Download Material Analysis of Amorphous Silicon Solar Cells 11/17/11 John

Material Analysis of
Amorphous Silicon
Solar Cells
11/17/11
John Majzner
Levi Weiss
Andrew Kom
Class of Vikram Dalal, EE 332, Iowa State University, Ames, Iowa 50010
Abstract
Amorphous silicon (a-Si), is one of
the many new technologies that might have
Another way to reduce the number of
dangling bonds is to pre-bake the transparent
film substrate.
the ability to dominate the solar cell market.
Amorphous silicon is an attractive material
The temperature dependence of a-Si
because it is relatively cheap, light weight,
is very unique. For most solar cell
and durable. However, at the same time, it
materials, as temperature increases, the
is much less efficient than other forms of Si
efficiency decreases. However, for a-Si, as
based solar cells. This paper is an analysis
temperature increases, the efficiency will
of the physical properties, manipulation, and
also increase. This makes amorphous
implementation of a-Si.
silicon more useful in hotter environments,
which is common for real world applications
The thickness of the a-Si cell is much
of solar cells.
thinner than crystalline silicon. This smaller
thickness causes a decrease in the absorption
Beyond being able to efficiently
of incident solar rays. This can be remedied
work in hot environments, it is also very
by putting multiple layers of junctions on
useful for harsh environments due to its
top of each other.
durability. Amorphous silicon is also used in
many space program related projects since
If the annealing temperature of a-Si
goes above 250°C, then the the film will
its light weight makes it ideal for many
space related needs.
begin to lose conductivity as well as increase
the initial dangling bond density. These
Another way to boost a-Si is to use
dangling bonds make the material less
graded doping and help increase the electric
efficient, but can be filled by adding
field in a a-Si cell. This electric field
hydrogen to them, through a process call
boosted by the graded doping will increase
hydrogenation. The hydrogen seals the gap
and leaves the Si molecule completed.
the flow of electrons, and help improve the
characteristics of the cell.
As for the future of amorphous
Benefits of Amorphous Si
There are a number of particularly
silicon, outside of solar cell technology, is
that it will be largely used in LEDs for both
important characteristics that make the less
lighting and television sets. The a-Si is
efficient amorphous silicon solar cells
made into dots that will, depending on the
size, give a maximum photoluminescence at
valuable and useful. The first of these is cost
[3].
short wavelengths that crystalline silicon
cannot emit. This makes it so the LED can
flash in different colors and be highly
efficient.
Amorphous silicon can be produced
at a much lower cost than any crystalline or
polycrystalline solar cell available. The
materials used to produce a-Si solar cells
Introduction
Due to the number of different
materials that can be used for solar cells, a
clear identification of each material’s unique
cost significantly less money than either of
the more efficient models. Also, the ability
to be produce a-Si in a large roll and in mass
quantities contributes to this lower cost [3].
properties must be analyzed. This paper
will discuss basic information about physical
properties of a-Si and widely used
manipulation methods used to increase
efficiency.
The reason it can be produced rollto-roll is directly related to the material’s
flexible nature. Since amorphous silicon has
no orderly structure, it can be disturbed by
flexing it. This property can be utilized for
A general understanding of what
makes a-Si a unique in the solar cell market
many projects that include irregularly
shaped structures [3].
is that it is a low cost, light weight, durable,
and low efficiency material [1]. It is
generally not used alone, but in layers or in
combination with other materials, to
increase its effectiveness [2].
Arguably the most important
attribute when considering the benefits of aSi is weight. In many cases, weight is a
very important factor on whether solar
power is a feasible option. For many space
related projects, light weight solar power is
light will bend and scatter as it travels
ideal, since it reduces the weight that has to
though the cell. The Image clearly shows its
be propelled [3].
interaction of light with the different layers
and its eventually be absorbed in the i-layer
The flexibility and lightweight of a-
[4]. The p and n layers induce an electric
Si leads to another useful trait of being able
field in the intrinsic layer which allows
to put the cell on a large range of surfaces.
movement of carriers as seen in figure B.
What is meant by this is that it can be put
The intrinsic layer acts as the depletion
onto fabrics, textiles, magnetic backing, and
region between the n and p doped layers [5].
almost any other material one would want.
This allows for more flexibility in what it
can be used for [3].
Finally, its ruggedness is another
very good quality of a-Si. Amorphous
silicon is very rugged, because of how it is
made. The fact that a single amorphous
silicon panel includes many interconnected
cells allows a different path for electrons to
travel if the panel were to be punctured. In
most cases, any crystalline or polycrystalline
Figure A, The different layers of a solar cell and how
panel would significantly decrease in its
light interacts with them [6] .
ability to produce, or lose it entirely if
punctured or damaged in any way [3].
a-Si Design
Figure A shows the basic design of a
typical a-Si cell. In addition, it shows how
other word, each cell is its own structure and
is separate from the others in the panel. In aSi however, no such requirement exists.
This means that you may have multiple cells
that are built connected together. The reason
that this is good, is that it allows one to have
a cell get damaged, but still have all the
others work around the bad cell, which
means the output is not affected as much as
Figure B, The energy band diagram for an
amorphous silicon solar setup.
if you were to damage a crystalline cell in a
crystalline based panel.
Differences Between Crystalline and
Amorphous Si Solar Technology
Beyond the simple aesthetic
differences between the two, such as look
and weight, there exist important differences
between the two technologies. To begin
with, if one were to examine a crystalline
panel and an amorphous silicon panel sideby-side, one would notice something right
away. In many amorphous silicon based
panels, the “fingers” extend a short ways,
but do not extend across the panel as they do
in crystalline. This is because crystalline
Figure (?), Crystalline Silicon Based Solar
http://www.monocrystallinesolarpanels.com/china156mmx156mm_high_efficiency_monocryst
alline_solar_cells-118314.html
Figure (?), Amorphous Silicon Based Solar
Energy Bands in Amorphous Silicon
panel require crystalline structure, which
means that the material must be clearly
transitioned from one cell to another. In
According to [5], what is perhaps the
most important characteristic of amorphous
silicon is the band gap change. In crystalline
silicon, the band gap can be represented by
figure D. Figure D is the indirect band gap
with a much larger direct band gap energy.
The indirect gap for Silicon is 1.1eV, while
direct band gap can be on the range of 2.2eV
[7]. The disorder found in amorphous silicon
causes it to develop a direct band gap
(Figure C) that can range from 1.6-1.8eV.
Having a direct band gap allows the
amorphous silicon to have a much higher
absorption coefficient. This results in an
improved ability to produce energy in lower
light conditions.
Figure D, Inderect Energy Band [8] .
Hydrogenating the a-Si
Effectively, when amorphous silicon
is made, it has a large number of defects that
come from the disorder of the silicon atoms.
These defects, referred to as dangling bonds,
are silicon atoms that have bonded with 3
other silicon atoms, but not with a 4th [9].
This defect significantly decreases the
efficiency of solar conversion [10]. To
counteract this result, hydrogen is
introduced into the material. The hydrogen,
Figure C, Direct Energy Band [8] .
to fill its outer state, is attracted to the open
silicon bond. This attraction leads to the
silicon bond being filled. This process
significantly cuts down on the number of
defect states as can be seen in Figure E, but
cannot delete them entirely [3].
the dangling bond density. Figure F shows a
example of the plasma deposition system
used for adding the hydrogen to a-Si to
transform it into a-Si:H.
Figure E, Diagram of the hydrogenated amorphous
silicon [11] .
Figure F, Diagram of a plasma deposition system
[1] reports that hydrogenated
[1] .
amorphous silicon (a-Si :H) is a useful
photovoltaic material based on the easiness
in which it is manufactured, as well as some
Physics Manipulation
Using hydrogen to remove dangling
attractive properties; a high absorption
bonds was first reported by Anderson and
coefficient, high photoconductivity, and p-n
Spear [12]. This single change from a-Si to
controllablility.
a-Si:H has made amorphous silicon one of
the top contenders in the material war of the
These a-Si:H films are produced
solar cells. Further manipulation of a-Si
through plasma deposition from monosilane
allows for a more efficient solar cell and
(SiH4), which is diluted by hydrogen to 10
making it a better material. Generally, as
mol%. The films are heated to improve the
you add more H into the cell, the bandgap
photoconductivity and doping efficiency,
increases [13]. To get more H into the cell,
with a cap of 250°C. If the temperature of
you can raise the pressure and or lower the
the anneal goes over 250°C, the film will
temperature during deposition.
begin to lose conductivity as well as increase
The electric field E in the i-layer will
greatly affect the cell performance through
not require new equipment, but can
improve the solar cell.
the fill factor. Having control over E within
the i-layer will allow for a more efficient
design. If the i-layer becomes too thick, the
E is deformed and reduced [14].
Thickness
[18] states that a-Si films can be
much thinner than crystalline Si devices,
with a thickness of 500 nm compared to tens
Creating a buffer of a-SiC:H between
of hundreds of microns. However, there are
the p-layer, pc-Si, and the i-layer, a-Si:H,
a large amount of defects in a-Si:H films
improves the performance due to the
that limit the minority carrier diffusion
repelling of AEc [15].
lengths to 100 nm. Since a-Si:H cells are
fabricated even thinner than regular a-Si, the
Having a higher open circuit voltage
(Voc) in the top layer of a triple-junction a-
incident absorption of solar radiation is
reduced even further.
Si solar cell is better then having a higher
band gap with a-SiC alloys [16]. The
[2] states that multi-junction solar
highest Voc that could be obtained was
cells are used in a-Si cells to obtain higher
1.04V when it was deposited at 150°C [16].
efficiencies. However, a major roadblock
for this is that it is has not been possible to
When we pre-bake the transparent
film substrate, the impurity content of the aSi will be reduced [17]. This process will
reduce the number of dangling bonds for
hydrogen and make it a more effective solar
cell. This manipulation is a low cost
example of how to better create a-Si. These
types of changes are incredibly useful
because it will not increase the cost and do
make a cell with a bandgap lower than 1.5
eV. With microcrystalline silicon
technology combined with a-Si technology,
however, thin film materials with 1.12 eV
bandgaps can be made.
Temperature
Photovoltaic (PV) modules are very
dependent of temperature and irradiance. In
humid environments, such as Thailand, the
efficiency of a-Si changes to 103.86% of
Figure G, Data points showing the relation of
normal [19]. This shows that the hotter the
temperature and Output of an a-Si solar cell [19] .
temperature, the more effective the solar cell
can be. From this we can deduce that within
colder environments, the effective efficiency
of the cell will drop.
The IV curve for a-Si keeps the same
shape as most solar cell materials. Figure H
shows the IV curve of the 11 segmented
Si:H. The optimum points are shown as X
One thing that sets a-Si apart from
other materials is that the PV module shows
marks on the graph. The optimum open
circuit voltage for this module is ~.8V.
much better performance at higher
temperatures relative to other types of PV
modules [20]. The efficiency of most
materials decreases as temperature
increases, but the efficiency of a-Si
increases as temperature increases [19].
The material a-Si has little temperature
dependence when it is operating in a
equilibrium state, but it will have a large
temperature dependence in short periods of
time [20]. Figure G shows a real world
relationship between temperature and power
Figure H, IV curve of a 11 segmented a-Si [21] .
output.
Light Spectrum Effects
According to [22], a 550 nm
wavelength is the most intensive part of the
solar spectrum, as seen in figure I. Because
this is the most intensive part of the
spectrum, it will be the least reflective
wavelength by %, as seen in figure J. From
figure J, we can also see that reflection is
segmented a-Si:H single junction module
not a simple function of decay vs
has an efficiency of 8.7% [21].
wavelength, but will increase and decay at a
specific spectrum.
Light trapping is essential for
increasing efficiency of a-Si thin film cells.
While using the light trapping material Zinc
Oxide (ZnO), a stable efficiency of 9.47%
could be obtained with an 11 segmented aSi:H [21]. To compare this to the highest
confirmed efficiency of other materials see
Figure I, Light spectrum to amount adsorbed [22] .
figure K. Some ways to increase the light
trapping is using graded doping, using a
high reflection substrate, and multi-junction
a-Si cells [13].
Figure J, Light spectrum to reflection % [22] .
Efficiencies
Using a glass substrate, a single a-Si
cell has a efficiency of 5.54 % with a fill
factor of 0.54 and a open circuit voltage of
0.9 [23]. Efficiency for flexible solar cells
is 4.03% with a fill factor of 0.5 and a open
circuit voltage of 0.82 [23]. An 11
situations. Figure L shows 3 different types
of junctions.
Figure L, The physical difference of junctions [25] .
Graded Doping
Graded doping can be used to
Figure K, highest confirmed efficiency of solar cell
increase the electric field, which will help
for multiple materials taken at 1000W/m^2 at 25C
surface recombination and electron
[24] .
collection. Graded doping also increases the
amount of light trapped [13]. Since there is
Device design
The design of a-Si is highly
dependent on the application of use. Since
a-Si is easy to manipulate, it is an attractive
material because there are many types of
manipulations that can apply to different
more doping on one side of the cell then the
other, a larger variation of charge will take
place, therefore increasing the electric field.
However, this increased electric field will be
non-continuous because it is an a-Si material
and the doping is graded [22].
Figure N IV curve of graded doping a-Si cell [26] .
An example of a graded doping cell
is shown below in figure M. It is possible to
put a light grading on the junction as well as
Plastics
Using a method of plasma enhanced
the substrate, which could lead to an 18%
chemical vapor deposition (PECVD),
increase in the max current density (Jmax)
amorphous silicon can be deposited at low
[26]. The IV curve for the graded bandgap
temperatures of 100°C [23]. Using low
is shown below in figure N. It gives a fill
temperatures for depositing can lower costs
factor of .68 V, an open circuit voltage
and allow the solar cells to be deposited on
(Voc) of .8 V, which are good values for a
high temperature deposition.
materials that cannot normally be used due
to temperature constraints, such as plastics.
The thickness of plastic substrates
can be much thinner then glass substrates
while keeping their integrity [23]. Using
plastics can lead to thinner and more light
weight products. These thin-film plastics
allow for non-planar shaping and greatly
increases their ability to be applied in the
real world [27].
Figure M, Graded doping of a-Si cell [26] .
LED technology
[28] shows that a likely material to
be used for full-color flat panel displays in
the future is silicon based light emitting
diodes (LEDs). Silicon LEDs have fullcolor emission, complementary metal oxide
semiconductor compatibility, as well as
cheap fabrication and system feasibility.
The major issue with using silicon LEDs is
E(eV) = Ebulk + C/d^2.
the tuning of short wavelength emission
colors. To remedy the situation, a-Si is
where Ebulk is the a-Si bandgap, d is the
used. a-Si’s luminescence efficiency is
size of the dot, and C is the confinement
larger than crystalline silicon because of its
parameter. If you adjust the dot size to
structural disorder. It also has a band gap of
certain values, the intensity of the
1.6 eV, which is larger than the 1.1 eV of
photoluminescence will be at a maximum at
crystalline silicon.
the short wavelengths that are the issue, as
shown in figure P.
The a-Si is made into a quantum
dot. As the quantum dot size increases, the
photoluminescence energy decreases, as
shown by figure O.
Figure O, Varying photoluminescence with change in
dot size [28] .
Assuming that the potential barrier is
infinite, the energy gap E of the a-Si
quantum dot is
Figure P, Varying wavelengths due to varying dot
sizes [28] .
High temperature anneal technology
Another new idea has recently
New methods of implementation are
sprung up for improving the efficiency of
created every day. Placing this new
amorphous silicon based solar cells. In a
technology in places it will not be noticed
recent experiment conducted in Colorado,
but still work effectively has become a large
annealing was conducted in temperatures
part of the challenge to using solar cells.
ranging from 300-400 degrees Celsius. In
One example of implementation is
current situation, annealing is done well
below 300 degrees C. In the case where it is
using a semitransparent insulated laminated
glass and placing a solar cell of a-Si on it so
well below 300, the initial efficiency starts
that will not be normal noticed and can
out high, but quickly degrades, settling at an
generated a power of 42W/m^2 as seen in
efficiency of about 6%. The reason this
figure Q [6] .
happens is because there are light-induced
dangling bonds that form due to hydrogen
atoms separating from the silicon after light
is introduced [29].
The effect of having a higher
annealing temperature is that it decreases the
number of light-induced dangling bonds, but
at the same time also decreases the initial
efficiency. What this results in is a higher
efficiency after its sun soaking period. The
main draw back of amorphous silicon based
Figure Q, Application of a-Si solar cell in Germany
[6] .
solar is its low efficiency, so this could be a
very significant discovery for the future
[29].
Weight of a-Si
A power to weight ration of 275
mW/g can be obtained from integrated type
Glass solar cell technology
a-Si solar cells [17]. This light weight
property is something that makes a-Si a
highly desirable material. The average
without buying very expensive temperature
house hold uses around 1000 watts of power
resilient equipment. New technologies are
[6]. To give you a idea of how much weight
still coming out that are focusing on
of solar cells would be needed for each
increasing the efficiency and lowering the
house 1000 [W] / 257 [W/kg]=3.64 [Kg] (~8
cost of a-Si cells, with a goal of making
pounds) of a-Si solar cells to make a
them more competitive with respect to other
household run during the day at peak energy
materials.
demand hours. This shows that it is not only
possible for people to functionally live with
cells on their roof, but practical.
Two simple, and cheap, changes to
make a-Si more effective is to annealing at
around 250°C and pre-bake the transparent
In Iowa the average cost of energy is
film substrate. Both of these will lower the
.0729 [$/kWh] [30]. For each hour the solar
number of dangling bonds and lessen the
cell is active the amount of money people
need to add hydrogen to the material.
save is .0729 [$/kWh] * .257 [kW/kg] =
.018735 [$/h]. This may seem small for
each hour, but the savings will build up for
each hour the solar cell is working. The
money saved will also increase as energy
costs go up.
When humans move to the stars the
cost of going to space is coming from from
weight. Having a weight to power officiant
of 275 mW/g on earth, that will only go up
in space. The future of the human race is
space and the cost to get items into space is
Conclusion
With its ability to gain efficiency in
due to weight of each item. Then a-Si is the
best material for solar cells in space travel.
heat and its wide use in the world today, aSi may be the best choice of solar cell
Development for a-Si is not just for
materials. New technology allowing a-Si to
solar cells, but also LED lighting. The
be created at temperatures as low as 100°C
research into LED lights will only build on
can make it even cheaper to produce and
the understanding of a-Si making it a more
allow new companies to create solar cells
viable matiral for practical uses.
silicon more useful in hotter environments,
The future of solar cells is not
necessarily based on the efficiency of the
which is common for real world applications
of solar cells.
cell, but its cost to power output. If people
begin to buy a-Si solar cells and use them in
Beyond being able to efficiently
the public sector, (e.g. a-Si cells on roofs),
work in hot environments, it is also very
a-Si could shine. However, for this to
useful for harsh environments due to its
become a reality, a-Si would have to either
durability. Amorphous silicon is also used in
become cheaper, or become much more
many space program related projects since
efficient.
its light weight makes it ideal for many
space related needs.
Another way to boost a-Si is to use
graded doping and help increase the electric
field in a a-Si cell. This electric field
boosted by the graded doping will increase
The thickness of the a-Si cell is much
thinner than crystalline silicon. This smaller
the flow of electrons, and help improve the
characteristics of the cell.
thickness causes a decrease in the absorption
of incident solar rays. This can be remedied
As for the future of amorphous
by putting multiple layers of junctions on
silicon, outside of solar cell technology, is
top of each other.
that it will be largely used in LEDs for both
lighting and television sets. The a-Si is
The temperature dependence of a-Si
is very unique. For most solar cell
made into dots that will, depending on the
size, give a maximum photoluminescence at
materials, as temperature increases, the
short wavelengths that crystalline silicon
efficiency decreases. However, for a-Si, as
cannot emit. This makes it so the LED can
temperature increases, the efficiency will
flash in different colors and be highly
also increase. This makes amorphous
efficient.
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