Tandem organic light-emitting diodes (OLEDs) with the buffer-modified fullerene/copper phthalocyanine (LiF/C60/CuPc/MoO3) as interconnector have been investigated. The properties of tandem OLEDs are dependent on electric field, which is induced by electric-field-dependence of charge generation (by electric filed and photons) and separation in such an interconnector structure. At low electric field, because of less generated-charges and light-absorption of C60/CuPc film, current efficiency and brightness of tandem OLEDs with two emissive units exhibit less than twice higher than those of conventional single-unit device. But more than twofold enhancement of current efficiency and brightness is observed at higher electric field due to a large number of charges generated in the interconnector. Meanwhile, the performance of tandem devices is affected by the thickness of C60 and CuPc. The optimal thickness is related to their exciton diffusion length. (C) 2014 Elsevier B.V. All rights reserved.