Amine Gas‐Induced Reversible Optical Bleaching of Bismuth‐Based Lead‐Free Perovskite Thin Films

Abstract Reversible optical property changes in lead‐free perovskites have recently received great interest due to their potential applications in smart windows, sensors, data encryption, and various on‐demand devices. However, it is challenging to achieve remarkable color changes in their thin films. Here, methylamine gas (CH₃NH₂, MA⁰) induced switchable optical bleaching of bismuth (Bi)‐based perovskite films is demonstrated for the first time. By exposure to an MA⁰ atmosphere, the color of Cs₂AgBiBr₆ (CABB) films changes from yellow to transparent, and the color of Cs₃Bi₂I₉ (CBI) films changes from dark red to transparent. More interestingly, the underlying reason is found to be the interactions between MA⁰ and Bi³⁺ with the formation of an amorphous liquefied transparent intermediate phase, which is different from that of lead‐based perovskite systems. Moreover, the generality of this approach is demonstrated with other amine gases, including ethylamine (C₂H₅NH₂, EA⁰) and butylamine (CH₃(CH₂)₃NH₂, BA⁰), and another compound, Cs₃Sb₂I₉, by observing a similar reversible optical bleaching phenomenon. The potential for the application of CABB and CBI films in switchable smart windows is investigated. This study provides valuable insights into the interactions between amine gases and lead‐free perovskites, opening up new possibilities for high‐efficiency optoelectronic and stimuli‐responsive applications of these emerging Bi‐based materials.