TITLE:
Donor-Doping Optimization of In2S3 Buffer Layers in CIGS Solar Cells: A TCAD Diagnostic of Transport-Recombination-Leakage Competition via Rs and Rsh
AUTHORS:
Youssou Gning, Marcel Biagui, Moussa Toure, Aly Toure, Mamadou Lamine Samb
KEYWORDS:
CIGS Solar Cells, In2S3 Buffer Layer, Donor Concentration, TCAD Simulation, SILVACO ATLAS, Interface Recombination, Series Resistance, Shunt Resistance
JOURNAL NAME:
Journal of Materials Science and Chemical Engineering,
Vol.14 No.4,
April
2,
2026
ABSTRACT: This work presents a comprehensive numerical study aimed at optimizing the performance of CIGS (Cu(In, Ga)Se2) solar cells incorporating indium sulfide (In2S3) 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 ND in the In2S3 buffer layer, varied from 1016 to 7 × 1018 cm−3. Its influence was evaluated through the main photovoltaic parameters: short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (η), as well as the parasitic resistances (Rs and Rsh). The results reveal a non-monotonic dependence of device performance on ND, highlighting the existence of an optimal trade-off between transport improvement and recombination enhancement. A maximum efficiency of 19.3% is obtained at ND = 6 × 1016 cm−3. In the low-doping regime (~1016 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 ND increases toward the intermediate range (1016 to 7 × 1016 cm−3), enhanced conductivity improves electron transport, reduces Rs, and promotes better current collection, leading to simultaneous gains in JSC and FF. Beyond the optimum, however, performance degradation becomes dominant. At higher donor concentrations (≳5 × 1017 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 VOC, indicating that recombination mechanisms increasingly dominate over resistive improvements. Although FF may remain high at large ND due to reduced Rs and increased Rsh, 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 VOC.