Ajuts: All authors are grateful for support from COST Action MP 1307, "StableNextSol". This article is based upon work from COST Action StableNextSol project MP1307, supported by COST (European Cooperation in Science and Technology). RM, TF, DF, RR and HH are grateful for financial support within the frame of "AIMS in OPV" junior research group by the BMBF, Germany. L.C. is thankful to ENEA (Ente Nazionale Energia e Ambiente) and the Italian Ministry of Foreign Affairs for a visitor post-doc fellowship to BGU. FG and MD are grateful for financial support by the National Center for Research and Development within the Project POSCIS under Grant No. LIDER/09/129/L-3/11/NCBR/2012). IVF and EAK acknowlegde financial support from Adelis Foudation. N. Blaubach is acknowledged for ICP measurements. MP thanks the Bundesministerium für Bildung und Forschung, Germany (BMBF FKZ 03EK3507).

We report a cooperative study conducted between different laboratories to investigate organic solar cell degradation with respect to P3HT material properties and different solar cell architectures. Various batches of P3HT were collected from different suppliers reflecting commercial availability as well as properties variability. Among the materials properties explicitly considered were the molar mass, dispersity, regio-regularity, impurities by trace metals and intrinsic doping evaluated from radical concentrations. Each of the participating laboratories contributing test devices applied their own layer stack, i.e. their own device architecture and layout. This variation was appreciated as another parameter for evaluation. Even though a large amount of devices failed due to extrinsic degradation effects, indeed, some materials properties were found to be more important than others for obtaining long lifetimes and high stability of P3HT-based polymer solar cells.