ZnO_1−xS_x Solid Solution as Potential Buffer Layer Materials for Cu₂ZnSnS₄‐Based Thin Film Solar Cells: Structural and Interfacial Properties

Abstract The current low conversion efficiency of Cu₂ZnSnS₄ (CZTS)‐based thin film solar cells is mainly blamed on the high carrier recombination via interface states at the absorber‐buffer. In this work, ZnO_1−xS_x solid solution is exploited as the potential buffer layer materials to solve this issue by using density functional theory calculations. With the varying of solid solubility, the lattice constant of ZnO_1−xS_x solid solution follows the first‐order Vegard's Law, while its bandgap follows the second‐order Vegard's Law. Based on the systematical analysis of crystal structure and electronic properties of ZnO_1−xS_x solid solution, ZnO_0.375S_0.625 is screened as the optimal buffer layer material for CZTS‐based thin film solar cells, owing to the suitable bandgap (2.33 eV) and the smallest lattice mismatch (<7%). The CZTS/ZnO_0.375S_0.625 interface has less harmful interface state, small conduction band offset (−0.02 eV) and band bending (−0.23 eV). Moreover, its band alignment belongs to the staggered Type‐II heterojunction, and its built‐in electric field is in the same direction as the carrier migration, which is very favorable for the efficient separation and fast transfer of photogenerated carriers. Therefore, these interface features and effects of CZTS/ZnO_0.375S_0.625 heterojunction can significantly boost the photovoltaic performance of CZTS‐based thin film solar cells.