Extended Data Fig. 2: Incident-angle-dependent XRD measurements.

a, Calculated probing depth as a function of X-ray incident angle (α_i). b, Calculated (100) Bragg peak position of α-FA_1-xCs_xPbI₃ as a function of Cs⁺ content. c, d, GIXRD patterns for (c) pristine and (d) IPA α-FA_1-xCs_xPbI₃ films with different probing depths. By increasing the α_i of X-ray radiation, we observed a shift towards lower angles in the (100) diffraction peak signal. The phenomenon we attributed to a lattice constant reduction in the top-zone of pristine film; and a nominal formula of FA_0.7Cs_0.3PbI₃ is derived here according to Vegard’s law. In contrast, the IPA α-FA_1-xCs_xPbI₃ films exhibited a constant peak position, indicating a uniform composition distribution vertically. Notably, these results were further corroborated by the XPS depth profiles in Supplementary Note 2.