New finding could lead to cheaper, efficient solar cells

Researchers have found an easier method to harness the power of light-capturing nanomaterials that could lead to cheaper and more efficient solar cells.
By applying an innovative theoretical analysis to observations from a first-of-its-kind experimental setup, researchers created a methodology that solar engineers can use to determine the electricity-producing potential for any arrangement of metallic nanoparticles.
"One of the interesting phenomena that occurs when you shine light on a metallic nanoparticle or nanostructure is that you can excite some subset of electrons in the metal to a much higher energy level," said Bob Zheng, Rice University's Laboratory for Nanophotonics (LANP) graduate student.

Scientists call these 'hot carriers' or 'hot electrons'.
Co-author Naomi Halas, professor at Rice University said hot electrons can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces.
Today's photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium.

Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides.
"We can tune plasmonic structures to capture light across the entire solar spectrum," Halas said.
"The efficiency of semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors," Halas said.

Postdoctoral research associate Alejandro Manjavacas of LANP conducted work that offered insight into the underlying physics of hot-electron-production in plasmonic-based devices.
"To make use of the photon's energy, it must be absorbed rather than scattered back out," Manjavacas said.
Zheng's experimental setup filtered high-energy hot electrons from their less-energetic counterparts.
Zheng created two types of plasmonic devices, consisting of a plasmonic gold nanowire on a layer of titanium dioxide.
In the first setup, the gold sat directly on the semiconductor, and in the second, a thin layer of pure titanium was placed between the gold and the titanium dioxide.
The first setup created a microelectronic structure that allowed only hot electrons to pass from the gold to the semiconductor. The second setup allowed all electrons to pass.

"The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system," said Manjavacas.
"In particular, we found that hot electrons were not correlated with total absorption. They were driven by a different, plasmonic mechanism known as field-intensity enhancement," Manjavacas said.
"This research provides a route to increasing the efficiency of plasmonic hot-carrier devices and shows that they can be useful for converting sunlight into usable electricity," Halas said.
The study was published in the journal Nature Communications.