Boosting solar cell efficiency: Engineers design new optical element to sort sunlight

Jun 11, 2014
A thin rectangular layer called a polychromat can be integrated into the cover glass of a solar panel. This layer sorts sunlight into colors can be absorbed by solar cells to increase their efficiency without increasing the cost, according to a new study by University of Utah electrical engineers. Credit: Dan Hixson, University of Utah

(Phys.org) —University of Utah electrical engineers have designed a thin layer made of a transparent plastic or glass that sorts and concentrates sunlight to boost the overall efficiency of solar cells by up to 50 percent. This layer, called a polychromat, can be integrated into the cover glass of a solar panel. It could also be used to boost power efficiency in a cellphone or improve low light conditions for a camera.

"Currently, high-efficiency are very expensive because they have to be carefully manufactured in a complex environment and are only cost-effective for space or defense applications like the Mars Rover," says Rajesh Menon, a Utah Science Technology and Research (USTAR) assistant professor of electrical and computer engineering at the U. "We have designed a very cheap optical element that can be incorporated into the of a solar panel that will separate sunlight into various colors."

Solar cells absorb light from the sun and convert it into electricity. Despite its tremendous potential as a limitless resource of energy, solar power is currently a small fraction of the global energy supply, due to its high cost compared with conventional power sources. In addition, challenges in materials have further limited solar power's wide reach.

Solar cell performance is directly linked to the efficiency of converting sunlight into electricity. Solar cells operate on the concept that one absorbed bundle of light from the sun, called a photon, generates electrical charge carriers in a layer of material within the solar cell that then becomes electricity.

However, sunlight is made up of different wavelengths of light, ranging from ultraviolet to visible to infrared. Light at different wavelengths is made of photons at different energies. Conventional solar cells only absorb a narrow range of wavelengths very efficiently. The energy at other wavelengths is not absorbed at all or is converted into waste heat rather than electricity. As a result, a solar cell can only convert so many photons into electricity, up to a theoretical limit of about 33.5 percent efficiency (called the Shockley-Queissner limit).

In this new study, Menon and electrical engineering graduate student Peng Wang designed a polychromat. The polychromat was 50 millimeters wide by 10 millimeters long, with 3 micrometer wide grooves to sort incoming light. The polychromat was made using photolithography for this study, but Menon says it can now be made cheaply by creating a mold of the polychromat and then stamping it out like a DVD.

University of Utah electrical engineers Peng Wang and Rajesh Menon have designed a thin layer made of a transparent plastic or glass that boosts the overall efficiency of solar cells by up to 50 percent. This layer could also be used to boost power efficiency in a cell phone, or improve low light conditions for a camera. Credit: Dan Hixson, University of Utah

The team placed the polychromat on top of a photovoltaic device, which is a device that generates a voltage when exposed to energy, especially light. The photovoltaic device is made of two absorber layers: gallium indium phosphide to absorb visible light, and gallium arsenide to absorb infrared light. When the University of Utah polychromat was added, the increased by 16 percent.

"These colors can be absorbed by appropriate solar cells to increase the efficiency of the overall process without increasing the cost," says Menon.

The researchers also developed computer simulations of a polychromat placed on a solar cell with eight different absorber layers to show a theoretical efficiency greater than 50 percent.

"With our approach, you can almost arbitrarily select the bands of light—within the laws of physics—to give more flexibility in the design of the panel," says Menon. "With a normal lens it's very difficult to actually separate the colors, and with a prism it's difficult to get the separated light exactly where you want it."

Recently, the University of Utah became the first university in the country to sponsor a community solar program that offers community members the opportunity to purchase discounted rooftop and installation for their homes. The polychromats from this study are not being used in the community program.

Menon says the next step to using these polychromats in commercial solar cells is working with solar cell manufacturers. He says this could lead to high-efficiency solar cells in another 5 or 10 years. In the meantime, Menon and his team will test this new technology at the characterization facility at the National Renewable Energy Laboratory.

"The solar cells in the Mars Rover are very efficient but can't be expected to be widely used on rooftops due to their high cost," says Menon. "We've figured out the optics, but incorporation into commercial photovoltaic devices is difficult today because few manufacturers can handle the multiple materials needed, so that's the next big challenge."

Menon says the polychromat concept could also be used to generate a brighter, more power-efficient LCD display in a cellphone, or to boost colors in a camera under low conditions. These applications are being explored through Menon's startup company, PointSpectrum Corporation.

This new study by Menon and Wang appears online today in the journal Progress in Photovoltaics: Research and Applications.

Explore further: Novel NIST laser system mimics sunlight to test solar cell efficiency

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antialias_physorg
5 / 5 (1) Jun 12, 2014
An advantage this may give is: Since this allows for parts of the receiving array to be specific for the low end of the spectrum without taking away valuable realestate from the high energy parts such solar cells would be better at producing energy with ambient light (e.g during overcast days) than current cells are.

cost-effective for space or defense applications

I'm wondering: what does 'cost effective' mean in terms of 'defense applications' (unless we're talking space based defense applications)?

/Gripe mode: on/
bundle of light from the sun, called a photon

"Quantum"? Yes. "Packet"? OK. "Bundle"? Really?
Next we'll have a "heap" of light.
/Gripe-mode: off/
RealScience
not rated yet Jun 12, 2014
@AA - Unless they have found a new loophole in the laws of optics, they won't be able to split diffuse light cleanly and efficiently onto different areas.

As for cost effective for defense applications, the highest-efficiency single-layer cells (Alta's ultra-thin epitaxial lift-off GaAs cells) are targeted at terrestrial defense markets. Given the high 'cost' of carrying batteries, the higher cost of the GaAs cells (as opposed to silicon cells) is worthwhile.

(For space I am skeptical unless/until they can match the efficiency of the stacked multi-junction cells, or unless launch costs come way down.)
antialias_physorg
not rated yet Jun 12, 2014
split diffuse light cleanly and efficiently onto different areas.

Since the band gap in the cells is quantized they don't need to. The just need to be good enough so that the energy of photons in stripe n are of an energy that lies below the bandgap of the cells in stripe n-1 and not below the bandgap of the cell in stripe n. Even if the edges aren't perfectly aligned that still means a lot of gain over letting all photons hit a single cell (or even a stacked multijunction cell)
RealScience
not rated yet Jun 12, 2014
@AA: truly diffuse light can't by any known means be split by spectrum onto area without losing efficiency (e.g. one could have multiple reflections inside an optical element that has patches next to the sub-cells that are wave-length selective mirrors that only let through frequencies that those sub-cells can use efficiently, but for diffuse light a significant amount of the light would bounce back out the way it came in, costing efficiency).

If the light is not 'split' (which is what the system in the article appears to do) there are loopholes:
Multi-junction don't 'split' the light, but stack the sub-cells so that all are in the light-path and can selectively absorb photons they can use efficiently and allow those below their band-gap through to other sub-cells.
Also, if one is allowed to absorb the photons and then re-emit them at slightly lower frequencies, then re-emission can be highly directional and one could in theory make a very efficient re-emitter of 'sorted' photons'.
RealScience
not rated yet Jun 12, 2014
@AA - I highly recommend reading works by Roland Winston of UC Merced on this general topic.
antialias_physorg
not rated yet Jun 12, 2014
truly diffuse light can't by any known means be split by spectrum onto area

Light on an overcast day is stll mostly directed.
one could have multiple reflections inside an optical element that has patches next to the sub-cells that are wave-length selective mirrors that only let through frequencies

That would be pointless. It would be even worse that just using filters (which are doubly pointless because the idea here is to use the full spectrum and not just the part that happens to coincide with the bandgap of a cell you're using).
Multi-junction don't 'split' the light, but stack the sub-cells

Which makes them expensive. And the transition through the stacks isn't without losses, either. The point here is to make stuff cheap.
Also, if one is allowed to absorb the photons and then re-emit them at slightly lower frequencies

That's being tried - but it has the flew of lower effciency - which the stuff in the article is trying to avoid

RealScience
not rated yet Jun 12, 2014
Light on an overcast day is still mostly directed.


This may be a difference in how you and I interpret 'overcast'.

Picking Google's first hit on its definition:
Overcast: of the sky or weather) marked by a covering of gray clouds; dull. "a chilly overcast day"
synonyms: cloudy, clouded (over), sunless, darkened, dark, gray, black, leaden, heavy, dull, murky, dismal, dreary

This matches how I think of overcast - a day such as today was here.
I checked outside and the light was highly diffuse and not mostly directional.

However I understand that you live in Germany, and overcast there may refer to something a bit lighter. It sounds like in what you refer to as overcast is not as dark, and hence the light might be more directional.

- continued -

RealScience
not rated yet Jun 12, 2014
one could have multiple reflections inside an optical element that has patches next to the sub-cells that are wave-length selective mirrors that only let through frequencies

That would be pointless. It would be even worse that just using filters (which are doubly pointless because the idea here is to use the full spectrum and not just the part that happens to coincide with the bandgap of a cell you're using).


I wasn't proposing a practical system, I was explaining the exceptions to the "can't split non-directional light onto areas without efficiency loss".
You misunderstand, so I probably didn't explain it well.

The wave-length-selective mirrors would be for light exiting to the various cells.
It actually works in theory if the light is concentrated so that the entrance is small compared to the cell area see US patent 6689949, but it is not economical for un-concentrated light.

- continued -
RealScience
not rated yet Jun 12, 2014
My reference to multi-junction cells was also pointing out a loophole to the splitting rule, rather than proposing a practical system (although concentration may yet make multi-junction cells cost-competitive).

Likewise my pointing out the frequency down-conversion loophole (which I have not seen used for splitting although I have seen it used to get around a similar non-directional light
rule in luminescent concentrators.

I was simply trying to be precise in what I meant by "truly diffuse light can't by any known means be split by spectrum onto area without losing efficiency" by ALSO pointing out the the very similar things that I know of that I was NOT covering in the statement.