For bulk-heterojunction photovoltaic cells fabricated from conjugated polymers and a fullerene derivative, the relation between the open-circuit voltage (V-oc) and the oxidation potential for different conjugated polymers is studied. A linear relation between V-oc and the oxidation potential is found (see figure). Based on this relation, the energy-conversion efficiency of a bulk-heterojunction solar cell is derived as a function of the bandgap and the energy levels of the conjugated polymer.