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 Ip and electron affinity Ea), we investigated the origin of open circuit voltage (Voc) of the devices. Our results indicate that the magnitude of Voc is governed by the difference between Ea of the fullerene and Ip 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.

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