Top-emission organic light emitting devices (TEOLED) with a mixed single layer by mixing of electron transport materials, hole transport material and dope material were investigated. With an optimized mixing ratio of PyPySPyPy:Alq3:alpha-NPD: Rubrene [25:50:25:1], maximum power efficiency, 3.45 lm/W, and maximum luminance, 7,000 cd/m(2) was obtained. Compared to the conventional heterostructure device, the performance was improved with a factor of more than two. The dependence of organic layer thickness on device performance was also investigated and the result showed that 100nm was an appropriate thickness for good performance of mixed single layer TEOLED. In addition, the maximum luminance and power efficiency were obtained at 20,000 cd/m(2) and 7.5 lm/W, respectively, when TPD layer of 45nm was capped onto top metal electrode.