Rudorffite Silver-Bismuth Iodides: Emerging Eco-Friendly Wide-Bandgap Absorbers for Indoor Photovoltaics

Indoor photovoltaics (IPVs) are poised to play a pivotal role in powering low-consumption electronics, including wireless sensors and Internet of Things (IoT) devices, by harvesting energy from ambient light. Among emerging absorbers, silver-bismuth iodide rudorffites (Ag_xBi_yI_x+3y) have attracted increasing attention as eco-friendly, wide-bandgap semiconductors offering strong visible absorption, intrinsic thermal and ambient stability, and the absence of toxic Pb. Recent years are rapid progress, with indoor power conversion efficiencies reaching ≈5% under 1000 lx light-emitting diode illumination. These advances are enabled by improved understanding of polymorphism, defect states, and charge-carrier dynamics, coupled with innovations in film fabrication via both solution and vapor processing. Strategies such as hot-air-assisted crystallization, compositional tuning, and hole transport material engineering have proven particularly effective in enhancing device performance and stability. This review summarizes the crystallographic, optical, and electronic properties of Ag-Bi-I rudorffites, compares fabrication approaches, and highlights recent device demonstrations, including semi-transparent and planar architectures. Remaining challenges-such as mitigating carrier localization, reducing deep defect densities, achieving scalable fabrication, and ensuring long-term stability-are discussed, along with opportunities for integration into practical IPV systems. Continued research may establish rudorffites as a sustainable, commercially viable alternative to Pb-based indoor photovoltaic technologies.