Optimization of (Cu2Sn)xZn3(1−x)S3/CdS pn junction photoelectrodes for solar water reduction

Abstract

Surface modification of p-type Cu₂ZnSnS₄ films with n-type semiconductors is an efficient way to enhance the properties for photoelectrochemical water reduction. However, the detailed optimization of the pn junction photoelectrodes and examination of the underlying mechanism are seldom reported. Herein, Cu₂ZnSnS₄-derived (Cu₂Sn)_0.45Zn_1.65S₃ (CTZS) nanocrystal films were first fabricated, and subsequently coated with n-type CdS layers to form CTZS/CdS photoelectrodes. To obtain an insight into the pn junction, we have examined the depletion region widths that extended into both CTZS and CdS layers. It was revealed that the CdS layer was fully depleted and the CTZS layer was partially depleted in CTZS/CdS photoelectrodes. Consequently, increased CTZS thickness led to the decreased charge separation, and increased CdS thickness resulted in the improved charge separation. Owing to the balance of light absorption and charge separation, CTZS/CdS films with moderate thicknesses of CTZS and CdS layers showed the highest photocurrent. Meanwhile, the annealing treatment for CTZS/CdS film was indispensable for the improved property. After Pt modification, the incident photon to current conversion efficiency could reach 24.7% at 450 nm, which was among the best values for Cu₂ZnSnS₄-based photocathodes. This work is expected to provide general guidance for exploring efficient photoelectrodes with pn junction structure.