We have successfully demonstrated efficient single-layer organic light-emitting devices (OLEDs) with a current efficiency of 31.38 cd A⁻¹. The efficiencies still remain as high as 31.36 cd A⁻¹, 30.76 cd A⁻¹ and 29.98 cd A⁻¹ at the luminance of 1000 cd m⁻², 5000 cd m⁻² and 10 000 cd m⁻². The key feature of the device concept is uniformly doping iridium-bis-(4,6-difluorophenyl-pyridinato-N,C2)-picolinate (FIrPic) into a single organic layer to balance the transport of charge carriers. To better understand the mechanism of single-layer OLEDs, the position of the recombination zone and the influence of FIrPic on the transport properties are also studied in detail. Our work clearly reveals that the performance of the single-layer OLEDs can be dramatically improved by intentionally doping with a phosphorescent dye to balance the transport of charge carriers. This novel and versatile device concept provides a promising simple method to achieve high performance single-layer OLEDs.