Flexible Cu2ZnSn(S,Se)4 thin film solar cells with lithium doping via doctor blading

Lithium doping is beneficial for enhancing the performance of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. However, the conventional doping strategy of spin-coating of the precursor ink containing Li source suffers from mass loss due to Li redissolution during the layer-by-layer deposition. In this study, we report an effective Li-doping strategy for preparing CZTSSe thin film on a flexible Mo foil substrate via the doctor-blading approach. In addition, we investigate the effect of Li doping on device performance. The grain size becomes larger with increasing Li-doping content, which could be attributed to the Li–Se liquid phase formation during the selenization process. Moreover, lithium can enter the CZTSSe lattice, and it tends to accumulate on the surface, passivating the defects and improving the functionality of the p–n junction. Thus, an appropriate Li-doping content enhances device performance. Furthermore, the strategy of codoping with Li and Na is preliminarily explored. Interestingly, the results reveal that introducing Na enhances the Raman signal of S vibration at 328 cm−1 compared with that of the 10%-Li sample. The efficiency of the flexible CZTSSe solar cells is further improved to 7.59%. Thus, this work provides a simple and effective strategy for Li doping via the doctor-blading method and demonstrates the interplaying mechanism between the codoped Li and Na.