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Investigations on anodic photocurrent loss processes in dye sensitized solar cells: comparison between nanocrystalline SnO2 and TiO2 films
4 October 2002
Chemical Physics Letters 364 (2002) 297–302 www.elsevier.com/locate/cplett
Investigations on anodic photocurrent loss processes in dye sensitized solar cells: comparison between nanocrystalline SnO2 and TiO2 films Yasuhiro Tachibana a
a,*
, Kohjiro Hara a, Shingo Takano b, Kazuhiro Sayama a, Hironori Arakawa a,1
Photoreaction Control Research Centre (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan b Sumitomo Osaka Cement Co. Ltd., 585 Toyotomi, Funabashi, Chiba 274-8601, Japan Received 23 July 2002; in final form 23 July 2002
Abstract Anodic photocurrent loss processes have been investigated for dye sensitized nanocrystalline SnO2 and TiO2 solar cells by means of electrical bias dependent photocurrent spectroscopy. Electron injection efficiency is not dependent on visible excitation wavelengths under electrical bias application for both films. Under an identical cell configuration, the unsensitised SnO2 film exhibited cathodic photocurrent generation while no cathodic photocurrent was detected for the TiO2 film. The origins of these observations are discussed. The experimental results suggest that the electron transfer reaction from the SnO2 to the electrolyte must be accelerated at the SnO2 surface by the bias application. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Nanocrystalline semiconductor films sensitised by dyes have acquired considerable interests in fundamental studies of a semiconductor/electrolyte interface [1] and technological applications such as photovoltaic devices [1,2]. The best performance of the dye sensitized solar cell to date has been achieved by a combination of a TiO2 film and
*
1
Corresponding author. Fax: +81-0-298-61-6771. E-mail address: [email protected] (Y. Tachibana). Also corresponding author.
a ruthenium dye with an overall energy conversion efficiency of 10% [2]. The functions of the solar cell are characterized by several parameters, such as the short circuit photocurrent (Jsc ), the open circuit photovoltage (Voc ), the fill factor (FF) and the incident photonto-current conversion efficiency (IPCE). The IPCE results from a product of light harvesting efficiency (LHE), electron injection efficiency and charge collection efficiency [2,3]. If electron injection and charge collection efficiencies are constant throughout whole visible wavelengths, the IPCE spectrum can be directly correlated with an absorption spectrum of a specific material. If an LHE
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 1 0 - 6
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spectrum is known, dependence of the electron transfer efficiency on the excitation wavelength can be determined. Thus, the photocurrent generation or loss mechanism can be identified by an analysis of the IPCE spectrum. The photocurrent loss is generally observed under electrical bias application to a solar cell owing to dark current flow. However, in reality, the photocurrent tends to decrease at lower voltages than expected. This is recognized as an anomalous photoeffect [1]. This non-ideal behaviour was observed for the dye sensitised solar cell under illumination [4,5], implying the existence of the photocurrent loss processes. Intensive studies have been carried out to search for an alternative semiconductor to the TiO2 . Although composite semiconducting materials [6,7] as well as single materials [8,9] were employed, they appeared to be inferior to the TiO2 due mainly to a lower open circuit photovoltage and fill factor. Such a low voltage can be related to the large reverse dark current or the photocurrent loss, or both. However, the mechanism of these loss processes has not yet been established in detail. In this Letter, we present comparative studies of IPCE spectra under electrical bias application to the solar cells using different semiconductor films. Nanocrystalline SnO2 films were chosen because relatively less photocurrent and photovoltage were obtained compared to the TiO2 cells [2,8]. Identification of the origins reducing the photocurrent and the photovoltage should provide the key parameters improving the solar cell performance.
2. Experimental Anatase TiO2 nanocrystalline films were prepared using a screen printing technique as described previously [10]. The paste includes 60 wt% acidic solution (average diameter: 14 nm) and 40 wt% basic solution (average diameter: 34 nm). Rutile SnO2 films were prepared by the similar way. Sol– gel synthesized small SnO2 nanoparticles (diameter: 10–20 nm) and commercial large particles (Kishida Chemical) with a diameter of 0.5–1.0 lm were mixed in the ratio of 8:2 to prepare the paste. The film was deposited onto a fluorine-doped SnO2 (FTO) con-
ducting glass (Nippon Sheet Glass, Japan, sheet resistance: 8–10 X=cm2 Þ, and calcined at 500 °C for 1 h in air. The sintered film (thickness: 8–12 lm for the TiO2 and 8 lm for the SnO2 ) was immediately sensitized by the ruthenium dye, ðtetrabutylammoniumÞ2 cis-(2,20 -bipyridyl-4-COOH, (40 -COO-)2 ðNCSÞ2 ruthenium(II) (Solaronix, Switzerland), and then employed to fabricate a sandwich-type solar cell as described previously [10]. IPCE measurements were conducted under applications of external electrical bias and AM1.5 simulated white light bias using a photocurrent measurement system (Bunko Keiki, CEP-99W). This condition was distinctively employed to monitor the photocurrent under the cell operation. Monochromatic light with a constant number of photons ð1016 =cm2 s) except for < 370 nm was modulated by a mechanical shutter with a repetition rate of 2 Hz under these biases and the photocurrent was measured using a lock-in amplifier. This repetition rate was determined since the modulation larger than 10 Hz resulted in a lower IPCE due probably to the slower electron trap filling in the film [11]. The IPCE was calculated following the definition [2,3] as below: IPCEðkÞ ¼ 1240 Jph =ðk F ðkÞÞ; where k is the monochromatic light wavelength, Jph is the photocurrent density and F ðk) is the incident light power density at wavelength k. The potential was applied negatively to the working electrode against the counter electrode, however, for simplicity the data are shown with positive values. Indistinguishable IPCE spectra at the short circuit were observed before and after a set of bias measurements to confirm the reproducibility. The IPCE data were not corrected for reflection and absorption loss by the FTO. Current–voltage (I– V) characteristics of the cell were measured using an AM1.5 simulated Xe lamp light source (Yamashita Denso K.K., YSS-150A, light power: 100 mW=cm2 Þ.
3. Results and discussions The I–V characteristics of the solar cells are shown in Fig. 1. The resultant cell factors: Jsc , Voc
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Fig. 1. The I–V characteristics of dye sensitised semiconductor solar cells. The photo responses were observed for the TiO2 film ( –) and the SnO2 film (- - - - - - -) under a simulated AM1.5 solar spectrum. The dark I–V curves for the cells based on the TiO2 film ( Þ and the SnO2 film ( – –) are shown for comparison.
Fig. 2. IPCEs normalized to unity as a function of electrical bias applied to the dye sensitised TiO2 solar cell. This normalization is based on the assumption that the photocurrent is identical under the short circuit at any wavelengths. The inset shows IPCE spectra under the electrical bias application.
and FF are 13.5 mA/cm2 , 0.72 V and 0.74 for the dye sensitized TiO2 cell while the SnO2 cell exhibits 9.5 mA/cm2 , 0.38 V and 0.46, respectively. A crossover of photo and dark curves was observed at 0.42 V for the SnO2 cell, indicating anomalous photoeffects [1]. It is apparent that the photocurrent observed for the SnO2 electrode decreases in the voltage range of 0–0.2 V where the dark current is not significantly affected. Note that the photo and dark curves for an FTO only exhibit negligible current generation at < 0:6 V. In this study, the IPCEs were obtained for the photocurrent induced only by the monochromatic light since the current generated by the electrical and white light bias applications was eliminated by the lock-in amplifier. The inset of Fig. 2 shows the IPCE spectra observed under different electrical biases applied to a dye sensitised TiO2 cell. Comparison of these IPCEs at different wavelengths was achieved by normalizing and plotting the data as a function of applied bias as presented in Fig. 2. The photocurrent responses were almost identical over a wavelength range of 450–750 nm with a sharp decrease at an applied bias of > 0:5 V. The
half-value of its maximum is located around 0.7 V (near the open circuit voltage), in consistence with the previous report [4]. In contrast, the lower IPCE was observed at 350 nm, in which case holes as well as electrons can be generated in the TiO2 by the band gap excitation. The generated holes may create additional electron transfer paths from the TiO2 to the electrolyte, increasing the reverse current (or charge recombination). Since the electron transport kinetics (following the electron injection) should be identical at any excitation wavelengths and a number of injected electrons are normalized at the short circuit (Fig. 2), these results suggest that the electron injection efficiency is independent of an excitation wavelength upon the electrical bias application. Assuming the electrolyte redox potential is +0.2 V vs SCE [12], the bias application of 1.0 V leads to a Fermi level of the TiO2 at 0:8 V vs SCE corresponding roughly to the dye excited state potential [12]. The electron injection rate is sufficiently fast even at this Fermi level; for example, the halfinjection time of 10 ps was observed at about 0:75 V vs SCE [13]. Moreover, the charge
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recombination between the electron and the oxidized dye is relatively slow in the electrolyte employed here [14]. Therefore, it can be concluded that the IPCE decrease upon the bias application originates most likely from an acceleration of the charge recombination rate between the injected electron and the electrolyte. The electrical bias dependent IPCEs for a dye sensitised SnO2 cell are presented in Fig. 3. With the application of > 0:4V, the negative IPCE was observed at 300–450 nm, indicating cathodic photocurrent generation (see the inset). Normalized IPCE data clearly show large cathodic photocurrent at 350 nm while the photoanodic responses are indistinguishable in a wavelength range of 470–720 nm. Note that the bias application of > 0:5 V results in decrease in this cathodic photocurrent, however, the origin of this reduction is not clear at the present. It is obvious that anodic photocurrent decreases even with the bias application of 0.1 V, which is consistent with the decrease in the current density shown in Fig. 1. Kamat et al. [8] reported that the yield of the electron injection from the dye to the SnO2 elec-
Fig. 3. Normalised IPCEs as a function of electrical bias applied to the cell based on nanocrystalline SnO2 films sensitised by the dye. The normalization method is the same as the TiO2 presented in Fig. 2. The inset shows the bias dependent IPCE spectra.
trode was not altered by the bias application of > 0:35 V vs SCE corresponding to < 0:55 V in our study. The IPCE loss can therefore be attributed to a light induced acceleration of the charge recombination kinetics between the electron and the oxidized dye or the electrolyte. The cathodic photocurrent was observed only for the dye sensitised SnO2 electrode in the UV wavelength region. This observation was investigated in more detail by measurements of bias dependent IPCEs for unsensitised TiO2 and SnO2 electrodes. Fig. 4 shows the results plotted in the same scale for ease of comparison. The TiO2 film indicates an identical spectral shape with a maximum at 340 nm at any bias applications. No cathodic photocurrent was revealed with the bias application of 0.9–1.5 V (except for negligible photocurrent flow). Lindquist and Vidarsson [15] found using a polycrystalline TiO2 that the cathodic current decreases as irradiated light intensity increases. The light intensity ð< 1016 photons/ cm2 s) we employed here may be too large to detect the cathodic photocurrent. In contrast, the SnO2 electrode exhibits remarkable difference in the IPCE spectra. The anodic photocurrent spectra are similar to the absorption spectrum of the SnO2 film [16], and thus this current generation can be ascribed to the SnO2 band gap excitation. The cathodic photocurrent appears as the bias is applied larger than 0.2 V, and the spectral maximum is shifted from 330 to 370 nm. The spectrum obtained at 0.4 V is equivalent to a calculated current density of 0.9 mA/cm2 using the standard AM1.5 spectrum [17]. This current density reduces the Jsc by 1.8 mA/cm2 if photons are not absorbed by the attached dye in this wavelength range. We consider here, two reaction processes responsible for the cathodic photocurrent generation, that is, an interfacial cathodic electron transfer following an SnO2 intraband excitation or an I photodissociation. 3 Regarding the former case, the absorption attributable to the intra-band excitation appears in a visible wavelength range under the negative bias application [6]. However, the extinction coefficient may be too low to explain this large negative IPCE at 0.4 V. The latter case is more favourable because the cathodic IPCE spectra are similar to the
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cathodic current was observed at the FTO, this reverse reaction can be enhanced by a large surface area of the SnO2 nanoporous film. Further detailed studies are necessary to clarify these issues. Nevertheless, in any case, the electron transfer reaction at the SnO2 surface must be considerably accelerated compared to the TiO2 surface under the bias application, and due to this mechanism the photocurrent decreases even if the electron injection from the dye to the SnO2 is efficient [8]. From the above discussions, we conclude that the choice of semiconductor films may be crucial to control the charge recombination kinetics and thus the anodic photocurrent. In addition, the absorption by the dye rather than the semiconductor or the electrolyte in a lower wavelength region is critical to raise the photocurrent under the bias application.
Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy Trade and Industry (METI).
References
Fig. 4. Electrical bias dependent IPCE spectra obtained from (a) the unsensitised TiO2 film, and (b) the unsensitised SnO2 film. absorption spectrum of I 3 [18,19]. The I3 excitation results in the formation of a di-iodide ion [20] which may induce the electron transfer reaction from the semiconductor to the electrolyte [21,22]. Moreover, Matsuda and Matsumoto [19] recently reported that the reaction at the FTO promotes cathodic photocurrent in the highly concentrated I 3 electrolyte. Although in our study no significant
[1] K. Rajeshwar, in: S. Licht (Ed.), Semiconductor Electrodes and Photoelectrochemistry, Wiley-VCH, Weinheim, 2002. [2] M.K. Nazeeruddin, P. Pechy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Gr€atzel, J. Am. Chem. Soc. 123 (2001) 1613. [3] G. Boschloo, A. Goossens, J. Phys. Chem. 100 (1996) 19489. [4] J. van de Lagemaat, N.G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044. [5] S. Sodergren, A. Hagfeldt, J. Olsson, S.E. Lindquist, J. Phys. Chem. 98 (1994) 5552. [6] S. Chappel, S.G. Chen, A. Zaban, Langmuir 18 (2002) 3336. [7] K. Tennakone, G. Kumara, I.R.M. Kottegoda, V.P.S. Perera, Chem. Commun. (1999) 15. [8] P.V. Kamat, I. Bedja, S. Hotchandani, L.K. Patterson, J. Phys. Chem. 100 (1996) 4900. [9] F. Lenzmann, J. Krueger, S. Burnside, K. Brooks, M. Gr€atzel, D. Gal, S. Ruhle, D. Cahen, J. Phys. Chem. B 105 (2001) 6347. [10] Y. Tachibana, K. Hara, K. Sayama, H. Arakawa, Chem. Mater. 14 (2002) 2527.
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[11] T. Trupke, P. Wurfel, I. Uhlendorf, J. Phys. Chem. B 104 (2000) 11484. [12] A. Hagfeldt, M. Gr€atzel, Chem. Rev. 95 (1995) 49. [13] Y. Tachibana, S.A. Haque, I.P. Mercer, J.E. Moser, D.R. Klug, J.R. Durrant, J. Phys. Chem. B 105 (2001) 7424. [14] S.A. Haque, Y. Tachibana, R. Willis, J.E. Moser, M. Gr€ atzel, D.R. Klug, J.R. Durrant, J. Phys. Chem. B 104 (2000) 538. [15] S.E. Lindquist, H. Vidarsson, J. Mol. Catal. 38 (1986) 131. [16] J.J. Zhu, Z.H. Lu, S.T. Aruna, D. Aurbach, A. Gedanken, Chem. Mat. 12 (2000) 2557.
[17] L.D. Partain, Solar Cells and Their Applications, John Wiley and Sons, New York, 1995. [18] M. Mizuno, J. Tanaka, I. Harada, J. Phys. Chem. 85 (1981) 1789. [19] T. Matsuda, H. Matsumoto, Electrochemistry 70 (2002) 446. [20] E. Gershgoren, U. Banin, S. Ruhman, J. Phys. Chem. A 102 (1998) 9. [21] N.W. Duffy, L.M. Peter, R.M.G. Rajapakse, K.G.U. Wijayantha, J. Phys. Chem. B 104 (2000) 8916. [22] S.Y. Huang, G. Schlichthorl, A.J. Nozik, M. Gr€atzel, A.J. Frank, J. Phys. Chem. B 101 (1997) 2576.
Chemical Physics Letters 364 (2002) 297–302 www.elsevier.com/locate/cplett
Investigations on anodic photocurrent loss processes in dye sensitized solar cells: comparison between nanocrystalline SnO2 and TiO2 films Yasuhiro Tachibana a
a,*
, Kohjiro Hara a, Shingo Takano b, Kazuhiro Sayama a, Hironori Arakawa a,1
Photoreaction Control Research Centre (PCRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan b Sumitomo Osaka Cement Co. Ltd., 585 Toyotomi, Funabashi, Chiba 274-8601, Japan Received 23 July 2002; in final form 23 July 2002
Abstract Anodic photocurrent loss processes have been investigated for dye sensitized nanocrystalline SnO2 and TiO2 solar cells by means of electrical bias dependent photocurrent spectroscopy. Electron injection efficiency is not dependent on visible excitation wavelengths under electrical bias application for both films. Under an identical cell configuration, the unsensitised SnO2 film exhibited cathodic photocurrent generation while no cathodic photocurrent was detected for the TiO2 film. The origins of these observations are discussed. The experimental results suggest that the electron transfer reaction from the SnO2 to the electrolyte must be accelerated at the SnO2 surface by the bias application. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Nanocrystalline semiconductor films sensitised by dyes have acquired considerable interests in fundamental studies of a semiconductor/electrolyte interface [1] and technological applications such as photovoltaic devices [1,2]. The best performance of the dye sensitized solar cell to date has been achieved by a combination of a TiO2 film and
*
1
Corresponding author. Fax: +81-0-298-61-6771. E-mail address: [email protected] (Y. Tachibana). Also corresponding author.
a ruthenium dye with an overall energy conversion efficiency of 10% [2]. The functions of the solar cell are characterized by several parameters, such as the short circuit photocurrent (Jsc ), the open circuit photovoltage (Voc ), the fill factor (FF) and the incident photonto-current conversion efficiency (IPCE). The IPCE results from a product of light harvesting efficiency (LHE), electron injection efficiency and charge collection efficiency [2,3]. If electron injection and charge collection efficiencies are constant throughout whole visible wavelengths, the IPCE spectrum can be directly correlated with an absorption spectrum of a specific material. If an LHE
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 1 0 - 6
298
Y. Tachibana et al. / Chemical Physics Letters 364 (2002) 297–302
spectrum is known, dependence of the electron transfer efficiency on the excitation wavelength can be determined. Thus, the photocurrent generation or loss mechanism can be identified by an analysis of the IPCE spectrum. The photocurrent loss is generally observed under electrical bias application to a solar cell owing to dark current flow. However, in reality, the photocurrent tends to decrease at lower voltages than expected. This is recognized as an anomalous photoeffect [1]. This non-ideal behaviour was observed for the dye sensitised solar cell under illumination [4,5], implying the existence of the photocurrent loss processes. Intensive studies have been carried out to search for an alternative semiconductor to the TiO2 . Although composite semiconducting materials [6,7] as well as single materials [8,9] were employed, they appeared to be inferior to the TiO2 due mainly to a lower open circuit photovoltage and fill factor. Such a low voltage can be related to the large reverse dark current or the photocurrent loss, or both. However, the mechanism of these loss processes has not yet been established in detail. In this Letter, we present comparative studies of IPCE spectra under electrical bias application to the solar cells using different semiconductor films. Nanocrystalline SnO2 films were chosen because relatively less photocurrent and photovoltage were obtained compared to the TiO2 cells [2,8]. Identification of the origins reducing the photocurrent and the photovoltage should provide the key parameters improving the solar cell performance.
2. Experimental Anatase TiO2 nanocrystalline films were prepared using a screen printing technique as described previously [10]. The paste includes 60 wt% acidic solution (average diameter: 14 nm) and 40 wt% basic solution (average diameter: 34 nm). Rutile SnO2 films were prepared by the similar way. Sol– gel synthesized small SnO2 nanoparticles (diameter: 10–20 nm) and commercial large particles (Kishida Chemical) with a diameter of 0.5–1.0 lm were mixed in the ratio of 8:2 to prepare the paste. The film was deposited onto a fluorine-doped SnO2 (FTO) con-
ducting glass (Nippon Sheet Glass, Japan, sheet resistance: 8–10 X=cm2 Þ, and calcined at 500 °C for 1 h in air. The sintered film (thickness: 8–12 lm for the TiO2 and 8 lm for the SnO2 ) was immediately sensitized by the ruthenium dye, ðtetrabutylammoniumÞ2 cis-(2,20 -bipyridyl-4-COOH, (40 -COO-)2 ðNCSÞ2 ruthenium(II) (Solaronix, Switzerland), and then employed to fabricate a sandwich-type solar cell as described previously [10]. IPCE measurements were conducted under applications of external electrical bias and AM1.5 simulated white light bias using a photocurrent measurement system (Bunko Keiki, CEP-99W). This condition was distinctively employed to monitor the photocurrent under the cell operation. Monochromatic light with a constant number of photons ð1016 =cm2 s) except for < 370 nm was modulated by a mechanical shutter with a repetition rate of 2 Hz under these biases and the photocurrent was measured using a lock-in amplifier. This repetition rate was determined since the modulation larger than 10 Hz resulted in a lower IPCE due probably to the slower electron trap filling in the film [11]. The IPCE was calculated following the definition [2,3] as below: IPCEðkÞ ¼ 1240 Jph =ðk F ðkÞÞ; where k is the monochromatic light wavelength, Jph is the photocurrent density and F ðk) is the incident light power density at wavelength k. The potential was applied negatively to the working electrode against the counter electrode, however, for simplicity the data are shown with positive values. Indistinguishable IPCE spectra at the short circuit were observed before and after a set of bias measurements to confirm the reproducibility. The IPCE data were not corrected for reflection and absorption loss by the FTO. Current–voltage (I– V) characteristics of the cell were measured using an AM1.5 simulated Xe lamp light source (Yamashita Denso K.K., YSS-150A, light power: 100 mW=cm2 Þ.
3. Results and discussions The I–V characteristics of the solar cells are shown in Fig. 1. The resultant cell factors: Jsc , Voc
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Fig. 1. The I–V characteristics of dye sensitised semiconductor solar cells. The photo responses were observed for the TiO2 film ( –) and the SnO2 film (- - - - - - -) under a simulated AM1.5 solar spectrum. The dark I–V curves for the cells based on the TiO2 film ( Þ and the SnO2 film ( – –) are shown for comparison.
Fig. 2. IPCEs normalized to unity as a function of electrical bias applied to the dye sensitised TiO2 solar cell. This normalization is based on the assumption that the photocurrent is identical under the short circuit at any wavelengths. The inset shows IPCE spectra under the electrical bias application.
and FF are 13.5 mA/cm2 , 0.72 V and 0.74 for the dye sensitized TiO2 cell while the SnO2 cell exhibits 9.5 mA/cm2 , 0.38 V and 0.46, respectively. A crossover of photo and dark curves was observed at 0.42 V for the SnO2 cell, indicating anomalous photoeffects [1]. It is apparent that the photocurrent observed for the SnO2 electrode decreases in the voltage range of 0–0.2 V where the dark current is not significantly affected. Note that the photo and dark curves for an FTO only exhibit negligible current generation at < 0:6 V. In this study, the IPCEs were obtained for the photocurrent induced only by the monochromatic light since the current generated by the electrical and white light bias applications was eliminated by the lock-in amplifier. The inset of Fig. 2 shows the IPCE spectra observed under different electrical biases applied to a dye sensitised TiO2 cell. Comparison of these IPCEs at different wavelengths was achieved by normalizing and plotting the data as a function of applied bias as presented in Fig. 2. The photocurrent responses were almost identical over a wavelength range of 450–750 nm with a sharp decrease at an applied bias of > 0:5 V. The
half-value of its maximum is located around 0.7 V (near the open circuit voltage), in consistence with the previous report [4]. In contrast, the lower IPCE was observed at 350 nm, in which case holes as well as electrons can be generated in the TiO2 by the band gap excitation. The generated holes may create additional electron transfer paths from the TiO2 to the electrolyte, increasing the reverse current (or charge recombination). Since the electron transport kinetics (following the electron injection) should be identical at any excitation wavelengths and a number of injected electrons are normalized at the short circuit (Fig. 2), these results suggest that the electron injection efficiency is independent of an excitation wavelength upon the electrical bias application. Assuming the electrolyte redox potential is +0.2 V vs SCE [12], the bias application of 1.0 V leads to a Fermi level of the TiO2 at 0:8 V vs SCE corresponding roughly to the dye excited state potential [12]. The electron injection rate is sufficiently fast even at this Fermi level; for example, the halfinjection time of 10 ps was observed at about 0:75 V vs SCE [13]. Moreover, the charge
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recombination between the electron and the oxidized dye is relatively slow in the electrolyte employed here [14]. Therefore, it can be concluded that the IPCE decrease upon the bias application originates most likely from an acceleration of the charge recombination rate between the injected electron and the electrolyte. The electrical bias dependent IPCEs for a dye sensitised SnO2 cell are presented in Fig. 3. With the application of > 0:4V, the negative IPCE was observed at 300–450 nm, indicating cathodic photocurrent generation (see the inset). Normalized IPCE data clearly show large cathodic photocurrent at 350 nm while the photoanodic responses are indistinguishable in a wavelength range of 470–720 nm. Note that the bias application of > 0:5 V results in decrease in this cathodic photocurrent, however, the origin of this reduction is not clear at the present. It is obvious that anodic photocurrent decreases even with the bias application of 0.1 V, which is consistent with the decrease in the current density shown in Fig. 1. Kamat et al. [8] reported that the yield of the electron injection from the dye to the SnO2 elec-
Fig. 3. Normalised IPCEs as a function of electrical bias applied to the cell based on nanocrystalline SnO2 films sensitised by the dye. The normalization method is the same as the TiO2 presented in Fig. 2. The inset shows the bias dependent IPCE spectra.
trode was not altered by the bias application of > 0:35 V vs SCE corresponding to < 0:55 V in our study. The IPCE loss can therefore be attributed to a light induced acceleration of the charge recombination kinetics between the electron and the oxidized dye or the electrolyte. The cathodic photocurrent was observed only for the dye sensitised SnO2 electrode in the UV wavelength region. This observation was investigated in more detail by measurements of bias dependent IPCEs for unsensitised TiO2 and SnO2 electrodes. Fig. 4 shows the results plotted in the same scale for ease of comparison. The TiO2 film indicates an identical spectral shape with a maximum at 340 nm at any bias applications. No cathodic photocurrent was revealed with the bias application of 0.9–1.5 V (except for negligible photocurrent flow). Lindquist and Vidarsson [15] found using a polycrystalline TiO2 that the cathodic current decreases as irradiated light intensity increases. The light intensity ð< 1016 photons/ cm2 s) we employed here may be too large to detect the cathodic photocurrent. In contrast, the SnO2 electrode exhibits remarkable difference in the IPCE spectra. The anodic photocurrent spectra are similar to the absorption spectrum of the SnO2 film [16], and thus this current generation can be ascribed to the SnO2 band gap excitation. The cathodic photocurrent appears as the bias is applied larger than 0.2 V, and the spectral maximum is shifted from 330 to 370 nm. The spectrum obtained at 0.4 V is equivalent to a calculated current density of 0.9 mA/cm2 using the standard AM1.5 spectrum [17]. This current density reduces the Jsc by 1.8 mA/cm2 if photons are not absorbed by the attached dye in this wavelength range. We consider here, two reaction processes responsible for the cathodic photocurrent generation, that is, an interfacial cathodic electron transfer following an SnO2 intraband excitation or an I photodissociation. 3 Regarding the former case, the absorption attributable to the intra-band excitation appears in a visible wavelength range under the negative bias application [6]. However, the extinction coefficient may be too low to explain this large negative IPCE at 0.4 V. The latter case is more favourable because the cathodic IPCE spectra are similar to the
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cathodic current was observed at the FTO, this reverse reaction can be enhanced by a large surface area of the SnO2 nanoporous film. Further detailed studies are necessary to clarify these issues. Nevertheless, in any case, the electron transfer reaction at the SnO2 surface must be considerably accelerated compared to the TiO2 surface under the bias application, and due to this mechanism the photocurrent decreases even if the electron injection from the dye to the SnO2 is efficient [8]. From the above discussions, we conclude that the choice of semiconductor films may be crucial to control the charge recombination kinetics and thus the anodic photocurrent. In addition, the absorption by the dye rather than the semiconductor or the electrolyte in a lower wavelength region is critical to raise the photocurrent under the bias application.
Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy Trade and Industry (METI).
References
Fig. 4. Electrical bias dependent IPCE spectra obtained from (a) the unsensitised TiO2 film, and (b) the unsensitised SnO2 film. absorption spectrum of I 3 [18,19]. The I3 excitation results in the formation of a di-iodide ion [20] which may induce the electron transfer reaction from the semiconductor to the electrolyte [21,22]. Moreover, Matsuda and Matsumoto [19] recently reported that the reaction at the FTO promotes cathodic photocurrent in the highly concentrated I 3 electrolyte. Although in our study no significant
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