Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which did not filter electrons based on energy level, included a thin layer of titanium between the gold and the titanium dioxide. CREDIT: B. Zheng/Rice University
Researchers from Rice University’s Laboratory for Nanophotonics (LANP) have developed a novel method that could be used to exploit the power of light-capturing nanomaterials to reduce costs and increase the efficiency of photovoltaic solar cells.
The latest breakthrough would allow solar-panel designers to integrate the light-capturing nanomaterials into their future designs. The US is aiming to reduce the cost of solar electricity to 6 cents/kWh. While the growth of domestic solar-energy industry reached 34% in 2014, basic technical breakthroughs are required to meet the national goal of the US.
LANP postdoctoral research associate Alejandro Manjavacas and graduate student Bob Zheng applied a new hypothetical study to observations from a first-ever experimental setup and ultimately developed an approach that would allow solar engineers to establish the electricity-generating potential for any arrangement of metallic nanoparticles.
The research team studied the light-capturing nanomaterials such as metallic nanoparticles, which are capable of changing light into plasmons. Plasmons are electron waves that flow across the surface of particles, for instance, the latest LANP plasmonic study has revolutionized solar-powered steam production, color-display technology, and eye-mimicking color sensors.
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, Scientists call these ‘hot carriers’ or ‘hot electrons.
Zheng, who works with LANP Director and Naomi Halas, study co-author
According to Halas, Rice University’s Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, hot electrons hold more significance in solar-energy applications as they can be utilized to promote chemical reactions on inert metal surfaces or to develop devices that generate direct current.
Present-day photovoltaic cells are highly efficient and employ a combination of semiconductors produced from indium and gallium, which are rare and costly elements. Halas stated that if highly efficient light-gathering plasmonic nanostructures are integrated with cost-effective semiconductors, such as metal oxides, then production cost can be considerably reduced. Such plasmonic nanostructures would also have optical characteristics, which can be accurately controlled by altering the shape of these nanostructures. Although this plasmonic approach was tested before, it did not result in much success.
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. Plasmonic-based photovoltaics have typically had low efficiencies, and it hasn’t been entirely clear whether those arose from fundamental physical limitations or from less-than-optimal designs.
Zheng
Halas and Zheng further stated that Manjavacas, a theoretical physicist who was part of the group of LANP scientist Peter Nordlander, contributed towards the new research, which provides a better understanding of the mechanism of hot-electron-production in devices made of plasmonics.
Manjavacas said, “To make use of the photon’s energy, it must be absorbed rather than scattered back out. For this reason, much previous theoretical work had focused on understanding the total absorption of the plasmonic system.”
Manjavacas further added that such kind of work originated from a breakthrough experiment performed by Ali Sobhani, also a Rice graduate student. In this experiment, the absorption was focused close to a metal semiconductor interface.
“From this perspective, one can determine the total number of electrons produced, but it provides no way of determining how many of those electrons are actually useful, high-energy, hot electrons,” Manjavacas said.
According to Manjavacas, Zheng’s data provided a better analysis because in his experimental setup, high-energy hot electrons were selectively filtered from the less-energetic hot electrons. In order to achieve this, two kinds of plasmonic devices were developed by Zheng, with each device containing a plasmonic gold nanowire placed on top of a titanium dioxide layer. In the original setup, the gold was allowed to sit directly on the semiconductor, while in the second setup, a pure titanium layer was placed between the titanium dioxide and the gold. The original setup produced a Schottky barrier, a kind of microelectronic structure that enabled only hot electrons to travel from the gold to the semiconductor, while all electrons were allowed to pass in the second setup.
The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system. 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
Halas said, “This is an important step toward the realization of plasmonic technologies for solar photovoltaics. 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.”
The LANP research team is focused on developing methods to boost the field-intensity improvement of photonic structures for various applications, such as single-molecule sensing. Manjavacas and Zheng informed that additional tests are being carried out to alter their system so that hot electrons output can be optimized.
Other co-authors of the study include Michael McClain and Hangqi Zhao from Rice University. The Office of Naval Research, the Welch Foundation, and the Air Force Office of Science and Research funded the study.
The study results have appeared in the July issue of Nature Communications.
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