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January 31, 2011

The Solar future is nanodomes and plasmonics


Acting like a waffle iron, silicon nanodomes, each about 300 nanometers in diameter and 200 nanometers tall, imprint a honeycomb pattern of nanoscale dimples into a layer of metal within the solar cell.

Stanford engineers dance with plasmonics to yield new direction for thin, inexpensive solar cells.

In an article published in Advance Energy Materials, the Stanford/EPFL team announced a new type of thin solar cell that could offer a new direction for the field. They succeeded in harnessing plasmonics – an emerging branch of science and technology – to more effectively trap light within thin solar cells to improve performance and push them one step closer to daily reality.



Titania within the solar cell is imprinted by the silicon nanodomes like a waffle imprinted by the iron.

"Plasmonics makes it much easier to improve the efficiency of solar cells," said McGehee, an associate professor of materials science and engineering at Stanford.

"Using plasmonics we can absorb the light in thinner films than ever before," McGehee said. "The thinner the film, the closer the charged particles are to the electrodes. In essence, more electrons can make it to the electrode to become electricity."

Plasmonics is the study of the interaction of light and metal. Under precise circumstances, these interactions create a flow of high-frequency, dense electrical waves rather than electron particles. The electronic pulse travels in extremely fast waves of greater and lesser density, like sound through the air.

The lightbulb moment for the team came when they imprinted a honeycomb pattern of nanoscale dimples into a layer of metal 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 members spread a thin layer of batter on a transparent, electrically conductive base. This batter is mostly titania, a semi-porous metal that is also transparent to light. Next, they use 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. First, it acts as a mirror, scattering unabsorbed light back into the dye for another shot at collection. Second, the light interacts with the silver nanodomes to produce plasmonic 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 enter and pass through the transparent base and the titania (the waffle), at which point some photons would be absorbed by the light-sensitive dye (the butter), creating an electric current. Most of the remaining photons would 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, 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.

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, we might soon find ourselves in the age of personal solar cells.

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