Origin of Stark Signals Induced by Continuous Photoirradiation for

Jul 8, 2014 - ABSTRACT: It has recently been reported that characteristic signals resulting from a Stark effect are included in the spectra of photoin...
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Origin of Stark Signals Induced by Continuous Photoirradiation for Working Dye-Sensitized Solar Cells Revealed by Photoinduced Absorption Measurements Katsuichi Kanemoto,*,† Shinya Domoto,† and Hideki Hashimoto†,‡ †

Department of Physics, Graduate School of Science, and ‡The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: It has recently been reported that characteristic signals resulting from a Stark effect are included in the spectra of photoinduced absorption (PIA) measurements for dyesensitized solar cells (DSCs), and the origin of the Stark effect has been discussed in several papers. Here, we explore the origin of the Stark signal observed under continuous photoirradiation from results of continuous wave (cw)-PIA experiments for several control DSCs of ruthenium-based sensitizers (N719). Typical line shape resulting from the Stark effect was observed in the quadrature phase of cw-PIA spectra under short-circuit conditions. The Stark signals are found to arise in relation to stable photocurrent and be independent of the presence of particular electrolyte ions. Time-resolved PIA measurements demonstrate that the Stark signals rise and decay in time scales much slower than the response time of injected electrons in TiO2. We conclude that redox actions of a pair of I−/I3− are responsible for the Stark effect under continuous photoirradiation and conjecture that the observed Stark signals are due to local electric fields acting on neutral dyes given by complexes consisting of dyes and iodine molecules or iodine-based ions. We propose to use Stark signals to monitor electrostatic environments around neutral dyes in working DSCs.

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sensitizers.12 To establish methods of using the Stark signals as such a monitoring tool, extensive research on the origin of the signals are still required. When applying spectroscopy to complicated systems consisting of many processes, as in the case of DSCs, techniques to enable decomposition of overlapping signals are desired. For this purpose, a cw-PIA technique combined with dual-phase lock-in detection is effective as it can decompose overlapping signals by adjusting the signal phase. The cw-PIA technique is further effective for research on solar cells as it presents spectroscopic information under quasi-cw-photoirradiation that can simulate conditions of solar irradiation. In this article, cw-PIA techniques by dual-phase lock-in detection are applied to working DSCs based on (Bu 4N) 2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) (N719), one of the most representative sensitizers of DSCs. We demonstrate that use of the spectroscopic techniques enables selective detection of Stark signals embedded in overlapping PIA signals consisting of some spectral components. It is revealed via several control experiments that the Stark signals observed under continuous photoirradiation result

INTRODUCTION Since the first report by O’Regan and Grätzel in 1991,1 dyesensitized solar cells (DSCs) have attracted much interest as potential low-cost alternatives to conventional photovoltaic systems. The operation of DSC is known to consist of many processes occurring at dye-sensitizers, a mesoporous oxide semiconductor and electrolyte ions. The underlying physics and chemistry in the cell operation are thus complicated and individual processes are difficult to resolve, which should be one of the reasons of exiguous progress in the energyconversion efficiency. To achieve significant progress in the efficiency, re-exploring the individual processes in the DSC operation is required. One of the most direct methods of examining physical and chemical processes in DSC is spectroscopy, and it has indeed been employed for the research of operation processes.2,3 It has recently been reported that characteristic spectral shifts in ground-state absorption signals are observed from working DSCs in continuous wave (cw) and transient photoinduced absorption (PIA) measurements.4−15 The signals were mostly interpreted as resulting from a Stark effect16−18 and often suggested to be caused by the local electric field acting on sensitizers.8,9,12−14 Particularly, the interpretation on the local electric field has recently been developed into the use of the Stark signals to monitor the electric field environment around © 2014 American Chemical Society

Received: March 28, 2014 Revised: July 8, 2014 Published: July 8, 2014 17260

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Figure 1. (a) Steady-state absorption spectra for the N719-DSC and the N719-film deposited on a glass substrate. (b) Differential of the spectra in (a). The horizontal broken lines indicate the position of baselines for each spectrum. Cw-PIA spectra for the N719-based DSC under the opencircuit (OC) (c) and short-circuit (SC) conditions (d).

from electric fields acting on neutral sensitizers given by iodine molecules or iodine-based ions and not by photoinjected electrons of TiO2 as was suggested from PIA experiments using pulsed photoexcitation.8,9,15 By this finding, Stark signals are proposed to be used as a monitoring tool of electronic couplings between sensitizers and iodine in working DSCs. The coupled states responsible for the Stark signals are expected to be produced in regeneration processes. Therefore, research by monitoring the Stark signals can present new insights into understanding the regeneration processes in DSCs.

lamp and detected with a Si photodiode after passing a monochromator. The cw-PIA signals were measured by monitoring the probe signal using a dual-phase lock-in amplifier, which yielded in-phase and quadrature-phase signal outputs. We note that “in-phase” results does not mean zero phase against the reference phase but means a phase adjusted to decompose overlapping PIA signals. Time-resolved PIA signals were measured using a digital oscilloscope for the laser modulation of 20 Hz. The time resolution of the experiments was about 10 μs. All measurements were performed at room temperature.

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EXPERIMENTAL SECTION DSCs used were fabricated by the following procedures. Nanocrystalline TiO2 paste (Solaronix, particle size 15−20 nm) was cast as mesoporus films on the transparent FTO glass conductive substrate. The TiO2 films were sintered at 450 °C for 25 min and used as a working electrode. N719 (Aldrich) was employed as a dye-sensitizer. Sensitization was achieved by immersing the TiO2 films in the N719 solution in ethanol (0.4 mg/mL) for 1 day. An alcohol-based paint containing a chemical platinum precursor (Platisol T, Solaronix) was deposited on another FTO substrate, and the sample was annealed at 450 °C for 10 min to obtain a transparent platinum layer. The FTO substrate with the platinum layer was used as a counter electrode. Electrolyte solutions for a normal cell consisted of 0.1 M lithium iodine (LiI), 0.1 M iodine, 0.6 M tetrabutyl-ammoniumiodide (TBAI) and 0.5 M 4-tert-butylpyridine (TBP) using 3-methoxypropionitrile as a solvent. DSCs were fabricated by injecting the electrolyte solution between the working and counter electrodes and by contacting the two electrodes face-to-face. The active area of the cell was 0.126 cm2. The cells were finally encapsulated with an epoxy resin before optical measurements. The energy-conversion efficiency of the cell was typically about 4−5% (Supporting Information). For cw-PIA measurements, a diode-pumped solid-state (DPSS) laser with cw 473 nm output (CNI) was used for photoexcitation. The laser output (290 mW/cm2) was modulated using an optical chopper at 70 Hz. A probe beam for the measurements was produced using a tungsten−halogen

RESULTS AND DISCUSSION The steady-state absorption spectra of a complete cell and a film of N719 and their differential spectra are shown in panels a and b of Figure 1, respectively. The N719 film gives an absorption peak at 2.28 eV, whereas the absorption peak of N719 in the cell is blue-shifted by about 0.05 eV against that of the film. This shift was mainly caused by addition of redox electrolyte ions. Cw-PIA measurements were performed for the complete N719 cell. The PIA spectra of the cell under the open-circuit (OC) and short-circuit (SC) conditions are shown in panels c and d of Figure 1, respectively. In general, when a cw-PIA spectrum measured using a dual-phase lock-in technique consists of a single component of photoexcitations, the amplitude of quadrature-phase signals can be made zero by adjusting the lock-in phase. The PIA spectra under the OC condition consisted of nearly only the in-phase components, suggesting that the observed components in the spectrum exhibit time responses similar to the modulation of photoexcitation laser (70 Hz). A downward peak around 2.20−2.25 eV observed in the in-phase OC- and SC-PIA spectra is close to the absorption peak in Figure 1a and assigned to the photobleaching signals of N719. The in-phase PIA spectra also have an upward peak around 1.6 eV. As reported previously,19−23 the peak is assigned to the absorption signal of cations of N719 generated by injection of electrons into the TiO2 layer. Also, the in-phase OC-PIA spectrum exhibits positive ΔOD signals below 2 eV spreading over broad energy 17261

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Figure 2. Cw-PIA spectra for a DSC lacking Pt layer under the short-circuit (SC) condition (a) and under the applied bias of −1 V (b). (c) Comparison of current−voltage characteristics between the normal (with Pt) and Pt-lacking (no Pt) DSCs measured under continuous laser irradiation at 473 nm (290 mW/cm2). (d) Cw-PIA spectra under the SC condition for the cell fabricated under 0.7 M TBAI and no LiI, indicating substitution of TBAI for LiI.

features of Stark effects in the OC- PIA spectrum in Figure 1c may be explained by changes in total unidirectional electric fields between SC and OC conditions.27 However, as demonstrated by comparing the spectra in Figure 1b,d, the spectral peak of the quadrature component is blue-shifted by ca. 0.025 eV, close to the report by Ardo et al. Particularly, a similar blue shift was observed as common features over several different N719 cells. We therefore conclude that the observed Stark signals are caused predominantly by the local electric field acting on neutral N719 dyes. Several control experiments were performed to investigate the origin of the Stark signals. First, we compared spectral features between the normal cell discussed above and the cell fabricated without a platinum layer. The PIA spectra for the Ptlacking cell under the SC condition and under bias of −1 V are shown in panels a and b of Figure 2, respectively. We also show in Figure 2c a comparison of current−voltage (J-V) characteristics between the normal and Pt-lacking cells measured under continuous laser irradiation. Contrary to the normal cell exhibiting a typical performance of photovoltaic devices, the Ptlacking cell shows unusual photovoltaic features with a negligibly small current under the SC condition and a current under −1 V equivalent to the SC current of the normal cell. Related to the difference in the J-V characteristic, different voltage responses of PIA signals are identified from the two cells. The Pt-lacking cell under the SC condition exhibits an inphase PIA spectrum consisting of the bleaching and absorption signals of N719 dyes and the absorption signals of TiO2electrons, whose line shape is very similar to that of the normal cell under the “OC” condition shown in Figure 1c. Also, the PIA line shape of the Pt-lacking cell under −1 V is similar to that of the normal cell under the SC condition, indicating observation of the Stark signals in the Pt-lacking cell. These features identified from the Pt-lacking cell indicate that the Stark signals are observed not from the condition of shorting circuits (under 0 V) but from the occurrence of stable photocurrent.

regions. The signals are assigned to injected electrons generated in the TiO2 layer.15,24 Contrary to the OC-PIA spectrum, the SC-PIA spectrum always gave nonzero quadrature signals for every lock-in phase, indicating coexistence of two or more components that exhibit different phase responses to the modulation of photoexcitation. We particularly identified, in time-resolved measurements using an oscilloscope, the presence of a spectral component obviously differing in the time dependence from other components, as shown later. The spectral component of different time dependence was correlated with the component observed in the quadrature SC-PIA spectrum. The quadrature spectrum has positive peaks around 1.90 and 2.27 eV, whose spectral features are obviously different from those of the in-phase spectrum. Compared with the spectra in Figure 1b, the line shape of the quadrature spectrum resembles the differential absorption spectra of the N719 film and cell although their peak position is somewhat different, details of which are discussed later. Spectral features similar to those of differential absorption spectra have been observed from DSCs and shown to result from a Stark effect.6−9 The spectrum of the quadrature component is thus concluded to result from the Stark effect. We note here that, by selecting appropriate lock-in phase, the Stark signal was able to be almost entirely decomposed out of the whole PIA spectrum. This is a definite advantageous feature of the cw-PIA technique over transient absorption measurements that occasionally suffer from spectral overlap of several transient signals. As the origin of electric field causing the Stark shift, Ardo et al. proposed a local electric field acting on neutral dyes for sensitized TiO2 nanocrystallite system with ruthenium-polypyridyl compounds.8,9 Evidence for the presence of the local electric field was described to be a large spectral shift (215 cm−1 or 0.027 eV) observed in Stark signals.8,9 In addition to the local field, a unidirectional built-in electric field within the film can also be the origin of the Stark effect, as is often reported for organic film semiconductors.25,26 Actually, the absence of 17262

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the laser excitation switched on and off using a chopper and recorded at 2.1 eV, where the absorption signal from the oxidized dyes is nearly negligible, as shown in Figure 1d. The measurement at 2.1 eV thus enables pure observations of the time response of the Stark signal. The observed time evolution indicates that the Stark signal gently rises and decays for the onand off- laser switching, respectively. The time constants for the rise and decay assuming a single exponential response were 15 and 27 ms, respectively. The difference in the time constant may be due to a delay of response in the redox system to the on and off switching. The observed positive TR signals (ΔT > 0) correspond to apparent bleaching of PIA signals. Similar observations were previously reported and interpreted as being long-lived bleaching components of the ground-state dyes.10 The TR signals shown here were identified to be correlated with the quadrature component exhibiting a typical Stark line shape. The result in Figure 3a thus represents time evolution of the Stark signals. It has been suggested that local electric fields responsible for the Stark signals are induced by electrons injected into TiO2.8,9,15 Therefore, we also examined the time-resolved PIA signals for the normal cell at 1.24, 1.30, and 1.38 eV, where absorption signals from injected electrons of TiO2 are expected to be dominant. The results shown in Figure 3b indicate that all the TR signals consist of rapid absorption components and slow bleaching components. Although the rapid absorption is obviously attributed to injected electrons of TiO2, the origin of the bleaching component is unclear. Also, the slow rise of the bleaching signals is similar to that of the Stark signals shown in Figure 3a. The slow bleaching signals are thus expected to result either from slow recovery of the injected electrons of TiO2 or simply from long tail components of Stark signals. The three TR data during the on-excitation in Figure 3b were fitted using a function of A1 exp(−t/τ1) − A2 exp(−t /τ2), where Ai and τi are the intensity and time constant, respectively, for i = 1, 2. The former (i = 1) and latter (i = 2) terms correspond to the absorption and bleaching components, respectively; τ1 = 1.1 ms and τ2 = 15 ms were used. The best-fit Ai values are shown in Table 1, and the result of the fit for the TR signals at 1.38 eV is

We also performed several PIA measurements for some control cells to examine the influence of electrolyte ions on the observed Stark signals. We show in Figure 2d as an example PIA spectra under the SC condition for a cell fabricated with 0.7 M TBAI and no LiI, indicating substitution of TBAI for LiI. The observed spectra are found to resemble those of the normal cell under the SC condition. We note that PIA spectra were also similar under the OC condition between the two cells. It has been previously reported that small cations such as L+ and H+ work for screening local electric fields acting on neutral dyes and that the diffusion of the cations is associated with the disappearance of the Stark effects.8,9,15 However, in this study, Li+ ions do not directly contribute to the observed Stark effect. Similar PIA spectra were also observed in the case of cells lacking TBP. On the contrary, Stark signals were not observed from a TiO2 film sensitized with N719 without injecting electrolyte ions (see Supporting Information). We particularly note that the absence of Stark signals in the Ptlacking cell under the SC conditions mentioned above was identified from the cell containing the same components of electrolyte ions as the normal cell. This confirms that presence of particular ions is not the direct cause of the Stark signals. The results of these control experiments are summarized as follows: the Stark signals observed from the normal DSC in this study are accompanied by effective photovoltaic operations of the DSC based on the action of redox pairs (I−/I3−). The influence of the Stark signals on the cells is further examined based on time-resolved (TR) measurements for the normal cell using an oscilloscope, the result of which is shown in Figure 3a. The data of TR experiments were measured for

Table 1. Best-Fit Values When Using a Function of A1 exp(−t/τ1) − A2 exp(−t /τ2) for the Time-Resolved Results of the On-Excitation shown in Figure 3b, Where τ1 = 1.1 ms and τ2 = 15 ms Were Used energy (eV)

1.24

1.30

1.38

A1 A2

1.1 0.19

0.92 0.26

0.81 0.37

depicted in the inset of Figure 3b. The determined values of intensity indicate that although the absorption intensity of the TiO2-electrons (A1) decreases with the increase of photon energy, the bleaching signals then increase, suggesting no correlation with each other. The bleaching signals are thus concluded to result from the long tail components of Stark signals. The obtained conclusion is important when considering the origin of the Stark signals. Comparison of Figure 3a,b indicates that the Stark signals appear to be independent of the TiO2electrons. It has been suggested that the local electric fields responsible for the Stark effect are caused by the TiO2electrons.8,9 However, the results after 30 ms in Figure 3a,b demonstrate that, despite almost complete recovery of the

Figure 3. (a) Results of time-resolved (TR) PIA experiments for the normal N719-based DSC measured at 2.1 eV for the laser excitation switched on and off using a chopper (20 Hz). Solid red curves indicate result of fit using a single exponential function. (b) TR-PIA results for the same cell measured at 1.24, 1.30, and 1.38 eV in the same conditions as in panel a. The inset shows an enlarged image of the result at 1.38 eV. The solid red curve is the best-fit result using a function of A1 exp(−t/τ1) − A2 exp(− t /τ2) where Ai (i = 1, 2) is the intensity, τ1 = 1.1 ms, and τ2 = 15 ms. 17263

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tion. The peak power of the pulse-excitation is much larger than that of the cw-laser used in this study; indeed, the intensity of induced absorption signals is one or two orders of magnitude larger than that of this study. It is therefore suggested that the difference of photoexcitation conditions would be the main reasons for the different observations. However, we emphasize that continuous photoexcitation using a cw-laser must be more appropriate to discuss the effect of Stark signals on DSCs in view of actual operating conditions of solar cells. Differently from the cases mentioned above, Cappel et al. observed Stark shifts of dye-sensitized TiO2 films in air from cw-PIA techniques similar to those of this study.7 Accurate comparison of features of Stark signals would be difficult because of differences in experimental conditions between the previous study and this study. However, in the previous study, the intensity of PIA signals is much larger and the modulation frequency of PIA experiments (9.3 Hz) is much smaller than the present study. Therefore, there is a possibility that experiments of much slower time scales and/or stronger continuous photoexcitation could give rise to Stark signals even in the case of sensitized TiO2 films. Regardless of the actual nature of the N719-complexes proposed here, it is obvious that Stark signals occur by electrostatic effects on neutral N719 by iodine molecules or iodine-based ions. This indicates that Stark signals can be used as a monitoring tool of electrostatic environments around neutral dye molecules in DSCs. Particularly, the formation of dye−iodine complexes can lead to reductions in the performance of DSCs.28 We note that there are no effective methods for monitoring the formation of such complexes and evaluating the electrostatic environment of neutral dyes directly from in situ analyses of working DSCs. The Stark signals are thus expected to provide unique information on working DSCs. Moreover, research directly associating the Stark signals with the cell performance are of interest and indeed currently under way.

TiO2-electrons to the ground state, the Stark signals still exist. This result denies the participation of the TiO2-electrons as the origin of Stark signals observed in this study. From the obtained findings, we discuss the origin of the Stark signals observed in this study. Important consequences are that the signals are induced by stable photocurrent and not directly dependent on the presence of TiO2-electrons or particular electrolyte ions. Also, occurrence of the signals was much slower than initial photoexcitation events such as generation of TiO2-electrons and dye-cations. From these features, we conclude that normal actions of redox pairs (I−/I3−) are related to the Stark signals. Because the rise of the Stark signals is slow, the signals should result from products during regeneration processes. Moreover, as described above, the Stark signals were found to be caused by the local electric field acting on neutral dyes. It is therefore suggested that certain complexes containing weakly coupled neutral dyes would be formed during the regeneration and the dyes are forced by short-range local electric fields within the complexes. In this view, accumulation processes of the complexes occurring involved by photocurrent determine the observed slow rise of the Stark signals. Also, the absence of the Stark signals under the OC condition is explained by the accumulation processes being unprogressed because of insufficient photocurrent. In the DSC cycle accepted generally, neutral dyes are expected to be regenerated together with I3−.2 Actually, I3− and iodine molecules, being in equilibrium with each other, can be present around neutral dyes and form complexes with the dyes. Therefore, we conjecture that Stark signals are caused by the local electric fields from I3− or iodine molecules acting on neutral N719. Related to this conjecture, it was recently suggested that complexes are formed between N719 and iodine and that they are present at concentrations much higher than that free iodine in DSCs.28 Moreover, it was shown that the N719-iodine complexes give rise to a blue shift in the steadystate absorption peaks of N719,28 which is consistent with the observations in Stark signals shown in this study. These reported features demonstrate our conjecture to be reliable. In this mechanism of Stark effect, it is expected that dyes are randomly oriented with respect to the TiO2 surface and definite strength of local fields from iodine ions or molecules act on the neutral dyes via binds of iodine to the thiocyanate group.28 The definite strength of fields probably result in a unidirectional absorption shift corresponding to the Stark shift in this study. This view is similar to the suggestion by Ardo et al. for the Stark shift by electrons in TiO2.9 We here discuss reasons for different observations of Stark signals between previous reports and this study. Important differences in the observations are as follows: (i) The time scale of Stark signals in previous reports ranges from picoseconds to microseconds,8,12−14,29 which is much faster than that of this study. (ii) Stark signals of previous studies were primarily explained by local electric fields from electrons of TiO2.8,14 (iii) Stark shifts were obtained also from dye-sensitized TiO2 films, noncomplete DSCs, in the previous studies.7,12 For these previous results, Stark signals in this study were observed in millisecond time scales, found to be not caused by TiO2electrons, and not observed in the case of OC-conditions. We first emphasize that all charged species can potentially induce this type of spectral shifts; hence, the Stark shift might occur or disappear depending on experimental conditions. We particularly note that most of the previous results were obtained from transient absorption measurements using pulsed photoexcita-

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CONCLUSIONS The origin of the Stark signal was investigated using cw-PIA experiments for several control DSCs based on N719. Typical line shape resulting from Stark effects was selectively observed in the quadrature component of cw-PIA spectra under shortcircuit conditions, whereas the Stark line shape was not observed in the open-circuit conditions. The Stark signals were found to be induced by generation of photocurrent and independent of the presence of particular electrolyte ions. Time-resolved PIA measurements demonstrated that the Stark signals rise and decay in time scales much slower than the timeresponse of injected electrons in TiO2, indicating no correlation of the TiO2-electrons with the Stark signals under continuous irradiation. We conclude that effective actions of a redox pair of I−/I3− are related to the Stark effect. We conjecture that complexes are formed between neutral dyes and iodine molecules or iodine-based ions during regeneration processes and that Stark signals are caused by local electric fields acting on neutral dyes within the complexes. We propose to use Stark signals to monitor electrostatic environments around neutral dyes in working DSCs.

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ASSOCIATED CONTENT

S Supporting Information *

Plot of current−voltage characteristic of the N719-DSC and PIA spectra for the N719-DSC before injecting redox 17264

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(14) Burdziński, G.; Karolczakab, J.; Ziółek, M. Dynamics of Local Stark Effect Observed for a Complete D149 Dye-Sensitized Solar Cell. Phys. Chem. Chem. Phys. 2013, 15, 3889−3896. (15) Song, W.; Luo, H.; Hanson, K.; Concepcion, J. J.; Brennaman, M. K.; Meyer, T. J. Visualization of Cation Diffusion at the TiO2 Interface in Dye Sensitized Photoelectrosynthesis Cells (DSPEC). Energy Environ. Sci. 2013, 6, 1240−1248. (16) Stark, J. Observation of the Separation of Spectral Lines by an Electric Field. Nature (London, U.K.) 1914, 92, 401−401. (17) Bublitz, G. U.; Boxer, S. G. STARK SPECTROSCOPY: Applications in Chemistry, Biology and Materials Science. Annu. Rev. Phys. Chem. 1997, 48, 213−242. (18) Boxer, S. G. Stark Realities. J. Phys. Chem. B 2009, 113, 2972− 2983. (19) Das, S.; Kamat, P. V. Spectral Characterization of the OneElectron Oxidation Product of cis-Bis(isothiocyanato)bis(4,4′-dicarboxylato-2,2′-bipyridyl) Ruthenium(II) Complex Using Pulse Radiolysis. J. Phys. Chem. B 1998, 102, 8954−8957. (20) Tachibana, Y.; Moser, J. E.; Grätzel, M.; Klug, D. R.; Durrant, J. R. Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films. J. Phys. Chem. 1996, 100, 20056−20062. (21) Pelet, S.; Moser, J. E.; Grätzel, M. Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2. J. Phys. Chem. B 2000, 104, 1791−1795. (22) Boschloo, G.; Hagfeldt, A. Photoinduced Absorption Spectroscopy of Dye-Sensitized Nanostructured TiO2. Chem. Phys. Lett. 2003, 370, 381−386. (23) Boschloo, G.; Hagfeldt, A. Photoinduced Absorption Spectroscopy as a Tool in the Study of Dye-Sensitized Solar Cells. Inorg. Chim. Acta 2008, 361, 729−734. (24) Redmond, G.; Fitzmaurice, D. Spectroscopic Determination of Flatband Potentials for Polycrystalline Titania Electrodes in Nonaqueous Solvents. J. Phys. Chem. 1993, 97, 1426−1430. (25) Campbell, I. H.; Hagler, T. W.; Smith, D. L.; Ferraris, J. P. Direct Measurement of Conjugated Polymer Electronic Excitation Energies Using Metal/Polymer/Metal Structures. Phys. Rev. Lett. 1996, 76, 1900−1903. (26) Kanemoto, K.; Ogata, A.; Inoue, N.; Kusumoto, T.; Hashimoto, H.; Akai, I.; Karasawa, T. Direct Optical Probing of Negative Carriers from an Operating [6,6]-Phenyl C61 Butyric Acid Methyl Ester Diode. Appl. Phys. Lett. 2010, 97, 033307−1−3. (27) Kanemoto, K.; Domoto, S. Spectroscopic Investigations on Stark Components Observed in Photoinduced Absorption Measurements for Dye-Sensitized Solar Cells. Thin Solid Films 2014, 554, 226−229. (28) Li, X.; Reynal, A.; Barnes, P.; Humphry-Baker, R.; Zakeeruddin, S. M.; De Angelis, F.; O’Regan, B. C. Measured Binding Coefficients for Iodine and Ruthenium Dyes; Implications for Recombination in Dye Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 15421− 15428. (29) O’Donnell, R. M.; Ardo, S.; Meyer, G. J. Charge-Screening Kinetics at Sensitized TiO2 Interfaces. J. Phys. Chem. Lett. 2013, 4, 2817−2821.

electrolyte. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-6-6605-2550. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-aid (24656065) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.

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REFERENCES

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dx.doi.org/10.1021/jp503085f | J. Phys. Chem. C 2014, 118, 17260−17265