Thursday, March 12, 2009
Cheap, Durable Nonsilicon Solar Cells
An advanced dye boosts the efficiency of dye-sensitized solar cells.
By Prachi Patel-Predd
Dye-sensitized solar cells could make solar power more affordable:
they are cheaper to make than conventional silicon solar cells and can
easily be printed on flexible surfaces. But there's a catch: creating
efficient cells of this type has required dyes made of the precious
metal ruthenium and volatile electrolytes. Now researchers at the
Chinese Academy of Sciences have replaced both of these materials in a
new kind of dye-sensitized solar cell that is not only highly
efficient: it also promises to be even cheaper and more durable.
The key to the advance is a new organic dye molecule designed by
chemistry professor Peng Wang and his colleagues. Organic dyes reduce
the cost of making the cells because they are more abundant and
cheaper to obtain than ruthenium compounds. The researchers also use a
different type of electrolyte called an ionic liquid. This produces a
more robust solar cell: the electrolytes that are currently used
contain organic solvents that can evaporate and leak out at high
temperatures. The ionic liquid can also be used with plastic, opening
up the possibility of flexible solar cells. "We demonstrated for the
first time that an all-organic dye can be employed to make stable,
solvent-free cells exhibiting a high efficiency comparable to
ruthenium dyes," Wang says.
The researchers set a new efficiency record for cells incorporating
organic dye molecules. To compete with conventional solar cells, dye-
sensitized ones need to be at least 10 percent efficient at converting
light into electricity. Wang and his colleagues achieved 9.8 percent
efficiency with the new organic dyes. "When you get up to high
efficiencies, small increases matter," says Michael Grtzel, a
chemistry professor at the cole Polytechnique Fdrale de Lausanne,
in Switzerland, who invented dye-sensitized solar cells. He says that
it's exciting to see researchers "getting so close to 10 percent with
organic dyes, which is a magic number."
When the researchers pair the organic dye with an ionic fluid, the
efficiency drops to 8.1 percent. But this is still a significant
advance, Grtzel says. He published work last year showing similar
cells that were 7.2 percent efficient. But while pairing nonvolatile
electrolytes with ruthenium dyes, he has achieved 10 percent. "We
thought we would never get more than 1 percent 10 years ago," he says.
In a dye-sensitized solar cell, dye-coated semiconductor nanoparticles
are sandwiched, along with the electrolyte, between two glass plates.
The dye molecules absorb light and generate electrons, which are
transferred to the semiconductor and on to the external circuit.
Meanwhile, positively charged holes go to the electrolyte. The big
problem with organic dyes in the past has been the difficulty of
keeping the charges separated: they tend to recombine and lower
The dye molecule that Wang and his colleagues designed increases the
cell's efficiency in three ways: it quickly shuttles electrons to the
semiconductor particles; it keeps the electrons and holes from
recombining; and it covers a broader spectrum of light, absorbing more
red light than was possible before.
At least two companies are commercializing dye-sensitized solar cells.
These devices are up to 11 percent efficient and use ruthenium dyes
and a volatile electrolyte. In October 2008, Dyesol opened a factory
in Queanbeyan, Australia, to make tiles that can be integrated into
building facades. G24 Innovations, in Cardiff, U.K., is making solar
chargers for mobile phones.
Wang says that his new work, published online in the journal Chemical
Communications, could make the technology cheaper and open up broader
applications. "At the moment, the use of toxic and volatile solvents
in high-efficiency cells is a big hurdle for the large-scale
application of dye-sensitized solar cells," he says.
The researchers are working to boost efficiency even more. Wang says
that will involve "mainly extending the spectral response of
sensitizers to the infrared and the design of better solvent-free
Meanwhile, Grtzel and his colleagues at Lausanne have set themselves
a lofty goal. They plan to reach efficiencies of 14 percent by the end
of next year using nonvolatile electrolytes. As for the dye, either
ruthenium-based dyes or organic dyes could win the race, he says. And
he is optimistic about cheaper organic dyes. "If you see how
efficiencies of both have gone up, the slope is steeper for organic
molecules," Grtzel says. "If you extrapolate, it could be a year or
so before they overtake."