| Summary: | 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<sub>bi</sub> is the driving force for charge separation and determines the open-circuit voltage V<sub>oc</sub>. In former works, V<sub>bi</sub> was estimated by measuring V<sub>oc</sub> 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<sub>bi</sub> and V<sub>oc</sub>. In our experiments on indium tin oxide/poly(3,4-ethylendioxythiophene) : poly(styrene-sulfonate)/poly(2 -methoxy-5-(3′,7′-dimethyloktyloxy)-p-phenylene-vinylene) : 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C<sub>61</sub>/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<sub>oc</sub> exceeded V<sub>bi</sub> which is revealing of semipermeable membranes. © 2005 Elsevier B.V. All rights reserved.
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