Molecular design of host materials based on triphenylamine/oxadiazole hybrids for excellent deep-red phosphorescent organic light-emitting diodes

A series of triphenylamine/oxadiazole hybrids, p-TPA-p-OXD(1a), o-TPA-p-OXD (1b), p-TPA-mOXD (2a) and o-TPA-m-OXD (2b),were designed, synthesized and characterized as bipolar transport host materials for deep-red phosphorescent organic light-emitting diodes (OLEDs). The ortho-TPA linked hybrids (1b and 2b) show less intramolecular charge transfer, blue-shifted emission, wider energy gap, and higher triplet energy as compared to their para-TPA linked analogues (1a and 2a). Phosphorescent organic light-emitting devices (PHOLEDs) fabricated by using the four hybrids as the hosts and the red emitter bis(1-phenylisoquinolinato)(acetylacetonate) iridium [(piq)(2)Ir(acac)] as the guest exhibit much higher EL performances with maximum external quantum efficiencies of 9.8-21.6% and lower turn-on voltages (2.7-3.1 V) compared with the reference device with common 4,4'-bis(N-carbazolyl) biphenyl (CBP) as a host material (4.3%, 5.3 V). The external quantum efficiency of 21.6% achieved by using o-TPA-m-OXD as host is the highest for deep-red electrophosphorescence with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.68, 0.32) reported in the literature to date. Green electrophosphorescence devices by using Ir(ppy)(3) as guest and 1b, 2a and 2b as hosts also show excellent EL performances with maximum external quantum efficiencies of 17.1-19.6%. This work demonstrates that tradeoffs among bipolar property, triplet energy, energy gap and energy level can be realized through judicious molecular design for a host in phosphorescent OLEDs.