Orange-emitting phosphorescent copolymers containing iridium complexes and bis(carbazolyl)fluorene groups in their side chains are employed as the emissive layer in multilayer organic light-emitting diodes (OLEDs). The efficiency of the OLED devices is optimized by varying characteristics of the copolymers: the molecular weight, the iridium loading level, and the nature and length of the linker between the side chains and the polymer backbone. A maximum efficiency of 4.9 +/- 0.4%, 8.8 +/- 0.7 cd A(-1) at 100 cd m(-2) is achieved with an optimized copolymer.