The role of impurities on the electron paramagnetic resonance response of polarons in conjugated polymers.

Poly(3-hexylthiophene) prepared by oxidative coupling using ferric chloride and with varying residual impurity levels of Fe, was used to study the temperature dependence of charge carriers in conjugated polymers. The temperature was ramped from room temperature to 110 K and Electron Paramagnetic Resonance (EPR) data for the iron impurity as well as the polymer were obtained. The doubly integrated area of the dispersion curves due to the polymer and the Fe impurity were calculated and show that the former exhibits a Curie Law behavior while the latter exhibits an exponential behavior on a background of immobile charge carriers. The exponential behavior is characteristic of a charge transfer complex (CTC) with a thermally activated excited state whose activation energy was calculated to be: E-CTC = (3.3 +/- 0.5) x 10(-2) eV and the background component which is independent of temperature was: S-0 = (7.5 +/- 0.5) x 10(16) spins/g.