The Bridge - Issue 2, 2021 - 28

Feature
A Review of Schottky Junction Solar Cells
Solar energy sources occupy about 6% of the total
energy from renewable sources. The availability,
accessibility, and low price of solar energy as compared
to other renewables has to led to increasing demand.
The use of solar energy is growing across the globe,
in both small-scale research and development
applications and large-scale manufacturing processes.
Examples of successful use of solar energy include
the Tesla solar roof, portable solar chargers, and offgrid
solar-powered manned and unmanned vehicles.
Solar energy has proven to be an effective means of
supplying electricity to remote and developing areas
underserved by traditional power grids.
II. Current State of Solar Cells
Increased use of solar energy to meet the world's
energy demands has led to large investments and
research in this area during the last few decades.
However, the greatest two challenges have been
to (1) increase the power conversion efficiency
(PCE) of the solar cells, while (2) reducing their cost
for large-scale fabrication [3]. In order to increase
the maximum power conversion efficiency in solar
cells, most research has been on the optimization
of the solar cell structure and development of new
semiconductor materials that can be used in a solar
cell that can provide a higher efficiency. The level
of photon absorption and the solar cell's power
conversion efficiency is determined depending on the
semiconductor's bandgap. In 1961, physicists William
Shockley and Hans Joachim Queisser calculated the
maximum theoretical power conversion efficiency
of solar cells achievable from a simple p-n junction
structure [4].
As can be seen in Figure 2.a, a maximum theoretical
PCE of around 32% is calculated for solar cells made
using semiconductor with bandgap energy around 1.1
eV, under typical solar illumination condition of AM1.5,
equivalent to 100 mW/cm2. It should be noted that
only 32% of solar energy is converted into electrical
power and the rest is either not absorbed (below
bandgap photons), thermally wasted due to relaxation
of hot photons with energy higher than the bandgap,
reflected from the top surface of the solar cells, or nonradiatively
recombined, resulting into huge losses in
power conversion efficiency of the solar cell, Figure 2.b.
Figure 2. (a) Shockley-Queisser detailed balance limit and maximum
theoretical Power Conversion Efficiency (PCE) in a simple p-n junction solar
cell, (b) breakdown of other loss mechanisms contributing to lowered PCE [4].
During the last two decades, there have been
numerous efforts in order to reduce the electrical,
optical, and thermal losses in the solar cells, with the
aim of increasing their power conversion efficiency.
Silicon solar cells are dominating the solar energy
market mainly due to two reasons: 1) the maturity
of the technology for the development of Silicon
solar cells, and 2) availability and abundancy of the
semiconductor materials, resulting in lower production
cost for large-scale fabrication of these devices.
Moreover, a reliable power conversion efficiency of
silicon, had made it a favorable choice for design
and fabrication of industry-scale photovoltaic panels.
However, due to the relatively low power conversion
efficiency (around 25% in practice) of silicon-based
solar cells, it has become necessary to investigate
other suitable materials which can replace Silicon with
better performance, higher PCE, and possibly similar
or lower cost. Figure 3 shows the research completed
since 1993 done to investigate different materials
and structures to achieve higher power conversion
efficiency higher than 25% achieved for Silicon solar
cells [5]. Copper indium gallium selenide (CIGS)
with tunable bandgap energy of 1-1.7 eV is used to
fabricate thin-film flexible solar cells. In [6], CIGS solar
cells are fabricated on flexible copper foil and graphene
has been used as hole transport layer, resulting into
power conversion efficiency of 9.91%. Recently, T. Kato
et al. at Solar Frontier Inc. have achieved a record high
power conversion efficiency of 22.9% in development
of a 1 cm2 CIGS (Cu(In1-x
Gax)(Se,S)2
) solar cells, in
which alkali treatment using cesium on the solar cells
has led to a boost in device performance [7].
Cadmium telluride (CdTe) based solar cells are also
of interest due to their potential lower production cost
than silicon-based solar cells, and comparable power
conversion efficiency with that of Silicon-based devices.
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The Bridge - Issue 2, 2021

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