Designing a Transparent CdIn 2 S 4 /In 2 S 3 Bulk‐Heterojunction Photoanode Integrated with a Perovskite Solar Cell for Unbiased Water Splitting

The integration of photoelectrochemical photoanodes and solar cells to build an unbiased solar‐to‐hydrogen (STH) conversion system provides a promising way to solve the energy crisis. The key point is to develop highly transparent photoanodes, while its bulk separation efficiency (η sep. ) and surface injection efficiency are as high as possible. To resolve this contradiction, first a novel CdIn 2 S 4 /In 2 S 3 bulk heterojunctions in the interior of nanosheets is designed as a photoanode with high transparency and an ultrahigh η sep. up to 90%. Furthermore, decorating the ultrathin amorphous SnO 2 layer by atomic layer deposition, the surface oxygen‐evolution kinetics of the photoanode are increased significantly. As a result, the onset potential of the photoanode shifts negatively to 0.02 V vs RHE, and the photocurrent density boosts to 2.98 mA cm −2 at 1.23 V vs RHE, which is ten times higher than that of pristine CdIn 2 S 4 . Such a high‐performance photoanode enables the integrated metal sulfide photoanode–perovskite solar cell system to deliver a STH conversion efficiency of 3.3%.