Power/Performance Bits: April 14

Undoped polymer ink
Researchers at Linköping University, Chalmers University of Technology, University of Washington, University of Cologne, Chiba University, and Yunnan University developed an organic ink for printable electronics that doesn’t need to be doped for good conductivity.

“We normally dope our organic polymers to improve their conductivity and the device performance. The process is stable for a while, but the material degenerates and the substances we use as doping agents can eventually leach out. This is something that we want to avoid at any cost in, for example, bioelectronic applications, where the organic electronic components can give huge benefits in wearable electronics and as implants in the body”, said Simone Fabiano, head of the Organic Nanoelectronics and associate professor at Linköping University.

The conducting ink is comprised of two different polymers that allow for spontaneous transfer of charge between them. “The phenomenon of spontaneous charge transfer has been demonstrated before, but only for single crystals on a laboratory scale. No one has shown anything that could be used at an industrial scale. Polymers consist of large and stable molecules that are easy to deposit from solution, and that’s why they are well suited for large-scale use as ink in printed electronics”, added Fabiano.

In addition to being relatively cheap, no foreign substances leach out from the new polymer mixture. It can remain stable for an extended time and withstands high temperatures.

Plus, the polymer mixture should be suitable for demanding applications, Fabiano said. “The discovery of this phenomenon opens completely new possibilities for improving the performance of light-emitting diodes and solar cells. This is also the case for other thermoelectric applications, and not least for research within wearable and close-body electronics.”

Bonding GaN with diamond
Researchers at Georgia Institute of Technology and Meisei University developed a room-temperature bonding technique for integrating wide bandgap materials such as gallium nitride (GaN) with thermally-conducting materials such as diamond.

The technique, called surface-activated bonding, could improve the cooling effect on GaN devices, which would lead to higher power levels, longer device lifetime, improved reliability and reduced manufacturing costs.

In surface-activated bonding, an ion source in a high vacuum environment is used to clean the diamond and GaN surfaces, which activates the surfaces by creating dangling bonds. Introducing small amounts of silicon into the ion beams facilitates forming strong atomic bonds at room temperature, allowing the direct bonding of the GaN and single-crystal diamond that allows the fabrication of high-electron-mobility transistors (HEMTs).

“This technique allows us to place high thermal conductivity materials much closer to the active device regions in gallium nitride,” said Samuel Graham, professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. “The performance allows us to maximize the performance for gallium nitride on diamond systems. This will allow engineers to custom design future semiconductors for better multifunctional operation.”

The interface layer that results from GaN to single-crystal diamond is 4nm thick, which allows up to two times more efficient heat dissipation compared to state-of-the-art GaN-on-diamond HEMTs by eliminating the low-quality diamond left over from nanocrystalline diamond growth.

Cross-section bright-field high-resolution STEM images of GaN-diamond interfaces bonded by the surface activated bonding technique. (Credit: Zhe Cheng, Georgia Tech)

“In the currently used growth technique, you don’t really reach the high thermal conductivity properties of the microcrystalline diamond layer until you are a few microns away from the interface,” Graham said. “The materials near the interface just don’t have good thermal properties. This bonding technique allows us to start with ultra-high thermal conductivity diamond right at the interface.”

The process can be done at room temperature, reducing the thermal stress applied to the devices from as much as 900 megapascals (MPa) to less than 100 MPa with the room temperature technique.

The method can be used with other semiconductor materials, such as gallium oxide, and other thermal conductors, such as silicon carbide. “This new pathway gives us the ability to mix and match materials,” said Graham. “This can provide us with great electrical properties, but the clear advantage is a vastly superior thermal interface. We believe this will prove to be the best technology available so far for integrating wide bandgap materials with thermally-conducting substrates.”

The researchers believe the technique could have applications for wireless transmitters, radars, satellite equipment and other high-power and high-frequency electronic devices.

Silicon microwire solar
Researchers at Ulsan National Institute of Science and Technology (UNIST) and Daegu Gyeongbuk Institute of Science and Technology (DGIST) developed a flexible, transparent solar cell made from silicon microwire composites.

In the new solar cell, cylindrical silicon rods are embedded in a flexible and transparent polymer material in a hexagonal array.

First, an aluminum oxide layer was applied to the etched silicon microwires using ALD to passivate the surface. Spin-coating was used to embed the microwire array in the PDMS polymer. Residue was removed before peeling the array off the silicon wafer.

As the visible lights passes through the polymer material without silicon rods, it appears entirely transparent to the human eye.

While making solar cells transparent reduces the amount of solar radiation absorbed, decreasing efficiency, the researchers changed the shape of the silicon microwire tip, slanting it dramatically for increased light absorption while maintaining transparency. Based on the analysis of the light absorption mechanism in the silicon rods, the team designed the light reflected from the top of the bar to be absorbed by the bar next to it.

Tests with the solar cell showed a conversion efficiency of 8% at 10% average transmittance. The team sees applications in building-integrated photovoltaics, vehicle glass, and portable IoT devices.