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 (Eg) change. At the absorber surface and subsurface, an upward Ec and Ev band bending structure was observed at the GBs, and the carrier separation was improved. At the absorber center, both upward Ec and Ev and downward Ec-upward Ev 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 Ec and Ev band bending structure at the GBs is desirable. To minimize carrier recombination loss and improve the solar cell characteristics, it is necessary to form a conduction band minimum (Ec) and valence band maximum (Ev) upward band bending structure over the entire absorber at the grain boundaries and to form a current path in the intragrains.image