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Boosting Solar Cells with Nanowires

Solar module makers are struggling even as cost points are at a bare minimum (and arguably unsustainably so) — and yet lower prices persist, especially in lower-end crystalline silicon (c-Si) modules. However, there is still improvement to be made in efficiency, says Dave Epstein, CEO of Sol Voltaics. More efficient panels can both be sold at higher prices and installed with fewer panels to reduce the cost profile of an entire project. The economics improves for everyone.

"We are exiting the era of cost reduction," he said, "and entering the era of efficiency." 

Research began more than a decade ago at Lund University in Sweden, and is now being launched as a startup: technology that selectively adds gallium-arsenide (GaAs) nanowires to solar panels, boosting their efficiency by up to 25 percent, the company claims. That improvement directly translates to better output and lower prices, improving the economics for the module manufacturers. It also reduces total installed costs by 15-20 percent (fewer panels with increased efficiency, even if they cost a little more), which improves the economics for everyone, Epstein said.

A solar module with 17 percent efficiency at $0.60/W, with Sol Voltaics' technology, could transform into a 22 percent efficiency module costing $0.75/W, according to the company. In the field, that turns a 100-MW installation into a 125-MW installation.

Nanomaterial additions to solar substrates typically have been produced through an epitaxial process, slowly grown in a furnace as crystals either in place on a substrate or separated and sorted separately in a batch process, neither cost-effective nor scalable. The company's Aerotaxy process suspends the active materials in carrier gas streams in a reactor, where precise control of temperature, pressure, flow rates, and mix causes them to grow much faster than epitaxial processes, and with reliable size and consistency (around 1-2 μm tall). They are then suspended and stored in a liquid. That liquid, called SolInk, is deposited onto the solar silicon substrate via an inkjet-type process, followed by a polymer layer and transparent conductive oxide (TCO) layer.

One trick to solve is managing the nanowire alignment. Two methods can be applied to solve that, Epstein noted: one is an electric field, another is chemical self-assembly. Coyly, he said "both have yielded promising results, but we have our direction."

The result is what the company and Lund University described in a Science paper earlier this year: an effect called "wave-concentrated PV," where the nanowires capture and concentrate lower-frequency light waves — a separate active layer, each acting as a separate solar cell. (GaAs is already used in much higher-efficiency cells for niche applications where cost isn't the key differentiator, such as satellites.) Longer-wavelength light goes through the transparent conductive oxide (TCO) layer to be converted by the silicon. They showed (and the Fraunhofer Institute confirmed) that indium phosphide (InP) nanowires covering just 12 percent of the substrate surface produced a solar cell with 13.8 percent efficiency. Epstein says the ideal coverage range is between 10-15 percent, with 12 percent the initial sweetspot; "somewhere above 15 percent you get diminishing returns," he said.

For module manufacturers, all they'll need is "a couple of pieces of equipment" that are "rather inexpensive," for deposition and the TCO layer, adding only a penny or two in capital expense to the line, Epstein said.

Others have tried to apply nanoparticles (nanotubes, nanowires, quantum dots), but not with active layers, Epstein said. "People know there's a use for them, but so far nobody has been able to crack how to make them cost-effective," he said. "Honestly there's been great research, but it's far from commercializable."

The company has raised $11 million to date from private investors including Industrifonden, Foundation Asset Management of Sweden, Teknoinvest, Provider, Nano Future Invest and Scatec Energy of Norway; it's also received public funding from others including the European Union, Vinnova, and Nordic Innovation Center. The company says it will raise $10-$20 million this year.

This year Sol Voltaics wants to continue to work the Aerotaxy process and pilot equipment. Next year the plan is to develop production-quality equipment scalable to a 100-MW machine, and tie up with a module manufacturer (or more), and partner with an equipment manufacturer familiar with making tools for solar PV manufacturing. By 2015 the company hopes to have a module-maker customer in pilot production, delivering them to market in small (~10 MW) quantities. Full-scale production is planned for 2016, defined as one or more 100-MW lines running their equipment.

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