Session C5
Paper#127
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THE ROLE OF FOUR-JUNCTION PHOTOVOLTAIC CELLS IN
EFFICIENTLY HARNESSING SOLAR ENERGY
Katey Paraskiewicz, [email protected], Sanchez 5:00, Claire Jackson, [email protected], Sanchez 5:00
Abstract – Today, rapid technological advancements within
the energy industry make solar power the most widely-used
alternative energy. The tool used to convert the sun’s rays
into usable energy is called a photovoltaic (PV) cell. Also
referred to as solar panels, PV cells convert sunlight into
electricity through the stimulation of electrons. Earlier
generations of solar cells consist of a layer of crystalline
silicon. The newest generation of photovoltaic cells, however,
feature four junctions, rather than the industry standard of
one. This revolutionary four-junction design handles a wider
range of light’s wavelengths thus allowing for the most
efficient absorption of sunlight the industry has ever seen.
Together, the prism-like shape of the cell and the integration
of multiple semiconducting materials allow for light to be
absorbed at a wider range of angles and wavelengths. The
design, process, and shape of the four-junction PV cell work
together to break barriers in the realm of solar power
efficiency. Within this paper, we will discuss solar energy as
a sustainable alternative to nonrenewable energy sources. We
will further this discussion by exploring the technical design
of the four-junction photovoltaic cell and the role it plays in
advancing the efficiency of the solar energy industry.
Key Words – Bandgap, photovoltaic cell, renewable energy,
semiconductor, tunnel junction, wavelength
THE SIGNIFICANCE OF PHOTOVOLTAIC
CELLS
Renewable Energy vs. Nonrenewable Energy
The push within the scientific community to harness
renewable energies is felt more strongly today than in any
other time in history. The movement stems from the
overwhelming extraction and consumption of nonrenewable
energy sources called fossil fuels. Fossil fuels, such as coal,
natural gas, and oil, negatively affect the environment. When
burned, they release gases such as carbon dioxide, methane,
sulfur dioxide, and nitrogen oxides [1]. When these gases
release, they become trapped in the earth’s atmosphere and
contribute to air pollution, the deterioration of the ozone layer,
University of Pittsburgh Swanson School of
Engineering 3-30-2017
and climate change. These are merely some of the deficits that
lead scientists, environmentalists, and civilians alike to
harness safer, alternative forms of energy.
As opposed to nonrenewable energy sources that utilize
earth’s finite resources, renewable energy works by
harnessing or mimicking earth’s natural processes to produce
energy in an eco-friendly way. Today, popular forms of
renewable energy include solar power, wind power,
hydroelectric energy, geothermal energy, and fuel cells [2].
The benefits of renewable energy include very few harmful
emissions, improved environmental quality, and improved
public health.
Harnessing Solar Energy
“Solar energy is the cleanest and most abundant renewable
energy source available…” claims the Solar Energy Industries
Association [3]. This claim is supported by the fact that solar
energy is indefinitely renewable because it is solely
dependent on the nuclear fusion power from the sun’s core.
As one of the most sustainable sources of energy, the sun’s
core is expected to continue burning for the next 5 million
years [4]. With such vast and long-term possibilities, solar
energy has the potential to greatly reduce the consumption of
fossil fuels. Because of this potential, this paper will
concentrate on the most modern way to convert sunlight into
electricity.
Though there are several ways to harness solar energy,
including concentrating solar power and solar
heating/cooling, this paper will focus exclusively on the field
of photovoltaics. The word ‘photovoltaic’ comes from the
process of converting photons into voltage, also known as the
photovoltaic effect [5]. Photovoltaic cells are commonly
referred to as solar cells.
But how exactly does a traditional photovoltaic cell
transform sunlight into usable electricity? Understanding this
information is crucial for later discussions within this paper.
This transformative process begins when the sun’s rays strike
the top of a photovoltaic cell. Next, a thin, semi-conductor
layer is paired with another layer in order to create an electric
field. This field exists at the p-n junction, or boundary. One
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Claire Jackson
side of this junction is positively charged and the other is
negatively charged. The sun’s beams excite the electrons
within the semiconductor material. The electric field provides
momentum and direction for these now excited electrons [6].
The electrons can then be captured by the use of a circuit.
release harmful gases that contribute to air pollution, such as
smog and acid rain. Once this pollution reaches the ozone
layer of earth’s atmosphere, the harmful gases dismantle the
ozone molecule (O3). As this layer becomes weaker and
weaker, dangerous amounts of ultraviolet B rays reach earth.
These rays can cause cancer, disrupt animal growth and
behavior, and contribute to global warming. By implementing
solar power, the consumption of nonrenewable energy
sources will decrease meaning dangerous carbon emissions
will decrease as well.
The solar energy industry also has strict recycling
guidelines for photovoltaic cells. These federal and state
guidelines help to reduce waste by repurposing the cells. Most
photovoltaic cells has a life expectancy of around 30 years [3].
Solar energy gained popularity in the 1980’s, which means
that most cells are still operating at effective levels. As these
solar panels reach their end-of-life, a waste issue could arise.
If solar power is to truly benefit the environment, the panels
must be repurposed rather than thrown into a landfill.
Harvesting and recycling a panel’s components, such as the
glass sheets and aluminum frame, reduces the cost of future
cells as well as reduces waste. Recycling PV cells also helps
sustain earth’s supply of semiconducting metals, such as
silicon, germanium, and gallium.
Redesigning PV Cells to Advance the Field of Solar
Power
Photovoltaic cells are commonly classified into three
generations. These generations are based on the materials
with which the cells are built and the processes used to create
them. The first generation consists of crystalline silicon
wafer-like cells, while the second consists of thin-film
semiconductor cells [7]. The third generation combines the
advantages of both previous generations. Some examples of
third-gen PV cells are organic polymer-based cells, hot carrier
cells, and multijunction cells.
These generations are a product of engineers’, scientists’,
and environmentalists’ efforts to continue producing
photovoltaic cells with higher and higher efficiency. It is this
research and design that pushes the solar industry into the
future. The multijunction PV cell has recently proved itself as
the third generation cell with the most promise. A specific
example of a multijunction cell is a four-junction PV cell.
According to IEEE, a four-junction cell developed in
Australia recently broke the world record for solar cell
efficiency under unconcentrated light [8]. In the next sections,
we will continue to discuss the role of four-junction
photovoltaic cells in efficiently harnessing solar energy.
Advantages of Solar Energy
Making the switch to solar energy is becoming
increasingly important as the rate of pollution entering earth’s
atmosphere continues to rise and the availability of
nonrenewable energy sources decreases [1]. Harnessing solar
energy does not cause near as much pollution as current
means of producing energy. With pollution being a lingering
threat in our world today, solar energy is becoming more and
more of a widely accepted sustainable alternative method to
fossil fuels [1].
One advantage of solar energy is reduced cost of
electricity. Photovoltaic cells today are able to convert
sunlight into electricity that can be immediately consumed.
This is not only efficient, but also cost effective. Using
electricity from solar cells makes a home or business less
reliant on electric companies, which in turn decreases their
electricity bills. The reduced cost of electricity has the
potential to benefit homeowners, as well as corporations and
companies. In addition, solar cells tend to pay back their
worth after a while. Over time, the reduction in electricity
costs an investor experiences will add up to the purchase price
of the cells themselves.
Another advantage of solar energy is its versatility. Solar
energy has many different purposes and applications. It can
be used to power ventilation systems, pools, pumps, lighting,
and cars. There are also remote applications, such as portable
HARNESSING SOLAR ENERGY
Sustainability of Solar Energy
Solar energy is one of the most sustainable forms of
renewable energy. This claim is supported firstly by solar
energy’s ability to preserve earth’s natural resources. A
widespread implementation of solar energy would directly
reduce the consumption of earth’s finite resources such as
fossil fuels, coal, and natural gas. With 85% of today’s energy
industry dominated by use of these nonrenewable sources, it
is clear that such consumption cannot be maintained for much
longer [1]. Whatever their timetable for complete depletion
may be, these resources cannot keep up with the growing
energy demands. Instead, society needs a long-term,
sustainable alternative energy source. Solar power presents
itself as such. The availability of the sun’s energy is nearly
limitless and the tools needed to harness it are already in
place.
Solar power also works to sustain our atmosphere, an
attribute that goes hand-in-hand with sustaining earth’s
natural resources. When coal and fossil fuels are burned, they
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devices that harness the suns power to perform simple tasks
such as charge a cell phone [9].
Another advantage, especially from a residential
standpoint, is that there are no moving parts involved in most
solar power devices. Therefore, the panels are not noisy and
seldom break down. This makes them even more convenient
and less of a hassle. Regarding panel maintenance, they
require very little cleaning, and the cleaning they do require
happens very seldom.
Another issue is that solar energy is an irregular energy
source; there is night and day. During the course of 24 hours,
sunlight is limited. This means that solar panels can only
operate at their peak efficiency during the day. To compensate
for the hours when the sun is not out and the panels are not
absorbing much energy, there must be a way to store energy
for use in non-peak times. Energy storage apparatuses are
very expensive, and drive the price of solar energy systems
even higher.
Although solar power is greener than nonrenewable
energy, it is still not completely eco-friendly. While the
energy itself does not cause pollution, the manufacturing,
transportation and installation of solar energy products does
[9]. The production process of making solar panels emits trace
amounts of greenhouse gasses [9]. Transporting solar panels
from manufacturing sites to installation sites happens via
truck or airplane, both of which contribute to air pollution by
emitting carbon monoxide. Another downside is that solar
energy apparatuses require space. For example, to power and
entire city with solar energy, a large plot of land would be
required to house the solar panels.
Disadvantages of Solar Energy
Although the advantages of solar energy are vast, there are
disadvantages as well. Solar panels and other renewable
energy sources are expensive. They are made of efficient
materials and require complex designs in order to successfully
convert solar energy into electricity. Although they eventually
pay for themselves, the initial purchasing price often acts as a
barrier to those interested in solar power.
A disadvantage of photovoltaic systems for homeowners
is that some houses are just unfit for solar panels. For
example, the house could be shaded, or positioned in a way
that is not conducive to absorbing light. However, shared
solar power systems, as seen in Figure 1, easily fix this issue
by allowing homeowners to invest in and share a ‘community
solar garden.’ These gardens are placed where the sun can
efficiently reach the solar panels. This allows multiple
households to reap the benefits of solar energy and consume
the electricity produced. Solar gardens are beneficial to
homeowners because the price of materials and installation is
only a fraction of what it would be for an individual-home
setup [10].
DESIGN ASPECTS: FOUR-JUNCTION VS
TRADITIONAL
A Traditional Design and Its Flaws
Today, the solar energy industry is dominated by singlecrystalline silicon photovoltaic cells, which are first
generation. Contrary to what the abundance of singlecrystalline silicon PV cells would indicate, there are many
flaws associated with them.
One downfall of producing silicon PV cells, whether they
be monocrystalline or polycrystalline, is the high cost of
production. In order to reach their maximum efficiency, the
crystalline structures must be of very high purity levels. This
means the structures must have few deformities and
contaminates. High purity levels correlate with high cost.
Poor quality leads to oxidation of the silicon which inhibits
the flow of excited electrons within the cell. Another issue
with silicon PV cells is their range of absorption. The
absorption coefficient, defined as 𝛼 = 4𝜋𝑘/𝜆, decreases as
the wavelength of the photon increases. A photon can be
thought of as a “packet” of energy. Photons are measured by
their wavelength, λ. As a result, long-wavelength photons
with λ > 1.2 μm are hardly absorbed [11]. Another downfall,
grain boundaries, lead to yet another decrease in efficiency.
Grain boundaries, or small defects in the crystalline structure,
also inhibit the electron flow. The amount of grain boundaries
is higher for polycrystalline silicon PV cells.
With such flaws, it is apparent that the solar power
industry needs new, unique, and powerful ways to collect and
FIGURE 1 [10]
Figure 1 shows how solar gardens work to benefit the
investors/subscribers
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utilize the sun’s energy. The third, newest generation of
photovoltaic cells offers a concrete solution to many of the
issues with first and second generations. Of all third
generation cells, our focus remains on the four-junction cell.
New and Improved: A Four-Junction Design
The key advantage of four junction photovoltaic cells is
their ability to absorb a wider range of wavelengths than
traditional PV cells. The new design has a light conversion
efficiency far ahead of today’s commercial solar cells.
Breaking the world record, four-junction cells achieve an
impressive 34.5% efficiency. Current commercial solar
panels shy in comparison, offering only 16% to 18%
efficiency [8]. The multiple junctions create this leap in
efficiency.
As the name may indicate, four-junction PV cells consist
of four single junctions layered on top of each other. These
layers are connected by tunnel junctions, whose job is to
provide a low-resistance connection site. The four main layers
are made of semiconducting elements and compounds such as
gallium arsenide, germanium, gallium indium phosphide, and
gallium arsenic iodide. The benefit of having four different
materials is that each one absorbs different wavelengths of
sunlight, which explains why the efficiency has increased so
greatly with this design. Also, the prism-like shape allows the
cell to collect light from a wider range of angles.
The layers are connected electrically in series, which
means that the same current flows through all of them, like a
circuit. They are layered based on the value of the material’s
bandgap. Put simply, a bandgap is an energy range in a solid
in which no electron can exist. Why are semiconductors ideal
materials for photovoltaics? The length of their bandgap is
nonzero, yet narrow enough for excited electrons to jump
across when struck by photons. After they leave their ground
state, they join the stream of electricity. The bandgap of a
material determines what portion of the solar spectrum it can
absorb. A semiconductor will not absorb photons less than
their bandgaps. This is why the four junctions are ordered by
decreasing bandgap values, as depicted in Figure 2. For
example, germanium (bandgap ≈ 0.67 eV) is usually the
bottom layer. The photons with the longest wavelengths are
absorbed by the top junctions, while the medium and shortest
wavelengths filter through the cell. This process of photon
filtration is how multijunction PV cells are able to convert
more solar energy into electricity than traditional cells. More
junctions correlate to higher efficiency.
FIGURE 2 [12]
Figure 2 shows the layering of a typical four-junction
photovoltaic cell.
PRACTICAL APPLICATIONS
The practicality of solar energy is high, as it can be
relatively affordable and efficient. When looking specifically
at the four junction cells, however, the practicality is more out
of reach for domestic use. They are hypothetically as
pragmatic as current solar panels, but they are much more
expensive due to the structural design and extensive materials
used. Four junction cells are still worth pursuing, however,
because of their increased efficiency. Since these cells convert
energy directly to electricity, the possible applications are
endless. For example, the four junction cells can be used to
heat pools through the process of solar water heating. Solar
water heaters come in a wide variety of designs, all including
a collector and storage tank, and all using the sun's thermal
energy to heat water. They are typically described according
to the type of collector and circulation system. For example,
a batch collector, also called Integrated Collector-Storage
(ICS) systems, heat water in dark tubes or tanks that are within
an insulated box. The water is then stored there until it needs
to be extracted. Water can remain in this collector for long
periods of time. The water stored is scalding. To protect
against the scalding water that would come out of the tap, a
tempering valve is there to mix in cold water before it is
delivered to the tap. This particular system is preferred in
climates where it rarely freezes, because it has no freeze
protection. However, there are other types of systems which
include freeze protection that are more sufficient for cold
weather climates. An example of a circulation system is an
active, or forced circulation system. Most common in the
United States, active, or forced-circulation systems use
electric pumps, controllers and valves to move water from the
collectors to the storage tank [13]. Also, four junction cells
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can be used for solar lighting. Solar lighting can be used for
street lights, solar garden lights, solar flood lights, and several
others. Solar lighting functions as it does because of the
photovoltaic effect, as mentioned previously these PV cells
can directly convert the suns light into an electric current.
That directly created current is what powers the lights. For
example, in solar powered garden lights they are able to
charge during the day when it is sunny. Next, when it begins
to get dark a photoreceptor on the light detects the darkness
and the light uses it’s the energy it has been converting all day
to power the light until the charge is used up [14]. Solar
pumping is another real-world application of solar energy the
process of which is shown below in figure 3 [15].
implemented. When PV cells are used to power network
towers (see figure 4), there is less pollution and less
maintenance required.
`
FIGURE 4 [13]
Figure 4 shows remote systems which PV cells are used
to produce power where backup generators are not a
feasible option.
As mentioned previously, solar AC pumping is a largescale application. AC pumping consists of three parts: the AC
pump, the pump controller, and solar panels. These are huge
systems and cost millions of dollars. These types of solar
pumps are often seen used for large farming applications and
other types of agriculture. A final, and very practical use of
PV cells, remote surveillance units. Remote surveillance units
are used in national parks and other remote locations without
direct access to electricity. These are essentially solarpowered cameras used to survey areas and reduce illegal
dumping in protected areas. In remote surveillance, solar
panels are mounted on top of the surveillance device facing
the most beneficial direction as to get the most energy out of
the sun. This is an incredibly reliable, cost and energy
efficient way to survey remote areas [13]. Also, since solar
panels do not emit air pollution, these solar-powered cameras
aid in sustaining the local wildlife. PV cells are also used to
power the satellites we have orbiting in space, so NASA is an
organization that puts PV cells to practical uses.
FIGURE 3 [13]
Figure 3 shows the process and parts of a solar pumping
system more in depth.
There are two types of solar pumping systems: the AC solar
pumping system and the DC solar pumping system. DC solar
pumping systems are made to handle much smaller tasks.
They are reliable, and cheap because they are for more
domestic purposes. AC pumps, however, are a better fit for
commercial applications. Lastly, another practical
application, mentioned above, is solar gardens. Solar gardens
could be implemented in communities everywhere, urban or
rural. It would decrease the greenhouse gasses emitted in the
area while simultaneously lessening the electric bill for locals,
or those invested in the solar garden.
As opposed to domestic applications, the practicality is
high for governments and corporations looking to reduce their
carbon footprint by utilizing renewable energy. Four junction
cells can be used in the business of telecommunication
networks [13]. Currently, network towers get their energy
supply from diesel generators. These towers are remote, emit
air pollution, and don’t allow for backup generators. This is a
perfect instance where solar energy and solar panels could be
IN CONCLUSION: THE ROLE OF FOURJUNCTION PV CELLS IN THE FUTURE OF
RENEWABLE ENERGY
As the consumption of fossil fuels for energy increases, so
does the amount of pollution in earth’s atmosphere. Because
of this, researchers and engineers today are developing newer,
cleaner ways to create consumable energy. Therefore, solar
energy and modern photovoltaic cells are more relevant than
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Claire Jackson
ever. Today’s emphasis on harnessing renewable energies
allows the field of solar energy to expand.
Solar energy is among the cleanest and most sustainable
sources of renewable energy. It is sustainable not only
because it relies solely on the nuclear fusion from the sun’s
core, but also because solar energy encourages the
preservation of resources and reduction of greenhouse gas
emissions. Though there are a multitude of ways to harness
the sun’s power, photovoltaic cells are one of the most
efficient, popular ways. The new design of these cells far
outperforms all of its predecessors. By increasing the cell’s
ability to absorb rays and yield energy, it more than triples
previous efficiencies. The new four junction design is more
complex than previous designs, and allows for sunlight
absorption to be maximized. The new design consists of a new
prism-like shape, four different semiconducting materials,
and four junctions ordered by decreasing bandgap values.
When considering four-junction cells as a viable
alternative to traditional cells, it is important to acknowledge
their advantages and recognize their disadvantages. Some of
the main advantages of this new four-junction cell design are
its increased efficiency, its sustainability, and its maintainable
nature. Overall, the benefits of solar energy and four-junction
PV cells outweigh the downfalls. When considering the
environment, it is without question that solar energy is
preferable to the nonrenewable energy sources upon which
today’s society relies so heavily. The increase of efficiency is
incredible, and the growth of the field of renewable in general
is something beneficial to everyone.
What do four-junction photovoltaic cells mean for the
future of solar energy? These third generation cells represent
how far the solar energy industry has come, and are a good
indication of how much further it can go. The four-junction
design acts as a stepping stone is the evolution of photovoltaic
cells. As engineers and researchers team up to make solar
energy more accessible and efficient, they will continue to
build upon and recreate this design. This process will
hopefully establish solar energy an everyday, widely-used
energy source.
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ACKNOWLEDGMENTS
We would like to acknowledge Beth Newborg for coming
into our class to tell us about the paper, as well as the librarian
for coming in and enlightening us about the resources
available to us. We’d like to thank our writing instructor for
the tough love, as well as the Writing Center for being there
for us when no one else was. Lastly, thank you to our peers
for reading through our paper and giving your input.
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