In Situ Photoconductivity Behavior of C₆₀ Thin Films: Wavelength, Temperature, Oxygen Effect

R. Ross, S. Kazaoui, and N. Minami
[Solid State Commun., Vol.90, p.623, 1994]

Uv-visible absorption, luminescence (excitation, emission), photoconductivity and conductivity measurements have been performed on polycrystalline C₆₀ thin films, in situ or exposed to oxygen. The shape of the emission spectrum is irrespective of the excitation energy, indicating that internal conversion takes place to the lowest localized electronic state. sigma_d and sigma_ph in the high temperature regime (T>270K) are thermally activated Ea(sigma_d)=0.50±0.05 and Ea(sigma_ph)=0.20±0.05eV whereas at low temperature regime (T<270K) they are almost temperature independent, for all excitation wavelength above the photoconductivity edge (Eg_el approx 1.7eV or lambda approx 730nm). Carrier photogeneration and transport processes are invoked in the context of localized electronic states. Oxygen effects on sigma_d and sigma_ph are interpreted. The figure 1 shows the experimental spectrum obtained for C₆₀ thin film kept strictly in vacuum after deposition, exposed to oxygen (T=298K, 1atm O₂, 12hours) and heated oxygen exposed film (the previously oxygen exposed sample is heated under vacuum at T=380K for 4h). This result indicates that oxygen has a strong electronic interaction with C₆₀ sample once it adsorbs and diffuses into the solid thin film. Most of the oxygen effect on sigma_ph and sigma_d can be reversed by heating the sample at 380K under vacuum (p<10^-6 Torr) for 4 hours, but the final state has a slightly lower photoconductivity and conductivity. As the oxygen effect on these properties is mostly reversible, photoassisted transformations are excluded, at least with the experimental conditions used here.