Evolution inspires more efficient solar cell design: Geometric pattern maximizes time light is trapped in solar cell

January 25, 2013
McCormick researchers have designed a geometrically-patterned light scattering layer that could make solar cells more efficient and less expensive.

(Phys.org)—The sun's energy is virtually limitless, but harnessing its electricity with today's single-crystal silicon solar cells is extremely expensive—10 times pricier than coal, according to some estimates. Organic solar cells—polymer solar cells that use organic materials to absorb light and convert it into electricity—could be a solution, but current designs suffer because polymers have less-than-optimal electrical properties.

Researchers at Northwestern University have now developed a new design for that could lead to more efficient, less expensive solar power. Instead of attempting to increase efficiency by altering the thickness of the solar cell's —a tactic that has preciously garnered mixed results—the researchers sought to design the of the scattering layer to maximize the amount of time light remained trapped within the cell.

Using a mathematical based on natural evolution, the researchers pinpointed a specific geometrical pattern that is optimal for capturing and holding light in thin-cell organic .

The resulting design exhibited a three-fold increase over the Yablonovitch Limit, a developed in the 1980s that statistically describes how long a photon can be trapped in a semiconductor.

A paper about the results, "Highly Efficient Light-Trapping Structure Design Inspired by Natural Evolution," was published January 3 in Scientific Reports, a publication of Nature.

In the newly designed organic solar cell, light first enters a 100-nanometer-thick "scattering layer," a geometrically-patterned dielectric layer designed to maximize the amount of light transmitted into the cell. The light is then transmitted to the active layer, where it is converted into electricity.

"We wanted to determine the geometry for the scattering layer that would give us optimal performance," said Cheng Sun, assistant professor of mechanical engineering in Northwestern's McCormick School of Engineering and Applied Science and co-author of the paper. "But with so many possibilities, it's difficult to know where to start, so we looked to laws of natural selection to guide us."

The researchers employed a genetic algorithm, a search process that mimics the process of natural evolution, explained Wei Chen, Wilson-Cook Professor in Engineering Design and professor of mechanical engineering at McCormick and co-investigator of the research.

"Due to the highly nonlinear and irregular behavior of the system, you must use an intelligent approach to find the optimal solution," Chen said. "Our approach is based on the biologically evolutionary process of survival of the fittest."

The researchers began with dozens of random design elements, then "mated" and analyzed their offspring to determine their particular light-trapping performance. This process was carried out over more than 20 generations and also accounted for evolutionary principles of crossover and genetic mutation.

The resulting pattern will be fabricated with partners at Argonne National Laboratory.

Explore further: Nano-sandwich technique slims down solar cells, improves efficiency

More information: www.nature.com/srep/2013/13010 … /full/srep01025.html

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2.8 / 5 (6) Jan 25, 2013
Trial and error is a perfectly legitimate design strategy that does not require intelligence. The artificial iteration and selection system in which this trial and error took place *in this instance* was obviously intelligently designed.
5 / 5 (3) Jan 25, 2013
Does this have to be used with organic cells or could it be used with silicon cells as well?
2.3 / 5 (3) Jan 25, 2013
Does this have to be used with organic cells or could it be used with silicon cells as well?

Looking at the setup, there is no reason you couldnt replace the active layer with whatever material you desired, but one should take into account the optimal spectrum needed for absorption and make sure this scatters well, I imagine the design is married to this parameter.

1.4 / 5 (8) Jan 25, 2013
Seems like a finite-element-type computer simulation of mirror patterns using a genetic algorithm and millions of parallel 'experiments', would somehow be able to find the patterns that maximize both facet (mirror) size and number of light bounces from a range of incident angles. Then we would know sort of from "experimental mathematics" what shapes work best. Seems to me like spiral-seashell shapes would work best but also have smallest facet size because of the continuously curving surface, and would be prohibitively expensive to micro-fab millions of them. Probably need to search for patterns that match certain crystal forms, since we can 'grow crystals', from even single chain molecules as the starting points.
1 / 5 (4) Jan 25, 2013
They're approaching meta-material photon energy converters that shift frequencies over a wide range by scattering nano-scale quarter and half-lambda equivalent sized detectors across the surface while, in so doing, increasing the effective surface area of the photon absorbing surface, enabling down-scaling of the size of an entire solar cell array, the ultimate goal after all.
1.6 / 5 (9) Jan 25, 2013
$per watt is now coming down consistently over time. Even if. It takes200 years, solar power will one day reach a true tipping point where only impoverished cou.tries use coal.
4.2 / 5 (5) Jan 25, 2013
Trial and error is a perfectly legitimate design strategy

But that's not what they did. They used a genetic algorithm which is
Trial and error
selection of best fit as a starting point for the next round.

This isn't a random approach but a (self-) directed walk with thrown-in variability.
3 / 5 (2) Jan 25, 2013
The 10x is a very old number (or a very cloudy location). It is currently ~3x to 4x in a moderately sunny location, if you ignore that:
coal is a consumed resource and ignore the CO2, mercury, fly ash, etc. from coal (accounting for which would very roughly double the price of coal),
and on the other side ignore that photovoltaics are intermittent (which will limit the amount useable unless low-cost storage is solved).

Jeddy is correct that the price differential will disappear, and in more like 10 to 15 years than 200 years at current cost trends.

This will leave only lack of storage as a limiter of solar's potential in all but a few ultra-cloudy regions. Whether and when storage will be solved remains to be seen, but it is a non-trivial challenge.
1.7 / 5 (7) Jan 25, 2013
Do you ant to know a Secret...... Here it is.. Slow down the Photons, AND prevent the Photons from Escaping....... HOW, by adding a Reflector to the internal side. Use a Clear Liquid Film inside and it acts like a Mirror... OR, a Crystal Layer that Multiplys the Photon into many Photons.....
1 / 5 (7) Jan 25, 2013
The increasing of surface area of solar cells may increase their efficiency but it increases the speed of their contamination, oxidation and photodegradation too. As usually, every improvement comes with its own price - you as a customer may be fooled with it after few years.
1.2 / 5 (6) Jan 25, 2013
You Can Also Enhance the Density of Photons into a Laser with Optics, Solar Panels can be altered, You Can Create Tiny Lenses like tiny Magnifying Glasses that Compact Photons as a way to create Photon Beams.
5 / 5 (2) Jan 25, 2013
@ichard - those aren't secrets. People do indeed put reflectors under some films to increase the light path. And there are experiments on layers that down-convert high energy photons into multiple lower energy photons (but crystals aren't magic so the total photon energy is not increased).
5 / 5 (2) Jan 25, 2013
@ichard, - yes, luminescent concentrators are related to lasers. Lenses don't create photon beams, but they can concentrate the light (and this is used in CPV).
But none of these are secrets, and they all have cost/benefit tradeoffs.
3.7 / 5 (3) Jan 26, 2013
$per watt is now coming down consistently over time. Even if. It takes200 years, solar power will one day reach a true tipping point where only impoverished cou.tries use coal.

Hope so, because now it's the US and China who burn all the coal to the atmosphere...
2.5 / 5 (8) Jan 26, 2013
Gotta wonder if creationists would boycott such a product "inspired by evolution". Of course not, because they are self-serving hypocrites, having learned such hypocritical behavior from their anti-humanity anti-curiosity doofy religion(s) designed to kill kill kill.
1 / 5 (4) Jan 27, 2013
Wasn't there some article here not very long ago about scientists doing research on material surfaces that absorb all visible light and look jet black.... Maybe those scientists should get together with these scientists. It might save them some design time in their work.

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