High Efficiency Cu₂ZnSn(S,Se)₄ Solar Cells with Shallow Li_Zn Acceptor Defects Enabled by Solution‐Based Li Post‐Deposition Treatment

Abstract Lithium incorporation in kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) materials has been experimentally proven effective in improving electronic quality for application in photovoltaic devices. Herein, a feasible and effective solution‐based lithium post‐deposition treatment (PDT), enabling further efficiency improvement on the high‐performance baseline is reported and the dominant mechanism for this improvement is proposed. In this way, lithium is uniformly incorporated into grain interiors (GIs) without segregation at grain boundaries (GBs), which can occupy the Zn sites with a high solubility in the CZTSSe matrix, producing high density of Li_Zn antisites with shallower acceptor levels than the intrinsic dominant defect (Cu_Zn antisites). As a result, CZTSSe absorber with better p‐type doping is obtained, leading to a pronounced enhancement in fill factor and a corresponding gain in open‐circuit voltage and short‐circuit current and consequently a significant efficiency boost from 9.3% to 10.7%. This work provides a feasible alternative alkali‐PDT treatment for chalcogenide semiconductors and promotes a better understanding of the mechanism of Li incorporation in kesterite materials.