Phosphorescence from iridium complexes doped into polymer blends

Abstract

Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, $\mathrm{tris}\text{[9,9-}\mathrm{dihexyl}\text{-2-(}\mathrm{phenyl}{\text{-4}}^{\prime }\mathrm{-(-}\mathrm{pyridin}{\text{-2}}^{″}-\mathrm{yl}\mathrm{))}\mathrm{fluorene}]$ iridium (III) $[\mathrm{Ir}(\mathrm{DPPF}{)}_3\mathrm{],}$ 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 $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ and an evident rising feature observed in films with 1 wt % $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ in the range of 578 to 615 nm within a 200 ns timescale indicate that efficient Förster energy transfer from exciplex to $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ occurs. The electrophosphorescent light-emitting diodes fabricated with PVK-PBD doped with $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ have external quantum efficiency of 8% ph/el, luminous efficiency of 29 cd/A and brightness greater than 3500 cd/m² at 1 wt % $\mathrm{Ir}(\mathrm{DPPF}{)}_3.$ The devices exhibited no electroluminescence (EL) emission from PVK or PBD even at a low concentration of $\mathrm{Ir}(\mathrm{DPPF}{)}_3$ (0.1 wt %), which indicates that the dominant mechanism in EL is charge trapping rather than energy transfer.