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Dye solar cell

Press reports: Newswire today /March 2006
Nanowires Could Lead to Improved Solar Cells

Honolulu, HI, United States, 03/06/2006

With an increased focus on alternative sources of cheap, abundant, clean energy,
solar cells are receiving lots of attention. Researchers are now on the brink of improving
the efficiency of solar cells through nanowires.

The dye sensitized solar cell (DSSC) is one of the most important developments in
photovoltaics in the last two decades. Excitonic solar cells, such as organic, hybrid organic
and inorganic solar cells are promising devices for inexpensive, large-scale solar energy
conversion. DSSCs are an exciting variant of the most efficient and stable of the excitonic
photovoltaic devices.

Untreated TiO2 absorbs light only in the UV region, but when the surface becomes modified
with dye molecules, these can absorb light in the visible range and then transfer the excited
electron to the particle. Back in 1991, Graetzel et. al came up with the methodology to
dye-sensitize colloidal TiO2 film as a way to fabricate low-cost, high-efficiency solar cells

Central to today's DSSCs is a thick titanium dioxide (TiO2) nanoparticle film that provides
a large surface area for the adsorption of light-harvesting molecules. One drawback of
nanoparticle DSSCs is their reliance on trap-limited diffusion for electron transport, a slow
mechanism that can limit device efficiency, especially at longer wavelengths.

To improve electron transport in these solar cells, while maintaining the high surface area
needed for dye adsorption, two researchers have designed alternate semiconductor
morphologies, such as arrays of nanowires and a combination of nanowires and nanoparticles,
to provide a direct path to the electrode via the semiconductor conduction band. Such
structures, that increases the rate of electron transport, may provide a means to improve
the quantum efficiency of DSSCs in the red region of the spectrum, where their performance
is currently limited.

The paper is titled "Dye-sensitized solar cells based on semiconductor morphologies with
ZnO nanowires" and will be published in the March 23, 2006 edition of Solar Energy
Materials and Solar Cells. Professor Eray Aydil from the University of Minnesota, co-author
of the paper together with Jason Baxter from the UC Santa Barbara, explained to Nanowerk:

"Detailed research into the way DSSCs work has shown that transport and recombination
of electrons in the nanoparticle TiO2 network are coupled. Researchers from NREL have
shown that this interdependency may be due to transport limited recombination; that
is any increase in transport rates also result in an increase in the recombination rate
with no net change in the cell performance."

"However, the use of single crystal nanowires may allow electron transport via extended
states in the conduction band rather than by a series of hops between trap states" says
Aydil. Thus the interdependency between the transport and recombination may be
removed in the case of nanowire DSSCs.

In addition, if nanowires achieve electron transport rates that are significantly faster
than transport rates in nanoparticle films, significant flexibility in choosing the hole-transport
medium could be gained since faster recombination rates could be tolerated. "One could
also make thicker films and increase optical density in the regions of the solar spectrum
where the dye absorption decreases" says Aydil. "These possibilities are the driving force
behind pursuing nanowire-based dye sensitized solar cells."

By Michael Berger, Copyright 2006 Nanowerk LLC

Posted by Kuppuswamy Kalyanasundaram at 11:00
TiO2 nanowire based DSSC
Preparation and Microstructure of TiO2-Nanowire Dispersed Composite
Electrode for Dye-Sensitized Solar Cells

Yoshikazu Suzuki (Institute of Advanced Energy, Kyoto University)
(author of over 50 articles and the book "Fine Ceramics Technical Strategy: from
the viewpoint of MOT")

"Several types of TiO2 nanowires (TiO2 (B) or anatase phases) have been
prepared by hydrothermal processes followed by post-heat treatments in air.
TiO2-nanowire dispersed TiO2 nanoparticle matrix porous composite
(TiO2 NP/TiO2 NW) electrodes were prepared for dye-sensitized solar cells (DSC)
in order to improve light-to-electricity conversion efficiency. Effect of one-dimensional
nanostructure (electron expressway concept) are discussed in detail."
Posted by Kuppuswamy Kalyanasundaram at 9:27
Titania nanotubes-based DSSCs can have efficiencies upto 15%
Quotes from press reports:

A solar cell, made of titania nanotubes and natural dye, may be the answer to
making solar electricity production cost-effective, according to a Penn State researcher
(Dr. Craig Grimes, professor of electrical engineering and materials science and engineering)

The researchers are instead looking at titania nanotubes to replace the particulate
coatings in dye sensitive solar cells and, their initial attempt produced about 3 percent
conversion of solar energy to electricity, they report in today's issue of Nano Letters. T
he researcher's inability to grow longer titania nanotubes, constrained the solar conversion rate.

"I think we can reach a 15 percent conversion rate with these cells, and other researchers
do as well," says Grimes. "That is 15 percent with a relatively easy fabrication system that
is commercially viable."

for full report see the source article at
Posted by Kuppuswamy Kalyanasundaram at 9:15