BiVO4/WO3/SnO2 Double-Heterojunction Photoanode with Enhanced Charge Separation and Visible-Transparency for Bias-Free Solar Water-Splitting with a Perovskite Solar Cell

Coupling dissimilar oxides in heterostruetures allows the engineering of interfacial, optical, charge separation/transport and transfer properties of photoanodes for photo electrochemical (PEC) water splitting. Here, we demonstrate a double-heterojunction concept based on a BiVO4/WO3/SnO2 triple-layer planar heterojunction (TPH) photoanode, which shows simultaneous improvements in the charge transport (similar to 93% at 1.23 V vs RHE) and transmittance at-longer wavelengths (>500 nm). The TPH photoanode was prepared by a facile solution method: a porous SnO2 film was, first deposited on a fluorine-doped tin oxide (FTO)/glass-substrate followed by WO3 deposition, leading to the formation of a double layer of dense WO3 and a WO3/SnO2 mixture at the bottom. Subsequently, a BiVO4 nanoparticle film was deposited by spin coating. Importantly, the WO3/(WO3+SnO2) composite bottom layer forms a disordered heterojunction enabling intimate contact, lower interfacial resistance, and efficient charge transport/transfer. In addition, the top BiVO4/WO3 heterojunction layer improves light absorption and charge separation. The resultant TPH photoanode shows greatly improved internal quantum efficiency (similar to 80%) and PEC water oxidation performance, (similar to 3.1 mA/cm(2) at 1.23 V vs RHE) compared to the previously reported BiVO4/WO3 photoanodes. The PEC performance was , further improved by a reactive-ion etching treatment and CoOx electrocatalyst deposition. Finally, we demonstrated a bias-free and stable solar water-splitting by constructing a tandem PEC device with a perovskite solar cell (STH similar to 3.5%).