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Future solar PV


Joyce Laird

When US President Obama gave his recent State of the Union Speech, he demonstrated (all be it rhetorically at this stage) that he understands the need for innovation in clean energy.

This will have gone down well in the corridors of the US' flagship renewable energy research and innovation hub – the US National Renewable Energy Laboratory (NREL). But which areas of PV – still very much at the R&D stage – does the organisation think this potential new lease of life for innovation will stimulate?

Top down

Dr. Larry Kazmerski oversees the development of measurement and characterisation of renewable energy technologies and energy efficiency technologies and practices.

He believes that very high end concentrating solar devices are getting a lot of attention. These include GaAs [gallium arsenide], GaAlAs [gallium aluminum arsenide], GaInAsP [gallium indium arsenide phosphide], and InSb [indium antimonide] as PV converters. These concentrator technologies are primarily aimed at large, utility-scale applications for high solar-insolation, he says.

“These triple junction solar cells – developed for NASA – are very expensive, but also very robust and efficient. Using lower-cost optics for high concentration, only one one-thousandth of an actual solar cell is needed to become a highly-effective power generator for large installations of 50 MW plus,” he says.

“Just recently we had confirmed a concentrator solar cell from Spire at 42.4% efficiency. While the actual cell belongs to Spire, all the scientific development, testing and so forth was done at NREL.”

Another interesting future technology is dye-sensitised solar cells, he says. These are more complex, but they involve a lot of chemistry and have proven 11% efficiency in the lab. “This comes out of Sharp in Japan. The main component is titanium oxide. TiO2 is the same material [found] in white paint,” Kazmerski says.

“It's very abundant, inert and safe. This is nanotechnology where they take TiO2 and make micro-formations from the material, put it together with a liquid dye and form the solar cell. Maybe someday you'll be able to go to your big box home store, buy a bucket and just paint on.”

Pure science at NREL

Matt Beard, senior scientist, Chemical and Materials Sciences Center can be found in the basic energy division of NREL. “Our area is strictly science – how to change the paradigm of converting solar radiation into usable energy,” he says.

“What our group has been working on for many years is to understand how to circumvent energy-losses in a solar cell. High energy photons (photon energy greater than the semiconductor bandgap) that are absorbed lose most of that energy to heat. What we want to do is see if we can use that excess energy before it cools. In a bulk material, that cooling or loss of energy happens very fast on the picosecond (10-12 seconds) time scale,” he explains.

Quantum dots may be the answer: “When you get down in that size range [2-10 nm], the properties of materials start to change because of the quantum confinement effects – which is why they are called quantum dots; because they are confined in 3 dimensions,” he says. “Generally they are spherical in shape but they are highly crystalline. So they retain the crystalline structure of the bulk material. Quantum dots are grown in solution.

“After they are grown in solution they can then be processed as inks. They are usually processed at much lower temperature than a bulk material would be. There are cost advantages and you get new properties that depend on the size of a nanomaterial,” Beard says. “You can tune the size of the nanomaterial by adjusting your reaction for longer times or under different conditions. Being able to tune the properties of these quantum dots is a key advantage. Size determines the electro-optical properties of the material.”

He adds that the tuning aspect has limits because there is always a range for a given material. Beard is looking at using lead selenide and lead sulphide quantum dots, which are very well known bulk materials: “In their bulk form they are commonly used for infra red detectors because they have very low band gaps. But when you make quantum dots, that band gap opens up, which is what you need for solar. Other advantageous properties appear as well,” he says.

“What we are trying to do is use the quantum dots as the actual absorber layer in a solar cell, but still retain the useful quantum behaviour of the material. We have a dual effort going on. First we want to understand and learn how to use this excess energy more efficiently, to understand the basic physics. Then we want to be able to take those isolated quantum dots and put them into a prototype solar cell and demonstrate that we can actually collect multiple carriers.

“Finally, our goal is to develop a framework for a quantum dot-based solar cell that is air stable and that can be moved out for beta testing,” he adds. “We have shown that you can make solar cells from quantum dots, and that they are very stable in air, but we have not demonstrated a working solar cell with the multiple exciton affect. That's the stage we are at right now. We're going from the isolated quantum dots to putting those into working solar cells.”

The process he has been working on lately is called multiple exciton generation: “In bulk material, this is called carrier multiplication. The idea is that you have a high energy photon and instead of producing one carrier, it can produce multiple carriers if it has the right amount of energy. Therefore, if you have this high energy photon you can use that energy more efficiently. We have found that within isolated quantum dots, this process is about [twice as] efficient [than] in a bulk material.”

“Material utilisation has become the name of the game in thin-film silicon…”
- Dr Martha Symko-Davies, NREL

What Beard is hoping for is that this will fit into a thin film approach. The cost would be extremely low, and the efficiency would be extremely high, he believes.

Incubator

Dr. Martha Symko-Davies is senior program manager of the PV Technology Incubator program at NREL. This program was developed to help fledgling companies move from prototype to pilot scale production in 18 months. The Department of Energy funding is often available to these small startups.

“It's fast and very leading edge,” she says. “We incubate about 10 companies a year. If starting up, you need to do it quickly because this industry is evolving fast and it's easy to become yesterday's news,” she says.

The program is holistic when it comes to PV. All conversion technologies are welcome: CdTe, CIGS – all thin-film and all crystalline, nanostructures, organic PV (OPV) and concentrating PV (CPV). Each has special challenges. The goal is to improve performance and reduce cost with new materials and/or technologies, processes or a combination.

Symko-Davies points to the kerfless area as one of their recent success stories: “1366 Technologies started their work with us, and was recently awarded US$4 million from ARPA-e (Advanced Research Projects Agency) of the US Department of Energy to develop a kerfless technology,” she says.

“As everyone in the industry knows, typically when cutting silicon a lot is wasted to the cutting process itself. This new technology is a process that does not use any of these traditional methods and greatly reduces that kerf.

“There's a lot of room for improvement in many technologies. Material utilisation has become the name of the game in thin-film silicon. We have a couple of companies we fund in that area,” she says.

Aside from all the standard technologies the Seed Fund Program at NREL is investigating long-term commercialisation strategies, quantum dots, intermediate band solar cells and novel III-V CPV cells. Symko-Davies says that this is not just within the remits of the companies they fund. NREL scientists are working on all of these areas, separate from the incubator project.

“When we work with the incubator companies, we team up and we put them together with our research teams and they have a facility space called a PDIL (process development and integration laboratory). We also have an amazing measurement and characterisation facility to characterise and facilitate R&D obstacles that can be tailored to the emerging disruptive technologies”.

Smart solar buildings

So what does NREL's Kazmerski believe the future for PV could be?

“One of the beauties of science is that we have so many visionaries,” he says: “If you don't stretch goals, you never get ahead. I see the whole market of PV eventually not being panels on a roof or a utility scale farm, it will eventually become building integrated to create PV ‘smart' structures that are totally self sustaining. All parts of a building will provide architectural value and also provide energy to run everything inside the structure. Future photovoltaics will be integrated into the buildings themselves, not something slapped onto the roof.”

This extract is taken from a longer article published in the November/December 2010 issue of Renewable Energy Focus U.S. magazine.


About:

Joyce Laird has an extensive background writing about the electronics industry; semiconductor development, R&D, wafer/foundry/IP and device integration into high density circuit designs.


Renewable Energy Focus, Volume 12, Issue 1, January-February 2011, Pages 14-15

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