Capillary Flow Control in Vacuum Drying for Uniform Pixel Thin Films in Inkjet-Printed OLEDs: Effects of Pumping Speed and Binary Solvent Ratios

Eliminating the coffee-ring effect caused by capillary flow is crucial for achieving uniform thin films in inkjet-printed display pixels. This study investigates methods to suppress capillary flow influence during the vacuum drying process to form uniform inkjet-printed OLED thin films. A drying model comprising four specific points and three distinct stages was proposed to analyze the correlation among capillary flow, vacuum pumping speed, and binary solvent ratios. These stages were closely monitored in real time by using a microscope, revealing two critical parameters influencing capillary flow. First, increasing the vacuum pumping speed suppressed capillary flow, enhancing the thin film uniformity. Second, the lifetime of Evaporation II (the period between the formation of a flat surface and the four-contact-line state) was identified as a key factor. As the boiling point of the mixed solvent increased, the vacuum level at the onset of Evaporation II decreased, shortening its lifetime. By introducing the dimensionless time factor f_cr associated with the coffee-ring effect, it was determined that minimizing the lifetime of Evaporation II effectively reduces capillary flow influence. The results demonstrated a 63.6% reduction in film thickness deviation and a 432% increase in f_cr, highlighting the suppression of the capillary flow. Furthermore, inkjet-printed organic light-emitting diode (OLED) devices fabricated under optimized conditions exhibited a 31.8% improvement in luminous efficiency compared to the lowest. These findings provide a comprehensive understanding of capillary flow dynamics and offer an effective strategy for improving thin film uniformity and OLED device performance.