We have synthesized three star-shaped 1,3,5-triazine derivatives-2,4,6-tris(biphenyl-3-yl)-1,3, 5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3,5-triazine (T3T), and 2,4,6-tris(9,9'0-spirobifluorene-2-yl)-1,3,5-triazine (TST)-as new electron transport (ET)-type host materials for green phosphorescent organic light-emitting devices. The morphological, thermal, and photophysical properties and the electron mobilities of these ET-type host materials are influenced by the nature of the aryl substituents attached to the triazene core. The meta-meta linkage between the 1,3,5-triazine core and the peripheral aryl moieties in T2T and T3T limited the effective extension of their pi conjugation, leading to high triplet energies of 2.80 and 2.69 eV, respectively. Time-of-flight mobility measurements revealed the good electron mobilities for these compounds (each > 10(-4) cm(2) V-1 s(-1)), following the order T3T > TST > T2T. The device incorporating T2T as the host, doped with (PPy)(2)Ir(acac) and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TBPI) as the ET layer, achieved a high external quantum efficiency (eta(ext)) of 17.5% and a power efficiency (eta(p)) of 59.0 lm W-1. For the same device configuration, the T3T-based device provided values of eta(ext) and eta(p) of 14.4% and 50.6 lm W-1, respectively; the TST-based device provided values of 5.1% and 12.3 lm W-1, respectively. We ascribe the superior performance of the T2T-based devices to balanced charge recombination; we ascribe the poor efficiencies of the TST-based devices to its relatively low triplet energy (2.54 eV), which did not allow efficient confinement of the triplet excitons on the green phosphorescent emitter (PPy)(2)Ir(acac).
