Modulating Band Offset through Interface Engineering of CuSnSe-Based Heterojunctions for Efficient Charge Separation and Collection.

Cu₂SnSe₃ (CTSe) shows promise due to its wide solar absorption and tunable band gap, though low efficiency caused by interface recombination and crystallinity issues remains a challenge. In this study, a systematic and facile synthesis method for CTSe nanoparticles (NPs) is demonstrated, accompanied by an in-depth analysis of their growth mechanisms. A comprehensive experimental and theoretical investigation is conducted to explore the structural, compositional, optoelectronic, and band alignment properties of p-type CTSe NPs as a solar absorber, along with n-type CdSe and ZnSe NPs as buffer layers. Additionally, the band edge positions of the synthesized NPs are estimated using cyclic voltammetry (CV), UV photoelectron spectroscopy (UPS), and density functional theory (DFT), enabling the modulation of band offsets through interface engineering. The investigation revealed a staggered type-II band alignment at the CTSe/CdSe heterojunction, characterized by a minimal conduction band offset (CBO) of 0.06 eV. The findings from CV and UPS measurement supported by density functional theory-based calculations, suggests effective charge carrier separation and transport at the interface. The CTSe/CdSe heterojunction exhibited Schottky I-V characteristics, demonstrating a current of 1 mA in dark conditions. These findings demonstrate CTSe NPs' potential as an efficient absorber in thin-film solar cells, addressing interface recombination losses and improving performance.