<p>The growing demand for environmentally benign and stable photovoltaic technologies has intensified research into lead-free double perovskites as alternatives to conventional lead-based absorbers. In this work, a systematic design, simulation, and optimization study of Cs₂CuBiBr₆-based double perovskite solar cells is carried out using pristine and Al-doped TiO₂ as electron transport layers. Device performance is investigated through SCAPS-1D simulations with a focus on band alignment, charge transport, recombination mechanisms, defect densities, and contact energetics. The results demonstrate that Al incorporation into TiO₂ significantly enhances electron extraction, reduces interfacial recombination losses, and improves energy-level alignment, leading to a substantial increase in power conversion efficiency from 26.25% for pristine TiO₂-based devices to 30.55% for Al-doped TiO₂ architectures. Comprehensive parametric analyses further identify series resistance, absorber thickness, and interfacial defect states as critical factors governing device efficiency and open-circuit voltage. Overall, this study reports record-level simulated performance for Cs₂CuBiBr₆-based solar cells and provides physically grounded design guidelines for the development of stable, scalable, and environmentally sustainable lead-free photovoltaic technologies.</p>