Boost Voc of pure sulfide kesterite solar cell via a double CZTS layer stacks

•CZTS solar cells with Voc of ~740 mV and efficiency of ~7.6% are obtained via double CZTS layer stacks.•Doping gradient in the CZTS absorber is the key for Voc improvement.•Voids within CZTS absorber and severe band tailing problem lead to the reduced Jsc.

Low Voc is the major factor limiting the current efficiency of pure sulfide Cu2ZnSnS4 (CZTS) thin film solar cells. In this work, stacking two CZTS layers with different majority doping levels is employed to increase Voc. With this double-layer structure, the obtained highest efficiency is 7.57% with Voc of 734 mV, Jsc of 15.4 mA/cm2 and FF of 67.0%. Compared to the traditional device with single CZTS layer (Efficiency: 7.62%, Voc: 665 mV, Jsc: 20.0 mA/cm2, FF: 57.3%), the average Voc has been increased by 70-80 mV. However the Jsc was decreased due to voids within the CZTS and severe band tailing problem, limiting the further boost of efficiency for double-layer CZTS solar cell.