Improvement of Cu₂ZnSn(S,Se)₄ Solar Cells by Adding <i>N</i>,<i>N</i>‐Dimethylformamide to the Dimethyl Sulfoxide‐Based Precursor Ink

(CZTSSe) solar cells based on dimethyl sulfoxide (DMSO) Cu-Zn-Sn-S precursor ink have seen tremendous progress in recent years. However, the wettability between the ink and Mo substrate is poor, owing to the high viscosity of the highly concentrated Cu-Zn-Sn-S ink. Herein, a solvent engineering process is proposed in which N,N-dimethylformamide (DMF) is added into the DMSO-based Cu-Zn-Sn-S ink for the deposition of CZTSSe thin-film absorbers in air. The addition of DMF significantly improves the wettability between the precursor ink and Mo substrate. The DMF/(DMF+DMSO) ratio also plays a critical role in determining the crystal quality of the resulting CZTSSe absorber and the device performance. The grain size of CZTSSe thin films increases with increasing DMF/(DMF+DMSO) ratio. Particularly, large grains through…