Electrochemical self-doping in intrinsically ionic, thermally activated delayed fluorescence materials enables high-performance light-emitting electrochemical cells

Intrinsically ionic, thermally activated delayed fluorescence (TADF) materials are promising active materials for solid-state light-emitting electrochemical cells (LECs), but the origin for their superior device performances has remained elusive and how to further boost their device performances toward real world applications has remained an open question. Here, three TADF materials containing the same TADF unit have been prepared, which include a neutral one (R), an ionic one with an imidazolium cation being anchored to the acceptor through an alkyl chain (E1), and an intrinsically ionic one with an imidazolium cation being directly anchored to the acceptor (E2). By comparison between R, E1 and E2, it is revealed that n-type electrochemical self-doping occurs in E2 upon electron-injection onto its acceptor in operating LECs, due to in-situ formed, closely bound [electronimidazolium cation] pair, which significantly enhances the device performance. By enhancing the chemical stability of the material and centering the carrier recombination zone in the active layer, the green LECs afford peak brightness up to 1461 cd m(-2), external quantum efficiencies (EQEs) up to 9.1%, and half-lifetimes up to 371 h at constant current of 50 A m(-2). Peak brightness/EQE/half-lifetime at 322 cd m(-2)/10.1%/similar to 1800 h has further been achieved at constant current of 10 A m(-2). These LECs represent the most efficient, bright, and stable TADF LECs reported so far. The work clarifies the great advantage and huge potential of intrinsically ionic TADF materials, with unique electrochemical self-doping capacity, for fabricating efficient, bright and stable lightemitting devices.