Hole versus electron transport in fullerenes

We have carried out density functional theory (DFT) calculations to derive the microscopic parameters that describe the hole and electron transport properties in C₆₀ and C₇₀ and their derivatives. The computed relaxation energies and the results of normal-mode analyses point out that there are no significant differences between the electron-vibration couplings and hole-vibration couplings in the C₆₀ systems, where the differences between the reorganization energies for hole and electron transfers do not exceed 10 meV, except in the case of the parent C₆₀ system where this difference is about 40 meV. The DFT estimates of the electronic couplings for holes and electrons in C₆₀ and its derivatives are also found to be very similar. Thus, our theoretical results confirm the conclusions of recent experimental investigations that underline that the C₆₀ fullerenes should not be regarded as just n-type transporting materials but in fact are intrinsically ambipolar. The DFT calculations performed for C₇₀ show that the reorganization energy for hole transport is slightly lower (5 meV) than for electron transport and this difference is even larger, up to 100 meV, in the functionalized C₇₀ systems. These results suggest that hole mobilities in C₇₀ and its derivatives could also be comparable or even larger than the corresponding electron mobilities. In addition, the DFT calculations indicate that, with respect to unsubstituted fullerenes, the derivatization has a much larger impact on the ionization potential energies than on the electron affinity energies, which are both reduces; therefore, chemical modifications could be further exploited to decrease the intrinsically high IP values of C₆₀ and C₇₀ and lead to a better match with the workfunctions of common electrodes, which would facilitate hole injection.