Improving open-circuit voltage deficit by interface passivation via Li treatment in Cu₂ZnSn(S,Se)₄ solar cells

The incorporation of light alkali elements in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells has attracted attention as an effective method to enhance the solar cell performance; such use increases open-circuit voltage (V_OC) through defect passivation. In this study, LiF post-deposition treatment (PDT) was used for Li-doping. A 0.7 nm thick LiF-doped layer was deposited on a CZTSSe absorber layer, and both layers were annealed at 200 °C, which resulted in the diffusion of Li into the absorber layer. CZTSSe thin-film solar cell devices were mechanically dimple-ground to investigate the band alignment at the CdS/CZTSSe interface. The lateral work function at the revealed CZTSSe surface was obtained using Kelvin probe force microscopy, and the decrease in the work function on the front side of CZTSSe was observed. The work function difference between the bulk and the interface decreased by about 77 mV in a CZTSSe sample subjected to LiF PDT, resulting in a lower conduction band offset at the CdS/CZTSSe interface. Furthermore, the photoinduced surface voltage increased by 89.5 mV after Li diffusion indicating the reduced trap density. Adjustment of the band alignment in a favorable way by modified defect types reduced carrier recombination, resulting in the enhancement of V_OC from 0.461 V to 0.488 V and the best cell efficiency of 10.4% was achieved.