Highly efficient ionic thermally activated delayed fluorescence emitters with short exciton lifetimes towards High-Performance Solution-Processed OLEDs

Ionic thermally activated delayed fluorescence (iTADF) materials are newly emerging as promising candidates for constructing solution -processed electroluminescence devices. Developing iTADF materials with high emission efficiency, short exciton lifetime, and fast reverse intersystem crossing (RISC) is crucial for achieving stable and efficient organic light -emitting diodes (OLEDs), but it remains challenging. Here we report two high-performance iTADF emitters, namely DMAC-TPPO[BF4] and DMAC-TPPS[BF4], which consist of a conventional acridine as the electron -donor and newly developed oxygen/sulfur-fused arylphosphonium derivatives as the electronacceptor. DMAC-TPPO[BF4] and DMAC-TPPS[BF4] exhibit high photoluminescence quantum yields of 77 % and 83 %, coupled with short exciton lifetimes of 1.2 and 1.1 mu s, and RISC rate constants reaching up to 2.60 x 106 and 2.73 x 106 s-1, respectively. Theoretical and experimental investigations demonstrate that the imbedding of bridging heteroatoms into the phosphonium cation is pivotal for enhancing emission efficiency through molecular skeleton rigidification, as well as for accelerating the RISC process by capitalizing on the non-metal heavy -atom effect. Eventually, solution -processed OLEDs utilizing DMAC-TPPS[BF4] realize a peak external quantum efficiency (EQE) of 17.8 % and show a minor efficiency roll -off of 7.3 % (EQE = 16.5 %) even at a practical high luminance of 1000 cd/m2.