Phase Engineering Reinforced Energy Transfer for High‐Performance Blue Perovskite Light‐Emitting Diodes

Abstract Layered metal‐halide perovskites, a category of self‐assembled quantum wells, are of paramount importance in emerging photonic sources, such as lasers and light‐emitting diodes (LEDs). Despite high trap density in two‐dimensional (2D) perovskites, efficient non‐radiative energy funneling from wide‐ to narrow‐bandgap components, sustained by the Förster resonance energy transfer (FRET) mechanism, contributes to efficient luminescence by light or electrical injection. Herein, it is demonstrated that bandgap extension of layered perovskites to the blue‐emitting regime will cause sluggish and inefficient FRET, stemming from the tiny spectral overlap between different phases. Motivated by the importance of blue LEDs and inefficient energy transfer in materials with phase polydispersity, wide‐bandgap quasi‐2D perovskites with narrow phase distribution, improved crystallinity, and the pure crystal orientation perpendicular to the charge transport layer are developed. Based on this emitter, high‐performance blue perovskite LEDs with improved electroluminescence (EL) external quantum efficiency (EQE) of 7.9% at 478 nm, a narrow full width at half‐maximum (FWHM) of 22 nm and a more stable EL spectra are achieved. These results provide an important insight into spectrally stable and efficient blue emitters and EL devices based on perovskites.