We applied co-evaporation films (BAlq:Alq(3)) as an emission layer of OLED and studied their energy carrier transfer process. Energy was partially transferred from excited BAlq to Alq(3) by PL spectra. We also investigated its light-emission mechanism using a co-evaporation layer in OLED. Energy transfer occurs from NPD to Alq(3) or suppression of electron injection into NPD takes place. Furthermore, the hole is injected into the BAlq within 20-nm distance from the NPD/BAlq interface injected holes can move to as much as 10 nm from the interface. Doping Alq(3) in BAlq changes the number of electrons that are injected into BAlq because of its electron trapping against BAlq. We also suggest that the rate constant of energy transfer from NPD to Alq(3) is Shorter than the radiative deactivation time of NPD. The number of NPD excitons generated by recombination is less than that of deactivating Alq(3). (c) 2005 Published by Elsevier B.V.