Manufacturing Bits: Aug. 23

Inexpensive roll-to-roll solar manufacturing process; green GaN LEDs.


Rolling Out Solar Power…Literally
An International team of researchers have developed solar cells that can be added onto a roll of flexible plastic in liquid form, bringing the same kind of economies of production to the solar industry as rolls of paper and ink did for newspapers more than a century ago.

Using a roll-to-roll processing method, the team was able to achieve a power conversion efficiency of more than 9.5%. The commercial target is 10. More expensive spin-coated films can achieve about 11% conversion rates. (In comparison, conventional roof-top solar cells achieve conversion rates in the low-20% range, but so far cost has been a deterrent for mass adoption without government subsidies.)

“The ‘rule of thumb’ has been that high-volume polymer solar cells should look just like those made in the lab in terms of structure, organization and shape at the nanometer scale,” Lee Richter, a NIST physicist who works on functional polymers, said in a statement. “Our experiments indicate that the requirements are much more flexible than assumed, allowing for greater structural variability without significantly sacrificing conversion efficiency.” 

The real breakthrough is the coating, which can be applied in layers 25 microns thick using a blade at 194 degrees Fahrenheit, which makes it suitable for roll-to-roll processing.

The research, spearheaded by the National Institute of Standards, includes an international team of researchers from the Technical University of Denmark, Hong Kong University, Saudi Arabia’s King Abdullah University of Science and Technology, and North Carolina State University.

Polymer cells from a solar park demo at the Technical University of Denmark. Source: NIST

Brighter Green GaN LEDs
Gallium nitride is receiving a lot of attention these days. It’s one of the wide band gap III-V materials that has been suggested as a replacement for CMOS at advanced process nodes. It also has seen commercial use in LEDs in the red to ultra-violet spectrum, when combined with either indium or aluminum.

Now, researchers at the University of Illinois at Urbana-Champaign, have figured out a way to use the material to develop brighter, more efficient green LEDs, as well. According to the school, GaN forms both hexagonal and cubic crystals, but the hexagonal crystal isn’t good for light output because an internal electrical field prevents the negative ions and positive holes from combining. Cubic crystals don’t share that problem, but until now the only way to create them was through molecular-beam epitaxy, which is slower and more expensive than metal-organic CVD.

“Our cubic GaN does not have an internal electric field that separates the charge carriers—the holes and electrons,” said Richard Liu, a graduate student in the electrical and computer engineering department at the school. “So, they can overlap and when that happens, the electrons and holes combine faster to produce light.”
Researchers say this approach also could eliminate the “droop” effect in LEDs, whereby light output declines over time. Improved green LEDs also could have an impact on white LEDs, which can be created through color mixing.

Hexagonal-to-cubic phase transformation. The scale bars represent 100 nm in all images. (a) Cross sectional and (b) Top-view SEM images of cubic GaN grown on U-grooved Si(100). (c) Cross sectional and (d) Top-view EBSD images of cubic GaN grown on U-grooved Si(100), showing cubic GaN in blue, and hexagonal GaN in red. Source: University of Illinois.

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