Fullerene-oligophenylenevinylene hybrids: Synthesis, electronic properties, and incorporation in photovoltaic devices

Fullerene derivatives in which an oligophenylenevinylene (OPV) group is attached to C-60 through a pyrrolidine ring have been prepared by 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding aldehydes and sarcosine. Electrochemical and photophysical studies have revealed that ground-state electronic interactions between the covalently bonded OPV moiety and the fullerene sphere are small. The photophysical investigations have also shown that both in dichloromethane and benzonitrile solution an efficient singlet-singlet OPV --> C-60 photoinduced energy-transfer process takes place, and occurrence of electron transfer, if any, is by far negligible relative to energy transfer. The C-60-OPV derivatives have been incorporated in photovoltaic devices, and a photocurrent could be observed showing that photoinduced electron transfer does take place under these conditions. However, the efficiency of the devices is limited by the fact that photoinduced electron transfer from the OPV moiety to the C-60 Sphere must compete with an efficient energy transfer. The latter process, as studied in solution, leads to the population of the fullerene lowest singlet excited state, found to lie slightly lower in energy than the charge-separated state expected to yield electron/hole pairs. Thus, only a small part of the absorbed Light is able to contribute effectively to the photocurrent.