Origin of the open circuit voltage in conjugated polymer-fullerene photovoltaic cells

We fabricated polymer-fullerene photovoltaic (PV) devices using various conjugated polymers, poly[2-methoxy-5-(2'-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV), poly[2-{4-(3,7-dimethyloctyloxy)-phenoxy}-1,4-phenylenevinyl-ene] (p-DMOP-PPV), and their copolymer system, p-DMOP-co-MEH-PPV. By comparing PV characteristics of such devices with a systematic change of the energy levels (ionization potential I-p and electron affinity E-a), we investigated the origin of open circuit voltage (V-oc) of the devices. Our results indicate that the magnitude of V-oc is governed by the difference between Ea of the fullerene and I-p of the polymer, which is not consistent with previous consideration of the work function difference between electrodes as in conventional metal-insulator-metal type devices.