(171a) Solution-Processed, Lead-Free Metal Iodide Thin Films for Photovoltaic Applications

We have synthesized and deposited metal iodide films of materials such as BiI₃ and SbI₃ thin films were using solution-based methods. We have found that Lewis bases such as THF and DMF readily form Lewis acid-base adducts with BiI₃, which gives them high solubility in these solvents at high concentrations (<400 mg/mL) suitable for spincoating thin films of BiI₃. We find that exposure to solvent during processing can have a dramatic effect on film morphology. For example, solvent annealing BiI₃ thin films under relatively mild temperatures (â?¤ 100 °C) resulted in pronounced grain growth and recrystallization of the film. The thin films were chemically stable for at least several months and can withstand extended (several hours) annealing at temperatures below 150 °C. Annealing in air after spincoating was found to be beneficial to photovoltaic device performance due to the formation of a BiOI layer at the BiI₃ surface which facilitates hole transport. In non-optimized BiI₃ solar cells, we have achieved power conversion efficiencies of 1.0 % demonstrating the potential of BiI₃ as a nontoxic, all-inorganic metal halide photovoltaic absorber layer.