J. Phys. Chem. C 2010, 114, 13113–13117
13113
Light Intensity Independent Electron Transport and Slow Charge Recombination in Dye-Sensitized In2O3 Solar Cells: In Contrast to the Case of TiO2 Shogo Mori* and Akihiro Asano DiVision of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan ReceiVed: March 3, 2010; ReVised Manuscript ReceiVed: June 8, 2010
Dye-sensitized solar cells (DSCs) were fabricated from In2O3 nanoparticles with an electrolyte containing the I-/I3- redox couple. Current transients were measured by a pulsed laser and a diode laser, showing light intensity independent current transients, which have not been seen from dye-sensitized TiO2, ZnO, and SnO2 solar cells. The response of the current transients was altered when bias potential was applied, suggesting that the electron transport is not due to diffusion. On the other hand, the lifetime of injected electrons in the In2O3, while showing longer values than electrons in TiO2, displayed similar light intensity dependence to that observed in TiO2 DSCs, suggesting the interfacial electron transfer occurs in the Marcus normal region. Introduction Dye-sensitized solar cells (DSCs) consist of dye-adsorbed porous electrodes immersed in an electrolyte containing a redox couple, sandwiched between transparent conductive glass and platinum electrodes. The electrodes have been prepared mostly from nanosized TiO2 particles because of their suitable conduction band edge potential (Ecb) relative to the LUMO levels of sensitizing dyes. In the DSCs, electrons are excited in the dyes and injected into the conduction band of TiO2, while the resulting dye cations are reduced by I- in the electrolyte. Since the porous electrode is surrounded by a high concentration of cations, the transport of the injected electrons in the porous TiO2 electrode has been interpreted with ambipolar diffusion.1,2 The diffusion coefficients (D) have been measured by various groups, and show values a few orders of magnitude lower than those of single crystal TiO2. Diffusion coefficients can be derived for example from current transients,3–5 open circuit voltage transients,6 and surface photovoltage transients,7 induced by perturbation of the irradiated light intensity. The current responses were well fitted by the solution of a diffusion equation,4,8 and the responses were not influenced by applied bias potential,4 indicating diffusion limited electron transport. Derived diffusion coefficients increased with the increase of light intensity. These observations have been modeled with intraband charge traps.9–11 The electron lifetime (τ) of the injected electron in the TiO2 is another important parameter, and has also been examined by various groups. In contrast to the diffusion coefficient, the lifetime deceases with the increase of light intensity.12 The D and τ responses to perturbations in light intensity seem to be inversely related,13 leading a model of trap limited recombianion.14–16 The existence of a high density of intraband charge traps has been expected for nanoporous TiO2 electrodes, due to their high surface area17,18 and large number of boundaries.17,19 Dye-sensitized solar cells have been fabricated not only from TiO2, but also from other metal oxides, such as ZnO,20,21 SnO2,22 and In2O3.23 One of motivations of using these materials comes from their higher electron mobility compared to that of TiO2. For SnO2 and In2O3, their lower Ecb, that is, more positive * To whom correspondence should be addressed. E-mail: shogmori@ shinshu-u.ac.jp.
potential, than those of TiO2 and ZnO are another attractive property,23 since lower Ecb values can sometimes be required to allow injection from dyes having lower LUMO levels. Employing SnO2 and In2O3 may be essential for some sensitizers, especially for the dyes absorbing infrared light. The diffusion coefficients and lifetimes have been examined for DSCs employing ZnO20 and SnO2,22,24 as well as DSCs constructed by using various TiO2 electrode preparation methods.17,25 To date, all of them have displayed similar light intensity dependence, suggesting the trapping model would be applicable in general to nanoporous metal oxide materials. In this paper, we show light intensity independent electron transport in dye-sensitized In2O3 solar cells. In addition, longer electron lifetimes in the In2O3 DSCs compared to the TiO2 DSCs are demonstrated. Experimental Section Dye-sensitized solar cells were prepared by standard processes (Supporting Information). A paste containing In2O3 was prepared by dispersing In2O3 nanoparticles (nanopowder, mean radius, 15-25 nm, Aldrich) in distilled water with acetyl acetone, Triton-X, and PEG20000. The TiO2 paste (Nanoxide-T) was purchased from Solaronix. A standard Ru complex dye (N719) was used for all the samples. The area of the sensitized films was ca. 0.1 cm2. The electrolyte consisted of 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI), 0.5 M 4-tert-butylpyridine (tBP), and 0.05 M I2, in dehydrated acetonitrile. I-V characteristics were measured under simulated one sun conditions (AM1.5, 100 mW/cm,2 YSS-100A, Yamashita Denso) with a mask. Electron diffusion coefficients and lifetimes were measured by stepped light induced photocurrent and voltage transients.5 In short, the whole working area of the DSCs was irradiated by a diode laser (635 nm, Lablaser, Coherent), and a small proportion of the laser intensity was stepped down. Induced transients were measured by a multimeter with data storage (AD7461A, Advantest). Time constants were obtained by fitting to a single exponential function. Thin electrodes were used to generate photoelectrons uniformly through the film. Pulsed laser induced current transients were measured by Nd:YAG laser (Spectra-Physics, INDI-50, 7 ns, 532 nm, 10 Hz), an oscil-
10.1021/jp1019203 2010 American Chemical Society Published on Web 07/08/2010
13114
J. Phys. Chem. C, Vol. 114, No. 30, 2010
Mori and Asano
Figure 2. I-V curves of DSCs under one sun conditions. Electrodes were N719 sensitized In2O3 and TiO2. Thicknesses were ca. 2.8 µm.
Figure 1. SEM image of In2O3 electrode.
loscope (Tekronix TDS3052), and a current amplifier (Stanford Research Systems, SR570). A mechanical shutter was used to irradiate a pulse with sufficient interval between measurements to avoid the residual electrons by a previous pulse. The laser was applied from the Pt side of the DSCs. Thick electrodes were used for this measurement to generate photoelectrons mostly at the Pt side of the electrode and to observe the transient time clearly. Time constants were obtained by fitting a single exponential function to the decay of the current.8 Bias potential was applied by a potentiostat with a two-electrode configuration. These measurements were repeated under various laser intensities down to that corresponding to about 1/50 of one sun conditions. Electron densities (n) at open circuit were measured by a charge extraction method.26 Measurements were repeated again under various light intensities to obtain the relationship between Voc and n. Electron density was estimated from the charge obtained by numerical integration of the transients divided by the volume of the DSC electrodes. Thin electrode DSCs were used for this measurement. Impedance spectra of the DSCs were measured by a potentiostat and a frequency response analyzer (Solartron 1287, 1255B) under various light irradiations (Halogen lamp) at opencircuit conditions. The amplitude was 10 mV and frequency range was 100 K to 0.02 Hz. Impedance spectra were fitted with a simple series connection of a resistance and the unit of parallel connection of charge transfer resistance and constant phase element. The equivalent circuit was applied only for the spectra showing no Warburg feature. Results and Discussion Figure 1 shows an SEM image of the In2O3 electrode. Figure 2 shows I-V curves of the solar cells prepared from this electrode and a standard TiO2 electrode. Table 1 summarizes the electrode properties and I-V characteristics of the DSCs under one sun conditions (incident photon to current conversion efficiency is shown in Figure S1, Supporting Information). The In2O3 DSC showed a fill factor (FF) lower than 25%, implying the driving force and/or mechanism of charge transport differs under different circuit conditions. The low FF was also seen with 1/10 of the light intensity, suggesting that the FF is not limited by electrolyte diffusion. Dark I-V curves (Figure 2) showed diode characteristics, and thus the low FF is not due to a shunt. The low Jsc of the In2O3 cell was partially due to the lower roughness factors, that is, a lower amount of dye resulting
from less available surface area, and the lower FF, but was not due to a reduced injection efficiency. Figure 3 shows the time constant obtained from current transients induced by stepped decrease of light intensity at short circuit condition with various initial laser intensities. While the current transients became slower for the TiO2 DSCs with a decrease of the initial light intensity, little change was seen with In2O3. Current transients were also measured with different pulsed laser intensities, which also demonstrated no light intensity dependence for In2O3 cells (Figure 4). Note that the RC time constant of the system is expected to be of the order of 0.1 ms, which is obtained by the product of the series resistance (up to ∼100 Ohm) of the system and the capacitance of FTO (10 µF/cm2 multiplied by the cell area). The current spike observed from the TiO2 cell in Figure 4 is probably the current due to reorganization of charges at the contact27 and the time constant was determined to be 0.2 ms. Therefore, the transient measurements observed here are not limited by the RC time constant. Figure 5 shows the current transients induced by a laser pulse under different applied potentials. The applied bias had little influence on the current transients for the TiO2 DSCs in the range of -0.3 to 0.3 V vs the redox potential of I-/I3-, which is consistent with a previous report.4,28 This indicates that the electrons in the TiO2 electrodes are highly screened by the electrolyte, and thus the external potential did not induce potential distribution in the TiO2. In contrast, the In2O3 DSC’s transients were significantly influenced by the external potential, suggesting that the current is electric field driven. We note that the electron transport in the TiO2 seems partially influenced by potential difference due to the Fermi level gradient in the electrode. However, for the In2O3 DSCs we conclude that the much stronger influence of applied bias on the transients indicates that the current is mostly due to drift at working circuit conditions. Since the electron transit time became longer when the circuit condition was changed from short to open circuit, the electron collection efficiency is expected to be dependent on the conditions. This appears to be correlated with the low FF less than 0.25 observed from the In2O3 DSCs. Figure 6 shows the electron lifetime as a function of electron density in the DSCs. In contrast to the current transients, the lifetime in the DSCs/In2O3 showed similar light intensity dependence and larger values at matched electron densities in comparison to those in the TiO2 DSCs. Such noninversely related electron transport time and lifetime has not been observed in DSCs employing other oxides such as TiO2, ZnO, and SnO2. Impedance spectra of the solar cells were also measured (Figure 7) to provide further confirmation of the above results. Magnified plots for the high-frequency region are shown in
Fabrication of Dye-Sensitized Solar Cells
J. Phys. Chem. C, Vol. 114, No. 30, 2010 13115
TABLE 1: Physical Properties of Porous Electrodes and I-V Characteristics under One Sun Conditions In2O3 TiO2 a
thickness/µm
BET area/m2 g-1
porosity
roughness factor
Jsc/mA cm-2
Voc/V
FF
efficiency/%
2.2 2.8
25 77a
73 60a
285 336
3.9 5.8
0.41 0.76
0.20 0.70
0.32 3.1
Data from Nakade, S. et al. Electrochem. Commun. 2003, 5, 804.
Figure 3. Time constant of current transients measured from DSCs. Currents were induced by stepped light intensity. Electrodes were N719 sensitized In2O3 (circle) and TiO2 (diamond). Thicknesses were ca. 2.8 µm.
Figure S2 (Supporting Information). Series resistance for both In2O3 DSCs and TiO2 DSCs is around 20 Ohms. For the In2O3 DSCs, no Warburg feature, which is related to electron diffusion in the semiconductor,16,29 was observed. In contrast, the TiO2 DSCs did display a Warburg feature under low light intensity (or at low Fermi levels in the TiO2). The frequency giving the top of the large arc of the impedance spectra was related to the lifetime observed by photovoltage decay though τ ) 1/ω. Thus, the arc was assigned to the impedance of interfacial charge
Figure 6. Electron lifetime in DSCs. Electrodes were N719 sensitized In2O3 (circle) and TiO2 (diamond). Thicknesses were ca. 2.8 µm.
transfer. Fitted values of the interfacial charge transfer resistance (Rc) are plotted in Figure S3 (Supporting Information) as a function of Fermi level with respect to the values of Ecb, which were assumed to be 0.45 and 0.85 V (with respect to the electrolyte redox potential) for In2O3 and TiO2, respectively. The values of Rc for In2O3 cells were higher than those of the TiO2 cells, which is consistent with the results obtained from the photovoltage transients. The reported conduction band edge of In2O3 is located ca. 0.6 V more positive than that of TiO2.23,30 At matched electron
Figure 4. Current transients from DSCs induced by a nanosecond laser pulse. Intensity of the laser pulse was varied by ND filters. Thicknesses were ca. 11.5 and 12.7 µm for In2O3 and TiO2, respectively. The transient becomes slower with the decrease of the pulse intensity for TiO2 cells, while the same transient was observed regardless of the pulse intensity for In2O3 cells.
Figure 5. Current transients from DSCs induced by a pulsed laser under various bias potentials applied on the DSCs. Electrodes were N719 sensitized In2O3 (a) and TiO2 (b). Thicknesses were ca. 11.5 and 12.7 µm for In2O3 and TiO2, respectively. An ND filter (ND25) was used for In2O3.
13116
J. Phys. Chem. C, Vol. 114, No. 30, 2010
Mori and Asano
Figure 7. Impedance spectra of DSC/In2O3 (a) and/TiO2 (b) under different light intensities.
Figure 8. Open circuit voltage as a function of electron density of the DSCs. Electrodes were N719 sensitized In2O3 (circle) and TiO2 (diamond).
density in the DSCs, the Voc for In2O3 DSCs was ca. 0.4 V lower than that for TiO2 DSCs (Figure 8). The Voc scales logarithmically with electron density with respect to the Ecb. At the same electron density, the Fermi levels with respect to the Ecb are expected to be similar for both materials. Thus, the different Voc at the same electron density suggests the difference in Ecb between the TiO2 and In2O3, which is in agreement with previous reports. Note that we have assumed similar trap densities and distribution between the two materials here, whearas the different slopes in Figure 8 suggest a small difference exists in the distribution. However, the different distribution is too small to explain the difference in the Voc at matched electron density. Electron lifetime in the DSCs can be affected by two different mechanisms, which include trap limited and interfacial transfer limited processes. To explain the long electron lifetime observed with In2O3 DSCs, interfacial transfer seems to be having an affect on the lifetime. Using Marcus theory, the longer lifetime can be interpreted as arising from a smaller free energy difference between the conduction band electrons and I3(schematic diagram shown in Figure S4, Supporting Information). Light intensity dependent lifetime behavior for TiO2 DSCs has been interpreted as arising from trap limited recombination.14,31 This model describes the density of conduction band electron as determined by the rates of trapping and detrapping, with recombination occurring when conduction band electrons meet acceptors at the TiO2/dye/electrolyte interface. For the case of In2O3, this trap limited recombination model seems to explain the observed light intensity dependence. For the case of the light intensity independent current transients, a trap limited diffusion model cannot explain the results. Current transients were also measured for different
electrode thicknesses and immediate current peaks were found from both thin and thick In2O3 cells. In contrast, the TiO2 cells showed a slower current peak time with the thicker cell (Figure S5, Supporting Information). This implies that there are nearly free electrons in the In2O3 at short circuit under dark conditions, i.e., the observed transient current was due to the electrons located close to FTO. The existence of free electrons could be due to the small difference between the Fermi level of FTO and Ecb of In2O3, resulting in the possibility of metallic behavior for In2O3. When positive bias was applied to the In2O3 cells (Figure 5a), the transients become diffusion-like. This could be due to fewer electrons in the In2O3 film. Note that although the potential difference and the electron density are related in some sense, their influences on the transport seem to be considered separately. Another possible reason could be a smaller dielectric constant and/or faster electron transport than the dielectric relaxation time of In2O3. We would like to point out that similar behavior was seen for SnO2 DSCs when the Ecb was shifted positively by employing an electrolyte containing LiI and I2,32 and was also seen for WO3 DSCs,33 which have a similar conduction band edge potential to In2O3. These results show that the light intensity independent and bias potential dependent current observed with In2O3 are not intrinsic properties of In2O3, but seem to be generally associated with the potential difference between FTO and Ecb, and/or the mobility and dielectric constant of the semiconductor. In practice, to improve the FF, this mechanism needs to be clarified. Conclusions Dye-sensitized In2O3 solar cells (DSCs) using an electrolyte containing the I-/I3- redox couple showed light intensity independent but bias potential dependent current transients. This suggests that the electron transport in these devices is not due to diffusion, indicating that a high concentration of electrolyte species does not guarantee screening in the nanoporous electrodes. The potential dependent current transients seem to be related to the very low fill factor measured for the In2O3 DSCs. The electron lifetime in In2O3 DSCs also showed longer values than those of the TiO2 DSCs. These results imply that the potential difference between the conduction band edge and redox couples has significant influence on both the electron transport and recombination. Acknowledgment. We thank Mr. Shingo Takano for stimulating this work. This paper is dedicated to the first principal, Chotaro Harizuka, on the occasion of the 100th anniversary of Faculty of Textile Science and Technology, Shinshu University. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
Fabrication of Dye-Sensitized Solar Cells Supporting Information Available: IPCE, impedance spectra, and transient currents. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kopidakis, N.; Schiff, E. A.; Park, N. G.; Van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (2) Nakade, S.; Kambe, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 9150. (3) Cao, F.; Oskam, G.; Meyer, G. J.; Searson, P. C. J. Phys. Chem. 1996, 100, 17021. (4) Solbrand, A.; Henningsson, A.; So¨dergren, S.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1999, 103, 1078. (5) Nakade, S.; Kanzaki, T.; Wada, Y.; Yanagida, S. Langmuir 2005, 21, 10803. (6) Nakade, S.; Saito, Y.; Kubo, W.; Kanzaki, T.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2004, 108, 1628. (7) Anta, J. A.; Mora-Sero, I.; Dittrich, T.; Bisquert, J. J. Phys. Chem. C 2007, 111, 13997. (8) Nakade, S.; Kubo, W.; Saito, Y.; Kanzaki, T.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 14244. (9) Nelson, J. Physical ReView B - Condensed Matter and Materials Physics 1999, 59, 15374. (10) Van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2001, 105, 11194. (11) Bisquert, J. J. Phys. Chem. C 2007, 111, 17163. (12) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (13) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949. (14) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. B: Condens. Matter Mater. Phys. 2001, 63, 2053211. (15) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. J. Am. Chem. Soc. 2004, 126, 13550. (16) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero, I.; Garcia-Belmonte, G.; Gimenez, S. J. Phys. Chem. C 2009, 113, 17278. (17) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607.
J. Phys. Chem. C, Vol. 114, No. 30, 2010 13117 (18) Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 1. (19) Mori, S.; Sunahara, K.; Fukai, Y.; Kanzaki, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. C 2008, 112, 20505. (20) Quintana, M.; Edvinsson, T.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035. (21) Willis, R. L.; Olson, C.; O’Regan, B.; Lutz, T.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 7605. (22) Fukai, Y.; Kondo, Y.; Mori, S.; Suzuki, E. Electrochem. Commun. 2007, 9, 1439. (23) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (24) Green, A. N. M.; Palomares, E.; Haque, S. A.; Kroon, J. M.; Durrant, J. R. J. Phys. Chem. B 2005, 109, 12525. (25) 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. (26) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. Electrochem. Commun. 2000, 2, 658. (27) Solbrand, A.; Lindstro¨m, H.; Rensmo, H.; Hagfeldt, A.; Lindquist, S. E.; So¨dergren, S. J. Phys. Chem. B 1997, 101, 2514. (28) It has been reported that the electron transient time depends on the light intensity. In other words, the transient time depends on the electron density, and this means the transient also should be changed by applied potential, which can control the electron density in the electrode. This is true when the applied potential results in the injection of electrons from FTO to TiO2. For the case of the TiO2 cell in Figure 5, there was little injection, and thus the electron density in the TiO2 was just the one generated by a laser pulse for all applied bias conditions. (29) He, C.; Zhao, L.; Zheng, Z.; Lu, F. J. Phys. Chem. C 2008, 112, 18730. (30) Furube, A.; Murai, M.; Watanabe, S.; Hara, K.; Katoh, R.; Tachiya, M. J. Photochem. Photobiol., A 2006, 182, 273. (31) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 11307. (32) Fukai, Y., Mori, S. Unpublished results. (33) Hara, K. Manuscript is in preparation.
JP1019203
