The electronic properties, carrier injection, and transport into poly(9,9-dioctylfluorene) (PFO), PFO end-capped with hole-transporting 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-κ2NC3′]iridium(III), 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.