Abstract
Dye-sensitized solar cell (DSSC) has received increasing interest, which achieved moderate conversion efficiency using low cost material and simple manufacturing apparatus. The high efficiencies of DSSCs have been achieve using TiO2 nanocrystalline fabricated on heavy, rigid, and expensive fluorine-doped tin-oxide (FTO) glass. Metal foil substrates enable extension of DSSCs to novel applications because they are thin, lightweight and flexible. Ti foil is an excellent alternative due to its superior physical and chemical properties. Long-term stability and enhancement of conversion efficiency of DSSC are two important subjects for industrializing DSSCs. The recombination occurs at the Ti substrate/electrolyte is one of the factors that limit the conversion efficiency. In order to improve photovoltaic performance, it is essential to suppress the recombination loss at the Ti substrate. With back-illuminated construction, partial incident light is absorbed by counter electrode and electrolyte. The improvement of light harvesting efficiency is particularly important for Ti-based DSSC. This study introduced a feasible and efficient method to prepare blocking layer. The surface of Ti substrate could be transformed into TiO2 thin underlayer by direct oxidation method. The nature of Ti metal was utilized to fabricate underlayer, and no tricky coating process was required. Introducing underlayer into photoelectrode could reduce the recombination with triiodide ion in the electrolyte. A sponge-like and conformal TiO2 underlayer was successfully fabricated by using hydrogen peroxide oxidation Ti foil. This underlayer serves as a charge recombination barrier layer at the nanocrystalline TiO2/substrate interface, and suppresses recombination reaction. This sponge-like TiO2 underlayer increases the electrical contact area between the Ti substrate and nanocrystalline TiO2 helping nanocrystalline TiO2 attach to the Ti substrate. This study compares the performance of DSSCs that were subjected to different Ti surface treatments. Electrochemical impedance spectroscopy results confirm that the proposed sponge-like TiO2 underlayer increased the open-current voltage (VOC) and fill factor (FF) due to prolonged electron life time (�輑ff), and minimized resistance at TiO2/Ti interface (RCT). By using hydrogen peroxide (H2O2) with a basic NH4OH agent, a thin TiO2 layer with a grooved structure was formed on Ti substrate, and the Ti substrate was textured. This grooved TiO2 thin layer also increased the electrical contact area at the nanocrystalline TiO2/Ti substrate interface, leading to reduced charge transfer resistance and improved fill factor (FF) of dye-sensitized solar cells. The TiO2 underlayer can also serve as a charge recombination barrier layer at the Ti substrate/electrolyte interface. Compared with DSSCs with non-treated and H2O2-treated Ti substrates, the DSSC with H2O2/NH4OH-treated Ti substrate showed increased conversion efficiency with a significant improvement in short-circuit current density (JSC). Reflection UV-vis spectroscopy and incident photon-to-current efficiency confirmed that the increased JSC was the result of a consistent reflection spectrum with Ru complex dye absorption. Surface modification by H2O2/NH4OH combined with optimized thickness of blocking layer and minimized gap in two electrodes achieved a high efficiency of 7.28 %. The degradation mechanism of Ti substrate-based DSSCs was studied after a thermal aging test. The deteriorated component of Ti-based DSSCs was clarified by chemical impedance spectroscopy and scanning electron microscope. This indicated that an unfavorable reaction occurred on the Pt counter electrode, leading to a decrease of the fill factor. The device components, that is, counter electrode and electrolyte, were separated from the cell to trace the degradation factor. The factors for catalytic ability degradation of counter electrode were analyzed by cyclic voltammetry. These results indicate that I2 and Li+ coupled with water led to an unfavorable reaction on Pt counter electrode, and that water content in the electrolyte may accelerate Pt degradation.