Stimulation of electrical conductivity in a π-conjugated polymeric conductor with infrared light

Abstract

Irradiation with infrared light is found to stimulate the electrical conductivity of a film of an organic polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in conductivity is found to be linear in the intensity of the irradiation $\text{(4–400\hspace{0.17em}}{\mathrm{mW/cm}}^2\mathrm{).}$ Both frequency and time domain measurements reveal that the change in resistance induced by irradiation, relaxes according to ${\text{ΔR(t)∝(1/t)}}^{\text{0.6}},$ with t as the time after excitation. As a possible mechanism for this relaxation, we model the diffusion of heat from the polymer film to the supporting glass substrate. Assuming that the change in resistance is linear with the raise in temperature caused by the infrared irradiation, one predicts a ${\text{ΔR(t)∝(1/t)}}^{\text{0.5}}$ dependence. The similarity between the model and experimental 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. Electrical characterization is complemented with optical measurements. These show infrared-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 $\mathrm{(\Delta T/T)}$ induced by the infrared pulse show a very short-lived component with a lifetime close to the instrumental resolution (∼500 fs). The rapid response is followed by a slow component that decays according to ${\text{(ΔT/T)(t)∝(1/t)}}^{\text{0.65}}.$ 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.