The Bridge - Issue 2, 2021 - 16

Feature
Modeling of organic semiconductor conduction parameters
with reference to inorganic semiconductors
field effect transistors (OFETs) and light emitting
diodes (OLEDs) playing an increasingly important
role in modern technology. Further advantages of
organic semiconductors compared to conventional
silicon-based materials include mechanical flexibility,
lightweight and easy and inexpensive solution process
ability. Continuous refinement of design strategies and
fabrication techniques allowed them to now routinely
display impressive charge carrier nobilities of over 1
cm2
V-1
s-1
s-1
in
V-1
in
Figure 3: Density of states (DOS) as a function of the energy (E) for
metallic and organic semiconductors.
The effective density of state assumes that the density
of states is located at the positions of Ec
as EL, and EH
, and Ev
, respectively as depicted in the same
figure. Finally, the energy level diagram of the organic
semiconductor is shown in Figure 4, where χ is the
electron affinity, Ef
is the Fermi-level, Eg
Figure 2: Typical aggregates and crystal packing motives of the
π-conjugated cores: (A) lamellar π-π stacking motifs with onedimensional
charge carrier channel, (B) brick-stone or brick-wall (also
called β-sheet) arrangement with two-dimensional π-π stacking; (C)
γ-packing with slipped face-to-face π-π stacking; (D) herringbone face-toedge
packing without face-to-face π-π overlap.
The energy band structure of the
organic semiconductor
As the organic materials are composed of molecules
weakly bound to each other therefore, the energy
band structure will be that of the molecules with
limited splitting of the energy levels to form narrow
bands. The energy band structure was investigated
using the density function theory. It is found that
valence and conduction bands are formed with
Gaussian distribution of the density of states around
the conduction band edge Ec
= EL, the LUMO level
and the valence band edge the HOMO level. Ev
and Nv
= EH
The conduction band and the valence band have
effective density of states Nc
.
, respectively. This
description of the energy band structure is similar to
that of the inorganic semiconductors.
A typical density of state DOS(E) as a function of the
energy E is shown in Figure 3 for an organic material
and inorganic semiconductor for sake of comparison.
THE BRIDGE
and φ is the work function of the materials.
So, formally the energy band diagram of organic
semiconductor is similar to that of inorganic
semiconductor [4].
Carriers in equilibrium in the
organic semiconductor
The intrinsic concentration ni
Generally, in any semiconductor there are two types of
mobile charges: the electrons in the conduction band
and the holes in the valence band. If the material is
pure and intrinsic the source of the electrons and holes
will be the thermal generation of electron hole pairs.
The concentration of the electrons will be equal to that
of the holes n0
n0 p0=ni [5]. The intrinsic concentration is given by; ni
NCNv exp(-Eg/KT), where K is the Boltzmann constant
2
and T is the absolute temperature. This law is general
and applicable for all semiconductors.
=p0=ni according to the mass action law,
2
=
is the band gap
as well
The DOS of the organic materials is Gaussian while
that of the inorganic is a square root function of energy
difference from the band edge.
V-1 s-1, reaching as high as 20-40 cm2
single crystals and even hundreds of cm2
ultrapure samples at low temperatures.
Figure 4: The energy level diagram of the organic semiconductor.
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The Bridge - Issue 2, 2021

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