Understanding Microstructure and Charge Transport in Semicrystalline Polythiophenes

Book excerpt: Semiconducting polymers are a promising class of organic electronic materials, with the potential to have a large impact in the field of macroelectronics. In this thesis, we focus on understanding the relationship between microstructure and charge transport in semicrystalline polythiophenes. A method is presented for the measurement of complete pole figures of polymer thin films using an area detector, allowing for the first time quantitative characterization of crystalline texture and degree of crystallinity. Thin film transistors are used to measure electrical characteristics, and charge transport behavior is modeled according to the Mobility Edge (ME) model. These characterization methods are first used to investigate the effect of substrate surface treatment and thermal annealing on the microstructure of polythiophene thin films, and the effect of microstructural details on charge transport. Next, we investigate the semicrystalline microstructure in confined polythiophene films. Pole figures are used to quantify a decrease in the degree of crystallinity of films with decreasing thickness, accompanied by an improvement in crystalline texture. Next, we investigate the influence of the degree of regioregularity, molecular weight and the processing solvent on microstructure (degree of crystallinity and texture) and charge transport in high mobility P3HT thin films. Surprisingly, when processing conditions are optimized, even a polymer with moderate regioregularity can form a highly textured film with high charge carrier mobility. Finally, we use films of P3HT with engineered, anisotropic in-plane microstructure to understand the importance and mechanism of transport across grain boundaries in these semicrystalline films. Results from this study provide the first experimental evidence for the application of a percolation model for charge transport in high molecular weight semicrystalline polymer semiconductors. Understanding how characteristics of the polymer as well as details of the processing conditions can affect the film microstructure and device performance is important for future materials design and device fabrication.