Influence of Iodination Methods at Room Temperature on Crystallinity and Electrical Property of CuI Thin Films

Copper(I) iodide (CuI) is an attractive p-type transparent conductor for electronics and optoelectronics, yet its device-level performance is highly sensitive to the iodination route that defines conversion uniformity, crystallinity, and grain structure. In this work, we systematically compare three room-temperature iodination pathways vapor, solid, and liquid iodination using identical Cu precursor films and quantify how each method alters grain size and the resulting transparency–conductivity trade-off. X-ray diffraction peak broadening and scanning electron microscope analysis reveal iodination-dependent grain-growth behavior, indicating that the iodine supply mode controls nucleation density and crystallite/grain development. These microstructural differences directly translate into distinct optoelectronic metrics: solid iodination provides the highest transmittance, whereas liquid iodination delivers the lowest sheet resistance, while vapor iodination exhibits comparatively poorer performance due to less favorable conversion and morphology. By correlating grain characteristics with transmittance and sheet resistance, we establish a processing-structure-property guideline and propose a practical room-temperature CuI synthesis protocol that enables route selection and optimization for application-specific transparent p-type electrodes.