Tuning the Electron-Acceptor Properties of [60]Fullerene by Tailored Functionalization for Application in Bulk Heterojunction Solar Cells

The synthesis of a series of fullerene derivatives designed to act as photoactive acceptor materials in polymer:fullerene photovoltaic blends has been reported. DFT calculations, which provide the LUMO energy values as a function of the fullerene substitution pattern, were employed in the design of these compounds. The functionalizations focused on the type of addition (1,2- or 1,4-addition) and electronic properties of the addends. Cyclic voltammetry determined the LUMO energies to be in good agreement with the calculated values. The photovoltaic efficiencies of the solar cells, composed of poly(3-hexylthiophene) (P3HT):fullerene blends, were measured. EPR spectroscopy was used to characterize the magnetic interactions and symmetry properties of radical anions and the excited triplet states of the derivatives. An explanation of the observed LUMO energy shifts based on the symmetry change in the frontier orbitals of the derivatives has been proposed.