Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, tris9,9-dihexyl-2-(phenyl-4′-(-pyridin-2″-yl))fluorene iridium (III) Ir(DPPF)_(3), is investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy. A redshifted PL and slow fluorescence decay are due to the formation of an exciplex in PVK-PBD blends. A decrease in intensity in polymer blends observed at 425 nm with increasing concentrations of Ir(DPPF)_(3) and an evident rising feature observed in films with 1 wt Ir(DPPF)_(3) in the range of 578 to 615 nm within a 200 ns timescale indicate that efficient Forster energy transfer from exciplex to Ir(DPPF)_(3) occurs. The electrophosphorescent light-emitting diodes fabricated with PVK-PBD doped with Ir(DPPF)_(3) have external quantum efficiency of 8 ph/el, luminous efficiency of 29 cd/A and brightness greater than 3500 cd/m~(2) at 1 wt Ir(DPPF)_(3). The devices exhibited no electroluminescence (EL) emission from PVK or PBD even at a low concentration of Ir(DPPF)_(3) (0.1 wt ), which indicates that the dominant mechanism in EL is charge trapping rather than energy transfer.
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