Materials Science and Engineering Department
Stanford University, Stanford CA 94305
Semicrystalline conjugated polymers have attracted much interest as disruptive materials for flexible, low-cost and printed electronics. Indeed, these polymers can be used as semiconductors in thin-film transistors, light-emitting diodes, solar cells and sensors. Furthermore, they have recently been made in stretchable forms. From the materials perspective, it has been known for decades that their electronic performance, as measured by carrier mobility, is very strongly dependent on the film microstructure. One of the goals of this field is to learn how to manipulate the microstructure through processing. In spite of this recognized fundamental need, very little is known about the crystallization processes in these polymers, which are crucial in microstructure formation. We used a model poly(thiophene), poly(3-hexyl-ethyl-thiophene)-(P3EHT)- to perform an in-depth, multi-technique study of crystallization kinetics and its effect on charge transport. P3EHT can be melted at temperatures below its decomposition temperature and when quenched to room-temperature it crystallizes slowly. We performed synchrotron-based x-ray diffraction, UV-Visible spectroscopy, solid-state NMR and charge transport measurements during the crystallization process. X-ray diffraction allows us to determine the structure of the unit cell and the relative degree of crystallinity of the film. I will show that transport improves with a percolation-like behavior when the distance between crystallites is approximately equal to the persistence length of the polymer. Furthermore, by combining these techniques we demonstrate that the crystallization kinetics follows a 1-D Avrami model and we extract the relevant kinetic parameters.