Indium(I) iodide (InI) is an inorganic compound with the chemical formula InI. It is a red crystalline solid that is sensitive to air and moisture. It is a source of indium(I) ions, which are relatively rare compared to the more common indium(III) state. The primary and most significant application of indium(I) iodide is as a crucial precursor material in the synthesis of perovskite semiconductors, particularly for photovoltaic applications. It is used in the fabrication of lead-free perovskite solar cells (PSC), such as those based on Cs2AgBiBr6 double perovskites or other indium-containing compositions like MA3In2I9. Its role is essential in tuning the bandgap, improving stability, and exploring new material systems for next-generation optoelectronic devices. Beyond photovoltaics, indium(I) iodide serves as a specialized reagent in synthetic inorganic and organometallic chemistry. It is used to prepare other indium complexes and can act as a catalyst or reactant in specific organic transformations. Due to its air and moisture sensitivity, it requires handling under an inert atmosphere, such as argon or nitrogen.