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A Meta-Analysis on Solar Cell Technologies
Farid Mohammadi 2017-10-10
i
Independent degree project - first cycle
International Bachelor Program in Electronics
A Meta-Analysis on Solar Cell Technologies
Farid Mohammadi
A Meta-Analysis on Solar Cell Technologies
Farid Mohammadi 2017-10-10
ii
MID SWEDEN UNIVERSITY
Department of Information Technology and Media
Electronics Design
Examiner: Benny Thörnberg, [email protected]
Supervisor: Göran Thungström, [email protected]
Author: Farid Mohammadi, [email protected]
Degree Programme: International Bachelor Programme in Electronics, 180
credits
Main field of study: Electronics
Semester, Year: Autumn 2017
A Meta-Analysis on Solar Cell Technologies
Farid Mohammadi 2017-10-10
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Abstract
The objective of this study is analysing the characteristics of five different solar
cell technologies regarding their efficiency, fill factor, cost and environmental
impacts and comparing their improvement records over years considering their
efficiency. The five solar cell technologies of interest are amorphous silicon,
monocrystalline silicon, polycrystalline silicon, cupper indium gallium selenide
thin film and cadmium telluride thin film. The structure and manufacturing
process of each of cell technologies were discussed. The study was conducted
by the aid of available scientific reports regarding the electrical characteristics
of different solar cell technologies. The extracted information regarding
efficiency rate and fill factor was analysed using graphs and significant findings
are discussed. The five technologies are also compared regarding their cost and
ease of fabrication and their impacts on environment and recycling challenges.
The result of this study is suggesting the most promising technology that may
be the optimal option for further investment and research.
Keywords: Solar cell, Photovoltaics, Amorphous silicon, Monocrystalline
silicon, Polycrystalline silicon, Cupper indium gallium selenide, Cadmium
telluride, Thin film, Efficiency, Fill factor
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Acknowledgements
I want to express my gratitude to my supervisor Dr. Göran Thungström for his
guidance, patience and continuous support.
I would like to pay my regards to Dr. Benny Thörnberg for the useful
comments and remarks through the process of writing this thesis.
I wish to present my special thanks to the former vice-president of the student
union, Mr. Reza Moossavi who coordinated my studying procedure at the
university when I was not in Sweden.
I would like to thank my friends for their support. They supported me greatly
and were always willing to help me.
Finally, I should express my profound gratitude to my family for providing me
with unfailing support and continuous encouragement throughout my years of
study.
Thank you,
Farid Mohammadi
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Farid Mohammadi 2017-10-10
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Table of Contents
Abstract .............................................................................................................. iii
Acknowledgements ............................................................................................. iv
List of Tables ..................................................................................................... vii
List of Figures .................................................................................................. viii
List of Equations ................................................................................................. ix
Terminology ......................................................................................................... x
Acronyms / Abbreviations ............................................................................... x
1 Introduction ...................................................................................................... 1
1.1 Background and problem motivation ........................................................ 1
1.2 Overall aim ................................................................................................ 2
1.3 Scope ......................................................................................................... 2
1.4 Concrete and verifiable goals .................................................................... 2
1.5 Outline ....................................................................................................... 3
1.6 Contributions ............................................................................................. 3
2 Theory .............................................................................................................. 4
2.1 Solar cell technologies .............................................................................. 5
2.1.1 Amorphous Silicon solar cells ............................................................ 6
2.1.2 Monocrystalline Silicon solar cells ..................................................... 8
2.1.3 Polycrystalline Silicon solar cells ....................................................... 9
2.1.4 Copper indium gallium selenide thin-film solar cells ...................... 11
2.1.5 Cadmium telluride thin film solar cells ............................................ xii
2.2 Electrical characteristics .......................................................................... 13
2.2.1 The ideal solar cell and solar cell in practice .................................... 13
2.2.2 Quantum efficiency and spectral response ....................................... 14
2.2.2 Power ................................................................................................ 15
2.2.3 Efficiency rate ................................................................................... 16
3 Methodology .................................................................................................. 18
4 Implementation .............................................................................................. 19
4.1 Efficiency rate and fill factor .................................................................. 19
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4.2 Cost and ease of fabrication .................................................................... 23
4.3 Environmental aspects ............................................................................. 24
5 Results ............................................................................................................ 25
5.1 Efficiency ................................................................................................ 25
5.2 Fill factor ................................................................................................. 27
5.3 Cost .......................................................................................................... 28
5.4 Environmental aspects ............................................................................. 29
6 Discussion ...................................................................................................... 30
7 Conclusion ..................................................................................................... 31
7.1 Social and ethical aspects ......................................................................... 31
8 Future Work ................................................................................................... 32
References .......................................................................................................... 33
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List of Tables
Table 1. Amorphous Silicon solar cell efficiency
Table 2. Monocrystalline silicon solar cell efficiency
Table 3. Polycrystalline silicon solar cell efficiency
Table 4. Cupper indium gallium selenide solar cell efficiency
Table 5. Cadmium telluride solar cell efficiency
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List of Figures
Figure 1. Photovoltaic – Solar cell system
Figure 2. The structure of crystalline silicon solar cell
Figure 3. Schematic of allotropic forms of
silicon: monocrystalline, polycrystalline, and amorphous silicon
Figure 4. Solar Calculator
Figure 5. Amorphous Silicon Solar panels
Figure 6. Crystal of Czochralski grown silicon
Figure 7. Solar panel made of octagonal monocrystalline silicon cells
Figure 8. Comparing polycrystalline (left) to monocrystalline (right) solar cells
Figure 9. CIGS cell on a flexible plastic backing
Figure 10. Fully monolithically integrated flexible solar module (consisting of 6
cells) based on cadmium telluride
Figure 11. The equivalent circuit of an ideal solar cell (full lines)
Figure 12. The I–V characteristic of an ideal solar cell (up) and the power
produced by the cell (down)
Figure13. FF can also be interpreted graphically as the ratio of the rectangular
areas
Figure 14. Printer manufacturing solar cells
Figure 15. Graph of Latest Confirmed Efficiency
Figure 16. Graph of efficiency over years
Figure 17. Graph of average efficiency improvement
Figure 18. Graph of fill factor over years
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List of Equations
Equation 1. Shockley solar cell
Equation 2. Open-circuit voltage of the solar cell
Equation 3. Photo-generated current
Equation 4. Fill factor
Equation 5. Maximum efficiency of the solar cell
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Terminology
Acronyms / Abbreviations
a-Si Amorphous Silicon
CIGS Copper Indium Gallium Selenide
CT Cadmium Telluride
FF Fill Factor
mono-Si Monocrystalline Silicon
poly-Si Polycrystalline Silicon
A Meta-Analysis on Solar Cell Technologies
Farid Mohammadi 2017-10-10
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1 Introduction
In recent years, solar energy production has become an important technology as a
sustainable source of energy. Solar cell technology has gone from a solely
scientific experiment into a practical but expensive technology and later to a more
affordable way of mass production of energy. Considering the enormous amount of
energy that is emitted from the sun every single day, scientists have always
dreamed of harvesting this abundant, free and promising source of energy. The sun
like most of the other stars is mostly made of hydrogen and helium in plasma state
and the energy comes from a process that is called nuclear fusion. Solar radiation is
energy in the form of electromagnetic radiation from the infrared (long) to
the ultraviolet (short) wavelengths. There are different technologies that harvest the
energy from the sun. Solar heating, photovoltaics, solar thermal energy, solar
architecture, molten salt power plants and artificial photosynthesis are the main
technologies [1]. Photovoltaics technology intends to transform the solar energy to
electricity. This research tries to compare some of these technologies by studying
different research papers and journals and to understand the pros and cons of each
of these technologies. Understanding the evolution of each of these technologies
can give us a very strong perspective of their future and the whole energy industry
in general.
1.1 Background and problem motivation
Energy has always been a concern for human beings. Energy is all around the
universe but we need to have it in hand to work and perform tasks. As our
civilization moved to more sophisticated and industrial era, constant supply of
affordable energy became and continues to be a crucial concept. In fact, the
demand for energy is ever-growing. Burning natural substances and biomass
including coal, dates to ancient times. However, the industrial mass use of fossil
fuel started in early 18th century. Knowing this can help us understand that it is
time to look for new forms of energy. Specially that fossil fuels are contaminating
our planet and risking our own life as well as chances of running out of them.
Solar energy as one the most appealing and promising forms of energy is on the
way of becoming a major source of energy in the market. Governments in different
countries are allocating more money to renewable energy especially solar energy.
Among the renewable energy sources and according to the budget of the
department of energy of the United States of America solar energy alone has an
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appropriation of $55.5 million funding in 2016 compared to $17.7 million for wind
energy, $17.8 to Hydrogen technology and $4.1 million for water technology. [2]
This research studies some forms of photovoltaic technologies as a method of
converting solar energy into electrical energy.
1.2 Overall aim
This research aims to give a perspective on some of the main solar cell
technologies and compare the efficiency, affordability and challenges in their
manufacturing, use and disposal and suggest some of these technologies as better
options for more investment and research.
1.3 Scope
The following technologies are discussed in this study:
Amorphous silicon solar cells
Monocrystalline silicon solar cells
Polycrystalline silicon solar cells
Copper indium gallium selenide thin film solar cells
Cadmium telluride thin film solar cells
These technologies are only investigated in the form of single-junction cells in non-
concentrator modules.
1.4 Concrete and verifiable goals
The aim of this study is to investigate five main solar cell technologies: amorphous
silicon solar cells, monocrystalline silicon solar cells, polycrystalline silicon solar
cells, copper indium gallium selenide thin film solar cells, cadmium telluride thin
film solar cells. The purpose is suggesting the most suitable technology for more
public and private research and investment. These technologies are studied
regarding the following aspects:
Efficiency and fill factor
The improvement of Efficiency and fill factor over years
Ease of fabrication and cost
Environmental impacts, disposal and recycling challenges
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1.5 Outline
This report is organized using different chapters. Chapter 1 is an introduction of
this work, explaining the background, aim, scope and goals of this study. Chapter 2
contains the theoretical information about five different solar cell technologies of
interests and their manufacture and explains the concepts of efficiency and fill
factor. Chapter 3 describes the methodology and outline of the thesis. Chapter 4
represents all the gathered data from different scientific reports and presents them
in tables. Chapter 5 explains the results of comparing the data about these five
technologies with graphical representation of the data using Matlab and Excel. In
Chapter 6 the overall conclusion of the study is discussed and social and ethical
aspects of this work are mentioned. Chapter 7 suggests future work for a more
complete prospect of solar cell technologies.
1.6 Contributions
This research has been done as an individual independent degree project.
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2 Theory
“The photoelectric effect was first noted by a French physicist, Edmund Bequerel,
in 1839, who found that certain materials would produce small amounts of electric
current when exposed to light.” [3]
Later, and in 1905 Albert Einstein described the nature of light and the
photoelectric effect on which this technology is based. “The Nobel prize in physics
in 1921 was awarded to Albert Einstein "for his services to Theoretical Physics,
and especially for his discovery of the law of the photoelectric effect".”[4]
There are different materials and techniques in the solar cell industry. New
scientific improvements and inventions in industrial section has always encouraged
us to try different ways for achieving the same purpose. Solar cell as a promising
way of energy supply has many different technologies. The possible market for
each of these technologies also makes it more challenging to make the optimum
choice in a given situation. When choosing a solar technology, among many factors
we need to consider the energy and cost that is spent on the fabrication, the
efficiency and suitability of the technology for a certain function and the
environmental impact of that technology when the end of its life comes and needs
to go back to nature or be recycled.
Figure 1: Photovoltaic – Solar cell system [5]
Solar energy is very appealing as a solar panel in function is not causing any noise,
is not consuming any fuel and is not emitting any toxic gas in the environment.
However, the fabrication process of solar panels can be very expensive, energy
A Meta-Analysis on Solar Cell Technologies
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consuming and release toxic gas and heat in the environment. Moreover, when it
comes to disposing time there are things to be considered such as any toxic
material that may exist in the structure of the panel. These are some examples of
the complications of solar technology and why they are not replacing other energy
technologies all at once. Once these problems are solved, we can enjoy the
enormous energy that comes to our planet from the sun every single day.
Solar cells follow the same basic
structure as many other
semiconductor devices which
consist of p-type and n-type layers
and junction depletion layer.
The energy from light absorption
creates an electron-hole pair and
then the electrons and holes are
directed to the negative and positive
sides due to structure of the device.
Figure 2 shows this process in
crystalline silicon solar cell. “After
light absorption, the minority
carriers (electrons) diffuse to the
junction where they are swept across
by the strong built-in electric field.
The electrical power is collected by
metal contacts to the front and back
of the cell” [6].
Figure 2: The structure of crystalline silicon solar cell [6]
This flow of electrons can be used on a load system and provide electrical energy
in form of direct current.
2.1 Solar cell technologies
The technologies which are studied in this research are explained briefly in the
following sections regarding their material, production and characteristics.
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2.1.1 Amorphous Silicon solar cells
Amorphous silicon (a-Si) refers to silicon in its non-crystalline form. Making
amorphous silicon is done through deposition process from gas.
This form of silicon as a semiconductor is used in solar panels and thin-films. It has
the positive characteristic of flexibility so it can be used on many different
substrates.
Figure 3: Schematic of allotropic forms of silicon: monocrystalline, polycrystalline, and
amorphous silicon [7]
Amorphous silicon cells have a relatively low efficiency. The efficiency can be as
low as 6% and in lab conditions as much as 12.5% [8].
Amorphous silicon cells are used in
calculators and other low power
applications because of their low
power output. However, this problem
may be solved by making stacks of
several amorphous silicon solar cells
[8].
Figure 4: Solar Calculator [9]
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Figure 5: Amorphous Silicon Solar panels [10]
Another possible solution to this problem is introducing hydrogen during the
fabrication of amorphous silicon. By doing so, photoconductivity is significantly
improved and doping is made possible [11].
Other than being a good option for different material substrates, amorphous silicon
solar panels do not contain any toxic material which makes them environmentally
favourable. The process of production does not need much energy compared to
other techniques and the amount of silicon that is used is less than many other
technologies. Amorphous silicon cells show a relatively good performance in poor
lighting conditions. This makes them a good option for cloudy areas; it also takes
advantage from the early and late hours of day when the sun is not fully shining on
it. Amorphous silicon solar cells are widely used in thin film technology due to
their flexibility and thinness; however, additional procedures are used to enhance
the efficiency.
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2.1.2 Monocrystalline Silicon solar cells
Monocrystalline silicon or single crystalline silicon is
often made in a process called Czochralski which is
named after the polish scientist who invented this
method in 1915. This process needs the high-purity
silicon to be melted in very high temperatures. Then
a seed-crystal is entered in the melted silicon and
moved upward while being rotated at the same time.
This process which needs very precise supervision
and control of temperature, rotation and movement of
the central rod, will result in the formation of a big
single crystal of silicon. Different experiments have
led to electrical efficiencies as high as 22%.
Figure 6: Crystal of Czochralski grown silicon [12]
Crystalline silicon cells are the most used type of cells in the market.
Monocrystalline cells are now 55% of this portion while multicrystalline cells
comprise 45%. The percentage was different before fast commercially production
of multicrystalline silicon. For example, in 1995 monocrystalline silicon was 60%
of this market. [13]
Considering the thick layer of silicon that is used in this cell and the high energy
that is needed in the production process, this technology is very costly. Silicon
wafers are cut in several layers as single pieces of crystal. As they need to be
placed on a module panel next to each other, the circular wafers should be reshaped
to an optimal shape in order to save area on the panel. The cylindrical ingot of
silicon is cut from sides and this gives the unique octagonal shape of the
monocrystalline solar cells. The silicon cut-outs need to be send back to the
production process and this adds to the costs of the production of this type of cells.
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Figure 7: Solar panel made of octagonal monocrystalline silicon cells [14]
Although crystalline silicon cells are fragile, they can withstand the harsh condition
in space and high efficiency makes them a good option for space travel. Specially
that unlike the commercial market, the high cost of monocrystalline silicon cell is
not an issue in the space agencies.
However, many researchers are trying to find ways to reduce the cost of this
product. One solution is using thinner silicon wafers. This would obviously reduce
the efficiency of the cell. To compensate this efficiency loss, a technique is used
which is called “backside passivation” or “back surface field” (BSF) By using this
method a stable efficiency of 17% has been achieved [15].
2.1.3 Polycrystalline Silicon solar cells
Polycrystalline silicon which are alternatively called multicrystalline silicon cells
are another silicon based technology. Crystalline silicon cells are the most widely
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used photovoltaic type now [16]. This is mostly thanks to polycrystalline silicon
cells which have a lower cost of production compared to monocrystalline cells. The
polycrystalline silicon production unlike monocrystalline silicon cells does not
require the Czochralski process. Therefore, the temperature of the production
process is lower. Raw silicon is melted and poured in a mould and cooled down.
This creates a multicrystalline structure of the silicon. This silicon later can be cut
into perfectly square shape wafers.
According to the International Energy Agency only between 2012 and 2013 there
has been 22% cost reduction on the production of polycrystalline silicon cells [16].
This shows the rapid improvement in polycrystalline solar cell production. The
perfect cut of silicon wafers help to reduce any silicon waste unlike the
monocrystalline cells. However, the lower purity of silicon means lower efficiency.
This makes polycrystalline cells requiring more area than monocrystalline cells to
deliver the same amount of power.
Figure 8: Comparing polycrystalline (left) to monocrystalline (right) solar cells [17]
Differentiating the two types of crystalline silicon cells is easy by their shape and
colour. As mentioned earlier monocrystalline cells have an octagonal shape. They
also have a dark and solid colour which is from the single crystal structure. On the
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other hand, polycrystalline cells have a perfect square shape and blue colour of
different shining crystals in the structure.
Figure 8 compares the two cells based on their physical appearance.
2.1.4 Copper indium gallium selenide thin-film solar cells
This technology was first introduced with copper, indium and diselenide (CuInSe2).
Cupper indium diselenide was also used in thin-film structures. Later, substituting
some indium atoms by gallium significantly increased the efficiency. CIGS has
shown an efficiency of 22% in the laboratory and 14% in commercial products.
Figure 9: CIGS cell on a flexible plastic backing.[18]
The high light absorption of CIGS makes it the best option for thin-film technology.
Similar to amorphous silicon, CIGS is flexible and can be deposited on different
substrates.
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CIGS can be used to produce low cost electricity by using roll to roll processing
and reducing the balance-of-system costs. [19]
Another advantage of CIGS solar cells is the fact that unlike the crystalline cells
they can be installed on almost all surfaces including the flexible ones. This opens
a new and wide range of applications for this type of cells. Considering the low
cost of production, CIGS thin film technology is a very promising technology.
For some years the disadvantage of this technology was the low efficiency of some
thin film products which was an important factor to consider. This meant that, for
having equal energy harvest as in crystalline cells, we needed bigger surfaces and
more solar cell modules. This problem was compromised and compensated by the
fact that having solar cells in the form of thin-film gives the chance of installing
them on unprecedented surfaces such as buildings and city structures. Much
research is being done to improve the efficiency of CIGS thin-film solar cells
which will be mentioned later in this paper and the current efficiency rate of this
type of cell can compete with other technologies.
2.1.5 Cadmium telluride thin film solar cells
Cadmium telluride thin film is another attempt to use a thin layer of semiconductor
on flexible substrates to reduce the cost. After amorphous silicon thin film,
cadmium telluride is the most common thin film technology in the market.
However, it has only 5% of the total photovoltaic market. [20] Cadmium telluride
solar technologies are expected to expand in the following years.
This technology is appealing as cadmium is a side product of many mining
processes including zinc and although it is a toxic heavy metal, it is anyway
available in industry. Tellurium has a similar condition. Although being a rare
element to reach and mildly toxic, it can be obtained as by-product of gold, copper
and lead refineries. Handling the toxicity of cadmium telluride based products will
be discussed further on in this paper.
Figure 10: Fully monolithically integrated flexible
solar module (consisting of 6 cells) based on
cadmium telluride [21]
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For a long time, the name of thin film solar cells was associated by low efficiency.
But this is not true anymore as the industry has improved drastically in the past
years. The solar energy company First Solar announced an efficiency of 22.1 in
2015. [22]. Considering the low cost of the cadmium telluride thin film solar cells,
if the efficiency problem is solved on large scale solar modules, this technology
may be one the most promising solar technologies.
2.2 Electrical characteristics
This section briefly explains the concepts of an ideal and non-ideal solar cell and
the efficiency of a solar cell.
2.2.1 The ideal solar cell and solar cell in practice
As it was discussed earlier, solar cell can absorb light photons and use the energy to
move the electrons in the semiconductor device in the direction that is defined by
the structure of the device. The continuation of this process can provide a direct
current. The following figure depicts how a solar cell may be represented.
Figure 11: The equivalent circuit of an ideal solar cell (full lines). Non-ideal components are
shown by the dotted line [23]
Shockley solar cell equation explains the I-V characteristics of the solar cell as
following:
Equation 1: Shockley solar cell equation [24]
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“…where kB is the Boltzmann constant, T is the absolute temperature, q ( > 0) is
the electron charge, and V is the voltage at the terminals of the cell.” [24] IO is the
diode saturation current. This means that a solar cell is very much like a rectifier
diode when in dark. Iph is the current that is dependent on the light which the cell is
receiving and the photon flux incident. The relation between this current and the
wavelength of the light is discussed as quantum efficiency or spectral response
which will be discussed shortly in the next section.
In the ideal case, the short-circuit current is equal to photo-generated current which
means: Isc = Iph
In this case the open-circuit voltage is given by:
Equation 2: Open-circuit voltage of the solar cell [25]
2.2.2 Quantum efficiency and spectral response
Quantum efficiency is calculated by comparing the number of electrons produced
in the external circuit and the number of incident photons of a given wavelength.
[26] External quantum efficiency (EQE(λ)) can be defined by considering all the
impinging photons on the surface of the cell and Internal quantum efficiency
(IQE(λ)) is defined when only the photons that are not reflected are taken into
account. [26] The photo-generated can be calculated after knowing the internal
quantum efficiency by:
Equation 3: Photo-generated current [26]
where Φ(λ) is the photon influx incident at wavelength λ and R(λ) is the reflection
coefficient from the top surface.
“The spectral response (denoted by SR (λ), with the unit A/W) is defined as the
ratio of the photocurrent generated by a solar cell under monochromatic
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illumination of a given wavelength to the value of the spectral irradiance at the
same wavelength.” [26] The spectral irradiance is a function of energy which gives
the power density at a wavelength. [27]
2.2.2 Power
If the open-circuit and maximum
voltage (Voc and Vmax) as well as
short-circuit and maximum current
(Isc and Imax) are measured, the power
(P = IV) can be calculated. In an ideal
case Isc is equal to photo-generated
current. The following figure shows
the I-V characteristic and the
generated power of an ideal solar cell.
Figure 12: The I–V characteristic of an ideal
solar cell (up) and the power produced by the
cell (down). The power generated at the
maximum power point is equal to the shaded
rectangle. [28]
By having the maximum current and maximum voltage we can calculate the
maximum power (Pmax); by using both the open circuit voltage and short circuit
current, we can calculate theoretical power (PT). [29]
A good way of measuring the quality of the solar cell is fill factor. Fill factor (FF)
is calculated as the ratio of these two powers:
𝐹𝐹 =𝑃𝑚𝑎𝑥
𝑃𝑇=
𝐼𝑚𝑎𝑥 . 𝑉𝑚𝑎𝑥
𝐼𝑠𝑐 . 𝑉𝑜𝑐
Equation 4: Fill factor [23, 29]
If we look back at figure 11 we can see a series resistance and a parallel resistance.
Ideally the series resistance is zero and the parallel resistance is infinity.
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A larger fill factor is always desirable
which corresponds to an I-V sweep
that is more square-like. [29] “Typical
fill factors range from 0.5 to 0.82.”
[29]
Figure 13: FF can also be interpreted graphically
as the ratio of the rectangular areas [29]
2.2.3 Efficiency rate
Efficiency of a solar cell in practice is subject to many different factors. These may
include the temperature, light intensity, light angle, latitude and other climate
conditions such as air clearance. Each of these factors has its own set of formulas
and calculation techniques. However, we can define the efficiency of a solar cell in
standard test conditions. This is actually the most common way of comparing the
performance of different solar cells.
Different standard test conditions are defined for terrestrial panels and space panels
where some factors such as air mass and temperature are different. “Terrestrial
solar cells are measured under AM1.5 conditions and at a temperature of 25°C.”
[30] AM1.5 is an air mass coefficient.
Efficiency rate is the ratio of the input power which comes from the light source
(for example the sun) and the output power of the solar cell. If we use the
maximum output power, we can calculate the maximum efficiency of the solar cell.
The following equation shows how maximum efficiency rate is measured:
𝜂 𝑚𝑎𝑥
=𝑃𝑚𝑎𝑥
𝑃𝑖𝑛
Equation 5: Maximum efficiency of the solar cell [29]
For calculating the input power, we first need to know the irradiance of the incident
light. Irradiance of a light source is the power that is received from the source in
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any moment on a unit of area and is measured in W/m2. [31, 32] If this number is
multiplied by the area of the cell, the result will be the input power.
The electromagnetic energy from the sun is divided across the electromagnetic
spectrum and is reduced until it gets to the earth’s surface due to absorption and
scattering in the atmosphere. Maximum surface irradiance at the sea level is
assumed to be approximately 1000W/m2. [33]
For example, the input power of a solar cell with area of 1m2 is 1000W. But in
practice this input power may be less due to the angel of radiation or different light
intensity. This number is calculated assuming a perpendicular exposure to light.
But this angle changes during the day. The length of the path that the sun light
needs to take can also vary in different times of the year. [33] All these factors need
to be considered when we want to assume a value for solar irradiance in outdoor
test conditions.
The light intensity can also cause a temperature increase in the cell. This indeed
can have a negative impact and efficiency tend to decrease by increase in the
temperature after a certain point. A temperature coefficient is defined for most of
commercial solar cells. For example, if a cell has temperature coefficient of -0.5%,
it means that the efficiency decreases by 0.5% for every 1⁰C increase in
temperature.
However, in this study all the results are taken from tests which are done in
standard test conditions and there are no changes in radiant energy and temperature
of the cell in these results.
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3 Methodology
This paper is intended to show the differences between different solar technologies
regarding some factors that have been measured and studied since the introduction
of these technologies. The following factors were chosen as they are considered the
most important ones:
Efficiency rate and fill factor
Ease of fabrication and cost
Environmentally friendly for disposal and recycling time
Careful attention has been paid in the choice of reliable and valid data from
different scientific and academic research papers and industrial and commercial
specification of products. Compatibility of the test conditions were considered.
To study the efficiency rate of solar cells, the result of tests from many different
test centres were considered and compared. These records have been published in a
series of research publications called “Progress in photovoltaics: research and
applications” in different versions. The following information has been extracted
from 21 consecutive issues. The information that was only related to the
technologies of interest were considered and the studied cell types were single
junction and non-concentrated.
A separate table was made for each solar cell technology containing the
information about the efficiency rate, fill factor, the year of the test and the name of
the test centre. However, the fill factor value was not available for the last year of
some tables.
In the result section and based on the information form these tables, some
corresponding graphs were made by Matlab and the changes were discussed.
Secondary graphs were used to show the significant findings from this data.
The cost of production for the cells was discussed briefly and the defining elements
of cost were mentioned. In the result section, the more favourable technologies
were mentioned regarding their cost and ease of fabrication.
The environmental aspects of each solar cell were discussed based on the
information from related literature and online resources. This discussion includes
the threats to environment and the methods of preventing them.
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4 Implementation
Choosing a specific solar cell technology for a specific application requires certain
consideration. Furthermore, a company’s decision makers need to know the overall
view of a certain technology to decide where to invest the money and a country’s
policy makers need to decide which technologies have a more promising prospect
and is worth of more resources for research and development.
This section consists of three main parts. As mentioned earlier, efficiency rate and
fill factor, ease of fabrication and cost and finally being environmentally friendly
for disposal time or recycling time are covered.
4.1 Efficiency rate and fill factor
As it was discussed earlier, efficiency rate and space efficiency are interconnected
and efficiency rate is indeed calculated using a factor of the area. Efficiency rates
of different solar cell types have been tested in many different occasions and by
different test centres. In most cases the fill factor of the solar cell is also tested and
recorded.
Table 1: Amorphous Silicon solar cell efficiency Table [34, 35, 36, 37, 38, 39, 40]
Percent of
Efficiency
Percent of
Fill Factor
Year Test Centre
10.4 66 1998 National Renewable Energy Laboratory (NREL)
10.5 67.9 2012 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
10.9 68.3 2013 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
11.6 66.2 2013 European Solar Test Installation (ESTI)
12.2 68.8 2014 European Solar Test Installation (ESTI)
12.3 69.9 2014 European Solar Test Installation (ESTI)
14 - 2016 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
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Table 1 shows the efficiency rate, fill factor, year and test centre for amorphous
silicon solar cell.
Table 2 shows the efficiency rate, fill factor, year and test centre for
monocrystalline silicon solar cell.
Table 2: monocrystalline silicon solar cell efficiency Table [34, 41, 42, 43, 40]
Percent of
Efficiency
Percent of
Fill Factor
Year Test Centre
22.7 80.3 1996 Sandia National Laboratories
22.9 80.3 1996 Sandia National Laboratories
23.8 81.6 2016 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
24.2 80.1 2016 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
25.3 - 2016 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
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Table 3: Polycrystalline silicon solar cell efficiency table [34, 40, 41, 44, 45, 46, 47, 48, 49, 50,
51, 52]
Percent of
Efficiency
Percent of
Fill Factor
Year Test Centre
15.5 78 1994 Sandia National Laboratories
17 75 2009 European Solar Test Installation (ESTI)
17.3 76.1 2010 Japanese National Institute of Advanced
Industrial Science and Technology (AIST)
17.55 75.3 2010 European Solar Test Installation (ESTI)
17.8 75.7 2011 European Solar Test Installation (ESTI)
18.2 76.7 2011 European Solar Test Installation (ESTI)
18.5 76.2 2012 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
19.2 78.93 2015 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
19.5 77.4 2015 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
19.9 79.5 2016 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
21.9 - 2016 Fraunhofer Institut für Solare Energiesysteme
(FhG-ISE)
Table 3 shows the efficiency rate, fill factor, year and test centre for polycrystalline
silicon solar cell.
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Table 4: Cupper indium gallium selenide solar cell efficiency table [34, 40, 41, 45, 46, 53]
Percent of
Efficiency
Percent of
Fill Factor
Year Test Centre
13.5 68.9 2002 National Renewable Energy Laboratory (NREL)
13.8 71.2 2010 National Renewable Energy Laboratory (NREL)
15.7 72.5 2010 National Renewable Energy Laboratory (NREL)
22.6 77.5 2016 Centre for Solar Energy and Hydrogen Research
Baden-Württemberg (ZSW)
Table 4 shows the efficiency rate, fill factor, year and test centre for cupper indium
gallium selenide solar cell.
Table 5: Cadmium telluride solar cell efficiency table [34, 37, 40, 41, 47, 49, 50, 54]
Percent of
Efficiency
Percent of
Fill Factor
Year Test Centre
10.9 62.3 2000 National Renewable Energy Laboratory (NREL)
12.8 71.4 2011 National Renewable Energy Laboratory (NREL)
15.3 72.9 2012 National Renewable Energy Laboratory (NREL)
16.1 74.8 2013 National Renewable Energy Laboratory (NREL)
17.5 76.6 2014 National Renewable Energy Laboratory (NREL)
18.6 74.2 2015 National Renewable Energy Laboratory (NREL)
22.1 - 2016 First Solar
Table 5 shows the efficiency rate, fill factor, year and test centre for Cadmium
telluride solar cell.
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4.2 Cost and ease of fabrication
The cost that the end user - which can be a residential building - pays is calculated
in ¢/kWh (cent kilo watt hour) which is considering the cost over the lifetime of the
solar cell. This price can be affected a lot by the location of the installation place.
The number of days with adequate sunshine and the angel of the solar beams are
very important and that is why the final cost of electricity made by solar cells can
vary in different parts of the world or even a country. [55] The other factors that
can define the final cost of the electrical energy from solar cells are installation
cost, degradation rate and battery. Cost of material is also different for different
countries and companies. The market also follows its own interests and concerns
regarding the demand and supply. End users may also benefit from public
subsidies.
The cost of solar cell production from manufacturing point of view is based on the
amount of used material, the energy consumption and complexity of the production
process. According to the production process of the five solar cell technologies of
interest, crystalline silicon cells use a lot of silicon compared to amorphous silicon
cells. However, they also have a higher efficiency rate. The efficiency of
amorphous cells can be increased by methods like stacking. But the stacking
process increases the cost. So, more affordable stacking techniques may make
amorphous silicon an acceptable option.
Monocrystalline silicon cells use Czochralski crystal growing process which costs
a lot as the process is done in very high temperatures.
Thin film solar cell of type cupper indium gallium selenide, cadmium telluride and
amorphous silicon have an extra benefit other than the lower amount of material
consumption. This benefit is the possibility of producing the modules as large rolls
of thin film via high-speed processes which is a key factor in lower price of the
new photovoltaics. [56]
Figure 14: Printer manufacturing solar cells [57]
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4.3 Environmental aspects
The technologies which are studied in this research are first considered regarding
the existence of toxic material in their production. All the silicon based
technologies which are amorphous silicon, monocrystalline and polycrystalline
cells are considered to be safe in this regards as they do not contain any toxic
material.
Cadmium telluride cell however contains cadmium which is a toxic material and
tellurium which is mildly toxic [58]. This is not however a repelling factor for the
use of cadmium telluride cells. Tellurium is rarely mined directly and is mostly the
by-product of gold and copper refineries. “Globally, primary tellurium sources are
large-tonnage, low-grade ores from copper and copper-gold porphyry-type
deposits, as a by-product of copper refining.” [59]
Cadmium as a toxic material has a similar situation. Cadmium is a by-product of
zinc refineries. [60]
Cupper indium gallium selenide cells contain indium as a mildly toxic element
[58]. However, indium as well is a by-product of zinc. [60]
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5 Results
5.1 Efficiency
As we can see from the tables in chapter 4, these solar technologies have not been
introduced and tested in the same year. They have not been tested for a similar
number of times either. However, these tables contain the highest independently
confirmed efficiency rate at the time and can give an overall understating of the
efficiency of these technologies and the way they have progressed during time.
First, the latest rate for the efficiency of these technologies are extracted from the
above tables and compared in a graph in figure 15:
Figure 15: Graph of Latest Confirmed Efficiency
The highest efficiency rate belongs to monocrystalline silicon solar cell with 25.3%
and lowest rate belongs to amorphous silicon solar cell with only 14%.
Polycrystalline silicon with 21.9%, cadmium telluride with 22.1% and cupper
indium gallium selenide with 22.6 stay in between while showing a similar
efficiency rate.
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For studying the technology improvement of each of these technologies and
comparing them with each other, all the data form the five tables have been
demonstrated in a graph in Figure 16 using Matlab software.
Figure 16: Graph of efficiency over years
It is clear within the time scope of this study, monocrystalline has always had a
higher efficiency rate compared to other technologies and amorphous silicon has
had the lowest.
The level of improvement for each of these technologies is different during years.
Amorphous silicon cell has improved from 10.4% in 1998 to 14% in 2016 which
shows an average of 0.2% improvement in a year.
Monocrystalline silicon cell has improved from 22.7% in 1996 to 25.3 in 2016
which shows an average of 0.13% improvement per year.
Polycrystalline silicon cell has an increased efficiency rate from 15.5% in 1994 to
21.9 in 2016 which is an average of 0.29% improvement per year.
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Cupper indium gallium selenide cell from 13.5% efficiency rate in 2002 has
improved to 22.6% in 2016 which shows 0.65% in average for each year.
Cadmium telluride cell with 10.9% efficiency rate in 2000 has improved to 22.1%
efficiency in 2016 which shows an average of 0.7% increase per year.
Figure 17 shows a graph that compares the average efficiency growth in a year for
the five types of solar cell.
Figure 17: Graph of average efficiency improvement
It is clear form this graph that the cadmium telluride cell with 0.7% improvement
per year has had the fastest improvement rate during the time scope and
monocrystalline silicon cell has had only 0.13% efficiency improvement per year
in its time scope making it the slowest technology in efficiency improvement
compared to others.
5.2 Fill factor
Looking back at tables 1 to 5 can show us how the fill factor has changed during
the years of study. The graph in Figure 18 is based on the fill factor percentage
information from each of the solar cell technologies.
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Figure 18: Graph of fill factor over years
This graph shows that the fill factor improvement over years of almost all of the
five technologies do not follow the same consistent increasing pattern as in
efficiency. This may be due to testing different products from different companies
in each of these cases. However, a general behaviour of the each of the graphs
shows that the fill factor from high to low is in the following order:
Monocrystalline silicon, polycrystalline silicon, cupper indium gallium selenide,
cadmium telluride and finally amorphous silicon. Cadmium telluride shows the
most improvement in fill factor during the years of study.
5.3 Cost
Crystalline silicon cells consume a lot of silicon in their manufacturing.
Monocrystalline silicon cell also needs high temperature fabrication process that
can increase the cost.
Thin film solar cells generally use 1/100 of photovoltaic material that is needed in
crystalline cells. [61]
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Thin film solar cells can be produced using high speed roll to roll process which
significantly reduces the cost.
5.4 Environmental aspects
Having the toxic elements as the essential material for the new generation of thin-
film solar cells opens a new discussion. Dependency of renewable energy
technology on by-product material makes it unreliable and encourages companies
to start their own mining programs with the purpose of having direct access to the
resources. [60] The increasing demand for new generation of solar cell can cause a
new challenge of handling more toxic material.
Considering all the possible hazardous material existing in solar cells as well as
lead (Pb) that may be present in silicon based cells as soldering material, it is clear
that solar cells need a systematic recycling system. [62] Normally after 20 to 30
years the solar cells come to end of life. The recycling is done in incinerators. This
will cause a fraction of the toxic material going to the air as gas and the rest stays
in the ash. By the use of municipal waste incinerators and electrostatic precipitators
the amount of the toxic material that is released as gas can be a fraction as small as
0.5% [62] Currently research is being done considering the leach out of some
material from the ash to the soil. [63]
Considering cadmium telluride cells containing both cadmium and tellurium as
toxic elements, it poses a higher risk compared to cupper indium gallium selenide
cells which contain only indium which is mildly toxic. The silicon based solar cells
are believed to be safe except for presence of lead (Pb) in the structure of the cell.
Monocrystalline cells need Czochralski crystal growing method which consumes
more energy compared to other technologies. Therefore, the fabrication process can
leave more harmful emissions into the environment.
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6 Discussion
By looking back at figure 16 in Chapter 5.1, some further details can be seen. The
number of entries for most of the cell technologies increases in and after 2010.
These entries correspond to more frequent test being done on the efficiency of
these technologies. This is visible for poly-Si, CT and a-Si. They also show a rapid
improvement in the efficiency during this period.
Considering that the material in these cells had not been changed during these
years, the improvement in cell fabrication can be the reason for this. One of the
techniques that improves the efficiency of a solar cell is anti-reflective coating.
Anti-reflective coating is a way to reduce reflection and increase light absorption in
the solar cell. The principle of this technique is the same in other optical devices
such as camera lenses and glasses. The anti-reflective coating of a solar cell helps
to increase the amount of photon energy that is absorbed by the cell, hence
increasing the efficiency. Anti-reflective coatings are very common in silicon based
cells as well as other types of cells. Some of the material that are used in making
anti-reflective coating are:
Titanium Oxide (TiO2), Indium Tin Oxide (ITO), Si3N4, Al2O3, Silicon Oxide
(SiO2), Ta2O5, Silicon Nitride (a-SiNx:H) and a-Si:C:H. [64, 65, 66]
Chemically etched porous silicon is also used as an anti-reflection coating for high
efficiency solar cells. [67] Furthermore, anti-reflective coating has protective
properties for the cell. [68] This can improve the durability of the solar cells and
decrease the cost for the end user.
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7 Conclusion
Photovoltaics are already a very important source of electrical energy in the world.
They can bring energy where traditional sources of energy are not available or
affordable. Global solar electrical energy capacity by the end of 2016 was 301 GW
which was 33% higher compared to 2015. [69] This shows the rapid growth in this
industry. Photovoltaics are also very attractive due to having little or no negative
environmental effects on our planet. However, the capacity of solar electrical
energy is not fully exploited and despite the growth in the functionality and
efficiency of these cells, they are not yet the main electrical energy source due to
their limitations. According to Solar Power Europe and World Energy Council
solar power is providing only more than 1% of the world’s electricity demand. [70,
71]
Cupper indium gallium selenide thin film and cadmium telluride thin film show the
best future prospect. This is based on their current high efficiency, the relative high
growth in the past years, affordability and ease of manufacturing process. They
also benefit from being applicable on flexible substrates. A fact that increases their
suitability for different situations. Their toxic elements however, need to be treated
very carefully during the recycling time.
7.1 Social and ethical aspects
Having a better understating of solar electrical energy industry can increase the
sustainability of human society in different regions. With sufficient investment and
schemes, solar electrical energy can bring clean and reliable energy to areas with
more deprived population and cleaner life place for big industrialized cities.
Choosing the right technology and investing in it does not require as much
resources as fossil fuel does which in some countries is in form of a monopoly in
the hands of the government. Solar electrical energy can bring more justice and
equality to the human society by giving the chance to many public and private
companies of any size to contribute in energy sector.
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8 Future Work
Considering the vast variety of solar cell types, further studies may be done by
analysing more types of photovoltaics. Gallium selenide solar cells and organic
solar cells are some considerable cell types for further study.
The amount of improvement in each of the technologies studied here by adding
multiple junctions and concentrators and considering the efficiency, cost and
durability of the cells can be subject of further studies.
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