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Cuprous(Ⅰ) iodide (CuI) is widely used as a conducting material in various optoelectronic devices. However, defect properties and instabilities lead to changes in the semiconductor properties of CuI, which seriously affects the stability of related optoelectronic devices. Therefore, in this paper, the decomposition and defect state properties of CuI films are investigated in depth. CuI was prepared by a simple iodination method and belongs to the zinc blende type structure, preferentially crystallizing along the (111) direction. Microscopic tests reveal that grains in CuI film are tightly bonded without obvious cracks or holes, and the grain size reaches 1.231 μm. In the subsequent spectral analysis, multiple emission peaks (free exciton and defect emission peak) were obtained by Gaussian fitting. The two major point defects (Cu vacancy (V Cu) and I vacancy (V I)) in CuI were analyzed theoretically, and the calculated defect energy levels matched the energies of the emission peaks. The results of Gaussian waveform fitting and density functional calculations indicate the presence of V Cu defects and V I defects in CuI. Significant variations in emission and electrical properties were observed in successive tests, indicating the relatively poor long-term stability of the CuI samples. The changes in the emission characteristics are mainly manifested by the weakening of the 416 nm emission and the enhancement of the 720 nm emission, while the changes in the electrical characteristics are mainly manifested by the increasing resistance. The above changes are attributed to the concentration of defect states, mainly due to the increase of V I defects and the decrease of V Cu defects, hence the severe iodine decomposition occurs in the CuI samples. To inhibit the decomposition, three schemes (multiple iodination, multilayer iodination, and encapsulation) were proposed, and relevant experiments were conducted. • Study on the relationship between the optoelectronic properties of CuI and semiconductor defects. • Demonstrated the presence of iodine decomposition in CuI samples. • Methods to inhibit the decomposition of CuI were investigated. [ABSTRACT FROM AUTHOR]