The Bridge - Issue 2, 2021 - 23

Feature
Modeling organic semiconductor metallic contact and optoelectronic
parameters with reference to inorganic semiconductors
Which means that the incident light intensity I(0) at
the surface of the material decays exponentially with
the distance x from the surface. The intensity decays
to (1/e) at x = 1/ α. Therefore, the inverse of α. is the
average penetration depth of the light in the material,
or may be better named the average absorption length.
The absorption coefficient depends on the wavelength
of the incident light. It is well established now that light
is composed of energy quanta called photons. These
photons interact with the electrons and atoms of the
material. The photon energy is given by:
Where h is Planck's constant and f is the photon
frequency. On the other side photons are wave packets
having a wave length λ, and velocity C = 3 x 108
that C = h f. Therefore, Eph = h (C/λ). The photon energy
important observation is that α of the organic materials
rises rapidly to maximum value then decreases again at
short wavelengths. This absorption coefficient curve can
be accounted for by the energy band structure of the
organic materials discussed previously in section 3.
α
wavelength λ
Figure 4: Typical shape of the absorption curves of the organic materials
compared to silicon
m/s such
is inversely proportional to the light wavelength. Now,
we are in a position to understand how the light affects
the material. Only valence electrons can absorb photons
when they can acquire sufficient energy to overcome the
energy gap of the material. More specifically, photons
can excite an electron from the valence band, causing it
to bind with a free hole and create an exciton (italicize
the word). This process, denoted as photo-generation of
excitons, is illustrated in Figure 3.
EL
Eph
Eg
EH
Figure 3: Photo-generation of excitons
From Figure 3, the required minimum photon energy
for the photo-generation process Eph ≥ Eg. Energy
rich photons, where e.g., Eph ~ 2 Eg cannot produce
2 excitons except with a very small probability as this
process requires the collision of photons with two valence
electrons simultaneously if it is a direct process.
The absorption coefficient α is a function of the incident
wavelength of the photons and it is a material property.
Figure 4 shows typical shape of the absorption coefficient
of different organic semiconductors as a function of the
wavelength λ. The energy gap of the material is also
given. It is clear from this figure that materials absorb
light appreciably (α > 100/cm) only if Eph ≥ Eg. A second
This simple relation is of a very critical importance for
solar cell design. As the semiconductors are expensive
one has to use the minimum quantity of them to perform
the absorption function. In this concern, one needs
thicker crystalline Si-layers as it has the lowest α.
The required thickness to absorb the incident solar
radiation by organic semiconductors is on the order of one
hundred nanometers. Thin film structures are needed.
Excitons in organic semiconductors
As described earlier, incident photons create bound
electron-hole pairs known as excitons. These excitons
have a specific lifetime called the exciton lifetime which
is the time that an exciton lasts on the average before
it disappears by recombination. Because excitons are
neutral they move by diffusion inside the molecules
HKN.ORG
23
For sake of comparison the absorption curve of silicon
is plotted on the same graph for the organic materials.
It is important to notice that the absorption coefficient
of the organic material is much greater than that of
silicon. The organic material has a band-pass absorption
characteristic, in contrast to the high-pass absorption
characteristic of the metallic material. The narrow band
absorption of the organic materials limits it generated
photocurrent. Therefore, this property could be considered
inferior to the metallic semiconductors.
The longer-wave length photons with smaller α need
a thicker layer of the semiconductor to be absorbed
otherwise they can be transmitted across the layer.
Generally, the minimum semiconductor layer thickness to
absorb light can be expressed by:
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The Bridge - Issue 2, 2021

Table of Contents for the Digital Edition of The Bridge - Issue 2, 2021

Contents
The Bridge - Issue 2, 2021 - Cover1
The Bridge - Issue 2, 2021 - Cover2
The Bridge - Issue 2, 2021 - Contents
The Bridge - Issue 2, 2021 - 4
The Bridge - Issue 2, 2021 - 5
The Bridge - Issue 2, 2021 - 6
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The Bridge - Issue 2, 2021 - Cover3
The Bridge - Issue 2, 2021 - Cover4
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