Effect of NaCl on the luminescent behavior of CsI thin films

In recent years, several reports have shed light on the growing interest in modulating the optoelectronic properties of cesium iodide (CsI) thin films for enabling their application in low-cost and eco-friendly luminescent devices. However, traditional techniques used for such modification like annealing, doping, and nano-structuring, often demand high temperatures or vacuum processing, limiting their large-scale scalability. Herein, we report a simple, eco-friendly method to tailor the luminescent properties of CsI thin films by incorporating NaCl via a simple aqueous based, low-temperature (∼50 °C) process. CsI/NaCl films prepared in a 1 : 1 molar ratio of CsI and NaCl, exhibited strong UV-excited photoluminescence at ∼415 nm, with significantly longer radiative decay times (τ₃ = 63.2 ns) than pure CsI films (τ₃ = 27.94 ns), indicating suppressed non-radiative recombination likely due to partial defect passivation by Na⁺ ions at grain boundaries or surfaces. Moreover, a weaker broad emission, attributed to deep trap states, was also observed at ∼531 nm under this UV excitation of ∼365 nm wavelength. Notably, humidity-controlled studies revealed that both PL intensity and emission wavelength increase with relative humidity (RH) up to ∼50%, then decline at higher humidity due to moisture-induced defects and Na⁺ ion migration. Furthermore, XRD and SEM/EDS analyses confirmed mixed-phase domains and interface-rich regions that contributed to such moisture sensitive behavior of these films. Overall, this low-cost, solution-based strategy offers a scalable route to tune optical properties and stability in halide films via green technology for optoelectronic applications.