ORGANIC SEMICONDUCTORS Christian Ohrt, Thomas Preuße ...

ORGANIC SEMICONDUCTORS
Christian Ohrt, Thomas Preuße
Faculty of Enginering, University of Kiel
Kaiserstraße 2 24143 Kiel Germany
ABSTRACT: This paper deals with a short introduction into the field of organic semiconductors. Its part of a
presentation, which is hold in the course material science seminar for the fifth semester students of the bachelor
material science study. We want to give a overview about the history, the structures and bondings, the opctical
properties, the conductive properties and some applications of organic semiconductors in our present time for
example OLEDs. Then we will show some advantages of using organic semiconductors in comparison to typical
inorganic semiconductors like Si and GaAs. The semiconductor technology is one of the most important deployments
in the world and is required in nearly all applications of high technology. One way to improve the properties of this
technology can be to use organic semiconductors.
1 Historical background
The first discovery of organic semiconductors was
made in the year 1963 by D. E. Weiss and the members of
his group. They discovered high conductivity in ioine-
”doped” oxidized polypyrrole blacks. They could achieve
a conductivity of 1 Siemens/cm, which is equivalent to a
resistance of 1 Ohm/cm. In 1965 they were able du reduce
the resistance 0.03 Ohm/cm with other conductive
polymers. Unfortunately this work was lost until recently.
In 1974 John McGuiness together with his group built a
voltage-controlled organic-polymer switch, which consist
of melanin, a self-doped mixed copolymer of oxidized
polyacetylene, polypynole and polyaniline. This material
emitted a flash of light when it switched. In the year 1977
conductivity in polymers was rediscovered and reported
by Alan J. Heeger, Alan G. McDiamid and Hideki
Shirakawa. They reported high conductivity in similarly
oxidized and iodine-doped polyacetylene. In the year 2000
they received the Nobel Prize in Chemistry “For the
discovery and development of conductive organic
polymers", which was leading to some disputes, because
of the earlier discovery in this field especially by D. E.
Weiss.
2 Properties of organic semiconductors
2.1 Basic properties
Organic semiconductors are low molecular weight
materials with fundamentally different bondings as their
inorganic counterparts. Organic semiconductors have a
conjugated π-electron system beeing formed by the by the
pZ-orbitals of sp² hybridized C-Atoms in the molecules.
The π–bonding is significant weaker then σ-bonding and
the energy gap of π– π* transitions is about 1.5 to 3 eV,
but the energy gap can be controlled by the degree of
conjugation in the molecule.
π-electron system beeing formed by the by the
pZ-orbitals of sp² hybridized C-Atom
The consequences: Organic semiconductors have a
reduced hardness and a lower melting point as inorganic
semiconductors and much weaker delocalization of
electronic wave functions of neighbouring molecules
which has direct appplications for optical properties and
charge carrier transport.
2.2 Optical properties
Organic semiconductors have well-defined spin states,
the so called singlet and triplet state. These two states
derive from quantum mechanics, more precisely from the
multiplicity. The singlet state is responsible for
fluorescence and the triplet state is responsible for
phosphorescence. To describe phosphorescence we need
the term of intersystem crossing. Intersystem crossing
means, that an electron changes its spin, after it was
excited by a photon. After the intersystem crossing, it can
not reduce its state of energy by spontan emission like in
flourenscence, the electron changes the system again and
than it can go back to the origin energy state by emiting
weak radiation, the so called phosphorescence.

Singlet and triplet states
2.3. Conductive Properties
Several kinds of carriers mediate conductivity in organic
semiconductors. These include π-electrons and unpaired
electrons. Almost all organic solids are insulators. But
when their molecules have π-conjugate systems, electrons
can move in the so called π-electron cloud. In charge
transfer complexes, even unpaired electrons can stay
stable for a long time, and are the carriers. This type of
semiconductor is also obtained by pairing an electron
donor molecule and an electron acceptor molecule. Thats
how you can dope organic semiconductors.
3 Applications
3.1 OLEDs
An organic light-emitting diode (OLED) is a lightemitting
diode (LED) whose emissive electroluminescent
layer is composed of a film of organic compounds. The
layer usually contains a polymer substance that allows
suitable organic compounds to be deposited. They are
deposited in rows and columns onto a flat carrier by a
simple "printing" process. The resulting matrix of pixels
can emit light of different colours.
Such systems can be used in television screens,
computer displays, portable system screens, advertising,
information and indication. OLEDs can also be used in
light sources for general space illumination, and largearea
light-emitting elements. OLEDs typically emit less
light per area than inorganic solid-state based LEDs
which are usually designed for use as point-light sources.
Operation mode of an OLED
4 Advantages and disadvantages of organic
semiconductors
One of the main reasons for developing organic
semiconductors instead of inorganic is their potential for
lower costs than silicon devices . Also their mechanical
properties (less weight, more flexible and elastic, easier
to produce in form of thin layers by pecial printing
techniques) are some advantages over their inorganic
counterparts. Disadvantages are their higher chemically
activity, which means, that they need more protection by
encapsulation. Because of lower melting point they are
more temperature sensitive in comparison to inorganic
semiconductors and finally is the process of the charger
transfer mechanisms not understood for 100 %.
5 Future Trends
In the present time OLED displays are still very
expensive as you can see at the new XEL-1 by Sony.
With a cost of 1700$ and a display with only 11 inch the
new OLED displys are still to expensive. But OLED
displys have some advantages. For example: The displays
of the XEL-1 from Sony is only 2,5 cm thick.