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Documents jugés importants
2010
Closing in on a carbon-based solar cell
juillet
Les boîtes quantiques et l'amélioration de l'efficacité des panneaux solaires (ADIT)

     A l'heure actuelle, les cellules photovoltaïques demeurent relativement peu efficaces dans l'opération de conversion de la lumière en électricité. En dépit des cellules à multi-jonctions qui permettent de convertir plus de 40% de l'énergie incidente et de matériaux semiconducteurs plus simples possédant en moyenne une efficacité de 20% suite à l'industrialisation à grande échelle, ces cellules ne convertissent toujours qu'un tiers de l'énergie incidente. Cependant, il se pourrait qu'une des limitations physiques à l'amélioration du rendement des cellules soit en passe d'être contournée: il s'agit des pertes par chaleur. Des nanocristaux à base de matériaux semiconducteurs permettant d'éviter les fuites d'électrons trop énergétiques seraient le fruit des travaux de chercheurs des universités du Minnesota et du Texas [1].
     Rappelons que les cellules solaires sont réalisées à partir de matériaux semiconducteurs en raison des propriétés bien particulières des ces matériaux. Lorsqu'un photon possédant la bonne longueur d'onde vient rencontrer un tel composant, celui-ci libère un électron qui sera ensuite à l'origine avec ses congénères du courant électrique. Néanmoins, nombre de ces électrons libérés se dissipent sous forme de chaleur au lieu de participer au flux électrique global. Des travaux de ces équipes ont déjà montré par le passé que des nanocristaux à base de matériaux semiconducteurs pouvaient en effet "ralentir" ces électrons "surchauffés". Par voie de conséquence, ces nanocristaux, aussi appelés boîtes quantiques (quantum dots) [2], seraient susceptibles d'augmenter l'efficacité des cellules solaires. Les résultats de leurs recherches montrent que c'est en pratique bien le cas. En plus de capturer les électrons énergétiques, les boîtes quantiques permettent aussi de les transmettre à un matériau receveur tel que le dioxyde de titane usuellement employé dans les cellules solaires conventionnelles. 		suite:
     En pratique, ce transfert se déroule en moins de 50 femtosecondes. Du fait de l'extrême rapidité du transfert, moins d'électrons sont perdus sous forme de chaleur et l'efficacité théorique de la cellule atteindrait 66%. Malheureusement, cet avantage remarquable des boîtes quantiques doit encore être intégré dans l'architecture d'une cellule solaire. La prochaine étape consiste à montrer que les électrons capturés par ce procédé et le courant qui en découle peuvent être transmis dans un câble comme c'est le cas dans toutes les cellules. En effet, il faudra alors réaliser un câble assez petit pour connecter une cellule photovoltaïque à base de boîtes quantiques, dont le diamètre n'excéderait pas 6,7 nanomètres, et qui ne dissiperait pas de manière trop importante l'énergie transmise. Cette étape, la plus délicate, risque de prendre plusieurs années avant d'être réalisée mais les chimistes de ces universités ont déjà ouvert le champ à de nombreuses pistes d'amélioration des performances des cellules solaires [3].
     Certaines startups telles que Magnolia Solar [4] se sont déjà lancées dans la course. La société sponsorisée par le Département de l'Energie américain (DOE) à hauteur d'un million de dollars avance main dans la main avec le département de Nano-Ingénierie de l'université d'Albanie à New-York.
     Les percées telles que les boîtes quantiques ne représentent qu'une des facettes du futur du solaire. Il y a à l'heure actuelle d'innombrables pistes en cours d'exploration. Soulignons les plus importantes: les revêtements haute performance qui permettent d'augmenter l'efficacité en éliminant partiellement les réflexions parasites, les systèmes de suivi de la trajectoire du soleil. Mais à l'heure actuelle, nul ne peut prédire laquelle franchira les portes des laboratoires...
Sources:
- [1] Hot-Electron Transfer from Semiconductor Nanocrystals, Science June 18 2010
- [2] How Quantum Dots Work Evident Technologies website
- [3] Quantum Dots Could Boost Solar Efficiency by 100%, Scientific American, June 20 2010
- [4] Magnolia Solar Brings Nano-Engineered, Non-Toxic, Low Cost Thin Film to the Table, Scientific American, March 6 2010.
Rédacteur:
Marion Franc, mfranc.ambassadeUS@gmail.com
Origine:
BE Etats-Unis numéro 215 (9/07/2010) - Ambassade de France aux Etats-Unis / ADIT
juin
Capturing "Hot" Electrons to Double Solar Power
Friday, June 18, 2010
     Researchers demonstrate that high-energy electrons lost in conventional solar cells can be captured.
By Katherine Bourzac

     There's a limit on the conversion efficiency of a conventional solar cell. No matter how it's tweaked, it can only convert 31% of the light that hits it into usable electrical current. That's because there's a broad spectrum of wavelengths in sunlight, and some of it has more energy than the active material in the solar cell can handle. High-energy light hits the active material in a solar cell and knocks loose electrons that have a similarly high energy--then these electrons rapidly lose that excess energy as heat.
     Physicists know that if they could capture "hot electrons", they could more than double the efficiency of solar cells. The problem is that they lose their energy in a picosecond. Now, researchers have for the first time demonstrated that it's possible to capture hot electrons while they're still in their high energy state, before that heat loss happens.

      Careful design at the nanoscale is key. Instead of a conventional bulk semiconductor, the researchers used quantum dots, because these nanomaterials can confine electrons over a longer timescale. "Nanomaterials can keep electrons electrons hot for a longer period of time, so that you can get them out," says Xiaoyang Zhu, professor of chemistry at the University of Texas, Austin.
     The confinement is great--until you want to get the hot electrons out. "The electron likes to stay inside the nanomaterial, so you need to make an extremely strong interaction with another material" that will conduct the electrons out of the quantum dot, Zhu says. His group coated the quantum dots with a very thin layer of an electrical conductor, and were meticulous about the quality of the interface between that material and the quantum dots.
     So now it's possible to get hot electrons out, but one major problem remains. Those hot electrons require new device designs that prevent them from simply losing their energy to heat once they enter the metal wire of an electrical circuit. "We hope to inspire people to work on the engineering," says Zhu.
     This research was published this week in the journal Science.
avril
Contact: David Bricker
brickerd@indiana.edu
812-856-9035
Indiana University


IMAGE: This is a 2-D view of a graphene sheet (black) and attached sidegroups (blue) that IU Bloomington chemist Liang-shi Li and his collaborators devised. In reality, each sidegroup rotates 90 degrees or so out of graphene's plane. The three blue, tail-like hydrocarbons of each sidegroup have great freedom of movement, but two are likely to hover over the graphene, making it very unlikely that one graphene sheet will touch another.
Credit: Image by Liang-shi Li
Usage Restrictions: None


IMAGE: Two graphene molecules (dark grey) are caged by sidegroups (blue) attached to each graphene sheet. The sidegroups help prevent the graphene sheets from stacking, as they are prone to do.

BLOOMINGTON, Ind. -- To make large sheets of carbon available for light collection, Indiana University Bloomington chemists have devised an unusual solution -- attach what amounts to a 3-D bramble patch to each side of the carbon sheet. Using that method, the scientists say they were able to dissolve sheets containing as many as 168 carbon atoms, a first.
     The scientists' report, online today (April 9), will appear in a future issue of Nano Letters, an American Chemical Society journal.
     "Our interest stems from wanting to find an alternative, readily available material that can efficiently absorb sunlight," said chemist Liang-shi Li, who led the research. "At the moment the most common materials for absorbing light in solar cells are silicon and compounds containing ruthenium. Each has disadvantages."
     Their main disadvantage is cost and long-term availability. Ruthenium-based solar cells can potentially be cheaper than silicon-based ones, but ruthenium is a rare metal on Earth, as rare as platinum, and will run out quickly when the demand increases.
     Carbon is cheap and abundant, and in the form of graphene, capable of absorbing a wide range of light frequencies. Graphene is essentially the same stuff as graphite (pencil lead), except graphene is a single sheet of carbon, one atom thick. Graphene shows promise as an effective, cheap-to-produce, and less toxic alternative to other materials currently used in solar cells. But it has also vexed scientists.
     For a sheet of graphene to be of any use in collecting photons of light, the sheet must be big. To use the absorbed solar energy for electricity, however, the sheet can't be too big.

suite:
     Unfortunately, scientists find large sheets of graphene difficult to work with, and their sizes even harder to control. The bigger the graphene sheet, the stickier it is, making it more likely to attract and glom onto other graphene sheets. Multiple layers of graphene may be good for taking notes, but they also prevent electricity.
     Chemists and engineers experimenting with graphene have come up with a whole host of strategies for keeping single graphene sheets separate. The most effective solution prior to the Nano Letters paper has been breaking up graphite (top-down) into sheets and wrap polymers around them to make them isolated from one another. But this makes graphene sheets with random sizes that are too large for light absorption for solar cells.
     Li and his collaborators tried a different idea. By attaching a semi-rigid, semi-flexible, three-dimensional sidegroup to the sides of the graphene, they were able to keep graphene sheets as big as 168 carbon atoms from adhering to one another. With this method, they could make the graphene sheets from smaller molecules (bottom-up) so that they are uniform in size. To the scientists' knowledge, it is the biggest stable graphene sheet ever made with the bottom-up approach.
     The sidegroup consists of a hexagonal carbon ring and three long, barbed tails made of carbon and hydrogen. Because the graphene sheet is rigid, the sidegroup ring is forced to rotate about 90 degrees relative to the plane of the graphene. The three brambly tails are free to whip about, but two of them will tend to enclose the graphene sheet to which they are attached.
     The tails don't merely act as a cage, however. They also serve as a handle for the organic solvent so that the entire structure can be dissolved. Li and his colleagues were able to dissolve 30 mg of the species per 30 mL of solvent.
     "In this paper, we found a new way to make graphene soluble," Li said. "This is just as important as the relatively large size of the graphene itself."
     To test the effectiveness of their graphene light acceptor, the scientists constructed rudimentary solar cells using titanium dioxide as an electron acceptor. The scientists were able to achieve a 200-microampere-per-square-cm current density and an open-circuit voltage of 0.48 volts. The graphene sheets absorbed a significant amount of light in the visible to near-infrared range (200 to 900 nm or so) with peak absorption occurring at 591 nm.
     The scientists are in the process of redesigning the graphene sheets with sticky ends that bind to titanium dioxide, which will improve the efficiency of the solar cells.
     "Harvesting energy from the sun is a prerequisite step," Li said. "How to turn the energy into electricity is the next. We think we have a good start."
     PhD students Xin Yan and Xiao Cui and postdoctoral fellow Binsong Li also contributed to this research. It was funded by grants from the National Science Foundation and the American Chemical Society Petroleum Research Fund.
     To speak with Liang-shi Li, please contact David Bricker, University Communciations, at 812-856-9035 or brickerd@indiana.edu.
     "Large, Solution-Processable Graphene Quantum Dots as Light Absorbers for Photovoltaics," Nano Letters (Articles ASAP)
Voir nos documents "quantum dots":
Cellules solaires PV plus efficaces grâce aux nanocristaux semiconducteurs & Le CIGS 2.0 d'Applied Quantum Technology, ADIT, juin 2010:
AUSTIN, Texas - Conventional solar cell efficiency could be increased from the current limit of 30% to more than 60%, suggests new research on semiconductor nanocrystals, or quantum dots, led by chemist Xiaoyang Zhu at The University of Texas at Austin.
Etats - Unis: des cellules photovoltaïques "Arc en Ciel", ADIT, mars 2008:
... Les scientifiques utilisent ces quantum dots de Cadmium Selenide (CdSe) semiconducteurs plutôt que d'autres matériaux car ils présentent ...
Etats-Unis, Silicon Nanocrystals for Superefficient Solar Cells, août 2007:
Souped-up silicon: A micrograph of a seven-nanometer chunk of crystalline silicon, called a nanocrystal or quantum dot. Such structures could dramatically ...
Etats - Unis, Cheap Nano Solar Cells, mars 2007:
13 août 2009 ... Another approach, which has been demonstrated experimentally by Kamat, is to coat the nanoparticles with quantum dots--tiny semiconductor ...
Norvège, Une nouvelle génération de cellules solaires, février 2006:
Des chercheurs de l'Universite de Trondheim (NTNU) s'intéressent à la conception d'une nouvelle génération de cellules solaires. (...) La technologie est basée sur les boîtes quantiques ou "quantum dots"...
Espagne, Etude d'un système d'énergie solaire plus économique, janvier 2006:
La technologie est basée sur les boîtes quantiques ou "quantum dots". Ces nanocristaux ont la faculté d'absorber une partie de la lumière infrarouge que les ...