Plasmonics trap the light fantastic

STANFORD (US) — A new type of thin solar cell harnesses plasmonics to more effectively trap light and improve performance.

“Plasmonics makes it much easier to improve the efficiency of solar cells,” says Mike McGehee, associate professor of materials science and engineering at Stanford University.

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.

“Using plasmonics we can absorb the light in thinner films than ever before.  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.”

The research is reported in the journal Advanced Energy Materials.

The scientists’ lightbulb moment 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 the waffle, the scientists 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 and then layered on a light-sensitive dye that oozes into the dimples and pores of the waffle. Lastly, the engineers added 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.

The domes of silver are crucial. Without them, reflectors won’t produce the desired effect. And the nanodomes must be just the right diameter and height, and spaced just so, to fully optimize the plasmonics.

Photons enter and pass through the transparent base and the titania (the waffle), at which point some photons are absorbed by the light-sensitive dye creating an electric current.

Most of the remaining photons would hit the silver back reflector 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.

Titania within the solar cell is imprinted by the silicon nanodomes like a waffle imprinted by the iron. (Credit: Michael McGehee)

It is easy to see why researchers are focused on thin-film solar technology, McGehee says. In recent years, the focus has been directed toward these lightweight, flexible cells that use photosensitive dyes to generate electricity, because of their many advantages.

They are less energy intensive and less costly to produce, flowing like newsprint off huge roll presses. They are thinner even than other “thin” solar cells. They are also printable on flexible bases that can be rolled up and taken virtually anywhere. Many use non-toxic, abundantly available materials, as well, a huge plus in the push for sustainability.

Dye-sensitized solar cells are not without challenges, however. First off, the very best convert only a small percentage of light into electricity—about 8 percent.

The bulkier commercial technologies available today have reached 25 percent efficiency, and certain advanced applications have topped 40 percent. And then there is durability. The latest thin solar cell will last about seven years under continuous exposure to the elements. Not bad until you consider that 20 to 30 years is the commercial standard.

Both efficiency and reliability will have to improve. Nonetheless, McGehee believes 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, personal solar cells may be a possibility.

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