Vapor deposition and stability of the lead-free halide double perovskite Cs₂AgInBr_6−xCl_x

All inorganic halide double perovskite Cs₂AgInX₆ (X = Cl, Br) is a potential lead-free alternative to CsPbX₃ for optoelectronic applications. However, Cs₂AgInBr₆ thin films were shown to be thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices and decomposing into Cs₂AgBr₃, Cs₃In₂Br₉, AgBr, and InBr₃ upon cooling from synthesis temperatures (∼150 °C) to room temperature. In contrast, Cs₂AgInCl₆ is stable at room temperature and in the ambient. Herein, we synthesize thin films of the mixed halide Cs₂AgInBr_6−xCl_x (x = 1-5) via vapor deposition by coevaporating CsX, AgX, and InX₃ and subsequently annealing the resulting films. Surprisingly, none of the halide alloys were stable in ambient air. The films remain cubic Cs₂AgInBr_6−xCl_x from tens of minutes to a few hours in the ambient air, decomposing slowly and eventually completely after a day to Cs₂Ag(Cl,Br)₃, Cs₃In₂(Cl,Br)₉, In(Cl,Br)_3, and Ag(Cl,Br). Storing the films in a nitrogen-filled glovebox slows the decomposition but does not stop it for Cs₂AgInBr₅Cl and Cs₂AgInBr₃Cl₃, suggesting humidity is a factor in the decomposition. However, Cs₂AgInBrCl₅ was an exception and remained stable in the nitrogen-filled glovebox for at least three months.