The Bridge - Issue 2, 2021 - 25

Feature
Modeling organic semiconductor metallic contact and optoelectronic
parameters with reference to inorganic semiconductors
Table 1: Examples of organic materials
bandwidth. The non-fullerene could be built to have an
optical absorption curve complementing that of the donor
material to render the whole donor acceptor complex
a wideband optical absorber. This led to appreciable
increase in the photo-conversion efficiency of solar cells
[4, 5]. In this case the donor must accept holes and
reject electrons back to perform a role similar to that of
the acceptor facilitating the dissociation of the excitons
generated in the acceptor material. Examples of the
acceptor semiconductor are listed in Table 1.
The transport layers
To produce a built-in field in the active zone of the donor
acceptor region for further collection of the electrons
and holes separated at the donor acceptor interface, one
contacts the acceptor layer by an electron transport layer
ETL and the donor layer by a hole transport layer HTL.
Both layers must be chosen to be good conveyor of the
electrons and holes to the respective metal electrode. So
they have to have high conductivity and proper energy
level diagram matching the donor and acceptor layers
and making at the same time ohmic contacts to the
metallic electrodes. Examples of the HTL and the ETL are
given in the Table 1.
Conclusions
This paper demonstrates that organic semiconductors
can be described by the same phenomenological
parameters as inorganic semiconductors [19], albeit
with different mathematical expressions and ranges
of values. With slight-to-moderate modifications,
simulators for inorganic devices can be used to
simulate organic devices.
Notable points about organic semiconductors
include the following:
* They are molecular in nature, with short-term order;
in contrast to inorganic materials, which can have
long-term order in crystallineform.
* They have relatively small mobilities, because of their
amorphous nature.
* Despite being narrow-band absorbers of light, they
have high absorption coefficients.
* Therefore, they need thin layers to absorb the incident
light in their absorption band.
* Their dielectric constant is low, leading to excitons
with high binding energy. This reduces collection
efficiency of the charge carriers, which in turn reduces
the net available energy of the solar cell.
* This problem has been solved through the use of
donor and acceptor polymer materials.
In conclusion, despite some shortcomings, organic
materials can be skillfully utilized to create electronic
devices with unique properties. The road to widespread
use of these materials in the electronic industry is long,
but shows great promise.
HKN.ORG
25
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The Bridge - Issue 2, 2021

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