High-performance electrophosphorescent light-emitting diodes (LEDs) were demonstrated with tris-[9,9-dihexyl-2-(pyridinyl-2') fluorenel iridium(III) [Ir(DPF)(3)], tris-{9,9-dihexyl-2-[phenyl-4'-(-pyridin-2"-yl)] fluorene} iridium(III) [Ir(DPPF)(3)], and tris-[2,5-bis-2'-(9,9'-dihexylfluorene) iridium] [Ir(HFP)(3)] as guests and poly(vinylcarbazole) (PVK) blended with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazoI (PBD), poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), and poly(9,9-dihexylfluorene)-co-2,5-dicyanophenylene (PF3CNP1) as hosts. The devices made with PVK-PBD exhibited the highest external quantum efficiency QEext), luminous efficiency (LE) and luminance (L). For example, yellowish green emission from PVK-PBD doped with Ir(DPF)(3) was observed with QE(ext) = 10% ph/el, LE = 36 cd/A, and L > 8300 cd/m(2), and red emission from PVK-PBD doped with Ir(DPPF)(3) was observed with QE(ext) = 5% ph/el, LE = 7.2 cd/A, and L > 2700 cd/m(2). Red electrophosphorescent LEDs with a low turn-on voltage (5 V), QE(ext)= 4.5% ph/el, LE = 6.2 cd/A, and L > 1000 cd/m(2) were achieved with the conjugated polymer, PFO, as the host and Ir(HFP)(3) as the guest. Electrophosphorescent LEDs fabricated with the conjugated copolymer PF3CNP1 doped with Ir(HFP)(3) exhibited QE(ext) = 1.5% ph/el and LE = 3 cd/A with L = 2200 cd/m(2). These devices exhibited good operational stability under DC drive at room temperature. Forster energy transfer played a minor role in achieving the high efficiencies in these electrophosphorescent devices; direct sequential charge trapping appeared to be the main operating mechanism. These results demonstrated that high-performance electrophosphorescence can be obtained from polymer-based LEDs that are fabricated by processing the active materials directly from solution. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2691-2705, 2003