The electronic properties, carrier injection, and transport into poly(9,9-dioctylfluorene) (PFO), PFO end-capped with holetransporting moieties (HTM), PFO-HTM, and PFO end-capped with electron-transporting moieties (ETM), PFO-ETM, were investigated. The data demonstrate that charge injection and transport can be tuned by end-capping with HTM and ETM, without significantly altering the electronic properties of the conjugated backbone. End-capping with ETM resulted in more closely balanced charge injection and transport. Single-layer electrophosphorescent light-emitting diodes (LEDs), fabricated from PFO, PFO-HTM and PFO-ETM as hosts and tris[2,5-bis-2'-(9',9'-dihexylfluorene)pyridine-kappa(2)NC(3')]iridium(I II), Ir(HFP)(3) as the guest, emitted red light with brightnesses of 2040 cd m(-2), 1940 cd m(-2) and 2490 cd m(-2) at 290 mA cm(-2) (16 V) and with luminance efficiencies of 1.4 cd A(-1), 1.4 cd A(-1) and 1.8 cd A(-1) at 4.5 mA cm(-2) for PFO, PFO-HTM, and PFO-ETM, respectively.