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  • Painting the roof with solar ink

Photonics Spectra
Nov 2009
Anne L. Fischer, Senior Editor,

Covering rooftops with solar cells came one step closer to reality recently when JA Solar of Yangzhou, China, announced plans to commercialize technology developed by Innovalight, a San Francisco Bay-area company that makes a silicon solar “ink.”

The Innovalight concept puts a twist on traditional silicon solar manufacturing. The difference between making regular silicon solar cells and those with solar “ink” is that the ink process uses high-throughput atmospheric ink-jet manufacturing rather than low-throughput vacuum-based processing. Silicon ink is made by a process in which tiny silicon nanoparticles are suspended in a liquid that can be applied to rigid and flexible substrates.

Silicon solar paint manufactured by Innovalight using an ink-jet process recently demonstrated an 18 percent conversion efficiency. Photo courtesy of Innovalight.

What’s most significant about this process, according to Innovalight CEO Conrad Burke, is that it produces nanoparticles that maintain their low melting temperatures while still remaining small enough to stay in solution. He added that, “By the nature of how small they are, we can exploit their quantum effects,” and he noted that this quality may ultimately give rise to other long-range opportunities with quantum dots or silicon nanoparticles, such as higher efficiencies.

Recently Innovalight demonstrated the technology to have an almost 18.5 percent conversion efficiency. An efficiency of 18 percent was independently certified by the US Department of Energy’s National Renewable Energy Laboratory (NREL) and the Fraunhofer Institute for Solar Energy Systems in Germany but, as Burke pointed out, because of the time lag, there’s a difference between where they are currently and what’s being measured by independent labs. He expects the technology can go well over 20 percent in the coming 12 months.

In Brian Korgel’s lab at the University of Texas, researchers are working on a CIGS-based solar ink that can be spray-painted onto a variety of surfaces. Photo courtesy of the University of Texas at Austin.

Interestingly, one of the founders of Innovalight, Brian Korgel, has moved his research from silicon-based solar ink to copper indium gallium selenide (CIGS). He cites several advantages that CIGS has over silicon, including the fact that it’s a direct bandgap semiconductor, which ultimately uses less material in manufacturing. His team at the University of Texas at Austin has achieved efficiencies of 1 percent and is shooting for 10 before going the commercial route. Korgel estimates that it will take from three to five years before that happens, but if it works, he said, the inks could be applied to windows, effectively turning them into solar collectors.

CIGS on foil

At another Bay-area start-up back in 2002, researchers were already working with CIGS-based roll-to-roll processing. San Jose-based Nanosolar also uses a high-throughput printing process rather than a vacuum technique, but the company utilizes aluminum foil as the substrate and bottom electrode of the cell, with a wrap-through back contact and thin, transparent top electrode. NREL certified the active-area foil efficiency at about 15 percent, according to Keith Emery, device performance team leader at NREL.

Nanosolar recently announced completion of its panel assembly factory in Germany, which is expected to manufacture one panel every 10 seconds, with a potential annual capacity of 640 MW.

conversion efficiency
In a pumped laser system, the ratio of output energy to pump energy.
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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