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energy R&D, on the move: solar PV


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energy R&D, on the move: solar PV


Professor Vladimir Bulovic on the Progress in Photovoltaic Technology

Renewable solar power, which has the potential to enhance the energy security and environmental performance of the US energy system, is a reality today. The extremely low cost of the conventional baseload power generation that is otherwise available to Americans, however, means that renewable technologieswhether centralized or distributedrequire a continued strong R&D effort to compete economically without subsidies.

But here is why so many observers are optimistic about photovoltaics (PV): things are getting better, and fast. Take a pencil to the historical solar PV cost per kilowatt-hour curve, and extend its gradual declining slope just five or ten years into the future. Before you know it, the pencil will start getting close to zero, and well within the cost range of many conventional power-generation technologiesall with little to no fuel requirements or operational pollution. While there are legitimate questions about the sustainability of this decline (i.e., how much of it is the result of genuine innovation and economies of scale and how much of it is a result of low-cost dumping of PV panels by a Chinese manufacturing glut), there is no doubt that trends are moving rapidly in the right direction. 

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Solar PV - Available Today


If you compare the cost of solar fifteen years ago to the cost of solar today, it has been reduced dramatically by a factor of five. And we’re expecting that in the future there will be another factor of three at a minimum. There is nothing that is fundamentally stopping us from getting the cost down.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

Solar PV - Available Today


If you compare the cost of solar fifteen years ago to the cost of solar today, it has been reduced dramatically by a factor of five. And we’re expecting that in the future there will be another factor of three at a minimum. There is nothing that is fundamentally stopping us from getting the cost down.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

photos: DOE/flickr

High-efficiency monocrystalline rear-junction silicon cells

 

The SunPower Corporation is a multibillion-dollar company that recently saw a major investment from Total New Energy S.A. Its high-efficiency solar PV cells are found on hundreds of thousands of US households and businesses and NASA aircraft. But behind the headlines, this Silicon Valley-based, Stanford professor-founded clean-energy startup has actually been selling—and quietly refining—its silicon wares for nearly thirty years.

Richard Swanson was a graduate student in the Stanford electrical engineering department in the 1970s when he began investigating how the PV technology then reserved for use in satellites could be reduced from its truly atmospheric cost of over $70 per watt. While many were looking to thin-films or other radically new technology pathways, his “point-diffused contact” cell designs aimed to improve efficiency by concentrating light and capturing the current produced at the back of the cell. This increased the amount of light available for conversion to electricity, reduced the need for expensive wiring throughout the cell, and helped open the door to automating manufacturing.

By the 1980s, Swanson, now running his own lab as a Stanford professor, was supported in his work by both the US DOE and Electric Power Research Institute (EPRI), the electric power industry’s collaborative research organization. In founding SunPower, Swanson and his team pivoted their technical approach to follow the small but growing PV market: fabricating increasingly large wafers, thinning cells, and using wire saws to reduce manufacturing costs. Incremental improvements have actually paid off: world-record monocrystalline cell efficiencies based on Swanson’s innovations have gradually risen to now exceed 24 percent. Moreover, panel costs continue to fall as production increases— what economists call “learning curves.” Today, “Swanson’s Law” refers to the trend of module prices falling by approximately 20 percent for every doubling of cumulative worldwide production. 

Hear Richard Swanson discuss PV at the 2013 Global Climate and Energy Project research symposium

Related: Inter-institution PV research collaboration at Stanford →


 
The DOE “Sunshot” team started what they called the “Michael Jordan” program. How can you get to 23 percent efficiency from silicon PV? 23 percent being Michael Jordan’s uniform number. There are issues about material quality. How many impurities do you have? What’s the mobility? What kind of recombinations are you getting? How do you improve the contacts? I don’t think we know all the answers to that. That’s a research problem that will have a direct implication on manufacturing.
— Arun Majumdar, Google vice president for energy*
 
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Solar PV - Near at Hand


This is absolutely critical so we're hell bent on getting this done.

— Arun Majumdar, Google vice president for energy*

Solar PV - Near at Hand


This is absolutely critical so we're hell bent on getting this done.

— Arun Majumdar, Google vice president for energy*

Nanocrystalline-silicon shells improve thin solar panel light absorption 

Stanford

Visitors to Statuary Hall in the US Capitol may have experienced a curious acoustic feature that allows a person to whisper softly at one side of the cavernous, half-domed room and for another on the other side to hear every syllable. Sound is whisked around the semicircular perimeter of the room almost without flaw. The phe- nomenon is known as a whispering gallery. A team of engineers has now created tiny hollow spheres of PV nanocrystalline-silicon and harnessed physics to do for light what whispering galleries do for sound. The results could dramatically reduce materials usage and processing cost in the production of solar panels.

“Nanocrystalline-silicon is a great PV material. It has a high electrical efficiency and is durable in the harsh sun,” says Shanhui Fan, a professor of electrical engineering at Stanford. The downfall of nanocrystalline-silicon, however, has been its relatively poor absorption of light, which requires thick layering that takes a long time to manufacture.

The engineers call their spheres nanoshells. Producing the shells takes a bit of engineering magic. The researchers first create tiny balls of silica—the same material that makes glass—and coat them with a layer of silicon. They then etch away the glass center using hydrofluoric acid that does not affect the silicon, leaving behind the all-important light-sensitive shell. These shells form optical whispering galleries that capture and recirculate the light. “The light gets trapped inside the nanoshells,” says Stanford’s Yi Cui, an associate professor of materials science engineering. “It circulates round and round rather than passing through and this is very desirable for solar applications.”

Credit: Andrew Myers for the Stanford School of Engineering, 2012

Vertical PV panel structures reduce installation cost 

MIT

Intensive research around the world has focused on improving the performance of solar PV cells and bringing down their cost. But little attention has been paid to the best ways of arranging those cells. Now, a team of MIT researchers has come up with a new approach: building cubes or towers that extend the solar cells upward in three-dimensional configurations.

The results from the structures they have tested show power output ranging from double to more than twenty times that of fixed flat panels with the same base area. Researchers saw the biggest boosts in power in the situations where improvements are most needed: in locations far from the equator, in winter months, and on cloudier days. The new findings are based on both computer modeling and outdoor testing of real modules.

“I think this concept could become an important part of the future of PVs,” says the team’s leader, Jeffrey Grossman, associate professor of material science and engineering. The time is ripe for such an innovation, Grossman adds, because solar cells have become less expensive than accompanying support structures, wiring, and installation. Self-supporting 3-D shapes could even create new schemes for residential or commercial PV installation, and the increased energy density could facilitate the use of cheaper thin-film materials in area-limited applications. As the cost of the cells themselves continues to decline more quickly than these other costs, they say, the advantages of 3-D systems will grow accordingly.

Credit: David Chandler, ©Massachusetts Institute of Technology, used with permission, 2012

Associate Professor Yi Cui, Stanford

Associate Professor Yi Cui, Stanford

Associate Professor Jeffrey Grossman, MIT

Associate Professor Jeffrey Grossman, MIT


 
We have worked on organic solar cells for a long time. These are based on molecules that can be sprayed onto a flexible plastic substrate at very low cost. When I started ten years ago, efficiency was down at a couple of percent. But this is rising very fast. And now the world record efficiency is up to eleven percent and no signs that it’s slowing down. So we think we can make a lot of progress there.
— Michael McGehee, Stanford professor of materials science and engineering
 
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Solar PV - On the Horizon


The proliferation of solar is about scale . . . immense scale along with the speed and cost of installation. Today, we can deploy infrastructure at scale and speed while meeting cost constraints. As we strive to further enhance solar’s attractiveness, we need to learn to install it as economically as we roll out other infrastructure like roads.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

Solar PV - On the Horizon


The proliferation of solar is about scale . . . immense scale along with the speed and cost of installation. Today, we can deploy infrastructure at scale and speed while meeting cost constraints. As we strive to further enhance solar’s attractiveness, we need to learn to install it as economically as we roll out other infrastructure like roads.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

Thin-film organic polymer flexible solar cells 

MIT

Using a novel process involving moderate temperatures and no liquids, a team of researchers at MIT has printed PV cells on tissue paper, printer paper, newsprint, textiles, and even plastic food wrap. These solar devices have features that make them ideal not only for integrating into consumer products but also for shipping to remote regions of the world where energy demand is growing rapidly and there is no power grid in sight.

Today’s PVs are typically fragile and must be moved with care and installed by trained experts to avoid damage. More robust PVs have been made on flexible materials such as plastic, but thus far, they have not been entirely successful: problems have stemmed largely from the anode (the positive electrode), which has a tendency to crack or lift off when the surface on which it is mounted is bent.

But anodes that are lightweight, flexible, and adhere well have now been fabricated by Karen K. Gleason, professor of chemical engineering, and her colleague Vladimir Bulovic, MIT School of Engineering associate dean for innovation, with their “Paper PV Team.” Key to their success is a process called oxidative chemical vapor deposition, or oCVD. Invented by Gleason, oCVD improves on conventional CVD, a well-known method of depositing a thin coating of one material on the surface of another, by adding an oxidant and carefully selecting starting materials to enable “gentler” heat and atmospheric operating conditions inside a vacuum chamber. The oCVD approach is designed especially for making thin films from organic polymers—carbon-containing molecules that are composed of repeating structural units and offer desirable traits including low cost, good electrical conductivity, and good mechanical properties that allow them to be flexed, stretched, and even folded.

Credit: Nancy Stauffer, ©Massachusetts Institute of Technology, 2011

Printable inorganic thin-film flexible solar cells 

UT Austin

Novel nanocrystal inks could provide a new materials platform for creating PV devices with high efficiency on a variety of uncon- ventional substrates. Thus far, research has focused on CIGS nanocrystals as the primary light absorber material. The resulting efficiencies of this process are good, but it requires very expensive, high-temperature sintering that is not easily scaled to large-area devices and large-manufacturing production capacities, preventing commercial adoption. Nanocrystal inks, however, provide a degree of processing flexibility that cannot be obtained using processes that require high temperature.

Professor Brian Korgel’s research group at the University of Texas has pioneered the synthesis of “ink-like” colloidal CIGS nanocrystals for PV devices. He showed that CIGS quaternary compounds could be made in high yield with good size control and dispersibility using colloidal, solution-phase methods. Over time, the Korgel research team has worked to develop and improve the efficiencies of such nanocrystal devices from less than 0.5 percent to now almost 4 percent. Moreover, this new semiconductor absorber layer can be deposited at room temperature, under ambient pressure, in air.

While CIGS from printed nanocrystals in Korgel’s group have reached 7 to 8 percent efficiency with selenization at over 500 degrees Celsius, the team’s ongoing research goal is to achieve efficiencies greater than 5 percent without the need for expensive high-temperature processing and the need for selenium vapor.

Credit: Carey W. King for the University of Texas Austin Energy Institute, 2013 

Organic, tandem PVs 

University of Michigan

Imagine paints that can collect solar energy to power a car or living room windows that harvest the sun’s rays to generate electricity for a home. A research team at the University of Michigan is analyzing the efficiency, reliability, and potential of organic PV technology for widespread commercial application.

Professor Steve Forrest’s work examines the promise of capturing and distributing the power of the sun via materials that are lighter, more pliable, and less expensive than current silicon-based PV systems. “The type of organic materials we use are not very different from the inks in an inkjet printer or the dyes used in clothing,” he says. “Some are very good semiconductors. In principle, they can be put down very cheaply on plastic films, metal foils, and other flexible substrates.”

The research is based on proprietary small-molecule systems developed by Forrest with Global Photonic Energy Corporation, where he serves as a research partner. These systems incorporate semitransparent, organic materials stacked in a tandem architecture. This arrangement maximizes the ability to capture photons—the bundles of sunlight energy—passing through the cells. The tandem structure, Forrest estimates, achieves about 30 percent more efficiency than single-cell architecture.

While Forrest said the technology remains “next generation,” its potential is far-reaching. “Commodity electricity generation is a pretty long step from here,” he says. “That requires a very proven and mature technology. Still, there are a lot of interesting niches that could be exploited before that— coating on windows and coatings on car surface, for example. From there, you could move into rooftop residential and then to commodity generation solar farms."

Credit: Amy Mast for the University of Michigan Energy Institute 

Professors Karen Gleason and Vladimir Bulovic. MIT

Professors Karen Gleason and Vladimir Bulovic. MIT

Professor Brian Korgel, UT Austin

Professor Brian Korgel, UT Austin

Professor Steve Forrest, University of Michigan

Professor Steve Forrest, University of Michigan

 

 
The heart of our business, first, is research and development.
— John Deutch, MIT professor of the institute