Friday, 26 July 2013

Photon funnel could direct and regulate light into solar cells

Photon funnel could direct and
regulate light into solar cells
Illustration of the light-harvesting
funnel array that directs energy to a
focal point along diverse routes, and
then slowly off-loads the energy to a
solar cell or other device. Credit:
Raymond Ziessel, et al. ©2013
American Chemical Society
It has often been said that solar cells
are like artificial versions of the
photosynthetic apparatuses found in
plants, such as leaves, since both
harvest sunlight. But nature's leaves
can do something that most solar
cells cannot do: protect themselves
against photochemical damage from
overexposure to sunlight.
In an attempt to protect artificial
light-harvesting devices from sun
damage , chemists have designed a
funnel-shaped molecular-scale array
that harvests photons, spreads the
energy around the array, and off-
loads the energy at a relatively slow
rate to a solar cell or other device.
By regulating the amount of energy
that enters the solar cell, the new
array could extend the lifetime of the
solar cell, which must function in
harsh conditions associated with
prolonged exposure to sunlight.
The researchers, Raymond Ziessel,
Gilles Ulrich, and Alexandre Haefele
at the University of Strasbourg in
France, along with Anthony Harriman
at Newcastle University in the UK,
have published their paper on their
artificial light-harvesting array in a
recent issue of the Journal of the
American Chemical Society .
"UV light is harmful to the cells and
to the supporting structure,"
Harriman told Phys.org . "Photons are
lost by way of annihilation, and
optimal performance requires a
steady flux of photons. This is even
more important for water-splitting
devices, which is where we see our
light harvester having real
applications."
The new array consists of 21 Bodipy
("boron-dipyrromethene") dyes,
which are highly fluorescent dyes
known for their good light absorption
and emission. The Bodipy dyes are
arranged in a funnel-like design that
converges onto a focal point . When
exposed to light, the array guides the
excitation energy from incident
photons through the funnel through a
series of cascading energy transfer
steps until the energy reaches the
focal point.
The most important feature of the
design is its ability to self-regulate its
energy. When the focal point is in an
excited state, further energy transfer
to the focal point is restricted. In
order to increase the amount of
energy that reaches the focal point,
the topology of the array provides
diverse travel routes for the energy
to ensure different arrival times. The
strategy involves redistributing excess
energy within the array until the
focal point is no longer "saturated."
This mechanism for protecting
against overexposure to sunlight is
not strictly based on the mechanisms
used by plants. In nature, various
different mechanisms have evolved
for this purpose, although the details
of these mechanisms are still under
active debate.
While the properties of the new array
are intriguing, the scientists add that
the actual synthesis is also state-of-
the-art. Using Bodipy dyes as
building blocks allows certainty about
the emergent structure, unlike when
using other molecules, such as
dendrimers, where it is difficult to
assure complete growth with each
layer.
In the future, the molecular-scale
funnel could protect solar cells by
functioning as a sensitizer; that is,
transferring energy in a controlled
way to the solar cells or other
external devices. The array also
provides a benefit in stability
compared with using a mixture of
compounds. And although the array
restricts energy transfer , it does not
decrease solar cell efficiency.
"At present, the limiting efficiency is
coupling together the two systems,"
Harriman said. "In principle, there
should be no decrease in efficiency.
The real advantage will come from
using a large-area collector and a
small-area solar cell."
In the future, the researchers plan to
improve the transfer of photons from
the array to the solar cell.
"We are trying to build systems
where the photons move easily from
cluster to cluster before being
trapped by the solar cell," Harriman
said. "Also, we are looking into ways
to push the photons towards the
solar cell, rather than rely on random
migrations. This kind of quantum
coherence might be important in
certain cases in nature but is way
beyond the current capability of
artificial systems. We have ideas on
how to improve and we foresee rapid
progress in this field."
More information: Raymond Ziessel,
et al. "An Artificial Light-Harvesting
Array Constructed from Multiple
Bodipy Dyes." Journal of the
American Chemical Society . DOI:
10.1021/ja4049306

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