Dye-Sensitized Solar Cells and Solar Cell Performance [1 ed.] 9781622572212, 9781612096339

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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

ENERGY SCIENCE, ENGINEERING AND TECHNOLOGY

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DYE-SENSITIZED SOLAR CELLS AND SOLAR CELL PERFORMANCE

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ENERGY SCIENCE, ENGINEERING AND TECHNOLOGY

DYE-SENSITIZED SOLAR CELLS AND SOLAR CELL PERFORMANCE

MICHAEL R. TRAVINO

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EDITOR

New York Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Dye-sensitized solar cells and solar cell performance / editor, Michael R. Travino. p. cm. Includes bibliographical references and index. ISBN:  (eBook) 1. Dye-sensitized solar cells. I. Travino, Michael R. TK2960.D94 2011 621.31'244--dc22 2010054286

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CONTENTS Preface

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Chapter 1

vii Recent Development of Organic Photovoltaic Cells: Materials and Device Physics Fujun Zhang, Weihua Tang, Linyi Bian, Zuliang Zhuo, Zheng Xu and Yongsheng Wang

1

Chapter 2

Advanced Large Area Thin Film Si Solar Cells Fan Yang

41

Chapter 3

Photovoltage Improvement for Dye-sensitized Solar Cells Zhijun Ning and He Tian

71

Chapter 4

Environmental Performance of Photovoltaic Cells L. Reijnders

99

Chapter 5

Solar Simulator Modified to Test PV Cells P. Sansoni, D. Fontani, D. Jafrancesco, L. Mercatelli, F. Francini, D. Ferruzzi, A. Romano and M. Pellegrino

Chapter 6

Drying Characteristics of Lemons and Dates under Indirect Type Forced Convection Solar Drying and Natural Open Sun Drying Sabah A. Abdul-Wahab

Chapter 7

Chapter 8

Chapter 9

Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells with Cu2O, CuInS2, Diamond, Porphyrin and ExcitonDiffusion Blocking Layer Takeo Oku, Ryosuke Motoyoshi, Akihiro Takeda, Akihiko Nagata, Tatsuya Noma, Atsushi Suzuki, Kenji Kikuch, Shiomi Kikuchi, Balachandran Jeyadevan and Jhon Cuya

115

137

155

Enhancement of Solar Cell Performance Using Surface Morphology Modification J. Y. Chen, C. K. Huang, H. H. Lin and K. W. Sun

177

Improving the Efficiency and Stability of TiO2 and ZnO Cells Sensitized with Low-cost Organic Dyes Myrsini Giannouli

221

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vi Chapter 10

Contents Present Scenario of Solid State Photoelectrochemical Solar Cell and Dye Sensitized Solar Cell Using PEO-Based Polymer Electrolytes Pramod K. Singh and Bhaskar Bhattacharya

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Index

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PREFACE This new book presents current research in the study of dye-sensitized solar cells and solar cell performance. Topics discussed include advanced large area thin film Si solar cells; photovoltage improvement for dye-sensitized solar cells; environmental performance of photovoltaic cells; solar simulators modified to test PV cells and improving the efficiency and stability of TiO2 and ZnO cells sensitized with low-cost organic dyes. Chapter 1 - In this chapter, the authors would like to introduce recent development of organic photovoltaic cells from the several aspects, including the development of novel polymer, research works on the device physics, inverted photovoltaic cells and possible approaches in optimization of organic photovoltaic cell. Chapter 2 - Photovoltaic cells are being commercialized at an increasing speed as a means of providing clean and renewable energy source. Thin film Si (including both amorphous Si, a-Si and microcrystalline Si, µc-Si) solar cells are an idea option for low-cost mass production of photovoltaic (PV) modules with the widely available, non-toxic source material, Si, and mature thin film deposition technology. This chapter introduces the current research and production status of thin film Si solar cells. It aims to provide audience with general semiconductor and PV background an in-depth knowledge of state-of-the-art thin film Si solar cells, including many practical issues in the fabrication and field application of largearea modules. This chapter introduces the fundamental thin film PV device physics, the state-of-the-art large scale solar panel manufacturing and the field performance of thin film Si solar cells. To begin with, the basic layer-by-layer cell structure and general photonic energy conversion are introduced. Then the authors talk about industrial fabrication of large area Si thin film solar panels, including the chemical vapor phase deposition (CVD) of Si layers, growth of transparent conductive oxide (TCO) and metal contacts, laser scribing and encapsulation of the solar panels. Typical solar panel manufacturing flow of layer deposition, including batch process, continuous process, and hybrid process are compared with examples. In addition, various practical issues related to the fabrication of large-area solar panels and efficiency improvement are addressed. The authors discuss the decrease of panel efficiency with increasing panel size, and the degradation of initial panel efficiency (Staebler-Wronski effect). Finally, the field performance of Si thin film solar cells is discussed in the context of changing temperature and illumination intensity. Various aspects of performance enhancement of large area thin film Si solar cells are addressed in different sections of this chapter.

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viii

Michael R. Travino

Chapter 3 - The efficiency improvement of dye-sensitized solar cells (DSCs) is critical for their commercialization and replacement of traditional silica solar cells. Generally, the energy conversion efficiency of DSCs is determined by the short-circuit photocurrent, the open-circuit photovoltage and fill factor. During the development of DSCs, it is now generally recognized that the photovoltage is the principal factor that restricts the further improvement of DSCs. In this chapter, the authors will firstly describe the fundamental energy conversion mechanism of DSCs and deduce from the theoretical point of view the factors that affect the photovoltage. Afterward, the relationship between the photovoltage and the main components of DSCs, which include the sensitizer, photoanode, electrolyte, coadsorbent and additive, will be outlined and analyzed. Meanwhile, the main strategies to improve the photovoltage of DSCs, for example, enhancing the blocking effect of the sensitizer, the addition of additives, introducing inorganic barrier on the TiO2 film, employing highly ordered nanostructure photoanode and electrolyte with lower energy level will be presented. Chapter 4 - Substantial terrestrial production of photovoltaic energy started with the commercial introduction of crystalline silicium (Si)-based solar cells. Crystalline Si-based cells still dominate the current market for photovoltaics, but a variety of other photovoltaic cells is currently in use and under development. These cells vary as to composition, solar energy conversion efficiency and expected service life. The environmental performance of these solar cells can be evaluated in a number of ways. Here two approaches to environmental performance evaluation are chosen. The first focuses on the ability of photovoltaic cells to sustainably provide a considerable fraction of current and future energy demand. Evaluated in this way, it is hard to beat Si-based solar cells. The second approach is based on life cycle assessment, using a specified quantity of electricity to be produced for > 20 years as a functional unit. Proper evaluation by life cycle assessment is far from easy, as the solar cells vary widely as to their stage of development. This may give rise to estimates of environmental performance that are biased in favour of solar cells which are characterized by longstanding commercial production. To counteract such bias, assumptions about technology development, scale of production and learning curves can be included in life cycle assessment, which allow for estimates about the future performance of solar cells. Currently available life cycle assessments of photovoltaic cells, which do not include constraints on future resource availability, are reviewed. It turns out that, as evaluated by life cycle assessment, thin film cells do relatively well regarding energy pay-back time. Whether nanoparticulate cells, as evaluated by life cycle assessment, can outperform Si-based cells in power production over > 20 years, is strongly dependent on future developments in service life and efficiency. If future resource availability would be included in life cycle assessment, it would seem that thin film Si-based cells would be hard to beat. Chapter 5 - Possible adaptations for a solar simulator Yamashita YSS-200 were studied and experimented with the aim of allowing the test of concentration photovoltaic cells (CPV cells). Three different solutions were simulated and compared, using Zemax and Lambda Research TracePro, finally selecting the most suitable one. The modification consisted in a supplementary optical system that was externally added to the existing layout. The auxiliary system was easily demountable, thus allowing a rapid restoring of the original configuration. The selected optical system was realised and suitable optical tests verified that it satisfied the requirements on irradiance level and irradiance uniformity. Spectral measurements confirmed

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Preface

ix

that the employed materials did not significantly modify the spectral distribution of the light emitted by the solar simulator. Chapter 6 - An indirect type forced convection solar dryer unit was designed and built at the College of Engineering, Sultan Qaboos University (SQU) in Muscat (Oman). The unit consists of a solar air heater (i.e., solar collector with corrugations) and a cabinet acting as a drying chamber (i.e., basically a batch dryer). It was manufactured with locally available materials and can be used for drying various agricultural products like fruits and vegetables. In this study, the solar dryer was tested under controlled conditions for drying lemons and dates as the test samples. The dryer unit was investigated experimentally and its performance was evaluated under the climatic conditions of Oman (21° 00 N, 57º 00 E). For comparison purposes, natural traditional sun drying experiments (i.e., solar dryer under open sun with natural convection) was also conducted at the same time. For this effect, the results of drying under the open sun and indirect type forced convection solar drying was drawn under identical weather conditions during summer conditions in June, 2009. The performance of the solar dryers was computed and expressed in terms of the moisture evaporation (crop mass during drying). It was found that the quality of lemons and dates after drying was better and drying time was less in the indirect type forced convection solar dryer in comparison to open sun drying. It is hoped that this study may be useful for further development work. Chapter 7 - C60-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and porphyrin, Cu2O, CuInS2 and diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals. Chapter 8 - Solar cell development has become more and more important due to the increase of worldwide energy demands, and conventional energy resources such as fossil fuels, will be exhausted soon. A key factor in applications and effectiveness of solar cell is to enhance its energy conversion efficiency. In this chapter, the authors reviewed and demonstrated innovative approaches to improve conversion efficiency of silicon solar cells by modifying surface morphology of the devices. Because more than 30% of incident light is reflected from the silicon surface back to the air, an anti-reflection (AR) layer is a typical type of coating which can be applied to the surface to reduce light reflection and to increase light absorption. Surface-relief gratings with sizes smaller than the wavelength of light, named sub-wavelength structures (SWS), can behave as antireflection surfaces. Using a mechanically continuous wavelike grating, the subwavelength structured grating acts as a surface possessing a gradually and continuously changing refractive index profile from the air to the substrate. The authors demonstrate polymer sheets with sub-wavelength AR structures using spin-coating replication and hotembossing techniques with applications on silicon solar cells. The techniques provide simple and low-cost means for large-scale production of AR layers and to improve solar cell performance.

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Michael R. Travino

On the other end of the spectrum, the results of the fabrication of ZnO nanorod and selfaggregated nanoparticles as the AR layers are also presented in this chapter. The vertically aligned and solution-grown ZnO nanorod arrays were deposited on the surface of the Si solar cells as the AR layer. The authors found that the nanorod morphology, controlled through synthetic chemistry, has a great effect on the AR layer performance. It’s also known that nanoparticle monolayers composed of metal, silica or polystyrene have the characteristics of surface plasmon resonances and reducing light reflection on surface. Various solar cells substructures with the deposition of nanoparticles have been proved to be able to enhance the photocurrent, acceptance angles, and to reduce the surface reflection within specific wavelength (700-1100 nm) range. The authors demonstrate an appropriate and well-operated application of self-assembled nanoparticle layers on solar cell surface, which increases the light absorption and enhances the photocurrent so that the light conversion efficiency of the cells can be improved. The techniques not only provide enhancement of the light-harvesting capability of the device but also with a minimum cost. Chapter 9 - The most efficient sensitizers for wide bandgap semiconductors are metalloorganic ruthenium complexes due to their high charge-transfer to TiO2 and light absorption in the visible spectrum. Ruthenium complexes however, are expensive and their use renders the resulting solar cells costly. Several simple organic dyes, such as xanthene dyes also yield satisfactory efficiencies, especially when used for sensitizing ZnO films. These dyes are inexpensive and do not rely on the availability of precious metals such as ruthenium. They also have high extinction coefficients and their molecular structures contain adequate anchoring groups to be adsorbed onto the oxide surface. However, solar cells developed using simple organic dyes tend to have drawbacks, such as low long-term stability. In this chapter, several of the parameters affecting the efficiency and stability of photovoltaic cells sensitized with simple organic dyes are investigated and an attempt is made to improve the stability and overall performance of these cells. To this aim, the characteristics of various nanostructured thin films used in dye-sensitized solar cells are examined. Parameters such as the morphology of the films are considered, as these factors greatly affect the efficiency and stability of dye-sensitized solar cells. Experimental results for dye-sensitized solar cells are presented in order to examine some of the major factors affecting the efficiency and the stability of such cells. Nanostructured ZnO, TiO2 as well as composite ZnO/TiO2 thin films were prepared and sensitized using simple organic dyes. Novel multi-component electrolytes for dye-sensitized solar cells were also developed. The effects of these electrolytes on the efficiency and stability of the cells were investigated and it was found that the combined properties of the materials used in these electrolytes enhance cell efficiency and stability. Chapter 10 - Due to energy crisis in coming future many efforts are directed towards alternate sources. Solar energy is accepted as novel substitute to the conventional sources of energy. Out of the long list of various types of solar cells solid state photoelectrochemical solar cell (SSPEC) and dye sensitized solar cells (DSSC) are now emerging area since it shows promise as an alternative for costly crystalline solar cell. This chapter provides a common platform to SSPEC and DSSC using polymer electrolyte particularly on PEO-based polymer electrolytes. Among various polymer electrolytes available for solar cell applications, most frequently used polymer is PEO (polyethylene oxide). Due to numerous advantageous properties of PEO it is frequently used as electrolyte in both SSPEC as well as DSSC. In DSSC, so far high efficiency (more than 11%) could be obtained only using volatile

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liquid electrolyte which suffers many disadvantages like corrosion, leakage, evaporation. The PEO based solid polymer proves its importance and could be used to solve the problems stated above. The recent developments in solar cell using modified PEO electrolytes by adding nano size inorganic fillers, blending with low molecular weight polymers and ionic liquid (IL) are discussed in detail. The role of ionic liquids in modifying the electrical, structural and photoelectrochemical properties of PEO based polymer electrolytes are also suggested.

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In: Dye-Sensitized Solar Cells and Solar Cell Performance ISBN: 978-1-61209-633-9 Editor: Michael R. Travino ©2012 Nova Science Publishers, Inc.

Chapter 1

RECENT DEVELOPMENT OF ORGANIC PHOTOVOLTAIC CELLS: MATERIALS AND DEVICE PHYSICS Fujun Zhang1,*, Weihua Tang1,*, Linyi Bian2, Zuliang Zhuo1, Zheng Xu1 and Yongsheng Wang1 1

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Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, People’s Republic of China 2 Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

ABSTRACT In this chapter, we would like to introduce recent development of organic photovoltaic cells from the several aspects, including the development of novel polymer, research works on the device physics, inverted photovoltaic cells and possible approaches in optimization of organic photovoltaic cell.

Keywords: Organic photovoltaic cells, materials, interlayer, bulk-heterojunction

INTRODUCTION The effective conversion of solar energy into electricity has attracted intense scientific interest for solving the rising energy crisis. Organic photovoltaic cells (OPVs), a kind of green energy source, show great potential application due to low production costs, mechanical flexibility devices by using simple techniques with low environmental impact and the *

Corresponding authors: [email protected] (Fujun); [email protected] (Weihua)

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versatility in organic materials design[1-4]. It is known that the operation of OPVs mainly involves the five following steps: (i) the light absorption by the active layer; (ii) the formation of exciton and subsequent diffusion to the interface of donor and acceptor; (iii) the exciton dissociation into the electrons and holes; (iv) free charge carriers transport in their individual pathway or layer; (v) charge extraction by their corresponding electrodes. In the past years, the key parameter, power conversion efficiencies (PCE), is up to 5% under the standard solar spectrum, AM1.5G [5-7]. The PCE of photovoltaic cells are co-determined by the open circuit voltage (Voc), the fill factor (FF) and the short circuit density (Jsc). Researchers have made great efforts in both developing new organic materials with low optical band gap and designing different structural cells for harvesting exciton in the visible light range. Solution-processing of π-conjugated materials (including polymers and oligomers) based OPVs onto flexible plastic substrates represent a potential platform for continuous, large-scale printing of thin film photovoltaics [8-11]. Rapid development of this technology has led to growing interest in polymer-based photovoltaic cells in academic and industrial laboratories and has been the subject of multiple recent reviews[9, 11-19]. These devices are promising in terms of low-cost power generation, simplicity of fabrication and versatility in structure modification. The structure modification of polymers has offered wide possibilities to tune their structural properties (such as rigidity, conjugation length, and molecule-to-molecule interactions) and physical properties (including solubility, molecular weight, bandgap and molecular orbital energy levels). This ability to design and synthesize molecules and then integrate them into organic–organic and inorganic–organic composites provides a unique pathway in the design of materials for novel devices. The most common polymer solar cells (PSCs) are fabricated as the bulk-heterojunction (BHJ) devices, where a photoactive layer is casted from a mixture solution of polymeric donors and soluble fullerene-based electron acceptor and sandwiched between two electrodes with different work functions [20]. When the polymeric donor is excited, the electron promoted to the lowest unoccupied molecular orbital (LUMO) will lower its energy by moving to the LUMO of the acceptor. Under the built-in electric field caused by the contacts, opposite charges in the photoactive layer are separated, with the holes being transported in the donor phase and the electrons in the acceptor. In this way, the blend can be considered as a network of donor–acceptor heterojunctions that allows efficient charge separation and balanced bipolar transport throughout its whole volume. Remarkably, the power conversion efficiency (PCE, defined as the maximum power produced by a PV cell divided by the power of incident light) of the PSCs has been pushed to more than 5% from 0.1% after a decade’s intensive interdisciplinary research. The current workhorse materials employed for PSCs are regioregular poly(3hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). This material combination has given the highest reported PCE values of 4%~5% by several groups [21-23]. Theoretically, the PCE of polymer photovoltaic cells can be further improved (ca 10%) by implementing new materials and exploring new device architecture [24] after addressing several fundamental issues such as bandgap, interfaces and charge transfer[22, 2426]. In the first section, we will update the recent progress in pursuit of high performance BHJ OPVs with newly developed conjugated polymers, especially low bandgap polymers from a viewpoint of material chemists. We will elaborate the correlation of polymer chemical structures with their properties including absorption spectra, bandgap, energy levels, mobilities, and photovoltaic performance. The analysis of structure-property relationship may

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Recent Development of Organic Photovoltaic Cells

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provide some hints in rational design of polymer structures and reasonable evaluation of their photovoltaic performance. In addition, the development of different structured (i.e. linear, star-shaped and dentritic) conjugated oligomers for BHJ photovoltaic cells is also summarized to give a vivid picture of how oligomers chemical structures strongly affect their properties such as absorption, bandgap, energy levels and photovoltaic performance. In the second section, we would like to present our device physical researches on the performance of OPVs.

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i)

We present herein efficient BHJ OPVs via mixing poly[2,7-(9,9-dihexylfluorene)-altbithiophene] (F6T2) and PCBM with variable weight ratios. The photo-physics and morphology of F6T2:PCBM blend films and the electrical characteristics of their corresponding single cells were studied in details by changing PCBM concentration. With the optimized F6T2:PCBM weight ratio (1:2), the single cell exhibits a highest power conversion efficiency of 2.46% due to the balance of light absorption and charge transport. ii) The influence of the ultraviolet (UV)–ozone treatment of indium tin oxide (ITO) surface and the active layer post-annealing treatment on the performance of organic photovoltaic cells were investigated. Bulk heterojunction organic photovoltaic cells based on the blend of P3HT:PCBM were fabricated. It is found that the devices with the UV–ozone treatment for 5 min on ITO substrates show the better performance, compared with the devices without this treatment. The results demonstrate that the short-circuit current density (Jsc) and fill factor (FF) could be improved by the postannealing treatment. The devices with both treatments together show the best performance, with the increase of Jsc from 2.68mA/cm2 to 4.13mA/cm2 and the enhancement of FF from 32.2% to 38.8%. Therefore, the power conversion efficiency is improved from 0.62% to 1.08%. iii) The effect of ultra thin molybdenum trioxide (MoO3) layer thickness inserted between ITO substrate and CuPc layer on the performance of organic photovoltaic devices (OPVs) was studied. Experimental results demonstrate that the short-circuit current density (Jsc) was decreased slightly with the increase of MoO3 thickness, meanwhile, the fill factor (FF) was increased from 53.5% to 57.7%, respectively, leading to the improved power conversion efficiency with the optimal thickness of MoO3 (1 nm). The experimental results also reveal that the Ohmic contact is formed with the deposition of MoO3. Further, the effect of MoO3 layer was checked from the variation of OPVs performance under different illumination intensity. It was found that the MoO3 layer could effectively prevent the exciton quenching at the ITO anode side, resulting in the small variation of FF for the devices with MoO3 layer compared to the device without MoO3 under high illumination intensity. In the third section, the development of inverted organic photovoltaic cells was introduced. Inverted small molecule organic photovoltaic cells were fabricated with a Ca modified indium tin oxide (ITO) substrates as the cathode and molybdenum trioxide (MoO3) interlayer modified Ag as anode. The Ca and MoO3 layers were found to be critical to the device performance. The insertion of Ca forms Ohmic contact between ITO and fullerene (C60), significantly enhancing the fill factor and further improvement of efficiency by 65%compared to the device without Ca layer. MoO3 interlayer has multiple functions as

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anode buffer layer, effectively preventing the exciton quenching, and reducing the interfacial resistance. A maximum PCE of 0.64% is achieved under illumination 100 mW/cm2. Finally, the possible routes toward the improvement of OPVs were summarized.

1. LOW BANDGAP POLYMER APPROACH IN OPVS 1.1. One-dimensional Low Bandgap Polymers OPVs

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Although P3HT demonstrates many advantages as electron donating material, its OPV application is limited by its relatively wide bandgap ~2.0 eV and thus lack broad absorption profile to collect a large fraction of the solar spectrum (ca ≤30%) [9, 27, 28]. Furthermore, a relatively high-lying HOMO level sets the maximum cell voltage to 0.60-0.65V and represents a potential cause of instability in atmospheric conditions [28]. To harvest solar energy over a broader spectrum organic, it is highly desirable to develop conjugated polymers with broader absorptions through narrowing their optical bandgap. A library of low bandgap polymers have been developed over the years to better harvest the solar spectrum, especially in the 1.4-1.9eV region.

Figure 1. Chemical structure of low bandgap polymers.

To get low band gap polymer, a powerful strategy in designing low band gap conjugated polymers is to alternate a conjugated electron-rich donor (D) unit and a conjugated electrondeficient acceptor (A) unit in the same polymer backbone. With alternation of D-A units, the

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bandgap of the polymer can be effectively reduced[29]. The PCE of devices performance of the polymers and PCBM is related to Voc and Jsc as a linear relationship. The magnitude of the band gap and the energy positions of the HOMO and LUMO energy levels are the most important characteristics for determining the optical and electrical properties of a given conjugated polymer. These in turn greatly affect the ultimate photovoltaic performance. The most straightforward way to reduce the band gap is simply by either raising the HOMO or lowering the LUMO level of the polymer or by compressing the two levels closer together simultaneously. But it is not the only one under consideration. The accepter material should be taken into account. Voc is determined by the energy level difference between the LUMO of the donor polymer and PCBM. So the lower HOMO level will better reach the attainable Voc, whereas reduction in a polymer’s bandgap by lifting up the HOMO level will inevitably result in a loss of Voc. On the other hand, the LUMO level of p-type materials has to be at least 0.3eV higher than that level of the fullerene derivatives to guarantee the formation of a downhill driving force for the electron transfer[30, 31]. Jsc is related to the hole and electron transfer. So reduction in a polymer’s bandgap by reducing the LOMO level will inevitably result in a loss of Jsc. That is to say, it is difficult to obtain high Jsc and Voc simultaneity. The device performance with low band gap polymer is generally poor. With alternation of D-A units, a large spectrum of one-dimensional polymers reported have been developed, with some demonstrating excellent photovoltaic performance. The electron-donating unit generally includes fluorene, carbazole, thiophene and their derivatives. The electron-accepting moieties usually adopt 2,1,3-benzothiadiazole and other electronwithdrawing units. To achieve high PCE in polymer BHJ cells, one should take into account that the structures, absorption wavelengths, bandgap and charge carrier mobility of the polymers. Some successful BHJ OPVs with low bandgap polymers are summarized in Figure 1. The efficiency of BHJ devices can be further improved by replacing acceptor C60 PCBM with its higher fullerene analogue C70PCBM (PC71BM), which has lower symmetry and allows more transitions[32]. Enhancement is mainly attributed to C70’s stronger light absorption in the visible region than that of C60. Compared with P3HT, low bandgap polymers have lower bandgap (Eg 20 years as a functional unit. Proper evaluation by life cycle assessment is far from easy, as the solar cells vary widely as to their stage of development. This may give rise to estimates of environmental performance that are biased in favour of solar cells which are characterized by longstanding commercial production. To counteract such bias, assumptions about technology development, scale of production and learning curves can be included in life cycle assessment, which allow for estimates about the future performance of solar cells. Currently available life cycle assessments of photovoltaic cells, which do not include constraints on future resource availability, are reviewed. It turns out that, as evaluated by life cycle assessment, thin film cells do relatively well regarding energy pay-back time. Whether nanoparticulate cells, as evaluated by life cycle assessment, can outperform Si-based cells in power production over > 20 years, is strongly dependent on future developments in service life *

Tel: + 31-20-5256206; Fax: + 31-20- 5257431; E-mail: [email protected]

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L. Reijnders and efficiency. If future resource availability would be included in life cycle assessment, it would seem that thin film Si-based cells would be hard to beat.

Keywords: photovoltaic cell, environmental performance, life cycle assessment, sustainability

INTRODUCTION Substantial terrestrial production of photovoltaic (PV) energy started with the commercial introduction of crystalline Si-based solar cells (Goetzeberger et al. 2003). Crystalline Sibased, including multicrystalline, cells still dominate the current market for photovoltaic cells (Raugei and Frankl 2009), but a variety of other photovoltaic cells is currently in use and under development. The most important photovoltaic cells which are currently in use or under development are included in Box 1 (Goetzeberger et al. 2003; Takiguchi and Morita 2009). Box 1. Most important types of photovoltaic cells currently in use or under development

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-Si-based photovoltaic cells : • monocrystalline Si, • multicrystalline (mc)Si, • amorphous Si, • thin film Si -II-V type cells (cells with elements from the second and fifth group in the periodic system): • Copper indium disulfide/diselenide (CIS) thin film, • Cadmium Telluride (CdTe) thin film, • Cu2S/CdS, • CdTe/CuS -III-V type cells ( cells with elements from the third and fifth group in the periodic system): • GaAs, • InP, • GaInP/GaAs thin film • •

CuInGaSe2, CuInSe2

•

Dye-sensitized or Grätzel cells, based on TiO2 nanoparticles

•

Organic nanoparticulate cells, based on fullerene nanoparticles

Differences between photovoltaic cells often regard cell composition, as reflected in Box 1. But there also may be other differences. One of the latter regards the presence or absence of heat recovery for useful purposes. When such recovery is present, solar cells are usually called hybrid cells (Azzopardi and Mutale 2010). Also PV systems may be fixed or mounted

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on tracking devices (Perpinan et al. 2009). Here, with one exception (Nishimura et al. 2010), only fixed solar cells solely producing electricity will be considered. Given the proliferation in the types of photovoltaic cells, comparative studies are useful. In this chapter the focus will be on comparisons regarding environmental performance. Firstly, it will be discussed what environmental performance actually means. From this discussion two approaches emerge. The first focuses on the ability of photovoltaic cells to sustainably provide a considerable fraction of current and future energy demand. The second approach is based on life cycle assessment, using a specified quantity of electricity to be produced over a specified period of time as a functional unit. Subsequently, available studies regarding these two approaches of environmental performance evaluation of photovoltaic cells will de discussed. And from this discussion conclusions will be drawn as to the relative environmental performance of solar cells.

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MEASURING ENVIRONMENTAL PERFORMANCE There is a wide variety of operational definitions of environmental performance (e.g. Jasch 2000; Brent and Visser 2002; Hermann et al. 2007; Munksgaard et al. 2007) and no generally agreed-upon way to measure the environmental performance of solar cells. However, in dealing with environmental performance, approaches are often based on the use of environmental life cycle assessment or simplified versions thereof. This is also a clear possibility for solar cells (e.g. Fthenakis and Kim 2009, 2010). Other evaluations of environmental performance are rather strategic in nature. One way to strategically evaluate environmental performance is to focus on sustainability (e.g. Fthenakis 2009), the potential for indefinite future use. Grossmann et al. (2010) have proposed a related way to strategically evaluate the environmental performance of solar cells. They focus on the ability to supply a considerable fraction of present and future energy demand, the ready availability of the materials needed and non-prohibitive land demand. This proposal is in the strategic context of picking ‘winning energy technologies’. In this chapter the focus will be firstly on strategic environmental performance evaluation by focussing on the ability to sustainably provide for a considerable fraction of current and future energy demand. Thereafter life cycle assessment of solar cells will be discussed.

ABILITY TO SUSTAINABLY PROVIDE FOR A CONSIDERABLE FRACTION OF CURRENT AND FUTURE ENERGY DEMAND The potential for indefinite future use, is not problematical for the source of the electricity produced by solar cells; the flux of solar energy to the Earth. This flux will remain at least at the current level for a period of up to 5.109 years and the supply of solar energy will exceed during these years current worldwide energy consumption by a factor in the order of 103, when one takes into account the current solar energy to electricity conversion efficiencies of mature photovoltaic technologies (Lewis and Nocera 2006; Reijnders 2009; Reijnders and Huijbregts 2009). This means that photovoltaic cells can in principle provide a considerable

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fraction of present and future energy demand, when materials for their production remain available for an indefinite period. Thus the focus of the strategic evaluation will be on the materials needed for photovoltaic activity. It may be noted, though, that, apart from those, other materials are needed for the production of the complete PV systems. These may include materials which are geochemically scarce (in the order of 10-4% -10-6% of the Earth’s crust) and have a limited resource base, such as Ag and Sn which are used in solders (Fleischer 1954; Dunham 1978; Nakamura et al. 2008; Izard and Müller 2010). The indefinite availability of materials for photovoltaic cells may turn out to be relatively problematic for the geochemically scarce elements (metals and metalloids) used in II-V and III-V photovoltaic cells (see Box 1), such as Cd, Te, Se, Ga, In and As (Fthenakis 2009). Their concentrations in the Earth’s crust are relatively low, roughly in the order of 10-2 to 10-6% (Fleischer 1954; Durnham 1978). These elements are moreover usually generated as by-products of the minerals processing, e.g. for the production of base-metals (Shah et al. 1999). In the nanoparticulate PV cells considered here, geochemically scarce elements with limited resource stocks, such as Sn, In, Pt and Ru, are used (Kleijn and van der Voet 2010; Reijnders 2010a). Highly efficient recycling of photovoltaic cells may be helpful in improving the resource base of geochemically scarce elements (Fthenakis 2009). However, the requirement of indefinite future availability to sustain a considerable fraction of energy demand, is to the advantage of elements that are more abundant (Shah et al. 1999). The abundance in the Earth’s crust of Ti, used in dye-sensitized or Grätzel cells, is about 0.5% (Fleischer 1954; Durnham 1978), which is much larger that for the geochemically scarce elements mentioned before. Si, however, is the second most common element. About 27.7% of the Earth’s crust is Si (Lutgens and Tarbuck 2000). Other elements which may be included in Si-based cells as dopants are present in very low amounts, which in principle, e.g. in the case of B, might even be sourced from the Si purification process (Green 2009; Liang et al. 2010). Also, the inputs in recycling processes as they have been developed to date for cells with geochemically scarce elements appear to be relatively large if compared with the input in the recycling of conventional Si-based cells (Müller et al. 2004; Berger et al. 2010). Beyond the metals used in PV cells (e.g. Sn and Pt), recycling moreover appears to be problematical for nanoparticulate solar cells (Reijnders 2010a). It has been stressed that purification of Si as it is present in the Earth’s crust to fitness for use in solar cells requires a large input of energy (Ftenakis et al. 2008). It may be noted that all materials included in solar cells for their photoactivity tend to require large inputs of energy (Mohr et al. 2007, 2009; Fternakis et al. 2009; Reijnders 2010a). Comparatively speaking, even the current energy input in Si purification seems rather modest, as evidenced by the calculations of cumulative energy demand for thin film solar cells on the basis of CdTe, CuInSe2, GaInP/GaAs and Si (Alsema 2000; Raugei et al. 2007; Fthenaki et al. 2008; Garcia-Valverde et al. 2010; Meijer 2010). Furthermore, the input of energy in supplying Si for solar cells is a target for very substantial reductions (de Wild-Scholten and Alsema 2004; Raugei and Frankl 2009; Takiguchi and Morita 2009). Major options to achieve this reduction target include the use of lower-grade (‘solar grade’) Si stock of acceptable purity and the use of Si ribbons for the production of non-thin film cells (de Wild- Scholten and Alsema 2004; Raugei and Frankl 2009; Takiguchi and Morita 2009; Liang et al. 2010). Moreover, one may note that an adequate supply of energy to purify Si and make that available for solar cell production might in the future come from solar energy (Mohr et al. 2009).

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From this one may conclude that it is hard to beat Si-based PV cells as to providing a considerable fraction of present and future energy demand.

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LIFE CYCLE ASSESSMENT OF PHOTOVOLTAIC CELLS Photovoltaic cells have a cradle (resource extraction) and a grave (final disposal). Between the two is the life cycle. A life cycle may include one product, but may also extend to a series of products. For instance solar cells may be subject to recycling, leading to constituent materials being included in new PV cells (de Wild-Scholten and Alsema 2004; Reijnders 2010a). Life cycle assessment or –analysis has been originally developed for assessing current products. Apart from analyzing the status quo, life cycle assessments may also deal with changes in demand for, or supply of, products and with novel products. The latter type of assessment has been called consequential, as distinguished from the analysis of status quo products, which has been called attributional (Sanden and Kalström, 2007; Frischknecht et al. 2009). The assessment of novel products has also occasionally been called: prospective attributional (Hospido et al. 2010; Song and Lee 2010). Different data can be needed in attributional and consequential life cycle assessment. Whereas in attributional life cycle assessment one e.g. uses electricity data reflecting current power production, in consequential life one needs data regarding changes in electricity supply. For the short term, assessing a marginal change in capacity of current electricity supply may suffice to deal with changes in electricity supply. When the longer term is at stake, major changes in energy supply, including (complex) new sets of energy supply technologies should be assessed (Lund et al. 2010). When novel products go beyond existing components, materials and processes, knowledge often partly or fully relates to the research and development stage or to a limited production stage. These stages reflect immature technologies. Comparing these with products of (much) more mature technologies may be unfair, as maturing technologies are optimized and tend to allow for better resource efficiency and a lower environmental impact (Jungbluth et al. 2005; Pehnt 2006; Wernet et al. 2010). Also, novel products may in the future be subject to currently uncommon environmental improvement options and may have to operate under conditions that diverge from those that are currently common (Sanden and Kalström 2007; Frischknecht et al. 2009). The latter conditions may e.g. include constraints in resource availability which currently do not exist, new infrastructures, budget constraints, higher resource costs which are conducive to resource efficiency and strict caps on greenhouse gas emissions. A solution to such divergence from ‘business as usual’ may be found in assuming technological trajectories and/or constructing scenarios which include assumptions about the environmental performance of future mature technologies under particular conditions (de Wild-Scholten and Alsema 2004; Jungbluth et al. 2005; Frischknecht et al. 2009; Mohr et al. 2009; Jorquera et al. 2010; Spatari et al. 2010). It should be realized that the assumptions involved lead to considerable uncertainty regarding the outcomes of consequential life cycle assessments, as these assumptions may be at variance with ‘real life’ in the future. Life cycle analysis or assessment is generally divided in four stages, see Box 2 (Rehbitzer et al. 2004).

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In the goal and scope definition stage, the aim and the subject of the life cycle assessment are determined. This implies the establishment of ‘system boundaries’ and usually the definition of a ‘functional unit’. System boundaries refer to what is included and excluded in life cycle assessment. Thus, boundaries between technological systems and significant and insignificant processes are drawn. For instance: several types of photovoltaic cells are often applied while using modules. In establishing system boundaries, a decision should be made on the inclusion or exclusion thereof. Also the solar cell lifecycle will usually include transport, but the system boundaries may be drawn in such a way that making and maintaining means of transport and infrastructure for transport are excluded from life cycle analysis. A functional unit is usually a quantitative description of the service performance of a product. Examples thereof are ‘the production of 25 years of electricity with a power of 1 Watt-peak’ or ‘the production of 1 kWh of electricity’. In this chapter the focus is primarily on electricity production for > 20 years, which is currently common for solar cells integrated in the built environment (e.g. on roofs). Box 2. The four stages of life cycle assessment • • • •

goal and scope definition inventory analysis impact assessment, and interpretation

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Box 3. Aspects of environmental impact which can be considered in life cycle assessment (Guinee, 2002; Rehbitzer et al. 2004) Aspects of environmental impact which can be considered in life cycle assessment: • Resource depletion (abiotic, biotic) • Desiccation • Land use; effects of land use on ecosystems and landscape • Change in albedo • Greenhouse gas emissions • Impact on the ozone layer • Acidification • Photo-oxidant formation • Eutrophication or nutrification • Human toxicity • Ecotoxicity • Nuisance (odor, noise) • Radiation • Casualties • Waste heat

The inventory analysis gathers the necessary data regarding the extraction of resources and emissions for all processes involved in the product life cycle. In full life cycle assessment the inventories cover emissions and resource extractions which may lead to a wide range of impacts (see Box 3).

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The outcome of the inventory stage is a list with all extractions of resources and emissions of substances causally linked to the functional unit considered. This list may be dependent on assumptions about future production processes and product performance. The next stage in the life cycle assessment is impact assessment. This implies: characterization. Characterization serves to aggregate the extractions of resources and emissions in the impact categories outlined in Box 3. For instance, emissions of greenhouse gases may be aggregated as emissions of CO2 equivalents. The interpretation stage connects the outcome of the impact assessment to the real world. In this stage the uncertainties involved should be addressed. And conclusions (such as in specified respects product A has a lower environmental impact that product B) may be drawn (Rehbitzer et al. 2010). Not all life cycle analyses of solar cells are straightforward in all respects. The lack of reliable data may be a major problem regarding solar cells which are currently under development (Mohr et al. 2007; Ito et al. 2008; Mohr et al. 2009; Roes et al. 2009; Meijer et al. 2010; Reijnders 2010a). Another problem with life cycle assessment concerns nanoparticulate cells (Reijnders 2010a). This problem is that so far no generally agreed-upon way has been found to deal with the health hazards and risks associated with nanoparticles (Bauer et al. 2008). An important reason for this is that human toxicity and ecotoxicity are normally linked to the emitted mass of substances, whereas the determinants of nanoparticle hazards are rather such factors as number of particles and surface characteristics (Reijnders 2010b). An additional problem is that there may be size-dependent non-linearities in the relation between diameter and hazard to human health (Reijnders, 2010b). The same may hold for ecotoxicity (Reijnders, 2010b).

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MAIN FINDINGS OF LIFE CYCLE ASSESSMENT Firstly, comparative studies of a number of aspects of solar cell life cycles relevant to environmental performance will be discussed. Thereafter the focus will be on available life cycle assessments of the types of photovoltaic cells mentioned in Box 1. None of the life cycle assessments which will be considered here has included constraints regarding the future availability of natural resources, including geochemically scarce elements.

ENERGY PAY-BACK TIME A main outcome of life cycle assessments is the energy pay-back time of photovoltaic cells. In calculating the energy pay-back time, one divides the energetic value of the electricity output of the solar cell by the cumulative energy demand of the life cycle. Cumulative energy demand is presently a major determinant of the life cycle environmental burden of products such as solar cells (Huijbregts et al. 2006). Current energy demand is largely supplied by fossil fuels and fossil fuel use is a major contributor to several of the aspects of environmental impact mentioned in Box 3 (Huijbregts et al. 2006). This may change in the future when fossil fuels are to be replaced by other sources of energy (Mohr et al. 2009).

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The energy pay-back time is of course dependent on the input of solar radiation, which may be different dependent on the siting of the solar cells. All the solar cells considered here have energetic pay-back times below their expected service life in temperate and (sub)tropical climates. When there is no recycling of solar cells, thin film cells do relatively well regarding their energy pay-back time and nanoparticulate cells do relatively poorly, if compared with conventional Si-based cells when currently achievable service life and efficiency are considered (Alsema 1998; Alsema 2000; Knapp and Jester 2001; Meijer et al. 2003; Jungbluth 2005; Fthenakis and Alsema. 2006; Raugei et al. 2007; Ftenakis et al. 2008; Raugei and Frankl 2009; Roes et al. 2009; Garcia-Valverde et al. Meijer et al 2010; Reijnders 2010a; Sherwani et al. 2010). The combination of solar cells with concentrators of solar radiation, as have been developed for crystalline Si cells and III/V cells may improve the energy pay-back time of fixed solar cells (McConnell 2005; Fthenakis and Kim 2010). The ongoing development of thin film systems tends to further reduce the energy payback time. As it stands, the thin film CdTe and Si solar cells would seem to outperform the thin film CuInSe2 (CIS) and GaInP/GaAs cells in energy pay-back time (Alsema 2000; Raugei et al. 2007; Fthenakis et al. 2008; Garcia-Valverde et al. 2010; Meijer 2010). In the case of CIS cells it has been assumed that they have a service life of > 20 years, an assumption which may not hold in a warm and humid climate (Shah et al. 1999). However, with thin film cells still under development and recycling not being considered so far, it is not easy to predict which thin film cell will outperform the others, environmentally when all thin film technologies have matured.

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EMISSIONS Several authors have studied emissions linked to solar cell life cycles in a comparative way. All these studies have considered applications in industrialized countries. This implies assumptions about the handling of relatively toxic substances such as As and Cd which may not hold universally (Shah et al. 1999). Mohr et al. (2007) studied emissions linked to GaAs and thin film GaInP/GaAs solar modules in comparison with multicrystalline (mc)Si-based cells in the Netherlands with conventional energy supply for module production. Their study was a full LCA with the categories indicated in Box 1 all covered. Aggregated at a level of impact on human and ecosystem health, thin film GaInP/GaAs modules did somewhat worse than mc Si modules, whereas GaAs modules did somewhat worse regarding the impact on human health and somewhat better as to the impact on ecosystem health. Much was done to account for the further development GaAs and GaInP/GaAs solar cells, but as the actual development may not be the same as the development assumed by Mohr et al. (2007), there may be a divergence between the findings of Mohr et al. (2007) and those that will occur in the real world. Ftenakis et al. (2008) studied life cycle greenhouse gas emissions, criteria pollutant emissions, and heavy metal emissions from four types of Si based (non-thin film) photovoltaic solar cells and of thin film CdTe cells (status 2004-2006), with conventional energy supply for solar cell production. In their study CdTe thin film cell performed best,

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with a 89-98% emission reduction for the three categories of pollutants, if compared with conventional electricity production. Thin film GaInP/GaAs solar modules and mcSi modules were studied again by Mohr et al. (2009), assuming that electricity supply would come from the same solar cells. It was found that by introducing PV electricity supply the reductions in toxicity burdens were small to negligible, but that the emission of greenhouse gases linked to thin film GaInP/GaAs solar modules was reduced by as factor 4.9 and of mcSi modules by a factor 2.5. There were also substantial reductions in acidification, eutrophication, ozone layer deterioration and photochemical oxidation linked to photovoltaic electricity supply. Overall the life cycle burden of emissions linked to thin film GaInP/GaAs solar modules turned out to be lower than the life cycle burden of emissions linked to mcSi modules (Mohr et al. 2009). Meijer et al. (2010) studied a thin film Si-based system (nanocrystalline Si superimposed on amorphous Si). Environmental burdens linked to emissions were found to be substantially lower than those of standard mcSi cells for emissions affecting human health, but an increase was found for emissions negatively affecting ecosystems.

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LAND USE Life cycle land use for the generation of electricity has been evaluated by Fthenakis and Kim (2009). They focused on land use of a variety of Si-based photovoltaic cells, while comparing these with other ‘renewables’ such as wind- and biomass-based electricity and hydropower. Biomass-based electricity production had the largest life cycle land use requirements, followed by hydropower. Wind did substantially better but multicrystalline Sibased photovoltaic systems outperformed wind electricity. The best performance in this respect came from photovoltaic cells integrated in building (roofs, facades). It might be expected that, in view of their relatively low solar energy conversion efficiencies, thin film cells and nanoparticulate cells will perform worse than mcSi solar cells, whereas the more efficient III/V cells may do better.

SI-BASED PHOTOVOLTAIC CELLS Conventional crystalline, including multicrystalline, Si-based PV cells have an estimated service life (with a production > 80% of their original power) of at least 25 years (Raugei and Frankl 2009). The environmental performance of such cells, which are applied while using modules, is still improving, and further improvements are expected, linked to the use of ribbon Si, technological learning and recycling of cells (Raugei and Frankl 2009). Si-based solar cells using thin layer techniques are under development. Such cells, which do not need modules but may e.g. be applied as layer on roofing materials, may achieve a lifetime of about 20 years. In that case, they may substantially outperform mcSi cells environmentally as assessed by life cycle assessment, when recycling is not considered (Meijer et al. 2010).

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II/V SOLAR CELLS Modules with thin film CuInSe2 (CIS) cells have been studied by life cycle assessment in a comparative way by Raugei et al. (2007), while not considering recycling. CdTe cells outperformed CIS cells as to energy pay-back time and fresh water ecotoxicity (Raugei et al. 2007). CdTe thin films have been found to have a better environmental performance than current conventional mcSi-based solar cells, when recycling was not included in the life cycle (Fthenakis et al. 2008).

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III/V SOLAR CELLS III/V solar cells have been developed because higher solar radiation-to-electricity conversion efficiencies can be achieved than with Si-based solar cells (Mohr et al. 2007, 2009). Typical examples of III/V solar cells are the GaAs and the thin film GaInP/GaAs photovoltatic cells (Mohr et al. 2007, 2009). Comparing their life cycle environmental performance with Si-based cells is not easy because III/V cells are at an earlier stage of development than Si-based cells. Nishimura et al. (2010) compared photovoltaic systems with mcSi cells with a combination of a concentrator and III/V cells mounted on a tracking systems, to be applied in the Gobi desert (China) and Toyohashi (Japan) as to energy payback times. Recycling was not considered. The mcSi-based system outperformed the III/V system; this was mainly attributed to the high energy cost of the tracking system (Nishimura et al. 2010). Mohr et al. (2007) studied fixed modules with GaAs, thin film GaInP/GaAs and mcSi PV cells sited in the Netherlands, making assumptions about further development of III/V cells and assuming no recycling. The life cycle environmental burdens of the modules studied were found to be rather similar (Mohr et al. 2007). A comparison based on scenario analyses, while not considering recycling, suggested that in the Netherlands the thin film GaInP/GaAs photovoltaic modules may outperform conventional mc Si-based modules environmentally, when their life cycle electricity demand is sourced from the same PV modules (Mohr et al. 2009).

DYE-SENSITIZED AND ORGANIC NANOPARTICULATE PHOTOVOLTAIC CELLS Nanoparticulate cells are currently under development with dye-sensitized, or Grätzel, solar cells and organic nanoparticulate PV cells as front runners. Though it has been claimed that nanoparticulate cells can outperform mcSi-based solar cells there are a number of factors which are not conducive to better environmental performance, when the functional unit supposes solar power production over periods > 20 years (Reijnders 2010a) These include limited recyclability, relatively short service life, lower solar radiation-to-electricity conversion efficiency and relatively high inputs in nanoparticle production (Reijnders 2010a). The environmental benefit for dye-sensitized and organic nanoparticulate cells can not be substantiated when presently realistic service lifes and efficiencies are considered (Garcia-

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Valverde et al. 2010; Reijnders 2010a). Whether they can outperform Si-based cells in the future in power production over > 20 years, is strongly dependent on developments in service life and efficiency (Roes et al. 2009; Reijnders 2010a). When solar cells have a relatively short service life, it may be easier for the dye-sensitized and organic nanoparticulate cells to outperform the other solar cells considered here in the future.

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CONCLUSIONS Here, two approaches have been chosen to evaluate the environmental performance of photovoltaic cells. The first focused on the ability of photovoltaic cells to sustainably provide a considerable fraction of current and future energy demand. Evaluated in this way, it is hard to beat Si-based solar cells. The second approach is based on life cycle assessment, using a specified quantity of electricity to be produced over > 20 years as a functional unit. Proper evaluation by life cycle assessment is far from easy, as the solar cells vary widely as to their stage of development. This may give rise to estimates of environmental performance that are biased in favour of solar cells which are subject to longstanding commercial production on a large scale. To counteract such bias, assumptions about technology development, scale of production and learning curves can be included in life cycle assessment, which will allow for unbiased estimates about the future performance of solar cells. Currently available life cycle assessments of photovoltaic cells have been reviewed. It turns out that, while not considering recycling, thin film cells do relatively well regarding energy payback times, with CdTe and Si thin film cells outperforming CuInSe2 and GaInP/GaAs thin film cells. However, thin film cells are still under development and it is not clear which mature thin film PV cell will prevail, as evaluated by life cycle assessment. It should be noted, though, that none of the available life cycle assessments has taken into account the future availability of natural resources, such as geochemically scarce elements. Whether nanoparticulate cells can outperform Si-based cells in the future in power production over > 20 years, is strongly dependent on developments in lifetime and efficiency (Roes et al. 2009; Reijnders 2010a). When applications of solar cells have a relatively short service life it may be easier for the dye-sensitized and organic nanoparticulate cells to outperform the other solar cells considered here in the future. If future resource availability would be included in life cycle assessment, it would seem that thin film Si-based cells would be hard to beat.

REFERENCES Alsema, E. 1998. Energy requirements of thin-film solar cell modules – a review. Renewable and Sustainable Energy Reviews 2: 387-415. Alsema, E. A. 2000. Energy pay-back time and CO2 emissions of PV systems. Progress in Photovoltaics. Research and Applications 8: 17-25. Azzopardi, B., Mutale, J. 2010. Life cycle analysis for future photovoltaic systems using hybrid solar cells. Renewable and Sustainable Energy Reviews 14: 1130-1134.

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Bauer, C., Buchgeister, J. Hischier, R., Poganietz, W.R., Schebek, L., Warsen, J. 2008. Towards a framework for life cycle thinking in the assessment in nanotechnology. Journal of Cleaner Production 16: 910-926. Berger, W., Simon, F., Weimann, K., Alsema, E.A. (2010) A novel approach fort he recycling of thin film photovoltaic modules. Resources, Conservation and Recycling 54: 711-718. Brent, A.C., Visser, J.K. 2005. An environmental resource impact indicator for life cycle management in the manufacturing industry. Journal of Cleaner Production 13: 557-565. De Wild-Scholten, M., Alsema, E. 2004. Toward cleaner solar PV. Refocus September/October: 46-49. Durham, K. 1978. World supply of non fuel minerals. The geological constraints. Resources Policy June: 92-99. Fleischer, M. 1954. The abundance and distribution of the chemical elements in the earth’s crust. Journal of Chemical Education 31: 446-455. Fthenakis, V., Alsema, E. 2006. Photovoltaics energy payback times, greenhouse gas emissions and external costs. Progress in Photovoltaics: Research and Applications 14: 275-280. Fthenakis, V.M., Kim, H.C., Alsema, E. 2008. Emissions from photovoltaic life cycles. Environmental Science & Technology 42: 2168-2174. Fthenakis, V. 2009. Sustainability of photovoltaics: the case for thin film solar cells. Renewable and Sustainable Energy Reviews 13: 2746-2750. Fthenakis, V., Kim, H.C. 2009. Land use and electricity generation: a life-cycle analysis. Renewable and Sustainable Energy Reviews 13: 1465-1474. Fthenakis, V.M., Kim, H.C. 2010. Photovoltaics: life cycle analyses. Solar Energy DOI: 10.1016/j.solener.2009.10.002. Fthenakis, V., Wang, W., Kim, H.C. 2010. Life cycle inventory analysis of the production of metals used in photovoltaics. Renewable and Sustainable Energy Reviews 13: 493-517. Frischknecht, R., Büsser, S., Krewitt, W. 2009. Environmental assessment of future technologies: how to trim LCA to fit this goal? International Journal of Life Cycle Assessment 14: 584-588. Garcia-Valverde, R., Cherni, J.A. Urbina, A. 2010. Life cycle analysis of organic photovoltaic technologies. Progress in Photovoltaics. Research and Applications18: 535558. Goetzberger, A., Hebling, C., Schock, H. 2003. Photovoltaic materials, history, status and outlook. Materials Science and Engineering R 40: 1-46. Green, M.A. 2009. The path to 25% silicon solar cell efficiency: history of silicon cell evolution. Progress in Photovoltaics: Research and Applications 17: 183-189. Grossmann, W.D., Grossmann, I., Steininger, K. 2010. Indicators to determine winning renewable energy technologies with an application to photovoltaics. Environmental Science & Technology 44: 4849-4855. Guinee, J.B. (editor) 2002. Handbook on Life Cycle Assessment. Kluwer Academic Publishers. Dordrecht, the Netherlands. Hermann, B.G., Kroeze, C., Jawjit, W. 2007. Assessing environmental performance by combining life cycle assessment, multi-criteria analysis and environmental performance indicators. Journal of Cleaner Production 15: 1787-1796.

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Hospido, A., Davis, J., Berlin, J., Sonesson, U. 2010. A review of methodological issues affecting LCA of novel food products. International Journal of Life Cycle Assessment 15: 44-52. Huijbregts, M.A.J., Rombouts, L.J.A., Hellweg, S., Frischknecht, R., Hendriks, J., van de Meent, D., Ragas, A.J.M., Reijnders, L., Struijs, J. 2006. Is cumulative fossil energy demand a useful indicator for the environmental performance of products? Environmental Science & Technology 40: 641-648. Ito, M., Kato, K., Komoto, K. Kichimi, T., Kurukoa, K. 2008. A comparative study on cost and life cycle analysis for 100 MW very large scale PV (VLS-PV) systems in deserts using m-Si, a-Si, CdTe, and CIS modules. Progress in Photovoltaics: Research and Applications 16: 17-30. Izard, C.F., Müller, D.B. 2010. Tracking the devil’s metal: historical global and Contemporary US tin cycles. Resources, Conservation and Recycling 54: 1436-1441. Jasch, C. 2000. Environmental performance evaluation and indicators. Journal of Cleaner Production 8: 79-88. Jorquera, O., Kiperstock, A., Sales, E.A., Embirucu, M., Ghirardi, M.L. 2010. Comparative energy life cycle-analyses of microalgal biomass production in open ponds and photobioreactors. Bioresource Technology 101: 1406-1413. Jungbluth, N. 2005. Life cycle assessment of crystalline photovoltaic s in the Swiss Ecoinvent database. Progress in Photovoltaics: Research and Applications 13: 429-446. Jungbluth, N., Bauer, C., Dones, R., Frischknecht, R. 2005. Life cycle assessment for emerging technologies: case studies for photovoltaic and wind power. International Journal of Life Cycle Assessment 10: 24-34. Kleijn, R. van der Voet, E. 2010. Resource constraints in a hydrogen economy based on renewable energy sources: an exploration. Renewable and Sustainable Energy Reviews 14: 2784-2785. Knapp, K., Jester, T. 2001. Empirical investigation of the energy payback time for photovoltaic modules. Solar Energy 71: 165-172. Lewis, N.S, Nocera, D.G. 2006. Powering the planet; chemical challenges in solar energy utilization. Proceedings of the National Academy of Sciences of the USA 43: 1572915735. Liang, Z,C, Chen, D.M., Liang, X.Q., Yang, Z.J., Shi, J. 2010. Crystalline Si solar cells based on solar grade silicon materials. Renewable Energy 35: 2297-2300. Lund, H., Mathiesen, B.V., Christensen, P., Schmidt, J.H. 2010. Energy system analysis of marginal electricity supply in consequential LCA. International Journal of Life Cycle Assessment 15: 260-271. Lutgens, F.K., Tarbuck, E.J. 2000. Essentials of Geology. 7th edition. Prentice Hall, Upper Saddle River N.Y. (USA). McConnell, R. 2005. Concentrator photovoltaic technologies. Refocus July/August: 35-39. Meijer, A., Huijbregts, M.A.J., Schermer, J.J., Reijnders, L. (2003) Life cycle assessment of photovoltaic modules: comparisons of mc-Si, InGAP and InGaP-mc-Si solar modules. Progress in Photovoltaics: Research and Applications 11: 275-283. Meijer, A., Mohr, N.J., Huijbregts, M.A.J., Reijnders, L. 2010. Life cycle assessment of Helianthos solar cell laminate. OTB Research Institute, Delft, the Netherlands.

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Mohr, M.J., Schermer, J.J., Huijbregts, M.A.J., Meijer, A., Reijnders, L. 2007. Life cycle assessment of thin film GaAs and GaInP/GaAs solar modules. Progress in Photovoltaics: Research and Applications 15: 163-179 . Mohr, N.J., Meijer, A., Huijbregts, M.A.J., Reijnders, L. 2009. Environmental impact of thin film GaInP/GaAs and multicrystalline silicon solar cell modules produced with solar electricity. International Journal of Life Cycle Assessment 14: 225-235. Müller, A., Wambach, K., Alsema, A.E. 2004. Reduction of environmental impacts of PV by the recycling process of Deutsche Solar. EUROPV 2004 conference. Slovenia, 15-20 October . Munksgaard, J., Christoffersen, L.B., Keiding, H., Pedersen, O.G, Jensen, T.S. 2007. An environmental performance index for products reflecting damage costs. Ecological Economics 64: 119-130. Nakamura, S., Murakami, S., Nakajima, K, Nagasaka, T. 2008. Hybrid input-output approach to metal production and its application to the introduction of lead-free solders. Environmental Science & Technology 42: 3843-3848. Nishimura, A., Hayashi, Y., Tanaka, K., Hirota, M., Kato, S., Ito, M., Araki, K., Hu, E.J. 2010. Lifecycle assessment and evaluation of energy payback time on high concentration photovoltaic power generation system. Applied Energy 87: 2797-2807. Pehnt, M. 2006. Dynamic life cycle assessment (LCA) of renewable energy technologies. Renewable Energy 31: 55-71. Perpinan, O., Lorenzo, E., Castro, M.A., Eyras, R. 2009. Energy payback time of grid connected PV systems: comparison between tracking and fixed systems. Progress in Photovoltaics: Research and Applications 17: 137-147. Raugei, M., Bargigli, S., Ulgiati, S. 2007. Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly Si. Energy 32: 13101318. Raugei, M, Frankl, P. 2009. Life cycle impacts and costs of photovoltaic systems: current state of the art and future outlooks. Energy 34: 392-399. Rebitzer, G., Ekball, T., Frischknecht, R. Hunkeler, D., Norris, G., Rydberg, T. Schmidt, W., Suh, S., Weidema, B., Pennington, D.W. 2004. Life cycle assessment. Part I: Framework, goal and scope definition, inventory analysis and applications. Environment International 30: 701-720. Reijnders, L. 2009. Fuels for the future. Journal of Integrative Environmental Sciences 6: 279-294. Reijnders, L. Huijbregts, M.A.J. 2009. Biofuels for Road Transport. A seed to wheel perspective. Springer, London . Reijnders, L. 2010a. Design issues for improved environmental performance of dyesensitized and organic nanoparticulate organic cells. Journal of Cleaner Production 18: 3017-312. Reijnders, L. 2010b. Hazards of TiO2 and amorphous SiO2 nanoparticles. In: Khan, H. (ed.) Toxic effects of nanomaterials. www.bentham.org/ebooks . Roes, A.L., Alsema, E.A., Blok, K. Patel, M.K. 2009. Ex-ante environmental and economic evaluation of polymer photovoltaics. Progress in Photovoltaics: Research and Applications 17: 372-393. Sanden, B., Kalström, M. 2007. Positive and negative feedback in consequential life cycle assessment. Journal of Cleaner Production 15: 1469-1481.

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Shah, A., Torres, P., Tscharner, R., Wyrsch, N., Keppner, H. 1999. Photovoltaic technology: the case for thin-film solar cells. Science 285: 692-699. Sherwani, A.F., Usmani, J.A., Varun (2010) Life cycle assessment of solar PV based electricity generation systems: a review. Renewable and Sustainable Energy Reviews 14: 540-544. Song, J., Lee, K. 2010. Development of a low carbon product design system based on embedded GHG emissions. Resources, Conservation and Recycling 54: 547-556. Spatari, S., Bagley, D.M., McLean, H.L. 2010. Life cycle evaluation of emerging lignocellulosic ethanol conversion technologies. Bioresource Technology 101: 654-667. Takiguchi, H., Morita, K. 2009. Sustainability of silicon feedstock for a low carbon society. Sustainability Science 4: 117-131. Wernet, G., Conradt, S., Isenring, H.P., Jimenez-Gonzales, C., Hungerbühler, K. 2010. Life cycle production of fine chemical production: a case study of pharmaceutical synthesis. International Journal of Life Cycle Assessment 15: 294-303.

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In: Dye-Sensitized Solar Cells and Solar Cell Performance ISBN: 978-1-61209-633-9 Editor: Michael R. Travino ©2012 Nova Science Publishers, Inc.

Chapter 5

SOLAR SIMULATOR MODIFIED TO TEST PV CELLS P. Sansoni1,*, D. Fontani1, D. Jafrancesco1, L. Mercatelli1, F. Francini1, D. Ferruzzi1, A. Romano2 and M. Pellegrino2 1

CNR-INOA Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6 – Firenze - 50125, Italy 2 ENEA Centro Ricerche Portici (NA) Via Vecchio Macello – 80055 Portici (NA), Italy

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ABSTRACT Possible adaptations for a solar simulator Yamashita YSS-200 were studied and experimented with the aim of allowing the test of concentration photovoltaic cells (CPV cells). Three different solutions were simulated and compared, using Zemax and Lambda Research TracePro, finally selecting the most suitable one. The modification consisted in a supplementary optical system that was externally added to the existing layout. The auxiliary system was easily demountable, thus allowing a rapid restoring of the original configuration. The selected optical system was realised and suitable optical tests verified that it satisfied the requirements on irradiance level and irradiance uniformity. Spectral measurements confirmed that the employed materials did not significantly modify the spectral distribution of the light emitted by the solar simulator.

INTRODUCTION Solar simulators were developed to be used for laboratory experimentations on plane solar panels and cells. In general a solar simulator is a device that can reproduce, on limited areas, the conditions of solar irradiation with specific spectral characteristics and irradiance uniformity. Due to the recent technological improvements in Concentrating PhotoVoltaics (CPV systems), there is an increasing request of employing solar simulators to test CPV *

Tel. +39-055-23081; Fax. +39-055-2337755; Email: [email protected]

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components. However a standard solar simulator (for solar panels) typically cannot generate the requested values of irradiance, hence it was useful to design and realize a modified solar simulator, suitable for CPV testing. Since solar simulators systems are very expensive, it could be advantageous to have the possibility of using the same simulator facility to analyse both plane and concentrating PV cells. The photovoltaic cells for concentrating systems (CPV cells) are usually designed to receive irradiance values significantly higher than the solar standard. The cell concentration ratio typically exceeds 100 and most of employed CPV cells work with concentrations in the range of 500-1000. In addition there is another crucial aspect that must be taken into account, especially for the test of medium size cells (diameter around 10 mm). In order to minimise the thermal stresses and to maximise the collection efficiency, the CPV cells require adequate irradiance uniformity over the cell surface. This chapter presents analyses and modifications of a solar simulator Yamashita YSS200 to allow tests of PV cells with concentration ratio 100-200. The work was developed employing both Zemax and Lambda Research TracePro programmes. Zemax is“classical” optical design software: due to the fact that it runs both in sequential and non-sequential mode, it permits to evaluate aberrations of the optical system and spot diagram characteristics. TracePro is lighting simulation software that works only in non-sequential mode: it generates very accurate maps of power density on the working plane. The concurrent use of these software packages allows a better evaluation of simulation results both of the actual configuration and of the planned improvement. In particular Zemax supplies information on the general optical performance of the system; while TracePro gives us reliable final data about power and irradiance distributions on the working areas, representing the crucial quantities of the task.

2. MODEL OF THE SOLAR SIMULATOR IN THE ACTUAL CONFIGURATION 2.1. Introduction The first step for the optical design of the solar simulator modification is to model the instrument in the actual configuration [1,2]. On the base of the specifications, the solar simulator Yamashita YSS-200 generates, at the distance of 440 mm from the external surface of the collimation lens (working distance), a uniformly illuminated area of square shape with dimensions 200 mm x 200 mm. In this area the irradiance uniformity is within 3% and the average irradiance is adjustable, with a maximum around 100 mW/cm2. The spectral distribution is generated by a 2500 W Xenon lamp, filtered by the Air Mass 1.5 filter.

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2.2. Performance Verification A preliminary work was the verification of instrument performance levels in terms of irradiance. The radiant flux was measured using an Ophir Nova Power Meter mounting the 2A-SH head, which has a flux limit of 30 W. This detector works in the wavelength range between 0.19 and 20 microns (with nearly constant spectral response and mean efficiency around 0.92); therefore it completely covers the range of interest (between 0.4 and 1 micron). The diameter of the aperture of the detector head is 10.0 mm. In correspondence of the maximum of lamp current (83 A) on the actual working surface (200 mm x 200 mm at 440 mm from the lens) we obtain the following values for radiant flux and irradiance: • •

centre of the real working area: flux 96.0 mW, irradiance 132.9 mW/cm2 corners of the real working area (mean): flux 99.9 mW, irradiance 138.3 mW/cm2.

2.3. Model of the Current System The main components of the simulator Yamashita YSS-200 are a source, a homogeniser and a final lens collimating the beam. A scheme of the entire optical system of the solar simulator Yamashita YSS-200 is shown in Figure 1.

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2

2

3

4

5 1 

1: lamp + reflector  3: air mass filter    5: collimating lens 

2: flat mirrors  4: homogeniser 

Figure 1. Overall scheme of the solar simulator.

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As is often happens with commercial instrumentation, we did have neither the structural drawing, nor the complete optical scheme of the solar simulator. Therefore to develop a model we had to obtain the component dimensions and the optical characteristics directly from the mechanical measurements or indirectly from the photometric results. In practice the employed methodology was as follows: •

• •

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•

determining, as accurately as possible, the mechanical dimensions of the instrument internal components, selecting some “changeable parameters” (i.e. mechanical dimensions or optical characteristics only approximately measured or assessed); carrying out a first-approximation model of the solar simulator by means of TracePro software; by means of an iterative procedure, setting suitable values of the “changeable parameters” in the approximated model in order to obtain, in the ray tracing simulation, both flux and irradiance distribution equal to the reference data; finally obtaining a TracePro model of the solar simulator, to be used as stating point to develop the modified design.

Figure 2 presents a scheme of the solar simulator source, which is constituted by a 2500 W Xenon lamp, surrounded by an aluminised reflector. The axis of the lamp is coincident with the reflector axis. The reflector has a length of 145 mm and a maximum diameter of 291 mm; the lamp has the discharge zone at 38 mm from the reflector base and its length is 344 mm. The shape of the lamp reflector could be either parabolic or elliptical and the measurement tolerance on the device made acceptable each of these shapes. Due to the fact that a typical solar simulator includes an elliptical reflector, we chose this solution; moreover, we set the focal lengths of the reflector as “changeable parameters”, due to the practical difficulties to assess them. The user manual indicates that the instrument contains a filter corresponding to Air mass 1.5. After this filter, the beam hits the homogeniser, composed of a 5x5 array of identical little biconvex lenses with side 10 mm (whole dimension of the homogeniser: 50 x 50 x 54.2 mm3). It is important to remark that it was almost impossible to model this device without disassembling it: for example, we did not realise if a little lens is a unique solid object or if it includes two piano-convex lenses. In addition, it was not easy to establish the focal length of each little lens: it seemed to be about 14 mm from the lens surface. The focal length of the lenses of the homogeniser was set to be one of the “changeable parameters” for the model of the instrument. The collimating lens has focal length 475.9 mm (measured by a spherometer), diameter 380 mm and thickness (at the edge) 65 mm. The type of optical material used to realise the lens was unknown, but it is reasonable to guess that it was quartz (fused silica), in order to maintain the lens transmission also in the UV region. Summarising, the “changeable parameters” of the model were as follows: • •

focal lengths of the reflector (please note that it is a single parameter, due to the relationship between the positions of the foci of an ellipse); focal length of the little lenses composing the beam homogeniser;

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45.0

105.0 57.0 38.0

291.2

Ellipsoid focal length 1 ≅ 58 focal length 2 ≅ 450 thickness = 1.0

170.0 10.0

145.0 R ≅ 20.0 38.0

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≅80.0 Figure 2. Lamp and reflector, longitudinal section.

Utilising the mentioned data, we were able to develop the first-approximation model of the simulator. Then we modified the “changeable parameters” to approach the correct TracePro model: we launched a simulation; we collected the results of total flux and irradiance distribution on the working plane and we compared them with the measured data. In practice, if there were significant differences between simulated and measured values, we modified the “changeable parameters” in this way: • •

if the difference was on the irradiance distributions we acted on the focal length of the lenses of the beam homogeniser; if the difference was on the flux values we operated on the focal length of the reflector.

Then, using an iterative procedure, we finally obtained an acceptable TracePro model of the solar simulator. Obviously, we could not estimate how much it approaches the real device: we could only affirm that it replicates the actual results of flux and irradiance distributions on the working plane.

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Fiigure 3a. A TracePro model off the Solar Simuulator: top 3D view. v

Fiigure 3b. Side view v of the Solaar Simulator YS SS-200.

Some exam mples of lateraal and 3D view ws of TracePro simulations are presentedd in Figures 3aa-d. The plots show an opptical design model reprodducing the sollar simulator Yamashita Y YSS-200 with a good approxximation. The obtained model is suuitable also foor Zemax, therrefore we deveeloped an optiical design, inn Zemax sequeential mode, of o the solar sim mulator YSS-2200. The main purpose was to reproduuce the outpuut beam withh optical chaaracteristics appproaching thee features of thhe real beam, which were measured m and analysed a also in terms of sppatial homogeeneity and anggular divergennce distribution. The main difficulty d encoountered in thhe optical projject developm ment was that we did not know k the actuual conicity vaalue of the enntrance mirrorr. In these cases to obtain thhe Zemax optiical simulationn we inserted, at the first orrder, the vario ous optical elements composing the solaar simulator system. s Succeessively we inntroduced the curvature radiii and the aperrtures, only paartially measurred. After having located thhe apertures at a the measureed distances, we adjusted the parameters trying to find f a self-

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coonsistent opticcal configuratiion with the coorrect focal lenngths and the desired field of o view. At thhis point, we considered c thee various waveelengths, betw ween 400 nm and a 1000 nm, to t take into acccount also thee chromatic hoomogeneity of the output beeam. Finally wee optimised the t conicity of o the mirrorr, finding a good correction of the geeometrical abeerrations and reaching r accepptable optical quality of thee output beam.. The limits inn beam homog geneity and anngular divergeence are represented by the characteristiccs, declared foor the solar sim mulator or meeasured on the instrument. The compatibbility betweenn simulated feeatures and meeasured valuess should obvioously be withinn the measureement errors.

3. MODIFFICATIONS OF THE OPTICAL SYSTEM Y 3.1. Introducction

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a we decided to Since the detailed opticcal scheme off the instrumeent was not available, model it startin m ng from the dimensional evaluations of o the optical elements. Affter having deeveloped an approximate a T TracePro moddel, it was iterratively corrected in order to reach a saatisfying agreeement with thhe measured results. r The reeference data were the totaal flux and irrradiance distributions meassured at a definned distance, corresponding c g to the workinng plane of thhe solar simulaator.

Fiigure 3c-d. Fron nt 3D view (c) and a back 3D vieew (d) of the sim mulator.

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The final result was a TracePro model of the solar simulator Yamashita YSS-200, which was also translated into a Zemax simulation. It reproduces the actual optical layout of the instrument and represents the starting point for the optical design of the possible modifications. The simulator adaptation had the purpose of obtaining, over two new working areas AS=11x11 mm2 and AL=15.6x15.6 mm2, the following features: • • • •

minimum value of irradiance 20 W/cm2 on a squared working area of 11 x 11 mm2 (case S); minimum value of irradiance 10 W/cm2 on a squared working area of 15.6 x 15.6 mm2 (case L); irradiance uniformity better than 50%; spectral distribution of lighting on the working plane substantially unchanged in the wavelength range 400 nm – 1000 nm.

The fundamental parameters for the optical system of the solar simulator are collected power (the most important) and irradiance uniformity on the working area [3]. Other important features that our modified system should maintain are as follows: •

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• •

the beam must have a sufficient degree of collimation, to avoid that a little displacement of the working plane causes relevant modifications of the irradiance distribution; the modification must not be definitive; the modification must be easily removable, so allowing a fast exchange between standard and modified configuration.

Besides, it is essential to note that the system does not have to generate an image; this last feature suggested to avoid optical systems of high quality (and high cost) addressed to imaging. Considering the previous list of restrictions, we decided to use an external device to be added to the solar simulator, which consisted in a lens system after the collimating lens of the simulator. The first lens of the auxiliary system had obviously to be large as the actual beam; then, in order to maintain the low costs, we decided to utilise a commercial plastic Fresnel lens. An additional advantage of a Fresnel lens, made of plastic, is its reduced weight with respect to an equivalent glass lens. Consequently the mechanical support structure and the solar simulator structure, to which the first one is connected, must bear a reduced additional weight. The second optical element of the auxiliary system is a custom-made traditional glass lens, which permits to have, on the new working areas the expected irradiance (10 W/m2 for AL and 20 W/cm2 for AS). Nevertheless the first step was to eliminate the beam homogeniser, which became useless considering the new uniformity requirements of the modified solar simulator. A further advantage of the homogeniser removal was to have higher values of radiant flux on the new working areas.

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Figure 4. Optical layouts of Solution_1, case S (left), case L (right).

The choice of the Fresnel lens was very limited, due to the fact that its diameter ought to be at least 260 mm (equal to the diameter of the beam near the collimating lens) and its focal length should be as short as possible, in order to reduce the length of the auxiliary system. The secondary glass lens had to be designed in dependence on the characteristics of the Fresnel lens and its optical design was performed by mean of the Zemax software. Three optical configurations were investigated as possible modifications of the solar simulator Yamashita YSS-200. All of them include a Fresnel lens and a secondary glass lens [4]. The new working surfaces did not correspond to the focal plane and their positions were determined to reach the best trade-off between power and irradiance uniformity requirements [5,6]. The proposed solutions differ for the type of employed Fresnel lens and for the converging or diverging secondary lens. This latter, in particular, could be different, inside the same solution, depending on the extent of the working surface to be lighted of side 11 or 15.6 mm.

3.2. Solution_1 (Squared Fresnel Lens Edmund, Diverging Lenses in Glass) Solution_1 includes a Fresnel Lens Edmund L32-597 (overall size 279.4 x 279.4 mm, effective size 266.7 x 266.7 mm, curvature radius 416.6 mm, width 2.3 mm, 1.97 lines/mm) and two diverging lenses. The secondary diverging lenses are separately employed: one to obtain the irradiance of 20 W/cm2 on the working area AS (11 x 11 mm2), the other to obtain 10 W/cm2 in the working area AL (15.6 x 15.6 mm2). Figure 4 reports the two layouts of Solution_1. The two secondary lenses are made of BK7, they have diameter 70 mm (active zone), the curvature radii are RS=-500 mm (for 20 W/cm2) and RL=-80 mm (for 10 W/cm2). Both lenses are located at a distance d of d=136.8 mm from the Fresnel lens. The working surface is positioned at dS=99 mm from the glass lens for case S and at dL=111.5 mm for case L. The configurations of Solution_1 in Figure 4 represent a preliminary optimisation of the optical system, which basically combines a Fresnel lens with a secondary lens. The idea was to find a combination allowing illuminating the working surface simultaneously matching the

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power requirements and maximising the uniformity, in the wavelength range of interest. The two layouts, for case L and Case S, include the same Fresnel lens Edmund L32-597 always combined to a diverging lens. Keeping unchanged the external dimensions of the total optical system, the use of a diverging lens permits to have configurations with longer overall focal length. Hence the system has a higher f-number (f/#=f/D; f=focal length; D=entrance pupil diameter) with a consequent reduction of optical aberrations. In particular it is useful to have a reduction of the longitudinal chromatic aberration, which otherwise could lead to a dishomogeneity over the working surface, as a function on the various wavelengths. From a point of view of the geometrical aberrations, to obtain a correct image we ought to consider systems that even out of focus are still homogeneous as light distribution and thus appropriate to illuminate the working areas. However an improvement in the focal length of the system would generate larger images, so a comparative evaluation of the different layouts is necessary to optimise the configuration. Solution_1 represents best compromise between the superior limit for the focal lengths and the most compact layout, thus minimising the overall system longitudinal size and the distances from the Fresnel lens. The distance between working surface and Fresnel lens is less than 250 mm for case L and less than 240 mm for case S. The analysis of the optical performance, carried out both with Zemax and TracePro, evidenced that Solution_1 is compatible with the requirements. The main advantage of Solution_1 is compactness, but it can be improved for the quality of the obtained illumination, as next solutions will show.

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3.3. Solution_2 (Round Fresnel Lens Germanov-Simon, Converging / Diverging Lenses in Glass) Solution_2 includes a round Fresnel Lens GS 9268 (overall size 304.8 mm, useful zone extended to the whole surface, curvature radius 609.6 mm, width 2.0 mm, 1.97 lines/mm) and two secondary BK7 lenses that are separately used. Figure 5 illustrates the configurations of Solution_2. A plane-convex converging lens with curvature radius RS=400 mm and diameter 110 mm generates an irradiance of 20 W/cm2 on the AS surface. The converging lens is positioned at d1=247.7 mm from the Fresnel lens, with the working surface AS at dS=94 mm from the glass lens. A plane-concave diverging lens with curvature radius RL=-250 mm and diameter 110 mm allows us to obtain 10 W/cm2 on the AL surface. The distance of the diverging lens from the Fresnel lens is d2=245 mm and the working surface AL is located at dL=144 mm from the lens. In general in the optical configurations of Solution_2 the total focal length and the longitudinal dimension of the overall system result increased with respect of the corresponding layouts of Solution_1. For case L the total distance between AL and Fresnel lens reaches the value of 389 mm; while for case S it results 342 mm.

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Figure 5. Optical layouts of Solution_2, case S (left), case L (right).

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The strategy was to simultaneously enlarge the single focal lengths, and consequently the distances from the Fresnel lens, with the aim of broadening the total focal length of the entire system. The positive consequences, for both optical systems designed to obtain 10W/cm2 or 10 W/cm2, were reduction of geometrical aberrations and improvement of the illumination uniformity on the working planes. The major advantage of Solution_2 is enhanced optical quality, while its drawback is lower compactness, with respect to Solution_1.

Figure 6. Optical layouts of Solution_2, case S (left), case L (right).

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3.4. Solution_3 (Round Fresnel lens Germanov-Simon, Converging Lenses in Glass) Solution_3 includes a round Fresnel Lens GS 9268 (the same lens used in Solution_2) and two secondary converging lenses. The two glass lenses are separately employed to obtain 20 W/cm2 or 10 W/cm2 of irradiance, respectively on the surfaces AS and AL. The layouts of Solution_3 are presented in Figure 6. For case S (20 W/cm2) we use a lens made of BK7 of diameter 110.0 mm, with an active zone of diameter 110.0 mm; it is a plane-convex lens placed with its convexity towards the Fresnel lens. The curvature radius is RS=100.0 mm and the central thickness is 20.0 mm. The distance from the lens centre to the Fresnel lens is d=237.7 mm. The working area AS of size 11 x 11 mm2 is located at dS=76.5 mm from the glass lens. For case L (10 W/cm2) we utilise a BK7 plane-convex lens of diameter 110.0 mm, with total and active zones of diameter 110.0 mm; it is a plane-convex lens placed with its convexity in the direction of the Fresnel lens. The curvature radius is RL=220.0 mm, the central thickness is 10.0 mm and the distance between lens centre and Fresnel lens is d=237.7 mm. The working area AL of size 15.6 x 15.6 mm2 is placed at dL=114.0 mm from the glass lens. If compared to the layouts of Solution_2, the optical systems of Solution_3 have slightly reduced their overall longitudinal dimensions. In the configuration for case L, the total distance from the Fresnel lens is 352 mm and it is 314 mm for case S. Solution_3 represents a trade-off between the previous two solutions, keeping an elevated illumination quality in both case L and case S. However the main advantage of Solution_3 is that these optical systems employ lenses with better shape and proportions. A more favourable ratio between lens diameter and curvature radius allows a reduction of the aberrations. The disadvantage of Solution_3 is a longer longitudinal system dimension, with respect to Solution_1; but the essential advantage is an improvement in the quality of the resulting lighting.

3.5. Simulations: Evaluation of the Optical Quality All solutions were developed maximising the irradiance uniformity and matching the irradiance requirements. For each solution we examined Zemax spot diagram, Zemax image diagram and TracePro irradiance map. The simulations with Zemax were carried out imposing a starting value for the total radiant flux of 40 W (over the area 200 x 200 mm2), to take approximately into account also the reflections on the surfaces of the optical components (4% of loss for each surface). The calculation is not very accurate, because Zemax does not consider the real reflection on the surfaces and the simulated source, unlike the real source, is spatially and angularly uniform. Besides, to have a more realistic evaluation for the energetic efficiency of the system, we carried out some simulations with the software Lambda Research TracePro, which is specific software for lighting simulation. In this case the simulated source is very similar to the real source and the software takes into account the real values of reflection losses on the surfaces.

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It is important to remark that all proposed solutions are in agreement with the requested specifications, discussed in Section 3.1. The requirements impose that the total flux obtained on the working areas from the simulations exceeds 24.2 W for case S and 24.3 W for case L.

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Figure 7. Spot diagram for case S: Solution_1 (left), 2 (centre), 3 (right).

Figure 8. Image diagram for case S: Solution_1 (left), 2 (centre), 3 (right). Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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Figure 9. Irradiance map for case S: Solution_1 (left), 2 (centre), 3 (right).

Figure 10. Spot diagram for case L: Solution_1 (left), 2 (centre), 3 (right).

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Figure 11. Image diagram for case L: Solution_1 (left), 2 (centre), 3 (right).

Figure 12. Irradiance map for case L: Solution_1 (left), 2 (centre), 3 (right).

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Figures 7-12 present some examples of results in order to compare the three proposed solutions. The Zemax spot diagrams are shown in Figure 7 and Figure 10, for case S (AS=11x11 mm2, 20 W/cm2) and case L (AL=15.6x15.6 mm2, 10 W/cm2), respectively. Analogously Figure 8 and Figure 11 report the Zemax image diagrams, for case S and case L, respectively. Finally the irradiance map generated by TracePro is shown in Figure 9 and Figure 12, for case S and case L, respectively. In all Figures 7-12 the left plot corresponds to Solution_1, the right plot pertains to Solution_2, while the bottom plot refers to Solution_3. Each spot diagram presented in Figure 7 is compared to the area AS (plotted as back frame); analogously every spot diagram in Figure 10 is referred to AL. The figure area corresponds to AS for Figures 8-9 and to AL for Figures 11-12. The spot diagram allows to estimate the chromatic aberrations of the system, considering three different ray tracing wavelengths (400 nm, 637 nm and 1000 nm), respectively corresponding to the blue, green and red colours plotted in the figure. It is useful to note that the position of the working area does not correspond to the image location: this permits us to obtain better irradiance uniformity. The image diagram allows evaluating the energetic efficiency of the system over the working surface, using as parameter the total flux calculated on the test area from the simulations. The irradiance map provides very accurate information about the power density distribution over the working area. The main purpose is to estimate the level of irradiance uniformity on the working areas. Trace Pro maps more reliable to check the matching with the required irradiance level. The calculations for the image diagrams provide also the corresponding numerical data. As exemplificative results, for Figure 8 we report the values of the radiant flux on the working area AS: 27.6 W (Solution_1), 31.4 W (Solution_2) and 29.1 W (Solution_3). It could be interesting to numerically compare the Zemax image diagram to the TracePro irradiance map. Considering the image diagrams of Solution_3 (right plots of Figures 8 and 11) , the values of the radiant flux over the working surface are 29.1 W for case S and 31.8 W for case L. The corresponding TracePro values, pertaining to the irradiance maps (right plots of Figures 9 and 12), are slightly different: 26.7 W for case S and 28.6 W for case L. These discrepancies are due to the randomisation of the non-sequential ray tracing and to the minor attitude of Zemax to this type of analysis. Nevertheless the Zemax image diagrams can be correctly used if they are employed in reciprocal comparison.

3.6. Analysis of Simulation Results Comparing the results for case L and case S, for Solution_1 and Solution_3 the plots appear quite similar, both in TracePro and Zemax. On the contrary, for Solution_2 the results are visually different, due to the introduction of a converging (case S) or diverging (case L) secondary lens. In general all image diagrams for Solution_1 and Solution_3 reach almost the same levels. Solution_1 offers acceptable irradiance uniformity, especially considering that, in general, every real irradiance distribution is more smoothed than the simulated result, but

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there is some chromatic aberration. The value of radiant flux exceeds the requirements, even if it is lower with respect to the other solutions. Solution_1 is the most compact one. Solution_2 allows to achieve a higher light concentration, hence a higher flux (with respect to the previous one), however the irradiance uniformity achievable in case S is lower. Even considering the smoothing, the differences in irradiance among the zones inside the image (simulated by TracePro) are quite marked. The irradiance uniformity is fairly good for case L. Solution_3 provides good values of radiant flux (still lower than in Solution_2) and of irradiance uniformity. The chromatic aberration is very low and it avoids the presence of coloured zones. Nevertheless its great advantage is the elevated uniformity of the irradiance map, both for case L and case S. In conclusion the system of Solution_3 appears to satisfy, in the most complete way, the initial requirements that the modified solar simulator should guarantee.

4. OPTICAL PERFORMANCE CONTROL ON THE MODIFIED SOLAR SIMULATOR YSS-200

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4.1. Realised Modifications and Simulator Set-up The solar simulator Yamashita YSS-200 was modified using Solution_3, illustrated in detail in Section 3.4. Since the total collected power is a mandatory requirement, we considered the possibility that, due to imperfections on the surfaces or wrong evaluation of the simulator layout, the power on the working surface could be less than the expected values. Consequently we finally decided to introduce a further lens with characteristics similar to the lens of Solution_3, but with higher optical power, and we optically manufactured it. Moreover, from the measurements for case S (20 W/cm2 on 11.0 x 11.0 mm2), we estimated that the requirements of case L (10 W/cm2 on 15.6 x 15.6 mm2) could be achieved using the lens realised for case S. The only difference was to change the distance between glass lens and working surface; so we avoided the realisation of the lens with curvature radius 220.0 mm. In order to test the optical performance of the modified solar simulator, some suitable measurements were carried out considering both S and L cases. Since some preliminary tests evidenced that the same lens satisfies, at different distances, the requirements of case L and case S, the validation tests were performed only with the lens with curvature radius 100 mm. This can be a great advantage for the end user, because the possibility of using the same lens for both working areas and power levels avoids dismounting and realigning. Table 1. Radiant flux and irradiance measured on AS and AL. Case L S

Working surface area [mm2] 15.6 x 15.6 11.0 x 11.0

Glass lens – working surface distance [mm] 114 76.5

Radiant Flux [W] 27.8 27.0

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Irradiance [W/cm2] 11.3 21.6

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4.2. Power Measurements The power measurements were performed using a OPHIR NOVA power meter (mounting the FL250 head) and two Aluminium diaphragms. The first diaphragm had a squared aperture of side 11.0 mm (to check the new working area AS 11 x 11 mm2), while the other had a squared aperture of side 15.6 mm (to verify the new working area AL 15.6 x 15.6 mm2). The diaphragms were manufactured in order to avoid unwanted reflections on the aperture borders. Table 1 presents the results for Radiant Flux and Irradiance measured on the working areas.

4.3. Uniformity Measurements

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The uniformity is evaluated by irradiance variation measurements along two orthogonal directions, approximately centred in the point of irradiance maximum [8]. In addition to the OPHIR NOVA Power Meter, with 2A-SH head, we employed a circular diaphragm of diameter 2.0 mm, placed on the head detector. In the data elaboration, the radiant flux value was divided by the diaphragm area in order to obtain the irradiance value. The lamp was warmed up at least two hours before starting the measurements, to ensure source stability. The radiant flux was measured at a fixed distance from the centre; the final value was calculated averaging the four data measured in the two directions of the two perpendicular axes. Figure 13 reports the measurements of the irradiance distribution over the working area AS for the lens with curvature radius 100 mm.

Figure 13. Light distribution over the working area AS.

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To assess the uniformity U, we considered the following two points: •

the beam centre (corresponding to the point of irradiance maximum Ee1)

•

the quantity Ee2 measured at a distance of 5.5 mm from the centre for case S or 7.8 mm for case L.

Thus the uniformity U is obtained as:

U=

E e1 − E e 2 E e1 + E e 2

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It is important to consider that the diameter of the measurement area is 2 mm: this value was sufficient and did not need to be reduced to improve the detection precision. The uniformity U results 0.23, for case S. For case L the quantity Ee2 is measured at a distance of 7.8 mm from the centre. The uniformity values, over the plane AL in the pertaining working conditions (obtained by suitably displacing the working plane), resulted to be of the same order of case S.

Figure 14. Comparison of the spectra before and after introduction of the concentrating system.

4.4. Variation of the Spectral Composition of the Emitted Light The employed materials, acrylic plastic and BK7 glass, do not strongly affect the spectral composition of the transmitted radiation in the range 400 nm - 1000 nm. Hence in this wavelength range they do not introduce large variations in the spectral emission of the solar simulator. Nevertheless the emission spectrum of the solar simulator was measured in

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presence and in absence of the modification of Solution_3, which is in practice a concentrating system. The measurements were performed in the wavelength range 300 nm 1100 nm. The results are presented in Figure 14: the values are normalised imposing the equivalence of the data pertaining to the wavelength 600 nm. There are some small differences between the spectral emissions with and without the concentrating system of Solution_3, especially for wavelengths over 700 nm. This discrepancy is probably created by the automatic gain system of the spectral radiometer.

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CONCLUSION This chapter discussed the reproduction of a real solar simulator and the development of some possible optical modifications to allow the test of PhotoVoltaic cells for solar concentration. The first step of the work was to assess an instrument model suitable both for lighting simulation software (TracePro) and for optical design software (Zemax). The examined instrument was the simulator Yamashita YSS-200, whose actual optical schemes were not available. Obviously starting from our approximated model of the solar simulator, there are some intrinsic errors that affect the methodology and consequently the proposed solutions. The second phase of the work was devoted to study and optimise three possible optical systems to adapt the solar simulator to test CPV cells. The result was obtained removing the beam homogenizer and introducing in the existing layout an auxiliary optical system, which was externally added to the solar simulator. The supplementary device has the advantage of being easily demountable, thus allowing to rapidly restoring the original configuration. We finally selected the most fitting solution, which was realised and suitable optical tests verified that it satisfied the requirements of power and irradiance uniformity. It is important to remark that the materials foreseen for the realisation of these components (acrylic plastic and glass) do not significantly alter the spectral distribution (between 400 nm and 1000 nm) in radiation transmission, as spectral measurements confirmed. All proposed solutions are in agreement with the requested specifications addressed to allow the experimentation of CPV cells for low-medium concentration ratios (100-200). In any case the source of the solar simulator Yamashita YSS-200 does not permit its application for tests at higher solar concentrations (>200x), except strongly reducing the working area. Furthermore to perform accurate measurements on high concentration PV cells it would be essential to optically optimise light spectrum and chromaticity.

REFERENCES [1] [2]

Fain, D.L. Appl. Opt. 1964, vol.3, n.12, 1389-1396. "Design considerations for precision solar simulation". Powell, I. Appl. Opt. 1980, vol.19, n.2, 329-334. "New concept for q system suitable for solar simulation".

Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Solar Simulator Modified to Test PV Cells [3] [4] [5] [6]

Domínguez, C.; Antón, I.; Sala, G. Frontiers Opt. 2008, OSA Technical Digest, JThA1. "Concentrator Photovoltaics Solar Simulator". Lentz, R.; Suzuki, A. "Nonimaging Fresnel lenses"; Springer-Verlag - Berlin, Springer: Heidelberg, GE, 2001. Mouroulis, P.; Macdonald, J. "Geometrical Optics and Optical Design"; Oxford University Press: Oxford, UK, 1997. Shannon, R. R. "The art and science of optical design"; Cambridge University Press: Cambridge, UK, 1997. "Handbook of plastic optics"; S. Baumer Ed. Wiley – VCH: Weinheim, GE, 2005. Francini, F.; Fontani, D.; Jafrancesco, D.; Mercatelli, L.; Sansoni, P (2006) "Optical control of sunlight concentrators" SPIE Proc. High and Low Concentration for Solar Electric Applications vol. 6339, 63390E.

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[7] [8]

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In: Dye-Sensitized Solar Cells and Solar Cell Performance ISBN: 978-1-61209-633-9 Editor: Michael R. Travino ©2012 Nova Science Publishers, Inc.

Chapter 6

DRYING CHARACTERISTICS OF LEMONS AND DATES UNDER INDIRECT TYPE FORCED CONVECTION SOLAR DRYING AND NATURAL OPEN SUN DRYING Sabah A. Abdul-Wahab* Mechanical and Industrial Engineering Department, College of Engineering, Sultan Qaboos University, P.O. Box 33, Al Khoud 123, Oman

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ABSTRACT An indirect type forced convection solar dryer unit was designed and built at the College of Engineering, Sultan Qaboos University (SQU) in Muscat (Oman). The unit consists of a solar air heater (i.e., solar collector with corrugations) and a cabinet acting as a drying chamber (i.e., basically a batch dryer). It was manufactured with locally available materials and can be used for drying various agricultural products like fruits and vegetables. In this study, the solar dryer was tested under controlled conditions for drying lemons and dates as the test samples. The dryer unit was investigated experimentally and its performance was evaluated under the climatic conditions of Oman (21° 00 N, 57º 00 E). For comparison purposes, natural traditional sun drying experiments (i.e., solar dryer under open sun with natural convection) was also conducted at the same time. For this effect, the results of drying under the open sun and indirect type forced convection solar drying was drawn under identical weather conditions during summer conditions in June, 2009. The performance of the solar dryers was computed and expressed in terms of the moisture evaporation (crop mass during drying). It was found that the quality of lemons and dates after drying was better and drying time was less in the indirect type forced convection solar dryer in comparison to open sun drying. It is hoped that this study may be useful for further development work.

Keywords: Drying; lemons; dates; forced convection solar dryer; open sun drying; Oman

*

Corresponding author. Tel.: +968 2414 1360; fax: +968 2414 1316. E-mail address: [email protected]

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1. INTRODUCTION The purpose of drying an agricultural product is to reduce its moisture content to a level that prevents its deterioration. For this reason, the drying requirements of agricultural products vary from product to another. The moisture to be removed from a particular agricultural product is determined by the initial moisture content of the product and the safe storage moisture content of the desired products [1]. In solar drying, the products have yet another requirement as to whether or not they can be exposed to direct sunlight. Traditionally, farmers used the open sun drying which achieves natural drying by using solar radiation, ambient temperature, natural wind, and relative humidity of ambient air. It is usually done by allowing crops to dry naturally in the field, or to spread grain and fruit out in the sun after harvesting [2]. The crops can then reach higher temperatures in natural open sun and left there for a number of days to dry. Hence, the energy requirements for open sun drying are available in the ambient environment and no capital investment in equipment is required. However, this process of natural drying has some serious limitations. This is because using this process in hostile climatic conditions leads to severe losses in the quantity and quality of dried product. Due to lack of sufficient preservation methods, farmers have to spread the product in thin layer on paved ground or on mats where this is exposed to sun, dust, dirt, wind and atmospheric pollution [3]. Because of these limitations, the quality of the resulting dried product can be degraded significantly. All these limitations can be eliminated by using a solar energy dryer which uses air collector to collect solar energy. The purpose of using the solar dryer is to supply more heat to the product than that available naturally under ambient conditions. This results in increasing significantly the vapor pressure of the crop moisture. Hence, moisture migration from the crop is improved. The solar energy dryer also sufficiently decrease the relative humidity of the drying air and so its moisture-carrying capability increases. There are many advantages for using the solar energy dryer. By using a solar dryer, the drying time can be shortened by 65% compared to sun drying because the inside of the dryer is warmer than the outside air. In addition, the quality of the dried product can be improved in terms of hygiene, cleanliness, safe moisture content, color, and taste. The product is also completely protected from rain, wind, dust, pollution, and insects. Further, the product will not be exposed directly to any kind of harmful electromagnetic radiation or electromagnetic poles [4]. Solar drying technology is a complex process of simultaneous heat and mass transfer. It involves the transport of moisture to the surface of the material and subsequent evaporation of the moisture by thermal heating [5]. Perfecting this technology, therefore, was targeting by several researchers to adopt and to design the right type of solar dryer. Ekechukwa and Norton [6] reviewed many solar dryers and compared their performance and applicability in rural areas. Pangavhane and Sawhney [7] reviewed the research and development work on various types of solar dryers, with a special attention given to the dryers used for grape drying. A review of the parameters involved in the testing and evaluation of different types of solar food dryers and results in the literature can be found in Leon et al. [8]. They presented a detailed classification of available solar food dryers based on the design of system components and the mode of utilization of solar energy.

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Several experimental studies in drying field were performed on the development and testing of various types of solar dryers to demonstrate their characteristics and performances [1, 9-22]. The issues related to the comparison of the benefits and costs of substituting open-sun drying with solar drying were also addressed [23]. In their study, Purohit et al. [23] attempted to make a simple framework to facilitate a comparison of the financial feasibility of solar drying as against open sun drying. Further, studies on the drying process and the cost economic analysis under open sun and solar biomass (hybrid) drying can be found in Prasad et al. [3]. Research efforts were also focused on both theoretical and experimental investigations of solar dryers to compare the predicted results with the experimental data. Examples of such studies can be found in Jain and Tiwari [5]; Sodha et al. [24]; Hachemi et al. [25]; Queiroz and Nebra [26]; Yaldiz et al. [27]; Karim and Hawlader [28]; and Ait Mohamed et al. [29]. The feasibility of the solar dryer depends largely upon the crop to be dried and the climatic conditions. In general, there are many factors affecting the drying rate of the agricultural products [8, 13, 19, 30-33]. Leon et al. [8] pointed out that product size, density, moisture content, drying air temperature, relative humidity and air velocity are some of the major parameters affecting the drying time and dried product quality. According to Prasad et al. [3], the drying rate depends on a number of external parameters (solar radiation, ambient temperature, wind velocity, and relative humidity) and internal parameters (initial moisture content, type of the product, mass of the product per unit exposed area, etc.). The complexity of the mechanisms of the drying process and the variable character of the products prevent single model to represent all the situations of the drying. These make experimental drying tests are very important and useful to determine the characteristics of the solar dryers. Thus, this work was undertaken to investigate experimentally the use of solar energy for drying purposes in the Sultanate of Oman, particularly lemons and dates. An indirect type forced convection solar dryer was designed and fabricated. The dryer was tested experimentally under the climatic conditions of Oman. The performance of the traditional sun drying under similar climatic conditions was also investigated and compared with the performance of the indirect type forced convection solar dryer. The indirect type forced convection solar dryer and the natural open sun drying were tested side-by-side in order to allow direct comparison of their experimental performance. Experiments for drying for both lemons and dates were performed.

2. EXPERIMENTAL 2.1. Experimental Set-up Figure 1 illustrates the essential features of the solar dryer system used in this study. It consists of four main parts: the solar collector (air heater to heat the incoming ambient air using solar radiation), the drying chamber (cabinet in which products to be dried are spread on a number of trays), flexible hose connectors, and the circulation fan (i.e., air flow is forced convection). The photograph of actually fabricated solar dryer is presented in Figure 2. The

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whole assembly of the solar collector was oriented towards south to maximize the solar radiation incident on the plate collector. The solar collector part, which shown in Figure 3 (120 cm, 70 cm, and 20 cm), is a single pass air-heater that is used for supplying hot air to the drying chamber. It collects energy from the sun to supply the required heat for drying. It consists of an absorber (painted matte black) with corrugated black absorber plate, glass cover, insulation and frame. The interior of the dryer is painted with dull black paint for absorption of solar radiation. The air is forced to the solar collector and is passed through the absorber plate to increase its temperature. It is then left the solar collector to the cabinet where the products are placed. The glass cover plate (4 mm thick, 1.18 m x 0.68 m) is fixed on the frame of the solar collector at a distance of 0.04 m above the absorber surface for solar energy interception. The collector frame is made of locally available thick wood and at the bottom plywood is provided to support the absorber plate. It has four supporters (70 cm, 5 cm, and 5 cm).

Figure 1. Systematic drawings of the solar dryer.

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Figure 2. Photograph of fabricated solar dryer in the field.

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Figure 4. The drying chamber (cabinet) part.

The drying chamber part, which shown in Figure 4, is basically a batch cabinet dryer. It is a drying cabinet in which the product to be dried is placed. It comprises of shelves, three removable trays (i.e., product beds where the product is placed) and door. The drying trays (40 cm, 40 cm, 40 cm) are made from wire mesh base with a fairly open structure to allow drying air to pass through the product and to prevent the product items from falling into the drying chamber by giving the support to the trays. The trays are kept on the wooden frame fixed to the inner side walls of the drying chamber. They can be easily removed from the dryer for the purpose of cleaning and the product loading and unloading. They overlap each other to prevent air leakages when they are closed. Also, there is a small hole at the top of the drying chamber with a diameter of 3.8 cm to allow the air to exit from the cabinet. The circulation fan (HJ 310 model, 12V DC, 15 cm diameter) is fixed at the fan box for the purpose of controlling the air flow rate. It forces the hot air from the solar flat plate collector, through flexible connector hose, to the drying chamber part. There are two flexible connector hoses in the solar dryer system that is used mainly for the purpose of connecting between its main parts. One of them is used to connect between the fan’s box and the solar collector part while the other connects between the solar collector part and the drying chamber part.

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2.2. Instrumentation The mass loss of the product during the drying process is carried out using a digital display electronic balance of 0.001 precision (Model Kern 572, metal casing). Four thermocouples (K type) are used to measure temperatures at different points of the solar dryer system. The first one is used to measure the ambient temperature of air. The second one is fixed to measure the temperature at the outer surface of the absorber plate (i.e., upper surface exposed to the sun). The remaining two thermocouples are fixed in the cabinet to measure the dry bulk temperature of air at its entrance and its exit. All thermocouples are calibrated using standard thermometers. These thermocouples is further connected to a multi point data recorder (i.e., thermocouples reader, model PM 8238) to record temperatures in interval of one hour.

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2.3. Experimental Procedure All experiments were conducted under meteorological conditions of Oman, Muscat. The solar drying experiments were carried out during June 2009 (i.e., good sunshine during the harvest season). The dryer was designed to operate with only solar radiation as the main source of energy. In the drying experiments, both lemons and dates were used as the test samples in the dryer unit. During the drying experiments, the weather was sunny and no rain appeared. There was no shading due to trees, buildings or other structures around the solar collector. The product to be dried was uniformly distributed evenly on the metallic mesh of the drying trays of the drying chamber. When the solar dryer was operating, solar radiation was passed through the glazing transparent cover sheet of the solar air collector, and was then absorbed by the suspended black absorber. Air entered the solar collector and was heated by the black absorber. Due to the trapped energy, the air around the absorber was heated and rise into the drying chamber. A negative pressure was created by the rising air which resulted in drawing additional fresh air up through the inlet side of the solar collector. This air was then heated and the process continued. As a result, a continuous flow of heated air taken place over/through the product placed on the perforated trays. This heated air entered the drying chamber below the perforated trays and flowed upwards through the samples. The hot air dehydrated the product, picked up its moisture, and got out through the vent at the top of the drying chamber. The door of the dryer was properly closed and sealed to ensure no leakage of hot air. The product was weighed to determine the amount of water evaporated by the product (humidity lost). Therefore, during each drying experiment, the reduction in moisture content of the product was determined by weighing the product at every hour. The weight of the products on the trays was measured by removing it from the drying chamber for approximately 10-15 s. The weight measurements were continued to be undertaken every hour at regular intervals between 7:00 a.m to 7:00 p.m. The dry bulk temperature of the ambient air and inside the drying chamber, were also measured simultaneously every hour. All these measurements were continued to be undertaken until constant weights of the products were observed.

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1. ANALYSIS The performance of the solar dryer was evaluated by calculating the moisture content of the product. The initial moisture content of the product was identified by placing a known weight of the product in the oven at 110 °C for 16 hours. The initial moisture content on wet basis (wb) is the weight of moisture present in the product per unit weight of fresh product and was computed as Initial moisture content, MCi (%wb) = (Wi-Wd)/Wi

(1)

Where MCi is the initial moisture content on wet basis at t = 0 (%wb) Wi is the weight of the fresh product at t = 0 (g) Wd is the weight of dry product taken from oven (g) The initial moisture content on dry basis (db) is the weight of moisture present in the product per unit weight of dry product and was computed as Initial moisture content, MCi (%db) = (Wi-Wd)/Wd

(2)

The instantaneous moisture content (on dry base) of the sample at any given time (t) during the drying process was calculated from Eq. (3).

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Mt = ([(Mi +1) (Wt/Wi)] -1) 100%

(3)

Where Mt is the instantaneous moisture content of the product to be dried at any time (%db) Mi is the initial moisture content of the product to be dried (%db) Wi is the initial weight of the product to be dried (g) Wt is the weight of product to be dried at any given time (g). It was calculated by weighing the drying tray with its load of sample at any time during the drying process.

4. RESULTS AND DISCUSSION 4.1. Lemons The initial moisture content of the lemons was calculated according to Eq. (1) and the results are shown in Table 1. It can be seen from Table 1 that the average initial moisture content of lemons on wet basis was determined experimentally to be 83.2%. Drying experiment was conducted for 1000 g of lemon samples loaded in the trays of the drying chamber of the forced convection solar dryer. For comparison purposes, natural open sun drying experiment was also conducted simultaneously on 1000 g of lemon samples that were exposed directly to the open sun. Both experimental tests commenced at about 07:00 a.m. local time of Oman in two consecutive days (16-17 June 2009). Therefore, the indirect

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type forced convection solar dryer and the natural open sun drying were tested and evaluated simultaneously under the same climatic conditions. An overview of the results of these tests is presented in Table 2. Table 1. The weight (g) and moisture content (%wb) for four samples of lemons Sample number Wi (g) Wd (g) 1 43.4 6.8 2 27.0 4.2 3 28.5 5.2 4 30.4 5.5 Average initial moisture content of lemon samples on wet basis

MCi (%wb) 84.33 84.44 81.75 81.91 83.2

MCi is the initial moisture content on wet basis at t = 0 (%wb). Wi is the weight of the fresh product at t = 0 (g), Wd is the weight of dry product taken from oven (g).

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Table 2. Experimental results of lemons in two consecutive days (16-17 June 2009) MoP or Wt is the mass of product to be dried at any given time (g)

Time Hour

Ta °C

Tp °C

7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0

36.5 39.7 40.6 42.0 43.1 44.2 44.9 44.7 43.8 42.5 41.1 37.8 35.7 37.1 40.2 41.6 42.8 43.7 44.6 45.1 44.8 43.9 42.7 41.0 36.3 35.9

47.3 54.2 59.4 66.7 69.8 71.6 72.4 72.7 70.4 65.3 63.5 54.7 46.9 48.1 55.7 59.8 67.4 70.7 73.2 73.8 72.9 72.1 68.3 64.6 59.3 47.5

cabinet

Tout, cabinet °C

Indirect type forced convection solar dryer MoP or Wt MC MC g %wb %db

°C 42.6 48.3 51.2 55.7 58.4 61.3 62.6 62.2 61.4 57.7 52.3 47.9 41.7 43.2 49.7 52.3 56.1 61.2 63.5 64.2 63.9 62.7 59.2 53.4 48.7 42.8

41.1 46.2 48.4 51.6 54.1 57.5 58.4 58.1 57.3 54.5 50.5 46.5 42.6 42.6 47.6 50.5 54.2 59.3 60.1 61.9 62.0 62.1 57.9 52.2 47.6 42.1

1000 982.7 947.2 903.6 849.2 766.8 699.8 616.9 578.2 545.4 521.2 520.6 520.5 518.2 504.2 479.2 446.7 404.2 379.4 346.8 331.8 321.0 315.2 304.1 302.4 301.8

Tin,

83.2 82.9 82.4 81.7 80.8 79.1 77.6 75.3 74.1 72.9 72.04 72.02 72.02 71.9 71.3 70.3 68.8 66.6 65.1 63.1 62.1 61.2 60.8 59.9 59.8 59.8

495.4 486.8 469.1 447.5 420.5 379.6 346.3 305.2 286.0 269.7 257.7 257.4 257.3 256.2 249.3 236.9 220.7 199.6 187.3 171.2 163.7 158.3 155.5 149.9 149.1 148.8

Natural pen sun drying MoP or Wt g 1000 985.8 954.6 919.6 869.2 830.4 764.4 718.2 683.2 657.6 614.6 613.4 613.4 612.8 598.8 577.3 553.6 530.8 500.7 468.9 437.0 412.6 400.3 394.4 394.0 394.0

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MC %wb

MC %db

83.2 83.0 82.5 81.9 81.1 80.4 79.1 78.0 77.1 76.4 75.2 75.2 75.2 75.1 74.7 74.0 73.2 72.4 71.2 69.8 68.3 67.0 66.4 66.0 66.0 66.0

495.4 488.3 472.9 455.5 430.5 411.2 378.4 355.5 338.1 325.4 304.1 303.5 303.5 303.2 296.2 285.6 273.8 262.5 247.5 231.8 215.9 203.8 197.7 194.8 194.6 194.6

146

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Temperature (°C)

80 70 60 50 40 30 7

9

11 13 15 17 19 8

Ta

Tp

10 12 14 16 18

Time (Hour)

Tin, cabinet

Tout, cabinet

Figure 5. Variations of temperatures when drying 1000 g of lemon samples for two consective days, 1617 June 2009.

Solar radiation (W/m2)

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1200 1000 800 600 400 200 0 7

9

11 13 15 17 19

8

10 12 14 16 18

Time (Hour) Figure 6. Solar radiation values when drying 1000 g of lemon samples for two consective days, 16-17 June 2009.

The comparison between the temperature profiles when drying 1000 g of lemon samples in two consecutive days (16-17 June 2009) is illustrated in Figure 5. It was found that temperatures increased with the time of day until reaching their maximum values at noon. Then the temperatures observed to be decreased. It can be seen from Figure 5 that the maximum temperature occurred at the same time due to the dependency of the product drying

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temperature on the temperature of the air entering the drying chamber, which in turn closely followed the solar radiation incident at the time of measurement (Figure 6). It should be noted that the plate of the solar collector tended to have a higher temperature profile. It can be seen that the temperature of the drying air inside the drying chamber decreased in the vertical direction. Such diurnal variations in temperatures were expected as some of the heat was consumed in evaporation of moisture from the product. For the inlet air temperature of 36.5 °C during the first day, the maximum air temperature at the dryer inlet was recorded as 62.6 °C at the solar irradiance level of 1010 W/m2. Also, for the inlet air temperature of 37.1 °C during the second day, the maximum air temperature at the dryer inlet was recorded as 64.2 °C at the solar irradiance level of 1020 W/m2. The comparison between the mass of lemon profiles for the indirect type forced convection solar dryer and the open sun drying is given in Figure 7. It is clear that the two profiles showed the same patterns and their only difference was their magnitudes. As seen from Figure 7, the lowest product weight reduction was seen with the case of using natural open sun drying. It was observed after 9 hours of drying (at 16:00) that the weight of the product was reduced from 1000 g to 545.4 g (454.6 g) for the case of using an indirect type forced convection solar dryer, and from 1000 g to 657.6 g (342.4 g) for the case of using natural open sun drying. In the light of this, the drying achieved with the indirect type forced convection solar dryer was better than that of the natural open sun drying. Variations of the moisture content with time, calculated using Eq. (3), when drying 1000 g of lemon samples in two consecutive days (16-17 June 2009) are illustrated in Figure 8 for both an indirect type forced convection solar dryer and the natural open sun drying. It is clear that the patterns of the moisture evaporation for the indirect type forced convection solar dryer and the natural open sun drying were identical to each other and their only difference was their magnitudes. The moisture content of the product decreased exponentially with increasing in drying time. The evaporated mass of water decreased as the drying time increased because at the initial stages of drying, the moisture removal was rapid, following the constant drying rate period. It is also seen from Figure 8 that the drying time was shorter and the drying rate was more when using indirect type forced convection solar dryer as compared with using the natural open sun drying. Within drying in two consecutive days, the moisture content reduction from an initial value of 83.12%wb to about 59.8%wb was achieved with indirect type forced convection solar dryer compared with moisture content reduction to about 66.0%wb achieved with using the natural open sun drying. This corresponded to about 25 h of insolation.

4.2. Dates A serious of experiments was also conducted for dates. The initial moisture content of the dates was calculated according to Equation (1) and the results are shown in Table 3. It can be seen from Table 3 that the average initial moisture content of dates on wet basis was determined experimentally to be 15.4%.

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Mass of product (g)

1200 1000 800 600 400 200 0 7

9 11 13 15 17 19 8 10 12 14 16 18

Time (hour)

MoP in the solar dryer

Moisture content (%wb)

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Figure 7. Variation of the mass of the product with time for both indirect type forced convection solar dryer and natural open sun drying when drying 1000 g of lemon samples for two consective days, 16-17 June 2009.

85 80 75 70 65 60 55 50 7

9

11 13 15 17 19

8

10 12 14 16 18

Time (Hour) MC (%wb) in the solar dryer MC (%wb) in the natural open sun drying Figure 8. Variation of moisture content with time for both an indirect type forced convection solar dryer and natural open sun drying when drying 1000 g of lemon samples for two consective days, 16-17 June 2009.

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Table 3. The weight (g) and moisture content (%wb) for four samples of dates (Tamr stage) Sample number Wi (g) Wd (g) 1 5.3 4.6 2 6.3 5.2 3 5.6 4.7 4 6.7 5.7 Average initial moisture content of date samples on wet basis

MCi (%wb) 13.21 17.46 16.07 14.92 15.4

MCi is the initial moisture content on wet basis at t = 0 (%wb). Wi is the weight of the fresh product at t = 0 (g). Wd is the weight of dry product taken from oven (g).

Drying experiment was conducted for 83.1 g of dates samples loaded in the trays of the drying chamber of the forced convection solar dryer. Drying experiment was also conducted on 83.1 g of lemon samples that were exposed directly to natural open sun drying for comparison purposes. The experimental tests commenced at about 07:00 a.m. local time of Oman in two consecutive days (10-11 June 2009). Thus both the indirect type forced convection solar dryer and the natural open sun drying were tested and evaluated simultaneously under the same climatic conditions. An overview of the results of these tests is presented in Table 4.

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Table 4. Experimental results of dates in two consecutive days (10-11 June 2009)

Time Hour 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 7.0 8.0 9.0 10.0 11.0 12.0

Ta °C 41.2 43.1 45.5 44.6 45.4 46.3 46.0 46.2 42.3 40.5 37.1 41.3 42.8 43.1 44.6 44.3 44.2

Tp °C 51.4 55.7 62.0 64.4 70.0 73.5 72.6 70.0 67.4 62.3 57.0 51.6 59.9 63.9 68.2 70.1 71.2

Tin, cabinet °C 46.7 52.1 56.9 60.7 64.8 66.2 65.0 64.0 61.7 57.8 52.8 47.1 55.0 59.2 62.0 63.9 64.8

Tout, cabinet °C 45.6 50.3 54.5 57.6 61.6 61.9 63.9 63.1 60.2 57.1 52.4 46.5 53.7 57.9 61.6 63.4 64.3

Indirect type forced convection solar dryer MoP or MC MC Wt g %wb %db 83.1 15.60 18.5 83.1 15.50 18.4 82.9 15.40 18.3 82.5 15.30 18.2 81.8 15.20 18.1 81.1 15.10 17.9 80.6 15.09 17.8 80.2 15.02 17.7 80.0 14.98 17.6 79.9 14.97 17.6 79.9 14.97 17.6 79.9 14.97 17.6 79.6 14.92 17.5 79.1 14.88 17.4 78.5 14.73 17.3 78.0 14.65 17.2 77.6 14.58 17.1

Natural pen sun drying MoP or Wt g 83.1 82.9 82.8 82.6 82.3 82.0 81.6 81.2 81.1 81.0 81.0 81.0 80.9 80.6 80.4 80.4 79.8

MoP or Wt is the mass of product to be dried at any given time (g).

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MC %wb 15.60 15.57 15.55 15.52 15.47 15.42 15.35 15.29 15.27 15.25 15.25 15.25 15.23 15.18 15.15 15.10 15.05

MC %db 18.5 18.4 18.4 18.4 18.3 18.2 18.1 18.1 18.0 18.0 18.0 18.0 17.9 17.9 17.8 17.7 17.7

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Temperature (°C)

80 70 60 50 40 30 7 8 9 10 11 12 13 14 15 16 17 7 8 9 10 11 12 Time (Hour) Ta

Tp

Tin, cabinet

Tout, cabinet

Solar radiation (W/m2)

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Figure 9. Variations of temperatures when drying 83.1 g of dates samples for two consective days, 1011 June 2009.

1200 1000 800 600 400 200 0 7 8 9 10 11 12 13 14 15 16 17 7 8 9 10 11 12

Time (Hour) Figure 10. Solar radiation values when drying 83.1 g of dates samples for two consective days, 10-11 June 2009.

Figure 9 illustrates variations of the various temperatures of the dates for two consecutive days, 10-11 June 2009. It can be seen that the temperatures increased with time of day until reaching their maximum values at noon, and then the temperatures observed to be decreased. Such diurnal variations in temperatures were expected as they closely followed the insolation pattern (Figure 10). Looking at Figure 9, it can be seen in the first day that the maximum

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temperatures of ambient, cabinet and plate were 46.3 °C, 66.2 °C and 73.5 °C, respectively. For the second day, they were 44.6 °C, 62.0 °C and 68.2 °C, respectively. For the inlet air temperature of 41.2 °C during the first day, the maximum air temperature at the dryer inlet was recorded as 66.2 °C at the solar irradiance level of 1000 W/m2. Also, for the inlet air temperature of 41.3 °C during the second day, the maximum air temperature at the dryer inlet was recorded as 64.8 °C at the solar irradiance level of 1030 W/m2. Variations of the product weight with time for an indirect type forced convection solar dryer and natural open sun drying when drying 83.1 g of dates are shown in Figure 11. It can be seen that there was a difference in the weight loss of the dried material was observed with the two dryers. The weight loss of dates was 6.62% for the indirect type forced convection solar dryer whereas it was only 3.97% for the natural open sun drying. This gives good notation that drying using the solar dryer will effectively reduce the drying time. Hourly variation of the product moisture content for an indirect type forced convection solar dryer and natural open sun drying is presented in Figure 12. As seen from Figure 12, the higher product moisture reduction was seen with the case of using an indirect type forced convection solar dryer than that of using natural open sun drying. It was observed after 8 hours of drying (at 15.00) that the moisture content was reduced from 15.6%wb to 14.98%wb for the case of using an indirect type forced convection solar dryer, and from 15.60%wb to 15.27%wb for the case of using natural open sun drying.

84

Mass of product (g)

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83 82 81 80 79 78 77 76 7

8

9 10 11 12 13 14 15 16 17 7

8

9 10 11 12

Time (hour) MoP in the solar dryer Figure 11. Variation of the mass of the product with time for both indirect type forced convection solar dryer and natural open sun drying when drying 83.1 g of dates samples for two consective days, 10-11 June 2009.

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Moisture content (%wb)

15.8 15.6 15.4 15.2 15 14.8 14.6 14.4 14.2 14 7 8 9 10 11 12 13 14 15 16 17 7 8 9 10 11 12

Time (Hour) MC (%wb) in the solar dryer MC (%wb) in the natural open sun drying

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Figure 12. Variation of moisture content with time for both an indirect type forced convection solar dryer and natural open sun drying when drying 83.1 g of dates samples for two consective days, 10-11 June 2009.

5. SUMMARY AND CONCLUSIONS An indirect type forced convection solar dryer unit was designed and fabricated to investigate and study its performance under the climatic conditions of Muscat, Sultanate of Oman. Due to its indirect mode of heating, the developed solar dryer is very much useful for drying of herbal products, which are sensitive to direct sunlight. In this study, the dryer was tested to dry both lemons and dates as the test samples. The drying experiments were performed for the summer during the June 2009. The natural open sun drying was also used to investigate its performance for comparison purposes. Both the developed indirect type forced convection solar dryer and the natural open sun drying were tested and evaluated under the same climatic conditions. The results indicated that the designed indirect type forced convection solar dryer presented a greater advantage, from the point of view of time of drying and the quality of the product. The drying with solar dryer was much shorter than the natural open sun drying, but still was too long. Hence, more studies are recommended to improve the performance of the solar dryer.

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Sabah A. Abdul-Wahab Torres-Reyes E, Navarrete-Gonzalez JJ, Ibarra-Salazar BA. Thermodynamic method for designing dryers operated by flat-plate solar collectors. Renewable Energy 2002;26:649-660. Karim MA, Hawlader MNA. Development of solar air collectors for drying applications. Energy Conversion and management 2004;45:329-344. Shanmugam V, Natarajan E. Experimental investigation of forced convection and desiccant integrated solar dryer. Renewable Energy 2006;31:1239-1251. Gbaha P, Andoh HY, Saraka JK, Koua BK, Toure S. Experimental investigation of a solar dryer with natural convective heat flow. Renewable Energy 2007;32:1817-1829. Sreekumar A, Manikantan PE, Vijayakumar KP. Performance of indirect solar cabinet dryer. Energy Conversion and management 2008;49:1388-1395. Purohit P, Kumar A, Kandpal TC. Solar drying vs. open sun drying: A framework for financial evaluation. Solar Energy 2006;80:1568-1579. Sodha MS, Dang A, Bansal PK, Sharma SB. An analytical and experimental study of open sun drying and a cabinet type dryer. Energy Conversion and Management 1985;25(3):263-271. Hachemi A, Abed B, Asnoun A. Theoretical and experimental study of solar dryer. Renewable Energy 1998;13(4):439-451. Queiroz MR, Nebra SA. Theoretical and experimental analysis of the drying kinetics of bananas. Journal Food Engineering 2001;47:127-132. Yaldiz O, Ertekin C, Uzun HI. Mathematical modeling of thin layer solar drying of sultana grapes. Energy 2001;26:457-465. Karim MA, Hawlader MNA. Mathematical modeling and experimental investigation of tropical fruits drying. Heat and Mass Transfer 2005;48:4914-4925. Ait Mohamed L, Kouhila M, Jamali A, Lahsasni S, Kechaou N, Mahrouz M. Single layer solar drying behavior of Citrus aurantium leaves under forced convection. Energy Conversion and Management 2005;46:1473-1483. Bolin HR, Petruccia V, Fuller G. Characteristics of mechanically harvested raisins produced by dehydration and by field drying. Journal Food Science 1975;40:10361038. Eissen W, Muhlbauer W, Kutzbach HD. Solar drying of grapes. Energy Drying Technol 1985;3(1):63-74. Bains MS, Ramaswamy HS, Lo KV. Tray drying of apple puree. Journal of Food Engineering 1989;9(3):195-201. Bennamoun L, Belhamri A. Design and simulation of a solar dryer for agriculture products. Journal of Food Engineering 2003;59:259-266.

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Chapter 7

FABRICATION AND CHARACTERIZATION OF C60-BASED BULK HETEROJUNCTION SOLAR CELLS WITH CU2O, CUINS2, DIAMOND, PORPHYRIN AND EXCITON-DIFFUSION BLOCKING LAYER Takeo Oku*, Ryosuke Motoyoshi, Akihiro Takeda, Akihiko Nagata, Tatsuya Noma, Atsushi Suzuki, Kenji Kikuch, Shiomi Kikuchi, Balachandran Jeyadevan and Jhon Cuya

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Department of Materials Science, The University of Shiga Prefecture, Hassaka 2500, Hikone, Shiga 522-8533, Japan

ABSTRACT C60-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and porphyrin, Cu2O, CuInS2 and diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the excitondiffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals.

*

E-mail: [email protected]

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1. INTRODUCTION Carbon-based nanostructures such as fullerenes, nanocapsules, onions, nanopolyhedra, cones, cubes, and nanotubes have been widely reported and investigated [1–5]. These carbon (C) nanomaterials with hollow cage structures show different physical properties, and have the potential of studying materials of low dimensionality within an isolated environment. By controlling the size, layer numbers, helicity, compositions, and included clusters, the clusterincluded C nanocage structures with band-gap energy of 0–1.7 eV and nonmagnetism are expected to show various electronic, optical, and magnetic properties such as Coulomb blockade, photoluminescence, and superparamagnetism [3–5]. Recently, C60-based polymer/fullerene solar cells have been investigated and reported [6– 10]. These organic solar cells have a potential for use in lightweight, flexible, inexpensive and large-scale solar cells [11–13]. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor-acceptor (DA) proximity in the devices by using blends of donor-like and acceptor-like molecules or polymers, which are called DA bulk-heterojunction solar cells [14–16]. The purpose of the present work is to fabricate and characterize C60-based bulk heterojunction solar cells. In the present work, 5,10,15,20-Tetraphenyl-21,23H-porphin zinc (ZnTPP), Cu2O, CuInS2 (CIS) and diamond was used for p-type semiconductors, and C60 with excellent electron affinity was used for n-type semiconductors. Porphyrin has high optical absorption in the visible spectrum and high hole mobility [17–20], and was expected to form cocrystallites with C60 [21,22] that would be suitable for the bulk heterojunction structure [23,24]. Copper oxides (Cu2O) are known as one of the p-type oxide semiconductors with a direct transition band gap of 2.0 eV. I-III-VI group compounds, called chalcopyrite, are expected as next generation solar cell materials, and CIS is one of the representative chalcopyrite compounds. Chalcopyrite compounds have advantages of high optical absorption and high resistivity to cosmic rays compared to conventional silicon solar cells [25,26]. In addition, they have a band structure of direct transition, which shows high quantum efficiency. The second purpose is to investigate an effect of exciton-diffusion blocking layer (EBL). 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is a perylene derivative with a simple structure, which was reported to be used for solar cells [27]. In the present work, PTCDA was used as the EBL for ZnTPP:C60 bulk heterojunction solar cells. EBL prevents hole transfer between active layer and anode, and improvement of conversion efficiency was expected by an introduction of the EBL. Device structures were produced, and efficiencies and optical absorption were investigated. The present work will indicate a guideline for new-type organic-inorganic solar cells using C60.

2. EXPERIMENTAL PROCEDURES A schematic diagram of the present C60-based bulk heterojunction solar cells is shown in Figure 1. A thin layer of polyethylenedioxythiophen doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ~10 Ω/□). The PEDOT:PSS has a role as an electron blocking

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 157 layer for hole transport. Then, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of C60 (Material Technologies Research Ltd., 99.98%), ZnTPP (Sigma Aldrich) in 1 mL o-dichlorobenzene (Nacalai Tesque, Inc., 99%). Total weight of ZnTPP:C60 was 18 mg, and weight ratio of ZnTPP:C60 was 3:7. Cu2O nanoparticles are synthesized by reducing copper-amine complex with 1-heptanol in the presence of tetramethyl ammonium hydroxide. The details of the synthesis scheme are as follows: specific amounts of copper acetate and oleylamine are introduced into 1-heptanol and heated under a nitrogen atmosphere at 120 °C for 1 h to remove the moisture in the system. Then, hot tetramethyl ammonium hydroxide (TMAOH) dissolved in 1-hepatanol is introduced and the suspension is heated at 150 °C for 2 h. Next, the suspension is cooled to room temperature and Cu2O nanoparticles are recovered by centrifuging. Finally, the Cu2O nanoparticles are washed with organic solvents to remove excess oleylamine and dispersed in toluene. A thin layer of PEDOT:PSS was spin-coated on pre-cleaned ITO glass plates. After annealing at 100 °C for 10 min in N2 atmosphere, semiconductor layers were prepared on a PEDOT:PSS layer by spin-coating using a mixed solution of a toluene dispersion with Cu2O nanoparticles, and a C60 solution (16 mg/mL) with C60 powder in o-dichlorobenzene [28]. CIS solution for p-type semiconductors were produced by dissolving CuI (Sigma Aldrich Corp., 99.99%) and InCl3 (Sigma Aldrich Corp., 99.99%) in a mixture of triphenylphosphite (1 mL) (Sigma Aldrich Corp., 97%) and acetonitrile (2 mL) (Nacalai Tesque, Inc., 99.5%), dropping bis(trimethylsilyl)sulfide (Tokyo Chemical Industry Co., Ltd., >95%) [29,30]. The solution for n-type semiconductors was prepared by dissolving C60 in o-dichlorobenzene. A thin layer of PEDOT:PSS was spin-coated on a pre-cleaned fluorine dope tin oxide (FTO) glass plates (Asahi Glass, ~9.3 Ω/□). Then, semiconductor layers were prepared on a PEDOT:PSS layer by spin coating, and annealed at 120 ºC for 10 min in N2 atmosphere. The FTO was used because of the high temperature annealing process. The thickness of the blended device was ~150 nm. To increase efficiencies, PTCDA with a thickness of ~20 nm was also added over the active layers as shown in Figure 1. After annealing at 100 °C for 30 min in N2 atmosphere, PTCDA (Wako Pure Chemical Industries Ltd.) was evaporated between active layer and metal layer. Finally, aluminum (Al) metal contacts were evaporated as a top electrode, and annealed at 140 °C for 20 min in N2 atmosphere. Current density-voltage (J-V) characteristics (Hokuto Denko Corp., HSV-100) of the solar cells were measured both in the dark and under illumination at 100mW/cm2 by using an AM 1.5 solar simulator (San-ei Electric, XES-301S) in N2 atmosphere. The solar cells were illuminated through the side of the ITO substrates, and the illuminated area is 0.16 cm2. Optical absorption of the solar cells was investigated by means of UV-visible spectroscopy (Hitachi U-4100). Transmission electron microscope (TEM) observation was carried out by a 200 kV TEM (Hitachi H-8100). The microstructures were also investigated by X-ray diffraction (XRD, Philips X’ Pert-MPD System). The isolated molecular structures were optimized by ab-initio molecular orbital calculations using Gaussian 03. Conditions in the present calculation were as follows: calculation type (FOPT), calculation method (RHF) and basis set (6-31G). Electronic structures such as energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and electron densities were investigated.

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-

+

PTCDA Al (Electrode)

p-type

n-type

ZnTTP

C60

Cu2O

C60

CuInS2

C60

Diamond

C60

Bulk Heterojunction PEDOT:PSS ITO or FTO Glass substrate Figure 1. Structure of C60-based bulk heterojunction solar cells.

Current density (mAcm-2)

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0.10 0.00 -0.10

ZnTPP:C60

-0.20 -0.30 -0.40

ZnTPP:C60/PTCDA

-0.50 -0.60 -0.70 0

0.1

0.2

0.3

0.4

Voltage (eV) Figure 2. Measured J-V characteristic of ZnTPP:C60 bulkheterojunction solar cells under illumination.

3. ZNTPP:C60 SOLAR CELLS Measured J-V characteristic of ZnTPP:C60 bulk heterojunction solar cells under illumination is shown in Figure 2. The bulk heterojunction indicates a one layered composite structures with p- and n-type semiconductors, which is denoted as ZnTPP:C60. Effects of PTCDA addition to the ZnTPP:C60 bulk heterojunction solar cells were also investigated, which is denoted as ZnTPP:C60/PTCDA. Each structure shows a characteristic curve for open circuit voltage and short circuit current density, and measured parameters of the present solar

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 159 cells are summarized in Table 1. Power conversion efficiency, fill factor, short-circuit current density and open-circuit voltage are denoted as η, FF, JSC, and VOC, respectively. As shown in Figure 2 and Table 1, current density of ZnTPP:C60 increased by PTCDA addition, and the best efficiency was obtained for the ZnTPP:C60/PTCDA sample. Figure 3 shows the optical absorption of C60, ZnTPP, ZnTPP:C60 and ZnTPP:C60/PTCDA bulk heterojunction solar cells. The ZnTPP:C60/PTCDA structure provided higher photoabsorption in the range of 300 to 800 nm (which correspond to 4.0 and 1.5 eV, respectively), compared to the ZnTPP:C60 structure. Exciton migration of C60 can be efficiently suppressed by use of PTCDA [23], and exciton would be generated for both ZnTPP/C60 and C60/PTCDA interfaces, which results in the increase of conversion efficiency, as listed in Table 1. X-ray diffraction patterns of ZnTPP and ZnTPP:C60 bulk heterojunction layers are shown in Figure 4(a) and 4(b), respectively. In Figure 4(a), diffraction peaks corresponding to ZnTPP crystal are observed. After formation of ZnTPP:C60 bulk heterojunction layer, the diffraction peaks corresponding to ZnTPP disappeared, and C60 peaks are observed as shown in Figure 4(b). In addition, a new diffraction peak is observed as indicated by an arrow, which would be believed to be porphyrin/C60 cocrystallites [21,22]. Table 1. Measured parameters of the present solar cells

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Sample ZnTTP:C60/PTCDA ZnTTP:C60 Cu2O:C60 CuInS2:C60 Diamond:C60

VOC (V) 0.33 0.30 0.17 0.18 0.023

JSC (mAcm−2) 0.62 0.074 0.11 0.016 0.0053

FF 0.38 0.26 0.23 0.28 0.35

η (%) 7.8 × 10−2 5.8 × 10−3 4.3×10−3 8.0 × 10−4 4.3 × 10−5

Figure 3. Absorbance spectrum of (a) C60, ZnTPP and (b) ZnTPP:C60 bulk heterojunction solar cells.

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(a)

Intensity (Arb. Units)

Intensity (Arb. Units)

(b)

Diffraction angle

Diffraction angle

Fiigure 4. X-ray diffraction d patteern of (a) ZnTPP P and (b) ZnTP PP:C60 bulk heteerojunction layeer.

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Figure 5 iss an electron diffraction d patttern of ZnTPP P:C60 bulk heterojunction layer, taken allong the [-123 3] direction off C60. A twin structure withh the (112) tw win plane is observed o in Fiigure 5, as indicated i by a dotted linee. Diffractionn spots whichh would corrrespond to coocrystallites of ZnTPP:C60 are a also observved as indicateed by arrows.

Fiigure 5. Electro on diffraction paattern of ZnTPP P:C60 bulk heterrojunction layerr.

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Figure 6. Calculated LUMO and HOMO levels of (a) ZnTPP (b) C60 and (c) PTCDA.

Electronic structures of the molecules were calculated, and energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Figure 6. HOMO levels in Figure 6(a), electrons are localized around the pyrrole rings of the ZnTPP. Energy levels of LUMO of C60 and PTCDA are also shown in Figure 6(b) and 6(c), respectively. The separated carriers would transfer from ZnTPP to C60, and from ZnTPP/C60 to PTCDA. An energy level diagram of the ZnTPP/C60/PTCDA solar cell is summarized in Figure 7. Previously reported values [7,8] were used for the energy levels of the figures by adjusting to the present work. The incident direction of light is from the ITO side. Energy barrier would exist near the semiconductor/metal interface. Electronic charge-transfer separation was caused by light irradiation from the ITO substrate side. Electrons are transported to an Al electrode, and holes are transported to an ITO substrate. The VOC of organic solar cells is reported to be determined by the energy gap between HOMO of donor molecule and LUMO of accepter molecule, and a relation between VOC and polymer oxidation potential is VOC = (1/e)(|EZnTPPHOMO| – |EC60LUMO|) – 0.3 (V), where e is the elementary charge [31]. The value of 0.3 V is an empirical factor, and this is enough for efficient charge separation [32]. The present experimental data of VOC indicated smaller compared to the calculated ones from the equation, which might be due to the voltage descent at the metal/semiconductor interface, and control of the energy levels is also important to increase the efficiency. In the present work, efficiencies of the solar cells were increased by addition of PTCDA layers, which would work as the exciton-diffusion blocking layer for porphyrin:C60 bulk heterojunction solar cells. The PTCDA layers prevents hole transfer between the porphyrin:C60 active layer and aluminum, and the conversion efficiencies were improved.

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Fiigure 7. Energy y level diagram of ZnTPP/C60/P PTCDA solar ceell.

Energy (eV V)

0.4

Absorbance

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0.5

4.0 0 3.5

3.0

2.5

2.0

C 2O Cu C60 C60

0.3

0.2

0.1

0 300

400

500 600 Wavelengtth (nm)

700

Fiigure 8. Opticall absorption of solar s cells with Cu2O:C60 struucture.

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800 0

Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 163

C60 111

C60 311 C60 222

C60 220

C60 331

Intensity (a.u.)

C60 420 Cu2O 110 C60 422 C60 511 Cu2O 111 Cu2O 200

10

20

30 2θ(degree)

40

50

Figure 9. XRD pattern of Cu2O:C60 thin film. a

b

Cu2O 310 Cu2O 220

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Cu2O 200 Cu2O 111

000

20 nm

Figure 10. (a) TEM image and (b) electron diffraction pattern of Cu2O:C60 structure.

Since the microstructure of ZnTPP and C60 bulk heterojunction layer is strongly dependent on the weight ratio of these, it is necessary to control the microstructure to form cocrystallites of ZnTPP:C60. In the present work, higher efficiencies were obtained for the ZnTPP:C60 sample with the weight ratio of 3:7, which would be suitable for the cocrystallite formation, as observed for weak reflections in X-ray and electron diffraction patterns. Recombination of electrons of C60 and holes of ZnTPP would occur in the bulk heterojunction layer with intermittent cocrystallite structure. If continuous cocrystallite structures form perpendicular to the thin films, it is believed that the recombination of

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electrons and holes could be suppressed, which would lead to improvement of conversion efficiency.

4. CU2O:C60 SOLAR CELLS

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The measured parameters of the solar cells are summarized in Table 1. A solar cell with a Cu2O:C60 structure provided η of 4.3×10−3 %, FF of 0.23, JSC of 0.11 mAcm−2 and VOC of 0.17 V [28]. Figure 8 shows measured optical absorption of the solar cells. The Cu2O:C60 structure shows high absorption at 300 nm and 800 nm. The absorption peaks around 324 nm for the Cu2O:C60 structure were due to Cu2O, and the absorption peak around 362 nm and 476 nm correspond to C60. All the crystalline components in the Cu2O:C60 thin films were investigated by XRD, as shown in Figure 9. Diffraction peaks corresponding to Cu2O and C60 were observed for the Cu2O:C60 thin film, and they consisted of a cuprite phase with cubic system (space group of Pn3m and lattice parameters of a = b = c = 0.4250 nm). The particle size was estimated using Scherrer’s equation: D=0.9λ/Bcosθ, where λ, B, and θ represent the wavelength of the X-ray source, the full width at half maximum (FWHM), and the Bragg angle, respectively. The crystallite sizes of Cu2O and C60 were determined to be 7.2 nm and 25.7 nm, respectively. Figure 10(a) and 10(b) shows a TEM image and a selected area diffraction pattern of the Cu2O:C60 thin film annealed at 100 °C, respectively. The TEM image indicated aggregated Cu2O nanocrystals with the size of 10–20 nm. The line broadening of the Debye-Scherrer rings in Figure 10(b) indicates nanocrystal structures of Cu2O. C60 peaks [33] were not observed in this pattern, which would be due to dispersion of C60 crystals. Optimization of the nanocomposite structure with Cu2O and C60 would increase the efficiencies of the solar cells.

e-2.9eV

e-

hν

-4.7eV

-5.0eV -4.9eV

h＋

-4.3eV -4.5eV

h＋ -6.2eV Nanocomposite

ITO

PEDOT: Cu O 2 PSS

C60

Al

Figure 11. Energy level diagram of ITO/PEDOT:PSS/Cu2O:C60/Al bulk heterojunction solar cells.

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 165 0. 7 0. 6 0. 5 0. 4 0. 3 0. 2 0. 1 0 300

400

500

600

700

800

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Figure 12. Optical absorption spectra of CIS/C60 heterojunction and bulk heterojunction solar cells.

Energy level diagram of Cu2O:C60 solar cells is summarized as shown in Figure 11. Previously reported values were used for the energy levels [9,34,35]. It has been reported that VOC is nearly proportional to the band gap of the semiconductors [36], and control of the energy levels is important to increase efficiency. Compared to a Si semiconductor with an indirect transition band structure, Cu2O with a direct transition band gap is more suitable for the optical absorption property. In addition, the ultrathin film of the Cu2O layers could provide efficient charge injection because of the high optical absorption. Although ZnO has been mainly used as a n-type oxide semiconductor for solar cells [37,38], C60 was applied instead of ZnO in the present work. The advantages of the present C60 are a good electronic acceptable material for solar cells and the simple film formation using a spin-coating method. Compared to previously reported Cu2O-based heterojunction solar cells [37], bulk heterojunction solar cells prepared by a spin-coating method without a vacuum system were investigated in the present work. The present bulk heterojunction solar cells have simple fabrication process and good cost performance. Cu2O-based solar cells prepared by a spin-coating method without vacuum evaporation were investigated in the present work. The low conversion efficiency of the present solar cells would be due to presence of Cu2O nanoparticle aggregation in the active layer. The defects produced by nanoparticle aggregation cause carrier recombination. Formation of the Cu2O:C60 thin films with homogeneous distributed Cu2O nanoparticles could increase the efficiency of solar cells.

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5. CUINS2:C60 SOLAR CELLS Measured parameters of a CuInS2:C60 bulk heterojunction structure are summarized in Table 1. A solar cell with CIS:C60 bulk heterojunction structure provided power convergent efficiency of 8.0×10−4%, fill factor of 0.28 and open-circuit voltage of 0.18 V. The p-n interfaces, which are photoelectron conversion areas were increased by using blend structures of p-type and n-type semiconductors. Figure 12 shows a measured optical absorption of the solar cells based on CIS. These solar cells show a wide optical absorption range from 400 to 800 nm, and the heterojunction solar cell show as higher optical absorption range from 350 nm to 550 nm than that of the bulk heterojunction. Since the FTO substrate was set as an incident side, the optical absorption of the CIS layer was high for the heterojunction structure. On the other hand, optical absorption of the bulk heterojunction would be lower compared to that of the heterojunction structure because C60 were mixed with the CIS layer. An X-ray diffraction pattern of CIS:C60 bulk heterojunction is shown in Figure 13. Several diffraction peaks are observed, which correspond to 112, 204 of CIS and 111, 220, 311, 222, 422, 511 of C60. The average particle sizes of CuInS2 and C60 were calculated from Scherrer’s formula to be 5 nm and 13 nm, respectively. The 204 peak of CIS is too small to be used for the calculation of the CIS grain size, and only one peak of 112 was used for the calculation.

C60 111

Cu

C60 311

Intensity (Arb. Units)

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S

10

In

C60 220 C60 222 CIS 112

C60 422 C60 511 CIS 204

20

30

40

Diffraction angle 2θ (degree) Figure 13. X-ray diffraction pattern of CIS:C60 thin film.

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 167 a

217 217 314 314 312 220 312 204 211 211

b

000

50 nm Figure 14. (a) TEM image and (b) electron diffraction pattern of CIS nanoparticles.

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Figure 14(a) is a TEM image of CIS, and many CIS particles are observed. Figure 14(b) is an electron diffraction pattern of CIS. Debye-Scherrer rings are observed in the diffraction pattern, which shows crystallite structures of CIS particles. An interfacial structure of CIS and C60 was observed by TEM as shown in Figure 15. Filtered Fourier transform of the HREM image of CIS:C60 bulk heterojunction layer is shown in Figure 15(a). Figure 15(b) is an inverse Fourier transform of (a), and arrows show the interface of CIS and C60. Lattice fringes of {101} of CIS and {111} of C60 were observed. The enlarged image of a part of C60 in (b) is shown in Fig 15(c). Arrangements of C60 molecules are observed in the image. CIS and C60 have size distribution, and the crystal sizes of them observed in the TEM image are larger compared to the averaged sizes. a

b

C60 200 C60 111

C60 {111}

C60 022

000

CIS 101

c

1.4nm

3nm

CIS {101}

Figure 15. (a) Filtered Fourier transform of HREM image of CIS:C60 bulk heterojunction layer. (b) Inverse Fourier transform of (a). (c) Enlarged image of a part of C60 in (b). Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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e-

e-

-4.1eV -4.7eV -5.0eV

h＋

hν

-4.3eV -4.5eV

-5.6eV

h＋

-6.2eV

Nanocomposite

FTO PEDOT CIS :PSS

C60

Al

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Figure 16. Energy level diagram of CIS:C60 solar cell.

Optimization of the nanocomposite structure with CIS and C60 would increase the efficiencies of the bulk heterojunction solar cell. From the present TEM observation, CIS and C60 were not mixed in a molecular scale. If the mixture structure of CIS and C60 is improved to a nanoscale, it is believed that the area of the p-n junction interfaces is increased, and the efficiency would be improved. In addition, it is important to search the most suitable mixture ratio of the p-type and n-type semiconductors for bulk heterojunction solar cells. An energy level diagram of CIS/C60 solar cells is summarized as shown in Figure 16. Previously reported values were used for the energy levels of the figures by adjusting them to the present work [13,27,39]. When light is incident from the FTO side, excitation by the light absorption happens in the p-n interface, and electrons and holes are produced by charge separation. Carriers would transport from –4.5 eV to –4.3 eV by hopping conduction. Improvement of the present bulk heterojunction solar cells would be possible by the introduction of a buffer layer, change of annealing conditions, and the improvement of the microstructure is also necessary to obtain high efficiency. The evaporation method provided high quality thin films, but a high vacuum and high temperature process are necessary. Although CISCuT method is a productive process, it requires a high temperature process [40]. On the other hand, the present spin coating method is simpler compared to the other formation methods. In addition, we can apply the spin coating method to plastic substrates without high vacuum and high temperature processes.

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 169

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6. DIAMOND:C60 SOLAR CELLS Measured J-V characteristic of diamond:C60 bulk heterojunction solar cell under illumination showed characteristic curve for open circuit voltage and short circuit current. A solar cell with the diamond:C60 structure provided power convergent efficiency of 4.3 × 10−5%, fill factor of 0.35, short circuit current of 5.3 μA/cm2 and open circuit voltage of 0.023 V, as listed in Table 1. Figure 17 shows optical absorption of the diamond:C60 bulk heterojunction solar cell. The diamond:C60 bulk heterojunction structure provided photo-absorption in the range of 350 to 500 nm, and shows high absorption at 339, 402 and 506 nm, which correspond to 3.7, 3.1 and 2.5 eV, respectively. Absorption peaks of the C60 were confirmed within the range from 300 to 400 nm, and an absorption peak of 506 nm corresponds to the diamond. X-ray diffraction pattern of diamond powder showed diffraction peaks of diamond powder, which were confirmed as 111, 220 and 311 of the diamond structure. A grain size of diamond powder was determined to be 12 nm, which was calculated by Scherrer’s equation. An increase of photo-absorption under the long wavelength would be due to the nanostructure of nanodiamond particles, which will be discussed later. An energy level diagram of diamond:C60 solar cell is summarized as shown in Figure 18. Previously reported values were also used for the energy levels [13]. An energy gap of diamond estimated from Figure 17, which corresponds to absorbance of 506nm, is used for the model. From a theoretical calculation [41], nanodiamonds are composed of three layers; a diamond core (sp3), a middle core (sp2+x) and a graphitized core (sp2). Therefore, a band gap of the nanodiamond is decreased by the existence of the sp2+x bonding [42,43]. The carrier transport mechanism is considered as follows; when light is incident from the ITO substrate, light absorption excitation occurs at the p-n heterojunction interface, and electrons and holes appear by charge separation. Then, the electrons transport through C60 toward the Al electrode, and the holes transport through PEDOT:PSS to the ITO substrate. Since it has been reported that Voc is nearly proportional to band gaps of semiconductors [36], control of energy levels is important to increase the efficiency. The low cell performance would be due to the insufficient dispersion of diamond and C60 in the composite layer, and further control of the nanocrystals is needed. An advantage for the bulk heterojunction structure is increased p-n heterojunction interface. However, due to disarray of the diamond:C60 microstructure, electrons and holes could not transport smoothly by carrier recombination at the C60/Al interface, and at the PEDOT:PSS/diamond interface, respectively. To solve these problems, introduction of a layer preventing carrier recombination and improvement of crystalline structure with few defects are needed. In the present work, an organic-inorganic hybrid solar cell was fabricated and characterized. For the carbon-based solar cells in previous works, thin films are fabricated by a CVD method. In the present work, solar cells with C60 as an organic semiconductor and diamond as an inorganic semiconductor were fabricated by a spin coating method, which is a low cost method. Boron nitride acts as p-type semiconductor, and diamond has a similar crystal structure as boron nitride. Combination of the present solar cells and nanomaterials such as diamond or boron nitride with various direct band gaps might be effective for increase of efficiencies [44]. The performance of the present solar cells would be dependent on the

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nanoscale structures of the organic-inorganic materials, and control of the structure should be investigated further. 0.4

4.0 3.5

3.0

2.0

2.5

0.3

0.2

0.1

0 300

400

500

600

700

Figure 17. Optical absorption spectrum of diamond:C60 bulk heterojunction solar cell.

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-3.0eV

e-4.3eV

-4.7eV -5.0eV

h＋

hν

-4.5eV

-5.6eV

h＋

-6.2eV

Nanocomposite

ITO

PEDOT: Diamond C 60 PSS

Al

Figure 18. Energy level diagram of diamond:C60 bulk heterojunction solar cell.

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Fabrication and Characterization of C60-Based Bulk Heterojunction Solar Cells … 171

CONCLUSION C60-based bulk heterojunction solar cells were fabricated and characterized. A device based on ZnTPP:C60/PTCDA provided η of 7.8 × 10−2%, FF of 0.38, JSC of 0.62 mA/cm2 and VOC of 0.33 V. Conversion efficiency was increased by introduction of PTCDA layer because the exciton migration of C60 can be efficiently suppressed by use of PTCDA. A device of bulk heterojunction structure based on Cu2O, CuInS2:C60 and diamond:C60 were also fabricated and characterized. Photovoltaic behavior including charge transfer and mobility can be described on the basis of the energy diagram of the bulk heterojunction solar cells from the present J-V measurements, optical absorption and structure analysis. Optimization of blended structures with C60 would increase the efficiencies of solar cells.

ACKNOWLEDGMENTS This work is partly supported by Takahashi Industrial and Economic Research Foundation, and Grant-in-Aid for Scientific Research, Ministry of Education, Science, Sports and Culture, Japan.

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In: Dye-Sensitized Solar Cells and Solar Cell Performance ISBN: 978-1-61209-633-9 Editor: Michael R. Travino ©2012 Nova Science Publishers, Inc.

Chapter 8

ENHANCEMENT OF SOLAR CELL PERFORMANCE USING SURFACE MORPHOLOGY MODIFICATION J. Y. Chen, C. K. Huang, H. H. Lin and K. W. Sun* Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan

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ABSTRACT Solar cell development has become more and more important due to the increase of worldwide energy demands, and conventional energy resources such as fossil fuels, will be exhausted soon. A key factor in applications and effectiveness of solar cell is to enhance its energy conversion efficiency. In this chapter, we reviewed and demonstrated innovative approaches to improve conversion efficiency of silicon solar cells by modifying surface morphology of the devices. Because more than 30% of incident light is reflected from the silicon surface back to the air, an anti-reflection (AR) layer is a typical type of coating which can be applied to the surface to reduce light reflection and to increase light absorption. Surface-relief gratings with sizes smaller than the wavelength of light, named sub-wavelength structures (SWS), can behave as antireflection surfaces. Using a mechanically continuous wavelike grating, the sub-wavelength structured grating acts as a surface possessing a gradually and continuously changing refractive index profile from the air to the substrate. We demonstrate polymer sheets with sub-wavelength AR structures using spin-coating replication and hot-embossing techniques with applications on silicon solar cells. The techniques provide simple and low-cost means for large-scale production of AR layers and to improve solar cell performance. On the other end of the spectrum, the results of the fabrication of ZnO nanorod and self-aggregated nanoparticles as the AR layers are also presented in this chapter. The vertically aligned and solution-grown ZnO nanorod arrays were deposited on the surface of the Si solar cells as the AR layer. We found that the nanorod morphology, controlled through synthetic chemistry, has a great effect on the AR layer performance. It’s also known that nanoparticle monolayers composed of metal, silica or polystyrene have the * Corresponding author; email: [email protected], Address: 1001 Dahseuh Rd. Shinchu, Taiwan 30010 Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

176

J. Y. Chen, C. K. Huang, H. H. Lin and K. W. Sun characteristics of surface plasmon resonances and reducing light reflection on surface. Various solar cells substructures with the deposition of nanoparticles have been proved to be able to enhance the photocurrent, acceptance angles, and to reduce the surface reflection within specific wavelength (700-1100 nm) range. We demonstrate an appropriate and well-operated application of self-assembled nanoparticle layers on solar cell surface, which increases the light absorption and enhances the photocurrent so that the light conversion efficiency of the cells can be improved. The techniques not only provide enhancement of the light-harvesting capability of the device but also with a minimum cost.

Keywords: antireflection, solar cell, nanoimprint, nanorod, nanoparticle

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1. INTRODUCTION AND BACKGROUND Because conventional energy resources such as fossil fuels will be exhausted soon, finding sufficient supplies of clean energy for the future is one of scientist’s most urgent challenges. Solar cell development has become more and more important due to the increase of worldwide energy demands. It would provide nearly twice the world’s consumption rate of fossil energy if covering 0.16% of the land on earth with 10% efficient solar conversion systems [1]. Various cost-competitive, relatively cheaper processing steps have been developed to make these solar cells more energy efficient. One of the general approaches to increase the conversion efficiency is by reducing the surface reflection of the incident light. For example, more than 30% of incident light is reflected from the silicon surface back to the air due to mismatch in the refractive index. The typical ways to avoid or reduce the surface reflection are surface texturization and AR coatings. Texturing of the front surface of silicon solar cells has been modeled and analyzed with reference to the reduction in reflection coefficient and increase in optical trapping. As shown in Figure 1-1, when perpendicularly incident light enters the sloped sides of the wells, part of the light is transmitted into the surface while most of the reflected portion is scattered to another point within the same well. Hence, the reflected light has a higher chance to enter the cell surface: this is the so-called double bounce effect, which can significantly reduce the cell surface reflection and increase the optical path in the substrate. In order to achieve good uniformity of pyramidal textured structure of the silicon surface, a mixture of NaOH/KOH and isopropyl alcohol (IPA) is generally used for texturization of mono crystalline solar cells. For multicrystalline silicon (mc-Si) solar cells, the standard alkaline solution of NaOH/KOH does not produce textured surface of good quality so as to give satisfactory open circuit voltage and efficiency. Nishimoto et al. [2] used acidic etchant for isotropic etching of mc-Si. They also established a model to simulate reflectance behavior of the spherical structure formed during acidic etching, and concluded that acidic texturing was superior to alkaline texturing for mc-Si. Recently, there have been reports on various methods of increasing silicon solar cell efficiency by improving light trapping in the periodic micrometer structure [3-6]. Yoji Saito et al. demonstrated the improvement in efficiency of the single crystalline silicon solar cells by texturing the front surface [5,6]. Figure 1-2 shows the reflectance spectral for single crystalline silicon surface textured in different conditions [5]. From the

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 177 plotted curves, we find a significant reduction in the reflection over the visible range when the cell surface was texturized using HNO3:HF:CH3COOH solution with a concentration under 5 % and a mixture ratio of 25:1:10. The shape of the textured surface defined by the etching solution type was hemispheric for isotropic etching and was vertical for anisotropic etching. The shape also determined the reflectance of the textured surface. The incident light that bounced back from the surface and hit the slope of the walls was reflected back into silicon again and contributed to the free carriers generation by the enhanced absorption. The conversion efficiency was increased from 11.51 % to 16.11% in compared to the untextured surface solar cell [6].

incident light

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Figure 1-1. The incident light will be trapped more easily by the textured surface.

Source: E. Manea, E. Budianu, M. Purica, D. Cristea, I. Cernica, R. Muller, and V. Moagar Poladian, Sol. Energy Materials and Sol. Cells 87 (2005) 423-431. Figure 1-2. Reflectance spectral dependence for single crystalline silicon surface, textured in different conditions. (1) textured surface by isotropical etching in HNO3:HF:CH3COOH, (2) textured surface by isotropical etching in HNO3:HF, (3) textured surface by anisotropical etching in TMAH, (4) silicon untextured surface [5].

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1778

J. Y. Chen, C. K. Huuang, H. H. Linn and K. W. Sun S

Fiigure 1-3. Technological flow for texturing sillicon surface.

The period dic honeycombb structures faabricated by photolithographhy and chemical etching were reported in Ref. [3-6]. The w T technologgical flow is shhown in Figurre1-3. Hole paatterns were trransferred to the silicon dioxide layerr by a photoolithographic process usinng positive phhotoresist. Th he patterned silicon dioxiide layer wass used as a masking layyer for the suubsequent etch hing on the sillicon substratee. These patterrned holes weere uniformly spaced s in a heexagonal layo out. Through thhe window oppened on the oxide, o the siliccon substrate was w etched inn an isotropic acid a solution to t form hexaggonal structurees with a maxiimum packingg density. A litttle over-etching beyond thhis point was carrier out during d the cell fabrication in i order to reeduce the unettched flat regioons on the cell surface. Figure 1-4 4(a) shows a scanning elecctron microscoopy (SEM) im mage of an over-etched o hooneycomb tex xtured surface in plan view and the Figurre 1-4(b) givess a perspectivve view [4]. Inn the extreme case of overr-etching, the wells can apppear as ideal hexagons from m the plan viiew with mostt of the walls collapsed exccept the cornerr regions. Thee other advantaage for this hooneycomb tex xturing methodd is that the different grains orientations have h minimum m effects on thhe final shapes of etch wellls. It was observed that all the wells givve very similaar texturing

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 179

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effect for each grain across the wafer, although a slight difference in the shape of the wells was observed. The grain boundaries on the wafer were barely observable under the microscope with low magnification lenses, which detect surface reflection more than the geometry of the surface structures.

Figure 1-4. SEM images of close to optimally etched surface: (a) plan view, including one of the metallization fingers and (b) perspective view. The spacing of the hexagons is 14 um in both cases [4].

The obtained texturing surfaces were characterized using spectrophotometric measurements. Figure 1-5 compares the measured total reflectance and external quantum efficiency of the honeycomb cell to the multi-crystalline silicon cell without front texturing surface and the high efficiency single crystal silicon cells with the inverted pyramid texture [3,4]. It can be seen that the honeycomb cell reduces surface reflectance in the short wavelength range from 400 nm to 800 nm. The double bounce effect is critical for this reflection reduction. From Figure 1-5, it clearly shows that the honeycomb cell displays a

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significantly higher long wavelength response than the planar cell. This is attributed to light trapping as well as the reduced front reflection. The honeycomb multi-crystalline cells demonstrated an energy conversion efficiency of 19.8% [3,4]. The honeycomb structures made major contributions to this efficiency improvement. This cell also gave a very high short-circuit current density of 38.1 mA/cm2. The novel honeycomb cell structure has considerably reduced the surface reflection and improved the light trapping inside the cell. The honeycomb cell also demonstrated a very high open-circuit voltage of 654 mV, which equals the highest reported value for multi-crystalline silicon solar cells. However, the periodic microstructure fabrication requires extremely high cost for mass production with the photolithography techniques.

Source: Jianhua Zhao, Aihua Wang, Patrick Campbell, and Martin A. Green, IEEE Transactions on Electron Devices, vol. 46, no. 10, (1999). Figure 1-5. Hemispherical reflection (lower curves) and external quantum efficiency (upper curves) as a function of wavelength for the three cells of honeycomb textured cell (hollow and filled squares), the planar multicrystalline cell (hollow and filled triangles) and the monocrystalline cell (hollow and filled cycles), all at 10。incidence [4].

AR coatings are widely utilized to suppress the light reflection of silicon solar cells. Currently most AR coatings for silicon solar cells are hydrogen containing silicon nitride (SiNx:H) layers deposited by plasma enhanced chemical vapor deposition (PECVD). The advantage of SiNx:H -layers over other AR-coatings like titanium dioxide is the ability of good surface and bulk passivation [7,8]. Besides of these benefits, the technique of PECVD deposition has several drawbacks. It is mainly the use of explosive gas and short service cycles. But other problems also appear in achieving uniform coating thickness on large scale inline coaters. This results in growing interest in different deposition techniques. In fact,

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 181 SiNx:H -layers only exhibit low reflection at wavelengths around 600 nm and the reflection is higher for other wavelengths [9].

2. SUB-WAVELENGTH NANOSTRUCTURES FOR ANTI-REFLECTION PURPOSES To achieve broadband AR coatings, it is necessary to get a graded refractive index structure that suppresses reflection over a broad range of light spectrum and angles of incidence. Surface-relief gratings with the size smaller than the wavelength of light, named sub-wavelength structure (SWS), behave as AR surfaces. Using a mechanically continuous wavelike grating (e.g., pyramidal, triangular, conical shapes), the SWS grating acts as a surface possessing a gradually and continuously changing refractive index profile from the air to the substrate. Deeper SWS grating can greatly enhance the AR effect, since the refractive index value changes smoothly and continuously. Periodic SWSs are discovered in nature such as moths and some butterflies, and have enormous inspirations for scientists to mimic them for optical applications [9-18]. The bio-mimetic anti-reflection structured (ARS) surface can be understood easily in terms of a thin film in which the refractive index changes gradually from the top of the structure to the bulk materials. The Fresnel reflection for a thin film coating in the interface of two materials is given as [(n1 −n2)/(n1 + n2)]2, where n1 and n2 are the refractive indices of the two materials. For the structured film with gradient refractive index, it can be regarded as a ARS surface with a reflectance composed of an infinite series of reflection at each increase progressively index. When light is incident upon the ARS surface, each reflection will carry a different phase because it comes from different depth of the substrate. If the transition takes Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

place over an optical distance of λ/2, there will be destructive interference and the reflectance will fall to zero [10]. In order to fabricate high performance artificial bio-mimetic ARS surfaces, three structural features are generally required: (1) the height of the ARS arrays h, (2) the period of the ARS arrays d and (3) the distance between the arrays s. To avoid scattering from the optical interface, the period d of the ARS structures has to be smaller than the wavelength of the incident light so that the array cannot be resolved by the incident light. If this condition is fulfilled, we can assume that at any depth the effective refractive index is the mean of that of the air and the bulk materials, weight in proportion to the volume of the materials at any depth. The condition that the ARS array will not be resolved by light is that the direction of the first diffracted order is over the horizon (i.e. at any angle greater than 90◦). That is to say, we require d < λ for normal incidence and d < λ/2 for oblique incidence if we consider only reflections. In the case of bio-mimetic ARS surface, the height of the ARS arrays h is another important factor to the performance of ARS surface. When the incidence light wavelengths were less than 2.5 h and greater than d at normal incidence, and for wavelengths greater than 2d at oblique incidence, the reflection of ARS surface can be very low. For ARS surfaces with fixed period and height, the distance between arrays s plays a less important role in the performance of the ARS surfaces. Therefore, for an ideal case of ARS surface, it should show tapered profile, at the same time, the period will be as fine as possible and the depth as great

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as possible, in order to give the widest possible bandwidth and almost omnidirectional antireflective properties [10].

Source: Y C Chang, GHMei, T WChang, TJ Wang, D Z Lin1 and C K Lee, Nanotechnology 18 (2007) 285303. Figure 2-1. SEM photography of the inverted pyramid structure: (a)50000x and (b)200000x magnification[12].

Many techniques base on the top-down lithography have been applied to fabrication ARS. Hadobas et al. reported that the reflection properties of 300 nm periodically structured Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Enhancement of Solar Cell Performance Using Surface Morphology Modification 183 silicon surfaces with depth varying between 35 and 190 nm, prepared by interference lithography, were examined in the range 200 nm 99% from 610 to 730 nm, with a transmittance maximum 99.25% at 660.5 nm (the grey line

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 187 in Figure 2-8(b)), while the transmission of planar silica substrate is < 93% (the black line in Figure 2-8(b)). The transmission of the AR surfaces is lower than that of planar silica substrates below 330 nm due to light scattering. The improved transmittance is in good agree with the reduced reflectance, which also demonstrates that the loss of light is very low.

Source: Chih-Hung Sun, Peng Jiang, and Bin Jiang, Applied Physics Letters 92, 061112 (2008).

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Figure 2-7. Measured reflectance spectra at normal incidence for a flat single-crystalline silicon substrate, a commercial single-crystalline silicon solar cell with SiNx ARC, and a 15 min etched silicon nipple array [9].

Source: Yunfeng Li, Junhu Zhang, Shoujun Zhu, Heping Dong, Fei Jia,Zhanhua Wang, Zhiqiang Sun, Liang Zhang, Yang Li, Haibo Li, Weiqing Xu,and Bai Yang, Adv. Mater. 2009, 21, 4731–4734. Figure 2-8. (a) Comparison of the specular reflection as a function of wavelength for a planar silica substrate (black line), single-sided (black dash line) and double-sided ARS surfaces (grey line). (b) Wavelength dependent transmission of planar silica substrate (black line), single-sided (black dash line) and double-sided ARS surfaces (grey line) [18].

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Figure 2-9. SEM images taken at 45° on (a) nanocone quartz substrate and (b) a-Si:H nanodome solar cells after deposition of multilayers of materials on nanocones. Scale bar 500 nm. (c) Schematic showing the cross-sectional structure of nanodome solar cells [17].

In practical applications, suppressing reflection and improving transmission of light are crucial for the performance of optical and electro-optical devices such as improving the efficiency of solar cells, increasing transmission, and eliminating ghost images or veil glare on flat panel displays or detectors. Zhu et al. demonstrated the fabrication of a-Si:H nanowire and nanocones (NCs) using an easily scalable and IC-compatible process. Fabrication of novel nanodome solar cells, which have periodic nanoscale modulation for all layers from the bottom substrate through the active absorber to the top transparent contact, was also demonstrated [16,17]. Figure 2-9 shows the SEM images of the nanocone substrate (a), aSi:H nanodome solar cells after deposition of multilayers of materials on nanocones (b), and the schematic of nanodome solar cells (c). Interestingly, for the a-Si:H NC arrays, the diameter of these NCs shrinks gradually from the root to the top, resulting in a graded transition of the effective refractive index. For this reason, the a-Si:H NC arrays demonstrate the best AR properties and the greatest absorption enhancement as well. The light absorption measurements with the different angle of incidence are shown in Figure 2-10. The unique geometry of the nanodomes is effective for AR and absorption enhancement. The AR effect is attributed to the tapered shape of nanodome structures, which has a better effective refractive index matching with the air. Comparison of conversion efficiency in the nanodome solar cell

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 189 and the flat film solar cell is shown in Figure 2-11. The nanodome device shows an efficiency of 5.9% while the flat device exhibits an efficiency of 4.7%.

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Figure 2-10. Integrating sphere measurement results of absorption under normal incidence (a): 30° angle of incidence (b), 60° angle of incidence (c) [17].

Source: Jia Zhu, Ching-Mei Hsu, Zongfu Yu, Shanhui Fan and Yi Cui, Nano Lett., 2010, 10 (6), pp 1979–1984. Figure 2-11. (a) Photographs of nanodome solar cells (left) and flat film solar cells (right). (b) Dark and light I-V curve of solar cell devices for nanodomes (left) and flat substrates (right) [17].

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3. ANTI-REFLECTION STRUCTURES USING REPLICATION TECHNIQUES 3.1. Nanoimprint in Anti-Reflection Application

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Nanoimprint lithography (NIL), which was first demonstrated by Chou et al. [19], is a lithography technique performed by pressing the patterned mold so that it makes contact with the polymer resist directly. The patterns on the mold will transfer to the polymer resist without any exposure source. Therefore, the diffraction effect of light can be ignored and limitation is dependent only on the pattern size of mold rather than the wavelength of exposure light. This technology provides a different way to fabricate nanostructures with easy processes, high throughput, and low cost. It is capable of replicating patterns with a linewidth below 10 nm in a parallel manner [20]. The methods of nanoimprint include mold fabrication, resist, material developments, and variations of NIL processes, which are summarized in the recent review [21]. Nanoimprint can not only create resist patterns, as in lithography, but can also imprint functional device structures in various polymers, which can lead to a wide range of applications in electronics, photonics, data storage, and biotechnology [22]. It was believed that the nanoimprint technique is well suited for the pattern transfer of sub-wavelength structures because they can be easily produced over a large area at low cost.

Figure 3-1. SEM image of Ni–Co mold after electroforming process (a), PMMA films with SWSs after the hot-embossing process at different molding temperatures of (b) 110°C; (c) 135°C; (d) 160°C [23]. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Enhancement of Solar Cell Performance Using Surface Morphology Modification 191 It’s an easy process to fabricate polymer sheets with sub-wavelength structures (SWS) using direct NIL [23-25]. Ting et al. demonstrated a feasible low-cost way to fabricate SWS PMMA films with large area [23]. The NIL molds were fabricated by a special electron cyclotron resonance plasma process with large area directly on silicon substrates. In order to increase the mold robustness, the Ni-Co metal was deposited on the silicon mold surface by electroplating process. The thermoplastic PMMA films, which had a glass transition temperature in the range of 100-110°C, were used as the workpieces in the hot-embossing

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step. The SEM image of Ni-Co mold and PMMA films with SWSs after the nanoimprint process at different molding temperatures are shown in Figure 3-1. The diameters of the nanostructure holes in the Ni–Co mold were from 100 to 150 nm and the heights were from 600 to 800 nm. When the molding temperature was increased to 135°C, PMMA with a high mobility began to flow into the holes of Ni-Co mold, therefore, the aspect ratios of nanostructures were increased to 2.6-3. The reflectivity of the PMMA films with SWSs was measured and was shown in Figure 3-2. The reflectivity of PMMA without SWSs was about 4.25–4.5% at the wavelength from 400 to 800 nm. The reflectivity of the PMMA films fabricated at the hot-embossing temperature of 160°C was lower than 0.5% with a high aspect ratio because the shapes of tapered angles could absorb the lights and decreased the reflectivity dramatically [23].

Source: Chia-Jen Ting, Meng-Chi Huang, Hung-Yin Tsai, Chang-Pin Chou1, and Chien-Chung Fu, Nanotechnology 19 (2008) 205301. Figure 3-2. The reflectivity of 2 mm thick PMMA sheets with/without SWS fabricated with different molding temperatures from 110 to 160°C [23].

The polymer sheets with SWS were also used to coat on the transparent substrate to reduce light reflection and to increase light transmission. For example, Linn et al. reports a simple self-assembly technique for fabricating AR coatings on glass substrate by replicating nipple arrays [24]. The mold was fabricated by spin-coating the nonclose-packed colloidal crystals. As shown in Figure 3-3, the polymer (ethoxylated trimethylolpropane triacrylate)

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nipple arrays coated glass slide was made. The resulting reflection shows significantly lower reflectivity (below 1% for the whole visible spectrum [24]) than that of a flat control sample.

Source: Nicholas C. Linn, Chih-Hung Sun, Peng Jiang, and Bin Jiang, Applied Physics Letters 91, 101108 (2007).

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Figure 3-3. Schematic illustration of the templating procedures for making moth-eye antireflection coatings [24].

Many studies have already shown that NIL is suitable for the replication of large-area AR structures. The polymer sheets with SWS fabricated by NIL were acted as etching mask to transfer SWS onto the substrate such as silicon dioxide [26], silicon [27], and AlInP [28]. Because only the first mask fabrication step requires more expensive lithographical processing such as electron beam lithography and two-beam laser interference photolithography, NIL provides a simple, low cost, and also fast method in creating a large variety of nanostructured materials on different substrates. A silicon dioxide surface consisting of periodically arranged nanopyramids was reported in Ref. [26]. The gap formed between the pyramids has an effect on the overall reflection, and it could be modified by NIL process. When the pyramids are side by side, the effective refractive index formed in the modified region is changed gradually from the refractive index of the base material to the refractive index of the surrounding material. However, a gap between the pyramids leads to a step in the gradual refractive index change. Effect of gap width between the pyramids on the reflectance on two incident angles and for three gap widths is shown in Figure 3-4 [26]. Here a is the base of the pyramids, the height h = 550 nm and the period d = 300 nm. The reflectance is calculated for normal incidence and for the incident angle at 40°. If the width of the gap is on the order of a couple of tens of nanometers, the effect on the reflectance is not yet significant. However, a gap with a width larger than 100 nm will increase the reflectance significantly.

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Source: B P¨aiv¨anranta, T Saastamoinen, and M Kuittinen, Nanotechnology 20 (2009) 375301. Figure 3-4. Effect of gap width between the pyramids on the reflectance on two incident angles and for three gap widths of 0, 50 and 100 nm corresponding to the pyramid bases of 300, 250 and 200 nm. The considered grating period is 300 nm [26].

Patterning the surface has certain advantages such as good AR properties in a broadband and a wide incident angle range, which is important in the application of the sunrise-to-sunset solar cells. Chen et al. fabricated the subwavelength scale AR moth-eye structures on silicon using a wafer-scale nanoimprint technique [27]. This high-quality imprinted pattern was achieved to cover with the entire 4 inch wafer, proving NIL can provide a high-quality nanoscale pattern with high throughput and low cost. An excellent AR property extended to large incident angles is shown in Figure 3-5 [27]. The reflection is less than 4% over the entire usable absorption range of crystalline silicon at an angle of incidence up to 45°. When the incident angle is larger than 45°, the average reflection increases rapidly with the incident angle because the average refractive index profile from air to silicon substrate changes too much from the case at normal incidence, but average reflection still less than 8% at 60°. Such a low-cost process for fabricating high-aspect-ratio silicon nanocones has a considerable potential for the all color, sunrise-to-sunset AR coatings for solar cells [27].

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Source: Q. Chen, G. Hubbard, P. A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang, and S. Abbott, Applied Physics Letters 94, 263118 (2009).

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Figure 3-5. Measured angle-dependent reflection spectra from NS1 at 10°, 15°, 30°, 45°, and 60°. The spectrum at 15° overlaps with that at 10° [27].

These SWS coatings also find important technological applications in optical devices and solar cells. For example, because AlInP has a very high band gap and high transparency, it is usually added as a front window on III-V multi-junction solar cell in order to passivate the emitter. Tommila et al. demonstrated NIL moth-eye antireflection coatings on a molecular beam epitaxy (MBE) grown AlInP/GaAs structure [28]. The moth-eye nanopatterns were fabricated by means of soft UV-NIL. The master template consisted of nanocones in a square array with a period of 300 nm, height of 190 nm and base diameter of 130 nm. It used SiNx as an etch mask for AlInP etching. The cone pattern was transferred from imprinted UV-NIL resist to the SiNx using dry etching and inductively coupled plasma reactive ion etching was utilized for the AlInP etching. The dimensions were tuned by changing the etching conditions as shown in Fig. 3-6 [28]. At normal incidence, the structures exhibited an average reflectivity of 2.7% measured in a spectral range of 450–1650 nm. Photoluminescence measurements of the emission from GaAs substrate suggested that the optical losses associated with the moth-eye pattern were low. Therefore, NIL offers a cost-effective approach to fabricate broadband AR coatings required in III–V high-efficiency multi-junction solar cells.

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Enhancement of Solar Cell Performance Using Surface Morphology Modification 195

Source: J. Tommila, V.Polojarvi, A. Aho, A.Tukiainen, J.Viheriala, J.Salmi, A. Schramm, J.M. Kontio, A. Turtiainen, T.Niemi, and M.Guina, Sol. Energy Materials and Sol. Cells 94 (2010) 1845-1848. Figure 3-6. SEM images of the AlInP nanocones [28].

Solar cells usually need a protective layer to protect them from external shocks and corrosion. This protective layer causes the reflection of sun light, which reduces the conversion efficiency. It is useful to reduce the reflection of transparent material using surface AR layer [18, 23, 24, 26, 28] as well as applying the AR layers to solar cells [29, 30]. Han et al. fabricated AR coating by NIL on polyvinyl chloride (PVC) films [29] and glass plates [30] as the protective layer on solar cell. Figure 3-7 shows the SEM image of Ni master mold, replicated PVC template, and imprinted glass plate [30]. The transmittance values were improved from 91% (bare glass plate) to 93% (single-side) and 95% (both-side patterned glass plates) in overall visible range by suppressing the reflection at the surface. Therefore, the solar cells conversion efficiency showed up to 2.5% increase with this protective layer [30].

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Source: Kang-Soo Han, Ju-Hyeon Shin, Heon Lee, Sol. Energy Materials and Sol. Cells 94 (2010) 583– 587. Figure 3-7. (a) SEM images of the Ni master mold, (b) replicated PVC template and (c) imprinted pattern on a glass plate [30].

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To reduce the reflection at the surface of concentrated photovoltaic device, a nanometer scale dot-pattern array was formed on the surface of GaInP/Ga(In)As/Ge solar cells using the NIL techniqu [31]. It formed a moth-eye pattern on GaAs tandem solar cells with a PVC mold. This technique is suitable to non-planar surface of GaAs, which has a front electrode. The GaAs based tandem type solar cells structure, which was consisted of front electrode, AR layer, GaAs based multi-junction and back electrode, as shown in Figure 3-8 [31]. The average reflection in the wavelength range from 300 to 850 nm of GaAs solar cell was decrease from 14.34% to 9.08% using the AR polymer sheet. By this way, the decrease of reflectance will be directly related to the increase of transmittance and the enhancement of conversion efficiency for GaAs solar cells. Consequently, the total conversion efficiency of GaAs cell was increase from 27.77% to 28.69% at 100 sun by the moth-eye AR structures on the surface [31].

Source: K.-S. Han, et al., Sol. Energy Materials and Sol. Cells (2010), doi: 10.1016/j.solmat. 2010.04.064. Figure 3-8. Schematic diagrams of solar cell structure (A) and the imprinting process (B) [31].

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3.2. Anti-Reflection Structures by Using Spin-Coating Replication / HotEmbossing Techniques

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Spin-coating replication/hot-embossing method had been employed to reproduce the AR structure [32-34]. As mention in Ref. [32], the patterned PMMA layer was peeled off from the Si template and directly transferred onto the roughness surface of a poly-Si P-N junction solar cell device to serve as the AR layer. It provides a simple and low-cost means for largescale use in the production of AR layers for improving solar cell performance. The flow charts of the mold fabrication and spin-coating replication/hot-embossing NIL processes are shown in Figure 3-9. A silicon wafer was used as the substrate for the hot-embossing NIL mold fabrication. A 50 nm SiO2 was deposited on the surface as the mask of the wet etching. The sample was then coated with resist and exposed to the electron beam. The patterns were first chemically etched in a solution mixture of HF and NH4F (1:6) to transfer the pattern on the SiO2 mask. Then, the entire mold was dipped into KOH solution. Due to the transverse etching of the acid solution and the anisotropic etching of KOH solution, the wet etching process resulted in different tapered nano-post arrays. The spin-coating replication/hotembossing techniques and a home-made pneumatic nanoimprintor were used to transfer the patterns on the molds to a PMMA layer on a Si template. A layer of PMMA was spin-coated on the mold. The substrate and mold were then combined and placed on the sample stage in the nanoimprintor. The PMMA films were hot-embossed by increase temperature and pressure. The pressure was relieved and the sample was de-molded after the stage has cooled down to room temperature.

Figure 3-9. Steps used for (a) the mold fabrication and (b) the spin-coating replication/hot-embossing processes [32]. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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The resulting patterns on the PMMA layers after the imprinting processes are shown in Figure 3-10. The nanostructures after the NIL have the same depth as the molds. The ability of the PMMA to fill completely the molds when it was spin-coated on the mold accounts for this. In Figure 3-11, the reflectance spectra and the comparisons of AR performance of the nanostructured PMMA layers with bare Si are shown. In contrast to the AR measurements from the molds, the nanostructured PMMA layer formed by the mold with a pitch of 1500 nm now has the lowest reflectivity for the entire wavelength range. In general, the tapered cone structure with a very steep angle and a polygon base can lead to the gradually and continuously change of the refractive index profile from the air to the substrate. Therefore, it can greatly reduce the reflection from the surface. Finally, the PMMA layers were peeled off from the Si templates and directly transferred onto the poly-Si solar cell roughness surface. The poly-Si solar cell with PMMA thin film was characterized under the Air mass 1.5 global (AM 1.5G) illumination condition and was compared to the cell that did not undergo the antireflection treatment. The measured current-voltage characteristics are shown in Figure 312. The short-circuit current was enhanced by 23% due to the PMMA nanostructured film. The light conversion efficiency of the solar cells was improved from 10.4% to 13.5% with the use of PMMA thin film as the AR layer [32].

Figure 3-10. SEM images of the replicated PMMA structures with pitches of (a) 700 nm (b) 1000 nm and (c) 1500 nm [32]. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

Enhancement of Solar Cell Performance Using Surface Morphology Modification 199

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Figure 3-11. Reflection spectra of the replicated PMMA structures with different pitches and bare Si [14].

Source: J.Y. Chen and K.W. Sun, Sol. Energy Materials and Sol. Cells 94 (2010) 629–633. Figure 3-12. Current–voltage characteristics of the solar cells with and without the PMMA layer [32].

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4. ALIGNED ZNO NANOROD ARRAYS AS THE ANTI-REFLECTION LAYER 4.1. Nanorods in Anti-Reflection Application

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This section will primarily review some bottom up fabrication techniques used for ARS in the nano-scale. For example, porous films can be deposited by physical vapor deposition. Thin films will grow with decreasing densities, compared to that of the bulk, when deposited at increasingly oblique or non-normal angle of incidence as shown in Figure 4-1 [35,36]. This principle, commonly called glancing angle deposition, is a physical deposition technique under conditions of obliquely incident vapor flux and limited adatom diffusion. It can be used to generate films with low densities, or high porosity, and hence low refractive index. The density, hence the refractive index, can be controlled at each surface or section of the growing film via the obliqueness (shown by the angle a, Figure 4-1d) of the growth flux. Hence, such a porous film can also have a graded refractive index as a function of its thickness (z). Such graded refractive index structures have been shown previously to reduce reflection. A graded refractive index of the film could be achieved by controlling the density, to match even a theoretical profile, say a Gaussian type, increasing the efficiency of the AR structure.

Source: J.J. Steele, M.J. Brett, J. Mater. Sci.: Mater. Electron 18 (2007) 367. S.R. Kennedy, M.J. Brett, Appl. Opt. 42 (2003) 4573. Figure 4-1. Schematic of the glancing angle deposition process showing geometrical shadowing and columnar growth. (a) At the initial stage, adatoms condense and nucleate on the substrate except over a certain portion, which is geometrically shadowed. (b) The resulting film consists of columns that grow out of these nuclei. (c) These columns are inclined in the direction of the vapour source [35]. (d) Schematic showing the variation of film density with the vapor incidence angle [36].

Yu et al. had demonstrated highly oriented indium-tin-oxide (ITO) nanocolumns by electron-beam evaporation with an obliquely incident nitrogen flux [37, 38] and were used to serve as a conductive AR layer for GaAs solar cells. As shown in Fig. 3-2(a), the columns were vertical grown and not adopt any specific direction in the initial formation on a silicon

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substrate. Afterward the ITO columns become uniformly oriented at the end of deposition, following the direction of the incident vapor flux as shown in Figure 4-2(b) and (c). This tapered-column profile is important to the reflective characteristics of the nanostructured ITO layer. Figure 3-2(d) shows uniformly distributed nanocolumns that can be prepared in singlestep deposition, up to an area of 2 cm by 3 cm.

Figure 4-2. SEM images of ITO nanocolumns deposited with obliquely incident nitrogen flux: (a) cross-sectional view of the initial column formation, (b) cross sectional view of the oriented columns at the end of deposition; c) tilted top view of the columns, showing tapered column profiles; d) uniformly distributed nanocolumns that can be prepared in single-step deposition, up to an area of 2 cm *3 cm [38].

The characteristic ITO nanocolumns offered omnidirectional and broadband AR properties for both s- and p-polarizations in the 350–900 nm wavelength range, and the reflection was drastically reduced up to an incidence angle of 70°, as shown in Figure 4-3. The ITO nanocolumns were deposited on the GaAs solar cells as the AR layer. Figure 3-4 shows the measured current-voltage characteristics. The short-circuit current was increased by 18% due to the reduced reflection. The conversion efficiency was enhanced by 28% with the ITO nanocolumn AR layer [38].

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Figure 4-3. (a) Measured and calculated reflectance spectra of ITO nanocolumns with different angle of incidence. (b) Measured reflection spectroscopy at incidence angles of 30° and 70° for both polarizations in the 350–900nm wavelength range [38].

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Source: P. Yu, C.-H. Chang, C.-H. Chiu, C.-S. Yang, J.-C. Yu, H.-C. Kuo, S.-H. Hsu, and Y.-C. Chang, Adv. Mater. 21 (2009) 1618-1621.

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Figure 4-4. Measured I–V characteristics of the GaAs solar cell with and without ITO nanocolumns AR layer [38].

Not only ITO but also other transparent conductive oxide materials, such as TiO2 [39] and ZnO [40,41], can be used to fabricate AR nanostructures. Glancing angle deposition technique was used to produce SiO2 nanorod films with 1.05 ≤ n ≤ 1.46 and TiO2 nanorod films with 1.3 ≤ n ≤ 2.7. To obtain a wider span of graded refractive index, the glancing angle deposition technique can be extended, with a simple switching of source, to produce layered structures of different materials [39]. A graded refractive index layer with 1.05 ≤ n ≤ 2.7 could be obtained by a layer consisted of TiO2 and SiO2 nanorod layers grown by the oblique angle deposition method using the electron-beam evaporation on a one side polished AlN substrate (n = 2.05). It was demonstrated to have near-perfect antireflection characteristics. AlN substrates have been used for the deposition of three layers of TiO2, followed by two layers of SiO2, to achieve a coating with a modified-quintic-index profile, as shown in Figure 4-5(a) [39]. The feature size of individual nanorods is smaller than 50 nm, that is, much smaller than the wavelength of visible light. It had significantly reduced the reflection by its black appearance (Figure 4-5(b)) in contrast to the common substrate materials. The wavelength dependent reflectivity was measured at normal incidence using a spectrophotometer over a wavelength range from 0.3 μm to 2 μm as shown in Figure 4-5(c) [39]. The results demonstrate that the graded-index coating has low reflectivity over the entire visible and near-infrared spectrum. The actual reflectivity values are below 1% for 0.5–1.5 μm wavelength regime, and show a minimum at ~ 0.78 μm that corresponds to the total film thickness.

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Source: Xi, J.-Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M.; Lin, S.-Y.; Liu, W.; Smart, J. A. Nat. Photon. 2007, 1, 176. Figure 4-5. (a) Cross-sectional SEM image of graded index coating, consisting of three TiO2 and two SiO2 nanorod layers, with a modified quintic index profile (inset). (b) Photograph of a gradientrefractive index AR coating on AlN, and specular surfaces of AlN, Si, and Al. (c) Wavelength dependence of theoretical (solid line) and measured (dashed line) reflectivity of the graded index coating at normal incidence [39].

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4.2. Growth of Vertically Aligned ZnO Nanorod Arrays as the Anti-Reflection Layer More recently, ZnO becomes attractive as a dielectric AR layer material because of its good transparency, appropriate refractive index, and ability to form textured coating via anisotropic growth. Various physical, chemical, and electrochemical deposition techniques have been explored to create oriented arrays of ZnO nanorod. Recently, the solution synthesis of ZnO nanorod arrays has been demonstrated as a simple, low temperature, and low-cost method [40-43]. The synthesized process of ZnO nanorod arrays using a two-step seeding and growth method. For example as mention in Ref. [41], the substrates were coated with a ZnO nanoparticle layer by the sol–gel preparation [44] as the seeding layer. First, the zinc acetate dihydrate and monoethanolamine were dissolved in the 2-methoxyethanol solution, which served as the coating solution, at room temperature. The clean substrates were spin-coated with the solution at to ensure a uniform coverage of seeds and then the substrates were heated over 300°C in air atmosphere for 1 h to yield layers of ZnO seeds. The solution for growing the ZnO nanorods was prepared by mixing the zinc nitrate hexahydrate with hexamethylenetetramine using the same molar concentration. After the two solutions were mixed, the substrates were immersed inside the solution at 90°C. After the growth process was completed, the substrates were cleaned in D.I. water to remove the residual salt and amino complex and finally dried in air. the nanorod morphology, controlled through synthetic chemistry such as growth time, spin-coating rates, and solution concentration, has a great effect on the AR layer performance. Figure 4-6 shows the SEM images of the vertically aligned ZnO nanorod arrays on Si substrates with growth time of 120, 180, 240, and 300 min when the growth solution with a molar concentration 0.02 M was used. The arrays consisted of nanorods with diameters of 50–60 nm and the lengths of the rods increased from 80 to 320 nm as the growth time was increased. In Figure 4-7, the reflectance spectra and the comparisons of AR performance of the above vertically aligned ZnO nanorod arrays with bare Si are shown. The figure shows that the ZnO nanorods with growth time of 180 min gave the lowest average reflection throughout the visible range among the others, and the reflectance was about 3%–4% at the wavelength of 667 nm. The increase in reflection as the growth time reached 300 min may be due to the larger inclined angles of the rods [41]. The variations in growth conditions strongly influenced the morphology of the textured ZnO nanorods and had a great effect on the AR layer performance because of the differences in lengths, densities, and diameters of the nanorods. The vertically aligned ZnO nanorod arrays were used to growth on the poly-Si solar cell as the AR layer. The poly-Si solar cell with ZnO nanorods was characterized under the air mass 1.5 global (AM 1.5G) illumination condition and was compared to the cell that did not undergo the AR treatment. The measured current– voltage characteristics are shown in Figure 4-8. The short-circuit current was enhanced by 20% due to the presence of the ZnO nanorod layer. The light conversion efficiency of the solar cells was improved from 10.4% to 12.8% with the use of aligned ZnO nanorod arrays as the AR layer [41].

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Figure 4-6. SEM images of the vertically aligned ZnO nanorod arrays on Si substrates with growth time of (a) 120, (b) 180, (c) 240, and (d) 300 min [41].

Figure 4-7. Reflectance of the solution-grown ZnO nanorods with different growth Time [41]. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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Source: J.Y. Chen, and K.W. Sun, Sol. Energy Materials and Sol. Cells 94 (2010) 930–934.

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Figure 4-8. I–V characteristics of the solar cells with and without the ZnO nanorod arrays AR layers [41].

5. ANTI-REFLECTION PROPERTIES OF SELF-AGGREGATED NANOPARTICLE LAYERS: EXPERIMENTS AND THEORETICAL MODEL Silicon (Si) is the leading material used in the commercial production of low-cost solar cells. The development cost of solar cells can be further reduced by either reducing the manufacturing costs or by increasing the solar cell efficiency. Various cost-competitive, relatively cheaper processing steps have been developed to make these solar cells more energy efficient. Due to the fact that nanoparticles have the characteristics of light-harvesting [42-45], plasmon resonances, and enhanced scattering [46], an appropriate and well-operated application of nanoparticles on the solar cells can therefore lead to high cell utility efficiency and provide low-cost production. Various solar cell substrates with nanoparticle deposits have been confirmed to enhance photocurrent [47] and acceptance angles [48], and to improve the reflection of a specific wavelength (700 nm -1100 nm) [49]. Recently, the scattering of incident light by nanoparticles has enabled the improved transmission of photons into the semiconductor active layers and the coupling of normally incident photons into the lateral, optically confined paths within the multiple-quantum-well waveguide layer, resulting in increased photon absorption, photo-current generation, and power conversion efficiency of the solar cells [47]. Improvements in short-circuit current density (Jsc) have also been observed in quantum dot solar cells using silica and Au

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nanoparticles [49]. The scattering provided by the nanoparticles is believed to be essential for coupling and partially trapping light in substrate radiation modes. Although metallic nanoparticles provide efficient light scattering through coupling with surface plasmons, they exhibit extraordinary absorption at the surface plasmon resonances [50], significantly decreasing the overall effect of the metallic nanoparticles [51,52]. Other than the metallic nanoparticles, dielectric nanoparticles with high dielectric permittivity can also be an alternative for sufficient light scattering. More importantly, some of the dielectric materials possess a relatively low dissipation level at visible wavelengths. Recently, the use of dielectric nanoparticles has been demonstrated theoretically to lead to similar and even higher enhancements compared with that of metallic nanoparticles [53]. Most of the previous research presented good nanoparticle performance with regard to flat-surface of devices. However, there is no report on the fabrication or tests on the properties of the textured surface of Si-based solar cell devices incorporated with selfassembled dielectric nanoparticles. In the following section, we demonstrate a comprehensive study of the nanoparticle-enchanced performance of polysilicon solar cells using dielectric nanomaterial (silica nanospheres) with particle sizes ranging from 100 nm to 500 nm. Nanoparticle colloidal crystals were fabricated on the solar cell surface to achieve the optimum anti-reflection property by controlling the spin rate, spin duration, and particle concentration on a custom-built spin coater. Optical properties of the self-assembled nanoparticle layers were also simulated based on the rigorous coupled-wave analysis (RCWA) method. The simulation results were compared with the experimental results.

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5.1. Efficiency Enhancement of Si Solar Cells by Using Self-Assembled Nanoparticles In this work, the poly-Si solar cell was prepared by following commercial fabrication processes. The p-type Si wafers were roughened using HF and HNO3 saw damage etch with no additional texturization. A 200 nm n-type layer was created on the texture by POCl3 diffusion using a centrotherm tube furnace to form the p-n junction, followed by the depositing of a thin anti-reflection layer of 80 nm SiNx using a centrotherm direct plasma PECVD furnace. The front and rear side finger and bus bar contacts were screen-printed with a standard, commercially available lead containing Ag paste, Al paste, and Ag+Al paste using a semi-automatic ATMASC 25PP printer. The cells were fired using a fast firing conveyor belt furnace at an optimal firing temperature of 850 oC to make the fingers and bus bars come into contact with the N- and P-type regions for maximum performance. Finally, the cell edges were isolated using a 532 nm Nd:YAG laser cutting tool. The schematic of the device structure with the self-assembled Silica nanoparticles (SNPs) is shown in Figure 5-1(a)-(b). SNPs purchased from the Golden Innovation Corp. with diameters of 100, 250, and 500 nm were first dispersed in a surfactant (mixture of EG/Ethanol/DI with a ratio of 35:35:30 by volume) to make a 10 wt% suspension solution. The nanoparticles were patterned on top of the commercially processed poly-Si solar cells mentioned above with a light absorption area of 1.5×1.5 cm2 using a custom-made spin coater. The spin coating process began at a spin rate of 350 rpm for 1 min, followed immediately by a faster spin rate of 3500 rpm for 30 s. The SEM image in Figure 5-2 indicates that the nanoparticles self-assembled into closely packed structures on the cell

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surface because of the van der Waals force [54] when the spin rates and the concentration of the SNP suspension solution in the recipe were properly adjusted.

Fig. 5-1. (a) The schematic of the top view from a poly-Si solar cell with silica nanoparticles coated on the light illuminating surface (1.5x1.5cm2) (b) side view of the device.

All the solar cell devices with and without the SNP treatment were evaluated at room temperature based on the illuminated current density versus voltage (J-V) characteristics, the external quantum efficiency (EQE), and the reflectance. The photocurrent was analyzed using a solar simulator under the Air Mass 1.5 Global (AM 1.5G) illumination condition (100 mW/cm2, 25 oC). The EQE was measured using an AM 1.5G standard spectrum and an Optosolar simulator (SR-150). The reflectance spectra of the samples were recorded using a UV-Visible-NIR spectrophotometer (Hitachi U-4100) for wavelengths ranging from 200 nm to 1200 nm.

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Figure 5-2. The SEM image of the self-assembled SNPs on the textured solar cell surface.

The enhanced light absorption, and hence the efficiency of light scattering into the silicon active layer, depends on the nanoparticle diameter, percentage surface coverage, and material permittivity. Based on earlier reports [52,53] on maximizing the photocurrent response of solar cells, to achieve the best anti-reflection property from the self-assembled SNPs, there should be certain combinations of the diameter and areal density of the SNPs. To determine the best combination, we experimented on SNPs with diameter values of 100, 250, and 500 nm and with different areal densities of SNP done by controlling the nanoparticle dose (i.e., 125, 250, 500, and 1000 μl, respectively). Figure 5-3 (a)-(b) shows the enlarged SEM images of the self-assembled SNP with particles sizes of 100, 250 and 500 nm at a fixed dose of 1000 μl. As shown in Figure 5-3(a), the SNP with a size of 100 nm formed a monolayer coverage through out the entire textured surface. For the 250 nm SNPs, the self-assembled structure was monolayered near the top of the textured surface but it had a multilayered structure at the bottom. However, for the 500 nm SNPs, the textured surface was almost filled by those particles due to their larger size. The flatter surface also increased the reflectance, which will be discussed later. As the particle dose was increased beyond 1000 μl, the surface became semi-opaque.

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Figure 5-3. The enlarged SEM images of the deposited SNPs on textured surface of cell with particle sizes of (a) 100nm, (b) 250nm and (c) 500nm at a nanoparticle dose of 1000 μl. The areal densities are ~ 2.51x1013, 1.61x1012 and 2.01x1011 in (a), (b) and (c), respectively.

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The maximum enhancement of the absorbed power in the poly-Si solar cells obtained after optimization of the size and surface coverage of SNPs is given in Figure 5-4. The best overall power conversion efficiency, η, increased from 11% to 12.3%. The increase in conversion efficiency of the solar cells with the SNP treatment were 1.3 %, 0.85 %, and 0.6 % for the 100, 250, and 500 nm sizes, respectively, at a dose volume of 250 μl. The efficiency increase of 1.3 % was achieved when the solar cell surface was covered with a monolayer of 100 nm silica nanoparticles with an areal density of around 1.1x1013/cm2. However, the enhancement factors began to drop when the nanoparticle dose was increased. This phenomenon was attributed to the substantial accumulation of nanoparticles, which eventually led to the self-assembled multilayer structures and flatter surface of the cell.

Figure 5-4. The gains in conversion efficiency of polysilicon solar cells after optimizing the size and coverage of SNPs.

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Figure 5-5 shows the current-voltage (J-V) characteristic of the solar cell at maximum enhancement. The performance of the cell without the SNP treatment is also presented in parallel for comparison. In the figure, the short-circuit current density (Jsc) increased from 36.1 mA/cm2 to 37.4 mA/cm2 with the 100 nm SNPs deposited on the textured surface. Note that the open-circuit voltage (Voc) and fill factor (FF) remained unaffected. Therefore, the enhancement of cell efficiency is most likely due to the increase in photocurrent and light absorption. The reflectance spectra of the solar cells coated with SNPs of different sizes for maximum enhancement are plotted in Figure 5-6. The reflectance spectrum of cells without SNPs is also displayed in parallel for comparison. The reflectance was reduced in the UV and IR regions for cells coated with self-assembled SNPs, whereas the change in reflectance in the visible region was insignificant. The EQE of the poly-Si solar cells coated with different sizes of self-assembled SNPs were measured relative to the cells without the SNP treatment, as shown in Figure 5-7. Clearly, the increase in photocurrent is due to the enhanced absorption of light in the UV and NIR regions by the active layer. The EQE measurements are in agreement with the reflectance results that light harvesting is improved in the wavelength ranges of UV and NIR due to the enhanced scattering by the self-assembled SNPs. In the best case, the EQE of the Poly-Si solar cells was improved by over 40% and 56% in the UV and NIR regions, respectively.

Figure 5-5. The J-V characteristics of the solar cells coated with the 100 nm SNPs in compared to devices without the nanoparticles.

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Figure 5-6. The reflectance spectra of solar cells at maximum performance with coated particle sizes of 100 nm, 250 nm and 500 nm. The spectrum of an untreated device is also presented for comparison.

Figure 5-7. The external Quantum efficiency measured under AM 1.5G illumination for solar cells at maximum performance with coated particle sizes of 100 nm, 250 nm 500 nm, and untreated. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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5.2. Modeling of Reflecttion Propertties of Self-A Aggregated d Silica N Nanoparticle e Layers

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mulated the suurface reflectaance of the To comparre with the exxperimental reesults, we sim ceells using the RCWA methhod based on a three-dimeensional modeel built accordding to the paarameters useed in this expperiment and in the SEM M images. Thee deposited SNPs S were asssumed to be spherical, close-packed, annd distributed in a square arrrangement onn top of the SiiNx layer. Perriodic and maatched electrom magnetic bouundary conditiions were appplied to the m model to acco ount for mulltiple scatteriing and cross-coupling off nearby nannoparticles. R Reflectance of the solar cell was computeed over the AM M 1.5G spectrrum, and the subsequent noormalization of the value was calculatted for the reeference surfa face without SNPs. S The coomparisons beetween measurrements and simulations s in the UV and NIR N regions arre shown in Fiigures 5-8 an nd 5-9. The simulated refflection of thhe surface cooated with SN NPs agrees reeasonably well with the experimental specctrum results.

Fiigure 5-8. The comparison c of the t simulated annd measured refflectance spectrra of the SNP trreated solar ceells in the UV reegion.

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Fiigure 5-9. The comparison c of the t simulated annd measured refflectance spectrra of the SNP trreated solar ceells in the NIR region. r

A simple interpretation of our findinggs is given as follows. Thee scattering cross-section (σ σsca) and absorrption cross-seection (σabs) of o a sub-wavelength sphericaal particle cann be written ass [53]

σ sca =

1 6π

4

2 (ε - 1) 1 2π ⎛ 2π ⎞ is the Ιm (α ) , where α = 4πR 3 ⎜ ⎟ α , and σ abs = λ ε +2 ⎝ λ ⎠

poolarizability of o the particlle. The particcle polarizabiility, α, contaains the denssity of the naanoparticle an nd the dielectrric function. Higher H order dipoles d appearr in nanopartiicles as the paarticle size gro ows and the dielectric d functtion changes. Therefore, thee scattering crross section off nanoparticlees increases with w the largerr volume and dielectric funnction. Althouugh the 500 nm m particle hass the largest polarizability p a among the thrree due to its larger size. However, H in thhe UV region,, the 100 nm particles are better “seen”” by the shortter wavelengthh and have m more chances to t scatter lightt. Therefore, the t surface cooated with 1000 nm particless shows the

1

loowest reflection because the σsca is dominated d byy the factor ( )4 . The monolayer

λ

diistribution of the 100 nm particles p also contributes c to the reduced surface s reflecttion. In the N region, thee σsca is determ NIR mined by the morphology m of nanoparticlee distribution because b the w wavelength is no longer a dominant facctor. Again, SNPs S of 100 nm in diameeter with a m monolayer distrribution give the t lowest refllectance than those t with muultilayered struuctures.

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CONCLUSION AND FUTURE PERSPECTIVES An untreated silicon solar cell only absorbs 67.4 percent of sunlight shone upon it — meaning that nearly one-third of that sunlight is reflected away and thus unharvestable. From an economic and efficiency perspective, this unharvested light is wasted and it creates a major barrier hampering the proliferation and widespread adoption of solar power. Availability of robust and reliable coatings is crucial for advancing the performance of PV cells. By developing a new AR coating that boosts the amount of sunlight captured by solar cells, research works has moved closer to realizing high-efficiency, cost-effective solar power. Applying an AR coating to the surface of PV cells will reduce these reflections and increase the module’s output power. Current commercial PV technologies convert 10%-20% of the incoming light to electricity. The same module with a suitable AR coating can deliver an additional 1 % or even higher power conversion. A product achieving higher conversion efficiency in a cost-effective manner can make solar modules more affordable.

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[43] D.M. Schaadt, B. Feng, E.T. Yu, Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles, Applied Physics Letters, 86 (2005) 063106. [44] S.H. Lim, W. Mar, P. Matheu, D. Derkacs, E.T. Yu, Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles, Journal of Applied Physics, 101 (2007) 104309. [45] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, Surface plasmon enhanced silicon solar cells, Journal of Applied Physics, 101 (2007) 093105. [46] H.R. Stuart, D.G. Hall, Absorption enhancement in silicon-on-insulator waveguides using metal island films, Applied Physics Letters, 69 (1996) 2327-2329. [47] D. Derkacs, W.V. Chen, P.M. Matheu, S.H. Lim, P.K.L. Yu, E.T. Yu, Nanoparticleinduced light scattering for improved performance of quantum-well solar cells, Applied Physics Letters, 93 (2008) 091107. [48] C.-P. Chen, P.-H. Lin, L.-Y. Chen, M.-Y. Ke, Y.-W. Cheng, J. Huang, Nanoparticlecoated n-ZnO/p-Si photodiodes with improved photoresponsivities and acceptance angles for potential solar cell applications, Nanotechnology, 20 (2009) 245204. [49] C.O. McPheeters, C.J. Hill, S.H. Lim, D. Derkacs, D.Z. Ting, E.T. Yu, Improved performance of In(Ga)As/GaAs quantum dot solar cells via light scattering by nanoparticles, Journal of Applied Physics, 106 (2009) 056101. [50] C.F.B.a.D.R. Huffman, Absorption and Scattering of Light by Small Particles, WileyInterscience, New York, (1983). [51] K. Nakayama, K. Tanabe, H.A. Atwater, Plasmonic nanoparticle enhanced light absorption in GaAs solar cells, Applied Physics Letters, 93 (2008) 121904. [52] Y.A. Akimov, W.S. Koh, K. Ostrikov, Enhancement of optical absorption in thin-film solar cells through the excitation of higher-order nanoparticle plasmon modes, Opt Express, 17 (2009) 10195-10205. [53] Y.A. Akimov, W.S. Koh, S.Y. Sian, S. Ren, Nanoparticle-enhanced thin film solar cells: Metallic or dielectric nanoparticles?, Applied Physics Letters, 96 (2010) 073111. [54] Y. Lalatonne, J. Richardi, M.P. Pileni, Van der Waals versus dipolar forces controlling mesoscopic organizations of magnetic nanocrystals, Nat Mater, 3 (2004) 121-125.

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Chapter 9

IMPROVING THE EFFICIENCY AND STABILITY OF TIO2 AND ZNO CELLS SENSITIZED WITH LOW-COST ORGANIC DYES Myrsini Giannouli* Author Affiliation: Energy and Environment Laboratory, Physics Department, University of Patras, 26500 Patras, Greece

ABSTRACT

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The most efficient sensitizers for wide bandgap semiconductors are metallo-organic ruthenium complexes due to their high charge-transfer to TiO2 and light absorption in the visible spectrum. Ruthenium complexes however, are expensive and their use renders the resulting solar cells costly. Several simple organic dyes, such as xanthene dyes also yield satisfactory efficiencies, especially when used for sensitizing ZnO films. These dyes are inexpensive and do not rely on the availability of precious metals such as ruthenium. They also have high extinction coefficients and their molecular structures contain adequate anchoring groups to be adsorbed onto the oxide surface. However, solar cells developed using simple organic dyes tend to have drawbacks, such as low long-term stability. In this chapter, several of the parameters affecting the efficiency and stability of photovoltaic cells sensitized with simple organic dyes are investigated and an attempt is made to improve the stability and overall performance of these cells. To this aim, the characteristics of various nanostructured thin films used in dye-sensitized solar cells are examined. Parameters such as the morphology of the films are considered, as these factors greatly affect the efficiency and stability of dye-sensitized solar cells. Experimental results for dye-sensitized solar cells are presented in order to examine some of the major factors affecting the efficiency and the stability of such cells. Nanostructured ZnO, TiO2 as well as composite ZnO/TiO2 thin films were prepared and sensitized using simple organic dyes. Novel multi-component electrolytes for dye*

Tel: +30 2610 997449, e-mail: [email protected]

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Myrsini Giannouli sensitized solar cells were also developed. The effects of these electrolytes on the efficiency and stability of the cells were investigated and it was found that the combined properties of the materials used in these electrolytes enhance cell efficiency and stability.

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INTRODUCTION In dye-sensitized solar cells the conversion of visible light to electricity is achieved through the spectral sensitization of wide bandgap semiconductors. Light is absorbed by dye molecules, which are adsorbed on the surface of the semiconductor, thus inducing charge separation. Excitation of the dye molecules results in electron injection into the conduction band of the semiconductor. For electron injection to occur, the excited electrons must be at higher energy level than the semiconductor conduction band. An electrolyte of high ionic strength is also used in dye-sensitized solar cells to facilitate charge transfer across the device. Dye sensitized solar cells, with highest efficiencies approximately 11%, (Gao et al., 2008) have gained attention over the last years and considerable progress towards improving their performance has being made. Novel electrolytes for dye-sensitized solar cells are being developed, which enhance cell efficiency and stability (Wang et al., 2010; Giannouli et al., 2010). In addition, recent studies have demonstrated the ability to replace the platinum in the photocathode by cobalt sulphide, which is more efficient and more stable (Wang et al. 2009), while highly-efficient photocathodes have been developed for tandem dye-sensitized solar cells (Nattestad et al., 2010). Scientists have also achieved a record light conversion efficiency of 8.2% in solvent-free dye-sensitized solar cells (Bai et al., 2008). The development of dye-sensitized solar cells of appreciable efficiency without the use of solvents will enable in the future the construction of large scale, inexpensive, flexible solar cells that are stable over long periods of light and heat exposure. The development and optimization of solar cells is of great interest, both commercially and scientifically. A large number of private companies produce solar cells and many of them specialize in the commercialization of new solar cell technologies, such as dye sensitized solar cells. However, dye sensitized devices are still not commercially available in large volumes. Disadvantages such as the low efficiency and stability of these cells pose a hindrance to their commercialization. Research studies that aim to reduce these drawbacks and optimize dye sensitized solar cells are very useful to the industry and pave the way for the production of high efficiency devices with low production costs. In this chapter, experimental results for dye-sensitized solar cells are presented in order to examine some of the major factors affecting the efficiency and stability of such cells. Nanostructured ZnO, TiO2 and composite ZnO/TiO2 thin films were prepared for this purpose. These films were sensitized with simple organic molecules, such as Rhodamine B, Rose Bengal and Coumarin 343. Several of the parameters affecting the stability of photovoltaic cells sensitized with organic dyes were examined and an attempt was made to improve the stability and overall performance of these cells. Novel types of electrolytes were also developed, which contain both propylene carbonate (PC) and ethylene glycol (EG) as solvent. The effects of the combined properties of the materials used in these multicomponent electrolytes on the efficiency and stability of the cells were investigated.

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METHODOLOGY

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The main aim of this work is to examine the characteristics of nanostructured thin films used in dye-sensitized solar cells. Parameters such as the morphology of the films were investigated, as these factors greatly affect the efficiency and stability of dye-sensitized solar cells (Rao and Dutta, 2008). To this aim, nanostructured ZnO, TiO2 and composite ZnO/TiO2 films were prepared as described in (Giannouli et al., 2010). Specifically, nanostructured thin films were deposited on a transparent conductive oxide (TCO) glass surface. The glass substrates that were used were coated with a fluorine doped tin oxide layer and are known as K-glass (SnO2:F sheet resistance of 16,7 Ω/sq, 80% transmittance in the visible, 0.38 cm glass thickness). Prior to film deposition the conductive glass substrates (size 2,5 × 3,0 cm2 ) were washed with detergent, rinsed with deionized water and ethanol and dried in an air stream, followed by calcination at 150oC for 1 hour. Commercial ZnO or TiO2 nanopowder (Aldrich) was used in order to create a colloidal paste that was spread on the glass surface. The powder was mixed with a small amount of distilled water containing acetyl acetone (10% v/v) (Nazeeruddin et al., 1993; Smestad, 1998) in order to prevent the coagulation of nanoparticles and improve the porosity of the film (Pichot et al., 2000). A small amount of Triton X-100 was added to the mixture to reduce surface tension and enable even spreading of the paste (Liu et al., 2010; Van der Zanden et al., 2000). In addition, composite ZnO/TiO2 films were developed. Previous studies have shown that the use of composite electrodes facilitates charge carrier separation (Tennakone et al., 2002). Composite thin films containing both ZnO and TiO2 were developed by mixing ZnO and TiO2 paste in different quantities. Specifically, 3 parts of ZnO paste were mixed with one part TiO2 in order to obtain a solution with ZnO/TiO2 ratio equal to 75/25 %. After preparation, the semiconductor oxide paste was spread on the conductive glass substrates via a doctor blade technique (see Figure 1). This method allows for the construction of films of varying thickness, which mainly depends on the amount of paste that is deposited on the film and on the concentration of the paste. The thickness of the resulting films was found to vary from 2 to 17 μm, as measured by a stylus XP-1 Ambios Technology profilometer. After air drying, the electrodes were annealed for 30 min at 450oC in air. Sintering enhances the electrical contact between the nanoparticles as well as between the nanoparticles and the conductive substrate (Shklover et al., 1997). The performance of dye-sensitized solar cells depends primarily on the properties of the dye which is used as sensitizer in relation to the semiconductor. The most efficient sensitizers for wide bandgap semiconductors are, in general, the well-known metallo-organic ruthenium complexes due to their high charge-transfer to the semiconductor film and light absorption in the visible spectrum (Grätzel, et al, 1991). Solar cells sensitized with such dyes not only yield high efficiencies, but also present improved stability compared to cells sensitized with simple organic molecules (Chen et al., 2006). Ruthenium complexes however, are expensive and their use renders the resulting solar cells costly. In addition, it has been reported (Guillén et al, 2008), that several simple organic dyes, and especially xanthene dyes (Eosin Y, Rose Bengal, etc.), yield efficiencies comparable to those achieved with ruthenium complexes, especially when used to sensitize ZnO films (Plank et al., 2009; Pradhan et al., 2007). Organic dyes such as these are inexpensive (Kroon et al., 2007), can be easily recycled (Lee et al., 2006) and do not rely on the availability of

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precious metals such as ruthenium. They also have high extinction coefficients and their molecular structures contain adequate anchoring groups to be adsorbed onto the oxide surface. Moreover, they are especially interesting when used in combination with ZnO due to the disadvantages of using Ru-based dyes with this oxide (Keis et al., 2000). The solar cells presented in this chapter were sensitized with cost-effective organic dyes, such as Bengal Rose, Coumarin 343 and Rhodamine B. Coating of the ZnO surface with dye was conducted by soaking the film in a solution of the dye for at least 12 h. The soaking time of the films was optimized according to the film thickness and afterwards any excess dye was washed off using a small amount of methanol. The type of electrolyte used also plays a fundamental part in the conversion efficiency of the solar cell. The electrolytes that were used for the fabrication of the solar cell devices were optimized to contain high ion concentrations and yield high solar cell efficiencies. For the purpose of this study four different types of electrolytes were considered: a standard-type electrolyte containing potassium iodide and iodine in propylene carbonate (PC) and three novel, multi-component electrolytes containing potassium iodide and iodine dissolved in varying mixtures of PC and EG (ethylene glycol). The standard-type electrolyte consists of 0.3M potassium iodide and 0.03M iodine in PC. Potassium iodide has low solubility in PC, which limits its concentration in the electrolyte. In order to increase the solubility of potassium iodide in the electrolyte, ethylene glycol was incorporated in the electrolyte solution. The new electrolytes that were developed contained potassium iodide, dissolved in a small amount of EG. Then, PC was added to the electrolyte in various quantities in order to obtain electrolyte solutions of different PC/EG concentrations. The following composite electrolytes were developed: An electrolyte containing 0.5M potassium iodide and 0.05M iodine in a 80% PC and 20% EG solvent. This electrolyte is henceforth referred to as the 80/20 electrolyte.

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• •

An electrolyte containing 0.5M potassium iodide and 0.05M iodine in a 90% PC and 10% EG solvent. This electrolyte is henceforth referred to as the 90/10 electrolyte. An electrolyte containing 0.5M potassium iodide and 0.05M iodine in a 95% PC and 5% EG solvent. This electrolyte is henceforth referred to as the 95/5 electrolyte.

It has been demonstrated (Giannouli et al., 2010) that the presence of EG in the electrolyte solution increases the solubility of potassium iodide as well as the presence of iodide ions, thus enhancing the current density of the cells. The performance of these electrolytes has been investigated in previous studies only when used in TiO2 cells sensitized with Rhodamine B. In the present chapter, the effectiveness of these novel electrolytes will also be studied when used in ZnO solar cells. The structure of the dye sensitized solar cells that was developed within the framework of this study is shown in Figure 2. The solar cells consist of a nanostructured mesoporous semiconductor film deposited on a glass or a flexible substrate, a platinum or platinised counter electrode and a liquid, gel or solid electrolyte containing a redox couple, which fills the space between the two electrodes. A monolayer of dye is adsorbed on the surface of the semiconductor and acts as sensitizer.

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S Semiconduct tor paste

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Fiigure 1. Doctor blade techniquue.

Counter electrodes e weere prepared through theermal decom mposition of a 5 mM heexachloroplatiinic acid in isoopropanol sollution. The sollution was sprread on SnO2: F covered coonductive glasss substrates (K-glass). Thee sensitized ZnnO electrode and a the counteer electrode w were assembled d as a sandwiich-type cell and a sealed toggether with sillicon. Electriccal contacts w were made on both electroddes using conductive adhessive copper taape. The electtrolyte was innserted in the cell with a syringe throuugh a small aperture, a whicch was then sealed s with siilicon.

S Semiconductor film

Semiiconductor fiilm

Fiigure 2: Structu ure of a dye senssitized solar celll.

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a) TiO2

b) ZnO

c) ZnO/TiO2 Figure 3. SEM images of a) ZnO, b) TiO2 and c) ZnO/TiO2 films.

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The properties of the solar cells were studied, with emphasis on their efficiency and stability. To this aim, the devices were illuminated with a 50 W halogen lamp, which provides illumination intensity equal to 708 W/m2. Current – Voltage (I – V) curves of the cells were obtained, using a computer-controlled AMEL Function Generator. The most important characteristics of the solar cells can be obtained from these I–V curves, such as the open circuit voltage, the short circuit current, the fill factor and the efficiency of the cell. The aforementioned properties were measured at regular intervals after the preparation of the cells, in order to evaluate the stability of the devices over time. Dark current measurements were also conducted for TiO2, ZnO and composite ZnO/TiO2 solar cells. Dark current arises when triiodide ions from the electrolyte draw electrons from the semiconductor, reducing the triiodide to iodide. Since electrons are removed from the semiconductor, the overall current produced in the photovoltaic cell is reduced by a small amount (Xu et al., 2007).

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RESULTS AND DISCUSSION The morphology of the ZnO films was determined using Scanning Electron Microscope (SEM) images of the semiconductor films. Figure 3 shows examples of typical TiO2, ZnO and ZnO/TiO2 films developed throughout this study. A TiO2 film is presented in Figure 3a, showing, in general, an even surface of TiO2 nanoparticles. Small cracks, such as the one appearing at the bottom end of this figure, appeared on the surface of some of the films. These cracks were formed in TiO2 films mainly during the drying process of the film, before sintering. Film thickness also plays an important role in the formation of cracks, as thicker films tend to have more cracks. It was observed that the presence of cracks on the surface of the films results to a decrease in the efficiency of the cells. This occurs mainly because the formation of cracks on the film’s surface reduces electrical contact between TiO2 nanoparticles. Also, if the cracks reach to the conducting substrate, electron recombination will be increased due to contact of the electrolyte with the conducting substrate. For the above reasons, TiO2 films were developed via a process that limits the formation of cracks on the surface of the films (Syrrokostas et al., 2009). Figure 3b shows a SEM image of a typical ZnO film developed during this study. From this figure it can be observed that the films have high porosity and consist of a fairly uniform layer of ZnO nanoparticles. Finally, Figure 3c shows a composite ZnO/TiO2 film with ZnO/TiO2 ratio equal to 75/25%. Several white spots can be observed throughout the surface of this film. The elemental composition of these spots was identified through SEM analysis and it was found that they consist of ZnO nanoparticles. Darker areas that appear on the surface of the film were found to consist of TiO2 nanoparticles. Figure 4 presents examples of the Current – Voltage characteristics (I-V curves) of ZnO films sensitized with Rose Bengal, Rhodamine B and Coumarine 343. The results displayed in this figure are representative of the majority of the cells prepared during the present study. From Figure 4 it can be observed that the voltage does not vary considerably for each type of sensitizer used. The current density however, was consistently higher for cells sensitized with Coumarine 343 than for corresponding cells sensitized with Rhodamine B or Rose Bengal. Cells sensitized with Rhodamine B had the lowest current densities and, as a result, yielded

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the lowest efficiencies. In general, the current density of cells sensitized with Rhodamine B was found to be 20-25% lower than that of cells sensitized with Rose Bengal and 25-30% lower than that of cells sensitized with Coumarine 343.

2,5

Rhodamine B Rose Bengal Coumarine 343

[Current density [mA/cm2]

2

1,5

1

0,5

0 0

0,1

0,2

0,3

0,4

0,5

0,6

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Voltage [v] Figure 4. Current-Voltage characteristics of ZnO cells sensitized with Rhodamine B, Rose Bengal and Coumarin 343.

As mentioned above, the type of electrolyte used affects considerably the performance of solar cells. The aforementioned composite electrolytes were compared against a standard-type electrolyte, for both ZnO and two TiO2 solar cells. The Current-Voltage characteristics of two ZnO cells, one filled with the 95/5 electrolyte and the other with the standard electrolyte, are shown in Figure 5. Also the performance of a TiO2 cell filled with the 95/5 electrolyte is shown in this figure for comparison, while all cells are sensitized with Rhodamine B. As shown in this figure, ZnO cells filled with the 95/5 electrolyte yielded higher efficiency values than TiO2 cells. Also, both ZnO and TiO2 cells filled with the 95/5 electrolyte tended to have higher efficiencies than cells filled with the standard electrolyte. In general, the efficiency of ZnO cells filled with the 95/5 electrolyte was found to be 5-10% higher than the efficiency of corresponding cells filled with the standard electrolyte. TiO2 cells filled with the 95/5 electrolyte had considerably higher efficiencies than the ones filled with the standard electrolyte (up to 58% higher). This is highlighted in Table 1, which shows the efficiency of TiO2 cells filled with each composite electrolyte, with respect to the efficiency of cells filled with the standard electrolyte. It is apparent from Table 1, that the highest efficiency was achieved with solar cells filled with the 90/10 electrolyte. The uncertainties in the results

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shown in Table 1 originate from differences in the solar cells, mainly regarding the thickness and the uniformity of the TiO2 films (Giannouli et al., 2010).

[Current density [mA/cm 2]

1,8 1,5 1,2

ZnO 95/5 ZnO standard TiO2 95/5

0,9 0,6 0,3 0 0

0,1

0,2

0,3

0,4

0,5

Voltage [V] Figure 5. Current-Voltage characteristics of ZnO cells filled with the 95/5 electrolyte and the standard electrolyte and sensitized with Rhodamine B.

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Table 1. Efficiency of TiO2 cells filled with various composite electrolytes with respect to the corresponding values obtained using the standard electrolyte

Efficiency [%] with respect to the standard electrolyte

80/20 electrolyte

90/10 electrolyte

95/5 electrolyte

48-56

62-70

50-58

The stability of the solar cells is also a very important parameter that needs to be considered in order to determine their overall performance. In general, the efficiency of dyesensitized solar cells decreases considerably with time and that is one of the main limitations of this emerging technology (Tributsch, 2004). Figure 6 shows the evolution of the cell efficiency over a 30 day period for two typical TiO2 cells, one filled with the standard electrolyte and one filled with the 95/5 electrolyte. Both cells were sensitized with Rhodamine B. From this figure it is apparent that both TiO2 cells yield the highest values of the efficiency at the 2nd or 3rd day after cell preparation. The reason for this is that permeation of the electrolyte occurs gradually due to the porous nature of the films (Longo et al., 2003). It was also observed that, after the first three days, cell efficiency drops gradually. Specifically, the efficiency follows the trend of the short-circuit current density, which reaches its maximum value on the 2nd or 3rd day after cell preparation and then decreases. On

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the other hand, it was found that the value of the open-circuit voltage increases slightly with time, but does not vary as much as the value of the current and cannot compensate for the large drop in the current. The gradual decrease in the current that was observed after the 3rd day from cell preparation is mainly due to dye desorption and degradation. Many organic dyes undergo degradation when they are adsorbed on the surface of a semiconductor such as the TiO2, which can cause catalytic photodegradation. In addition, reactive oxygen species (mainly oxygen ions) are formed with time, which react with oxidized dye molecules under visible or ultraviolet light irradiation in the presence of TiO2 particles (Zhang et al., 1997; Wu et al., 1998). As a result, the number of oxidized dye molecules is reduced. Dye degradation depends also on the adsorption sites (Tributsch, 2004) and constitutes a major problem not only for the simple organic dyes, but also for the high efficient metal complexes (Macht et al., 2002). In the case of the 95/5 electrolyte, the drop in the efficiency is not as steep as that of the cell containing the standard electrolyte. Specifically, 30 days after cell preparation the cell containing the standard electrolyte exhibited a higher decrease in its efficiency by approximately 15% than the cell filled with the 95/5 electrolyte. It has been shown (Giannouli et al, 2010) that other composite electrolytes containing ethylene glycol, such as the 95/5 electrolyte, enhance cell stability. Cells filled with the composite electrolytes described here maintain high efficiencies for a longer time under illumination and also yield high efficiencies for a longer time after cell preparation than cells filled with a standard-type electrolyte. 0,8 Standard electrolyte 95/5 electrolyte

0,7

Efficiency [%]

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0,6 0,5 0,4 0,3 0,2 0,1 0 Day 1

Day 5

Day 9

Day 13 Day 17 Day 21 Day 25 Day 29 Number of Days

Figure 6. Evolution of cell efficiency with time over a 30 day period for a TiO2 cell filled with the standard electrolyte and a TiO2 cell filled with the 95/5 electrolyte.

As mentioned above, the main reason for the decline in the efficiency of dye sensitized solar cells with time is the degradation of the dye used to sensitize the films. For ZnO films in Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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particular, protons originating from the dye may cause the dissolution of Zn surface atoms (Keis et al., 2002). These Zn atoms then react with the protons from the dye, forming Zn+2/dye complexes: ZnO + 2H+ → Zn+2 + H2O The above reaction leads to the formation of inactive dye molecules which limit charge carrier injection and reduces the efficiency of the cells. The dissolution of Zn atoms depends on a number of factors, such as the dye concentration, the sensitization time and the pH of the dye solution (Bahnemann 1993). In the present work, a comparison was conducted of the rate at which the efficiency of the cells declines between ZnO and TiO2 cells. Figure 7 shows a typical example of the decrease in the current density of a ZnO cell with time for 30 days after cell preparation. From this figure it can be observed that the most abrupt decrease in the current density of the ZnO cells occurs between the first and the second day after cell preparation. This effect is mainly due to the aforementioned dye degradation. By comparing the results presented in Figures 6 and 7, it becomes apparent that TiO2 films have higher stability than ZnO films. The efficiency of ZnO solar cells is reduced rapidly with time, while TiO2 cells maintain relatively high efficiencies for some time after cells preparation. Specifically, it was observed that ZnO cells experience a drop of up to 80% in their efficiency 30 days after preparation, while TiO2 cells lose approximately 50% of their original efficiency during the same time. On the other hand, the efficiencies achieved during the first days after cell preparation were considerably higher for ZnO than for TiO2 cells. 1,2

0,8

Efficiency [%]

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1

0,6

0,4

0,2

0 Day1

Day5

Day9

Day13

Day17

Day21

Day25

Day29

Days after cell preparation Figure 7. Evolution of cell efficiency with time over a 30 day period for a ZnO cell sensitized with Rhodamine B.

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As mentioned above, composite ZnO/TiO2 films with ZnO/TiO2 ratio equal to 75/25% were developed, with the aim to improve the properties of dye sensitized solar cells. Figure 8 shows the efficiency of a composite solar cell over a 30 day period. The performance of the composite cell in this figure is compared against the performance of a plain ZnO solar cell. Both cells were sensitized with sensitized with Rhodamine B. From Figure 8, it is apparent that the efficiency of the ZnO cell on the day it was assembled is considerably higher than that of the composite cell. However, the efficiency of the composite cell does not decrease as rapidly with time, so that at the end of the 30 day period, the composite cell has higher efficiency than the plain ZnO cell. Dark current measurements were also conducted for ZnO, TiO2 and ZnO/TiO2 cells. Figure 9 shows dark current results for three cells, one for each type of semiconductor. All cells shown in this figure were sensitized with Rhodamine B. It was found that the composite cells have the lowest dark current, while the TiO2 cells have the highest. The higher dark current in the TiO2 cells, even at lower potentials, indicates a faster recombination of electrons with triiodide ions compared to ZnO and composite cells (Senevirathne et al., 2008). 1,2

ZnO ZnO/TiO2

0,8

Efficiency [%]

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1

0,6

0,4

0,2

0 Day1

Day5

Day9

Day13

Day17

Day21

Day25

Day29

Days after cell preparation Figure 8. Cell efficiency over a 30 day period for a ZnO and a composite ZnO/TiO2 cell sensitized with Rhodamine B.

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0,1

0,2

0,3

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231 0,5

Current density [mA/cm2]

0 -0,01 -0,02 -0,03

ZnO TiO2 ZnO/TiO2

-0,04 -0,05 -0,06 -0,07

Voltage [mV] Figure 9. Dark current characteristics of ZnO, TiO2 and ZnO/TiO2 cells sensitized with Rhodamine B.

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CONCLUSION The main purpose of the study was to investigate methods for enhancing the stability and the performance of cells sensitized with simple organic dyes. To this aim, ZnO, TiO2 and composite ZnO/TiO2 films were prepared and sensitized with the simple organic molecules Rhodamine B, Rose Bengal and Coumarin 343. The solar cells that yielded the highest efficiencies were the ones sensitized with Coumarin 343 followed by cells sensitized with Rose Bengal. Emphasis was also placed on assessing the properties of solar cells filled with various types of composite electrolytes. The composite electrolytes presented here contain both propylene carbonate and ethylene glycol as a solvent. The effects of each type of electrolyte in the efficiency and stability of the cells were investigated. It was observed that the combined properties of the two solvents in the multi-component electrolytes enhance the efficiency and improve the stability of the solar cells. Finally, a comparative assessment of the properties of ZnO, TiO2 and composite ZnO/TiO2 films was conducted. It was observed that, in general, ZnO cells sensitized with these simple dyes yield higher efficiency than corresponding TiO2 cells. However, TiO2 cells exhibit higher stability than ZnO cells. Composite ZnO/TiO2 cells do not yield as high efficiencies as ZnO cells immediately after preparation, but have improved stability, so that a month after cell preparation, composite cells have higher efficiency than ZnO cells.

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REFERENCES Bahnemann, D.W. Israel J. Chemistry 1993, Vol. 33 pp. 115. Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S.; Grätzel, M. Nature Materials 2008, Vol. 7 p. 626. Chen, Z.; Tang, Y.; Zhang, L.; Luo, L. Electrochimica Acta 2006 Vol. 51 pp. 5870. Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S.; Grätzel, M. Chemical communications 2008, Vol. 23 pp. 2635–7. Giannouli, M.; Syrrokostas, G.; Yianoulis, P. Progress in Photovoltaics 2010, Vol. 18, pp. 128-136. Grätzel, M.; O'Regan, B. Nature 1991, Vol. 353 p737. Guillén, E.; Casanueva, F.; Anta, J.; Vega-Poot, A.; Oskam, G.; Alcántara, R.; FernándezLorenzo, C.; Martín-Calleja, J. Journal of Photochemistry and Photobiology A: Chemistry 2008, Vol. 200 p. 64. Keis, K.; Hagfeldt, A.; Keis, K.; Bauer, C.; Boschloo, G.; Hagfeldt, A.; Westermark, K.; Rensmob, H.; Siegbahn, H. Journal of Photochemistry and Photobiology A: Chemistry 2002, Vol. 148 p. 57. Kroon, J.; Bakker, N.; Smit, H.; Liska, P.; Thampi, K.; Wang, P.; Zakeerudin, S.; Grätzel, M.; Hinsch, A.; Hore, S.; Würfel, U.; Sastrawan, R.; Durrant, J.; Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. Prog in Photovol: Res Appl 2007, Vol. 15 p.51. Lee, W.; Okada, H.; Wakahara, A.; Yoshida, A. Ceramics International 2006, Vol. 32 p. 495. Liu, Y.; Wang, H.; Shen, H.; Chen, W. Applied Energy 2010, Vol. 87 pp. 436–441. Longo, C.; Freitas, J.; De Paoli, M.A. Journal of Photochemistry and Photobiology, A: Chemistry 2003, Vol. 159 pp.33–39. Macht, B.; Turrion, M.; Barkschat, A.; Salvador, P.; Ellmer, K.; Tributsch, H. Sol. Energy Mater. Sol. Cells 2002, Vol. 73 pp. 163 – 173. Nattestad, A.; Mozer, A.; Fischer, M.; Cheng, Y.; Mishra, A.; Bäuerle, P.; and Bach, U. Nature Materials 2010, Vol. 9 pp. 31 – 35. Nazeeruddin, M.K.; Kay, A.; Rodicio, I.; Humpbry-Baker, R.; Miiller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J Am Chem Soc 1993, Vol. 115 p. 6382. Pichot, F.; Pitts, R.; Gregg, B. Langmuir 2000, Vol. 16 p. 5626. Plank, N.; Howard, I.; Rao, A.; Wilson, M.; Ducati, C.; Mane, R.; Bendall, J.; Louca, R.; Greenham, N.; Miura, H.; Friend, R.; Snaith, H., and Welland, M. Journal of Physical Chemistry C. 2009, Vol. 43 pp. 18515. Pradhan, B.; Batabyal, S.; Pal, A. Solar Energy Materials & Solar Cells 2007, Vol. 91 p. 769. Rao, R.; Dutta, V. Nanotechnology 2008, Vol. 19 pp. 445712 - 445721. Senevirathne, M.; Pitigala, P.; Sivakumar, V.; Jayaweera, P.; Perera, A.; Tennakone, K. Journal of Photochemistry and Photobiology A: Chemistry 2008, Vol. 195 pp. 364–367. Shklover, V.; Nazeeruddin, M.K.; Zakeeruddin, S.M.; Barbe, C.; Ka,y A.; Haibach, T.; Steurer, W.; Hermann, R.; Nissen, H.U.; Grätzel, M. Chem Mater 1997, Vol. 9 p. 430. Smestad G. Sol Energy Mater Sol Cells 1998, Vol. 55 p. 157. Syrrokostas, G.; Giannouli, M.; Yianoulis, P. Renewable Energy 2009, Vol. 30 pp. 17591764.

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Tennakone, K.; Bandaranayake, P.K.; Jayaweera, P.V.; Konno, A.; Kumara, G. Physica E 2002, Vol. 14, pp. 190 – 196. Tributsch, H. Coordination Chemistry Reviews 2004, Vol. 248 p. 1511. Van der Zanden, B.; Goossens, A. J Phys Chem B 2000, Vol. 104 pp. 7171. Wang, M.; Anghel, A.; Marsan, B.; Cevey Ha, N.; Pootrakulchote, N.; Zakeeruddin S.; and Gratzel, M. J Am Chem Soc 2009, Vol. 131 pp. 15976–15977. Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J Phys Chem B 1998, Vol. 102 pp. 58455851. Xu, W.; Dai, S.; Hu, L.; Zhang, C.; Xiao, S.; Luo, X.; Jing, W.; Wang, K. Plasma Science and Technology 2007, Vol. 9 pp. 554-559. Zhang, F.; Zhao, J.; Zang, L.; Shen, T.; Hidaka, H.; Pelizzetti, E., Serpone, N. J Mol Catal A: Chem. 1997, Vol. 120 pp. 173-178.

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In: Dye-Sensitized Solar Cells and Solar Cell Performance ISBN: 978-1-61209-633-9 Editor: Michael R. Travino ©2012 Nova Science Publishers, Inc.

Chapter 10

PRESENT SCENARIO OF SOLID STATE PHOTOELECTROCHEMICAL SOLAR CELL AND DYE SENSITIZED SOLAR CELL USING PEO-BASED POLYMER ELECTROLYTES Pramod K. Singh* and Bhaskar Bhattacharya≠ Department of Physics, School of Engineering & Technology, Sharda University, Greater Noida, India

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ABSTRACT Due to energy crisis in coming future many efforts are directed towards alternate sources. Solar energy is accepted as novel substitute to the conventional sources of energy. Out of the long list of various types of solar cells solid state photoelectrochemical solar cell (SSPEC) and dye sensitized solar cells (DSSC) are now emerging area since it shows promise as an alternative for costly crystalline solar cell. This chapter provides a common platform to SSPEC and DSSC using polymer electrolyte particularly on PEObased polymer electrolytes. Among various polymer electrolytes available for solar cell applications, most frequently used polymer is PEO (polyethylene oxide). Due to numerous advantageous properties of PEO it is frequently used as electrolyte in both SSPEC as well as DSSC. In DSSC, so far high efficiency (more than 11%) could be obtained only using volatile liquid electrolyte which suffers many disadvantages like corrosion, leakage, evaporation. The PEO based solid polymer proves its importance and could be used to solve the problems stated above. The recent developments in solar cell using modified PEO electrolytes by adding nano size inorganic fillers, blending with low molecular weight polymers and ionic liquid (IL) are discussed in detail. The role of ionic liquids in modifying the electrical, structural and photoelectrochemical properties of PEO based polymer electrolytes are also suggested. * E-mail address: [email protected] (Pramod K. Singh) ≠ E-mail address: [email protected] (Bhaskar Bhattacharya) Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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Keywords: Polymer electrolytes; Photoelectrochemical solar cell, Ionic liquid; Ionic conductivity; SEM; TEM; Dye sensitized solar cell

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1. INTRODUCTION One of the prime areas of present worldwide research is related with the “crisis of energy in coming future”. Fossils fuels, supplying ~ 80% of all energy consumed worldwide are facing rapid resource depletion. Because of a growing demand for energy, there is an urgent need of environmentally friend sustainable energy technologies. Renewable energy, which includes solar energy is a novel alternate and seems to be a promising candidate to solve this problem. In photovoltaic industry, today the main barrier is directly related to the high cost of the electricity generated by solid state solar cell (SSSC) based on crystalline silicon. Crystalline silicon accounted for nearly 74% of solar cell production but due to high cost the modern research is diverted towards low cost alternatives. Shortage of Si-based raw materials to manufacture solar cells is also coming just around the corner. Therefore, new types of low cost solar cells are anticipated [1,2]. As far as electrochemical applications using polymer electrolyte is concerned, polyethylene oxide (PEO) comes straight forward due to conductive properties. Due to other useful properties like ease in film formation, excellent complexation with ionic salts, low glass transition temperature etc. it is frequently used in lithium batteries, supercapacitors, photoelectrochromic display devices [3-5]. It is well known that polyethers, like PEO, may give conducting solutions when mixed with an alkali metal salt. Solvating capabilities of the PEO based polymer are due to the unpaired electrons on the ether oxygen atoms, which act as donors for the alkali cations. The higher transference number for the anions observed in these systems, which is not desirable in the applications where the cations are the active species (like in lithium rechargeable batteries), is very attractive in electrochemical photovoltaic cells where the anions react at the photoelectrode. Hence, several attempts have been made to develop the solar cell using PEO- based solid state electrolytes.

FTO with Pt-coating Solid PEO/IL electrolyte Nanoporous TiO2 with dye

FTO with blocking layer (a)

(b)

Figure. 1. Schematic configuration of solid state photoelectrochemical solar cell (SSPEC, Fig 1a) and dye sensitized solar cell (DSSC, Fig. 1b) [5, 83]. Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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In this chapter, we have focused our attention around solid state photoelectrochemical solar cell (SSPEC) and dye sensitized solar cell (DSSC) using solid polymer electrolyte based on polyethylene oxide (PEO) based polymer. The recent advancement and future prospect of SSPEC and DSSC using PEO-polymer electrolyte are also presented in detail. The applications of modified PEO-polymer electrolytes by adding different additives like inorganic fillers, plasticizers, ionic liquid etc. in SSPEC and DSSC are also presented. The basic difference between SSPEC and DSSC are shown in Figure 1. SSPEC contains a semiconducting substrates onto which a polymer electrolyte containing redox couple is sandwiched (Fig. 1a) while in DSSC a layer of dye that works as sensitizer is soaked onto the surface of wide band gap porous semiconducting electrodes (Fig. 1b) and finally polymer electrolyte containing redox couple is sandwiched between dye sensitized porous semiconducting electrode and platinized counter electrode. The primary difference is in the light absorbing material and its energetics. However, the role of the polymer electrolyte remains almost the same as a channel for the redox couple. In subsequent paragraphs, we discuss the basic principle of the two and functioning of PEO based polymer electrolytes therein.

2. BASIC PRINCIPLE OF SOLID STATE PHOTOELECTROCHEMICAL SOLAR CELL AND DYE SENSITIZED SOLAR CELL

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2.1. Principle of Solid State Photoelectrochemical (SSPEC) Solar Cell After the discovery of photoelectric effect, the researchers and engineers have been infatuated with the idea of converting light into electric power or chemical fuels. Their common dream is to capture the energy that is freely available from sunlight and turn it into the valuable and strategically important asset that is electric power. Photovoltaic devices are based on the concept of charge separation at a single junction, hetero-junction between two different type (n- and p-type) semiconductor or semiconductor-metal (Schottky) junctions. The foundation of modern photo-electrochemistry, marking its change from a mere support of photography to a thriving research direction on its own, was laid down by the work of many famous groups [6-10] who presents the detailed electrochemical and photoelectrochemical studies of the semiconductor–electrolyte interface. Research on photoelectrochemical cells went through a frantic period after the oil crisis in 1973, which stimulated a worldwide quest for alternative energy sources. In photo-electrochemical cells, the junctions are semiconductor-electrolyte interfaces. The simplest device consists of a semiconducting electrode, a metallic electrode and an electrolyte as shown in Fig. 1a. The operation of a photo-electrochemical cell could be explained on the basis of energy level diagram shown in Figure 2. In electrolyte, the energy at which electrons must be provided to drive the electrochemical reaction is known as the redox potential and is usually referenced to the normal hydrogen electrode (NHE) or saturated calomel electrode (SCE). The energy position at which the conduction and valence bands for n- and p-type semiconductors respectively intercept the solid electrolyte interface is known as the flat band potential Vfb. The flat band potential is a very useful quantity in photo-electrochemistry as it facilitates location of the

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energetic position of the valence and conduction band edge of a given semiconductor material. It is obtained by measuring the capacity of the semiconductor-electrolyte junction. The semiconductor can be used as a light sensitive anode or cathode depending on whether it is n- or p- type, respectively.

Fe2+/Fe3+

e-

Eg e-

n-type semiconducting

Simple electrolyte

anode

Metallic cathode

e-

Fe2+/Fe3+

e-

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Eg

Metallic anode

Simple electrolyte

p-type semiconducting anode

Figure 2. The energy level diagram showing conduction mechanism in SSPEC.

The operational principle of SSPEC is shown in Figure 2. Photons of energy exceeding that of the band gap generate electron–hole pairs, which are separated by the electric field present in the space-charge layer. The negative charge carriers move through the bulk of the semiconductor to the current collector and the external circuit. The positive holes are driven to the surface where they are scavenged by the reduced form of the redox relay molecule and oxidizing it. The oxidized form is reduced back to redox relay molecule by the electrons that re-enter the cell from the external circuit. Much of the work on regenerative cells has focused on electron-doped (n-type) II/VI or III/V semiconductors using electrolytes based on sulphide/polysulphide, vanadium(II)/vanadium(III) or I2/I– redox couples. Conversion efficiencies of up to 19.6% have been reported for multijunction regenerative cells [6].

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2.2. Principle of Dye Sensitized Solar Cell (DSSC)

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Professor M. Grätzel of EPFL, Switzerland introduced a new type of solar cell in year 1991 which consists a nanoporous wideband semiconductors films immersed in dye solution and made the breakthrough in photovoltaic area. This solar cell is termed as Grätzel cell or dye sensitized solar cell (DSSC). The academic and commercial interests have been paid on DSSC for their high efficiency, their potential low-cost and simple assemble technique. Dye-sensitized solar cell is composed of nano-crystalline semiconductor oxide film electrode, dye sensitizers, electrolytes, counter electrode and transparent conducting substrate. Typically, dye-derived nano-crystalline semiconductor films were used as photo-anode, platinized counter electrode, filled with electrolyte solution containing I3 −/I− redox couple in organic solvent. The operating mechanism of the solar cells is shown in Figure 3. Under the irradiation of sunlight, the dye molecules became photo-excited and ultrafastly injected an electron into the conduction band of the semiconductor electrode, then the original state of the dye is subsequently restored by electron donation from the electrolyte, usually the solution of an organic solvent or ionic liquid solvent containing the I3 −/I− redox system.

Figure 3. Structure and operating principle of dye sensitized solar cell.

The regeneration of the sensitizer by iodide intercepts the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated, in turn, by reduction of triiodide at the counter electrode, the circuit being completed through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the semiconductor electrode and the redox potential of the electrolyte. Overall, electric power is generated without permanent chemical transformation.

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3. STATUS OF SOLID STATE PHOTOELECTROCHEMICAL CELL (SSPEC) USING PEO POLYMER ELECTROLYTE The direct conversion of solar energy to electricity by using a semiconductor/electrolyte interface has been demonstrated by Gerischer and Goberecht [6] and by Ellis et al. [7]. The Gerischer cell consist of n-CdSe single crystal photoanode and a doped SnO2 cathode dipped in an aqueous alkaline electrolyte containing the Fe(CN)64−/Fe(CN)63− redox couple. The energy conversion efficiency was 5% but the cell performance decreased rapidly due to decomposition of the illuminated semiconductor electrode and evaporation of solvent. Since then various modifications have been tried on each component of PEC and many good reviews can be seen. These problems are partly solved by replacing this electrolyte by solid polymer electrolytes. Initial application of polymer in photoelectrochemical solar cell (PEC) based on polyethylene oxide (PEO) was performed by Skotheim [8] in year 1981 using PEO:KI:I2 polymer electrolyte system with n-Si and indium tin oxide (ITO) electrodes. Although this cell could solve the volatility and corrosion problems but their photocurrent (Jsc) and fill factor (FF) remained very poor i.e. 20 µA cm-2 and 0.25 respectively. This is due to a high recombination rate at semiconductor interface or low ion mobilities [9]. Skotheim and Inganas [10] later showed that the energy barrier to hole transfer problem could be overcome by use of special coating of Pt on the silicon surface, and using these they obtained Jsc up to 10 mA cm-2 at 1 sun irradiation intensity. Rao et a. [11, 12] studied the charge discharge behaviour of cell with configuration Na/PEO:salt:I2/Carbon+ electrolyte. Mohamed et al. [13] reported a solid state photoelectrochemical cell using PEO:NaI with different salts. Yohannes et al. [14] fabricated a SSPEC using PEO complexed with I-/I3- redox couple while Bhattacharya et al. [15] reported a SSPEC using PEO:NH4I/I2 solid polymer electrolyte. Following same strategy recently Arof et al. [4,16] constructed cells with configuration ZnSe/PEO-Chitosan:NH4I/I2, ZnTe/PEO-Chitosan:NH4I/I2 blend electrolytes. However, the performance of SSPEC reported above are still low (< 1%) in comparison with the same cells using liquid electrolyte. The possible reasons are i. ii. iii.

Low ionic conductivity (σ) in comparison to liquid electrolyte. Rubbery type morphological structure of these polymer electrolytes which reduces the contact area between electrode and electrolyte, i.e., the active interface. High charge recombination rate at semiconductor interface

4. STATUS OF DYE SENSITIZED SOLAR CELL (DSSC) USING SOLID PEO BASED POLYMER ELECTROLYTE The future of photovoltaic is believed to be dye-sensitized solar cell (DSSC), which is relatively new, with completely different approach. In DSSC (Fig. 1b), visible light energy could be converted into electrical energy through charge separation in sensitizer dyes adsorbed on a wide band gap semiconductor. It comprises a transparent conducting oxide (TCO) electrode, a platinum coated counter electrode and an electrolyte containing redox

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couple sandwiched between these electrodes [17]. One unique characteristic of this solar cell is the ease with which it is produced and relatively good efficiency using low cast materials [18,19]. Additionally, it shows 10-20 % more electricity than conventional crystalline Sisolar cell module in large scale outdoor performance [20]. In DSSC, the popular alternatives which are commonly being tested, to replace the liquid electrolytes are

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i. ii. iii.

Hole conducting solid electrolytes Gel/quasi solid state electrolytes Polymeric solid state electrolytes

The solid electrolytes (i) and (ii) stated above are not being discussed here as each of these electrolytes has their own broad area and not in the scope of our discussion. The polymeric solid electrolytes (iii) are of special interest because of its many advantageous properties like low cost, easy thin film formation and overall good device performance. The highest reported efficient DSSC contains volatile organic solvent which has still a drawback for long term practical operation [17-19]. Moreover, the corrosion of iodine on Pt electrolyte, leakage and evaporation of solvent are also additional barriers. Using solid polymer electrolyte one can overcome these problems. Among all polymer electrolytes, PEO-based polymer electrolyte has already shown excellent performance in different electrochemical application area. The use of PEO based electrolyte in PEC also indicated its possibilities in photovoltaic applications. In DSSC, it is also considered as a novel candidate due to better stability, performance and hence large number of review articles are already available in literature [1,21,22]. Passing through literature it is quite difficult to distinguish between DSSC based on low molecular weight PEO (also known as oligomer electrolytes) and high molecular weight PEO. In this chapter, we present the schematic presentation of DSSC developed to date using PEO-based polymer electrolytes. To avoid confusion, in present chapter we divide the PEO based DSSC in the following two categories i. ii.

DSSC containing PEO with low molecular weight (molecular weight less than 50000 g/mol.; oligomers; liquid in nature at room temperature). DSSC using high molecular weight PEO (molecular weight more than 1000000 g/mol.; solid powder at room temperature).

4.1. DSSC Using Low Molecular Weight PEO (Oligomers) as Electrolyte After the successful demonstration of DSSC by Prof. Grätzel group in year 1991, many extensive investigations have been carried out in all aspects of DSSC. As far as solid PEOpolymer electrolyte is concerned, it is well known that due to high crystallinity of PEO it is not easy to get perfect soaking of this electrolyte at electrode. This results to poor electrode – electrolyte contact and hence the overall DSSC performance is reduces [21,22]. To resolve this problem Prof. Kang group has proposed oligomer based approach (low molecular weight PEO) [23-28]. They used a series of low molecular weight PEO based oligomers with various iodide sources such as ionic liquid (IL), potassium iodide (KI), sodium iodide (NaI), lithium iodide (LiI) and obtained high efficient DSSC. To obtain high ionic conductive polymer

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electrolyte this group added a variety of additives like fumed silica, silica nanoparticles, glutaraldehyde (GA), propylene carbonate (PC) and ethylene carbonate (EC) in low molecular weight PEO which enhances short circuit current density (Jsc) and overall efficiency. They also demonstrated that for better interfacial contact, it is necessary that the coil size of PEO should be less than the pore size of nanoporous TiO2 electrode (~ 16 nm). The coil size, as estimated by the radius of gyration of the polymer varies with its molecular weight/ chain length. The coil size of PEO oligomer with a molecular weight (Mw) of 1000 g/mol (PEG1000) in solution is less than 3 nm and hence can easily penetrate through the pores of the mesoporous titania. As a result good interfacial contact with deep penetration could be obtained which resulted to high efficient DSSC (3% to 6% at 100 mW cm-2). Following this strategy Ren et al. [29] developed PEO based oligomer electrolytes having Mw 2000 and 1500 g/mol (PEO2000, PEO1500) containing plasticizers (PC, EC) as additives. Based on this electrolyte a DSSC with efficiency 3.6% at 27 mW cm-2 has been reported. Recently Akhtar et al. [30,31] used PEG based oligomers having Mw ~10000, 20000 g/mol (PEG10000, PEG20000) and TiO2 nanotube (TNT), heteropolyacid (HPA) as additives to develop efficient DSSC having efficiency 4.43% and 3.1% at 100 mW cm-2 respectively. The overall data using oligomer approach electrolyte are listed in Table 1.

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4.2. DSSC Using High Molecular Weight PEO as Electrolyte (Without Any Additives) The first DSSC based on high molecular weight (Mw = 1.3x106 g/mol) PEO – based solid electrolyte without any dopants was reported in year 1999 by Nogueira et al. [32] in which they used a modified polymer PEO-epychlomer (mention as PEC here after) with sodium iodide (NaI) and iodine (I2) as redox couple and conducting poly (o-methoxy aniline) as sensitizer. The reported efficiency of their cell was 1.6x10-4 % at 120 mW cm-2. Such a low efficiency can be attributed to i. ii. iii.

highly crystalline matrix of polymer low ionic conductivity (σ) of polymer electrolyte (~ 10-5 S cm-1) in comparison with liquid electrolyte where σ value lies between 10-1 to 10-2. incomplete wetting of the semiconductor nanoparticles at the anode by the polymer electrolyte

Later this group used same polymer electrolyte i.e. PEO-epychlomer (PEC) and ruthenium based dyes. The best DSSC efficiency they reported are 1.6 % and 2.6 % at 100 mW cm-2 and 10 mW cm-2 light intensity respectively [33-35]. In year 2005, Kim et al. [36] reported a DSSC consisting PEO/NaI:I2 polymer electrolyte and ruthenium based dye which shows efficiency of 0.07 % at 10 mW cm-2 light intensity. As we have discussed above, according to model proposed by Prof. Kang group [23-28] in high molecular weight PEO (Mw ~ 1000 000 g/mol) the approximate coil size is ~ 63 nm and ~ 19 nm for molecular weight 100 000 g/mol while the estimated TiO2 pore size is 10-15 nm. Due to this reason for high molecular weight PEO, it is not easy to penetrate inside the TiO2 pore which resulted low efficiency. However same group have reported [37] an efficient DSSC (2.04 % at 100 mW cm-2 light intensity) containing high molecular weight PEO:KI/I2 system (Mw =1000

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000 g/mol). The collective dada of DSSC using above said polymer electrolyte systems are shown in Table 2. Table 1.The collective data of DSSC using low molecular weight PEO (oligomer) based polymer electrolytes System

Additives

σ (S/cm) PEG1000:IL:I2 --~10-3 PEO+PPG:KI:I2 --~10-5 PEGDME:IL/KI:I2 Silica ~10-3 PEG1000:KI:I2 GA ~10-3 PEGDME:IL/KI:I2 ---~ 10-3 PC,EC ~10-3 PEO2000: LiI:I2 PEG100000:LiI:I2 TNT, TBP ~10-3 PEO20000:LiI:I2 HPA, TBP ~10-3

Jsc (mA/cm2) 9.53 11.2 9.58 9.48 15.24 2.8 9.3 9.7

Voc (Volt) 0.57 0.72 0.67 0.64 0.62 0.58 0.72 0.52

FF (%) 62 48 70 60 66 60 65 65

η (%) 3.34 3.84 4.50 3.64 5.88 3.60 4.43 3.10

Irr. (mW/cm2) 100 100 100 100 100 27 100 100

Ref. 23 25 26 27 28 29 30 31

4.3. DSSC Using High Molecular Weight PEO as Electrolyte (with Additives)

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The DSSC proposed by Nogueira et al. [33] clearly shows that overall efficiency has already reached the limit for the system based on polymer-iodide salt/iodine complex. The use of only high molecular weight PEO as the electrolyte cannot show higher values than this. For further improvement in efficiency of DSSC it is necessary to modify the electrolytes by adding popular additives. In this connections many additives such as inorganic nanoparticles, plasticizers, copolymers, ionic liquid etc have been tried which shows that further improvement of DSSC efficiency could be possible. Table 2. The collective data of DSSCs using high molecular weight PEO- based polymer electrolytes without adding any additives System PEOepychlomer:NaI:I2 (PEC) PEOepychlomer:NaI:I2 PEO:NaI:I2 PEO:KI:I2

Jsc Voc Salt σ (wt%) (S/cm) (mA/cm2) (Volt) NaI 1.0x10 -5 0.012 0.048

FF (%) 32

Ref. η Irr. (%) (mW/cm2) 1.6x10-4 120 32

NaI

1.5x10 -5 0.5-4.2

0.74-0.82 73-47 2.6-1.6 10,100

33,34

NaI KI

1.6x10 -6 0.5 8.3x10 -5 6.12

0.54 0.59

36 37

26 56

0.07 2.04

10 100

4.3.1. Adding Nano Size-Inorganic Fillers as Additive in Polymer Electrolyte The high crystallinity and too low ambient conductivity of PEO- based electrolyte acting as barrier to use it in DSSC as electrolyte. The additions of nano-inorganic fillers are well known successful approaches to enhance conductivity and mechanical properties of polymer electrolyte film which is necessary for device application [38-40]. It is generally recognized that the improvement of ionic conductivity (σ) in PEO-polymer electrolytes comes from the

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suppression of crystallinity and the promotion of amorphous region. In this direction, Dr. Falaras group [41-44] have already proposed highly efficient DSSC based on nanocrystalline porous TiO2 film, ruthenium complexes based different dyes, Pt counter electrode and solid polymer electrolytes PEO-LiI:I2/titania as an electrolyte. This group could successfully reduce the crystallinity of PEO polymer electrolyte and reported that introduction of titania nanoparticles reduces crystallinity which in turn enhances ionic conductivity (σ) as well as provides a three dimensional path (Fig. 4) for easy movement of redox couple (I-/I3-) into polymer matrix and hence they achieved efficiency as high as 4.2% at 65.6 mW cm-2 [43].

Figure 4. Two dimensional AFM topographic images of PEO:LiI/I2+titania composite polymer electrolyte system. The arrow indicates the distribution of titania particle [43].

Following this approach recently Chen et al. [45] reported an efficient DSSC (4.06 % at 75 mW cm-2) using nanoparticle of TiO2 as filler in a blend-modified PEO-PVDF polymer electrolyte matrix.

4.3.2. Adding Plasticizers as Additive As discuss in our earlier section in PEO polymer electrolyte based DSSC, the one of the main approach was directed towards the reduction of crystallinity of PEO or lower its glass transition [46,47]. The plasticizer, usually a low molecular weight polyether or carbonate is incorporated in small amounts into polymeric matrix to increase its segmental motion which is closely associated with glass transition temperature. The plasticizers introduce a degree of disorder in polymer matrix which is necessary for further improvement in electrical conductivity (σ) and its device application. In DSSC using PEO polymer electrolytes many popular plasticizers like polyethylene glycol (PEG), polypropylene glycol (PPG), ethylene carbonate (EC), 1, 2-dimethoxyethane (DME), γ-butyrolactone (BL) and propylene carbonate (PC) etc already shown its importance in enhancing ionic conductivity by reducing crystallinity [46-52]. To introduce more amorphous reason into PEO matrix, it is necessary to introduce a certain degree of disorder in the structure. Adding plasticizers is a novel approach

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which fulfills the w t requiremeent needed foor efficient DSSC D using polymer electrrolyte [46]. H Haque et al. [4 47] reported an a efficient DS SSC which shhows high effficiency 5.3% at 10 mW cm m-2 using plassticizers EC&P PC in 1:1 ratioo in PEO:NaII:I2 system. Laater Ileperumaa et al. [48] ussed PC, EC plasticizers p dooped PEO:KII:I2 polymer electrolyte e sysstem which shows s high -3 -1 -2 coonductivity (2 2.2x 10 S cm m ) and DSSC C efficiency 0..6 % at 100 mW m cm . Anaandan et al. [449,50] doped known plasticcizers like hetteropolyacid (HPA), ( benziddine (Bz) in PVDF:KI:I P 2 annd PEC:KI:I2+NPT + polymer electrolyte matrix m and achhieved efficienncies were 2.77%, 3.43% reespectively at 15 mW cm-2. Using same strategy, s Ganesan et al. [51,52] used diphenyl amine (D DPA), 2,6-bis((N-pyrazolyl) pyridine (BN NPP) into PEO O:KI:I2 polymeer electrolyte matrix and reeported efficieencies were 6.5 and 8.8% at a 60 and 80 mW cm-2 lighht intensity reespectively. Frreitas et al. [5 53,54] reporteed an efficientt (efficiency 3% 3 at 100 mW W cm-2) stablle (upto 30 daays) DSSC using plasticizers γ-butyrolaactone (BL) in i PEC containing NaI or LiI and I2 syystem. Recenttly an efficiennt DSSC (4.033% at 100 mW W cm-2) was claimed by Flores F et al. [555] applying plasticizer poly(ethyleneg p glycol)dibenzooate (PEG-diB B) in PEO-eepychlomer (P PEC):NaI:I2 matrix. m Additioonally this grouup successfullly demonstrated a large area (4.5 cm2) D DSSC module in which theyy connected 13 cells in seriies (Fig. 5). Thhey obtained an average effficiency of 0..9 % per cell operated o in ouutdoor condition [56]. Beneedetti et al. dem monstrated -2 ann efficient DSSC (efficienncy 3.7% at 100 mW cm m ) employingg PEO copolyymer:LiI:I2 ellectrolyte conttaining plasticcizers γ-BL annd 12-crown-4 ether as addditives [57]. The T overall inncrease in effficiency of thhe polymer ellectrolyte systems stated above a are bassed on the reeduction in cry ystallinity of PEO polymerr matrix by addding known plasticizers p sttated above annd good ionic conductivity (σ) ( (10-5 to 100-3 S cm-1).

Fiigure 5. DSSC module m where 16 1 solar cells co onnected in seriies at irradiation n of a 50 W fluo orescent laamp [56].

4.4. Polymerr Blend Elecctrolyte o polymer in PEO matrix iss a well know wn method to further f lower Tg value of Blending of PE EO which is a known baarrier for condductivity enhaancement [444-46]. For bettter charge trransfer, the electrolyte e muust allow faast movementt of charges through it, i.e., good coonductivity. The T conductivvity mainly arises a from the rapid segmental motioon and the innteraction betw ween the cation and the doonor atom off the main struucture. The main m focus,

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therefore, was to make the redox reaction and a swift charge transfer. It has been established that the amorphous region of the polymer-salt complex provides the conduction pathway and is mainly responsible for the ion conduction in a polymer-salt complex. Therefore, much effort has been directed toward reducing the crystallinity of the polymer (PEO). Different approaches have been made to improve the conductivity of the polymer electrolyte. The modification is made such that the mechanical stability is retained and the conductivity gets enhanced. The use of polyphospazenes and polysiloxanes, which results in a comblike structure, has been sought as the route for such modifications [58,59]. Polysiloxanes, with a glass transition temperature of -127 °C, much lower than that of PEO, can enhance the chain flexibility of the oxyethylene chain. As a result, the mobility of the ions should be improved thereby enhancing the conductivity. The PEO as the side chains attached with the siloxane backbone results in the formation of a comblike structure. The high degree of segmental motion in such a structure having the main chain with a polymer of low glass transition temperature will thus result in better conductivity. Lee et al [60] has reported such a blend and sorted the blending (holding) limit of the PDMS in high Tg polyethylene oxide. The limit and its changes were explored using thermal studies and conductivity justifying the number of charge carriers available for conduction upon blending. PEO/PDMS blend complexed with LiI was found to show conductivity close to 10-3 S cm-1 at 303 0K (Fig. 6). The addition of PDMS in PEO has been shown to reduce the crystallinity of the matrix. The conductivity is found to depend more on the number of charge carriers. The dye-sensitized solar cells fabricated using the nanoporous TiO2 electrode and the solid polymer blend have been shown photo conversion efficiencies of 0.48% and 1.35% at 100 and 10 mW cm-2, respectively [60]. Another blend using the same chemical composition of the monomers with different chain lengths was also explored. PEO of high molecular weight was blended with PEG of molecular weight 200. The blend was doped with a series of iodine salts. Salts were chosen with the consideration that the size of cation varies but the charge state (+1) remains as such. Thus the effect of the size of the cation gets well reflected and some definition for the choice of cation vis-à-vis salt can be made. Cell properties of the PEO-based polymer blend electrolyte with different cation iodide salts (MI), the cations (M+) being Li+, Na+, K+, NH4+, EMI+, and HMI+ with wide variation in their ionic radii were studied. The Li+ ion as the smallest ion and the largest (almost 5 times) as HMI+. All the blended samples showed higher conductivity and less crystallinity in comparison with their only PEO counterparts. The relative crystallinity of the blend, as obtained from the DSC analysis, drops to ~70% of that of pure PEO and even more when salt is added to it. Accordingly the conductivity of the films jumps by more than 3 orders of magnitude with its maximum value 9.2 x 10-5 S cm-1 for the EMII doped blend. It can be seen that these cells show much better efficiency than those fabricated using only PEO and respective iodide salts (Fig. 7). It is generally believed that the conductivity of the electrolyte with smaller cation should be higher, possibly due to the higher mobility of smaller cations. However, our results contradicted this belief. We found that the conductivity of the films with smaller cation was less compared to that with the larger cation. It is known that the total conductivity is given by the relation σ = nqμ

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(1)

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where n is the number of dissociated charge carriers in the matrix, q is the charge carried by them, and μ is the mobility of the carriers. Therefore, any change in the number of charge carriers and/or in the mobility will result in changing the total conductivity value. We have calculated the relative number of dissociated (free) charge carriers available for conduction to verify the contribution of n or μ in the increase in the conductivity with the size of the cations. We have assumed that the salt gets completely dissociated when dissolved in the polymer matrix. According to the electrolyte dissociation model [61], the number of dissociated charge carriers (n) is given as n = n0 exp{ -U/2εkBT}

(2)

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where, U is the dissociation energy of the salt and ε is the dielectric constant of the matrix at temperature T. The dielectric constants of the samples were calculated at 1 MHz frequency from the impedance data. Using the dielectric data and the above equation, it was found that the number of charge carriers, in this case, has almost no role in controlling the conductivity. The change in the electrolyte properties have thus been attributed to the change in the mobility of the ions due to the change in the crystallinity of the polymer electrolyte. The highest efficiency obtained in the series is 2.05% with the Li+ doped PEO:PEG blend. For cations with different sizes the efficiency varied as it varies in case of liquid electrolytes. The major role in drop is found to be due to the photocurrent than the voltage. Interestingly it is observed that as the size of the cation was increased the efficiency decreased [62]. The change in the solar cell performance has been explained in the last section of this article.

Figure 6. Ionic conductivity at 303 K of PEO:PDMS blends.

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Figure 7. The solar cell characteristics under AM 1.5 illumination. The inset indicates the salts used in the solid polymer electrolyte as the electrolyte for the DSSC.

4.4.1. Plasticizer with Nanofiller in Polymer Blend Electrolyte In literature adding different nanofillers (nano size particles of inorganic materials) into blend plasticized PEO polymer electrolyte matrix is also popular approach to achieve high efficient DSSC. In this continuation, Prof. Zhao’s group [63-66] reported a series of papers based on nanoparticles of SiO2 and carbon as additive in PEO-PVDF and PVDF-HFP polymer electrolytes plasticized with PC and DME. Later they showed that ultrasonic treatment and adding some other additives (water and ethanol) could also be a novel approach to enhance DSSC efficiency. The collective data of DSSC that fall within this category are shown in Table 3.

4.5. Ionic Liquid as Additive in Dye Sensitized Solar Cell Ionic liquids (ILs) are kind of new materials which prove its importance in different area of materials science [67-69]. Recently, there has been increased interest in research towards using ILs in various electrochemical applications [67-69]. Due to variety of useful properties researchers frequently used it in different area and hence it is not possible to cover the application of these IL in a single topic. In present chapter we are concentrated around the use of ILs in modification of PEO polymer electrolyte and their possible application in dye sensitized solar cell (DSSC). In DSSC, ILs are mostly uses as a replacement of volatile organic solvents [70-77]. In DSSC using PEO polymer electrolyte, ILs are considered as novel candidate because it has ionic conduction property (composed of ions) as well as other useful properties like low vapor pressure, non flammability. Apart from these advantages the liquid nature at room

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temperature of most of these ILs is the biggest disadvantage. Therefore, in order to develop solid electrolyte, one of the novel method is to dope these ILs in polymer electrolyte matrix. Table 3. The collective data of DSSC using high molecular weight PEO and PEO-blend polymer electrolytes with plasticizers as additives System

Plasticizer

σ (S/cm) PC, EC ~10-5 HPA ~10-5 Bz ~10-4 DPA ~10-5 BNPP ~10-5 PC, EC ~10-3 γ-BL ~10-4 γ-PEG-diB ~10-4 crown ether, γ-BL ~10-3 PC, EC,γ-BL,PEG ~10-3 PC,DME ~10-4

PEC:NaI:I2 PVDF:KI:I2 PEC:KI:I2 PEO: KI:I2 PEO:KI:I2 PEO:KI:I2 PEC:NaI:I2 PEC:NaI:I2 PEC:LiI:I2 PEO:PS PEO:PVDF :LiI:I2+*NPC PEO:PVDF-HFP PC,DME :LiI:I2+ *NPS PEO:PVDFPC,DME HFP:LiI:I2 +*NPT PEO:PVDF:LiI:I2 PC,DME + *NPT

Jsc (mA/cm2) 6.10 3.90 5.53 10.2 21.3 0.67 9.0 9.6 11.4 2.0 7.90

Voc (Volt) 0.80 0.42 0.37 0.81 0.70 0.67 0.76 0.84 0.78 0.60 0.67

FF (%) 53 25 25 47 47 --47 49 42 68 58

Η (%) 2.5 2.7 3.4 6.5 8.8 0.6 3.0 4.0 3.7 1.3 4.8

Irr. Ref. (mW/cm2) 100 46 15 49 15 50 60 51 80 52 60 53 100 54 100 55 100 57 60 58 65.2 59

~10-3

6.39

0.54

65

3.6

62.5

60

~10-4

7.34

0.53

67

4.2

65.2

61

~10-3

12.3

0.56

51

3.9

93.2

62

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*NPT, *NPS and *NPC are for nanoparticles of TiO2, SiO2 and carbon respectively

For developing efficient DSSC our group have already developed a series of novel polymer electrolytes based on “low viscosity IL-doped high molecular weight PEO polymer electrolyte” [78-92]. It is known that ionic conductivity (σ) and mobility (µ) of polymer electrolyte is closely related with viscosity (η) by the equation µ = q/6πηr

(3)

Following equations 1, 2 and 3 it is clear that mobility (µ) or ionic conductivity (σ) are inversely proportional to viscosity (η) and hence low viscosity IL is preferable condition for high ionic conductivity (σ). In market variety of ILs are available according to their various properties. We have tested a series of low viscosity ILs doped into high molecular weight PEO polymer electrolyte system containing different iodide salts for DSSC application. The low viscosity ILs used by our group are 1-ethyl 3-methylimidazolium thiocyante (EMImSCN), 1-ethyl 3-methylimidazolium dycynamide (EMImDCN), (1-ethyl 3methylimidazolium bis(trifluoromethylsulfonyl)imide) (EMImTFSI), 1-ethyl 3methylimidazolium trifluoromethanesulfonate (EMImTFO). It is well known that IL are composed of ions i.e. cations as well as anions that can be dissociated easily. Hence, dispersal of these low viscosity ILs modified the electrical

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properties of PEO polymer electrolyte by providing additional number of charge carriers contributing to the conductivity. Figure 8 shows the variation of ionic conductivity with the values of IL concentration in PEO. It is depicted that adding ILs into polymer electrolyte matrix (PEO:KI:I2 in present case) the ionic conductivity increases by manifold. The enhancement in ionic conductivity by doping IL could be explained by knowing the facts the ionic conductivity is governed by equation (1). This equation clearly indicates that increasing number of charge carries or mobility is favorable condition for ionic conductivity enhancement. Dispersal of low viscosity ILs satisfied both the conditions stated above. Since it is composed of ions and hence adding IL provided the additional charge carriers and on the other hand low viscosity of IL provided low viscous polymer matrix for easy ion movement by reducing crystallinity (affirmed by DSC stated below). From Fig. 8 it is clear that more is the amount of IL in the PEO, better is the ionic conductivity. It is also noted that beyond certain concentration it is not possible to get free standing polymer film due to the decrease in the crystallinity of the PEO. We have carried out our experimental studies only within the range of free standing polymer film and obtained different conductivity maxima for all ionic liquids used in present study.

Ionic conductivity (S/cm)

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10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

EM im SC N EM Im TFSI EM Im D C N EM Im TFO

0

20

40

60

80

100

120

IL (w t%) Figure 8. Variation of ionic conductivity (σ) with amount of ionic liquid (IL) added in IL doped solid polymer electrolyte films.

To further affirm the role of low viscosity IL doping into PEO polymere electrolyte matrix we have carried out differential scanning calorimetry (DSC) experiment. Figure 9 shows the typical example of DSC curve of PEO:KI:I2 system doped with low viscosity IL, 1ethyl 3-methylimidazolium thiocyanate (EMImSCN). The relative percentage crystallinity (χ%) has been calculated by assuming that of the pure PEO being 100% with the equation χ=ΔHf/ΔHfo

(4)

where ΔHf and ΔHfo are defined as heat of fusion of the doped complex and pristine sample respectively. The calculated parameters using DSC curves are listed in Table 4. From this Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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Heat Flow (W/g)

figure and table it is clear that adding low viscosity IL both the heat of fusion (ΔHf) and the melting temperature (Tm) decreases (Fig. 9b). It is also noted that with higher IL content the values of ΔHf and Tm decreases further. In addition, the melting endotherm broadening is also observed at higher IL concentration (Fig. 9c). Both the reduced melting temperature and the broadening of the melting endotherm affirmed that adding low viscosity could suppress the crystallinity of PEO polymer electrolyte. The regular arrangement of the PEO chain partially opens up in the presence of ILs. More is the IL concentration; more is the disorder/ amorphicity. Similar observations have been noted in other low viscosity ILs–PEO polymer electrolyte systems.

c b a

20

30

40

50

60

70

80

90

0

Temperature ( C )

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Figure 9. DSC thermograms of (a) PEO:KI:I2 (b) PEO:KI:I2+40 wt% EMImSCN ionic liquid and (c) PEO:KI:I2+80 wt% EMImSCN polymer electrolyte systems with heating rate of 5 0C/min.

(a)

(b)

Figure 10. Schematic diagram showing effect of doping of low viscosity IL into polymer electrolyte matrix.

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In general, it is believed that the ionic conductivity (σ) increases as the degree of crystallinity decreases. This phenomenon can be understood as schematically shown in Fig. 10. In Fig. 10a when PEO is complexed with KI, it shows partially crystalline nature. The ordering of the polymer chains in localized regions result to the crystallinity of the films. When the IL is added to this system (Fig. 10b), it further modifies the crystallinity. The formerly ordered coils (crystalline region) are expected to get partially disordered, in other words less regular arrangement of the coils. Hence, the overall crystallinity of the films gets reduced (as evidence by DSC). It is reported that for conduction ions always prefer amorphous regions and thus the ionic conductivity gets enhanced. This decrease in crystallinity (increase in amorphicity) improves the charge transfer mechanism in the device and hence the cell performance gets improved. Table 4. The calculated values of percentage crystallinity ( %) along with the melting temperature (Tm) and corresponding heat of fusion ( Hf) in PEO:KI:I2 polymer electrolyte system doped with low viscosity IL (EMImSCN)

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Composition (wt%) PEO:KI: I2 (80:20:2.0) PEO:KI:I2 + 40 % IL PEO:KI:I2 + 80 % IL

(a)

Tm (0C) 60.3 51.7 45.0

ΔHf (J/g) 86.3 29.0 16.1

χ (%) 45.9 15.4 8.60

σ (S cm-1) 8.80 x 10-6 1.90 x 10-5 7.62 x 10-4

(b)

Figure 11. Three dimensional AFM images of (a) PEO:KI:I2 and (b) PEO:KI:I2 + 40 wt% EMImDCN free standing IL-polymer electrolyte films in tapping mode.

Doping of low viscosity IL also affected surface property of polymer electrolyte matrix. Figure 11 shows the tapping mode atomic force microscopy (AFM) images of pure polymer electrolyte (PEO:KI:I2) and IL (EMImDCN) doped polymer electrolyte film. In absence of IL the polymer electrolyte film (PEO:KI:I2) shows crater-valley type rough surface (Fig. 11a) with surface roughness RMS = 14.45 nm. Incorporation of IL into PEO:KI:I2 matrix modify its structure (Fig. 11b) in such a way that the intensity of crater-valley decreases and surface

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seeems almost smooth with reduced surface roughness of 7.22 nm. n Such a decrease d in rooughness by adding a IL cleaarly indicates that t ionic liquuid (IL) incorpporate well with polymer ellectrolyte mattrix and provide relatively smooth surfacce matrix. Thhis smooth surrface could heelp in making g a good contaact at electrolyyte-electrode interface i whicch further improves solar ceell efficiency. In case of thhe other ILs studied by uss, similar surfface modificaations were obbserved and could c be direcctly correlatedd to the plasticcizing effect due d to the low w viscosity IL Ls. onfirmation of o surface modifications m and crystallinity changees due to Further co inncorporation of o ILs in PEO O were done with w the polarrized optical microscope m (P POM). The PO OM micrograaphs are shownn in Fig. 12. It is noted thatt pure PEO fillm (Fig. 12a) shows s well seemicrystalline nature in whhich large sizze spherulites are tightly innterconnected with each otther. Adding NaI N and I2 intoo PEO matrixx (Fig. 12b) thhe spherulite siize becomes small s while am morphous region (black porrtion) increasees. It was alsoo noticed that doping of low w viscosity IL L into polym mer electrolyyte matrix (P PEO:NaI:I2+IL L) shows fuurther improvvement in am morphicity where w blackishh portion inccreases drastiically (Fig. 12c). 1 This shhows good aggreement with h our DSC annd ionic conduuctivity measuurements. It is i also observved that the siize and distrib bution of the sppherulites are random and have h wide variation. This iss the reason foor the widenin ng and shiftingg of the meltinng peaks whichh we could notice under DSC.

(a))

(b)

(c)

Fiigure 12. Polariized optical miccroscopy (POM M) of (a) pure PE EO (b) PEO:NaII:I2 and (c) PEO O:NaI:I2+IL po olymer electroly yte matrix.

Role of IL Ls in the modiification of thhe electrochem mical properties of the PEO O has been exxplored by thee cyclic voltam mmetry in arggon (Ar) atmoosphere. The electrochemiccal reaction off low viscositty ionic liquidd, polymer eleectrolyte conttaining iodidee/iodine was carried c out. Fiigure 13 show ws typical cycclic voltammoograms of thee KI:I2:IL andd PEO:KI:I2+IIL polymer ellectrolyte mem mbrane. IL (E EMImTFSI) doped d polymeer electrolyte membrane shhows (solid linne) two well defined redoxx peaks. The first peak is assigned a to thhe oxidation of o iodide to trriiodide: 3I- -2 2e→ I3-. At hiigher potentiaal the electro-ggenerated triioodide oxidizess to iodine: I3-- e- → 3/2 I2 and shows seecond peak. The T IL (EMIm mTFSI) and PE EO are electroochemically innactive in scan nned electrochhemical windoow. In absencee of polymer, KI:I K 2:IL givess two redox peeak (dotted cu urve). But at lower l potentiaal, it shows onnly one oxidaation peak whiich may be duue to limited solubility of the t iodide saltt in organic solvent. Interestingly when polymer is cooated on electtrode (PEO:K KI:I2+IL), peakk observed at lower potenttial was arounnd 100 mV

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positively shifted and peak at higher potential was about 100 mV negatively shifted. This shifting of the peaks in the film form may be attributed to interaction between IL, polymer matrix and iodide/iodine complex. This interaction of IL and polymer electrolyte containing redox couple has been observed in our all low viscosity ILs-PEO polymer electrolyte system. The photoelectrochemical performance of dye sensitized solar cell (DSSC) was calculated by the following equations: FF =Vmax . Jmax / Voc .Jsc

(5)

η =Vmax .Jmax / Pin × 100 =VocJsc FF / Pin × 100

(6)

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where FF is fill factor, η is the light-to-electricity conversion efficiency, Jsc is the shortcircuit current density (mAcm−2), Voc is the open-circuit voltage (V), Pin is the incident light power, and Jmax (mAcm−2) and Vmax (V) are the current and voltage in the J–V curve respectively at the point of maximum power output. The current density versus voltage (J–V) characteristic of the DSSC following two-step casting was evaluated at one sun condition (100 mWcm−2 at AM1.5) and shown in Fig. 14.

Figure 13. Cyclic voltammograms of KI:I2:IL (dotted red line) and PEO:KI:I2+IL (solid black line) operated at 50 mV/S scan rate.

The overall collective data of low viscosity ILs doped PEO polymer electrolytes are shown in Table 5. It is clear that doping low viscosity IL enhances the overall value of DSSC efficiency. The doping of IL enhances the ionic conductivity of PEO polymer electrolyte matrix by suppressing the crystallinity which was affirmed by the DSC measurement and surface features under POM, SEM and AFM. The short circuit current density value Jsc is directly related with ionic conductivity of polymer electrolyte [1,21,22]. In almost all low viscosity ILs-PEO electrolyte systems we observed one to two order of σ enhancement by simply IL doping which directly enhances the values of Jsc and overall DSSC efficiency.

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It is also noted that for efficient DSSC using this low viscosity ILs-PEO system apart from the conductivity enhancement the fabrication method of DSSC is also important. In different IL-PEO system the conductivity maxima (mentioned above in conductivity explanation) observed at different composition (reason not clear). Bhattacharya et al. [93] reported a DSSC using PEO:KI:I2+EMImSCN system showing efficiency 0.63 % at 100 mW/cm2 light intensity. Using same electrolyte and composition but different approach (two step casting of electrolyte in our case) we have achieved an efficiency of 1.29 % at 100 mW/cm2 [81]. This enhanced efficiency could be observed due to two step casting process, high humidity and better ionic conductivity. Additionally the two step process may improve electrode-electrolyte interface contact (stated below) and overall efficiency.

Figure 14. J–V curve of the DSSC using maximum conductivity PEO doped with an ionic liquid 1methyl 3-propyl imidazolium iodide (PMII). IL-doped solid polymer electrolyte at 100mWcm−2.

Table 5. The ionic conductivity ( ) and photovoltaic parameters of polymer electrolyte with and without low viscosities ionic liquids Composition σ (wt%) (S cm-1) *PEO:KI:I2 (80:20:2) 8.80x10-6 *PEO:KI:I2+80wt%EMImSCN7.62x10-4 PEO:KI:I2 (75:25:2.5) 2.02x 10-5 *PEO:KI:I2+80wt%EMImSCN2.25x10-5 PEO:KI:I2+30wt% EMImTFSI 8.82x10-5 PEO:KI:I2+40 wt%EMImDCN 4.72x10-4 PEO:NaI:I2 (87.5:12.5:1.25) 2.02x10-6 PEO:NaI:I2+80wt%EMImTFO 4.72x10-5 PEO:NaI:I2+40wt%EmImDCN 3.76×10 5

Jsc (mA cm-2) 0.22 1.88 2.47 1.89 4.02 5.08 1.51 5.65 1.43

Voc (Volts) 0.74 0.63 0.82 0.65 0.77 0.81 0.83 0.79 0.83

FF (%) 77.4 50.7 50.8 52.0 56.0 49.0 61.2 55.0 61.2

Η (%) 0.1 0.6 1.04 0.63 1.75 2.00 0.76 2.45 0.74

Ref. 79 79 80 80 82 83 84 84 88

* represents the DSSC using one step casting (solid film between electrodes) of IL-PEO polymer electrolyte

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The roles of these low viscosities ILs on charge transfer process under illumination are shown in Fig. 15a. It is known that DSSC using solid polymer electrolyte resulted in low efficiency because of incomplete wetting of the electrodes by polymer electrolyte or poor interfacial contact. The doping of low viscosity IL could reduce in crystallinity (as discussed above) and hence improved the interfacial contact area (electrode-electrolyte interface) is expected. This provided good redox couple mobility within low viscous IL/polymer electrolyte matrix which might contribute favourably for the better charge transfer and high photocurrent generation. Moreover, low viscous IL/polymer matrix could also assist in the electron transfer from polymer-IL matrix towards dyes absorbed nanoporous TiO2 electrode (Fig. 15b) which would certainly enhance the Jsc and overall efficiency.

(a)

(b)

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Figure 15. Schematic presentation of the DSSC (WE/TiO2/D/PE+IL/CE), showing mechanism of electron transfer at electrode-electrolyte interface (a) without IL and (b) the interface region with improved contact due to addition of IL where D is dye, PE is polymer electrolyte.

5. PERFORMANCE AND STABILITY As mentioned in preceding sections stability of the device is a big challenge. However, only few papers focus on this issue. The cells suffer from degradation of various components and the combinations as well. Degradation of the dye due to exposure of the UV is probably the major in this list. But such problems are not specific to the polymer electrolyte based systems. In the following lines we discuss our efforts towards the study of the degradation of polymer electrolyte based cells and sort out the reason for that. To be more specific, we studied the response of the cells mentioned in Ref. [62]. These cells show stable performance in the dry outdoor atmosphere (tested for temperature 20-30°C and RH 34%-45%) and good repeatability of the parameters were recorded. However, when exposed to a humid atmosphere (more than 60%) the response of the cells in terms of their Jsc, Voc and hence the efficiency has been noted to decrease with time. After 24 h they stabilize to lower values. We could observe some fractal-like patterns in these cells with the naked eye which are supposed to be responsible for degradation. Since fractal growth in similar systems has already been observed [94]; such growth is known to hamper the device performance (Fig. 16). Similar observations have been seen in our other cells [86]. So with the presumption of growth of iodine fractals and also to understand the decrease in the cell parameters we have scanned the

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cells at different regions under POM. Figure 17 shows different regions of the cell. A regular spherulitic structure surrounded with amorphous regions [95] of the polymer could be seen outside the interface area (Figure 17a). This outside region has no voids or free space left by the polymer, covering the region uniformly. Near the interface, the concentration of the spherulites is found to be high and moderately uniform coverage of the electrode could be observed (Fig. 17b). As we approach the central region of the interface (Fig. 17c), the polymer coverage decreases. Some parts of the interface even become polymer-free and hence the actual active interface area becomes less than that covered by the dye soaked TiO2 layer. Near the central region, only a finger-like film could be seen (Fig. 17d). These fingerlike structures appear to be very similar to the fractal-like growths. However, in our system, these are not the iodine fractals but the polymer film itself in web shape. This has been verified by the FTIR studies of the scrap obtained after opening up a cell. The FTIR spectra were similar to that of the polymer electrolyte as shown in [60]. This removal of the electrolyte is due to the shrinkage of the polymer film with time. PEO is well known to show humidity dependent characteristics. The cells, after fabrication, when left in the room ambient with RH more than 60%, get exposed to the humid atmosphere (as expected for the practical sites). The polymer, after absorbing water from the atmosphere, becomes relatively less viscous and its amorphous parts shrink randomly around the spherulites. In other words, the high humid atmosphere allows a re-distribution of the crystalline parts or spherulites. In the regions away from the interface, the polymer finds its edges free and can shrink easily. Therefore, no void is left. However, when the polymer is constrained between the glass plates it cannot shrink freely leaving behind parts of the electrode/interface area without any polymer electrolyte. Thus the actual interface area becomes much smaller than the area of the TiO2 which is generally used for estimating the efficiency of the cells. Similar observations have been noted for the Si/PEO interface earlier. It may be recalled that we have adopted the two step method of casting the polymer films for cell fabrication.

Figure 16. Photographs of the DSSC using PEO:PMII ionic liquid/I2 solid polymer electrolyte at 70% RH. The fractal formation is clearly viewed in photograph.

Therefore, the first layer of the polymer which is due to the dilute solution and very thin is expected to remain intact. We could not notice any change in this layer by POM. For further probing we opened a cell and scanned its surface under SEM. Figure 18 shows the SEM image of a part of the working electrode exposing the shrunk rib-like polymer film. The first

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laayer of the po olymer could also be seen around this web like figuures and appeears a little sccratched aroun nd the thickerr parts (black in Fig. 18a) possibly p due to t the openingg up of the ceells. When observed from an oblique angle, a the clusstering of the polymer layeer could be cllearly seen an nd the presencce of the bottoom layer is also confirmed (Fig. 18b). This T bottom laayer is responsible for the residual, r thouggh small, efficciency of the cells. This thhick second laayer which iss also flexiblee at room temperature unndergoes shrinnkage whereaas the first reemains intact.

Fiigure 17. The POM pictures off the DSSC at different d regionss of the cells: (aa) outside the in nterface; (b) neear the interfacee; (c) interface closer c to centrall region; (d) cen ntral part of thee interface.

(a)

(b b)

Fiigure 18. (a) SE EM images of th he opened cell showing s the shrrunk polymer. (b b) Oblique view w showing th he presence of th he bottom layerr (marked by arrrow). Dye-Sensitized Solar Cells and Solar Cell Performance, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook Central,

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CONCLUSIONS The discussions and results presented above confirms that to achieve better conductivity of the PEO based polymer electrolytes it is necessary to reduce the crystallinity of PEO. The addition of various inorganic fillers, blending or by incorporation of low viscosity molten salts (ionic liquids) can result to drastic reduction in crystallinity and thereby increase in the conductivity. The highly conducting (modified) polymer electrolyte shows surface features which are ideal for proper wetting of the electrode and good interface formation. However, conductivity is not the sole factor for the better performance of the devices. The replacement of liquid electrolytes by such polymer electrolytes shows stable behavior of the solar cells. In case of dye sensitized solar cell (DSSC), the performance of IL doped PEO electrolytes are found to be very stable and show appreciable efficiency. Further possibilities of improvements are open and needs to be explored properly.

ACKNOWLEDGMENTS This work was supported by the School of Engineering & Technology, Sharda University, G. Noida, India.

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[88] Singh, P. K., Jadhav, N. A., Mishra, S. K., Singh, U. P. and Bhattacharya, B (2010). Application of ionic liquid doped solid polymer electrolyte. Ionics, 16, 645-648 [89] Singh, P. K., Bhattacharya, B. and Nagarale, R. K (2010). Effect of nano TiO2 dispersion on PEO polymer electrolyte property. Journal of Applied Polymer Science, 118, 2976-2980 [90] Singh, P. K. (2011). Importance of ionic liquid doped solid polymer (PVPI) electrolyte. International Journal of Sustainable Energy, 30,270-276 [91] Singh, P. K., Bhattacharya, B., Mehra, R. M. and Rhee, H. W. (2011). Plasticizer doped ionic liquid incorporated solid polymer electrolytes for photovoltaic application. Current Applied Physics, 11, 616-619 [92] Singh, P. K., Tomar, S. K., Pandey, S. P., Rhee, H. W. and Bhattacharya, B (2011). Porous nanocrystalline TiO2 electrode and poly (N-methyl 4-vinylpyridine iodide)ionic liquid solid polymer electrolyte for device application. International Journal of Nanotechnology (Accepted) [93] Bhattacharya, B., Tomar, S. K. and Park, J.K. (2007). A nanoporous TiO2 electrode and new ionic liquid doped solid polymer electrolyte for dye sensitized solar cell application. Nanotechnology, 18, 485711-485714 [94] Chandra, A. (1996). Anion clustering and fractal pattern growth in ion conducting polymeric matrix. Solid State Ionics, 86-88, 1437-1442 [95] Lee, J. Y., Bhattacharya, B., Kim, Y. H., Jung, H.T. and Park, J. K. (2009). Self degradation of polymer electrolyte based dye-sensitized solar cells and their remedy. Solid State Communications, 149, 307-309

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INDEX

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A absorption spectra, 2, 5, 17, 34, 87, 88, 165 acetic acid, 80 acetone, 221 acetonitrile, 82, 157 acid, 2, 80, 83, 87, 90, 156, 178, 197, 223 acidic, 85, 86, 176 acrylate, 261 adaptations, viii, 115, 122 additives, viii, 71, 75, 83, 86, 87, 91, 237, 242, 243, 245, 248, 249 adsorption, 78, 82, 83, 84, 85, 86, 228 AFM, 244, 252, 254 aggregation, 76, 81, 83, 84, 86, 165 agriculture, 154 air temperature, 139, 147, 151 alkane, 89 alternative energy, 237 ambient air, 138, 139, 143 amine, 157, 245 amino, 87, 205 ammonium, 157 anchoring, x, 76, 219, 222 aniline, 242, 261 anisotropy, 6 annealing, 3, 6, 19, 21, 22, 30, 31, 32, 36, 37, 38, 61, 157, 168 anodization, 173 anti-reflection (AR), ix, 175 ARC, 186, 187 argon, 253 aromatic hydrocarbons, 14 artificial intelligence, 57, 58 Asian countries, 42 assessment, viii, 99, 100, 101, 103, 104, 105, 107, 108, 109, 110, 111, 112, 113, 231 atmosphere, 55, 157, 205, 253, 256

atomic force, 252 atoms, 10, 19, 45, 61, 87, 229, 236 Au nanoparticles, 208

B band gap, 2, 4, 5, 11, 28, 29, 45, 46, 47, 50, 62, 65, 85, 156, 165, 169, 194, 237, 238, 240 bandgap semiconductors, x, 219, 220, 221 barriers, 241 base, viii, 2, 8, 19, 34, 36, 39, 86, 99, 100, 102, 103, 106, 107, 108, 109, 116, 118, 142, 144, 169, 182, 192, 194, 198, 208, 236, 264 batch process, vii, 41, 55, 56, 57, 58, 59 batteries, 236, 261 Beijing, 1, 32 benefits, 48, 139, 180 bias, viii, 27, 54, 76, 79, 99, 109 binding energy, 17 biomass, 107, 111, 139, 153 biopolymer, 264 biotechnology, 190 biotic, 104 blend films, 3, 17 blends, 15, 18, 29, 31, 34, 36, 39, 156, 171, 173, 247 bonding, 169 bonds, 8, 45, 61, 62, 65, 76, 172 branching, 11 Britain, 68 bulk materials, 42, 181 burn, 54 by-products, 102

C cadmium, 259 calorimetry, 250

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carbon, 19, 25, 46, 72, 113, 156, 169, 171, 248, 249, 263 carbon atoms, 19 carbon dioxide, 72 carboxyl, 81, 86 case studies, 111 case study, 113 casting, 30, 183, 254, 255, 257 category a, 248 cation, 6, 10, 80, 245, 246, 247 cell surface, x, 116, 176, 177, 178, 208, 209, 210, 211 ceramics, 232, 261 challenges, 43, 111, 176 charge density, 87 chemical, vii, 2, 10, 16, 17, 22, 25, 26, 41, 42, 47, 48, 110, 111, 113, 173, 178, 180, 183, 205, 237, 239, 246 chemical etching, 178 chemical structures, 2, 16, 17, 22, 26 chemical vapor deposition, 42, 48, 180 chemical vapor phase deposition (CVD), vii, 41, 48, 49, 51, 52, 57, 62, 65, 169 China, 1, 32, 71, 92, 93, 108 chitosan, 259, 260 chlorobenzene, 7, 30 chloroform, 30 circulation, 139, 142 CIS, 64, 66, 100, 106, 108, 111, 112, 156, 157, 165, 166, 167, 168 clean energy, 72, 176 cleaning, 49, 58, 142 climates, 65, 106 clustering, 258, 265 clusters, 30, 45, 57, 156, 174 coatings, 59, 85, 176, 180, 181, 186, 191, 192, 193, 194, 216 cobalt, 220 College of Engineering, ix, 137 color, 138, 186, 193 combustion, 72 commercial, viii, 42, 99, 100, 109, 118, 122, 186, 187, 207, 208, 216, 239 communication, 39, 69 compatibility, 45, 121 competition, 22, 26 complexity, 139 composites, 2, 38, 172 composition, viii, 99, 100, 133, 171, 225, 246, 255, 262 compounds, 13, 14, 85, 156 concentration photovoltaic cells (CPV cells), viii, 115

concentration ratios, 134 conductance, 46 conduction, 45, 47, 60, 72, 75, 168, 220, 237, 238, 239, 246, 247, 248, 252 conductivity, 28, 47, 61, 236, 240, 242, 243, 244, 245, 246, 247, 249, 250, 252, 253, 254, 255, 259, 261, 262 conductor, 32 conference, 69, 112 configuration, viii, 8, 29, 46, 47, 48, 55, 56, 57, 72, 76, 78, 81, 115, 116, 121, 122, 124, 126, 134, 236, 240 confinement, 21, 57, 61 conjugation, 2, 78, 81, 89, 91, 262 constituent materials, 103 construction, 6, 31, 220, 221 consumption, 101, 176 contamination, 25, 27, 37 continuous process, vii, 41, 55, 56, 57 cooling, 55, 58 coordination, 172 copolymers, 5, 36, 243, 245 copper, 64, 157, 173, 223 corrosion, xi, 64, 195, 235, 240, 241 cosmic rays, 156 cost, v, vii, ix, x, 2, 32, 33, 41, 42, 43, 47, 48, 50, 55, 57, 60, 65, 66, 108, 111, 122, 139, 165, 169, 175, 176, 180, 183, 190, 191, 192, 193, 194, 197, 205, 207, 216, 219, 222, 236, 239, 241 covering, 17, 85, 176, 257 cracks, 225 crop, ix, 137, 138, 139, 153 crop drying, 153 crops, 138, 153 crown, 245, 249 crust, 102, 110 crystal structure, 38, 169 crystalline, viii, x, xi, 21, 30, 42, 45, 47, 60, 64, 65, 66, 85, 99, 100, 106, 107, 111, 164, 169, 172, 176, 177, 179, 186, 187, 193, 235, 236, 239, 241, 242, 252, 257 crystalline Si-based cells, viii, 99 crystallinity, 19, 31, 51, 55, 61, 241, 243, 244, 246, 247, 250, 251, 252, 253, 254, 256, 259 crystallites, 45, 47, 62 crystals, 18, 21, 30, 31, 164, 186, 191, 208 cubic system, 164 cure, 59 customers, 44, 57 cycles, 58, 105, 106, 110, 111, 180

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D dark conductivity, 61 database, 111 decay, 72, 76, 79, 82, 90 decomposition, 223, 240 defects, 27, 54, 165, 169 degenerate, 47 degradation, vii, 6, 25, 30, 37, 41, 43, 46, 61, 62, 64, 228, 229, 256, 265 degree of crystallinity, 31, 252 dehydration, 153, 154 density functional theory, 90 deposition, vii, x, 3, 6, 19, 21, 27, 28, 41, 42, 43, 46, 48, 49, 50, 51, 54, 55, 56, 57, 58, 61, 62, 65, 176, 180, 183, 188, 200, 201, 203, 205, 221 deposition rate, 49, 55, 61 deposits, 207 depth, vii, 41, 181, 183, 198 derivatives, 5, 8, 10, 14, 15, 17, 34, 35, 86 desorption, 228 detection, 133 detonation, 174 developing countries, 153 deviation, 51 diaphragm, 132 dielectric constant, 247, 262 dielectric permittivity, 208 differential scanning, 250 differential scanning calorimetry (DSC), 90, 246, 250, 251, 252, 253, 254, 260 diffraction, ix, 155, 157, 159, 160, 163, 164, 166, 167, 169, 190 diffusion, ix, 2, 31, 32, 38, 46, 47, 51, 55, 86, 155, 156, 161, 172, 200, 208 diffusion process, 46 dimensionality, 156 diodes, 11, 25, 35, 37, 38, 45, 172 direct adsorption, 86 disorder, 244, 251 dispersion, 157, 164, 169, 265 displacement, 122 dissociation, 2, 22, 36, 247 distilled water, 221 distribution, ix, 22, 27, 51, 110, 115, 116, 118, 119, 120, 122, 124, 130, 132, 134, 167, 215, 244, 253, 257 divergence, 103, 106, 120, 121 diversity, 58 DOI, 94, 96, 110 donors, 2, 6, 8, 9, 10, 15, 17, 34, 36, 76, 172, 173, 236 dopants, 102, 242

doping, 46, 47, 85, 250, 251, 253, 254, 256 DOT, 14, 15 double bonds, 8 drawing, 118, 143 dream, 237 drug delivery, 10 drying, ix, 48, 137, 138, 139, 140, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 221, 225 durability, 48 dye sensitized solar cells (DSSC), x, 235 dyes, vii, x, 10, 35, 72, 75, 76, 79, 80, 81, 87, 89, 219, 220, 221, 222, 228, 231, 240, 242, 244, 256 dye-sensitized solar cells (DSCs), viii, 71, 72

E economic evaluation, 112 ecosystem, 106 Elam, 95 electric field, 2, 22, 36, 46, 238 electrical conductivity, 47, 244 electrical properties, 5, 45, 250 electrical resistance, 47 electricity, viii, 1, 99, 101, 103, 104, 105, 107, 108, 109, 110, 111, 112, 113, 216, 220, 236, 240, 241, 254 electrochemical deposition, 205 electrochemistry, 237 electrodes, 2, 25, 47, 85, 86, 91, 221, 222, 223, 237, 240, 241, 255, 256 electroluminescence, 35 electrolyte, viii, x, 33, 71, 72, 75, 76, 77, 80, 81, 86, 87, 88, 90, 91, 220, 222, 223, 225, 226, 227, 228, 231, 235, 236, 237, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 259, 260, 261, 262, 263, 264, 265 electromagnetic, 138, 214 electron, ix, 2, 4, 5, 6, 9, 11, 14, 15, 18, 23, 25, 27, 30, 32, 37, 45, 47, 72, 73, 74, 75, 76, 77, 79, 80, 81, 82, 83, 85, 86, 87, 90, 91, 155, 156, 157, 160, 163, 167, 171, 172, 173, 178, 183, 191, 192, 197, 200, 203, 220, 225, 238, 239, 256 electron beam lithography, 183, 192 electron cyclotron resonance, 191 electron diffraction, ix, 155, 160, 163, 167 electron microscopy, ix, 155, 173, 178 electrons, 2, 18, 45, 65, 72, 74, 75, 78, 80, 82, 83, 85, 89, 161, 163, 168, 169, 220, 225, 230, 236, 237, 238 electroplating, 191 embossing, ix, 175, 190, 191, 197 emission, 13, 53, 72, 107, 133, 194

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encapsulation, vii, 32, 41, 43, 45, 59, 64 energy, vii, viii, ix, x, 1, 2, 4, 5, 9, 10, 11, 15, 17, 22, 24, 25, 26, 28, 31, 34, 36, 41, 45, 46, 50, 53, 55, 58, 66, 71, 72, 73, 74, 75, 76, 77, 78, 81, 82, 83, 84, 85, 86, 87, 88, 89, 91, 92, 99, 100, 101, 102, 103, 105, 106, 107, 108, 109, 110, 111, 112, 138, 139, 140, 143, 153, 155, 156, 157, 161, 165, 168, 169, 171, 172, 173, 175, 176, 180, 207, 220, 235, 236, 237, 238, 240, 247, 259 energy consumption, 101 energy efficiency, 89 energy input, 102 energy supply, 66, 103, 106 energy transfer, 10 engineering, xi, 62, 65, 91 environment, 64, 104, 138, 156 environmental impact, 1, 103, 104, 105, 112 ester, 2 etching, 176, 177, 178, 183, 184, 185, 186, 192, 194, 197 ethanol, 82, 113, 221, 248 ethylene, 59, 89, 220, 222, 228, 231, 242, 244, 260, 261, 262, 263, 264 ethylene glycol, 220, 222, 228, 231, 261 ethylene oxide, 260, 261, 262, 263, 264 Europe, 67, 69 evaporation, ix, xi, 6, 30, 53, 137, 138, 147, 165, 168, 200, 203, 235, 240, 241 evolution, 110, 227 excitation, 50, 168, 169, 218 exciton, ix, 2, 3, 4, 17, 19, 22, 23, 24, 29, 31, 32, 38, 155, 156, 159, 161, 171, 172 experimental condition, 6 exposure, 61, 190, 220, 256 external costs, 110 external shocks, 195 extinction, x, 74, 219, 222 extraction, 2, 23, 25, 103, 104

F fabrication, vii, x, 2, 6, 29, 31, 38, 41, 42, 43, 48, 60, 65, 66, 82, 165, 172, 173, 175, 178, 180, 182, 183, 188, 190, 192, 197, 200, 208, 222, 255, 257 farmers, 138 feedstock, 113 Fermi level, 19, 26, 47, 74, 75, 239 fillers, xi, 235, 237, 243, 259, 261 film formation, 165, 236, 241 film thickness, 38, 51, 61, 203, 222 films, x, 3, 17, 19, 21, 29, 30, 31, 34, 38, 42, 45, 46, 48, 50, 51, 55, 59, 61, 62, 64, 65, 85, 86, 108, 163, 164, 165, 168, 169, 173, 190, 191, 195, 197,

200, 203, 218, 219, 220, 221, 222, 224, 225, 227, 228, 229, 230, 231, 239, 246, 250, 252, 257 financial, 92, 139, 154 first generation, 42 flammability, 248 flexibility, 1, 57, 246 fluorescence, 15, 35 fluorine, 157, 221 foils, 45 food, 111, 138, 153 food products, 111 force, 5, 80, 82, 209, 252 formation, ix, 2, 5, 10, 23, 30, 76, 77, 81, 82, 86, 104, 155, 159, 163, 165, 168, 200, 201, 225, 229, 236, 241, 246, 257, 259 formula, 166 fouling, 58 fractal growth, 256 free volume, 21 freedom, 58 fruits, ix, 137, 154 fuel cell, 259, 263 fullerene, 2, 3, 5, 18, 30, 33, 34, 36, 38, 83, 100, 156, 171, 172, 173 functionalization, 10 fusion, 250, 252

G gel, 174, 205, 222, 261, 262, 263 general semiconductor, vii, 41 geometry, 6, 8, 179, 188 Germany, 69 glass transition, 191, 236, 244, 246 glass transition temperature, 191, 236, 244, 246 global warming, 72 glycol, 220, 222, 228, 231, 244, 261 gold nanoparticles, 172, 218 grading, 47 grain boundaries, 45, 85, 179 grain size, 166, 169 gratings, ix, 175, 181 Great Britain, 68 Greece, 219 greenhouse, 103, 105, 106, 107, 110, 153 greenhouse gases, 105, 107 growth, vii, 21, 31, 41, 45, 47, 48, 49, 50, 51, 52, 55, 61, 66, 200, 205, 206, 256, 265 growth rate, 21, 31, 51 growth time, 205, 206

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H halogen, 88, 225 harvesting, x, 2, 15, 138, 176, 207, 212 hazards, 105 health, 105, 106, 107 heating rate, 251 height, 181, 184, 192, 194 helicity, 156 heterojunction solar cells, ix, 33, 34, 35, 36, 38, 39, 83, 155, 156, 158, 159, 161, 164, 165, 168, 171, 172, 173 HFP, 248, 249, 263 high charge-transfer, x, 219, 221 history, 43, 110 homogeneity, 120, 121 human, 72, 105, 106, 107 human health, 105, 106, 107 humidity, 138, 139, 143, 255, 257 hybrid, vii, 8, 14, 32, 35, 36, 41, 55, 56, 57, 100, 109, 139, 153, 169, 173, 263 hybrid cell, 100 hybrid process, vii, 41, 55, 57 hydrocarbons, 14 hydrogen, 47, 111, 180, 237 hydroxide, 157 hygiene, 138

inorganic fillers, xi, 235, 237, 243, 259 insects, 138 insertion, 3, 13, 22, 23, 24, 26, 28 inspections, 48 insulation, 59, 76, 140 integration, 58 interface, 2, 21, 22, 23, 26, 27, 28, 30, 32, 39, 46, 47, 51, 64, 161, 167, 168, 169, 181, 183, 186, 237, 240, 253, 255, 256, 257, 258, 259 interfacial layer, 25, 38 interference, 181, 183, 192 investment, 57, 138 iodine, 75, 81, 88, 90, 222, 241, 242, 243, 246, 253, 256 ionic conduction, 248 ionic liquid (IL), xi, 235, 241, 250, 253 ionization, 19 ions, 75, 81, 83, 85, 89, 91, 184, 222, 225, 228, 230, 246, 247, 248, 249, 252 IR transmission, 51 Iraq, 153 iridium, 81 irradiation, 5, 6, 7, 8, 53, 64, 74, 75, 89, 90, 91, 115, 161, 228, 239, 240, 245, 263 Islam, 92 Israel, 232 Italy, 67, 68, 115 I-V curves, 225

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I illumination, vii, 3, 4, 5, 8, 10, 18, 23, 24, 26, 42, 61, 62, 65, 91, 124, 125, 126, 157, 158, 169, 198, 205, 209, 213, 225, 228, 239, 248, 256 images, 122, 124, 179, 184, 185, 188, 195, 196, 198, 201, 205, 206, 210, 211, 214, 224, 225, 244, 252, 258 impact assessment, 104, 105 imprinting, 196, 198 improvements, xi, 21, 61, 65, 85, 92, 107, 115, 156, 259 in transition, 28 incidence, 180, 181, 186, 187, 188, 189, 192, 193, 194, 200, 201, 202, 203, 204 Incident Photon-to-current Conversion Efficiency (IPCE), xi India, 235, 259 indium, 3, 19, 36, 38, 47, 64, 100, 156, 173, 200, 240 industrialized countries, 106 industries, 42 industry, 48, 72, 110, 220, 236 infrastructure, 104 initial state, 61

J Japan, 68, 108, 155, 171

K kinetics, 76, 80, 154, 262 KOH, 176, 197

L Lambda Research TracePro, viii, 115, 116, 126 lamination, 59 landscape, 104 laser scribing, vii, 41, 43, 52, 53, 54, 59, 65 lattice parameters, 164 lead, 6, 25, 72, 74, 78, 80, 85, 86, 87, 91, 103, 104, 112, 124, 164, 190, 198, 207, 208 leakage, xi, 27, 54, 143, 235, 241 learning, viii, 99, 107, 109 lending, xi, 235, 259 lens, 116, 117, 118, 122, 123, 124, 125, 126, 130, 131, 132

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life cycle, viii, 99, 100, 101, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112 lifetime, 43, 46, 59, 64, 76, 77, 78, 80, 89, 107, 109 ligand, 172 light, ix, x, 2, 3, 5, 7, 9, 10, 11, 15, 17, 18, 19, 25, 27, 28, 29, 31, 32, 35, 37, 38, 43, 45, 46, 47, 48, 51, 61, 63, 64, 65, 66, 69, 74, 76, 86, 91, 115, 124, 131, 134, 147, 161, 168, 169, 172, 175, 176, 177, 180, 181, 183, 187, 188, 189, 190, 191, 195, 198, 203, 205, 207, 208, 209, 210, 212, 215, 216, 218, 219, 220, 221, 228, 237, 238, 240, 242, 245, 254, 255, 259 light conditions, 64 light emitting diode, 37 light scattering, 48, 51, 187, 208, 210, 218 light transmission, 191 light-emitting diodes, 11, 25, 35, 37, 38 liquids, xi, 235, 248, 250, 255, 259, 264 lithium, 86, 89, 91, 236, 241, 261 lithography, 182, 183, 185, 190, 192 Luo, 92, 95, 96, 232, 233

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M macromolecules, 35 magnetic properties, 156 magnitude, 5, 246 majority, 62, 225 management, 110, 153, 154 manipulation, 55 manufacturing, vii, 41, 42, 43, 48, 56, 66, 110, 207 market share, 42, 45, 66 Marx, 39 masking, 178 mass, vii, ix, 41, 47, 55, 74, 105, 118, 137, 138, 139, 143, 145, 147, 148, 149, 151, 180, 198, 205 mass loss, 143 materials, ix, x, 1, 2, 5, 8, 9, 10, 11, 14, 15, 22, 25, 26, 32, 35, 42, 43, 46, 57, 62, 65, 66, 69, 75, 85, 101, 102, 103, 107, 110, 111, 115, 133, 134, 137, 156, 170, 171, 181, 183, 185, 188, 192, 203, 208, 220, 236, 241, 248, 259 materials science, 248 matrix, 45, 62, 242, 244, 245, 247, 248, 249, 250, 251, 252, 253, 254, 256, 265 mature thin film deposition technology, vii, 41 measurements, viii, xi, 23, 25, 28, 38, 62, 90, 115, 118, 131, 132, 134, 143, 171, 179, 188, 194, 198, 212, 214, 225, 230, 253, 254 mechanical properties, 243 melting temperature, 251, 252 melts, 264 membranes, 259, 263, 264

metal complexes, 228 metal contacts, vii, 41, 157 metal nanoparticles, 218 metal oxides, 25, 28, 36 metallo-organic ruthenium, x, 219, 221 metals, x, 18, 25, 26, 37, 102, 110, 219, 222 meter, 69, 132 methanol, 222 methodology, 118, 134 microcrystalline, vii, 6, 41, 45, 47 micrometer, 176 microscope, 53, 157, 179, 253 microscopy, ix, 155, 173, 178, 252, 253 microspheres, 173 microstructure, 19, 30, 45, 163, 168, 169, 180 microstructures, 157 migration, 138, 159, 171 Ministry of Education, 1, 171 missions, 109 mixing, 3, 205, 221 model system, 80 modifications, 116, 122, 123, 134, 240, 246, 253 modules, vii, 41, 43, 44, 47, 54, 59, 60, 64, 65, 104, 106, 107, 108, 109, 110, 111, 112, 174, 216, 260 moisture, ix, 25, 59, 64, 137, 138, 139, 143, 144, 145, 147, 148, 149, 151, 152, 157 moisture content, 138, 139, 143, 144, 145, 147, 148, 149, 151, 152 mold, 190, 191, 195, 196, 197, 198 molecular beam, 194 molecular beam epitaxy, 194 molecular structure, x, 14, 15, 81, 87, 88, 89, 157, 219, 222 molecular weight, xi, 2, 29, 38, 235, 241, 242, 243, 244, 246, 249, 261 molecules, ix, 2, 75, 76, 77, 155, 156, 161, 167, 220, 221, 228, 229, 231, 239 molybdenum, 3, 38 monolayer, 26, 183, 184, 186, 210, 211, 215, 222 monomers, 29, 246 Moon, 93, 96 morphology, ix, x, 3, 14, 30, 31, 34, 39, 52, 54, 85, 172, 175, 205, 215, 219, 221, 225 multilayered structure, 210, 215

N nanocrystals, ix, 31, 72, 155, 164, 169, 174, 218 nanofibers, 85 nanohorns, 174 nanoimprint, 176, 190, 191, 193 nanomaterials, 112, 156, 169 nanometer scale, 196

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Index nanometers, 31, 86, 192 nanoparticles, x, 76, 100, 105, 112, 157, 165, 167, 172, 173, 174, 175, 207, 208, 209, 211, 212, 214, 215, 217, 218, 221, 225, 242, 243, 244, 248, 249, 260, 263 nanorods, 32, 39, 85, 92, 172, 203, 205, 206 nanoscale structures, 170 nanostructured materials, 192 nanostructures, 31, 32, 85, 156, 174, 190, 191, 198, 203 nanotechnology, xi, 110 nanotube, 85, 86, 242, 261 nanowires, 21, 32, 85 natural resources, 105, 109 Netherlands, 99, 106, 108, 110, 111 next generation, 156 nickel, 38 NIR, 51, 209, 212, 214, 215 nitrogen, 29, 86, 157, 200, 201 nodes, 35 non-toxic source material, vii, 41 North America, 65 novel polymer, research, vii, 1 NPS, 249 NPT, 245, 249 nucleation, 47 nuclei, 200

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O o-dichlorobenzene, 157 oil, 237 oligomers, 2, 3, 9, 33, 173, 241 operations, 57, 58 optical microscopy, 253 optical properties, ix, 47, 155 optical systems, 122, 125, 126, 134 optimal performance, 31 optimization, vii, 1, 25, 29, 54, 66, 211, 220 organic compounds, 85 organic photovoltaic cells, vii, 1, 3, 32, 38 organic solvents, 30, 157, 248 oscillation, 47 oxidation, 25, 107, 161, 253 oxygen, 19, 25, 59, 228, 236 ozone, 3, 19, 20, 21, 36, 104, 107

P parallel, 55, 190, 212 passivation, 180 PEO (polyethylene oxide), x, 235

273

performance indicator, 110 permeation, 227 permittivity, 208, 210 perylene, ix, 14, 36, 77, 155, 156, 172 pharmaceutical, 113 Philadelphia, 68 photoconductivity, 61 photodegradation, 228 photoelectron spectroscopy, 19 photolithography, 178, 180, 192 photoluminescence, 10, 32, 156 photonics, 190 photons, 21, 31, 65, 207 photoresponse, 9 photovoltaic (PV) modules, vii, 41 photovoltaic cells, vii, viii, x, 1, 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 18, 25, 31, 32, 34, 36, 38, 39, 41, 99, 100, 101, 102, 103, 104, 105, 107, 109, 115, 116, 172, 173, 219, 220, 236 photovoltaic devices, xi, 3, 35, 36, 38, 39, 171 photovoltaics, 37, 66, 67, 68, 109, 110, 111, 112, 135, 232, 260 physical characteristics, 46 physical properties, 2, 156 physicochemical properties, 47 physics, vii, 1, 3, 33, 36, 41 piano, 118 pitch, 198 plants, 66, 156 plasticizer, 244 plastics, 45 platform, x, 2, 8, 235 platinum, 72, 220, 222, 240 PMMA, 190, 191, 197, 198, 199 polarity, 81 polarizability, 215 pollutants, 107 pollution, 138 polycyclic aromatic hydrocarbon, 14 polyether, 244 polymer blends, 34 polymer chain, 252 polymer composites, 172 polymer electrolytes, x, 235, 237, 240, 241, 243, 244, 248, 249, 254, 259, 260, 261, 262, 265 polymer films, 34, 257 polymer materials, 5, 6, 32 polymer matrix, 244, 247, 250, 254, 256 polymer oxidation, 161 polymer structure, 3, 31 polymers, xi, 2, 4, 5, 14, 29, 31, 33, 156, 173, 190, 235 polypropylene, 244

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polystyrene, x, 156, 175, 184, 185, 186 polyvinyl chloride, 195 ponds, 111 porosity, 200, 221, 225 porphyrins, 172 potassium, 222, 241 power generation, 2, 33, 42, 65, 112 power plants, 156 preparation, 205, 221, 225, 227, 228, 229, 231 preservation, 138 process gas, 50 product design, 113 product life cycle, 104 product performance, 105 production costs, 1, 220 profilometer, 221 proliferation, 101, 216 propylene, 220, 222, 231, 242, 244 protection, 27, 28, 59, 83, 85 protons, 229 prototype, 6 purification, 102 purity, 6, 28, 102 PV background, vii, 41 PVC, 195, 196 pyrimidine, 86 pyrolysis, 48

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Q quality assurance, 54, 55 quality control, 48 quantum dot, 32, 207, 218 quartz, 118, 188

R radiation, 46, 74, 106, 108, 133, 134, 138, 139, 140, 143, 146, 147, 150, 208 radio, 50 radius, 19, 31, 123, 124, 126, 131, 132, 242 radius of gyration, 242 Ramadan, 153 raw materials, 236 reactants, 59 reactions, 11 reactive oxygen, 228 recombination, 27, 34, 46, 61, 62, 72, 75, 76, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 163, 165, 169, 225, 230, 240 recombination processes, 75, 77, 83 recovery, 100

recycling, 102, 103, 106, 107, 108, 109, 110, 112 red shift, 83 redistribution, 24 reflectance spectra, 176, 185, 187, 198, 202, 205, 209, 212, 213, 214, 215 reflectivity, 191, 192, 194, 198, 203, 204 refractive index, ix, 47, 175, 176, 181, 183, 188, 192, 193, 198, 200, 203, 204, 205 refractive indices, 47, 181 regeneration, 75, 239 regulations, 45 renewable energy, vii, xi, 41, 110, 111, 112 reparation, 205 replication, ix, 175, 192, 197 reproduction, 134 requirements, viii, 51, 107, 109, 115, 122, 123, 124, 126, 127, 131, 134, 138 resistance, 4, 19, 21, 23, 24, 27, 29, 31, 46, 47, 51, 54, 60, 64, 65, 66, 221 resource availability, viii, 99, 103, 109 resources, ix, 72, 104, 105, 109, 175, 176 response, 117, 172, 180, 210, 256 restrictions, 51, 122 RIE, 184, 186 rings, 6, 14, 161, 164, 167 risks, 105 rods, 205 room temperature, 45, 75, 157, 197, 205, 209, 241, 249, 258 root, 46, 188 root-mean-square, 46 roughness, 46, 47, 48, 51, 197, 198, 252 routes, 4 rules, 34, 36 rural areas, 138 ruthenium, x, 79, 219, 221, 242, 244 ruthenium complexes, x, 219, 221

S safety, 45, 61 salts, 236, 240, 246, 248, 249, 259, 261, 263 scaling, 50, 61 scanning calorimetry, 250 scanning electron microscopy, 178 scatter, 47, 215 scattering, 15, 47, 48, 51, 181, 183, 187, 207, 210, 212, 214, 215, 217, 218 scope, 104, 112, 241 second generation, 42 seeding, 205 segregation, 21 self-aggregated nanoparticles, x, 175

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Index self-assembly, 15, 32, 183, 191 self-organization, 15, 21, 34 semiconductor, vii, xi, 28, 32, 41, 42, 45, 47, 90, 157, 161, 165, 169, 173, 207, 218, 220, 221, 222, 225, 228, 230, 237, 238, 239, 240, 242, 260, 262 semiconductors, ix, x, 35, 85, 155, 156, 157, 158, 165, 166, 168, 169, 219, 220, 221, 237, 238, 239 sensitivity, 25, 28, 30, 64 sensitization, 220, 229 sensors, 10 shape, 6, 11, 76, 78, 116, 118, 126, 177, 179, 184, 188, 257 shock, 54, 59 showing, 65, 77, 88, 186, 188, 200, 201, 225, 238, 251, 255, 256, 258 Si, vii, viii, x, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 59, 60, 61, 62, 63, 64, 65, 66, 99, 100, 102, 103, 106, 107, 108, 109, 111, 112, 165, 175, 176, 186, 188, 197, 198, 199, 204, 205, 206, 207, 208, 209, 211, 212, 214, 218, 236, 240, 241, 243, 257, 260 Si thin film solar panels, vii, 41 side chain, 5, 246 silica, viii, x, 71, 118, 175, 185, 186, 187, 207, 208, 209, 211, 242, 260 silicon, ix, 6, 35, 45, 46, 47, 51, 53, 64, 69, 72, 110, 111, 112, 113, 156, 175, 176, 177, 178, 179, 180, 183, 184, 185, 186, 187, 191, 192, 193, 197, 200, 210, 216, 217, 218, 223, 236, 240 simulation, 116, 118, 119, 120, 122, 126, 127, 130, 134, 153, 154, 208, 214 sintering, 85, 225 SNP, 209, 210, 211, 212, 214, 215 sodium, 64, 241, 242 solar cells solid state photoelectrochemical solar cell (SSPEC), x, 235 solar collectors, 154 solid state, x, 14, 15, 21, 42, 89, 173, 235, 236, 237, 240, 241, 262, 263 solubility, 2, 6, 10, 11, 30, 82, 222, 253 solution, x, xi, 2, 6, 7, 8, 9, 10, 11, 15, 21, 30, 33, 34, 37, 39, 61, 82, 86, 103, 118, 123, 126, 134, 157, 172, 175, 176, 177, 178, 197, 205, 206, 208, 221, 222, 223, 229, 239, 242, 257 solvents, 30, 38, 157, 220, 231, 248 species, 75, 228, 236, 263 specifications, 116, 127, 134 spectroscopy, 19, 157, 202, 218 spherulite, 253 spin, ix, 7, 8, 30, 38, 156, 157, 165, 168, 169, 175, 183, 186, 191, 197, 198, 205, 208 stability, vii, x, xi, 38, 43, 51, 66, 89, 132, 219, 220, 221, 225, 227, 228, 229, 231, 241, 246, 256, 263

stable states, 55 state, vii, x, xi, 10, 14, 15, 21, 41, 42, 45, 53, 65, 72, 89, 90, 112, 173, 235, 236, 237, 239, 240, 241, 246, 259, 260, 261, 262, 263 state-of-the-art thin film Si solar cells, vii, 41 states, 28, 38, 45, 55, 61, 74, 81 steel, 44, 55, 57 storage, 138, 190 structure, vii, 2, 6, 8, 9, 14, 15, 16, 17, 22, 25, 31, 38, 41, 43, 44, 45, 47, 55, 57, 60, 61, 65, 66, 73, 74, 78, 79, 81, 82, 83, 84, 85, 88, 89, 90, 91, 122, 142, 156, 158, 159, 160, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 176, 180, 181, 182, 183, 184, 186, 188, 194, 196, 197, 198, 200, 208, 210, 222, 240, 244, 245, 252, 257 substrates, ix, 2, 3, 6, 19, 26, 27, 29, 44, 45, 47, 48, 49, 50, 51, 55, 57, 59, 60, 61, 161, 166, 169, 173, 175, 176, 178, 181, 183, 184, 186, 187, 188, 191, 192, 193, 194, 197, 198, 200, 201, 203, 208, 221, 222, 225, 237, 239 sub-wavelength structures (SWS), ix, 175, 191 Sultan Qaboos University (SQU), ix, 137 Sun, v, 39, 45, 68, 77, 78, 82, 88, 90, 92, 93, 94, 96, 137, 172, 175, 185, 187, 192, 199, 207, 216, 217, 264 sun drying experiments, ix, 137 suppression, 80, 244 surface area, 131 surface modification, 253 surface structure, 179 surface tension, 221 surfactant, 208 sustainability, 100, 101 sustainable energy, 72, 236 Sweden, 71 Switzerland, 239 symmetry, 5 synchronization, 57, 58 synthesis, 113, 157, 205 system analysis, 111

T Taiwan, 175 target, 102 TBP, 86, 87, 91, 243 techniques, ix, x, 1, 6, 28, 33, 48, 60, 65, 66, 107, 175, 176, 180, 182, 183, 197, 200, 205 technologies, 43, 48, 49, 65, 101, 103, 106, 110, 183, 216 technology, vii, viii, 2, 41, 42, 43, 48, 64, 66, 72, 99, 109, 113, 138, 153, 190, 227

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Index

temperature, vii, 19, 23, 30, 37, 42, 43, 45, 47, 49, 50, 51, 61, 62, 64, 65, 75, 138, 139, 140, 143, 146, 151, 157, 168, 191, 197, 205, 208, 209, 236, 241, 244, 246, 247, 249, 251, 252, 256, 258 temperature annealing, 30, 37, 157 temperature dependence, 62 tension, 221 testing, 62, 64, 116, 138, 139, 153 texture, 47, 52, 179, 184, 208 thermal decomposition, 223 thermal treatment, 21 thermograms, 251 thin films, x, 38, 42, 48, 108, 163, 164, 165, 168, 169, 219, 220, 221 three-dimensional model, 214 tin, 3, 19, 36, 38, 47, 51, 65, 72, 111, 156, 157, 200, 221, 240 tin oxide, 3, 51, 72, 156, 157, 221, 240 titania, 242, 244, 261, 262 titanium, 39, 180 toluene, 30, 157 total energy, 87, 89 toxic substances, 106 toxicity, 42, 104, 105, 107 TPA, 8, 84, 85, 89 trade, 123, 126 trade-off, 123, 126 transference, 236 transformation, 184, 239 transistor, 45 transition metal, 25, 28 transition temperature, 191, 236, 244, 246 transmission, ix, 46, 47, 48, 51, 118, 134, 155, 186, 187, 188, 191, 207 transmission electron microscopy (TEM), ix, 155, 157, 163, 164, 167, 168, 236 transparency, 27, 28, 47, 51, 194, 205 transparent conductive oxide (TCO), vii, 41, 221 transport, 2, 3, 6, 8, 9, 18, 23, 28, 29, 30, 31, 32, 46, 72, 74, 86, 88, 104, 138, 157, 168, 169 transport processes, 74 transportation, 6, 32, 90 treatment, 3, 19, 20, 21, 28, 30, 31, 32, 36, 172, 198, 205, 209, 211, 212, 248 tunneling, 22, 29 twist, 6 two step method, 257

U United States (USA), 41, 65, 111, 174 utility costs, 57

V valence, 22, 91, 173, 237 valuation, 154 vanadium, 238 vapor, vii, 41, 42, 48, 138, 180, 200, 201, 248 vapor phase deposition, vii, 41 variations, 30, 31, 55, 133, 147, 150, 190, 205 vegetables, ix, 137 velocity, 81, 139 versatility, 2, 32 vinylidene fluoride, 263 viscosity, 249, 250, 251, 252, 253, 254, 255, 256, 259, 264 volatility, 240

W Washington, 69 water, 108, 143, 147, 174, 205, 221, 248, 257 wavelengths, 5, 21, 53, 86, 121, 124, 130, 134, 181, 186, 208, 209 weight loss, 151 weight ratio, 3, 10, 15, 17, 18, 19, 30, 157, 163 weight reduction, 147 wells, 176, 178 Western Europe, 69 wetting, 242, 256, 259 wide band gap, 46, 237, 240

X X-ray diffraction (XRD), ix, 155, 157, 159, 160, 163, 164, 166, 169

Y yield, x, 43, 53, 54, 81, 205, 219, 221, 222, 227, 228, 231

Z Zemax, viii, 115, 116, 120, 122, 123, 124, 126, 130, 134 zinc oxide (ZnO), v, vii, x, 32, 37, 39, 47, 48, 51, 52, 64, 76, 85, 86, 165, 173, 175, 200, 203, 205, 206, 207, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 264 ZnO nanorods, 32, 39, 205, 206

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