In this paper, highly efficient phosphorescent organic lighting emitting diodes (PhOELDs) with low efficiency roll-off are demonstrated by using a unilateral homogenous device structure with wide band-gap material 4, 4', 4″-tri(N-carbazolyl)-triphenylamine (TCTA) as hole transporting layer and emitting layer (EML). The optimized blue device exhibits a high power efficiency of 40 lm/W, external quantum efficiency of 19.2% and current efficiency of 37.7 cd/A. More importantly, the device exhibits a low efficiency roll-off at 1000 cd/m2. In addition, the white homogenous PhOLEDs only exhibits the efficiency roll-off 5.6% and 17.5%, corresponding to the brightness of 1000 and 5000 cd/m2 respectively. These interesting results demonstrate that the simple unilateral homogenous device structure is a promising way to enhance the device efficiency and reduce the efficiency roll-off.