Numerical Optimization of Sb2Se3 Thin-Film Solar Cells Using SCAPS-1D: Band Alignment, Defect Engineering, and High-Efficiency Device Design

Antimony selenide (Sb2Se3) has emerged as a compelling earth-abundant, low-toxicity absorber for next-generation thin-film photovoltaics owing to its quasi-one-dimensional (Q1D) ribbon structure, near-ideal direct band gap (~1.1-1.3 eV), high absorption coefficient (>1e5 cm-1), and benign grain boundaries. Despite these merits, the best experimental devices remain limited to ~10% power conversion efficiency (PCE), far below the Shockley-Queisser limit, primarily because of a severe open-circuit voltage (Voc) deficit rooted in unfavourable band alignment and bulk/interface recombination. A predictive, physically transparent device-design framework that simultaneously resolves these loss channels is still lacking. Here, a comprehensive one-dimensional drift-diffusion analysis is performed using the Solar Cell Capacitance Simulator (SCAPS-1D) for a substrate-configuration ITO/i-ZnO/buffer/Sb2Se3/HTL/Au architecture. The model is first calibrated against a fabricated CdS/Sb2Se3 reference cell (PCE ~ 7.1%), then advanced through a staged optimization of absorber thickness, carrier and defect densities, and band gap, followed by buffer/conduction-band-offset (CBO) engineering, hole-transport-layer (HTL) and back-surface-field selection, and control of parasitic resistances, interface defects, and temperature. The analysis identifies a defect-tolerance window (bulk defect density <= 1e14 cm-3), an optimal 1.0-1.1 um absorber doped at ~1e16 cm-3, and a spike CBO of +0.1 to +0.3 eV (best realized with a Zn(O,S) buffer) that suppresses interface recombination. Introducing a Cu2O HTL/back-surface field raises the built-in potential and minimizes back-contact recombination. The optimized device attains PCE = 21.6%, Voc = 0.812 V, Jsc = 32.4 mA/cm2, and FF = 82.1%, a threefold gain over the baseline. The study provides quantitative design rules and a transferable methodology for Cd-free, high-efficiency antimony-chalcogenide photovoltaics and indoor-light harvesters.