Ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([ BMIm]Cl) is doped into PEDOT: PSS hole-injecting layer to adjust the electroluminescent performance of OLEDs. The secondarily IL-doped PEDOT: PSS films are characterized in detail by SEM, XPS and UPS. The UPS results indicated the decrease of ionization potential of PEDOT: PSS due to the secondary IL-doping. The SEM observation showed the phase transition of IL-doped PEDOT: PSS films from the crazing texture to smoothing surface and serious phase separation. As a result, the IL-doped PEDOT: PSS allows the optimal luminous efficiency to reach a maximum value of 4.27 cd/A, which is about 5.0 times as 0.85 cd/A in the IL-undoped PEDOT: PSS hole-injecting layer. (C) 2015 Elsevier Ltd. All rights reserved.