Vapor deposition and stability of the lead-free halide double perovskite Cs2AgInBr6−<i>x</i>Cl<i>x</i>

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 Cs2AgInBr6−xClx (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 Cs2AgInBr6−xClx 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.