Investigating Dopant Intercalation into Semiconducting Polymer Crystals by TEM, Rutherford Backscattering and Polarized UV-vis-NIR Spectroscopy

Doping of polymer semiconductors is a central topic in plastic electronics and especially in the design of new better performing thermoelectric materials. Sequential doping of highly ordered polymer semiconductors is widely used to tune the charge carrier density that determines the thermoelectric properties. Various studies have demonstrated that dopant molecules intercalate into the layers of side chains in the crystals of polymer semiconductors.(1-3) It is therefore essential to clarify the way dopants are incorporated into the crystal lattice of the pristine polymer and how this is related to the dopant size and electron affinity. A combination of electron diffraction and polarized UV-vis-NIR spectroscopy on highly oriented P3HT and PBTTT thin films allows to quantify the amount of intercalated dopant molecules as well as the molecular orientation of dopants such as F4TCNQ, F6TCNNQ with respect to the polymer chains. A mapping of the orientational distribution of dopant molecules in the polymer crystal can thus be obtained. In the case of P3HT/F6TCNNQ, modeling of the diffraction patterns of doped crystals identifies the stoichiometry of the doped polymer phase and confirms the dopant orientation obtained from spectroscopy measurements as well as the lattice parameter variation induced by intercalation. In the case of Magic blue, Rutherford Backscattering is used to quantify the dopant amount in the films, Polarized UV-vis-NIR helps determine the diffusion coefficients of the dopant into the P3HT matrix whereas TEM demonstrates preferential location of MB in the amorphous phase of P3HT. It is demonstrated that the highest charge conductivities (up to 3000 S/cm) are observed in oriented P3HT films when the amorphous phase of P3HT is selectively doped with MB.<br/>References.<br/>(1) Untilova, V.; Biskup, T.; Biniek, L.; Vijayakumar, V.; Brinkmann, M. Control of Chain Alignment and Crystallization Helps Enhance Charge Conductivities and Thermoelectric Power Factors in Sequentially Doped P3HT:F4TCNQ Films. Macromolecules 2020, 53 (7), 2441–2453.<br/>(2) V. Untilova, H. Zeng, P. Durand, P. Allgayer, L. Herrmann, N. Leclerc and M.Brinkmann. Intercalation and ordering of F6TCNNQ and F4TCNQ dopants in regioregular poly(3-hexylthiophene) crystals: impact on anisotropic thermoelectric properties of oriented thin films Macromolecules 2021, 54, 13, 6073-6084.<br/>(3) Vijayakumar, V.; Zhong, Y.; Untilova, V.; Bahri, M.; Herrmann, L.; Biniek, L.; Leclerc, N.; Brinkmann, M. Bringing Conducting Polymers to High Order: Toward Conductivities beyond 105 S cm−1 and Thermoelectric Power Factors of 2 mW m−1 K−2. Advanced Energy Materials 2019, 9, 1900266.