Enhancement of Open-Circuit Voltage in Cu₂ZnSn(S,Se)₄ Solar Cells with Defect Passivation via LiF Post-Deposition Treatment

Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising candidate for low-cost, non-toxic, and highly stable alternatives to traditional light-absorbing materials. The incorporation of light alkali metals in CZTSSe solar cells has considered as a possible way to address limited power conversion efficiency caused by a significant open-circuit voltage deficit. In this study, fine interface engineering with effective passivation of detrimental defects via LiF post-deposition treatment (LiF PDT) was reported.[1] Three samples, the sample without a LiF layer, the sample with an as-deposited 0.7 nm LiF layer, and the sample with an 0.7 nm LiF layer followed by additional annealing at 200 ^oC, were prepared. Secondary ion mass spectroscopy shows that the diffusion of Li occurred only with additional heat treatment. Meanwhile, as-deposited LiF was lost during the CdS deposition process. Energy dispersive spectroscopy shows the possibility of Zn(S,Se) secondary phase migration to the surface of the absorber through additional heat treatment. To investigate the interface properties directly, mechanical dimpling of solar-cell devices was utilized. Uniform grinding condition was confirmed through Raman spectroscopy. Kevin probe force microscopy reveals that the work function difference between bulk and interface reduced as the sample was subjected to the LiF PDT process, resulting in a lower conduction band offset at the CdS/CZTSSe interface. Furthermore, an increase in surface photovoltage signal indicates reduced defect-assisted carrier recombination through Li incorporation. Li modified the dominant defect types from Cu_Zn and Zn_Cu to shallower Li_Zn and Li_Cu antisites, preventing recombination losses and promoting charge transport. As a result, the sample with LiF PDT process achieved 10.4 % efficiency with significant enhancement of <i>V</i>_OC.<br/><br/>[1] G. Lim et al., <i>Journal of Materials Chemistry A</i> (2023), In press