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′-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 π 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 cm²V⁻¹ s⁻¹), following the order T3T > TST > T2T. The device incorporating T2T as the host, doped with (PPy)₂Ir(acac) and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TBPI) as the ET layer, achieved a high external quantum efficiency (η_ext) of 17.5% and a power efficiency (η_p) of 59.0 lm W⁻¹. For the same device configuration, the T3T-based device provided values of η_ext and η_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)₂Ir(acac).