Investigation of Band Alignment at CZTS/CdS and CZTS/ZnS Heterojunction Interface Utilizing X-Ray Photoelectron Spectroscopy

Band offsets limit charge carrier mobility at the interface and diminish power conversion efficiency in heterojunction solar cell devices. As a buffer layer, CdS often serves as an n-type heterojunction counterpart to p-type CZTS. ZnS could replace CdS due to its better lattice matching with CZTS. This study looks at how bands line up at heterojunction interfaces, specifically CZTS/CdS and CZTS/ZnS, to see if it is possible to use a ZnS buffer instead of CdS with a CZTS absorber. On substrates made of soda-lime glass, thin films were deposited. We deposited thin films of CZTS, CdS, and ZnS using sol-gel spin coating, CBD, and SILAR methods, following known protocols. XRD, FESEM, EDS, and NIR spectroscopy with UV-Vis all verified that thin films were present had the right optical and microstructural properties. We prepared two additional samples (CdS on CZTS and ZnS on CZTS) to investigate band alignment at heterojunctions. We conducted additional SCAPS-1D simulations to compare and verify the experimental results. A ZnS buffer produced a ’spike-like' conduction band offset (CBO), indicating effective carrier transfer. The observed CBO was +0.75 eV. SILAR-deposited ZnS thin films with spike-like CBOs may replace the CdS buffer layer.