Epitaxial β-Ga2O3 with tailored UV response for high-performance and stable perovskite solar cells

Ultraviolet exposure accelerates degradation of perovskite absorbers and charge-transport interfaces, whereas internal ultraviolet-protection layers can complicate fabrication and affect charge extraction. Here, Si-doped epitaxial β-Ga2O3 (β-Ga2O3: Si) films were grown on sapphire by metal–organic chemical vapor deposition and used as external encapsulation covers for inverted p–i–n perovskite solar cells. Si incorporation preserves the monoclinic β-Ga2O3 phase and comparable film thickness, while promoting grain coalescence, reducing surface roughness, and improving large-area optical uniformity. More importantly, it tailors the ultraviolet response of the β-Ga2O3: Si/sapphire structure, lowering transmittance across the 200–400 nm range, with the value at 254 nm decreasing from 48.83% to 30.15%. The altered ultraviolet transmission is accompanied by redistributed optical constants, a modified surface electronic configuration, a more lattice oxygen dominated surface environment, and enhanced 254 nm ultraviolet photoresponse. The β-Ga2O3:Si/sapphire cover also shows uniform surface mechanical response, effective environmental barrier performance, and thermal stability. Perovskite solar cells employing this cover deliver an initial power conversion efficiency of 26.16% and retain 85.86% of their initial efficiency after 1000 h of continuous 254 nm irradiation, compared with 49.00% for bare-sapphire-encapsulated controls. These results establish β-Ga2O3: Si/sapphire as a multifunctional external cover for ultraviolet spectral management and durable operation of perovskite solar cells.