| Conjugated polymers include some of the most promising candidates for the active layer of low-cost thin-film transistors (TFTs) and bulk heterojunction photovoltaic (PV) cells. The charge carrier mobility of these conjugated polymers is the key materials property limiting the performance of both of these devices. This thesis outlines a fundamental investigation of the charge transport and morphology of the first high mobility conjugated polymer, regioregular poly(3-hexylthiophene) (P3HT). The charge carrier mobility in TFTs was found to increase by four orders-of-magnitude as the molecular weight (MW) of P3HT is increased from 3000 g/mole to 36,000 g/mole. P3HT films with different MWs provided an ideal system for correlating morphological changes in conjugated polymers to resulting changes in charge transport. Atomic force microscopy, x-ray diffraction and grazing incidence x-ray scattering (GIXS) were used to measure changes in the crystallinity and crystal orientation associated with varying the spin-casting solvent, annealing conditions, substrate surface treatment, and drop-casting at a constant MW. The GIXS results showed that at a constant MW in both low- and high-MW films, the mobility correlated to the strength of in-plane pi-stacking. When comparing different MWs, however, this correlation broke down. Rocking curves on samples with a chemically modified surface showed highly oriented crystals that were nucleated from the substrate and correlate with variations in charge transport. Switching to low-MW P3HT improves the overall crystallinity, the intensity of in-plane pi-stacking, and the concentration of highly oriented crystals, but the mobility is more than a factor of 100 lower than high-MW P3HT. These counterintuitive results clearly show that the charge carrier mobility of conjugated polymers is coupled to several different aspects of the morphology. In the case of the low-MW films, the strong driving force for ordering creates grain boundaries that isolate the ordered regions from their neighbors. Whereas in high-MW films, the long chains connect the small ordered regions and provide a clear pathway for charges to move through the film. These results were used to develop a model for relating charge transport and structure that can be used as a guide for the development of new, improved chemical structures. |
