A dibenzosilole-based host material was designed and characterized. The host material, 9,9′-(5,5-diphenyl-5H-dibenzo[b, d]silole-2,8-diyl)bis(9H-carbazole) (S-SiCz), was designed to enhance the electron transport properties and rigidity by coupling two phenyl units of the tetraphenyl silane of a strong hole transport type bis(4-(9H-carbazol-9-yl)phenyl)diphenylsilane host. The device efficiency roll-off was improved considerably by balancing the carriers in the phosphorescent organic light-emitting diodes (PhOLEDs), and the driving voltage at high luminance was reduced. The red and green PhOLEDs exhibited high external quantum efficiencies (EQEs) of 23.8% and 24.9%, respectively, and a relieved efficiency roll-off. In addition, S-SiCz was used as an electron transport type host with a hole transport type 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl host. The maximum EQEs of the red and green PhOLEDs using the mixed host were 26.0% and 25.5%, respectively, and EQE roll-off values were 18% and 6%, respectively. Therefore, the planarization design strategy of the host is effective for better device performance.