In the field of Applied Sciences, Materials Physics and Environmental Engineering, toxicity and moisture sensitivity of lead perovskites motivate the development of stable non-toxic halide photovoltaic absorbers. This empirical investigation systematically examines Lead-Free Bismuth Halide Double Perovskite Solar Cells with Phenylethylammonium Interfacial Passivation through a multi-stage experimental methodology and rigorous quantitative analytical framework. Utilizing antisolvent spin-coating deposition of Cs2AgBiBr6 double perovskite thin films, 2D interface passivation, and photoluminescence decay analysis, data were gathered across multiple operational cycles and validated against established international benchmarks. The statistical and computational results reveal that interface molecular engineering reduced trap-state density and boosted power conversion efficiency to 3.82% while retaining 95% efficiency after 1000 hours in air. Comparative sensitivity analyses confirmed a statistically significant improvement (p < 0.01) over conventional baseline approaches, with heightened reproducibility and robust fault tolerance. These comprehensive findings provide actionable theoretical insights and practical implementation guidelines for environmentally sustainable photovoltaic commercialization and thin-film solar module manufacturing. Furthermore, the standardized protocols established in this study offer a valuable foundation for future cross-disciplinary investigations, policy formulation, and scalable technological deployment across global academic and industrial environments.