Zinc oxide nanospears could lead to better solar cells

August 17, 2009--By growing (and precisely aligning) microscopic, spear-shaped zinc oxide crystals on a surface of single-crystal silicon, researchers at Missouri University of Science and Technology (Rolla, MO) may have developed a method to make solar cells more efficient.

August 17, 2009--By growing (and precisely aligning) microscopic, spear-shaped zinc oxide crystals on a surface of single-crystal silicon, researchers at Missouri University of Science and Technology (Rolla, MO) may have developed a method to make solar cells more efficient. Jay A. Switzer and his colleagues report in the journal Chemistry of Materials that their simple, inexpensive process could also lead to new materials for UV lasers, solid-state lighting, and piezoelectric devices.1

"It's kind of like growing rock-candy crystals on a string," says Switzer. But instead of using sugar water and string, Switzer's team grows the zinc oxide "nanospears" on single-crystal silicon placed in a beaker filled with an alkaline solution saturated with zinc ions. The process yields tilted single-crystal rods growing from the silicon surface; the spears are 100 nm to 200 nm in diameter and about 1 micron in length. The research was reported August 11 in Chemistry of Materials' online ASAP ("as soon as publishable") section and will appear in an upcoming issue.

Two materials, two absorption ranges
Zinc oxide, a semiconductor, both absorbs and emits light, so it could be used in solar cells to absorb sunshine as well as in lasers or solid-state lighting as an emitter of light. Silicon is also a semiconductor, but it absorbs light in a different part of the spectrum than zinc oxide. By growing zinc oxide on top of the silicon, "you're putting two semiconductors on top of each other," thereby widening the spectrum from which a solar cell could draw light, Switzer says.

Previous efforts to grow zinc oxide on silicon have been limited to expensive ultra-high-vacuum methods, and because of silicon's high reactivity, it's been impossible to deposit the zinc oxide directly, without the use of a third material as a buffer. In addition, previous attempts to align the two materials epitaxially have been unsuccessful until now. By tilting the nanospears 51 degrees, Switzer and his team have reduced the mismatch from 40% to just 0.2%, a near-perfect alignment. Epitaxially aligning the zinc oxide and silicon is important to ensure higher efficiency, Switzer says.

The research is supported through a four-year, $700,000 grant from the Department of Energy's Office of Basic Energy Sciences, Materials Sciences and Engineering Division.


REFERENCE

1. Guojun Mu et al., Chem. Mater., Article ASAP, DOI: 10.1021/cm9010019
http://pubs.acs.org/doi/abs/10.1021/cm9010019
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--posted by John Wallace, johnw@pennwell.com

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