Highly Efficient Organic Light-Emitting Diodes based on the Red Phosphorescent Emitter Sensitized by the Green Emitting Iridium Complex

We demonstrated the red phosphorescent organic light-emitting diodes with sensitizer, iridium(III) tris(2-phenylpyridine) [Ir(ppy)(3)]. The devices with sensitizer showed a significant improvement in a luminance and efficiency compared to those without sensitizer. Moreover, the electroluminescent (EL) spectra of devices with sensitizer show a similar emission peak with photoluminescent (PL) spectra of iridium (III) bis(2-phenylquinoline)(acetylaceton) [Ir(pq)(2)(acac)] under the optimized doping ratio of Ir(pq)(2)(acac) and Ir(ppy)(3). It is because that the triplet metal-to-ligand charge transfer (MLCT) state of Ir(ppy)(3) is higher than that of Ir(pq)(2)(acac)and the intra-molecular energy transfer (IMET) time (within ns time scale) from the ppy-centered (MLCT)-M-3 state to the pq-centered (MLCT)-M-3 state is shorter than the radiative lifetime (within mu s time scale) of Ir(ppy)(3).