Measurements and evaluation of dye-sensitized solar cell performance

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews xxx (2012) xxx–xxx

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Invited review

Measurements and evaluation of dye-sensitized solar cell performance Katsuhiko Takagi ∗ , Shinichi Magaino, Hidenori Saito, Tomoko Aoki, Daisuke Aoki Kanagawa Academy of Science and Technology (KAST), Sakato 3-2-1, Takatsu, Kawasaki 213-0012, Japan

a r t i c l e

i n f o

Article history: Received 29 May 2012 Received in revised form 23 August 2012 Accepted 24 August 2012 Available online xxx Keywords: Dye-sensitized solar cell (DSC) Irradiance Responsivity Induced Photon-to-Current Efficiency (IPCE) Ionic liquid electrolyte

a b s t r a c t Measurement and evaluation methods for the performance of dye-sensitized solar cells (DSCs), of which the mechanism for photocurrent generation is quite different from that of silicon-type solar cells, are reviewed here and a relevant method proposed. The slow response times and nonlinearity of DSC photocurrents against the light intensity (irradiance) at wavelengths of incident light are profoundly influenced by their characteristic working principles since photocurrent generation for DSCs is more complicated than for Si-type solar cells. DSCs work not only by the physical process of an electron in solid-state TiO2 but also diffusion processes in the fluid electrolytes in contrast to only the simple solidstate physical process of a charge separation at the p–n junction of the interface for Si-type solar cells. In addition, newly developed DSCs are prepared by such elemental materials as sensitizers, electrolytes and semiconductors of diverse morphologies. In this respect, it is essential to establish a comprehensive and relevant method for the correct spectral measurement of the responsivity and performance of a wide range of DSCs which may include cells involving various kinds of electrolytic media. In this review, DSC electrolyte media with such disparate viscosities as a typical organic solvent, 3-methoxypropionitrile (Cell A), or an ionic liquid (Cell B) are introduced and analytical methods such as the AC method is compared with the DC method to gauge spectral responsivity. IPCE measurements were carried out by adjusting the chopping frequency low enough to obtain a steady state current under illumination conditions similar to those under practical use. Our studies revealed that when sufficient time is allowed for complete photocurrent generation, especially for DSCs involving an ionic liquid, I–V measurements which take this time allowance into consideration show them to perform satisfactorily. In fact, in an extreme example, I–V measurements of a DSC with an ionic liquid electrolyte can take over 50 min before correct data can be obtained. Thus, standards for the evaluation of DSCs need to be established separately from those for Si-type solar cells to avoid incorrect and incomplete comparisons. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Working principles of dye-sensitized solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaluation of DSC performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Reference cells and pseudo-reference cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of the DSC Incident Photon-to-Current Conversion Efficiency (IPCE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DSC performance measurements by a solar simulator (SS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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∗ Corresponding author. Tel.: +81 44 819 2020; fax: +81 44 819 2038. E-mail address: [email protected] (K. Takagi). 1389-5567/$20.00 © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochemrev.2012.08.003

Please cite this article in press as: K. Takagi, et al., Measurements and evaluation of dye-sensitized solar cell performance, J. Photochem. Photobiol. C: Photochem. Rev. (2012), http://dx.doi.org/10.1016/j.jphotochemrev.2012.08.003

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Katsuhiko Takagi (Dr.Eng.) is a senior researcher at the Kanagawa Academy of Science and Technology (KAST) after joining as an Executive Director in April, 2008. From 2010 onwards, he is the project leader of the research program in “Development of Analysis & Evaluation Standards for the Performance and Basic Material Characteristics of Organic Photovoltaics (OPV)”. He received his Ph.D. and became a professor at Nagoya University where he specialized in organic photochemistry, particularly in utilizing inorganic templates such as mesoporous silica, clay minerals, and titania nanosheets for the construction of organic and inorganic hybrid materials (1971–2005). Shinichi Magaino is presently the Executive Director of Kanagawa Academy of Science and Technology (KAST), Kawasaki, Japan. He received his Doctor of Engineering degree from Waseda University in 1988. He worked as a researcher at the Kanagawa Industrial Technology Center (KITC) from 1974 and was promoted to its Executive Director in 2009. His research interests lie in the field of electrochemistry.

Hidenori Saito is the chief researcher at the Materials Characterization Center of the Kanagawa Academy of Science and Technology (KAST). He received his Masters of Engineering degree in 1991 from Kanto Gakuin University under the supervision of Prof. Koji Shimizu. From 2010 onwards, he works using investigative techniques such as FE-SEM in the analysis and evaluation of Organic Photovoltaics for the Funding Program for World-Leading Innovative R&D in Science and Technology (FIRST) project.

Tomoko Aoki is an engineer at the Materials Characterization Center of the Kanagawa Academy of Science and Technology (KAST). She received her M.Sc. in 1999 from Tokyo Institute of Technology under the direction of Prof. Yoko Kaizu. She worked as a government officer at Science and Technology Agency of the Ministry of Education, Sports, Science and Technology (MEXT/STA) from 1999 to 2004 before moving to KAST. From 2010 onwards, she is a researcher with the project, “Development of Analysis & Evaluation Standards for the Performance and Basic Material Characteristics of Organic Photovoltaics (OPV)”.

Daisuke Aoki is an engineer at the Materials Characterization Center of the Kanagawa Academy of Science and Technology (KAST). He received his M.Sc. in 2006 from Tokyo Institute of Technology under the direction of Prof. Tomokazu Iyoda. From 2010 onwards, he is a researcher with the project, “Development of Analysis & Evaluation Standards for the Performance and Basic Material Characteristics of Organic Photovoltaics (OPV)”.

1. Introduction Over half a century has passed since semiconductor-based solar cells were first used as electric sources for mobile electrical instruments such as calculators, portable radios and cassette recorders and, since then, on-site power generation for home use has also become more widespread. In the aftermath of the devastating 2011 Tohoku earthquake of 9.0 magnitude which caused a tsunami to damage and incapacitate the Fukushima nuclear power plant reactors, it has become ever more urgent to develop renewable yet

sustainable alternative energy sources such as solar cells for electrical power. Newer type solar cells with improved performance and functionality are continuously being developed, even as the use of silicon-type solar cells has gradually increased worldwide [1–10]. Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules have been presented and, since July 2011, guidelines for the inclusion of these results have been outlined and new entries reviewed [11]. Among them, dye-sensitized solar cells (DSCs) composed of Ru-complexed dyes, TiO2 particles, and I− with ca. 7% light-toelectricity conversion efficiency () (developed by Grätzel et al. [12]), have steadily improved their long-term stability and shown higher performance in their light conversion efficiencies [13–15]. At present, the highest efficiency of over 11% for a unit cell with an area of 1-cm2 has been reported by Koide et al. of the Sharp Corporation [16]. Actually, an efficiency of 11.4% has been reported in 2009 by the Japanese National Institute for Material Science (NIMS) [17], however, this could not be recognized as a class record due to its small unit cell area of 0.23 cm2 . A DSC with the highest record of 9.9% for a modular size cell (17-cm2 ) consisting of eight parallel-unit cells has been fabricated by the Sony Corporation [18]. The study of DSCs using semiconductors such as ZnO, Cu2 O sensitized by Rose Bengal (RB), and Rhodamin B (RhB), was initiated by Gerischer et al. in 1968 [19–26]. Tsubomura et al. have also reported the preparation and performance of DSCs of multiporous ZnO, RhB, and I− , although the conversion efficiency was rather low (∼2.5%) [27–29]. Moreover, despite limitations in ultraviolet light irradiation of shorter than ca. 390 nm, the pioneering work performed by Fujishima and Honda on light-to-electricity conversion accompanied by the evolution of H2 and O2 from water [30] has been instrumental in the intensified research and development of DSCs. Here, it should be noted that light-to-electricity conversion devices have yet to be constructed for the oxidative decomposition of water with visible light. This is because the open circuit voltage (Voc ) of a DSC is insufficient for the oxidative decomposition of H2 O (Eredox 1.23 V vs. NHE, pH 0), as illustrated in Fig. 1. The theoretical conversion efficiency of DSCs is calculated to be lower than 33% with a Voc of 1.0–1.5 V, which is comparable to the case of silicon solar cells with 29% theoretical efficiency and a Voc of 0.77 V [31,32]. Here, the predicted efficiency and Voc are obtained by assuming that: (a) the incident light is completely absorbed by the sensitizer above the energy level of the conduction band; (b) quantitative transformation of the excited energy into the electron; and (c) the complete flow of the formed electrons into an external circuit. Under these conditions, DSCs are assumed to be theoretically superior to Si-type solar cells. Indeed, the findings of such high performance DSCs by Tsubomura [27] and Grätzel [14] have opened the path to next-generation DSC solar cells which, unlike conventional Si-type solar cells, possess such advantageous characteristics as: (i) flexibility; (ii) transparency in the visible light region; (iii) low preparation cost by a printing technique; (iv) light and mobile handling; and (v) colorful design. In addition, Segawa et al. have prepared a new functionalized DSC with TiO2 in combination with tungsten oxide as the second semiconductor which is capable of storing and releasing photocurrents under dark conditions [33,34]. However, in spite of these favorable features, DSCs still exhibit problems that need to be resolved since they generally include volatile liquid electrolytic solution and tend to lose considerable solvent by leakage which results in lower cell function with long-term use, although solid-state DSCs have been developed to attain good durability [35–40]. DSCs are, thus, still under continuing investigation and development in order to stabilize and improve their performance and durability.

Please cite this article in press as: K. Takagi, et al., Measurements and evaluation of dye-sensitized solar cell performance, J. Photochem. Photobiol. C: Photochem. Rev. (2012), http://dx.doi.org/10.1016/j.jphotochemrev.2012.08.003

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Fig. 1. Dye-sensitized solar cells (DSCs): (a) schematic illustration of the working principles; and (b) I–V plots to estimate the relationship (Eq. (5)) of the open circuit voltage (Voc ), short-circuit current (Jsc ), fill factor (FF), and photoelectric conversion (), where Pmax denotes the maximum point of output. The I3 − ion formed by h+ oxidation of the I− ion diffuses in the electrolyte to the cathodic counter electrode.

DSCs being developed using non-volatile ionic liquids as the electrolytes are expected to improve durability and, in fact, in an acceleration test of up to 15 years under STC conditions (AM 1.5 G, 100 mW/cm2 , 25 ◦ C), the drop in performance from the initial stage was less than ca. 20% [41]. As a typical ionic liquid, 1,3-dialkyl substituted imidazolium salt with bis(trifluoromethyl) imide anion, has often been reported as the electrolyte mostly in combination with LiI and I2 as the source of I− or I3 − [42–49]. Ionic liquids are, indeed, thermally quite stable [50] and have non-volatile and highly charged characteristics, although originally hydrophobic and immiscible with water [51]. In general, one of their significant physical properties is their role as an excellent hole transport material [52,53]. H2 O in an electrolyte system, however, will negatively affect the long-term stability of DSCs. And particularly for plastic substrate-based systems, although expensive barrier layers are necessary, an ionic electrolyte have been shown to keep humidity out of DSCs for over 10–20 years or even longer [54]. Although ionic liquids are suitable for DSCs in this sense, they exhibit considerable viscosity which suppresses the diffusion of the redox species such as I− or I3 − , thus, lowering DSC performance as compared with using a typical organic solvent [55,56]. For example, EMImTCB viscosity is reported to decrease from  = 22 cp at 20 ◦ C to 11.4 cp at 40 ◦ C and 6.8 cp at 60 ◦ C [57]. Due to the slow response upon light irradiation of DSCs containing an ionic liquid, it has been extremely difficult to accurately evaluate and measure their performance. Although some guidelines have been tentatively proposed by the Optoelectronic Industry and Technology Development Association (OITDA, Japan) [58–60], a standardized evaluation or procedural method for DSC performance has yet to be officially established. The present review describes the measurement and evaluation methods for DSCs involving an ionic liquid. The following issues need to be addressed in analyzing and evaluating DSC performance: (i) the long response time necessary for photocurrent generation; (ii) nonlinearity between the light intensity (irradiance) and photocurrent; and (iii) unavailability of reliable and durable reference cells to calibrate the light source for irradiation of the DSCs. These problems and the slow response of the output of light-induced electricity make it difficult to standardize or unify evaluation methods which are now carried out independently by various manufacturers. In contrast to DSCs, international standards for Si-type solar cells have been established and the guidelines published from the International Electrotechnical Commission [61]. In Japan, the JIS (Japanese Industrial Standard) has been established for the

evaluation of c-Si [62] and a-Si solar cells [63] and these guidelines are well aligned to the above international standards. This article aims to survey the evaluation methods of DSCs carried out in Japan while proposing the development of more relevant and comprehensive evaluation and procedural standards in order to improve quality as well as accelerate their acceptance and widespread use, just as c-Si or a-Si solar cells are manufactured to comply with JIS standards and have, thus, become more widely accepted.

2. Working principles of dye-sensitized solar cells A typical DSC is composed of organic dyes such as a Rucomplexed dye, TiO2 particles, I− , and an appropriate electrolytic solvent, as shown in Fig. 1a, where the open-circuit potential (Voc ) is estimated at 0.75 V from the excitation energy of S (e.g., 1.1 V vs. NHE for N3) and the redox potential of I− /I3 − [64],[65]. Excitation of the sensitizer (S) (e.g., a Ru-complexed dye such as N3, N719, or N749) adsorbed on the TiO2 nano-particle surface by visible light in the range of 400–800 nm results in the excitation of an electron to an excited S1 state, subsequently transferring the electron very rapidly into the conduction bands of the TiO2 particles with a rate constant of 1012 –1015 s−1 [66]. The migrated electron diffuses among the TiO2 particles to reach a transparent (i.e., anode) electrode, while I3 − formed by an electron transfer from an iodide ion (I− ) to the one-electron oxidized dye accepts an electron at the counter electrode (i.e., cathode). The mechanism of electric current generation, therefore, involves the diffusion of I− and I3 − in the solvent, the viscosity of which can significantly affect the response time for photon-to-current conversion. This is in stark contrast to the case of Si-type solar cells where the responsivity of the charge separation at the p–n junction of the Si semiconductor is a rapid process. Fig. 2 shows the transient photocurrent waveform, Isc , under modulated monochromatic light illumination at 550 nm: (a) a crystalline Si solar cell; and (b) a dye-sensitized solar cell (DSC). For the DSC, the transient photocurrent (Isc ) tends to rise slowly and takes some time to attain a saturated state with light of low irradiance. In this case, the Isc increases with an increase in the monochromatic light intensities from 0.25 mW to 5 mW at 550 nm. This is in stark contrast to the case of a Si-type solar cell where the transient photocurrent density instantly reaches a saturated state upon light irradiation. This is presumably because electron diffusion in TiO2 particle aggregates may be much slower than that in crystalline

Please cite this article in press as: K. Takagi, et al., Measurements and evaluation of dye-sensitized solar cell performance, J. Photochem. Photobiol. C: Photochem. Rev. (2012), http://dx.doi.org/10.1016/j.jphotochemrev.2012.08.003

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Fig. 2. Waveforms of the transient photocurrent, Isc (a.u.), irradiated by a modulated monochromatic light at 550 nm: a crystalline Si solar cell (upper wave) and a DSC (lower wave) with a chopping frequency of 0.3–85 Hz.

silicon and, in fact, the diffusion constant has been reported to be 1 cm2 /s for a Si-type solar cell and <5 × 10−4 cm2 /s for a DSC [67]. It has originally been reported that TiO2 particles are almost non-conductive, however, electron diffusion through the TiO2 particles becomes much more accelerated with exposure to strong light since the trapping holes are buried by the great amount of electrons generated [68–70]. Indeed, a nanoporous-structured TiO2 electrode of fine particles possessing a roughness of >1000 shows characteristic large electrostatic capacity which results in a longer time lapse to attain a steady state of saturation for electron capacity, thus, requiring long response times (Fig. 2, lower plots). On the other hand, a Si-solar cell (Fig. 2, upper plots) shows fast photocurrent generation upon increasing the incident irradiance, i.e., electron diffusion spreads easily through the silicon semiconductor to the electrode, allowing rapid photocurrent generation. A DSC, thus, has a unique electron transfer mechanism with a large time constant for light-induced electric generation and, therefore, the same criteria as Si-type solar cells should not be applied. 3. Evaluation of DSC performance 3.1. Light sources There are three basic international certification organizations for the testing and verification of photovoltaic cell (PV) performance as well as Standard Test Conditions (STC) for calibration of air mass, sunlight intensity and cell temperature at AM 1.5 G (G stands for global), 100 mW/cm2 and 25 ◦ C, respectively, as the reference solar light, i.e.: (1) The National Renewable Energy Laboratory (NREL), USA; (2) Physikalische-Technische Bundesanstalt (PTB), Germany [71]; and (3) the National Institute of Advanced Industrial Science and Technology (AIST), Japan. The normalized standard solar simulator (SSst ) at AIST (Japan) has been installed for the testing and certification of photovoltaic cells since steady and stable light at STC (AM 1.5 G, 100 mW/cm2 , 25 ◦ C) is difficult to obtain from natural sunlight due to the changeable climate throughout the country. In Japan, the standard solar spectrum for a reference solar simulator (SS) has been determined by the guidelines of JISC8912 and 8933, the standards and conditions of which are aligned with the international standard, IEC60904-9. It should be noted that since the conversion efficiency () is deeply dependent on the spectral distribution of the incident irradiance, another SS (i.e., SSmeas ) to analyze DSCs needs to be calibrated by the standard sunlight SS (i.e., SSst ), especially for the coincidence of their spectral distributions. However, at present, the total photocurrents summarized over all of the wavelengths of the solar spectrum (Jsc ) are regulated within coincidence errors of 25% by

Fig. 3. Spectral irradiance distribution of solar simulators (SSst ): (a) Red signal: dual light source type SS equipped with Xe and halogen lamps; (b) Green signal: single light source type SS equipped with a Xe lamp; and (c) Blue signal: AM 1.5 G reference cell. It should be noted that the SS spectrum of the green signal exhibits several sharp emissions at around 800–1000 nm.

IEC60904/JISC8912 Class A and JISC8942 Class MA for the single light source type SS with a Xe lamp and the dual light source type SS with Xe and halogen lamps, respectively, the spectral distributions of which are compared in Fig. 3. 3.2. Reference cells and pseudo-reference cells The most principal index for photovoltaic cell performance is the maximum output of electricity under STC, i.e., when exposed to normalized standard sunlight (AM 1.5 G, irradiance of 100 mW/cm2 at 25 ◦ C). The output of the PV should be highly dependent on the spectral irradiance of the incident sunlight. Hence, a reference cell which possesses relative spectral responsivity identical or closely similar to that of the PV is required to estimate the incident irradiance of the light source. A scale for photovoltaic performance has been provided by the International Reference Cell Calibration Program of the World Photovoltaic Scale (WPVS) [72]. The traceability of the solar light intensities of the SSmeas to be used under STC can be confirmed by their calibration with SSst . When SSmeas is used for the normalization data of DSC performance (such as Jsc , Voc , FF and ) by its I–V measurements, the traceability of SSmeas to SSst should be confirmed. For calibration of SSmeas by SSst , the overall short-circuit density (Jsc ) of the reference cell measured by SSmeas is calibrated by that of SSst , as shown in Eq. (1), where I () and SR () is the measured spectral response and spectral irradiance at the wavelength () of the AM 1.5 G standard, respectively [73].



Jsc =

I()SR()d

(1)

Since long-term functional stability and durability cannot be ensured due to the annealing effects of DSCs, structurally similar DSCs cannot be used as reference cells to estimate the shortcircuit photocurrent density, Jsc , based on the normalized standard SSst . As an alternative choice for a reference cell for thin film a-Si type solar cells, a structurally different but spectrally analogous reference cell, a c-Si solar cell equipped with appropriate optical filters, is internationally recognized. Similarly, for DSCs, a c-Si solar cell equipped with appropriate optical filters was prepared as a pseudo-reference cell involving N719 or black dye (N749). Here, it should be pointed out that the above reference cells (or pseudo-reference cells in the case of DSCs) should be calibrated by a formally recognized organization, considering the uncertainty caused by spectral mismatch between the Incident Photon-toCurrent Conversion Efficiency (IPCE) of the reference cell and DSC to be analyzed. Here, IPCE is expressed by the percentage ratio of

Please cite this article in press as: K. Takagi, et al., Measurements and evaluation of dye-sensitized solar cell performance, J. Photochem. Photobiol. C: Photochem. Rev. (2012), http://dx.doi.org/10.1016/j.jphotochemrev.2012.08.003

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K. Takagi et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews xxx (2012) xxx–xxx Table 1 Spectral-mismatch percentages of DSCs estimated by Eq. (2) with c-Si and a-Si solar cells as the reference cells. Solar simulator (SS)

Spectral mismatch (%)

Single light source (Xenon) Dual light source (Xenon and Halogen)

c-Si

a-Si

4.34 0.12

−1.34 −0.47

Fig. 4. Relative spectral responsivity of a-Si, c-Si and a dye-sensitized solar cell (DSC) as reference cells.

Jsc to the irradiance on the basis of the energy unit, while spectral responsivity (SR) is determined on the basis of the energy unit. The spectral mismatch (%) is expressed in Eq. (2) and the corrected Jsc ’s (mA/cm2 ) are shown in Eq. (3). Spectral mismatch (%)

   s ()Q2 ()d · m ()Q1 ()d  = 1−  × 100 

m ()Q2 ()d ·

 Jsc (s) = Jsc (m) 

s ()Q2 ()d ·

s ()Q1 ()d

 m ()Q1 ()d 

m ()Q2 ()d ·

s ()Q1 ()d

(2)

(3)

where Jsc (s) and Jsc (m) are designated as the short-circuit photocurrent densities for the photovoltaic cells to be estimated under reference solar light (AM 1.5 G) and to be measured by the SSmeas calibrated by the (pseudo) reference cells, respectively. s () and m () indicate the spectral irradiance of the relative standard sunlight at AM 1.5 G and the spectral irradiance of the SSmeas , respectively. Q2 () and Q1 () indicate the relative spectral responsivities of the reference cell and the cell to be analyzed, respectively. In the calibration of the SSmeas irradiance using the reference cell, the spectral mismatch errors are restricted to within 2% for Si solar cells [16]. Care is required in spectral mismatch analysis of DSCs since a SS with a single light source (Xe lamp) tends to overestimate the Jsc of DSCs exhibiting light absorption in the range of 800 nm and 1000 nm due to the presence of several spike signals within this range, as shown in Fig. 3. Indeed, Table 1 indicates that a dual light source of a combination of Xenon and Halogen lamps affords a spectral mismatch percentage that is less than the Xenon single light source and comparisons of the spectral responsivities for c-Si, a-Si and DSCs are shown in Fig. 4. Also, for accurate measurements of the IPCE, it is necessary to adjust the coincidence of the SSmeas spectral distribution and the DSC spectral responsivity. Pseudo-reference cells are necessary to more precisely calibrate DSCs involving sensitizers active for longer wavelength regions since the development of newer DSCs is increasingly being reported. However, it has not been easy for the following reasons: (i) the diversity of the organic dyes used as the sensitizers; (ii) the need for further extension of effective incident light toward

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near-infrared (NIR) regions; and (iii) problems in long term stability and reliability. The DSCs to be analyzed can also be used as the reference cell and their IPCEs are meaningful only in the case of their long term stability and reliability against contact with outer moisture or oxygen gas. At present, pseudo-reference cells in combination with an appropriate SS have been reported to be useful for the suppression of spectral mismatch errors, thus, making them applicable for the calibration of the N3-type DSC. In addition, a dye with a terpyridine ligand, a “black dye”, absorbing a longer wavelength of up to 900 nm has been reported as being satisfactorily calibrated by a pseudo-reference c-Si solar cell with a KG-3 filter, rather than a KG-5 filter, with a thickness of 1 or 2 mm. In this case, the thickness of the filter must be carefully selected in order to ensure as less spectral mismatch errors as possible [68]. Here, it should be noted that experimental errors caused by multi-reflection between a pseudo-reference cell surface (mainly from the filter) and the output lens of the SS used need to be considered [58]. Since the errors are based on the structural materials of the pseudo-reference cells, it is important to set up both the pseudo-cells and the DSC to be analyzed in an irradiation area where there is as little contribution of their multi-reflections as possible by considering the following conditions: (i) the longest distance of the outer lens of the SS and the target cell as possible; and (ii) the most homogeneous light intensity of the SS in the irradiation area as possible. 4. Measurement of the DSC Incident Photon-to-Current Conversion Efficiency (IPCE) Spectral responsivity of solar cells, i.e., the spectral sensitivity of the generated photocurrent toward the light intensity at the wavelength of incident light is based on a single photon, which is known as the quantum yield or the “Incident Photon-to-Current Conversion Efficiency (IPCE)”. Since the IPCE is mainly dependent on the absorption spectra of the dye sensitizers, especially in the case of DSCs, efficiency is an important factor in evaluating DSC performance. The IPCE is expressed by the sum of the short-circuit currents (Jph ) generated against a single photon (, nm) of the solar irradiance (mW/cm2 ), as shown in Eq. (4). IPCE (%) =

1.240 × 105 × Jph ×˚

(4)

DSCs are generally known to possess a slow response for photocurrent generation by incident light, therefore, Jsc measurements can only be accurately assessed if sufficient time is allowed to complete photocurrent generation, in contrast to the case of c-Si solar cells. There are two methods which can satisfactorily measure the IPCE of DSCs, i.e., the “AC method” and “DC method”. Of these two, the AC method has been put forward in the ASTM E1021 norm as the standard procedure for spectral response measurement of solar cells [74]. Spectral response is measured using pulsed monochromatic illumination with chopped light beams under continuous white bias irradiation. A lock-in amplifier is used to distinguish current response to the pulsed monochromatic illumination from the total output current. The slow response of DSCs is most probably related to the slow diffusion rate of electrons across the TiO2 film [67,69,75–83] and the limitations in diffusion of the ions in the electrolyte [76,77,84]. Considering this slow response, the chopping frequency of the monochromatic beams to measure the spectral responsivity should be determined carefully to avoid underestimation of the short-circuit current when the standard AC method is applied according to the ASTM E1021 norm. Hara et al. [68] have stated that the chopping frequency of the monochromatic beams should be in the range of 1–2 Hz under continuous bias illumination of 1 Sun (AM 1.5 G, 100 mW/cm2 ). The DC method measures spectral

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Fig. 5. IPCE plots of DSCs with Z907 dye and an organic solvent as the electrolyte: (a) by the DC method carried out in the monochromatic irradiance range of 1.0 × 1016 to 0.1 × 1016 photons/cm2 s; and (b) by the AC method in the chopping frequency range of 1.3–85 Hz under illumination of 1 Sun bias light.

response to continuous irradiation using monochromatic light in the absence of the above bias light [85,86]. Hohl-Ebinger et al. [78] have reported that the spectral responsivity obtained by the AC method with a chopping frequency of less than 1 Hz under 1/2 Sun bias and the spectral responsivity obtained by the DC method are in excellent agreement. Similar results have been reported by Hara et al. [68] and Sommeling et al. [69], however, changes in the spectral responsivity with the bias light intensity have also been reported [69,79]. A number of research groups have reported that the response time of a DSC may deviate significantly, depending on the kind of elemental materials used in DSCs such as the electrolytic solvents [56–68,74,77,84]. The IPCE plotting of DSCs was carried out in our lab by both the AC and DC methods. The AC method was performed by changing the irradiance of the incident light with a white bias light of 1 Sun with a rock-in-amplifier while the DC method was performed by scanning the wavelength of the incident monochromatic light and monitoring the short-circuit currents (Jsc ) at 10 nm intervals in the range of 300–800 nm, as shown in Fig. 5. When a conventional DSC of Z907 with an organic solvent as the electrolyte was used, spectral responsivity (IPCE) obtained by the DC method was carried out in the range of 1.0 × 1015 to 1.0 × 1016 photons/cm2 s of monochromatic irradiance and by the AC method in the range of 1.3–85 Hz under illumination of 1 Sun bias light. A good coincidence in the IPCE between both methods was observed only when the AC method was performed with a chopping frequency of lower than 6.3 Hz. It was observed that the IPCE could be satisfactorily measured under low chopping frequency which allows sufficient irradiance conditions to complete photon generation in both the AC and DC methods.

Two kinds of DSCs involving different electrolyte media were investigated: a typical organic solvent, 3-methoxypropionitrile (MPN) (Cell A), and an ionic liquid (Cell B), in order to study the dependency of the electrolyte medium on the transient shortcircuit current densities (Figs. 6 and 7) [87], where the current densities are normalized by the steady state values after stepwise application of monochromatic light between 550 and 750 nm [84,88]. These Figures show that the response times with irradiance of 4.0 mW/cm2 are much shorter than that with 0.05 mW/cm2 for both cells. This may be due to the slow diffusion rate of the electrons across the TiO2 film, as discussed above, in which Cell B takes longer to complete photogeneration, i.e., to attain a steady state, than Cell A, although the current density increases for Cell B more rapidly than for Cell A at the early stage of low irradiance (0.05 mW/cm2 ) [89]. The electrochemical impedance spectra of Cells A and B were studied at open-circuit conditions under 1 Sun irradiation, as shown in Fig. 8, where the spectra of both cells are composed of three semicircles [90]. It has been reported that the semicircle in high frequency is attributed to a charge transfer at the counter electrode, a circle in intermediate-frequency is associated with both the electron transport in the mesoscopic TiO2 film and back reaction at the TiO2 /electrolyte interface, and a semicircle in low-frequency reflects the diffusion of I3 − in the electrolyte [91–98]. As shown in Fig. 8, the semicircle in low frequency was larger for Cell B than for Cell A. These results indicate that the diffusion rate of I3 − in the ionic liquid electrolyte is lower than that for the MPN containing an electrolyte. The slow response of Cell B to attain a saturated point can be explained by the slow diffusion rate of I3 − . The characteristic frequency at the top of the semicircle in the intermediate frequency is higher for Cell B than for Cell A

Fig. 6. Transient short-circuit current densities of a DSC of a typical organic solvent (MPN) electrolyte normalized by steady state values after stepwise application of monochromatic irradiation at wavelengths of: (a) 550 nm; and (b) 750 nm.

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Fig. 7. Transient short-circuit current densities of a DSC of an ionic liquid electrolyte normalized by steady state values after stepwise application of monochromatic irradiation at wavelengths of: (a) 550 nm; and (b) 750 nm.

Fig. 8. Electrochemical impedance spectra of Cells A (MPN: blue line) and B (ionic liquid: red line) obtained at open-circuit conditions under 1 Sun irradiation.

since it is proportional to the lifetime reciprocal of the electrons in the TiO2 film. From these results, the rapid current increase of Cell B is assumed to be associated with the short electron lifetime of an ionic liquid-based cell. Fabregat-Santiag et al. [99] have also suggested that the electron lifetime is much longer for the fluid solvent-containing electrolyte than for the ionic liquid electrolyte, while the diffusion rate of I3 − was much lower in the ionic-liquid electrolyte than in the solvent-containing electrolyte, as shown by impedance spectroscopy measurements of the DSCs. In fact, the

ionic liquid was more viscous compared to MPN and a 30-fold difference in their viscosities was measured at ambient temperatures. Herein, it is noteworthy to observe that ionic liquids have excellent properties such as high ionic conductivity, non-flammability, thermal stability and a wide potential window suitable for photovoltaic cells [100,101]. Fig. 9 shows that the spectral responsivities of Cells A and B were almost unchanged by irradiation of continuous monochromatic light in the range of 0.05–4.0 mW/cm2 light intensity by the DC method without bias light [86]. That is, a linear relationship between Jsc and the irradiance in an 80-fold variation of the light intensity for both cells was observed. This fact implies that the linear relationship between the Jsc and irradiance is hardly affected by the low diffusion rate of I3 − in an ionic liquid electrolyte. The spectral responsivities of Cell A (MPN) and Cell B (ionic liquid) were determined as a response to pulsed monochromic light, the intensity of which was 4 mW/cm2 in the absence of bias light, which are shown in Figs. 10 and 11, respectively. Upon irradiation at 750 nm, the chopping frequency to obtain a steady state value was lower than 1.3 Hz for Cell A and lower than 0.3 Hz for Cell B, although the steady state of Jsc at 550 nm was almost the same for both cells at a chopping frequency lower than 1.3 Hz. These slow responses upon irradiation at 750 nm may be caused by the low diffusion rate of the electrons across the TiO2 film which is related to the electron density (light intensity) due to the low absorbance of the dye sensitizer, resulting in a decrease in the current response [102–104]. The overall short circuit current densities of Cells A and B estimated from Eq. (1) using the spectral response were compared with those measured under 1 Sun irradiation with a SS (Table 2). The short-circuit current densities estimated using the spectral response measured by the AC method under 1 Sun irradiation were

Fig. 9. Linear relationship between Jsc and irradiance in the range of 0.05–4.0 mM/cm2 upon monochromatic illumination of the DSCs having an electrolyte of: (a) a typical organic solvent (MPN); and (b) an ionic liquid.

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Fig. 10. Transient short-circuit currents of Cell A as a response to pulsed monochromic light with an intensity of 4 mW/cm2 , as determined in the absence of bias light at wavelengths of: (a) 550 nm; and (b) 750 nm.

Fig. 11. Transient short-circuit currents of Cell B as a response to pulsed monochromic light with an intensity of 4 mW/cm2 , as determined in the absence of bias light at wavelengths of: (a) 550 nm; and (b) 750 nm.

Table 2 Overall short-circuit current densities of Cells A and B estimated by Eq. (1) with the spectral response (Jscc ) and those measured under 1 Sun irradiation with the solar simulator (Jscm ). Cell

Jscm (mA/cm2 )

Jscc (AC method) (mA/cm2 )

Jscc (DC method) (mA/cm2 )

A B

16.83 12.33

16.66 12.28

17.50 13.37

in good agreement with those measured by a SS for both cells and measurement by the DC method without bias light afforded higher values than the AC method. These results suggest that spectral responsivity measurement should be performed under illumination conditions as similar to practical use as possible. 5. DSC performance measurements by a solar simulator (SS) The photoelectric conversion () of DSCs is measured by a SS and the efficiency () is denoted in Eq. (5), as follows: =

Voc × Jsc × FF × 100 Prad

(5)

where the Voc (V), Jsc (mA/cm2 ), Prad (mW/cm2 ), and FF represent the open-circuit voltage, short-circuit current, incident light

intensity (100 mW/cm2 at STC), and fill factor, respectively. Fig. 1b shows the typical I–V plots to illustrate DSC performance, i.e., Voc , Jsc , and FF as well as the conversion efficiency (). Reliable and comprehensive I–V measurements of DSCs basically lies on the coincidence of the two curves from the forward and reverse directions of the applied voltage by monitoring the short-circuit photocurrent (Jsc ). Koide et al. have reported that the dependence of the measurement of the transient photocurrent on the sweep directions and sampling delay time may be explained by the longer time constant of DSCs. To improve accuracy, measurements should be carried out with a sampling delay time exceeding several seconds. However, it was also found that the average value of the efficiency measured by the two sweeping directions is constant when the sampling delay time is longer than 40 ms [105]. Table 3 summarizes comparisons of the conversion efficiencies () estimated by I–V measurements of DSCs from the forward and reverse directions. Here, the total scanning time requires 0–200 s for one cycle if a photocurrent is measured with applying a bias voltage in the range of −0.1 V and 0.8 V. The sweep time is, thus, an important factor in determining a relevant standard evaluation method. I–V measurements which would take an excessively long time to complete are not practical and would not be widely accepted. On the other hand, the response time should be long enough to generate complete photon-to-electricity conversion, resulting

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Table 3 DSC performance estimated by I–V measurements by changing the delay times and monitoring the forward and reverse scanning directions. Scan direction

Sweep time (s)

Jsc (mA/cm2 )

Voc (V)

FF

Efficiency (%)

Forward (Isc → Voc )

0.4 1.2 2.2 10 20 100 200

16.12 16.02 15.97 15.75 15.68 15.48 15.29

0.648 0.657 0.661 0.665 0.666 0.665 0.664

0.511 0.548 0.557 0.571 0.575 0.578 0.579

5.34 5.76 5.88 5.99 6.00 5.96 5.88

Reverse (Voc → Isc )

0.4 1.2 2.2 10 20 100 200

15.99 15.81 15.75 15.59 15.53 15.26 15.09

0.672 0.674 0.673 0.667 0.665 0.664 0.664

0.649 0.599 0.583 0.577 0.581 0.590 0.593

6.97 6.38 6.18 6.00 6.00 5.98 5.94

in a good coincidence between the forward and reverse I–V curves. Many factors affect the responsive time in I–V measurements of DSCs such as: (a) the layer thickness and size of the TiO2 particles; (b) viscosity of the electrolyte medium; and (c) whether a solid or liquid electrolyte is used, in other words, the diffusion length of the DSC devices. Regarding the origin of these characteristics in DSC performance, Hara et al. have reported that no coincidence between the plots of the forward and reverse scanning are observed by the applied voltage in the case of materials possessing large electrostatic capacitance [106]. It is assumed that a multiporous TiO2 electrode may have a large electrostatic capacitance due to the

remarkably large surface area. In this sense, the generated electrons may be trapped in the surface, taking a long time to flow into the anode electrode and, thus, making it hard to respond quickly to the rapid scanning of the voltage [68]. There was a striking difference in the responsive times of Cell A involving MPN and Cell B with an ionic liquid as the electrolyte, showing ca. 30-fold difference in their viscosities [87]. Figs. 12 and 13 show a dependence of the I–V curves of Cells A and B, respectively, on the scanning sweep times toward the forward (red square: Isc → Voc ) and reverse scan directions (blue rhombus: Voc → Isc ). Upon changing the sweep time from 0.4 to 200 s, for Cell A, the forward and reverse scan directions of the I–V curves

Fig. 12. Dependence of I–V curve of DSC (Cell A) with a MPN electrolyte on the forward (solid line: Isc → Voc ) and reverse sweep directions (broken line: Voc → Isc ) with sweep times between −0.1 V to +0.8 V at: (a) 0.4 s (b) 2.2 s; (c) 20 s; and (d) 200 s.

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Fig. 13. Dependence of I–V curve of DSC (Cell B) with an ionic liquid electrolyte on the forward (solid line: Isc → Voc ) and reverse sweep directions (broken line: Voc → Isc ) with sweep times between −0.1 V to +0.8 V at: (a) 0.4 s; (b) 2.2 s; (c) 20 s; and (d) 200 s.

Fig. 14. Dependence of the efficiency () on the sweep times toward the forward (blue rhombus: Isc → Voc ) and reverse scan directions (red square: Voc → Isc ) with sweep times between −0.1 V to +0.8 V for DSCs involving the electrolyte: (a) MPN; and (b) an ionic liquid.

became smoothly coincident when the sweep time was longer than ca. 20 s, whereas, Cell B was observed to take a much longer time for coincidence in both scan directions, clearly showing the reliability of the I–V measurements. Moreover, Fig. 14 shows that the conversion efficiency () of Cell A is coincident at ca. 10 s for the scanning sweep times in the forward and reverse scan directions, whereas that for Cell B was ca. 500 s. This was attributed to the difference in the diffusion rates of ions I− or I3 − in the electrolyte, MPN and the ionic liquid [89]. Although nonvolatile electrolytes such as ionic liquids are durable for DSCs and secure against performance degradation, the response time of photocurrent generation for the I–V plots is, unfortunately, rather slow in measuring DSC performance.

6. Conclusions Dye-sensitized solar cells (DSCs) have been reviewed from the viewpoint of the correct measurement and evaluation of their performance. The slow response times and nonlinearity of DSC photocurrents against the light intensity (irradiance) at wavelengths of incident light are profoundly influenced by their characteristic working principles. I–V measurements of DSCs, especially those including an ionic liquid, showed that they are capable of performing satisfactorily if sufficient time is allowed to complete photocurrent generation. However, in an extreme example, a DSC involving an ionic liquid can take over 50 min for measurements of the I–V curves to show coincidence of the forward and reverse

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applied voltage swings. Novel instruments and techniques to measure the I–V curves by considerably shortening the sweep times for DSCs with slow diffusion processes for photocurrent generation are strongly desired in order to correctly evaluate DSC performance and make them more widely available. In our investigations, the AC method using white bias light can be proposed as the most suitable in reflecting DSC performance under practical working conditions. Acknowledgments The authors would like to acknowledge and express appreciation for financial support from the New Energy and Industrial Technology Development Organization (NEDO) of the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) of Japan. The authors would also like to express sincere gratitude to Professor Akira Fujishima, President of Tokyo University of Science, for his guidance and encouragement in these research projects. References [1] A. Toyoshima, S. Uchida, All About Solar Cells – From Semiconductors to DyeSensitized, Technical Information Institute Co., LTD, Tokyo, 2007 (in Japanese). [2] T. Miyasaka (Ed.), Photovoltaic Cells of New Concepts and Manufacturing Processes, CMC Publishing, Co., Tokyo, Japan, 2009 (in Japanese). [3] H. Arakawa (Ed.), Recent Advances in Research and Development for DyeSensitized Solar Cells II, CMC Publishing, Co., Tokyo, 2007 (in Japanese). [4] S. Mori, S. Yanagida, in: T. Soga (Ed.), Nanostructured Materials for Solar Energy Conversion, Elsevier, London, 2006, pp. 193–226. [5] H. Segawa, S. Uchida, Modularization, Material Development, and Evaluation Technology of DSCs, Gijutukyoiku, Tokyo, 2010 (in Japanese). [6] T. Miyasaka (Chairman of the Conference), Proceedings of the 6th Aceanian Conference on Dye-sensitized and Organic Solar Cells (DSC-OPV6), Beppu, Japan, October 17–18, 2011. [7] L. Han, et al., Proceedings of the 4th World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii, May 7–12, 2006. [8] G. Chmiet, et al., Proceedings of the 2nd World Conference on Photovoltaic Energy Conversion, Vienna, Austria, May 14–18, 1998. [9] T. Toyoda, et al., Proceedings of 1st Conference on Renewable Energy 2006, Makuhari, Japan, October 14–18, 2006. [10] T. Meyer, et al., Proceedings CD of 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11–18, 2003. [11] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovolt.: Res. Appl. 20 (2012) 12–20. [12] J. Desilvestro, M. Grätzel, L. Kavan, J.E. Moser, J. Augustynski, J. Am. Chem. Soc. 107 (1985) 2988–2990. [13] N. Vlachopoulos, P. Liska, J. Augustynski, M. Grätzel, J. Am. Chem. Soc. 110 (1988) 1216–1220. [14] B. O’Regan, M. Grätzel, Nature 335 (1991) 737–740. [15] M. Grätzel, Nature 414 (2001) 338–344. [16] N. Koide, R. Yamanaka, H. Katayama, MRS Proc. 1211 (2009), 1211-R12-O2. [17] http://www.nims.go.jp/eng/news/press/2011/08/p201108250.html [18] http://www.sony.co.jp/SonyInfo/technology/technology/theme/solar 01.html [19] H. Gerischer, M.E. Michel-Beyerle, F. Rebentrost, H. Tributsch, Electrochim. Acta 13 (1968) 1509–1515. [20] H. Gerischer, H. Tributsch, Ber. Bunsen-Ges. Phys. Chem. 72 (1968) 437–445. [21] H. Gerischer, H. Tributsch, Ber. Bunsen-Ges. Phys. Chem. 73 (1969) 251–260. [22] H. Tributsch, Ber. Bunsen-Ges. Phys. Chem. 73 (1969) 582–590. [23] H. Gerischer, H. Tributsch, Ber. Bunsen-Ges. Phys. Chem. 73 (1969) 850–854. [24] H. Gerischer, Photochem. Photobiol. 16 (1972) 243–260. [25] H. Gerischer, H. Selzle, Electrochim. Acta 18 (1973) 799–805. [26] B. Pettinger, H.R. Schöppel, H. Gerischer, Ber. Bunsen-Ges. Phys. Chem. 77 (1973) 960–966. [27] H. Tsubomura, M. Matsumura, Y. Nomura, T. Amamiya, Nature 261 (1976) 402–403. [28] M. Matsumura, S. Matsudaira, H. Tsubomura, M. Takata, H. Yanagida, Yogyo Kyokaishi 87 (1979) 169–178. [29] M. Matsumura, S. Matsudaira, H. Tsubomura, M. Takata, H. Yanagida, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 415–421. [30] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [31] G. Smestad, Sol. Energy Mater. Sol. Cells 32 (1994) 273–288. [32] M. Grätzel, Abstract of The First Conference of Future Generation Photovoltaic Technologies, NREL, Denver, CO, USA, March 24, 1997. [33] Y. Saito, A. Ogawa, S. Uchida, T. Kubo, H. Segawa, Chem. Lett. 39 (2010) 488–489. [34] Y. Saito, S. Uchida, T. Kubo, H. Segawa, Thin Solid Films 518 (2010) 3033–3036. [35] K. Tannakone, G.R.R.A. Kurama, A.R. Kuramasinghe, K.G.U. Wijantha, P.M. Sirimanne, Semicond. Sci. Technol. 10 (1995) 1689–1693. [36] B. O’Regan, D.T. Schwartz, Chem. Mater. 10 (1998) 1501–1509.

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