The mixed layer for an emitting layer (EML) consists of two materials 4,4 ',4 ''-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) without host material. A series of EML structures were fabricated with three types of single mixed layers (mixing ratio 2:1, 1:2) and double mixed layers (mixing ratio 2:1/1:2), and the EML of using double mixed layer was optimized. The maximum luminous efficiency (LE) and quantum efficiency (QE) were 62.07 cd/A, 18.92%, respectively. Moreover, the roll-off ratio of LE was 20.42% at 20,000 cd/m(2). Interestingly, this result shows the roll-off ratio was reduced roughly 50% compared to the reference device.