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Stepped Light-Induced Transient Measurements of Photocurrent and Voltage in Dye-Sensitized Solar Cells: Application for Highly Viscous Electrolyte Systems Shogo Nakade,† Taisuke Kanzaki,† Yuji Wada,† and Shozo Yanagida*,‡ Material and Life Science, Graduate School of Engineering, and Center for Advanced Science and Innovation, Osaka University, Suita, Osaka 565-0871, Japan Received May 11, 2005. In Final Form: August 2, 2005 To measure electron diffusion coefficients (D) and electron lifetimes (τ) of dye-sensitized solar cells (DSC), we introduced stepped light-induced transient measurements of photocurrent and voltage (SLIMPCV), which can simplify the optical setup and reduce measurement time in comparison to conventional time-of-flight and frequency-modulated measurements. The method was applied to investigate the influence of the viscosity of a thermally stable high-boiling-point solvent on the energy conversion efficiency of DSCs. By systematic study of the influence of the viscosity, the species of cations as the counter charge of I-/I3-, and the concentrations of electrolytes, we concluded that a lower dye cation reduction rate due to slower iodine diffusion is a limiting factor for a highly viscous electrolyte system. On the other hand, comparable values of D and increased values of τ were observed in a highly viscous electrolyte. By employing 0.5 M TBAI and 0.05 M I2 in propylene carbonate, the efficiency of the DSC became comparable to that of a DSC using conventional electrolytes consisting of LiI, imidazolium iodide, and 4-tert-butylpyridine in methoxyacetonitrile. The simultaneous evaluation of D and τ through the appropriately simple measurement realizes fast optimization of the efficient and reliable DSC composed of thermally stable but often viscous electrolytes.
1. Introduction Currently, the dye-sensitized solar cell (DSC) is one of the most active research subjects in photoelectrochemistry.1-15 DSCs are comprised of dye-adsorbed nanoporous TiO2 film soaked in electrolytes, sandwiched by a transparent conductive oxide (TCO) substrate and a platinum counter electrode. In the cell, light is absorbed by the dye and excited electrons are injected into the TiO2. * Corresponding author. Phone: +81-6-6879-7351. Fax: +816-6879-7351. E-mail:
[email protected]. † Graduate School of Engineering. ‡ Center for Advanced Science and Innovation. (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (3) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.;, Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 15, 6382. (4) Liu, Y.; Hagfeld, A.; Xiao, X.-R.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells, 1998, 55, 267. (5) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (6) Kanzaki, T.; Nakade, S.; Wada, Y.; Yanagida, S. J. Mater. Chem., submitted for publication, 2005. (7) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (8) Tennakone, K.; Kumara, G. R. R. A.; Kumarashinghe, A. R.; Kottegoda, I. R. M.; Wijayantha, K. G. U.; Perera, V. P. S. J. Phys. D: Appl. Phys. 1998, 31, 1492. (9) Wang, P.; Dai, Q.; Zakeeruddin, S. M.; Forsyth, M.; MacFarlane, D. R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 13590. (10) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374. (11) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109 3480. (12) Cao, F.; Oskam, G.; Meyer, G. J.; Searson, P. C. J. Phys. Chem. B 1996, 100, 17021. (13) Dlocik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281. (14) Kopidakis, N.; Schiff, E. A.; Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (15) Nakade, S.; Matsuda, M.; Kambe, S.; Saito, Y.; Kitamura, T.; Sakata, T.; Wada, Y.; Mori, H.; Yanagida, S. J. Phys. Chem. B 2002, 106, 10004.
The electrons diffuse in the TiO2 and are collected by the TCO. In parallel, the resulting hole in the dye is reduced by a redox couple and is brought to the counter electrode by diffusion. Electrolytes typically consist of several cations such as Li and imidazolium cations and additives such as tert-butylpyridine.3 Cations and additives are chosen to achieve a high injection yield from the dye to the TiO24 and high open circuit voltage.3,5,6 For solvents, acetonitrile (AN) has been commonly used because of its low viscosity and high dielectric constant. Those parameters are related with the ion conductivity and solubility of the salts. On the other hand, AN has a low boiling temperature of 81.6 °C, which makes it difficult to keep the electrolytes sealed for a long period. Sealing of electrolytes is one of the largest issues for the practical use of the DSCs. Thus, great efforts have been made to solidify or replace the liquid electrolyte.7-10 For the case of molten salts, their high viscosity requires higher redox couple concentrations to provide enough ion conductivity, and it causes other issues, e.g., an increase of undesired light absorption by the redox couple and a decrease of electron lifetime (τ).10 To increase τ in DSCs using AN, we found that employing bulky cations such as tetra-n-butylammonium cation increased the τ.11 When highly viscous solvents are employed to obtain thermally stable electrolytes, a further increase of τ can also be expected due to the decreased mobility of triiodide in the viscous environment. The aim of this paper is to examine the influence of solvents, the species of cations, and the electrolyte concentrations on DSCs performance and to obtain a guide to select appropriate cations for high boiling point and then highly viscous solvent electrolytes. For the purpose, we selected propylene carbonate, which is commonly available as an electrochemical solvent, and whose boiling point is 242 °C, and compared the effect of the cations between Li+ and TBA+ on DSC performance, since the cations have a large difference in their size and adsorption properties.11
10.1021/la051257j CCC: $30.25 © 2005 American Chemical Society Published on Web 10/05/2005
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2. Measurements of Electron Diffusion Coefficients and Lifetime by Stepped Light-Induced Transients of Photocurrent and Voltage First, we introduce a modified method to measure electron diffusion coefficients in DSCs. To measure D, pulsed-laser-induced current transients or intensitymodulated photocurrent spectroscopy has been used.5,10-15 The motivation of the modification was to simplify the optical setup and reduce the measurement time, which are desired especially accompanied with the optimization of electrolytes for DSCs. Our method uses a current transient induced by a stepwise change of the laser light intensity. At a short circuit condition of the DSC, instant reduction of partial light intensity induces the current transient, i.e., the short circuit current (Jsc) corresponding to the initial light intensity decreases to a constant value corresponding to the reduced light intensity. The time to reach the constant value depends on the electron diffusion coefficients. The straightforward approach is to solve a diffusion equation with appropriate boundary and initial conditions and fit the solution to the measured transients. However, at short circuit, the condition of the TCO/TiO2 interface is not fully understood yet. Here, instead of solving the diffusion equation, we will follow the approach used by Kopidakis et al.,14 i.e.,
Table 1. Properties of Solvents
acetonitrile PC a
boiling point °C
viscosity cP
dielectric constant
81.65 242
0.284a 1.38b,c
36.64 66.14
At 50 °C. b At 40 °C. c Only c is from ref 17, otherwise from ref
16.
the Fermi level of the TiO2 electrode, Voc scales with log(n/n0), where n0 is the electron density in the dark. When ∆n is small, ∆log(∆n/n0) can be approximated to be proportional to ∆n, by using the Taylor expansion. In the case of our measurements, ∆n is less than 4% of the total electron density. Thus, for the case of a small stepwise change of light intensity, Voc should be proportional to eq 4. Then, the lifetime can be derived by the fits of eq 4 to the voltage transient. The Fourier transform of eq 4 relates to the spectrum of the intensity-modulated photovoltage spectroscopy (IMVS). 3. Experiments
(4)
For stepped light-induced transient measurements, a diode laser (Coherent, LabLaser, λ ) 635 nm) was employed for both the photocurrent and voltage transients. The absorption coefficient of the N719 dye at this wavelength is so small that nearly uniform electron generation along the thickness of the DSC can be expected. To irradiate the whole area of the DSCs, the laser beam was expanded by a lens, and an aperture was placed in front of the solar cell. Note that current transients did not depend on the size of the laser spot when the spot radius was much bigger than the thickness of the TiO2. On the other hand, photovoltage transients became slow if the spot size was smaller than the cell area of the DSC. The transients were induced by the stepwise change of the laser intensity, which was controlled by a function generator (Toho Technical Research; the rate of change was 5 × 105 V/s). Typically, the laser was operated at the voltage of 3.10 V and stepped down to 3.05 or 3.00 V, which provides a laser power of approximately 7.1-6.9 or 6.6 mW, respectively. A set of ND filters was used to change the laser intensity. Current transients were monitored by a digital storage oscilloscope through a current amplifier (SR570, Stanford Research Systems). For photovoltage transients, a differential amplifier (NF Electronic Instruments, 5307) with a gain of 20 times was used to monitor a few millivolts of decay of the photovoltage. To compare the results obtained by the method above, identical samples were measured with pulsed-laser-induced current transients employing a Nd:YAG laser (532 nm, 7 ns pulse) under continuous irradiation of a He-Ne laser, whose intensity is much larger than that of the pulsed laser and with IMVS employing the diode laser used above and a lock-in-amplifier (SR810, Stanford Research Systems).10,11 Dye-sensitized solar cells were fabricated with the previously published method.15 Nanoporous TiO2 films were prepared on transparent conductive oxide (TCO) (Nippon Sheet Glass, SnO2/ F, 8 ohm/sq) from TiO2 nanoparticles (Nippon Aerosil, P25) by annealing at 450 °C in air for 30 min. Dyes were adsorbed by dipping the film into a solution containing 0.5 mM Ru dye (Bu4N)2[Ru(Hdcbpy)2(NCS)2] (known as N719, Solaronix). Solar cells were made by placing a Pt-sputtered TCO on the TiO2 film, and the electrolyte was introduced from a small hole drilled on the counter electrode. Typical cell area was approximately 0.15 cm2. The standard electrolyte consisted of 0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, and 0.5 M 4-tertbutylpyridine in methoxyacetonitrile. To study the influence of viscosity, we prepared electrolytes with various concentrations of LiI or TBAI with 0.05 M I2 in PC or AN and fabricated DSCs with them. The properties of the solvents are listed in Table 1.16,17
where A is ∆n, which is the difference of the electron densities under the initial and final laser irradiation conditions. When the open circuit voltage (Voc) scales with
(16) Lide, D. R., Ed. Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2003. (17) Hechler, G. Chem.-Ing.-Tech. 1971, 43903.
D ) (L/2)2/tH
(1)
where tH is the time to extract half of the excess electrons and L is the thickness of the electrode. The excess electrons are the difference between the amount of electrons in the DSC under the initial and final light intensities. The excess electrons can be obtained by the numerical integration of the current transient. For the case where the transients can be fitted with an exponential function, exp(-t/τC), the relationship between tH and τC is tH ) 0.693τC. Thus, using the fitted constant of τC, the diffusion coefficient can be obtained by
D ) L2/(2.77τC)
(2)
Note that fitting with the exponential function is empirical, and it is not derived analytically. The electron lifetime at open circuit has been measured from the photovoltage response of DSCs against the perturbation of light intensity. Under the conditions of uniform electron generation and constant D and τ values against the position x in the DSCs, the rate equation of electron density, n(x,t) becomes
dn(t)/dt ) G(t) - n(t)/τ
(3)
where G(t) is the electron generation rate. When the light intensity is stepped down G(t) ) Gi when t < 0 and G(t) ) Gf when t g 0, where Gi and Gf are the generation rates corresponding to the light intensity before and after the stepwise change. By setting the final intensity as zero, i.e., neglecting the time-independent electron density by Gf, the G(t) in eq 3 is dropped. Note that eq 3 is valid only under the small perturbation of electron density, since τ has electron density dependence. The solution of eq 3 is simply
n(t) ) A exp(-t/τ)
Dye-Sensitized Solar Cells
Figure 1. Typical current responses of DSCs against the different stepped laser intensities. The thickness of the TiO2 was 6.1 µm, and the area was 0.14 cm2. Initial short circuit currents were (a) 268, (b) 75, (c) 30, and (d) 8 µA. The inset shows the electron diffusion coefficients in DSCs obtained by two different light perturbation methods. The short circuit current of the inset shows the steady current generated by the initial intensity of the diode laser (step) and by the intensity of the He-Ne laser (pulse).
4. Results and Discussion 4.1. Validity of the Method Using Stepwise Change of Laser Intensity. To check the validity of the modified method, we measured identical DSCs using the standard electrolyte by the two different methods and compared the results. Figure 1 shows the current transients induced by the stepwise change of laser intensities. Less than 10% of the initial Jsc was reduced by the change. The time to reach a constant Jsc became longer as the initial laser intensity was decreased. All the current transients were fitted well with a single-exponential function. The values of D under different initial light intensities, derived by eq 2, are shown in the inset of Figure 1. The two closed symbols in the inset correspond to two samples prepared by the same procedure. Excellent agreement was obtained with the values derived by the pulsed-laser-induced current transients from identical samples. We note that if the initial condition could be approximated as a uniform distribution of electron density in the TiO2 electrode, i.e., n(x,0) is a constant, and there would be no boundary at the TiO2/TCO interface, the diffusion equation can be solved analytically, and n(x,t) can be expressed using the error function. Then, current transients can be obtained by D∂n(x,t)/∂x|x)0. We have fitted the analytical solution to the measured transients, and the result was very poor, especially at the early phase of the transients. This result implies that the approximation of the initial condition was not appropriate and/or the RC time constant, which would be on the order of 10-4 s,18,19 originating at the TiO2/TCO interface limited the fast electron flow at the initial stage of the transient. For Voc transients induced by the same laser used for the current transients, all transients were fitted well by eq 4 (Figure 2). Duffy et al., reported that photovoltage transients induced by a pulsed laser at λ ) 532 nm had non-single-exponential decay.20 At the wavelength of 532 nm, the dye absorbs the light strongly so that higher concentrations of electrons are generated near the TiO2/ TCO interface. For this case, Voc decay is caused by the recombination and diffusion moving away from the (18) Franco, G.; Gehring, J.; Peter, L. M.; Ponomarev, E. A.; Uhlendorf, I. J. Phys. Chem. B 1999, 103, 692. (19) Nakade, S.; Saito, Y.; Kubo, W.; Kanzaki, T.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2004, 108, 1628. (20) Duffy, N. W.; Peter, L. M.; Wijayantha, K. G. U. Electrochem. Commun. 2000, 2, 262.
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Figure 2. Typical open circuit voltage transients induced by stepped laser intensity. The initial voltages were (a) 0.65, (b) 0.60, (c) 0.57, and (d) 0.51 V. The inset shows the electron lifetime obtained with two different light perturbation methods. The thickness of the TiO2 films was about 6 µm. Measurements were performed at open circuit, and the bottom axis shows the Jsc under the same intensity used for the Voc transient measurements.
interface. The single-exponential decay observed at the 635 nm laser excitation here suggests that uniform electron distribution was obtained so that the influence of electron diffusion can be ignored. The inset of Figure 2 plots the electron lifetimes derived by the method and by intensity-modulated photovoltage spectroscopy. The values were in good agreement between the two methods. The typical measurement time to obtain the inset was 15 min by SLIM, while 90-120 min was needed by IMVS. Since the time to measure the single transient is at most a few seconds, the measurement time by SLIM can be even shortened by automating the tedious procedures, such as changing the ND filter and recording the value of Jsc, with a personal computer. 4.2. I-V Characteristics. I-V characteristics of the DSCs with various concentrations of LiI and TBAI in PC or AN are summarized in Figure 3. All were measured under one-sun conditions (100 mWcm-2, AM 1.5). All samples were dissolved with 50 mM I2, and the TiO2 thickness was 6.0 ( 0.4 µm. The highest efficiency was 5.2% from 0.5 M TBAI with AN. Among the electrolytes using PC, it was 4.3% at 0.5 M TBAI. The value was comparable to the 4.5% efficiency of DSCs prepared with the standard electrolyte in methoxylacetonitrile. In comparison between Li+ and TBA+, Li+ gave larger Jsc values. However, the lower Voc, which was caused by the positive shift of the conduction band potential due to Li+ adsorption on the TiO2 surface, limited the efficiency.5,11 The lower fill factor of the DSCs with Li+ seemed also consistent with the adsorption properties of Li+, i.e., a decreased amount of mobile cations by the adsorption. In comparison between AN and PC, the Jsc of the DSCs using PC was lower than that using AN, and the difference was decreased with the increase of the electrolyte concentration. 4.3. Influence of Viscosity on D and τ. Figure 4 shows the values of D in DSCs using PC and AN, which were obtained by stepped light-induced photocurrent transients. Within experimental error, which is ca. (20% of the mean value, no difference was observed between the two solvents. On the basis of the ambipolar diffusion, the diffusion coefficient in DSC can be approximated to that of the electron, when the density of positive charges is much larger than that of electrons.14 Thus, the decreased ion diffusion in PC did not influence the D as observed in Figure 4.
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Figure 5. Electron lifetime at open circuit condition in DSCs using various concentrations of LiI or TBAI in AN or PC; 50 mM I2 was dissolved for all conditions. The bottom axis shows the Jsc measured at the same intensity as that used for the lifetime measurements at open circuit.
Figure 3. Current-voltage characteristics of DSCs using various concentrations of LiI or TBAI in AN or PC; 50 mM I2 was dissolved for all conditions. Measurements were performed under simulated AM 1.5 conditions at 100 mW/cm2 of irradiation power.
Figure 4. Electron diffusion coefficients in DSCs with Li or TBA in AN or PC. Concentrations of LiI or TBAI were 0.5 M, and 0.05 M I2 was dissolved for all conditions.
The electron lifetimes in the DSCs using Li+ or TBA+ in PC or AN are plotted in Figure 5. The DSCs with PC showed a longer electron lifetime than that with AN. Since there are two possible recombination paths in DSCs, eq 3 can be more precisely written by
n n ∂n )G- ∂t τI τD
(5)
where τI is the lifetime determined with I3- and τD is the lifetime determined with dye+. The measurable lifetime here is the reciprocal of (1/τI + 1/τD). The τI is influenced by the concentration of I3-, the distance between I3- and the TiO2 surface, and the collision frequency between e-TiO2 and I3-, and the τD is influenced by the concentration of
dye+, which is related with the collision frequency between dye+ and I-, and the concentration of I-. When dye+ is reduced by I- faster than by electrons in TiO2, τD becomes large, and the term of n/τD can be dropped from eq 5. Here, the difference of the reorganization energy due to the difference of the two solvents was about 30 mV, and thus, we neglected the influence. When PC is employed instead of AN, the difference of viscosity should directly reflect as the decrease of the collision frequency between e-TiO2 and I3-. This would increase the τI by the same degree of the viscosity difference. When a high concentration of LiI was dissolved, where the term of n/τD can be ignored, the difference of the lifetime was comparable to that of the viscosity. Other conditions examined here resulted in a moderate increase of the lifetime, suggesting that n/τD cannot be ignored. Kinetic competition of the dye+ reduction rate in PC has been studied by Montanari et al., and they reported that more than 30 mM I- with 0.1 M Li+ was needed to dominate the reaction with dye+ by I-.21 As for the influence of the cation on the dye+ reduction, Pelet et al. found that employing TBA+ resulted in 1 order of degree slower dye+ reduction rate than Li+.22 On the basis of these reports, more than 300 mM TBAI would be needed for efficient dye+ reduction in PC. This is consistent with the observed increase of Jsc with the increase of TBAI up to 0.5 M in PC. In comparison between Li+ and TBA+ in PC, the lifetime at 0.3 M TBAI showed shorter values than that with 0.3 M LiI,23 agreeing also to the slower dye+ reduction with TBA+. Another influence of viscosity on the lifetime may be that the lower mobility of the anions in PC would increase the surface concentration of I3-, which is formed by the reduction of dye+ by I-. This could also limit the increase of the electron lifetime in PC. 4.4. Optimization Guide of DSC. The previous section suggested that the dye+ reduction rate in PC was more critical than in AN, requiring higher I- concentration, while increases of τ without decreasing D were confirmed. In Figure 3, the lower efficiency of DSCs using PC was due to the lower Jsc and fill factor. In comparison between (21) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203. (22) Pelet, S.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2000, 104, 1791. (23) In a previous paper, we reported that TBA+ showed a longer lifetime than Li+. This was explained with the location of the cations, i.e., either outside or inside the layer of the adsorbed dye (ref 11). At the concentration of 0.5 M or above in AN, the reproducibility of the results is good. At the concentration of 0.1 M, the results with TBA+ had a larger deviation among different batches of samples. This is probably caused by the difference of the dye adsorption condition, e.g., the lifetime decreases with the decrease of the amount of adsorbed dye, and the measurements are sensitive at the low concentration of TBAI.
Dye-Sensitized Solar Cells
Li+ and TBA+, Jsc and Voc are inversely related. Thus, one direction of optimization should be to mix the two cations at the appropriate ratio to obtain large Jsc and Voc. Addition of Li+ into TBA+/PC would also help increase τ by increasing the dye+ reduction rate. To increase the fill factor, the use of TBAI with more than 0.5 M may not contribute to the increase of the conductivity in the pore of the TiO2 electrode, because of the difficulty in their infiltration in the pore due to the large size of the cation. Thus, the addition of smaller-sized and less adsorptive cations, such as imidazolium cations, would be preferred for this purpose. For the DSCs’ electrolytes in low viscosity solvents, LiI, imidazolium iodide, and tert-butyl pyridine have been the common composition and have been applied to various types of DSCs without optimization. Recently, for AN, we found an alternative composition using quaternary ammonium cation, which increased the electron lifetime and, consequently, the energy conversion efficiency of DSCs.6 For highly nonvolatile but viscous electrolytes, the results here strongly indicate that there is a large room for optimization to achieve both high energy conversion efficiency and long-term durability of DSCs. 5. Conclusions We introduced stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV) to measure electron diffusion coefficients and lifetimes. It
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was confirmed that the measured values by the methods were identical with those obtained by pulsed-laser-induced current transients and intensity-modulated photovoltage spectroscopy, while the new methods required simpler optical setup and shorter measurement time. Performance of the DSC using a highly viscous electrolyte was analyzed by SLIM-PCV. By systematic study of the influence of solvent viscosity, the species of cations, and the electrolyte concentrations, we concluded that the dye cation reduction rate is the limiting factor for highly viscous electrolytes, while comparable electron diffusion coefficients and a longer electron lifetime were observed. These results suggest that by controlling the dye+ reduction rate and taking advantage of the longer electron lifetime, further optimization of the electrolyte composition and concentrations in highly viscous solvents should be possible. The quick analysis of D and τ by SLIM-PCV will contribute especially for this purpose. Acknowledgment. This research was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry and by the International Joint Research Grant Program (No.2002RB048) from the New Energy and Industrial Technology Develop Organization (NEDO). LA051257J
