Light‐Driven Defect Engineering of Oxide Heterojunctions via Concurrent Sn Redistribution and Photocrystallization for Transparent, Flexible, Self‐Powered UV Photodetectors

ABSTRACT Low‐temperature defect engineering is essential for flexible oxide optoelectronics but remains challenging because vacuum‐deposited ultrathin oxide heterojunctions often exhibit limited crystallinity and abundant defect states. Here, we demonstrate a UV‐driven defect‐engineering strategy for flexible, transparent, and self‐powered ultraviolet (UV) photodetectors based on SnO 2 /Cu 2 O heterojunctions. High‐power UV illumination simultaneously induces photocrystallization of amorphous SnO 2 and photon‐assisted redistribution of interfacial Sn 4+ ions into the Cu 2 O layer. The diffused Sn 4+ ions effectively passivate Cu‐related defects, whereas photocrystallization increases the relative oxygen‐vacancy fraction in SnO 2 , synergistically enhancing charge transport, suppressing charge recombination, and reinforcing light absorption across the heterojunction. In addition, highly transparent poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate (C‐PEDOT: PSS) top electrodes enable top‐side UV illumination while eliminating parasitic UV absorption by the polymer substrate. As a result, the self‐powered photodetectors achieve a responsivity of 122.4 mA W −1 and an estimated Johnson‐noise‐limited noise equivalent power (NEP) of 4.68 × 10 −14 W Hz −0.5 , representing more than a fivefold enhancement over untreated devices. Furthermore, a lightweight large‐area transparent device directly mounted on an indoor window enables reliable real‐time UV index monitoring under changing weather conditions. These findings highlight UV‐driven Sn 4+ redistribution as a versatile low‐temperature defect‐engineering strategy for transparent and flexible oxide optoelectronic sensing platforms.