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A Neat Little Trick Doubles
Plastic Solar-Cell Efficiency
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by John Emsley
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Chemistry
Top Ten Papers
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Rank
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Papers
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Cites
Jul-Aug 08
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Rank
May-Jun 08
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1
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Y. Kamihara, et al.,
"Iron-based layered superconductor
La[O1-xFx]FeAs (x
= 0.05-0.12) with Tc = 26
K," J. Am. Chem. Soc.,
130(11): 3296-7, 19 March 2008. [Tokyo
Inst. Technol., Yokohama, Japan] *273SL
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62
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2
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2
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J. Peet, et al.,
"Efficiency enhancement in low-bandgap
polymer solar cells by processing with
alkane dithiols," Nature
Mater., 6(7): 497-500, July 2007.
[U. Calif., Santa Barbara] *184NH
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27
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†
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3
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Y. Kamihara, et al.,
"Iron-based layered superconductor:
LaOFeP," J. Am. Chem. Soc.,
128(31): 10012-3, 9 August 2006. [Tokyo
Inst. Technol., Yokohama, Japan] *069NI
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26
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†
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4
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X.L. Lie, et al., "Chemically
derived, ultrasmooth graphene
nanoribbon semiconductors,"
Science, 319(5867): 1229-32,
29 February 2008. [Stanford U., CA]
*267SX
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21
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†
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5
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T. Watanabe, et al.,
"Nickel-based oxyphosphide
superconductor with a layered crystal
structure, LaNiOP," Inorganic
Chem., 46(19): 7719-21, 17
September 2007. [Tokyo Inst. Technol.,
Yokohama, Japan] *209EJ
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20
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†
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6
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M. Dinca, et al., "Hydrogen
storage in a microporous metal-organic
framework with exposed Mn2+
coordination sites," J. Am. Chem.
Soc., 128(51): 16876-83, 27
December 2007. [6 U.S. institutions]
*118KQ
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17
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6
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7
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D. Enders, et al.,
"Organocatalytic one-pot asymmetric
synthesis of functionalized tricyclic
carbon frameworks from a
triple-cascade/Diels-Alder sequence,"
Angew. Chem. Int. Ed., 46(3):
467-9, 2007. [RWTH Aachen, Germany]
*125PJ
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17
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†
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8
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Y. Hayashi, et al.,
"Diphenylprolinol silyl ether as a
catalyst in an enantioselective,
catalytic, tandem Michael/Henry
reaction for the control of four
stereocenters," Angew. Chem. Int.
Ed., 46(26): 4922-5, 2007. [Tokyo
U. Sci., Japan] *186IV
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17
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†
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9
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J.E. Green, et al., "A
160-kilobit molecular electronic memory
patterned at 1011bits per
square centimetre," Nature,
445(7126): 414-7, 25 January 2007.
[Caltech, Pasadena; U. Calif., Los
Angeles; Ohio St. U., Columbus] *128WD
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15
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5
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10
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C. Soci, et al., "ZnO nanowire
UV photodetectors with high internal
gain," Nano Lett., 7(4):
1003-9, April 2007. [U. Calif., San
Diego] *155TG
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15
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†
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SOURCE:
Thomson Reuter's
Hot Papers
Database. Read the
Legend.
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Superconductors continue to head the top of the chemistry Hot Ten:
witness papers #1 and #3, both of which were reported on in the previous
issue, and #5, which is from the same stable. Also high in the list is
paper #4, which is at the other end of electron mobility, i.e.,
semiconductors. The new entrant, however, paper #2—which is also
about electricity—offers practical applications in the form of more
efficient solar panels.
If in 50 years time the world wishes to enjoy the lifestyle we are used to,
but without our dependence on fossil fuels, then it will have to find ways
of harvesting energy from the sun. Turning its bounteous supply of photons
into moving electrons will be a vital part of making the world independent
of polluting forms of power. A major step in making this possible will be
plastic solar cells, and making these more efficient is the
essence of paper #2.
This comes from the Center for Polymers and Organic Solids at the
University of California at Santa Barbara and reports the remarkable
findings of a group led by Nobel Laureate
Alan Heeger and
Guillermo Bazan. They have discovered that adding
1,8-octanedithiol to the solution from which plastic solar cells are
made almost doubles their power-conversion efficiency from 2.8% to 5.5%.
(Other dithiols also increased the efficiency but not quite so
markedly.) The method is operationally very simple, and once the film
has formed then the additive disappears, and it seems that there is no
need either to optimize materials or to change the way the cells are
fabricated.
Polymer solar cells combine fullerenes with polymer semiconductors, the
former releasing electrons and the latter providing the "holes." The
electrons and holes then migrate to their respective electrodes and produce
the current. Of course electrons and holes near the interface simply move
towards each other and combine to no net effect, but this is inherent in
the morphology that results from the spin-casting method of manufacture.
Improving this morphology has been achieved by Heeger and Bazan, and the
method requires only small concentrations of 1,8-octanedithiol to be added
to the solutions from which the heterojunctions films are cast. These were
fabricated from the photo-donor organic polymer poly(3-hexylthiophene)
(a.k.a. P3HT) with the acceptor being the fullerene derivative
[6,6]-phenylC61- butyric acid methyl ester
(C61-PCBM). The drawback from solar cells produced from these
components is that they do not respond to thermal annealing or to slow
solvent evaporation after the film has been cast.
The answer which Heeger and Bazan provide is both a simple and versatile
method for tailoring the heterojunction morphology in systems where thermal
annealing is not effective. They also point out that their approach even
works on a system in which polymer crystallinity is not observed.
The addition of a mere 24 mg per liter of 1,8-octanedithiol to the
chlorobenzene solution from which the films were cast had a remarkable
effect. Moreover, after these had been dried in vacuum they were examined
by FTIR and Raman spectroscopy, and these showed no thiol residues to be
present. More than 250 samples of the plastic solar films were made and
more than 1,000 devices were constructed for testing. The most efficient of
these had a polymer/fullerene ratio of between 1:2 and 1:3 and attained a
power-conversion efficiency of 5.5% under illumination of 100
mWcm-2 generating a current of 16.2 mAcm-2 and a
voltage of 0.62V.
Currently Bazan is researching how the different environments brought about
by the additive influence the way the internal components of the
photovoltaic film assemble.
As he tells Science Watch: "We have found that the nano-scale
arrangement of the molecular components is critical for effective charge
generation and collection. However, very little known is known about what
the optimal spatial organization of the components is, and even if we knew
that, we still don’t know how to get there!"
Nevertheless, paper #2 predicts that significant improvements are likely to
be forthcoming and to lead to even more efficient solar-energy
conversion.
Dr. John Emsley is based at the Department of Chemistry, Cambridge
University, U.K.
Keywords: solar cells, polymer solar cells, plastic solar cells, Alan
Heeger, Guillermo Bazan, solar panels.
What's Hot In... : What's Hot in Chemistry - Menu : A Neat Little Trick Doubles Plastic Solar-Cell Efficiency
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