Method to Protect Charge Recombination in the Back-Contact Dye ...

Method to Protect Charge Recombination in the Back-Contact Dye-Sensitized Solar Cell Beomjin Yoo,1,2 Kang-Jin Kim,2 Doh-Kwon Lee,1 Kyungkon Kim,1 Min Jae Ko,1,* Yong Hyun Kim,3 Won Mok Kim,3 and Nam-Gyu Park4,* 1

Solar Cell Center, Materials Science and Technology Division, Korea Institute of Science and Technology (KIST), Seoul, 136-791, Korea 2 Department of Chemistry, Korea University, Seoul 136-701, Korea 3 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea 4 School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea *[email protected]

Abstract: We prepared a back-contact dye-sensitized solar cell and investigated effect of the sputter deposited thin TiO2 film on the backcontact ITO electrode on photovoltaic property. The nanocrystalline TiO2 layer with thickness of about 11 µm formed on a plain glass substrate in the back-contact structure showed higher optical transmittance than that formed on an ITO-coated glass substrate, which led to an improved photocurrent density by about 6.3%. However, photovoltage was found to decrease from 817 mV to 773 mV. The photovoltage recovered after deposition of a 35 nm-thick thin TiO2 film on the surface of the back-contact ITO electrode. Little difference in time constant for electron transport was found for the back-contact ITO electrodes with and without the sputter deposited thin TiO2 film. Whereas, time constant for charge recombination increased after introduction of the thin TiO2 film, indicating that such a thin TiO2 film protected back electron transfer, associated with the recovery of photovoltage. As the result of the improved photocurrent density without deterioration of photovoltage, the back-contact dye-sensitized solar cell exhibited 13.6% higher efficiency than the ITO-coated glass substrate-based dye-sensitized solar cell. ©2010 Optical Society of America OCIS codes: (000.2700) General science.

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B. O’Regan, and M. Grätzel, “A low cost, high efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature 353(6346), 737–740 (1991). M. Grätzel, “Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells,” J. Photochem. Photobiol. Chem. 164(1–3), 3–14 (2004). M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, and M. Grätzel, “Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers,” J. Am. Chem. Soc. 127(48), 16835–16847 (2005). Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, and L. Han, “Dye-sensitized solar cells with conversion efficiency of 11.1%,” Jpn. J. Appl. Phys. 45(25), 638–640 (2006). N.-G. Park, and K. Kim, “Transparent solar cells based on dye-sensitized nanocrystalline semiconductors,” Phys. Status Solidi 205(8), 1895–1904 (2008) (a). R. M. Swanson, Proc. 17th IEEE Photovoltaics Specialists Conf., 1984, p1294. J. M. Kroon, N. J. Bakker, H. J. P. Smit, P. Liska, K. R. Thampi, P. Wang, S. M. Zakeeruddin, M. Grätzel, A. Hinsch, S. Hore, U. Würfel, R. Sastrawan, J. R. Durrant, E. Palomares, H. Pettersson, T. Gruszecki, J. Walter, K. Skupien, and G. E. Tulloch, “Nanocrystalline dye-sensitized solar cells having maximum performance,” Prog. Photovolt. Res. Appl. 15(1), 1–18 (2007). N. Fuke, A. Fukui, Y. Chiba, R. Komiya, R. Yamanaka, and L. Han, “Back contact dye-sensitized solar cells,” Jpn. J. Appl. Phys. 46(18), L420–L422 (2007). Y. Kashiwa, Y. Yoshida, and S. Hayase, “All-metal-electrode-dye sensitized solar cells (transparent conductive oxide-less dye sensitized solar cell) consisting of thick and porous Ti electrode with straight pores,” Appl. Phys. Lett. 92(3), 033308 (2008).

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Received 18 Jun 2010; revised 24 Jul 2010; accepted 27 Jul 2010; published 17 Aug 2010

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10. N. Fuke, A. Fukui, R. Komiya, A. Islam, Y. Chiba, M. Yanagida, R. Yamanaka, and L. Han, “New approach to low-cost dye sensitized solar cells with back contact electrodes,” Chem. Mater. 20(15), 4974–4979 (2008). 11. N. Fuke, A. Fukui, A. Islam, R. Komiya, R. Yamanaka, L. Han, and H. Harima, “Electron transfer in back contact dye-sensitized solar cell,” J. Appl. Phys. 104(6), 064307 (2008). 12. B. Yoo, K. Kim, S. H. Lee, W. M. Kim, and N.-G. Park, “ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells,” Sol. Energy Mater. Sol. Cells 92(8), 873–877 (2008). 13. B. Yoo, K.-J. Kim, S.-Y. Bang, M. J. Ko, K. Kim, and N.-G. Park, “Chemically deposited blocking layers on FTO substrates: Effect of precursor concentration on photovoltaic performance of dye-sensitized solar cells,” J. Electroanal. Chem. 638(1), 161–166 (2010). 14. H.-J. Koo, J. Park, B. Yoo, K. Yoo, K. Kim, and N.-G. Park, “Size-dependant scattering efficiency in dyesensitized solar cell,” Inorg. Chim. Acta 361(3), 677–683 (2008). 15. B. Yoo, K. Kim, D.-K. Lee, M. J. Ko, H. Lee, Y. H. Kim, W. M. Kim, and N.-G. Park, “Enhanced charge collection efficiency by thin-TiO2-film deposition on FTO-coated ITO conductive oxide in dye-sensitized solar cells,” J. Mater. Chem. 20(21), 4392–4398 (2010). 16. S. Ito, M. K. Nazeeruddin, P. Liska, P. Comte, R. Charvet, P. Péchy, M. Jirousek, A. Kay, S. M. Zakeeruddin, and M. Grätzel, “Photovoltaic characterization of dye-sensitized solar cells: effect of device masking on conversion efficiency,” Prog. Photovolt. Res. Appl. 14(7), 589–601 (2006). 17. J. Park, H.-J. Koo, B. Yoo, K. Yoo, K. Kim, W. Choi, and N.-G. Park, “On the I-V measurement of dyesensitized solar cell: Effect of cell geometry on photovoltaic parameters,” Sol. Energy Mater. Sol. Cells 91(18), 1749–1754 (2007). 18. N. Kopidakis, K. D. Benkstein, J. van de Lagemaat, and A. J. Frank, “Transport-limited recombination of photocarriers in dye-sensitized nanocrystalline TiO2 solar cells,” J. Phys. Chem. B 107(41), 11307–11315 (2003). 19. K. D. Benkstein, N. Kopidakis, J. van de Lagemaat, and A. J. Frank, “Influence of the prercolation network geometry on electron transport in dye-sensitized titanium dioxide solar cells,” J. Phys. Chem. B 107(31), 7759– 7767 (2003).

1. Introduction Conventional dye-sensitized solar cell is composed of a negative electrode comprising dyecoated nanocrystalline TiO2 particles deposited on a transparent conductive oxide (TCO) substrate, a positive electrode containing thin film of platinum, and a triiodide/iodide based redox electrolyte. Since the first report on dye-sensitized solar cell (DSSC) in 1991 [1], progresses in both fundamentals and applications have been substantially made. As a result, solar-to-electricity conversion efficiency of above 11% was demonstrated [2–5]. Photocurrent density in solar cell is usually resulted from the collective measure of light harvesting, charge separation and charge collection efficiencies. In case of wafer-based silicon solar cell, back-contact structure has been proposed in order to improve light harvesting efficiency by minimizing the loss from light reflection at front electrode [6]. Similarly, backcontact concept has been applied to dye-sensitized solar cell. Kroon et al. reported first the back contact structure in dye-sensitized solar cell and efficiency of 3.6% was achieved [7]. Fuke et al. and Kashiwa et al. reported on the FTO (fluorine-doped tin oxide)-less glass based dye-sensitized solar cells, where titanium metal was used as charge collector and efficiencies around 7.4% were obtained [8,9]. Improved efficiency of 8.4% was reported recently, where life time of photo-injected electrons was found to be long enough for them to reach back contact titanium metal electrode from the intensity modulated photovoltage spectroscopy study [10]. One can expect that photocurrent density will be higher in the TCO-less backcontact structure than in the conventional DSSC using TCO-coated substrate. However, contrary to the expectation, photocurrent density of the back-contact structure was observed to be mostly lower than that of the conventional one and its difference was pronounced as the TiO2 film thickness increased [11]. This was interpreted by the fact that electron diffusion was slower in the back contact structure than in the conventional structure due to longer distance for the photo-injected electrons to reach the back contact metal electrode [11]. Fill factor and open-circuit voltage were also found to be slightly deteriorated [9]. Since metal electrode is directly exposed to the liquid electrolyte, poor fill factor and lowered voltage could arise from recombination at metal/electrolyte interface. However, little attention has been paid to a probable electron recombination at metal/electrolyte interface in the back-contact DSSC structure. Moreover, from the application point of view, the unique property of transparency in DSSC may be lost because of the use of opaque metal. In spite that the back-contact structure using FTO-less substrate is originally designed to gain optical transmittance and thereby improve photovoltaic property, its photovoltaic property turns out to be a little bit

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worse than the conventional DSSC. Therefore, careful investigations are further required to improve photovoltaic property in the back-contact DSSC structure. We report here on the preparation and photovoltaic property of the back-contact DSSC having a plain glass substrate and a transparent ITO (indium tin oxide) layer as a back-contact charge collector. We have investigated the effect of deposition of thin film passivation layers on the back-contact electrode on the photovoltaic performance. Studies on morphology and electron transport/recombination behavior are carried out with scanning electron microscopy and transient photocurrent-voltage spectroscopy. The reason of choosing ITO as a backcontact electrode instead of metal electrode is to keep the transparent property of DSSC. Recently we reported the thermally stable ITO substrate treated with a thin antimony-doped tin oxide layer [12]. Electrical property of ITO has been known to be deteriorated at temperature above 300 °C. For this reason, ITO itself has not been considered as electrode material for the conventional DSSC structure since temperature as high as 500 °C is required to prepare TiO2 film. However, ITO is one of good candidates for the back contact DSSC since high temperature thermal annealing process is not required to prepare the back-contact electrode in the plain glass substrate-based DSCC. 2. Experimental Nanocrystalline TiO2 film on the TCO-less glass substrate was prepared as follows. Prior to preparation of nanocrystalline TiO2 film, a thin blocking layer was first deposited on a plain glass substrate from 0.15 M Ti(IV) bis(ethylacetoacetato) diisopropoxide in 1-butanal by spin coating at 2000 rpm, followed by annealing at 500 °C in air for 30 min, as described elsewhere [13]. Nanocrystalline TiO2 particles were prepared according to the method described elsewhere [14]. Screen-printable paste was made by mixing TiO2 nanoparticles, ethyl cellulose (Aldrich), lauric acid (Fluka), and terpinol (Fluka) in the nominal weight ratio of 1:0.3:0.1:4. A thin TiO2 layer was first deposited on the blocking layer-coated glass substrate using the screen printable paste, which was dried in air at ambient temperature for 10 min. A thick TiO2 layer was overcoated on the dried TiO2 layer, which was followed by annealing at 500°C for 30 min in air. The resulting thickness of the TiO2 film was about 11.2 ± 0.12 µm as determined by an Alpha-step IQ surface profiler (KLA Tencor). The ambient temperature-dried thin TiO2 layer was found to improve the adhesion of nanocryatlline TiO2 film on the glass substrate. A back-contact ITO layer was deposited on the 500 °C-annealed TiO2 film by using by radio frequency (rf) magnetron sputtering technique, where an ITO target with the In2O3 to SnO2 ratio of 90:10 by weight was used and the substrate temperature was 300 °C. The sputter deposition was carried out under a working pressure of 0.16 Pa at rf power of 50 W and the substrate was rotated at a constant speed of 12 rpm. To study the effect of passivation layer on the ITO layer, a very thin TiO2 layer was coated on the ITO electrode by rf magnetron sputter deposition at the substrate temperature of 300 °C, according to the method described previously [15]. The photoanodes were therefore arranged in the following order: glass/nanocrystalline TiO2/ITO/with and without thin film TiO2. For dye adsorption, the resulting photoanode was immersed in 0.5 mM ethanol solution of N719 dye (Ru[LL'(NCS)2], where L = 2,2'-bypyridyl-4,4'-dicarboxylic acid, L' = 2,2'bypyridyl-4,4'-ditetrabutylammonium carboxylate) at ambient temperature. Pt counter electrode was prepared by spreading the 7 mM H2PtCl6·6H2O 2-propanol solution on a FTO glass (Pilkington, 8 Ω/sq, 2.3 mm thick), which was followed by annealing at 400 °C for 20 min. The photoanode and the Pt counter electrode were sealed with Surlyn 1702 (Dupont). We used the electrolyte comprising 0.7 M 1-propyl-3-methyl immidazoliuim iodide, 0.03 M I2 (Aldrich), 0.05 M guanidinium thiocyanate (Aldrich) and 0.5 M 4-tert-butylpyridine (Aldrich) in acetonitrile (Aldrich) and valeronitrile (Aldrich) (85:15 v/v). The active area of dye-coated TiO2 film was measured by an image analysis program equipped with a CCD camera (Moticam 1000). Surface and cross-sectional morphologies were studied by a field emission scanning electron microscope (FE-SEM, Nova NanoSEM 200). Chemical analysis was performed by depth profiling with Auger electron spectroscopic technique (AES, Scanning Auger #130346 - $15.00 USD

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Nanoprobe PHI-700 & LC-TOFMS LECO) at the etching rate of 15 nm/min (estimated with SiO2). Transmittance spectra were measured with a Perkin-Elmer Lambda 35 UV/vis spectrometer. Photocurrent-voltage (J-V) curves were measured using a Yamashita Denso YSS-200A solar simulator (class-A) equipped with a 1600 W Xenon lamp as a white light source and an AM 1.5G filter. Light intensity was adjusted with an NREL-calibrated Si solar cell with KG-2 filter for approximating one sun light intensity. During photocurrent-voltage measurement, DSSCs were coverd with a black mask with an aperture to avoid additional light coming through lateral space [16,17]. Incident photon-to-current conversion efficiency (IPCE) was measured as a function of wavelength from 300 to 800 nm using a specially designed IPCE system for the dye-sensitized solar cell (PV measurements, Inc.). A 75 W xenon lamp was used as a light source for generating monochromatic beam. A calibration was performed using a silicon photodiode, which was calibrated using the NIST-calibrated photodiode G425 as a standard, and IPCE values were collected under bias light at a low chopping speed of 10 Hz. The time constants for photo-injected electron transport and recombination were measured by using photocurrent and photovoltage transient induced by small-intensity laser pulse superimposed on bias light. The cells were probed with a weak laser pulse at 532 nm superimposed on a relatively large, back ground (bias) illumination at 680 nm [18,19]. The bias light was illuminated by a 0.5 W diode laser (B&W TEK Inc., Model: BWF1-670300E/55370). The intensity of the bias light was adjusted using ND filters (neutral density filters). The 680 nm bias light is only weakly absorbed by the dye, and therefore the injected electrons are introduced into a narrow spatial region of the film, corresponding to where the probe light enters the film. A 30 mW frequency-doubled Nd:YAG laser (Laser-Export Co. Ltd. Model: LCS-DTL-314QT) (λ = 532 nm, pulse duration 10 ns) was used as probe light. The photocurrent transients were obtained by using a Stanford Research Systems model SR570 low-noise current preamplifier, amplified by a Stanford Research Systems model SR560 low-noise preamplifier, and recorded on Tektronics TDS 3054B digital phosphore oscilloscope 500MHz 5GS/s DPO. The photovoltage transient were obtained by using a Stanford Research System model SR570 low-noise preamplifier, and then recorded on Tektronics TDS 3054B digital phosphore oscilloscope 500 MHz 5 GS/s DPO. The response time of the preamplifier at the range of photocurrent and photovoltages used was less than 1 ms. Time constants were obtained by fitting the observed data with exponential function, exp(-t/τ), where t is time and τ is the characteristic time constants. 3. Results and discussion Figure 1 shows a schematic structure of a back-contact dye-sensitized solar cell where the TiO2 layer is deposited on a plain (non conducting) glass substrate and the electrical conducting ITO layer forms on the top of the TiO2 film. Such a working electrode configuration is mainly different from the conventional DSSC that adopts a TCO-coated substrate for nanocrystalline TiO2 layer. The counter electrode is composed of thin layer of platinum on a FTO glass substrate. ITO-coated glass has been hardly adopted as a substrate for the TiO2 layer because its conductivity increases after annealing the TiO2 layer at 500 °C [12]. However, deterioration of electrical conductivity of ITO can be avoided since we deposit the ITO layer on the 500 °C-annealed TiO2 layer. It is required that ITO layer has pores to some extent for dye adsorption procedure and electrolyte diffusion. However, such porous structure had better not affect electrical conductivity. Morphology control of the ITO layer is therefore one of important aspects in the back contact DSSC.

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Fig. 1. Schematic structure of a back-contact dye-sensitized solar cell, where charge collecting ITO is placed on the the nanocrystalline TiO2 layer coated on a non-conducting plain glass substrate.

Figure 2 shows scanning electron microscopy (SEM) images for the nanocryatlline TiO2 (referred to as nc-TiO2 hereafter) layer on a non-conducting plain glass substrate (a and b in Fig. 2), the ITO layer deposited on nc-TiO2 layer on a plain glass substrate (c and d in Fig. 2) and the very thin TiO2 film (referred to as TF-TiO2 hereafter) deposited on the ITO-coated ncTiO2 layer (e and f in Fig. 2). Figure 2(a) shows a typical mesoporous structure of nanocrystalline TiO2 layer and the nc-TiO2 layer thickness is determined to be about 11 µm from the cross sectional SEM in Fig. 2(b). ITO deposition covers the nc-TiO2 layer as can be seen in Fig. 2(c), however, porous nature is somewhat maintained. The deposition of TF-TiO2 on the ITO layer from sputtering method dose not alter significantly the ITO surface morphology (Fig. 2(e)), which indicates that porous nature is sustained.

Fig. 2. Surface (a-c) and cross-sectional (d-f) scanning electron micrograph (SEM) images for the bare nc-TiO2 film; (a and d), the back-contact ITO film deposited on the nc-TiO2 film (d and e), and the TF-TiO2 film deposited on the back-contact ITO layer on the nc-TiO2 film (c and f).

Figure 3 shows the cross sectional SEM and the chemical concentration from AES depth profile for the TF-TiO2 coated ITO layer on nc-TiO2 film. When viewed at high magnification it is clear that the ITO layer forms on the nc-TiO2 layer with columnar structure and thickness of about 170-200 nm (Fig. 3(a)). AES measurement is performed since a TF-TiO2 overlayer on the ITO layer is not clearly seen in the SEM image. Figure 3(b) shows the atomic concentration as a function of sputter time. The Auger depth profiling with rate of 20 nm/min revels the presence of Ti, In, Sn and oxygen, indicating that TiO2 forms on the ITO layer and its thickness is determined to be about 35 nm. The ITO layer thickness from the depth profile

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analysis is confirmed to be about 200 nm, which is consistent with the thickness observed from the cross sectional SEM.

Fig. 3. (a) Cross-sectional SEM image of the nc-TiO2/ITO/TF-TiO2 layer on a plain glass substrate. (b) Atomic concentration with respect to sputter time obtained by AES depth profiling of the nc-TiO2/ITO/TF-TiO2 layer. Depth profiling rate was 20 nm/min.

Figure 4 shows photocurrent-voltage curves of DSSCs having working electrodes based on the conventional DSSC structure (ITO-coated substrate/nc-TiO2) and the back-contact structure (plain glass substrate/nc-TiO2/ITO with and without TF-TiO2 on ITO). Photovoltaic parameters are summarized in Table 1. As can be seen in Fig. 4, the plain glass/nc-TiO2/ITO electrode shows photocurrent density of 12.92 mA/cm2 that is higher than that of 12.15 mA/cm2 for the ITO-coated substrat/nc-TiO2 electrode, while photovoltage (773 mV) of the former is lower than that (817 mV) of the latter. Fill factor is higher for the back-contact structure (0.672) than for the conventional structure (0.639). As a result, the back-contact electrode exhibits the conversion efficiency of 6.71% that is slightly higher than the efficiency of 6.34% from the conventional structure. Introduction of TF-TiO2 on the back-contact ITO improves photovoltage from 773 mV to 815 mV without any deterioration of photocurrent density. The deposition of thin film TiO2 layer on the ITO back contact eventually leads to increase in conversion efficiency from 6.71% to 7.20%. From the photocurrent-voltage curves, it is pointed that the back-contact structure shows higher photocurrent but lower voltage than the conventional structure and the lowered voltage is recovered by deposition of thin TiO2 layer on the back-contact ITO.

Fig. 4. Photocurrent-voltage and dark current-voltage curves for the conventional ITO-coatedglass-based DSSC and the back-contact DSSCs with and without a TF-TiO2 film. ITO deposition on a glass substrate was performed at the same condition as the back contact ITO deposition.

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Table 1. Shot-circuit photocurrent (JSC), open-circuit photovoltage (VOC), filll factor (FF) and overall conversion efficiency (η) of dye-sensitized solar cellsa with conventional structure and back-contact structure. Data were average values from 3 cells. Photoelectrode Type

JSC (mA/cm2)

VOC (mV)

FF

η (%)

0.639 ± 6.34 ± 0.004 0.03 0.672 ± 6.71 ± Back-contact ITO w/o TF-TiO2 12.92 ± 0.07 773 ± 9 0.002 0.05 0.677 ± 7.20 ± Back-contact ITO w/ TF-TiO2 13.05 ± 0.06 815 ± 8 0.003 0.03 a Photocurrent and voltage were measured with the dye-sensitized solar cell having an aperture mask under AM 1.5G one sun light intensity (100 mW/cm2). ITO-coated glass

12.15 ± 0.05

817 ± 5

Figure 5(a) shows IPCE spectra of the back-contact structure without TF-TiO2 and the conventional structure. In Fig. 5(b) optical transmittance is compared, where transmittance is measured with the substrates being in contact with the electrolyte whose composition is identical with the electrolyte used for the real device. The IPCE data are higher for the plain glass-based back-contact electrode than for the ITO-coated glass substrate, which is associated with the higher transmittance characteristics for the plain glass substrate as shown in Fig. 5(b). Therefore, higher photocurrent density obtained from the back-contact structure is due to better optical transmittance. The lowered photovoltage is probably related to the increased dark current as can be seen in Fig. 4, which implies that back electron transfer may occur at the back-contact ITO/electrolyte interface. Since ITO is entirely exposed to electrolyte at the opposite side of ITO being in contact with nc-TiO2, we have thought such recombination may arise from the ITO/electrolyte interface. For this reason, thin film TiO2 layer is deposited on the ITO layer.

Fig. 5. (a) Incident photon-to-current conversion efficiency (IPCE) spectra as a function of wavelength for the back-contact DSSCs with and without a TF-TiO2 film. (b) Comparison of transmittance of Vycor glasses coated with and without an ITO film. Measurement was performed in the presence of redox electrolyte.

Figure 6 compares time constants for electron transport through the TiO2 network and time constants for electron recombination from the bulk TiO2 and/or the back-contact ITO to the electrolyte for the back-contact structures with and without thin film TiO2 layer. Both the back-contact ITO electrodes with and without TF-TiO2 shows similar time constant for electron transport, which suggests that little difference in electron diffusion rate in the mesoporous nc-TiO2 network, regardless of TF-TiO2. On the other hand, time constant for recombination becomes slow after TF-TiO2 deposition, which indicates that back reaction of photo-injected electrons are retarded by covering the ITO surface exposed to electrolyte. This seems to be closely related to the improvement of voltage from 773 mV to 815 mV.

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Fig. 6. (a) Time constants for electron transport (τc) and (b) time constants for charge recombination (τR) as a function of light intensity, represented by photocurrent density, for the back-contact ITO charge collector with and without a TF-TiO2 film.

4. Conclusion A back-contact dye-sensitized solar cell with a plain glass substrate was found to be better in terms of optical transmittance than the conventional dye-sensitized solar cell structure employing an ITO-coated glass substrate. As a result, higher photocurrent density was observed due to the optical gain. However, open-circuit voltage was found to be deteriorated, which was recovered by deposition of a 35 nm-thick TiO2 film on the surface of the backcontact ITO electrode. From our observations, compared to the conventional structure, it is expected that the back-contact structure is superior in terms of light harvesting efficiency but inferior in terms of charge recombination. Charge recombination, however, can be suppressed by deposition of thin TiO2 layer on the back-contact TCO electrode. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Education, Science and Technology (MEST) of Korea under contracts No. R31-2008-000-10029-0 (WCU program) and in part by National Agenda Project (NAP) funded by the Korea Institute of Science and technology.

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