This study presents a comprehensive SCAPS-1D simulation of an ultra-thin CIGSe/CdS/i-ZnO/ITO solar cell with a 420 nm absorber layer, focusing on the influence of key physical parameters and back surface field engineering. The effects of acceptor doping density in CIGSe (N a = 1013 to 1018 cm-3), interface defect density (N i-t = 109 to 1018 cm-3), bulk defect density (N t = 1012 to 1020 cm-3), and electron affinity (χ = 4.35-4.65 eV) were systematically investigated. Increasing N a significantly enhanced device performance by strengthening the internal electric field and increasing the carrier concentration, thereby improving V oc, fill factor, and efficiency. In contrast, elevated interface and bulk defect densities led to severe recombination losses and significant degradation of all photovoltaic parameters. Optimal band alignment was obtained at χ ≈ 4.35 eV, corresponding to a slight negative conduction-band offset that facilitates carrier transport and suppresses recombination. Recombination analysis showed stable performance of the radiative recombination coefficient over the range 10-16 to 10-8 cm3 s-1, while Auger recombination became dominant at coefficients above 10-23 cm6 s-1. Among the investigated back surface field layers, Cu2O provided the best performance due to its wide band gap (2.2 eV) and strong back-surface electric field, yielding a maximum simulated efficiency of ∼40.3% with V oc = 0.817 V, J sc = 30.03 mA cm-2, and FF = 82.88%. Capacitance-voltage and Mott-Schottky analyses revealed that capacitance increases from 57.6 to 109.9 nF cm-2 with increasing N a, and the built-in potential ranges from 0.80 to 1.32 V, confirming enhanced junction properties. These results provide practical guidelines for optimizing ultra-thin CIGSe solar cells through defect control, band alignment tuning, and back surface field design.
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