Chemistry of Materials, 2019, vol 31, 14, pp. 4990-4998
Assembling halide perovskites into layered structures holds promise for addressing chemical and phase stability challenges; however, several other challenges need to be addressed to create efficient and stable halide perovskite devices. Layered halide perovskites (LHPs) suffer from a broad distribution of layer thicknesses and band gaps within thin films. Reducing polydispersity could substantially improve charge transport within LHP films and the performance of LHP-based solar cells. Herein, we focused on layering α-CsPbI3 ((C4H9NH3)2Csn–1PbnI3n+1) thin films. We found that (C4H9NH3)2Csn–1PbnI3n+1 with nominal layer thicknesses of n = 1, 2, 3, and 4 can be deposited at temperatures as low as 100 °C, substantially below the phase-transition temperature of bulk α-CsPbI3. Furthermore, we demonstrated that incorporating solvents with high complexation strength into the precursor solution promotes the formation of intermediate phases within the thin film, slowing down LHP crystallite nucleation, eventually resulting in improved phase purity. By reducing decomposition rates through combined use of solvent complexes and thermal processing methods, we fabricated (C4H9NH3)2CsPb2I7 films that had improved phase purity, crystallinity, and film morphology. We also demonstrate that the improved phase purity resulted in photoluminescence with a maximum intensity corresponding to the targeted n = 2 phase. This work represents a step toward highly stable LHP thin films with narrow site-energy distribution.