Extended Data Fig. 2: X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering (GIWAXS) characterisations.

a, XRD patterns of the control, MA, PEA, t-BA films deposited on silicon substrates. The asterisk symbols denote the peaks of silicon. The perovskite reflections in the XRD pattern from the MA film are shifted to lower angles suggesting some MA⁺ incorporation into the A-site of the 3D perovskite phase. The additional peaks for the MA film are assigned to Cs₄PbBr₆, which are also present in the PEA film. The broad, weak perovskite diffraction peaks from the PEA film indicate the reduced perovskite crystallinity along the measurement axis. b, Grazing-incidence XRD patterns of the control, MA, PEA, t-BA films in the 2θ range of 8°-23.5°. c-f, GIWAXS patterns of the control (c), MA (d), PEA (e), t-BA (f) films deposited on silicon substrates. With the control and MA films, isotropic Debye-Scherrer scattering rings are observed, indicating grains are randomly oriented, as is typical for neat CsPbBr₃. With the larger alkylammonium cations, two different CsPbBr₃ texture directions are dominant. GIWAXS measurements further reveal that Cs₄PbBr₆ is present to some extent in all films (Supplementary Fig. 10, 11), and in the MA and t-BA samples the Cs₄PbBr₆ phase exhibits strong orientation. We infer from these data that the t-BA film exhibits optimal crystallinity, and uniquely benefits from both CsPbBr₃ and Cs₄PbBr₆ domains being highly oriented.