Power/Performance Bits: April 11

High-efficiency silicon photodetector

Electrical engineers at the University of California, Davis, and W&WSens Devices, Inc. built a new type of high-efficiency photodetector that could be monolithically integrated with silicon electronics.

The new detector uses tapered holes in a silicon wafer to divert photons sideways, preserving the speed of thin-layer silicon and the efficiency of a thicker layer. The group built an experimental photodetector and solar cell using the new technology. The photodetector can convert data from optical to electronics at 20 gigabytes per second with a quantum efficiency of 50%, the fastest yet reported for a device of this efficiency.

“We’re trying to take advantage of silicon for something silicon cannot usually do,” said Saif Islam, professor of electrical and computer engineering at UC Davis. Existing high-speed photodetector devices use materials such as gallium arsenide, which is about ten times as efficient as silicon at the same scale and wavelength, but is significantly more expensive. “If we don’t need to add non-silicon components and can monolithically integrate with electronics into a single silicon chip, the receivers become much cheaper.”

The team began by experimenting with ways to increase the efficiency of silicon by adding tiny pillars or columns, then holes to the silicon wafer. After two years of experiments, they settled on a pattern of holes that taper towards the bottom.

Tapered black holes in silicon, about 1 micrometer in size, can trap photons and act as a photodetector for high-speed data connections. (Source: Saif Islam, UC Davis)

“We came up with a technology that bends the incoming light laterally through thin silicon,” Islam said.

The idea is that photons enter the holes and get pulled sideways into the silicon. The wafer itself is about two microns thick, but because they move sideways, the photons travel through 30 to 40 microns of silicon.

The holes-based device could also potentially work with a wider range of wavelengths of light than current technology. The team sees the device as having potential to speed up data center communications.

More stable perovskite solar cells

Researchers at the Ulsan National Institute of Science and Technology (UNIST) and Korea Research Institute of Chemical Technology (KRICT) developed a new cost-efficient way to produce inorganic-organic hybrid perovskite solar cells with record performance.

The team’s perovskite cells managed both high efficiency, at 21.2%, and high photostability using Lanthanum (La)-doped BaSnO3 (LBSO) photoelectrode materials synthesized under very mild conditions. They used methylammonium lead iodide (MAPbI3) perovskite materials for the solar cells.

Perovskite solar cells’ major weakness is their low photostability, breaking down rapidly when exposed to normal conditions. For commercial viability, improving the lifetime of perovskite cells is necessary.

The new material retained 93% of its initial performance after 1,000 hours of exposure to sunlight. The synthesis of the photoelectrode material can also proceed at less than 200°C, which is much lower than that of conventional photoelectrode materials (high temperature over 900°C), making fabrication much easier.

The LBSO powder prepared in the study, the solution dispersed in the solvent, and the thin film coated on the substrate. (Source: UNIST)

Additionally, the team proposed a new solar cell manufacturing methodology, which they call the ‘Hot-Pressing Method’. This method tightly adheres two objects by applying temperature and pressure, which the researchers suggest would allow the production of low-cost, high efficiency and stable perovskite solar cells.

“This study combines the newly-synthesized photoelectrode material and the hot-pressing method to lower the manufacturing cost to less than half of the existing silicon solar cells,” said Professor Sang-Il Seok of Energy and Chemical Engineering at UNIST.

The research team envisions that this method and platform will significantly contribute to accelerate the commercialization of perovskite solar cells.

Solar antennas

Startup company NovaSolix claims to have developed a solar technology that is twice as efficient as photovoltaics. The rectifying antenna system, which uses carbon nanotube antennas, can convert a broader spectrum of light waves to electricity.

The current rectenna design uses quartz based chips on which millions of nanometer diameter carbon nanotubes are grown per square inch. These form low impedance antennas coupled to diodes with petahertz cutoff frequencies, the fastest diodes ever built, enough to capture and rectify up to 80% of sunlight from infrared down to ultraviolet.

By replacing the quartz with flexible rolled glass, NovaSolix plans to create a continuous high volume roll to roll manufacturing process using micron level printing and maskless self-aligning lithography. The company’s early estimates indicate the associated costs of manufacturing the technology could be 80-90% percent less than PV.

The technology builds on studies regarding carbon nanotube rectennas and asymmetric MoC diodes from Georgia Tech, the California Institute of Technology, and the University of Liverpool.

Initial applications will include fixed wing aircraft, drones and satellites, according to the company. Prototypes will be available Q4 2017.