Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming co-hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.