Limiting effects of conduction band offset and defect states on high efficiency CZTSSe solar cell

We investigated limitation factors of high efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, where the CZTSSe absorbers were made by using sulfo-selenization process. CZTSSe absorbers with two S/(S+Se) ratios, ~ 0.12 (Se-rich) and ~ 0.22 (S-increased), were prepared by varying the sulfo-selenization temperature. The Se-rich CZTSSe solar cells were found to have larger conduction band offset (CBO) between the absorber and the buffer, which was reflected in the kinked J-V curves at low temperatures. Considering that the larger CBO prevents electron transport from absorber to buffer and resultantly reduces short circuit current and fill factor, it could be possible limitation factor of the high efficiency solar cell. Contrary to Se-rich solar cells, S-increased solar cells showed reduced CBO and no kinked J-V curve. However, deep defects were found to be generated, which induced defect centers of charge recombination both at interface and in bulk of the absorber. The larger CBO in Se-rich CZTSSe solar cell and deep defects in S-increased CZTSSe solar cell are observed even in ~ 12% efficiency solar cells. Thus, we believe that these possible limitation factors should be resolved to achieve high efficiency kesterite CZTSSe solar cell above 12%.