The Bridge - Issue 2, 2021 - 29

Feature
A Review of Schottky Junction Solar Cells
inverted structure, encapsulation using an
insulating material, and other optimized processing
steps, which will be discussed accordingly in the
subsequent sections.
Figure 3. Achieved practical efficiency of solar cells with different
semiconductor materials and structures. The red dashed line represents
the average PCE of Silicon solar cells [5].
First Solar Inc. announced in 2014 to have built CdTebased
solar cells with record high PCE of 21% certified
for industrial and large-scale fabrication [8]. However,
due to the environmental concerns because of
potential toxicity of Cd, and also the usage of rare-earth
element such as tellurium, CdTe may face difficulties
and limitations in large-scale fabrication over a long
period of time [9].
Figure 4. Experimental setup for electrical characterization of solar cells
under standard illumination condition.
In recent years, organic or hybrid solar cells, in which
an organic-based material (usually polymer) is used
in either one or all light-absorbing active layers of
the solar cell structure, are being investigated due
to their interesting electrical and optical properties,
such as transparency, flexibility, lightweight, and more
importantly, easier fabrication process, which enables
large-scale fabrication of these solar cells. In [10], a
new polymer donor, called as PBDB-T-SF is used to
fabricate fullerene-free organic solar cells with thickness
of around 100-200 nm and record high PCE of 1213.1%.
One of the major disadvantages of organic
solar cells is short-lifetime of these devices, mainly
due to deterioration of the organic material which can
possibly reduce the rate of photo-generated current.
There are several solutions in order to address the
short lifetime of organic solar cells, such as using
III. Schottky Junction Solar Cells
Multi-junction solar cells with sub-cells made of
semiconductors with different bandgap energies, are
usually based on p-n junction. In this structure, the
potential difference between Fermi levels of an n- and
p-type semiconductor leads to creation of an electric
field and built-in voltage at the junction interface.
Photo-generated carriers are then separated by the
electric field and extracted from the depletion region,
leading to flow of the current. In a multi-junction
structure, each sub-cell is basically a single p-n junction,
where photons with the energy near to the band edge
of the semiconductor are absorbed.
However, there are several theoretical and practical
limitations in the design and fabrication of multijunction
solar cells based on p-n structure in order
to achieve high power conversion efficiency. For
example, the current mismatch between different
layers in a multijunction solar cells leads to severe
degradation in the efficiency of these devices. Also,
producing the required doping level, especially p-type
doping, for wide-bandgap semiconductors has often
been accompanied with difficulties, such as impurity
incorporation and crystal defects formation, all which
can act as recombination centers within the material,
and thus negatively impact the photovoltaic response
of the solar cell [11].
Figure 5. Valence and
conduction energy bands
for some of the widebandgap
III-V compound
semiconductors. Hole
localization due to deep
valence band leads to
difficulty in producing
high p-type conductivity
in these wide-bandgap
semiconductors.
Figure 5 shows the energy levels of conduction
and valence bands of some of the wide-bandgap
compound semiconductors. In these semiconductors,
deep valence band leads to localization of holes with
high ionization energy. Therefore, they cannot be used
as free charge carriers in solar cells and contribute to
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The Bridge - Issue 2, 2021

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