Power/Performance Bits: Feb. 20

Wireless TENG
Researchers at Clemson University developed a wireless triboelectric nanogenerator, or W-TENG, that can also act as a battery-free remote.

The key to triboelectric nanogenerators is using materials that are opposite in their affinity for electrons so they generate a voltage when brought in contact with each other.

For the W-TENG, one electrode was constructed of a multipart fiber made of graphene and a biodegradable polymer known as polylactic acid (PLA). PLA on its own is good for separating positive and negative charges, but not so great at conducting electricity, which is why it was paired it with graphene. Teflon was used for the other electrode.

“We use Teflon because it has a lot of fluorine groups that are highly electronegative, whereas the graphene-PLA is highly electropositive. That’s a good way to juxtapose and create high voltages,” said Ramakrishna Podila, an assistant professor of physics at Clemson.

The W-TENG is 3-D printed out of a graphene-PLA nanofiber (A), creating the bottom electrode of the technology (B). A Teflon sheet is then added as the top electrode (C). (Source: Adv. Energy Mater. 2017, 1702736 / Clemson)

The device generates a maximum of 3,000 volts, a high enough voltage that the W-TENG generates an electric field around itself that can be sensed wirelessly. Its electrical energy can also be stored wirelessly in capacitors and batteries.

“It cannot only give you energy, but you can use the electric field also as an actuated remote. For example, you can tap the W-TENG and use its electric field as a ‘button’ to open your garage door, or you could activate a security system — all without a battery, passively and wirelessly,” said Sai Sunil Mallineni, a Ph.D. student in physics and astronomy at Clemson.

The researchers are in talks with industrial partners to begin integrating the W-TENG into energy applications. More research is also being done to replace the Teflon electrode with a more environmentally friendly electronegative material.

Waking up devices
Engineers at Stanford University developed a new method for extending the battery life of wireless devices: a wake-up receiver that turns on a device in response to incoming ultrasonic signals.

By working at a significantly smaller wavelength and switching from radio waves to ultrasound, this receiver is much smaller than similar wake-up receivers that respond to radio signals, according to the team, while operating at extremely low power and with extended range.

Once attached to a device, the wake-up receiver listens for a unique ultrasonic pattern that tells it when to turn the device on. It needs only a very small amount of power to maintain this constant listening, so it still saves energy overall while extending the battery life of the larger device. A well-designed wake-up receiver also allows the device to be turned on from a significant distance.

Amin Arbabian, assistant professor of electrical engineering, right, and graduate student Angad Rekhi demonstrate their ultrasonic wake-up receiver and the circuit boards used to test its performance. (Image credit: Arbabian Lab)

Designing the device posed a number of challenges, said Amin Arbabian, assistant professor of electrical engineering at Stanford. “Scaling down wake-up receivers in size and power consumption while maintaining or extending range is a fundamental challenge,” he said. “But this challenge is worth pursuing, because solving this problem can enable scalable networks of wake-up receivers working in our everyday environment.”

In order to miniaturize the wake-up receiver and drive down the amount of power it consumes, the researchers made use of highly sensitive ultrasonic transducers, which convert analog sound input to electrical signals. With that technology, the researchers designed a system that can detect a wake-up signature with as little as 1 nanowatt of signal power.

More stable perovskites
Researchers at the National Renewable Energy Laboratory (NREL) created a new design for an environmentally stable, high-efficiency perovskite solar cell. While perovskites show promising efficiencies, rapid degradation is a challenge to commercialization.

The typical design of a perovskite solar cell sandwiches the perovskite between a hole transport material, a thin film of an organic molecule called spiro-OMeTAD that’s doped with lithium ions and an electron transport layer made of titanium dioxide, or TiO2. This type of solar cell experiences an almost immediate 20% drop in efficiency and then steadily declines as it became more unstable.

“What we are trying to do is eliminate the weakest links in the solar cell,” said Joseph Luther, a senior scientist in the Chemical Materials and Nanoscience team at NREL. The researchers theorized that replacing the layer of spiro-OMeTAD could stop the initial drop in efficiency in the cell. The lithium ions within the spiro-OMeTAD film move uncontrollably throughout the device and absorb water. The free movement of the ions and the presence of water causes the cells to degrade.

A new molecule, nicknamed EH44 and developed at the Colorado School of Mines, was incorporated as a replacement to spiro-OMeTAD because it repels water and doesn’t contain lithium. “Those two benefits led us to believe this material would be a better replacement,” Luther said.

The use of EH44 as the top layer resolved the later more gradual degradation but did not solve the initial fast decreases that were seen in the cell’s efficiency. The researchers tried another approach, this time swapping the cell’s bottom layer of TiO2 for one with tin oxide (SnO2). With both EH44 and SnO2 in place, as well as stable replacements to the perovskite material and metal electrodes, the solar cell efficiency remained steady. The experiment found that the new SnO2 layer resolved the chemical makeup issues seen in the perovskite layer when deposited onto the original TiO2 film.

“This study reveals how to make the devices far more stable,” Luther said. “It shows us that each of the layers in the cell can play an important role in degradation, not just the active perovskite layer.”

The cell held onto 94% of its starting efficiency after 1,000 hours of continuous use under ambient conditions, but more testing is needed to see whether it can survive the 20 year lifetime expected of solar panels.