Halide Mixing Determines the Organic Structure in 2D Layered Perovskites

Layered 2D perovskite surface coatings are one of the most effective ways to improve the stability of lead halide perovskite solar cells. The packing of the organic spacer cation determines the stability and crystallinity of the layered perovskites. However, the structure determination of the light organic cations remains a challenge, especially for polycrystalline and disordered mixed-halide materials. Here, we determine the atomic-level structure of the phenylethylammonium (PEA+) spacer cations in mixed iodide-bromide layered perovskites by NMR, supported by density functional theory (DFT) chemical shift calculations. In particular, we find from 1H-13C correlation experiments at 21.1 T with 100 kHz magic-angle spinning that mixed-halide compositions with as little as 25% Br adopt the alternating (bromide-like) arrangement of the PEA+ cations. Only a sample with 10% of bromide adopts a nonalternating (iodide-like) arrangement. By comparing the impact of the cation arrangement on the inorganic lattice, we discuss the implications for the optoelectronic properties.