Design Predictions of n–n Heterojunction Based Photoanode for Efficient Unbiased Overall Solar Water Splitting

There are multiple requirements for maximizing the efficiency of photoelectrochemical overall water splitting using a single n‐type semiconducting photoanode. Many visible‐light‐responsive photoanode materials have been developed but overall water splitting by such materials without external bias has hardly been demonstrated. Herein, the aim is to scale the impact of performance improvement by an n–n heterojunction photoanode comprising an n‐type thin modification surface layer over an n‐type photon absorber, using a finite element method. For the n‐type photon absorber, the semiconductor properties are initially set to those of a well‐studied visible–light‐absorbing Ta3N5, and key properties are varied. It is demonstrated that an appropriate donor density and thickness of the thin surface layer induce adequate energy band bending in the photon absorber, resulting in a dramatic reduction of the onset potential of the photoanode. Thus, even though the quality of the photoanode is poor (e.g., the donor density is high (≈1 × 1019 cm−3)), the optimized surface layer enables the n–n heterojunction photoanode to facilitate bias‐free solar water splitting. These exhaustive investigations could provide design guidelines for obtaining effective photoanodes using an n‐type surface modification layer, such as cocatalysts and hole collection layers. Conditions of an n‐type semiconductor modification layer with physical properties suitable for an oxygen evolution reaction (OER) photoanode have been numerically simulated. Adjusting the donor density in the modification layer and visible‐light‐responsive photon absorber is important for achieving both a negative onset potential and a high current density. The thin modification layer with low donor density leads to bias‐free OER.