Interface engineering approach of in-air-processed Sb₂S₃solar cells enabling 7.5% AM 1.5G device efficiency and an 18% indoor milestone performance

Among the wide range of emerging absorber materials under development, Sb₂S₃, with its optimal bandgap of 1.7 eV and distinctive anisotropic properties, stands out as a material offering an excellent trade-off between intrinsic stability, cost-effective deposition, and high performance under both, AM 1.5G and indoor illumination. While current strategies focus on absorber optimization, interface engineering remains largely unexplored. In this work, we introduce, for the first time, a ZnO interfacial layer deposited via ultrasonic spray pyrolysis (USP) in air at the TiO₂/Sb₂S₃ interface. This innovation extends to a fully cadmium-free device architecture, in which all key layers—TiO₂ electron transport layer, ZnO interlayer, and Sb₂S₃ absorber—are processed entirely via USP under ambient conditions. A record efficiency of 7.5% under AM 1.5G illumination and an 18% indoor milestone performance is demonstrated for a TiO₂-based Sb₂S₃ solar cell platform, featuring a 150 nm thick absorber—the thinnest Sb₂S₃ absorber delivering such performance to date. Comprehensive characterization reveals the critical role of the ZnO interfacial layer, highlighting its impact on absorber grain size, interface and bulk defects, and device functionality. We propose refinements to indoor measurement protocols, accounting for variations in source temperature and incident power, paving the way for reliable indoor PV performance evaluation.