Origin of the stretched-exponential hole relaxation in regioregular poly(3-hexylthiophene)

It is shown that the time-dependence of a sufficiently low concentration of excess positive charges in the conjugated polymer regioregular poly(3-hexylthiophene) (RR-P3HT) is governed by the stretched-exponential (Kohlrausch) relation. The decay is attributed to the recombination of holes with negatively charged impurities in the sample. The stretch parameter is found to increase linearly with temperature, which can be attributed to dispersive hole transport in an exponential energy landscape. Extension of the recombination kinetics to include second-order effects (observed at higher initial hole concentrations) leads to a generalized Kohlrausch law. In the absence of impurities, a power-law type time-dependence is predicted.