Numerical Optimization of Thickness and Optical Band Gap of Absorber and Buffer Layers in Earth-Abundant Cu₂ZnSnS₄ Thin-Film Solar Cells

In this work, numerical simulations were employed to examine the influence of thickness and band gap energy of the Cu₂ZnSnS₄ (CZTS) absorber and Zn(O,S) buffer layer on the performance of the earth-abundant and nontoxic Mo/Cu₂ZnSnS₄/Zn(O,S)/i-ZnO/ZnO:Al structure. Firstly, simulation was performed on the CZTS-based solar cell with experimental values of thickness (610 nm) and band gap energy (1.51 eV) obtained for the absorber layer CZTS. We found an open-circuit voltage V_oc = 1.17 V, a short-circuit current density J_sc = 23.26 mA/cm², a fill factor FF = 57.31%, and a conversion efficiency η = 15.61%. Then we varied the thickness (from 500 nm to 3000 nm) and gap (from 1.40 eV to 1.60 eV) of CZTS thin film and concluded that the optimized thickness and band gap energy were 2400 nm and 1.48 eV, respectively. Finally, we used these values to found the optimal performance of the device. The optimized results were FF = 21.24%, J_sc = 28.05 mA /cm², V_oc = 3.63 V, and η = 21.64%. It is noted that the solar cell performance remained stable by varying the thickness and gap energy of the Zn(O,S) buffer layer.