To obtain photovoltaic devices based on electron donating conjugated polymers with a higher efficiency, a major breakthrough was realised by mixing the polymers with a suitable electron acceptor, thereby enhancing the rate for photo-induced charge generation by several orders. State-of-the-art organic bulk hetero-junction photovoltaic cells are based on an interpenetrating donor-acceptor network in the bulk to form efficient nanostructured p-n junctions in the organic materials. Devices made with 'Gilch' poly(2-methoxy-5-(3',7'-dimethyl-octyloxy))-p-phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)phenyl-C-61-butyric-acid (PCBM) (a soluble C60 derivative) as an electron acceptor yielded the highest efficiency until now in this class of devices. A power conversion efficiency of approximately eta(c) greater than or equal to 2.5% (electrical power out/incident light power) under AM 1.5 illumination was reported. The 'gilch' route is a direct synthetic route. The 'sulphinyl' route is a promising. indirect precursor-route towards MDMO-PPV. Due to the non-symmetric monomer, so-called 'head-to-head' and 'tail-to-tail' additions are excluded to a higher level in comparison to the 'gilch' route. This difference between both materials makes them interesting candidates to compare them in the state-of-the-art photovoltaic devices. Preliminary results indicate that the 'sulphinyl' MDMO-PPV/PCBM bulk hetero-junction solar cells attain a power conversion efficiency of nearly eta(e)=3% (electrical power out/incident light power), have a higher fill factor, incident photon per converted electron value (IPCE) and short circuit current. It is indicated that the observed solar cell characteristics are related to the defect level of the conjugated polymer used.