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

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 mobilit...