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Silicon Variant Optimized for Solar Absorption

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WASHINGTON, Nov. 17, 2014 — Natural silicon’s indirect bandgap prevent its use, as is, in next-generation solar technologies. Creating different forms of the material to alter the bandgap could change that.

A team from the Carnegie Institution for Science has developed a new type of silicon using a quasi-direct bandgap that falls within the required range for solar absorption.

This new silicon allotrope consists of an open framework — a zeolite-type structure, rather than a conventional diamond structure — that is comprised of channels with five-, six- and eight-membered silicon rings.

Viewed through the channels of Si24, the new zeolite-type allotrope of silicon has an open framework comprised of five-, six- and eight-membered sp3-bonded silicon rings. Courtesy of DR. Timothy Strobel/Carnegie Institution for Science.

“This is an excellent example of experimental and theoretical collaboration,” said Dr. Duck Young Kim, a research scientist at Carnegie’s Geophysical Laboratory. “Advanced electronic structure theory and experiment have converged to deliver a real material with exciting prospects.”

A two-step process was used to create the new silicon. First, a silicon-sodium compound (Na4Si24) was formed under high pressure. Next, the “sodium was removed from the precursor by a thermal ‘degassing’ process,” the researchers wrote in a study.

The silicon band structure contains open channels along the crystallographic A-axis that are formed from six- and eight-membered sp3 silicon rings. This new allotrope possesses a quasi-direct bandgap near 1.3 eV.

The resulting Si24 (pure silicon allotrope) was found to be stable at ambient pressure to at least 842 °F (450 °C).

“Using the unique tool of high pressure, we can access novel structures with real potential to solve standing materials challenges,” said Dr. Timothy Strobel, a research scientist at the Carnegie Geophysical Lab. “Here we demonstrate previously unknown properties for silicon, but our methodology is readily extendible to entirely different classes of materials. These new structures remain stable at atmospheric pressure, so larger-volume scaling strategies may be entirely possible.”

The work was funded by DARPA and the Energy Frontier Research Center’s EFree program.

The research was published in Nature Materials (doi: 10.1038/nmat4140). 

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Nov 2014

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