Phosphorescence from iridium complexes doped into polymer blends

Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, tris[9,9-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/m2 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. (C) 2004 American Institute of Physics.