Surprising Steps Forward
in Solar Cells Thanks to Nano
Chemistry
by John Emsley
Chemistry
Top Ten Papers
Rank
Papers
Cites
This Period Nov-Dec
08
Rank
Last Period
Sep-Oct 08
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
108
1
2
C. de la Cruz, et al.,
"Magnetic order close to
superconductivity in the
iron-based layered
LaO1-x
FxFeAs systems,"
Nature, 453(7197):
899-902, 12 June 2008. [6 U.S.
and China institutions] *311WV
62
2
3
H. Takahashi, et al.,
"Superconductivity at 43 K in
an iron-based layered compound
LaO1-x Fx
FeAs," Nature,
453(7193): 376-8, 15 May 2008.
[Nihon U., Tokyo, Japan; Tokyo
Inst. Technol., Japan] *301AI
39
3
4
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
35
†
5
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
31
†
6
X.D. Wang, et al.,
"Direct-current nanogenerator
driven by ultrasonic waves,"
Science, 316(5821):
102-5, 6 April 2007. [Georgia
Inst. Tech., Atlanta] *153XD
27
†
7
X.L. Li, et al.,
"Chemically derived,
ultrasmooth
graphene nanoribbon
semiconductors,"
Science, 319(5867):
1229-32, 29 February 2008.
[Stanford U., CA] *267SX
22
10
8
K. Zhu, et al.,
"Enhanced charge-collection
efficiencies and light
scattering in dye-sensitized
solar cells using oriented
TiO2 nanotube
arrays," Nano Lett.,
7(1): 69-74, January 2007.
[Natl. Renewable Energy Lab.,
Golden, CO] *124PK
20
†
9
N. Tian, et al.,
"Synthesis of tetrahexahedral
platinum nanocrystals with
high-index facets and high
electro-oxidation activity,"
Science, 316(5825):
732-5, 4 May 2007. [Xiamen U.,
China; Georgia Tech, Atlanta]
*163RR
19
†
10
A.I. Hochbaum, et al.,
"Enhanced thermoelectric
performance of rough silicon
wires," Nature,
451(7175): 163-7, 10 January
2008. [U. Calif., Berkeley;
Lawrence Berkeley Natl. Lab.,
CA] *249GA
Papers #5 and #8 report on the benefits of
nanomaterials—silicon and titanium dioxide (TiO2),
respectively—in boosting the performance of solar
cells.
Miniature solar cells could power microelectronic systems, and
downsizing to this scale requires nano-electronic components, but
so far such cells have low efficiency and poor stability. However,
the coaxial nanowires of paper #5 overcome these drawbacks. They are the work of
Charles Lieber’s group at Harvard
University and they consist of a p-type silicon nanowire core
surrounded by an n-type silicon sheath. Electrons and holes are
generated by light in the layer of silicon between them and are
swept into the n-shell and p-core, respectively, by a built-in
electric field.
The coaxial wires were grown in three stages starting with a
vapor-liquid-solid method for the 300 nanometer (nm) diameter core.
This was coated by a chemical vapor deposition with a layer of pure
silicon and finally with a phosphine-doped outer layer. Metal
contacts were fabricated onto the p-core and outer n-layer. The new
wires maintained their performance for seven months and the Harvard
group connected them both in parallel and in series to demonstrate
their versatility to drive large loads. The efficiencies of the
devices were only 2-3%, but Lieber suggests this could be improved.
The research described in paper #8 offers significant benefits to
solar cells of the Grätzel type. In these cells, light is
captured by dye molecules which release electrons into a metal
oxide and holes into an electrolyte, and provided they successfully
migrate to opposite electrodes they will produce an electric
current; if they recombine before doing so they produce no current.
To be effective, charge collection at the electrodes has to be much
faster than recombination within the electrolyte. Paper #8 shows
charge collection can be greatly boosted by means of
TiO2 nanotubes.
Arthur Frank of the National Renewable Energy Laboratory in
Colorado led the group, and #8 reports both the microstructure and
the way electrons behave in dye-sensitized solar cells which
incorporate the TiO2 nanotubes. These
were prepared from electrochemically anodized
titanium foil and were investigated by scanning and
transmission electron microscopy. This showed them to consist
of closely packed nanotubes which were several micrometers in
length, with wall thickness of around 10 nm and pore diameters
of around 30 nm.
The nanotubes are all oriented in the same direction and
perpendicular to the underlying titanium foil on which they were
formed. The dye in these new cells was the same ruthenium complex
used in many other Grätzel type cells, and this was adsorbed
into the nanotubes. The electrolyte into which the holes were
released was 1-hexyl-2,3-dimethylimidazolium iodide in
methoxyproprionitrile. The cells were then illuminated with light.
Transport times in the new cells was similar to the earlier cells
which were based on randomly packed nanocrystallite films, but the
recombination was 10 times slower so they generated more current.
Also, the absorption of light by the ruthenium complex was enhanced
due to stronger light scattering within the nanotube arrays
compared to that of the traditional Grätzel cells with their
nanocrystallite-based films.
More recently Frank has published a method of removing structural
disorder from his nanotube arrays (K. Zhu, et al.,
Nano Lett., 7[12]: 3739-46, 2007) and a way of growing
p-type semiconductors in the nano pores of n-type materials such as
n-TiO2 (Q. Wang, et al., Nano Lett.,
9[2]: 806-13, 2009). Currently he is working to understand the
electron dynamics in ordered
mesoporous films with
the aim of increasing the efficiency and stability of sensitized
solar cells.
As Frank tells Science Watch, "The future is bright for
the development of commercial high-efficiency, low-cost solar cells
based on the sensitization phenomenon, and ordered nanoporous
electrode architectures. I believe that by developing the
scientific underpinning to exploit the unique properties of
dye-sensitized mesoscopic systems, an efficiency of 15% is
attainable for single-junction devices. We may even develop
sensitized nanostructure systems with efficiencies beyond the
theoretical (Shockley-Queisser) limit of 32% by incorporating
third-generation concepts based on quantum
confinement."
Dr. John Emsley is based at the Department of Chemistry,
Cambridge University, U.K.
KEYWORDS: SOLAR CELLS, NANOMATERIALS, NANOCHEMISTRY, CHARLES
LIEBER, NANOWIRES, SILICON, TITANIUM DIOXIDE, ARTHUR FRANK,
MESOPOROUS FILMS.