Scientists Grease The Next Generation Of Organic Semiconductors

Carnegie Mellon University chemists have found that grease can make some innovative plastics vastly better electrical conductors. This discovery, published June 25 in Advanced Materials, outlines a chemical process that could fuel the next generation of tiny switches for transistors in RFID tags, flexible screen displays, and debit or key cards. "This research brings us closer to developing organic semiconductors with electrical and physical properties far superior to those that exist today," principal investigator Richard D. McCullough, professor of chemistry and dean of the Mellon College of Science at Carnegie Mellon, said in a statement. The new process involves adding "grease" by either chemically combining an inherently-conducting polymer (ICP) with a grease-like chemical, or by depositing hybrid material called a block copolymer onto a greased platform. On the surface layer of a transistor, ICPs make good electrical conductors that provide the switch element for a transistor to turn on and off. But ICPs are by nature brittle. To counter this brittleness, scientists chemically link ICPs with grease-like elastic polymers to make block copolymers. While they provide much-needed flexibility, elastic polymers insulate rather than conduct electricity. Block copolymers that contain grease-like polymers are less effective electrical conductors than pure ICPs. Yet in the right processing setting, the opposite can hold true, the Carnegie Mellon scientists now report. It just depends how you treat a transistor's silicon dioxide base layer. As part of the study, the Carnegie Mellon team tested four block copolymers, each with a different ratio of insulating elastic polymer to conducting polymer. When they applied thin films of these different polymers to untreated silicon dioxide, they found the greater the overall amount of insulating polymer in the final film, the worse that film performed in conducting an electric charge. The result is a flexible switch layer that doesn't work very well. But when the scientists pretreated the transistor's silicon dioxide platform with OTS-8 — a chemical that creates a grease-like coating — they found that transistors incorporating any of the four block copolymers conducted an electric charge with remarkable ease, even when the insulating polymer constituted more than half of the applied block copolymer. "Something amazing is happening at the molecular interface between our block copolymer and the OTS-8-treated surface so the block copolymers self- assemble with great precision," research associate Genevieve Sauve said in a statement. "In fact, we think that the grease-like, insulating polymer in the material and the grease-coated surface both somehow exert important effects in driving this self-assembly." Block copolymers with up to 57 percent insulating polymer performed 10 times better on OTS-8-treated surfaces than they did on untreated surfaces, according to the investigators. More importantly, the block copolymers were nearly equal in their performance to ICPs alone on treated surfaces, according to McCullough. "This is the first report that copolymers are good organic semiconductors," McCullough said. "These results mean that we could soon design devices that are both flexible and highly functional."