New multi-junction solar cell could break efficiency barrier

January 14, 2013
This is a schematic diagram of a multi-junction solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency. Credit: US Naval Research Laboratory

U.S. Naval Research Laboratory scientists in the Electronics Technology and Science Division, in collaboration with the Imperial College London and MicroLink Devices, Inc., Niles, Ill., have proposed a novel triple-junction solar cell with the potential to break the 50 percent conversion efficiency barrier, which is the current goal in multi-junction photovoltaic development.

"This research has produced a novel, realistically achievable, lattice-matched, multi-junction solar cell design with the potential to break the 50 percent power conversion efficiency mark under concentrated illumination," said Robert Walters, Ph.D., NRL research physicist. "At present, the world record triple-junction is 44 percent under concentration and it is generally accepted that a major technology breakthrough will be required for the efficiency of these cells to increase much further."

In multi-junction (MJ) solar cells, each junction is 'tuned' to different in the solar spectrum to increase efficiency. High bandgap is used to absorb the short wavelength radiation with longer wavelength parts transmitted to subsequent semiconductors. In theory, an infinite-junction cell could obtain a maximum power conversion percentage of nearly 87 percent. The challenge is to develop a semiconductor material system that can attain a wide range of bandgaps and be grown with high crystalline quality.

By exploring novel semiconductor materials and applying band structure engineering, via strain-balanced , the NRL research team has produced a design for a MJ solar cell that can achieve direct band gaps from 0.7 to 1.8 (eV) with materials that are all lattice-matched to an (InP) substrate.

"Having all lattice-matched materials with this wide range of band gaps is the key to breaking the current world record" adds Walters. "It is well known that materials lattice-matched to InP can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band-gap."

The primary innovation enabling this new path to high efficiency is the identification of InAlAsSb quaternary alloys as a high band gap material layer that can be grown lattice-matched to InP. Drawing from their experience with Sb-based compounds for detector and laser applications, NRL scientists modeled the band structure of InAlAsSb and showed that this material could potentially achieve a direct band-gap as high as 1.8eV. With this result, and using a model that includes both radiative and non-radiative recombination, the NRL scientists created a solar cell design that is a potential route to over 50 percent under concentrated solar illumination.

Recently awarded a U.S. Department of Energy (DoE), Advanced Research Projects Agency-Energy (ARPA-E) project, NRL scientists, working with MicroLink and Rochester Institute of Technology, Rochester, N.Y., will execute a three year materials and device development program to realize this new solar cell technology.

Explore further: IMEC obtains record conversion efficiency of 24.7% for GaAs solar cells on Ge substrate

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3.7 / 5 (3) Jan 14, 2013
"...break efficiency barrier" AND the bank!
not rated yet Jan 14, 2013
Are they seriously proposing to use metal stripes on top to conduct current? This would diminish the efficiency due to the shadowing of the stripes..
5 / 5 (2) Jan 14, 2013
Are they seriously proposing to use metal stripes on top to conduct current? This would diminish the efficiency due to the shadowing of the stripes..

Yes but it won't block the light at all. I suspect that the strips of metal will be at a distance apart what will permit the light to pass. (390 to 700 nm for visible light) Just like the screen that allows you to see into the microwave but doesn't let the microwaves out. The holds in the screen are smaller then the wavelength.
not rated yet Jan 14, 2013
@deatopmmg - when light is concentrated ~1000x, current triple junction cells are not expensive (~$0.25/W in volume). Even if this were twice the cost it would not break the bank.

@hb - today's triple junction cells use metal stripes on top. However the picture is not to scale; the metal lines only block 3% to 5% of the light.

@mrlewish: right idea for the future (still experimental.
But the microwave screen is not a good example because it does block much of the light. You are probably referring to the plasmonic mesh discussed in physorg a few weeks ago:
1 / 5 (1) Jan 14, 2013
Why is efficiency so important? Isn't it rather the ratio between the lifespan and the cost of production that matters? Because to me it seems that if I can build dirt cheap solar panels that live long, I don't care if they are not efficient. I'll just buy and use more of them, possibly with the money I earned selling the electricity collected by those I already own.
not rated yet Jan 14, 2013
Because higher efficiency reduces the panel area needed, which for a solar farm proportionately reduces the land cost, the permitting cost, the species relocation cost, the foundation cost, the steel cost, the installation labor, and also reduces the cleaning and other maintenance costs.

Even for a home installation high efficiency reduces installation costs (or allows more power from a given roof space).

(Years ago cell cost per watt mattered more than efficiency because the cells dominated the panel cost and the panels dominated the total cost, but cell and panel prices have dropped so much that panels are now typically 1/3 to 1/4 the cost of a solar farm and only ~1/5 the cost of a rooftop installation.)
not rated yet Jan 15, 2013
Efficiency also does matter to satellites where you want to keep the weight low, to make sending them cheaper.
1 / 5 (1) Jan 15, 2013
Isn't it rather the ratio between the lifespan and the cost of production that matters?

- In large installations maintenance and (re)installation outweigh the cost of the solar cells.
- For anything where installation cost or mass is the deciding factor (in space, remote areas, ...) highest efficiency per kg is paramount.
- For anything where space is limited high efficiency is the deciding factor (laptop/phone/tablet recharging mats. Home installations with limited roof space. Highrises with many parties and little roof space (i.e. in cities in general), etc.)

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