Lead iodide thin films deposited by sputtering: The effect of deposition temperature on the optical and structural properties

Lead iodide (PbI₂) is a 2D layered semiconductor used in several electronic applications, such as solar cells, X-ray, and gamma-ray detectors. Most of its properties have been reported for monocrystals or polycrystalline thick films used in high-energy photon detectors. As for thin films used in other optoelectronic devices, the reported properties are limited to the conditions adopted in manufacturing the devices. Furthermore, very little is known about the properties of films deposited by sputtering. Here, we investigate the optical and structural properties of PbI₂ thin films deposited by rf-sputtering a PbI₂ target. The deposition temperature significantly influences the film's properties, as determined by X-ray, scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-vis, and Raman spectroscopy. A common characteristic at all temperatures was forming metallic lead (Pb) segregated in the surface of films, with concentration depending on the deposition temperature. These lead clusters were successfully converted into PbI₂ using an iodination process, allowing the synthesis of pure PbI₂ films without lead segregation. The activation energy for the reaction between Pb clusters and iodine vapor was determined by adopting the Arrhenius equation. It was also observed that converting PbI₂ film into perovskite through the two-step process, by immersion of the PbI₂ film into methylammonium iodide solution, transforms the segregated lead into perovskite. The sputtering technique allows the deposition of uniform films over large areas compatible with roll-to-roll processes, which are desired to produce large-area detectors and perovskite solar cells.