Cathode dependence of the open-circuit voltage of polymer:fullerene bulk heterojunction solar cells

摘要

The open-circuit voltage (V_(OC)) of bulk-heterojunction solar cells based on 6,6-phenyl C_(61)-butyric acid methyl ester (PCBM) as electron acceptor and poly2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene (OC_(1)C_(10)-PPV) as an electron donor has been investigated. In contrast to the present understanding, it is now demonstrated that for non-ohmic contacts the experimental V_(OC) is determined by the work function difference of the electrodes. A total variation of more than 0.5 V of the V_(OC) was observed by variation of the negative electrode (cathode) work function. For ohmic contacts the V_(OC) is governed by the LUMO and HOMO levels of the acceptor and donor, respectively, which pin the Fermi levels of the cathode and anode. The band bending created by accumulated charges at an ohmic contact produce a considerable loss in V_(OC) of 0.2 V at room temperature. The experimentally observed voltage loss in V_(OC) of 0.38 V due to the presence of ohmic contacts at both interfaces strongly limits the maximum open-circuit voltage of OC_(1)C_(10)-PPV/PCBM solar cells.