In order to improve photon harvesting, two small molecule organic solar cells are placed in series on top of each other. These stacked cells need an efficient recombination center in between both cells. In this study we test vacuum deposited metal layers as recombination centers with pentacene and buckminsterfullerene (C-60) as donor and acceptor, respectively. S-shaped curves are visible in the I V characteristics when using thin layers of alluminium, indicating a barrier for extraction inside the device. Thin metal layers of gold or silver result in an increased open-circuit voltage without the appearance of these S-shaped features. (c) 2006 Elsevier B.V. All rights reserved.