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Organic Solar Cells

for New Applications

Feel free to ask!

We are happy to provide competent

advice, without obligation and specific

to your application.

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: Andreas.Gombert@ise.fraunhofer.de

Dr Michael Niggemann

Tel: +49 (0) 7 61/2 03-47 98Fax: +49 (0) 7 61/2 03-48 01

E-mail: Michael.Niggemann@ise.fraunhofer.de

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.