Doping is one of the most powerful methods amongst the various performance improvement ways. Doping affects the energy levels of the host layer by the energy level of the dopant. This allows the energy bandgap to be adjusted to a desired level and thus generates light corresponding to that energy level. Alternatively, it can act as an energy barrier between the interfaces to change the flow of carriers. In this study, the voltage dependences of undoped and doped devices were observed. Bis(2-phenylquinoline) iridium(III) (acetylacetonate) (Ir(pq)(2)acac) was doped in 4,4'-N, N'-dicarbazole-biphenyl (CBP) as the emission layer. The light intensity changes with the doping concentration, and the efficiency was also studied. When a high voltage was applied, the effect of triplet-triplet annihilation (TTA) adversely affected the electron-hole recombination. We analyzed the optimal operating conditions and the effect of doping concentration on OLEDs.