Article pubs.acs.org/JPCC
Simultaneous Monitoring of Photoinduced Absorption Signals and Short-Circuit Photocurrent during Photoexcitation in Dye-Sensitized Solar Cells Katsuichi Kanemoto,* Daichi Takatsuki, Hitomi Nakatani, and Shinya Domoto Department of Physics, Graduate School of Science, Osaka City University 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *
ABSTRACT: Continuous-wave photoinduced absorption (PIA) measurements were performed simultaneously with photovoltage or photocurrent (PC) measurements to monitor slow transfer processes of electron carriers in operating dyesensitized solar cells (DSCs). Time-resolved simultaneous experiments demonstrate that the rise curve of PC during photoexcitation is delayed against that of PIA signals. The delay is analyzed by a model that considers effects of electron transfer on PIA signals and shown to be determined by transfer time of electrons over the TiO2 film. On the contrary, no delay is found between the decay curves at excitation−off of PIA signals and PC, suggesting that the transfer kinetics of TiO2 electron varies after the electrons reach the transparent electrode. These spectral features indicate that the time-resolved PIA signals of TiO2 electron under short-circuit conditions are primarily determined by its slow transfer process and that the simultaneous PIA experiments are effective to study the slow electron-transfer processes of DSCs.
1. INTRODUCTION Dye-sensitized solar cells (DSCs) have attracted much attention over the last two decades as promising low-cost alternatives to conventional silicon-based photovoltaics.1−4 Evolved from the DSCs with liquid electrolytes, solid-state solar cells based on perovskite materials have recently made remarkable progresses in the conversion efficiency comparable to that of silicon cells.5,6 Further improvements in power conversion efficiently and longterm stability are currently required toward practical application in both DSCs and the perovskite cells. For the improvements, establishing an effective monitoring method of electronic states under cell operation must be significant. The operation process of DSCs is typically investigated using electrical measurements such as photocurrent (PC)7 and photovoltage (PV),7,8 impedance measurements,9−11 and spectroscopic techniques.12 Among them, only spectroscopic techniques can potentially discriminate signals from different materials and are hence effective for studying the operation processes of DSCs and the perovskite cells that consist of many ingredients.13−16 Many studies have thus been reported concerning spectroscopic properties of DSCs, and dynamics of carrier injection from dyes into TiO2 have been revealed.12 However, most of the studies were performed for film samples without electrodes, and the spectroscopic measurements were hardly conducted simultaneously with electric measurements such as PC and PV. Studies by simultaneous spectroscopic measurements with PC and PV were previously reported by Anderson et al.17 The experiments performed by short pulse © XXXX American Chemical Society
photoirradiation exhibited a weak correlation between the spectroscopic and electric signals. However, by the short pulse excitation, slow processes of cell operation hardly progress during the pulse duration. This would be disadvantageous to the research for DSCs and the perovskite cells dominated by slow operation processes. We thus here focus on photoinduced absorption (PIA) measurements combined with continuouswave (cw) photoexcitation. The PIA techniques have been applied to DSCs18−20 and perovskite solar cells21,22 and shown to be effective for studying behaviors of long-lived photogenerated species. This technique is particularly important in that it enables spectroscopic research under simulated solar cell operating conditions via quasi-cw photoexcitation. We show the results of cw-PIA experiments for electron carriers of TiO2 in operating DSCs of ruthenium-based sensitizers (N719). The PIA experiments were performed simultaneously with PC measurements in the short-circuit (SC) condition or with PV-measurements in the open-circuit (OC) condition. The results demonstrate clear correlation between the spectroscopic signals and electric signals, indicating suitability for monitoring behaviors of electron carriers during the DSC operation. However, delay is found in the rise process of PC against that of the PIA signals during photoexcitation, whereas no delay is observed in the decay process at the moment Received: April 11, 2017 Revised: May 26, 2017 Published: May 26, 2017 A
DOI: 10.1021/acs.jpcc.7b03419 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Photoinduced (PIA) spectra for the N719-dye-sensitized solar cells (DSCs) under the open-circuit (OC) (a) and short-circuit (SC) conditions (b). (c) Spectra obtained from bias-modulation spectroscopic measurement in the dark condition for the FTO electrode sandwiching only solvent (3-methoxypropionitrile). Ch1 and Ch2 in the figures indicate the signal phases in the lock-in measurements.
fabricated by injecting 3-methoxypropionitrile between two FTO electrodes facing each other. The signals were measured under the dark condition by applying an alternating bias to the sample and recording an intensity change in the transmitted probe light from the sample using the same setup as in the cwPIA measurements. Time resolved PIA signals were measured simultaneously with PC or PV using a digital oscilloscope for the laser modulation of 20.1 Hz. The measurements were repeated for accumulation. The time resolution of the experiments was ∼10 μs. All measurements were performed at room temperature.
of off-excitation. A simple model is proposed for analyzing the rise process and demonstrates the delay to be caused by the slowtransfer process of TiO2 electrons. The difference in the delay between the on- and off-excitation processes is found to occur because the slow-transfer processes vary after the electrons reach electrodes via transport inside the TiO2 film. Such difference in the transfer cannot be identified from typical transient absorption measurements by short pulse photoexcitation. The simultaneous cw-PIA techniques are thus suitable for research of DSCs and perovskite cells dominated by slow-transfer processes and hereafter expected to be employed for finding drawbacks of practical cells under operation.
3. RESULTS AND DISCUSSION 3.1. Correlation between Spectroscopic and Electrical Signals. The PIA spectra for the N719 DSC under the OC and SC conditions are shown in Figure 1a,b, respectively. Both spectra exhibit different spectral features between the two phase components. In general, coexistent components with different lifetimes could exhibit a different phase response depending on the difference in the response rate to modulated photoexcitation. Thus the observed phase structures of PIA spectra indicate coexistence of some different species. A downward peak around 2.20 to 2.25 eV in the OC- and SC-PIA spectra of ch1 is assigned to the photobleaching signals of N719. A peak around 1.6 eV in the ch1 of SC-PIA spectrum is assigned to the absorption signal of cations of N719 generated by injection of electrons into the TiO2 layer.13,18−20,23,24 The ch2-spectrum of OC-PIA is similar to the spectrum of electrons in the TiO2 layer25,26 and mainly given by the TiO2 electrons. The ch2-spectrum of SC-PIA is similar to the differential absorption spectrum of N719 and corresponds to Stark signals induced by local electric fields acting on neutral N719.20,27−29 The aim of this research is to explore behaviors of carriers under solar cell operation by spectroscopic measurements. For this purpose, we first specify an appropriate energy point of detection enabling spectral monitoring of TiO2 electrons. It was previously reported that the photoinduced absorption signals ranging from 950 (1.30 eV) to 1020 nm (1.21 eV) can be assigned to the TiO2 electron alone.17 However, we note the presence of a weak peak at 1.15 eV found in the ch1-spectrum of SC-PIA. This peak is usually absent in the PIA spectrum for a film sample of N719 adsorbed on TiO2 without electrodes (Figure S2). We attempted to measure absorption signals from FTO electrodes using bias-modulation spectroscopic techniques.30,31 The measurements were performed for the FTO electrodes sandwiching only solvent under the dark condition. The result shown in Figure 1c demonstrates presence of an absorption peak at 1.15 eV, and thus the peak is assigned to the absorption signal of carriers generated in the FTO electrode. Therefore,
2. MATERIALS AND MEASUREMENTS DSCs used were fabricated by the following procedures. Monocrystalline TiO2 paste (Solaronix, particle size 15−20 nm) was cast as mesoporous 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. (Bu4N)2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl) (N719, Aldrich) was employed as a dye-sensitizer. Sensitization was achieved by immersing the TiO2 films in the N719 solution in ethanol (0.42 mg/mL) for 1 day. An alcohol-based paint containing a chemical platinum precursor (Platisol T, Solaronix) was deposited on another FTO substrate (Solaronics) and 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 cell consisted of 0.1 M lithium iodine (LiI), 0.1 M iodine, 0.6 M tetrabutylammoniuiodide (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. The energy-conversion efficiency of the cell was typically ∼5% (see 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 (250 mW/cm 2 ) was modulated using an acousto-optic (AO) modulator. A probe beam for the measurements was produced using a tungsten/ halogen lamp and detected with a Si photodiode for the visible region and an InGaAs detector for the near-infrared region after passing a monochromator. The cw-PIA signals were measured by monitoring the probe signal using a dual-phase lock-in amplifier, yielding ch1 and ch2 components of signal. We note that the ch1 component does not mean zero phase against the reference phase but was adjusted to decompose overlapping PIA signal components. The spectra of cw-PIA were measured at 70 Hz. A sample for measuring spectroscopic signals from FTO was B
DOI: 10.1021/acs.jpcc.7b03419 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. Results of simultaneous time-resolved photovoltage (a) and PIA measurements (b) in the on- and off- processes of photoexcitation for the DSC under OC condition. Results of simultaneous photocurrent (c) and PIA measurements (d) under SC condition. Comparison of rise curve (e) and decay curve (f) between the PIA signal and photocurrent.
for the rise process and treating G(t) as zero for the decay process.31,34 The generation process (>1012 s−1) is very fast on the time scale of the present PIA observations (t > 10 μs), enabling us to regard the G(t) term as being constant (= G) during continuous photoirradiation. To build a realistic model for analyzing the TR-PIA response, we pay attention to several experimental findings. First, an obvious difference was found in the PIA-time response between the OC and SC conditions, suggesting that transfers of electrons affect the PIA time response. Besides, there was a delay in the rise process of PC against that of PIA. The delay should be associated with the transfer time of electron over the TiO2 film because the PC is expected to flow subsequently after the electron transfer. The generation of TiO2 electrons via photoexcitation occurs only at the interface with dyes. Electrons are thus initially absent at TiO2 sites distant from the interface, and the distant electrons should be generated by transfer of the interface electrons. We here assume that the transfer induces damping of electron density at the interface and introduce the transfer rate constant kt,0 as follows
spectroscopic measurements from 1.0 to 1.9 eV could include non-negligible contributions from the FTO electrode as well as from TiO2 electrons. The signal from FTO is nearly negligible at 0.964 eV, where signals from TiO2 electrons still exist as shown in the OC-PIA spectrum. Moreover, this energy is very distant from the Stark signals peaked at 2.2 eV, and their contribution is negligible. The energy of 0.964 eV was thus selected for monitoring behaviors of TiO2 electrons in time-resolved (TR) spectroscopic measurements, and all TR-PIA signals shown hereafter were measured at 0.964 eV. Figure 2a,b, respectively, shows the results of simultaneous TR-PV and PIA measurements in the on and off processes of photoexcitation for the N719 cell under OC condition. It has been shown by transient absorption measurements by pulse photoexcitation that generation of TiO2 electrons by photoinjection from dyes is very fast, so that the rate is beyond 1 × 1012 s−1.11,32,33 Nonetheless, the TiO2 electrons in Figure 2b grow slowly and exhibit similar time response to that of PV. Figure 2c,d, respectively, shows the results of simultaneous TR-PC and PIA measurements in the on and off processes of photoexcitation under SC condition. The rise and decay curves under SC condition become much faster than those under OC condition. Importantly, an excellent correlation is found between the TRPIA signals and PC under SC condition. This indicates the suitability of the simultaneous measurements for monitoring electron carriers under cell operation. However, as shown in Figure 2e,f, the rise curve of PIA is slightly delayed relative to that of PC, although the decay curve of PIA resembles that of PC. 3.2. Model for Considering Difference in the TR-PIA and PC. Correlation of the observed PIA and PC responses is here discussed. The rate equation for the TiO2 electrons is described as follows with ne being the electron density
dne = G(t ) − kdne dt
kd = k r + k t,0
(2)
where kr is the recombination rate constant (Figure 3a). Under these conditions, the electron density at the interface ne,0 in the rise process is simply calculated as ne,0 =
G [1 − e−(kr + k t,0)t ] k r + k t,0
(3)
These electrons reside in a conduction band or at trap sites in TiO2 spreading over long distance.35 Hence the kt,0 term is probably induced by the electron transfers between layered conduction bands. Similar damping by electron transfers can further take place. The rate equation for the next layer electrons (the density ne,1 and transfer rate kt,1) is then given by
(1)
dne,1
where G(t) is the generation rate and includes contributions from the photoexcitation intensity, the absorption cross section of dye, and the transfer efficiency of dye to TiO2 electrons. kd is the decay rate constant of the electron. The relative intensity of PIA signal in TiO2 electrons, proportional to ne, can be calculated from the solution of eq 1 by considering time evolution of G(t)
dt
= k t,0ne,0 − k t,1ne,1
(4)
Here the recombination rate kr was neglected for simplicity because the recombination with I3− ions should be reduced inside the TiO2 film. Also, the back-transfer from the next layer was neglected. Using eq 3, the solution of eq 4 is calculated by C
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∞) is given by Gkt,0/{(kr + kt,0)kt,i}. The electron transfer in nanocrystalline TiO2 films has been generally believed to occur by diffusion36 because plenty of ions cancel internal electric field. For incorporating the effect of diffusion, gradient in electron density is assumed to be present under SC condition,35 and we employ the following relation among successive layers (Supporting Information) k t, i > k t, i − 1 > ... > k t,1 > k t,0
(6)
The time responses of PC and PIA signals under the SC condition are discussed based on the above model. The spectral response of TiO2 electrons is different depending on the distance from the interface with dyes, and the PIA intensity is f proportional to ∑i = 0 ne , i , where f is used for the final layer. f
The rise process of SC-PIA signals was thus fitted using ∑i = 0 ne , i . Also the ne,f electrons are located at the interface with the FTO electrode. Therefore, neglecting the time delay of response between the ne,f electrons and the electrons in electrodes, the time response of PC can be predicted by that of ne,f. The results of fit for the rise processes of PC and SC-PIA using the functions of f ne,f and ∑i = 0 ne , i are displayed in Figure 3b,c, respectively. In these fits, we assumed that kt,i (i = 0, 1, ...) simply decreases linearly with i to satisfy the condition in eq 6. Using kr = 400 s−1 and f = 3, the results of fit using kt,0 = 1800 s−1 reproduced well rise processes of both SC-PIA and PC (see the Supporting Information for details). In reality, photogeneration of the ne,0 electrons would also occur in the TiO2 surface near the interface with FTO electrodes, and such electrons could undergo more rapid transfer to FTO. This model thus describes only averaged picture of electron transfers from spatially distributed ne,0 electrons. However, nonetheless, the observed delay between SC-PIA signals and PC is well explained by this model, indicating that the delay is determined by transfer time of electrons over the TiO2 film. The time delay between SC-PIA signals and PC shown in Figure 2e corresponds to ∼50 μs. From the model developed above, this delay results from the rise-curve delay between f ∑i = 0 ne , i and ne,f. As shown in Figure S3, the magnitude of delay
Figure 3. (a) Schematics of model used for considering correlation between time-resolved PIA signals and photocurrent. Results of fit for the rise processes of photocurrent using a function of ne,f (b) and PIA f signals using ∑i = 0 ne , i (c).
ne,1 =
Gk t,0 ⎡ 1 − e−k t,1t e−(kr + k t,0)t − e−k t,1t ⎤ ⎢ ⎥ − k r + k t,0 ⎣⎢ k t,1 k t,1 − k t,0 − k r ⎥⎦
(5)
The first term of eq 5 indicates the rise of electron density depending on the transfer rate kt,1 and the second term induces a delay of the rise. The ne,1 electrons can also transfer to next layers (ne,2), and such transfers may occur successively to generate electrons of the density ne,i and the transfer rate kt,i (i = 3, 4, 5, ...). For the parameters, the electron density in the steady state (t →
Figure 4. Comparison of time-resolved photocurrent (a) and SC-PIA signals (b) under the 3.1 (3.1I0) and 2.0 times (2.0I0) stronger intensity relative to those under 250 mW/cm2 excitation intensity (= I0). Results of fit for the rise processes of photocurrent using a function of ne,f (c) and PIA signals using f ∑i = 0 ne , i (d). Comparison of intensity-dependent decay curves of photocurrent (f) and SC-PIA signals (g). D
DOI: 10.1021/acs.jpcc.7b03419 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C in the rise curve of ne,i increases with increasing i. When assuming f the rise curve of ∑i = 0 ne , i to be approximated by an average of rise curves among ne,i (i = 0, 1, ...f), double the time delay between SC-PIA and PC (100 μs) is close to the time delay between ne,0 and ne,f. Importantly, the delay between ne,0 and ne,f roughly corresponds to a simple measure of the total transfer time of electrons over the TiO2 film. For the average thickness of the TiO2 film (13 μm), the velocity of electron is then calculated as 0.13 m/s. Unlike the case of the rise process, the decay process shown in Figure 2f did not exhibit a delay between PC and SC-PIA. The difference between the rise and decay processes is probably attributed to the difference in the magnitude of ne,f at the moments of excitation-ON and -OFF. Namely, negligible ne,f at the light-ON moment results in the delay of PC, but ne,f is nearly in its equilibrium value at the light-OFF moment, allowing immediate electron transfer to electrodes with negligible delay. Also, the decay curves of SC-PIA and PC were found to resemble f each other. This suggests ne,f to be nearly proportional to ∑i = 0 ne , i during the decay process. Such a proportional relation can be obtained when the electrons decay with maintaining the density ratio among all ne,i components. At the light-off moment, injection from dyes into TiO2 electrons stops, and the electron transfer is driven by transfer of ne,f electrons to the FTO electrode. Therefore, the observed decay kinetics suggests that when the electron transfer is driven by the transfer to the electron, all ne,i components uniformly decay with maintaining the density ratio. 3.3. Photoexcitation Intensity Dependence. It has often been reported that kinetics of spectroscopic signals varies depending on photoexcitation intensity.36,37 The photoexcitation intensity-dependence of cw-PIA signals is here discussed for the results measured under SC conditions. Figure 4a,b shows the results of TR measurements of PC and SC-PIA, respectively, under 2.0 (2.0I0) and 3.1 times (3.1I0) stronger intensity based on those under 250 mW/cm2 excitation (= I0) shown above. The PC linearly increases against the photoexcitation intensity, but PC under the 3.1I0-photoexcitation weakly decreases after reaching a peak value around 5 ms with a time constant of 24 ms despite continuous photoirradiation. Similar decreases in PC were previously reported,38,39 and such decrease was more evident in DSCs with cobalt complex-based redox electrolytes.38 Because the motion of the cobalt-complex in solution is known to be slow,40−42 the observed PC decrease is attributed to delay of redox cycle reactions in iodine electrolytes. Strong photoexcitation could thus result in measurements under irregular operating conditions. The rise processes of TR-PIA signals and PC are compared in Figure 4c for the results under the 3.1I0-photoexcitation: For the 2.0I0 photoexcitation, see Figure S4. Delay of PC against the PIA signals is observed in both excitations similarly to the case of the I0 excitation. The same model as that used for the I0 excitation was also employed for the 2.0I0 and 3.1I0 photoexcitations. The results of fit for PC and the PIA signals under the 3.1I0 photoexcitation are shown in Figure 4d,e, respectively (for the 2.0I0 photoexcitation, see Figure S5). Both the PC and PIA signals were well reproduced only by changing kt,0 (2200 and 2400(s−1) for 2.0I0 and 3.1I0 photoexcitations) and using the same parameter of kr and f as that for the I0 photoexcitation. These consequences confirm suitability of the model for explaining behaviors of electrons in DSCs under the SC condition. It has been reported that electron diffusion
coefficients in DSCs are enhanced by increasing the photoexcitation intensity.43−45 The determined k0 values are consistent with the previous references. Figure 4f,g shows comparison of decay processes in PC and the PIA signal, respectively, among the results under the I0, 2.0I0, and 3.1I0 photoexcitations. Both Figures show the decay rates of PC and the PIA signal to be nearly independent of the photoexcitation intensity, differing from the trend observed during the light-ON process. The intensity-enhanced kt,0 at lightON is attributed to increased ne,0 by stronger photoexcitation, resulting in enhanced diffusion. Such enhancements are not expected when the photoexcitation stops and the ne,f electrons drive the electron transfer. The intensity-independent decay curves thus fairly agree with the prediction above that the ne,f electrons drive the electron transfer. 3.4. Difference from Short Pulse Photoexcitation Measurements. We finally discuss difference of the information obtained from the cw-PIA and transient absorption (TA) measurements for working DSCs. The TA technique using typically a short photoexcitation pulse is powerful to explore generation dynamics of TiO2 electrons that are believed to occur on subpicosecond time scale. However, for the detection, high peak-intensity pulse much greater than usual solar cell operation intensity is typically required for photoexcitation. The strong photoexcitation is practically unsuitable for research of working DSCs because electron transports generally depend on photoexcitation intensity,43−45 as shown in the previous section. Besides, the pulse duration is generally much shorter than the time scale of DSC operation. The case of using the short pulse can be simulated by applying a time response function resulting from the pulse excitation to G(t) in eq 1. As simply predicted, the kinetics of electron carriers is then primarily given by a fast response component from the carrier generation. Slow signal components are only partly given because slow events slightly develop during the pulse excitation. cw-PIA techniques by longduration photoexcitation are advantageous for research of slow motions. The techniques are particularly effective for DSCs where slow processes are dominant and can be applied for monitoring dye-regeneration and cell-degradation processes during photoexcitation. Such experiments are currently underway.
4. CONCLUSIONS Correlation between electric signals and spectroscopic signals of TiO2 electrons in operating DSCs was investigated by simultaneous TR-measurements. The energy point for detecting PIA signals of TiO2 electrons was first specified by considering the spectroscopic signals from the FTO electrode. The simultaneous experiments demonstrated that the rise curve of PC during photoexcitation is delayed against that of PIA signals. A model for considering effects of electron transfer on PIA signals was built and revealed that the delay is due to slow transfers of electrons in the TiO2 film. On the contrary, no delay was observed between the decay curves of PIA and PC at excitation off, indicating that the transfer kinetics varies after the electrons reach the FTO electrodes. These conclusions agree well with the intensity dependence of the simultaneous measurements. The obtained findings confirm the importance of the simultaneous PIA experiments for studying electron transport of DSCs dominated by slow electron transfer processes. E
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(14) Montanari, I.; Nelson, J.; Durrant, J. R. Iodide Electron Transfer Kinetics In Dye-Sensitized Nanocrystalline TiO2 Films. J. Phys. Chem. B 2002, 106, 12203−12210. (15) Zarick, H. F.; Erwin, W. R.; Boulesbaa, A.; Hurd, O. K.; Webb, J. A.; Puretzky, A. A.; Geohegan, D. B.; Bardhan, R. Improving Light Harvesting in Dye-Sensitized Solar Cells Using Hybrid Bimetallic Nanostructures. ACS Photonics 2016, 3, 385−394. (16) Zarick, H. F.; Boulesbaa, A.; Puretzky, A. A.; Talbert, E. M.; DeBra, Z. R.; Soetan, N.; Geohegan, D. B.; Bardhan, R. Ultrafast Carrier Dynamics in Bimetallic Nanostructure-Enhanced Methylammonium Lead Bromide Perovskites. Nanoscale 2017, 9, 1475−1483. (17) Anderson, A. Y.; Barnes, P. R. F.; Durrant, J. R.; O’Regan, B. C. Simultaneous Transient Absorption and Transient Electrical Measurements on Operating Dye-Sensitized Solar Cells: Elucidating the Intermediates in Iodide Oxidation. J. Phys. Chem. C 2010, 114, 1953− 1958. (18) Boschloo, G.; Hagfeldt, A. Photoinduced Absorption Spectroscopy of Dye-Sensitized Nanostructured TiO2. Chem. Phys. Lett. 2003, 370, 381−386. (19) 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. (20) Kanemoto, K.; Domoto, S.; Hashimoto, H. Origin of Stark Signals Induced by Continuous Photoirradiation for Working Dye-Sensitized Solar Cells Revealed by Photoinduced Absorption Measurements. J. Phys. Chem. C 2014, 118, 17260−17265. (21) Roiati, V.; Mosconi, E.; Listorti, A.; Colella, S.; Gigli, G.; De Angelis, F. Stark Effect in Perovskite/TiO2 Solar Cells: Evidence of Local Interfacial Order. Nano Lett. 2014, 14, 2168−2174. (22) Pazoki, M.; Jacobsson, T. J.; Kullgren, J.; Johansson, E. M. J.; Hagfeldt, A.; Boschloo, G.; Edvinsson, T. Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials. ACS Nano 2017, 11, 2823−2834. (23) 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. (24) 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. (25) Redmond, G.; Fitzmaurice, D. Spectroscopic Determination of Flatband Potentials for Polycrystalline Titania Electrodes in Nonaqueous Solvents. J. Phys. Chem. 1993, 97, 1426−1430. (26) 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. (27) Ardo, S.; Sun, Y.; Staniszewski, A.; Castellano, F. N.; Meyer, G. J. Stark Effects after Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites. J. Am. Chem. Soc. 2010, 132, 6696− 6709. (28) Cappel, U. B.; Feldt, S. M.; Schoneboom, J.; Hagfeldt, A.; Boschloo, G. The Influence of Local Electric Fields on Photoinduced Absorption in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 9096−9101. (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. (30) Kanemoto, K.; Yasui, M.; Kosumi, D.; Ogata, A.; Sugisaki, M.; Karasawa, T.; Akai, I.; Hashimoto, H. Morphology-Dependent Carrier and Exciton Generations in Regioregular Poly(3-hexylthiophene) Polymer Diodes as Revealed by Bleaching Spectroscopy. Phys. Rev. Lett. 2009, 103, 187402. (31) Kanemoto, K.; Yasui, M.; Higuchi, T.; Kosumi, D.; Akai, I.; Karasawa, T.; Hashimoto, H. Spectroscopic Investigation Of Excitons, Photocarriers, and Bias-Induced Carriers in Regioregular Poly(3Alkylthiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 205203.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03419. Photocurrent−voltage characteristics, cw-PIA spectrum for the N719 film on TiO2, supplementary explanation for model fit, and PIA results under 2.0I0 photoexcitation. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid (no. 26620207) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.
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REFERENCES
(1) O'Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737− 740. (2) Ning, Z. J.; Fu, Y.; Tian, H. Improvement of Dye-Sensitized Solar Cells: What We Know and What We Need To Know. Energy Environ. Sci. 2010, 3, 1170−1181. (3) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (4) Peter, L. M. The Gratzel Cell: Where Next? J. Phys. Chem. Lett. 2011, 2, 1861−1867. (5) Park, N. G.; Gratzel, M.; Miyasaka, T.; Zhu, K.; Emery, K. Towards Stable and Commercially Available Perovskite Solar Cells. Nature Energy 2016, 1, 16152. (6) Tan, H. R.; Jain, A.; Voznyy, O.; Lan, X. Z.; García de Arquer, F. P. G.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M. J.; Zhang, B.; Zhao, Y. C.; et al. Efficient and Stable Solution-Processed Planar Perovskite Solar Cells via Contact Passivation. Science 2017, 355, 722−726. (7) Barnes, P. R. F.; Miettunen, K.; Li, X. E.; Anderson, A. Y.; Bessho, T.; Gratzel, M.; O’Regan, B. C. Interpretation of Optoelectronic Transient and Charge Extraction Measurements in Dye-Sensitized Solar Cells. Adv. Mater. 2013, 25, 1881−1922. (8) O’Regan, B. C.; Bakker, K.; Kroeze, J.; Smit, H.; Sommeling, P.; Durrant, J. R. Measuring Charge Transport from Transient Photovoltage Rise Times. A New Tool to Investigate Electron Transport in Nanoparticle Films. J. Phys. Chem. B 2006, 110, 17155−17160. (9) Wang, Q.; Moser, J. E.; Gratzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945−14953. (10) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D. B.; Zakeeruddin, S. M.; Gratzel, M. Correlation between Photovoltaic Performance and Impedance Spectroscopy of DyeSensitized Solar Cells Based on Ionic Liquids. J. Phys. Chem. C 2007, 111, 6550−6560. (11) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero, I.; GarciaBelmonte, G.; Barea, E. M.; Palomares, E. A Review of Recent Results on Electrochemical Determination of The Density of Electronic States of Nanostructured Metal-Oxide Semiconductors and Organic Hole Conductors. Inorg. Chim. Acta 2008, 361, 684−698. (12) Listorti, A.; O’Regan, B.; Durrant, J. R. Electron Transfer Dynamics in Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3381− 3399. (13) 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. F
DOI: 10.1021/acs.jpcc.7b03419 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (32) Anderson, N. A.; Lian, T. Q. Ultrafast Electron Transfer at the Molecule-Semiconductor Nanoparticle Interface. Annu. Rev. Phys. Chem. 2005, 56, 491−519. (33) Ardo, S.; Meyer, G. J. Photodriven Heterogeneous Charge Transfer With Transition-Metal Compounds Anchored to TiO2 Semiconductor Surfaces. Chem. Soc. Rev. 2009, 38 (1), 115−164. (34) Botta, C.; Luzzati, S.; Tubino, R.; Bradley, D. D. C.; Friend, R. H. Photoinduced Absorption of Polymer-Solutions. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 14809−14817. (35) Peter, L. ″Sticky Electrons″ Transport and Interfacial Transfer. of Electrons in the Dye-Sensitized Solar Cell. Acc. Chem. Res. 2009, 42, 1839−1847. (36) Kopidakis, N.; Schiff, E. A.; Park, N. G.; van de Lagemaat, J.; Frank, A. J. Ambipolar Diffusion of Photocarriers in Electrolyte-Filled, Nanoporous TiO2. J. Phys. Chem. B 2000, 104, 3930−3936. (37) Solbrand, A.; Henningsson, A.; Sodergren, S.; Lindstrom, H.; Hagfeldt, A.; Lindquist, S. E. Charge Transport Properties in DyeSensitized Nanostructured TiO2 Thin Film Electrodes Studied by Photoinduced Current Transients. J. Phys. Chem. B 1999, 103, 1078− 1083. (38) Nelson, J. J.; Amick, T. J.; Elliott, C. M. Mass Transport of Polypyridyl Cobalt Complexes in Dye-Sensitized Solar Cells with Mesoporous TiO2 Photoanodes. J. Phys. Chem. C 2008, 112 (46), 18255−18263. (39) Tress, W.; Corvers, S.; Leo, K.; Riede, M. Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S-Shaped Current-Voltage Characteristics. Adv. Energy Mater. 2013, 3, 873−880. (40) Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J. E.; Gratzel, M. An Alternative Efficient Redox Couple for the Dye-Sensitized Solar Cell System. Chem. - Eur. J. 2003, 9, 3756−3763. (41) Cameron, P. J.; Peter, L. M.; Zakeeruddin, S. M.; Gratzel, M. Electrochemical Studies of the Co(Iii)/Co(Ii) (Dbbip)(2) Redox Couple as A Mediator for Dye-Sensitized Nanocrystalline Solar Cells. Coord. Chem. Rev. 2004, 248 (13−14), 1447−1453. (42) Cazzanti, S.; Caramori, S.; Argazzi, R.; Elliott, C. M.; Bignozzi, C. A. Efficient Non-Corrosive Electron-Transfer Mediator Mixtures for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2006, 128, 9996−9997. (43) Nelson, J. Continuous-Time Random-Walk Model of Electron Transport in Nanocrystalline TiO2 Electrodes. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15374−15380. (44) Nakade, S.; Matsuda, M.; Kambe, S.; Saito, Y.; Kitamura, T.; Sakata, T.; Wada, Y.; Mori, H.; Yanagida, S. Dependence of TiO2 Nanoparticle Preparation Methods and Annealing Temperature on the Efficiency of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2002, 106, 10004−10010. (45) Bisquert, J. Chemical Diffusion Coefficient of Electrons in Nanostructured Semiconductor Electrodes and Dye-Sensitized Solar Cells. J. Phys. Chem. B 2004, 108, 2323−2332.
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DOI: 10.1021/acs.jpcc.7b03419 J. Phys. Chem. C XXXX, XXX, XXX−XXX
