Quantum Dots from Argonne

Dr. Seth B. Darling, Scientist for the Center for Nanoscale Materials at Argonne says that his team’s research is focused on developing a new type of solar concentrator that is based on inorganic semiconductor quantum dots. Quantum dots are basically a small chunk of a semiconductor that is on the order of 2-10 nanometer (nm). These hold some advantages: They are highly stable and they also have high quantum yield so they can be very efficient at absorbing and re-emitting light.

The inorganic quantum dots don’t have the re-absorption problems of organic dyes and as a bonus, Darling says that they are very tunable. These inorganic dots can be tuned to work through all parts of the solar spectrum.

“First, we are working on a different type of inorganic, semiconductor quantum dot. This new dot allows us to make a greater separation between absorption and emission properties. We can basically tune those two independently to absorb light from virtually any part of the spectrum, then tune the emission so that it doesn’t overlap with the absorption,” he says.

“Secondly, we have what I’ll call this global optimization of parameters: How thick is the slab going to be? How large an area is it going to be? What is the composition of the material used to make up the slab? What concentration of luminescent particles to use? What are the right energies for absorption and emission to match up to the commercially available PV cells?”

Darling’s team is doing what is called a Monte Carlo ray tracing simulation where the path of the photons of light traveling through the slab can be tracked in a computer. Using this method, he can see how many photons get captured by the photovoltaic cell at the end. This supports the best optimization of all parameters to find the optimal system design and get the most light to the cell. “We’ve developed a code that will do that analysis. Then once we know the optimal parameters, we have the synthesis capabilities to make those quantum dots that have those properties,” he says.

“Creating quantum dots is all chemistry. Precursor inorganic materials can be purchased in fairly large quantities and they are mixed together with various organic molecules in solution. There they grow very small nanoparticles of inorganic materials that are a core of inorganic material capped with organic molecules. This is a very low cost process, and yet highly scalable,” he adds.

Basically the quantum dots are mixed with a liquid polymer before it is set. Then it is poured into a mold in the desired shape. “Not only is this very low cost, it works in diffuse light. There’s no tracking needed,” Darling says. “We can take this idea and go further with it by making it into a multi-layer stack. Then you can split up the light spectrum into different pieces. Transparent concentrator slabs cover the same area that expensive PV panels would and only smaller solar panels on the side are needed. What you’re really getting at is the geometric gain ratio: How much area are you collecting light over vs. how many PV panels. That is a parameter that can be optimized, but roughly we are talking about a ratio of 20 to one.”

Optimizing CdTE at CSU

Dr. Walajabad Sampath, Mechanical Engineering Professor at Colorado State University, teaches classes like any other professor, but he has a double life: He developed the research and technology that led to the creation of Abound Solar. At one time, his lab actually ‘was’ Abound Soar, although he was always based at the university.

Today, he is still hot on the scientific research trail to develop better and more efficient cadmium telluride (CdTe) thin-film solar cells. “Currently a CdTe cell has a layer of cadmium sulfide and a layer of cadmium telluride.

But there are compounds in this family that are three elements, and those that are four elements and so forth. If you think of making layers with those materials, not just the simple binary types, then there are many opportunities to improve efficiency,” he explains. “We are testing these other materials in our research. We have built a system to both make and test these complex materials.”

Sampath says that his scientists are currently working on a special machine to deposit these new materials for testing. “It is a deposition system similar to those used, but it is a more advanced system designed specifically for this type of research. The materials are not something we need to do extensive research on. It’s more the processing step for final testing,” he says. “If we succeed with this it will only be a simple modification to any CdTe production line, not a whole new thing. The idea is to save cost not increase it. And it will bring down cost in two ways: It will increase cell efficiency by up to 30% and bring down manufacturing cost by 50%.”

Top European R&D Now in the USA

“The Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Freiburg, Germany, is Europe’s largest solar energy research institute, comparable with NREL with regards to size, scope and scientific output,” says Dr. Christian Hoepfner, Director of Technical Operations for Fraunhofer CSE, the two-year old USA offspring of Fraunhofer ISE.

“We believe that the US solar market is going to be increasingly important, which is why we have started new PV research centers in the USA.”

The Fraunhofer CSE research encompasses all forms of PV module technology and also supports two other research groups: Energy efficiency technology for buildings and a new group which covers distributed electrical energy systems, or ‘Smart Grid’ technology.

“We are developing test procedures that go far beyond the certification testing that is currently required. We work with companies that have new and exciting materials to encapsulate and seal the modules, and we are helping them to understand how to apply those materials to PV. Our goal is to understand all of the nuances of these new materials and how they interact with the actual solar cell materials; what impact they may have on the solar module structure and so forth. We want to make sure only the very best combinations get into the market,” Hoepfner says.

“We can take any material and partner with a company to show them what the challenges are in the PV module area and test to see if their material is a good fit. We can help them set up their internal protocols to make sure they can come up with a material for this market that is worth the expansion, or not. We are able to provide material scientists who are experts in this area to steer them in the right direction. We have a facility where we can actually make PV modules, test all aspects and then destroy them to see in detail what worked, what did not, and to discover why. It is important to always know why something did work as well as why something else did not.”

Next year, Fraunhofer is opening a new PV module certification lab in Albuquerque, New Mexico, called the CFV Solar Test Laboratory. It has both the lab and the outdoor space for lifetime and liability testing of all solar modules.

Nanotubes at GIT

Jud Ready, Ph.D. Sr. Research Engineer & Adj. Professor at the Georgia Institute of Technology reports that they are developing a 3D PV cell that uses carbon nanotubes as a support scaffold and back electrical contact for the cell. “This structure offers significant ‘light trapping’ capabilities and can be used in various PV systems including CdTe, CdSe [cadmium selenide], a-Si [amorphous silicon] and CIGS [copper indium gallium selenide].

The benefit of this 3D structure is that it ‘orthogonalizes’ carrier extraction and light absorption to benefit both – whereas planar systems must optimize one typically to the detriment of the other,” he explains. Ready says that textured PV cells have been around for many years, but previous technology used an etching procedure to remove material from the wafer. “Our technology was the first to demonstrate this texturing and light-trapping structure in an additive manner and that is the basis for our patents that have been awarded in China, Korea, and Australia. The US patent should issue this year. Others are pending in India, Japan and the EU.”

“Due to the light trapping structure, I see this used in numerous PV markets; for example transportation or other emplacements where a fixed array or where a tracking array would offer benefits but is not fiscally viable in terms of cost (residential) or in terms of weight/maintenance (aerospace),” he adds.

A New Look at Honeywell

Honeywell has been supplying barrier films to the pharmaceutical and food industries for more than 30 years and is now applying that expertise to the photovoltaic industry. Jerry Buchanan, Global Business Manager, says that the challenge to the module industry is to reduce a module’s cost and improve its performance.

“Our most recent backing system launch, PowerShield® PV270 starts to address some of those issues, but our long-term view is that total PV packaging plays a critical role in the performance of any module. Once a module is put together, it’s the packaging that protects it for 25 to 30 years.”

Buchanan believes that packaging solutions for the module is absolutely critical: “We spend a lot of time prototyping and building full-scale modules, doing aging simulation and looking at power output – which is ultimately how you judge the value of what you bring to the industry.”

He notes that there is a gap between where module manufacturers guarantees are at, and where the performance of the modules actually could be. “We think there might be some interactions between the packaging materials and the contacts, the lines, and the actual structure of the cell. We believe we can influence these interactions using our packaging solutions for a more robust long-term solar product,” Buchanan says.

The biggest challenge when looking at packaging is finding the right solar modules to work with. “Regardless of technology, we need to make sure we are using reliable and well proven cell sources and be very consistent in the construction of the module. We supervise and are very careful regarding how we select companies and what we work on.”

“Crystalline is the long term contender, but we haven’t lost sight of CIGS, CIS [copper indium selenide], CdTe, and we think there are some real challenges there. I think we are the best solution to address the barrier issues that all of those companies will face as they try to remove either one or both sides of those sheets of glass,” he adds.

Window Photovoltaics

New Energy Technologies Inc, Burtonsville, MD, is working on a very exciting technology called SolarWindow™ that is a coating that generates electricity from both natural (sun) and artificial (light bulbs) light. “Simply stated, SolarWindow is capable of working on exterior and interior applications. Our demonstrations show substantially superior performance over current thin-film and solar photovoltaic technologies at generating electricity from artificial light, an important advantage over conventional solar technologies which are limited in their capacity of full functionality. Conventional thin-film and silicon solar PV requires direct sunlight to produce electricity,” John A. Conklin, CEO says.

Their current market focus is toward new construction, replacement windows, and both structural and architectural glass, but Conklin says that other potential markets are being considered and evaluated.

“We believe SolarWindow may provide opportunities for space, portable, remote, and supplemental power applications for use by manufactures, integrators, and installers in the future,” Conklin says.

NREL Overview from the TOP

Dr. Larry Kazmerski is an IEEE Life Fellow and the Executive Director of Science and Technology Partnerships at the National Renewable Energy Laboratory (NREL) where he oversees the development of measurement and characterization of renewable energy technologies and energy efficiency technologies and practices.

He says 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], InSb [indium antimonide] as PV converters because they have exceptional performance with potential to convert more than a third of the sun’s terrestrial power into electricity. These concentrator technologies are primarily aimed at large, utility-scale applications for high solar-insolation.

“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 highly effective power generators for large installations 50 MW+. Several major companies are currently investing in researching this technology,” he says.

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

Another interesting future technology includes die-sensitized solar cells. These are a little more complex, but they still 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 as 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. These futuristic solar cells are targeted to paint onto roofs, walls or other structures. Maybe someday you’ll be able to go to your big box home store and buy a bucket and just paint it on.”

Pure Science

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 how to convert 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 utilize that excess energy before it cools. In a bulk material that cooling, that loss of energy, happens very fast on the picoseconds (10-12 s) 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 three 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 nano-material,” Beard says. “You can tune the size of the nano-material 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 sulfide quantum dots which are very well known bulk materials. “In their bulk form they are commonly used for infrared 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 behavior 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 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. “An exciton is just an electron/hole pair. However, if in a quantum dot, they’re confined to that quantum dot so they’re called an exciton. An exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge,” he explains.

“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 utilize that energy more efficiently. We found that within isolated quantum dots, this process is about 2x more efficient than in a bulk material.”

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.

NREL ‘Incubates’ New Companies

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. We incubate about 10 companies a year. We just closed a solicitation for Letters of Interest and will bring a new round of companies into the portfolio. 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, nano-structures, 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. “A company, 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 utilization 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 commercialization strategies, quantum dots, intermediate band solar cells and novel III-V CPV cells. Symko-Davies says that this is not just with 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 characterization facility to characterize and facilitate R&D obstacles that can be tailored to the emerging disruptive technologies.”

A Final Thought

Kazmerski wraps it all up by saying: “One of the beauties of science is that we have so many visionaries. If you don’t stretch goals, you never get ahead. I see the whole market of photovoltaics eventually not being panels on a roof or putting up utility farms, 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.”

As long as there are scientists like these, it is a sure thing that PV will be continual evolution in technology. Perhaps Kazmerski’s prophecy of a world of “smart solar buildings” may not be too far in the future.

About the Author:

Based in California, Joyce Laird has been writing for a wide range of industrial magazines for over a decade. Her extensive background in the semiconductor industry created a perfect transition to covering developments in photovoltaics.

Renewable Energy Focus U.S., Issue 3 November/December 2010.