Effect of the disorder/order phase transition on the electrical and photoelectrical properties of C60 thin films

Abstract

This article reports the temperature dependence of the structural and photoelectrical properties of C60 thin films with varying crystal structure and oxygen content near the orientational disorder/order phase transition at about 260 K. X-ray diffraction data demonstrate that highly crystalline oxygen-free C60 films undergo the first-order phase transition at 252 K with a lattice parameter discontinuity Δa/a of 0.22%; no discontinuity was observed in C60 films with coexisting amorphous and nanocrystalline phases. A strong effect of the phase transition on the dark- and photoconductivity in those films is presented: In highly crystalline C60 films both dark- and photoconductivity increase as the temperature is lowered through the transition region. However, opposite temperature changes of the conductivity are observed in nanocrystalline/amorphous C60 films. In situ exposure of samples to oxygen gas suppresses any critical behavior. The experimental results are interpreted in terms of trap-controlled mobility of the dominant charge carriers such that the temperature changes of the conductivity are attributed to variations in the concentration, energy distribution, and population of the carrier traps at the phase transition. Differences in the parameters of the traps, which are associated with the freeze-out of the molecular orientations, may also explain the strong variations in the temperature dependence of the conductivity observed in our C60 films and reported by other authors.