Cu-Doped NiO _x as an Effective Hole-Selective Layer for a High-Performance Sb ₂ Se ₃ Photocathode for Photoelectrochemical Water Splitting

Although antimony triselenide (Sb ₂ Se ₃ ) has been intensively investigated as a low-cost p-type semiconductor for photoelectrochemical (PEC) water splitting, most previous studies focused on only the top interface of Sb ₂ Se ₃ photocathodes. Herein, a solution-processed Cu-doped NiO _x (Cu:NiO _x ) thin film is proposed as an effective bottom contact layer for the Sb ₂ Se ₃ photocathode. The photocurrent density of the Sb ₂ Se ₃ photocathode is improved to a record-high level of 17.5 mA cm ^-2 upon the insertion of Cu:NiO _x capable of blocking the recombination at the back interface, while facilitating hole extraction. Electrochemical impedance spectroscopy and intensity-modulated photocurrent spectroscopy, in conjunction with other observations, indicate that the enhanced photocurrent is due to the improved quality of the bottom contact without a noticeable change in the top interface. This study not only provides new insight into the role of the bottom contact layer in photocathodes, but also is an important step toward efficient PEC H ₂ production via a solution-processable Earth-abundant photoelectrode.