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

TLDR

In bulk‑heterojunction solar cells with ohmic contacts, the open‑circuit voltage is set by the donor HOMO and acceptor LUMO levels, but charge‑induced band bending can reduce VOC by about 0.2 V at room temperature. The study shows that for non‑ohmic contacts the open‑circuit voltage of PCBM/OC1C10‑PPV cells is governed by the electrode work‑function difference, with cathode changes producing over 0.5 V variation, while ohmic contacts impose a 0.38 V loss that caps the maximum VOC.

Abstract

The open-circuit voltage (VOC) of bulk-heterojunction solar cells based on [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor and poly[2-methoxy-5(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-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 VOC is determined by the work function difference of the electrodes. A total variation of more than 0.5 V of the VOC was observed by variation of the negative electrode (cathode) work function. For ohmic contacts the VOC 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 VOC of 0.2 V at room temperature. The experimentally observed voltage loss in VOC of 0.38 V due to the presence of ohmic contacts at both interfaces strongly limits the maximum open-circuit voltage of OC1C10-PPV/PCBM solar cells.