Modifying Surface Termination by Bidentate Chelating Strategy Enables 13.77% Efficient Kesterite Solar Cells.

Surfaces display discontinuities in the kesterite-based polycrystalline films can produce large defect densities, including strained and dangling bonds. These physical defects tend to introduce electronic defects and surface states, which can greatly promote nonradiative recombination of electron-hole pairs and damage device performance. Here, an effective chelation strategy is reported to suppress these harmful physical defects related to unterminated Cu, Zn, and Sn sites by modifying the surface of Cu2ZnSn(S,Se)4 (CZTSSe) films with sodium diethyldithiocarbamate (NaDDTC). The conjoint theoretical calculations and experimental results reveal that the NaDDTC molecules can be coordinate to surface metal sites of CZTSSe films via robust bidentate chelating interactions, effectively reducing surface undercoordinated defects and passivating the electron trap states. Consequently, the solar cell efficiency of the NaDDTC-treated device is increased to as high as 13.77% under 100 mW cm-2 illumination, with significant improvement in fill factor and open-circuit voltage. This surface chelation strategy provides strong surface termination and defect passivation for further development and application of kesterite-based photovoltaics.