Ohio State study: Silver nanoparticles give polymersolar cells a boost

COLUMBUS, Ohio -- Small bits of metalmay play a new role in solar power.

Researchers at Ohio State University areexperimenting with polymer semiconductors that absorb the sun's energy andgenerate electricity. The goal: lighter, cheaper, and more-flexible solarcells.

They have now discovered that adding tiny bits ofsilver to the plastic boosts the materials' electrical currentgeneration.

Paul Berger, professor of electrical and computerengineering and professor of physics at Ohio State, led the team thatreported the results online in the journal Solar Energy Materials andSolar Cells.

Berger and his team measured the amount of lightabsorbed and the current density–theamount of electrical currentgenerated per square centimeter–genenerated by an experimental solarcell polymer with and without silver nano-particles.

Without silver, the material generated 6.2milli-amps per square centimeter. With silver, it generated 7.0–anincrease ofalmost 12 percent.

The small silver particles help the polymer capturea wider range of wavelengths of sunlight than would normally be possible,which in turn increases the current output, Berger explained.

He added that with further work, this technologycould go a long way toward making polymer solar cells commerciallyviable.

"The light absorption of polymer solar cells isinadequate today," he said."The top-performing materials have an overall efficiency of about 5percent.Even with therelatively low production cost of polymers compared to other solar cellmaterials, you'd still have to boost that efficiency to at least 10percent to turn a profit. One way to do that would be to expand the rangeof wavelengths that they absorb. Current polymers only absorb a smallportion of the incident sunlight."

The new fabrication technique involves encasingeach silver particle in an ultra-thin polymer layer–a different polymerthan the light-absorbing polymer that makes up the solar cell–beforedepositing them below the light-absorbing polymer; the coating preventsthe silver particles from clumping, but also allows them to self-assembleinto a dense and regular mosaic pattern that Berger believes is key toenhancing the light absorption.

Even though the silver particles allow the materialto produce 12 percent more electrical current, that improvement may nottranslate directly into a12 percentincrease in overall solar cell efficiency. Many factors effect efficiency,and some energy can be lost.

Still, the silver nanoparticles could boost theoverall efficiency of virtually any kind of solar cell–those made frompolymers or other semiconductor materials. Berger and his colleagues arenow studying other nanoparticle formulations that would increase thatnumber further.

"By changing the organic coating, we could changethe spacing of the particles and alter the size of each particle. Byfine-tuning the mosaic pattern, we could move the enhanced absorption todifferent wavelengths, and thus get even more of an improvement. I thinkwe can get several percent more," he said.

The semiconductor polymer captures more lightbecause the metal nanoparticles absorb light that would normally bewasted. This extra light energy excites electrons in the metal particles,creating electron waves called plasmons -- a cross between plasma andphotons. The plasmons dance across the surface, depositing energy insidethe solar cell that would otherwise be lost.

Researchers have been looking for a way to generateplasmons in solar cells without greatly increasing the difficulty and costof manufacture. Given that his technique uses simple fabrication equipmentat room temperature, and given that the silver particles self-assemblebased only on the chemistry of the coating, Berger feels that anylaboratory could easily make use of this finding.

"Not only do week seek better efficiency, but alsolower costs too," he added.

His co-authors on the paper include student Woo-JunYoon, who is conducting this work for his doctoral degree; FernandoTeixeira, associate professor of electrical and computer engineering; andJiwen Liu, Thirumalai Durasisamy, Rao Revur, and Suvankar Sengupa–all ofMetaMateria Partners, LLC, formerly of Columbus, Ohio, which coated thesilver nano-particles with polymer.

This work was funded by the Wright Center forPhotovoltaics Innovation and Commercialization, and the Institute forMaterials Research at Ohio State.
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For enhanced coverage,see: http://researchnews.osu.edu/archive/solarsilver.htm