Improvement of Cu2ZnSn(S,Se)4 Solar Cells by Adding N,N-Dimethylformamide to the Dimethyl Sulfoxide-Based Precursor Ink

Cu2ZnSn(S,Se)(4) (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 the whole cross section can be achieved with 20 % DMF addition. Accordingly, the power conversion efficiency of the device increases from 6.5 % to 8.6 % under AM 1.5G illumination. However, the efficiency decreases to 5.4 % when the DMF content is further increased to 30 %. Interface recombination and back contact barrier are found to be the main limitations of these devices.