Spatially optimized quaternary phosphine oxide host materials for high-efficiency blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

Abstract

Two quaternary ambipolar phosphine oxide host materials, 9-(4-(9-(4′-(2-(diphenylphosphoryl)phenoxy)biphenyl-4-yl)-9H-fluoren-9-yl)phenyl)-9H-carbazole (9CzFDPESPO) and 9-(4-(9-(3′-(diphenylphosphoryl)-4′-(2-(diphenylphosphoryl)phenoxy)biphenyl-4-yl)-9H-fluoren-9-yl)phenyl)-9H-carbazole (9CzFDPEPO), were designed and prepared with mixed indirect and multi-insulating linkages to investigate the spatial effect on the electroluminescence performance of high-energy-gap hosts in blue phosphorescence and thermally activated delayed fluorescence light-emitting diodes (PHOLEDs and TADF OLEDs), in comparison to a ternary analogue 9-(4-(9-(4-(diphenylphosphoryl)phenyl)-9H-fluoren-9-yl)phenyl)-9H-carbazole (9CzFSPO). The donor–acceptor (D–A) distance in 9CzFDPESPO is elongated through the involvement of diphenylene as a π-extender, while the second phosphine oxide acceptor on diphenylene of 9CzFDPEPO is utilized to make its D–A distance comparable to that of 9CzFSPO. The single-molecular optoelectronic properties of these three hosts were uniform, owing to the effectively suppressed intramolecular electronic coupling by indirect and multi-insulating linkages. Their similar high triplet energy of 3.0 eV and the suitable energy levels of the highest occupied and the lowest unoccupied molecular orbitals around −6.1 and −2.5 eV, respectively, provide effective energy transfer and carrier injection. In contrast, 9CzFDPESPO shows the highest hole mobility owing to its longest D–A distance, directly indicating the spatial effect. As expected, with the most effectively suppressed spatial effect, 9CzFDPESPO endowed its blue phosphorescence and TADF devices with a state-of-the-art performance, e.g. external quantum efficiencies of 22.5% and 16.7%, respectively, which were the best results for quaternary host materials reported so far.