Rubrene mixing has been shown to be an effect mean for enhancing both the open circuit voltage (Voc) and the short-circuit current (Jsc) of copper-phthalocyanine (CuPc)/fullerene (C-60) based solar cell. While the increase in Jsc can be readily explained by the additional rubrene absorption and the introduction of a bulk heterojunction; causes for Voc increase are still not clear. The energy offset between the highest occupied molecular orbital (HOMO) level of donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor (HOMOD-LUMOA) at the CuPc/C-60 interface was found to increase substantially upon rubrene mixing in either side of the interface. As the HOMOD-LUMOA is generally considered to limit the Voc, its increase agrees well with the device results. Energy level bending and associated built-in electric fields were also observed and their possible implications to device performance are discussed. (C) 2011 Elsevier B. V. All rights reserved.