A nondoped organic system of fullerene (C60)/pentacene was investigated as a connecting unit for green electroluminescent tandem organic light emitting devices with two identical emissive units consisting of (N,N'-di(naphthalene-1-yl)-N,N'-diphthalbenzidine)/tris(8-hydroxyquinoli ne aluminum) (Alq(3)) that exhibited better current efficiency and current density characteristics over conventional devices. At 20 mA/cm(2), the current efficiency of the tandem devices using a nondoped organic layer of C60/pentacene was about 3.4 cd/A, almost 34% improvement over that of the corresponding conventional devices. The connecting unit showed a superior optical transparency, resulting in a minimal microcavity effect in the devices. Interface dipole and band bending on both sides of the C60/pentacene interface was suggested for the formation of an intrinsic p-n junction, which is a prerequisite of a connecting unit, leading to a significant performance improvement in the tandem devices. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3247354] All rights reserved.