Power/Performance Bits: Jan. 30

Wavy display transistors; crumpled graphene batteries; solar windows.


Wavy display architecture
Researchers at KAUST developed a new transistor architecture for flexible ultrahigh resolution devices aimed at boosting the performance of the display circuitry.

Flat-panel displays use thin-film transistors, acting as switches, to control the electric current that activates individual pixels consisting of LEDs or liquid crystals. A higher field-effect mobility of the channel material can improve resolution and frame rate.

Amorphous-oxide semiconductors, such as zinc oxide and indium-gallium zinc oxide, have provided transistor channels with modest mobility. Scaling down these transistors is expensive and introduces flaws known as short-channel effects that increase their power consumption and degrade their performance, according to Muhammad Hussain, a professor of electrical engineering at KAUST.

As an alternative, the team designed non-planar vertical semiconductor fin-like structures that are laterally interconnected to form wavy transistor arrays. The researchers opted for zinc oxide as the active channel material and generated the wavy architecture on a silicon substrate before transferring it onto a flexible soft polymer support using a low temperature process.

Wavy transistor arrays for flexible screens. (Source: Muhammad M. Hussain/KAUST)

With the vertical orientation, the researchers widened the transistors by 70% without expanding their occupied pixel area, doubling the transistor performance. The wavy arrays exhibited reduced short-channel effects and higher turn-on voltage stability compared to their planar equivalents.

In a proof-of-concept experiment, they could drive flexible LEDs at twice the output power as their conventional counterparts. “The LEDs were brighter without increasing power consumption,” said Hussain.

The team sees wavy transistor arrays as a path towards personal electronics with dynamically reconfigurable displays.

Crumpled graphene for batteries
Researchers at Northwestern University propose a method to make lighter, higher capacity lithium-metal batteries by utilizing crumpled graphene balls to prevent the growth of dendrites.

Dendrites are a major problem for lithium batteries. During charging and discharging, the metal grows metal filaments that can degrade the battery’s performance or short-circuit, possibly causing fires.

A current workaround to the issue of dendrites is to use a porous scaffold, such as those made from carbon materials, on which lithium preferentially deposits. Then when the battery is charging, lithium can deposit along the surface of the scaffold, avoiding dendrite growth. This, however, introduces a new problem. As lithium deposits onto and then dissolves from the porous support as the battery cycles, its volume fluctuates significantly. This volume fluctuation induces stress that could break the porous support.

Instead, the team turned to a scaffold made from crumpled graphene balls, which can not only prevent dendrite growth but can also survive the stress from the fluctuating volume of lithium.

Crumpled graphene balls are novel ultrafine particles that resemble crumpled paper balls. The team created them by atomizing a dispersion of graphene-based sheets into tiny water droplets. When the water droplets evaporated, they generated a capillary force that crumpled the sheets into miniaturized balls.

Jiaxing Huang discovered crumpled graphene balls six years ago. (Source: Jiaxing Huang/Northwestern)

“One general philosophy for making something that can maintain high stress is to make it so strong that it’s unbreakable,” said Jiaxing Huang, professor of materials science and engineering at Northwestern. “Our strategy is based on an opposite idea. Instead of trying to make it unbreakable, our scaffold is made of loosely stacked particles that can readily restack.”

The crumpled graphene scaffold accommodates the fluctuation of lithium as it cycles between the anode and cathode. The crumpled balls can move apart when lithium deposits and then readily assemble back together when the lithium is depleted. Because miniature balls are conductive and allow lithium ions to flow rapidly along their surface, the scaffold creates a continuously conductive, dynamic, porous network for lithium.

Compared to batteries that use graphite as the host material in the anode, the team’s solution is much lighter weight and can stabilize a higher load of lithium during cycling. Whereas typical batteries encapsulate lithium that is just tens of microns thick, this battery holds lithium stacked 150 microns high. A provisional patent has been filed for the technology.

Solar windows
Scientists at the Lawrence Berkeley National Laboratory have a new approach to solar windows that darken automatically by incorporating a perovskite that can be reversibly switched between a transparent state and a non-transparent state, without degrading its electronic properties.

The researchers were investigating the phase transition of the inorganic halide perovskite, which “can essentially change from one crystal structure to another when we slightly change the temperature or introduce a little water vapor,” said Peidong Yang of Berkeley Lab and professor at UC Berkeley.

When the material changes its crystal structure, it changes from transparent to non-transparent. “These two states have the exact same composition but very different crystal structures,” he said. “That was very interesting to us. So you can easily manipulate it in such a way that is not readily available in existing conventional semiconductors.”

In the course of research into improving solar cell stability in the organic-inorganic hybrid perovskite methylammonium lead iodide, they tried using cesium to replace the methylammonium.

“The chemical stability improved dramatically, but unfortunately the phase was not stable,” said Letian Dou, an assistant professor at Purdue University. “It transformed into the low-T [temperature] phase. It was a drawback, but then we turned it into something that’s unique and useful.”

Sped up video show materials going from low-T to high-T phase. (Source: Berkeley Lab)

The material is triggered to transition from the low-T to high-T phase (or from transparent to non-transparent) by applying heat. In the lab, the temperature required was about 100 degrees Celsius. They are working to bring it down to 60 C.

Jia Lin, a Berkeley Lab postdoctoral fellow, said moisture, or humidity, was used in the lab to trigger the reverse transition. “The amount of moisture needed depends on the composition and the transition time desired,” he said. “For example, more bromide makes the material more stable, so the same humidity would require longer time to transform from the high-T to low-T state.”

The solar cell is fully reversible and doesn’t show color fade or performance degradation over repeated cycles. The researchers will continue to work on developing alternative ways to trigger the reverse transition, such as by applying voltage, or engineering the source of the moisture.

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