High-efficiency ultra-thin CIGSe solar cells: defect engineering and back-surface field design

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.