Efficient light emission was obtained in a silole-based organic light-emitting diode. A high luminous current efficiency of 20 cd/A, corresponding to an external quantum efficiency of 8%, was achieved. The apparent violation of the upper theoretical limit of 5.5% for the external quantum efficiency of a singlet emitter is discussed. With a suitably designed cathode, a high power efficiency of ∼14 lm/W was obtained. A strong dependence of the power efficiency on the thickness of Alq₃ layer is also observed and explained.