A series of 1,3,5-triazine-core-containing electron-transport materials (ETMs) are reported as an undoped electron-transport layer (ETL), whose energy levels can be tuned by introducing pyridine rings on the periphery of the molecule and also orientation of nitrogen. An unprecedented low operating voltage of 2.42 V, corresponding to the emitting photon energy (hv), was achieved for the fac-tris(2-phenylpyridine) iridium (Ir(PPy)3) based green phosphorescent OLEDs at 100 cd m(-2) without consumption of their efficiency. Moreover, the threshold voltage for electroluminescence can be even 0.1 similar to 0.2 V lower than the minimum value of hv/e.