Superconductors and Super Solar Cells

What's Hot in Chemistry 2011

by John Emsley

Three new papers enter the current Hot Ten. Paper #5 reports a new high-temperature superconductor. Paper #7 reports a new type of solar cell and demonstrates its use in some simple devices. Paper #10 reports a new conducting material for use in such cells.

The so-called "iron age of superconductivity" began with the discovery, by Yoichi Kamihara of the Tokyo Institute of Technology, that lanthanum oxide iron arsenide (LaOFeAs) displayed this behavior despite incorporating iron (see Science Watch, Nov-Dec 2008). It had hitherto been assumed that this magnetic element would exclude its being incorporated into superconductors.

Paper #5 goes one step further, showing that even a simple binary compound, iron selenide, can have superconducting ability. It qualifies as a "high temperature" superconductor because it exhibits this property as high as 37 K (–236 °C) albeit only under pressures of 10 GPa (c. 100,000 atmospheres). This is far from being the highest temperature for a superconductor. The mixed metal oxide HgBa2Ca2Cu3O8 remains superconducting up to 134 K (–139 °C).

The new superconductor has the composition Fe1.01Se with a ß crystal form. It came as a result of collaborative work at five institutes, namely the University of Mainz and the Max-Plank-Institute at Mainz in Germany, Princeton University in New Jersey, the Institute of Physical Chemistry at Warsaw, Poland, and the University of Paderborn, also in Germany.

David Cardwell holding a bulk single grain sample of superconducting Y-Ba-Cu-O being levitated by a permanent magnet. From the interview, "David Cardwell on Superconducting Materials with Industrial Applications," within the Special Topic of High-Temperature Superconductors.

ß-Fe1.01Se poses some interesting questions, as yet only partly answered. It is thought that the superconductivity arises from layers in which the iron atoms are tetrahedrally surrounded by selenium atoms, and that these tetrahedra have shared edges. These layers are interspersed by other layers. But why does its superconductivity increase under pressure? The answer would appear to lie in the structural deformation which this causes to the interlinked tetrahedral layers.

Paper #7 concerns conduction under more normal temperatures, but without the need for metals. The July-August 2011 issue of Science Watch dealt with a method of mass-producing 30-inch wide graphene films on a transparent sheet of polyethylene terephthalate (PET). The graphene was transferred from a substrate to the PET by passing the two through rollers, as in offset printing, but heated at 120 °C. The resulting product met the requirements of strong adhesion, transparency, and conductivity, and it was tested as a touch-screen panel and shown to be remarkably robust despite being flexible.

Paper #7 also uses this roll-to-roll technique, and it comes from the Technical University of Denmark at Roskilde, being the work of Frederik Krebs, Jan Fyenbo, and Mikkel Jørgensen. It reports the production of "printed" solar cells which can fuel small hand-held devices by charging a lithium ion battery sufficient to power a tiny flashlight. These were handed out as freebies at a conference, and some were donated to schoolchildren in Zambia so that they might use them as reading lamps.

The cells were printed as parallel strips of lines 5 mm wide onto PET coated with the transparent conducting material ITO (indium tin oxide). The strips consisted of zinc oxide, P3HT/PCBM, the conducting polymer PEDOT:PSS, and silver. P3HT/PCBM is poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester and PEDOT:PSS is poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate).
Finally, paper #10 offers an alternative material to the semiconducting polymers used as the donor materials in organic solar cells. Polymers would seem to be an obvious choice for making these, on account of their chemical composition being in the form of molecular strands like tiny wires along which electrons can flow via delocalized bonding. However, these polymers have weaknesses such as their inconsistent performance and the difficulty in making them as pure as required for optimum behaviour.

Enter the flexible film announced by Thuc-Quyen Nguyen and his team at the University of California, Santa Barbara. This material is formed from a diketopyrrolopyrrole donor, processed with a fullerene acceptor, and the resulting conductive film performs well and without the drawbacks associated with conducting polymers.
The above papers leads one to hope that maybe one day it will be possible to generate electricity from cheap solar panels located in tropical desert regions and transport it via superconducting power lines to the cold cities of the north. One day.End

Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.

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What's Hot in Chemistry
Rank Paper Cites This Period
May-Jun 11
Rank Last Period
Mar-Apr 11
Y.Y. Liang, et al., "For the bright future—Bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%," Adv. Materials, 22(20): E135-8, 25 May 2010.  [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   *612IK 49 2
S. Bae, et al., "Roll-to-roll production of 30-inch graphene films for transparent electrodes," Nature Nanotech., 5(8): 574-8, August 2010.  [8 South Korean, Singaporean, and Japanese institutions]  *635BZ 40 3
3 F.C. Krebs, T. Tromholt, M. Jorgensen, "Upscaling of polymer solar cell fabrication using full roll-to-roll processing," Nanoscale, 2(6): 873-86, June 2010. [Tech. U. Denmark, Roskilde]  *608ML 33 8
C.H. Lu, et al., "A graphene platform for sensing biomolecules," Angew. Chem. Int. Ed., 48(26): 4785-7, 2009. [Fuzhou U., China; First Inst. Oceanography, Qingdao, China]  *464GX 30 7
S. Medvedev, et al., "Electronic and magnetic phase diagram of ß-Fe1.01Se with superconductivity at 36.7 K under pressure," Nature Materials, 8(8): 630-3, August 2009. 
[6 European and U.S. institutions]  *474ML
6 Y.Y. Liang, et al., "Highly efficient solar cell polymers developed via fine-tuning of structural and electronic properties," J. Am. Chem. Soc., 131(22): 7792-9, 10 June 1009. [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   *460HD 26 4
7 F.C. Krebs, J. Fyenbo, M. Jorgensen, "Product integration of compact roll-to-roll processed polymer solar cell nodules: methods and manufacturing using flexographic printing, slot-die coating and rotary screen printing," J. Mater. Chem., 20(41): 8994-9001, 7 November 2010. [Tech. U. Denmark, Roskilde; Mekoprint Electronics, Stovring, Denmark]  *663GP 25
J.F. Li, et al., "Shell-isolated nanoparticle-enhanced Raman spectroscopy," Nature, 464(7287): 392-5, 18 March 2010. [Xiamen U., China; Georgia Tech, Atlanta]  *570FG 22 9
9 M. Zhou, Y. Zhai, S. Dong, "Electrochemical sensing and biosensing platform based on chemically reduced graphene oxide," Anal. Chem., 81(14): 5603-13, 15 July 2009.  [Chinese Acad. Sci., Changchun, China]  *472MF 21
10 B. Walker, et al., "Nanoscale phase separation and high photovoltaic efficiency in solution-processed, small-molecule bulk heterojunction solar cells," Adv. Funct. Mater., 19(19): 3063-9, 9 October 2009.  [U. Calif., Santa Barbara]  *510ZJ 21
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend.




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