To determine the actual absorption in the photoactive layer of a plastic solar cell, e.g. consisting of blend of poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) and a methanofullerene, [6,6]-Phenyl C-61-butyric acid methyl ester (PCBM), a matrix formalism for the light propagation in this multi-layer system is applied. This calculation results in an upper limit for the incident photon to collected electron (IPCE) conversion efficiency for a given internal quantum efficiency. Comparisons with experimental results as well as conclusions for the optimal layer thickness are drawn. (C) 2003 Elsevier B.V. All rights reserved.