Photovoltaic devices made from metallo-phthalocyanine/fullerene (M-Pc/C(60)) with 5,6,11,12-tetraphenylnaphthacene (rubrene) as an additive are fabricated and characterized. The effect of rubrene is examined for 4 different M-phthalocyanines - H(2)Pc, ZnPc, ClInPc, and VOPc - to represent 4 different valencies of the central moiety of M-Pc. In each case, rubrene has shown a notable increase in the open circuit voltage and in the case of the ClInPc and VOPc results in an increase in the overall power conversion efficiency. Through measurement of external quantum efficiency, it is shown that the increased efficiency is due to increased V(oc) and not due to the photocurrent contribution from the complementary absorption profile of rubrene. Finally, the photostability of rubrene-based cells is studied, showing that unencapsulated devices decay rapidly in air as a result of the formation of rubrene peroxide, as evidenced by significant decay of the UV-vis absorption and direct measurement of the cell performance over a time period as short as several minutes. (C) 2011 Elsevier B.V. All rights reserved.