Although the mixed-host organic light emitting diode (OLED) has been known for improved performance, the charge injection and mobility may not be uniform in the light emitting layer (EML) hence affecting the exciton formation and recombination. In this work, we investigate the effects of concentration profiles of electron- and hole-transport materials in EML on the charge injection and confinement as a result of in-situ formation of transport layers by thermal evaporation of a pre-mixed source. Experimental results show that the OLED performance depends on the continuously graded concentration profiles, and are correlated to numerical simulations which reveal charge confinement, spatial shift of recombination zone and lowering of hole mobility. (C) 2016 Elsevier B.V. All rights reserved.