Employing Overlayers To Improve the Performance of Cu₂BaSnS₄ Thin Film based Photoelectrochemical Water Reduction Devices

Earth-abundant copper-barium-thiostannate Cu₂BaSnS₄ (CBTS)-based thin films have recently been reported to exhibit the optoelectronic and defect properties suitable as absorbers for photoelectrochemical (PEC) water splitting and the top cell of tandem photovoltaic solar cells. However, the photocurrents of CBTS-based PEC devices are still much lower than the theoretical value, partially due to ineffective charge collection at CBTS/water interface and instability of CBTS in electrolytes. Here, we report on overcoming these issues by employing overlayer engineering. We find that CdS/ZnO/TiO₂ overlayers can significant-ly improve the PEC performance, achieving saturated cathodic photocurrents up to 7.8 mA cm^-2 at the potential of -0.10 V versus reversible hydrogen electrode (RHE) in a neutral electrolyte solution, which is much higher than the best bare CBTS film attaining a photocurrent of 4.8 mA cm^-2 at the potential of -0.2 V versus RHE. Finally, our results suggest a viable approach for improving the performance of CBTS-based PEC cells.