CN engineered electron transport type hosts for high efficiency and extended lifetime in blue thermally activated delayed fluorescent organic light-emitting diodes

Abstract

Electron transport type host materials for blue thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) were synthesized by engineering the electron withdrawing CN units in the molecular structure. The two host materials having twisted molecular structure by ortho orientation of dibenzofuran and CN modified carbazole were developed as the electron transport type hosts to obtain high triplet energy for blue TADF application. Two CN units were introduced in the molecular structure for electron transport properties. One host had one CN unit in the phenyl core and one CN unit in the carbazole, and the other host had two CN units in the carbazole. The two hosts showed similar triplet energy above 3.0 eV, but different lowest unoccupied molecular orbital (LUMO) energy levels and orbital distributions. The host with one CN unit in the phenyl core showed superior device performances such as EQE over 20% and device lifetime of 32.2 h up to 80% of initial luminance of 500 cd m⁻² as the electron transport type host in the mixed host because of deep LUMO level suppressing electron trapping.