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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
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MULTI-JUNCTION PHOTOVOLTAIC CELLS FOR THERMOPHOTOVOLTAIC ENERGY SYSTEMS
David Lloyd (dil24@pitt.edu, Sanchez 10:00), Yasmin Daukoru (yad20@pitt.edu, Mahboobin 10:00)
Abstract— In 1954, Bell Laboratories developed the first
silicon based solar cell.[1] This early attempt was merely a
novelty, as it was far too expensive to be used widespread,
but it demonstrated that the power carried by light could be
harnessed. As shown by the many places solar power is
utilized today, from powering homes to aerospace
applications, Bell laid the path for the growth of a field with
great potential. Solar power is the most bountiful alternative
energy, as the sun is constantly showering the earth with
light. Yet as of 2012, almost 60 years after the development of
that first system, only about 1.1% of the world’s energy needs
were filled by solar power.[2] This is because the majority of
solar cells in use are only capable of converting 10-15% of
the energy carried in photons of light into usable energy.[3]
The energy is there for the taking, but collecting methods
need to be improved. To be a viable source of energy, solar
cells need to be either inexpensive enough to justify low
efficiency, or efficient enough to lower the cost of the cell
within an energy-generating system.
A promising means of reducing fossil fuel use and promoting
the use of alternative energies is through the generation of
electricity by a thermophotovoltaic system (TPV). TPVs have
the potential to deliver energy on a utility and industrial
scale.[5] Realizing this potential relies on increasing energy
conversion by improving the efficiency of photovoltaic cells.
Multi-junction cells, by using several materials and
REFERENCES
[1] G. Knier. (2002). “How do Photovoltaics Work?” NASA
Science. (online article). http://science.nasa.gov/sciencenews/science-at-nasa/2002/solarcells/
[2] “2014 Key World Energy Statistics.” (2014).
International Agency of Energy. (online publication).
http://www.iea.org/publications/freepublications/publication/
keyworld2014.pdf
[3] K. Pickerel. (2015). “What are the different types of solar
modules?” Solar Power World. (online article).
University of Pittsburgh Swanson School of Engineering 1
2016/01/29
arrangements, address sources of energy loss, allowing for
recorded efficiencies upwards of 40%.[4] Further
development of multi-junction photovoltaic cells in turn
lowers barriers to large scale electrical generation by TPVs.
Like traditional photovoltaic systems, multi-junction cells
convert the energy carried in photons of light into electric
current when the photons strike the cell, knocking free
electrons. The idea behind multi-junction cells is that by
using multiple (typically 3-5) base materials in a cell and
linking them together, more energy in the light spectrum can
be converted to electricity.[1][6] With such impressive
efficiency, this technology needs to be further explored and
developed. With the ability to harness considerably more
power from the sun, it would be possible to lower the world’s
dependence on fossil fuel, therefore preventing further
pollution and harm to the environment.
This paper will describe the capabilities of TPVs,
establishing the solar cell as the primary component for
improvement in a photovoltaic energy generating system. The
structural and material properties of multi-junction cells will
be compared to popular silicon-based cells and cells reliant
on a narrow range of materials selection. Finally this paper
will discuss the economic and social benefits of advancing
TPVs.
http://www.solarpowerworldonline.com/2015/07/what-arethe-different-types-of-solar-modules/
[4] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E.
D. Dunlop. (2016). "Solar cell efficiency tables (version 47)."
Progress in Photovoltaics: Research and Applications.
(online article). http://dx.doi.org/10.1002/pip.2728
[5] C. Ferrari, F. Melino, M. Pinelli, P. R. Spina, and M.
Venturini.
(2014).
"Overview
and
Status
of
Thermophotovoltaic Systems." Energy Procedia. (online
article).
http://www.sciencedirect.com/science/article/pii/S187661021
4000198 p.161
David Lloyd
Yasmin Daukoru
[6] S. Chen, L. Zhu, M. Yoshita, T. Mochizuki, C. Kim, H.
Akiyama, et al. "Thorough subcells diagnosis in a multijunction solar cell via absolute electroluminescenceefficiency measurements." (2015). Scientific Reports. (online
article).
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4296303/
requirements to be met by solar cells for use in a TPV
system. The solar cells suggested by the author (Silicon,
GaSb, etc) are based on material properties such as low band
gap and ease of cooling. The advantages of these materials
make will be used to describe the multi-junction cell an ideal
candidate since it can combine the best of different several
materials and configurations.
ANNOTATED BIBLIOGRAPHY
A. Datas. (2015). "Optimum semiconductor bandgaps in
single junction and multi-junction thermophotovoltaic
converters," Solar Energy Materials and Solar Cells. (online
article).http://www.sciencedirect.com/science/article/pii/S092
7024814006461
This
research
article
discusses
multi-junction
thermophotovoltaic cells as a means “to maximize both the
efficiency and the power density” of thermophotovoltaic
systems (TPVs). The article gives the advantages of TPVs.
We will use the article’s analysis of TPV power density and
efficiency needs to describe why multi-junction cells can
improve both needs by providing the optimal band gap of
light absorption.
A. Bagher, M. Vahid, M. Mohsen. (2015). “Types of Solar
Cells and Application.” American Journal of Optics and
Photonics. (online article). doi: 10.11648/j.ajop.20150305.17
This article provides an overview of all different solar cell
types to date by material, material properties (light
absorption, cost, method of production, band gap, etc), and
configuration. Since the article includes upcoming solar cell
types, its breadth will be used to compare the electrical and
economic performance of multi-junction cells with existing
and newer solar cells.
C. Breyer and A. Gerlach. (2013). "Global overview on gridparity." Progress in Photovoltaics: Research and
Applications.
(conference
paper).
http://dx.doi.org/10.1002/pip.1254
This article describes the photovoltaic industry’s trend
towards a lower cost per energy in energy production versus
fossil fuel power plants. In our paper, we will integrate the
advantages of TPV electrical generation with the potential in
reach of PVs in general. Since TPVs can cogenerate
electricity and heat with varied energy inputs, TPVs are as
capable of displacing fossil fuels as other PV systems.
C. Ferrari, F. Melino, M. Pinelli, P. R. Spina, and M.
Venturini.
(2014).
"Overview
and
Status
of
Thermophotovoltaic Systems." Energy Procedia. (online
article).
http://www.sciencedirect.com/science/article/pii/S187661021
4000198 p.161
This article provides an overview of the current status of
TPVs and how their components work. We will use this
article to describe how TPVs work and explain the difference
between TPV systems and PV systems.
L. M. Fraas. (2014, June 8-13). "Economic potential for
thermophotovoltaic electric power generation in the steel
industry." 2014 IEEE 40th Photovoltaic Specialist
Conference
(PVSC).
(conference
paper).
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=69
25031
This article gives advantages of thermophotovoltaic cells
over solar cells—namely 24-hour operation from non-solar
thermal energy inputs and generation of electricity from
recovered thermal energy. This article will be used in our
paper to present an industrial application of TPVs.
S. Chen, L. Zhu, M. Yoshita, T. Mochizuki, C. Kim, H.
Akiyama, et al. "Thorough subcells diagnosis in a multijunction solar cell via absolute electroluminescenceefficiency measurements." (2015). Scientific Reports. (online
article).
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4296303/
This article proposes a new method for measuring the
current-voltage relationship within individual subcells, and
various sources of energy loss without referencing previous
measurements. In addition to providing an assessment of
multi-junction cells’ capabilities, we will use this article to
provide details of how multi-junction cells can be evaluated
to avoid overly optimistic efficiencies.
M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D.
Dunlop. (2016). "Solar cell efficiency tables (version 47)."
Progress in Photovoltaics: Research and Applications.
(online article). http://dx.doi.org/10.1002/pip.2728
This article provides the recorded efficiencies of PV cells
by type. We will use this article as in our comparison of
multi-junction cells to other PV cells as well as to explain
why multi-junction cells are the best choice of converter
within a TPV.
H. Daneshvar, R. Prinja, and N. P. Kherani. (2015).
"Thermophotovoltaics: Fundamentals, challenges and
prospects."
Applied
Energy.
(online
article).
http://search.ebscohost.com/login.aspx?direct=true&AuthTyp
e=ip,uid&db=aph&AN=110511533&scope=site
While the article prioritizes the radiator and emitter over
the solar cell as key components, the article does give
K. Kaygusuz. (2009). “Environmental Impacts of the Solar
2
David Lloyd
Yasmin Daukoru
Energy Systems.” Energy Source, Part A: Recovery,
Utilization, and Environmental Effects. (online article).
http://www.tandfonline.com/doi/full/10.1080/155670308020
89664
This article endorses solar energy systems as an
alternative to conventional fossil fuels in meeting increasing
energy demands and decreasing greenhouse gas emissions.
By anticipating criticisms and negative impacts of solar
systems, this source will allow us to recommend actions to
minimize the negative effects of implementing solar systems
such as increasing cell efficiency to offset the energy and
materials intensive processes used to manufacture the solar
cells.
bandgap not being absorbed, and absorbed light above the
bandgap producing heat. We will use this article to articulate
common pitfalls faced by PV cells and how a multi-junction
architecture partially alleviates these pitfalls.
R. Singh, G. F. Alapatt, and A. Lakhtakia. (2013). "Making
Solar Cells a Reality in Every Home: Opportunities and
Challenges for Photovoltaic Device Design." IEEE Journal of
the
Electron
Devices
Societ.
(online
article).
http://ieeexplore.ieee.org/xpls/icp.jsp?arnumber=6589128#se
c7
This article presents current barriers to solar cells (apart
from silicon and thin-film cells) entering widespread use. The
authors review current research trends in solar cells
technology in boosting efficiency beyond that of single cell
architecture, with an eye towards self-proposed guidelines for
emerging solar cell types to displace silicon cells. This source
provides evidence, from economic and performance-based
standpoints, of the utility of developing multi-junction cells
independently of our focus on TPVs.
“2014 Key World Energy Statistics.” (2014). International
Agency
of
Energy.
(online
publication).
http://www.iea.org/publications/freepublications/publication/
keyworld2014.pdf
This publication provides multiple statistics of the world’s
energy usage broken down by region, fuel types, prices, fuel
use, and much more. Of particular interest are the sections
dealing with electricity generation by fuel type and CO 2
emissions by fuel type. The electricity section will be used to
provide a quantitative measure of how much electricity TPVs
will have to generate either alone or coupled with other
energy processes to provide electricity and reduce fossil
fuels’ share of greenhouse gas emissions.
G. Knier. (2002). “How do Photovoltaics Work?” NASA
Science. (online article). http://science.nasa.gov/sciencenews/science-at-nasa/2002/solarcells/
This article provides a basic overview of how PV cells
work. In light of more detailed sources within our
bibliography, this source will be used as an introduction
preceding a more detailed explanation of how PV cells
function.
K. Pickerel. (2015). “What are the different types of solar
modules?” Solar Power World. (online magazine article).
http://www.solarpowerworldonline.com/2015/07/what-arethe-different-types-of-solar-modules/
This magazine article for a general, consumer audience
briefly discusses silicon PVs and their market dominance. As
explained in the article, silicon based PVs are the most costeffective given their efficiency and durability. We will
incorporate this source in a comparison of advantages and
disadvantages of multi-junction cells versus other PV cells.
A. Polman and H. A. Atwater. (2012). "Photonic design
principles for ultrahigh-efficiency photovoltaics," Nature
Materials.
(online
article).
http://dx.doi.org/10.1038/nmat3263
This article discusses factors limiting the efficiencies of
current photovoltaic cells to below their thermodynamic
limits. Sources of energy loss within a PV cell are due to
insufficient concentrated light, light below the cell material’s
3