Stable, color-tunable 2D SCN-based perovskites: revealing the critical influence of an asymmetric pseudo-halide on constituent ions.

Two-dimensional (2D) layered perovskites (An+1BnX3n+1, n = 1, 2, …) have recently attracted significant research interest because of their enhanced ambient stability compared to their conventional 3D counterparts. In addition to the common A-site cation engineering, using an asymmetric pseudo-halide anion, SCN-, in the anion X-site has been recently proven to be another effective approach to constitute 2D perovskites. Among these, 2D (MA)2Pb(SCN)2I2 was the most widely investigated and was considered to be a promising material owing to its good optoelectronic properties; however, its poor stability has aroused concerns in recent researches. In this study, systematical composition engineering of A2Pb(SCN)2X2 (A = FA+, MA+, Cs+ and X = Br-, I-) was conducted. Our results revealed that the linear SCN- anion dictates critical restrictions on the constituent ions of its derived 2D framework (PbX4(SCN)2), which has not yet been extensively discussed. We demonstrated that using a smaller Cs+ cation can afford a more favorable 2D structure compared with the MA+ cation. Cs2Pb(SCN)2I2 was revealed to possess improved stability and photo-response compared to (MA)2Pb(SCN)2I2. Interestingly, Cs2Pb(SCN)2I2 and (MA)2Pb(SCN)2I2 appear to possess distinct electronic band structures. This is indicated by their discrepant photoluminescence spectra, in which the former exhibits a rather intense singlet emission at room temperature in contrast with the latter, which shows a dominant emission associated with triplet or defective states. Furthermore, using a smaller Cs+ cation enables facile replacement of a smaller halide anion. A series of mixed-halide 2D Cs2Pb(SCN)2(I1-xBrx)2 (x = 0, 1/3, 1/2, 2/3, 1) with varying vivid colors was explored by both calculation and experimental efforts to corroborate the enhanced stability when the x value increases. The results revealed in this study might represent a novel discovery of an inherent trait of the 2D SCN-based perovskites and also suggest that the all-inorganic 2D Cs2Pb(SCN)2X2 perovskite system is a promising class of materials with good stability and color-tunability that deserves further exploration.