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Wednesday, 9 November 2011

Nanowires could be solution for high performance solar cells

University of Illinois
Nov 9, 2011
InGaAs: Solar cells (bottom) made with arrays of nanowires. Engineers can tune the performance by using nanowires of differing composition and thickness (top). | Graphic by Xiuling Li

Tiny wires could help engineers realize high-performance solar cells and other electronics, according to University of Illinois researchers.

The research group, led by electrical and computer engineering professor Xiuling Li, developed a technique to integrate compound semiconductor nanowires on silicon wafers, overcoming key challenges in device production. The team published its results in the journal Nano Letters.

Semiconductors in the III-V (pronounced three-five) group are promising for devices that change light to electricity and vice-versa, such as high-end solar cells or lasers. However, they don’t integrate with silicon seamlessly, which is a problem since silicon is the most ubiquitous device platform. Each material has a specific distance between the atoms in the crystal, known as the lattice constant.

“The biggest challenge has been that III-V semiconductors and silicon do not have the same lattice constants,” Li said. “They cannot be stacked on top of each other in a straightforward way without generating dislocations, which can be thought of as atomic scale cracks.”

When the crystal lattices don’t line up, there is a mismatch between the materials. Researchers usually deposit III-V materials on top of silicon wafers in a thin film that covers the wafer, but the mismatch causes strain and introduces defects, degrading the device performance.
Instead of a thin film, the Illinois team grew a densely packed array of nanowires, tiny strands of III-V semiconductor that grow up vertically from the silicon wafer.

“The nanowire geometry offers a lot more freedom from lattice-matching restrictions by dissipating the mismatch strain energy laterally through the sidewalls,” Li said.
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