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

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.