Ligand‐Induced Cation–π Interactions Enable High‐Efficiency, Bright, and Spectrally Stable Rec. 2020 Pure‐Red Perovskite Light‐Emitting Diodes

Achieving high‐performance perovskite light‐emitting diodes (PeLEDs) with pure‐red electroluminescence for practical applications remains a critical challenge because of the problematic luminescence property and spectral instability of existing emitters. Herein, high‐efficiency Rec. 2020 pure‐red PeLEDs, simultaneously exhibiting exceptional brightness and spectral stability, based on CsPb(Br/I)3 perovskite nanocrystals (NCs) capping with aromatic amino acid ligands featuring cation–π interactions, are reported. It is proven that strong cation–π interactions between the PbI6‐octahedra of perovskite units and the electron‐rich indole ring of tryptophan (TRP) molecules not only chemically polish the imperfect surface sites, but also markedly increase the binding affinity of the ligand molecules, leading to high photoluminescence quantum yields and greatly enhanced spectral stability of the CsPb(Br/I)3 NCs. Moreover, the incorporation of small‐size aromatic TRP ligands ensures superior charge‐transport properties of the assembled emissive layers. The resultant devices emitting at around 635 nm demonstrate a champion external quantum efficiency of 22.8%, a max luminance of 12 910 cd m−2, and outstanding spectral stability, representing one of the best‐performing Rec. 2020 pure‐red PeLEDs achieved so far. A small‐size aromatic amino acid of tryptophan (TRP), featuring strong cation–π interactions, is introduced to "clean" the imperfect surface sites of the CsPb(Br/I)3 perovskite nanocrystals (NCs). The as‐formed TRP‐NCs demonstrate increased luminescence efficiency and obviously enhanced spectral stability, which enables the achievement of Rec.2020 pure‐red perovskite‐based light‐emitting diodes exhibiting high external quantum efficiency along with superior brightness and spectral stability.