The main issues associated with these materials are low stability against thermal and photoinduced degradation (e.g., MAPbI 3 is sensitive to light and heat, (10, 11) moisture, (12) and oxygen (12)) and the potential environmental impact associated with scale up of new lead-based technologies, a concern which is shared with other families of functional materials such as ferroelectrics. (4, 6, 7) Furthermore, films can be cast from solution (8) or vapor deposited (9) and contain inexpensive, earth-abundant elements. This stems from their possession of a suitable band gap as set by the Shockley-Queisser limit, (1) high absorption coefficients suitable for thin film technology, (2) good carrier mobilities, (3-5) and low recombination rates. Lead halide based perovskites and perovskite-related semiconductors, particularly methylammonium lead iodide (MAPbI 3), are of great interest due to their remarkable performance in photovoltaic devices. AgBiI 4 shows good stability toward the AM1.5 solar spectrum when kept in a sealed environment and is thermally stable below 90 ☌. Band edge states closely resemble those of BiI 3 however, the p-type nature of AgBiI 4 with low carrier concentration is more similar to MAPbI 3 than the n-type BiI 3. The indirect optical band gap of AgBiI 4 is measured to be 1.63(1) eV, comparable to the indirect band gap of 1.69(1) eV measured for BiI 3 and smaller than that reported for other bismuth halides, suggesting that structures with a close-packed iodide sublattice may give narrower band gaps than those with perovskite structures. In addition, we demonstrate the existence a noncubic CdCl 2-type polymorph by isolation of nontwinned single crystals. The structure of AgBiI 4 is based unambiguously upon a cubic close packed iodide sublattice, but it presents an unusual crystallographic problem: we show that the reported structure, a cubic defect-spinel, cannot be distinguished from a metrically cubic layered structure analogous to CdCl 2 using either powder or single crystal X-ray crystallography. AgBiI 4 powder, crystals, and polycrystalline films were synthesized by sealed tube solid state reactions, chemical vapor transport (CVT), and solution processing, respectively, and their structural, optical and electronic properties are reported.
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