Power/Performance Bits: July 27

Amplifying light for lidar; harvesting energy from Wi-Fi; gold for tandem solar cells.


Amplifying light for lidar
Engineers at University of Texas at Austin and University of Virginia developed a light detector that can amplify weak light signals and reduce noise to improve the accuracy of lidar.

“Autonomous vehicles send out laser signals that bounce off objects to tell you how far away you are. Not much light comes back, so if your detector is putting out more noise than the signal coming in you get nothing,” said Joe Campbell, professor of electrical and computer engineering at the University of Virginia School of Engineering.

The new avalanche photodiode uses a staircase-like alignment, in which electrons roll down physical steps in energy, multiplying with each step to create a stronger electrical current for light detection.

“The electron is like a marble rolling down a flight of stairs,” said Seth Bank, professor in the Cockrell School’s Department of Electrical and Computer Engineering at UT Austin. “Each time the marble rolls off a step, it drops and crashes into the next one. In our case, the electron does the same thing, but each collision releases enough energy to actually free another electron. We may start with one electron, but falling off each step doubles the number of electrons: 1, 2, 4, 8, and so on.”

Electrons multiply as they roll down the “staircase ” as part of the avalanche photodiode. (Credit: University of Texas at Austin)

The team said that adding steps increases sensitivity and consistency of the device, while the consistent multiplication of electrons with each step makes the electrical signals from the detector more dependable, even in low light conditions.

“The less random the multiplication is, the weaker the signals you can pick out from the background,” said Bank. “For example, that could allow you to look out to greater distances with a laser radar system for autonomous vehicles.”

Key to the device is a new way of growing materials, Banks added. Instead of growing materials with atoms randomly distributed, they created layered allows composed of binary compounds stacked on top of each other.

The staircase avalanche photodiode can operate at room temperature. The team plans to continue refining the process to add more steps. They are also working with a semiconductor company on commercialization.

In addition, they plan to combine the multi-step staircase device with a different avalanche photodiode they previously built that is sensitive to near-infrared light. This could have applications in fiber-optic communications and thermal imaging. “This should give us the best of both worlds: response to a wider range of colors and greater sensitivity to weak signals because of the lower noise amplification that comes naturally from the staircase architecture,” Bank said.

Harvesting energy from Wi-Fi
Researchers at the National University of Singapore and Tohoku University developed a device that uses spin-torque oscillators (STOs) to harvest energy from 2.4GHz Wi-Fi signals and wirelessly power an LED without need for a battery.

“We are surrounded by Wi-Fi signals, but when we are not using them to access the Internet, they are inactive, and this is a huge waste. Our latest result is a step towards turning readily-available 2.4GHz radio waves into a green source of energy, hence reducing the need for batteries to power electronics that we use regularly. In this way, small electric gadgets and sensors can be powered wirelessly by using radio frequency waves as part of the Internet of Things. With the advent of smart homes and cities, our work could give rise to energy-efficient applications in communication, computing, and neuromorphic systems,” said Professor Yang Hyunsoo from the NUS Department of Electrical and Computer Engineering.

The device has an array of eight STOs connected in series. Using this array, the 2.4 GHz electromagnetic radio waves that Wi-Fi uses was converted into a direct voltage signal, which was then transmitted to a capacitor to light up a 1.6-volt LED. When the capacitor was charged for five seconds, it was able to light up the same LED for one minute after the wireless power was switched off.

A chip embedded with about 50 spin-torque oscillators. (Credit: National University of Singapore)

The researchers also compared series design with parallel design for on-chip STO systems and found that the parallel configuration is more useful for wireless transmission due to better time-domain stability, spectral noise behavior, and control over impedance mismatch. Series connections have an advantage for energy harvesting due to the additive effect of the diode-voltage from STOs.

“Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronizing state of coupled STOs using injection locking from an external radio-frequency source. These results are important for prospective applications of synchronized STOs, such as fast-speed neuromorphic computing,” said Dr Raghav Sharma of the Department of Electrical and Computer Engineering at NUS.

Next, the researchers are looking to increase the number of STOs in the array to improve energy harvesting capability of the device. They are also planning to test their energy harvesters for wirelessly charging other useful electronic devices and sensors and looking for industry collaboration for wireless charging and wireless signal detection systems.

Gold for tandem solar cells
Researchers from Pennsylvania State University, Shaanxi Normal University, and Hubei University created a semitransparent electrode for perovskite solar cells that can be coupled with traditional silicon cells in a tandem device.

“We’ve shown we can make electrodes from a very thin, almost few atomic layers of gold,” said Shashank Priya, associate vice president for research and professor of materials science and engineering at Penn State. “The thin gold layer has high electrical conductivity and at the same time it doesn’t interfere with the cell’s ability to absorb sunlight.”

Ultrathin gold films as a transparent electrode for perovskite solar cells has been explored before, but challenges in fabrication have led to poor conductivity. By using chromium as a seed layer, the gold was able to form in a continuous ultrathin layer, providing good conductivity.

“Normally, if you grow a thin layer of something like gold, the nanoparticles will couple together and gather like small islands,” said Dong Yang, assistant research professor of materials science and engineering at Penn State. “Chromium has a large surface energy that provides a good place for the gold to grow on top of, and it actually allows the gold to form a continuous thin film.”

The perovskite solar cell that the team developed achieved 19.8% efficiency, a record for a semitransparent cell. And when combined with a traditional silicon solar cell, the tandem device achieved 28.3% efficiency, up from 23.3% from the silicon cell alone.

“A 5% improvement in efficiency is giant,” Priya said. “This basically means you are converting about 50 watts more sunlight for every square meter of solar cell material. Solar farms can consist of thousands of modules, so that adds up to a lot of electricity, and that’s a big breakthrough.”

They also found that the perovskite cells made with the gold electrodes were stable and maintained high efficiency in lab tests. Ultimately, the researchers hope this is a step toward completely transparent solar cells.

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