Organic solar cells made of metal-organic frameworks are highly efficient in producing charge carriers.
Credit: Wöll/KIT
Researchers at KIT have developed a material
suited for photovoltaics. For the first time, a functioning organic
solar cell consisting of a single component has been produced on the
basis of metal-organic framework compounds (MOFs). The material is
highly elastic and might also be used for the flexible coating of
clothes and deformable components. This development success is presented
on the front page of the journal Angewandte Chemie International Edition.
"We have opened the door to a new room," says Professor Christof
Wöll, Director of KIT Institute of Functional Interfaces (IFG). "This
new application of metal-organic framework compounds is the beginning
only. The end of this development line is far from being reached," the
physicist emphasizes.
Metal-organic frameworks, briefly called MOFs, consist of two basic
elements, metal node points and organic molecules, which are assembled
to form microporous, crystalline materials. For about a decade, MOFs
have been attracting considerable interest of researchers, because their
functionality can be adjusted by varying the components. "A number of
properties of the material can be changed," Wöll explains. So far, more
than 20,000 different MOF types have been developed and used mostly for
the storage or separation of gases.
The team of scientists under the direction of KIT has now produced
MOFs based on porphyrines. These porphyrine-based MOFs have highly
interesting photophysical properties: Apart from a high efficiency in
producing charge carriers, a high mobility of the latter is observed.
Computations made by the group of Professor Thomas Heine from Jacobs
University Bremen, which is also involved in the project, suggest that
the excellent properties of the solar cell result from an additional
mechanism -- the formation of indirect band gaps -- that plays an
important role in photovoltaics. Nature uses porphyrines as universal
molecules e.g. in hemoglobin and chlorophyll, where these organic dyes
convert light into chemical energy. A metal-organic solar cell produced
on the basis of this novel porphyrine-MOF is now presented by the
researchers in the journal Angewandte Chemie (Applied Chemistry). The
contribution is entitled "Photoinduzierte Erzeugung von Ladungsträgern
in epitaktischen MOF-Dünnschichten: hohe Leistung aufgrund einer
indirekten elektronischen Bandlücke?" (photo-induced generation of
charge carriers in epitactic MOF-thin layers: high efficiency resulting
from an indirect electronic band gap?).
"The clou is that we just need a single organic molecule in the solar
cell," Wöll says. The researchers expect that the photovoltaic capacity
of the material may be increased considerably in the future by filling
the pores in the crystalline lattice structure with molecules that can
release and take up electric charges.
By means of a process developed at KIT, the crystalline frameworks
grow in layers on a transparent, conductive carrier surface and form a
homogeneous thin film, so-called SURMOFs. "The SURMOF process is suited
in principle for a continuous manufacturing process and also allows for
the coating of larger plastic carrier surfaces," Wöll says. Thanks to
their mechanical properties, MOF thin films of a few hundred nanometers
in thickness can be used for flexible solar cells or for the coating of
clothing material or deformable components. While the demand for
technical systems converting sunlight into electricity is increasing,
organic materials represent a highly interesting alternative to silicon
that has to be processed at high costs before it can be used for the
photoactive layer of a solar cell.
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Karlsruhe Institute of Technology.
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