Cation Vacancy‐Mediated Ultrafast Hole Transport in CuBi₂O₄ Photocathodes

As a promising photocathode candidate, tetragonal CuBi₂O₄ (CBO) has been studied extensively in recent years. As an intrinsically p-type material, its acceptor sites originate from the cation vacancies, which are also a potential cause of hindered hole utilization in photocathodes. In this study, the ultrafast transport dynamics of the valence band hole states in CBO photocathodes were investigated by varying their atomic composition and manipulating the p-type character. As a comprehensive ultrafast optical transient absorption spectroscopy (TAS) investigation of compositionally manipulated CBO that combines both ex situ and in situ TAS experiments with photoelectrochemical (PEC) performance tests, the study reveals the polaron formation tendencies of the valence band (VB) holes at cationic vacancy sites. Therefore, it draws a complete picture of the ultrafast hole transport dynamics and provides valuable insights into the hindrance of the photocurrent generated in CBO.