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Solar power from silver ‘fractal trees’

UC DAVIS (US) — Microscopic “fractal trees” grown from silver could be the basis of a new type of solar cell, chemists say.

Fractals are patterns that repeat over multiple length scales. In this case, branches of silver 1-50th the width of a human hair are themselves branched, and smaller branches grow on those branches, forming a treelike pattern.

“We expect these structures will allow us to make better, more efficient solar cells,” says Frank Osterloh, professor of chemistry at the University of California, Davis.

In a solar cell application, the silver trees are coated with light-absorbing polymers. When light particles (photons) hit the polymer coat, they produce short-lived electrons and holes in the polymer. The positively charged holes are collected through the silver branches, while the electrons move to the counterelectrode to create an electrical potential.

Osterloh compared the structures to real trees, which use a fractal structure of branches to twigs to spread a wide canopy of leaves for sunlight collection. Similarly, the nanosized silver trees will have a large surface area.

Osterloh’s lab will fabricate the cells, which will be characterized by collaborators Sean Shaheen at the University of Denver and Richard Taylor at the University of Oregon. Boaz Ilan, UC Merced, will carry out computer modeling on the systems.

Funding came from the Research Corporation for Scientific Advancement, which has funded Osterloh’s solar energy research since 2010. The new project was one of three “spontaneous proposals” funded during the organization’s investigators meeting last fall.

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