All posts by Daniel Davies

Stanford scientists design solar cells that can capture 97% of light

A material developed by Stanford University scientists may lead to a paradigm shift in the design and fabrication of solar cells.

Currently solar panels are built with a metal wire grid that carries electricity to and from the device. But these wires also prevent sunlight from reaching the solar cell’s semiconductor – which converts sunlight into electricity, and is sandwiched between the metal wires.

The research team have discovered how to hide the reflective metal wires and funnel light directly to the semiconductor below.

“Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light,” said the lead author of the study Vijay Narasimhan. “Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells.”

The material utilised by the Stanford research team is capable of absorbing 97% of light; that represents a 20 to 22% increase on conventional solar technology.

The system developed to collect sunlight works by placing a 16-nanometer-thick film of gold on a flat sheet of silicon. The gold film is perforated with an array of nanosized square holes, and once the silicon is immersed in a solution of hydrogen peroxide and hydrofluoric acid, the silicon nanopillars emerge through the holes in the gold film.

Narasimhan compares the nanopillar design to a colander in a typical kitchen. “When you turn on the faucet, not all of the water makes it through the holes in the colander, ” he said.

“But if you were to put a tiny funnel on top of each hole, most of the water would flow straight through with no problem. That’s essentially what our structure does: the nanopillars act as funnels that capture light and guide it into the silicon substrate through the holes in the metal grid.”

Image courtesy of Vijay Narasimhan, Stanford University

Image courtesy of Vijay Narasimhan, Stanford University

In order to test the effectiveness of the new solar panels, the nanopillars were put through a series of simulations and experiments.

The researchers discovered that the nanopillar architecture works with contacts made of silver, platinum, nickel and other metals.

“We call them covert contacts, because the metal hides in the shadows of the silicon nanopillars,” said the study’s co-author Ruby Lai. “It doesn’t matter what type of metal you put in there. It will be nearly invisible to incoming light.”

Full findings from the research are available in the journal ACS Nano.

Researchers discover particles missed in quantum theory’s development

A particle missed during the initial development of quantum theory has been found thanks to researchers’ exploration of the “material universe” .

An international team of researchers have theorised that a particle, called the type-II Weyl fermion, exists in a material known as tungsten ditelluride (WTe2), which the researchers liken to a “material universe” because it contains several particles, some of which exist under normal conditions in our universe and others that may exist only in specialized types of crystals.

These crystals can be grown in the laboratory, so experiments can be done to look for the newly predicted Weyl fermion in WTe2 and another candidate material, molybdenum ditelluride (MoTe2).

Image courtesy of B. Andrei Bernevig et al

Image courtesy of B. Andrei Bernevig et al

“Even more intriguing is the perspective of finding more ‘elementary’ particles in other condensed matter systems,” the researchers say.

“What kind of other particles can be hidden in the infinite variety of material universes? The large variety of emergent fermions in these materials has only begun to be unravelled.”

The researchers assume that the particle’s existence was missed by physicist Hermann Weyl, during the initial development of quantum theory 85 years ago, because it violated a fundamental rule called Lorentz symmetry that does not apply to the materials where the new type of fermion arises from.

In recent years, researchers have discovered that such a “material universe” could host all other particles of relativistic quantum field theory. Three quasiparticles, the Dirac, Majorana and Weyl fermions, were discovered in such materials, despite the fact that the latter two had long been elusive in experiments.

Particle3

“One’s imagination can go further and wonder whether particles that are unknown to relativistic quantum field theory can arise in condensed matter,” says Princeton University associate professor of Physics B Andrei Bernevig.

“One may wonder,” Soluyanov said, “if it is possible that some material universes host non-relativistic ‘elementary’ particles that are not Lorentz-symmetric?”

The existence of the type-II Weyl fermion and the behaviour it exhibits suggests a range of potential applications, from low-energy devices to efficient transistors.

The full version of the researchers’ findings – Type II Weyl Semimetals – can be found in the journal Nature.