Power/Performance Bits: Feb. 19

Flexible energy harvesting rectenna
Researchers from MIT, Universidad Politécnica de Madrid, University Carlos III of Madrid, Boston University, University of Southern California, and the Army Research Laboratory created a flexible rectenna capable of converting energy from Wi-Fi signals into electricity to power small devices and sensors.

The device uses a flexible RF antenna to capture electromagnetic waves as AC waveforms, which are fed to a rectifier. While traditional rectennas often use silicon or gallium arsenide for the rectifier, they are rigid and too expensive to use in applications covering large areas, such as the surfaces of buildings.

Instead, the team turned to molybdenum disulfide (MoS2), a 2D semiconductor just three atoms thick. When exposed to certain chemicals, the atoms of MoS2 rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material.

“By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” said Xu Zhang, a postdoc at MIT who will soon join Carnegie Mellon University as an assistant professor.

According to the researchers, the parasitic capacitance of their Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, making it much faster at signal conversion and allowing it to capture and convert up to 10 gigahertz of wireless signals.

“Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” added Zhang.

The researchers can see the rectenna powering flexible, wearable electronics and building-wide IoT sensors. Another application could be in powering the data communications of implantable and swallowable medical devices, said Jesús Grajal, a researcher at the Technical University of Madrid. “Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die. It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.”

In experiments, the researchers’ device produced about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). The maximum output efficiency for the current device stands at 40%, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the MoS2 rectifier is about 30%. For reference, today’s best silicon and gallium arsenide rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50% to 60%.

The device can be fabricated in a roll-to-roll process. The team plans to build more complex systems as well as improve its efficiency.

All-photonic quantum repeater
Researchers from Osaka University, University of Toronto, University of Toyama, and NTT Basic Research Laboratories completed a proof-of-principle experiment for a key element of long-distance quantum communication: all-photonic quantum repeaters.

A quantum Internet could enable much more secure communication through a technique called quantum key distribution, or QKD. QKD relies on the principle that sensing or measuring the state of a quantum system disturbs that system. Thus, third-party eavesdropping would leave behind a clearly detectable trace, and the communication can be aborted before any sensitive information is lost.

While this has been demonstrated in small-scale systems, on a large scale it would require the use of fiber optic cables. As light travels through long distance cables, it loses potency. Currently, long-distance fiber optic cables have repeaters at regular intervals along the line to boost and amplify the signal, but they don’t work for quantum information.

The repeaters that do work for quantum information require storage of the quantum state at the repeater sites, making the repeaters much more error prone, difficult to build, and very expensive because they often operate at cryogenic temperatures.

The team says the new repeaters, however, eliminate many of these problems.

“We have developed all-photonic repeaters that allow time-reversed adaptive Bell measurement,” said Hoi-Kwong Lo, a professor of electrical and computer engineering and physics. “Because these repeaters are all-optical, they offer advantages that traditional — quantum-memory-based matter — repeaters do not. For example, this method could work at room temperature.”

The repeater also does not require large-scale optical switches or quantum error correcting codes.

“An all-optical network is a promising form of infrastructure for fast and energy-efficient communication that is required for a future quantum internet,” added Lo. “Our work helps pave the way toward this future.”

More stable perovskite solar
Scientists at the Okinawa Institute of Science and Technology developed a method to improve both the stability and scalability of perovskite solar cells.

While perovskite solar cells (PSCs) have reached high efficiency levels at a low cost, they degrade quickly in real-life conditions. “We need solar modules that can last for at least 5 to 10 years. For now, the lifetime of PSCs is much shorter,” said Longbin Qiu, a postdoctoral scholar in the OIST Energy Materials and Surface Sciences Unit.

Photo credit: Okinawa Institute of Science and Technology Graduate University (OIST)

To improve this, the team looked to the PSC’s electron transport layer. This layer, which controls the flow of electrons, is typically made of titanium dioxide. But when exposed to sunlight, it reacts with the perovskite active layer and degrades the device.

Tin oxide has been proposed as a replacement for the titanium oxide layer, but thus far had not been incorporated into a large scale device. By using sputtering deposition, the researchers produced smooth layers with a uniform thickness over a large area.

Using the tin dioxide, the solar cell reached an efficiency of over 20%. To show scalability, the team fabricated 5 by 5 centimeter solar modules with a designated area of 22.8 square-centimeters. The resulting devices showed over 12% efficiency.

The researchers plan to continue optimizing their PSC design with the goal of producing large-scale solar modules with improved efficiency. The team also experiments with flexible, transparent solar devices and aims to apply their optimized PSC design in solar windows, curtains, backpacks and deployable charging units. They expect commercialization to be viable in the next few years.