Annealed Polycrystalline TiO₂ Interlayer of the n-Si/TiO₂/Ni Photoanode for Efficient Photoelectrochemical Water Splitting

High photovoltage generation from a photoelectrode is important for efficient solar-driven water splitting. Here, we report a thermal treatment process that greatly enhances photovoltage generation from an n-Si/TiO2/Ni photoanode. By selectively annealing the TiO2 interlayer, the photoanode generates a high photovoltage of 570 mV, which is very competitive as compared with photovoltages produced using other similar metal–insulator–semiconductor structures with earth-abundant metal catalysts. Different annealing conditions and junction layer thicknesses were systematically investigated. It is found that the optimal annealing temperature occurs between 500 and 600 °C. Within this temperature range, the deposited amorphous Ti is converted into polycrystalline anatase phase TiO2. The optimal annealing time scales linearly with TiO2 thickness and inversely with annealing temperature. The large photovoltage generation is attributed to the reduced defect states and improved junction barrier height by the annealed TiO2 interlayer. This study demonstrates that thermal annealing offers an attractive approach to modify the TiO2 interlayer material’s properties for photovoltage optimization.