The electrical and the optical properties of organic light-emitting devices (OLEDs) utilizing a 4,7-diphenyl-1,10-phenanthroline (BPhen) and aluminum tris(8-hydroxyquinoline) (Alq(3)) co-host electron transport layer (ETL) has been investigated to clarify the origin of the luminance efficiency enhancement. The luminance efficiency of the OLEDs was significantly enhanced by utilizing a BPhen:Alq(3) co-host ETL The electron-only-devices for OLEDs with a BPhen:Alq(3) co-host ETL or an Alq(3) ETL were fabricated to investigate the electron injection and transport properties in the co-host or single ETL The enhancement of the luminescence efficiency in OLEDs fabricated utilizing a co-host ETL consisting of a BPhen and an Alq(3) were attributed to the decrease in the electron injection barrier and the increase in the hole blocking barrier. The origin of the luminescence efficiency enhancement in OLEDs fabricated utilizing a co-host ETL consisting of a Men and an Alq(3) were described on the basis of the experimental results. (C) 2009 Elsevier B.V. All rights reserved.