We report the conjugated polymer P( tBu-CBP) as a host with high triplet energy ( E-T 2.53 eV) and suitable HOMO ( 5.3 eV) and LUMO ( 2.04 eV) energy levels. Upon doping with green and red emission Ir-complexes, it gives devices with high luminous and external quantum efficiencies for green emission ( 23.7 cd/A, 6.57%) and for red emission ( 5.1 cd/A, 4.23%), respectively, and low turn-on voltage ( 3 V). For both devices, the efficiencies are higher than those of the corresponding devices with the same backbone P( 3,6-Cz) as a host by a factor of 4, even though the latter has an E-T ( 2.6 eV) slightly higher than that of the former. The results reflect that, in phosphorescent devices, the difference in E-T between the host and guest is not the only factor that determines the device efficiency, and the present side group modification via the 9 position of carbazole also plays an important role, which allows a tuning of HOMO and LUMO levels to provide more balance in electron and hole fluxes and provides prevention from formation of excimer.