The Bridge - Issue 2, 2021 - 24

Feature
Modeling organic semiconductor metallic contact and optoelectronic
parameters with reference to inorganic semiconductors
where they were created by light. So, the range of their
motion is limited by the molecule sizes. It turns out that
the diffusion length of the excitons is about few tens of
nanometers in polycrystalline materials [3]. In contrast the
photo-generated excitons in the metallic semiconductors
are readily dissociated by the thermal energy as they are
weakly bound thanks to the high dielectric screening.
In order to calculate the dissociation energy of the
exciton, one can model the electron and its hole in
the exciton as a modified hydrogen atom as shown in
Figure 5. This model is similar to the hydrogen atom,
except that the electron moves with an effective mass
mn
in a medium with a relative dielectric constant ɛr
instead of ɛr
> 1
= 1 in a hydrogen atom. Then according to
Bohr atomic model for the hydrogen atom, the required
dissociation energy Ei
Where mex
is the effective mass of exciton composed of
electrons and holes. It is given by:
Where EH
atom. For, metallic semiconductor as silicon, ɛr
is the energy required to ionize the hydrogen
=12, so
that the dissociation energy is of the order of 0.05eV.
While as for an organic semiconductor with a relative
dielectric constant of 4 the dissociation energy amounts
to 0.45 eV. When the effective mass is taken into
consideration with the effective mass ratio is greater than
one for the organic semiconductors, the binding energy
or the exciton can be as high as one eV.
of the exciton can be expressed by:
Figure 5: Hydrogen atom model of excitons where they move in a
screening medium
This high binding energy of the excitons imparts a great
problem for dissociating the excitons since to collect their
electrons and holes they must be free. It is now clear that
the good optical properties of the organic semiconductors
of high absorption coefficient and low reflectance are
offset by short lifetime, high binding energy of the
excitons and narrow band absorption. It is so that the
THE BRIDGE
small lifetime of the excitons can reduce the collection
efficiency of the charge carriers and the high binding
energy will be subtracted from the net available
potential energy of the collected carriers in case of
solar cell applications, in addition the narrow band
absorption band.
Functional organic semiconductors
To overcome the previously described functional shortage,
some solutions have been devised by producing specific
functional materials. In this section, these functional
materials will be outlined.
The donor polymer materials
The donor polymer materials are organic
semiconductors with long periodic molecules such
as P3HT. They effectively absorb the incident solar
radiation in their absorption range, producing excitons
that diffuse in and across their long-chain molecules
and across them. Their long molecules lead to exciton
diffusion lengths that are limited to a few tens of
nanometers, despite being relatively long.
The acceptor organic semiconductors
These materials have large affinity to the electrons
such that when they brought in contact to the donor
molecules, they will attract the electrons of the excitons
formed in the donor materials. So, their LUMO level
must lie under the LUMO of the donor molecules by an
amount of energy greater than the dissociation energy of
the excitons. In this way they will constitute an effective
sink for the electrons of the excitons generated in the
donor molecules. They must reflect the holes back to the
donor absorber to suppress their recombination in the
electron transport layer.
The concept of the acceptor material directly contacting
the molecules of the donor molecules is a basic concept
leading to the efficient dissociation of the excitons. Really
it enabled the realization of efficient separation method
for the generated electron hole pairs in the excitons. By
transferring the electrons of the excitons to the acceptor
molecules its chance for recombination becomes very
small especially when one further transfers the electrons
and holes far away from each other by collecting electric
field. There are two classes of acceptor materials; the
fullerene and the non-fullerene organic molecules. The
fullerene is expensive, less stable in the environment and
possesses weak optical absorption coefficient with narrow
https://hkn.ieee.org/ https://hkn.ieee.org/
The Bridge - Issue 2, 2021

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