Molecular Engineering Enables TADF Emitters Well Suitable for Non-Doped OLEDs with External Quantum Efficiency of Nearly 30%

Non-doped organic light-emitting diodes (OLEDs) are particularly appealing due to the merit of the extremely simple device structure. However, the performance of non-doped OLEDs is usually far inferior than that of doped devices, mainly due to the lack of desirable emitters. An ideal emitter for non-doped OLEDs should not merely be highly emissive in the host matrix but also be capable of delivering excellent properties in its condensed state. Herein, through molecular engineering, an "axial and equatorial carbazolyl extension" approach to manipulate the molecular packing behavior is developed, and thus, the emitter is awarded with superior properties in its neat film. Based on this approach, through simply modifying the conventional acridine donor in a thermally activated delayed fluorescence (TADF) emitter with carbazole moieties in the way of hyper-conjugation, two new TADF emitters with the molecular skeleton being extended both horizontally and vertically by carbazole moieties are constructed. The resulted TADF emitters reveal superb properties with simultaneous excellent thermal and morphological stabilities, photophysical behaviors, and charge transporting ability in their neat film. Owing to the merits of these synergistic superior properties, highly efficient non-doped green emissive OLED with the state-of-the-art external quantum efficiency of nearly 30% is realized.