Recently, efficiency of organic solar cells (OSCs) with conjugated polymers and fullerene derivatives has been increased, in most cases by improving their short-circuit current density. Regarding the other photovoltaic parameters, the open-circuit voltage (Vac) remains controversial. In this study, we investigated the relation between the V(OC) and the energy difference (Delta E) between the highest occupied molecular orbital of the donor material and the lowest unoccupied molecular orbital of the acceptor. The simple linear relation between V(OC) and AE was not observed in the polymer-based OSCs. There must be some factor that decreased the V(OC) of the cells. (C) 2008 Elsevier B.V. All rights reserved.