Dynamics of photoexcited carriers in pristine oxygen-free C60 films: Study of the time-resolved transient photoconductivity

We study the dynamics of photoexcited carriers in pristine oxygen-free C60 films using the measurement of the transient photoconductivity (PC). The nonexponential relaxation of photocarriers in oxygen-free C60 can be described by the multiple trapping model. The carrier recombination kinetics is characteristic of bimolecular recombination at high temperature. As the thermally-activated long-lived PC component freezes out at the low temperature regime, a monomolecular recombination prevails and the carrier relaxation approaches an exponential decay. The peak transient PC exhibits a local maximum at T=240 K, near a first-order structural phase transition temperature, indicating that evaporated film of oxygen-free C60 contains regions of structurally ordered C60. The disorder in a pristine C60 film is manifested by the existence of a large, thermally-activated, long-lived PC component.