The Bridge - Issue 2, 2021 - 22

Feature
Modeling organic semiconductor metallic contact and optoelectronic
parameters with reference to inorganic semiconductors
Here, Nc
is the effective density of states corresponding
to the energy at the bottom of the conduction band, Nt
the total trapped electron density, and l= Tt/T is the ratio
is
of distributed traps to the free carriers.
The electron traps lie near the conduction band which
is here the LUMO level while the hole traps lie near the
valence band which is HOMO level. So, in principle there
are hole traps and electron traps that affect electron
mobility and hole mobility. There are also deep lying
traps in the energy gap which can act as traps for both
electrons and holes.
The deep lying traps can act as recombination centers,
affecting the concentration of electrons and holes; while
the electron traps affect the mobility of one charge type.
As for the conduction mechanisms, every one has
its characteristic I-V relation, especially the voltage
dependence and the thickness dependence of the active
material in the diode, D. They can be used as indicators
of the dominating mechanism.
Then by curve fitting of the current as a function of D, one
can determine the physical parameters of the material.
One can make additional measurements to verify some
of the estimated parameters from the fitting such as the
I-V measurements.
In case of doped organic semiconductor either
unintentionally or intentionally the material will possess
free carries and has a definitive Fermi level. In this case
the material behaves as an ordinary semiconductor and
builds Schottky contacts.
In this case the M-S contacts will follow the theory of
Schottky junctions:
Where Is
is the reverse saturation current, ɳ is the ideality
factor. For the evidence for these findings please follow
the Link in ref. [2].
Figure 2 shows a typical I-V curve of an organic
semiconductor diode. It demonstrates the typical space
charge limited current at the intermediate current density.
In summary, when one applies the voltage on the
MOSM diode the current follows the drift current then
turns to space charge limited current and then at high
applied voltage the current saturates into the thermionic
injection limited current. In case of doping the organic
semiconductor the MOSM behaves as a conventional
Schottky diode.
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Figure 2: Typical I-V characteristic of an organic MOSM diode showing
different mechanism of conduction.
The optoelectronic properties of
organic materials
We are interested in understanding the response of the
semiconductor to light, i.e., the photoelectric effect. If
a light beam is incident on a semiconductor material,
part of the light energy will be reflected, a part will be
absorbed inside the material after refraction and the
remaining part will be transmitted. According to the
energy conservation law:
Where I is the incident light energy, and R, A, and T,
respectively, are the portions reflected, absorbed,
and transmitted.
Reflection occurs because of the change of the wave
impedance, Zw
The reflectance ρ is given by:
Where Zwa is the wave impedance of the air,
Zwa=√μ0
, and µ= µo
=√(μ⁄ε) at the surface of the material.
⁄ε0. ε is the dielectric constant and µ is
the permeability. It is assumed that light is falling
perpendicular to the surface. For a typical organic
material with ε equal around; 4 εo
, ρ = 1/9,
which is much less than that of silicon with ρ = 0.3, an
appreciably larger than that of the organic materials. From
this reflection point of view, the organic materials are
performing better.
The absorption of the light energy is characterized by the
absorption coefficient α, where
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The Bridge - Issue 2, 2021

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