3.17% efficient Cu2ZnSnS4–BiVO4 integrated tandem cell for standalone overall solar water splitting†
A 7.27% efficient Cu2ZnSnS4-based solar water splitting photocathode with long term stability of 24 hours was first reported by modification of a HfO2/CdS/HfO2 sandwich buffer layer. The effect of the deposited HfO2 atomic layer film at the two interfaces of CdS/CZTS and Pt/CdS or the electrolyte on the photoelectrochemical photocurrent, photovoltage and stability of the CZTS-based photocathode was systemically investigated with the following methods: time course observation of micromorphology transformation trends, light intensity-dependent Voc values (to reflect the variation in interfacial carrier recombination) and EIS spectra (for determining the changes in resistance that influence the charge transfer and trapping inside of the bulk semiconductors or surface/interfaces of the photocathode). The HfO2 layer between the CZTS and CdS interfaces significantly decreased the carrier recombination ratio inside the photocathode, and the HfO2 layer between the CdS and Pt or the electrolyte efficiently optimized the electrode/electrolyte interface that facilitates the charge transfer without being trapped. Furthermore, the CZTS–BiVO4 photocathode with a photoanode tandem cell was fabricated, and its unbiased solar to hydrogen conversion efficiency was achieved 3.17%, while a large 5 × 5 cm integrated CZTS photocathode in tandem with the BiVO4 photoanode with superior long term stability over 60 hours was assembled for the first time.