A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C-60, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I-V characteristics of several other devices were investigated. It is found that the presence of C-60 in a bulk heterojunction solar cell introduces fill factor reducing shunting paths. (C) 2000 Elsevier Science B.V. All rights reserved.