Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Ionic thermally activated delayed fluorescence (TADF) emitters are rarely investigated due to their poor photoluminescence and electroluminescence performance. Herein, highly efficient ionic TADF emitters with charged donor-acceptor (D-A+) and D-A+-D architectures are designed, innovatively based on the phosphonium cation electron acceptor. The symmetric D-A+-D compound in doped film exhibits a high photoluminescence quantum yield of 0.91 and a short emission lifetime of 1.43 microse-conds. Partially solution-processed organic light -emitting diodes based on these ionic TADF emitters achieve a maximum external quantum efficiency (EQE) of 18.3% and a peak luminance of 14,532 can-delas per square meter (cd/m2) and show a small efficiency roll-off of 7.1% (EQE = 17%) at a practical high luminance of 1000 cd/m2. These results demon-strate the high potential of phosphonium cations as promising electron acceptors to construct TADF emitters for high-performance electroluminescence devices. The current study opens up an appealing way for future exploitation of high-efficiency ionic TADF materials.