Fabrication of polymer light-emitting diodes based on emission from the phosphorescent molecule fac-tris(2-phenylpyridine) iridium doped into a poly(N-vinyl carbazole) host are reported. For single-layered devices with magnesium-silver cathodes, the luminance efficiency at 20 mA/cm(2) was measured as 8.7 cd/A. This efficiency could be increased by over a factor of two by incorporation of evaporated small-molecule layers into the device structure. Significant increases in device efficiency were also obtained without these evaporated layers by modification of the electrodes. Incorporation of 3,4-poly(ethylene dioxythiophene):poly(styrene sulfonate) at the anode improved the device efficiency but had little impact on drive voltage. Insertion of lithium fluoride at the cathode resulted in no improvement in performance for magnesium-silver and aluminum cathodes, but a significant improvement was realized in efficiency and drive voltage for calcium-aluminum cathodes. Excellent device performance was observed for all three cathode metals used in conjunction with cesium fluoride. Through optimization of the electrodes and emitter-layer thickness, devices exhibiting efficiencies as high as 37.3 cd/A are realized. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2715-2725, 2003