Polarization Dependence of Charge Conduction in Conjugated Polymer Films Investigated with Time-Resolved Terahertz Spectroscopy

Room-temperature time-domain terahertz (TDS) and time-resolved terahertz (TRTS) spectroscopic methods are employed to measure carrier mobility and charge generation efficiency in thin-film semiconductor polymers. Interrogation of the dependence on excitation and probe polarizations yields insight into the underlying material properties that guide charge transport. We apply terahertz polarization anisotropy probes to analyze charge conduction in preparations of the copolymer PCDTPT, consisting of alternating cyclopenta-dithiophene (donor) and thiadiazolo-pyridine (acceptor) units. Comparisons are made among films of different ordering and morphologies, including aligned films prepared by blade coating, a near-isotropic dropcast film, and isotropic liquid dispersion. They are further contrasted with their population dynamics ascertained through transient absorption and the traditional photoconductive polymer poly-3-hexylthiophene. Polarization anisotropy is observed as preferential charge conduction along the backbone propagation direction of PCDTPT, with various factors disproportionately influencing directional mobility and charge pair yield. PCDTPT exhibits unexpectedly strong conductivity when isolated in toluene dispersion. Quantitative comparisons yield a better understanding of polaron/free-charge relaxation and transfer mechanisms and illustrate dynamics among photoexcited charge carriers and their motion and diffusion through different material morphologies.