In Situ Spectroscopic Ellipsometry for Thermochromic CsPbI₃ Phase Evolution Portfolio

Abstract

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.

Simplifying Molecular Transport in Polyelectrolyte Multilayer Thin Films

“…[ 88,141 ] Apart from these optical methods, ellipsometry can also be used for measuring thickness of the wide range of thin films. [ 142–146 ] Using ellipsometry variation in the PEM, thickness can be analyzed for studying salt induced molecular exchange or changes with respect to polymer concentration, pH, temperature, etc. [ 113,147–149 ] But, ellipsometry require specialized substrates and complex modelling for data interpretation.…”

Hybrid Perovskite Degradation from an Optical Perspective: A Spectroscopic Ellipsometry Study from the Deep Ultraviolet to the Middle Infrared

“…A similar effect is observed in the MA‐free perovskite, with another kink appearing at around 2.85 eV (Figure 8c). This coincides with the onset of a strong absorption peak in “yellow phase” non‐perovskite δ‐CsPbI 3 , [ 27,43 ] suggesting the formation of this phase in addition to PbI 2 as the films degrade. For comparison purposes, the k spectra for PbI 2 and δ‐CsPbI 3 are shown in Figure 8d.…”

“…A prominent concern is the rate at which MA and FA are depleted at temperatures of 100 °C and above, with substantial effects already after 30–60 min. Given that most standard solution processing recipes require annealing at similar temperatures and durations [ 1–10,17,20,23,27,34–37,42 ] (including the ones used in this work), concerns regarding the actual stoichiometry of pristine perovskite films can arise. Indeed, the data presented here suggests that, depending on annealing time, it is likely that some of the organic cations will have been lost during fabrication, leading to final film stoichiometries that differ significantly from the perovskite stock solutions.…”

“…Typical film thicknesses were in the range of 400–550 nm. One or two additional Tauc–Lorentz oscillators were considered to account for the formation of PbI 2 and δ‐CsPbI 3 as the perovskites degraded, with absorption onsets around 2.4 [ 7 ] and 2.85 eV, [ 6,27 ] respectively. These features can be observed in Figure 8.…”

“…Spectroscopic ellipsometry has been used widely for the characterization of perovskites, [ 1,7,8,23–26 ] including studies on the effects of thermally induced degradation. [ 17,27 ] Expanding on these works, this study employs a very wide spectral range, spanning from the deep ultraviolet (DUV) to the middle infrared (MIR). In this way, changes in the concentration of the organic cations can be inferred from their vibrational absorption bands in the MIR, while the formation of PbI 2 and non‐perovskite δ‐CsPbI 3 as degradation by‐products was modeled from their effects in the DUV to near infrared (NIR).…”