Light absorption induced band bending in p-type Cu₂(Zn,Sn)(S,Se)₄ thin-film photovoltaic cells: local Raman imaging and scanning probe microscopy

Kesterite-structured Cu₂ZnSn(S,Se)₄ (CZTSSe) is one of the most promising materials for highly efficient and low-cost thin-film solar cells because of its appropriate optical and electrical properties. The best efficiency of CZTSSe solar cells is 12.6%, which was obtained using a hydrazine process, while the materials used in this study show efficiency of 12.3%. Note that the CZTSSe thin-films were grown by sputtering and subsequent sulfur-selenium treatment. Raman spectroscopy imaging was utilized to measure local built-in voltage and its local composition. According to the surface potential results, we were able to observe upward band bending near grain boundaries, meaning intra-grains take a role as current path by collected electrons. Unlike Cu(In,Ga)Se₂, defects in CZTSSe thin-films have deep energy levels, which can be charge carrier recombination center and cannot be suitable for a current path. By measuring surface photo-voltage, we observed the significant changes in energy band bending under illumination, implying the existence of these interfacial states. The main contribution of the research is related to the realization of new way for carrier separation enhancing power conversion efficiency of the kesterite-based thin-film solar cells.