Stretchable Electronics
December 6, 2012, Printed Electronics USA, Santa Clara, CA—Yong Tack Hong from Seoul National University presented results of their work in ink printed stretchable electronics for displays and sensors. This work is based on silver doped carbon nana-tubes and other thin films for the OTFTs on stretchable substrates.
They are working on various printing technologies to get an assortment of features for evaluation. The key characteristics are thickness, resolution, overall throughput, and the ability to hold alignment. The parameters they check are conductivity, cycling stability, and stretchability. All of the technologies are getting better in all phases of development.
The rationale for stretchable substrates is that many surfaces are not planar, and application of electronics to those surfaces requires conformal films. Silver is a good material for the TFTs and the carbon nano-tubes to enhance conductivity. The piezo inkjet printer is a drop-on-demand device that an deliver picoliter droplets. The greatest area of concern is the applied surface roughness.
They have found that the organic TFTs need a smooth surface to get repeatable performance and controllable work functions. The topologies are evolving to attain greater performance, with patterned source, drain, and gate structures. The devices are self-aligned for better manufacturability and separated with Teflon to improve parameters.
They attain good stretchability by depositing the conductive materials on pre-stretched materials. When the tension is relaxed, the conductors form wavy structures which are then passivated. The silver conductors get from 14 to 37 ohms change in resistance over 1k stretch cycles of up to 20 percent strain. The carbon is used when transparency is needed, and the resistance is a function of the number of print layers. They have achieved over 100 percent strain with a 5-layer conductor.
One application for the stretchable materials is for strain sensors. The resistance increases with increased strain for metal thin films and the resistance decreases for conductive nano-particles. A hybrid material gets a good resistance change with high strain, up to 150 percent strain. Their strain gauge gets repeatable resistance changes in their tests. The hybrid material needs work on cycling stability.
One consideration for stretch displays is that the display should maintain resolution with size changes. To address this issue, they developed a way to increase the number of lit pixels as the material is stretched.
