While these carbon-based cells, which use organic polymers or small molecules as semiconductors, are much thinner and less expensive to produce than conventional solar cells made with inorganic silicon wafers, they still lag behind in their ability to efficiently convert sunlight into electricity.

Now, the University of California, Los Angeles (UCLA) researchers and their colleagues from China and Japan have shown that by incorporating gold nanoparticles into these organic photovoltaics they can significantly improve the cells' power conversion.

The team of researchers, led by Yang Yang, a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science and director of the Nano Renewable Energy Center at UCLA's California NanoSystems Institute, demonstrate how they sandwiched a layer of gold nanoparticles between two light-absorbing subcells in a tandem polymer solar cell in order to harvest a greater fraction of the solar spectrum.

They found that by employing the interconnecting gold-nanoparticle layer, they were able to enhance power conversion by as much as 20 percent.

The gold nanoparticles create a strong electromagnetic field inside the thin organic photovoltaic layers by a plasmonic effect, which concentrates light so that much more of it can be absorbed by the subcells.

“We have successfully demonstrated a highly efficient plasmonic polymer tandem solar cell by simply incorporating gold nanoparticles layer between two subcells," Yang said. "The plasmonic effect happening in the middle of the interconnecting layer can enhance both the top and bottom subcells simultaneously - a sweet spot - leading to an improvement in the power conversion efficiency of the tandem solar cell from 5.22 percent to 6.24 percent. The enhancement ratio is as high as 20 percent.”

Experimental and theoretical results demonstrate that the enhancement effect was attained from local near-field enhancement of the gold nanoparticles.

The results show that the plasmonic effect has great potential for the future development of polymer solar cells. The team's proposed interlayer structures as an open platform can be applied to various polymer materials, opening up opportunities for highly efficient, multi-stacked tandem solar cells.

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