Noble phenoxaphosphine-based bipolar hosts for blue OLEDs using thermally-activated delayed fluorescence

Abstract

Novel thermally-activated delayed fluorescence (TADF) host materials were designed for blue electrophosphorescence by combining electron donor acridine derivatives with the electron acceptor phenoxaphosphine (OPO) unit using the density functional theory. We obtained the energies of the first singlet (S1) and first triplet (T1) excited states of these TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using a dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of the TD-DFT. Using DFT and TD-DFT calculations, the large separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gives a small energy difference (ΔEST) between the S1 and T1 state. These host molecules could retain high triplet energy, and they showed great potential for blue phosphorescent organic light-emitting diodes (OLEDs). We demonstrated that these molecules are promising host materials with a lower barrier for hole and electron injection; these also provide balanced charge transport for both hole and electron and small ΔEST.