SCIENCE & TECHNOLOGY TRENDS 5

Trends of Research and Development of Dye-Sensitized Solar Cells

Jin Ka w a k i t a Associated Fellow

through interconnection with system power and 1 Introduction combination with batteries. Establish reliability as an industrial product. Efforts to establish a low-carbon society have been 3) Fostering of social infrastructures, use initiated by reducing dependence on fossil fuels infrastructures, and use environment including petroleum, coal, and natural gas, based on Build a recycling and reuse framework. Design such renewable energy as sunlight and wind power. As systems through government-business cooperation. part of the effort, the Agency for Natural Resources 4) Industrial evolution and international and Energy (ANRE) has set out a goal to significantly competitiveness increase the extent to which solar power generation Promote the procurement of raw materials in is introduced in Japan tenfold by 2020 and 40-fold by overseas markets. Develop overseas production bases. 2030.[1] With a goal to develop solar power generation Foster human resources. as a major energy source by 2030, the New Energy In addition to these challenges, research and and Industrial Technology Development Organization development have been desired of new concepts such (NEDO) has created the Photovoltaic Roadmap as a quantum dot and new structures such as a tandem Toward 2030 (PV2030), a guideline for technological structure. Ultra-high-efficiency solar cells with light- progress in Japan. Aiming to develop photovoltaic collecting and other new systems have also been generation as a principle technology to support CO2 anticipated. reduction by 2050 with a contribution to not only This report introduces the trends of research and Japan but the global community as well, PV2030 development for dye-sensitized solar cells, of which has also been revised to further promote the use of Japan has particularly strong capabilities and which photovoltaic generation and maintain the Japanese have color variations, cost advantages, and other industry’s international competitiveness (PV2030+[2]). features that are not found in other types of solar cells, To promote the use of photovoltaic generation, in light of promoting the extent to which photovoltaic PV2030+ aims to accomplish the following: generation is generally introduced, as mentioned 1) Improved cost efficiency of solar cell modules, previously. including cost reduction Develop technologies for manufacturing solar cell 2 Current Situation of Dye-sensitized modules and high performance system devices at Solar Cells low costs. Inexpensive system design. Simplified installation work. Extend the system life to further 2-1 Comparison of Solar Cell Materials improve cost efficiency. Table 1 shows the types and characteristics of solar 2) Transformation into usable energy for the sake cell materials for comparison. In terms of energy of expanded use and applications conversion efficiency and long-term reliability, the Establish system-use technologies for eliminating mainstream solar cells at present are silicon-based. For the mismatch between generation and power demand the sake of promoting the extent to which photovoltaic

70 QUARTERLY REVIEW No.35 /April 2010

Table 1 : Types and Characteristics of Solar Cell Materials (Comparison of Single-Junction Cells, as of May 2009) Measured Cost Disadvantages Structure/ Type Material conversion competitive- Advantages (Necessary process [3] efficiency (%) ness improvements) Not suited for mass production; High cost, n-type Si layer doped High efficiency, high variable raw material Single-crystal Si on single-crystal 25.0 × reliability p r i c e , l i t t l e r o o m p-type Si layer for improvement in conversion efficiency Lower cost than Lower efficiency than Si-based n-type Si layer doped Polycrystalline single-crystal Si; s i n g l e - c r y s t a l S i; on polycrystalline 20.4 △ Si High efficiency, high Variable raw material p-type Si layer reliability price Relatively small use p-layer, i layer, and Lower efficiency than of Si material; Lower Amorphous Si n layer deposited by 9.5 △ single-crystal Si; Light cost than single- CVD process degradation crystal Si High efficiency; Low deposition rate; GaAs Metal-organic CVD 26.1 × Endure radiations in Using toxic As; High space cost A variety of production methods; Using highly toxic Compound- p-type CdTe Optimum band Cd; Dependent on based CdTe polycrystalline layer 16.7 △ gap for generation; t h e a m o u n t o f Te on n-type CdS layer Lower cost than resources single-crystal Si Vapor deposition of High optical D e p e n d e n t o n I n CIS/CIGS 19.4 △ CIS/CIGS layers absorbance resources Capable of production by simple process in Dye, Place dye-absorbed Dye- open air; Colorable, U l t r a v i o l e t semiconductor, TiO electrode in 10.4 ○ sensitized 2 transparent; degradation electrolyte electrolyte Maintain generation characteristics under room light etc. Little thickness; Apply mixture of Capable of U l t r a v i o l e t Organic thin Fullerene, p-type polymer and 5.2 ○ manufacturing d e g r a d a t i o n; L o w film based polymer n-type fullerene etc. by inexpensive efficiency application process Prepared by the STFC generation is used in the future, the challenge is to 2) Colorable, transparent reduce the material and process costs significantly The use of dye and its wide selection allow colored from the current 46 yen/kWh. Crystalline silicon solar cells and transparent cells. cells are used in large quantities, but have a unstable 3) Flexible thin structure cost factor, namely price fluctuations due to material Using aggregates of fine particles of photoelectric supply. The problem with amorphous silicon solar conversion materials, the solar cells can be formed as cells is low energy conversion efficiency. Non-silicon flexible thin films. compound semiconductors are under development, 4) Generation characteristics insusceptible to the whereas such materials have essential problems, incident angle and intensity of the sunlight including resource depletion and toxicity in the long Generation characteristics can be maintained even term. in a weak light condition, such as under faint light in Unlike the foregoing cells, dye-sensitized solar cells the morning and evening and when indoors. have the following advantages: 5) Lighter weight 1) Capable of production in a simple way Plastic substrates can be used to reduce the weight of No vacuum process is required for manufacturing. solar cells and panels. The solar cells and panels can be produced in a With these advantages, dye-sensitized solar cells simple way in open air. This means a significant cost can be installed in locations where appearance is reduction of 1/5 to 1/10 as compared to silicon solar important and other solar cells are hardly applicable, cells. such as the glass panes and inner and outer walls of a building, the sunroof and outer panels of an

71 Figure 1: Prototype Models of Dye-Sensitized Solar Cell Panels FigureMounted 1: Prototype on arched Models roof (left; of Dye-SensitizedindicatedSCIENCE by & the TECHNOLOGY Solar arrow) Cell Panels DecoratedTRENDS for interior use (right) Mounted on arched roof (left; indicated by the arrow) Decorated for interior use (right) Mounted on arched roof (left; indicated by the arrow) Decorated for interior use (right)

Figure 2: Cell Structure (left) and Principle of Operation (right) of Dye-Sensitized Solar Cell 光電極 Photoelectrode Source : Reference[5] 対向電極 Counter electrode 太陽光 Sunlight 色素 Dye Source: Reference4] Source: Reference5] 酸化チタン Titanium oxide 4] Source : Reference[4] 5] Source: Reference電解液 Electrolyte Source: Reference Figure 1 : Prototype Models of Dye-Sensitized Solar Cell Panels 起電力 Electromotive force

Photoelectrode Counter electrode Titanium oxide Counter electrode

Dye

Dye Titanium oxide Sunlight Electrolyte Electromotive force Electrolyte Sunlight

Prepared by the STFC based on Reference7] Figure 2 : Cell Structure (left) and Principle of Operation (right) of Dye-Sensitized Solar Cell Prepared by the STFC based on Reference[7]

automobile, and the enclosure of a cellular phone. This 2-2 Trends of Dye-Sensitized Solar Cells allows the creation of new markets with expanded demand. Figure 1 shows examples of prototype 2-2-1 Principle of Operation models for dye-sensitized solar cell panels. Such A dye-sensitized solar cell is one of the solar cells panels can be installed on the colored arched roof of a that uses organic dyes to gain photovoltaic force. It is garage, taking advantage of the excellent design and also called a Grätzel cell after the inventor, a professor drainage performance. The panels can be variously of the Swiss Federal Institute of Technology in freely decorated for room walls, windows, and interior Lausanne (EPFL: Ecoles Polytechniques Fédérales de use. Lausanne). Figure 2 shows the cell structure and the principle of operation for the dye-sensitized solar cell. The light incident on the transparent electrode (photo- electrode) excites the dye in the cell from the ground

72 科学技術動向 2009 年 12 月号

年前半時点での最大実測値は、セ いる。現時点では、実用化サイズ くアプローチに秀でている。一方、 ル変換効率で 11.2%（シャープ株 で 8％の変換効率が実現でき、安定 日本の研究機関は、民間企業と大 式会社発表）、モジュール効率 した製造技術が確立できることが、 学や独立行政法人との共同研究に 8.4%（ソニー株式会社発表）となっ 市場化できる最低ラインと考えら よる色素開発やセルデバイス化と ている。これまでの変換効率の推 れる。一方、モジュール変換効率 いった応用研究的手法が奏功して 移および今後の開発目標を図表 4 が15%となった場合には7円／ いる。現在ではセル、モジュール に示す。 kWh の発電コストが可能になると ともに日本の研究成果が変換効率 モジュール効率についても、 の試算もある 11）。今後のセルおよ の世界最高値を示している。また、 2008 年以降に 8% を超える値が報 びモジュールの変換効率の開発目 今後の主たる技術課題である電解 告されるようになり、NEDO によ 標を達成するための技術課題と対 質の擬固体化やプラスチック基板 る PV2030+ における 2010 年にお 応策については、3 ─ 2 章で後述する。 の開発に関しても、日本は世界を ける開発目標値をすでに凌いでい リードしている。日本がこれまで る。 2-2-3 日本と世界との比較 色素増感太陽電池の研究開発にお FigureFigure なお、変換効率については、 4: 4: Numbers Numbers of of Papers Papers on on Dye-Sensitized （独） Dye-Sensitized この領域は研究開発のレベルと Solar Solar Cells Cells and and All All Solar Solar Cells Cellsいて優勢である要因としては、以 by by Country Country (Major(Major産業技術総合研究所のような評価 Countries) Countries) いう観点では、グレッツェル教授 下が挙げられる。 標準を有する機関による評価数値Dye-Sensitized Solar Cells率いる EPFL および日本の複数の 1） ナノテクノロジー・材料分野 色色素素増増感感太太陽陽電電池池 Dye-Sensitized Solar Cells が最も信頼性が高いが、一般的に 研究機関が世界をリードしている。 の研究開発力が国際的に高い 17）。 論文数 NumberNumber of ofpapers papers 論文は、それぞれの研究機関が独自に数 EPFL はレーザー分光による解析 電気化学では本多・藤嶋効果を Japan Japan計測した数値が通用する場合が多 や半導体理論、色素の分子軌道計 含む光電気化学に関連する基礎 China Chinaいため、図表 4 では両方を示して 算といった基礎研究的手法に基づ 研究が質・量ともに充実してい SouthSouth Korea Korea QUARTERLY REVIEW No.35 /April 2010 た。 U.S.U.S. 図表 4 色素増感太陽電池の変換効率の推移および今後の研究開発目標 2） 光電極やセルを作製する際に 日本の「ものづくり技術 18）」の 強 SwitzerlandSwitzerland20 ◇： Cellそれぞれの研究機関が独 conversion efficiency independently measured自に計測したセル変換効 by each individual research率 institute みが発揮された。 GermanyGermany Cell conversion efficiency measured by ○：（独）産業技術総合研究所 3） 国家プロジェクトにおいて、 France evaluation-standard institute, such as New

France ) Energyなどの評価標準機関が計 and Industrial Technology Development Organization測したセル変換効率 (NEDO) 複数の研究機関出身の研究者が Years 1980 (% – 2003 Years 1980 – 2003 J □：（独）Module産業技術総合研究所な conversion efficiency measured by一同に介しコラボレーションす 10 S J evaluation-standard institute such as NEDO Years 2004 – 2009 どの評価標準機関が計測し Years 2004 – 2009 N J ることによって得られた要素技 S G たモジュール変換効率 ᄌ឵ല₸ J 術開発の成果を、研究者が出身 G ●：（独）NEDONEDO target in cellによるセル変 conversion efficiency 換効率目標値 太陽電池 SolarSolarJ Cells Cells 研究機関の独自の技術と融合さ 太陽電池 NEDO target in module conversion efficiency

Conversion efficiency(%) ■：（独）NEDO によるモジュー せ、それぞれ発展させることが U.S.U.S. 0 ル変換効率目標値 1970 1980 1990 2000 2010 2020 2030 できた。 JapanJapan Yearᐕ アルファベットSymbols JJ: ：日本Japan 研究論文については、色素増感 GermanyGermany S: Switzerland S：スイスG: Germany 太陽電池および太陽電池全般の国 ChinaChina GN:：ドイツ Netherlands N： オランダ 別研究論文数（上位国）を1980 年と France France Figure 3 : Changes in Conversion Efficiency of Dye-Sensitized Solar Cell and Future 2004 年から現在までに分けて図表 参考文献Goals of Research2）、3）、12 ～1and6） Developmentを基に科学技術動向研究センターにて作成 SouthSouth Korea Korea Prepared by the STFC based on References5[2,3,12-16] に 示 す。 こ の 結 果 は Thomson SwitzerlandSwitzerland Reuters 社 ISI の デ ー タ ベ ー ス 図表 5 色素増感太陽電池および太陽電池全般に関する国別研究論文数（上位国） Web of Knowledge を用いて検索し Dye-Sensitized Solar Cells Solar Cells たものであり、検索に用いたキー ワードは「dye ─ sensitized solar □ Years 1980 – 2003 □ Years 1980 – 2003 ■ Years 2004 – 2009 ■ Years 2004cell」および「Solar – 2009 cell」である。色 素増感太陽電池の研究開発は太陽 電池全般に比べて、日本の研究論 Number of papers

Number of papers 文数が多い。これは複数の大学、

U.S. Japan China 公的研究機関、民間企業が発表し U.S. France China Japan France Germany Germany South KoreaSwitzerlandているためである、しかし、近年、 South Korea Switzerland 中国・韓国からの論文も急増して PreparedPrepared by by the the STFC STFC Figure 4 : Numbers of Papers on Dye-Sensitized科学技術動向研究センターにて作成 Solar Cells and All Solar Cells by いる。これは日本と同様に、この Country (Major Countries) 14 Prepared by the STFC state to an excited state, whereby an electron ( e- ) is of the dye molecules, so that electrons are moved by - formed. The e passes through titanium oxide (TiO2) a phenomenon similar to that of water flowing from to reach the transparent electrode, and flows into the a high to low level. In the latter point of view, the external circuit. Meanwhile, the loss of e- in the dye is main electron transfer proceeds preferentially because supplemented with iodine ions ( 3I - ) in the electrolyte. the electron transfer rates in the respective reaction At this point, 3I - are transformed (oxidized) into iodine processes are more than 10 times higher than in the - - - ( I3 = I2 + I ), and then return to 3I (reduction) by reverse reactions and sub reactions. Suppose that receiving e- that is supplied from the counter electrode. sunlight that had a higher energy than the difference Such an electron transfer has been accounted for in between the ground level and excited level of the dye terms of the energy level and electron transfer rate. is supplied and converted into as much electricity as The former provides the explanation that the energy possible. In such cases, the theoretical calculation of level of the conduction band of titanium oxide and the energy conversion efficiency is 33%.[6] the redox level of iodine ions and iodine should be situated between the excited level and ground level

73 SCIENCE & TECHNOLOGY TRENDS

2-2-2 History of Research and Development technological challenges and measures for achieving In the early 1970s, research was conducted on photo- the goals of development of the cell and module sensitizers for photographic use. The research involved conversion efficiencies in the future will be described the absorption of dye on oxide semiconductors for in Section 3-2. the purpose of quantifying the dye-based spectral sensitization phenomena. This paved the way for the 2-2-3 Comparison Between Japan and the World extraction of a photocurrent to an external circuit In this field of research and development, EPFL, led through the electrodes.[8] The first dye-sensitized by Professor Grätzel, and several Japanese research solar cell present to the world, a cell from which institutes are leading the world. EPFL excels at an electric current can be taken out based on the approaches based on basic research methods, such as electromotive force between the two electrodes, was a laser spectroscopic analysis, semiconductor theories, developed in 1976 by Professor Tsubomura et al. at and dye molecular orbital calculation. The Japanese Osaka University, using porous zinc oxide as the dye research institutes are successful in pursuing applied support. The energy conversion efficiency was 2.5%.[9] research methods, such as the development of dyes and In 1991, Professor Grätzel et al. at EPFL, Switzerland, cell devices in collaboration with private companies, presented a prototype of the current dye-sensitized universities, and independent administrative solar cell, which had an improved conversion institutions. At present, Japanese research efficiency of 7.12%.[10] The findings of the Grätzel's achievements provide the world's highest records research are characterized as follows: both in the cell and module conversion efficiencies. 1) Titanium oxide nanoparticles were used to increase Japan is also leading the world in developing quasi- the dye absorption area to a large scale. solid electrolytes and plastic substrates, which are 2) Ru-based dyes with a wide range of optical major technological challenges for the future. Due to absorption were developed. the following factors, Japan has been in the leading 3) A cell structure with relatively small loss of position in research and development of dye-sensitized conversion efficiency was devised. solar cells: Research efforts continued as will be described in 1) The internationally high capabilities of research and the chapter on technological trends. In the first half development in the fields of nanotechnologies and of 2009, the maximum cell conversion efficiency in materials.[17] In electrochemistry, basic researches actual measurement was 11.2% (announced by Sharp on photo-electrochemistry, including the Honda- Corporation) and the maximum module efficiency Fujishima effect, have been rich in both quality and was 8.4% (announced by Sony Corporation). Figure quantity. 3 shows the past changes in the conversion efficiency 2) The advantages of the Japanese monozukuri (basic and future goals of development. manufacturing) technologies[18] have been utilized in Since 2008, module efficiencies of above 8% have manufacturing photo-electrodes and cells. been reported, already exceeding the development 3) There have been national projects in which goal as of 2010 in NEDO PV2030+. researchers from different research institutes gather The most reliable evaluation values on the to collaborate. The results of development of the conversion efficiencies are presented by authorizing respective technologies have been successfully institutes with the industrial standard, such as the merged with the original technologies of their own National Institute of Advanced Industrial Science research institutes for further improvement. and Technology (AIST), whereas numerical values Figure 4 shows the number of papers on dye- independently obtained by individual research sensitized solar cells and all solar cells from each facilities are often accepted. Figure 3 shows both major country, from 1980 to 2003 and since 2004. values. At present, practical-sized products with a These results were obtained from the Thomson conversion efficiency of 8%, accompanied by reliable Reuters’ ISI Web of Knowledge database, with the manufacturing techniques, are considered to be the search keywords “dye-sensitized solar cell” and “solar minimum requirement for the market. According to cell.” Japan has more papers on dye-sensitized solar estimates, a module conversion efficiency of 15% cell than on any other solar cells. The reason is that allows generation cost as low as 7 yen/kWh.[11] The more than one university, public research organization,

74 QUARTERLY REVIEW No.35 /April 2010

Table 2 : Major Publicly Funded Research Programs on Dye-Sensitized Solar Cells

Country/region Program/project Period NEDO: Research and Development of Photovoltaic Generation Years 2006 – 2009 System Technologies Japan JST: Basic Research Program "Creative Research for Clean Energy Years 2009 – 2016 Generation Using Solar Energy" Europe FP7 : ROBUST DSC project Years 2007 – 2010 U.S. DOE: Solar Energy Technologies Program Years 2007 – 2011 NOTE) FP7: The E.U. 7th Framework Programme for Research DOE: Department of Energy

Prepared by the STFC and private company published the papers. In recent described for a total of six items, including the four years, papers from China and South Korea have major parts of the cell structure shown in Figure 2 and been increasing sharply. This appears to be due to two articles on device configuration. positive investment in this field of research[19] and the background of the monozukuri technologies as in (1) Photo-electrode Japan. As mentioned above, one of the characteristics of Table 2 shows major publicly-funded research the Grätzel cell is that the photo-electrode is made programs in Japan, the United States, and Europe. of nano particles of titanium oxide (TiO2) with a Dye-sensitized solar cells are regarded as next- porous structure as the support for dye absorption. generation solar cells close to practical use, and given Layers of titanium oxide with different particle research grants in all of Japan, the United States, and sizes can be stacked to provide a light confinement Europe. effect (Professor Arakawa et al. at Tokyo University In regard to commercialization, G24 Innovations of Science). This has been combined with the light (U.K.) and Solaronix SA (Switzerland) have already scattering from star-shaped particles of titanium released commercial products. Dyesol (Australia) oxide to provide a cell conversion efficiency of above has been making efforts toward commercial 10% (Sumitomo Osaka Cement Co., Ltd.).[11] An production, including the delivery of modules to improvement of the conversion efficiency over a wide public organizations. Konarka Technologies, Inc. range of wavelengths by TiCl4 surface treatment (U.S.) aims to prepare flexible products for the market. (Grätzel et al.) has been reported.[21] Research has In Japan, Nissha Printing Co., Ltd. has announced also been made to improve the conductivity of TiO2 that it will ship out samples in 2010,[20] but has yet through morphological control, such as the formation to reach commercialization. Prototype models have of nanotubes, as well as to coat the surface of TiO2 been released by Aisin Seiki Co., Ltd., Toyota Central with a different type of oxide, such as niobium oxide R&D Labs., Inc., Fujikura Ltd., Sony Corporation, (Nb2O5), thereby suppressing electron leakage from TDK Corporation, Rohm Co., Ltd., Hitachi Maxwell, the titanium oxide to the electrolyte solution. Research Ltd., and Peccell Technologies, Inc. The development papers have been published for oxide semiconductor for practical use is continuing. Because Grätzel's materials other than titanium oxide and composite basic patent (Switzerland) expired on April 12, 2008, materials with other oxide semiconductors for the sake efforts toward commercialization and practical use are of improved charge separation efficiency. Such efforts, expected to be more active in the future. however, have not been successful in providing characteristics superior to those of simple titanium 3 Trends of Research and oxide. As for the method of manufacturing the photo- Development of Dye-Sensitized electrode, the uniform formation of the electrode is Solar Cells essential to favorable characteristics. After various attempts, screen printing is considered to be suitable 3-1 Component Technologies in terms of mass production. Research and development of the respective technologies of dye-sensitized solar cells will be

75 Figure 5: Action Spectrum of Dye-Sensitized Cells With Different Ruthenium (Ru) Dyes 波長 Wavelength SCIENCE & TECHNOLOGY TRENDS 量子効率 Quantum efficiency

←Improved Ru-bipyridine -carbonyl complex dye

←Ru-bipyridine-carbonyl complex dye ←TiO2 only (without dye) Quantum efficiency/IPCE %

Wavelength /nm Figure 5 : Action Spectrum of Dye-Sensitized Cells With Different22] Ruthenium (Ru) Dyes Prepared by the STFC based on Reference [22] Prepared by the STFC based on Reference

[23] (2) Dye efficiency and longer life. Studies have also been Dye-sensitized solar cells use the dye to expand made on the stability of the dye. Ruthenium dyes the available rangeFigure of wavelengths 6: Prototype in the Examplesspectrum of haveDye-Sensitized a turnover (the possible Solar numberCell Modules of photovoltaic of sunlight. ThePlastic dye therefore substrate plays module the essential (left) andconversions large module per dye molecule) for outdoor of above use ten (right) million, role substantially. The dye that was first used in the which compares with ten years of light irradiation.[24] Grätzel cell is a ruthenium (Ru) bipyridine complex with a carboxyl group. This dye can efficiently absorb (3) Electrolyte visible light in a wavelength range of up to 800 nm. The electrolyte in a dye-sensitized solar cell has The carboxyl group provides chemical bonding to a redox potential that determines the potential the surfaces of titanium oxide particles. The resulting of the cell's positive electrode. The electrolyte is advantage was the smooth injection of electrons indispensable for the sake of electron transfer in the from the dye to the titanium oxide.[22] Figure 5 shows electrolyte, based on the physical diffusion of redox the action spectrum of dye-sensitized cells with pairs. The highest conversion efficiency ever has been different ruthenium (Ru) dyes. Professor Grätzel et achieved by an electrolyte solution of acetonitrile in al. then changed the substituent group and found which iodine ions and iodine are dissolved. Having a dye commonly called black dye, which absorbs a low evaporating pressure, acetonitrile is prone to the spectrum of up to 900 nm with a photoelectric evaporation and drops in the conversion efficiency at conversion efficiency as high as 80% for incident high temperatures or with long-term use. Techniques monochromatic light. The cell using this dye records for sealing the electrolyte in the cell have thus been the highest performance at present. Since32] ruthenium developed. The solidification of the electrolyte is an expensive Source:metal, ruthenium-free Reference dyes have also is also required due to concern about physical been developed. However, no dye has been reported damage to the cell. A quasi-solid electrolyte made to surpass ruthenium in performance. It should be of a combination of a nonvolatile ionic liquid and a noted that AIST (Professor Arakawa et al., currently gel with a conversion efficiency of above 7% was at Tokyo University of Science) presented a coumarin reported (Professor Hayase et al. at Kyushu Institute organic dye that contains no metallic element and of Technology).[25] Aiming at a fully solid electrolyte, exhibits a conversion efficiency of around 8%. A there have been researches on the use of inorganic carbazole dye presented by AIST senior researcher compounds such as CuI and CuSCN, conductive Hara et al. provides an improved cell conversion polymers such as polypyrrole, low-molecular

76 QUARTERLY REVIEW No.35 /April 2010 materials such as Triphenyldiamine, and amorphous flexibility to the cell (Professor Miyasaka et al. at Toin organic compounds such as OMe-TAD. University of Yokohama). For higher output, solar cells need to have an (4) Counter Electrode increased area for light reception. A single cell of a The counter electrode plays the role of returning greater size, however, typically has a higher substrate electrons that are generated at the photo-electrode resistance with a significant drop in output per unit and delivered through the external circuit, back to area. This requires more than one cell to be connected the electrolyte. Since the electrolyte is corrosive, the for upsizing (modularization). Various methods have counter electrode requires high corrosion resistance been examined to connect cells to each other. A as well as a high reaction rate when reducing iodine module with connecting grids achieved a conversion inFigure the electrolyte 5: Action to Spectrum an iodide of ion. Dye-Sensitized Considering Cellsefficiency With Different of 9.0% Ruthenium(Professor Arakawa (Ru) Dyes et al. at Tokyo the波 balance長 Wavelength between these factors, a conductive University of Science).[29] glass量子 electrode効率 coatedQuantum with efficiency platinum (Pt) has been For cell and module stabilities, a single small cell used heretofore. Carbon electrodes and conductive that is stable under simulated solar light for more than polymers have been examined as an alternative to 7000 hours was reported.[30] Figure 6 shows prototype Improved Ru-bipyridine ← [31] expensive Pt, whereas such materials do not come-carbonyl up complex examples dye of a module with a plastic substrate to Pt in terms of the reduction rate. (Peccell Technologies, Inc.) and a large module for outdoor use[32] (Fujikura Ltd.). (5) Cell encapsulation/modularization For a better dye-sensitized solar cell, it is 3-2 Technological Challenges for the Future and understandably necessary to improve← theRu-bipyridine-carbonyl overall Measures of Dye-sensitized Solar Cells complex dye performance of the←TiO cell2 only device. Dr. Han of Sharp (without dye) Corporation (currently of the National Institute Future challenges in technological development for Materials Science) et al. clarified the losses in for dye-sensitized solar cells are summarized in the the respective components of a cell by an internal following three points. resistance analysis, and achieved the world's highest performance at present through loss-reducing 1) Improved energy conversion efficiency [26] approaches. AIST ((then) Managing Director22] Much is still unknown about the sources of loss, SugiharaPrepared et al.) by successfully the STFC basedimproved on theReference conversion including the reaction mechanism. To improve the efficiency up to 11% by stacking a plurality of cells in conversion efficiency, it is first necessary to discover [27] tandem. Attempts to improve efficiency were also a dye that can increase the number of photons made to encapsulate a cell with optical nanofibers.[28] absorbed and the range of absorption in the solar Figure 6: Prototype Examples of Dye-Sensitized Solar Cell Modules The glass substrate can be replaced with plastic to add spectrum. It is also important to match the redox Plastic substrate module (left) and large module for outdoor use (right)

Plastic substrate module (left) and large module for outdoor use (right) Source: Reference32] Figure 6 : Prototype Examples of Dye-Sensitized Solar Cell Modules [32] Source: Reference

77 Figure 7: Example of Roll-to-Roll Process for Electronic Paper (1) Application Film substrate Transparent electrode Embossing roll die Microcup Figure 7: Example of Roll-to-Roll Process for Electronic Paper (1) Application (2) Micro-embossing Film substrate (3) Injection and sealing of liquid and pigment Transparent electrode Adhesive Embossing roll die (4) Lamination and cutting Microcup Cutter (2) Micro-embossing Completed electrophoretic film (3) Injection and sealing of liquid and pigment Adhesive (4) Lamination and cutting Cutter SCIENCE & TECHNOLOGY TRENDS Completed electrophoretic film

(4) Lamination and cutting Adhesive Cutter (3) Injection and sealing of Completed electrophoretic liquid and pigment film

(1) Application Source: Reference34] Microcup

Film substrate (2) Micro-embossing Transparent electrode Embossing roll die

34] Source:Figure Reference 7 : Example of Roll-to-Roll Process for Electronic Paper Figure 8: Principle of Operation of Dye-Sensitive Solar Cell That Can StoreSource: Energy Reference [34] (Left: photovoltaic generation; center: photovoltaic charge; right: discharge)

Figure 8: Principle of Operation of Dye-Sensitive Solar Cell That Can Store Energy (Left: photovoltaic generation; center: photovoltaic charge; right: discharge)

(Left: photovoltaic generation; center: photovoltaic charge; right: discharge) 35] Source: ReferenceFigure 8 : Principle of Operation of Dye-Sensitive Solar Cell That Can Store Energy [35] Source: Reference potential of the electrolyte to that of the dye. For 2) Long-term reliability research and development,Source: computational Reference simulation35] In view of reliability and long-term stability, it is can be effectively used to design optimum dyes desired to verify and improve the cell conversion and electrolytes. Meanwhile, one of the approaches efficiency and durability. An established method considered to be effective in designing the photo- of making a contact at the interfaces between the electrode, including the dye, is to clarify the electrolyte and the electrodes is required for a relationship between the structure and properties solidified electrolyte in particular. The mechanism of of the electrode materials and the charge-transfer dye desorption and the mechanism of reaction of the mechanism,[33] and design the optimum electrode electrolyte's composition change, which contribute to structure. As for the counter electrode, it is necessary degraded long-term stability of a dye-sensitized solar to clarify the relationship between the corrosion cell, also require further elucidation. behavior of candidate materials in the electrolyte and the catalyst activity of the reduction reaction 3) Higher throughput of the electrolyte, and narrow down the candidate As employed herein, the throughput refers to materials. Such development needs to be followed by the amount of products that are produced from optimization of light management in which the light raw materials in a given time. The cell production confinement effect of the photo-electrode is extended process provides improved throughput by virtue of to a cell level, including the glass substrate, electrolyte, the advantage of using no vacuum system. A roll- and counter electrode. To improve module efficiency, to-roll process is being considered. For example, a sophisticated module design and fine processing circuit pattern is printed on a roll of substrate as large technologies are also needed. as several hundreds of meters in length and around 1 m in width. The substrate is laminated with a roll of sealing film or the like before being rewound

78 Figure 9: Mechanism of Water Decomposition Reaction Simulating Two Stages of Photoexcitation and Charge Transfer Photosynthesis is also called the “Z scheme” after the form of the electron flow, including two stages of photoexcitation and charge transfer caused by sunlight.

Redox potential (V) with respect to standard hydrogen electrode

WO3 の価電子帯 WO3 valence band 太陽光 Sunlight

WO3 の伝導帯 WO3 conduction band

色素増感 TiO2 Dye-sensitized TiO2 色素の HOMO Dye HOMO 色素の LOMOQUARTERLY Dye LOMOREVIEW No.35 /April 2010

Dye LOMO

WO3 conduction Sunlight band

Dye HOMO Dye-sensitized

Sunlight

WO3 valence band

Redox potential (V) with respect to standard hydrogen electrode PreparedPhotosynthesis by the isSTFC also called based the on “Z Referencescheme” after36] the form of the electron flow, including two stages of photoexcitation and charge transfer caused by sunlight. Figure 9 : Mechanism of Water Decomposition Reaction Simulating Two Stages of Photoexcitation and Charge Transfer Prepared by the STFC based on Reference[36] on another reel. The substrate is passed through Figure 8 shows the principle of operation that is the production machines continuously. The mutual proposed as an energy storable dye-sensitized solar connection of the production machines significantly cell (Professor Segawa at Tokyo University).[35] In saves the labor and devices for transportation. Figure this example, the dye-sensitized solar cell includes 7 shows an example of a roll-to-roll process for a conductive polymer charge storable electrode electronic paper.[34] aside from the ordinary photo-electrode and counter The cold film formation of the photo-electrode electrode. Photovoltaic function is performed between can be effectively combined with print-based fine the photo-electrode and the counter electrode as in patterning. The process of absorption of the dye into ordinary dye-sensitized solar cells. For charging, the titanium oxide, which is the key to the electrode photo-electrode and the charge storable electrode production, also needs to be clarified for the sake of are connected so that electrons occurring from the faster production processes. photo-electrode are accumulated into the charge storable electrode through an external circuit. With 3-3 Other Developments the accumulation of electrons in the charge storable Photochemical cells in general, including dye- electrode, anions are released into the electrolyte sensitized solar cells, are characterized by how the to maintain the charge balance. For discharge, the photoelectric conversion reaction involves a redox charge storable electrode and the counter electrode reaction. Taking advantage of the characteristic, are connected to discharge electrons from the charge electrons or holes occurring from the electrodes or storable electrode while anions are accumulated ions in the electrolyte can be utilized to implement a again. The discharged electrons are passed through charge-storing function, like a secondary battery or the external circuit, and consumed in the counter capacitor in the solar cells. This means the possibility electrode for the reduction reaction of iodide ions in of achieving power assurance in the dark within a the electrolyte. single cell, which is one of the challenges common to If a system can be constructed so that the electrons all solar cells. exchanged between the cell electrodes through the

79 SCIENCE & TECHNOLOGY TRENDS external circuit are consumed by new electrochemical 3) Fostering of social infrastructures, use reactions at the respective electrodes, then an infrastructures, and use environment artificial photosynthesis may become possible. For Dye-sensitized solar cells contain no toxic substance example, Figure 9 shows the mechanism of a water as a cell component material. The materials are decomposition reaction that simulates two stages relatively easy to separate and recover, which is of photoexcitation and charge transfer, which are advantageous in view of a recycling and reuse among the characteristics of photosynthesis, using a framework for solar cell panels. dye-sensitized titanium oxide (TiO2) electrode (by 4) Industrial evolution and international Professor Grätzel).[36] The first stage of photoexcitation competitiveness reaction at the tungsten oxide (WO3) electrode initially There is as yet no particular problem in procuring produces an electron and a hole. The hole decomposes the raw materials of dye-sensitized solar cells water into oxygen. The electron is transported from overseas markets. The Asian regions have a through the medium within the same system (such background suited for introducing the monozukuri as ions in the solution) to reach the dye-sensitized technologies, which is advantageous to the TiO2 electrode. The electron at the dye-sensitized development of production bases in Asia. Japan's TiO2 electrode is raised to a higher energy level by superior capabilities for research and development the second stage of photoexcitation reaction, whereby of materials and nanotechnologies can be utilized to water is decomposed into hydrogen. The two stages secure international competitiveness of the product of photoexcitation and charge transfer are referred to technologies on dye-sensitized solar cells. as “Z scheme” since they look like “Z” sideways. To Dye-sensitized solar cells are collections of achieve this artificial photosynthesis, it is necessary various technologies. The distinct characteristics, to improve the efficiency of the light-based charge unlike those of other solar cells, require a broader separation, develop a medium that transports charges spectrum of views and ideas in terms of research within the system with high efficiency, and develop and development and the way of applications. an electrode catalyst that promotes the CO2 reduction For the sake of improved cell characteristics and reaction. industrial evolution, various fields of specialization therefore need to be merged. One good example is the 4 Conclusion integration between amorphous silicon, which accepts all types of base materials, and the plastic thin film In view of the photovoltaic generation discussed in technologies, whereby an amorphous solar cell film Chapter 1, dye-sensitized solar cells have the following of little thickness, light weight, and high flexibility potentials: was successfully developed.[37] Dye-sensitized solar 1) Improved cost efficiency of solar cell modules, cells are expected to benefit from the exchange of including cost reduction researchers between research fields such as organic Dye-sensitized solar cells require no vacuum complex chemistry and micromechanics, and system for manufacturing, and thus have an essential electrochemistry and chemical engineering, to name a advantage in terms of production cost. There is still few. The fields of materials and print engineering can a lot of room for improvement in the photoelectric be integrated for improved production efficiency. Cell conversion efficiency. A significant improvement module designers and interior designers can cooperate in the cost efficiency of solar cell modules can be to broaden the applications, taking advantage of the expected. dye-sensitized solar cells. Such integration of different 2) Transformation into usable energy for the sake fields of specialization requires mediators and of expanded use and applications intermediary settings. I hope for more frameworks Using various types of dyes, dye-sensitized solar to facilitate exchanges among researchers, engineers, cells have wide color variations for excellent decorative and people who are capable of setting the directions of potential. The photovoltaic phenomenon in a dye- research and development. sensitized solar cell involves a redox reaction, and the ions involved can be used for the development of an energy storable solar cell and artificial photosynthesis.

80 QUARTERLY REVIEW No.35 /April 2010

References

[1] Agency for Natural Resources and Energy, “Action Plan for Promoting the Introduction of Solar Power Generation” [2] https://app3.infoc.nedo.go.jp/informations/koubo/kaiken/BE/nedopressorder.2009-06-08.2039491773/kadai. pdf [3] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Progress in Photovoltaics: Research and Applications, 17 (2009), p. 320 [4] http://techon.nikkeibp.co.jp/article/NEWS/20080930/158787/ [5] http://www.sony.co.jp/Products/SC-HP/cx_pal/vol80/pdf/sideview80.pdf [6] M. Grätzel, Abstract of the first conference of Future Generation Photovoltaic Technologies, NREL [7] http://www.mase.nagasaki-u.ac.jp/search/energy.html [8] Akira Fujishima, Eiji Hayashitani, Kenichi Honda, Seisan Kenkyu [Industrial Science], 23 (1971), p. 363 [9] H. Tsubomura, Nature, 261 (1976), p. 402 [10] B. O'Regan, M. Grätzel, Nature, 353 (1991), p. 737 [11] Hironori Arakawa (ed.), Recent Advances in Research and Development for Dye-Sensitized Solar Cells II, CMC Publishing, 2007, p. 9 [12] M. A. Green, K. Emery, K. Bücher, D. L. King, S. Igari, Progress in Photovoltaics: Research and Applications, 6 (1998), p. 265 [13] M. A. Green, K. Emery, K. Bücher, D. L. King, S. Igari, Progress in Photovoltaics: Research and Applications, 7 (1999), p. 321 [14] M. A. Green, K. Emery, D. L. King, S. Igari, W. Warta, Progress in Photovoltaics: Research and Applications, 10 (2002), p. 355 [15] M. A. Green, K. Emery, D. L. King, Y. Hishikawa, W. Warta, Progress in Photovoltaics: Research and Applications, 14 (2006), p. 455 [16] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Progress in Photovoltaics: Research and Applications, 16 (2008), p. 435 [17] http://www8.cao.go.jp/cstp/tyousakai/kihon/haihu17/siryo2-4-1.pdf [18] http://ja.wikipedia.org/wiki/%E3%82%82%E3%81%AE%E3%81%A5%E3%81%8F%E3%82%8A#. E4.BC.81.E6.A5.AD.E3.81.AB.E3.81.8A.E3.81.91.E3.82.8B.E3.80.8C.E3.82.82.E3.81.AE.E3.81.A5.E3.81.8F. E3.82.8A.E3.80.8D [19] http://www.spc.jst.go.jp/report/200801/report_tera1.html [20] http://www.nissha.co.jp/news/news_images/2009/20090914prelease.pdf [21] C. J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Grätzel, Journal of the American Ceramic Society, 80 (1977), p. 3167 [22] Hironori Arakawa, Catalysts and Catalysis, 44 (2002), p. 190 [23] http://www.aist.go.jp/aist_j/press_release/pr2008/pr20081119/pr20081119.html [24] M. Grätzel, Proceedings of Renewable Energy, 2006, p. 9 [25] Shuji Hayase, Hiroyasu Sumino, Shinji Murai, Tomo Mikoshiba, IEICE Technical Report, 101 (2001), p. 27 [26] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Jpn. J. Appl. Phys., 45 (2006), L638 [27] http://www.aist.go.jp/aist_j/press_release/pr2008/pr20080304/pr20080304.html [28] B. Weintraub, Y. Wei, Z. L. Wang, Angew. Chem. Int. Ed., 48 (2009), p. 1 [29] H. Arakawa et al. , Proceedings of WCPEC-4, 2006, 179 [30] Hironori Arakawa (ed.), Recent Advances in Research and Development for Dye-Sensitized Solar Cells, CMC Publishing, 2001, p. 33 [31] http://innovation.nikkeibp.co.jp/etb/20080228-00.html [32] http://www.fujikura.co.jp/rd/field/mt.html [33] M. Kawakita et al. , J. Mater. Res., 24 (2009), p. 1417

81 色素増感太陽電池の研究開発動向

34） http://techon.nikkeibp.co.jp/article/FEATURE/20090205/165220/ 35） http://jstshingi.jp/2005/pdf/102602.pdf 36） M. Grätzel, Nature, 414（2001）pp. 338 SCIENCE & TECHNOLOGY TRENDS 37） http://www.meti.go.jp/policy/economy/gijutsu_kakushin/innovation_policy/pdf/report.pdf [34] http://techon.nikkeibp.co.jp/article/FEATURE/20090205/165220/ [35] http://jstshingi.jp/2005/pdf/102602.pdf [36] M. Grätzel, Nature, 414 (2001), p. 338 執筆者プロフィール[37] http://www.meti.go.jp/policy/economy/gijutsu_kakushin/innovation_policy/pdf/report.pdf

Profile

Jin川喜多 仁 Kawakita Affiliated Fellow, STFC 科学技術動向研究センター 客員研究官 Senior Researcher, National Institute for Materials Science 独立行政法人 物質・材料研究機構 主幹研究員 http://www.nims.jp/photovoltaics_center/Jpn/members/KAWAKITAJin.htm http://www.nims.jp/photovoltaics_center/Jpn/members/KAWAKITAJin.htm

Specialized専門は電気化 in学 。electrochemistry.リチウム電池正極材 料Engagedの充放電挙 in動 やresearch金属材料 のon腐 食the挙 動chargeの解明 、and discharge behavior of lithium防食用 battery表面処理 プpositiveロセスの electrode開発、光電 変materials換材料の 特and性向 the上に corrosion従事。 behavior of metallic materials, as well as電 in気 化development学的手法を用い ofた 材anticorrosive料設計と機能性 surface材料の創 製treatmentに取り組ん でprocessesいる。 and photovoltaic materials with improved properties. Kawakita is working on electrochemical material design techniques and the production of new functional materials.

(Original Japanese version: published in December 2009)

Science & Technology Trends December 2009 21

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