Impact of Regioregularity on Alignment and Thermoelectric Properties of Sequentially Doped and Oriented Films of Poly(3-hexylthiophene)
Shubhradip Guchait, Laurent Herrmann, Karim Kadri, Nicolas Leclerc, François Tran Van, Martin Brinkmann
Abstract
Alignment of conjugated polymers such as regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) is an effective means to enhance charge transport and thermoelectric properties of thin films doped with F 4 TCNQ. In this contribution, we investigate the impact of P3HT regioregularity (RR) on the alignment achieved in thin films by high-temperature rubbing and the resulting thermoelectric properties in sequentially doped and aligned films. The structure and thermoelectric properties of doped P3HT thin films are investigated by a combination of transmission electron microscopy (electron diffraction), polarized UV–vis–NIR spectroscopy, and thermoelectric measurements. Despite limited thermomechanical properties, rubbing at 165 °C generates order and alignment in 71% RR P3HT (R71-P3HT) with a typical lamellar stacking of P3HT chains similar to that of the smectic-like phase of RR-P3HT. Order is further enhanced in aligned R71-P3HT films by doping with F 4 TCNQ. Intercalation of dopants in the ordered domains of R71-P3HT induces a reorganization of polythiophene backbones within individual π-stacks with lattice parameter variations equivalent to those observed for RR-P3HT. Despite ordering induced by both rubbing at 165 °C and doping, charge conductivity of oriented R71-P3HT in the chain direction remains 50 times below that of oriented RR-P3HT. The strong blue-shift of the polaronic bands in R71-P3HT versus RR-P3HT indicates that polarons are strongly localized in both ordered and amorphous zones of R71-P3HT, explaining the modest charge conductivity of 3–4 S/cm observed in the chain direction.