Temperature‐Dependent Photoluminescence and Grain Size Control in a Prototypical Lead‐Free Bismuth Halide Perovskite

Lead‐free perovskites are non‐toxic solution‐processed semiconductors that promise applications in solar cells, light‐emitting diodes, and advanced photodetectors with broad and easily tunable spectral sensitivity. However, understanding excitonic properties and growth conditions are key for the development of efficient and stable materials. This study explores the temperature‐dependent photoluminescence (PL) and the influence of anti‐solvent on crystal size in methylammonium bismuth iodide (MA 3 Bi 2 I 9 ), a prototypical bismuth‐based lead‐free perovskite. Using a one‐step spin‐coating method, low‐dimensional hexagonal MA 3 Bi 2 I 9 crystals with controllable grain sizes ranging from 1.4 to 36 μm are developed, employing chlorobenzene as an anti‐solvent. Temperature‐dependent micro‐PL measurements reveal multi‐component PL emissions below 199 K and interestingly, below 144 K, bound excitonic recombination dominates the PL spectra. The observed free and bound exciton recombination peaks are consistent with the MA 3 Bi 2 I 9 exciton binding energy of ≈340 meV reported in earlier studies. Observed temperature‐dependent PL spectra provide deeper insights into the excitonic properties of MA 3 Bi 2 I 9 , which have strong correlation with optoelectronic device performance, thereby providing a quick tool to assess as‐synthesized lead‐free perovskite films for defect‐site density and binding energy. Furthermore, the potential of concentration‐dependent anti‐solvent engineering in obtaining controllable bismuth perovskite crystal grain sizes for optoelectronic applications is underscored.