Researchers at Penn State University are looking into easy, low-cost and adaptable processing methods of producing organic conducting polymer circuits.
‘For plastic circuits we cannot use the old processing,’ says Dr. Qing Wang, assistant professor of materials science and engineering. ‘Photolithography and silicon technologies require harsh environments and plastics cannot hold up to them.’
Wang, working with Ziqi Liang and Kun Li, graduate students in materials science and engineering, are looking into novel processing methods for production of organic conducting polymer circuits. One method that is low cost, easy to do, fast and adaptable to large areas and non-flat surfaces, is micro contact printing.
‘We use conducting polymers that are functionalised,’ Wang told attendees at the annual meeting of the American Chemical Society in New York. ‘They have functional groups attached that allow them to be soluble and to attach to the surface.’
The researchers used poly (p-phenylene vinylene), PPV, which was modified by adding alkyoxy side chains and amino end groups. Altering the polymer allows it to dissolve in a variety of organic solvents. The amines act as reaction points where the polymer can attach to another chemical.
Attachment is important as most polymers are slippery and do not want to adhere to surfaces. In conventional ink printing, ink is held onto the paper by surface interactions, but not by chemical reactions. When printing a plastic electronic device, surface interactions are not strong enough to hold the polymer ‘ink’ onto the surface.
Wang used a gold substrate onto which an organic acid, 16-mercaptohexadecanoic acid, was placed in a self-assembled monolayer. This single layer of molecules of MHA provides specific chemical groups to which the amino end groups of the polymer can attach.
In conventional printing, ink is placed on the plate and then the ink and paper are brought together for a very short time during which the wet ink is transferred to the paper. When printing polymers on organic acid coated gold, the process is different. The researchers used a pliable stamp of the sub-micron pattern they wish to transfer. They then applied the polymer ‘ink’ to the stamp surface and dried it. The stamp and the substrate are held in contact for 30 minutes while the polymer transfers to the substrate.
Because the stamp is pliable, this printing method is applicable to curved surfaces. A wide variety of opto-electric devices are possible, including light-emitting diodes, field effect transistors, lasers, solar cells and chemical and biological sensors.
Wang has investigated the resulting patterns using a variety of macroscopic techniques to ensure that the pattern created on the surface is continuous and usable. Micro contact printing does create patterns with some defects, but the researchers believe that the resultant product is usable.
‘Micro-printed patterns of conducting polymer need to be used in applications where some defects can be tolerated,’ says Wang. ‘Although we do optimise the printed pattern as much as possible.’