Atomic structure and structural disorder vs. device efficiency in Kesterite monograin solar cells with S/(S+Se)≈0.8

Abstract

Cu-poor/Zn-rich Cu₂ZnSn(S,Se)₄ (CZTSSe) monograins with S/(S+Se) = 0.8 were investigated using neutron powder diffraction to directly quantify Cu/Zn disorder and intrinsic point defects. According to their chemical composition, the monograins show a mixture of the off-stoichiometry types A and B. Monograins with a predominantly A-type off-stoichiometry exhibit reduced Cu/Zn disorder and increased V_Cu concentrations, whereas Zn_Sn defect concentration increases with increasing B-type off-stoichiometry. Zn_Cu antisite defects, belonging to both A and B off-stoichiometry types increase with increasing deviation from stoichiometry, and the combined defect concentration is lowest towards the stoichiometric point and A-type line, correlating with highest photovoltaic efficiency. Opposing trends are observed between optical band gap and solar cell stability, indicating that monograin solar cells with the same anion ratio but lower band gap energy are more stable. By correlating the atomic-scale defect scenario to macroscopic device performance, these results provide a quantitative framework for defect engineering in kesterite-type absorbers.