Power/Performance Bits: June 19

Tandem solar reaches 25.2% efficiency
In the push for ever-more efficient solar panels, researchers are turning to tandem, or double-junction, photovoltaics. Tandem solar panels use two different types of solar cell capable of absorbing different wavelengths of light stacked on top of each other to maximize the conversion of light rays into electrical power.

Recently, two groups have reached a record high efficiency of 25.2% using a perovskite cell in combination with a silicon cell. Perovskite efficiently converts blue and green light, while silicon is better at converting red and infra-red light.

Helmholtz-Zentrum Berlin, Oxford PV, and the University of Oxford built a 1 cm² perovskite-silicon tandem solar cell with a two-terminal design, which the team says will support the ease of integration into module fabrication and photovoltaic systems. The two-terminal design also eliminates the additional materials, weight and power invertor challenges of four-terminal tandem cells.

Oxford PV says the technology is reaching commercial potential. The company also has an industrial pilot line producing commercial sized 156mm x 156mm perovskite-silicon tandem solar cells for validation by a development partner.

A group from EPFL and CSEM took a different approach, aiming to reduce the cost of two-layer systems by integrating a perovskite cell directly on top of the standard silicon cell without polishing the silicon. Their method adds only a few extra steps to the current silicon-cell production process.

“Silicon’s surface consists of a series of pyramids measuring around 5 microns, which trap light and prevent it from being reflected. However, the surface texture makes it hard to deposit a homogeneous film of perovskite,” explained Quentin Jeangros of EPFL. When the perovskite is deposited in liquid form, as it usually is, it accumulates in the valleys between the pyramids while leaving the peaks uncovered, leading to short circuits.

Silicon’s pyramids covered with perovskite. (Source: EPFL)

To get around the problem, the team used evaporation methods to form an inorganic base layer that fully covers the pyramids. That layer is porous, enabling it to retain the liquid organic solution that is then added using the spin-coating thin-film deposition technique. The researchers subsequently heated the substrate to a relatively low temperature of 150°C to crystallize a homogeneous film of perovskite on top of the silicon pyramids.

Battery-free implants
Researchers at MIT and Brigham and Women’s Hospital developed small, battery-less implantable devices that can be powered wirelessly. The implants are powered by radio frequency waves, which can safely pass through human tissues. In tests in animals, the researchers showed that the waves can power devices located 10 centimeters deep in tissue, from a distance of 1 meter.

Because they do not require a battery, the devices can be tiny. In this study, the researchers tested a prototype about the size of a grain of rice, but they anticipate that it could be made even smaller.

“Even though these tiny implantable devices have no batteries, we can now communicate with them from a distance outside the body. This opens up entirely new types of medical applications,” said Fadel Adib, an assistant professor in MIT’s Media Lab.

Wirelessly powering implantable devices with radio waves emitted by antennas outside the body has proven challenging, as radio waves tend to dissipate as they pass through the body, so they end up being too weak to supply enough power. To overcome that, the researchers devised a system that they call “In Vivo Networking” (IVN). This system relies on an array of antennas that emit radio waves of slightly different frequencies. As the radio waves travel, they overlap and combine in different ways. At certain points, where the high points of the waves overlap, they can provide enough energy to power an implanted sensor.

In this study, the researchers tested a prototype about the size of a grain of rice, but they anticipate that it could be made even smaller. (Source: Felice Frankel, edited by MIT News)

“We chose frequencies that are slightly different from each other, and in doing so, we know that at some point in time these are going to reach their highs at the same time. When they reach their highs at the same time, they are able to overcome the energy threshold needed to power the device,” Adib said.

With the new system, the researchers don’t need to know the exact location of the sensors in the body, as the power is transmitted over a large area. This also means that they can power multiple devices at once. At the same time that the sensors receive a burst of power, they also receive a signal telling them to relay information back to the antenna. This signal could also be used to stimulate release of a drug, a burst of electricity, or a pulse of light, the researchers say.

Aside from health and medical uses, the team sees the potential to improve RFID applications in other areas such as inventory control, retail analytics, and “smart” environments, allowing for longer-distance object tracking and communication.

The researchers are now working on making the power delivery more efficient and transferring it over greater distances.

Magnetic insulator interconnects
Physicists at Rutgers University and Tsinghua University demonstrated a way to conduct electricity between transistors without energy loss using a special mix of materials with magnetic and insulator properties.

“This material, although it’s much diluted in terms of magnetic properties, can still behave like a magnet and conducts electricity at low temperature without energy loss,” said Weida Wu, associate professor in the Department of Physics and Astronomy at Rutgers University-New Brunswick. “At least in principle, if you can make it work at a higher temperature, you can use it for electronic interconnections within silicon chips used in computers and other devices.”

An exotic magnetic insulator conducts electricity along its edges without energy loss. The M stands for magnetization of the magnet, and this shows the magnetization reversal process (red to blue and vice versa). (Source: Image: Wenbo Wang/Rutgers University-New Brunswick)

The team combined chromium and vanadium as magnetic elements with an insulator consisting of bismuth, antimony and tellurium. When electrons in this material are aligned in one direction, an electric current can only flow along its edges in one direction, leading to zero energy loss.

While they see this material, called a quantum anomalous Hall insulator, as a path toward conducting electricity in electronics with maximum efficiency, there’s much progress yet to be made. The material, only conducts electricity without energy loss when the temperature is close to absolute zero: minus 459.67 degrees Fahrenheit. Next steps would include demonstrating the phenomenon at a much higher and more practical temperature for electronics, along with building a platform for quantum computing.