Hole versus electron transport in fullerenes

Abstract

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.