Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics

Bismuth‐based hybrid perovskites are candidates for lead‐free and air‐stable photovoltaics, but poor surface morphologies and a high band‐gap energy have previously limited these hybrid perovskites. A new materials processing strategy to produce enhanced bismuth‐based thin‐film photovoltaic absorbers by incorporation of monovalent silver cations into iodobismuthates is presented. Solution‐processed AgBi2I7 thin films are prepared by spin‐coating silver and bismuth precursors dissolved in n‐butylamine and annealing under an N2 atmosphere. X‐ray diffraction analysis reveals the pure cubic structure (Fd3m) with lattice parameters of a=b=c=12.223 Å. The resultant AgBi2I7 thin films exhibit dense and pinhole‐free surface morphologies with grains ranging in size from 200–800 nm and a low band gap of 1.87 eV suitable for photovoltaic applications. Initial studies produce solar power conversion efficiencies of 1.22 % and excellent stability over at least 10 days under ambient conditions. Solution‐processed AgBi2I7 thin films are developed and deployed to produce air‐stable and lead‐free photovoltaic devices. The AgBi2I7 thin film crystallized in the cubic phase with dense and pinhole‐free surface morphologies, and has an optical absorption that covers the visible spectrum and into the near‐infrared region. The AgBi2I7 devices showed stable operation over 10 days under ambient conditions.