In this work, a simulation of the CZTSSe (Cu2ZnSn(S,Se)4) solar cell with Al/ZnO:Al/ZnO(i)/CdS/CZTSSe/Mo structure have been studied using the SCAPS-ID (Solar Cell Capacitance Simulator in one Dimension). The simulation results have been validated with real experimental results. Next, a novel structure is proposed in which a back surface field (BSF) layer is inserted in order to boost the solar cell performance, a CZTSSe layer has been used as (BSF) layer. The efficiency of CZTSSe solar cell increases from 12,3 to 15,3 % by inserting a BSF layer. Finally, an optimization of different physical parameters (Thickness and doping concentration) of BSF layer has been done, and the optimum values are determined. In this research work, the proposed structure of CZTSSe solar cells with optimum parameters showed higher functional properties. The maximum value of efficiency achieved was 16,98 % with <$EJ sub sc ~=~36,31> mA/cm<^>2, <$EV sub ac ~=~0,69> V, and FF = 69,60 % under 1,5G AM illumination.