The laser produces silky smooth microchannels, improving thin film array efficiency, and potentially reducing production costs.  (Source: Purdue University School of Mechanical Engineering image/Yung Shin)
Thin film solar power gets another incremental boost

Scientists at Purdue University [press release] have completed another significant work aimed at improving the efficiency of solar panels.  Researchers used lasers to scribe microchannels improving the efficiency of inter-cell power transfer, and the overall efficiency of a mounted multi-cell thin-film solar cell mat.

I.  Why Solar?

The Sun is one of the Earth's two great potential energy sources (nuclear power being the other).  While it may be convenient to store solar energy in a portable form via technologies like algae biofuel, ultimately nuclear and solar will be the primary drivers of a Space Age high-tech nation.

Thirty years ago solar power existed, but it was virtually useless for any practical purpose due to sky-high costs.  Today it's still a bit too expensive to serve as a comfortable replacement for coal, but costs have plunged to the level where it can be contemplated as a contributor to overall national power generation. Within twenty or thirty years we may arrive at a point where solar is among the most cost-effective power technologies.  

But to get there the slow and arduous pace of iterative improvements must be maintained.

II. What's in a Connector?

Much work has gone into improving the base efficiency of solar cells and find optimal materials for the cells.  Purdue University's Center for Laser-Based Manufacturing instead looked at another source of efficiency and power loss -- cell interconnects.

Typically solar cells are manufactured similar to microprocessors, using vapor deposition techniques on a semiconductor substrate.  Metal interconnects were used to link cells.

But more recently thin film solar cells have been taking off.  Thin films can be made to be flexible and transparent.  And they use less material, making them potentially cheaper.  However, they require new processes to create.  Instead of bulky metal interconnects, they use tiny interconnects called "microchannels" to electrically connect individual cells allowing thin film cell arrays to be created.

Traditionally these interconnects were produced by a mechanical stylus.  The result frequently featured efficiency dropping imperfections and took a great deal of time, raising production costs.

Past studies had suggested using lasers as a replacement.  The Purdue team reports being the first to successfully execute this approach, using an ultra-precise "ultrashort pulse laser".

Yung Shin [profile], a Purdue professor of mechanical engineering who led the study remarks, "The efficiency of solar cells depends largely on how accurate your scribing of microchannels is. If they are made as accurately as possibly, efficiency goes up."

Using the cutting edge laser, the team cut away chunks of material using a process called "cold ablation".  Where similar efforts had failed in the past was that the cutting process was too slow and heated the material, creating defects and damage.  

The ultra-fast laser, though, creates pulses lasting only picoseconds.  It chips along creating a damage-free ultra-smooth, sharply defined microchannel.

Describes Professor Shin, "It creates very clean microchannels on the surface of each layer. 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." 

III.  What's Next for Pulse-Laser Microchanneling?

Currently thin film cells account for 20 percent of the global market in terms of watts generated.  In two years -- by 2013 -- that total is expected to rise to 31 percent.

Thus you can expect to see this microchanneling technique to be incorporated into production designs sooner rather than later.

The researchers have already completed the most important phase of their project, funded by a $425,000 USD National Science Foundation (NSF) grant -- getting the basic process working.  

Now they're continuing research to try to better understand how the fast pulse method works on a microscopic level.  That could yield clues as to how to refine the technique even further.  It would also likely allow the team to patent their technique, removing the final roadblock to commercialization.

The team has published [abstract] a paper on their work in the Proceedings of the 2011 NSF Engineering Research and Innovation Conference in January.

The paper's authors include Professor Shin; Gary Cheng [profile], an industrial engineering professor; and graduate students Wenqian Hu, Martin Yi Zhang and Seunghyun Lee.

The total project is expected to last three years and will hopefully continue to offer rewarding breakthroughs in terms of improved efficiency and reduced production costs.

DailyTech is in the process of contacting the authors of this work about further details about potential commercialization.

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