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Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other
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TRANSPARENT SOLAR PANELS IN SOLAR WINDOWS AND CELLULAR
PHONES
Melissa Barbour, [email protected], Mena 3:00, Anthony Jacob, [email protected], Mahboobin 10:00
Abstract—Conventional solar concentrators have limitations.
They require great quantities of space, the right position to
catch the light, and are aesthetically unpleasing. But a new
design seeks to remedy all three problems at once by
developing a transparent solar concentration system that can
be seamlessly integrated into any building or technological
device with transparent surfaces.
Professionals have been experimenting with designs
for see-through solar panels for some time. But just recently, a
team at Michigan State developed solar panels that absorb
only light we can not see. By collecting only light from the
invisible spectrum, it ensures that to our eyes these solar panels
look just like panes of glass.
The possible applications of transparent solar panels
are numerous. The most widely researched application is
known as a “solar window”. However, while the idea is
exciting, widespread implementation is practically unlikely as
solar panels (no matter their opacity) are most efficient at a
specific angle that could not be met when mounted to the sides
of building. In addition, they can only be mounted to the south
side of a building and they will most likely be shaded by
another building.
A more practical option for this technology has been
suggested by its creator, who sees additional implementation
possibilities in mobile devices such as phones or e-readers.
Indeed, cell phone companies have already attempted the
addition of solar panels behind the touch screen. They have
been unsuccessful because opaque solar panels must be placed
behind the screen. By the time the light gets to the solar panel,
most of it has been absorbed by the display. Transparent panels
have been suggested as an obvious solution. They could be
placed in front of the display without obstructing the view and
thereby be in a better position to power the device. Some
speculate that the addition of a transparent solar cell could be
enough to keep a device independently powered indefinitely.
Key Words— Construction, Energy, Renewable, Solar,
Sustainable
University of Pittsburgh, Swanson School of Engineering
Submission Date: 03.03.2017
1
RELEVANCE
Conventional solar concentration systems have many
limitations. They require great quantities of space, the right
position to catch the sun’s light, and are aesthetically
unpleasing. As Margaret Rhodes writes for Wired Magazine,
“Standard solar panels live on rooftops, need to tilt at a certain
angle, and, most problematically, are an eyesore” [1]. This has
hindered implementation in environments with space
constraints and kept solar power from living up to its full
potential. But a new solar panel design seeks to remedy both
problems at once with a transparent solar concentration system
that can be seamlessly integrated into any building or
technological device with transparent surfaces.
THE PROGRESSION OF THE SOLAR
PANEL
Traditional Solar Panels
The history of solar cells dates back to 1876, where a
professor and student discovered that when selenium was
exposed to light, it produced electricity. A little less than one
hundred years later, the silicon solar cell was discovered. The
cell produced enough electricity and was efficient enough to
run small electrical devices such as alarm clocks. As years went
on, these solar cells became available commercially but were
still too expensive for everyday people. Slowly, efforts were
taken to try and reduce cost. In the 1970s, Exxon Corporation
financed research to create solar cells from lower-grade silicon
and cheaper materials as an attempt to reduce cost. The federal
government also passed several solar friendly bills and created
the National Renewable Energy Laboratory in 1974, which is
now the United States’ primary lab for renewable energy and
energy efficiency research and development [2].
As technology and efficiency of solar cells increased,
solar power became more popular. Finally, in 2005, residential
and commercial solar panels started becoming more popular.
These panels had many benefits and thus were a large step in
the right direction. However, these traditional solar panels have
many shortcomings. For example, one major problem with the
Melissa Barbour
Anthony Jacob
addition, these panels are beginning to be integrated into
consumer products, such as mobile phones [5].
One problem with this technology is efficiency. Solar
cell efficiency refers to the portion of energy in the form of
sunlight that can be converted via photovoltaics into electricity.
The solar conversion efficiency of a standard Luminescent
Solar Concentrator (LSC), the technology behind these
transparent solar panels, is around one percent. Solar
technologies can have efficiencies ranging from eight percent
to forty-five percent. A commercial rooftop solar panel has an
efficiency of about fifteen percent. Ideally, the efficiency of
these LSCs can be increased by greater than five percent with
the introduction of new designs [5].
Researchers at M.I.T. attempted a different approach
to solving this efficiency problem by proposing to make a
photovoltaic solar cell that would absorb all the energy from
the sun except the part that allows us to see. Researchers
showed that this design should realistically be able to reach
over twelve percent efficiency, a rating comparable to that of
existing commercial solar panels [6].
The biggest obstacle between these LSCs and getting
them in the commercial market is their longevity. In
commercial applications such as window coatings, the solar
cells need to continue performing well for years. According to
Vladimir Bulović, a professor of electrical engineering at
M.I.T., work to extend the lifetime of related products has
made good strides. With many industries tackling the same
issue, he believes that this engineering problem should be
solved in the coming years, and their solar cells should be
guaranteed to have a commercially viable lifespan [7].
However, despite these drawbacks, one major benefit
to these LSCs is the potential they have in reducing costs of
solar technology. Since these LSCs concentrate the energy
absorbed into a smaller area, the number of expensive solar
cells required dramatically decreases, reducing the overall cost
of the module [7].
Besides that, what makes these transparent panels
attractive for the near future, is that they can be added to things
that are already being deployed, and they won’t require
devoting vast new areas to collecting solar energy. With this
technology, usable areas already exist in the surfaces
surrounding us.
traditional solar panel is installation area. Many residential
areas and companies would like to utilize solar power, but these
solar panels require a large area for the system to be efficient
in providing electricity on a constant basis. Another concern
with these panels is their cost. The cost of purchasing and
installing solar panels always becomes a disadvantage when
the topic of solar energy comes up, and these traditional panels
support that argument. The panels must also be in the right
position to catch the sunlight and absorb its energy, and the
weather conditions must be adequate in order for these panels
to do their job. Finally, another concern with these panels is
that they are aesthetically unpleasing. Solar arrays speak
loudly, and many people are reluctant to install a large solar
array on their otherwise attractive building. With all these
shortcomings, it becomes more difficult for the traditional solar
panels to progress successfully.
Colored Solar Panels
One alternative design for solar panels involves
collecting only one color of light and allowing all others to pass
through. This gives the panels the appearance of being tinted
the same color as the light being collected. These “stained
glass” panels are cheaper than the traditional black and nearly
match the opaque panels’ efficiency [1]. However, the tinted
light that these panels produce has its own aesthetic
disadvantages. While every plain window being replaced with
stained glass may sound appealing, Richard Lunt, one of the
researchers, has found otherwise. “’No one wants to sit behind
colored glass. It makes for a very colorful environment, like
working in a disco’” [4]. Lunt is on a team at Michigan State
that worked to develop a new type of solar panel that collects
only the light we can not see.
Transparent Solar Panels
Transparent panels are a step ahead of the traditional
solar panel. Researchers are making transparent solar cells that
could turn everyday products such as windows and electronic
devices into power generating sources. Introduced around
2014, these panels absorb infrared and ultraviolet light.
Because we cannot see ultraviolet or infrared light, these panels
appear transparent to the human eye, even while concentrating
sunlight. Researches have deposited coatings of their solar cells
on various materials and have used them to run electronic
displays using surrounding light, all while using simple room
temperature methods. One of the biggest advantages of the
transparent solar cells is that they can be deposited on any
surface without obscuring the look of the underlying material.
Coating a skyscraper with these transparent photovoltaic cells
is estimated to be able to provide for more than a quarter of the
building’s energy without changing the building’s appearance.
Not only that, but the cells would block much of the infrared
radiation, a large part of the sunlight that heats up the room.
This would cut down on air conditioning needs, further
reducing energy use and operating costs of the building. In
THE TECHNOLOGY BEHIND
TRANSPARENT SOLAR PANELS
Luminescent Solar Concentrators
The main technology behind these transparent panels
is the Luminescent Solar Concentrator (LSC). LSCs “collect
ambient light from a broad range of angles and concentrate the
captured light onto photovoltaic (PV) cells” [8]. The concept
for LSCs has been around for over 30 years. However, recent
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Melissa Barbour
Anthony Jacob
advances in phosphorescent and fluorescent efficiencies have
allowed for an improvement in the LSC’s power conversion
efficiency.
FIGURE 2 [6]
This schematic diagram shows the key components in this
transparent photovoltaic device.
The main components of this design, pictured above,
consist of the plastic or glass, the dye molecules, and solar
cells. The plate consists of multiple layers coated with particles
of dye. The thickest layer, at the bottom, is comprised of the
plastic, glass, or any other transparent substrate being coated.
Near the top, are multiple layers of photovoltaic coating. At the
core of this coating are the two active layers. These layers
consist of absorptive semiconductor materials that interact
once they are excited by the sunlight. They then create an
electric field that causes the current to flow. Surrounding those
layers are electrodes connected to an external circuit, which
carry the current out of the device. Since these electrodes are
transparent, an additional layer can be added to the back of the
cell to reflect sunlight of selected wavelengths. Miles Barr,
president and CTO of Ubiquitous Energy, Inc., says of these
transparent panels, “We use a combination of molecular
engineering, optical design, and device optimization—a
holistic approach to designing the transparent device” [6]. This
goes to show that the depth put into this design is what
differentiates it significantly from the traditional solar panel.
Between the thick plastic/glass layer and the
photovoltaic layer is a critically placed gap. This gap allows the
solar cell to be transparent to the human eye. However, this also
means that the cell does not capture all the incident
energy. “We do let the visible photons [light particles] pass
through, allowing them to efficiently light the room. But we try
to catch all of the photons in the infrared and ultraviolet,” says
Bulović. “We try not to let any of those photons get through”
[6]. Current versions of this cell transmit more than seventy
percent of the visible light, which is within the same range of
the tinted glass used in windows of buildings. However, their
power conversion efficiency is low. This design should
realistically be able to reach twelve percent efficiency. Richard
Lunt supports this, mentioning that by simply “stacking” the
transparent solar cells, they could reach an efficiency of ten
FIGURE 1 [3]
Transparent panel containing organic salts that
absorbs UV and infrared light and emits infrared so that
the panel appears transparent.
Basically, these LSCs consist of a glass or plastic
plate coated with dyes that absorb sunlight and emit light at
longer wavelengths, as pictured above. The LSCs employ
novel nanocrystal polymer blends that allow for selective
ultraviolet light harvesting that results in a high degree of
visible light. A substantial part of the longer wavelength light
is trapped by a process called total internal reflection and
guided to the edges of the plate, where it is then absorbed by
small area PV cells. Total internal reflection is the reflection of
the total amount of incident light at the boundary between two
media. During this process, as the angle of incidence increases
between the light and the plate, more of the light is reflected,
thus spreading to the edges of the plate [7]. Previous efforts to
construct similar transparent power producing surfaces have
focused on, “optically thin photovoltaics that have a significant
tinting or limited transmission” or “focusing optic systems
using direct light only that requires bulky solar tracking or
optics” [7]. These efforts are flawed, leading to significant
tinting and limited transmission, defeating the purpose of these
transparent panels. This leads to the now more recent
technology behind this LSC design, which focuses on
selectively absorbing near-infrared photovoltaics and
exploiting
the
excitonic
character
of
molecular
semiconductors.
Components of the LSC and the Technology
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Melissa Barbour
Anthony Jacob
which is going to shade that side of the building’” [9]. The
numerous hindrances to implementation create a lot of doubt
as to the viability of solar windows.
percent while still maintaining the ability to transmit light.
They have already demonstrated that an array of transparent
cells integrated in series can power the liquid crystal display on
a clock, while relying solely on ambient light [6].
The combination of the plastic/glass and the dyes acts
as a waveguide. A waveguide is any device that traps light and
then moves those light waves along a path to a particular
destination. In the case of the LSC, the dyes absorb the light
that hit the plastic/glass. The sun’s energy is transferred to the
dye, and the electrons in those molecules become excited and
jump to a higher energy level. When the electrons fall back to
a lower energy level, the dye molecules release that energy into
the carrier sheet, where it gets stuck. During total internal
reflection, the light bounces around in the material, ultimately
making its way to the outer surface. A type of aluminum called
tris is added to the mix of dye molecules. These aluminum
molecules cause the dyes to emit light waves at a frequency the
dyes can't absorb. In this way, no light is lost to re-absorption
as it makes its way to the solar cells at the edges of the glass.
At the outer surface, the solar cells are waiting to absorb the
light and generate electricity. By concentrating the light to the
edges, a much smaller amount of photovoltaic material is
needed which helps to reduce overall cost of the panel [7].
Solar Phones
A more practical option for transparent solar panel
technology has been suggested by its creator, who sees possible
implementation in “tall buildings with lots of windows or any
kind of mobile device that demands high aesthetic quality like
a phone or e-reader” [4]. Indeed, cell phone companies have
already attempted the addition of solar panels. Several years
ago, rumors surfaced that the iPhone 6 or 7 would include a
“screen with a laminate of film containing solar cells” [5], but
these claims never came to fruition. Many cell phone carriers
have experimented with the idea of integrating solar panels
behind the display screen. However, as Motorola engineer Zili
Li explained to New Scientist, they have been unsuccessful
“because the LCD [display] absorbs most of the incoming
light before it can reach the [opaque] solar cell” [10].
Transparent panels have been suggested as an obvious solution.
They could be placed in front of the LCD display without
obstructing the view, and thereby be in a better position to
power the device. Some speculate that the addition of a
transparent solar cell could be enough to keep a device
independently powered indefinitely.
A constantly charged device would be revolutionary.
Nancy Stauffer writes for the MIT Energy Initiative that
“Enabling such devices to gather energy from ambient light
and recharge their own batteries will provide significant
benefits, including added convenience, greater freedom from
the power grid, and a better user experience” [6]. However, the
implications of this technology reach far beyond the personal
inconvenience of having to charge our electronic devices. In
many third world countries, mobile phones are the primary
device on which people carry out important transactions, like
banking. New scientist reports that in developing areas of Asia
and Africa farmers use mobile phones to check market prices
in many different areas so they know how best to sell their own
crops. In Sierra Leone, coffee traders want to cut costs by
paying their farmers electronically [11]. But they cannot
implement this business practice if there is uncertainty about
whether or not any given farmer will have an active phone. In
many of these remote areas, the electricity needed to power
phones is intermittent at best. “An estimated 500 to 650 million
cellphone users are off-grid” [11] according to that same
article. This means that these people must walk miles to a
charging station and then pay for its use. One company
working to solve this problem, Buffalo Grid, reports that “In
rural economies, about 50 per cent of the money spent on
mobile phones is actually spent on charging them. ... That is
some of the most expensive electricity in the world” [11].
Lessening or even eliminating dependence on an unreliable
grid or expensive charging stations could simplify the lives of
many people and make technology accessible to many more for
the first time.
APPLICATIONS
Solar Windows
The possible applications of transparent solar panels
are numerous. One expert working to implement colored solar
panels in Denmark imagined that “Bus stops, typically made
from three to four walls of transparent glass, could easily
harvest energy. Park benches could be transformed
into colored pieces of furniture and double as charging
stations. Outdoor concert stages could catch enough light to
then power the restroom and dining facilities” [1]. Many solar
technology experts focus on what are called “solar windows”.
The implementation involves covering or replacing common
glass with transparent solar panels. In this way, almost any
building could generate its own electricity. The Journal of
Applied Physics notes that “windows, skylights, and
greenhouse panels” [8] are all possible targets for this new
technology. There are several startups working to scale up to
market scale production and integration of solar windows.
Theoretically, implementation of enough solar windows could
even make a building self-sustaining. However, while the
possibility is exciting, such widespread implementation is
practically unlikely as solar panels (no matter their opacity) are
most efficient under very specific conditions. As reported by
Yale Environment 360, “‘The optimal installation for solar is
you want it to be facing south, you want a slight tilt to it, and
you want good solar access, so you don’t want anything to
shade those panels,’ says [MJ] Shiao, of GTM Research. ‘The
problem with skyscrapers is suddenly you’re putting them in
vertical orientation, there’s only one south side to the building,
and chances are that skyscraper is next to another skyscraper,
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Melissa Barbour
Anthony Jacob
technology will bring electricity to emerging countries that
have not been covered by the grid and lessen the environmental
impact of first world energy addicts. After years of progress,
the advancements made in photovoltaic technology have made
it possible for many see-through surfaces to become energy
collectors. Whichever mediums locations are chosen to use
transparent solar panel, this could prove to be a very significant
step in reducing our dependence on nonrenewable sources of
energy. With more remote countries entering the twenty first
century and all countries developing sustainable energy for the
twenty second, the future is clear.
THE FUTURE
Making it easier for these farmers to do business with
more people will not only improve their standard of living but
also decrease the environmental impact of farming. For years,
farmers in remote areas have been increasingly resorting to
unsustainable farming techniques to increase output. The
ability of farmers to instantly check market prices in other areas
will keep the companies who buy their produce honest. Fair
wages will reduce the desperation felt by small farmers. It is
likely that this could, in turn, slow the spread of
environmentally unconscious practices like slash and burn
farming.
It is important for all engineering endeavors to take
sustainability into account. Merriam-Webster defines
sustainability as “of, relating to, or being a method of
harvesting or using a resource so that the resource is not
depleted or permanently damaged” [12]. Experts have
concluded that non-renewable energies, such as fossil fuels, do
not meet that definition and are, therefore, unsustainable. A
2012 BBC study estimated that if consumption of these
resources continued at the current rate, the last fossil fuel
reserves would be empty by 2054 [13]. This highlights the
importance of innovative renewable energy technologies. New
designs, such as the transparent solar cell, that increase the
viability of renewable energy will play an important role in
powering our world going forward.
The increased implementation possibilities of the seethrough solar panel are an important contribution to the
expansion of sustainable energy. Making renewable energy
available to more people could greatly slow the depletion of
non-renewable energy sources. Reducing our dependence on
these sources is vitally important as we near the point where
relying on them will no longer be possible.
Furthermore, increased implementation potential is
not the only advantage of clear solar panels when it comes to
sustainability. Detractors of current solar technology cite the
environmental risks of a potential leak. Toxic substances such
as Cadmium, lead, and sulfuric acid, have the potential to harm
the environment if accidentally released [3]. Many of these
pollutants will still be present with the transparent technology
as they are not found in the panel but in the battery bank.
However, transparent solar technology does not include
cadmium sealed inside the panel, and thereby poses less of a
threat to the environment than the opaque design.
Because of the increased implementation possibilities
and the lack of toxic material in the panel, the clear solar cell
holds a lot of potential as far as sustainability efforts are
concerned. It and technologies like it are likely to receive more
attention in coming years as necessity forces us towards
sustainability.
SOURCES
[1] M. Rhodes. “Clear Solar Panels Double as Highway Sound
Barriers.”
Wired.
08.04.15.
Accessed
1.10.17.
https://www.wired.com/2015/08/clear-solar-panels-doublehighway-sound-barriers/
[2] “A History of Solar Cells: How Technology has Evolved.”
Solar Power Authority. 07.13.16. Accessed 03.01.17.
https://www.solarpowerauthority.com/a-history-of-solar-cells/
[3] N. Gromicko. “Disadvantages of Solar Energy.”
InterNACHI.
2017.
Accessed
03.01.17.
https://www.nachi.org/disadvantages-solar-energy.htm
[4] J. Wallace. “Solar Concentrators: Colorless dopants make
luminescent solar concentrator transparent as a window.” Laser
Focus World. 10.02.14. Accessed 1.10.17.
http://www.laserfocusworld.com/articles/print/volume50/issue-10/world-news/solar-concentrators-colorlessdopants-make-luminescent-solar-concentrator-transparent-asa-window.html
[5] K. Boehrer. “Researchers Develop Transparent Solar
Concentrator That Could Cover Windows, Electronics.” The
Huffington
Post.
08.24.14.
Accessed
1.10.17.
http://www.huffingtonpost.com/2014/08/24/transparent-solarconcentrator_n_5700544.html
[6] N. Stauffer. “Transparent Solar Cells.” MITe. 06.20.13.
Accessed 03.01.17.
http://energy.mit.edu/news/transparent-solar-cells/
[7] R. Lunt. “Transparent Luminescent Solar Concentrators for
Large- Area Solar Windows Enabled by Massive Stokes-Shift
Nanocluster Phosphors.” Advanced Energy Materials.
04.25.13. Accessed 1.10.17.
https://www.deepdyve.com/lp/wiley/transparent-luminescentsolar-concentrators-for-large-area-solar-dYZXF06GWo
[8] S. Leow. “Analyzing luminescent solar concentrators with
front-facing photovoltaic cells using weighted Monte Carlo ray
tracing.” Journal of Applied Physics. 06.07.13. Accessed
03.01.17.
http://aip.scitation.org/doi/full/10.1063/1.4807413
[9] D. Levitan. “Will Solar Windows Transform Buildings to
Energy Producers?” Yale Environment 360. 05.03.12.
Accessed 1.10.17.
LASTING IMPLICATIONS
In conclusion, the transparent solar panel is poised to
make renewable energy more accessible than ever. This
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Anthony Jacob
http://e360.yale.edu/feature/will_solar_windows_transform_b
uildings_to_energy_producers/2524/
[10] P. Marks. “Self-Powered Displays Keep Gadgets Live.”
New
Scientist.
05.05.07.
Accessed
1.10.17.
https://www.newscientist.com/article/mg19426026-100-selfpowered-displays-keep-gadgets-alive/
[11] A. Ananthaswamy. “Send a Text Message to Charge Your
Cellphone.” New Scientist. 03.06.13. Accessed 1.11.17
https://www.newscientist.com/article/mg21729075.500-senda-text-message-to-charge-your-cellphone/
[12] “Sustainability.” Merriam Webster. Accessed 3.22.17
https://www.merriam-webster.com/dictionary/sustainability
[13] D. Swain. “Stock Check.” BBC. 06.12. Accessed 3.24.17
http://www.bbc.com/future/bespoke/BBCF_infoData_stock_c
heck.pdf
ACKNOWLEDGEMENTS
We would like to thank our writing instructor Rachel
McTernan for helping with our specificity. We would also like
to thank the dedicated professionals who work to make our
lives more sustainable every day.
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