In-situ boundary bridging unlocks multi-grain-domain carrier diffusion in polycrystalline metal halide perovskites

Charge transport and extraction in polycrystalline perovskite films are often hindered by inefficient carrier transfer across grain domain boundaries (GDBs). Herein, we present a universal post-treatment strategy leveraging supramolecular crown ether-assisted slow release and precise delivery of Rb⁺ cations to GDBs, achieving in-situ GDB bridging. The solid-state nuclear magnetic resonance (NMR), transmission electron microscopic (TEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses confirm that Rb⁺ forms a non-perovskite phase, primarily localized at the surface and GDBs. Ultrafast time-resolved photoluminescence mapping revealed accelerated carrier diffusion across the grain boundaries for the Rb⁺-treated perovskite thin films which enables photo-generated charge carriers to travels over two grain domain boundaries before recombination. As a result, perovskite solar cells treated with this strategy achieved a champion efficiency of 26.02% (certified as 25.77%) and demonstrated remarkable stability, retaining 99.2% of their initial efficiency after 1300 h of continuous one-sun illumination under maximum power point tracking (ISOS-L-1I).