Light-emitting diodes based on phosphorescent guest/polymeric host systems

Fabrication of polymer light-emitting diodes based on emission from the phosphorescent molecule fac tris(2-phenylpyridine) iridium doped into a poly(vinyl carbazole) host are reported. Several spin-coating solvents were evaluated for deposition of the polymer layer; toluene and chlorobenzene were found to consistently produce device-quality films with sufficient incorporation of the dopant. For single-layered devices with ${\mathrm{Mg}}_{\text{0.9}}{\mathrm{Ag}}_{\text{0.1}}$ cathodes, the luminance efficiency at 20 mA/cm² was measured to be 8.7 Cd/A for devices processed from chlorobenzene. This efficiency could be increased by over a factor of two with a trilayered device geometry consisting of the doped polymer layer, a hole-blocking layer, and electron transport layer. Further increases in efficiency, up to 30 Cd/A and 8.5% external quantum efficiency, were observed when a second dopant of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was added to the polymer emitter layer.