Electrophosphorescence from a polymer guest-host system with an iridium complex as guest: Forster energy transfer and charge trapping

We report high-efficiency green electrophosphorescent light-emitting diodes obtained by using tris[9,9-dihexyl-2-(phenyl-4'(-pyridin-2"-yl))fluorene]iridium(III) (Ir(DPPF)(3)) as the guest, and a blend of poly(vihylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) as the host. The electrophosphorescent emission is characteristic of Ir(DPPF)(3), with its maximum at 550 nm. An external quantum efficiency of 8 % photons per electron and luminous efficiency of 29 cd A(-1), with maximum brightness of 3500 cd m(-2), were achieved at 1 wt.% concentration of Ir(DPPF)(3). The devices exhibited no emission from PVK or PBD, even at the lowest concentration of Ir(DPPF)(3) (0.1 wt.-%). The results indicate that Forster energy transfer plays a minor role in achieving high efficiencies in these devices. Direct charge trapping appears to be the main operating mechanism.