Thursday, January 23, 2014

From a carpet of nanorods to a thin film solar cell absorber within a few seconds

Research teams at the HZB and at the University of Limerick, Ireland, have discovered a novel solid state reaction which lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound. At the EDDI Beamline at BESSY II, the scientists could observe this process in real-time when heating the sample: in a few seconds Kesterite grains formed. The size of the grains was found to depend on the heating rate. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin film solar cells. These findings have now been published in the journal “Nature Communications”.

Grain formation during growth of kesterite solar cells observed in real-time
As starting material for the formation of the kesterite film serves a “carpet of nanorods”: With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick have fabricated films of highly ordered wurtzite nanorods, which have exactly the same composition as kesterite Cu2ZnSnS4. With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold could now observe how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. “It is interesting to see that the complete formation of the kesterite film is so fast”, says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form – instead of a few larger grains. Additionally, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.
Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model which can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.
Novel synthesis pathway for thin film semiconductors with controlled morphology
The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu2ZnSnS4-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se2 chalcopyrite solar cells which already achieved efficiencies above 20%. Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements which are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. “But we continue to focus on kesterites, because this is a really exciting topic at the moment.”
The results have been published in Nature communications doi: 10.1038/ncomms4133

Thursday, January 16, 2014

New hybrid molecules could lead to materials that function at the nanoscale

"It is possible to finely tune the properties of molecules through chemical synthesis to achieve just the right balance of properties needed," says Jonathan Rudick, an assistant professor of chemistry at Stony Brook University. "For example, through chemical synthesis, we can select ranges of the solar spectrum that a molecule will absorb, which has been essential for progress made in the area of organic molecules for solar power."
The National Science Foundation (NSF)-funded scientist is studying a class of molecules known as dendrons, highly branched molecules shaped like wedges or cones, which pack together to form circular or spherical assemblies with nanoscale dimensions. His group aims to develop a new class of nanoscale materials that can be processed like conventional synthetic polymers, yet retain the high structured order found in proteins.
One potential benefit of their work could be in developing a low-cost, low-weight and compact material that could be used to purify large volumes of water, and prove valuable in developing countries where potable water is difficult to find. It also could be useful in large scale water treatment facilities "where you need to be able to purify large volumes quickly, and the less membrane it takes to do that, the better," he says.
This requires creating the tiniest of channels for the water to pass through, which is not as simple as it sounds.
"The composition lining of the hole determines whether the water will go through," he says. "When you get a hole down to being the size of a molecule, then the interactions between the atoms in the water molecule and the atoms that line the hole become critical as to whether or not the water will go through. It's not like shooting water through a faucet."
Dendrons pose a special challenge in that "there is very little order to how the atoms are arranged within their assembly," making it difficult for scientists to manipulate the atoms, Rudick says.
However, peptides, on the other hand, another class of molecules "can take on a helical conformation, in which the atoms are arranged like a spiral staircase," with known locations for each atom, he explains. "Because the location of each atom in the helical molecule is known, we can accurately anticipate the positions of atoms in bundles of helical peptides."
Dendronized helix bundle assemblies also could have a major impact in the development of molecular materials for solar power, he says.
"The active components in organic photovoltaic materials are organic molecules that can absorb light called chromophores," he explains. "The arrangement of chromophores in a film plays an important role in determining whether an absorbed photon of light is transformed into energy we can use.
"Furthermore, the best arrangement of chromophores is not yet known, and will likely vary depending on the particular chromophore being used," he adds. "By incorporating chromophores within the helical bundle portion of our hybrid molecular materials, we will be able to systematically explore how to optimize the performance of solar conversion ."

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Wednesday, January 15, 2014

Improving solar efficiencies by 'inverse opal' structure

Researchers have shown how to increase the efficiency of thin-film solar cells, a technology that could bring low-cost solar energy. The approach uses 3-D "photonic crystals" to absorb more sunlight than conventional thin-film cells.

The synthetic crystals possess a structure called an "inverse opal" to make use of and enhance properties found in the gemstones to reflect, diffract and bend incoming sunlight.
"Usually, in thin-film silicon solar cells much of the sunlight comes right back out, but using our approach the light comes in and it is diffracted, causing it to propagate in a parallel path within the film," said Peter Bermel, an assistant professor in Purdue University's School of Electrical and Computer Engineering and Birck Nanotechnology Center.
Compared to solar cells made of silicon wafers, cost is reduced 100 times for the thin films. However, they are less efficient.
"The question is, can we make up that lower efficiency by introducing new approaches to light trapping for thin film solar cells?" Bermel said. "Can we combine low cost and high performance?"
The researchers are the first to demonstrate incorporation of the 3-D photonic crystals to increase light trapping in crystalline silicon solar cells. Experimental findings indicate roughly a 10 percent increase in efficiency over conventional silicon thin films, with further potential for improvement.
The technology is better at absorbing and harvesting near-infrared light.
"A major reason thin-film silicon solar cells have lower efficiency is that they don't absorb near-infrared light very effectively," Bermel said. "Light in the near-infrared range is important because there is a lot of solar energy in that wavelength range and also because silicon can convert near infrared light to energy if it can absorb it, but thin films don't fully absorb it."
Findings were detailed in a research paper appearing in October in the peer-reviewed scientific journal Advanced Optical Materials.
The researchers created inverse opals using a process called meniscus-driven self-assembly.
"You could make them to custom order or design, and we decided to make them for solar cells in order to improve absorption of light," Qi said.
Silicon has for many years been the dominant material used in solar cells. However, solar cells made of thick monocrystalline silicon wafers are too expensive to be practical for widespread application. This limitation has driven recent innovation in multicrystalline and thin-film silicon solar cells.
"Our premise is to use only 1 percent as much material as a silicon wafer using these thin films of crystalline silicon," Qi said.
Applications for thin-film solar cells include generating electricity for utilities and the home, as well as smaller-scale applications such as mobile charging of electronic devices.
Natural opals create rainbow patterns caused when different wavelengths of light are diffracted at different angles. Opals are made of solid silica spheres in a matrix of some other material. The new synthetic structures are called inverse opals because they consist of hollow spheres of air surrounded by silicon.
The researchers first build a standard opal structure. The spheres are placed in a solution, which evaporates, leaving the self-assembled structure.
"As it evaporates the spheres get stacked on top of the substrate right at the meniscus, the interface between the liquid and air," Varghese said.
Manufacturers now increase light absorption by etching or depositing random textures on the thin films.
"We think it is best to combine both the textured randomness as well as ordered structure," Bermel said. "The texture helps well with some wavelengths and the ordered structure will help with others."
More information: Varghese, L. T., Xuan, Y., Niu, B., Fan, L., Bermel, P. and Qi, M. (2013), "Enhanced Photon Management of Thin-Film Silicon Solar Cells Using Inverse Opal Photonic Crystals with 3D Photonic Bandgaps." Advanced Optical Materials, 1: 692–698. DOI: 10.1002/adom.201300254

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Wednesday, January 8, 2014

Increasing solar efficiencies by reducing different between HOMO & LUMO level

A common way to increase the solar cell efficiencies is to adjust the differences between HOMO and LUMO level of the polymer so that the excitons can be harvested with minimal lost. A fluorine atom is usually added to polymer backbone but it is a multistep process and has a considerable fabrications cost.   
A team of chemists led by Jianhui Hou from the Chinese Academy of Sciences created a polymer known as PBT-OP from two commercially available monomers and one easily synthesized monomer. Wei Ma, a post-doctoral physics researcher from NC State and corresponding author on a paper describing the research, conducted the X-ray analysis of the polymer's structure and the donor:acceptor morphology.
PBT-OP was not only easier to make than other commonly used polymers, but a simple manipulation of its chemical structure gave it a lower HOMO level than had been seen in other polymers with the same molecular backbone. PBT-OP showed an open circuit voltage (the voltage available from a solar cell) value of 0.78 volts, a 36 percent increase over the ~ 0.6 volt average from similar polymers.

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Monday, December 16, 2013

Researchers split water into hydrogen, oxygen using light, nanoparticles

Researchers split water into hydrogen, oxygen using light, nanoparticles

Researchers from the University of Houston have found a catalyst that can quickly generate hydrogen from water using sunlight, potentially creating a clean and renewable source of energy.
Their research, published online Sunday in Nature Nanotechnology, involved the use of cobalt oxide nanoparticles to split water into hydrogen and oxygen.
Jiming Bao, lead author of the paper and an assistant professor in the Department of Electrical and Computer Engineering at UH, said the research discovered a new photocatalyst and demonstrated the potential of nanotechnology in engineering a material's property, although more work remains to be done.
Bao said photocatalytic water-splitting experiments have been tried since the 1970s, but this was the first to use cobalt oxide and the first to use neutral water under visible light at a high energy conversion efficiency without co-catalysts or sacrificial chemicals. The project involved researchers from UH, along with those from Sam Houston State University, the Chinese Academy of Sciences, Texas State University, Carl Zeiss Microscopy LLC, and Sichuan University.
Researchers prepared the nanoparticles in two ways, using femtosecond laser ablation and through mechanical ball milling. Despite some differences, Bao said both worked equally well.
Different sources of light were used, ranging from a laser to white light simulating the solar spectrum. He said he would expect the reaction to work equally well using natural sunlight.
Once the nanoparticles are added and light applied, the water separates into hydrogen and oxygen almost immediately, producing twice as much hydrogen as oxygen, as expected from the 2:1 hydrogen to oxygen ratio in H2O water molecules, Bao said.
The experiment has potential as a source of renewable fuel, but at a solar-to-hydrogen efficiency rate of around 5 percent, the conversion rate is still too low to be commercially viable. Bao suggested a more feasible efficiency rate would be about 10 percent, meaning that 10 percent of the incident solar energy will be converted to hydrogen chemical energy by the process.
Other issues remain to be resolved, as well, including reducing costs and extending the lifespan of cobalt oxide nanoparticles, which the researchers found became deactivated after about an hour of reaction.
"It degrades too quickly," said Bao, who also has appointments in materials engineering and the Department of Chemistry.
The work, supported by the Welch Foundation, will lead to future research, he said, including the question of why cobalt oxide nanoparticles have such a short lifespan, and questions involving chemical and electronic properties of the material.

University of Houston (2013, December 15). Researchers split water into hydrogen, oxygen using light, nanoparticlesScienceDaily. Retrieved December 16, 2013, from­/releases/2013/12/131215160904.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily%2Ftop_news%2Ftop_science+%28ScienceDaily%3A+Top+News+--+Top+Science%29

Thursday, November 21, 2013

Hydrogen cars could be headed to showroom near you

Cars that run on hydrogen and exhaust only water vapor are emerging to challenge electric vehicles as the world's transportation of the future.

At auto shows on two continents Wednesday, three automakers were unveiling hydrogen fuel cell vehicles to be delivered to the general public as early as spring of next year.
Korea's Hyundai Motor Co. will be the first to the mass market in the U.S. with a hydrogen-powered Tucson small SUV for lease next spring. Details were to come later Wednesday at the Los Angeles Auto Show. Honda also revealed plans in Los Angeles for a car due out in 2015. Earlier, at the Tokyo Motor Show, Toyota promised a mass-produced fuel cell car by 2015 in Japan and 2016 in the U.S.
Hydrogen cars are appealing because unlike electric vehicles, they have the range of a typical gasoline car and can be refueled quickly. Experts say the industry also has overcome safety and reliability concerns that have hindered distribution in the past.
But hydrogen cars still have a glaring downside—refueling stations are scarce, and costly to build. Critics say the cars are still a long way from mass production.
Consumers can expect costs in line with some luxury models. In Tokyo, Toyota promised a price of 5 million yen ($50,000) to 10 million yen ($100,000), and as close to the lower figure as possible. That's comparable to its Lexus sedans, but a range that makes the once space-age experiment with fuel cells more credible.
Even as battery-powered and hybrid-electric cars took on conventional gasoline models in the past decade, automakers continued research into hydrogen fuel cells, said Paul Mutolo, director of external partnerships for the Cornell University Energy Materials Center. Manufacturers now are limited only by costs and the lack of filling stations, he said.
Hydrogen cars, Mutolo said, have an advantage over battery-powered electric cars because drivers don't have to worry about running out of electricity and having to wait hours for recharging. "It's very similar to the kind of behavior that drivers have come to expect from their gasoline cars," he said.
Hydrogen fuel cells use a complex chemical process to separate electrons and protons in hydrogen gas molecules. The electrons move toward a positive pole, and the movement creates electricity. That powers a car's electric motor, which turns the wheels. "You're literally ripping the electrons from inside the molecule, generating electricity," Mutolo said.
Since the hydrogen isn't burned, there's no pollution. Instead, oxygen also is pumped into the system, and when it meets the hydrogen ions and electrons, that creates water and heat. The only byproduct is water. A fuel cell produces only about one volt of electricity, so many are stacked to create enough juice.
Hydrogen costs as little as $3 for an amount needed to power a car the same distance as a gallon of gasoline, Mutolo said.
Hyundai's plan includes leasing the hydrogen SUVs starting in the Los Angeles area, where most of the state's nine refueling stations are located. California lawmakers have allocated $100 million to build 100 more.
Mutolo estimates it will take at least 10 years for stations to spread nationwide.
Manufacturers likely will lose money on hydrogen cars at first, but costs will decrease as precious metals are reduced in the fuel cells, he said.
Toyota said its new fuel cell vehicle will be for ordinary customers, not just officials and celebrities. The car will go on sale in Japan in 2015 and within a year later in Europe and U.S.
Toyota's fuel cell car is on display as a "concept" model called FCV at the Tokyo show, where alternative fuel is grabbing the spotlight. The FCV looks ready to hit the streets, similar to the Prius gas-electric hybrid.
Honda, which has leased about two-dozen fuel cell cars since 2005, took the wraps off a futuristic-looking FCEV concept vehicle in Los Angeles. The concept vehicle shows the style of a 300-mile range fuel cell car that will be marketed in the U.S. and Japan in 2015 and in Europe after that. Honda wouldn't say if it will be offered for lease or purchase.
All major automakers, including General Motors Co. and Daimler, have been working on fuel cells for decades. But the prospect of reaching showrooms was not very real until recently.
Skeptics say hydrogen-fueling stations are more expensive than electric car charging stations, partly because electricity is almost everywhere and new and safe ways for producing, storing and transferring hydrogen will be needed.
Carlos Ghosn, chief executive of Nissan Motor Co., which has bet heavily on electric vehicles for its future, is one vocal skeptic.
"Having a prototype is easy. The challenge is mass-marketing," he told reporters. He said he did not see a mass-market fuel cell as viable before 2020.

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