The influence of materials work function on the open circuit voltage of plastic solar cells

Conjugated polymeryfullerene plastic solar cells of the first generation were consisting of two distinct layers, made of the donor polymer and of the acceptor fullerene, respectively, sandwiched between two metal contacts. By mixing the polymer and the fullerene components, thus replacing a single flat junction with an interpenetrating network bulk-heterojunction, the device efficiency was dramatically improved. As a further step to proceed with the development of plastic solar cells, we developed the bulk diffusion bilayer approach, allowing the creation of donor-acceptor diffused interfaces with less restrictions to the phase compatibility of the two components. For a novel series of fullerenes, the bulk diffusion bilayer approach is shown to yield devices with comparable efficiencies as the blend bulk heterojunction approach. Bulk-heterojunction devices show unusually high open circuit voltage (VOC) values. These values cannot be explained by the metal-insulator-metal (MIM) model that has been often used for organic light emitting diodes. In order to investigate the origin of the VOC in bulk-heterojunction plastic solar cells, we have prepared PPV based devices varying both the metal negative contact and the fullerene acceptor. Fullerene derivatives with varying acceptor strength, (i.e. the first reduction potential) were used as electron acceptors in bulk-heterojunction plastic solar cells produced with the blend as well as the diffusion bilayer approach. The open circuit voltage of the devices was found to correlate directly with the acceptor strength of the fullerenes, while it was rather insensitive to variations in the workfunction of the negative electrode metal. These results suggest that the quasi-Fermi level of the fullerene pins the Fermi level of the evaporated negative metal contact.