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%).This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea under contracts NRF-2012M3A7B4049986 (Nano-Material Technology Development Program), NRF-2016M3D1A1027664 (Creative Materials Discovery Program), and NRF-2014R1A4A1008474 (Basic Research Lab Program). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01053785)