Above 10% efficiency earth-abundant Cu2ZnSn(S,Se)(4) solar cells by introducing alkali metal fluoride nanolayers as electron-selective contacts

Abstract

The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterite based Cu2ZnSn(S,Se)(4) solar cells by simply introducing alkali metal fluoride nanolayers (similar to several nm NaF, or LiF) to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front ITO decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency (similar to 11.5% in the cell effective area) CZTSSe cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well.