MMaterialsgateNEWS 2012/12/07

Energy: Tiny structure gives big boost to solar power

Princeton researchers have found a simple and economic way to nearly triple the efficiency of organic solar cells, the cheap and flexible plastic devices that many scientists believe could be the future of solar power.

The researchers, led by electrical engineer Stephen Chou, were able to increase the efficiency 175 percent by using a nanostructured "sandwich" of metal and plastic that collects and traps light. Chou said the technology also should increase the efficiency of conventional inorganic solar collectors, such as standard silicon solar panels, although he cautioned that his team has not yet completed research with inorganic devices.

Chou said the research team used nanotechnology to overcome two primary challenges that cause solar cells to lose energy: light reflecting from the cell, and the inability to fully capture light that enters the cell.

With their new metallic sandwich, the researchers were able to address both problems. The sandwich – called a subwavelength plasmonic cavity – has an extraordinary ability to dampen reflection and trap light. The new technique allowed Chou's team to create a solar cell that only reflects about 4 percent of light and absorbs as much as 96 percent. It demonstrates 52 percent higher efficiency in converting light to electrical energy than a conventional solar cell.

That is for direct sunlight. The structure achieves even more efficiency for light that strikes the solar cell at large angles, which occurs on cloudy days or when the cell is not directly facing the sun. By capturing these angled rays, the new structure boosts efficiency by an additional 81 percent, leading to the 175 percent total increase.

The physics behind the innovation is formidably complex. But the device structure, in concept, is fairly simple.

The top layer, known as the window layer, of the new solar cell uses an incredibly fine metal mesh: the metal is 30 nanometers thick, and each hole is 175 nanometers in diameter and 25 nanometers apart. (A nanometer is a billionth of a meter and about one hundred-thousandth of human hair). This mesh replaces the conventional window layer typically made of a material called indium-tin-oxide (ITO).

The mesh window layer is placed very close to the bottom layer of the sandwich, the same metal film used in conventional solar cells. In between the two metal sheets is a thin strip of semiconducting material used in solar panels. It can be any type – silicon, plastic or gallium arsenide – although Chou's team used an 85-nanometer-thick plastic.

The solar cell's features – the spacing of the mesh, the thickness of the sandwich, the diameter of the holes – are all smaller than the wavelength of the light being collected. This is critical because light behaves in very unusual ways in sub-wavelength structures. Chou's team discovered that using these subwavelength structures allowed them to create a trap in which light enters, with almost no reflection, and does not leave.

"It is like a black hole for light," Chou said. "It traps it."

The team calls the system a "plasmonic cavity with subwavelength hole array" or PlaCSH. Photos of the surface of the PlaCSH solar cells demonstrate this light-absorbing effect: under sunlight, a standard solar power cell looks tinted in color due to light reflecting from its surface, but the PlaCSH looks deep black because of the extremely low light reflection.

The researchers expected an increase in efficiency from the technique, "but clearly the increase we found was beyond our expectations," Chou said.

Chou and graduate student Wei Ding reported their findings in the journal Optics Express, published online Nov. 28, 2012. Their work was supported in part by the Defense Advanced Research Projects Agency, the Office of Naval Research and the National Science Foundation.

The researchers said the PlaCSH solar cells can be manufactured cost-effectively in wallpaper-size sheets. Chou's lab used "nanoimprint," a low-cost nanofabrication technique Chou invented 16 years ago, which embosses nanostructures over a large area, like printing a newspaper.

Beside the innovative design, the work involved optimizing the system. Getting the structure exactly right "is critical to achieving high efficiency," said Ding, a graduate student in electrical engineering.

Chou said that the development could have a number of applications depending on the type of solar collector. In this series of experiments, Chou and Ding worked with solar cells made from plastic, called organic solar cells. Plastic is cheap and malleable and the technology has great promise, but it has been limited in commercial use because of organic solar cells' low efficiency.

In addition to a direct boost to the cells' efficiency, the new nanostructured metal film also replaces the current ITO electrode that is the most expensive part of most current organic solar cells.

"PlaCSH also is extremely bendable," Chou said. "The mechanical property of ITO is like glass; it is very brittle."

Source: Princeton University, Engineering School – 06.12.2012.

Investigated and edited by:

Dr.-Ing. Christoph Konetschny, Inhaber und Gründer von Materialsgate
Büro für Material- und Technologieberatung
The investigation and editing of this document was performed with best care and attention.
For the accuracy, validity, availability and applicability of the given information, we take no liability.
Please discuss the suitability concerning your specific application with the experts of the named company or organization.

You want additional material or technology investigations concerning this subject?

Materialsgate is leading in material consulting and material investigation.
Feel free to use our established consulting services

MMore on this topic

For the first time, a silicon-based optical fiber with solar-cell capabilities has been developed that has been shown to be scalable to many meters in length. The research opens the door to the possibility of weaving together solar-cell silicon wires to create flexible, curved, or twisted solar fabrics.

The findings by an international team of chemists, physicists, and engineers, led by John Badding, a professor of chemistry at Penn State University, will be posted by the journal Advanced Materials in an early online edition on 6 December 2012 and will be published on a future date in the journal's print edition. The team's new findings build on earlier work addressing the challenge of merging optical fibers with electronic chips -- silicon-based integrated circuits that serve as the building blocks for most semiconductor electronic devices such as solar cells, computers, and cell phones. Rather than merge a flat chip with a round optical fiber, the team found a way to build a... more read more

Rice University scientists have unveiled a revolutionary new technology that uses nanoparticles to convert solar energy directly into steam. The new “solar steam” method from Rice’s Laboratory for Nanophotonics (LANP) is so effective it can even produce steam from icy cold water.

Details of the solar steam method were published online today in ACS Nano. The technology has an overall energy efficiency of 24 percent. Photovoltaic solar panels, by comparison, typically have an overall energy efficiency around 15 percent. However, the inventors of solar steam said they expect the first uses of the new technology will not be for electricity generation but rather for sanitation and water purification in developing countries. “This is about a lot more than electricity,” said LANP Director Naomi Halas, the lead scientist on the project. “With this technology, we are beginning to think about solar thermal power in a completely different way.” The efficiency of solar... more read more

Photovoltaic cell efficiency may soon get a big boost, thanks to next-generation antireflection coatings crafted from nanomaterials capable of cutting down on the amount of light reflected away from a cell's surface.

Materials boasting a "tunable" refractive index have been developed within the past few years, and they show tremendous potential for photovoltaic applications. Professor E. Fred Schubert, of Rensselaer Polytechnic Institute's Department of Electrical, Computer, and Systems Engineering, is investigating ways to exploit this newly gained controllability and will present his findings at the upcoming AVS 59th International Symposium and Exhibition, held Oct. 28 - Nov. 2, in Tampa, Fla. The refractive index is the property of a material that changes the speed of light, and is computed as the ratio of the speed of light in a vacuum to the speed of light through the material. Among... more read more

Scientists are reporting development of a new transparent solar cell, an advance toward giving windows in homes and other buildings the ability to generate electricity while still allowing people to see outside. Their report appears in the journal ACS Nano.

Yang Yang, Rui Zhu, Paul S. Weiss and colleagues explain that there has been intense world-wide interest in so-called polymer solar cells (PSCs), which are made from plastic-like materials. PSCs are lightweight and flexible and can be produced in high volume at low cost. That interest extends to producing transparent PSCs. However, previous versions of transparent PSCs have had many disadvantages, which the team set out to correct. They describe a new kind of PSC that produces energy by absorbing mainly infrared light, not visible light, making the cells 66 percent transparent to the human eye. They made the device from a photoactive plastic that converts infrared light into an electrical... more read more

MaterialsgateNEWSLETTER

Partner of the Week

Search in MaterialsgateNEWS

MaterialsgateFAIR:
LET YOURSELF BE INSPIRED