Dye-sensitized Solar Cells Using Ionic Liquid-based Electrolytes Hiroshi Matsui, Kenichi Okada, Dr. Takuya Kawashima, Tetsuya Ezure and Dr. Nobuo Tanabe Because of low fabrication costs, simple manufacturing process, using no toxic materials and so on, a dye-sensitized solar cell (DSC) composed of nanocrystalline TiO2, organic dyes and electrolytes is expected to be a large-scale prevalent solar cell. It is necessary to improve cell stability and develop large sized cell fabrication technology for its practical use. In this report, developments of DSC using an ionic liquid electrolyte and its upscaling technology are introduced. For application of ionic liquids as electrolytes of DSC, short-circuit current (Jsc) and open-circuit voltage (Voc) of cells using ionic liquids were evaluated as a function of I−/I3− redox couples in the electrolyte. At optimized electrolyte composition, η=4.5 % of energy conversion efficiency was obtained. Based on the result, a 100 × 100 mm2 sized DSC using an ionic liquid electrolyte was fabricated and η=2.7 % (based on active area) was obtained. Fabrication of a DSC using an ion-gel electrolyte, which was a gelated ionic liquid electrolyte, was also described. 1. Introduction From standpoints of consideration for environment conservation and worries about exhaustion of fossil fuels, researches and developments for various type of solar cell are conducted extensively in recent years. It is necessary to lower module costs as low as commercial power to be diffused. Among these studies, a dye-sensitized solar cell (DSC)1) is expected as a promising energy source in the future because of low fabrication costs, simple manufacturing process, using no toxic materials and so on. 2. Structure and Technical Tasks trode. 2.2 Technical Tasks in Developing a DSC For the practical use of DSCs, following subjects should be developed such as improving of long-term stability, increasing of energy conversion efficiency, enlargement of cells and so on. In this report, we particularly focused attention on a DSC using an ionic liquid electrolyte for improvement of long-term stability and its upscaling technique. The cell usually employs an electrolyte solution using a volatile organic solvent such as acetonitrile. Therefore, it is necessary to prevent electrolyte from evaporation of a solvent. During long-term operation, 2.1 Cell Structure As shown in Fig.1, a porous layer made of metal oxide semiconductor (e.g., TiO 2) nanoparticles is formed on a transparent conductive substrate by sintering. A monolayer of dye is chemically adsorbed at the surface of nanoparticles. Counter-electrode (Pt deposited glass substrate) is arranged face-to-face. An electrolyte solution contains redox couple such as I− /I3− is filled up into an intervening space between both electrodes. Via absorption of a photon, the dye changes from the ground state to the excited state. Then, it injects an excited electron into the conduction band of the semiconductor. Oxidized dyes are reduced by iodide ions (I−) in the electrolyte. The triiodide ions (I3−) are also reduced at the counter-elec48 e− − e e− 3.2eV − − e e TiO2 e− e− Conductive transparent electrode e− Counter− electrode Electrolyte e− − Dye I /I3 − Dye/TiO2 nano-particles 100nm Fig. 1. Structure of a Dye-sensitized Solar Cell. H3C + N S − CF3 = Fujikura Technical Review, 2004 N = Typical cells used in this study were prepared as follows. An ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIm-TFSI as shown in Fig. 2) was used. Electrolyte solution using an ionic liquid (simply called as "ionic liquid electrolyte" hereafter) was composed of 1-ethyl-3methylimidazolium iodide (EMIm-I), I2, LiI, and 4tert-butylpyridine (TBP) that was dissolved in EMImTFSI. In the case of the ion-gel electrolyte preparation, poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) and the ionic liquid electrolyte were dissolved in tetrahydrofuran. By casting this solution, the ion-gel electrolyte was packed into an intervening space between photo-electrode and counter-electrode. Electrolyte solution using a volatile solvent (simply called as "volatile electrolyte" hereafter), as a reference, was composed of 1,2-dimethyl-3-propylimidazolium iodide, I2, LiI, and TBP that was dissolved in methoxyacetonitrile. A photo-electrode was prepared as follows. TiO2 paste was coated on a sheet glass substrate with transparent conductive oxide (TCO) using doctor-blade technique. Nanoxide-T (Solaronix SA) was used as TiO2 paste for a standard condition. O − F3C − S − N = 3. Experimental O = solvent losses occur owing to its high volatilities resulting in decrease in cell performance. There are many researches to improve cell stability by replacing such an electrolyte with non-volatile one2) - 13). As one of those researches, an ionic liquid seems to be suitable for an electrolyte of the cell. The ionic liquid is a molten salt with low melting point below room temperature, and it has some unique characters such as non-volatility, non-flammability, and thermal and electrochemical stability and so on. By application of ionic liquids to DSCs, noticeable improvements of long-term and thermal stability were reported2), 4), 5). On the other hand, in the ionic liquid electrolyte, physical mass transfer is slowly than that in the volatile electrolyte because it is highly viscous3). For an ionic liquid type DSC, it is necessary to increase generation output and to develop a technique for filling a nano-porous layer with electrolytes uniformly. Solidification of an electrolyte layer is also expected to prevent the cell from electrolyte leakage when a cell is broken or a cell is in the manufacturing process. An ion-gel electrolyte, which is a chemically or physically gelated ionic liquid electrolyte by using of proper gelator, have been extensively studied4), 5), 8) 11) . Moreover, upscaling technology of a cell that has la-boratory size is necessary. For the upscaling, it is important to decrease an IR drop attributed to an internal resistance of the cell that will increase significantly with enlarging of a photo-electrode area. To realize high-performance DSC, overcoming of these subject matters must be indispensable. O O CH2CH3 Fig. 2. Structure of EMIm-TFSI. A fluorine-doped stannic oxide (FTO) was used as a TCO (8 -10 Ω/square). After drying the wet film on the substrate, the film was sintering at 450°C. A light reflecting layer consist of larger size TiO2 particles was prepared over the nano-porous TiO2 layer using same procedure. Sintering time was totally 60 minutes. The substrate with the TiO 2 layer was immersed overnight in a solution of dye (ruthenium (2, 2'-bipyridyl-4,4'-dicarboxylate)2 (NCS)2 as called N3 Dye) at room temperature. As a counter-electrode, TCO substrate on which platinum had been deposited by sputtering was used. In the case of the preparation of a photo-electrode for a 100 × 100 mm2 large sized cell, mixture of two kinds of TiO2 paste that consist of Nanoxide-T and another one (mixing ratio was 8 : 2) was used to avoid delaminating of TiO2 film. The latter was prepared by reported procedure 14) using TiO 2 particles; P25 (Nippon Aerosil). A TCO substrate for large sized cells had an FTO/ITO double layer as a TCO layer and current collecting grids on a sheet glass substrate to decrease resistance drastically. The FTO/ITO layer was prepared by spray pyrolysis deposition method (2-3 Ω/square)15), and current collecting grids were formed on a glass substrate directly by additive process for printed wiring boards. Measurement of I-V characteristics of cells was carried out using a potentiostat/gulvanostat and a DC electronic load under simulated solar light (AM 1.5, 100 mW/cm2). 4. Results and Discussion 4.1 Ionic Liquid-based Electrolyte Fig. 3 shows typical current-voltage curve of a cell of EMIm-TFSI system’s result. Energy conversion efficiency of η=4.5 % was obtained which was about 67 % of volatile electrolyte system’s result. For optimizing of electrolyte composition, short-circuit current (Jsc) and open-circuit voltage (Voc) of cells using an ionic liquid were evaluated as a function of I −/I 3− redox couples in the electrolyte. Theoretical voltage of DSC was defined as a difference between Fermilevel of semiconductor electrode under illumination and redox potential of the electrolyte1). However, few researches about the correlation between cell voltage and redox potential of electrolytes have been report49 ed at this time. Fig. 4 shows dependence of equilibrium potential and Voc on the concentration of iodine redox couples in ionic liquid electrolytes (measurement of equilibrium potential was carried out by Kawano and Watanabe, for example, Fall Meeting of the Electrochemical Society of Japan, 2001). Voc decreased with increasing concentration of I−/I 3− redox couples. Equilibrium potentials showed negaVolatile electrolyte type η=6.7% Current density (mA/cm2) 15 Ionic liquid; EMIm−TFSI type η=4.5% 10 5 0 0 200 400 600 Voltage (mV) 800 4.2 Cell Performances Using Ion-gel Electrolyte Equilibrium pontetial Open-circuit voltage 150 750 700 100 650 50 600 0 550 −50 500 −100 0 0.5 450 1.5 1 Open-circuit voltage (mV) − Equilibrium potential (mV vs. I /I3−) Fig. 3. I-V Characteristics of Test Cells Using an Ionic Liquid Type Electrolyte or a Volatile Type Electrolyte. 200 [I−]+[I3−] (mol/I) (EMIm−I concentration) Short-circuit current (mA) Fig. 4. Dependence of Equilibrium Potential and Open-circuit Voltage on the Concentration of Iodine Redox Couples in Ionic Liquid Type Electrolytes. 5 4 3 EMIm−I : I2 10 : 1 6:1 2:1 Gelation of the electrolyte solution was carried out using PVdF-HFP as a gelator. It was possible to form stable ion-gels by addition of 5 to 8 wt% PVdF-HFP (Fig. 6). Energy conversion efficiency of 3.8 % was obtained in a 5mm × 9mm sized cell using the ion-gel electrolyte, and it was about 85% of output compared with that of the ionic liquid type electrolyte system (Fig. 7). Some distinctive applications for electrode structures or cell manufacturing are expectable by solidification of electrolytes. Electrolyte solutions is injected into a cell through a small spout using capillary phenomenon, pressure difference etc. in general. In the case of an ionic liquid electrolyte, it is difficult to inject electrolytes into all over a cell by conventional techniques, since it is more viscous than conventional Ionic liquid electrolyte 2 1 0 0 0.5 1 1.5 2 [I−]+[I3−] (mol/I) (EMIm−I concentration) Fig. 5. Dependence of Short-circuit Current on the Concentration of Iodine Redox Couples in Ionic Liquid Type Electrolytes. 50 tive shift with increasing concentration of I−/I3− redox couples. There was a good correlation between Voc and equilibrium potential. Dependence of Jsc on the concentration of I−/I3− redox couples is shown in Fig. 5. Jsc increased with increasing concentration of I−/I3− redox couples to a maximum value. After that, it decreased. The increase of the photocurrent was attributed to increase of conductivity of electrolyte, the decrease of the current seems to be results of decrease of transparency of electrolyte solution which caused by increasing of light absorption of I3−. Watanabe et al. reported an enhancement of charge transfer rate; that was significantly when concentration of the redox species was high and, in addition, that of I− and I3− were comparable. That was due to the conjugation of physical diffusion and exchange reaction in the ionic liquid6), 7). At this time, the cell performance of our ionic liquid system has not beyond the one of conventional volatile electrolyte system. It would be improved by utilization of such a dense iodine composition and thin electrolyte layer. Ion-gel electrolyte Fig. 6. A View of an Ion-gel Electrolyte Prepared in a Glass Bottle Using PVdF-HFP as a Gelator. Current density (mA/cm2) 15 Ionic liquid type; η=4.5% Ion−gel type; η=3.8% 10 Ion−gel type (Ion−gel sheet method); η=2.4% 5 0 0 200 400 600 Voltage (mV) 800 Fig. 7. I-V Characteristics of Cells Using Various Type of Electrolyte. Electrode size was 5mm × 9 mm. Fig. 9. A View of 100 × 100 mm2 Ionic Liquid Type DSC with Current Collecting Grids. Counter−electrode TiO2 nano− porous layer TCO substrate • Penetration of an electrolyte • Adhesion of electrodes Fig. 8. Schematic Drawing of a Cell Manufacturing Process with Ion-gel Sheet. volatile electrolytes. We developed a new fabrication process of ion-gel sheet method, where ionic liquid was gelated and formed into a sheet and a cell was built up by sandwiching the ion-gel sheet (Fig. 8). An efficient manufacturing process by the roll-to-roll systems can be expected by the method in the future. A press condition was to load of 7 kgf/cm2 and 60 min. keep at 95 °C. The ion-gel liquefied at this temperature. By the process, energy conversion efficiency was 2.4% as shown in Fig. 7. 4.3 Large Sized DSC Using Ionic Liquid Type Cell Based on results as mentioned above, 100 × 100 mm2 sized cells using EMIm-TFSI as an electrolyte solvent were fabricated (Fig. 9). For the practical use of DSC, it is necessary to enlarge cells to the size of several square centimeter or more at least without significant decreasing of cell performance obtained by small size cells. However, in particular, conductivity of a transparent conductive oxide (TCO) layer on the glass substrate is not high enough, and only upscaling of a photo-electrode area results in decreasing cell performance significantly. Consequently, decreasing of internal resistance of a cell attributed to TCO is indispensable. TCO substrates for large sized cells need a properFujikura Technical Review, 2004 ITO/FTO double layer (as a TCO layer) Heat press Nickel metal grids (as current collecting grids) Glass substrate Fig. 10. Cross Sectional Structure of a Transparent Conductive Glass Electrode with Current Collecting Grids. 800 Electric current (mA) Ion−gel electrolyte sheet 600 Volatile electrolyte system η=4.3% (Total area) η=5.1%(Active area) Ionic liquid electolyte system η=2.3% (Total area) η=2.7%(Active area) Ion-gel electrolyte system η=2.0% (Total area) η=2.4%(Active area) 400 200 0 0 200 400 600 Voltage (mV) Without grids (Ionic liquid electrolyte system)* 800 η=0.2% Fig. 11. I-V Characteristics of 100 mm × 100 mm Sized Test Cells.* As a reference, without current collecting grids on a commercial FTO substrate (8~10Ω/square). ty not only highly conductive but also passive against a reaction chemically and electrochemically with the electrolyte including iodine redox couples. We developed high-conductive transparent glass electrode with FTO/ITO double layer and current collecting metal grids made of nickel (Fig. 10). Fig. 11 shows IV characteristics obtained by 100 × 100 mm2 sized cells. Prepared cells had following dimension; total photo-electrode area was 90 × 90 mm2, and 85 % of it was active area. Energy conversion efficiency was 2.7 51 % on the ionic liquid system, and 2.4 % on the ion-gel system based on the active area (2.3 % and 2.0 %, respectively, based on the total area). On the other hand, same size cell using normal TCO substrate that was not decreased resistance worked poor. In such a cell, a shape of I-V curve showed like straight line because of large internal resistance. As a result, it was found that a fabricated high-conductive transparent electrode could decrease internal resistance of the cell, and improved performance of large sized DSC. At this time, a gap of output between the ionic liquid system and the volatile solvent system was so large compared with results obtained by small sized cell studies. It was not yet completely make the most of optimized conditions, the study for improving cell performance is being continued. 5. Conclusion Photovoltaic performance of the cell using ionic liquid; EMIm-TFSI was evaluated as a function of I− / I3− redox couple contents. There was a good correlation between Voc and equilibrium potential of I − / I 3− redox couples in the electrolyte solution. Jsc increased with increasing concentrations of I− / I3− redox couples to a maximum value. After that, it decreased. The increase of the photocurrent was attributed to increase of conductivity of electrolytes, the decrease of the current seems to be results of decrease of transparency of the electrolyte solution. After optimization, energy conversion efficiency of η=4.5% was obtained. 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