To generate solar energy, a
solar
cell must have an electrode that is transparent. Currently
there are two materials which meet this requirement: indium tin
oxide (ITO), which is the preferred one, and fluorine tin oxide
(FTO), which is less effective. However, indium is rare and has to
be extracted from zinc and lead ores, of which it is a minor
component; production is less than 500 tons a year.
ITO and FTO are not without their drawbacks. They lack transparency
with respect to the infrared region of the spectrum, and this
restricts their ability to gather a wider range of solar energy.
They are unstable in the presence of acids and bases, and their
metal ions are prone to diffusing into the polymer layers thereby
reducing efficiency. Unless they are structurally perfect they
suffer from current leakage.
The figures below are from an interview with
coauthor
Hideo Hosono, in regards to paper #1. Click
figures for a larger view & description.
Graphene,
on the other hand, appears to have none of these
drawbacks—and it is cheap and sustainable. Graphene films are
transparent, electrically conducting, and can be made ultra-thin.
Paper #9 describes such an electrode, and one that is suitable for
solid-state dye-sensitized solar cells which harvest light over a
wider range of the spectrum. What is particularly important for
these titanium dioxide based solar cells is that the graphene films
are chemically more stable, especially to strong acids. The paper
comes from the Max Planck Institute for Polymer research at Mainz,
Germany.
Graphene sheets are produced from graphite starting with the acid
oxidation of graphite flakes. The oxygen-containing groups which
are formed make the product dispersible in water in which it can be
exposed to ultrasonification to separate it into thinner sheets.
These are then deposited on to a substrate such as quartz, and this
is done by simply dipping in the hot solution. The thickness of the
film can be varied by changing the temperature of the aqueous
medium.
The graphite oxide so obtained is an insulator but can be reduced
by heating to high temperatures in an atmosphere of argon and
hydrogen gas. (The absence of oxygenated groups in the product was
evidenced by IR spectroscopy.) The resulting graphene film was tens
of layers thick. One such film, which was 10 nm in width, was
observed to have transmittance of 71% at a wavelength of 500 nm
which may be lower than that of ITO’s 90% and FTO’s
82%. However, compared to ITO and FTO, the graphene film is
transparent to IR radiation. The films have a conductivity of 550 S
cm-1 which compares to that of graphite’s 1250 S
cm-1 and so they have the potential to act as
electrodes.
Currently leading the research at the Max Planck Institute are
Xinliang Feng and Klaus Müllen, and their recent papers
suggest more exciting developments. In Nanotechnology
(Y.Y. Liang, et al., 20[43]: no 434007, 2009) the group
reports an improved way of making the films which involves using
acetylene in the reduction of the graphite oxide, a method which
not only repairs defects within the sheets but also increases the
conductivity to 1425 S cm-1 while still maintaining high
optical transmittance.
In Advanced Materials (Q. Su, et al., 21[31]:
3191-5, 2009) they report the inclusion of large aromatic donor and
acceptor molecules to functionalize the graphene. This approach
stabilizes the graphene in aqueous dispersion and also enables it
to be deposited in monolayer and double-layer on substrates in
large quantities. When the graphene is then heated at around
1000° C, the aromatic molecules repair holes in the
film, thereby contributing to an improved conductivity of 1314 S
cm-1 which now exceeds that of ordinary graphene.
As Xinliang Feng tells Science Watch: "Our work is
possibly the most attractive application of graphene in future
electronics. We are currently improving the quality of graphene
film in terms of transmittance and conductivity, because these are
the crucial parameters for the window electrode replacement of
traditional ITO. I think that we are leading in this area of large
scale and cheap synthesis of transparent graphene electrodes. If
graphene electrodes can be fabricated by easy and cheap methods in
large quantities, then a big market for them can be
expected."
A sustainable future for solar panels now seems
assured.
Dr. John Emsley is based at the Department of Chemistry,
Cambridge University, U.K.
Chemistry
Top 10 Papers
Rank
Paper
Citations
This Period
(Jul-Aug 09)
Rank
Last Period
(May-Jun 09)
1
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
173
1
2
C. de la Cruz, et al.,
"Magnetic order close to
superconductivity in the iron-based
layered
LaO1-xFx FeAs
systems," Nature,
453(7197): 899-902, 12 June 2008.
[6 U.S. and China institutions]
*311WV
54
2
3
H. Takahashi, et al.,
"Superconductivity at 43 K in an
iron-based layered compound
LaO1-xFx
FeAs," Nature, 453(7193): 376-8, 15
May 2008. [Nihon U., Tokyo, Japan;
Tokyo Inst. Technol., Japan] *301AI
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
40
5
6
A.I. Hochbaum, et al.,
"Enhanced thermoelectric
performance of rough silicon
nanowires," Nature,
451(7175): 163-7, 10 January 2008.
[U. Calif., Berkeley; Lawrence
Berkeley Natl. Lab., CA] *249GA
36
7
7
S. Stankovich, et al.,
"Synthesis of
graphene-based
nanosheets via chemical reduction
of exfoliated graphite oxide,"
Carbon, 45(7): 1558-65,
June 2007. [Northwestern U.,
Evanston, IL; U. North Carolina,
Chapel Hill] *185XJ
36
6
8
B. Tian, et al., "Coaxial
silicon nanowires as
solar cells and nanoelectric
power sources," Nature,
7164(449): 885-9, 18 October 2007
[Harvard U., Cambridge, MA] *221LY
29
8
9
X. Wang, L. Zhi, K. Müllen,
"Transparent, conducive graphene
electrodes for dye-sensitized solar
cells," Nano Letters,
8(1): 323-7, January 2008. [Max
Planck Inst. Polymer Res., Mainz,
Germany] *249VI
25
†
10
A.I. Boukai, et al.,
"Silicon nanowires as efficient
thermoelectric materials,"
Nature, 451(7175): 168-71,
10 January 2008. [Caltech,
Pasadena] *249GA