2, KU Leuven, Leuven, , Belgium
Highly crystalline thin films of organic semiconductors offer great potential for the realization of high-performance, low-cost flexible electronics. These films are commonly fabricated using meniscus guided coating techniques such as solution shearing, dip coating or zone casting. In order to increase the film crystallinity, and thereby its carrier mobility, process optimization is usually carried through trial-and-error approaches to determine the right combination of semiconductor, solvent, temperature, surface treatment, coating speed and annealing treatment. Only few reports present systematic parameter studies that convey clear understanding and provide guidelines for future experiments.
Here, we present a systematic study over the impact of three important process parameters on the zone-casting of highly crystalline thin films. First, we explore the influence of ten different substrate surface treatments on thin film morphology and electrical characteristics. These surface treatments result in largely different surface energies and therefore largely different wetting envelopes. Nevertheless, we see that, as long as the solvent is fully wetting the substrate, the surface treatment only has a negligible influence on the morphology of the coated layers, but can still have a huge impact on the electrical characteristics. From this study, guidelines for choosing the right substrate-solvent combination can be extracted. Furthermore, we show how choosing the right solute concentration can result in larger processing windows and therefore higher coating speeds. Optimized coating speeds show similar electrical performances, while the drop in performance at higher speeds is less pronounced for solutions with higher solute concentrations. Finally, we also show the impact low temperature annealing has on the OTFT performance, resulting in a significant reduction of the threshold voltage. We attribute this to a beneficial reorganization of the organic semiconductor underneath the source and drain contact.
Combining these studies with our recently developed lateral homo-epitaxial fabrication technique, we demonstrate bottom-gate top contact thin film transistors based on highly crystalline C8-BTBT films with mobilities above 10 cm2/Vs and parameter spreads below 10% over 400 devices. These films behave almost twice as good as regularly evaporated poly-crystalline thin films, showing the potential of highly crystalline thin film transistors for large area electronics.