Role of molecular oxygen and other impurities in the electrical transportand dielectric properties ofC60films

We have used dielectric spectroscopy to measure the frequency and temperature dependence of the low-frequency (0.5 mHz--100 kHz) complex dielectric function \ensuremath{\varepsilon}(\ensuremath{\omega}) of thin ${\mathrm{C}}_{60}$ films. A small charge transfer occurs between the ${\mathrm{C}}_{60}$ and ${\mathrm{O}}_{2}$ molecules which occupy interstitial spaces of solid ${\mathrm{C}}_{60}$ exposed to oxygen (or ambient air). Due to the large size of the ${\mathrm{C}}_{60}$ molecules, this small charge transfer creates large dipole moments, which in turn are coupled to the applied ac electric field via a diffusion-controlled relaxation mechanism. This gives rise to a significant increase in the permittivity ${\mathrm{\ensuremath{\varepsilon}}}_{1}$ accompanied by a broad dielectric loss peak ${\mathrm{\ensuremath{\varepsilon}}}_{2}$ observed at \ensuremath{\sim}10--100 Hz. With increasing oxygenation, the interstitial sites become nearly fully occupied, interstitial hopping is inhibited, and the loss peaks, together with the enhanced polarization, disappear. Both tracer and chemical diffusion coefficients for the ${\mathrm{C}}_{60}$/${\mathrm{O}}_{2}$ system have been obtained purely from dielectric spectroscopy measurements.