Synthesis, Photophysical Properties, and Photovoltaic Devices of Oligo(<i>p</i>-phenylene vinylene)-fullerene Dyads

Abstract

The synthesis of a homologous series of oligo(p-phenylene vinylene)-fulleropyrrolidines (OPVn-C60, n = 1−4, where n is the number of phenyl rings) is described. The photophysical properties of these donor−acceptor dyads and the corresponding model compounds, α,ω-dimethyl-2,5-bis(2-(S)-methylbutoxy)-1,4-phenylene vinylene oligomers (OPVn, n = 2−4) and N-methylfulleropyrrolidine (MP-C60), are studied as a function of the conjugation length in solvents of different polarity and as thin films. Fast singlet energy transfer occurs after photoexcitation of the OPVn moiety of the dyads toward the fullerene moiety in an apolar solvent. Photoexcitation of the dyads in a polar solvent results in electron transfer for OPV3-C60 and OPV4-C60, and to some extent for OPV2-C60, but not for OPV1-C60. These results are compared to the results obtained for mixtures of OPVn and MP-C60 in the same solvents. The solvent-dependent change in free energy for charge separation of the donor−acceptor systems is calculated from the Weller equation, and the rate constants for energy and electron transfer are derived from the fluorescence lifetime and quenching. The results show that in a polar solvent electron transfer in these dyads is likely to occur via a two-step process, that is, a very fast singlet energy transfer prior to charge separation. In thin solid films of OPV3-C60 and OPV4-C60, a long-lived charge-separated state is formed after photoexcitation. The long lifetime in the film is attributed to the migration of charges to different molecules. A flexible photovoltaic device is prepared from OPV4-C60.