Schaub 4:00
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QUANTUM DOT PHOTOVOLTAIC CELLS
Christian Bottenfield (cgb17@pitt.edu)
bandgap energy of the common crystalline silicon cell
is about 1.1 eV (electron volts), while other solar cells
may have bandgap energies ranging from 1.0-1.6 eV.
Photovoltaic cells within this range are safe from
creating excess heat.
INTRODUCTION
The Next Generation of Solar Energy
Photovoltaic cells, commonly called solar cells,
serve an important role in reducing the world’s
dependency upon pollution-producing energy sources
such as coal and natural gases. Despite their reputation
as a sustainable alternative, solar cells still lack the
efficiency necessary to replace cheaper modes of
energy production on a larger scale. A recent advance,
quantum dots, holds the potential to drastically
improve solar power technologies and provide a
transformative improvement to traditional silicon
photovoltaic cells. I have, through intensive research
of this topic, confirmed my interests in the research
aspect of engineering, particularly in the sustainable
energy field. The following will explore the specifics
of quantum dot solar cells, why it is important for
quantum dot solar cells to adhere to the code of ethics,
and why I believe this research paper develops
communication skills and personal interests within the
freshman engineering curriculum.
TYPICAL SILICON SOLAR CELL
Silicon cells consist of a p-type and an n-type silicon.
Between the two layers lies the p-n junction, through which
electrons pass, producing energy [2].
CURRENT SOLAR TECHNOLOGY
The typical solar cell consists of a conductor base to
provide an electrical contact to the rest of the array and
to support the boron-doped silicon wafer above.
Phosphorus is dispersed throughout this layer and
forms a p-n junction. This junction is a separation of
charges between a p-type and an n-type silicon in the
layer. The polarity of the silicon wafer grows as
photons excite electrons that eventually diffuse across
the p-n junction, creating a current carried by the
conductor base. Finally, an anti-reflective coating is
added along with the front surface contact, a grid of
minimum surface area that collects the electrons from
the p-n junction, enabling the cell to produce power
[1].
For a photovoltaic cell to work, the energy of an
incoming photon must equal the bandgap energy, or
the energy required to dislodge a valence electron.
Furthermore, the energy of the photon should not be
too great, because extra energy will be expended as
thermal energy that could overheat the cell. The
All light does not have a uniform energy, and solar
cells have specific ranges of light that they can absorb
based on the bandgap energies required. Sunlight can
range anywhere from the ultraviolet (~2.9 eV) to the
infrared (~0.5 eV) wavelengths. Thus, less than 45%
of the energy produced by the sun can be captured by
an array of solar cells, because the energies are either
to great or too small [3].
THE QUANTUM DOT SOLAR CELL
What is a Quantum Dot?
Quantum dots are essentially the dimensionless
analogues of two dimensional objects called quantum
wells, which are defined as potential regions where
electrons paired with electron holes are confined within
the crystalline structure of the quantum dot. An
electron hole is formed when an absorbed photon
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University of Pittsburgh, Swanson school of Engineering
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Christian Bottenfield
displaces an electron from the valence band into the
conductor band, resulting in an empty region.
Behaving like a point-positive charge, the electron hole
creates a Coulombic attraction between itself and the
negative electron [4].
PLOT OF QD RADIUS VS. ENERGY
The graph above demonstrates the inverse
relationship between the size (radius) of the quantum dot and
the energy [8].
DIAGRAM OF EXCITON PAIR
The diagram above shows the separation of an
electron and an electron hole across the band gap, thus
forming a confined exciton [5].
QD Solar Cell Efficiency
Quantum dots may provide a cheaper, more
efficient alternative to the modern silicon photovoltaic
cells. A paper written in the late 1990’s by Arthur
Nozik claimed that, while typical silicon solar cells
could produce only one exciton per photon, a quantum
dot photovoltaic device could produce two or more
excitons per photon. The potential to more than double
the energy output of current solar technology led
Victor Klimov of Los Alamos National Laboratory
found experimental proof that Nozik was right. His
lead-selenide, lead-telluride, and lead sulfide quantum
dot semiconductors yielded up to seven excitons per
photon, a principle called Multiple Exciton Generation
or MEG, when exposed to ultraviolet light. Despite
such promising results, Nozik’s team has not quite
created an effective solar array. The potential for
large-scale energy production is there, but the transfer
of electricity from the quantum dots to the conductor
proves to be very tricky. In quantum dots, the rate of
reabsorption of the electrons is much faster than in
typical materials.
The researches reached 2%
efficiency with an initial prototype and the maximum
efficiency to reach 42 percent, a vast improvement
over silicon’s maximum of 31 percent [9]. Another
research group, led by Dr. Jin Young Kim, attained
efficiencies up to 6% by using a polymer in
conjunction with the QD cells [10]. This pure research
into the efficiency of certain models is precisely the
type of research I might consider in the future. I am
more concerned with the underlying engineering
Physically, a quantum dot (QD) is a nanoparticle
semiconductor of transition metal material that
displays quantum optical effects. A nanoparticle can
only be classified as a quantum dot if the separation
between the electron and electron hole is close to the
Exciton Bohr Radius.
This specific distance,
approximately a few nanometers in most
semiconductors, is actually defined, somewhat
circularly, to be the radius required to attain quantum
confinement. When trying to model a quantum dot’s
behavior as its size varies, it is often useful to think of
it simply as a moving particle within a larger spherical
shell. The kinetic energy of a small particle bouncing
within a sphere would increase as the sphere decreased
in volume. Likewise, the energies of quantum dots are
size-dependent and are inversely proportional to size
[6]. Since size affects the energies of quantum dots and
energy relates to wavelength by the equation E=hc/λ,
the color emitted by the quantum dot also varies with
size. Smaller quantum dots absorb more energy, thus
shifting their emissions toward the violet end of the
spectrum while light from larger quantum dots are
closer to the red end of the spectrum [7].
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principles than I am with the final product.
SKEPTICISM
BUILDING A QD SOLAR CELL
Engineering Issues
Production
There is much skepticism about the usefulness of
quantum dots in solar cells due to disturbances from
the quantum dot’s local surroundings that could disrupt
internal quantum states.
Such disruptions could
include spectral diffusion, which is the fluctuation of
energy in a single quantum dot. An inconsistent
energy might not entirely disable a solar array from
producing energy, but it can lower the efficiency since
a QD solar cell needs fine tuning to absorb the correct
wavelengths of light. However, careful engineering of
the quantum dots can reduce this irregularity [13].
Another issue with QD solar cells is the transfer of
electricity from the quantum dot itself to the
conductors that carry charge to a power source.
Quantum dots tend to reabsorb electrons at a rate much
faster than normal silicon photovoltaic cells reabsorb
them, despite the sheer number of electrons produced
by MEG in quantum dots. This latter problem is the
larger concern, but few doubt the engineers will solve
this, as well. [9].
The allure of the quantum dot approach to solar
technology lies also in their production. Quantum dots
are far cheaper to produce than large silicon sheets,
because they are readily created by simple chemical
processes [9]. These processes include colloidal
synthesis, viral assembly, electrochemical assembly,
and high temperature dual injection.
Colloidal
synthesis is the most promising for commercial use and
is the favored method for creating quantum dots,
because it allows for large-scale production with minor
toxic waste. Viral assembly and electrochemical
assembly require a virus and electrochemical stimuli
respectively to build quantum dots from an engineering
template. These two methods, along with high
temperature dual injection, are not scalable enough for
possible commercial use and are limited to creating a
small number of quantum dots at a time [11].
Structure
Structurally, there are three leading theoretical
approaches to designing quantum dot solar cells:
photoelectrodes made of quantum dot arrays, QDsensitized TiO2 nanocrystalline structures, and organic
semiconductor polymer matrices. The first entails the
use of quantum dots in a three-dimensional array by
building the structure layer by layer from a base film of
quantum dots. The second method requires dye
molecules to attach to TiO2 particles that are dispersed
throughout the nanocrystal structures, acting as an
electron transport. [12], [10]. These dye molecules
become excited easily and enhance the overall
photovoltaic effect of the array. The final theoretical
approach involves the use of quantum dots in junction
with organic semiconductor polymers that can serve as
contact points to harvest electricity after excitation.
These theoretical ideas can be further combined with
the idea of a tandem solar cell. Tandem cells are solar
cells arranged in layers such that each layer is made to
absorb a different wavelength of light, thus increasing
the overall efficiency. Mathematically, a tandem solar
structure with enough layers to cover the entire
sunlight spectrum could obtain an efficiency of 66
percent, twice the efficiency of current silicon-based
solar cells [12].
Environmental Issues
Quantum dots also pose an environmental issue
because many contain heavy metals that have been
banned in many countries. For commercial viability,
quantum dots made for household use must not contain
lead or cadmium as do many current prototypes (CdSe
and PbSe quantum dots, for example). Luckily,
colloidal phosphor nanoparticles doped with rare earth
metals are proving to be just as effective while passing
EPA regulations. Additionally, industrial use of heavy
metals is more liberal than household use, so PbSe and
CdSe quantum dots will likely still exist commercially
[11].
WHY IT MATTERS
Personally, I feel the current solar cells have fallen
very short of their capabilities. The sun is the most
powerful object in the solar system, yet the devices we
have created to harness that energy are among the
weaker energy sources compared to hydroelectric
power, coal, natural gases, and nuclear power. The
maximum efficiency of silicon solar cells is half that of
QD solar cells, so why bother using a technology with
such a low theoretical output? Time and money would
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Christian Bottenfield
be much better spent on research into QD solar cells
that will return that investment in the long term. As for
which approach to the quantum photovoltaic device, I
prefer the photoelectrode idea because it directly
involves the idea of a tandem cell, so that all the
sunlight spectra may be utilized. It is partially these
types of thoughts that I believe contribute to a more
stimulating freshman engineering education, which
will be elaborated upon shortly.
objective analyses of work only if he or she has the
expertise and harbors no deceptive motives of any kind
[14], [15]. This sums up several of the more important
codes from both NSPE and AIChE. Quantum dot solar
technology should not suffer from misrepresentations
of facts, especially regarding its safety. In addition to
dangers to the public, skewed facts may also cause
monetary blunders. For example, a research paper may
falsely claim to have achieved a high percentage
efficiency for QD solar cells.
Based on that
misinformation, a company may invest in the
technology only to find that the efficiencies promised
are not attained, costing the business thousands of
dollars.
As for the rest of the code of ethics, I feel that
they are less applicable to QD solar cell technology
because they are geared towards generic situations in
the professional engineering community that do not
relate directly to a product or technology. Rather, they
detail ethical responsibilities in interpersonal
relationships among engineers. For example, NSPE’s
fourth statement denounces engineers who do not act
as trustworthy agents to employers [14]. Although this
code must be followed by those engineers working on
quantum dot solar technology, I do not think the
current state of QD technology has a large enough
commercial market to warrant this warning since it is
actually only in the research stage.
Granted,
researchers need “faithful agents or trustees” [14], but
the codes remaining are so general that they can be
summed up with such a statement as “Engineers shall
act with integrity in all academic, public, and
interpersonal situations.”
In my opinion, these
remaining codes more or less represent basic moral
expectations, many of which could bring about legal
confrontations if broken.
QD CELLS AND CODES OF ETHICS
To become viable for any application, QD solar
cells must obey the code of ethics for the NSPE
(National Society of Professional Engineers) and for
the specific engineering discipline encompassing the
QD technology.
The interdisciplinary nature of
quantum dot solar cells allows for much debate as to
which field it belongs to most, but here I have chosen
chemical engineering due to the concepts emphasized
in this paper. The chemical engineering code of ethics
was provided by AIChE (American Institute of
Chemical Engineers) and aligns itself closely with
NSPE’s code of ethics. The first and perhaps most
important code for any engineer states that the overall
well-being of the public is at the forefront of the
engineer’s professional duties [14], [15]. For quantum
dot solar cells, this means that safety to the public must
be confirmed before commercial use. As mentioned
previously, initial experimentation with quantum dots
involved heavy metals such as cadmium and lead [11].
These pose threats to the safety of the public and
therefore violate the first code of ethics. The results of
not recognizing the potential danger could lead to the
injury of buyers or producers and the contamination of
various media, further increasing the chance of public
harm. A technology, no matter how innovative and
economically fruitful, loses value to society when it
becomes a threat.
I would like to emphasize one specific canon
following the first statement of the NSPE code of
ethics that states, “Engineers shall not reveal facts,
data, or information without the prior consent of the
client or employer except as authorized or required by
law or this Code” [14]. Since quantum dot technology
is very recent many of the specific applications hold
patents that an engineer working in research in design
must be aware of. Other companies could have patents
on a certain usage which could result in lawsuits if the
engineers are not careful.
Furthermore, it is the duty of the engineers to give
EDUCATIONAL VALUE OF PROJECT
In retrospect, I believe writing this project benefits
the freshman engineering student in two important
ways. The first reason, for the development of
professional communication skills, is purely
educational, while the second reason is more personal.
Communication
Engineers, despite common misconceptions, do
actually need the ability to write clearly and
effectively. Effective writing constitutes concise,
appropriate wording for a professional audience
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Christian Bottenfield
without leaving a public audience entirely confused by
complex jargon. Writing is an engineer’s most
important mode of communication, especially for
engineers employed in research in design. According
to one article on engineering education,
quantum dot solar cells has increased my desire to
begin research in a similar field and has raised
questions about which engineering major would best
prepare me for such a career route. Aside from
researching new energy improvements and methods, I
would also like to use my engineering abilities to
improve the lives of people in third-world
communities, possibly using quantum dot solar
technology when efficiency and cost are at large-scale
viability.
“Engineers will have to communicate clearly and
persuasively in both speaking and writing with other
engineers and scientists, systems analysts, accountants,
and managers with and without technical training,
within their company and affiliated with multinational
parent, subsidiary, and client companies, with
regulatory agency personnel, and with the general
public” [16].
CONCLUSION
Although quantum dot solar cells have not yet
reached the power potentials of the silicon solar cells,
the theoretical efficiencies predict much higher results
within the near future. Increased efficiency combined
with cheaper production costs for household and
industrial buyers makes quantum dot technology a
promising commercial product. Quantum dot solar
cells represent a new generation of solar power
devices, the third generation, as opposed to the siliconbased second generation. Investment in the engineering
of quantum dot photovoltaic cells will allow solar
technology to support a larger part of the world’s
energy consumption and subtract from the use of
limited resources that pollute the air such as coal and
fossil fuels. Along those same lines, engineers of
quantum dot applications must adhere to the code of
ethics to ensure the safety of products for industry and
the public, as well as for integrity within the
professional work environment so that reliable research
and collaboration are possible.
Personally, this
research topic has provided a possible avenue for
further studies and rooted my interests within the area
of sustainable energy. Because of this project, I feel
more confident in my direction and choices as an
engineer. I urge any freshman engineering program to
implement such a project for the benefit of the
students’ communication skills and personal gains.
This clearly illustrates the breadth and importance
of communication necessary for the modern engineer.
These skills can be taught adequately through classes;
however, introducing communication early in the
freshman curriculum through this writing project
ingrains the ideas of communication from the start and
in an integrated way, rather than through an isolated
workshop. By integrated, I mean that our writing
combines the physical and intellectual process of
writing a professional paper with technical engineering
ideas.
Personal Interests
The second reason why I feel this writing project is
beneficial to freshman engineering students is because
it focuses a particular interest. Many freshman
engineers find themselves confused about which kind
of engineering they actually wish to pursue. This
assignment makes students choose a position on
current engineering dilemmas or advances, which gives
the student a starting point to begin assessing what
related topics he or she might also like. As for myself,
I have acquired a deep interest in quantum dot
technology and find myself acknowledging it as a
possible future research area.
REFERENCES
IMPACT OF THE PROJECT
[1] M. Young (Jan. 2010) “The Science of the Silicon
Solar Cell.” The Power of the Sun. (Website)
http://science.sbcc.edu/~physics/solar/sciencesegment/
[2] “Photovoltaics: Solar Electricity and Solar Cells in
Theory and Practice.” (May 5. 2011). (Website)
http://www.solarserver.com/knowledge/basicknowledge/photovoltaics.html
My choice of this topic actually began a year ago
while searching for topics to write about in my high
school physics class. That paper purely examined the
physics behind quantum dots but briefly mentioned the
many applications in engineering. For this paper I
chose an application in energy that most closely fit my
interests in energy research. My time spent studying
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Christian Bottenfield
[3] “Crystalline Silicon Photovoltaic Cells” Energy
Basics.
(Aug.
12,
2011).
(Website)
http://www.eere.energy.gov/basics/renewable_energy/c
rystalline_silicon.html
[4] D. J. Norris (1995) “Measurement and Assignment
of the Size-dependent Optical Spectrum in Cadmium
Selenide (CdSe) Quantum Dots.” (Online Article)
http://dspace.mit.edu/handle/1721.1/11129
[5] J. Pailee (Dec. 9, 2011) “Exciton Energy Levels”
(Website)
http://en.wikipedia.org/wiki/File:Exicton_energy_level
s.jpg
[6] E. van der Pol, N. Zijlstra (Dec. 22 2005)
“Quantum Optics: Quantum Dots.” (Online Blog)
http://physics.schooltool.nl/quantumoptics/qd.php
[7] “Quantum Dots and Nanoparticles” Exploring the
Nano
World.
(2008)
(Website)
http://mrsec.wisc.edu/Edetc/background/quantum_dots
/index.html
[8] L. Silvestri (Dec. 2000) “Optical Properties of
Excitons in Quantum Dots: Diffractions of an
Electromagnetic Plane Wave by a Spherical Quantum
Dot.” Journal of Physics and Chemistry of Solids. Vol.
61, no 12. pp.2043-2053
http://www.sciencedirect.com/science/article/pii/S0022
369700002067
[9] D. Talbot (2007) “TR10: Nanocharging Solar.” Ten
Emerging Technology Review. (Online Report)
http://www.technologyreview.com/article/407470/tr10nanocharging-solar/
[10] J. Y. Kim (July 7, 2007) “Efficient Tandem
Polymer Solar Cells Fabricated by All-Solution
Processing.” Science (Online Article). Vol.317.
http://www.dsf.unica.it/EOG/teaching/presentazioneop
toelettronica/Kim_Efficient%20tandem%20polymer%
20solar%20cells%20fabricated%20by%20allsolution%20processing_Science_2007.pdf
[11] “Quantum Dot Production” (Oct. 8, 2012)
(Website)
http://www.news-medical.net/health/Quantum-DotProduction.aspx
[12] A.J. Nozik (Aug. 25, 2009) “Semiconductor
Quantum Dots and Quantum Dot Arrays and
Applications of Multiple Exciton Generation to ThirdGeneration Photovoltaic Solar Cells.” Chem Review
2010. Vol. 110 No. 11 (Online Article)
http://www.chem.uci.edu/~lawm/Semiconductor%20q
uantum%20dots%20and%20quantum%20dot%20array
s%20and%20applications%20of%20multiple%20excit
on%20generation%20to%20thirdgeneration%20photovoltaic%20solar%20cells.pdf
[13] M. J. Fernee (July 18, 2011) “Quantum Dots
Shine Unsteadily.” Physics. (Print Article) Vol. 4, no
56.
DOI:
10.1103/Physics.4.56
http://physics.aps.org/articles/v4/56
[14] National Society of Professional Engineers (2012)
“NSPE Code of Ethics for Engineers.” (Website)
http://www.nspe.org/Ethics/CodeofEthics/index.html
[15] Iowa State University Chapter of American
Society for Chemical Engineers (2012) “American
Society of Chemical Engineers (AIChE) Code of
Ethics” (Website)
http://www.stuorg.iastate.edu/aiche/ethics.html
[16] A. Rugarcia (2000) “The Future of Engineering
Education: A Vision for a New Century.” (Online
Article)
http://www4.ncsu.edu/unity/lockers/users/f/felder/p
ublic/Papers/Quartet1.pdf
ACKNOWLEDGMENTS
Several individuals devoted their time and effort in
helping me finish this paper. Without them this paper
would have taken twice the time with half the quality.
My sincere thanks go to Derek Orr for his aid in
finding a topic for the paper. I thank my mother and
father, Tim and Gigi, for their moral support and
encouragement. Lastly, I give my thanks to my ENGR
0011 group members for giving their opinions on my
paper as well as their time to proofread it.
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