LONG-DECAY-TIME PHOTOCONDUCTIVITY IN HIGHLY DISORDERED CARBON-FIBERS

A photoconductivity with a rather long decay time (congruent-to 0.3 s at 290 K) is observed in a carbon material that is characterized by its huge specific surface area (1000 m2/g). The decay time is temperature dependent at low temperatures (< 100 K) with a small activation energy (3-5 meV), and is also dependent on the illumination power. A decrease in the mobility of the carriers during the illumination is strongly suggested. The relaxation of dangling bonds on the large surface area is proposed as the mechanism to screen the Coulomb attraction among the photocarriers and to build a recombination barrier, resulting in the long decay time. This photoconductivity is considered to be an effect analogous to the persistent photoconductivity in semiconductors, although some apparent differences are also observed.