The working principle of organic bulk-heterojunction solar cells is a widely discussed topic. For thin film solar cells it is commonly supposed that the built-in potential V-bi is the driving force for charge separation and determines the open-circuit voltage V-oc. In former works, V-bi was estimated by measuring V., in the saturation regime. To check the validity of this model, the direct measurement of the built-in potential is desirable. We have investigated the origin of the open-circuit voltage of organic bulk-heterojunction solar cells by means of electroabsorption spectroscopy. This technique allows measurement of the built-in potential directly and therefore permits an independent measurement of V-bi and V-oc. In our experiments on indium tin oxide/poly(3,4-ethylendioxythiophene):poly(styrene-sulfonate)/poly(2-met hoxy-5-(3',7'-dimethyloktyloxy)-p-phenylene-vinylene): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C-61/metal bulk-heterojunction solar cells no significant correlation between the open-circuit voltage and the built-in potential was observed. For certain electrode materials, i.e. gold and copper, V-oc exceeded V-bi which is revealing of semipermeable membranes. (c) 2005 Elsevier B.V. All rights reserved.