We carried out a detailed optimization of P3HT:PCBM polymer solar cells by variation of blending ratio, film thickness and annealing conditions. From our studies it became evident that the film thickness and the fullerene concentration are mutually dependent parameters, what the overall performance concerns. In detail, we revealed a clear relationship between film thickness, PCBM concentration and the blend film morphology. We varied the PCBM concentration in our polymer solar cells between 25% and 50%, and found the best results for 33.3% of PCBM. In this case, the optimum between competing processes like effective charge carrier separation and percolation path establishment was realized. In thicker films, the growth of PCBM aggregates via phase separation leads to formation of percolation paths and therefore improves the photocurrent. In contrast, for thinner films a high PCBM concentration is favourable to achieve optimal efficiencies. (C) 2008 Elsevier B.V. All rights reserved.