Fig. 1: Precise cation delivery and in situ inter-grain bridging.

a Defects are distributed at various locations in the film. b Cs⁺ cations captured by the lattice and distributed throughout the film. c Non-lattice-incorporated Rb⁺ cations were delivered precisely to A sites and achieve in situ formation of 1D Rb-based non-perovskite phase at films’ surface and GDB. d Room temperature ⁸⁷Rb NMR spectra of CE-Rb-treated perovskite films rinsed and unrinsed by mixed solvent (respectively referred to as rinsed and unrinsed CE-Rb), δ-RbPbI₃ and δ-FA_yRb_1-yPbI₃ samples acquired with central transition selective (CT-selective) Hahn echo and simulated spectrum of CE-Rb with the parameters from ref. ⁴⁰. e ¹H → ¹³C CP spectra of CE-Cs-treated and CE-Rb-treated perovskite films rinsed and unrinsed by mixed solvent, respectively, as well as the α-FAPbI₃ (containing MA⁺ as an additive) perovskite films at 100 K. f ToF-SIMS depth profiles for CE-Rb, Pb, and Sn cations. The inset is a schematic diagram of the CE-Rb-treated perovskite film sputtered by Ar⁺ cluster ion beam at different depths.