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Pb-free metal halide perovskites are excellent light-harvesting materials for photodetector applications because of their inherent remarkable optoelectronic properties with lower-temperature processability. However, to date, the realization of device-quality Pb-free metal halide films is still a great challenge. Here, using a single-step spin coating method, we have synthesized Pb-free methylammonium bismuth iodide [(CH3NH3)3Bi2I9 (MBI)] perovskite films using different solvents, such as γ-butyrolactone (GBL), 2-methoxyethanol (2ME), dimethyl sulfoxide (DMSO), and N,N-dimethyl formamide (DMF). Furthermore, MBI perovskite films were subjected to various characterization techniques to investigate and analyze morphological, optical, and structural properties. The formation of MBI thin films in various solvents was confirmed by XRD analysis and Raman spectroscopy. As evident from the XRD pattern, MBI belonged to the P63/mmc space group and had a hexagonal crystal structure. SEM analysis showed that the MBI film synthesized using different solvents has randomly oriented hexagonal-shaped grains parallel or orthogonal to the surface of the substrate. The synthesized perovskite films strongly absorbed light in the visible region with a higher absorption coefficient (∼105 cm–1) and an optical band gap of ∼1.85 to 2.0 eV, as evident from UV–vis and photoluminescence spectroscopy. TG analysis showed that MBI perovskites are thermally stable until ∼325 °C. It was found that MBI perovskite films prepared using a 2ME solvent have excellent substrate coverage with the smallest grain size and excellent optical properties. Finally, a photodetector was fabricated using an MBI perovskite film, exhibiting the fast rise and decay times of 0.79 and 0.36 s, with a responsivity of ∼8.0 × 10–2 mA/W and a detectivity of ∼5.0 × 108 Jones at a 0 V bias. The present results open up a new avenue for Pb-free, highly stable MBI perovskites for future self-biased photodetectors.

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