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 (Na = 1013 to 1018 cm−3), interface defect density (Ni–t = 109 to 1018 cm−3), bulk defect density (Nt = 1012 to 1020 cm−3), and electron affinity (χ = 4.35–4.65 eV) were systematically investigated. Increasing Na significantly enhanced device performance by strengthening the internal electric field and increasing the carrier concentration, thereby improving Voc, 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 Voc = 0.817 V, Jsc = 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 Na, 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.