Interplay of Nanostructure and Molecular Doping of Poly(3-hexylthiophene)

Abstract: The accelerating growth of the number of inter-connected small devices, which together make up the so-called Internet of Things, is increasing the need for autonomous power sources. Heat is an abundant and often wasted source of energy. Thermoelectric generators could be used to harvest this waste energy. Small devices could potentially be powered by low-grade heat sources using flexible plastic thermoelectric generators. This thesis discusses thermoelectric plastics and in particular the semiconducting polymer poly(3-hexylthiophene) (P3HT). P3HT is a model conjugated polymer that is commercially available and has become an important reference material for the study of optoelectronic processes in organic semiconductors. At first, I investigated isotropic thin films of P3HT doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). I chose doping from the vapour phase as this allowed me to disentangle the influence of polymer processing and doping. I demonstrate that by improving the degree of solid state order of P3HT it is possible to strongly increase the electrical conductivity, which enhances the thermoelectric power factor from 0.2 to 2.7 μW m-1 K-2. Secondly, I explored the impact of orientation on the thermoelectric properties of P3HT. I chose to study highly anisotropic thin films of P3HT, aligned using a high temperature rubbing technique. Further, I investigated free-standing bulk tapes that where uniaxially oriented through tensile drawing. Sequential doping from solution with F4TCNQ or a molybdenum dithiolene complex allowed me to preserve the anisotropy of both thin films and stretched tapes. I found that orientation of the polymer allows to further increase the thermoelectric properties in the direction of alignment. As a result, a power factor of 16 μW m-1 K-2 for tensile drawn tapes and ~ 100 μW m-1 K-2 for rubbed thin films is obtained. Furthermore, oriented P3HT tapes show no change in the glass transition temperature of about 20 C upon doping with a molybdenum dithiolene complex, which suggests that tensile drawing can be used to prepare flexible thermoelectric materials.