William Rance, a graduate student at the Colorado School of Mines, holds a polymer solar cell

Nanotech and Renewable Technologies

December 2007 / January 2008 By: Kevin Eber Volume 5 Number 6

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Renewables today are, of course, a buzzword in a number of circles. But renewables research is no recent phenomenon. The National Renewable Energy Laboratory (NREL), a pioneer in alternative fuels, recently celebrated its 30th anniversary in July. Its research over the past three decades has helped to create today's mature and quickly growing industries in solar power, wind power, biofuels, high-performance buildings and hybrid vehicles, but NREL researchers continue to look to the future.

The latest technological advances in biotechnology, materials and other fields offer fresh approaches to these technologies, presenting the possibility of creating new low-cost solutions to energy challenges. Among the latest scientific advances, nanotechnology is key to many of NREL's recent innovations.

NREL has been involved in nanotechnology since its inception and started investigating quantum dots—€”nanosized particles of semiconductors—€”in 1983. That research recently yielded a breakthrough that is described below. Nanotech really gained momentum in 1985 with the discovery of buckyballs or fullerenes—€”molecules of 60 carbon atoms in the shape of nanoscale soccer balls.
NREL holds a patent for its method of generating buckyballs in its high-flux solar furnace.

Over the years, researchers have employed nanotech in a wide range of technologies, including nanoparticle precursors, such as organometallic inks, which can be used like spray paint to apply metal grids onto solar cells or to create thin-film solar cells. The lab is exploring carbon nanotubes as a means of hydrogen storage.

Most recently, researchers have achieved breakthroughs in three applications of nanotechnology: creating solar cells from silicon quantum dots, employing nanostructures in solar cells made from plastics and in creating nanostructured anodes for lithium-ion batteries.

Quantum Dots and Silicon Solar Cells

In 2007, researchers announced that silicon quantum dots could generate more than one exciton per absorbed photon. Excitons consist of an electron bound loosely to a positive "hole" left behind by a freed electron, and once dissociated into an electron and hole, they can create an electrical current.
The finding has huge implications for solar cells, because it suggests a path toward solar cells that will more effectively convert the photons in sunlight into the electrons that flow out of the cells as electrical power.

The NREL team discovered the multiple-exciton effect using silicon quantum dots obtained from Innovalight, Inc., and exposing them to solar wavelengths less than 420 nm, that is, from violet visible light into the ultraviolet. Until this discovery, the creation of multiple excitons from one photon had only been observed using quantum dots of materials not commonly used in solar cells, presenting significant challenges for capitalizing on the discovery with an actual working device. Finding the phenomenon in silicon quantum dots avoids those challenges, because silicon is the material used for the great majority of solar cells that are sold today. The research also validated a prediction made by NREL Senior Research Fellow Arthur Nozik in 1997.

"This is a significant scientific advance," says Nozik, "but for this discovery to be converted into a useful and technologically important solar cell, it is necessary to dissociate the excitons and collect the resulting free electrons and holes with high efficiency. Doing so could produce a next-generation photovoltaic cell that exhibits both high power conversion efficiency and low cell cost."

In other words, it's one thing to find a material that efficiently produces excitons from sunlight, but it's another thing altogether to build a device that can collect those electrons and deliver them to the electrical terminals of the solar cell, rather than losing them as they recombine with other charge carriers in the solar cell. NREL, Innovalight, and other laboratories are tackling that challenge, and they know that the payoff could be great.

Calculations at NREL by Nozik and Scientist Mark Hanna have shown that the maximum theoretical conversion efficiency of such quantum dot solar cells would be about 44 percent under normal sunlight and about 68 percent under sunlight concentrated by a factor of 500 with special lenses or mirrors. That's a big leg up over today's conventional solar cells, which have maximum theoretical efficiencies of 33 percent and 40 percent, respectively, under the same solar conditions.

Improving the Efficiency of Organic Solar Cells

While Nozik and his team are working to build a better silicon solar cell, Sean Shaheen is leading an effort to produce inexpensive solar cells made from plastics that can conduct electricity. Like quantum dots, these organic solar cells generate excitons, which migrate to a nearby boundary and then separate, yielding free charges that can produce electrical current. Excitons won't migrate far, though, so these organic solar cells require nanostructures to help capture the excitons before they recombine with other charge carriers.

The lab has been working with Konarka Technologies to create inexpensive plastic solar cells with conversion efficiencies exceeding 5 percent, which is high for plastic solar cells. The trick is to incorporate fullerenes into the plastic to act as sites where excitons can separate into charge carriers.

Boosting the Capacity of Lithium-Ion Batteries

Nanotech could be a key factor in advancing solar cell technologies, but it could also play an important role in tomorrow's efficient vehicles. An NREL-funded effort to create metal oxide nanomaterials could significantly boost the capacity of today's lithium-ion batteries, the leading contender in next-generation battery technology for plug-in hybrid vehicles.

"With these new nanostructured materials, we've come close to achieving an energy storage capacity that is double the capacity of graphite, which is the commercially employed anode material," says scientist Anne Dillon. "Furthermore, no degradation has been observed upon significant laboratory testing, indicating the films are very durable."

Dillon has been working with Harv Mahan (and previously, with former employee Se-Hee Lee) to make anodes for lithium-ion batteries using nanoparticles of molybdenum oxide. The process starts with hot-wire chemical vapor deposition (CVD), a technique originally pioneered by NREL to deposit amorphous silicon films for solar cells, and subsequently adapted to deposit carbon nanotubes for a variety of applications. In this current adaptation of hot-wire CVD, a molybdenum filament is heated to a white-hot temperature, about 2,000°C, inside a quartz tube filled mostly with argon, an inert gas.

Introducing a small amount of oxygen causes the surface of the filament to evaporate, resulting in nanoparticles of molybdenum oxide, which form in the gas phase and condense on the quartz surface as a powder. Once removed, this powder can then be suspended in a methanol solution, and when a voltage is applied, it will deposit in thin, porous films.

Because the oxide films are actually made of millions of tiny particles, they have a high surface area, which allows them to act extremely efficiently as an anode, or negative terminal, of a lithium-ion battery. The team plans to apply a similar approach to the battery's positive terminal, or cathode.

One advantage of the nanoparticle terminals is their sponge-like texture, which allows them to easily expand and contract. That creates the potential to build a solid-state lithium-ion battery, replacing the battery's liquid electrolyte with a polymer.

As that polymer expands and contracts with temperature, the nanoparticle terminals could flex along with it; graphite, in contrast, is relatively brittle. The goal is to increase not only the energy capacity of the battery, but also the speed at which it can produce electricity, a factor known as its rate capability.

"If lithium-ion batteries are ever going to be used for plug-in hybrids, you need to improve the rate capability, and our nanostructured materials have shown a better rate," says Dillon.

All of these efforts are geared toward advances that could yield the next clean energy revolution, whether it is highly efficient silicon solar cells, ultra-cheap plastic solar cells, or high-capacity lithium-ion batteries. Any of these could be disruptive technologies: game-changing advances that turn the energy field upside down. And, like many of the technological advances that enrich our lives, nanotechnology will be a key factor in any of these discoveries.

The majority of this article was excerpted from NREL's 2006 Research Review, which was published in July. The 2004 Research Review also features more information about NREL's nanotech work. Both publications are available on the NREL Research Review Web site: www.nrel.gov/research_review/.

Kevin Eber is a writer for NREL and the. DOE Office of Energy Efficiency and Renewable Energy This work has been authored by an employee of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.