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Ultrafast laser scribes solar cells

Photonics Spectra
May 2011
Laura S. Marshall, Managing Editor, laura.marshall@photonics.com

A new manufacturing method using an ultrafast pulsing laser could make thin-film solar cell arrays a more efficient and less expensive means of power generation.

The current method of connecting solar panels into arrays that generate usable electricity involves mechanically scribing with a stylus, but this technique is less than ideal: Not only is it slow and expensive, but it also produces imperfect channels.

“The efficiency of solar cells depends largely on how accurate your scribing of microchannels is,” said Yung Shin, a professor of mechanical engineering and director of Purdue University’s Center for Laser-Based Manufacturing. “If they are made as accurately as possible, efficiency goes up.”

Shin and his team are working to increase solar cell efficiency using an ultrashort-pulse laser on thin-film solar cells to produce the microchannels, he said.

“Production costs of solar cells have been greatly reduced by making them out of thin films instead of wafers, but it is difficult to create high-quality microchannels in these thin films,” Shin said. “The mechanical scribing methods in commercial use do not create high-quality, well-defined channels.


A scanning electron microscope image shows a microchannel that was created using an ultrafast-pulsing laser. Courtesy of Purdue University School of Mechanical Engineering. Image/Yung Shin.


“Although laser scribing has been studied extensively, until now we haven’t been able to precisely control lasers to accurately create the microchannels to the exacting specifications required.”

The group’s research shows that the ultrafast laser pulses formed microchannels with sharp boundaries and precisely specified depths. The laser pulses last only a matter of picoseconds, so the laser does not cause heat damage to the thin film. It removes material precisely through cold ablation.

“It creates very clean microchannels on the surface of each layer,” Shin said. “You can do this at very high speed – meters per second – which is not possible with a mechanical scribe.

“This is very tricky because the laser must be precisely controlled so that it penetrates only one layer of the thin film at a time, and the layers are extremely thin. You can do that with this kind of laser because you have a very precise control of the depth, to about 10 to 20 nanometers.”

Approximately 20 percent of the global photovoltaic market in terms of watts generated is made up of thin-film solar cells, and experts predict that this will rise to 31 percent by 2013.

The research is led by Shin and Gary Cheng, an associate professor of industrial engineering. The work is funded through a three-year, $425,000 grant from the National Science Foundation.

A paper demonstrating the method’s feasibility was published in Proceedings of the 2011 NSF Engineering Research and Innovation Conference. The paper was written by Shin, Cheng andgraduate students Wenqian Hu, Martin Yi Zhang and Seunghyun Lee.


GLOSSARY
power
With respect to a lens, the reciprocal of its focal length. The term power, as applied to a telescope or microscope, often is used as an abbreviation for magnifying power.
scribing
The process of perforating a silicon or ceramic substrate with a series of tiny holes along which it will break. Nd:YAG or CO2 lasers are now routinely used.
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