Donor-Doping Optimization of In 2 S 3 Buffer Layers in CIGS Solar Cells: A TCAD Diagnostic of Transport-Recombination-Leakage Competition via R s and R sh

Abstract

This work presents a comprehensive numerical study aimed at optimizing the performance of CIGS (Cu(In, Ga)Se 2 ) solar cells incorporating indium sulfide (In 2 S 3 ) as a non-toxic buffer layer alternative to conventional CdS. Two-dimensional simulations were performed using the SILVACO ATLAS device simulator, based on the self-consistent solution of Poisson’s equation and carrier continuity equations within the drift-diffusion framework under standard AM1.5G illumination (100 mW∙cm − 2 ) at 300 K. The study focuses on the impact of the donor concentration N D in the In 2 S 3 buffer layer, varied from 10 16 to 7 × 10 18 cm − 3 . Its influence was evaluated through the main photovoltaic parameters: short-circuit current density ( J SC ), open-circuit voltage ( V OC ), fill factor ( FF ), and power conversion efficiency ( η ), as well as the parasitic resistances ( R s and R sh ). The results reveal a non-monotonic dependence of device performance on N D , highlighting the existence of an optimal trade-off between transport improvement and recombination enhancement. A maximum efficiency of 19.3% is obtained at N D = 6 × 10 16 cm − 3 . In the low-doping regime (~10 16 cm − 3 ), the efficiency remains limited (16.6%) mainly due to insufficient buffer-layer conductivity and relatively high series resistance, which constrain carrier extraction and reduce the fill factor. As N D increases toward the intermediate range (10 16 to 7 × 10 1 6 cm − 3 ), enhanced conductivity improves electron transport, reduces R s , and promotes better current collection, leading to simultaneous gains in J SC and FF . Beyond the optimum, however, performance degradation becomes dominant. At higher donor concentrations (≳5 × 10 17 cm − 3 ), η decreases and stabilizes around 14.0%, corresponding to an overall loss of approximately 27% compared with the optimum. This drop is primarily governed by the strong reduction of V OC , indicating that recombination mechanisms increasingly dominate over resistive improvements. Although FF may remain high at large N D due to reduced R s and increased R sh , these resistive benefits cannot compensate for the voltage loss. Overall, the analysis confirms that buffer-layer doping must be carefully optimized: moderate doping improves transport, whereas excessive doping irreversibly limits device efficiency by enhancing recombination and reducing V OC .