In Situ Spectroscopic Ellipsometry for Thermochromic CsPbI₃ Phase Evolution Portfolio

Thermochromism is a color change phenomenon for a compound when it undergoes a heating/cooling process. The observation of this phenomenon mostly relies on the naked eye, rather than an exact optical technique, which retards better new thermochromic material design and is detrimental to the understanding of its fundamental mechanism. In this paper, we present a facile and noninvasive in situ ellipsometric method to monitor the optical properties evolution of CsPbI₃ film in its thermochromic process with three notable phase transitions from room temperature to 340 °C. We monitored the in situ structure evolution of CsPbI₃ film by three techniques, namely, temperature-dependent X-ray diffraction, Raman spectroscopy, and spectroscopic ellipsometry. Four phases of CsPbI₃, that is, α, β, γ, and δ, could be detected in temperature-dependent spectroscopic ellipsometry measurement. Especially, we found the most expected “stable” α and γ phases show temperature-dependent bandgap evolution even in their phase-stable temperature region. The in situ spectroscopic ellipsometry therefore could function as a facile and noninvasive technique for detection of phase and band evolution with potential applications in smart windows, semiconductor, and optical devices.