A California company is using silicon ink patterned on top of silicon wafers to boost the efficiency of solar cells. The Sunnyvale, CA, firm Innovalight says that the inkjet process is a cheaper route to more-efficient solar power.
Inkjet solar: The inkjet printing process allows Innovalight to make silicon wafers that are thin enough to bend. Innovalight has partnered with solar-cell manufacturer JA Solar, headquartered in Shanghai, which plans to integrate the inkjet printing technology into its manufacturing lines. The resulting solar cells, which the company says have an efficiency of 18 percent, should be on the market by next year.
It's possible to increase the efficiency of solar cells by patterning silicon in a way that improves the absorption of high-energy, short-wavelength light. But this usually requires adding several etching steps to the manufacturing process, and this type of cell architecture "costs a lot of money to make using standard procedures," says Homer Antoniadis, chief technology officer at Innovalight.
Innovalight has not disclosed the costs of adding its ink to solar cells, but if the company's process achieves such gains in efficiency at a lower cost, it could help make solar a more competitive source of electricity. The company has developed silicon inks that can be patterned using inkjet printing technology developed by OTB Solar, a company headquartered in Eindhoven, Netherlands.
Innovalight's ink is a suspension that contains silicon nanocrystals. Although the recipe is proprietary, Antoniadis says the company has good control over two key factors: the size of the nanocrystals and their printability. By making the crystals just a few nanometers in diameter, the company has lowered the temperature required to bind them to the underlying wafer. The silicon ink also contains an organic compound that helps suspend the silicon, which otherwise tends to sink to the bottom, thus making the liquid compatible with inkjet printing. Because the process requires lower temperatures and can be performed on thinner silicon wafers than those used in conventional cell manufacturing, it helps bring the price down, says Innovalight's CEO Conrad Burke.
The 18 percent average efficiency of the solar cells made in Innovalight's pilot plant in California has been independently confirmed by the National Renewable Energy Laboratory. Without using the inkjet-printing technique, the efficiency of such cells would be about 16.5 percent to 17 percent, says Antoniadis.
Silicon solution: Innovalight’s ink solution contains silicon nanocrystals. "The record efficiency of a silicon solar cell is about 25 percent in the lab," says Matthew Beard, a senior scientist at the National Lab's Golden, CO, research center who was not involved in the testing. However, says Beard, "if they can [achieve 18 percent efficiency] at a lower cost, then that would be significant."
Cyrus Wadia, a research scientist at the Lawrence Berkeley National Laboratory, agrees, adding that because the efficiency boost is modest, "the linchpin is the cost." Wadia reckons that the silicon-ink printing would have to add less than 13 percent, approximately, to the total cost of the cell to be worthwhile.
The ultimate promise of silicon ink is that it could be used to make an entire solar cell, eliminating the need for a wafer altogether, says Wadia, who is developing nanomaterials with this same goal in mind.
"In principle, this is possible," says Antoniadis, "but it will require substantially more development and time." Innovalight has developed silicon inks that can conduct both positive and negative charges -- the key materials in a solar cell. But the company is pursuing the silicon-on-silicon application first, says Antoniadis, because "we need to deliver material value in the shortest possible time."
By Katherine Bourzac
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