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 per-ovskite solar cells. The packing of the organic spacer cation determines the stability and crystallinity of layered perov-skites. However, 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 non-alternating (iodide-like) ar-rangement. By comparing the impact of the cation arrangement on the inorganic lattice, we discuss the implications for the optoelectronic properties.