Cesium lead chloride (CsPbCl₃) is a promising wide-bandgap perovskite for ultraviolet optoelectronic devices. [1–3] However, the preparation of high-quality CsPbCl₃ films via low-temperature solution-process deposition is limited by the poor solubility of lead chloride and cesium chloride in common perovskite solvents such as dimethyl sulfoxide and N,N-dimethylformamide. [4] Halide exchange has emerged as a practical approach to address the solubility limitation and to convert solution-deposited CsPbBr₃ thin films into CsPbCl₃ films by replacing bromine ions with chlorine ions. [5] Yet, the depth-resolved dynamics of the exchange process remain poorly understood. Spectroscopic ellipsometry (SE) offers a nondestructive and depth-sensitive method to investigate the exchange process and provide accurate optical constants required for device modeling. Here, we employed an established optical dispersion model based on the Tauc-Lorentz approach combined with two harmonic oscillators, as reported in our previous study. [6] SE was used to monitor the halide exchange in CsPbBr₃ thin films exposed to chlorine vapors for 20 minutes, and data were recorded at 5-minute intervals. A multilayer optical model was further developed to resolve a CsPbCl₃-rich surface, a mixed CsPbBr_3-xCl_x region described by effective medium approximation, and a residual CsPbBr₃ bottom layer.  The halide exchange progress is detected by changes in the refractive index (n) and the extinction coefficient (k). The spectral features of the refractive index shift toward shorter wavelength (from ~522 nm to ~416 nm), and the extinction coefficient  moves from 512 nm to 413 nm. The latter corresponds to a bandgap increase from 2.3 to 3.0 eV, which is comparable with the reported value for CsPbCl₃. [6,7] These results demonstrate that SE provides a direct view of halide exchange dynamics and precise optical constants, both of which are essential for designing and optimizing CsPbCl₃ optoelectronic devices.

References

[1]         J. Lu et al., Advanced Materials 29, 1700400 (2017).

[2]         J. Zhang et al., RSC Adv. 7, 36722 (2017).

[3]         Y. Zhang et al., Angew Chem Int Ed 60, 9693 (2021).

[4]         A. Sadhanala et al., Nano Lett. 15, 6095 (2015).

[5]         G. Cen et al., (2020).

[6]         S. Khan et al., APL Energy 3, 026108 (2025).

[7]         L. Y. Bai et al., Journal of Luminescence 227, 117592 (2020).