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

Abstract 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 _x Bi _y I _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.