Symmetrical spirobi[xanthene] based locally asymmetrical phosphine oxide host for low-voltage-driven highly efficient white thermally activated delayed fluorescence diodes

Abstract The balance between the electrical performance and quenching suppression is crucial to develop the efficient host materials for full-thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLEDs). In this work, through introducing one bulky diphenylphosphine oxide (DPPO) group at 4 position of each xanthene ring in a symmetrical and orthogonal spirobi[xanthene] (SX) skeleton, 9,9′-spirobi[xanthene]-4,4′-diylbis(diphenylphosphine oxide) (SXDPO) was constructed, giving rise to the optimized intermolecular interactions. Besides the high triplet (T1) energy level of 2.94 eV, the highly rigid configuration of SXDPO simultaneously rendered the dramatically suppressed excited-state relaxation and the stable and uniform amorphous phase of its films before and after doping TADF dyes. It was interesting that SXDPO showed the complementary electrical properties to enhance the carrier transportation in the films. Therefore, SXDPO successfully achieved the balance between the electrical performance improvement and the collisional quenching suppression. As the result, SXDPO endowed its complementary TADF WOLEDs with the state-of-the-art performance, e.g. the turn-on voltage as low as 2.8 V and power efficiency beyond 60 lm W−1, which was one of the best comprehensive results reported so far for full-TADF WOLEDs.