We report an improvement of efficiency in red phosphorescent organic light-emitting diodes (PHOLEDs) based on a combination of heterojunction (HJ) structure and mixed host (MH) system using a phosphorescent red emitter: bis(2-phenylquinolinato)-acetylacetonate iridium III [Ir(pq)2(acac)] doped in 4,4,N,N'-dicarbazolebiphenyl (CBP) of hole transport type host material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) of electron transport type host material. This combination device resulted in an effective electron and hole balance and distribution of the recombination zone. Therefore, highly efficient red PHOLEDs with maximum luminous efficiency and external quantum efficiency of 21.93 cd/A and 14.09% were achieved. Moreover, the combination device showed a power efficiency of 9.51 lm/W, which is higher than 7.61 lm/W in the control device at a luminance of 1000 cd/m2.