Solution-Processed Cd-Substituted CZTS Photocathode for Efficient Solar Hydrogen Evolution from Neutral Water

Abstract

Cu-kesterite has the appropriate optical properties of a photocathode in a solar water-splitting system; however, its performance is limited by poor bulk and surface transport properties. In this work, a photocathode design consisting of solution-processed Cu2Cd0.4Zn0.6SnS4 (CCZTS) photoabsorber coated with CdS/TiMo/Pt is reported to yield a photocurrent of 17 mA cm−2 at 0 VRHE, which is at least 3 times higher than pristine Cu2ZnSnS4. X-ray photoelectron spectroscopy depth profiling and UV photoelectron spectroscopy measurements reveal a deeper valence band maximum for the CCZTS absorber, which results in a “spike-like” 0.13-eV conduction band offset when integrated with CdS. Rotating dipole Hall and femtosecond transient reflectivity measurements revealed an improved mobility and carrier lifetime of the CCZTS absorber. Our investigation shows that the improved photocurrent can be attributed to the improved bulk properties of CCZTS absorber. Cu-kesterite has the appropriate optical properties of a photocathode in a solar water-splitting system; however, its performance is often limited by poor bulk and surface transport properties. In this work, for the purpose of improving the bulk properties of Cu2ZnSnS4 (CZTS) thin film, we replaced Zn with Cd partially to prepare a pure sulfide Cu2Cd0.4Zn0.6SnS4 (CCZTS)-based photocathode via sol-gel method. Through Cd substitution, we are able to improve the properties of bulk CZTS absorber, leading to a 4-fold increase in photocurrent. This improvement is attributed to the better charge collection, which may be due to the reduction in band tailing or band-edge defects and improved carrier transport properties. The strategy to improve intrinsic semiconductor properties by cation substitution can be pursued for further improving the performance of photocathodes for solar water splitting as well as photoabsorbers for solar cell devices in general. CZTS is a promising photoabsorber as a photocathode for water reduction to produce hydrogen but its performance is often limited by the presence of a high concentration of defects. Here, substitution of Cd improves the bulk quality of CZTS, resulting in a photocurrent enhancement of 4 mA cm−2 to 17 mA cm−2 at 0 VRHE. This improvement is attributed to the better charge collection, which may be due to the reduction in band tailing or band-edge defects and improved carrier transport properties.