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Organic Solar Cells
for New Applications
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Fraunhofer Institute for
Solar Energy Systems ISE
Heidenhofstr. 279110 Freiburg
Germany
Tel.: +49 (0) 7 61/45 88-0
Fax: +49 (0) 7 61/45 88-90 00
www.ise.fraunhofer.de
Contact
Dr Andreas Gombert
Tel: +49 (0) 7 61/45 88-59 83
Fax: +49 (0) 7 61/45 88-99 83
E-mail: [email protected]
Dr Michael Niggemann
Tel: +49 (0) 7 61/2 03-47 98Fax: +49 (0) 7 61/2 03-48 01
E-mail: [email protected]
Organic solar cells represent a new
type of solar cells based on a com-
posite of organic semiconductors.
The low consumption of material and
the application of efficient production
techniques offer great potential for
cost-efficient production of these
solar cells. Further advantages are
the mechanical flexibility and the low
weight. The solar efficiency value of
organic solar cells is currently in therange between 3% and 5%.
One promising approach to increase
the efficiency is to tailor the electronic
properties of the organic semiconduc-
tors and to control the nanomor-
phology of the semiconductor com-
posite (Fig.1). First potential applica-
tions for organic solar cells are seen
to be in power supplies for mobile
consumer electronics. Further possibleapplications are power sources for
stand-alone sensor networks and
simple electronic circuits based on
organic electronic components. The
contribution of organic solar cells to
a sustainable energy supply is our
long-term goal.
Our R&D portfolio aims at the com-
mercialisation of organic solar cells.
This includes the following aspects:
- Novel solar cell architectures which
meet the requirements of low
material costs and cost-efficient
production
- Investigation and evaluation of
novel solar cell components
- Investigation of the long-term
stability of encapsulated devices.
August 2006
Fig. 2: Image of a test sample of an "ITO-free"
organic solar cell.
Fig. 1: Cross-section of an organic solar cell.
Idealised sketch of the composite of the
organic semiconductor donor and acceptor
components.
Acceptor phase
Donor phase
Transparent Anode
100-300nm
Cathode
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Fig. 3: Automated measurement setup
allowing efficient characterisation of
organic solar cells.
Fig. 4: "ITO-free" organic solar cell on
a flexible substrate (not encapsulated).
Fig. 5: Cross-section through an organic
photodiode with interdigitated nano-
electrodes of two different metals. The
distance between electrodes is about
400 nm (scanning electron micrograph).
Nanostructured Devices
The combination of nano-patterning
and micro-patterning techniques with
the use of organic semiconductors
opens up diverse possibilities forcreating novel organic electronic
devices. We are developing functional
substrates with vertically orientated,
interdigitated nano-electrodes for
organic photovoltaic devices.
For example, partly transparent devi-
ces with metallic electrodes can be
produced. Based on this architecture,
we have prepared an organic photo-
diode with an electrode separation of
400 nm (Fig. 5).
We chose two different metals with
appropiate workfunctions as the
electrode materials. The electrodes are
embedded in the photoactive layer of
the photodiode. This enables efficient
transport of the charge carriers. The
versatile concept of interdigitated
vertical nano-electrodes has great
potential for application in many other
components, such as organic light-emitting diodes, organic transistors
and sensors.
Solar Cell Concepts
Two essential requirements are to be
met for the production of cheap solar
cells: The use of cost-effective mate-
rials as well as efficient fabricationprocesses such as roll-to-roll coating
technology. To fulfill these require-
ments, we develop novel solar cell
architectures in which the expensive
transparent indium tin oxide (ITO)
electrode is replaced and efficient
coating and structuring is possible.
One of the ITO-free solar cell archi-
tectures developed by Fraunhofer ISE
is based on the substitution of the
ITO-electrode by a transparent poly-mer anode which is supported by
metal structures. In order to enable
efficient production technologies, the
deposition sequence of the electrodes
was inverted in comparison to the
standard organic solar cells. The solar
cell is built up on the cathode instead
of on the polymer anode (Fig. 2).
We have inverted the layer sequence
without any loss in power conversion
efficiency as compared to standard
devices.
Organic Solar Cells
for New Applications
Characterisation
The chemical tailoring of organic semi-
conductors offers a large potential
for increasing the solar cell efficiency
and stability. Therefore, the efficientscreening of novel organic semicon-
ductors and semiconductor combina-
tions as well as the process optimiza-
tion for novel solar cell architectures
will be an important issue for the
development of organic solar cells.
We have established a highly efficient
characterisation setup for organic
solar cells, incorporating the impor-
tant measurement techniques (Fig. 3).
The high characterisation throughput
in combination with an electronic labbook and a statistical evaluation of
the process and device parameters
enables efficient optimization of the
solar cells.