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

Abstract

A series of triphenylamine/oxadiazole hybrids, p-TPA-p-OXD (1a), o-TPA-p-OXD (1b), p-TPA-m-OXD (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)₂Ir(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'Éclairage (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.