Quantum Dots and Smart Materials the Future of Solar Energy

WASHINGTON, DC, August 16, 2005 (ENS) - Reactors heated by focused, concentrated sunlight in thermal towers that reach temperatures over 3,000 degrees Celsius ... nanostructured materials such as quantum dots ... solar cells that achieve 50 percent efficiency - these are not the stuff of science fiction. These technologies are predicted to be the reality of solar power by mid-century.

A workshop of 200 scientists from the United States, Europe and Asia met in April to examine the challenges to developing solar energy as a competitive energy source and to pinpoint the basic research directions that show promise. The cross-disciplinary group of solar energy scientists spanned academia, national laboratories, government, and industry.

Their report, issued Friday, identified 13 priority research directions with the “potential to produce revolutionary, not evolutionary, breakthroughs in materials and processes for solar energy utilization.”

More energy from sunlight strikes the Earth in one hour than all the energy consumed on the planet in a year, and world demand for energy is projected to more than double by 2050 and to more than triple by the end of the century, the scientists said.

They called sunlight is "a compelling solution" to our need for clean, abundant sources of energy because it is readily available, free from geopolitical tension, and poses no threat to our environment through pollution or to our climate through greenhouse gas emissions.

Emerging with "a sense of optimism" from the four day workshop, the scientists said the technology to bridge the gap between our present use of solar energy and its undeveloped potential "defines a grand challenge in energy research."

Bridging this gap requires revolutionary breakthroughs that come only from basic research, they said. "We must understand the fundamental principles of solar energy conversion and develop new materials that exploit them."

Workshop chair Dr. Nathan Lewis is a chemistry professor at the California Institute of Technology. (Photo courtesy CalTech)

Workshop participants discussed solar energy conversion systems in three categories - solar electricity, solar fuels and solar thermal systems.

Dr. George Crabtree is Materials Science Division director at Argonne National Laboratory. (Photo courtesy Argonne)

A major challenge is tapping the full spectrum of colors in solar radiation. The absorbing materials in the current generation of photocells and, artificial photosynthetic machines typically capture only a fraction of the wavelengths in sunlight.

Designing composite materials that effectively absorb all the colors in the solar spectrum for conversion to electricity, fuel, and heat would be a crosscutting breakthrough, the scientists said.

Then the captured solar energy "must be transported as excited electrons and holes from the absorber to chemical reaction sites for making fuel or to external circuits as electricity," the scientists explained.

"Nature transmits excited electrons and holes without energy loss through sophisticated assemblies of proteins whose function we are just beginning to understand with genome sequencing and structural biology," they said.

Today's rapid advances on the scientific frontiers of nanoscience and molecular biology provide a strong foundation for future breakthroughs in solar energy conversion, the workshop participants agreed.

An electronic circuit fabricated on a flexible plastic substrate. Solar cells are being developed that can be integrated with such organic electronic devices. (Photo Nicole Cappello courtesy Georgia Tech)

The vast majority of solar panels today are made of silicon. These devices are called first generation, and make for highly stable and efficient solar cells, but, because of the material processing necessary, it is expensive to make first generation solar cells, and levels of efficiency in electricity production range from around 10 to 20 percent.

A more recent alternative involves constructing solar cells using thin films with the potential to produce solar energy at a reduced cost. These thin film cells are called second generation, and are cheaper, but they have more difficulty absorbing radiation and are not very efficient.

Scientists have been seeking a third generation - a low cost semiconductor material that would have a tunable bandgap, allowing the manufacturer to control the absorptive properties of the solar cell. Quantum dots appear to fill the bill.

Quantum dots are semiconductor crystals typically between 1 and 10 nanometers in diameter, a nanometer being a billionth of a meter. Each quantum dot contains a tiny droplet of free electrons.

A contoured structure of quantum dots, with the blue dots extending farthest from the base. (Photo courtesy NREL Solid State Theory Group)

"Quantum dots are especially exciting for their tunable absorption wavelength, their quantum conversion efficiency above 100% through multiple-exciton generation, and their easy fabrication through self-assembly," the workshop scientists said.

Quantum dots can be made into flexible sheets, put into liquid form, or made to be transparent, and they cost relatively little compared with bulk silicon semiconductor material and thin films.

One manufacturer, Evident Technologies, says quantum dots can theoretically achieve the third generation goal of greater than 60 percent efficiency at $100 or less per square meter of paneling that would be necessary to make photovoltaic solar cells economically competitive with other forms of energy.

In addition to electric energy, solar radiation can be converted to heat energy, and the scientists concluded that solar thermal conversion can replace much of the heat now supplied by burning fossil fuels such as oil, gas and coal.

Solar concentrators focus sunlight collected over a large area to a line or spot where heat is collected in an absorber.

A 10-kilowatt prototype of the Advanced Dish Development System being evaluated at the National Solar Thermal Test Facility at Sandia National Laboratories in Albuquerque, New Mexico. May 2004. (Photo courtesy Sandia)

But research is needed to develop thermal storage materials that accumulate heat efficiently during sunny periods and release heat slowly during dark or cloudy periods, the scientists said.

Their report notes that progress in research proposed at the workshop also could lead to artificial "molecular machines" that turn sunlight into chemical fuel inexpensively and efficiently.

Basic research could enable scientists to coax cheap materials to perform as well as expensive materials, they said.

They believe "smart materials" could be developed based on nature’s ability to transfer captured solar energy with no energy loss. They forsee self-repairing solar conversion systems and new materials for high-capacity, slow-release thermal storage.

The Bush administration supports the development of solar energy, said Dr. Raymond Orbach, director of the U.S. Energy Department's Office of Science.

"The tax credits contained in the historic energy bill signed by President [George W.] Bush will greatly help expand the use of renewable energy," Orbach said. “This research will help improve a critical component of renewable energy, solar technology, in the future."