Advancing Basic Science for Humanity
Exploring the Energy Frontier
IN AN EFFORT to spur the development of new energy-related technology, the federal government has established 46 Energy Frontier Research Centers (EFRC) at universities, national laboratories, corporations and non-profit organizations. Scientists affiliated with the Kavli Prize or Kavli Institutes play leading roles in three of these EFRCs. Located at the California Institute of Technology, Columbia University, and the University of California, Santa Barbara, here is a look at these researchers and their centers.
Columbia University – Breaking the Solar Efficiency Barrier
Louis E. Brus, the Samuel Latham Mitchell Professor of Chemistry at Columbia University, is part of a select group – the first Kavli Prize laureates. With Sumio Iijima, Brus was awarded the inaugural Kavli Prize for nanoscience in 2008 for his pioneering work on nanocrystals, also known as “quantum dots.” Now he is scientific co-director (with Tony Heinz, professor of physics and electrical engineering) of Columbia University’s new EFRC. In his new role, Brus is putting his expertise to work on the problem of raising the efficiency of solar cells.
Photovoltaics, the direct conversion of sunlight into electricity, promises someday to become a source of inexhaustible clean energy, but first it must jump at least two hurdles. One is cost. Solar power is several times more expensive than electricity from conventional sources. The second is inefficiency. After decades of work on new and improved cells, the best cells currently produced commercially can convert only about 25% of the light that hits them into electricity.
Brus and his colleagues are attacking the efficiency challenge by studying the whole photovoltaic process, from generation of electric charges to their collection and transport. Over the five-year term of their EFRC grant, they will develop and test nanostructured materials to see which ones improve the process at each step.
The Columbia-based center, which also includes researchers from the University of Arkansas, University of Texas, Purdue University and the Brookhaven National Laboratory, sums up its work in three “research thrusts.” One focuses on charge generation, the transfer of photon energy to electrons and their corresponding “holes” (empty spaces in an atom’s valence band). Another examines charge collection, the conversion of the electrons’ excited state to usable electric current. The third looks at “carrier multiplication,” where a single photon generates more than one electron-hole pair (“exciton”) and hence more than one electrical charge. The overall goal is to push the inherent efficiency of solar cells beyond the Shockley-Queisser Limit, which is just over 30% for a cell made up of one semiconducting material.
“We have a number of schemes that might lead to a number of more efficient solar cells,” Brus says. Some of these make use of organic materials such as grapheme nanoribbons and carbon nanotubes. Nanocrystals, Brus’s specialty, also are at center stage. Brus’s discovery of nanocrystals in the early 1980’s was recognized worldwide with his receipt in 2008 of the Kavli Prize in Nanoscience in 2008. These structures, larger than normal molecules but smaller than bulk crystals, “are a possible chromophore for absorbing the sun’s light,” he says, possibly boosting solar-cell efficiency by concentrating photons more effectively than materials currently in use. Nanocrystals are very good at both absorbing and emitting light, which is why they have been put to use in biological imaging at the cellular level. Brus says the center will focus on understanding the scientific principles that allow the extraction of one electron and hole from each photo-excited nanocrystal. Nanocrystals are also involved in the search for methods of multiplying charge carriers – the center’s third thrust.
Will all this research produce a breakthrough? Brus says it’s more realistic to hope for “incremental gains in efficiency,” at least for the short-term. He sees solar-cell efficiency as a complex problem that will not be solved by a single invention. “This is not like the Manhattan Project,” Brus states. But in the longer run, he adds, “fundamental things can change.” Toward that end, the center will see seek new discoveries in nanoscience that might lead to new solar-cell design principles. This wide-ranging work on the fundamentals of photovoltaics could lay the groundwork for entirely new technologies.
California Institute of Technology – Guiding Light
Harry A. Atwater, a Caltech scientist and board member of the Kavli Nanoscience Institute (KNI) at the California Institute of Technology, leads “Light Material Interactions in Energy Conversion,” an EFRC that concentrates on the light-gathering aspect of solar energy. Atwater says the Center aims to “sculpt the flow of light through materials” by designing nanostructures that both absorb and steer sunlight.
This front-end focus on light manipulation is a departure from most solar-energy research, which has mostly concentrated on the back end – how best to get the light-energized electrons out of the solar cells and into electric lines. “The area of light-matter interactions has long been underemphasized in the solar energy community,” Atwater says. Research on light-gathering nanomaterials has been booming in other disciplines, such as computing and biological sciences, and Atwater wants to see what this fast-advancing technology can do for photovoltaics.
KNI has been a leader in nanophotonics, which studies how light can be used instead of electric current for devices such as sensors and micro-circuit switches. The young field of plasmonics is showing how metals can guide and scatter light at scales smaller than a wavelength of light. New “metamaterials” -- artificial materials with properties not readily found in nature – are able to direct light at the sub-wavelength scale and possibly alter its speed.
The new Caltech-based EFRC, which collaborates with Lawrence Berkeley National Laboratory and the University of Illinois, will tap into all of these rapidly developing fields. Crystals developed in nanophotonics, for instance, might be used in a so-called “rainbow concentrator” that breaks up sunlight into different wavelengths and guides the separated beams toward specialized cells. Plasmonics may open up new uses for the metal surfaces that are already part of solar cells. “Every solar cell has a metal contact,” says Atwater, and plasmonics may show the way for using these contacts – not just to pick up electrical current, but also to manipulate light.
“In our center we are in some sense agnostic about what material we use, because the general principles we are developing for light manipulation can be applied to almost all solar energy technologies,” Atwater says. He sees the EFRC as “a national resource for understanding light-matter interactions.”
Atwater’s new role leading a center on photovoltaics is a natural outgrowth of his scientific career, which from the start has been focused on solar energy. “The solar energy world is divided into the long-term true believers and those who have recently beaten a path to the doorstep,” he says. He is one of the true believers. Growing up in the 1970s in Pennsylvania, he experienced gasoline rationing and saw his school shut down for a month because of a shortage of heating oil. “That made a profound impression on me,” he recalls. He studied solar energy in the 1980s at MIT before joining the Caltech faculty in 1988, and has “retained an interest in solar energy at some level” throughout his career. In 2007, he co-founded Alta Devices Inc., which develops technology to cut the cost and raise the efficiency of thin-film photovoltaics.
University of California, Santa Barbara – Novel Materials from Land and Sea
The University of California, Santa Barbara’s (UCSB) EFRC, the Center for Energy-Efficient Materials, taps into the same cross-disciplinary research seen at UCSB’s Institute for Energy Efficiency (IEE). No surprise here, since a number of scientists and engineers are involved in both the Center and the IEE, and Center’s director, John E. Bowers, also leads the IEE.
Bowers, who is the Fred Kavli Chair in Nanotechnology at UCSB, says the new Center has “a more basic research focus” than the IEE, but it incorporates much of the IEE’s mission and talent. Its three primary research areas also are key topics at the IEE. One is the development of low-cost, high-efficiency materials for photovoltaics. These include flexible, paintable organic solar cells pioneered by Alan Heeger, a UCSB physicist and Nobel Laureate. In addition, the Center is looking at so-called “bio-inspired” materials for solar power, similar to the elegantly-woven silicon nano-structures created by sea sponges. The Center’s leading expert in this area is Daniel Morse, a UCSB professor of molecular, cellular and developmental biology.
Another focus is solid-state lighting. Researchers here include UCSB Materials Professor Shuji Nakamura, who pioneered the development of the first practical blue LED that helped lead to the development of white LED lighting. They hope to boost the output of LED lighting to more than 200 lumens per watt, about 10 times that of incandescent lights. Bowers says one of their challenges is to remedy the “droop” in efficiency of LED lighting in the green region of the spectrum.
The Center’s third research thrust is the study of novel materials for efficiency in both the production and use of energy. These include better heat conductors to solar-thermal generators and new materials that can be used to produce electricity from the waste heat from automobile engines.
UCSB has four collaborators at its EFRC. They are the National Renewable Energy Laboratories of the U.S. Department of Energy, the Los Alamos National Laboratory, UC Santa Cruz and Harvard University.
For more on Bowers and the IEE, see the recent Spotlight story: Efficiency Expert.