The current density-voltage (J-V) measurements on organic light-emitting devices (OLEDs) with and without a N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB):tris-(8-hydroxyquinoline) aluminum (Alq(3)) mixed layer were performed to investigate the carrier transport mechanism. The slopes of the double logarithmic J-V curves for the OLEDs with and without the mixed layer in the space-charge-limited current (SCLC) region were 6.5 and 2, respectively. The dramatic increase in the slope of the double logarithmic J-V curve for the OLEDs with the mixed layer was attributed to the decrease in the mobility difference of the charge carriers at the NPB/Alq(3) heterointerface resulting from the minimization of the energy barriers, and the carrier transport behaviors in the SCLC region were significantly affected by the unintentional virtual electrode, serving as a charge carrier reservoir, that formed at the heterointerface.