Ultrathin Monolayer Perovskites via Microstructure‐Induced Capillary Flow for Efficient Pure‐Blue Light‐Emitting Diodes

ABSTRACT Perovskite light‐emitting diodes (PeLEDs) have emerged as promising candidates for next‐generation displays. However, the performance of pure‐blue PeLEDs based on quasi‐2D perovskites remains challenging due to abundant inner grain boundaries resulting from their inevitable multi‐layered grain stacking. Here, we report a simple strategy to construct quasi‐2D perovskites with ultrathin monolayer structures by utilizing substrate microstructure‐induced capillary action for liquid film thinning and flattening. The resulted architecture features highly oriented ∼6.3 nm thick grains with eliminated inner transverse grain boundaries, suppressing carrier trapping and ion migration, and promoting radiative recombination. Adopting this structure, we demonstrated pure‐blue (466 nm, CIE: 0.132, 0.065) PeLED with a peak external quantum efficiency of 15.0% at 56.54 cd m −2 , and a maximum luminance of 947 cd m −2 accompanied by stable electroluminescence spectra. Our strategy provides a novel pathway for regulating perovskite film structures by utilizing microstructure‐induced surface effects and liquid film morphology modulation.