), 2425–2427. [88] O’Regan B, Schwartz D T, Large enhancement in photocurrent efficiency caused by UV illumination of the dye-sensitized heterojunction TiO2/RuLL ‘ NCS/CuSCN: Initiation and potential mechanisms. Chem Mater, 1998, 10(6), 1501–1509. [89] Kumara G R R A, Konno A, Senadeera G K R, et al., Dye-sensitized solar cell with the hole collector p-CuSCN deposited from a solution in n-propyl sulphide. Sol Energ Mat Sol Cells, 2001, 69(2): 195–199.

Chapter 4 Electrolyte used in dye-sensitized solar cells

263

[90] Kang J, Li W, Wang X, et al., Polymer electrolytes from PEO and novel quaternary ammonium iodides for dye-sensitized solar cells. Electrochim Acta, 2003, 48(17): 2487–2491. [91] Wang L, Fang S B, Lin Y, et al., A 7.72% efficient dye sensitized solar cell based on novel necklace-like polymer gel electrolyte containing latent chemically cross-linked gel electrolyte precursors. Chem Commun, 2005, (45): 5687–5689. [92] Murai S, Mikoshiba S, Sumino H, et al., Quasi-solid dye sensitised solar cells filled with phase-separated chemically cross-linked ionic gels. Chem Commun, 2003, (13): 1534–1535. [93] Ileperuma O A, Dissanayake M A K L, Somasundaram S, Dye-sensitised photoelectrochemical solar cells with polyacrylonitrile based solid polymer electrolytes. Electrochim Acta, 2002, 47(17), 2801–2807. [94] Wang P, Zakeeruddin S M, Exnar I, et al., High efficiency dye-sensitized nanocrystalline solar cells based on ionic liquid polymer gel electrolyte. Chem Commun, 2002, (24): 2972–2973. [95] Asano T, Kubo T, Nishikitani Y, Electrochemical properties of dye-sensitized solar cells fabricated with PVDF-type polymeric solid electrolytes. J Photoch Photobio A, 2004, 164(1–3), 111–115. [96] Kim D W, Jeong Y B, Kim S H, et al., Photovoltaic performance of dye-sensitized solar cell assembled with gel polymer electrolyte. J Power Sources, 2005, 149: 112–116. [97] Komiya R, Han L Y, Yamanaka R, et al., Highly efficient quasi-solid state dye-sensitized solar cell with ion conducting polymer electrolyte. J Photochem Photobio A-Chem, 2004, 164(1–3): 123–127. [98] Guo L, Dai S, Wang K, et al., P(VDF-HFP)-based gel electrolyte dye-sensitized nano-TiO2 thinfilm solar cells. Chem J Chin Univ, 2005, 10: 1934–1937. [99] Li W, Kang J, Lin Y, et al., Quasi-solid-state TiO2 nanocrystalline solar cells with polysiloxane gel network electrolyte. Chin Sci Bull, 2003, 48: 129–131. [100] Zhang C, Wang M, Zhou X, et al., Optimization of polymer electrolytes in dye-sensitized solar cells. Chin Sci Bull, 2004, 49(13): 1241–1243 [101] Kubo W, Murakoshi K, Kitamura T, et al., Fabrication of quasi-solid-state dye-sensitized TiO2 solar cells using low molecular weight gelators. Chem Lett, 1998(12): 1241–1242. [102] Shi C, Dai S, Wang K, et al. Research progress of electrolytes in dye-sensitized nano-thin film solar cells. Chem Bull, 2005, 68(1): W0001. [103] Mohmeyer N, Wang P, Schmidt H W, et al., Quasi-solid-state dye sensitized solar cells with 1, 3:2,4-di-O-benzylidene-D-sorbitol derivatives as low molecular weight organic gelators. J Mater Chem, 2004, 14(12): 1905–1909. [104] Kubo W, Murakoshi K, Kitamura T, et al., Quasi-solid-state dye-sensitized TiO2 solar cells: Effective charge transport in mesoporous space filled with gel electrolytes containing iodide and iodine. J Phys Chem B, 2001, 105(51): 12809–12815. [105] Murai S, Mikoshiba S, Sumino H, et al., Quasi-solid dye-sensitized solar cells containing chemically cross-linked gel – How to make gels with a small amount of gelator. J Photochem Photobio A-Chem, 2002, 148(1–3): 33–39. [106] Usui H, Matsui H, Tanabe N, et al., Improved dye-sensitized solar cells using ionic nanocomposite gel electrolytes. J Photochem Photobio A-Chem, 2004, 164(1–3): 97–101. [107] Lee H K, Chang S I, Yoon E, A capacitive proximity sensor in dual implementation with tactile imaging capability on a single flexible platform for robot assistant applications. MEMS 2006: 19th IEEE International Conference on Micro Electro Mechanical Systems, Technical Digest, 2006, 606–609. [108] Yang H, Cheng Y F, Li F Y, et al., Quasi-solid-state dye-sensitized solar cells based on mesoporous silica SBA-15 framework materials. Chin Phys Lett, 2005, 22(8): 2116–2118. [109] Yang H, Yu C Z, Song Q L, et al., High-temperature and long-term stable solid-state electrolyte for dye-sensitized solar cells by self-assembly. Chem Mater, 2006, 18(22): 5173–5177.

264

Xu Pan, Songyuan Dai

[110] Chen Z G, Yang H, Li X H, et al., Thermostable succinonitrile-based gel electrolyte for efficient, long-life dye-sensitized solar cells. J Mater Chem, 2007, 17(16): 1602–1607. [111] Chen Z G, Li F Y, Yang H, et al., A thermostable and long-term-stable ionic-liquid-based gel electrolyte for efficient dye-sensitized solar cells. ChemPhysChem, 2007, 8(9): 1293–1297. [112] Lin H, Zhuang D, Li X, et al. Research progress on solid electrolytes for dye-sensitized solar cells. Sci Tech Rev, 2007,(22): 63–67. [113] Lin H, Li X, Wang N, et al. Research status of electrolytes for dye-sensitized solar cells. World Sci-Tech R & D, 2006, (4): 41–45. [114] Tennakone K, Kumara G R R A, Kumarasinghe A R, et al., A Dye-sensitized nano-porous solidstate photovoltaic cell. Semicond Sci Technol, 1995, 10(12): 1689–1693. [115] Meng Q B, Takahashi K, Zhang X T, et al., Fabrication of an efficient solid-state dyesensitized solar cell. Langmuir, 2003, 19(9): 3572–3574. [116] Oregan B, Schwartz D T, Efficient photo-hole injection from adsorbed cyanine dyes into electrodeposited copper(i) thiocyanate thin-films. Chem Mater, 1995, 7(7), 1349–1354. [117] O’Regan B, Schwartz D T, Zakeeruddin S M, et al., Electrodeposited nanocomposite n-p heterojunctions for solid-state dye-sensitized photovoltaics. Adv Mater, 2000, 12(17): 1263–1267. [118] O’Regan B C, Lenzmann F, Charge transport and recombination in a nanoscale interpenetrating network of n-type and p-type semiconductors: Transient photocurrent and photovoltage studies of TiO2/Dye/CuSCN photovoltaic cells. J Phys Chem B, 2004, 108(14), 4342–4350. [119] Mahrov B, Hagfeldt A, Lenzmann F, et al., Comparison of charge accumulation and transport in nanostructured dye-sensitized solar cells with electrolyte or CuSCN as hole conductor. Sol Energy Mater Sol Cells, 2005, 88(4): 351–362. [120] O’Regan B C, Scully S, Mayer A C, et al., The effect of Al2O3 barrier layers in TiO2/Dye/CuSCN photovoltaic cells explored by recombination and DOS characterization using transient photovoltage measurements. J Phys Chem B, 2005, 109(10): 4616–4623. [121] Zhang X T, Sutanto I, Taguchi T, et al., Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell. Sol Energy Mat Sol C, 2003, 80(3): 315–326. [122] Tennakone K, Kumara G R R A, Kottegoda I R M, et al., A solid-state photovoltaic cell sensitized with a ruthenium bipyridyl complex. J Phy D-Appl Phy, 1998, 31(12): 1492–1496. [123] Tennakone K, Kumara G R R A, Kottegoda I R M, et al., Nanoporous n-TiO2/selenium/p-CuCNS photovoltaic cell. J Phy D-Appl Phys, 1998, 31(18): 2326–2330. [124] Kumara G R A, Konno A, Shiratsuchi K, et al., Dye-sensitized solid-state solar cells: Use of crystal growth inhibitors for deposition of the hole collector. Chem Mater, 2002, 14(3): 954–955. [125] Schmidt-Mende L, Kroeze J E, Durrant J R, et al., Effect of hydrocarbon chain length of amphiphilic ruthenium dyes on solid-state dye-sensitized photovoltaics. Nano Lett, 2005, 5(7): 1315–1320. [126] Snaith H J, Moule A J, Klein C, et al., Efficiency enhancements in solid-state hybrid solar cells via reduced charge recombination and increased light capture. Nano Lett, 2007, 7(11): 3372–3376. [127] Gebeyehu D, Brabec C J, Padinger F, et al., Solid state dye-sensitized TiO2 solar cells with poly(3-octylthiophene) as hole transport layer. Synth Met, 2001, 121(1–3): 1549–1550. [128] Murakoshi K, Kogure R, Wada Y, et al., Solid state dye-sensitized TiO2 solar cell with polypyrrole as hole transport layer. Chem Lett, 1997, 26(5): 471–472. [129] Saito Y, Kitamura T, Wada Y, et al., Poly(3,4-ethylenedioxythiophene) as a hole conductor in solid state dye sensitized solar cells. Synth Met, 2002, 131(1–3): 185–187. [130] Saito Y, Fukuri N, Senadeera R, et al., Solid state dye sensitized solar cells using in situ polymerized PEDOTs as hole conductor. Electrochem Commun, 2004, 6(1): 71–74.

Chapter 4 Electrolyte used in dye-sensitized solar cells

265

[131] Fukuri N, Saito Y, Kubo W, et al., Performance improvement of solid-state dye-sensitized solar cells fabricated using poly(3,4-ethylenedioxythiophene) and amphiphilic sensitizing dye. J Electrochem Soc, 2004, 151(10): A1745–A1748. [132] Xia J B, Masaki N, Lira-Cantu M, et al., Influence of doped anions on poly(3,4ethylenedioxythiophene) as hole conductors for iodine-free solid-state dye-sensitized solar cells. J Am Chem Soc, 2008, 130(4): 1258–1263. [133] Xia J B, Masaki N, Jiang K J, et al., The influence of doping ions on poly(3,4ethylenedioxythiophene) as a counter electrode of a dye-sensitized solar cell. J Mater Chem, 2007, 17(27): 2845–2850. [134] Kim Y, Lim J W, Sung Y E, et al., Photoelectrochemical oxidative polymerization of aniline and its application to transparent TiO2 solar cells. J Photochem Photobio A-Chem, 2009, 204(2–3): 110–114. [135] Zhang X H, Wang S M, Xu Z X, et al., Poly(o-phenylenediamine)/MWNTs composite film as a hole conductor in solid-state dye-sensitized solar cells. J Photochem Photobio A-Chem, 2008, 198(2–3): 288–292. [136] Ravirajan P, Peiro A M, Nazeeruddin M K, et al., Hybrid polymer/zinc oxide photovoltaic devices with vertically oriented ZnO nanorods and an amphiphilic molecular interface layer. J Phys Chem B, 2006, 110(15): 7635–7639. [137] Snaith H J, Gratzel M, Electron and hole transport through mesoporous TiO2 infiltrated with spiro-MeOTAD. Adv Mater, 2007, 19(21), 3643–3647. [138] Nogueira A F, Durrant J R, De Paoli M A, Dye-sensitized nanocrystalline solar cells employing a polymer electrolyte. Adv Mater, 2001, 13(11), 826–830. [139] Stergiopoulos T, Arabatzis I M, Katsaros G, et al., Binary polyethylene oxide/titania solidstate redox electrolyte for highly efficient nanocrystalline TiO2 photoelectrochemical cells. Nano Lett, 2002, 2(11): 1259–1261. [140] Kim J H, Kang M S, Kim Y J, et al., Dye-sensitized nanocrystalline solar cells based on composite polymer electrolytes containing fumed silica nanoparticles. Chem Commun, 2004, (14): 1662–1663. [141] Han H W, Liu W, Zhang J, et al., A hybrid poly(ethylene oxide)/poly(vinylidene fluoride)/TiO2 nanoparticle solid-state redox electrolyte for dye-sensitized nanocrystalline solar cells. Adv Funct Mater, 2005, 15(12): 1940–1944. [142] Wang H X, Li H, Xue B F, et al., Solid-state composite electrolyte Lil/3-hydroxypropionitrile/ SiO2 for dye-sensitized solar cells. J Am Chem Soc, 2005, 127(17): 6394–6401. [143] O’regan B, Grätzel M, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature, 1991, 353(6346), 737–740. [144] Hagfeldt A, Grätzel M, Light-induced redox reactions in nanocrystalline systems. Chem Rev, 1995, 95(1), 49–68. [145] Chiba Y, Islam A, Watanabe Y, et al., Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn J Appl Phys, 2006, 45(25): L638–L640. [146] Pichot F, Gregg B A, The photovoltage-determining mechanism in dye-sensitized solar cells. J Phys Chem B, 2000, 104(1), 6–10. [147] Zistler M, Wachter P, Wasserscheid P, et al., Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids. Electrochim Acta, 2006, 52(1): 161–169. [148] Gordon C M, McLean A J, Photoelectron transfer from excited-state ruthenium(II) tris (bipyridyl) to methylviologen in an ionic liquid. Chem Commun, 2000, (15): 1395–1396. [149] Andrews L, Prochaska E S, Loewenschuss A, Resonance raman and ultraviolet-absorption spectra of the triiodide ion produced by alkali iodide iodine argon matrix reactions. Inorg Chem, 1980, 19(2), 463–465.

266

Xu Pan, Songyuan Dai

[150] Ruff I, Korosiod I, Application of diffusion constant measurement to determination of rate constant of electron-exchange reactions. Inorg Chem, 1970, 9(1), 186–188. [151] Wang X G, Stanbury D M, Oxidation of iodide by a series of Fe(III) complexes in acetonitrile. Inorg Chem, 2006, 45(8), 3415–3423. [152] Nogueira A F, De Paoli M A, Montanari I, et al., Electron transfer dynamics in dye sensitized nanocrystalline solar cells using a polymer electrolyte. J Phys Chem B, 2001, 105(31): 7517–7524. [153] Wang X G, Stanbury D M, Oxidation of iodide by a series of Fe(III) complexes in acetonitrile. Inorg Chem, 2006, 45(8), 3415–3423. [154] Clifford J N, Palomares E, Nazeeruddin M K, et al., Dye dependent regeneration dynamics in dye sensitized nanocrystalline solar cells: Evidence for the formation of a ruthenium bipyridyl cation/iodide intermediate. J Phys Chem C, 2007, 111(17): 6561–6567. [155] Fitzmaurice D J, Frei H, Transient near-infrared spectroscopy of visible-light sensitized oxidation of I- at colloidal TiO2. Langmuir, 1991, 7(6), 1129–1137. [156] Kuciauskas D, Freund M S, Gray H B, et al., Electron transfer dynamics in nanocrystalline titanium dioxide solar cells sensitized with ruthenium or osmium polypyridyl complexes. J Phys Chem B, 2001, 105(2): 392–403. [157] Pelet S, Moser J E, Grätzel M, Cooperative effect of adsorbed cations and iodide on the interception of back electron transfer in the dye sensitization of nanocrystalline TiO2. J Phys Chem B, 2000, 104(8), 1791–1795. [158] Nakade S, Kanzaki T, Kubo W, et al., Role of electrolytes on charge recombination in dyesensitized TiO2 solar cell (1): The case of solar cells using the I–/I3– redox couple. J Phys Chem B, 2005, 109(8): 3480–3487. [159] Yanagida S, Yu Y H, Manseki K, Iodine/iodide-free dye-sensitized solar cells. Acc Chem Res, 2009, 42(11), 1827–1838. [160] Hara K, Horiguchi T, Kinoshita T, et al., Influence of electrolytes on the photovoltaic performance of organic dye-sensitized nanocrystalline TiO2 solar cells. Sol Energy Mater Sol Cells, 2001, 70(2): 151–161. [161] Wang Z S, Sayama K, Sugihara H, Efficient eosin Y dye-sensitized solar cell containing Br-/Br3- electrolyte. J Phys Chem B, 2005, 109(47), 22449–22455. [162] Hattori S, Wada Y, Yanagida S, et al., Blue copper model complexes with distorted tetragonal geometry acting as effective electron-transfer mediators in dye-sensitized solar cells. J Am Chem Soc, 2005, 127(26): 9648–9654. [163] Chen J, Xia J, Fan K, et al., A novel CuI-based iodine-free gel electrolyte for dye-sensitized solar cells. Electrochim Acta, 2011, 56(16): 5554–5560. [164] Tennakone K, Senadeera G K R, De Silva D B R A, et al., Highly stable dye-sensitized solidstate solar cell with the semiconductor 4CuBr 3S(C4H9)2 as the hole collector. Appl Phys Lett, 2000, 77(15): 2367–2369. [165] O’Regan B, Lenzmann F, Muis R, et al., A solid-state dye-sensitized solar cell fabricated with pressure-treated P25-TiO2 and CuSCN: Analysis of pore filling and IV characteristics. Chem Mater, 2002, 14(12): 5023–5029. [166] Snaith H J, Zakeeruddin S M, Wang Q, et al., Dye-sensitized solar cells incorporating a “liquid” hole-transporting material. Nano Lett, 2006, 6(9): 2000–2003. [167] Wang M K, Chamberland N, Breau L, et al., An organic redox electrolyte to rival triiodide/ iodide in dye-sensitized solar cells. Nat Chem, 2010, 2(5): 385–389. [168] Daeneke T, Kwon T H, Holmes A B, et al., High-efficiency dye-sensitized solar cells with ferrocene-based electrolytes. Nat Chem, 2011, 3(3): 211–215. [169] Li T C, Spokoyny A M, She C X, et al., Ni(III)/(IV) bis(dicarbollide) as a fast, noncorrosive redox shuttle for dye-sensitized solar cells. J Am Chem Soc, 2010, 132(13): 4580–4582.

Chapter 4 Electrolyte used in dye-sensitized solar cells

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[170] Yella A, Lee H W, Tsao H N, et al., Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science, 2011, 334(6060): 1203–1203. [171] Grätzel M, Dye-sensitized solar cells. J Photochem Photobio C: Photochem Rev, 2003, 4(2), 145–153. [172] Bisquert J, Vikhrenko V S, Interpretation of the time constants measured by kinetic techniques in nanostructured semiconductor electrodes and dye-sensitized solar cells. J Phys Chem B, 2004, 108(7), 2313–2322. [173] Schlichthorl G, Huang S Y, Sprague J, et al., Band edge movement and recombination kinetics in dye-sensitized nanocrystalline TiO2 solar cells: A study by intensity modulated photovoltage spectroscopy. J Phys Chem B, 1997, 101(41): 8141–8155. [174] Wang Z S, Zhou G, Effect of surface protonation of TiO2 on charge recombination and conduction band edge movement in dye-sensitized solar cells. J Phys Chem C, 2009, 113(34), 15417–15421. [175] Wang M K, Grätzel C, Moon S J, et al., Surface design in solid-state dye sensitized solar cells: Effects of zwitterionic co-adsorbents on photovoltaic performance. Adv Funct Mater, 2009, 19 (13): 2163–2172. [176] Kusama H, Orita H, Sugihara H, DFT investigation of the TiO2 band shift by nitrogencontaining heterocycle adsorption and implications on dye-sensitized solar cell performance. Sol Energy Mater Sol Cells, 2008, 92(1), 84–87. [177] Simpson S F, Harris J M, Raman-spectroscopy of the liquid solid interface – Monolayer and bilayer adsorption of pyridine on silica. J Phys Chem, 1990, 94(11), 4649–4654. [178] Hendra P J, Turner I D M, Loader E J, et al., Laser Raman-spectra of species adsorbed on oxide surfaces. II.. J Phys Chem, 1974, 78(3): 300–304. [179] Greijer H, Lindgren J, Hagfeldt A, Resonance Raman scattering of a dye-sensitized solar cell: Mechanism of thiocyanato ligand exchange. J Phys Chem B, 2001, 105(27), 6314–6320. [180] Cahen D, Hodes G, Grätzel M, et al., Nature of photovoltaic action in dye-sensitized solar cells. J Phys Chem B, 2000, 104(9): 2053–2059. [181] Gratzel M, Photoelectrochemical cells. Nature, 2001, 414(6861), 338–344. [182] Liu Y, Hagfeldt A, Xiao X R, et al., Investigation of influence of redox species on the interfacial energetics of a dye-sensitized nanoporous TiO2 solar cell. Sol Energy Mater Sol Cells, 1998, 55(3): 267–281. [183] Wagemaker M, Kentgens A P M, Mulder F M, Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase. Nature, 2002, 418(6896), 397–399. [184] Kambe S, Nakade S, Kitamura T, et al., Influence of the electrolytes on electron transport in mesoporous TiO2-electrolyte systems. J Phys Chem B, 2002, 106(11): 2967–2972. [185] Nakade S, Makimoto Y, Kubo W, et al., Roles of electrolytes on charge recombination in dyesensitized TiO2 solar cells (2): The case of solar cells using cobalt complex redox couples. J Phys Chem B, 2005, 109(8): 3488–3493. [186] Kopidakis N, Neale N R, Frank A J, Effect of an adsorbent on recombination and band-edge movement in dye-sensitized TiO2 solar cells: Evidence for surface passivation. J Phys Chem B, 2006, 110(25), 12485–12489. [187] Wang Z-S, Sugihara H, N3-sensitized TiO2 films: In situ proton exchange toward open-circuit photovoltage enhancement. Langmuir, 2006, 22(23), 9718–9722. [188] Gao R, Wang L, Ma B, et al., Mg(OOCCH3)2 Interface modification after sensitization to improve performance in quasi-solid dye-sensitized solar cells. Langmuir, 2010, 26(4): 2460–2465. [189] Zhao J, Yan F, Qiu L, et al., Benzimidazolyl functionalized ionic liquids as an additive for high performance dye-sensitized solar cells. Chem Commun, 2011, 47(41): 11516–11518.

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[190] Zhang S, Yang X, Zhang K, et al., Effects of 4-tert-butylpyridine on the quasi-Fermi levels of TiO2 films in the presence of different cations in dye-sensitized solar cells. Phys Chem Chem Phys, 2011, 13(43): 19310–19313. [191] Stergiopoulos T, Rozi E, Karagianni C S, et al., Influence of electrolyte co-additives on the performance of dye-sensitized solar cells. Nanoscale Res Lett, 2011, 6: 307. [192] Raja S, Satheeshkumar C, Rajakumar P, et al., Influence of triazole dendritic additives in electrolytes on dye-sensitized solar cell (DSSC) performance. J Mater Chem, 2011, 21(21): 7700–7704. [193] Marszalek M, Fei Z, Zhu D R, et al., Application of ionic liquids containing tricyanomethanide C(CN)3– or tetracyanoborate B(CN)4– anions in dye-sensitized solar cells. Inorg Chem, 2011, 50(22): 11561–11567. [194] Kisserwan H, Ghaddar T H, Dalton Trans, 2011, 40(15): 3877–3884. [195] Kusama H, Kurashige M, Arakawa H, Influence of nitrogen-containing heterocyclic additives in I–/I3– redox electrolytic solution on the performance of Ru-dye-sensitized nanocrystalline TiO2 solar cell. J Photochem Photobio A-Chem, 2005, 169(2), 169–176.

Xu Pan

Chapter 5 Counter electrode for dye-sensitized solar cells The role of a counter electrode is to collect electrons that pass from the photoanode through an external circuit transmission, and then transfer them to the electron acceptors in the electrolyte (liquid, colloid, or solid) to complete the cycle of electronic transport. Additionally, it has a catalytic effect –accelerating the electron exchange rate between the redox couple and the cathode in the electrolyte – thus improving the overall performance of the solar cell. A common redox couple in DSC electrolyte is I–/I3–; the reduction reaction of I3– on the surface of the electrode is I3− + 2e − ! 3I − In theory, in the open-circuit state (current value is zero), the maximum voltage that the DSC can provide depends on the redox potential of the electrolyte and the Fermi level of the photoanode semiconductor. However, when the DSC is in working condition, due to the loss of current in the process through the electrolyte, such as mass transfer overpotential, current loss at the surface of electrolyte/counter electrode and dynamics overpotential, the counter electrode potential is lower than the theoretical value in the open-circuit state. The ionic conductivity and duplet for transmission between the electrodes in the electrolyte are the main determining factors of this loss, which is known as the mass transfer overpotential (ηmt). At the same time, the catalytic reduction activity for redox duplet on the surface of electrode can also cause this loss, which is known as dynamic or charge exchange overpotential (ηct). The loss of total voltage on the counter electrode is the sum of the above, which is called the total counter electrode overpotential (ηCE) [1]. Transparent conductive glass counter electrode of FTO and ITO are widely used in the large area of DSC cells, but their catalytic reduction activity of I3– is bad; so the total counter electrode overpotential is larger and the low charge exchange rate leads to a large number of cells inside the energy consumption. Hence, it is necessary to make surface modification. At present, the most common electrode materials are the conductive substrate that is deposited as a layer of metal platinum on the surface. Other materials, such as carbon (e.g., graphite, carbon black, carbon nanotubes, graphene, etc.), polymers and compound materials have also been reported.

Xu Pan, Hefei Institutes of Physical Science, Chinese Academy of Sciences https://doi.org/10.1515/9783110344363-005

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5.1 Counter electrode and preparation method 5.1.1 Pt counter electrode Research shows that the reaction rate of I3– + 2e–→3I– on the platinum electrode is high [2]. Even a small amount of platinum (5–10 µg · cm−2) is enough to decrease the reduction reaction overpotential of I3–; the exchange current density can reach 20 mA · cm−2 [3]. Pt has a low super potential and a better catalytic activity to the redox electron pair in the electrolyte of most DSCs. The preparation process of Pt counter electrode is simple and it is widely used in preparing the counter electrode of dye-sensitized solar cells. However, due to the high cost, the dosage of Pt needs to be reduced and the catalytic activity needs to be improved so that the cost of counter electrode can be reduced effectively. At present, the main methods to prepare the Pt counter electrode are the electric deposition method, the magnetron sputtering method, the thermal decomposition method and so on. 5.1.1.1 Pt electrode prepared by electric deposition Pt nanoparticles on the Pt electrode are prepared by electrodeposition. They can be dispersed uniformly and compactly on the surface of the FTO conductive glass. Pt nanoparticles and FTO conductive glass substrate have strong adhesion, reduced surface defect, bright mirror surface, and good reflective performance, so the catalytic activity is high. Yoon et al. [4] prepared a Pt electrode by electrodeposition using the H2PtCl6 solution, which added non-ionic surface active agent as the stabilizer. The photoelectric conversion efficiency of the DSC that is assembled by this counter electrode can be 7.6%. Lin et al. [5] prepared a Pt electrode by the method of direct current sedimentary in 30 min and used the electrolyte 3-(2-amino ethyl amine) propyl-methyl dimethoxy silane. The electrode had low exchange resistance, less load Pt, and high effective specific surface area. The photoelectric conversion efficiency of the DSC assembled by this counter electrode can be 7.39%. Kim et al. [6] prepared a Pt electrode, one each with the direct current sedimentary method and the alternative current sedimentary method. The catalytic activity of the Pt electrode prepared by the first method was higher than the one developed by the second method, and the specific surface area was larger. However, the Pt electrode film prepared by the electrodeposition method is relatively thick, with high Pt content, small specific surface area, and the ability to adsorb I3– is weak, so the catalytic efficiency is low and the application of DSC in industrial production is limited.

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5.1.1.2 Pt electrode prepared by the magnetron sputtering technique Pt electrode prepared by the magnetron sputtering method has low electron exchange resistance, high surface area, strong adhesion between the Pt nanoparticles and FTO conductive glass substrate, but it has high Pt loading amount. Choi et al. [7] prepared a Pt electrode by the magnetron sputtering method with a protection of 0.67 Pa of argon. In this method, the Pt electrode has a larger effective area than the ordinary sputtering method. The DSC (6 cm × 4 cm) assembled by the Pt electrode prepared by the magnetron sputtering method with six blocks in series and 5 blocks in parallel, has an open-circuit voltage of 4.8 V, a short circuit current of 569 mA, and a photoelectric conversion efficiency of about 3.6%. Lee et al. [8] plated a layer of thin Pt film on ITO by sputtering as the counter electrode of DSC. Nano-structured Pt film not only has a high transmittance (75%), but also has low electron transfer resistance. In the case of positive exposure, the magnetron sputtering method using Pt film, with a nanosize thickness (1.4 nm) and the synergistic effect of reflective aluminum foil, can increase the cell efficiency of DSC from 6.8% to 7.9%. In order to reduce the cost, Kim et al. [9] prepared a Pt electrode with double components of Pt-NiO and Pt-TiO2 by the magnetron sputtering method. Compared with the conventional method, Pt electrode with double components can not only reduce the load amount of Pt, but also has a high specific surface area, thus improving the catalytic activity of Pt electrode and the photoelectric conversion efficiency of DSC. 5.1.1.3 Pt electrode prepared by the thermal decomposition method H2PtCl6 solution was daubed to conductive glass substrate in the thermal decomposition method, and under the condition of heating, H2PtCl6 decomposed into Pt nanoparticles. The preparation process is simple. The Pt nanoparticles prepared are relatively uniform and porous, which are easy to adsorb more electrolytes. So, the Pt electrode prepared by the thermal decomposition method has a good catalytic effect. However, the thermal decomposition method also has some disadvantages as following adhesion strength of Pt nanoparticles and FTO conductive glass substrate is weak and the high temperature thermal decomposition also increases the square resistance of FTO conductive glass. Hao et al. [10] prepared Pt electrodes on FTO conductive glass by the magnetron sputtering method, electrodeposition and thermal decomposition. It was found that the DSC assembled by the Pt electrode prepared by electrodeposition has the maximum output power, but the load amount of Pt is high, which does not adapt to the characteristic of economy. As the thermal decomposition method is a simple and it is possible to prepare a rapid and efficient Pt electrode, this method is presently commonly used to prepare the Pt electrode of DSC.

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5.1.1.4 Pt electrodes prepared by the chemical reduction method Compared to other preparation methods of Pt electrode, the chemical reduction method is very simple with a low processing temperature. Chen et al. [11] prepared a Pt electrode by the reduction method. Firstly, H2PtCl6 was dissolved in terpineol; the solution was screen printed on the surface of ITO-PEN, and dried at 80 °C for 2 h, and then NaBH4 solution was used to reduce Pt4+ at 40 °C. Subsequently, it was processed by two methods: the first method is water heating treatment process under atmospheric pressure. The electrode is placed in a non-sealed container, filled with 100 °C water, for 4 h to remove organic residue, and then the electrode was dried at 80 °C. The other method is to sinter the Pt electrode for 4 h at 100 °C. Compared to the first method, this method achieves higher electrocatalytic activity of prepared Pt electrode, lower charge transfer resistance, and better light transmittance (400–800 nm, 70%); the cell assembled with this electrode has an efficiency of 5.41%. Sun et al. [12] used polyols to reduce H2PtCl6 at less than 200 °C, and deposited the nano Pt film on substrates, such as conducting polymer films, ITO and polyimide. The DSC assembled by the electrode prepared by this method has an efficiency of 8.10%. This kind of preparation method is suitable for the preparation of Pt electrodes with large area on flexible organic substrates. 5.1.1.5 Plating Pt electrodes on other substrates The counter electrode prepared by the plating method traces Pt on the FTO conductive glass has a high catalytic performance to the reduction of I3–, but as a result of the large square resistance of FTO conductive glass, the fill factor of DSC is greatly reduced; thus, the photoelectric conversion efficiency of DSC is affected. Hence, it is of great significance to choose a substrate with good electrical conductivity for the preparation of the electrode to improve the photoelectric conversion efficiency of DSC. Many metals, such as stainless steel and nickel, are difficult to be directly applied in liquid DSC because of the corrosive effect of I3–/I– electrolyte. However, if we plate F doping SnO2 or carbon on these metals, they can be used as the substrate material of the electrode. Ma et al. [13, 14] sputtered a small amount of Pt on a stainless steel and nickel sheet to prepare a counter electrode; the DSCs showed a photoelectric transformation efficiency of 5.24% and 5.13%, respectively. Lin et al. [15] deposited a layer of the NiP alloy on the surface of FTO conductive glass, and prepared the Pt/NiP electrode with the thermal decomposed H2PtCl6. Compared with the conventional FTO conductive substrate of Pt electrode, Pt/NiP electrode can increase the light reflection, which can improve the efficiency of light collection. The DSC with the Pt/NiP counter electrode has a photoelectric conversion efficiency of 8.30%.

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5.1.2 Carbon counter electrode Mainly because of low preparation cost and high photoelectric conversion efficiency, DSC has attracted widespread attention. Although very small amounts of Pt plated on FTO conductive glass could provide a good catalytic effect, the cost of preparing a large area of the cell is still expensive. In order to reduce the production cost of DSC, people wish to use a cheap catalytic material, instead of Pt, to prepare the electrodes. Carbon materials own low price, stable performance and high electrical conductivity, and the catalytic activity of I3– reduction reaction is high; so carbon material have become a hotspot of electrode materials in the research of DSC. Currently, carbon materials used for counter electrode mainly include carbon black, activated carbon, graphite, nanocarbon powder, mesoporous carbon and carbon nanotube (CNT).

5.1.2.1 Common carbon electrode Imoto et al. [16] mixed carbon powder, carbon black, hydroxymethyl cellulose and 30% ethanol into a paste and evenly coated the mixture on an FTO conductive glass. Next, it was thermally treated at 180 °C to form common carbon electrode. The DSC assembled by this carbon electrode has a photoelectric conversion efficiency of 3.89%. Huang et al. [17] used only graphite as the catalyst for a counter electrode. The DSC assembled by the graphite electrode has a photoelectric conversion efficiency of 5.7%. Kay et al. [18] added 20% of carbon black to graphite and prepared carbon electrode. This carbon electrode has a square resistance of about 5 Ω/□, and it has a large specific surface area and a high catalytic activity in the reduction of I3– reaction. The DSC assembled by this carbon electrode has a photoelectric conversion efficiency of 6.67%. Murakami et al. [19] prepared high performance carbon electrodes with carbon black and TiO2 as raw materials. The filling factor and the photoelectric conversion efficiency are affected by the thickness of carbon black on the carbon electrode. When the thickness of the carbon black is 14.47 μm, they obtained the DSC with a current density of 16.8 mA cm–2, an open-circuit voltage of 789.8 mV, a fill factor of 0.685 and a PCE of 9.1%. Ramasamy et al. [20] deposited the nano carbon powder on the FTO conductive glass and prepared the counter electrode. The photoelectric conversion efficiency is 6.73% in the DSC, and it has a good stability. Lee et al. [21] made a counter electrode using nano carbon powder, and the DSC assembled by this electrode has a photoelectric conversion efficiency of 7.56%. After 60 days, the conversion efficiency remains at 84% of the original efficiency. Li et al. [22] prepared a mesoporous carbon electrode under the conditions of low temperature, by cladding a layer of carbon paste without organic adhesive on the FTO conductive substrate. The carbon paste was prepared by dispersing the activated

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carbon in a solution of stannic chloride (SnCl4) by the ball-milling method. In the ball milling process, the water-soluble stannic chloride is employed into a tetravalent stannate gel as a binder. The DSC assembled by this mesoporous carbon electrode has a PCE of 6.10%. Wang et al. [23] used mesoporous carbon as the counter electrode catalyst of DSC and found that the PCE can be increased by increasing the amount of the load carbon. When the amount of load carbon on the counter electrode is 339 μg · cm−2, the PCE is 6.18%. Ramasamy et al. [24] prepared a double holeordered mesoporous carbon with high a specific surface area (1,575 m2 · g−1) and used it as the counter electrode catalyst of DSC; the PCE was 7.46%. Electrochemical tests show that the electron transfer resistance between the ordered mesoporous carbon electrode and the electrolyte is low, hence the filling factor and the photoelectric conversion efficiency can be improved.

5.1.2.2 Carbon nanotube (CNT) electrode Carbon nanotubes can be divided into single-walled carbon nanotubes (SWCNT) and multiwalled carbon nanotube (MWCNT). A single-walled carbon nanotube is made up of a single layer of graphite sheet which is rolled up; a multiwalled carbon nanotube is composed of several coaxial arrangements of graphite sheets. Carbon nanotube materials have the characteristic of large specific surface area, high conductivity and good chemical stability. so, the counter electrode of DSC has attracted widespread attention. However, carbon nanotubes cannot provide high exchange current density. In order to achieve the catalytic activity comparable to Pt, we need to deposit more carbon nanotubes on the counter electrode. Imoto et al. [25] used SWCNT as the catalyst of the counter electrode, and when used in DSC, the PCE could be 4.5%. Ramasamy et al. [26] successfully sprayed the solution, matched by MWCNT, on the FTO conductive glass substrates and used it as the catalyst of I3– reduction reaction in DSC. The spraying quantity of MWCNT on the FTO conductive glass can influence the fill factor of the DSC. Under standard light intensity, the maximum fill factor is 0.62 and the photoelectric conversion efficiency reaches 7.59%. Lee et al. [27] used MWCNT as the catalyst of the counter electrode. MWCNT can accelerate the electron transfer at the interface between the electrolyte and the electrode hence the electronic exchange resistance decreases and the filling factor increases. Under standard illumination, the PCE of the DSC-applied MWCNT electrode reaches 7.7%. Nam et al. [28] sprayed and distilled the CNT at first, and then prepared the CNT electrode by the screen printing method and the chemical vapor deposition method. The PCE was more than 10%, and the performance of the CNT electrode prepared by the deposition method is better than the one prepared by the screen printing method.

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It is feasible to use carbon instead of Pt as the counter electrode catalyst of DSC, and carbon has the characteristics of low cost, heat resistant, corrosion resistant, simple manufacturing process, and a certain practical value. However compared with the Pt electrode, the carbon electrode has lower catalytic activity. The PCE of the DSC assembled by the carbon electrode is about 20% lower than the one assembled by the Pt electrode, and the bonding between carbon and the FTO conductive glass substrate is not strong. In order to overcome the defects of carbon materials as the counter electrode catalyst of DSC, we mix the carbon or carbon nanotubes into a solution of Pt. After sintering, the adhesion of carbon and the FTO conductive glass substrates can be increased. We also make the Pt nanoparticles loaded on carbon nanoparticles to reduce the amount of Pt so that the effective specific surface area of Pt nanoparticles can be increased.

5.1.2.3 Pt/carbon electrode Cai et al. [29] prepared a Pt/acetylene black electrode by the thermal decomposition of H2PtCl6 on the acetylene black substrate. Pt nanoparticles are uniformly dispersed on the surface of the acetylene black. The amount of Pt loading on the Pt/ acetylene black electrode is just 2.0 μg · cm−2, far lower than the one prepared by the conventional method (5–10 μg · cm−2). The electrochemical measurements show that the electron transfer resistance of Pt/acetylene black electrode is 1.48 Ω · cm−2, and the PTE can reach 8.6%. Li et al. [30] prepared a Pt/carbon black electrode by using H2PtCl6 reduced by NaBH4. This electrode has a very high catalytic activity to the reduction reaction of I3– and the PTE can reach 6.72%. In case it does not reduce the PCE of the DSC, the Pt/carbon black electrode can reduce the cost of DSC. Wang et al. [31] prepared mesoporous carbon with a specific surface area of 380 m2 · g−1. And then, H2PtCl6 can be reduced on the mesoporous carbon. The size of Pt nanoparticles (3.4 nm) is uniform. It can be highly dispersed on the mesoporous carbon and the PTE can reach 6.62%.

5.1.3 Other electrode materials 5.1.3.1 Conductive polymer electrode Conductive polymer has the characteristic of low cost, and simple preparation process, and doping and a change of the preparation process can improve the electrical conductivity, improving the catalytic activity to the electrolyte, so that it can replace the counter electrode made of precious metals, such as Pt. The study of conducting polymer as a catalyst for counter electrode will bring better prospects for

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the future development of DSC. Conductive polymers that are commonly are thiophene derivatives, polyaniline (PANI), polypyrrole, and so on. 5.1.3.1.1 Polythiophene and its derivatives In 1988, Groenendaal et al. [32] synthesized poly(3,4-ethylenedioxythiophene) (PEDOT). It has good light transmittance, high conductivity, and a high catalytic activity to the reduction of I3–. Saito et al. [33] daubed the PEDOT (PEDOT:TsO)-doped p-toluenesulfonic acid (TsO) to the surface of conductive glass, and then prepared a PEDOT:TsO electrode at a low temperature (110 °C). The PEDOT:TsO electrode has a high catalytic activity, and the catalytic activity increases with the increase in the thickness of the PEDOT:TsO film. When a 2 μm thick PEDOT:TsO film is applied to the ionic liquidbased DSC, the PCE can reach 3.93%. Lee et al. [34] found that with the increase in the ratio of imidazole/PEDOT, its roughness decreases and the conductivity of the PEDOT film increases. When the ratio of imidazole/PEDOT is 2:1, the PCE can reach 7.44%. The improving of cell efficiency may be due to the improvement in the active area of the PEDOT film and the catalytic activity enhancement of the PEDOT electrode. The mixture of PEDOT and polystyrene sulfonic acid (PSS) has the advantages of good water solubility, high light transmittance, and high catalytic activity. Its transparent property can also be used in the preparation of a clear flexible counter electrode. Shibatal et al. [35] prepared a polystyrene sulfonic acid base (PSS)-doped PEDOT (PEDOT:PSS) electrode. The short-circuit current density of the gel electrolyte DSC, assembled by the PEDOT:PSS electrode, can reach 11.3 mA · cm−2, and it increases with the increase in the thickness of the PEDOT:PSS film. The electrical conductivity of the pure PEDOT:PSS film is very low, which makes its application limited, but it can be greatly improved by doping different nanoparticles. Chen et al. [36] doped carbon black (0.1 wt%) to the PEDOT:PSS film dealing with dimethyl sulfoxide(DMSO), and the PCE of this PEDOT:PSS electrode can reach 5.81%. It is possible that the doping of DMSO improves the conductivity of PEDOT: PSS film, and the carbon black increases the effective active area of the PEDOT:PSS film, so that the catalytic activity of PEDOT:PSS film increases. Hong et al. [37] plated graphite-doped PEDOT:PSS film at room temperature on the indium tin oxide (ITO) substrate, doped by the method of spin coating, and used it as the counter electrode of DSC. ITO electrode, covered by mixture layer (containing 1 wt% of graphite) with a thickness of 60 nm, has a high transmittance (>80%) in the visible region and a high electrocatalytic activity. The DSC assembled by this electrode has a PCE of 4.5%. But in the same condition, the DSC assembled by the Pt electrode is 6.3%. Lee et al. [38] prepared poly(3,3-diethyl-3,4-dihydro-2H-thiophene-[3, 4-b] and [1,4] two oxygen-mixed heptyl ring) (PProDOT–Et2) film by in situ polymerization

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on the FTO conductive glass as the counter electrode by the electrochemical method and the photoelectric transformation efficiency can reach 7.88% by this method. 5.1.3.1.2 Polyaniline (PANI) electrode PANI has high electrical conductivity, acting as the common hole-transporting materials in the solar cell. Li et al. [39] prepared electrodes with PANI. PANI is mesoporous and its aperture is about 100 nm, so PANI electrode has small electron transfer resistance and high electric catalytic activity for reduction reaction of I3–. The photoelectric conversion efficiency of DSC assembled with the PANI electrode can reach 7.15%. Li Zuopeng et al. [40] discussed the influence of different doping ions (such as SO42–, ClO4–, BF4–, Cl–, and TsO–) on the microstructure and electrochemical activity of the PANI film, wherein the doped SO42– anion of the PANI film (PANI-SO4) has a mesoporous structure and the pore size is up to several microns. Comparing with the Pt electrode, the mesoporous-structured PANI-SO4 film has higher reduction current and smaller interface transfer resistance for I3–. Under standard light conditions, the photoelectric conversion efficiency of DSC assembled with PANI electrode was up to 5.6%, while under the same conditions, the photoelectric conversion efficiency of the DSC assembled with the Pt electrode was only 6.0%. At room temperature conditions, Zhang et al. [41] successfully plated nano PANI film with controllable thickness onto a conductive glass FTO using cyclic voltammograms. By optimizing the preparation conditions of PANI electrode, the short circuit current density of DSC, assembled with the PANI electrode, increased by 11.6%, relative to that of the DSC assembled with the Pt electrode. Through voltage regulator control technology, Qin et al. [42] electrodeposited PANI from H2SO4 (0.5 mol · L−1) solution containing 0.3 mol · L−1 to the surface of stainless steel, to prepare a low cost, tough electrode. The mesoporous structure of the PANI film makes the PANI electrode have high conductivity and excellent catalytic activity. Ameen et al. [43] used pure PANI nanofibers (PANI-NFs) and PANI-NFs (PANINFs-SFA) of chemical-doped sulfonamide (SFA) as electrode materials for an efficient DSC. Under standard lighting irradiation, the photoelectric conversion efficiency of DSC, using the PANI-NFs electrode, reached 4.0%. While the photoelectric conversion efficiency of DSC, assembled with PANI-NFs-SFA electrode, increased by 27%, reaching 5.5%. The reason may be that the PANI-NFs film doped by SFA increases the catalytic activity of PANI-NFs film for the reduction reaction of I3–. 5.1.3.1.3 Polypyrrole (PPy) electrode Wu et al. [44] synthesized polypyrrole (PPy) conductive polymers and prepared the PPy conductive polymer, uniformly and intimately, on the FTO conductive glass

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substrate to form a PPy electrode using the deposition method. PPy nanoparticles are mesoporous; their pore sizes are about 40–60 nm. PPy electrode has small interface transfer resistance and high electrocatalytic activity, the photoelectric conversion efficiency of DSC assembled with the PPy electrode was up to 7.66%. Makris et al. [45] used a constant potential deposition method to synthesis PPy with pyrrole monomer in solution and used PPy film as the DSC electrode catalytic material; its catalytic activity is 30% lower than that of the Pt electrode.

5.1.3.2 Compound electrodes Gong et al. [46] adopted the hydrothermal method for the in-situ growth of counter electrode thin film on conductive glass substrate. The DSC prepared with graphenelike Co 0.85 Se counter electrode attained an efficiency of 9.4%, compared with 8.64% of DSC with platinum counter electrode under the same condition, which is highest record cell conversion efficiency of DSC-based I–/I3– redox couple and nonplatinum counter electrodes. Using the same process to prepare the Ni0.85Se electrode, the cell efficiency of the DSC was 8.32%. Experiments show that the catalytic activity of the Co0.85Se counter electrode is higher than that for the platinum electrode, and its electrochemical stability is comparable with that of the platinumbased counter electrode, which is higher than that of Ni0.85Se. The catalytic activity of Ni0.85Se is inferior to platinum for iodine. Gong et al. [47] believed that catalytic activity of selenide may be associated with stoichiometric ratio, and they prepared the NiSe2 electrode. The results proved that its catalytic activity for the reduction reaction of I3– is higher than the platinum-based counter electrode, and the cell efficiency of DSC reached 8.69%, compared to 8.04% for platinum-based counter electrode. Rct of NiSe2 electrode increased only by 0.81 to 0.81 Ω · cm2, showing that it has excellent corrosion resistance for the electrolyte containing iodine. Hou et al. [48] calculated and studied the catalytic reduction mechanism of iodine redox couple and calculated the catalytic activity of different semiconductor materials using the first principles of quantum chemistry. Under the premise of theoretical speculation, Hou et al. successfully validated the correctness of the theoretical deduction by experiment; the efficiency of DSC using α-Fe2O3 as the counter electrode was up to 6.69%, compared with that of platinum-based counter electrode, which was 7.32%. Meanwhile, theoretical results show that the catalytic activity of TiO2, MnO2, SnO2, CeO2, ZrO2, La2O3, Al2O3 and Ga2O3 might be poor. Hou et al. selected Ta2O5, TiO2 and CeO2 for experimental verification; the result was consistent with the inference, proving the feasibility of the theoretical calculation. Copper oxide (CuO) is a kind of narrow band gap p-type semiconductor material for dye-sensitized solar cells. It has good photoelectric, chemical and catalytic properties. It is of low cost and has attracted much attention from researchers of DSC. Anandan et al. [49] placed copper foil (1 cm2), treated with hydrochloric acid,

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in a solution of ammonia (NH3) and sodium hydroxide (NaOH) with a certain concentration, over time and generated neat rows of CuO nanorods on the copper foil surface. They washed the rods with deionized water, dried, and got CuO electrode. The CuO electrode prepared in this manner has a larger surface area, lower production costs and excellent stability. The short circuit current, open-circuit voltage, fill factor and the PCE of DSC assembled with the CuO electrode were 0.45 mA, 546 mV, 0.17 and 0.29%, respectively. Xia et al. [50] have used vanadium pentoxide (V2O5) doped with aluminum (Al) as catalytic material for solid DSC counter electrode. The PCE of the DSC assembled with this new counter electrode reached 2.0%, which is comparable to solid DSC with precious metals electrodes, and thus the manufacturing cost of solid DSC was reduced. Hu et al. [51] adopted p-type semiconductor material with a high absorption coefficient, such as FeS2 (absorption coefficient is 5 × 105 cm−1, when λ ≤ 700 nm, band gap Eg = 0.90 eV) and FeS, as the DSC electrode. They prepared a thin FeSx film using the one-step pyrolysis on iron foil, in which the DSC-based FeS had a high cell efficiency of 1.32%. Wu et al. [52] systematically studied three kinds of nanostructures, which are the pre-transition metal compounds (carbides, nitrides and oxide) electrode materials to replace the expensive Pt catalyst. Among these materials, Cr 3 C 2 , CrN, VC (N), VN, TiC, TiC(N), TiN and V2O3 have shown very good catalytic activity to I3–/I–. Furthermore, they synthesized VC embedded in mesoporous carbon structural materials by in situ synthesis method, and used it for electrode catalyst material to obtain an efficiency of 7.63%. Under the same circumstances, the efficiency of the DSC with Pt electrode was 7.5%. For other redox (T2/T–) electrolytes, TiC and VC-MC exhibit a PCE higher than that of the Pt electrode.

5.1.3.3 Other metal counter electrodes Sapp et al. [53] prepared an Au electrode by thermal evaporation, which is to deposit 25 nm thick Cr on the FTO conductive glass, followed by a deposit of 150 nm thick Au, before assembling the solar cells. The results show that the Au electrode is superior to the Pt electrode. In the testing process, the Au electrode did not face the corrosion phenomenon. However, gold still belongs to the precious metals class. Apart from assembling cells with the I3–/I– electrolyte, the testing photoelectric performance was also very poor, with the energy conversion efficiency being only 1.3%. Ni metal is very cheap, relative to Pt and Au. Fan et al. [54] proposed that conductive glass-plated Ni can be used as a counter electrode of DSC. They prepared an Ni electrode using the electrodeposition method. In the experiment, they employed the I3–/I– as electrolyte and the conductive glass-plated Ni as the electrode.

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The open-circuit voltage of cell was 0.468 V, with the short circuit current of 5.23 mA · cm−2 and the fill factor of 0.15. The incident monochromatic photonelectron conversion efficiency (IPCE) was up to 41.19%. However, Ni has good catalysis, leading to a drop in the catalytic activity of Ni due to its slow reaction with the I3– electrolyte.

5.1.4 Counter electrodes of flexible DSC FTO conductive glass has good light transmission, but its large weight, property of breaking easily, and its difficult process, brought a lot of inconvenience to the practical application of DSC, especially in the preparation of large-area cells. Considering the portability and continuity of production of solar cells, a flexible conductive substrate based on plastic and metal foil has been developed. Because of its light weight, easy deformation capability, and low price, flexible DSC has attracted widespread attention. Wang et al. [55] deposited CoS on the surface of a flexible ITO/PEN film. Its catalytic reduction activity to I3– is higher than the thin Pt film deposited on the surface of ITO/PEN. The cell efficiency of DSC with the Z907 dye was up to 6.5%. Katusic et al. [56] adopted noble metals, Au and Pt, to mix into ITO film. The specific surface area reached 60 m2. g−1, obtained solid phase In2Pt and In2Au, the average particle radius was 3.5–5.5 nm, and the resistivity was 0.6 Ω · cm−2. Kawashim et al. [57] developed an FTO/ITO double-layer conductive film, with a square resistance of the double conductive film of about 1.4 × 10−4 Ω · m−2 and a visible light transmittance of 80%. The performance of the DSC based on the two-layer conductive film electrode is better than that based on the ITO film electrode. After sintering at 300–600 °C in air for 1h for FTO/ITO bilayer conductive film, the resistivity dropped by 10%. Longo [58] and Haque [59] deposited a small amount of Pt to make a counter electrode of flexible cell on polyester substrate coated ITO by magnetron sputtering. At room temperature, using mechanical pressure membrane, Lindstrom [60] pressed SnO2 particles doped with antimony onto the surface of a polyester sheet coated with an ITO conductive layer, to prepare electrodes. In summary, the choice of flexible substrate catalytic materials is restricted, because flexible substrate thermal stability is inferior to conductive glass. It may occur in bending deformation or oxidation in high temperature heating process in the deposition of catalyst. Therefore, a mature low temperature catalyst preparation technology is the precondition for flexible substrate material accumulates promotion.

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5.2 Oxidation reduction reaction on the surface of the counter electrode The electrode reaction is composed of a series of basic steps: ① Reaction species transfer to the surface of the electrode–mass transfer process in the electrolyte phase. ② The reaction species take part in the process of conversion before reaction on the surface of the electrode or in the liquid layer near the electrode surface, such as adsorption on the surface or some chemical reactions – the “front” surface conversion process. ③ With or losing electrons on the surface of the electrode the reaction product is generated which is the electrochemistry process. ④ “The transformation steps after the reaction” occur on the surface of the electrode or in the liquid layer near the electrode surface, such as desorption from the surface, the composite, decomposition, disproportionate or other chemical changes of reaction product which is “succedent” surface conversion steps. ⑤ The reactant separates from the electrode surface into the electrolyte or transfers to the inside of the electrode which is mass transfer steps in the electrolyte. For an electrode reaction, in addition to the series of point-step reactions to each other, it may also include a number of “parallel” fractional steps. There is a ratedetermining step in all of the electrode reactions, and it controls the electrode reaction rate. Under the assumption that there is a “qualified” rate-controlling step, the other distribution steps will be considered in thermodynamic equilibrium state. If a rate of the electrode reaction is controlled only by the rate of the electrochemistry step, it can be approximatively considered that the concentration of the reaction particle in the electrolyte is uniform and all surface conversion steps are in equilibrium. So, we can use the surface adsorption isotherm to calculate the surface adsorption capacity, and we can use the equilibrium constant to deal with the chemical transformation on the surface layer. It is assumed that the mass transfer rate in the liquid phase is the bottleneck of the electrode reaction rate; we need to consider the changes in the concentration of the reactive particles in a medium. The rate-determining step of the electrode reaction is not specific; it will change with the change of the reaction conditions. And the change of the rate-determining step can also cause change in the kinetic characteristics of the whole electrode reaction. Therefore, in the study of the catalytic mechanism of the counter electrode of the dye-sensitized solar cells, the determination of the rate determining step is a hot point which we will discuss in detail in Section 5.2.1.

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5.2.1 Principle of redox reaction on counter electrode In DSC, the catalytic reaction on the counter electrode is I3– + 2e–→3I–, which is the reverse reaction of photoanode, that is, I3– is reduced to I–, this reaction occurs on the interface between the electrolyte and the electrodes. Typically, the multiphase catalytic reaction includes the following main steps. ① the reactant molecules are adsorbed on the surface of the solid catalyst; ② the molecules or atoms adsorbed on the surface of a solid catalyst undergo electrochemical reaction; ③ the product molecules take the desorption from the surface of the solid catalyst and diffuse out of the surface of catalyst. In the DSC, the catalytic reaction of I3– on the Pt electrode has the characteristic of the electrode reaction and the heterogeneous catalytic reaction. Therefore, the catalytic reaction process of I 3– in the “Pt electrode/electrolyte” interface generally includes three stages. ① I3 – takes the path of the of electrolyte itself, diffusing from the dye to the surface area of the Pt electrode. ② I3– is absorbed on the surface of the Pt electrode in some way to produces I– through the chemical reaction. ③ I– desorpts from the surface of the Pt electrode, and then diffuses to the electrolyte body. The whole electrode reaction process is shown in Figure 5.1:

Figure 5.1: The electrode reaction process of I3–/I– in the “Pt electrode / electrolyte” interface.

Anneke et al. proposed the reaction mechanism of I3–/I– on pure Pt electrode as follows: (a) I– + Pt⇆I– (Pt) (I– adsorbent on the Pt) quick (b) I– (Pt)⇆I(Pt) + e– (I– oxidation) slow (c) I(Pt) + I(Pt)⇆I2 + 2Pt (I2 reduction) quick (d) I2 + I– ⇆I3– (I3– reduction) quick

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Because of the accepted quick process of adsorption and dissociation of Pt in the process(c), Anneke regarded the process(b) as the speed-determining step. This is similar to the Pt catalytic mechanism proposed by Vetter et al. [61] (H2SO4 as solvent) and Hauch et al. [62] (ACN as solvent), which considers the oxidation reaction of I– adsorbed on Pt as the speed-determining step. However, Dané et al. [2] (H2SO4 as solvent) and Macagno et al. [63] (ACN as solvent) proposed another catalytic reaction mechanism. According to their theory, the speed-determining step is not (b) or (c), but the process that the I adsorbed on Pt and the I– in the vicinity of Pt electrode lose electrons to generate I2; the process is shown as follows: IðPtÞ + I − !  I2 + Pt + e −

(5:1)

In conclusion, the effect of the different electrolytes on the catalytic mechanism of I3– on the Pt electrode is still debatable.

5.2.2 Characterization method for counter electrode reaction The counter electrodes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and other conventional methods, which are mainly focused on the characterization of the electrochemical reaction on the surface.

5.2.2.1 Electrochemical impedance studies Electrochemical impedance spectroscopy (EIS) is also a common method for the study of electrode-reaction kinetics. EIS is a small-amplitude alternating current potential wave of different frequencies, which is applied to the electrochemical system to measure the ratio (which is system impedance) of the AC electric potential and the current signal changes with the frequency of the sine wave ω and the phase angle Φ of the impedance changes with the ω. The basic idea of the study is to consider the electrochemical system as an equivalent circuit, which is composed in different ways, such as serial–parallel with primary elements of resistance (R), capacitance (C), inductance (L), and so on. Through the characterization of EIS, we can ensure that it constitutes a form of equivalent circuit as well as the sizes of every primary element. We can use the electrochemical significance of these elements to research the dynamics of the electrode process, the double layer and diffusion. EIS is widely used in the study of dye-sensitized solar cells. At open-circuit state, it is used to research the parameters, such as electron transport and dark reaction, and predict the performance of DSC. The results of the experiments show that the results

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obtained by EIS are consistent with the results of the transient technology, but the latter is not suitable for the interpretation of the aging phenomenon, due to the interference of the electronic recombination [65]. The outstanding contribution of EIS in the study of the electrode is shown in the determination of the charge-exchange resistance (RCT) of the counter electrode/electrolyte interface [3, 59, 60]. The relationship between the charge-exchange resistance (RCT) and the exchange current density (J0) is Rct = RT/ nFJ0, where R, T, n, and F are the mole gas constant, temperature, electron transfer number (n = 2), and the Faraday constant, respectively. Meanwhile, the exchange current density (J0) is closely related to the catalytic activity. So, we can determine the level of catalytic activity through the RCT information of EIS. A typical EIS spectrum of the DSC contains the Nyquist plot of three semicircles. These three semicircles are the electron transfer impedance (Z1) of the Pt counter electrode in the high-frequency region, the interface of the Ti/dye/electrolyte in the intermediate-frequency region, and the Nernst diffusion of the electrolyte in the low-frequency region. R1, R2, and R3 are the real parts of Z1, Z2, and Z3, respectively. Rh is the resistance of the high-frequency region, exceeding 106 Hz, which is proportionate to the square resistance of TCO. For a more accurate study of the electrode portion – removing the interference of other components of DSC in the study of the DSC counter electrode – we usually introduce the model of a symmetric thin-layer cell to form the “Pt electrode/electrolyte/Pt electrode” thin-layer cell. Figure 5.2 [62] is the structural sketch map of “Pt electrode/electrolyte/Pt electrode” thin-layer cell. The advantages of the thin-layer cell are as follows: the electric field distribution on the surface of the two electrodes is the same; because the cell is very thin, the convection phenomenon does not happen; high concentrations of ions in the electrolyte stop the creation of electric field in the internal of the cell; so the migration can be ignored, and diffusion is the only influential factor of material transport.

Figure 5.2: Construction of thin-layer cells of “Pt electrode/electrolyte/Pt electrode” .

Figure 5.3 shows the electrochemistry impedance spectroscopy of the Pt/FTO thinlayer cell. Figure 5.4 is the corresponding equivalent circuit diagram [66]. It is different from the electrochemistry impedance spectroscopy of DSC. The figure contains only

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two semicircles, Z1 and Z3. Because the Pt/electrolyte/Pt thin-layer cell contains two electrolyte/counter electrode interfaces, the electrode, Rct, measured according to the semicircle in the high-frequency region from the impedance spectra [67] is two times bigger than the Rct in the actual DSC.

Figure 5.3: Typical electrochemical impedance spectra of thin-layer cells of “Pt/ electrolyte/Pt” [66].

N

2Rct

Rs

1/2C Figure 5.4: The equivalent circuit above; N, Nernst diffusion impedance; Rct, charge exchange resistance of a single electrode; C, double-layer capacitance of single electrode; Rs, series resistor [66].

5.2.2.2 Steady-state polarization curve method [68] When current passes through the electrode, the phenomenon of electrode potential deviation from the equilibrium potential is called polarization. According to the different causes of polarization, it can be divided into two kinds: concentration polarization and electrochemistry polarization.

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5.2.2.2.1 Concentration polarization When the electrode reaction occurs, the phenomenon of the electrolyte concentration near the electrode surface being different from the main electrolyte concentration is called polarization. In the vicinity of the cathode, the cation is reduced rapidly, while the cation in the main electrolyte is too slow to diffuse to the vicinity of electrode, so the cathode potential is more negative than the reversible potential. In the vicinity of the anode, the electrode is oxidized or dissolved and the ion is too slow to leave, so the anode potential is more positive than a reversible potential. 5.2.2.2.2 Electrochemical polarization Electrode reaction is completely carried out over several steps; if the rate of the onestep reaction is slow, it will require a higher activation energy. In order to ensure a smooth progress of the electrode reaction, an additional voltage (known as electrochemical overpotential or activation overpotential) is required so that polarization phenomenon called the electrochemical polarization is produced. The change curve of the polarization potential and the polarization current is called the polarization curve. The shape and the variation law of the polarization curve reflect the dynamic characteristics of the electrochemical reaction process. According to the polarization curve, kinetic constants such as exchange current and transfer coefficient can be determined, and the control step of the electrode reaction process can be viewed. The electrode reaction is controlled by different steps, each of which owns its particular shape and variation of the polarization curves; it is briefly summarized as follows. (1) Features of polarization curve controlled by concentration diffusion Polarization characteristics are controlled by the concentration diffusion, in that, the cathode polarization increases slowly and the potential becomes smaller with the increase in the current density. However, when the current density increases to a certain limit, polarization increases significantly. The diffusion rate has nothing to do with the potential and the polarization curve is a horizontal line (Figure 5.5). (2) Features of polarization curve controlled by electrochemical reaction The current density of the controlled by electrochemical reaction increases quickly. At low current, it will show polarization, and the electrode potential becomes strongly negative. Thereafter, when the current density continues to increase, there is a slow change in polarization. (Figure 5.6)

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Figure 5.5: Polarization curve controlled by concentration diffusion.

Figure 5.6: Polarization curve controlled by electrochemical reaction.

(3) Features of polarization curve jointly controlled by concentration diffusion and electrochemical reaction The dotted line in Figure 5.7 is the polarization curve controlled by a simple diffusion concentration, the solid line is mainly the polarization curve controlled by the electrochemical reaction. When the current density is under the limiting current density, cathodic polarization process is gradually controlled from the electrochemical reaction to the hybrid zone (when I is in the range between 0.1 Id and 0.9 Id), and then converted into concentration diffusion.

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Figure 5.7: Polarization curve of jointly controlled by concentration diffusion and electrochemical reaction.

Tafel curves can be used to study the electron transport properties of I3–/I– on “Pt electrode/electrolyte” interfacial double electrical layer of the thin-layer cell; the polarization curves are obtained though an experiment according to the Butler–Volmer equation:    

αnF βnF φ − CR exp − φ (5:2) I = nFAks Co exp RT RT where I is electrode current, A is the surface active area of the electrodes, ks is the standard equilibrium constant, φ is the overpotential, Co is the concentration of reactants, CR is the concentration of the product, α is the anode transfer coefficient, and β is the cathode transfer coefficient. The Butler–Volmer equation describes the relationship of the electron exchange current and the electrode potential of the cathode and anode, and the set anode reaction and the cathode reaction take place on the same electrode. The exchange current is determined by the intersection of the cathode portion and the anode part of the steady state polarization curves. The group compared the steady state polarization of the thin-layer cell curves at different concentrations of I–, as shown in Figure 5.8; the analysis results of the steadystate polarization curves are shown in Table 5.1 [69]. As given in Table 5.1, the interface exchange current increases with the increasing concentration of I– in the electrolyte and, therefore, a high concentration, I–, can accelerate the I3–/I– transfer at the interface of the “Pt electrode/ electrolyte.”

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Figure 5.8: Effect of the concentration of I– on the steady-state polarization curves of the thin-layer cell. Electrolyte 1: 0.1 mol · L−1 LiI and MPN solution of 0.1 mol · L−1 I2,electrolyte 2: 0.2 mol · L−1 LiI and MPN solution of 0.1 mol · L−1 I2, electrolyte 3: 0.7 mol · L−1 LiI and MPN solution of 0.1 mol · L−1 I2. Table 5.1: Analysis results influenced by the steady-state polarization curves in Figure 5.8. Electrolyte

Exchange current (A)

Electrolyte  Electrolyte  Electrolyte 

. × − . × − . × −

5.2.2.3 Cyclic voltammetry Cyclic voltammetry (CV) is another important means in electrode reaction research. Three electrode systems are often used to study. The counter electrode of DSC serves as the working electrode, the Pt serves as the counter electrode, and Ag/Ag+ electrode serves as the reference electrode. Figure 5.9 [16] shows a cyclic voltammogram of the Pt counter electrode in the I2 + I– system; the solid line represents [I–]/[I2] = 1/9 and the dashed line represents [I–]/[I2] = 9/1. It contains two pairs of redox peaks, one pair of the relatively negative peaks corresponds to reaction (5.3) and the other pair of relatively positive peaks corresponds to reaction (5.4) [70]: I3 − + 2e − = 3I − 3I2 + 2e

−

= 2I3

−

ð5.3Þ ð5.4Þ

As shown in Figure 5.9, when [I–]/[I2] = 9/1, there is only one pair of redox peaks on the carbon electrode, while when [I–]/[I2] = 1/9, there are two pairs of redox peaks

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on the platinum electrodes, which may be caused by high multi-iodide ions, such as I5– forming at a high concentration of I2. At the same time, it can be seen that the reduction peak current density of I3– is lower on the glassy carbon electrode, compared to the platinum electrode. This indicates that the reaction rate is smaller on the glassy carbon electrode, that is, the redox charge transfer resistance, Rct, of I3–/ I– is larger on the glassy carbon electrode. [Carbon electrode]

[Pt electrode] 1.0

1.0 0.8

0.8 0.6 0.6

Current/mAcm-2

Current/mAcm-2

0.4 0.2 –0.0 –0.2 –0.4

0.4

0.2

0.0

–0.6 –0.8

–0.2 –1.0 –1.2 –800 –600 –400 –200

0

200 400 600 800 +

Electrode potential/mV vs. Fc /Fc

–0.4 –800 –600 –400–200

0

200 400 600 800

Electrode potential/mV vs. Fc+⁄ Fc

Figure 5.9: Cyclic voltammograms of the glassy carbon electrode and the platinum plate electrode: 5 mM LiI + acetonitrile solution of I2 with 0.1 M LiClO4 as the supporting electrolyte. The solid line represents [I–]/[I2] = 1/9 and the dashed line represents [I–]/[I2] = 9/1.

Tang et al. [69] has studied the catalytic reaction mechanism of I3– on the Pt electrode adopting the CV of the thin-layer cells. The electrolyte contains 0.1 mol · L−1 LiI and 0.1 mol · L−1 I2 in MPN solvent, because I2 + I − ! I3− the concentration of I– is very low in the electrolyte. A pair of oxidation (at about −0.4 V) reduction (at about +0.4 V) peak and a peak current of adsorption appear on the CV curves. As can be seen from Figure 5.10, with the increased scan rate, the oxidation peak potential (Epc) moves forward and the reduction peak potential (Epa) moves to the negative direction, and ΔEp of the redox reaction increases. According to the theory of CV, the redox peak current (Ip) will increase with the increase in scan rate. For a completely reversible redox reaction, ΔEp should be less than 60/n mV, and the oxidation (reduction) peak potential (Ep) will not change with the change in scan rate. And in a totally irreversible process, Ep and

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Figure 5.10: Cyclic voltammetry curve of thinlayer cells at different scan rates: 0.1 V · s−1 (curve 1), 0.2 V · s−1 (curve 2), 0.4 V · s−1 (curve 3), 0.6 V · s−1 (curve 4), 0.8 V · s−1 (curve 5), 1.0 V · s−1 (curve 6), electrolyte: 0.1 mol · L−1 LiI and 0.1 mol · L−1 I2 in MPN solvent.

the logarithm of the scan rate show a linear relationship; redox peaks cannot come in pairs. Thus, the redox reaction of I3–/I– belongs to the quasi reversible reaction at the “Pt electrode/electrolyte” interface. When pseudo halogen ions come in contact with the Pt electrode surface, they will be adsorbed onto the Pt electrode and then stabilized on the Pt electrode surface. This kind of contact adsorption is often called the “characteristic absorption.” The force involved in characteristic adsorption is beyond the electrostatic interactions and is associated with the chemical properties of the ion. When surface adsorption is present in the electrode reaction, Ep and Ip of the reaction are in accordance with the following equations [71]: RT lnðBO =BR Þ nF  Ip = n2 F 2 vΓo RT

0 Ep = Efb −

(5:5) (5:6)

where Ep is the adsorption equilibrium potential when the adsorption capacities of the reactant and product are not the same, and equal in value to the adsorption 0 is the adsorption equilibrium potential when the adsorpcurrent peak potential; Efb tion capacities of the reactant and product are the same and equal in value to the reduction peak potential; BO is the adsorption equilibrium constant of the reactants; BR is the adsorption equilibrium constant of the reaction product, n is the number of electrons in each molecule, F is the Faraday constant, R is the mole gas constant, T is the absolute temperature, and Γo is the adsorption amount of the reactants. The adsorption current peak is in front of the reduction current peak (as shown in Figure 5.10),

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0 and is called the “forward wave”, therefore, Ep is larger than Efb . Equation (5.7) can be obtained according to eq. (5.5):

BR iiBO

(5:7)

In the reduction reaction process of I3–, the reactant is I3– and the product is I–. Therefore, the adsorption ability of I– is greater than the adsorption ability of I3– on the Pt electrode, that is, BI − iiBI − . The adsorption current peak is behind the oxidation peak 3 current and is called the “post-wave” and we can get eq. (5.8) according to eq. (5.5): BO iiBR –

(5:8) –

–

In the oxidation process of I , the reactant is I and the reaction product is I3 . The adsorption ability of I– is better than that of I3– at the Pt electrode. Therefore, BI − iiBI − . The oxidation peak current (Ipc) and the corresponding reduction peak 3 (Ipa) have a linear relationship with scan rate (v) shown in Figure 5.11.

Figure 5.11: The relationship of Ip with scan rate on the cyclic voltammograms curve.

According to eq. (5.6), the electrolyte catalytic reaction process of I3– is controlled by the adsorption steps at low concentrations of I–. Because BI − iiBI − , the desorp3 tion of I– is very difficult at the Pt electrode, we suggest the following steps: (a) I3−bulk !I3−surf (b) I3−surf + Pt !I3− ðPtÞ (c) I3− ðPtÞ + Pt !I2 ðPtÞ + I − ðPtÞ (d) I2 ðPtÞ + e − !IðPtÞ + I − ðPtÞ (e) IðPtÞ + e − !I − ðPtÞ − (f) I − ðPtÞ !Isurf + Pt − ! − (g) Isurf Ibulk

(Diffusion of I–) (Adsorption of I– on Pt electrode) (Dissociation of I–) (Reduction of I) (Reduction of I) (Desorption of I– from Pt electrode) (Diffusion of I–)

Fast Fast Fast Fast Fast Slow Fast

This is consistent with the reaction mechanism previously mentioned; I3–/I– on a pure Pt electrode, except the difference of the rate-determining step.

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In short, CV is very mature and an important characterization method in electrode reaction study of dye-sensitized solar cells.

5.2.2.4 Electrochemical scanning microscope Recently, the electrochemical scanning microscope (SECM) aroused people’s concern in studying the catalytic activity of the electrodes. Figure 5.12 shows an inprinciple diagram of the SECM test. z

UME Glass Sealing

y

Pt wire

Bi-Potentiostat

x

Positioning Control

CE RE

Ultramicroelectrode (UME)

O e

R

Electrochem. Cell Substrate

Pt-tip Substrate

(a)

(b)

Figure 5.12: The principle diagram of SECM test [72].

SECM is mainly focused on characterization of electrical catalytic activity with respect to the use of a counter electrode. Huang et al. [71] characterized electrochemical catalytic activity of the dye-sensitized solar cell and the nanographite (NG)/ PANI counter electrode, adopting the SECM method. Experimental work on the SECM feedback mode, Fc/Fc+, serves as the redox couple; the approximation curve of the counter electrode shows a positive feedback because the NG/PANI film is a conductor. It is shown in Figure 5.13 that the NG/PANI normalized the positive feedback the approximation curve of sputtering at the Pt electrode. The theoretical curve with the kinetic parameters Λ at 5 and 1 (Λ = aks0/Dapp, a diameter of ultrafine Pt probe, Dapp apparent diffusion coefficient of Fc). By comparing the three curves of the counter electrodes, theoretical calculation is made on NG/PANI

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and the Pt counter electrode, in accordance with Λ = 5. The heterogeneous reaction rate constant ks0 of the counter electrode catalytic reaction is about 0.217 cm · s−1, and ks0 is PANI approximately 0.043. The larger value of ks0 indicates that the counter electrodes have better catalytic activity, that is, adding NGs improves the catalytic activity of the PANI film. Meanwhile, the NG/PANI film exhibits similar electrocatalytic activity with Pt electrodes.

Normalized tip current IT

PANI NG/PANI Pt =5 =1

Normalized distrance L Figure 5.13: PANI (20 mC cm−2), NG/PANI (20 mC cm−2), normalized positive feedback approach curve of sputtering Pt electrode, solid line and dashed line are theoretical curves with Λ at 5 and 1.

Over the past 10 years, although there have been rapid developments, the use of SECM in DSC is rarely investigated. However, SECM has a solid basis of quantitative theoretical analysis; its introduction will bring in new ideas in the electrode catalytic kinetics research of DSC.

References [1] [2] [3] [4]

[5]

Papageorgiou N. Counter-electrode function in nanocrystalline photoelectrochemical cell configurations. Coord Chem Rev, 2004, 248(13–14): 1421–1446. Dane L M, Janssen L J J, Hoogland J G. Iodine/iodide redox couple at a platinum electrode. Electrochim Acta, 1968, 13(3): 507–518. Papageorgiou N, Maier W F, Grätzel M. An iodine/triiodide reduction electrocatalyst for aqueous and organic media. J Electrochem Soc, 1997, 144(3): 876–884. Yoon S, Lee J, Kim S O, et al. Enhanced cyclability and surface characteristics of lithium batteries by Li-Mg co-deposition and addition of HF acid in electrolyte. Electrochim Acta, 2008, 53(5): 2501–2506. Lin C Y, Lin J Y, Wan C C, et al. High-performance and low platinum loading electrodeposited-Pt counter electrodes for dye-sensitized solar cells. Electrochim Acta, 2011, 56(5): 1941–1946.

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[6] [7]

[8]

[9] [10] [11]

[12]

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

[27]

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Kim S S, Nah Y C, Noh Y Y, et al. Electrodeposited Pt for cost-efficient and flexible dyesensitized solar cells. Electrochim Acta, 2006, 51(18): 3814–3819. Choi J Y, Hong J T, Seo H, et al. Optimal series-parallel connection method of dye-sensitized solar cell for Pt thin film deposition using a radio frequency sputter system. Thin Solid Films, 2008, 517(2): 963–966. Lee Y L, Chen C L, Chong L W, et al. A platinum counter electrode with high electrochemical activity and high transparency for dye-sensitized solar cells. Electrochem Commun, 2010, 12(11): 1662–1665. Kim S S, Park K W, Yum J H, et al. Dye-sensitized solar cells with Pt-NiO and Pt-TiO2 biphase counter electrodes. J Photochem Photobio, A, 2007, 189(2–3): 301–306. Hao S, Wu J, Lin J, et al. Platinum-modified photocathode and its application in nanocrystalline solar cells. Photographic Sci Photochem, 2004, 22(3): 175–182. Chen L L, Tan W W, Zhang J B, et al. Fabrication of high performance Pt counter electrodes on conductive plastic substrate for flexible dye-sensitized solar cells. Electrochim Acta, 2010, 55(11): 3721–3726. Sun K, Fan B H, Ouyang J Y. Nanostructured platinum films deposited by polyol reduction of a platinum precursor and their application as counter electrode of dye-sensitized solar cells. J Phys Chem C, 2010, 114(9): 4237–4244. Ma T L, Fang X M, Akiyama M, et al. Properties of several types of novel counter electrodes for dye-sensitized solar cells. J Electroanal Chem, 2004, 574(1): 77–83. Fang X M, Ma T L, Akiyama M, et al. Flexible counter electrodes based on metal sheet and polymer film for dye-sensitized solar cells. Thin Solid Films, 2005, 472(1–2): 242–245. Wang G Q, Lin R F, Lin Y, et al. A novel high-performance counter electrode for dye-sensitized solar cells. Electrochim Acta, 2005, 50(28): 5546–5552. Imoto K, Takahashi K, Yamaguchi T, et al. High-performance carbon counter electrode for dye-sensitized solar cells. Sol Energy Mater Sol Cells, 2003, 79(4): 459–469. Huang Z, Liu X H, Li K X, et al. Application of carbon materials as counter electrodes of dye-sensitized solar cells. Electrochem Commun, 2007, 9(4): 596–598. Kay A, Grätzel M. Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder. Sol Energy Mat Sol Cells, 1996, 44(1): 99–117. Murakami T N, Ito S, Wang Q, et al. Highly efficient dye-sensitized solar cells based on carbon black counter electrodes. J Electrochem Soc, 2006, 153(12): A2255–A2261. Ramasamy E, Lee W J, Lee D Y, et al. Nanocarbon counter electrode for dye sensitized solar cells. Appl Phys Lett, 2007, 90(17). Lee W J, Ramasamy E, Lee D Y, et al. Performance variation of carbon counter electrode based dye-sensitized solar cell. Sol Energy Mater Sol Cells, 2008, 92(7): 814–818. Li K X, Luo Y H, Yu Z X, et al. Low temperature fabrication of efficient porous carbon counter electrode for dye-sensitized solar cells. Electrochem Commun, 2009, 11(7): 1346–1349. Wang G Q, Xing W, Zhuo S P. Application of mesoporous carbon to counter electrode for dye-sensitized solar cells. J Power Sources, 2009, 194(1): 568–573. Ramasamy E, Chun J, Lee J. Soft-template synthesized ordered mesoporous carbon counter electrodes for dye-sensitized solar cells. Carbon, 2010, 48(15): 4563–4565. Imoto K, Suzuki M, Takahashi K, et al. Activated carbon counter electrode for dye-sensitized solar cell. Electrochemistry, 2003, 71(11): 944–946. Ramasamy E, Lee W J, Lee D Y, et al. Spray coated multi-wall carbon nanotube counter electrode for tri-iodide (I-3(-)) reduction in dye-sensitized solar cells. Electrochem Commun, 2008, 10(7): 1087–1089. Lee W J, Ramasamy E, Lee D Y, et al. Efficient Dye-sensitized cells with catalytic multiwall carbon nanotube counter electrodes. ACS Appl Mater Inter, 2009, 1(6): 1145–1149.

296

Xu Pan

[28] Nam J G, Park Y J, Kim B S, et al. Enhancement of the efficiency of dye-sensitized solar cell by utilizing carbon nanotube counter electrode. Scripta Mater, 2010, 62(3): 148–150. [29] Cai F S, Liang J, Tao Z H, et al. Low-Pt-loading acetylene-black cathode for high-efficient dye-sensitized solar cells. J Power Sources, 2008, 177(2): 631–636. [30] Li P J, Wu J H, Lin J M, et al. High-performance and low platinum loading Pt/Carbon black counter electrode for dye-sensitized solar cells. Sol Energy, 2009, 83(6): 845–849. [31] Hsu K F, Hu S K, Chen C H, et al. Formation mechanism of LiFePO4/C composite powders investigated by X-ray absorption spectroscopy. J Power Sources, 2009, 192(2): 660–667. [32] Groenendaal B L, Jonas F, Freitag D, et al. Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future. Adv Mater, 2000, 12(7): 481–494. [33] Saito Y, Kitamura T, Wada Y, et al. Application of poly(3,4-ethylenedioxythiophene) to counter electrode in dye-sensitized solar cells. Chem Lett, 2002, 10: 1060–1061. [34] Lee K M, Chiu W H, Wei H Y, et al. Effects of mesoscopic poly (3,4-ethylenedioxythiophene) films as counter electrodes for dye-sensitized solar cells. Thin Solid Films, 2010, 518(6): 1716–1721. [35] Shibata Y, Kato T, Kado T, et al. Quasi-solid dye sensitised solar cells filled with ionic liquid – increase in efficiencies by specific interaction between conductive polymers and gelators. Chem Commun, 2003, 21: 2730–2731. [36] Chen J G, Wei H Y, Ho K C. Using modified poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) film as a counter electrode in dye-sensitized solar cells. Sol Energy Mater Sol Cells, 2007, 91(15–16): 1472–1477. [37] Hong W J, Xu Y X, Lu G W, et al. Transparent graphene/PEDOT-PSS composite films as counter electrodes of dye-sensitized solar cells. Electrochem Commun, 2008, 10(10): 1555–1558. [38] Lee K M, Chen P Y, Hsu C Y, et al. A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells. J Power Sources, 2009, 188(1): 313–318. [39] Li Q H, Wu J H, Tang Q W, et al. Application of microporous polyaniline counter electrode for dye-sensitized solar cells. Electrochem Commun, 2008, 10(9): 1299–1302. [40] Li Z P, Ye B X, Hu X D, et al. Facile electropolymerized-PANI as counter electrode for low cost dye-sensitized solar cell. Electrochem Commun, 2009, 11(9): 1768–1771. [41] Zhang J, Hreid T, Li X X, et al. Nanostructured polyaniline counter electrode for dye-sensitized solar cells: fabrication and investigation of its electrochemical formation mechanism. Electrochim Acta, 2010, 55(11): 3664–3668. [42] Qin Q, Tao J, Yang Y. Preparation and characterization of polyaniline film on stainless steel by electrochemical polymerization as a counter electrode of DSSC. Synth Met, 2010, 160(11–12): 1167–1172. [43] Ameen S, Akhtar M S, Kim Y S, et al. Sulfamic acid-doped polyaniline nanofibers thin filmbased counter electrode: application in dye-sensitized solar cells. J Phys Chem C, 2010, 114(10): 4760–4764. [44] Wu J H, Li Q H, Fan L Q, et al. High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells. J Power Sources, 2008, 181(1): 172–176. [45] Makris T, Dracopoulos V, Stergiopoulos T, et al. A quasi solid-state dye-sensitized solar cell made of polypyrrole counter electrodes. Electrochim Acta, 2011, 56(5): 2004–2008. [46] Gong F, Wang H, Xu X, et al. In situ growth of Co0.85Se and Ni0.85Se on conductive substrates as high-performance counter electrodes for dye-sensitized solar cells. J Am Chem Soc, 2012, 134(26): 10953–10958. [47] Gong F, Xu X, Li Z Q, et al. NiSe2 as an efficient electrocatalyst for a Pt-free counter electrode of dye-sensitized solar cells. Chem Commun, 2013, 49(14): 1437–1439.

Chapter 5 Counter electrode for dye-sensitized solar cells

297

[48] Hou Y, Wang D, Yang X H. Rational screening low-cost counter electrodes for dye-sensitized solar cells. Nature Commun, 2013, 1583 [49] Anandan S, Wen X G, Yang S H. Room temperature growth of CuO nanorod arrays on copper and their application as a cathode in dye-sensitized solar cells. Mater Chem Phys, 2005, 93(1): 35–40. [50] Xia J B, Yuan C C, Yanagida S. Novel counter electrode V2O5/Al for solid dye-sensitized solar cells. ACS Appl Mater Inter, 2010, 2(7): 2136–2139. [51] Hu Y, Zheng Z, Jia H M, et al. Selective synthesis of FeS and FeS2 nanosheet films on iron substrates as novel photocathodes for tandem dye-sensitized solar cells. J Phys Chem C, 2008, 112(33): 13037–13042. [52] Wu M X, Lin X, Wang Y D, et al. Economical Pt-free catalysts for counter electrodes of dyesensitized solar cells. J Am Chem Soc, 2012, 134(7): 3419–3428. [53] Sapp S A, Elliott C M, Contado C, et al. Substituted polypyridine complexes of cobalt(II/III) as efficient electron-transfer mediators in dye-sensitized solar cells. J Am Chem Soc, 2002, 124(37): 11215–11222. [54] Fan L, Wu J, Huang Y, et al. Improvement of the performance of dye-sensitized TiO2 solar cells by cathode modification. Electron Comp Mater, 2003, 22(5): 1. [55] Wang M K, Anghel A M, Marsan B, et al. CoS Supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells. J Am Chem Soc, 2009, 131(44): 15976–15977. [56] Katusic S, Albers P, Kern R, et al. Prod-action and characterization of ITO-Pt semiconductor powder containing nanoscale noble metal particles catalytically active in dye-sensitized solar cells. Sol Energy Mater Sol Cells, 2006, 90(13): 1983–1999. [57] Kawashima T, Ezure T, Okada K, et al. FTO/ITO double-layered transparent conductive oxide for dye-sensitized solar cells. J Photochem Photobio, A, 2004, 164(1–3): 199–202. [58] Longo C, Freitas J, De Paoli M A. Performance and stability of TiO2/dye solar cells assembled with flexible electrodes and a polymer electrolyte. J Photochem Photobio A, 2003, 159(1): 33–39. [59] Haque S A, Palomares E, Upadhyaya H M, et al. Flexible dye sensitised nanocrystalline semiconductor solar cells. Chem Commun, 2003,24, 3008–3009. [60] Lindstrom H, Holmberg A, Magnusson E, et al. A new method for manufacturing nanostructured electrodes on plastic substrates. Nano Lett, 2001, 1(2): 97–100. [61] Vetter K J. Kinetik Der Elektrolytischen Abscheidung Von Wasserstoff Und Sauerstoff. Angew Chem Int Ed, 1961, 73(9): 277. [62] Hauch A, Georg A. Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochim Acta, 2001, 46(22): 3457–3466. [63] Macagno V A, Giordano M C, Arvia A J. Kinetics and mechanisms of electrochemical reactions on platinum with solutions of iodine-sodium iodide in acetonitrile. Electrochim Acta, 1969, 14(4): 335. [64] Tang Y T, Pan X, Zhang C N, et al. Influence of different electrolytes on the reaction mechanism of a triiodide/iodide redox couple on the platinized FTO glass electrode in dye sensitized solar cells. J Phys Chem C, 2010, 114: 4160–4167. [65] Wang Q, Moser J E, Grätzel M. Electrochemical impedance spectroscopic analysis of dyesensitized solar cells. J Phys Chem B, 2005, 109(31): 14945–14953. [66] Fang X M, Ma T L, Guan G Q, et al. Effect of the thickness of the Pt film coated on a counter electrode on the performance of a dye-sensitized solar cell. J Electroanal Chem, 2004, 570(2): 257–263. [67] Macdonald J R. Impedance spectroscopy and its use in analyzing the steady-state ac response of solid and liquid electrolytes. J Electroanal Chem, 1987, 223(1–2): 25–50.

298

Xu Pan

[68] Chen G, Wang G, Application of Electrochemical Methods; Chemical Industry Press: Beijing, 2003. [69] Tang Y, Research on counter electrode of dye-sensitized solar cells. Institute of Plasma Physics, Chinese Academy of Sciences, 2011. [70] Zha Q. Introduction to Electrode Process dynamics[M];Beijing: Science Press, 2002. [71] Figgemeier E, Kylberg W H, Bozic B. Scanning photoelectrochemical microscopy as versatile tool to investigate dye-sensitized nano-crystalline surfaces for solar cells – art. no. 619711. P Soc Photo-Opt Ins, 2006, 6197: 19711. [72] Huang K C, Huang J H, Wu C H, et al. Nanographite/polyaniline composite films as the counter electrodes for dye-sensitized solar cells. J Mater Chem, 2011, 21(28): 10384–10389.

Weiqing Liu, Songyuan Dai

Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells 6.1 The solid–solid contact interface 6.1.1 The property of solid–solid contact interface The main distinction between semiconductor and metal is the extent of occupation of the energy band; when the temperature is absolute 0 K, within the semiconductor, the valence band is completely filled with electrons, while the conduction band is not taken up by any electrons; in the meantime, the conductivity of the semiconductor is very low due to the absence of free carrier. When the temperature is higher than 0 K, electrons in the valence band will transfer into relatively higher energy conduction bands across the forbidden band under certain conditions. This transition process leaves a hole in the valence band and, simultaneously, adds an electron in the conduction band. The holes in the valence band and the electrons in the conduction band are free to move, so the conductivity of semiconductor is greatly enhanced. The electron system in the semiconductor has a unified Fermi level EF,s when it reaches the thermal equilibrium.

p

n

Ec Ev

Figure 6.1: Schematic diagram of pn junction.

The same kind of semiconductor material can form p-type and n-type semiconductors which have converse types of conduction by processes including the impurity-doped approach and so on. The hole and electron concentration in the n-type and p-type semiconductors are different. Before they contact, both the electron concentration and Fermi level of n-type semiconductor EFn,s is higher than those of the p-type EFp,s.

Weiqing Liu, Nanchang Hangkong University Songyuan Dai, North China Electric Power University https://doi.org/10.1515/9783110344363-006

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After they contact, the properties of contact interface are different from those of the interior semiconductor, electrons and holes will have diffusion motions: electrons will flow from the n-type semiconductor to the p-type one and the holes’ motion is opposite to it. Near the contact interface of the n-type semiconductor, the donor ions positively charged cannot move, and will form a positively charged area as the electron concentration reduces gradually; while in the vicinity of the contact interface of p-type semiconductor, the donor ions negatively charged cannot move, and will form a negatively charged area with electrons concentration reducing gradually. A space region, therefore, emerges with many oppositely charged electrons near the contact interface, as illustrated in Figure 6.1. In this region, a built-in electric field which directs from the positive charge to the negative charge counterpart also appears. Electrons and holes are drifted by the built-in electric field, which is opposite to the process involving the diffusion of electrons and holes, finally the Fermi levels of ntype semiconductor and the p-type semiconductor will be equal, and there will be no net flows of electrons and holes at the macro level. The electrons of the conduction band in n zone will encounter the barrier when entering the p zone, so the charged space region near the contact interface is also termed the barrier region. The Fermi level of the intrinsic semiconductor is located in the middle of the forbidden band, while the Fermi level of the n-type semiconductor EFn,s closes to the conduction band. Conversely, p-type EFp,s is near the valence band. When the pn junction is at the thermal equilibrium state, the two constituent parts have a uniform Fermi level EF,s; when both ends of the energy bands of the pn junction begin bending at the thermal equilibrium state, the electric potential difference VD between the two ends of space charge layer is called contact potential difference or the built-in potential of pn junction, and qVD, the amount of band bending is called the barrier height. The barrier height is equal to the difference in Fermi level between these two types of semiconductors before they contact: qVD = EFn , s − EFp , s The electron concentration in the n-type impurity region can be expressed as   EFn, s − EFi, s nn = ni exp kT

(6:1)

(6:2)

In the equation, ni is the intrinsic carrier concentration and EFi,s is the intrinsic Fermi level. Similarly, the hole concentration in the p-type impurity region can be expressed as   EFp, s − EFi, s (6:3) np = ni exp kT Finally, eq. (6.1) combining eq. (6.2) is substituted into eq. (6.3), and we can obtain the expression of VD:

Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells

   kT 1
ND NA VD = EFn, s − EFp, s = ln ni 2 q q

301

(6:4)

Eq. (6.4) shows VD is relevant to ND and NA, the dopant concentration of both sides of pn junction, and temperature as well as the width of band gap. The properties of heterojunction consisting of two different types of semiconductors and forming by their contact are different from those of the homojunction above. The contact interface properties of heterojunction depend on the electron affinity, bandwidth, and work function of these two types of semiconductors. When the contact interface formed by the contact of these two types of semiconductors is compared with the homojunction, there exist both similarities and differences. When the Fermi level of the two materials is different, the space charge regions are formed at the two sides of the contact interface as a consequence of the flows of electrons and holes, which results in bending in the energy band. This process is basically similar with that for the homojunction. But the two biggest distinctions lie in the electron affinity and work function, which gives rise to the discontinuity of the contact interface, and may lead to the appearances of spikes and fractures. The interaction between adjacent atoms in metal lattice makes the outermost electrons of the metal atoms easy to lose. There are many free electrons in the metal, so the metal generally has a high conductivity. The atomic energy levels in the lattice site are splitting and overlapping, and that will create the quasi-continuous energy bands. When the temperature is 0 K, the highest energy level occupied by the electrons is called Fermi level of metal EF,m, electrons are filled with all energy levels that lower than EF,m and no electron exists in the energy level higher than EF,m. When the temperature is higher than 0 K, some electrons in the vicinity of EF,m are subject to thermal excitation, and the electrons below the EF,m will transfer to the energy level higher than EF,m. Although the free electrons in the metal can move freely between the lattices, most of them cannot be separated from the metal. The electrons can escape from the metal only if they absorb energy to overcome the energy difference between the inside energy level and outside energy level of the metal. The minimum energy needed for an electron in the Fermi level EF,m to escape from the interior of the metal to a vacuum can be expressed by the work function of metal. The work function of a metal Φm is the difference between the stationary electron energy Evac and the Fermi level EF,m of the metal in the vacuum: Φm = Evac − EF, m

(6:5)

The difficulty for an electron to escape from the metal can be judged from the numerical value of the metal work function, in other words, the numerical value of metal work function represents metals’ capacity concerning bounding electrons: The greater the numerical value of metal work function is, the more difficult the electron’s escape

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becomes, the stronger the metal bound the electron; the smaller the numerical value of metal work function is, the easier the electrons escape from the metal, and the weaker the metal bound the electron. If the electrons in the semiconductor want to escape from it, they will need certain energy. Similarly, that energy can also be expressed by a work function Φs = Evac − EF, s

(6:6)

The position of the Fermi level in the semiconductor is related to the doping concentration, so the semiconductor work function is also related to doping. Except from the work function, the difference between the Ec and the Evac of the conduction band is called the electron affinity energy χs , signifying the minimum energy required for an electron escaping from the bottom of the conduction band in the semiconductor, the electron affinity energy χs can be expressed by χs = Evac − Ec

(6:7)

Figure 6.2: Several schematic diagrams of energy level’s variation before and after the contact of the metal and n-type semiconductor.

Figure 6.2 is several Schematic diagrams of energy level’s variation before and after the contact of the metal and N-type semiconductor. The contact interface energy level is not the same when there is a difference between the work function of metal and semiconductor: When Φm > Φs (the work function of the metal is greater than the work function of the semiconductor), the Fermi level of the semiconductor is higher than that of

Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells

303

the metal. The electrons in the semiconductor will flow to one side of the metal when these two contacts, and the positive charge remains in the semiconductor. The flow of electrons in these two phases reduces the electric potential of the metal but increases the electric potential of the semiconductor, finally, the Fermi level of the metal and the semiconductor are equal, and there is no net flow of the electrons at the macro level in the end. Consequently, the metal surface is negatively charged and the surface of the semiconductor is positively charged, and these two amounts of the charge are equal but the two surfaces are oppositely charged. Therefore, it is electrical neutral as a whole. A space charge region with positive charges is formed in the semiconductor layer by ionized donor, as the depletion layer, has relatively low electron concentration, and then the contact interface is a high resistance region also called barrier layer. A built-in electric potential is established, which just compensated for the difference in the Fermi energy levels or the work function between the metal and the semiconductor. When Φm < Φs (the work function of the metal is less than the work function of the semiconductor), the Fermi level of the semiconductor is lower than that of the metal. The electrons in the metal will flow to one side of the semiconductor when these two materials contacts. The flow of electrons in these two phases increases the electric potential of the metal while reduces that of the semiconductor, the Fermi level of the metal and the semiconductor are equal, and there is no net flow of the electrons at the macro level in the end. A space charge region with negative charges is formed on one side of the semiconductor and the electron concentration is higher than interior, which is called the accumulation layer. Therefore, the contact interface is a highly conductive region which is also termed an antibarrier layer. There is no absolutely perfect crystal in the world, the surface state will appear between the valence band and conduction band of semiconductor while the crystal periodic potential is destructed. Surface will accumulate a large number of charges when the surface state density is very high, then in the vicinity of the surface of the semiconductor is bound to have the appearance of the opposite charges. Under this situation, the semiconductor surface creates a potential barrier and the internal energy band bends before the semiconductor and metal contact. When the semiconductor contacts with the metal, the surface states can shield the semiconductor from the effect of metal contact; this makes the internal contact potential barrier height of the semiconductor determined by the property of the semiconductor surface instead of work function of metal. So when actually measuring potential barrier heights formed by the contacts of different metals with the same semiconductor, the potential barrier heights do not necessarily change with the metal work function. The junctions of semiconductor/semiconductor contact and metal/semiconductor contact have different properties of current and voltage under the bias voltage in different directions and this is called rectification characteristics. When the applied potential is applied to the semiconductor/semiconductor contact interface, the current-voltage equation of ideal pn junction is:

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    qV −1 J = J0 exp kT

(6:8)

  qV > > 1, eq. (6.8) When the pn junction is a under forward bias voltage, exp kT under a forward bias voltage can be simplified as:   qV (6:9) J = J0 exp kT The pn junction internal current density increases exponentially with the increase   of forward bias voltage, pn junction is in a conductive state. qV ≈ 0, eq. (6.8) When the pn junction is under reverse bias voltage, exp kT under a reverse bias voltage can be simplified as: J = − J0

(6:10)

A reverse saturation current that is irrelevant to the reverse voltage is formed in the interior of pn junction. This current is very small and the pn junction is in the cutoff state. When the applied potential is applied to the metal/semiconductor contact interface, voltage is mainly applied to the barrier layer because the contact interface is a high resistance region. When the bias voltage is applied and the bias voltage is in the same direction as the built-in potential, the potential barrier will be increased. The increase and decrease of the potential barrier destroy the original thermal equilibrium state of the charges at the contact interface. The Fermi level of metal and semiconductor materials is no longer at the same level. Assuming the flow of the electrons from semiconductor to metal is Js!m and that from metal to semiconductor is Js!m , the net current density of the two different directions in the metal/semiconductor junction is [1]: J = Js!m − Jm!s

(6:11)

Assuming the direction from metal to semiconductor is the positive current direction:       − qϕBn qVa * 2 exp −1 (6:12) J = A T exp kT kT here, A* ≡

4πqm*n k2 h3

A* is the effective Richardson constant for hot electron emission. Equation (6.12) also can be written in ordinary diode equation as:

(6:13)

Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells

    qVa −1 J = JsT exp kT in the equation, JsT is the reverse saturation current density:   − qϕBn JsT = A* T 2 exp kT

305

(6:14)

(6:15)

ϕBn is Schottky barrier height. When the metal/semiconductor junction is forward bias voltage, the potential barrier of contact interface reduces, then, the number of electrons flowing from the semiconductor to the metal is increased and the total number of electrons flowing in the opposite direction decreases. This process forms the forward current that from metal to semiconductor, and the larger the bias voltage is, the larger the potential barrier decreases, so the forward current increases in accordance with the increase of bias voltage. When the metal/semiconductor junction is in reverse bias voltage, barrier in the contact interface rises. At this time, the number of electrons from semiconductor to metal is reduced and the electrons from metal to semiconductor dominate the number of moving electrons, thus the reverse current is formed from the metal to the semiconductor. But the electrons in the metal have to overcome a considerably high barrier to enter the semiconductor and this barrier does not change with the bias voltage, so the reverse current is very small. The number of electrons flowing into the metal from the semiconductor is basically negligible and the reverse current is close to saturation when bias voltage is further improved. In addition to the rectification, the metal/semiconductor can also exert a nonrectifying effect, that is, Ohmic contact. Ohmic contact interfaces do not generate significant impedance, the voltage drop of the interface is much less than the other voltage drops and does not affect the current-voltage characteristics of the device when current passes through the Ohmic contact interface. Semiconductor devices are generally required to form a good ohmic contact at the contact point of the current input or output. An additional resistance to current was inevitably generated at the contact interface when the two-material contact. The contact resistance RC is defined as the reciprocal of current density derivation of voltage when v = 0:  RC =

∂J ∂V

 − 1

V=0

Ω cm − 2

(6:16)

When the two materials form an Ohmic contact, the value of RC should be smaller. For the rectifying contact formed by the lower semiconductor doping concentration, the current–voltage relationship is given by eq. (6.14). When the thermal emission current plays the main role in the metal semiconductor contact, the unitary contact resistance is

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 RC =

kT q

 + qϕ exp kT Bn A* T 2

(6:17)

From the equation above, the unitary contact resistance decreases rapidly following the decline of the barrier height.

6.1.2 The composition of the solid–solid contact interface of DSC The solid–solid contact interfaces of DSC exist in the both anode side and cathode side of the cell as shown in Figure 6.3. On the anode side, TiO2 paste printing on the conductive substrate can form TiO2 nanocrystal particles by sintering, and TCO/TiO2 interface are generated by the point contact between TiO2 nanocrystal particles and conductive substrate, while TiO2 thin films are connected by a myriad of TiO2 nanoparticles and the contact among particles can be seen as a contact face (TiO2/TiO2 interface). At the cathode side, the preparative counter electrode through carrying Pt on the conductive substrate can form Pt/TCO interface. And there are a large number of studies on the interfaces between the conductive substrate and TiO2 thin films (TCO/TiO2 interface) as well as on interfaces among TiO2 particles (TiO2/TiO2 interface), and few researches on the Pt/TCO contact interface (Pt/TCO interface).

Figure 6.3: Schematic diagrams of the solid–solid contact interface in DSC.

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6.1.3 The properties of the solid–solid contact interface of DSC The preparation of producing conductive substrate includes making the materials such as indium tin oxide and fluorine doped tin oxide coat the glass. This can be achieved, generally, by methods like chemical vapor deposition, cathode sputtering, sputtering pyrolysis, electron beam evaporation, and oxygen ion beam–assisted deposition. The phenomenon of degeneration emerges when semiconductor doping concentration exceeds certain quantity either the donor’s impurity or the recipient’s impurity is very large; that is, the Fermi level enters the valence band or conduction band. TCO(SnO2:F) used in the DSC has a high doping concentration (ND ≥ 1020 cm−3) and is a kind of degenerate semiconductor [4]. So, when TCO is in contact with TiO2, we can consider it as a Schottky contact. Comparing to the vacuum level, the work function of TiO2 is −5.15 eV, while the work function of TCO(SnO2:F) is −4.85 eV [5]. The Fermi level of TCO is 0.3 eV higher than that of the TiO2, and when they contact, electrons flow from TCO to TiO2 until their Fermi levels are in the same level. The TiO2’s conduction band bends down, and at the TiO2 side, a space charge region with negative charges appears as an accumulation layer, whose electric concentration is higher than that of interior TiO2. The contact interface of the TiO2 and TCO is a heterojunction, and the built-in potential forming between the TCO and TiO2 can just compensate for the difference between their Fermi energy levels. At present, there are still some controversies over the role of built-in potential on TCO/TiO2 contact interface. The TCO/TiO2 contact interface has long been considered as the position of photogenerated charges separation and origin of photovoltage. This case is similar to the traditional solid-state photovoltaic cells, and the DSC’s maximum photovoltage should be limited by the built-in potential of the interface. Willig estimates that the size of TCO/TiO2 built-in potential is about 0.7 eV, so they arrive at a conclusion that the biggest open-circuit voltage of DSC should be 0.7 V [6]. If this is the case, then the change of the work function of the conductive substrate material is bound to change the DSC’s open-circuit voltage. But in experiments that have been reported, the measured photovoltages using different work function of material such as ITO, Au and Pt as conductive substrate were not corresponding with the work function of conductive substrate material. At present, most researchers do not support the theory that the built-in field of TCO/TiO2 contact interface decides the open-circuit voltage [7]. Because there is no recombination inside TCO and TiO2 particles, and the current density flowing out from TiO2 thin film is equal to that flowing through TCO/TiO2 contact interface, so the height of Schottky contact barrier on TCO/TiO2 contact interface does not affect the short-circuit current. At the same time, no electron flows from the external circuit under the open-circuit condition, and the voltage drop does not exist in TCO/TiO2 interface. Therefore the opencircuit voltage, also, is not influenced by Schottky contact barrier, but the height of Schottky contact barrier exerts a big impact on fill factors [8].

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After sintering TiO2 paste, TiO2 particles will contact and form three-dimensional reticulated porous membrane. Because the materials contacted are of same natures, therefore, a built-in electric potential and space charge region may not emerge on the TiO2/TiO2 contact interface, but many lattice defects may exist. These defects will change in accordance with the sintering temperature, which will further affect electron transmission in the film [9]. Some factors, including the overlapped degrees among TiO2 particles, coordination number between adjacent particles and the change of film porosity and the surface area, can seriously affect the DSC’s properties [10]. In order to suppress the electron recombination, we often coat the surface of TiO2 with a layer of insulating material to form a barrier. Two cases of coating particles (before sintering and coating films after the sintering) affect the TiO2/TiO2 interface differently. When we first coat TiO2 particles to form core-shell structure particles, and then proceed to be sintering until porous films form, The TiO2/TiO2 contact interface will be separated physically by a high resistance layer. This will increase the difficulties for electrons to go through TiO2/TiO2 interface. However, if we coat the TiO2 thin film after its sintering, then electrons going through the TiO2/TiO2 contact interface will almost remain unaffected. Nevertheless studies have shown that electrons can tunnel through the neck area of two particles contacting, for this reason, the TiO2 thin film coating will block electron transfer among particles in the neck area. In addition, if the TiO2 thin films were prepared with the methods including low temperature sintering, filling in the binder or isostatic pressing and so on, the bad contact on TiO2/TiO2 interface can negatively affect the conversion efficiency of DSC.

6.2 The solid–liquid contact interface 6.2.1 The property of solid–liquid contact interface When the redox system exists in solution, the electrochemical potential of electrons after establishing balance can be expressed by [11]   c
e, redox = μ0e, redox + kT ox (6:18) μ cred Therein, μ0e, redox is the electrochemical potential under the standard condition, cox and cred represent, respectively, the concentrations of oxidation substance and the reduction substance. Similar to the solid electron system, it is rational to define the “Fermi level” of a redox system as EF,redox, and under the energy scale, the electrochemical potential of the solution is equal to EF, redox:
e, redox EF, redox = μ

(6:19)

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Electrons occupying state and unoccupied state of redox system in solution is corresponding to reduction state and oxidation state of redox system. Specially, both the energy states present the Gaussian distribution, and the distribution functions of reduction state and oxidation state, namely Dred and Dox [11–12], are as follows: "

ðE − EF, redox − λÞ2 Dox = exp − 4kTλ "

#

ðE − EF, redox + λÞ2 Dred = exp − 4kTλ

(6:20) # (6:21)

According to the Franck–Condon principle, only when the distributions of Dox and Dred in solution match with the occupying and the unoccupied electron state of the contact solid, the matches are likely to contribute to current (Figure 6.4).

Figure 6.4: The contact interface of semiconductor–redox system.

When the solid phase contacts the liquid phase, if electrochemical potential of charged particles in the two phases varies, the charged particles will shift between two phases, then on both sides of the solid–liquid interface, two layers having charges of opposite symbols will form [13]. When the solid phase of a metal material possessing good conductivity is in contact with liquid phase, for the body metal contains abundant free electrons, the electron’s increase or decrease is not enough to change the electron distribution inside of the metal. Thus, when the metal contacts electrolyte solution, the residual charges on the side of the metal almost focus on the surface. While the ionic distribution on the side of the electrolyte solution depends on electrolyte concentration and charge density on electrode surface: when the total concentration of electrolyte solution is fairly large (e.g., a few mol · L−1), the charge density on electrode surface is also very large, the remaining charges (ions) in the liquid phase also tend to be closely situated in the most inside of scattering layer of the interface. The distance between ions in surface layer and the electrode surface is approximately equal to or slightly greater than the solvated ion radius. If the ion concentration in solvent is not big

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enough or the charge density on electrode surface is relatively small, due to the interference of thermal motion, all remaining charges of solution are not likely to intensively arrange the innermost of the scatter layer. In this case, the distribution of residual charges in solution will have a certain degree of “dispersivity” [14]. When the solid electrode is a semiconductor since the concentration of electrons or holes inside the semiconductor is fairly low, when in contact with electrolyte solution, whether electrons are moving out of semiconductor or moving into it, all residual charges will not focus on the surface but form a space charge layer within a certain distance inside the semiconductor. So when a semiconductor is in contact with electrolyte solution, the double electric layers of both ends of the interface have certain dispersion. Concretely speaking, when the n-type semiconductor is in contact with electrolyte solution, if the Fermi level of a semiconductor is higher than that of the redox couple of electrolyte solution EF,redox, electrons will transfer from semiconductor to the electrolyte solution, and a depletion layer composed by donors forms on the semiconductor surface, and at this time the semiconductor energy band bends upward; while when the n-type semiconductor’s Fermi level is lower than that of the redox couple of electrolyte solution EF,redox, electrons will transfer from the electrolyte solution to the semiconductor, on which an accumulation layer composed by acceptors emerges, and at this time the semiconductor energy band bends downward. When the n-type semiconductor is in contact with electrolyte solution, and after residual charges on the semiconductor electrode surface are neutralized, semiconductor electrode is in a flat band state, as shown in Figure 6.5.

Figure 6.5: The energy-level structure of n-type semiconductor contacting with the electrolyte solution.

There will exists a space charge layer, a Helmholtz layer and a dispersion layer on both ends of interface after the semiconductor electrodes contact with electrolyte solution, and all the three layers have the capacitance characteristics of accommodating chargers. The semiconductor–electrolyte contact interface’s capacitance is also composed by the three capacitances in series, and in most cases the semiconductor–electrolyte

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contact interface’s capacitance is approximately equal to that of the space charge layer inside semiconductor. The variable relationship between the semiconductor– electrolyte contact interface’s capacitance and electrode potential accords with the Mott–Schottky equation [14]   1 1 2 kT (6:22) = = φ − φ − fb 2 CSC εsc e0 ND e0 CInterface in light of the equation, we can deduce that there is a simple linear relationship −2 −2 and electrode potential. Therefore, according to Cinterface − φ relabetween Cinterface tionships measured by experiments, the semiconductor doping concentration and the flat band potential can be concluded through measurements and data analyses. There are two special cases existing when the semiconductor electrodes polarize. First, when electrodes’ polarization mainly changes the potential distribution of the space charge layer, while the potential distribution of the Helmholtz layer and the scattering layer in solution remains unchanged the conduction band and valence band edge, relative to the reference electrode, remain constant, this situation is called “band edge pinning”; second, when electrodes’ polarization mainly changes the potential distribution of the Helmholtz layer, Fermi level relative to band edge is “pinning.” Two situations above both exist in the semiconductor–electrolyte solution system [15]. There are also additional electron states, namely “surface states,” existing on the semiconductor–electrolyte solution interface. The semiconductor lattices periodically arranging is stopped on the surface, making the surface atoms have the dangling orbitals outward, and this unsaturated bond are called the “dangling bonds.” Dangling bonds can show the effect of donor level, that is to say, dangling bonds’ electrons can jump into the conduction band, making the surface positively charged, and they can also capture electrons, making the surface negative charged to show the effect of acceptor level. The existence of surface state will affect the relationship between the surface capacitance and the electrode potential, and will also affect the Mott–Schottky measurement. The existence of high-density surface state affects the charges distribution on surface, blocking the outside influence on the space charge layer, even shows the electron energy level degeneracy and consequently exhibits metal properties. At the micro-level, describing the charges transfer mechanism of the semiconductor–electrolyte solution contact interface helps us to gain a deeper understanding the electrons transfer process of the semiconductor–electrolyte solution contact interface. When electrons transfer from the liquid to the solid, the anode current density is related to the (Dred) and the (Nun) in solid, and integrating all the energy can get anode current density, which is shown in eq. (6.23) [13].

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+ð∞

ia = k a

Dred ðE0 Þ · Nun ðE0 Þ · dE0

(6:23)

−∞

When electrons transfer from the solid to the liquid, the cathode current density is related to the (Noc) and the (Dox), and integrating all the energy can get anode current density, just as shown in the following equation: +ð∞

ic = − kc

Noc ðE0 Þ · Dox ðE0 Þ · dE0

(6:24)

−∞

Theoretically, the electron transfer relationship at the micro level can be concluded. Actually, using eqs. (6.23) and (6.24) to calculate the relationship also requires many simplifications and hypotheses. At the macro-level, we can intuitively understand the transfer process of the semiconductor–electrolyte solution contact interface through the I–V performance. The redox reaction on solid–liquid interface involves charges transfers between the solid–liquid two phases, and there is a close relationship between the reaction rate and electrode potential. Supposing that chemical reaction rate is ka under forward bias voltage and is kc under reverse bias voltage. When under forward bias voltage he reaction rate on electrode surface can be expressed by current density, and the corresponding anode current (ia) is αa F

ia = Fka cred e + RT E0

(6:25)

Here, F is Faraday constant, and ka is the reaction rate under reverse bias voltage, and cred is reducing substance concentration, and αa is transfer coefficient under forward bias voltage, and E0 is the electrode potential, the corresponding cathode current (ic) is shown in eq. (6.26), αc F

ic = − Fkc cox e − RT E0

(6:26)

In the equation, kc is the reaction rate under reverse bias voltage, and cox is reducing substance concentration, and αc is the transfer coefficient under reverse bias voltage. The total current density is the sum of two currents: Equation (6.27), αa F

i = Fka cred e + RT E − Fkc cox e −

ð1 − αa ÞF E0 RT

(6:27)

When the electrodes are not polarized and are in a balanced state, the current density absolute value of anode reaction and cathodic reaction in two directions is called the exchange current density: αa F

ji0 j = Fka cred e + RT E0 = Fkc cox e −

ð1 − αa ÞF E0 RT

(6:28)

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Under certain current density, over-potential replaces E0 (eq. (6.28)) and simultaneously, assuming the redox concentration of electrode surface is the same to bulk, at this time, the current-voltage equation can be expressed by Butler-Volmer equation [13]:   ð1 − αa ÞF αa F (6:29) i = i0 e + RT η − e − RT η

6.2.2 The composition of the solid–liquid contact interface of DSC During the preparation process of DSC, after the electrolyte solution is injected into pre-drilled hole at the cathode plate, the electrolyte solution can permeate into the two plates’ interspace between the photoanode and cathode in DSC to produce a solid–liquid contact interface. The solid–liquid contact interface inside DSC mainly has the following kinds: on the anode side, electrolyte solution will permeate into pores of the dye-sensitized porous TiO2 thin film and it will combine with the huge TiO2 surface area to generate the dye–TiO2/EL interface. Meanwhile, since the TiO2 thin film cannot completely cover the TCO interface, so on the photoanode side, parts close to the conductive substrate will form a TCO/EL contact interface; on the cathode side, when the electrolyte solution contacts the platinized conductive substrate surface, then the EL/Pt–TCO interface will take shape, as this is shown in Figure 6.6.

Figure 6.6: The solid–liquid contact interface of DSC.

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6.2.3 The property of the solid–liquid contact interface of DSC 6.2.3.1 The contact between TCO and the electrolyte solution In DSC, TCO can contact with the electrolyte solution to form TCO/EL contact interface. The TiO2 thin film will form the point contact array on conductive glass substrate after a high-temperature sintering, Gaps inevitably exist with the points contact, and at the bottom of which is TCO uncovered by TiO2. Studies have shown that the ratio of TCO area uncovered is related to the types of TiO2 paste; for example the ratio for that based on P25 pastes accounts for about 30% of the TCO area [16]. In addition, the sealing ring used in DSC normally is larger than TiO2 thin film area, then there is also bare TCO existing between the film and the sealing film. If the side length of a piece of square TiO2 thin film is 0.5 cm, and that of the square sealing film is 0.7 cm, assuming when the seal film is heat-sealed, side length change caused by the extension is 0.05 cm, then we can estimate the uncovered TCO accounts for 10% of the film area, and such a cell’s exposing TCO area is about 0.19 cm2. Relative to the cell area, the TCO/EL interface area cannot be ignored. A growing number of researches suggest that TCO/EL contact interface will also affect the DSC’s performance [17] and that the flat band potential of TCO(SnO2:F), relative to the redox potential in the electrolyte solution, is about − 0.23 V. In this case, on the TCO/EL contact interface, the TCO side will form a depletion layer, that is to say, the TCO energy bands bend upward and form the negative double electric layers [5] near the electrolyte solution surface.

6.2.3.2 The contact between semiconductor TiO2 and the electrolyte solution After the TiO2 thin film contacts with the electrolyte solution, its inner electrons will transfer to I3− of the electrolyte solution, until the TiO2 Fermi level equals the redox potential of electrolyte solution. Theoretically, in this situation, depletion layers may come into existence on TiO2 particles, but actually because the size of TiO2 particles is extremely small, thus, the particles are unable to maintain the space charge layer. One of the biggest characteristics of the thin film constituted by Nano-TiO2 particles is its huge surface area and easy to adsorb dye molecules. When the TiO2 adsorbs dyes and contacts with the electrolyte solution, the large surface area of TiO2 and the various components of dyes as well as the electrolytic liquid system determine the complex features of the dye–TiO2/EL interface. Common dyes usually contain carboxylic acid groups, which will release H+ when adsorbed on the TiO2 surface, and the released H+ may be adsorbed on the surface of TiO2. After being adsorbed, dye molecules will form a dipole double electric layer on the surface of TiO2, the dipole will increase the affinity of TiO2 electrons, which leads to the moving down of the conduction band. When TiO2 thin films, which have adsorbed dyes,

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dip into the electrolyte solution containing high concentrations of redox couple, positively charged ions in electrolyte solution can be adsorbed and even become embedded in the surface of TiO2. In addition, the negatively charged ions or additive molecules in the electrolyte can also be adsorbed or exist near the surface. Then a complex electric double layer which is composed of various ions forms on the dye–TiO2/EL interface. The types, sizes, charge quantities, adsorption abilities of ions and the competition between positive and negative ions in the electrolyte solution make the actual dye–TiO2/EL interface’s situation more complicated.

6.2.3.3 The contact of the counter electrode and the electrolyte solution TCO of high doping concentration can reveal certain metal properties, but its redox reaction catalytic capacity for electrolyte solution is not strong. If we want the redox reaction to proceed on TCO surface quickly, we must adhere a layer of Pt particles on TCO interface to increase its catalytic properties and reduce the over-potential. After being attached Pt particles, the conductivity and catalytic activity of TCO increase greatly consequently, electron transfer resistance can be reduced from 25 MΩ cm−2 to 2 Ωcm−2 [18]. When the Pt-TCO is in contact with the electrolyte solution, electrons will transfer to the electrolyte solution and form a Helmholtz layer on the EL/Pt-TCO contact interface. In eq. (6.18), Nernst equation can well describe the redox potential on the EL/Pt-TCO contact interface, when the concentrations of the redox components in electrolyte solution keeps consistent, then, the redox potential of the interface will keep consistent, too. In general, the redox potential of redox couple on electrolyte solution EL/Pt–TCO interface can be used as potential reference point.

6.3 The kinetics process of contact interface in frequency domain 6.3.1 The process of frequency domain and time domain There are mainly two methods in measuring charges transport and transfer on DSC: one is the measurement in the time domain; the other is in the frequency domain. Time domain measurement uses time as the a variable and measures the relationship between the signals and time, and as to measurement’s results, the horizontal axis represents time, while the vertical axis means the changes of the signal describing the signal function values at the different time. Frequency domain measurement uses frequency as a variable, and it tests the change of signals along with frequency, and when describing the results, the horizontal axis represents frequency and the vertical axis is the change of the signals describing signal function values under different

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frequencies. Time domain measurement is in consistent with people’s living space thinking, and this kind of measurement gets clearer relationship between signal and frequency. While the frequency domain measurement can get more signal information and a more profound understanding of the information of measurement signals. Time domain and frequency domain can be convertible through Fourier transform, and under certain conditions, a function using time as a variable can convert to a function using frequency as the independent variable through the Fourier transform, and similarly, we can also transform frequency domain function into time domain function through inverse Fourier transform.

6.3.2 The measurement methods of kinetic processes on contact interface 6.3.2.1 Methods of electrochemical impedance spectroscopy Electrochemical impedance spectroscopy is a kind of common electrochemical testing technology in the frequency domain, and this technology is also an important tool when studying the electrode process kinetics, electrode surface structure as well as the determination of the conductivity of the solid electrolyte. This method is to apply a small signal disturbance on the system to observe system’s responses to the disturbance when the system is in a steady state, and the method has many advantages [19]: the first merit lies in its experimental ability of high precision measurement, this is because the responses can be infinitely stable and we can have a mean value over a long time; the second virtue is that it can make the mathematical processing relatively simple through the linearization of current voltage characteristics; the third advantage is that it can be conducted to measure in a very wide range of frequency (10–4–106 Hz). There have been many researchers [20] analyzing the electrochemical impedance spectrum of DSC, and they have pointed out that in the Bode diagram, the peaks from low frequency to high frequency respectively correspond to the Nernst diffusion impedance in electrolyte, charge recombination impedance on the dye–TiO2/EL interface and charge transfer resistance on EL/Pt–TCO interface and so on, and the concrete contents will be described in Section 6.3.3.

6.3.2.2 The methods of intensity modulated photocurrent (photovoltage) spectroscopy IMPS/IMVS (intensity modulated photocurrent /photovoltage spectroscopy) is a method using sine modulated light to measure and analyze semiconductor electrode

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system in frequency domain, and the main principle of the method is that using monochromatic light illuminates (DC light) TiO2 electrodes, and incident light is composed of the DC background and smaller amplitude of modulation intensity. Using small amplitude of modulated light intensity (about 10% of DC light intensity) has three advantages: the first merit is that it enables the linearization of equations describing charge transfer, recombination and the capturing by the surface state, accordingly, the three equations can then be described by a first order kinetics equation; the second one is that under illumination of the incident light, it makes the bending of the semiconductor’s energy band, the space charge layer capacitance and majority carriers’ concentration is approximately similar with the counterparts under a dark balance; the third one is that the relationship between electron diffusion coefficient and the changes of light intensity can be observed for the diffusion coefficient is mainly determined by the DC light intensity. Since the photocurrent and photovoltage response to the external measurement is corresponding to the internal photocurrent flowing into porous films and cell’s internal recombination, therefore, under the circumstance of short circuit, IMPS provides the information regarding charge transfers and back reaction kinetics [21], and we can get the charge transfer time. Under the condition of open circuit, IMVS can be used to measure electron lifetime [4]. Two kinds of experimental methods provide a new perspective for understanding carrier transmission and recombination of DSC.

6.3.2.3 The photovoltage decay method under the open-circuit state In order to study the electron transport and interface recombination of DSC, Peter group in British [22, 23] first introduced a new experimental technology, namely, the open-circuit photovoltage decay – electric quantity extraction method. Its principle can be sketched as follows: At first, the light is illuminated on DSC in the open-circuit state for a period of time (such as 5 s) to make them reach a steady state, and at this time the photoelectrons’ production and recombination are in balance, then we quickly close the incident light, the open-circuit voltage of DSC under the dark state will decline over time, and in the process of the attenuation, if we make DSC short at any time points and then integrate the short circuit, then the rest electron power in the DSC at that point can be calculated. In the whole process of the photovoltage decay, the number of conduction electrons in nanocrystalline electrode is decreasing, and quasi-Fermi level also decreases continually from the position close to the conduction band. Figure 6.7 is the device diagram invented by Peter group [24] to realize the method. LED is used as the incidence light source, the potential/current control of DSC is achieved by operational amplifier, and the short-circuit current can be obtained by measuring the voltage drop on the resistance, and extractive electric quantity can be obtained by current integrator, and digital storage oscilloscope is used to record the signal change of the power and electric potential. According to the photovoltage

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DSO

PC

charge

integrator

voltage

current

measuring resistor

LED

DSC

switch timer Figure 6.7: Schematic diagram of the open-circuit photovoltage decay – electric quantity extraction experiment device.

attenuation-electric quantity extraction method, we get the attenuation relationship between electrical quantity and time, which further reflects the information of electrons in TiO2 nanocrystalline thin film recombining with oxidation-state species in the electrolyte solution. In addition, Peter group [24] pointed out that the energy state distribution of electron trap in DSC can be achieved without any additional assumptions according to this method. In the open-circuit photovoltage decay-electric quantity extraction method, certain intensity light is used to change the quasi-Fermi energy of electrons in the photoanode. Jennings and coworkers [25] let the DSC remain in a dark state, but first they applying voltage to the photoanode to make the current a stable value, then immediately close the circuit, and the rest electric quantity of the DSC at that point can be calculated by integrating the short-circuit current. Results have shown that the electric quantity value obtained by voltage control is well matched with the result of the open-circuit photovoltage decay-electric quantity extraction experiment, which suggests

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it is a powerful method of studying the quasi-Fermi level of nanocrystalline electrode with this alternative method. Based on the exponential relationship between open-circuit voltage and the conduction band electrons concentration of DSC, when using similar devices, electron life can be obtained just through simple open-circuit photovoltage decay [26], τn = −

  kB T dVoc − 1 dt q

(6:30)

in which Voc is the open-circuit voltage of the DSC. Using this method to measure electron lifetime has such advantages: first, electron lifetime value which is continually changing in accordance with the open-circuit voltage with a high resolution; second, the experiment method is relatively simple; third, the processing of experimental data is simplified. Zaban and coworkers [26] used this method to carry on experiments to measure electron lifetime of DSC, and observed electron lifetime exponentially declining as the open-circuit voltage increases. When the Voc reduces by about 0.6 V, and the electron lifetime will increase from 20 ms to 20 s, and the change reaches three orders of magnitude, which matches with the measurement results of the intensity modulation photovoltage spectrum. The recombination reaction series of electrons and oxidation-state species in electrolyte changes along with photovoltage, with a mean of 1.4, and the order changing with the photovoltage indicates that the recombination reaction mechanism is much more complex than the mechanism revealed by IMVS. Bisquert and coworkers [27] conduct a more complete analysis of the theoretical basis of open-circuit photovoltage decay method, which shows that this method relies on perfect theoretical analyses.

6.3.2.4 Short-circuit photocurrent method In order to study the quasi-Fermi level of TiO2 electrode under short-circuit conditions and compare it with the counterpart under open state, Boschloo and coworkers set up a new method, namely, short-circuit photocurrent method [28, 29], to be used on device similar to the open-circuit photovoltage attenuation device: the condition is that the DSC remains in a short-circuit condition, and first, the DSC is illuminated for a period of time to reach a steady state, then the incident light should be immediately turned off, At the same time, switching the DSC to the opencircuit state and monitoring the change of the DSC voltage. Obviously, the opencircuit voltage of the DSC will soon rise to a maximum value and then decrease, and the maximum voltage is called short-circuit voltage Vsc, and apparently Vsc is related to the electric quantity inside DSC. Accordingly, we can get internal information of quasi-Fermi level within TiO2 film under short-circuit conditions. In addition, when turning off the incident light, if the DSC is kept in short-circuit state and

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short-circuit current is measured, then by integrating time, electric quantity decay curve in the DSC can be obtained. In view of this, the method of short-circuit current and the method of open-circuit voltage decay are often used at the same time to conveniently research and compare electron quasi-Fermi levels of DSC under the condition of open circuit and short circuit.

6.3.2.5 Transient photocurrent (photovoltage) method The transient photocurrent is commonly used to measure electron diffusion coefficient. Solbrand and coworkers [30] pointed out that the transient photocurrent consists of two parts together: the contribution of diffusion current and electrostatic repulsion. Diffusion current (Idiff) increases as time(t) goes by, and the electrostatic repulsion attenuates like RC circuits. Kopidakis and coworkers [31] considered when carrier’s diffusion exceeds half the thickness of the DSC, we can think that half of the total charges are extracted and can use eq. (6.31) to calculate diffusion coefficient. D = ðL=2Þ2 =th

(6:31)

In eq. (6.32), D is diffusion coefficient, L is electrode thickness, and th is the time of extracting half of the total electrons. If the transient current can use exponential function exp(-t/τc) to fit, then th = 0.693 τc, and diffusion coefficient can be represented by eq. (6.32). D = L2 =ð2.77τc Þ

(6:32)

Recently, Nakade and coworkers [32] improve the method of traditional transient photocurrent method to simplify the optical devices and shorten the measurement time. Under the short-circuit condition, the initial photocurrent value corresponds to the initial incident intensity, when the incident light intensity begins to change the step, and the initial photocurrent will also begin to decay until it reaches the corresponding photocurrent value of the new light intensity, and the time of photocurrent reaching the new constant value depends on the electron diffusion coefficient. The calculating method of the diffusion coefficient is the same as above. Also, they establish the method of step change light inducing transient photovoltage [32]. And under the open-circuit condition, the relationship between electron concentration n(x,t) and time (t) is dnðtÞ=dt = GðtÞ − nðtÞ=τ

(6:33)

in which G(t) is electron production rate, and τ is electron lifetime, and within a range of small changes, electron concentration decays exponentially along with time. And another reason is that the open-circuit voltage Voc is proportional to ln(n/n0),

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that is, we can calculate electron lifetime according to the fact that step change light induces the changes of the transient voltage light. By using the method of the step change light inducing changes of transient photocurrent (photovoltage), Nakade and coworkers [32] systematically researched the influence of solvent viscosity, cationic types and electrolyte concentration, discovering that the reduction rate of dyes cations is a factor of restricting the improvement of DSC performance in the high-viscosity electrolyte system, and using this method for rapid determination of the diffusion coefficient and electron lifetime helps to achieve the optimization of electrolyte system.

6.3.3 The kinetic process on contact interface under the effect of modulated voltage DSC contains many processes of charges transport and transfer, and their kinetics constant speeds are different, spanning from 10−12 to 101 s. For such a photoelectric chemical system, a means having a wide test range is needed to intensively study the working mechanism of DSC [33]. Electrochemical impedance spectroscopy (EIS) is a kind of measurement technology in frequency domain, and first use light or bias voltage to make DSC in a steady state, then we apply a periodic modulation voltage signal to the steady-state system, compare and analyze the output and input signals to get all kinds of kinetic information [34]. The EIS can research the microscopic kinetic process of DSC within the scope of 10–3–106 Hz, meanwhile EIS is also widely applied to the researches on the new materials selection, preparation and optimization of DSC, greatly promoting the development of the DSC [35].

6.3.3.1 Light and dark states impedance characteristics research Studies have shown that all the photoelectric energy transfer processes inside the DSC are related to the contact interfaces among internal phases [36]. Based on the “sandwich” structure of DSC, several interfaces will form after contacts between two internal phases. There are two charge transport channels between the contact interfaces: the semiconductor thin layer with the electron transport mechanism and the electrolyte thin layer of the ion transport mechanism. Under light irradiation or bias voltage, charges will begin transferring processes on the thin layer as well as on the contact interfaces. There are mainly two test conditions of using EIS to study DSC: one is a dark condition, and the other is a light condition. The excitation signals are different under dark and light conditions, consequently, when DSC is changing from an open-circuit state to a short-circuit state, it is various that the electron injection sources, locations, charge transport and transfer directions and other kinetic information [20, 37] for the two conditions. Under the dark condition, when applying a negative bias voltage on

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TCO

TCO

TCO

TCO dye

dye

e-

TiO2

eI3-

I-

e-

I-

TiO2

I3- TCO

TCO

e-

I3-

dye e-

e-

I-

eTiO2

Figure 6.8: Internal charge transport and transfer process under light or with bias voltage under dark (a) With bias voltage under dark; (b) stead open circuit under light irradiation; (c) stead short circuit under light irradiation.

the photoanode conductive substrate, dyes in DSC are not excited, and electrons go into the inside of the thin film from the conductive substrate [10]. Electron concentration increases near the conductive substrate, then electrons transfer toward the TiO2 thin film by diffusion. Figure 6.8(a) is the charge flow direction: conductive substrate → sensitized TiO2 thin film → electrolyte solution → counter electrode → conductive substrate [37, 38]. Since DSC works under the light condition, and the internal processes under the dark state cannot really reflect the work state of DSC. Compared to the dark state testing, the internal charge flow direction under light condition testing is different, just as shown in Figure 6.8(b) and (c), the internal charge flow direction of DCS in open-circuit condition is as follows: dyes → sensitized TiO2 thin film → electrolyte solution →dyes; when in short-circuit state (electron collection efficiency is 100%), the charge flow direction of the light work cycle in DSC is as follows: sensitized TiO2 thin film → conductive substrate → counter electrode → electrolyte solution → sensitized TiO2 thin film [37, 38]. Researches by Liu and coworkers [38] have shown that the kinetic information being obtained from the impedances under light or dark conditions is not the same, which is due to the differences between the internal charge transport and transfer process.

6.3.3.2 The research on frequency response of DSC internal processes When the modulation voltage frequency is inflicted on the DSC changes from a high frequency to a low frequency, charges transfer on each interface inside the DSC and the information of charge transfer impedance on thin layer will appear in different frequency ranges. For EIS spectra of the actual DSC, several different sizes of semicircles will appear along with the changes of frequency. In the EIS theory, the emergence of a semicircle corresponds to a process, which means the emergence of a time constant [39]. In general, EIS spectrum of DSC can be divided into four frequency areas impedance according to the changes of frequency: pure resistance Rh in the extremely

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high-frequency area, the impedance Z1 in the high-frequency area, the impedance Z2 in the medium frequency area and the impedance Z3 in the low-frequency area, which is shown in Figure 6.9. -4.0

100-10 kHz 10-1 kHz 1 kHz-100Hz 100-10 Hz 10-1 Hz 1Hz-100mHz 100-20 mHz

-3.0

-2.0

Z1 Z2 Z3

Internal resistance Z2

-1.0

Z3

Z1

0.0 0

Rh

1

R1

2

R2 Z‘/

3

R34

5

Figure 6.9: DSC of EIS frequency response ranges [40].

Changing the properties of the DSC internal components and observing the changes of the EIS spectra can confirm the corresponding relationships between DSC internal processes and the four frequency areas impedance [40, 41]. Pure resistance Rh mainly results from the square resistance; high-frequency area impedance Z1 corresponds to the electron transfer process on the EL/Pt–TCO interface; medium frequency area Z2 is similar to the diode in DSC, which is related to the electron transfer process on the contact interface between TiO2 and electrolyte solution (dye–TiO2/EL); low-frequency impedance Z3 is related to ions diffusion on the dye–TiO2/EL interface and on the EL/Pt–TCO interface. Theoretically, the EIS spectra include conductive substrate/TiO2 (TCO/TiO2) interface, dye–TiO2/EL interface, EL/ Pt–TCO interface and the electrolyte layer impedance information, therefore, the EIS spectra should have four corresponding semicircles as shown in Figure 6.10. However in fact, the TCO/TiO2 interface impedance is covered by other processes, and is difficult to directly observe, and EIS spectrum only has dye–TiO2/EL interface, EL/ Pt–TCO interface and the electrolyte layer impedance information [20, 42]. Liu [43] has found that the impedance on the TCO/TiO2 interface appears and the kinetic information of the four processes can be seen at the same time under the negative bias voltage after blocking the effects of the electrolyte solution.

Weiqing Liu, Songyuan Dai

The series resistance

107 Z′′

324

Z′′

Z'

TCO/TiO2 interface

Z′′

Frequency/Hz

Z'

EL/Pt-TCO interface TiO2/EL interface

Z′′

Z'

Z′′

Z'

The Nernst impedance

Z'

10-2

Z′′

Z′′

(a)

Z' (b)

Z' (c)

Figure 6.10: The EIS spectra under various frequency. (a) Four semicircles appearing process in impedance spectrum with frequency increasing; (b) EIS spectra with four semicircles; (c) EIS spectra with three semicircles.

6.3.3.3 The mathematical model of the EIS After decades of development and research, the EIS theory model has been built gradually mainly targeted at DSC. When DSC is under light or the external bias voltage, the charges may be affected by driving force and may transport, transfer or accumulate at a special location. The resistance in charge transport and transfer processes is similar to the resistance in the electric circuit, and the accumulation of charges is a kind of energy storage, which is similar to capacitance in circuit. So the charge transfer and transport processes inside DSC can be equivalent to the circuit system consisting of the resistance and capacitance elements [41]. The transport of electrons in the TiO2 film and ions in electrolyte solution will be affected by some obstacles, and the impact of these resistances can be equivalent to the charge transfer resistance [44]:    q EF, redox − Ecb (6:34) V+ Rt = R0 exp − q kT where R0 is a constant; k is Boltzmann constant, T is temperature; Ecb is the lowest conduction band edge energy level, and EF,redox is redox potential. The ion diffusion in electrolyte solution will also be affected by some resistances; these resistances can also be represented by a diffusion impedance. In the electrolyte solution, compared with I3− concentration,I − concentration is higher, so it has no contribution to diffusion impedance, and only diffusion impedance of I3−

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makes a sense to the diffusion impedance. When the thickness of the electrolyte solution layer is certain, I3− concentration is the highest in the TiO2 thin film, and due to the high catalysis ability of counter electrodes, so we think the I3− concentration falls to 0 near the counter electrode. In this situation, diffusion impedance is Zn =

R3 ðiτel ωÞ1=2

tanh ðiτel ωÞ1=2

(6:35)

in which R3 = kT=ðn2 q2 c0 NDel δÞ is DC resistance; τel = δ2 =Del is the characteristic time constant of diffusion; Del is diffusion constant of I3− ; δ is diffusion length; k is Boltzmann constant; T is temperature; N is Avogadro constant; co is I3− concentration; and n is the charge transfer number. Electron transfer processes among multiple interfaces inside the DSC will also be hindered, and this hindrance is equivalent to a resistance element, namely electron transfer resistance. Conductive substrate generally used in DSC is a glass plated with a layer of F:SnO2, which has good conductivity. F:SnO2 is a kind of degenerate semiconductor, with high conductivity like metals [44]. Then when the exposed conductive glass is in contact with the electrolyte solution, charge transfer may happen. There are two directions in the processes: the electrons transfer from conductive glass to the electrolyte solution or from electrolyte solution to conductive glass. The current transfer on the TCO/EL interface can be obtained by Butler–Volmer [44, 45]     αqV ð1 − αÞqV − j0 exp − (6:36) j = j0 exp kT kT where j0 is exchange current density and α is a transfer factor. When the potential takes a derivative with respect to current, dV=dj has resistance significance, and according to the positive and negative potential there are two cases: ① When the potential is negative, transfer resistance Rct,TCO/EL can be expressed as follows: hαq i Rct, TCO=EL = R0 exp V (6:37) kT ② When the potential is positive, transfer resistance Rct,TCO/EL can be expressed as eq. (6.38).   ð1 − αÞq Rct, TCO=EL = R0 exp − V (6:38) kT The contacting interface between dye-sensitized TiO2 thin film and the electrolyte layer is the most important interface in DSC, directly related to the energy output of DSC. Generally, electrons will have a recombination reaction with the I3− in the electrolyte solution after passing the dye–TiO2/EL interface. The reaction mechanism on this interface is very complex; electrons can not only have a recombination

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reaction with I3− from the TiO2 conduction band, but also can react indirectly through localized states [4, 46]. When combining electrons recombination on the surfaces of TiO2 film and eyed-TiO2/EL, the interface impedance can be represented by the transport line model [34]:   h i Rt Rct, Dyed − TiO2=EL 1=2 tanh ðωr =ωd Þ1=2 ð1 + 1 + iω=ωr Þ1=2 Z= 1 + iω=ωr

(6:39)

in which ωr and ωd are the characteristic frequency of electron transport and interface recombination, respectively. In DSC, in order to increase the reactivity and reduce the influence of overpotential, conductive glasses carrying Pt are usually used to act as the counter electrodes. When the electrolyte solution diffusion does not restrict the counter electrode reaction, according to Butler–Volmer equation and the first-order Taylor formula, Rct, EL/Pt-TCO of EL/Pt–TCO interface can be approximately obtained [20, 47]: Rct, EL=Pt − TCO =

kT qj0

(6:40)

In DSC, charges generally accumulate on TiO2 thin film and on each contact interface to form capacitance. Both the TCO/EL interface capacitance and the EL/Pt–TCO interface capacitance can be represented by Helmholtz double electric layer capacitance [34, 48, 49]. Because of the porous characteristics of nanometer TiO2, the capacitance inside the TiO2 thin film is very complex. Because the nanoporous TiO2 film lacks space charge layers, so there is no space charge layer capacitance. Generally, there are two main types of capacitance in TiO2 thin film: one is the chemical capacitance inside the thin film (Cμ) and the other is Helmholtz double electric layer capacitance CH (CH has a sense only under high bias voltage) [49–51]. Chemical capacitance Cμ is the most important capacitance in the DSC and different from general plate capacitance, it is related to the electron concentration and electrochemical potential of the film. The electron accumulation in the thin film has two forms: some free electrons are in the conduction band and others are in the localized state. Thus Cμ can be divided into two parts: the conduction band capacitance and localized state capacitance [50]: Cμ = q2

∂ðnc + nl Þ = CμðcbÞ + CμðtrapÞ ∂μn

(6:41)

in which nc is the free electron concentration, nl is the localized state electron concentration, and μn is the electrochemical potential.

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6.3.3.4 Two electrodes and three electrodes measuring system DSC can be seen as a kind of photochemical cell containing two electrodes, so usually when using EIS to measure DSC, there will be two electrodes system; that is, working electrode is nanoporous thin-film electrode of DSC, while the reference electrode and auxiliary electrodes are counter electrodes. When researching electron transfer on the EL/Pt–TCO interface and the ion diffusion in the electrolyte solution, except from analyses on Nernst diffusion impedance in EIS low-frequency area, the method of using the two electrodes system in the “symmetrical thin-layer cell” (structure shown in Figure 6.11) to study can also be adopted [52]. The thinlayer cell adopting two identical platinum electrodes, can eliminate the effects of TiO2 thin film and directly research the electron transfer on the EL/Pt–TCO interface and ion diffusion process in the electrolyte solution. Platinum

ITO glass Wiring

Wiring

Spacer ITO glass

Electrolyte Rs

Z''

(a) Rct

Rd

CDL (b) charge-transfer Series resistance resistance

Rs

Rct

(c)

Nernst diffusion resistance

Rd

Z'

Figure 6.11: Two electrodes and three electrodes measuring system for DSC [52]. (a) Symmetrical thin layer cell; (b) equivalent circuit model; and (c) schematic diagram of typical EIS.

Since the two electrodes system lacks the reference electrode, the redox potential of the electrolyte solution is commonly used as a reference [4]. Hoshikawa et al. [53] designed the three electrodes glass tube–type DSC with reference electrode and studied the impedance behavior different from the conventional DSC structure by using EIS. Strictly speaking, this is a kind of photoelectrochemical cell containing dyed TiO2 electrode, which is not the traditional DSC. Because the structure of the three electrodes glass tube–type DSC is different from the conventional DSC,

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Weiqing Liu, Songyuan Dai

another realistic DSC with three electrodes has been designed, whose structure is shown in Figure 6.12 [54].

TiO2 electrode

Electrolyte

Reference electrode

Spacer

Counter electrode

1.5 mm

Figure 6.12: Three-electrode DSC structure schematic diagram [54].

Compared with the conventional DSC, the thickness of this electrolyte solution layer is different (1.5 mm), and a Pt wire is inserted in the electrolyte solution as a reference electrode to form a three-electrode DSC. Relevant literatures show that by using the three-electrode system, the contribution made by the working electrode and counter electrode to the impedance of the DSC can be clearly separated.

6.3.3.5 Research on equivalent circuit construction of DSC The EIS of DSC obtained by measuring can help deduce the internal processes by establishing suitable equivalent circuits according to the characteristics of the spectra. Since DSC is a photoelectric system with multi interfaces, which contains many charge transfer and transport processes. Many researchers have designed different equivalent circuits to simulate the working processes of DSC. At present, there are two most important equivalent circuits including the “ladder-like equivalent circuit” and the “transport line equivalent circuit.” The ladder-like equivalent circuit is much clear; it simplifies the internal charge transport and transfer processes of DSC to simple RC circuits, and then connects them to form a complete circuit according to their serial/parallel interrelationships. The equivalent circuit established by Frank et al. [55], which is a representative of the ladder-like equivalent circuits as shown in Figure 6.13. And the equivalent circuit can

Chapter 6 Photoelectrochemistry of interface in dye-sensitized solar cells

TCO/TiO2 interface RTi

329

TCO/electrolyte interface RCT

TCO

Cacc

Css

RCE

CH

R

Rs CCE

C

CE/electrolyte interface TiO2/electrolyte interface Figure 6.13: The ladder-like equivalent circuit (a) with TCO/TiO2 and dye–TiO2/EL interfaces; (b) with dye–TiO2/EL and EL/Pt–TCO interfaces [55].

be divided into several parts of RC circuits, and finally, these RC circuits are connected in series to form a circuit system. Because the DSC’s photoanode uses the nanoporous semiconductor thin film, the internal electrons of the film not only transport in the network structure, but also accompany the electron transfer processes on the dye–TiO2/EL interface. In this case, it is very difficult for the simple circuits to describe the transport process of the electrons and the interface’s transfer in the photoanode. In view of this, Bisquert et al. [34, 44] established the transport line equivalent circuits, and the process of charges transport and transfer inside the DSC were almost completely described (Figure 6.14(a)). The charges transport processes inside the DSC are divided into two channels: electrons transport channel inside the film and ions transport channel in the electrolyte solution. The electron exchanging between the two channels can all be represented by a series of RC subcircuits. The transport line equivalent circuit can be simplified differently under different external bias voltages (Figure 6.14(b) and (c)). The equivalent circuits above can be applied to the cases in which transport resistances are small in the electrolyte solution layer. For example, the commonly used liquid electrolyte solution. When the solid hole material is used as the electrolyte, the charge transport time in the electrolyte is long, and sometimes it even exceeds the electron lifetime [56]. In this case, a series of sub-charges transport resistances should

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Weiqing Liu, Songyuan Dai

Figure 6.14: Two-channel transmission line equivalent circuit. (a) Complete equivalent circuit; (b) TiO2 simplified equivalent circuit for insulation state; and (c) TiO2 simplified equivalent circuit for conductive state [33, 34].

be added to represent the charge transports in the electrolyte, instead of the Nernst diffusion impedance in the transport channel in the electrolyte solution [57, 58]. The transport line equivalent circuit with two channels is a classic DSC model, at present, when using EIS to research DSC, this equivalent circuit is mostly used. Since the TiO2 surface of DSC is covered with a dye layer, the dye layer may also contain electron transfer channels; based on this, the transport line equivalent circuit with three channels [59] can be established. The three-channel model shown in Figure 6.15 increases the transfer channel on the dye molecule layer based on the original twochannel model. Through the establishment of the three-channel transport line equivalent circuit and a series of detailed computational simulations, the information of electron transport on the dye layer can be obtained. Although the “ladder”-like equivalent circuit and transport line equivalent circuit have gained considerable success, there are also some shortcomings existing. Yong et al. [60] pointed out that the reported equivalent circuit cannot simultaneously fit

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331

metal oxide(2)

substrate dye layer(1)

electron energy

counter electrode oxide

Figure 6.15: Three-channel transmission line equivalent circuit. (a) Schematic diagram of the photoanode structure and (b) electron transport and transfer on the dye layer and semiconductor layer [59].

the I–V curve and the EIS spectrum: the transport line and the “ladder” equivalent circuit with no diode characteristics cannot be well fitted to the I–V curve. Although Han [61] designed an equivalent circuit containing a diode element, the circuit cannot fit the EIS curve completely. Based on this situation, Yong et al. developed a new DSC equivalent circuit containing two diodes as is shown in Figure 6.16: By setting the appropriate components parameters, the model succeeds in both fitting the I–V curve and EIS spectrum simultaneously [60]. Rrec RE

L RCE

Rs W

Iph Di

Dr ci

Rsh

V

Figure 6.16: The equivalent circuit with two diodes for DSC by Yong et al [60].

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Weiqing Liu, Songyuan Dai

6.3.3.6 The extraction of DSC impedance information and the analyses on kinetic processes After the equivalent circuit is developed, the parameters of the components in the equivalent circuit can be extracted by combining with the EIS data. The analyses on impedance information can be conducted by turning to relevant software to obtain the variation trends of all components, the trends change along with conditions including time, electric potential, and temperature. Equivcrt, ZsimpWin, Zview, and so on are the ideal software for fitting EIS data. A great deal of impedance information extracted by using the fitting software must be analyzed the internal dynamic process of DSC finally. And many parameters [62] including capacitance, transfer resistance, transport resistance, ion diffusion resistance, electron diffusion constant, electron lifetime, and so on can be extracted from the EIS measurement. After further data processing, characteristics including the electron transfer kinetics, the charge transport kinetics, the electron collection kinetics, the change of semiconductor energy level and localized state density distribution can be analyzed for DSC. Interface transfer kinetics: the analyses on the transfer process of electrons on an interface are simple, and the kinetic constant of the interface is usually described by using RC constant. The values of the resistance and capacitance of each semicircle in the high and medium frequency areas can be obtained through equivalent circuits fitting of EIS for DSC. After simple calculation, electron transfer time constant on the interface can be obtained and the interface transfer kinetics process can be analyzed [62, 63]. Charge transport kinetics: the transport process and the kinetic constant of ions in the electrolyte solution can be directly analyzed by using Nernst diffusion impedance. The transport resistance of electrons in the TiO2 thin film is in the junction area of the high-frequency and medium-frequency semicircles and it is similar to the 45-degree diagonal Warburg transport resistance [38, 44, 64]. It should be stressed that the resistance can only appear in a certain range of bias voltages [44]. The transport processes and the kinetic constants of the films can be analyzed if electron transfer resistance and capacitance are known. Electron collection kinetics: after the photogenerated electrons are injected from the dye to the TiO2, their transport process to the substrate are influenced by the recombination. The electrons collected finally are the result of the competition between the transport process and the recombination process, thus the electron collection efficiency is related to the electron transfer resistance Rt and the dye–TiO2/EL interface transfer resistance Rct [65]: ηc = 1 −

Rt Rct

(6:42)

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108 107 106 105 104 103 102 101 100 10-1 -0.2

3.0

Device A Device B Device C Device D

Device A Device B Device C Device D

2.5

C /(mF·cm )

R1/( ·cm2)

The energy level change of the semiconductor: the edge shift of the TiO2 film conduction band is a major factor for the change of open-circuit voltage Voc [4]. When the electric quantity in the film is certain, the difference in between the conduction band and quasi-Fermi level of the electron remains constant, and the Voc will change if the belt edge moves. By analyzing the relationship between the transport resistance and the bias voltage (eq. (6.34)) in the EIS, the moving information of the band edge can be obtained as is shown in Figure 6.17(a) [57].

2.0 1.5 1.0 0.5

-0.4

-0.6

0.0 -0.2

-0.8

-0.4

-0.6

-0.8

U/V

U/V

(b)

(a)

TiO conduction band

Energy level/eV vs vacuum

-4.0

electrons in Tranp at V -4.5 Device C Device D

V = 510 mV vs Spiro

-5.0 Ef dark, Sprio 0

2

4

6

8

10

12

DOS/(10 V/cm )

(c) Figure 6.17: (a) Electron transfer resistance; (b) variation of chemical capacitance with the applied bias voltage; and (c) density distribution of states of electron-level scale [57].

Localized state density distribution: Many localized states exist in the TiO2 crystals, which has a large impact on the kinetics of the electron transport and recombination processes [66]. The localized density g(E) has a direct relation with the chemical capacitance [57, 66]: ∂nt trap = q2 (6:43) ≈ q2 gðEÞ Cch ∂E

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Weiqing Liu, Songyuan Dai

According to eq. (6.43), the conclusion that the chemical capacitance of the film is related to the localized density of g(E) can be drawn. Then the chemical capacitance obtained may directly reflect the localized state density distribution (Figure 6.17(b) and (c)).

6.3.3.7 Study on the photoanode impedance of DSC Nanoporous semiconductor thin film is the main component of anodic, and its properties directly determine the efficiency of DSC. EIS can be used to aid designing the photoanode geometry, optimizing the structure of thin film, studying the surface modification and doping mechanism, selecting thin film materials and developing new types of photoanodes.

Figure 6.18: Different geometries of the DSC [67].

The existence of internal resistance in DSC system is inevitable. It is necessary to design the geometry of the photoanode to reduce energy loss. The photoanode geometry design includes the size, shape, and thickness of the photoactive area, and the relationship between geometry structure and internal resistance of photoanode can be obtained directly by EIS measurement. EIS Studies showed that distance between the TiO2 electrode and the ohm contact point directly determines the internal resistance of the DSC the fill factors, and further impacts the efficiency of DSC (Figure 6.18) [67]. TiO2 film thickness is one of the factors that determine the efficiency of the DSC [68]. When the film thickness increases, the electrons transfer resistance on the TiO2/EL interface decreases and the electron diffusion coefficient increases, but the electrons recombination rate [48, 62] will not be affected by the thickness of the film. Semiconductor thin film microstructure can be controlled by the selection of the pore forming agent, the particle size distribution, the sintering temperature and the use of adhesive. EIS can be used to analyze the influence of the film microstructures

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of semiconductor thin films with different preparation processes on the performance of the DSCs. The addition of poly(ethylene glycol) in the preparation process of TiO2 paste can significantly change the pore structure of the films. EIS studies showed that the surface area of low/high molecular weight polyethylene glycol-type thin film is small, but the porosity of the external film is large, which is beneficial to electron transport in the electrolyte solution [69]. In addition to the use in the preparation process of paste, organic pore forming agent can also be used in the paste to control the structure of the film. Kang [70] placed 70–100 nm organic recombination spherical doped into the TiO2 paste to control the film holes, when the ratio of organic recombination spherical doped weight ratio was 10%, the film with minimum interface transfer resistance and optimal electrolyte diffusion performance can be obtained. Changing the particle size distribution can also achieve the purpose of regulating rate of the film holes. The electron transport properties can form an inflection points while the electron recombination properties will be increased with the quantity small-sized particles added increasing [71]. The sintering temperature is an important parameter for TiO2 paste to change into thin film, which determines the degree of contact among the TiO2 particles. When the sintering temperature increases from 350 to 600 °C, the transfer resistance of the electrons on the dye–TiO2/EL interface is reduced from 60.37 to 27.90 Ω. However, the electron transport resistance first rises then descends, which is attributed to the change of surface state trend with the increasing temperature [9]. Producing the TiO2 thin films by using adhesive under low temperatures is an important research field direction of DSC, especially in the of flexible DSC research [72]. TTIP (Ti(IV) tetraisopropoxide) can be added to the nano-TiO2 paste as an adhesive and the semiconductor thin films can be prepared at a low temperature (