Dual-absorber photoelectrodes are attractive candidates for solar water splitting due to their broadened absorption spectrum and improved photovoltage compared to single-absorber systems. Here, we present a hybrid photoanode constructed by growing hematite (α-Fe₂O₃) film on silicon microwires (SiMWs); the film is demonstrated to be promising for photoelectrochemical (PEC) water splitting. We simultaneously introduce extrinsic (i.e., Sn) and intrinsic (i.e., oxygen vacancy) dopants, which lead to an increase of the photocurrent from 0.89 to 3.12 mA cm⁻² at 1.23 V_RHE (under AM 1.5G irradiation) and a cathodical shift of 0.7 V in the starting potential of the anode current along with a relatively high photocurrent at a low potential (e.g., 0.33 mA cm⁻² at 0.45 V_RHE). Moreover, the relative concentration of the oxygen vacancies in α-Fe₂O₃ can be modulated, and the corresponding influence on the PEC performance is investigated in detail.

Hematite (α-Fe₂O₃) material has outstanding advantages in the photoelectrochemical water splitting (PEC-WS) field, while its generally low photogenerated-carrier separation (η_sep) and transfer (η_tran) efficiencies severely impede its further application. An internal electric field can significantly enhance η_sep and η_tran efficiencies, but the effective scope urgently needs to be expanded. This work innovatively implements gradient doping into an α-Fe₂O₃ film to achieve a direction-controlled and full-space internal electric field throughout the entire photoanode. The carrier-transport dynamics and density functional theory (DFT) calculations demonstrate that the direction of the internal electric field of an α-Fe₂O₃ photoanode with gradient-decreasing doping is consistent with that of the photoanode/electrolyte junction. The synergy effect between them can substantially enhance the photogenerated-carrier separation and transfer efficiency. Compared with the case of gradient-increasing doping, the α-Fe₂O₃ photoanode with gradient-decreasing doping exhibits the largest saturated photocurrent density and an enhancement of 40.7% (55.2%) for η_sep (η_tran) at 1.23 V_RHE under AM 1.5G illumination. This work provides a facile and general gradient doping strategy to expand the effective scope of an internal electric field for boosting the photoelectric conversion performance.