Vertical plane depth-resolved surface potential and carrier separation characteristics in flexible CZTSSe solar cells with over 12% efficiency

Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells have resource distribution and economic advantages. The main cause of their low efficiency is carrier loss resulting from recombination of photo-generated electron and hole. To overcome this, it is important to understand their electron-hole behavior characteristics. To determine the carrier separation characteristics, we measured the surface potential and the local current in terms of the absorber depth. The elemental variation in the intragrains (IGs) and at the grain boundaries (GBs) caused a band edge shift and bandgap (E_g) change. At the absorber surface and subsurface, an upward E_c and E_v band bending structure was observed at the GBs, and the carrier separation was improved. At the absorber center, both upward E_c and E_v and downward E_c-upward E_v band bending structures were observed at the GBs, and the carrier separation was degraded. To improve the carrier separation and suppress carrier recombination, an upward E_c and E_v band bending structure at the GBs is desirable.