Host Engineering Based On Multiple Phosphorylation For Efficient Blue And White Tadf Organic Light-Emitting Diodes

It is challenging to develop host materials for efficient blue and white thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLED), since optoelectronic properties of host molecules should be accurately optimized to suppress triplet quenching. In order to investigate the influence of peripheral groups on host performance, we construct three phosphorylated carbazole hosts with 1-3 diphenylphsophine oxide (DPPO) substituents at 1,8 positions of carbazole and para position of N-phenyl, respectively named tBCzPSPO, tBCzPDPO and tBCzPTPO. It shows that DPPO groups at 1 and 8 positions change the electronic states and frontier molecular orbitals, due to the electron-withdrawing effect of P = O, while, DPPO at para-position of Nphenyl only provides additional steric hindrance. Furthermore, DPPO groups are involved in frontier molecular orbitals for carrier transport assistance, but excluded from the first triplet (T-1) state. Therefore, triphosphorylated tBCzPTPO is superior in triplet protection, giving rise to high photoluminescence quantum yields more than 80%. The single-emissive-layer blue and white TADF OLEDs based on tBCzPTPO further achieved the state-of-the-art external quantum efficiencies beyond 20% and power efficiencies up to 60 lm W-1.