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

Abstract

Coupling dissimilar\noxides in heterostructures allows the engineering of interfacial,\noptical, charge separation/transport and transfer properties of photoanodes\nfor photoelectrochemical (PEC) water splitting. Here, we demonstrate\na double-heterojunction concept based on a BiVO₄/WO₃/SnO₂ triple-layer planar heterojunction (TPH)\nphotoanode, which shows simultaneous improvements in the charge transport\n(∼93% at 1.23 V vs RHE) and transmittance at longer wavelengths\n(>500 nm). The TPH photoanode was prepared by a facile solution\nmethod: a porous SnO₂ film was first deposited on a fluorine-doped\ntin oxide (FTO)/glass substrate followed by WO₃ deposition,\nleading to the formation of a double layer of dense WO₃ and a WO₃/SnO₂ mixture at the bottom. Subsequently,\na BiVO₄ nanoparticle film was deposited by spin coating.\nImportantly, the WO₃/(WO₃+SnO₂) composite\nbottom layer forms a disordered heterojunction, enabling intimate\ncontact, lower interfacial resistance, and efficient charge transport/transfer.\nIn addition, the top BiVO₄/WO₃ heterojunction layer improves light absorption\nand charge separation. The resultant TPH photoanode shows greatly\nimproved internal quantum efficiency (∼80%) and PEC water oxidation\nperformance (∼3.1 mA/cm² at 1.23 V vs RHE) compared\nto the previously reported BiVO₄/WO₃ photoanodes.\nThe PEC performance was further improved by a reactive-ion etching\ntreatment and CoO_<i>x</i> electrocatalyst deposition.\nFinally, we demonstrated a bias-free and stable solar water-splitting\nby constructing a tandem PEC device with a perovskite solar cell (STH\n∼3.5%).