Influence of indoline dye and coadsorbate molecules on photovoltaic performance and recombination in dye-sensitized solar cells based on electrodeposited ZnO

Journal of Electroanalytical Chemistry 709 (2013) 10–18

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Influence of indoline dye and coadsorbate molecules on photovoltaic performance and recombination in dye-sensitized solar cells based on electrodeposited ZnO Melanie Rudolph a,b,1, Tsukasa Yoshida a,2, Derck Schlettwein b,⇑ a b

Environmental and Renewable Energy Systems Division, Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Institute of Applied Physics, Justus Liebig University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany

a r t i c l e

i n f o

Article history: Received 17 June 2013 Received in revised form 27 September 2013 Accepted 29 September 2013 Available online 10 October 2013 Keywords: Dye-sensitized solar cell Zinc oxide Indoline dye Coadsorption Recombination parameter Trap distribution

a b s t r a c t A detailed photoelectrochemical analysis of dye-sensitized solar cells based on electrodeposited porous ZnO, the indoline dye D149, cholic acid as optional coadsorbate, and a liquid iodide/triiodide electrolyte was performed. The effects of changes in the amount and chemical environment of the dye molecules on overall cell performance, distribution of trap states and recombination were studied. Short-circuit photocurrent, open-circuit photovoltage and cell efficiency improved with increasing amount of dye within the range investigated, while fill factors were lowered. The decline of the fill factor could be attenuated by coadsorbing cholic acid. Electrochemical impedance spectroscopy (EIS) was used to study the chemical capacitance, recombination resistance and series resistance in cells with different D149 loadings in the presence or absence of cholic acid. For increasing dye loading, the recombination parameter b and, in consequence, the recombination resistance around the maximum power point decreased, which explained the drop in the fill factors. In cells containing cholic acid, the conduction band edge was shifted upwards, the density of surface trap states was reduced, and the observed decrease of b was less pronounced compared to cells without coadsorbate. The charge transport in the electrolyte and the kinetics of charge transfer at the counter electrode were slowed down in cells with high dye content. A diode model accounting for non-linear recombination served to quantitatively link dye- and coadsorbate-induced changes in the voltage-dependence of the recombination resistance to the fill factor variation. Recombination with oxidized D149 molecules in aggregates and weak interaction between D149 and ZnO are discussed as physical origins of the observed trends. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction With current solar energy conversion efficiencies of up to 12.3% [1] dye-sensitized solar cells (DSCs) are on their way to become a viable low-cost alternative to conventional solar cells. The photoactive part of a typical DSC consists of a nanoparticulate titanium dioxide film that is sensitized with a Ru(II) dye [2,3]. Besides TiO2, zinc oxide has shown great potential as DSC photoelectrode material [4–8]. Nanostructured ZnO films suited for application in DSCs can be obtained at low temperatures by a variety of preparation methods [6,9–15], which makes them suitable for application in plastic DSCs [6,11–14,16] or in DSCs based on other ⇑ Corresponding author. Tel.: +49 641 9933 400. E-mail address: [email protected] (D. Schlettwein). Permanent address: Institute of Applied Physics, Justus Liebig University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany. 2 Present address: Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 9928510, Japan. 1

1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.09.028

temperature-sensitive flexible substrate materials like textiles [17,18]. Low-temperature electrochemical deposition in the presence of structure-directing agents like eosin Y is an attractive option to form mesoporous ZnO films on conductive glass, foil or textile fibers [6,15–18]. Pore size and internal structure of such electrodes are controlled by the concentration of eosin Y in the deposition bath and the deposition potential [6,15,19,20]. The classical Ru(II) dyes are difficult to apply as a sensitizer for ZnO because of unwanted chemical reactions between dye and ZnO [4,6]. On the other hand, sensitizers such as indoline dyes, which do not contain complexing agents and show lower acidity, have been found to be more suitable [8]. To date, the best conversion efficiency of 5.6% for DSCs based on electrodeposited ZnO has been attained using the double rhodanine indoline dye D149. The low temperature preparation of the ZnO nanostructure as well as the fact that indoline dyes like D149 exhibit high extinction coefficients while not containing rare metals render this material system a cost-efficient, non-toxic and sustainable alternative to conventional DSC consisting of energy-intensive nanoparticulate TiO2

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films and Ru(II) dyes. The use of indoline sensitizers like D149, however, brings about certain challenges, because they show a tendency of aggregation on semiconductor surfaces [21–25]. Since dye aggregation can worsen the photovoltaic performance [21,22,26,27], it is necessary to take steps to avoid it. Results of previous studies on DSCs with indoline dyes suggest that addition of the coadsorbate cholic acid or its derivatives to the dye adsorption solution presents an effective way to suppress dye aggregation, as it significantly improves the photovoltaic characteristics [21,22,26–28]. For example, coadsorption of cholic acid has been found to prevent a decline of the fill factor otherwise observed upon an overloading of electrodeposited nanostructured ZnO with D149 [21]. Further analysis of such cells by electrochemical impedance spectroscopy has indicated that devices without coadsorbate and with high dye loadings may show an energetically deeper distribution of surface trap states in the ZnO compared to cells containing less D149 along with cholic acid. It has been proposed that these observations resulted from formation of D149 aggregates in cells with high dye loadings without coadsorbate, and that these aggregates might have formed deep surface trap states which increased the rate of recombination in the voltage range around the maximum power point. The present study aimed at clarifying the reasons for the low fill factors in electrodeposited ZnO DSCs with high D149 loadings in more detail. We performed a comprehensive and systematic variation of the D149 loading in the presence and absence of cholic acid, respectively. Scanning electron microscopy served to examine the nanostructure of the electrodeposited porous ZnO films. The aggregation tendency of D149 on the ZnO films and the effect of the coadsorbate on the latter were assessed by solid-state UV/Vis absorption spectroscopy. The impact of the amount and chemical environment of the D149 molecules on the photoelectrochemical characteristics of the electrodeposited ZnO/D149 solar cells was characterized in detail by means of current-voltage characterization and electrochemical impedance spectroscopy (EIS). An emphasis was put on examining the consequences of the dye and cholic acid adsorption process on the distribution of trap states in the nanostructured ZnO, which was obtained via the chemical capacitance from EIS. Moreover, a detailed analysis of the recombination resistance served to study how monomeric or aggregated D149 as well as the coadsorbate influenced the rate of recombination at the interface between ZnO, the dye and the I =I 3 electrolyte. Efficiency-limiting phenomena caused by interaction between D149 and the electrolyte were revealed in the discussion of the series resistance determined by EIS.

2. Experimental Unless stated otherwise, chemicals were purchased from Wako, Sigma-Aldrich, Roth or Merck in ACS grade or higher, and used without further purification. Milli-Q water with a resistivity of 18.2 MX cm was utilized for all experiments. FTO- coated glass substrates (FTO = fluorine-doped SnO2) (Asahi glass, 10 X/h) were cleaned by sonicating in water, detergent (Inui-Syoji vista#50), acetone, and distilled 2-propanol consecutively. After treatment in a Filgen UV253H UV/ozone cleaner, a circular area of 6 mm diameter in the center of the FTO/glass pieces was defined as active area for the electrodeposition by means of photolithography using positive-type photoresist (Tokyo Ohka Kogyo). The samples were contacted and fixed on electrode holders designed to allow rotation of the substrates during electrodeposition. For parallel electrodeposition of ZnO on eight substrates, a custom-built setup [29] (Dainippon Screen) with a circular arrangement of eight rotating electrode holders in combination with a 16 channel potentiostat system (Bio-Logic, VMP3) was used. A RedRod (Radiometer) refer-

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ence electrode (0 mV vs. Ag/AgCl) was employed. A Pt wire served as counter electrode for pre-electrolysis and deposition of the compact ZnO layers (blocking layers), while a Zn wire counter electrode was used for deposition of the porous layers. The temperature of the deposition bath was kept at 80 °C. The eight rotating (500 rpm) working electrodes were activated for 30 min by preelectrolysis at 1.05 V vs. Ag/AgCl in O2-saturated 0.1 M aqueous KCl solution [30]. Subsequent addition of ZnCl2 to yield a Zn2+ concentration of 5 mM in the electrolyte triggered the deposition of the blocking layers, which was performed at the same potential of 1.05 V vs. Ag/AgCl for 10 min. Porous ZnO was then deposited on top of the blocking layers by adding the structure-directing agent eosin Y to the deposition bath to obtain a concentration of 300 lM, and applying a potential of 0.75 V vs. Ag/AgCl for 30 min. Scanning electron microscopy (SEM) images of cross sections of films not used for cell assembly were taken using a Hitachi S-4800 field emission scanning electron microscope. The porous film thickness of 14 samples was determined from the SEM cross section images and divided by the charge transferred during their electrodeposition to obtain a factor allowing conversion of transferred charge to film thickness. The porous film thickness of the samples used for cell assembly was then calculated from the charge transferred during their electrodeposition using this conversion factor. The film thickness thus obtained for the cells used for cell assembly was 4.2 ± 0.1 lm. Following electrodeposition, eosin Y was removed by immersing the films in aqueous KOH (Nacalai tesque) solution of pH 10.5 [31] for 24 h. The samples were rinsed with water, dried at 100 °C for 60 min and cleaned by UV/ozone for 30 min. The fully organic photosensitizer D149 (Chemicrea) was adsorbed to the surface of thus-pretreated porous ZnO films by immersing the samples into a 0.5 mM solution of D149 in a 1:1 (by volume) mixture of acetonitrile and tert-butanol at room temperature. In a widely identical subsequent series of experiments using a different batch of D149, the dyeing procedure was repeated for identical films, but with an additional 1 mM cholic acid in the dye solution. After 1, 2, 10 or 120 min, the films were removed from the solutions and rinsed with ethanol. UV/Vis absorption spectra of the D149-sensitized films were measured in transmission mode using a Hitachi U-4000 spectrophotometer or a tec5 LS-CH/LOE-USB UV/ Vis spectrometer system, both equipped with integrating spheres. Counter electrodes were prepared by sputter-depositing Pt on ATO-coated glass sheets (ATO = antimony-doped SnO2) (Geomatec, 5 X/h) for 5 min using a sputtering current of 24 mA. Sandwichtype solar cells were assembled by sealing the dye-sensitized ZnO working electrodes and the Pt-coated counter electrodes together using hot-melt foil (SurlynÒ, thickness 30 lm) and introducing the electrolyte consisting of 0.1 M I2 (Scharlau) and 1 M tetrapropylammonium iodide (TPAI) (Alfa Aesar) in a 4:1 (by volume) mixture of ethylene carbonate and acetonitrile through holes previously drilled in the counter electrodes. The holes were sealed using hot-melt foil and cover slips. The active area of the solar cells was 0.28 cm2. In the characterization of the solar cells, light intensities were adjusted using an EKO ML-020VM silicon pyranometer, an EKO LS-100 spectroradiometer, or a silicon solar cell provided by the Solid-state Electronics Engineering Laboratory at Gifu University. All detectors are based on silicon as absorber material, i.e. their spectral sensitivity is comparable. The illuminated cell area was limited to 0.2 cm2 using a black shadow mask. Current–voltage curves under simulated solar light (AM1.5G conditions), provided by a Yamashita Denso YSS-150 solar simulator or an LOT Oriel solar simulator LS0106, were measured using a Hokuto Denko HSV-100 potentiostat or a Zahner IM6 electrochemical workstation. For electrochemical impedance spectroscopy (EIS) measurements under illumination, a Solartron Impedance/

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Gain-Phase Analyzer 1260 combined with a Solartron potentiostat/ galvanostat 1287 and a 200W Xe arc lamp (Ushio), or a Zahner IM6 electrochemical workstation and an LOT Oriel solar simulator LS0106 were used. The amplitude of the voltage modulation was 10 mV and the frequency range was 100 mHz to 100 kHz. The light intensity was adjusted to 60 mW cm2 or to 100 mW cm2. The impedance spectra were fitted using the software ZView2. The applied d.c. cell voltage Vcell as a function of the resulting d.c. cell current Icell was corrected by the voltage drop at the series resistance R Icell V series ðIcell Þ ¼ 0 Rseries ðIÞdI, where Rseries = RFTO + RPt + RZd, to obtain the voltage VF corresponding to the Fermi level difference between the ZnO film and the electrolyte: VF(Icell) = Vcell(Icell)  Vseries(Icell). In order to obtain the internal current–voltage curves, (I vs. VF) of the cells, Vseries(Icell) was subtracted from the external V(I) characteristics that had been obtained by the current-voltage measurements under AM1.5G conditions [32,33]. The density of states (DOS) in the nanostructured ZnO films was calculated from the chemical capacitance Cl obtained by EIS using the formula DOS = Cl/(qAd[1  p]) [34,35], where q is the electron charge, A is the projected film area, d is the film thickness and p is the porosity (assumed to be 0.5 [20]). For each set of adsorption conditions (adsorption time, absence/ presence of coadsorbate) three films were prepared and analyzed by UV/Vis spectroscopy. Between one and three films per set of conditions were further characterized in DSC structures. Either averaged values or datasets representative for a given condition are discussed.

Fig. 1. Cross-sectional SEM image of a porous ZnO film obtained by electrodeposition from aqueous, O2-saturated solution containing 0.1 M KCl, 5 mM ZnCl2 and 300 lM eosin Y. Deposition was carried out at a potential of 0.75 V vs. Ag/AgCl.

3. Results and discussion 3.1. Structural and optical characteristics of the ZnO/D149 photoelectrodes In our previous reports on porous ZnO electrodeposited in the presence of the structure-directing agent eosin Y, a pore size of around 20 nm was typically achieved by adding 40–50 lM of eosin Y to the deposition bath [6,15,19]. A potential more negative than 0.95 V vs. Ag/AgCl was applied, which is sufficiently negative to produce highly nucleophilic reduced eosin Y molecules. This led to a fast and widely irreversible deposition of ZnO with incorporated eosin Y, which could be completely removed by soft alkaline treatment to expose the nanoporous ZnO matrix [31]. Deposition of ZnO/eosin Y hybrid films was possible at less negative potentials as well, which led to loading of eosin Y into ZnO in its oxidized form. In that case a smaller amount of eosin Y was incorporated and it was difficult to extract it by the alkaline treatment [36]. However, we found that an increase of the concentration of eosin Y in the deposition bath to a few hundreds of lM led to equivalent amounts of eosin Y in the hybrid films compared to films grown with the reduced eosin Y, and the loaded dye could be completely extracted. When the molar ratio of eosin Y exceeded about 0.8% with respect to that of ZnO, complete desorption was possible [37]. Such films exhibited a more homogeneous film thickness and an increased mechanical stability compared to porous ZnO deposited in the presence of reduced eosin Y. In the present study, we therefore prepared ZnO/eosin Y hybrid films with 300 lM of eosin Y in the bath and at 0.75 V vs. Ag/AgCl and subsequently desorbed the eosin Y. The SEM image in Fig. 1. shows that the present films show a columnar structure very similar to that of films deposited in earlier studies [6,15,19,35]. The average pore diameter is estimated to be in the range of 20–30 nm, which is in line with previous reports as well [19]. The porous ZnO structures were characterized by UV/Vis absorption spectroscopy after adsorption of D149 for 1, 2, 10 or 120 min with or without cholic acid in the dye solution (Fig. 2).

Fig. 2. UV/Vis absorption spectra of D149 in dimethylformamide (    ) and of D149 adsorbed on electrodeposited porous ZnO with ( ) or without (- - - -) the coadsorbate cholic acid. The ZnO films were immersed in the dye or dye/ coadsorbate solutions for 1, 2, 10 or 120 min. Increasing color depth indicates increasing adsorption time. Each of the curves represents the average of three spectra of samples prepared under identical conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Based on Lambert-Beer type behavior, we use the integrated absorR 700 nm bance absint ¼ 425 nm abs dk as a measure for the amount of dye in the films, since differences in the environment of a dye molecule might have an influence on details of the spectral shape but not on the intrinsic oscillator strength. The validity of this approach was verified for a parallel set of films by comparing the integrated absorbance with the amount of dye determined by dye desorption and optical analysis in solution (Fig. S1, Supplementary Information). By increasing the adsorption time from 1 min to 120 min, the amount of D149 in the electrodes increased by a factor of 4.5 in the presence of cholic acid, and by a factor of 3 in the absence of cholic acid (see absint in Table 1). For equal adsorption times, a larger amount of D149 was adsorbed in our series of electrodes sensitized with cholic acid in the dye solution compared to our series without cholic acid. This is not expected and must be due to the details of the experimental procedure. In the direct comparison (Fig. S1), a decrease of the saturation amount of D149 in the porous films by addition of cholic acid to the dye solution was found, as also reported earlier [21]. The UV/Vis absorption spectra of the ZnO films sensitized with D149 show features in the same wavelength range as the dye in solution, but differ in their shape because of dye–dye interaction (aggregation) or differences in the dielectric environment caused by adsorption to ZnO. Short adsorption times lead to spectra most closely resembling the spectrum of D149 in solution, while the

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Table 1 Amount of D149, represented by the integrated absorbance absint, present in the D149-sensitized ZnO films after immersing the films in D149 solution with or without cholic acid for various adsorption times tads. Trap distribution parameter a and conduction band edge shifts DEc/q (see text) for DSCs based on the D149-sensitized ZnO films. Each value of absint, a and DEc/q represents the average of one to three identically prepared films or cells, respectively. The largest observed deviation of an experimental value from the average is given as an estimated error. tads/ min

absint (±16)/ nm

a (±0.06)

DEc/q (±15)/ mV

Without cholic acid

1 2 10 120

97 108 197 258

0.37 0.36 0.33 0.27

33 37 40 34

With cholic acid

1 2 10 120

92 124 303 396

0.24 0.18 0.17 0.18

+11 0 (ref.) +24 +17

strongest deviations are found for long adsorption times without the coadsorbate cholic acid. The ZnO films sensitized for 10 min or 120 min without cholic acid show asymmetric main absorption bands that are red-shifted compared to the main band of the solution spectrum. Such peak shape has also been found in UV/Vis spectra of the indoline dye D102 on TiO2, and has been discussed as an indication for the presence of J-aggregates [23]. Comparing samples with equal adsorption times, it can be clearly seen that the use of the coadsorbate removes the asymmetry and red-shift of the main absorption band. Thus, the suppression of D149 aggregation by cholic acid, which has been discussed in previous studies based on efficiency improvements, is corroborated in this work by means of solid-state UV/Vis absorption spectroscopy measurements.

Fig. 3. Open-circuit voltages Voc (a; N,D), short-circuit current densities Jsc (a; j,h), conversion efficiencies g (b; d,s) and fill factors FF (b; ,e) of D149-sensitized ZnO solar cells with (filled symbols) or without (open symbols) coadsorbed cholic acid as a function of the integrated absorbance absint. Values are averages of the results obtained for one to three samples prepared using the same adsorption time, and error bars correspond to the largest deviation of individual values from the average.

3.2. Photovoltaic performance of ZnO/D149 solar cells The current–voltage characteristics of the ZnO/D149 solar cells with varied amounts of D149 either with or without the coadsorbate cholic acid are displayed in Fig. S2. The curves show a systematic increase of the short-circuit photocurrent density Jsc and the open-circuit photovoltage Voc as a function of the adsorption time. Comparing the characteristics of cells with and without cholic acid, quite similar values of Jsc of up to 11 mA cm2 are attained for the longest adsorption time of 120 min. The open-circuit photovoltage reaches a higher maximum value of 630 mV in the presence of cholic acid compared to 590 mV without coadsorbate. Although for each adsorption time, higher Voc are obtained with cholic acid, a more detailed analysis reveals that these changes are caused by an increased amount of adsorbed D149 (Fig. 2. and Table 1). Fig. 3 therefore shows the Voc, Jsc, fill factor FF and conversion efficiency g as a function of absint, which is used as a measure of the amount of D149. For both cells with and without the coadsorbate, Voc and Jsc increase with absint. The open-circuit photovoltage at a given amount of D149 in the ZnO films, however, is not influenced by the presence of cholic acid. For TiO2 sensitized with the Ru(II) dye N719, coadsorption of the cholic acid derivative chenodeoxycholic acid has been reported to cause an increase of Voc as a result of a shift of the TiO2 conduction band edge by about 80 mV. Possible shifts of the conduction band edge in the present cells are analyzed in detail in Section 3.3.2. While the coadsorbate does not affect the photovoltage achieved with a certain amount of dye adsorbed, it leads to a distinct flattening of the Jsc vs. absint plot. This indicates that cholic acid may either decrease the probability of electron injection from D149 to ZnO, or it may lead to more pronounced recombination (under short-circuit conditions) to the electrolyte or to oxidized D149 molecules. In line with the results

of our previous study [21], the fill factor shows a decay as a function of the amount of D149, which suggests that the presence of the D149 molecules promotes recombination in the voltage range around the maximum power point. The decrease of FF is rather weak in the presence of cholic acid despite larger dye loading. In cells without coadsorbate, FF shows a steep drop to significantly lower values even at smaller dye loadings. The detailed analysis of trap distribution and recombination (Section 3.3) reveals possible origins of the trends observed in the short-circuit photocurrent, open-circuit photovoltage and fill factor. Because of the clear improvement of Voc and Jsc with the amount of dye, the conversion efficiency g still increases (Fig. 3b) inspite of the decline of the fill factor. The overall benefit of the use of cholic acid as a coadsorbate is clearly reflected in a monotonous increase of g with the amount of dye to around 4%, as opposed to saturation of g at a lower value already at smaller amounts of dye in the absence of the coadsorbate. 3.3. Distribution of trap states and recombination in ZnO/D149 solar cells In order to gain a more thorough understanding of the influence of the D149 loading and the use of cholic acid as coadsorbate on the distribution of electronic trap states in the porous ZnO films and on recombination reactions in the ZnO/D149 solar cells, electrochemical impedance spectroscopy (EIS) measurements under illumination with white light were performed. The obtained spectra were fitted to an established equivalent circuit (examples in Fig. S3) [7,38]. All spectra measured at voltages of 0.5 V or more negative exhibited three semicircles: a high-frequency semicircle

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characteristic for charge transfer across and charge accumulation in the electrolyte/counter electrode interface (described by the equivalent circuit elements CPt and RPt), a mid-frequency semicircle characteristic for charge transfer across the D149-sensitized ZnO/electrolyte interface and charge accumulation in the porous ZnO structure (described by the recombination resistance Rrec and the chemical capacitance Cl) and a low-frequency semicircle associated with charge transport through the electrolyte (described by the circuit element Zd) [38]. Typically, the low-frequency feature could not be distinguished anymore for measurements at voltages less negative than 0.5 V. 3.3.1. Distribution of trap states In dye-sensitized nanostructured metal oxides, the chemical capacitance of electrons in the semiconductor consists of the sum of Cl of conduction band electrons and Cl of electrons in trap states [39], including bulk and surface traps. The measured capacitance is typically governed by the density of states in the band gap. It has been found that nanostructured TiO2 and ZnO usually exhibit exponential trap distributions [7,21,32,35,40], so that

C l ¼ C l;0 expða

qV F Þ kT

ð1Þ

Cl,0 is the voltage-independent part of the capacitance that is proportional to the total trap density Nt, q is the electron charge, kT is the thermal energy, a is the trap distribution parameter and VF is the voltage related to the Fermi level difference between D149sensitized ZnO and the electrolyte, VF = (EFn  Eredox)/q. In Fig. 4, the capacitance Cl is therefore plotted in a semilogarithmic plot against VF. The latter was determined by correcting the applied cell voltage by the voltage drop at the series resistance as described in the experimental section. As expected (Eq. (1)), Cl increases exponentially as VF becomes more negative. An overview of the obtained values of a for samples with and without cholic acid and with varied amount of D149 is given in Table 1. With values between 0.17 and 0.38, a is higher than the values typically found for DSCs based on nanoparticulate ZnO films (0.10–0.13) [7,35,41], which points to a flatter distribution of trap states [35]. For cells with coadsorbate, the chemical capacitance shows a weaker dependence on the voltage compared to cells without coadsorbate (Fig. 4), and hence the values of the trap distribution parameter a are smaller (Table 1). Since the cholic acid molecules are located on the surface of the nanostructured ZnO films, the observed effect can be ascribed to a change in the distribution of surface trap states. The fact that the to-

Fig. 4. Chemical capacitance and linear fits according to Eq. (1) of ZnO/D149 solar cells with (filled symbols, solid lines) and without (open symbols, broken lines) cholic acid obtained by EIS measurements. Increasing color depth indicates increasing adsorption times of 1, 2, 10 or 120 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tal chemical capacitance (related to bulk and surface traps) is notably influenced by surface modifications indicates that a significant share of the total amount of trap states is located at the surface of the nanostructured ZnO electrodes. In order to gain a clear picture of the influence of the adsorbed molecules on the distribution of surface states, it is useful to look at the density of states (DOS) for electrons in the semiconductor calculated from the chemical capacitance [34,35], which is shown in Fig. 5. This representation reveals that the addition of cholic acid leads to a drop in the density of states in the energy range qVF > 0.55 eV to about one half of the DOS of cells without cholic acid. Among the samples with coadsorbate, an increase of the adsorption time from 1 min to 120 min causes a slight decrease of the DOS, while the adsorption time hardly affects the density of states in cells that do not contain cholic acid. For energies qVF < 0.5 eV, the DOS appears to be slightly larger for cells with coadsorbate. Even though the nature of surface states in DSCs and the influence of adsorbed dye molecules, coadsorbates and electrolyte components on their energetic distribution have not yet been clarified in full detail, there is spectroscopic evidence and theoretical studies showing that adsorption of dye molecules or other molecules to the semiconductor surface can significantly reduce the number of surface trap states or shift their energy levels compared to the bare, non-modified surface [42–45]. Possible reasons given for this include passivation of surface traps by binding of adsorbates to coordinatively unsaturated surface atoms [43], and/or shifts of the semiconductor energy levels by the presence of surface dipoles formed upon adsorption of certain molecules [45]. The latter has been observed e.g. for adsorption of dyes with carboxylic acid functions on TiO2 [45]. Since the coadsorbate cholic acid possesses a carboxylic acid function, it is possible that the described changes in the distribution of trap states are related to formation of surface dipoles upon adsorption of cholic acid. In the present cells, an increase of the adsorption time only influences the DOS when cholic acid is present in the adsorption solution. In other words, the density of states of surface traps in the electrodeposited porous ZnO films depends on the amount of cholic acid adsorbed, while the amount of dye does not appear to be a significant factor. This may indicate that there is only weak interaction between D149 molecules and the surface of the electrodeposited ZnO films. Evidence for this has already been found in earlier studies on indoline dye-sensitized ZnO [46]. Apart from influencing the distribution of trap states, adsorption of dye molecules and coadsorbates may lead to a shift of the conduction band edge in the semiconductor [47–49]. For a series of cells with similar trap distribution parameter a, shifts of their

Fig. 5. Electronic density of states (DOS) calculated from the chemical capacitance for the nanostructured D149-sensitized ZnO films with (filled symbols) or without (open symbols) coadsorbate. Increasing color depth indicates increasing adsorption times of 1, 2, 10 or 120 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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capacitance curves along the voltage axis (relative to a reference sample) correspond to relative differences in the position of the conduction band edge [7,32]: VF(Cl,sample)  VF(Cl,ref) = DEc/q. Since in the present study the changes of a caused by coadsorption of cholic acid or variation of the adsorption time are not large compared to the differences we find between samples prepared under identical conditions (cf. Table 1), we consider an approximate determination of DEc/q from the chemical capacitance plots (Fig. 4) reasonable (cf. Fig. S4 showing the alignment of the Cl curves after shifting them by DEc/q). Comparing DEc/q of cells prepared in the presence of cholic acid with the values determined for cells without coadsorbate (Table 1), we find that cholic acid leads to a conduction band edge movement of up to 64 mV away from Eredox, which is also reflected in the DOS plots (Fig. 5). Considering the estimated error for DEc/q of ±15 mV, this result is comparable to the upward shift of the conduction band edge of 80 mV that has been found for Ru(II) dye sensitized TiO2 solar cells as a consequence of coadsorbing chenodeoxycholic acid [47]. The upward shift of the conduction band edge might cause a decrease in the electron injection efficiency and thereby explain the decreased quantum efficiency under short-circuit conditions observed for samples with cholic acid compared to cells without coadsorbate. 3.3.2. Recombination Recombination in the D149-sensitized ZnO solar cells was characterized by analyzing the recombination resistance Rrec obtained by EIS. Rrec depends on the transition probabilities of electrons from conduction band states or surface trap states to acceptor states (typically, I 3 in the electrolyte) and on the electron densities in the conduction band and in surface states [32,50]. Fig. 6(a) shows a plot of Rrec against VF for ZnO/D149 solar cells prepared with different dye adsorption times, with or without the coadsorbate cholic acid. As observed before for TiO2- as well as ZnO-based DSCs [7,21,32,35,40], Rrec decreases exponentially with more negative voltage. If an evaluation of the electron recombination probability in different cells is intended, Rrec ideally should be compared for the same occupancy of conduction band and surface trap states. For cells with the same trap distribution parameter a, Rrec is therefore typically plotted as a function of VF corrected by relative conduction band edge shifts [32]. Since different trap distributions were observed in the present samples (Figs. 4 and 5), Rrec is plotted directly against the experimental DOS [35,51] in Fig. 6(b). For DOS higher than about 1.5  1019 eV1 cm3, which include the DOS corresponding to open-circuit conditions (Figs. 3 and 5), Rrec shows an increase with the adsorption time (dye content) both for cells with and without cholic acid. This is in line with the open-circuit voltage Voc increasing with the dye loading as observed in Fig. 3 A larger Rrec for larger dye loadings has been reported in the literature before and has been ascribed to better coverage of the semiconductor surface by the dye molecules [52,53]. A better dye coverage leads to a larger distance between electrons in the semiconductor and oxidized species in the redox electrolyte and thus to a decreased probability of tunneling. Comparing the recombination resistance for samples with and without coadsorbate for a given adsorption time, we find that in the range of DOS larger than 2.5  1019 eV1 cm3 Rrec shows smaller values for the cells containing cholic acid, which is unexpected considering that in the present study the D149-sensitized ZnO films with coadsorbate contain higher amounts of D149 (Fig. 2). It suggests that cholic acid acts as a catalyst for recombination. An increased rate of recombination has also been found for TiO2-based solar cells with a Ru(II) sensitizer combined with chenodeoxycholic acid as coadsorbate [47]. Inspite of the increased rate of recombination, cells with cholic acid do not show lower Voc than cells without coadsorbate at the same dye loading (Fig. 3). The upward shift of the conduction band edge caused by

Fig. 6. Voltage-dependent recombination resistance Rrec with linear fits according to the equation in the inset (a), and Rrec as a function of the density of states (DOS) in the ZnO (b) for D149-sensitized ZnO solar cells with (filled symbols, solid lines) and without (open symbols, broken lines) cholic acid obtained by EIS measurements. Increasing color depth indicates increasing adsorption times of 1, 2, 10 or 120 min. The lines in (b) are a guide to the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

cholic acid (Table 1) thus compensates the effect of the decrease in the recombination resistance, resulting in an almost constant Voc. For a more detailed discussion of the fill factor, the recombination rate at smaller DOS (corresponding to less negative voltages) has to be taken into account. With decreasing DOS, values of Rrec become larger in cells with cholic acid compared to cells without cholic acid, which means that under these conditions the overall rate of recombination is reduced by the presence of the coadsorbate. Moreover, Rrec becomes less dependent on the dye loading, and at DOS of around 0.5  1019 eV1 cm3, the above trend of increasing recombination resistance with increasing dye loading even appears to be inverted: Rrec becomes smaller for longer adsorption times. This observation is in line with the decrease of the fill factor with increased amount of D149 seen in Fig. 3. The inverted trend of Rrec at low DOS is related to a decrease in the slope of the voltage-dependent recombination resistance for increasing adsorption time (Fig. 6(a)) and is discussed in detail below. 3.3.3. Voltage-dependence of the recombination resistance In order to quantify the observed change in slope of the recombination resistance, the semilogarithmic plots of Rrec vs. VF were fitted to the equation given in the inset of Fig. 6(a) to obtain the recombination parameter b for all cells [7,21,32,35,40]. Values of b for samples with the same adsorption time and with or without cholic acid were averaged and are displayed in Fig. 7 as a function of the amount of D149 in the films. The present values of b between 0.35 and 0.53 are comparable to 0.45–0.64 found for DSCs based on D149-sensitized ZnO in previous studies [7,35]. For cells without coadsorbate, the recombination parameter decreases by

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about 30% as the amount of D149 is increased by extension of the adsorption time from 1 min to 120 min. This effect is clearly weakened by the presence of cholic acid in the D149-sensitized ZnO films, where the same change in dye loading only leads to a 10% reduction of b. The use of the coadsorbate hence leads to higher recombination parameters b for long adsorption times. In other words, cholic acid weakens the increase of recombination in the lower voltage range observed when the amount of D149 is increased. b describes the voltage dependence of the recombination resistance, which is influenced by charge-transfer from a distribution of surface states. It has been derived that b can be expressed as b = 0.5 + ass, where ass is the trap distribution parameter of surface states (ss) [50]. Based on this, a correlation between b and the trap distribution parameter a = ass + abulk (cf. Eq. (1)) is expected. However, our detailed analysis of the trap distribution (Fig. 5) shows a pronounced drop of a by addition of cholic acid, while the coadsorbate leads to an increase of the recombination parameter b. Moreover, we do not find the amount of dye to influence a, whereas it has a considerable effect on b. Thus, in the cells investigated in this study, the voltage dependence of the recombination resistance does not correlate with the energy distribution of surface states in the porous ZnO film. Assuming an energy-independent rate constant for charge transfer (cf. Section 3.3.4) from surface states to any electron acceptor states, this suggests that it is rather the distribution of acceptor states which controls how the recombination rate changes with the position of the quasi-Fermi level of electrons. These acceptor states may be oxidized species in the electrolyte or oxidized D149 molecules. Since b and the fill factor decay with increasing amount of D149, we suggest that this decay is due to a growing surface concentration of oxidized dye molecules offering additional electron acceptor states in the lower voltage range and that this leads to a change in the main recombination path. Since cholic acid prevents adsorbed D149 molecules from aggregating (Fig. 2), the fact that it limits the drop in b and FF indicates that the increase of the total recombination rate around the maximum power point is mainly caused by recombination with oxidized D149 in aggregates.

3.3.4. Dependence of the fill factor on the recombination parameter b To clarify if the decay of b as a function of the dye loading fully explains the drop of the fill factor with the amount of dye (Fig. 3), it is useful to analyze the correlation between FF and b quantitatively and compare it to the correlation expected by a physical model. Apart from the rate of recombination, the series resistance of a DSC is a key factor influencing FF. Using the series resistance deter-

Table 2 Experimentally obtained internal and external fill factors, FF and FFint,exp, as well as internal fill factors FFint,calc calculated using Eq. (2) for solar cells built from ZnO films sensitized with D149 for different adsorption times tads, with or without the coadsorbate cholic acid. tads/min

FF

FFint,exp

FFint,calc

Without cholic acid

1 2 10 120

0.69 0.68 0.61 0.54

0.71 0.71 0.66 0.59

0.71 0.71 0.67 0.65

With cholic acid

1 2 10 120

0.66 0.63 0.59 0.57

0.69 0.66 0.63 0.62

0.71 0.71 0.70 0.69

mined by EIS, we determined the internal fill factors [32] FFint,exp corresponding to the experimental external fill factors FF (Table 2). Comparison of FFint,exp and FF shows that the series resistance reduces the fill factor by up to 10%, which is analyzed in more detail below. We then calculated the internal fill factors that are expected based on the Voc, VMPP (voltage in the maximum power point of the internal current–voltage curves) and recombination parameters b determined by I–V characterization and by EIS, respectively. For this purpose, an established diode model [32] that accounts for non-linear recombination by describing the recombination rate by Un = krec nbc (b-recombination model) was used:

FF int;calc ¼

  V MPP expðbqV oc =kTÞ  expðbqV MPP =kTÞ expðbqV oc =kTÞ  1 V oc

ð2Þ

Table 2 shows the calculated internal fill factors FFint,calc in comparison to the experimental external and internal fill factors. It can be seen that for small amounts of D149 in the films, the calculated values are well in accordance with FFint,calc. For larger amounts of dye, the ZnO/D149 cells shows smaller fill factors than expected according to the above model. A possible explanation for this could be that the recombination rate in the model is approximated by an empirical expression including a constant recombination rate constant krec and the model does not distinguish between recombination with the electrolyte and recombination with oxidized dye molecules. However, in the present cells krec may change depending on the voltage, as we found evidence for a change in the main recombination path in the cells with high dye loadings to oxidized D149 rather than to the electrolyte. A more accurate description could include an energy-dependent average rate constant rather than a constant one, as for example discussed by Wang et al. [40], which would also account for the energy-dependent change of the main recombination path that is indicated in the present study. 3.4. Series resistance

Fig. 7. Recombination parameter b (d,s) as a function of the integrated absorbance for solar cells containing D149-sensitized ZnO photoelectrodes with (filled symbols) and without (open symbols) cholic acid. b was determined from the slopes of linear fits of the recombination resistance (see Fig. 6(a)).

The overview of the internal and external fill factors (Table 2) shows that the series resistance Rseries constitutes a non-negligible factor limiting the efficiency of the ZnO/D149 solar cells analyzed in this study. Therefore, it is useful to study the individual contributions to the series resistance in more detail. Rseries is given by the sum of the resistance of the FTO-coated glass substrate (RFTO), the charge-transfer resistance at the platinum counter electrode (RPt) and the ohmic part of the electrolyte impedance (RZd): Rseries = RFTO + RPt + RZd. Fig. 8 shows that RFTO and RPt deliver the main contributions to Rseries in the range of positive d.c. cell current densities Jcell, which corresponds to the range relevant under operating conditions. While the substrate resistance is nearly constant over the whole range of cell currents investigated, RZd grows significantly in the negative current range, while the counter electrode

M. Rudolph et al. / Journal of Electroanalytical Chemistry 709 (2013) 10–18

Fig. 8. Contributions to the series resistance Rseries as a function of the d.c. cell current density Jcell resulting from the application of different bias voltages in the EIS measurements. The resistances determined by EIS are shown for the example of a solar cell built from a porous ZnO electrode sensitized with D149 for 120 min in the absence of cholic acid. RZd is the ohmic part of the electrolyte impedance, RS is the resistance of the FTO-coated glass substrate and RPt is the charge-transfer resistance at the counter electrode/electrolyte interface.

Fig. 9. Charge-transfer resistance at the Pt/FTO counter electrode for ZnO/D149 solar cells with (filled symbols) or without (open symbols) cholic acid as a function of the d.c. cell current density Jcell. Increasing color depth indicates increasing adsorption times of 1, 2, 10 or 120 min.

resistance increases towards positive Jcell. Comparing RPt for cells prepared with different adsorption times, Fig. 9, we find a growth of the counter electrode resistance with the adsorption time, which is particularly prominent for cells without coadsorbate. For cells without cholic acid, RPt is clearly smaller than for those with coadsorbate. The above observations suggest that a part of the D149 molecules dissolves in the redox electrolyte upon insertion of the latter into the solar cells. Some of the dissolved dye molecules could then adsorb on the counter electrode surface and thereby increase the charge-transfer resistance at the counter electrode/electrolyte interface. The fact that RPt is smaller in cells containing the coadsorbate may indicate that D149 is more stably bound to the ZnO surface if cholic acid is present. The above hypotheses are supported by the fact that the ohmic part of the electrolyte resistance, RZd, is influenced by the adsorption time and the presence or absence of cholic acid in a similar way as RPt is (Fig. S5).

17

coadsorbate cholic acid in the dye solution, was used to obtain films with significantly different amounts of D149. UV/Vis absorption spectra of electrodeposited, D149-sensitized ZnO electrodes without coadsorbate showed signs of dye aggregation for longer adsorption times (10 or 120 min). In cells built from these electrodes, an increase of the D149 loading led to an increase in the short-circuit photocurrent, open-circuit photovoltage and overall conversion efficiency. The fill factor, on the other hand, was significantly decreased by an increased dye loading, which limited a further increase of the efficiency. Impedance spectroscopy revealed two factors contributing to the observed decline of the fill factor: (1) an increased amount of D149 lowers the recombination parameter b and, hence, the recombination resistance in the range of densities of states (DOS) corresponding to voltages around the maximum power point and (2) the series resistance is increased, presumably by partial desorption of D149 molecules into the electrolyte. For DOS corresponding to voltages around the open-circuit voltage and beyond, however, D149 had a blocking effect on recombination, which was reflected in an increase of the recombination resistance and, as mentioned, of the open-circuit photovoltage with increasing amount of dye. When cholic acid was coadsorbed, aggregation of the dye D149 was suppressed, as seen by changes in the UV/Vis absorption spectra. For the same dye loading, higher fill factors compared to cells without coadsorbate were attained, because (1) the decrease of the recombination parameter with increasing amount of D149 in the intermediate voltage range was clearly weaker and (2) the series resistance remained low even for long adsorption times, indicating that D149 was more stably bound to ZnO. Cholic acid lowered the recombination resistance under open-circuit conditions. However, as it also caused a 60 mV upward shift of the conduction band edge, the open-circuit photovoltage remained largely unaffected by the increased rate of recombination. Possibly resulting from the upward shift of the conduction band edge, the short-circuit photocurrent showed a weaker increase with the amount of D149 when cholic acid was present, indicating a decreased electron injection rate. For moderate dye loading, cells using films without cholic acid therefore showed the highest efficiency. For cells containing a large dye loading necessary to achieve competitive short-circuit photocurrents, the beneficial effect of cholic acid on the fill factor led to clearly increased efficiency. The advantage of the use of this coadsorbate in electrodeposited D149-sensitized ZnO solar cells is thereby underlined. A lack of correlation between the distribution of surface states in the porous ZnO and the energy-dependence of the recombination rate demonstrated that the latter was rather governed by the energetic distribution of the ‘targets’ of recombination: oxidized species in the electrolyte or oxidized dye molecules adsorbed to ZnO. Based on the observed effects of the adsorption time and of the presence of the coadsorbate, we suggested that it is oxidized D149 molecules in aggregates which created additional electron acceptor states in the energy range crucial for the fill factor and thereby led to systematic changes in the energy-dependence of the recombination resistance as a function of the adsorption time. Cholic acid prevented such aggregation of D149 on the surface of the porous ZnO and therefore had a positive influence on the fill factor. Since D149 aggregates enhanced recombination it is concluded that the regeneration efficiency of oxidized dye molecules in aggregates by I ions in the electrolyte is decreased relative to monomeric oxidized dyes.

4. Summary and conclusions Acknowledgements We presented a detailed analysis of the photoelectrochemical properties of dye-sensitized solar cells based on electrodeposited ZnO sensitized with the indoline dye D149. Variation of the dye adsorption time between 1 min and 120 min, with or without the

This work has been supported by the Japanese New Energy and Industrial Technology Development Organization (NEDO) and the German Federal Ministry of Education and Research (BMBF). The

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authors would like to thank Dr. Miura (Chemicrea Japan) for supplying the D149 dye. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jelechem.2013. 09.028. References [1] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629–634. [2] B. O’Regan, M. Grätzel, Nature 353 (1991) 737–740. [3] A. Hagfeldt, M. Grätzel, Acc. Chem. Res. 33 (2000) 269–277. [4] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo, H. Siegbahn, J. Photochem. Photobio. A 148 (2002) 57–64. [5] M. Quintana, T. Edvinsson, A. Hagfeldt, G. Boschloo, J. Phys. Chem. C 111 (2007) 1035–1041. [6] T. Yoshida, J.B. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporté, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wöhrle, K. Funabiki, M. Matsui, H. Miura, H. Yanagi, Adv. Funct. Mater. 19 (2009) 17–43. [7] E. Guillén, L.M. Peter, J.A. Anta, J. Phys. Chem. C 115 (2011) 22622–22632. [8] J.A. Anta, E. Guillén, R. Tena-Zaera, J. Phys. Chem. C 116 (2012) 11413–11425. [9] J. Elias, C. Lévy-Clément, M. Bechelany, J. Michler, G.Y. Wang, Z. Wang, L. Philippe, Adv. Mater. 22 (2010) 1607–1612. [10] L.E. Greene, B.D. Yuhas, M. Law, D. Zitoun, P.D. Yang, Inorg. Chem. 45 (2006) 7535–7543. [11] H.W. Chen, C.Y. Lin, Y.H. Lai, J.G. Chen, C.C. Wang, C.W. Hu, C.Y. Hsu, R. Vittal, K.C. Ho, J. Power Sources 196 (2011) 4859–4864. [12] C.Y. Jiang, X.W. Sun, K.W. Tan, G.Q. Lo, A.K.K. Kyaw, D.L. Kwong, Appl. Phys. Lett. 92 (2008) 143101. [13] D. Wei, H.E. Unalan, D.X. Han, Q.X. Zhang, L. Niu, G. Amaratunga, T. Ryhanen, Nanotechnology 19 (2008) 424006. [14] X.Z. Liu, Y.H. Luo, H. Li, Y.Z. Fan, Z.X. Yu, Y. Lin, L.Q. Chen, Q.B. Meng, Chem. Commun. 27 (2007) 2847–2849. [15] T. Yoshida, T. Pauporté, D. Lincot, T. Oekermann, H. Minoura, J. Electrochem. Soc. 150 (2003) C608–C615. [16] T. Dentani, K.I. Nagasaka, K. Funabiki, J.Y. Jin, T. Yoshida, H. Minoura, M. Matsui, Dyes Pigments 77 (2008) 59–69. [17] T. Loewenstein, M. Rudolph, M. Mingebach, K. Strauch, Y. Zimmermann, A. Neudeck, S. Sensfuss, D. Schlettwein, ChemPhysChem 11 (2010) 783–788. [18] M. Rudolph, T. Loewenstein, E. Arndt, Y. Zimmermann, A. Neudeck, D. Schlettwein, Phys. Chem. Chem. Phys. 11 (2009) 3313–3319. [19] T. Pauporté, T. Yoshida, R. Cortès, M. Froment, A. Lincot, J. Phys. Chem. B 107 (2003) 10077–10082. [20] A. Goux, T. Pauporté, T. Yoshida, D. Lincot, Langmuir 22 (2006) 10545–10553. [21] Y. Sakuragi, X.F. Wang, H. Miura, M. Matsui, T. Yoshida, J. Photochem. Photobio. A 216 (2010) 1–7. [22] T. Horiuchi, H. Miura, K. Sumioka, S. Uchida, J. Am. Chem. Soc. 126 (2004) 12218–12219.

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53]

T. Horiuchi, H. Miura, S. Uchida, Chem. Commun. 24 (2003) 3036–3037. M. Pastore, F. De Angelis, ACS Nano 4 (2010) 556–562. A. E-Zohry, A. Orthaber, B. Zietz, J. Phys. Chem. C 116 (2012) 26144–26153. H.P. Lu, C.Y. Tsai, W.N. Yen, C.P. Hsieh, C.W. Lee, C.Y. Yeh, E.W.G. Diau, J. Phys. Chem. C 113 (2009) 20990–20997. A.R.K. Selvaraj, S. Hayase, J. Mol. Model. 18 (2012) 2099–2104. C. Magne, M. Urien, I. Ciofini, T. Tugsuz, T. Pauporté, R. Soc. Chem. Adv. 2 (2012) 11836–11842. T. Yane, A. Koyama, K. Hiramatsu, Y. Isogai, K. Ichinose, T. Yoshida, Electrochemistry 80 (2012) 891–897. K. Ichinose, Y. Kimikado, T. Yoshida, Electrochemistry 79 (2011) 146–155. T. Yoshida, M. Iwaya, H. Ando, T. Oekermann, K. Nonomura, D. Schlettwein, D. Wöhrle, H. Minoura, Chem. Commun. 4 (2004) 400–401. F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Sero9 , J. Bisquert, Phys. Chem. Chem. Phys. 13 (2011) 9083–9118. J. Halme, P. Vahermaa, K. Miettunen, P. Lund, Adv. Mater. 22 (2010) E210– E234. B.C. O’Regan, J.R. Durrant, P.M. Sommeling, N.J. Bakker, J. Phys. Chem. C 111 (2007) 14001–14010. C. Magne, T. Moehl, M. Urien, M. Grätzel, T. Pauporté, J. Mater. Chem. A 1 (2013) 2079–2088. D. Komatsu, J. Zhang, T. Yoshida, H. Minoura, Trans. Mater. Res. Soc. Jpn 32 (2006) 417–420. D. Komatsu, Dissertation, Gifu University, 2008. F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt, Sol. Energy Mater. Sol. Cells 87 (2005) 117–131. J. Bisquert, Phys. Chem. Chem. Phys. 5 (2003) 5360–5364. Q. Wang, S. Ito, M. Grätzel, F. Fabregat-Santiago, I. Mora-Sero9 , J. Bisquert, T. Bessho, H. Imai, J. Phys. Chem. B 110 (2006) 25210–25221. E. Guillén, E. Azaceta, L.M. Peter, A. Zukal, R. Tena-Zaera, J.A. Anta, Energy Environ. Sci. 4 (2011) 3400–3407. K. Schwanitz, U. Weiler, R. Hunger, T. Mayer, W. Jaegermann, J. Phys. Chem. C 111 (2007) 849–854. N.M. Dimitrijevic, Z.V. Saponjic, D.M. Bartels, M.C. Thurnauer, D.M. Tiede, T. Rajh, J. Phys. Chem. B 107 (2003) 7368–7375. F. Nunzi, E. Mosconi, L. Storchi, E. Ronca, A. Selloni, M. Grätzel, F. De Angelis, Energy Environ. Sci. 6 (2013) 1221–1229. K. Westermark, A. Henningsson, H. Rensmo, S. Södergren, H. Siegbahn, A. Hagfeldt, Chem. Phys. 285 (2002) 157–165. J. Falgenhauer, C. Richter, H. Miura, D. Schlettwein, ChemPhysChem 13 (2012) 2893–2897. N.R. Neale, N. Kopidakis, J. van de Lagemaat, M. Grätzel, A.J. Frank, J. Phys. Chem. B 109 (2005) 23183–23189. N. Kopidakis, N.R. Neale, A.J. Frank, J. Phys. Chem. B 110 (2006) 12485–12489. E. Ronca, M. Pastore, L. Belpassi, F. Tarantelli, F. De Angelis, Energy Environ. Sci. 6 (2013) 183–193. J. Bisquert, F. Fabregat-Santiago, I. Mora-Sero9 , G. Garcia-Belmonte, S. Gimenez, J. Phys. Chem. C 113 (2009) 17278–17290. K. Ben Aribia, T. Moehl, S.M. Zakeeruddin, M. Grätzel, Chem. Sci. 4 (2013) 454– 459. T. Marinado, K. Nonomura, J. Nissfolk, M.K. Karlsson, D.P. Hagberg, L.C. Sun, S. Mori, A. Hagfeldt, Langmuir 26 (2010) 2592–2598. M. Miyashita, K. Sunahara, T. Nishikawa, Y. Uemura, N. Koumura, K. Hara, A. Mori, T. Abe, E. Suzuki, S. Mori, J. Am. Chem. Soc. 130 (2008) 17874–17881.