Fabricating Flexible Dye-sensitized Solar Cells by the Electrophoretic Deposition Method
Date Issued
2008
Date
2008
Author(s)
Chang, Yuan-Ruei
Abstract
Recently, dye-sensitized solar cells (DSSCs) have been fabricated on the plastic substrates, including the indium-tin-oxide coated polyethylene naphthalate (ITO-PEN), which is a commercially available transparent conductive plastic substrate. However, ITO-PEN could not endure the high temperature (500 oC) treatment, which is a necessary step for sintering TiO2 in DSSCs to achieve high cell efficiency. Fabrication of flexible DSSCs at low temperature with good cell performance thus becomes an important issue. In this study, the electrophoretic deposition (EPD) method was used to deposit mesoporous TiO2 film onto the plastic substrate as the photo-anode, and platinum was sputtered on the FTO-glass as the counter electrode. Nanocrystalline titanium dioxide (Degussa, P-25) was well dispersed in isopropanol (IPA) and deposited potentio statically onto the plastic substrate by the EPD followed by sintering at low temperature (150 oC), then the crack-free mesoporous TiO2 film was obtained. The cell performance was improved by chemical post-treatment through the drop coating of Ti(ΙV) tetraisopropoxide (TTIP) solution on the TiO2 films. The effect of TTIP concentration on the cell performance was investigated by EIS analysis and dye loading analysis, and the optimum TTIP concentration was found to be 1.2x10-3 mole/cm3. To enhance the amount of dye loading within the film, different thicknesses of the TiO2 layers as photo-anodes were obtained by changing the deposition time in the EPD. It was found that the optimum cell performance can be achieved at the TiO2 film thickness of 11 micrometer. Greater film thickness would promote the recombination reaction between the injected electrons within the film and the redox couple in the electrolyte. This also would prevent the electrolyte’s penetration into the film, thus decrease the cell performance. In addition, it was found from the BET measurement that low -temperature sintering process could also enhance the connection among TiO2 particles, and decrease the grain boundary resistance among particles. After optimizing the sintering process, the cell efficiency of 3.4% was achieved. Q25 TiO2 particles possessing higher zeta potential in the solution were used to replace P25 TiO2 particles. It is expected that Q25 powders suspend better in the EPD and may produce more conductive photo-anode. However, the result showed that the photo-anode preparing from Q25 particles performed slightly lower efficiency, as compared to that prepared from P25 particles. This might be that Q25 particle’s surface contained more impurities and occupied the dye adsorbing sites. The results also showed that P25 powders’ suspension property dosen’t harm the photo-anode performance. The photo-anodes prepared with Q25 powder were carried out at different applied electric fields in the EPD, including 200, 300, 400 and 500 V. The higher the voltage was used, the lower the cell efficiency was obtained. Because the aggregation rate of the particle was proportional to the electric field during the deposition, higher electric field was found to produce looser TiO2 film thus exhibited poor cell performance. If large TiO2 particles were incorporated in the photo-anode, it could harvest incident light and increase cell efficiency. In this study, 200 nm TiO2 particles (U200) were added to the EPD cell to co-deposit with Q25 particles and got a TiO2 film with a higher light harvesting efficiency. However, from the AC impedance analysis, it was found that this photo-anode had high interanl electric resistance, as compared to the film made without any large particles. Due to its looser structure, the optimal cell efficiency was only achieved at 2.4%. On the other hand, preparation of the photo-anodes with bilayer structure (inner layer was prepared by Q25 particles, and outter layer was composed of Q25 particles and light scattering particles) was improved the cell efficiency from 3.3% to 3.9%. This is because bilayer structure not only maintain a low internal electric resistance in the nanocrystalline film, but also trap the incident light by the outer scattering layer, which was prepared by mixing 200 nm and 25 nm TiO2 particle. TiO2 film with bilayer structure was subjected to UV/Ozone treatment to remove the residual organic molecular on the TiO2 surface, and the cell efficiency was improved. The values of the open–circuit photovoltage, short-circuit photocurrent density, fill factor and sunlight -to-electric energy conversion efficiency achieved were 830 mV, 7.2 mA/cm2, 0.71 and 4.2%, respectively, under illumination with AM 1.5 (100 mWcm-2) simulated sunlight. When the platinum counter electrode was changed from FTO-glass to ITO-PEN, it performed the same cell efficiency of 4.2%.inally, plastic DSSC was sealed by UV glue to study the stability of the cell. The result showed that the cell stored at room temperature for three hours performed the same as its original states. However, because the UV glue was degraded by the electrolyte and evaporated, the cell efficiency decayed 3.8% at seven hours compared with the initial performance.
Subjects
DSSC
Plastic substrate
Electrophoretic deposition
AC impedance
Stability
Type
thesis
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