A Clean Break

By Anna Lynn Spitzer

Irvine, Calif., April 2009 -- The race is on to wean the U.S. from fossil fuels. Economic, environmental and security concerns are fueling scientific pursuit of technologies that will produce clean and renewable energy to meet growing demand.

Ben Raphael
Matt Law measures the electrical conductivity of a naocrystal film comprised of millions of quantum dot solar cells

The Obama Administration, vowing to double the nation’s supply of renewable energy in the next three years, is allocating $8 billion in the American Recovery and Reinvestment Plan to energy research. Several states are requiring local utilities to acquire a larger percentage of energy from renewable sources; California’s three major public utilities have been mandated to obtain 20 percent of their electricity from clean sources by the end of next year, and a full third of their supply by 2020.

It’s a challenge that ultimately will be measured in dollars and cents. To ensure widespread acceptance, new technologies must be cost-efficient as well as environmentally friendly.

Researchers at UC Irvine are hard at work. They are developing low-cost solar cells from nanomaterials to capture sunlight and turn it into electrical energy. They are re-engineering biofuel production to reduce manufacturing costs, and developing new techniques to burn coal more efficiently. They’re maximizing output on fuel cells that produce electricity with zero emissions. And they’re finding ways to recycle waste from nuclear power plants.

In addition, researchers at UCI’s Advanced Power and Energy Program are investigating several other approaches to generating and distributing power that will lead to a cleaner and more sustainable future.

The six articles in this series detail some of those efforts.

Genome Diagram
An argon laser lights up a nanowire array so Reg Penner can measure the sample's photoluminescence to determine its electronic properties

Let the Sunshine In
Silicon solar cells turn the unlimited energy of the sun into electricity without harming the environment. The technology comes with a steep price tag, however. 
Current solar cells require extremely pure silicon and costly fabrication techniques, putting solar electricity out of reach for most consumers. Nanoscale materials, on the other hand, are potentially cheap to manufacture. They can be produced in large quantities by low-temperature solution methods, processed into inks and deposited onto flexible substrates using roll-to-roll printing.

Calit2-affiliated researchers in UCI’s Center for Solar Energy in the School of Physical Sciences are confident they can develop more efficient, less costly alternatives using these novel materials and innovative construction methods.

Harnessing Sun Power

Solar cells are made from semiconductor materials that have two “bands” of energy where electrons can exist. The lower-energy valence band and the higher-energy conduction band are separated by a bandgap that is devoid of electrons. When a semiconductor absorbs photons from sunlight, negatively charged electrons in the valence band jump across the bandgap to the conduction band. This leaves empty states in the valence band called “holes” that act like positive charges.

Materials with small bandgaps absorb more sunlight and therefore produce a large current but at a low voltage, while a larger bandgap yields a higher voltage but less current. The challenge for researchers is to maximize voltage and current while minimizing cost.

Nanoscale materials could be the answer, according to Matt Law,chemistry assistant professor. Nanoscale generally refers to structures that are 100 nanometers (nm) or smaller, the equivalent of 1/500 the diameter of a human hair. Another way of looking at it: the size of a nanometer when compared to a meter is the same ratio as a marble compared to the size of the Earth.

Law is using these materials in different ways. One is “quantum dots,” a nanocrystal construction that can greatly increase the device’s current without negatively affecting its voltage.

Law’s lab is studying these nanocrystals made from semiconductors like lead selenide and tin telluride.

Although quantum dot solar cells are still in their infancy, Law sees a bright future. “Eventually, we hope to lower the cost per watt [of solar power] by about 10 times,” he says. “That would put it in the same ballpark as making electricity from coal or natural gas.”

Law’s group also experiments with nanorod solar cells. Nanorods are approximately 10-100 nm wide and can be grown vertically on a substrate, like trees in a forest. Sunlight can be absorbed along the entire length of the rod and electrical charges can be collected much more efficiently than with thin films made from the same material.

Other research in Law’s group focuses
on synthesizing new semiconductor materials from Earth-abundant elements like zinc, phosphorus and iron. These elements are readily available, non-toxic and cheap, but for various reasons have not been utilized successfully for solar applications. “A number of these have promise and we’re just beginning to learn how to control their physical and electrical properties,” Law says.

Wired for Efficiency

Reg Penner, center director and chemistry professor, uses another approach to solar energy. His group is investigating ultra-long nanowire arrays that will absorb sunlight on one
side and produce electricity through direct thermal-to-electrical energy conversion. These nanowires utilize the longer rays of the spectrum,generating a voltage spontaneously
when they’re heated at one end.
Although these materials are in use now, they are extremely inefficient, Penner says. Research indicates, however, that if the materials can be formed into wires that are less than
10 nm in diameter, efficiency can be increased enormously, a goal his group is pursuing.

Current testing shows bismuth telluride and lead telluride are the most viable materials. “You have to choose a material that can be efficient at 200 degrees Celsius,” says Penner. “Forming
it into nanowires should make it orders-of-magnitude more efficient.”

Fabrication is tricky. Thousands of the nanowires, each only 10-50 nm in width and millimeters in length, must be fashioned into a device. “The nanowires are very small and fragile,
and very, very long,” Penner says, “and we don’t want them to be in thermal contact with any surface because the thermal gradient needed to generate electricity will leak into the surface if
the nanowires touch it.”

So his research group developed and patented a process called “lithographically patterned nanowireelectrodeposition,” which allows the nanowires to be suspended across air gaps as they are deposited on
photoresist-covered glass wafers.

The devices could stand alone or be used in combination with photovoltaics, says Penner; both ideas will be evaluated.

Consumer devices could be a number of years down the road, but Law and Penner are taking their research one step at a time. “We’re just trying to figure out what works,” Penner says.