The Bridge - Issue 2, 2021 - 15

Feature
Modeling of organic semiconductor conduction parameters
with reference to inorganic semiconductors
It is found that the semiconductor parameters could
be the same in both classes of semiconductors but
their mathematical formulations and range of values
are different.
The semiconductor properties and parameters treated
in this paper are:
* The organic semiconductor atomic arrangement
* Arrangements of molecules in the organic materials
* The energy band structure of the organic semiconductor
* Carriers in equilibrium in the organic semiconductor
* The doping of the organic semiconductors
* Current conduction in organic semiconductors
* Recombination of mobile charge carriers in
organic semiconductors
* The semiconductor equations
The organic semiconductor atomic arrangements
The arrangements of the atoms inside the material
control its physical properties including the electronic
and optical properties. The atomic arrangements
depend on the chemical bonding between the
constituting elements of the material and their
electronic configuration.
The organic semiconductors are compounds having
atomic arrangement patterns as the organic materials.
The main feature of such a structure is that it is a
molecular one with strong covalent bonds inside the
molecules and weak inter molecular bonding mostly by
van der Wall forces. So, the organic semiconductors are
composed from organic molecules arranged randomly
or regularly or in a mixed fashion.
Figure 1 shows the atomic configuration of typical
organic semiconductor molecules. One sees that
thanks to the presence of the carbon atoms in the
molecules of the organic semiconductors, the bond
in the molecule core is covalent like that of metallic
silicon semiconductor. It is also noticed that all bonds
in the molecules are saturated and therefore the
intermolecular bonding will be of van der Waal dipolar
bonding which is weak.
Therefore, we find that these materials are highly ductile
and classified mechanically as plastic materials. So, they
have different mechanical properties from the metallic
semiconductors such as silicon which is hard and stiff.
There are thousands of possible variations in these
structures, including substitution of some carbons for
Figure 1: Typical molecular structures leading to OSC materials. Top, from
left: pentacene, α-sexithiophene (α-6 T), poly(3-hexylthiophene) or P3HT.
Bottom, from left, copper phthalocyanine, naphthalenetetracarboxylic
diimide (NTCDI), terthiophene tetracyanoquinodimethane. The first four
are hole carriers, and the last two electron carriers.
heteroatoms and appending vast libraries of side chains
that enable tuning of charge transport, processing
conditions, and chemical interactions [2].
Arrangements of molecules in the
organic materials
As the organic materials are composed of organic
molecules, then the geometrical arrangement of the
molecules in the materials affects much its electronic
and optical properties as they define the intermolecular
interaction. This issue is similar to the formation of
crystalline structures in inorganic semiconductors.
So, one can consider that the organic semiconductor is
basically a fine-grained semiconductor with a molecule
representing a grain. The charges can move easily
inside the molecule as consequence of its extended
valence structure, but at the molecule boundary there
will be potential barriers that impede the motion of
the charge carriers. So, it is clear that the transport of
charges between the molecules is the limiting factor
for this movement. This is similar to the transport
across the grain boundary in metallic semiconductors.
Many research efforts are spent to control the
intermolecular interaction to ease the motion
across the molecules of materials by controlling
their molecular arrangements during the deposition
process. The arrangement of the molecules ranges
from regularly ordered molecules to randomly
ordered molecules [3]. Figure 2 depicts some of
these arrangements.
Progress in fabrication, particularly solution processing,
and characterization of small-molecule and polymer
organic semiconductors could enhance these materials
and utilize them in industrial products such as organic
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The Bridge - Issue 2, 2021

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