Spacer Cation Engineering Enables Blue Quasi‐2D Perovskites to Achieve Highly Efficient and Spectrally Stable Electroluminescence

Abstract The combination of organic spacer cations and mixed‐halides to produce multiphase quasi‐2D perovskites is a promising strategy for fabricating blue perovskite light‐emitting diodes (PeLEDs). However, insufficient energy transfer, trap‐assisted recombination and exciton‐phonon coupling lead to significant non‐radiative losses. Here, a co‐spacer engineering strategy of binding guanidinium (GA + ) and ortho ‐fluorophenylethylamium (oF‐PEA + ) through hydrogen bonds is proposed to prepare blue mixed‐halide quasi‐2D perovskite films with high photoluminescence quantum yields (PLQYs). GA + with Lewis base characteristics reduces the trap states by defect passivation. Additionally, oF‐PEA + inhibits the rapid diffusion of GA + by hydrogen bonding interactions, which mitigates the formation of undesirable low‐dimensional phases and facilitates the growth of high‐dimensional emissive phases with a more concentrated distribution, resulting in efficient energy transfer of excitons and weaker exciton‐phonon coupling. These synergistic effects enable the blue perovskite films to achieve a PLQY as high as 91.5%. As a result, the fabricated blue PeLEDs show a remarkable external quantum efficiency of 21.1% at the stable emissionpeak of 489 nm with a narrow full width at half‐maximum of 19 nm.