Publishing their results in the ACS Nano letter under the title “Fully Printed Stretchable Thin-Film Transistors and Integrated Logic Circuits”, the researchers detail how they came to use a solution of unsorted carbon nanotubes (CNTs) to print source/drain/gate electrodes), a solution of high-purity semiconducting single-walled carbon nanotubes (sSWCNTs) for the channel semiconductor, and barium titanate (BaTiO3) nanoparticles dispersed in a solution of polydimethylsiloxane (PDMS) to enable the printing of a novel type of stretchable hybrid gate dielectric.
Using a direct printing process with these solutions, the researchers first fabricated thin-film transistors (TFTs) and characterised the devices under thousands of stretching cycles beyond 50%. While the use of CNTs for making stretchable conductive parts has already been reported (because the one-dimensional CNTs form a mesh structure with overlapping strands), the paper focused largely on the novel hybrid gate dielectric developed.
Choosing the right solvent and blending cubic phase BaTiO3 nanoparticles (about 50nm in diameter) with PDMS, the researchers were able to obtain a relative permittivity high enough (about 9 at a BaTiO3 volume content of 26%) to ensure a good gating strength, while endowing the printed dielectric blend good mechanical robustness and intrinsic stretchability.
Experimenting with the novel stretchable dielectric, the researchers printed parallel plate capacitors (with CNTs for the electrodes) and measured their capacitance-frequency characteristics, showing a capacitance virtually independent of frequency in the range of 100Hz-1MHz, with very little leakage (below 10nA/cm2 under a voltage of 200V (∼1MV/cm). “For a TFT with a channel footprint of 2000×200μm, such leakage current density corresponds to a gate leakage current of ∼40pA” they wrote to put these results in perspective. The stretchable TFTs printed on PDMS substrates had a typical channel length of 150 to 200μm and a channel width of 2500μm, and regardless of the tensile strain applied, they maintained very standard MOSFET characteristics with clear saturation regions in the output curves.
In fact, the transistor remained functional while being stretched in excess of 50%, and stretching was only limited by the rupture of PDMS substrates, noted the researchers. Under stretch, there is a slight degradation of device performance metrics, but those start to stabilize after hundreds of stretching cycles and then the difference between the transfer curves at 0% and 50% strains becomes practically negligible.
“The TFTs are stabilized and show almost strain-independent electrical performance after more than 1000 stretching cycles” the paper reports. Next, the researchers printed stretchable integrated logic circuits on PDMS, including inverter, NOR and NAND gates and demonstrated their operation while submitting the circuits to thousands of stretching cycles (beyond 50%) along any directions.
First author of the paper, postdoc Le Cai admitted that although the research was currently at the infant stage, the researchers were pursuing to patent the technology in the hope to commercialize it in the future.
With such printable and stretchable circuits, “the ultimate goal would be to fabricate large area, conformal and interactive electronic systems that can be used for wearable health monitoring, wallpaper displays, human-interactive soft robots and so on” Cai concluded.
More recently, Chuan Wang, assistant professor of electrical and computer engineering at the Michigan State University pushed the refinement further by combining these novel printable and stretchable circuits with layers of organic light emitting chemistries to create a fully stretchable OLED. The next step would be to create single OLED pixels, which are probably another year or two away.
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