Understanding the impact of polymer self-organization on the microstructure and charge transport in poly(3-hexylthiophene)

Conjugated polymers represent the next generation of conducting materials that will enable technological devices incorporating thin film transistors, photovoltaic (PV) cells etc. in a cost effective roll-to-roll manner. The charge carrier mobility, which characterizes the ease with which charges can be transported through the material, is the key metric by which these materials are gaged and is also a decisive factor that limits device performance. Given the impact of microstructure on charge transport, ordered self-assembly in polymeric semiconductors assumes paramount relevance. This thesis outlines a fundamental investigation of the correlations between the morphology and microstructure of a model conjugated polymer, poly(3-hexylthiophene) (P3HT), and its corresponding charge transport properties in an organic field effect transistor (OFET) geometry. Moreover, the evolution of the structural and electrical properties are mapped, which provide new insights into the self-assembly process.;The variation in the electrical properties is studied as film formation evolves as a function of solvent evaporation from a sessile P3HT solution droplet. The channel formation process is mapped using four contact field effect measurements. The channel formation study is complimented by interrogating the evolution of the polymer chain conformations using in situ Raman spectroscopy, which reveals the presence of an intermediate lyotropic liquid crystalline phase before film crystallization. The manifestation of the liquid crystalline phase offers a potential rationale to the mobility profiles recorded by the in situ electrical measurements. A joint investigation of both measurements reveals that the onset of channel current occurs well before polymer crystallization and that the subtle structural changes in the P3HT film continue to evolve even after crystallization, which further impact the observed drain current.;The large impact of polymer chain conformations on the drain current revealed in the above study provided substantial impetus to understand the role of single chain effects on macroscopic charge transport in P3HT. We report that differences in regioregularity of side chain attachment in poly(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient to induce dramatic changes in the electronic and morphological properties of the material. Casting the electronic absorption spectra in the framework of Spano's model reveals that the conjugation length of the polymer chain is surprisingly sensitive to regioregularity. This observation correlates well with the field effect mobilities that are attenuated by one to two orders of magnitude in the lower regioregularity polymer film. We suggest that the increased intrachain order coupled with a reduced fraction of grain boundaries in the higher RR film is responsible for the reported differences. These studies serve as important pieces in the microstructure-charge transport puzzle.;A greater insight into the correlation is obtained by devising a technique to tune the crystallinity of the P3HT films and correlating that with the field effect mobility. We shown the formation of ordered supramolecular precursors in P3HT solutions through the application of low intensity ultrasound. These precursors survive the casting process, resulting in a dramatic increase in the degree of crystallinity of the thin films obtained by spin coating. The crystallinity of the films is tunable, with a continuous evolution of meso-scale structures observed as a function of ultrasonic irradiation time. A multiphase solid state morphology is obtained that in turn results in a percolation type charge transport mechanism. This investigation is then extended to understand the role of process conditions, the solvent used and molecular parameters such as regioregularity.