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
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 on Friday 8 September 2006 at 11:00