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.

Flexible kesterite thin-film solar cells under stress

Understanding the stress-induced phenomena is essential for improving the long-term application of flexible solar cells to non-flat surfaces.Here,we investigated the electronic band structure and carrier transport mechanism of Cu2ZnSn(S,Se)4(CZTSSe)photovoltaic devices under mechanical stress.Highly efficient flexible CZTSSe devices were fabricated controlling the Na incorporation.The electronic structure of CZTSSe was deformed with stress as the band gap,valence band edge,and work function changed.Electrical properties of the bent CZTSSe surface were probed by Kelvin probe force microscopy and the CZTSSe with Na showed less degraded carrier transport compared to the CZTSSe without Na.The local open-circuit voltage(VOC)on the bent CZTSSe surface decreased due to limited carrier excitation.The reduction of local VOC occurred larger with convex bending than in concave bending,which is consistent with the degradation of device parameters.This study paves the way for understanding the stress-induced optoelectronic changes in flexible photovoltaic devices.