Self-Assembly in Supercritical Fluids—High Resolution Patterning on Curved and Flexible Surfaces

Organic electronic devices, especially those based on polymeric materials, offer the promise of highly flexible and stretchable circuitry and wearable electronics. However, manufacturing techniques for these promising devices has lagged behind materials development. Contrast this with small molecule devices on rigid substrates, where physical vapor deposition has already brought organic light emitting diodes into widespread use. Key advantages of this technique include thin film deposition without the need for complex in-situ chemical transformations as well as fine and highly reproducible patterning. We have developed a method of thin film formation that has all of the advantages of physical vapor deposition, while working with polymeric materials and enabling deposition on curved and flexible substrates. This technique leverages the unique properties of supercritical, near-critical, and/or subcritical fluids.¹ Counter to typical solutions, the saturation solubility of a solid solute first increases, and then decreases with increasing temperature near its critical point. We have leveraged this non-monotonic solubility behavior to deposit films onto a heated substrates. The decreasing solubility at the substrate surface simply precipitates the polymer from solution. Because the substrates are heated resistively, we can control the local heating by patterning resistive heating elements on the substrate. This we accomplished by performing photolithography on a substrate coated with indium tin oxide. When the lines were resistively heated, material deposited directly and selectively onto them. The technique also works when a thin polymer layer is inserted between the electrode array and the supercritical fluid, allowing the photolithographically patterned electrode array to serve as a patterning master for additive manufacturing onto thin and flexible materials. Moreover, the technique does not rely on line of sight mass transfer or cumbersome printing heads. We leveraged this advantage to deposit materials on to the curved interior of an elastomeric hemisphere approximately 3 mm in diameter.¹ The process will be shown to work with an aliphatic polymer and two types of organic semiconducting polymers.^{2, 3} We postulate that this is a generalizable and scalable manufacturing technique that will become vital in the development of high resolution polymer circuitry.<br/><br/>1. Yousefi, N.; Maala, J. J.; Louie, M.; Storback, J.; Kaake, L. G., Physical Supercritical Fluid Deposition: Patterning Solution Processable Materials on Curved and Flexible Surfaces. <i>ACS Appl Mater. Interfaces </i><b>2020,</b> <i>12</i>, 17949-17956.<br/>2. Yousefi, N.; Saeedi Saghez, B.; Pettipas, R. D.; Kelly, T. L.; Kaake, L. G., Physical supercritical fluid deposition of polymer films: controlling the crystallinity with pressure. <i>Mater. Chem. Front. </i><b>2021,</b> <i>5</i>, 1428-1437.<br/>3. Yousefi, N.; Saeedi Saghez, B.; Pettipas, R. D.; Kelly, T. L.; Kaake, L. G., The role of solvent additive in polymer crystallinity during physical supercritical fluid deposition. <i>New J. Chem. </i><b>2021,</b> <i>45</i>, 11786-11796.