Performance Degradation in Solution-Processed Cu2ZnSn(S,Se)4 Solar Cells Based on Different Oxidation States of Copper Salts

The performance stability of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is one of the key issues for its commercialization. Currently, Cu+ and Cu2+ salts have been successfully used to prepare CZTSSe solar cells, but comparative studies on the performance stability of the two types of solar cells are scarce. In this work, the performance degradation processes of CZTSSe solar cells, stored for 249 days under dry conditions, prepared respectively with Cu2+ and Cu+ salts (denoted Cu2+-cell and Cu+-cell, respectively) are investigated. It is found that the Cu+-cell has a higher initial PCE but worse performance stability than the Cu2+-cell. By calculation of the contribution percentage of VOC, JSC, and FF to the PCE and JL, Rs, Rsh, and (A, J0) to VOC, JSC, and FF for the two types of solar cells with the best initial PCE, it is concluded that the contribution percentages of JL, Rs, Rsh, and (A, J0) to a decreased PCE for the Cu2+-cell are nearly equal, while the decreased PCE for the Cu+-cell dominantly comes from a greatly decreased Rsh and sharply increased J0. The great decrease in Rsh is due to the formation of SnSe2, which may result from its chemical reaction pathway to fabricate the CZTSSe absorber. The sharp increase in J0 may be attributed to the formation of SnZn antisite defects. This work indicates that it is important to consider the Cu oxidation state of the salts to improve the stability of solution-processed CZTSSe solar cells.