Na-doping-induced modification of the Cu2ZnSn(S,Se)4/CdS heterojunction towards efficient solar cells

Abstract It is very important to understand why a small amount of alkali metal doping in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells can improve the conversion efficiency. In this work, Na-doped CZTSSe is prepared by a simple solution method, and then the effects on the surface properties of the absorber layer, the buffer layer growth, and the modifications of the solar cell performance induced by the Na doping are studied. The surface of the absorber layer is more Cu-depletion and less roughness due to the Na doping. In addition, the contact angle of the surface increases because of Na doping. As a consequence, the thickness of the CdS buffer layer is significantly reduced and the optical losses in the CdS buffer layer are decreased. The difference of quasi-Fermi levels (EFn – EFp) increases with a small amount of Na doping in the CZTSSe solar cell, so that open circuit voltage (VOC) increased significantly. This work offers new insights into the effects of Na doping on CZTSSe via a solution-based approach and provides a deeper understanding of the origin of the efficiency improvement of Na-doped CZTSSe thin film solar cells.