The future of solar: cleaner process, better cells

Future innovations in solar energy could be percolating in at least two Missouri S&T labs, where researchers Lifeng Zhang and Jay A. Switzer are working on separate projects.


For Zhang, an assistant professor of materials science and engineering, the research involves recycling silicon waste created through the manufacturing of solar cells. Switzer, meanwhile, is figuring out how to “grow” atomic-scale zinc oxide crystals on top of silicon, a process that could lead to more efficient solar cells.
While solar power is touted as a green alternative energy source, the manufacturing process creates a lot of waste. As much as half the silicon used to make solar wafers ends up discarded as a silicon slurry. “There’s no reason to waste it,” Zhang says. “We have to find a way to recycle it.”
To do so, the silicon powders must be separated from bigger, non-recyclable particles in the slurry. One promising method may involve electromagnetic separation. Silicon, like metal, is conductive in a liquid state. Zhang proposes liquefying it at high temperatures. He says electromagnetic equipment could be used to separate the non-conductive particles from the heated slurry. The silicon slurry would then be considered “clean.”
The project is officially funded for one year through the American Recovery and Reinvestment Act and support from private partners. If Zhang can prove his methods work, there is a good chance that the research will continue to receive support for another three years.
While Zhang works on cleaning up the process, Switzer is leading an effort to make a new nanomaterial that could be used for future solar cells. Switzer, the Donald L. Castleman/Foundation for Chemical Research Professor of Discovery in Missouri S&T’s chemistry department, reported recently in the journal Chemistry of Materials that his process for growing zinc oxide on silicon could also lead to new materials for ultraviolet lasers, solid-state lighting and piezoelectric devices.
Switzer likens the process to “kind of like growing rock candy crystals on a string.” Instead of using sugar water and string, however, the researcher grows the zinc oxide “nanospears” on the single-crystal silicon placed in a simple wet lab environment, using a beaker filled with an alkaline solution saturated with zinc ions. This process yields tilted, single-crystal, spear-shaped rods that grow out of the silicon surface, like tiny spikes.
Zinc oxide is a semiconductor that 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 at 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.
“You can absorb more light and possibly get more power out” with a zinc oxide-silicon solar cell, he says.
Switzer’s 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.
Switzer’s co-authors for the Chemistry of Materials paper are Guojun Mu and Rakesh V. Gudavarthy, both graduate students in chemistry, and Elizabeth A. Kulp, Chem’00, PhD Chem’09, a postdoctoral associate.

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