Photoconductivity intrans-polyacetylene: Transport and recombination of photogenerated charged excitations

Photoconductivity in unoriented Shirakawa-type polyacetylene as well as in highly oriented Durham-Graz-type polyacetylene has been measured. Both cw and transient experiments on different time scales have been carried out. The existence and the behavior of a fast and a slow component of the photoconductivity have been investigated in detail and the relationship between the two components has been elucidated. For the fast component we have observed a relaxation time of ${\tau }_{1/2}$≊100 ps, a migration distance of the carriers up to 400 Å, and a mobility of μ≊2 ${\mathrm{cm}}^2$/V s. The mobility of the slow component is 2 orders of magnitude lower. In the oriented samples two kinds of anisotropy of the photocurrent have been found, one with respect to the direction of the applied electric field with a value of ${\mathrm{\sigma }}_{\mathrm{\parallel }}$/${\mathrm{\sigma }}_{\mathrm{\perp }}$=50, and the other with respect to the polarization of the incident light. From a comparison of these data with those of photoinduced absorption measurements, we attribute the fast component mainly to polarons and the slow component to a hopping transport of charged solitonlike defects.