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Laser texturing yields solar efficiency boost

26 Oct 2011

SiOnyx says that texturing with a picosecond-pulsed laser boosts silicon cell efficiency and improves production uniformity.

SiOnyx, the Massachusetts company developing its “black silicon” laser texturing process to improve infrared performance for silicon-based image sensors, says that the same technology can significantly boost solar cell performance.

Through a collaboration with the International Solar Energy Research Center (ISC) in Konstanz, Germany, multicrystalline cells textured with a picosecond laser showed an efficiency 0.3% higher than standard cells, by improving infrared sensitivity.

As well as increasing the average efficiency to more than 17%, the texturing process also improved the uniformity of cell performance – potentially offering tighter process binning if the technology is adopted by a volume manufacturer.

SiOnyx, which presented the results of the study at the recent IEEE Photonics conference in Arlington, Virginia, says that it plans to submit a journal paper on the work by the end of this year.

Chris Vineis, director of solar technology at the company, told optics.org that the main benefit of the laser texturing process is to reduce the cell’s surface reflectance below that delivered by the industry’s standard wet chemical method.

“It also creates a superior form of light trapping, such that even thin silicon wafers can achieve very good efficiencies,” Vineis explained. In the work with ISC Konstanz, 150 micron-thick cells (20% thinner than normal) showed average efficiencies of 16.9%, a performance comparable with the industry standard, but with cells that could be produced 10-15% cheaper.

Another benefit is that the laser texturing process is completely independent of crystal grain orientation, Vineis says. “Thus, on multicrystalline silicon solar cells the process creates a highly-uniform, dark surface, in contrast to the standard wet chemical texturing techniques,” he added. “The improved uniformity results in tighter process binning (i.e., reduced process variation).”

Coherent investment
For the texturing step, SiOnyx used a Coherent Aethon tool, which is based around a Talisker picosecond laser source - although Vineis points out that customers can use whichever platform they prefer, as the SiOnyx process does not require a specific tool. Coherent is actually an investor in SiOnyx, having taken part in the company’s $12.5 million series B financing round in October 2010. Vulcan Capital, the venture investment firm founded by Microsoft co-founder Paul Allen, was another new investor at that time.

Vineis says that some initial trials have taken place with volume cell producers, and he is anticipating further interest towards eventual commercial deployment. The company is pursuing a licensing model, offering the technology as a drop-in solution for existing production lines.

“[Adoption] does not require any modification to the rest of their equipment or process,” Vineis pointed out. “Furthermore, we have a more advanced process that could create even larger efficiency boosts (over 1% absolute), and would integrate well with various solar cell architectures on manufacturers’ future roadmaps.”

Although the primary focus for SiOnyx remains infrared-enhanced image sensors, rather than solar cells, in September the company was awarded a $7 million contract for the development of portable photovoltaics by the US Defense Advanced Research Projects Agency (DARPA). That project is expected to run until March 2013.

The ‘black silicon’ process was first discovered in a laboratory at Harvard University, and SiOnyx was subsequently founded by Harvard’s Eric Mazur and Jim Carey in 2006. Following initial funding, the company raised $11 million through convertible securities in September 2007, ahead of the $12.5 million series B round last year.

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