The Bridge - Issue 2, 2021 - 30

Feature
A Review of Schottky Junction Solar Cells
photo-generated current [12]. This leads to difficulties
in producing highly efficient p+
or n+
doped layers to
be used as active, tunneling, or metallization contact
junctions in solar cells.
In Schottky junction solar cells, an interface between
a thin conducting film and a semiconductor, provides
the necessary depletion region (band bending) for
separation of photo-generated electron-hole pairs,
which leads to generation of photo-current. Figure
6 shows the difference of operation between a
conventional p-n junction and a Schottky junction
solar cell. An abrupt potential barrier is created at the
interface of this junction due to: 1) difference in the
energy levels between Fermi level of the metal and
conduction band of the semiconductor instead of the
transitional energy barrier observed at the interface of a
p-n junction, and 2) pinning of the edge of Fermi level
to specific energy levels.
Narrower depletion region in Schottky junction solar
cells leads to lower Schottky barrier height and higher
dark reverse leakage current, mainly due to thermionicfield-emission
(TFE) which results in a deterioration of
the photovoltaic characteristics. This typically reduces
the efficiency of Schottky junction solar cells compared
to p-n junction solar cells. Therefore, a trade-off exists
between the choosing the suitable material and the
structure of the solar cell in order to achieve the
optimized photovoltaic response in the Schottky
solar cells.
The concept of Schottky junction solar cells have been
widely used to design and fabricate high efficiency
solar cells. For example, researchers have fabricated
Schottky solar cells by deposition of 80 nm Indium
Tin Oxide (ITO) on n-GaAs substrate [14]. However, a
large dark reverse leakage current density (~10-2
mA/
), mainly caused by (1) thermionic emission at the
Schottky barrier, (2) higher carrier recombination rate
due to crystal defects at the interface, and (3) Schottky
barrier height inhomogeneity, limits the efficiency of
these solar cells to less than 1%.
cm2
Figure 6. Band structure of a typical (a) p-n junction, and (b) metalsemiconductor
Schottky junction solar cell at zero bias voltage.
The use of Schottky junction solar cells can eliminate
the need for the use of highly doped p-type
semiconductor, and therefore potentially improve
the photovoltaic response of the solar cell. Moreover,
manufacturing Schottky solar cells may prove to be
cost-effective and industry scalable, since fewer and
simpler processing steps are required that result into
lower total production cost [13].
The depletion region for a p-n and Schottky structure
can be calculated using:
Figure 7. Schematic
structure of Schottky
graphene/n-Silicon
Schottky solar cells
with 2nm ALD-grown
passivation layer [17].
The dark reverse leakage current at Schottky junctions
can be lowered by using passivation layer, which can
reduce the recombination of charge carriers [15]. Al2
with thickness of 5 nm has been used at Au/Ti/nGaAs
Schottky junctions and has reduced the leakage
current density from 7.3×10-8
to 1.2×10-14 mA/cm2
and increased the Schottky barrier height from 0.77 eV
to 1.18 eV [16]. Similarly, a 2 nm Al2
O3
has increased
where WD is the width of the depletion region, ԑs
is the permittivity of the semiconductor, Na
are acceptor and donor concentrations, φ is the
built-in potential, and Va
applied voltage.
THE BRIDGE
is the forward or reverse
and Nd
the power conversion efficiency of graphene/n-Silicon
Schottky solar cell from 7.2% to 8.7% [17]. Figure 7
shows the schematic structure of the Schottky solar
cell, where the application of Al2O3 passivation layer
has led to improvement in photovoltaic response.
Ghods et al. have similarly used the concept of fieldeffect
passivation for reducing the leakage current in
metal/n-GaAs Schottky junction solar cells [18]. In this
O3

https://www.eia.gov/energyexplained/renewable-sources/ https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf https://investor.firstsolar.com/news/press-release-details/2014/First-Solar-Builds-the-Highest-Efficiency-Thin-Film-PV-Cell-on-Record/default.aspx https://hkn.ieee.org/ https://hkn.ieee.org/

The Bridge - Issue 2, 2021

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