Highly efficient single-layer polymer light-emitting diodes that employ Nile Red as a fluorescent dye, with a green phosphorescent-sensitizer, bis(2-phenyl pyridinato-N,C2′) iridium (acetylacetonate) doped in a PVK/PBD host are demonstrated. The function of the phosphorescent sensitizer is to convert the triplet exciton into a singlet exciton during the energy transfer process. Therefore, ideally, all the excitons can be utilized. For comparison, devices with the same structure, but using a fluorescent sensitizer instead of a phosphorescent sensitizer, were fabricated. The efficiency of the phosphor-sensitized device is 6.4 cd/A, almost triple that of lumophor-sensitized or nonsensitized devices. This result indicates that not only singlet excitons but also triplet excitons are efficiently transferred from the host to the fluorescent dye when a phosphorescent material is used as a sensitizer.