WO₃ Coating Enhances the Performance of Cu₂O Photocathodes in Solar Water Splitting Cells

Cuprous oxide (Cu 2 O) is a promising photocathode material for unbiased solar water splitting cells. To improve the onset potential of Cu 2 O photocathodes, numerous efforts have been devoted to exploring a suitable n-type buffer layer material. However, previous studies mostly focused on optimizing the band alignment between the absorber and buffer layer, ignoring the role of surface states as reaction intermediates. Moreover, the high cost of Ga element and complicated multilayer process in current Ga 2 O 3 -based devices impede their large-scale application. In this work, the photoelectrochemical (PEC) performance of Cu 2 O photocathodes was investigated by coating an amorphous and crystalline WO 3 buffer layer with an annealing temperature of 100 and 200 °C, respectively. It is found that the onset potential of the Cu 2 O photocathode anodically shifts from 0.7 to 0.95 V RHE with an amorphous WO 3 coating, while a poorer PEC performance is observed on the electrodes with crystalline WO 3 coating. Further analysis indicates that the porous structure and photocapacitive surface states on the amorphous WO 3 layer provide a special electron-transfer pathway into the electrolyte, which is eliminated in the crystalline WO 3 layer. Our study offers a strategy for enhancing the photovoltage of photocathodes instead of optimizing band alignment.