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-resolvedphotoluminescence (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 Förster 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.