The performance of bulk-heterojunction solar cells made with poly(3-hexylthiophene) (P3HT) as the donor and phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor depends strongly on various factors in fabrication processes. This work studies the effect of the thickness of the P3HT:PCBM layer produced in the spin coating process on photovoltaic performances. Thickness dependent optical absorption in P3HT:PCBM layer is numerically modelled and the results are compared with experimental data. In addition, it is analyzed how degradation of air-exposed P3HT:PCBM blends depends oil the storage time in nitrogen atmosphere. (C) 2009 Elsevier B.V. All rights reserved,