Research Bits: May 24

Printed electronics: flexible OLED display, shellac ink, 2D material properties.


Printed flexible OLED display
Researchers from the University of Minnesota Twin Cities and Korea Institute of Industrial Technology used a customized 3D printer to print a flexible OLED display.

“OLED displays are usually produced in big, expensive, ultra-clean fabrication facilities,” said Michael McAlpine, a professor in the Department of Mechanical Engineering at University of Minnesota. “We wanted to see if we could basically condense all of that down and print an OLED display on our table-top 3D printer, which was custom built and costs about the same as a Tesla Model S.”

The team combined two different modes of printing. The electrodes, interconnects, insulation, and encapsulation were all extrusion printed, while the active layers were spray printed using the same 3D printer at room temperature. The display prototype was about 1.5 inches on each side and had 64 pixels. Every pixel worked and displayed light.

Additionally, the display prototype is flexible and could be packed in an encapsulating material.

The fully 3D-printed flexible organic light-emitting diode (OLED) display prototype is about 1.5 inches on each side and has 64 pixels. Every pixel works and displays light. The 3D-printed display is also flexible, which could make it useful for a wide variety of applications, such as foldable smartphone displays. (Credit: McAlpine Group, University of Minnesota)

“The device exhibited a relatively stable emission over the 2,000 bending cycles, suggesting that fully 3D printed OLEDs can potentially be used for important applications in soft electronics and wearable devices,” said Ruitao Su, a University of Minnesota mechanical engineering Ph.D. graduate who is now a postdoctoral researcher at MIT.

“The nice part about our research is that the manufacturing is all built in, so we’re not talking 20 years out with some ‘pie in the sky’ vision,” McAlpine said. “This is something that we actually manufactured in the lab, and it is not hard to imagine that you could translate this to printing all kinds of displays ourselves at home or on the go within just a few years, on a small portable printer.”

The next steps are to try 3D printing OLED displays with higher resolution and improved brightness.

Shellac ink for printed circuits
Researchers at Empa are developing more environmentally friendly conductive inks for printed, disposable electronics and sensors.

The metal-free, non-toxic, and biodegradable ink uses carbon as the conductive material. Elongated graphite platelets are mixed with tiny soot particles that establish electrical contact between these platelets, in a matrix made of shellac. Shellac is made from the excretions of scale insects and commonly used as a wood varnish. A key aspect of the material is that it is soluble in alcohol, which evaporates after the ink is applied.

To be used in screen printing and 3D printing, the ink must display shear thinning behavior, where it is viscous while at rest but becomes more fluid when force is applied. The researchers created numerous formulations, which differed in size of graphite platelets and ratio of carbon black. They ultimately found several variants of the ink that could be used in different 2D and 3D printing processes.

“The biggest challenge was to achieve high electrical conductivity and at the same time form a gel-like network of carbon, graphite and shellac,” said Xavier Aeby, a researcher at Empa’s Cellulose & Wood Materials lab.

In tests, the team was able to print a cuboid that contained 15 superimposed grids from a 3D printer made of strands 0.4mm in diameter. They also constructed a sensor for deformation that used a thin PET strip with an ink structure printed on it, whose electrical resistance changed precisely with varying degrees of bending. The researchers said that tests for tensile strength, stability under water, and other properties showed promising results.

Novel material for printed circuits: Two test cuboids one centimeter wide from the 3D printer. The printed electronic sensors can be seen in the background. (Credit: Empa)

“We hope that this ink system can be used for applications in sustainable printed electronics,” said Gustav Nyström, head of Empa’s Cellulose & Wood Materials lab, “for example, for conductive tracks and sensor elements in smart packaging and biomedical devices or in the field of food and environmental sensing.”

Printable 2D material properties
Researchers from Imperial College London, Politecnico di Torino, Trinity College, and University of Cambridge studied how electronic charge is transported in several inkjet-printed films of 2D materials, showing how it is controlled by changes in temperature, magnetic field, and electric field.

The aim of the work is to enable improved material selection for 2D materials that can be dispersed in solution and formulated into printable inks used to create ultra-think films that are flexible, semi-transparent, and with novel electronic properties.

The team investigated three typical types of 2D materials: graphene (a semimetal built from a single layer of carbon atoms), molybdenum disulphide (or MoS2, a semiconductor) and titanium carbide MXene (or Ti3C2, a metal) and mapped how the behavior of the electrical charge transport changed under these different conditions.

“Our results have a huge impact on the way we understand the transport through networks of two-dimensional materials, enabling not only the controlled design and engineering of future printed electronics based on 2D materials, but also new types of flexible electronic devices,” said Felice Torrisi, from the Department of Chemistry at Imperial. “For example, our work paves the way to reliable wearable devices suitable for biomedical applications, such as the remote monitoring of patients, or bio-implantable devices for long-term monitoring of degenerative diseases or healing processes.”

The researchers said that the relationships between 2D material type and the controls on electrical charge transport will help other researchers design printed and flexible 2D material devices with the properties they desire, based on how they need the electrical charge to act. It could also enable design of new types of electrical components.

“The fundamental understanding of how the electrons are transported through networks of 2D materials underpins the way we manufacture printed electronic components. By identifying the mechanisms responsible for such electronic transport, we will be able to achieve the optimum design of high-performance printed electronics,” said Renato Gonnelli, a professor from the Politecnico di Torino.

Adrees Arbab, from the Cambridge Graphene Centre and the Department of Chemistry at Imperial, said, “In addition, our study could unleash the new electronic and optoelectronic devices exploiting the innovative properties of graphene and other 2D materials, such as incredibly high mobility, optical transparency, and mechanical strength.”

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