Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/ Alq(3) (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm(2)). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al3+ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al)(x)(BCp)(y)](3x+)/Al has formed under electric field and the new cation [(Al)(x)(BCp)y](3x+) at the BCP/Al inter-face has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (< 1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting. (c) 2005 Elsevier B.V. All rights reserved.