PREPARATION OF Ti0 2 THIN FILM USING MODIFIED DOCTOR-BLADE METHOD FOR IMPROVEMENT OF DYE-SENSITIZED SOLAR CELL Tan-Phat Huynh', Thi-Thao Hoanq', Phuoc-Hiep Nguyen1 , Thanh-Nam Tran1 , The-Vinh Nguyen2,* 1Faculty of Materials Technology, 2Faculty of Environment, Hochiminh City University of Technology, Hochiminh City, Vietnam ABSTRACT We have demonstrated a modified doctor-blade method using a dense Ti02 paste in combination with moderate compression for preparation of Ti02 thin film. Dyesensitized solar cell (DSSC) with the working area of 2 0.25 cm using that novel Ti02 thin film was observed to show its superior photovoltaic performance at AM 1.5: 2, Vae of 0.77 V, -Jsc of 18.2 mAlcm FF of 0.50 and r, of 7.0 0/0. This overall conversion efficiency was approximately 2 times as much as that of a corresponding DSSC fabricated with a conventional Ti02 thin film (r, of 3.70/0). The modified doctor-blade method was also found to significantly improve the efficiencies of DSSCs with the working areas of 0.5, 1.0 2. and 5.0 cm paste in combination with moderate compression for improvement of DSSC performance. The effect of cell size including 5x5 mm2, 5x10 rnrrr', 5x20 mm2 and 2 5x 100 mm on the efficiency of DSSC was also investigated. EXPERIMENTAL Materials Commercial titanium dioxide powder (P25, Degussa) was used as Ti02 source. Ruthenium 535 bis-TEA dye or N-719 (Solaronix) as a sensitizer and other chemicals were analytical grade and used as received. The transparent conducting oxide (TCO) glass substrate was supplied by Solaronix (TC010-10, 8 O/sq, 80% transmittance in visible light). Ti02 thin film preparation INTRODUCTION Dye-sensitized solar cell (DSSC) has been known as a promising photovoltaic device to achieve moderate efficiency at ultra-low cost [1]. One of the main components of DSSC is Ti02 thin film that is conventionally prepared by using a doctor-blade method [1, 2]. In order to improve photovoltaic performance of DSSC, spray coating [3, 4], screen printing [5-7] and liquid phase crystal deposition (LPCD) [8] methods have been intensively studied for Ti02 thin films preparation. It was well agreed that the thickness and structure of Ti02 thin film were strongly affected by preparation method [2, 9]. For a given Ti02 material, the existence of an optimized thickness of Ti02 thin film is inherent for the most efficient charge storage and transfer in the film and therefore the highest overall conversion efficiency of DSSC [10]. In doctor-blade method, surfactants are commonly used in Ti02 paste preparation to increase porosity of the resulting Ti02 thin film. Nevertheless, high porosity of Ti02 thin film inherently decreases charge transfer between the Ti02 thin film and electrode. In this present work, Ti02 thin film was prepared by using a dense Ti02 978-1-4244-2950-9/09/$25.00 ©2009 IEEE Preparation of Ti02 thin film using a conventional doctor-blade method was described elsewhere [11]. For the modified doctor-blade method, a dense Ti02 paste was prepared as following: Ti02 powder (1 g) was blended with a mixture of polyethylene glycol (PEG, MW of 20,000, 0.5 g) and de-ionized water (1.5 ml) in a plastic bag under rolling of a steel pipe for 10 min. After that, the resulting Ti02 paste was coated on TCO glass to make a thin film using a doctor-blade method [11]. The obtained Ti02 thin film was then dried in air for 30 min. A drop of lubricating oil was put onto the film surface and stabilized under 100°C in an oven for 20 min to reduce cracks. Finally, the film was covered with 2 a flat glass and compressed at 50 kgf/cm for 30 s. The derived Ti02 thin film was calcined at 450°C for 2 h. Fabrication of DSSC The details of DSSC fabrication was illustrated in our previous studies [11, 12]. Briefly, the 2-electrode sandwich cell for photovoltaic measurement consisted of a dye adsorbed Ti02 electrode, a Pt coated TCO counter electrode, a spacer and an organic electrolyte (0.3 M Lil, 15 mM band 0.2 M tertbutylpyridine in 002168 acetonitrile). The resulting cells had active areas of 0.25, 0.50, 1.00 and 5.00 em'. Characterization of Ti02 thin film and DSSC Ti02 thin films were characterized using an optical microscopy (MML 1200, Kruss) and FE-SEM (JSM 7401, JEOL). The amount of adsorbed dye was determined by UV-Vis spectra (UV-530, JASCO) of the desorbed dye from Ti02 electrode with a solution of 1mM NaOH in an equimolar solution of ethanol and water. The electronic characterization of OSSC including dark current and photocurrent were measured by using a digital multimeter (Model 2400, Keithley). A 300 W Halogen lamp served as a light source and its intensity was calibrated using a Si-photodiode at intensity of 100 2 mW/cm • Surface and cross-section images of two kinds of Ti02 thin films using FE-SEM are shown in Fig. 2. The surface morphology of these films under large magnification is similar. Nevertheless, cross-section images reveal that the thickness of modified doctorblade thin film (ca. 22 IJm) is larger than that of conventional counterpart (ca. 18 IJm). This result could be due to the dense Ti02 paste used to prepare the modified doctor-blade thin film. In addition, Fig. 2d also shows that the modified doctor-blade thin film seems to present its compact structure as compared to the conventional counterpart (Fig.2b). RESULTS AND DISCUSSION Characterization of Ti02 thin film. Fig.2. FE-SEM images of surface (left) and crosssection (right) of Ti02 thin film prepared by conventional (a & b) and modified (c &d) doctorblade methods. Fig.1. Micrograph of Ti02 thin films prepared by (a) conventional and (b) modified doctor-blade methods. Fig. 1 shows the micrograph of Ti02 thin films prepared by conventional and modified doctor-blade methods. The Ti02 thin film prepared with modified doctor-blade method is perfectly smooth (Fig. 1b), which could be result from the presence of lubricating oil. The oil prevents quick evaporation of water in the Ti02 during the calcination process. Meanwhile, there are a lot of macro-cracks on the surface of Ti02 thin film prepared with conventional doctor-blade method. These macrocracks would result in decreasing of overall conversion efficiency of OSSC. 978-1-4244-2950-9/09/$25.00 ©2009 IEEE Table 1 presents the amount of adsorbed dye on Ti02 thin films. These data are quite consistent with those of Ti02 thin film thickness. The thicker the thin film, the higher the amount of adsorbed dye on its surface. Nevertheless, the compact structure of modified doctorblade Ti02 thin film could result in a non-linear relationship between the film thickness and amount of absorbed dye on its surface in comparison with conventional counterpart. Table 1. Amount of adsorbed dye on Ti02 thin films Preparation method Adsorbed dye [mollcm2j Conventional doctor-blade 9.04x10-1O Modified doctor-blade 9.60x10-1O 002169 Photovoltaic performance of osse. Table 2. Photovoltaic performance of OSSC 20.-------------------, Ti02 thin film preparation method - 0-0-0-0 - 0 _ 0 ' 0 15 ! ' 0 - "b 10 - 0 - 0 - 0 - 0 -0 _ 0 _ 0 £ ｾ 'D '0 ｏＫＭｾＮ ｾ Conwnti_ 0.0 10.3 47.3 3.7 18.2 50.0 7.0 ] Although the amount of adsorbed dye on modified doctor-blade thin film is not much higher than that on conventional counterpart, the efficiency of MOB-OSSC is definitely superior to that of COB-OSSC. These results imply that high density of excited electrons resulting from large amount of adsorbed dye on Ti02 thin film does not guarantee high overall conversion efficiency of a corresponding OSSC. The data in Fig. 2 and Table 2 also reveal that the thickness of Ti02 thin film is not a critical factor influencing the overall conversion efficiency of the resulting OSSC. Efficient charge transfer between Ti02 thin film and TCO glass resulting from the compact Ti02 thin film could produce superior photovoltaic performance of the corresponding OSSC. These results are well consistent with our previous study [11]. Optimization of Ti02 thin film thickness and structure are noteworthy for improvement of overall conversion efficiency of the resulting OSSC. """"'-1>1-:::\ Modified doctor-blade ae , 0.2 0.77 '1 [%] 2 0\ ' 0 -0- Modified doctor-blade FF [%] Jse [mAlcm \ - 0' 0 5 Conventional doctor-blade Vee M 0.76 0.4 ＮｩＧＺｮＭ 0.6 0.8 VM Fig.3. Typical graphs of photocurrent-voltage for OSSCs fabricated with two Ti02 thin films. The cell performance was measured with ca. 0.25 2 2 cm active area under 100mW/cm . The shortcircuit current -Jsc, open-circuit voltage Voe, fill factor FF, and overall conversion efficiency '1 are shown in Table 2. Fig. 3 shows the typical graphs of photocurrent-voltage (J-V) for the OSSCs fabricated with conventional and modified doctor-blade Ti02 thin films. Table 2 summarizes the cell performance data obtained from the J-V curve measurements. The OSSC fabricated with modified doctor-blade thin film (denoted as MOB-OSSC) shows significant improvement of photocurrent density (Fig. 3). As a consequence, its overall conversion efficiency is observed to increase approximately 2 times higher than that of OSSC fabricated with conventional doctor-blade thin film (denoted as COB-OSSC) as shown in Table 2. For the purpose of module construction, OSSCs with large working areas were also investigated in this study. Table 3 shows the photovoltaic performance of COBOSSC and MOB-OSSC with various kinds of working areas. Modified doctor-blade method is also found to significantly improve the overall conversion efficiency of MOB-OSSC with the working areas of 0.5, 1.0 and 5.0 2 cm • Table 3. Photovoltaic performance of COB-OSSC and MOB-OSSC with different working areas VeeM 2] Working Area [cm 2 Jsc [mAlcm ] FF[%] '1 [%] COBOSSC MOBOSSC COBOSSC MOBOSSC COBOSSC MOBOSSC COBOSSC MOBOSSC 0.5xO.5 (0.25) 0.76 0.77 10.3 18.2 47.3 50.0 3.7 7.0 0.5x1.0 (0.50) 0.72 0.72 7.9 14.7 49 .0 46.4 2.8 4.9 0.5x2.0 (1.00) 0.71 0.70 6.3 10.9 50.0 49 .9 2.2 3.8 0.5x10.0 (5.00) 0.70 0.70 4.2 6.4 25.0 47 .6 0.7 2.1 978-1-4244-2950-9/09/$25.00 ©2009 IEEE 002170 Colloidal Ti02 Films", Nature 353, 1991, pp. 737740. Vae of both types of DSSCs are observed comparable and slightly decreased as the working area increases 2. from 0.25 to 5.00 cm Meanwhile, -Jsc of CDB-DSSC is much lower than that of MDB-DSSC at all of working areas. The result could be ascribed to the compact and crack-free Ti02 thin film prepared by the modified doctor-blade method. Fill factors are found similar for two kinds of DSSCs and also for all of working areas 2. except for CDB-DSSC at 5.00 cm For a Ti02 thin film prepared by conventional doctor-blade method, too large area of the thin film is inherently followed by a poor surface morphology with a lot of cracks. Accordingly, charge recombination in a corresponding CDB-DSSC could cause substantial decrease in its fill factor. This in turn results in much decline of overall conversion 2 efficiency of CDB-DSSC at working area of 5.00 cm as shown in Table 3. [4] S. Fan et aI., "Influence of TiCI4 Treatment on Performance of Dye-sensitized Solar Cell Assembled with Nano-Ti02 Coating Deposited by Vacuum Cold Spraying", Rare 25,2006, pp. 163-168. CONCLUSIONS [5] T. Ma et aI., "Preparation and Properties of Nanostructured Ti02 Electrode by A Polymer Organic-medium Screen-printing Technique", 5, 2003, pp. 369-372. Ti02 thin film prepared by using a dense Ti02 paste and moderate compression was found to substantially increase the photovoltaic performance of the resulting 2, DSSC: Vae of 0.77 V, -Jsc of 18.2 mAlcm FF of 0.50 and r, of 7.0 0/0. The overall conversion efficiency was approximately 2 times as much as that of a corresponding DSSC fabricated with a conventional Ti02 thin film (n of 3.70/0). The improved performance of MDB-DSSC could be ascribed to the compact and crack-free Ti02 thin film prepared by the modified doctor-blade method. This novel method is also observed to substantially improve the overall conversion efficiency of the resulting DSSC with working areas of 2, 0.5, 1.0 and 5.0 cm which presents strong potential for DSSC module construction. Acknowledgements The authors gratefully acknowledge the financial support from the Hochiminh City Department of Science & Technology under the Contract no. 495/HD-SKHCN. We also thanks for the kind suggestions and helps of colleagues from Faculty of Materials Technology, Hochiminh City University of Technology and Faculty of Chemistry, Hochiminh City University of Natural Science. References [1] B. O'Regan, M. Gratzel, "A Low-cost, Highefficiency Solar Cell Based on Dye-sensitized 978-1-4244-2950-9/09/$25.00 ©2009 IEEE [2] Z. Wang, H. Arakawa, "Significant Influence of Ti02 Photoelectrode Morphology on The Energy Conversion Efficiency of N719 Dye-sensitized Solar Reviews 248, 2004, pp. 1381Cell" 1389. [3] S. Fan et aI., "Fabrication of Nano-Ti02 Coating for Dye-Sensitized Solar Cell by Vacuum Cold Spray Spraying at Room Temperature", 16,2007, pp.893-897. [6] E. Ramasamy et aI., "Portable, Parallel Grid Dyesensitized Solar Cell Module Prepared by Screen Power 165, 2007, pp. 446Printing", 449. [7] J. Han et aI., "Dye-sensitized Solid-state Solar Cells Fabricated by Screen-printed Ti02 Thin Film With Addition of Polystyrene Balls", Chinese 19,2008,pp.1004-1007. [8] Y. Masuda et aI., "Deposition Mechanism of Anatase Ti02 on Self-Assembled Monolayers from an Aqueous Solution", 15, 2003, pp. 2469-2476. [9] Y. Saito et al., "Morphology Control of Mesoporous Ti02 Nanocrystalline Films for Performance of DyeEnergy sensitized Solar Cells", 83,2004, pp. 1-13. [10] T.-V. Nguyen, et aI., "Charge Storage and Transfer in Dye-sensitized Solar Cells: A Study by st Electrochemical Impedance Spectroscopy", 31 IEEE PVSC, 2005, pp. 167-170. [11] T.-V. Nguyen, H.-C. Lee and O.-B. Yang, "The Effect of Pre-thermal Treatment of Ti02 Nanoparticles on the Performance of DyeEnergy sensitized Solar Cells", 90, 2006, pp. 967-981. [12] T.-V. Nguyen, et aI., "Electrodeposition of Ti02/Si02 Nanocomposite for Dye-sensitized Solar Cell", 81,2007, pp. 529-534. 002171