New materials for more powerful solar cells

Applying a thin film of metallic oxide significantly boosts the performance of solar panel cells--as recently demonstrated by Professor Federico Rosei and his team at the Énergie Matériaux Télécommunications Research Centre at Institut national de la recherche scientifique (INRS). The researchers have developed a new class of materials comprising elements such as bismuth, iron, chromium, and oxygen. These "multiferroic" materials absorb solar radiation and possess unique electrical and magnetic properties. This makes them highly promising for solar technology, and also potentially useful in devices like electronic sensors and flash memory drives.

The results of this research are discussed in an article published in Nature Photonics by researcher and lead author Riad Nechache.

The INRS research team discovered that by changing the conditions under which a thin film of these materials is applied, the wavelengths of light that are absorbed can be controlled. A triple-layer coating of these materials--barely 200 nanometres thick--captures different wavelengths of light. This coating converts much more light into electricity than previous trials conducted with a single layer of the same material. With a conversion efficiency of 8.1% reported by Nechache and his coauthors, this is a major breakthrough in the field.

The team currently envisions adding this coating to traditional single-crystal silicon solar cells (currently available on the market). They believe it could increase maximum solar efficiency by 18% to 24% while also boosting cell longevity. As this technology draws on a simplified structure and processes, as well as abundant and stable materials, new photovoltaic (PV) cells will be more powerful and cost less. This means that the INRS team's breakthrough may make it possible to reposition silicon PV cells at the forefront of the highly competitive solar energy market.

Blu-ray disc can be used to improve solar cell performance

o knew Blu-ray discs were so useful? Already one of the best ways to store high-definition movies and television shows because of their high-density data storage, Blu-ray discs also improve the performance of solar cells -- suggesting a second use for unwanted discs -- according to new research from Northwestern University.

An interdisciplinary research team has discovered that the pattern of information written on a Blu-ray disc -- and it doesn't matter if it's Jackie Chan's "Supercop" or the cartoon "Family Guy" -- works very well for improving light absorption across the solar spectrum. And better yet, the researchers know why.

"We had a hunch that Blu-ray discs might work for improving solar cells, and, to our delight, we found the existing patterns are already very good," said Jiaxing Huang, a materials chemist and an associate professor of materials science and engineering in the McCormick School of Engineering and Applied Science. "It's as if electrical engineers and computer scientists developing the Blu-ray technology have been subconsciously doing our jobs, too."

Blu-ray discs contain a higher density of data than DVDs or CDs, and it is this quasi-random pattern, perfected by engineers over decades for data storage, that, when transferred to the surface of solar cells, provides the right texture to improve the cells' light absorption and performance.

Working with Cheng Sun, an associate professor of mechanical engineering at McCormick, Huang and his team tested a wide range of movies and television shows stored on Blu-ray discs, including action movies, dramas, documentaries, cartoons and black-and-white content, and found the video content did not matter. All worked equally well for enhancing light absorption in solar cells.

The findings will be published Nov. 25 in the journal Nature Communications.

In the field of solar cells, it is known that if texture is placed on the surface of a solar cell, light is scattered more effectively, increasing a cell's efficiency. Scientists have long been searching for the most effective texture with a reasonable manufacturing cost.

The Northwestern researchers have demonstrated that a Blu-ray disc's strings of binary code 0s and 1s, embedded as islands and pits to store video information, give solar cells the near-optimal surface texture to improve their absorption over the broad spectrum of sunlight.

In their study, the researchers first selected the Jackie Chan movie "Supercop." They replicated the pattern on the active layer of a polymer solar cell and found the cell was more efficient than a control solar cell with a random pattern on its surface.

"We found a random pattern or texture does work better than no pattern, but a Blu-ray disc pattern is best of all," Huang said. "Then I wondered, why did it work? If you don't understand why, it's not good science."

Huang puzzled over the question of why for some time. One day, his wife, Shaorong Liu, a database engineer at IBM, suggested it likely had something to do with data compression. That was the insight Huang needed.

Huang and Sun then turned to McCormick colleague Dongning Guo, an expert in information theory, to investigate this idea. Guo is an associate professor of electrical engineering and computer science.

The researchers looked closely at the data processing algorithms in the Blu-ray standard and noted the algorithms serve two major purposes:

• Achieving as high a degree of compression as possible by converting the video signals into a seemingly random sequence of 0s and 1s; and
• Increasing error tolerance by adding controlled redundancy into the data sequence, which also limits the number of consecutive 0s and 1s.

These two purposes, the researchers said, have resulted in a quasi-random array of islands and pits (0s and 1s) with feature sizes between 150 and 525 nanometers. And this range, it turns out, works quite well for light-trapping applications over the entire solar spectrum.

The overall broadband absorption enhancement of a Blu-ray patterned solar cell was measured to be 21.8 percent, the researchers report.

"In addition to improving polymer solar cells, our simulation suggests the Blu-ray patterns could be broadly applied for light trapping in other kinds of solar cells," Sun said.

"It has been quite unexpected and truly thrilling to see new science coming out of the intersection of information theory, nanophotonics and materials science," Huang said

New materials for more powerful solar cells

Applying a thin film of metallic oxide significantly boosts the performance of solar panel cells--as recently demonstrated by Professor Federico Rosei and his team at the Énergie Matériaux Télécommunications Research Centre at Institut national de la recherche scientifique (INRS). The researchers have developed a new class of materials comprising elements such as bismuth, iron, chromium, and oxygen. These "multiferroic" materials absorb solar radiation and possess unique electrical and magnetic properties. This makes them highly promising for solar technology, and also potentially useful in devices like electronic sensors and flash memory drives.

The results of this research are discussed in an article published in Nature Photonics by researcher and lead author Riad Nechache.

The INRS research team discovered that by changing the conditions under which a thin film of these materials is applied, the wavelengths of light that are absorbed can be controlled. A triple-layer coating of these materials--barely 200 nanometres thick--captures different wavelengths of light. This coating converts much more light into electricity than previous trials conducted with a single layer of the same material. With a conversion efficiency of 8.1% reported by Nechache and his coauthors, this is a major breakthrough in the field.

The team currently envisions adding this coating to traditional single-crystal silicon solar cells (currently available on the market). They believe it could increase maximum solar efficiency by 18% to 24% while also boosting cell longevity. As this technology draws on a simplified structure and processes, as well as abundant and stable materials, new photovoltaic (PV) cells will be more powerful and cost less. This means that the INRS team's breakthrough may make it possible to reposition silicon PV cells at the forefront of the highly competitive solar energy market.

Cheaper silicon means cheaper solar cells

Researchers at the Norwegian University of Science and Technology have pioneered a new approach to manufacturing solar cells that requires less silicon and can accommodate silicon with more impurities than is currently the standard. Those changes mean that solar cells can be made much more cheaply than at present.

A new method of producing solar cells could reduce the amount of silicon per unit area by 90 per cent compared to the current standard. With the high prices of pure silicon, this will help cut the cost of solar power. 

“We’re using less expensive raw materials in smaller amounts, we have production fewer steps and have potentially lower total energy consumption,” PhD candidate Fredrik Martinsen and Professor Ursula Gibson of the Department of Physics at NTNU explain.

They recently published their technique in Scientific Reports.

Their processing technique allows them to make solar cells from silicon that is 1000 times less pure, and thus less expensive, than the current industry standard.

Glass fibres with a silicon core

The researchers’ solar cells are composed of silicon fibres coated in glass. A silicon core is inserted into a glass tube about 30 mm in diameter. This is then heated up so that the silicon melts and the glass softens. The tube is stretched out into a thin glass fibre filled with silicon. The process of heating and stretching makes the fibre up to 100 times thinner.

This is the widely accepted industrial method used to produce fibre optic cables. But researchers at the Department of Physics at NTNU, working with collaborators at Clemson University in the USA, are the first to use silicon-core fibres made this way in solar cells. The active part of these solar cells is the silicon core, which has a diameter of about 100 micrometres.

Lower energy consumption

This production method also enabled them to solve another problem: traditional solar cells require very pure silicon. The process of manufacturing pure silicon wafers is laborious, energy intensive and expensive. “We can use relatively dirty silicon, and the purification occurs naturally as part of the process of melting and re-solidifying in fibre form”, says Gibson. “This means that you save energy, and several steps in production.”

It is estimated to take roughly one-third of the energy to produce solar cells with this method compared to the traditional approach of producing silicon wafers.

Gibson has worked for several years to combine purification and solar cell production. She got the idea for the project after reading an article on silicon core fibres by John Ballato at Clemson University in South Carolina, who is at the forefront of research in fibre optics materials development.

“I saw that the method he described could also be used for solar cells,” she said, “and we developed a key technique at NTNU that improved the fibre quality.” Gibson and her research group began to work with Ballato, who is a co-author of the article published in Scientific Reports.

Silicon rods

The new type of solar cells are based on the vertical rod radial-junction design, which is a relatively new approach. The design uses less pure silicon that a planar cell, Martinsen explains, and then launches into a crash-course on the inner workings of a solar cell: photons of different wavelengths are absorbed in different layers of the silicon wafer. They generate free charges, or charge carriers, which are then separated to provide electrical energy.

These charges need to be close to the electrodes and close to the p-n junction to be captured. The p-n junction is the active region in the device - where different types of charge carriers are separated. If the charge is not captured, the energy dissipates and goes to heating up the solar cell itself.

In a traditional solar cell, the journey from where a charge is generated to the surface can be quite long. This means that highly purified silicon is required. But with silicon fibres, there is a junction all the way around the fibre. The distance from where the charge is generated to where it is captured is quite short. Charge carriers can be captured effectively, even when using impure silicon. 

“The vertical rod design still isn’t common in commercial use. Currently, silicon rods are produced using advanced and expensive nano-techniques that are difficult to scale,” Martinsen says. “But we’re using a tried and true industrial bulk processes, which can make production a lot cheaper.”

Potential

The power produced by prototype cells is not yet up to commercial standards. Contemporary solar cells have an efficiency of about 18 per cent. The prototype created by NTNU researchers has only reached about 3.6 per cent. Gibson and Martinsen still have faith in the potential of this production method, and are working to improve the design and fabrication processes.

“These are the first solar cells produced this way, using impure silicon. So it isn’t surprising that the power output isn’t very high,” says Martinsen. “It’s a little unfair to compare our method to conventional solar cells, which have had 40 years to fine-tune the entire production process. We’ve had a steep learning curve, but not all the steps of our process are fully developed yet. We’re the first people to show that you can make solar cells this way. The results are published, and the process is set in motion.”

The next step is to refine production, make larger and more effective solar cells, and couple multiple cells together.

New materials yield record efficiency polymer solar cells

Researchers from North Carolina State University and Hong Kong University of Science and Technology have found that temperature-controlled aggregation in a family of new semi-conducting polymers is the key to creating highly efficient organic solar cells that can be mass produced more cheaply. Their findings also open the door to experimentation with different chemical mixtures that comprise the active layers of the cells.

Polymer solar cells are a delicately controlled mixture of a polymer donor and a fullerene acceptor. The cell is created by adding a solvent to the polymer and fullerene until the mixture becomes a liquid, then spreading the liquid thinly onto a surface. As the solvent evaporates, the thin layer solidifies, with the donor material hardening into tiny, highly ordered "clumps" that are connected by other, disordered donor molecules, and the acceptor weaving around them. Currently the most efficient organic solar cells are manufactured using one of only two different fullerenes.

NC State physicist Harald Ade and postdoctoral researcher Wei Ma had previously studied the morphology of solar cells and found that the size scale of the clumps within the donor layer and the aggregation -- or interaction between neighboring molecules within the layers -- were the main drivers of solar cell efficiency.

In a paper published today in Nature Communications, Ade, Ma and a team of chemists from the Hong Kong University of Science and Technology led by He Yan show that size scale and aggregation within these devices are strongly temperature dependent. They also show that record efficiencies of up to 10.8 percent -- as opposed to the currently published 9.8 percent -- are achievable with the substitution of numerous fullerenes. Additionally, this performance can be achieved in thick film devices.

"Once we saw how temperature affected the aggregation and morphology of these solar cells, it allowed the chemists more freedom to play with different chemical compositions in the active layer," Ade says. "Yan's team demonstrated 10 percent efficiency with 10different mixtures, and in thicker films. So these solar cells could be compatible with existing methods of mass production, like slot die casting and roll-to-roll processing similar to newspaper printing, rather than the more expensive production methods currently in use that are required for thickness control.

"We hope that these findings will allow others to experiment with different polymer:fullerene blends, further increasing the efficiency of solar cells, decreasing their production costs and leading to a commercially viable alternative source of energy."