solar cells

Quantum dot solar cells break efficiency record, silicon in its sights

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Solar cells made with quantum dots have achieved a new record efficiency, and been made more stable at the same time

Solar cells made with quantum dots have achieved a new record efficiency, and been made more stable at the same time

One of the most promising, emerging solar cell technologies has received a major efficiency boost. Engineers at UNIST in South Korea have created quantum dot solar cells with a world record efficiency of 18.1%.

Quantum dots are essentially just tiny, circular semiconductor crystals that are incredibly efficient at absorbing and emitting light. The color of light they interact with can be set by changing their size, which makes them useful in display technologies or as sensors.

But where they might end up being most useful is in solar cells. Most commercial solar cells are made with bulk materials as the light-collecting layer, which means the whole surface absorbs the same wavelengths. But with quantum dots you can have multiple sizes that focus on a different part of the spectrum, boosting potential efficiency. As an added bonus, they’re cheap and easy to manufacture, and can even be made into a sprayable solution.

For the new study, researchers at UNIST tweaked the recipe a bit to improve the technology. Quantum dot solar cells made with organic materials have the highest theoretical efficiency, but unfortunately they suffer from defects that make them less stable in sunlight and weather – not ideal for devices designed to be out in the sun all day. To get around that, these solar cells are usually made with inorganic materials instead, but this limits their efficiency, the team says.

The UNIST team made their quantum dots out of an organic perovskite, and developed a new method for anchoring them to a substrate that allowed the dots to be placed closer together. This boosted the efficiency to a record-setting high of 18.1%, up from 16.6% in 2020. This record has been independently recognized by the National Renewable Energy Laboratory (NREL), which keeps an ongoing chart comparing the efficiency of different technologies.

Even better, the new solar cells were far more stable. They maintained their efficiency for 1,200 hours under normal conditions, and 300 hours at an elevated temperature of 80 °C (176 °F). They performed just as well after two years in storage.

Quantum dot solar cells still have a long way to go to catch up to the everyday silicon solar cells, the latter has had a half-century head-start and is rapidly approaching its theoretical maximum efficiency. Meanwhile, quantum dots have only really been in the lab since about 2010, when they had an efficiency of under 4%. Along with the efficiency gains, the inexpensive and simple manufacturing should help scale up the tech and make a wider range of surfaces photovoltaic.

Solar cell polymers with multiplied electrical output

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One challenge in improving the efficiency of solar cells is that some of the absorbed light energy is lost as heat. So scientists have been looking to design materials that can convert more of that energy into useful electricity. Now a team from the U.S. Department of Energy’s Brookhaven National Laboratory and Columbia University has paired up polymers that recover some of that lost energy by producing two electrical charge carriers per unit of light instead of the usual one.

Solar cell polymers with multiplied electrical output

“Critically, we show how this multiplication process can be made efficient on a single molecular polymer chain,” said physicist Matthew Sfeir, who led the research at Brookhaven Lab’s Center for Functional Nanomaterials, a DOE Office of Science User Facility. Having the two charges on the same molecule means the light-absorbing, energy-producing don’t have to be arrayed as perfect crystals to produce extra electrical charges. Instead, the self-contained materials work efficiently when dissolved in liquids, which opens the way for a wide range of industrial scale manufacturing processes, including “printing” solar-energy-producing material like ink.

The research is published as an Advance Online Publication in Nature Materials, January 12, 2015.

The concept of producing two charges from one unit of light is called “.” (Think of the fission that splits a single biological cell into two when cells multiply.) Devices based on this multiplication concept have the potential to break through the upper limit on the efficiency of so-called single junction , which is currently around 34 percent. The challenges go beyond doubling the electrical output of the solar cell materials, because these materials must be incorporated into actual current-producing devices. But the hope is that the more-efficient current-generating materials could be added on to existing solar cell materials and device structures, or spark new types of solar cell designs.

Most singlet fission materials explored so far result in twin charge carriers being produced on separate molecules. These only work well when the material is in a crystalline film with long-range order, where strong coupling results in an additional charge being produced on a neighboring molecule. Producing such high quality crystalline films and integrating them with solar cell manufacturing complicates the process.

Producing the twin charges on a single polymer molecule, in contrast, results in a material that’s compatible with a much wider variety of industrial processes.

The materials were designed and synthesized by a Columbia University team led by Professor Luis Campos, and analyzed at Brookhaven using specialized tools at the CFN and in the Chemistry Department. For Sfeir and Campos, the most fascinating part of the interdisciplinary project was exploring the electronic and chemical requirements that enable this multiplication process to occur efficiently.

“We expect a significant leap in the development of third-generation, hot-carrier solar cells,” said Campos. “This approach is especially promising because the materials’ design is modular and amenable to current synthetic strategies that are being explored in second-generation .”

Details of the materials’ analysis

At the CFN, Sfeir and Erik Busby (a postdoctoral fellow) used time-resolved optical spectroscopy to induce and quantify singlet fission in the various polymer compositions using a single laser photon. Xiaoyang Zhu of Columbia helped to understand the data and interpret results.

“We put light energy into a material with a laser pulse and watch what happens to that energy using a series of weaker light pulses – somewhat analogous to taking snapshots using a camera with a very fast shutter,” Sfeir said.

The team also studied the same process using “pulse radiolysis” in collaboration with John Miller, who runs the Laser-Electron Accelerator Facility.

“The differences observed between these two experiments allowed us to unambiguously identify singlet fission as the primary process responsible for the production of these twin charges,” Sfeir said.

With Qin Wu, the team also used a powerful computer cluster at the CFN to model these materials and understand the design requirements that were necessary for singlet fission to take place.

“The ideas for this project and supervision of the work were really shared between Brookhaven and Columbia,” Sfeir said. “It’s a great example of the kind of collaborative work that takes place at DOE user facilities like the CFN.”

The next steps for the CFN-Columbia team will be to test a large class of materials using the design framework they’ve identified, and then integrate some of these carbon-based polymer materials into functioning solar cells.

“Even though we have demonstrated the concept of multiplication in single molecules, the next challenge is to show we can harness the extra excitations in an operating device. This may be in conventional bulk type solar cells, or in third-generation concepts based on other inorganic (non-carbon) nanomaterials. The dream is to build hot-carrier solar cells that could be fully assembled using solution processing of our organic singlet fission materials.”

Efficiently harvesting hydrogen fuel from Sun using Earth-abundant materials

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Scientists have a new efficient way of producing hydrogen fuel from sunlight and water. By combining a pair of solar cells made with a mineral called perovskite and low cost electrodes, scientists have obtained a 12.3 percent conversion efficiency from solar energy to hydrogen, a record using Earth-abundant materials as opposed to rare metals.

When an electrical current is applied, water splits into hydrogen and oxygen.
Credit: EPFL / LPI / Alain Herzog

The race is on to optimize solar energy’s performance. More efficient silicon photovoltaic panels, dye-sensitized solar cells, concentrated cells and thermodynamic solar plants all pursue the same goal: to produce a maximum amount of electrons from sunlight. Those electrons can then be converted into electricity to turn on lights and power your refrigerator.

At the Laboratory of Photonics and Interfaces at EPFL, led by Michael Grätzel, where scientists invented dye solar cells that mimic photosynthesis in plants, they have also developed methods for generating fuels such as hydrogen through solar water splitting.

To do this, they either use photoelectrochemical cells that directly split water into hydrogen and oxygen when exposed to sunlight, or they combine electricity-generating cells with an electrolyzer that separates the water molecules.

By using the latter technique, Grätzel’s post-doctoral student Jingshan Luo and his colleagues were able to obtain a performance so spectacular that their achievement is being published today in the journal Science. Their device converts into hydrogen 12.3 percent of the energy diffused by the sun on perovskite absorbers — a compound that can be obtained in the laboratory from common materials, such as those used in conventional car batteries, eliminating the need for rare-earth metals in the production of usable hydrogen fuel.

Bottled sun

This high efficiency provides stiff competition for other techniques used to convert solar energy. But this method has several advantages over others:

“Both the perovskite used in the cells and the nickel and iron catalysts making up the electrodes require resources that are abundant on Earth and that are also cheap,” explained Jingshan Luo. “However, our electrodes work just as well as the expensive platinum-based models customarily used.”

On the other hand, the conversion of solar energy into hydrogen makes its storage possible, which addresses one of the biggest disadvantages faced by renewable electricity — the requirement to use it at the time it is produced.

“Once you have hydrogen, you store it in a bottle and you can do with it whatever you want to, whenever you want it,” said Michael Grätzel. Such a gas can indeed be burned — in a boiler or engine — releasing only water vapor. It can also pass into a fuel cell to generate electricity on demand. And the 12.3% conversion efficiency achieved at EPFL “will soon get even higher,” promised Grätzel.

More powerful cells

These high efficiency values are based on a characteristic of perovskite cells: their ability to generate an open circuit voltage greater than 1 V (silicon cells stop at 0.7 V, for comparison).

“A voltage of 1.7 V or more is required for water electrolysis to occur and to obtain exploitable gases,” explained Jingshan Luo. To get these numbers, three or more silicon cells are needed, whereas just two perovskite cells are enough. As a result, there is more efficiency with respect to the surface of the light absorbers required. “This is the first time we have been able to get hydrogen through electrolysis with only two cells!” Luo adds.

The profusion of tiny bubbles escaping from the electrodes as soon as the solar cells are exposed to light say it better than words ever could: the combination of sun and water paves a promising and effervescent way for developing the energy of the future.

Video: http://www.youtube.com/watch?v=hkGAqk-TXw8&feature=youtu.be

Story Source:

The above story is based on materials provided by Ecole Polytechnique Fédérale de Lausanne. Note: Materials may be edited for content and length.

Journal Reference:

  1. Jingshan Luo, Jeong-Hyeok Im, Matthew T. Mayer, Marcel Schreier, Mohammad Khaja Nazeeruddin, Nam-Gyu Park, S. David Tilley, Hong Jin Fan, and Michael Grätzel. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science, 26 September 2014: 1593-1596 DOI:10.1126/science.1258307

Recycling old batteries into solar cells.

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This could be a classic win-win solution: A system proposed by researchers at MIT recycles materials from discarded car batteries—a potential source of lead pollution—into new, long-lasting solar panels that provide emissions-free power.
Recycling old batteries into solar cells

The system is described in a paper in the journal Energy and Environmental Science, co-authored by professors Angela M. Belcher and Paula T. Hammond, graduate student Po-Yen Chen, and three others. It is based on a recent development in that makes use of a compound called perovskite—specifically, organolead halide perovskite—a technology that has rapidly progressed from initial experiments to a point where its efficiency is nearly competitive with that of other types of solar cells.

“It went from initial demonstrations to good efficiency in less than two years,” says Belcher, the W.M. Keck Professor of Energy at MIT. Already, perovskite-based photovoltaic cells have achieved power-conversion efficiency of more than 19 percent, which is close to that of many commercial .

Initial descriptions of the perovskite technology identified its use of lead, whose production from raw ores can produce toxic residues, as a drawback. But by using recycled lead from old , the manufacturing process can instead be used to divert toxic material from landfills and reuse it in photovoltaic panels that could go on producing power for decades.

Amazingly, because the perovskite photovoltaic material takes the form of a thin film just half a micrometer thick, the team’s analysis shows that the lead from a single car battery could produce enough to provide power for 30 households.

As an added advantage, the production of is a relatively simple and benign process. “It has the advantage of being a low-temperature process, and the number of steps is reduced” compared with the manufacture of conventional solar cells, Belcher says.

Those factors will help to make it “easy to get to large scale cheaply,” Chen adds.

Read more at: http://phys.org/news/2014-08-recycling-batteries-solar-cells.html#jCp

The perovskite lightbulb moment for solar power.

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perovskite on glass

Bright idea: a 330nm-thick film of organometal halide perovskite fabricated on a glass sheet. This film is the active element of new 15% efficient solar cells. Photograph: Boshu Zhang, Wong Choon Lim Glenn & Mingzhen Liu

The worst part of my job as a materials scientist is going to conferences. They are often turgid affairs conducted in the ballrooms of hotels so identical to one another that you can’t tell whether you are in Singapore or Manchester. The same speakers are there, for the most part droning on about the same thing they droned on about at the last conference. I should know, I am one of them.

But occasionally, just occasionally, someone says something so radically new that it causes you sit up and actually listen. Your neighbours are no longer fiddling with their smartphones; there is the proverbial buzz in the air.

This was the scene at the Materials Research Society conference in Bostonlast December, where a breakthrough in perovskite solar cells was announced. If perovskites mean nothing to you read on, as they may have a very big impact on your future fuel bill.

If we could capture approximately 1% of the sunlight falling on to the British Isles and turn it into electricity we would meet our current energy demands. The reason no one suggests doing this rather than building wind, nuclear or conventional power stations is the cost. We currently use silicon solar cells to turn sunlight into electricity but they are expensive and require subsidies.

Silicon is a poor conductor of electricity because all of its four outer electrons are bound up in the chemical bonds holding the crystal together. However, by adding a tiny amount of phosphorous, which has five outer electrons, you effectively add a free electron to the crystal and make it conduct moderately well. Similarly, you can add boron, which has only three outer electrons, and effectively do the same thing, only now the conducting charge is called an electron hole.

The magic comes when you put a phosphorous silicon layer next to a boron silicon layer: the holes and the electrons cancel each other out at the junction but create an electric field that means that electrons only like to flow in one direction across the junction. This is called a diode.

There are many flavours of diodes, each having a different junction architecture. Light-emitting diodes (LEDs) emit light when electrons flow across the junction but the opposite effect also works: light hitting the diode creates an electric current, and this is how a solar cell works.

Silicon solar cells are as intricately micro-engineered as the silicon chips in your smartphone or tablet, but instead of being the size of a postage stamp they are the size of a table. They are made in enormous billion-dollar clean-room and high-temperature fabrication facilities, which are expensive precisely because controlling the purity of the silicon and the doping levels of phosphorous and boron is not trivial. It is also energy intensive. Although prices have been coming down, fossil fuels are still so cheap it is very hard for silicon solar cells to compete.

Materials scientists have been exploring other semiconductor technologies for a long while, trying to find something cheaper and better than silicon. Until recently the best bet was dye-sensitised solar cells. These have been around since the 1980s, but they have not managed to make a big impact on the energy market because, although they are cheap because they don’t require billion-dollar clean-room facilities, their efficiency is generally low. This is where the new perovskite solar cells come in. There are exotic-sounding compounds, such as methylammonium trihalogen plumbates, that have a quite simple crystal structure called a perovskite. Like the dye-sensitised solar cells, these solar cells are easy and cheap to make, but they have another trick up their sleeve – they don’t need a complicated diode architecture to achieve high efficiencies.

Materials researchers in Oxford, led by Dr Henry Snaith, have recently shownthat they can make simple perovskite solar cells with efficiencies pushing 20%. This is big news, because 20% makes them competitive with existing commercial silicon solar cells while being much cheaper to make in high volumes. They are also more suitable for incorporating into roofing materials and glass panels than silicon and so have the clear potential of being as fundamental to our city architecture as steel, concrete and asphalt. In other words, they could well be the materials that will make it possible to collect the 1% of solar energy we need as a nation, at a cost that can compete with fossil fuels.

Hearing research results such as this makes you grudgingly admit conferences are worth going to, and indeed gets you wondering whether we might look back in 10 years and pinpoint this as the time the solar energy revolution really ignited. One of my industry colleagues believes so; after the talk he immediately Skyped his research group, told them to stop what they were doing and get working on perovskite solar cells. The race to commercialise them is on.

World’s most efficient nanoplasmonic solar cells developed.

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In a boon for the local solar industry, a team of researchers from Swinburne University of Technology and Suntech Power Holdings have developed the world’s most efficient broadband nanoplasmonic solar cells.

In a paper published in Nano Letters, the researchers describe how they have manufactured thin film solar cells with an absolute efficiency of 8.1 per cent.

The research was conducted under the auspices of the Victoria-Suntech Advanced Solar Facility (VSASF) at Swinburne. The group is working to dramatically increase the efficiency of thin film solar technology.

According to Swinburne Professor Min Gu, Director of the VSASF, thin film cells have attracted enormous research interest as a cheap alternative to bulk crystalline silicon cells. However, the significantly reduced thickness of their silicon layer makes it more difficult for them to absorb sunlight.

“Light trapping technology is of paramount importance to increase the performance of thin film solar cells and make them competitive with silicon cells,” Professor Gu said. “One of the main potential applications of the technology will be to cover conventional glass, enabling buildings and skyscrapers to be powered entirely by sunlight.”

The VSASF group has been improving thin film cell efficiency by embedding gold and silver nanoparticles into the cells. This increases the wavelength range of the absorbed light, improving the conversion of photons into electrons.

In their most efficient cells yet, the researchers went one step further, using what are known as nucleated or ‘bumpy’ nanoparticles.

Senior Research Fellow at Swinburne Dr Baohua Jia said: “The broadband plasmonic effect is an exciting discovery of the team. It is truly a collaborative outcome between Swinburne and Suntech over the last 12 months.”

Dr Jia believes that this new technology will have an important impact on the solar industry. “What we have found is that nanoparticles that have an uneven surface scatter light even further into a broadband wavelength range. This leads to greater absorption, and therefore improves the cell’s overall efficiency.

Professor Gu applauded the quick timeframe in which the research group has been able to achieve 8.1 per cent total efficiency, however he believes there is still considerable scope to improve the cells and transform the way the world sources energy.

“We are on a rapid upwards trajectory with our research and development. With our current rate of progress we expect to achieve 10 per cent efficiency by mid 2012,” he said. “We are well on track to reach the VSASF’s target to develop solar cells that are twice as efficient and run at half the cost of those currently available.”

Professor Gu said that another advantage of the group’s approach is that nanoparticle integration is inexpensive and easy to upscale and therefore can easily be transferred to the production line.

“We have been using Suntech solar cells from the outset, so it should be very straightforward to integrate the technology into mass manufacturing. We expect these cells to be commercially available by 2017.”

Suntech CEO Dr Zhengrong Shi said: “Our team has achieved an impressive milestone with the world record for the most efficient broadband nanoplasmonic thin-film cell. This is an important step in demonstrating the potential of nanotechnology in leading the next generation of solar cells.”

Source:Nano letters