Controlling Drying\nConditions in Vacuum for Uniform\nFilm Formation in Inkjet-Printed OLEDs

The drying process of inkjet-printed organic light-emitting\ndiodes\n(OLEDs) is influenced by both ink properties and external environmental\nfactors, which ultimately affect the film profile. First, we conducted\na detailed investigation of the drying time based on changes in the\nboiling point (BP) of mixed solvents and analyzed the correlation\nwith the film profile. Under atmospheric drying conditions in a nitrogen\n(N<sub>2</sub>) atmosphere, the increased drying time under capillary-driven\nflow leads to greater particle movement toward the edges, significantly\nincreasing the coffee-ring effect. Additionally, using a high-boiling-point\nsolvent mixture of ethyl 4-methylbenzoate (EMB) and 2-ethylhexyl benzoate\n(EHB), we produced uniform thin films both between and within pixels\n(inter and intrapixel uniformity) through a vacuum drying process.\nIn particular, we proposed a drying process model that divides the\ndrying of inkjet pixels into a microfluidic phase and a gelation phase.\nThrough five gelation phase-controlled vacuum drying experiments,\nthe morphology within the pixels was precisely investigated. By sufficiently\nremoving residual solvents after the microfluidic phase and then proceeding\nwith heating, we produced uniform thin films. Furthermore, we fabricated\nOLED devices using this gelation phase-controlled vacuum drying process,\nachieving uniform pixel emission and improved device performance.