Room-Temperature Synthesis and Photodetector Performance of All-Inorganic Lead-Free Cs _<b>3</b> Bi _<b>2</b> <i>X</i> _<b>9</b> ( <i>X</i> = Cl, Br, I) Perovskite Nanocrystals

All-inorganic halide perovskites have gained significant interest as environmentally friendly alternatives to Pb-based perovskites for optoelectronic applications. Here, we report the preparation of Pb-free inorganic Cs₃Bi₂<i>X</i>₉ (<i>X</i> = Br, I, Cl) perovskite using a simple room-temperature antisolvent recrystallization method on soda-lime glass substrates. We thoroughly analyzed and compared the structural, optical, and morphological properties of these films. X-ray diffraction analysis revealed that the Cs₃Bi₂Cl₉ perovskite has the largest crystallite size (51 nm) compared to the Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ perovskites. The perovskite films demonstrated enhanced crystallinity and exhibited hexagonal and orthorhombic phases with distinct space groups. Raman spectroscopy confirmed the successful formation of the Cs₃Bi₂<i>X</i>₉ phase. Field emission scanning electron microscopy analysis showed that Cs₃Bi₂<i>X</i>₉ exhibited irregular morphologies with noticeable voids across all samples. The surfaces were predominantly covered with uneven, spherical-like granules. The surface morphology displays dense packing with evident porosity and particle sizes ranging from submicrons to a few micrometers. The elemental analysis of the Cs₃Bi₂<i>X</i>₉ perovskite revealed the formation of nearly stoichiometric films. The light absorption edge of Cs₃Bi₂<i>X</i>₉ varied from 1.8 to 3.2 eV, depending on the halide composition. The formation of Cs₃Bi₂<i>X</i>₉ perovskites was confirmed using X-ray photoelectron spectroscopy analysis. Finally, we fabricated photodetectors based on Cs₃Bi₂<i>X</i>₉. Among these, the Cs₃Bi₂Cl₉ perovskite-based photodetector exhibited superior performance, with a detectivity of 29.6 × 10⁸ Jones, photoresponsivity of 1.14 mA/W, a fast rise time of 0.21 s, a decay time of 0.35 ms, and an internal quantum efficiency of 2.45 × 10^-1 %. These findings demonstrate the potential of Cs₃Bi₂Cl₉ perovskite-based photodetectors as promising candidates for next-generation, lead-free photodetector devices.