Plasmonics
Researchers in solar energy speak of a day when millions of otherwise fallow square meters of sun-drenched roofs, windows, deserts, and even clothing will be integrated with inexpensive solar cells that are many times thinner and lighter than the bulky rooftop panels familiar today. So, when your iPod is on the nod, you might plug it into your shirt to recharge. Lost in the Serengeti with a sapped cell phone? No problem, rolled in your backpack is lightweight solar pad. Sailing the seven seas and your GPS needs some juice? Hoist a solar sail and be one with the gods of geosynchronous orbit. It is not hard to envision a time when such technologies will be ubiquitous in our increasingly energy-hungry lives. That day may come a bit sooner thanks to a multidisciplinary team of Stanford engineers led by Mike McGehee, Yi Cui and Mark Brongersma, and joined by Michael Graetzel at the École Polytechnique Fédérale de Lausanne (EPFL).
Waves of Energy
The Stanford/EPFL team succeeded in harnessing plasmonics – an emerging branch of science and technology – to more effectively trap light within a type of thin solar cell to improve performance and push them one step closer to daily reality. “Plasmonics makes it much easier to improve the efficiency of solar cells,” explains McGehee, associate professor of Materials Science and Engineering at Stanford. McGehee is also director of CAMP – the Center for Advanced Molecular Photovoltaics – a multi-disciplinary, multi-university team tackling the challenges of thin-film solar cells from several angles. “Using plasmonics we can absorb the light in thinner films than ever before,” McGehee noted further. “The thinner the film, the less material it takes to make them, and the less they cost. Perhaps most important, the closer the charge carriers are to the electrodes. In essence, the electrons have a shorter distance to travel to become electricity.” Plasmonics is an emerging field of science and technology that aims to shrink current photonic devices to the nanoscale by using metals rather than dielectrics, like the glass in telecommunications fiber optics. Researchers are discovering that metal can exert new levels of control over light and are using this knowledge to tame photons in nanoscale devices. Everyone encounters the strong interaction of light and metal on a daily basis when looking into a bathroom mirror. The shiny metal in the mirror contains many electrons that freely move about. When light strikes a mirror, it creates electric fields that cause these electrons to oscillate, sending a wave of light back that you see as a reflection. When light illuminates a nano-patterned metallic structure, this electron oscillation scatters the light in many directions. “This scattering effect is particularly promising in solar cells. Plasmonics researchers are engineering increasingly complex metallic nanostructures to manipulate the flow of light at the nanoscale with ever-increasing control,” said Mark Brongersma, associate professor of Materials Science and Engineering at Stanford. Many fields of study utilize plasmonics. In computing, for instance, the field garners attention because the high frequencies might someday allow data transmission along metal wires at rates competitive with those of fiber optics. In solar cells, as this new finding shows, plasmonics can help photo-sensitive materials absorb more of the all-important light that strikes the cell.
Lightbulb Moment – A Perfect Solar Waffle
The lightbulb moment for the team came when they imprinted a honeycomb pattern of nanoscale dimples into a layer of metal oxide within the solar cell. Think of it as a nanoscale waffle, only the bumps on the waffle iron are domes rather than cubes – nanodomes to be exact, each only a few billionths of a meter across. To fashion their waffle, McGehee and team spread a thin layer of batter on a transparent, electrically conductive base. This batter is mostly titania, a semi-porous metal oxide that is also transparent to light. Next, they used their nano waffle iron to imprint the dimples into the batter. Next, they layer on some butter – a light-sensitive dye – which oozes into the dimples and pores of the waffle. Lastly, the engineers add some syrup – a layer of silver, which hardens almost immediately. When all those nanodimples fill up the result is a pattern of nanodomes on the light-ward side of the silver. This bumpy layer of silver has two primary benefits that improve the harvesting of light. First, it acts like a fogged mirror, scattering unabsorbed light in all directions back into the dye for another shot at collection. Second, the light interacts with the silver nanodomes to enable plasmonic light-concentrating effects. Those domes of silver are crucial. Reflectors without them will not produce the desired effect. And any old nanodomes won’t do either, they must be just the right diameter and height, and spaced just so, to fully optimize the plasmonics. If you imagine your nanoself observing one of these solar cells in slow motion, you would see photons entering and passing through the transparent base and the titania (the waffle), at which point some photons will be absorbed by the light-sensitive dye (the butter) creating electric current. The remainder will hit the silver back reflector (the hardened syrup) and bounce back into the solar cell. A certain portion of the photons that reach the silver layer, however, will strike the nanodomes and cause plasmonic waves to course outward. And there you have it – the first ever plasmonic dye-sensitized solar cell.
A Quest for Light
It is easy to see why researchers are focused on thin-film solar technology. In recent years, much hope and research has been directed toward these lightweight, flexible cells that use photo-sensitive dyes to generate electricity. These so-called “dye-sensitized solar cells” have many advantages: They are less energy intensive and less costly to produce, flowing like newsprint off of huge roll presses. They are thinner than other thin-film solar cells. They are also printable on flexible bases that can be rolled and taken virtually anywhere. Many use non-toxic, abundantly available materials, as well – huge plusses in the push for sustainability. Dye-sensitized solar cells are not without challenges, however. First off, the very best of these cells convert only a small percentage of the sunlight that reaches them into electricity – about 8 percent. The bulkier, silicon-based commercial technologies available today have reached 25 percent efficiency, and certain advanced applications have topped 40 percent; hence the push to increase the absorption of light. And then there is the question of durability. The latest thin solar cell will last about 7 years under continuous exposure to the elements. Not bad until you consider that 20 to 30 years is the commercial standard. Needless to say, work remains for thin-film solar cells. Both efficiency and reliability will have to improve if the field is to take off. Nonetheless, engineers like McGehee believe that if they can convert just 15 percent of the light into electricity – a figure that is not out of reach – and tease the lifespan to a decade or more, we might soon find ourselves in the age of personal solar cells. An advance like plasmonics just might provide the spark necessary to take the field down a new and exciting path.
A Matter of Economics
Cheaper and cleaner will be the keys. Coal-based power is plentiful and cheap, but also comes at a steep environmental cost in gouged landscapes and polluted skies. At today’s commercial rates, however, even the best solar alternatives cost five times more per kilowatt hour than coal. It is clear that economics, and not technology, are what stands between us and our solar future. McGehee and others are confident they can make thin solar cells more attractive. And the solar future will one day arrive, the question is only: How soon? While no one can answer for certain, you can rest assured that Mike McGehee and the team at CAMP will be dancing just as fast as they can, trapping the light fantastic.
Sharing Ideas, Harvesting Light
As promising as McGehee’s work is, this is really less the story of technology and more a tale of teamwork. The team at Center for Advanced Molecular Photovoltaics (CAMP) combines experts in quantum physics, materials, chemistry and photonics and many other sciences at noted universities across the globe to seek out lighter, more durable, more efficient materials, better and less costly production techniques, and new scientific avenues, all in a quest to harvest light. McGehee, for instance, is noted for his pioneering work on the physics and materials of thin-film solar technology. Cui is a rising star in nanotechnology. Brongersma is an expert in plasmonics. And, from half-a-world away, the man who introduced the world’s first dye-sensitized solar cell in 1991, EPFL’s Michael Graetzel, contributed his considerable expertise. “This innovation would not have been possible without a multidisciplinary approach,” says McGehee. “Engineers had developed plasmonic solar cells before, although they had not been very effective. Dye-sensitized cells have been around since Graetzel, almost exactly 20 years ago. Somehow, no one had merged the two to significantly improve a solar cell. This is an advance with roots in several fields – materials science, nanotechnology, physics, and photonics to name a few.”
