Direct Observation of Efficient Triplet–Triplet Energy Transfer in Phosphorescent Organic Light-Emitting Diode

We report on the energy transfer mechanism between the host and the iridium dopant in phosphorescent organic light-emitting diodes (PHOLEDs). The excited-state energies of the host and the dopant are carefully defined to discuss the energy transfer. The PHOLED using bis(benzo[ h ]quinolin-10-olato- kN , kO )beryllium(II) (Bebq 2 ) as the host and bis(4-pentafluorophenyl-2-phenylpyridine)(acetylacetonate)iridium(III) (m-PF-py) as the dopant shows a maximum external quantum efficiency (η ext ) of about 15%, whereas the photoluminescence quantum yield of the emitting layer film is only 19%. The high η ext of the PHOLED implies efficient triplet–triplet energy transfer from Bebq 2 to m-PF-py.