Tetrafullerene conjugates for all-organic photovoltaics

The synthesis of two new tetrafullerene nanoconjugates in which four C-60 units are covalently connected through different pi-conjugated oligomers (oligo(p-phenylene ethynylene) and oligo(p-phenylene vinylene)) is described. The photovoltaic (PV) response of these C-60-based conjugates was evaluated by using them as the only active material in organic solar cells, showing a low PV performance. Photophysical studies in solution demonstrated a very fast (similar to 10 ps) deactivation of the singlet excited state of the central core unit to produce both charge-separated species (i.e., C-60(center dot-)-oligomer(+center dot)-(C-60)(3) and C-60 centered singlet excited states). The charge-separated state recombines partly to the C-60 centered singlet state that undergoes subsequent intersystem crossing. Photophysical studies carried out in films support these data, exhibiting long-lived triplet excited states. For both tetrafullerene arrays, the low yield of long-lived charge carriers in thin films accounts for the limited PV response. On the contrary, utilizing the oligo(p-phenylene vinylene) centered precursor aldehyde as an electron donor and antennae unit and mixing with the well-known C-60 derivative PCBM, the photophysical studies in films show the formation of long-lived charges. The PV devices constructed from these mixtures showed a relatively high photocurrent of 2 mA cm(-2). The sharp contrast between the nanoconjugates and the physical blends tentatively was attributed to improved charge dissociation and the collection of more favorable energy levels in the blends as a result of partial aggregation of both of the components.