The effect of the host structure on the optimum doping concentration in red phosphorescent organic light-emitting diodes (PHOLEDs) was investigated. A mixed host of the phosphine oxide derivative (SPPO21) and 4.4',4 ''-tris(N-carbazolyl)triphenylamine (TCTA) was used as the host in the red PHOLED and the device performances were studied according to the doping concentration. The device performance of the red PHOLED was optimized at a doping concentration of 2% in the SPPO21 rich red PHOLED, while the device performance was optimized at a doping concentration of 6% in the device with 50% TCTA content. It was found that the optimum doping concentration was determined by the energy transfer and charge trapping. (c) 2011 Elsevier B.V. All rights reserved.