Power/Performance Bits: June 24

Solar-cell efficiency in one step
Rice University scientists have created a single-step process for producing highly efficient materials that let the maximum amount of sunlight reach a solar cell.

The Rice lab of chemist Andrew Barron found a simple way to etch nanoscale spikes into silicon that allows more than 99 percent of sunlight to reach the cells’ active elements, where it can be turned into electricity.

The more light absorbed by a solar panel’s active elements, the more power it will produce. But the light has to get there. Coatings in current use that protect the active elements let most light pass but reflect some as well. Various strategies have cut reflectance down to about 6 percent, the researchers said, but the anti-reflection is limited to a specific range of light, incident angle and wavelength.

Enter black silicon, so named because it reflects almost no light. Black silicon is simply silicon with a highly textured surface of nanoscale spikes or pores that are smaller than the wavelength of light. The texture allows the efficient collection of light from any angle — from sunrise to sunset.

The researchers replaced a two-step process that involved metal deposition and electroless chemical etching with a single step that works at room temperature.

Rice University scientists have reduced to one step the process to turn silicon wafers into the black silicon used in solar cells. The advance could cut costs associated with the production of solar cells. Here, a cross section shows inverted pyramids etched into silicon by a chemical mixture over eight hours. (Source: Rice University)

Rice University scientists have reduced to one step the process to turn silicon wafers into the black silicon used in solar cells. The advance could cut costs associated with the production of solar cells. Here, a top-down view shows pyramid-shaped pores etched into silicon over eight hours. (Source: Rice University)

A faster path to optical circuits
Scientists at EPFL have developed a fast and effective method for optimizing photonic crystal nanocavities that has led to the design of new-generation structures which may advance the future of optical circuits.

Just as electronic circuits work with electrical charges, optical circuits process pulses of light, which gives them a distinct advantage in terms of speed. Optical technologies are therefore the object of intense research, aiming to develop novel optical devices that can control the flow of light at the nanometer scale. As such, EPFL scientists have developed a new method that can optimally design a widely-used class of optical devices with unprecedented effectiveness. Their designs have been fabricated in the U.S., at the University of Rochester, and successfully tested in Italy, at the University of Pavia.

Measuring massless electrons
Individual electrons in graphene are massless, but when they move together, it’s a different story. Graphene, a one-atom-thick carbon sheet, has taken the world of physics by storm in part because its electrons behave as massless particles. Yet these electrons seem to have dual personalities. Phenomena observed in the field of graphene plasmonics suggest that when the electrons move collectively, they must exhibit mass.

After two years of effort, researchers led by Donhee Ham, Gordon McKay Professor of Electrical Engineering and Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS), and his student Hosang Yoon, have successfully measured the collective mass of ‘massless’ electrons in motion in graphene.

By shedding light on the fundamental kinetic properties of electrons in graphene, this research may also provide a basis for the creation of miniaturized circuits with tiny, graphene-based components.

A schematic of the experimental setup. Ham and Yoon measured the change in phase of a microwave signal sent through the graphene. (Source: Harvard SEAS)