Leveraging Process Fundamentals to Improve Semiconductor Thickness Control and Uniformity in Inkjet-Printed Schottky Diodes

Liquid-applied coating and printing methods are attractive options for the production of large-area, low-cost flexible electronics. However, controlling the deposited functional layer thickness and uniformity, particularly at submicrometer thicknesses, is challenging. This study focuses on thickness uniformity and control in Schottky diodes made by self-aligned capillarity-assisted lithography for electronics (SCALE). SCALE combines UV imprinting to structure a substrate surface and inkjet printing of functional inks to make flexible electronic devices. In the diode described here, the key functional layer is the poly(3-hexylthiophene-2,5-diyl) (P3HT) semiconductor, which was deposited from a 1,2-dichlorobenzene solution. Thin, uniform P3HT layers with no shorts are required for optimal diode performance. Thickness nonuniformities in the P3HT layer, including the coffee-ring effect and lack of planarization over adjacent electrode channels, occurred during drying. These nonuniformities were most severe when drying was carried out at elevated temperatures (≥50 °C). By drying P3HT layers at 23 °C, the film uniformity and planarization improved significantly, and the device yield was nearly 8× higher. P3HT layers less than 300 nm thick were demonstrated. The improvements in uniformity and planarization are discussed in terms of the competition between solvent evaporation and P3HT diffusion. Self-aligned, printed Schottky diodes demonstrated up to 4.0 × 104rectification ratio at ±1 V, minimal hysteresis, and ∼0.3 V turn-on voltage.