Insights into charge separation of heterojunction photoanodes: overlooked significance of directionality of built-in electric field.

The primary challenge in enhancing the efficiency of solar energy conversion lies in improving the charge separation efficiency for photocatalytic (PC)/photoelectrochemical (PEC) processes, and the construction of heterojunctions represents one of the most effective strategies for promoting charge separation in the bulk phase. However, further investigation is necessary to elucidate the impact of the heterojunction interface structure on charge separation. In this paper, CdS/CdO and CdO/CdS films were prepared using an in-situ phase transformation approach to achieve a well-defined heterojunction interface structure for comparison. The research findings reveal significant disparities in the initiation potential and charge separation efficiency between CdS/CdO and CdO/CdS, with CdS/CdO exhibiting superior PEC properties. Notably, the photocurrent density of CdS/CdO reaches 6.5 mA cm at 1.23 V. This can be attributed to the fact that the favorable alignment of the built-in electric field (BEF) in CdS/CdO with the applied bias direction facilitates efficient charge separation and transmission at the heterojunction interface, as supported by the experimental results, density functional theory (DFT) calculations and finite element analysis (FEA). This finding offers a novel insight for the design and fabrication of high-performance heterojunction photocatalysts and photoelectrocatalysts.