Highly oriented crystals at the buried interface in polythiophene thin-film transistors

Thin films of polymer semiconductors are being intensively investigated for large-area electronics applications such as light-emitting diodes, photovoltaic cells and thin-film transistors. Understanding the relationship between film morphology and charge transport is key to improving the performance of thin-film transistors. Here we use X-ray diffraction rocking curves to provide direct evidence for highly oriented crystals at the critical buried interface between the polymer and the dielectric where the current flows in thin-film transistors. Treating the substrate surface with self-assembled monolayers significantly varies the concentration of these crystals. We show that the polymer morphology at the buried interface can be different from that in the bulk of the thin films, and provide insight into the processes that limit charge transport in polythiophene films. These results are used to build a more complete model of the relationship between chain packing in polymer thin-films and charge transport.