Moisture-stimulated reversible thermochromic CsPbI3-xBrx films: In-situ spectroscopic-resolved structure and optical properties

Abstract We report a composition-dependent, moisture-stimulated reversible thermochromic phenomenon in CsPbI3-xBrx (0 ≤ x ≤ 2) films. The mechanism for the thermochromism is attributed to a reversible phase transition between perovskite (H-T phase) and nonperovksite (L-T phase) structure with the transition temperature points dependent on the I/Br ratio. Temperature and composition dependent thermochromic behavior of the films was recorded via in-situ x-ray diffraction, spectroscopic ellipsometry and digital photography in heating process. The color of the film changes from black to dark red with increasing Br content of 0 ≤ x ≤ 2 for the H-T phases, while chromically changed into yellowish-gray (semitransparent) to reddish-gray (transparent) for that of L-T phase. The three representative compositions of x = 0, 1, and 2 were selected as modelling compositions to illustrate the temperature-dependent opitcal constants evolution. We found two linear relationships: (i) T L - T → H - T = - 80 x + 340 (°C) for the phase transition point, and (ii) Δ E g C s P b I 3 - x Br x = 0.17 x + 1.76 ( e V ) for the bandgap evolution with composition. The composition dependent phase evolution and temperature-dependent optical parameters in CsPbI3-xBrx family not only provide the fundamental structure and thermal stability data for further understanding the relationship between composition and physical properties, but also important for the design and fabrication of effective devices for the promising optical, optoelectronic and thermochromic applications.