We report highly efficient red-light-emitting devices based on a fluorene-triphenylamine tris[4-(9-phenylfluoren-9-yl)phenyl]amine (TFTPA) host doped with an osmium phosphor. The sterically hindered fluorene peripheries provided a compatible environment for the osmium dopant and alleviated concentration quenching of the phosphor at high doping levels. Increasing the doping concentration from 7 to 21 wt % dramatically decreased the driving voltage of the TFTPA-based devices, leading to improved power efficiency. The 21 wt % Os-doped device exhibited maximum luminous and powder efficiencies of 29.9 cd/A and 25.2 lm/W, combined with high efficiencies at high brightness. At 1000 cd/m2 the efficiencies remained 29.2 cd/A and 22.2 lm/W.