Suppressing deep-trap formation in Cu₂ZnSnS₄-based solar cells

Cu₂ZnSnS₄ (CZTS) is a cheap, nontoxic, easy-to-synthesize, and stable solar cell absorber material. Despite these advantages over Si, GaAs, CdTe, CuIn_xGa_1-xSe₂, and hybrid perovskites, CZTS-based solar cells are plagued by low efficiencies (12.6%) compared to the Shockley-Queisser limit (33.7%). This 21.1% efficiency deficit was suggested recently to be due to the formation of defect clusters involving the Sn_Zn antisite, which induces deep-trap states and therefore promotes carrier recombination. In this talk, we present density functional theory calculations of defect thermodynamics to show that these deep-trap defect clusters can be suppressed in CZTS via Ge- and Se-codoping. Additionally, we will describe an electrostatic mechanism for the suppression of deep defects by Ge that can be readily applied to other kesterite-type absorber materials and therefore used to design next-generation CZTS-inspired solar cells.