Simulation of electrical conductivity in a pi-conjugated polymeric conductor with infrared light

Irradn. with IR light is found to stimulate the elec. cond. of a film of an org. polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in cond. is found to be linear in the intensity of the irradn. (4-400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradn., relaxes according to DR(t) ~ (1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, the authors model the diffusion of heat from the polymer film to the supporting glass substrate. By assuming that the change in resistance is linear with the raise in temp. caused by the IR irradn., one predicts a DR(t) ~ (1/t)0.5 dependence. The similarity between the model and exptl. behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Elec. characterization is complemented with optical measurements. These show IR-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (DT/T) induced by the IR pulse show a very short-lived component with a lifetime close to the instrumental resoln. (.apprx.500 fs). The rapid response is followed by a slow component that decays according to (DT/T)(t) ~ (1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scale