Hybrid Local and Charge Transfer Emitters Utilizing Hyperconjugation Effect Towards Solution‐Processed Ultra‐Deep‐Blue OLEDs with External Quantum Efficiency Approaching 12 %

Abstract

AbstractHybrid local and charge transfer (HLCT) excited state materials, which possess weak donor‐acceptor (D–A) pure organic structures, deserve one of the most promising efficient and stable blue emitters. Through high‐lying reverse intersystem crossing (hRISC) process, 75 % triplet excitons generated by electrical excitation could be harvested and utilized in organic light‐emitting diodes (OLEDs). However, there are still significant challenges to achieve high‐efficiency ultra‐deep‐blue HLCT emitters with low Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinate y values. Here, a series of novel blue HLCT emitters based on spiro[1,8‐diazafluorene‐9,2′‐imidazole] structure were designed and synthesized by fine‐tuning the spiro[fluorene‐9,2′‐imidazole] core structure in our previous work through heteroatom substitution and hyperconjugation effect. The target emitters were endowed with excellent photophysical and electrochemical merits, thermal stability and solution processibility. The solution‐processed OLED based on 4′,5′‐bis(4‐(9H‐carbazol‐9‐yl)phenyl)spiro[1,8‐diazafluorene‐9,2′‐imidazole] (NFIP‐CZ) achieved efficient ultra‐deep‐blue emission (CIEx,y=0.1581, 0.0422) with the maximum external quantum efficiency (EQEmax), maximum current efficiency (CEmax) and maximum power efficiency (PEmax) of 11.94 %, 4.07 cd ⋅ A−1 and 2.56 lm ⋅ W−1. The record EQE is a breakthrough in both solution‐processed and vacuum vapor deposition ultra‐deep‐blue HLCT OLEDs currently.