Organic light-emitting devices (OLEDs) are expected to be adopted as the next generation of general lighting because they are more efficient than fluorescent tubes and are mercury-free. The theoretical limit of operating voltage is generally believed to be equal to the energy gap, which corresponds to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the emitter molecule divided by the electron charge (e). Here, green OLEDs operating below a theoretical limit of the energy gap (E-g) voltage with high external quantum efficiency over 20% are demonstrated using fac-tris(2-phenylpyridine)iridium(III) with a peak emission wavelength of 523 nm, which is equivalent to a photon energy of 2.38 eV. An optimized OLED operates clearly below the theoretical limit of the E-g voltage at 2.38 V showing 100 cd m(-2) at 2.25 V and 5000 cd m(-2) at 2.95 V without any light outcoupling enhancement techniques.