Realizing high-performance bluish-green ionic thermally activated delayed fluorescence materials through counterion regulation

Ionic thermally activated delayed fluorescence (iTADF) materials have rarely been reported and applied in organic light emitting diodes (OLEDs) owing to their poor sublimability, photoluminescence (PL) and electro-luminescence (EL) performance. It is appealing and challenging to develop iTADF emitters for high-performance OLEDs based on a deep understanding of their structure-property relationship. In this work, we report three new iTADF materials, DMAC-TPP[Br], DMAC-TPP[BF4] and DMAC-TPP[BArF24], which have the same phosphonium-cation-based chromophore and different counter anions. These materials show excellent PL properties, e.g., high photoluminescence quantum yields (PLQYs), short exciton lifetimes and fast reverse intersystem crossing (RISC). The comparative studies reveal that the photophysical properties of these iTADF materials are slightly affected by the anions via molecular configuration, intra-/inter-molecular interactions and molecular stacking. More importantly, we find that the anions play a key role in determining the EL performance of the iTADF emitters. The change of anions can lead to 3-fold increased external quantum efficiency (EQE), over 21-fold increased luminance and significantly suppressed efficiency roll-offs for the iTADF-OLEDs. The solution-processed OLED based on DMAC-TPP[BF4] achieved a EQE of 15.1 % with small efficiency roll-offs of only 0.7 % and 10.6 % at luminance of 100 cd/m2 and 1000 cd/m2, and a maximum luminance of 10400 cd/m2. The significant difference in EL performance is ascribed to the different charge recombination governed by the migration and hole-trapping ability of anions under an electric field. These results motivate the further development of high-performance iTADF materials for EL applications by anion engineering.