We present a highly efficient organic light-emitting diode with a single-layer structure and air-stable Al cathode. The device consists of a spin-coated thin film of polystyrene in which rubrene and Alq(3) molecules are dispersed. By optimizing the concentration of Alq(3) and rubrene, a maximum external quantum efficiency of 2.0% photons/electron and a current efficiency of 8.5 cd/A at 22 V could be achieved in the device. The high electroluminescence efficiency is attributed to a combination of energy transfer from Alq(3) to rubrene, and direct carrier trapping and recombination on rubrene, followed by an efficient fluorescent emission from rubrene.