Power/Performance Bits: Aug. 7

Optical neural network; perovskite passivation; improving organic solar.


Optical neural network
Researchers at the National Institute of Standards and Technology (NIST) have made a silicon chip that distributes optical signals precisely across a miniature brain-like grid, showcasing a potential new design for neural networks.

Using light would eliminate interference due to electrical charge and the signals would travel faster and farther, said the researchers. “Light’s advantages could improve the performance of neural nets for scientific data analysis such as searches for Earth-like planets and quantum information science, and accelerate the development of highly intuitive control systems for autonomous vehicles,” NIST physicist Jeff Chiles said.

The NIST chip vertically stacks two layers of photonic waveguides. This 3D design enables complex routing schemes, which are necessary to mimic neural systems. Furthermore, this design can easily be extended to incorporate additional waveguiding layers when needed for more complex networks.

NIST’s grid-on-a-chip distributes light signals precisely, showcasing a potential new design for neural networks. The three-dimensional structure enables complex routing schemes, which are necessary to mimic the brain. Light could travel farther and faster than electrical signals. (Source: Chiles/NIST)

The stacked waveguides form a three-dimensional grid with 10 inputs or “upstream” neurons each connecting to 10 outputs or “downstream” neurons, for a total of 100 receivers. Fabricated on a silicon wafer, the waveguides are made of silicon nitride and are each 800nm wide and 400nm thick. Researchers created software to automatically generate signal routing, with adjustable levels of connectivity between the neurons.

Laser light was directed into the chip through an optical fiber. The goal was to route each input to every output group, following a selected distribution pattern for light intensity or power. Power levels represent the pattern and degree of connectivity in the circuit. The authors demonstrated two schemes for controlling output intensity: uniform (each output receives the same power) and a “bell curve” distribution (in which middle neurons receive the most power, while peripheral neurons receive less).

“We’ve really done two things here,” Chiles said. “We’ve begun to use the third dimension to enable more optical connectivity, and we’ve developed a new measurement technique to rapidly characterize many devices in a photonic system. Both advances are crucial as we begin to scale up to massive optoelectronic neural systems.”

Perovskite passivation
Researchers at the University of Washington boosted the performance of thin-film perovskite solar cells, beating even today’s best solar cell materials at emitting light.

“It may sound odd since solar cells absorb light and turn it into electricity, but the best solar cell materials are also great at emitting light,” said UW chemical engineering professor Hugh Hillhouse. “In fact, typically the more efficiently they emit light, the more voltage they generate.”

The solar material, a lead-halide perovskite, was chemically treated it through surface passivation, which treats imperfections and reduces the likelihood that the absorbed photons will end up wasted rather than converted to useful energy. The team used an organic compound known by its acronym TOPO for passivation.

“One large problem with perovskite solar cells is that too much absorbed sunlight was ending up as wasted heat, not useful electricity,” said co-author David Ginger, a UW professor of chemistry and chief scientist at the CEI. “We are hopeful that surface passivation strategies like this will help improve the performance and stability of perovskite solar cells.”

TOPO-treating a perovskite semiconductor significantly impacted both its internal and external photoluminescence quantum efficiencies — metrics used to determine how good a semiconducting material is at utilizing an absorbed photon’s energy rather than losing it as heat. TOPO-treating the perovskite increased the internal photoluminescence quantum efficiencies by tenfold — from 9.4 percent to nearly 92 percent.

An image of a back-reflector surface used by the researchers to test perovskite performance. Each quadrant is a different surface material — gold, titanium, palladium or a silica compound — upon which the perovskite material would be deposited for experiments. (Source: University of Washington)

“Our measurements observing the efficiency with which passivated hybrid perovskites absorb and emit light show that there are no inherent material flaws preventing further solar cell improvements,” said Ian Braly, who conducted this research as a doctoral student in chemical engineering at UW. “Further, by fitting the emission spectra to a theoretical model, we showed that these materials could generate voltages 97 percent of the theoretical maximum, equal to the world record gallium arsenide solar cell and much higher than record silicon cells that only reach 84 percent.”

The next steps involve demonstrating a similar chemical passivation that is compatible with easily manufactured electrodes and experimenting with other types of surface passivation.

Improving organic solar cells
Researchers from the University of Strasbourg, University of Lyon, Nazarbayev University, and Moscow Institute of Physics and Technology developed a way of boosting the efficiency of organic solar cells by incorporating fluorine atoms in the polymer.

By experimenting with various polymer modifications, the team increased cell efficiency from 3.7 to 10.2 percent. While this is still lower than commercial silicon photovoltaics, the gain in efficiency suggests polymer-based solar cells deserve further development.

The generic polymer used in the experiment has a rather complex molecular structure that consists of a chain of repeating units. Each of them includes sulfur heterocycles — rings made of one sulfur and four carbon atoms — and hydrocarbon side chains with a branched structure.

The structure of a repeating unit in the polymer chain without fluorine (left) and after fluorination (right). (Source: Elena Khavina/MIPT Press Office and the researchers)

The researchers produced a number of modifications of this polymer to find which one has better photovoltaic properties. They changed the structure by adding fluorine atoms and varying the length of the side chains. One polymer configuration proved to result in vastly superior properties. Namely, the cell efficiency and current output were several times higher.

The team then investigated the microscopic structure of the best-performing compound. X-ray analysis revealed polymer stacking to be more ordered. Also, the molecules were characterized by higher charge carrier mobility, an advantage for solar cells.

The organic solar cells can be manufactured in fewer stages, compared with conventional silicon photovoltaics, said Dimitri Ivanov, a professor at MIPT. The light-absorbing polymers can also function as a thin film, which means the solar panels need not be flat.

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