Tuning of Fullerene Materials for Organic Solar Cells: A Theoretical Study on the Properties of Defect Fullerenes C59 and C69

Hole transport and other fundamental properties of defect fullerenes C59 and C69 were investigated using density functional theory calculations. C59 and C69 isomers without a four-membered ring and three neighboring five-membered rings are generally stable. Formation of a carbon vacancy in C60 and C70 slightly increases the highest occupied molecular orbital energy and greatly decreases the lowest unoccupied molecular orbital energy, so that the energy gap decreases by 1 eV. The reorganization energies of all defect fullerenes are larger than those of the original C60 and C70 because of the localization of injected carriers around the vacancy. The reorganization energy of defect fullerenes is closely related to relaxation of the C–C bond of unsaturated C atoms.