Session A13
Paper 128
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 be provide complete analyses of all relevant data. If this paper is
used for any purpose other than these authors’ partial fulfillment of a writing requirement for first year (freshman)
engineering students at the University of Pittsburgh Swanson School of Engineering, the user does so at his or her own
risk.
IMPLEMENTATION OF COLLOIDAL QUANTUM-DOT SOLAR CELLS
AND THEIR APPLICATIONS
Katelyn Benson, [email protected], Vidic 2:00pm, Tyler Smith, [email protected], Vidic 2:00pm
Abstract- Solar energy has seen the emergence of the
colloidal quantum-dot solar cell, a novel form of photon
collection which offers the possibility of increasing the
theoretical efficiency of solar panels while also
substantially cutting the costs of production. Quantum dot
solar panels, the topic of this paper, are exciting due to
their ability to be tuned to a wider spectrum of
electromagnetic radiation and their ability to withstand
external damages through the use of ligands. Quantumdots are made of nanoscale particles, most often PbS, that
have organic ligands attached all around, creating a
molecular shield which protects the cell from outdoor
elements. In this paper, the technology, cost, importance,
and applications of colloidal quantum-dot solar cells will
be discussed. The focus of the research and the most
obvious application for colloidal quantum-dot solar cells
is a power source for homes and businesses. With
increasing uses and applications of any new technology,
scientists need to question whether it is significant and
ethical. As society continues to push towards
sustainability, solar energy is often brought up. Colloidal
quantum-dot solar cells serve as an environmentally
friendly solution to the power problem. The economical,
versatile applications prove that this technology will be
significant to the future of energy.
Key Words—Colloidal Quantum-Dot,
Ligands, Power, Solar Energy
Fossil
Fuels,
AN INTRODUCTION TO CQD
TECHNOLOGY
What are CQDs?
The emergence of innovative solar panel technology has
seen the birth of a novel form of photon capture: the
colloidal quantum-dot solar cell. According to Alternative
Energy’s Technology section, a technical blog that
regularly updates innovative breakthroughs in modern
science, these solar cells are comprised of light-sensitive
nanoparticles that are capable of capturing photons and
harnessing their energy [1]. Much like the standard silicon
solar panels used today, colloidal quantum-dot cells
(CQDs) utilize p-type and n-type semiconductors to create
a potential over which electrons shift, resulting in an
electrical current that can be harnessed and stored in
1
University of Pittsburgh, Swanson School of Engineering
Submission date: 03.31.2017
batteries. One of the most significant differences between
CQDs and current solar technology is the utilization of
such panels outdoors. Current panels have a tendency to
bind to oxygen, which results in a regression of production
as the panel ages. CQDs have been found to resist this
bonding to oxygen, and therefore are more practical for
outdoor usage, while also boasting higher device lifetime
expectancies [1].
CQDs are not yet at the same efficiency rate that solar
panels are, but technological breakthroughs are rapidly
advancing their efficiency. In 2014, MIT professors Mougi
Bawendi and Vladimir Bulovic found that these solar cells
transformed just 9% of sunlight into electricity, but it took
remarkably less time to reach this milestone than the
traditional silicon cells. Bawendi compares the 4-year
timespan it took CQDs to reach 9% efficiency to the more
than 60 years it took silicon cells to reach their current
efficiency rating of 15-25%, depending on which
configuration is considered, which is still well below the
theoretical limit. But the most notable benefit of CQDs
would be their proposed low cost and ease of production.
Although no exact figures were given, assistant researcher
and graduate student Chia-Hao Chuang noted that “[e]very
part of the cell, except the electrodes for now, can be
deposited at room temperature, in air”, something siliconbased panels cannot claim, and have been found to show
no signs of degradation after more than five months of
storage in open air [2]. The ability to withstand
degradation is attributed to the fact that these solar cells
are produced in solution, and often are coated with
complex polymers called ligands, which protect them from
the harsh effects of oxygen on the nanoparticles that
capture the photons.
How Do They Work?
CQDs have the same basic steps to producing electrical
energy as standard silicon solar cells do: collection,
potential change, and current maintenance. In accordance
with Dr. Yulan Fu and his associate professors’
experiments with patterned photovoltaic colloidal quantum
dots, to complete the collection phase, the CQD is tuned to
a certain range of wavelengths that it can accept [3]. The
greater the wavelength of electromagnetic radiation, the
lower the energy state. CQDs have a larger range of
accepted wavelengths than standard solar panels do, which
will be mentioned in greater detail in the coming sections.
Katelyn Benson
Tyler Smith
Since there are an infinite number of radiation rays emitted
from the sun at a time, rays are lined up precisely with the
opening in the CQD, allowing a photon to enter the
chamber. Photons may also be deflected off of other
objects to the chamber opening of a CQD as well. This
chamber is capable of being fine-tuned to a certain energy
acceptance level through the distances between each
particle, explained in later sections to be the electric
bandgap, which gives the CQD this larger range of
wavelength acceptance. Since the CQD is comprised of
nanoparticles, smaller than the microscopic level, an
emphasis can be placed on one photon being in the
chamber at a time to simplify explanations, although a
wave of protons enters the chamber of the CQD which
technically contains more than one photon. The
composition of the CQD, often lead (II) sulfide, is a
natural semiconductor, which leads to the next step of the
process: potential change.
Much like standard silicon solar panels, CQDs harness
the photons emitted by the sun into the atmosphere and
utilize their energy to create an electric potential. This
electric potential is caused by an electron-hole pair, which
essentially is just an electron being knocked loose and the
hole it creates from where it used to be. The photons from
the sun provide enough energy to knock loose an electron
in the p-type semiconductor, around 1.1 eV of energy,
which results in a positively charged atom. This electron
becomes known as a conduction electron and can move
around the crystalline structure, which provides a means of
current in the semiconductor. Consequently, the p-type
semiconductor that previously had lost an electron
becomes capable of accepting electrons from other atoms
of the semiconductor, again providing a means of current
through the material. When the p-type and n-type
semiconductors are coupled with electrodes in the CQDs,
this provides a means for the conduction electrons to
travel, and hence become electric current capable of being
harnessed [3].
The final step of the energy conversion process is
current maintenance. Electrons have a natural tendency to
flow towards positive charge, and because of this natural
phenomenon current is produced once the electrons reach
the conduction band of a semiconductor. The current in
the CQD is the string of conduction electrons that are
picked up by the electrode and sent by means of a wire to
a battery or other type of capacitor. Within these devices,
the electrons are stored as electrical energy. This electrical
energy can then be converted to any other form through
resistors, which can be anything from microwave ovens to
light bulbs. Not all of the free electrons are harnessed in a
solar cell, though. According to Professor Rod Nave, a
professor in Experimental Physics at Georgia State
University and author of the technical article “Band
Theory for Solids”, electrons must possess enough energy
to overcome the energy gap of the valence and conduction
bands [4].
The valence band of a solar cell is simply the group of
valence electrons of an atom or group of atoms that
requires an input of energy to release the electron [4]. The
conduction band is where the electrons travel to once they
are freed from the valence band. Once the electrons are
freed into the conduction band, they become charge
carriers and are able to flow in a current. But in order to
reach the conduction band, the electrons must gain enough
energy to “jump” from the valence band. This difference
in energy is referred to as the bandgap. Due to the
fundamental law that a charge is a quantized unit,
electrons must be at a certain energy level to make the
leap. There is no way for the electron to start to shift
towards the conduction band; it either has enough energy
to make the jump, as evidenced by Figure 1 below, or it
does not.
FIGURE 1 [5]
Example of bandgap, valence band, and conduction
band graph
Figure 1, depicted in chapter 10 of a technical book
“Introduction to Inorganic Chemistry” written by unlisted
graduate students at Penn State University, depicts the
energy levels necessary for an electron to make the leap
from the valence band to the conduction band, and in turn
become part of the electric current. When electrons do not
have enough energy to make the leap from one band to
another, they are stuck in the valence band of the
semiconductor and must await another input of energy to
charge them enough to reach satisfactory levels.
Range of Focus
One of the greatest aspects of CQDs is their ability to be
tuned to a wide range of electromagnetic radiation. This
radiation emitted from the sun is capable of producing
waves with tiny wavelengths, such as gamma rays, but
incredibly large frequencies to waves with very large
wavelengths, such as radio waves, with much smaller
frequencies. The amount of energy in a wave is
proportional to the wave’s frequency, so smaller, highfrequency radiation like gamma rays, x-rays, and
ultraviolet light have more energy than the larger, lowfrequency waves such as radio waves, microwaves, and
infrared radiation.
Currently, according to Hamamatsu Photonics LLC’s
research article “Characteristics and use of infrared
2
Katelyn Benson
Tyler Smith
detectors”, published at Stanford University, silicon-based
solar panels are capable of reaching only “the micrometer
(µm) range on a low-cost backing” [6], which involves
thin films of crystalline silicon placed on glass or plastic.
These types of panels are capable of handling visible
purple light, about 0.45 micrometers, to some smaller
infrared waves of about 1.1 micrometers, but nothing
much larger than that. In contrast, CQDs are capable of
handling larger waves than silicon-based panels. PbS
CQDs were capable of handling wavelengths from
approximately 0.35 micrometers to 2.75 micrometer [6],
values much larger than what silicon cells can handle.
According to Fu, over fifty percent of the sun’s energy is
radiated above 0.8 micrometers [3], signifying a huge
advantage in CQD technologies. By accepting a wider
range of wavelengths, CQDs are exposed to more
transformable energy, and hence will be able to output
more usable electrical energy even with lower efficiency
rates than modern-day solar technologies. See Figure 2
below.
not capable of making an electron-hole pair at all, deeming
it useless. Two possible solutions to this problem of
overly-energized photons would be to produce an
enhanced photovoltage or an enhanced photocurrent,
which is exactly what quantum dots provide to the solar
world.
To achieve an enhanced photovoltage, the hot electronhole pairs that are formed must be separated so that the hot
electron can be transported from the photo converter
before it loses its excess energy [8]. This transport must be
done very quickly, since electrons do not have a lot of
mass and hence can cool very quickly, according to the
University of British Colombia’s publishing of Newton’s
Law of Cooling, which states that “the rate of change of
the temperature of an object is proportional to the
difference between its own temperature and the ambient
temperature” [9]. Since the electron is at such a high state
of energy compared to its surroundings, it will tend to cool
quickly, so it is essential that the transportation of the
electron occurs as soon as possible.
As for the process of enhanced photocurrent, the hot
electron can be utilized to perform impact ionization in
which the electron comes into contact with another atom
in the semiconductor and creates a second electron-hole
pair of normal energy. This, in turn, results in two
electron-hole pairs being created from one high-energy
photon, which is noted to be the inverse of the Auger
effect by Nozik, which is the combination of two electronhole pairs into one single highly-energized electron-hole
pair [8]. Nozik notes that these methods could, in turn,
provide a theoretical conversion limit of 66%, over twice
that of Shockley and Quessier’s calculations.
To achieve these methods of enhanced photocurrents
and photovoltages, the utilization of CQDs in p-i-n cells is
a very useful technique, due to its ability to transport
electrons long distances. Much like typical p-type and ntype semiconductors, this configuration is simply a 3dimensional array of CQD cells that are very closely
placed to create small gaps between each [8]. These small
gaps, depicted in Figure 3 below, are called mini-bands,
and they provide the electrons a means of transportation
due to their ionization at lower energy levels than regular
bands. This allows electrons to travel longer distances due
to more ions in the bands being prevalent, and lower
energies are required to make the leap from the valence
band to the conduction band. This also will allow the
electron to retain more of its energy as it is transported,
maintaining the enhanced photovoltage for a longer period
of time.
FIGURE 2 [7]
Wavelength of Infrared and Visible Light waves
Figure 2, depicted in the Dutch website Welzijn
Kanker’s cancer patient radiation research article, zooms
in on only the infrared and visible light sections of the
electromagnetic spectrum. Infrared waves are categorized
into three sections: long waves, medium waves, and short
waves. Unlike silicon solar cells, which can only really
reach into the Short Wave IR range, CQDs are capable of
extending this range into the Medium Wave IR, opening
more opportunities to transform radiation from the sun.
MAKING THE CQDs
Configurations of CQDs
The main issue when determining the configuration of
solar cells is the theoretical limit that these configurations
have. According to Dr. AJ Nozik’s research on
photovoltaic CQDs, which was published by the U.S.
Department of Energy, the current theoretical limit
calculated by Shockley and Quessier in the 1960s is found
to be just 31% [8]. The reasoning behind the limit has to
do with the energy levels of the photons that are collected.
As previously stated, there lies a range in which photons
can be absorbed, with the minimum being about 1.1 eV
[3]. When the photons possess too much energy, they
produce electron-hole pairs that are too hot to function
properly, and the photon’s energy is then lost as heat to the
material. If there isn’t enough energy, then the photon is
FIGURE 3 [8]
p-i-n Type Junctions
3
Katelyn Benson
Tyler Smith
versus non-passivated semiconductors depicted in Figure 5
below. These particles, denoted as ligands, can be
anything from pure atoms to hybrid polymers. In a case
study provided by Dr. Yuehua Yang and his associates,
results of PbS CQD solar cells with three different types of
hybrid ligands were compared to a PbS CQD without a
ligand. Although the exact method of producing these
ligands falls more in the hands of chemists, Yang
emphasized the inexpensive nature of the solution when
compared to traditional means of producing silicon-based
solar panels [10].
By placing the solar cells so close to one another, the
distance necessary for the electron to travel is reduced
dramatically. This will allow the particle to retain its
energy rather than lose it as heat during transport.
Another possible configuration to achieve enhanced
photocurrent would be to suspend the CQDs in an organic
matrix of polymers that conduct both the electrons and the
holes away from one another, as evidenced by Figure 4
below. Certain blends of polymers have shown promise in
conducting both the electrons to one electrical contact and
the holes to another, thereby increasing the distance
between the two to ensure that the recombination of the
electron into the valence shell of the atom does not occur
[8]. This concept allows for a greater flow of current to
occur in the CQD and provides more free electrons to be
utilized as energy down the road than traditional means of
electron-hole pairs in silicon cells. When compared to
traditional theoretical limits of 31%, the CQD’s ability to
reach maximums in the 60%-range is quite exciting,
especially when it can be done by generally
straightforward means such as configuring cells in a 3dimensional matrix or blending polymers to attract
positive and negative particles.
FIGURE 5 [10]
Passivated vs. Non-Passivated PbS
In Figure 5, published alongside Yang’s research, the
left depicts a single PbS CQD that is passivated by many
ligands, discouraging the oxygenation of the solar cell. On
the right, a pure PbS is depicted to show what it looks like
before being passivated or oxygenated by the atmosphere.
Ligands in CQDs act as barriers to oxygenation. As
oxygen attaches to exposed points on the CQD crystalline,
the ability to transport free electrons through current
diminishes, greatly reducing the efficiency of the solar
cell. The ligands are able to bond to the CQD crystalline at
exposed points and protect it from the harmful process of
oxygenation, creating a sort of shield around the CQD,
increasing the device life by a significant amount when
compared to non-passivated cells. Figure 6 below depicts
the results of passivated versus non-passivated CQDs
when exposed to oxygen on a larger scale.
FIGURE 4 [8]
3-Dimensional Organic Polymer Matrix
Placing the CQD in a 3-dimensional matrix (depicted
in 2-dimensions here) allows more electric attraction to be
felt by both the electron and the hole in the pair. This
provides a means of assisting the electron across the
bandgap to make the leap to the conduction band, and also
helps the hole leap across to the valence band.
PASSIVATING CQDs WITH LIGANDS
FIGURE 6 [10]
Non-Passivated vs. Passivated CQDs with OPA
One of the most prominent benefits of the CQD is its
sustainability in outdoor conditions. While it was noted
that traditional silicon solar panels tend to bind to oxygen
when exposed to the outdoors, CQDs have shown promise
in being able to resist this deterioration [1]. The ability to
resist oxygenation is not within the material properties of
the semiconductors themselves, but the particles attached
to the semiconductors that surround them in a sort of
shielding manner, with a general comparison of passivated
On the left, CQDs that were not passivated with ligands
are shown after assembly. On the right, CQDs that were
passivated by OPA are shown. Notice how compact CQDs
with OPA are in comparison to the non-passivated cells.
Yang also found that absorption rates of CQDs
passivated with ligands were lower than those without
ligands. This idea is certainly plausible, as those without
4
Katelyn Benson
Tyler Smith
passivation do not have any barriers that may deflect the
photon from being captured by the cell. What is interesting
is the power conversion efficiency (PCE) of the passivated
cells compared to non-passivated ones. The CQD with the
shortest ligand attached to it, which in Dr. Yang’s case
was a mixture of cadmium chloride, oleic acid, and orthophthalaldehyde (OPA), whose chemical compound is
C8H6O2, had the greatest experimental PCE of 5.04%,
whereas non-passivated cells reached just 1.71% [10]. The
molecular composition of OPA is depicted in Figure 7
below.
CQDs may be lower than modern-day silicon panels, their
ability to reach wider ranges of radiation coupled with
their rapid rate of advancement provides CQDs with the
opportunity of surpassing known levels of energy
conversion in a much shorter amount of time than it took
silicon-based panels.
ETHICS SURROUNDING CQDs
Environmental Impact of Other Energy Sources
In the past few decades, scientists have noticed the
detrimental impact that many energy sources have had on
the environment. Many environmental studies, such as Dr.
Bernard’s, which appears in Environmental Health
Perspectives, have shown that burning fossil fuels
contributes to air pollution, which can lead to adverse
health effects, especially lung diseases [12]. Burning fossil
fuels gives rise to acid precipitation, the greenhouse effect,
and ozone layer depletion. In addition, climate change and
global warming are linked to burning fossil fuels, which
has drastically altered ecosystems throughout the world,
especially in the Arctic and Antarctic regions where global
warming is causing the ice caps to melt [13]. This causes
flooding in coastal settlements all over the world, and
could mean the disappearance of island communities. As a
result, environmental issues associated with energy
management have become a focus of scientific research.
FIGURE 7 [11]
Ortho-Phthalaldehyde
This chemical depiction of OPA, depicted on
Wikipedia’s Phthalaldehyde article, shows how simple the
ligand is in comparison to other long-chained polymers.
By being so compact, OPA allows photons to enter the
CQD much and limits the amount of deflection of the
particles.
This data provides evidence of how critical PCE is
rather than just raw absorption rates. Although a solar cell
may absorb a considerable more amount of energy than
another, the ability to maintain a high enough voltage to
provide the means of a current is the true measuring factor
of a cell.
Renewable Energy
Research cited in Dr. Hossain’s article featured in
International Journal of Energy Research has shown that
at the current rate of consumption, fossil fuels will
eventually be depleted [13]. Renewable energy sources,
such as solar power, aim to provide energy for homes,
factories, and transportation. Unlike burning fossil fuels,
renewable energy sources are better for the environment
because they harness energy from naturally occurring
phenomenon, such as wind, the sun, and running water,
and they can never run out. This is where colloidal
quantum dot solar cells come in; as a form of solar energy,
the CQDs harness energy from the sun, and convert it into
energy that can be used to power homes and businesses.
As opposed to fossil fuels, CQDs take advantage of energy
that is already present, and simply convert it into usable
energy [1]. CQDs do not contribute to harmful carbon
emissions, and thus provide a safer, cleaner energy
alternative. Photovoltaic cell implementation is becoming
more widespread as consumers realize the adverse effects
that fossil fuels have on the environment. As a promising
form of solar technology, CQDs provide an ethical
alternative to carbon-emitting fossil fuels.
ANALYSIS OF CQDs
CQDs provide the solar world with the potential to
become the staple of energy production. As the sun is a
continuous form of energy, the importance of harnessing
this energy is critical to sustainability. Current solar
technology certainly has its benefits, especially when it
comes to power conversion efficiencies and absorption
rates that trump the current levels of CQDs, but after more
than sixty years of innovation, these panels have yet to
even approach their theoretical limit. In contrast, colloidal
quantum-dot solar cells are a novel solution to the flaws in
traditional solar panels. Their resistance to outdoor
oxidation ensures they have a long device lifetime, and
their ability to adjust the electric bandgap between the
valence and conduction bands provides a wide range of
possible photons that could be captured and transformed
by the solar cell. This also allows the energy necessary for
electrons to jump from valence to conduction bands to be
specialized to certain forms of radiation, helping more
electrons make the leap to the conduction band rather than
being lost in the bandgap. Although the efficiencies of
APPLYING THE TECHNOLOGY
Implementation Methods
The possible applications of colloidal quantum dot
solar cells are diverse; this technology has the capability
5
Katelyn Benson
Tyler Smith
for a variety of uses. Currently, most people would
picture solar panels on the roof of a home, but CQDs have
the potential to be implemented in less conventional
ways. Researchers have begun exploring the possibility of
using photovoltaic panels in roadways and on the oceans,
to harness large quantities of solar energy without taking
up valuable land. Additionally, CQDs could appear on
satellites in space in the future [1]. The adaptability of
CQDs to different implementation methods promotes
widespread application of the technology as a power
source.
energy prices will drop, so overall, solar technology is less
expensive than nonrenewable energy sources [1]. In
addition, consumers need to take the environmental cost
into account. Widespread implementation of photovoltaic
panels can diminish the toll that burning fossil fuels takes
on the environment.
To decrease the initial price of installing CQDs, home
and business owners in the US can take advantage of
leasing plans [15]. Instead of paying thousands of dollars
up front to install the technology, consumers pay a yearly
leasing fee to utilize solar technology. The lessor also pays
to fix any broken panels and compensate for depreciation
of the panels. A study used California homes as an
example to weigh the costs and benefits of purchasing
versus leasing a photovoltaic system, and they found that
the yearly cost dropped from $2684 for a purchased
photovoltaic system, to $1663 for a leasing plan [15]. The
leasing plan allows solar energy to become a feasible
energy source for many more consumers.
Application of CQDs in Developing Countries
Since the industrial revolution of the nineteenth
century, humans have been using an increasing amount of
energy. Energy shortages are common, especially in
developing countries, where the rapidly growing urban
areas cannot keep up with the energy demand, and rural
areas are left with unreliable energy sources. This
phenomenon is specifically evident in India, which has the
eighth fastest growing economy in the world [14]. In a
study at CSIR-National Geophysical Research Institute, in
India, researchers investigated the potential impact of
photovoltaic system implementation in India. India needs
massive amounts of dependable energy to support its
rapidly growing economy and population, but 81 million
households
are
still
left
without
stable
electricity. Currently, the country relies heavily on
burning fossil fuels to power urban homes and factories;
seventy percent of India’s power comes from coal
[14]. However, as previously discussed, fossil fuels have
a harmful effect on the environment. Particularly in areas
of high population density, such as Indian cities, the
carbon emissions from the coal power plants could have
widespread health effects on the people, and cause climate
change throughout the globe.
Colloidal quantum dot solar cells could solve India’s
energy crisis. As a renewable energy source, CQDs can
reduce air pollution. Widespread implementation of
photovoltaic systems could serve as an effective energy
source for the growing populations in urban areas, as well
as a reliable energy source for the rural populations. In
countries like India, people do not just need electricity to
charge their cell phones or power microwaves. They are
in dire need for energy to power necessities like water
desalination systems and pasteurizers [14]. Fossil fuels are
a short-term solution to the energy demand, but solar
panels function as a long-term solution, that will be able to
carry on for future generations.
CQDs: A SUSTAINABLE FORM OF
ENERGY
When creating and implementing a new technology,
engineers need to consider the sustainability of the
technology. Sustainability could involve increasing
efficiency, solving environmental issues, improving
quality of life for consumers, and providing an affordable
energy option. The purpose of colloidal quantum dot solar
cells, the reason they were created, is to be sustainable in
these areas.
With regards to increasing efficiency, colloidal
quantum dots offer a new horizon for solar energy. Nozick
states that current solar panels have a theoretical limit of
absorption of just 31%, and after decades of experimenting
and innovating, these panels still have yet to reach this
ceiling [8]. Colloidal quantum-dot solar cells, on the other
hand, can reach values of up to 60-65%, a huge benefit
when compared to traditional means. Yet efficiency is not
just about absorption percentages, but also the
transformation of the energy from the photons into
electrical energy. As seen in Nozick’s trials, placing a
CQD in a polymer blend or a 3-dimensional matrix allows
the electron-hole pair to be created with much less energy,
which in turn allows more of the energy from the photon
to be transformed for practical uses [8]. Less energy
released by the electrons means less overheating of CQD
panels and in turn longer lifespans of the devices. Another
aspect of device functionality comes with the CQDs’
ability to withstand oxygenation. By passivating the
semiconductors with ligands, with one example from
Yang’s research being OPA, CQDs are protected from
external reactions with oxygen due to the ligand attaching
to and completely enclosing the cell [10]. The technology
within is not only sustainable to society by providing an
emissions-free form of energy, but it is also sustainable in
terms of device functionality.
The affordability of colloidal quantum-dot solar cells is
another benefit it has and adds to its practicality as a
sustainable source. As previously stated, CQDs are
Cost of Implementation
Although there are no exact figures yet on how much
CQDs will cost, Dr. Liu’s article on the cost of
photovoltaic technology states that sources agree that
CQDs will ultimately be an inexpensive form of energy
[15]. The initial application of the photovoltaic cells is
more expensive than fossil fuels, but solar technology is a
long-term investment. After installing CQDs, monthly
6
Katelyn Benson
Tyler Smith
passivated with ligands like OPA, which is done in
solution. The solution, according to Bawendi and
Bulovic’s research on colloidal quantum-dots, is an
inexpensive alternative to the precise and often expensive
environment that current silicon panels require for
production [2]. By cutting the costs of production,
innovators are providing a means for CQDs to be
implemented into society since prices of panel systems
will be lowered alongside the costs to the producers. One
of the key steps to implementing this technology into
society is to reduce the price consumers must pay; often
times solar energy is seen to be pricey, but as this
technology emerges and becomes easier and cheaper to
produce, that reputation may not be seen for much longer.
Unlike the burning of fossil fuels, which is the major
way that power companies harness energy, solar power is
renewable. As long as the sun continues to emit solar
radiation, photovoltaic cells will have an energy source.
The infinite capacity of solar power proves the
sustainability of CQDs. A major advantage of CQDs is
their positive effect on the environment. Burning fossil
fuels wreaks havoc on the atmosphere and ecosystems
around the globe, but clean energy like solar power
provides a sustainable alternative. The CQDs do not
contribute to climate change or ozone layer depletion, thus
eliminating this environmental issue. The reliability of
CQDs can improve the quality of life for consumers,
particularly in areas without a stable energy source
[14]. For example, urban areas in India need enormous
amounts of energy to fuel their growing economy, and
rural areas are often left without access to steady
power. Implementation of CQDs can solve the energy
problem in India by providing a sustainable energy
source. Using CQDs could greatly improve the quality of
life of people living in India, as well as other countries that
need a sustainable source of power. As an efficient,
affordable, and environmentally friendly technology, the
CQD proves to be a sustainable form of energy.
technology does not stop in India. Researchers have
studied the economic pros and cons of solar panel
implementation in California homes, and found that
photovoltaic systems can become economically feasible
for many homeowners, especially with a leasing plan
[15]. The steep cost of photovoltaic cell application held
many consumers back in the past, but with lower prices,
CQDs can be a widely-used technology. Researchers are
exploring many implementation methods and application
possibilities, which demonstrates the versatility of the
technology [1].
A Final Look at CQDs
As presented in this paper, colloidal quantum-dot
technology has the potential to change the way society
consumes energy. Efficiency is a huge issue in renewable
energy, because fossil fuels seem like the most competent
energy source. However, the increased efficiency that
CQDs bring to solar technology makes renewable energy a
more powerful opponent to fossil fuels. Switching to
CQDs could mean a great deal for the environment, as
well. A century ago, no one thought about climate change
or keeping the air clean. Now, with a newfound
awareness of the harmful environmental effects of burning
fossil fuels, scientists are pushing consumers to use
renewable energy sources, which harness preexisting
energy. The flexibility of the technology to adapt to
different implementation methods and applications also
demonstrates the significant impact that CQDs can have
on energy technology. To conclude, the efficiency of the
technology, positive environmental impact, and versatility
of colloidal quantum dot solar cells establish photovoltaic
technology as a significant, reliable energy source.
SOURCES
[1] “The Latest in Solar Technology.” Alternative Energy.
2016.
Accessed
1.9.17.
http://www.altenergy.org/renewables/solar/latest-solartechnology.html
[2] D. Chandler. “Improving a new breed of solar cells.”
Massachusetts Institute of Technology. 4.27.2014.
Accessed 1.10.17. http://news.mit.edu/2014/improvingnew-breed-solar-cells-0527
[3] Y. Fu, A. Dinku, Y. Hara. “Modeling photovoltaic
performance in periodic patterned colloidal quantum dot
solar cells.” Optics Express. 2015. Accessed 2.3.2017.
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-2315-A779&id=320004
[4] “Band Theory of Solids.” Georgia State University.
Accessed
2.28.2017.
http://hyperphysics.phyastr.gsu.edu/hbase/Solids/band.html#c2
[5] “Introduction to Inorganic Chemistry/Electric
Properties
of
Materials:
Superconductors
and
Semiconductors.” WikiBooks. 2.25.2017. Accessed
3.1.2017.
https://en.wikibooks.org/wiki/Introduction_to_Inorganic_
Chemistry/Electronic_Properties_of_Materials:_Supercon
ductors_and_Semiconductors
CQDs AND SOCIETY
Colloidal quantum dot solar cells have the potential to
greatly impact the way society consumes energy. CQDs
are a promising new form of solar technology, and
researchers are still working to make them more efficient
in absorbing sunlight and converting it to usable energy
[1]. The increased efficiency makes CQDs and
photovoltaic systems a stronger competitor to fossil fuels,
which still dominate the worldwide energy source. Unlike
fossil fuels, CQDs do not have adverse effects on the
environment, so switching to solar technology will
decrease carbon emissions and air pollution. A clean,
stable energy source is necessary, especially in urban areas
and rapidly growing countries. For example, the
expanding population in India needs a reliable energy
source to power factories and homes in cities, as well as
homes and farms in rural areas [14]. CQDs could be the
solution to the energy crisis in increasingly industrial
countries like India, to provide a stable energy source to
fuel the expansion. The possibilities for photovoltaic
7
Katelyn Benson
Tyler Smith
[6] “Characteristics and Use of Infrared Detectors.”
Hamamatsu K.K. 11.2004. Accessed 3.1.2017.
http://www.slac.stanford.edu/grp/arb/tn/arbvol5/AARD46
0.pdf
[7] “Welzijn Kanker patienten.” Accessed 3.1.2017.
http://welzijnkanker.nl/alle-artikelen/kanker-krijgen-doorstraling/
[8] A. Nozik. “Quantum dot solar cells.” National
Renewable Energy Laboratory. 6.19.2002. Accessed
2.6.2017.
http://www.sciencedirect.com/science/article/pii/S138694
7702003740
[9] “Newton’s Law of Cooling.” University of British
Colombia.
Accessed
3.1.2017.
http://www.ugrad.math.ubc.ca/coursedoc/math100/notes/d
iffeqs/cool.html
[10] Y. Yang, B. Zhao, Y. Gao, H. Liu. “Novel Hybrid
Ligands for Passivating PbS Colloidal Quantum Dots to
Enhance the Performance of Solar Cells.” Springer.
6.20.2015.
Accessed
1.11.17.
http://download.springer.com/static/pdf/777/art%253A10.
1007%252Fs40820-015-00464.pdf?originUrl=http%3A%2F%2Flink.springer.com%2Fa
rticle%2F10.1007%2Fs40820-015-00464&token2=exp=1484165846~acl=%2Fstatic%2Fpdf%2F7
77%2Fart%25253A10.1007%25252Fs40820-015-00464.pdf%3ForiginUrl%3Dhttp%253A%252F%252Flink.spri
nger.com%252Farticle%252F10.1007%252Fs40820-01500464*~hmac=1d4fa9428d97330b587b02766b60ac08a5bd364
ae56d58ddd5bcc94006d1389b
[11] “Phthalaldehyde.” Wikipedia. 9.6.2016. Accessed
2.28.2017. https://en.wikipedia.org/wiki/Phthalaldehyde
[12] S. Bernard, J. Samet, A. Grambsch, K. Ebi, I.
Romieu. “The potential impacts of climate variability and
change on air pollution-related health effects in the United
States.” Environmental Health Perspectives. 2011.
Accessed
2.26.2017.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1240667/
[13] F. Hossain. “Solar energy integration into advanced
building design for meeting energy demand and
environment problem.” International Journal of Energy
Research.
2016.
Accessed
1.11.17.
http://onlinelibrary.wiley.com/doi/10.1002/er.3525/pdf;jse
ssionid=0F5D3066D743145FBFFAB3C863C2A123.f02t0
4
[14] S. Manju, N. Sagar. “Progressing towards the
development of sustainable energy: A critical review on
the current status, applications, developmental barriers and
prospects of solar photovoltaic systems in India.”
Renewable and Sustainable Energy Reviews. April 2016.
http://www.sciencedirect.com/science/article/pii/S136403
2116310024
[15] X. Liu, E. G. O’Rear, W. E. Tyner, J. F. Pekny.
“Purchasing vs. leasing: A benefit-cost analysis of
residential solar PV panel use in California.” Renewable
Energy.
June
2014.
Accessed
1.10.17.
http://www.sciencedirect.com/science/article/pii/S096014
811400055X
ACKNOWLEDGEMENTS
This paper could not have been written without the help
of Kelly Appleton, a senior Mechanical Engineering
student at the University of Pittsburgh. Thank you, Kelly
for all of your support.
8