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

Abstract

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 poly[2-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.