We have used optical modeling to calculate whether optical absorption and corresponding current densities in solar cells consisting of PF10TBT:PCBM (1:4) can be enhanced, either in single junctions by applying optical spacers or by going to a tandem structure. The current densities are calculated from the optical absorption using experimentally determined IQE values. When an optical spacer is used, the thickness of the transparent electron transport layer (ETL) and hole transport layer (HTL) is very important. The absorption, and thus current density, in a single junction solar cell could be enhanced by 10% by reducing the HTL (PEDOT) thickness and by inserting a thin ETL (30 nm ZnO). The current density in a tandem consisting of two PF10TBT:PCBM blend layers is strongly dependent on the thickness of these blend layers. An increase of 20% is possible for a layer combination of 150 nm and 250 nm, combined with appropriate ETL and HTL layer thicknesses, and assuming no further losses. (c) 2007 Elsevier B.V. All rights reserved.