The impact of defect density on a CGS/CIGS tandem solar cell’s performance

Copper Indium Gallium Selenide (CIGS) solar cells became a leading thin-film photovoltaic device because of their stability, relatively low production costs, and high efficiency, particularly when used in tandem configurations. A monolithic tandem cell is proposed in this paper. Copper Gallium Selenide (CGS) and CIGS structures constitute the two p-n junctions of this cell. The studied tandem cell’s electrical parameters were optimized using a 2D Silvaco-Atlas simulator, considering the variation of the indium concentration, the temperature, and the thickness of the bottom cell’s absorber layer. Comparisons between the performance of the CGS/CIGS tandem solar cell, considering and excluding defect density, were then done for the optimal value of Indium concentration x = 0.81 which corresponds to the minimum value of defect density N_t = 1.66·10¹⁵ cm⁻³. When the density of material defects is considered, the efficiency of the proposed tandem cell attained 20.35 %, with an open-circuit voltage (V_OC) of 1.58 V, a short-circuit current density (J_SC) of 16.26 mA cm⁻² and a Fill Factor (FF) of 79.40 %. However, an efficiency of η = 32.58 % is obtained when the defect density is ignored, highlighting the impact of defect density on the overall performance of the monolithic CGS/CIGS tandem solar cell.