SOLAR CELLS James Laskey, Jessica Sirney, Elliot Taylor, Adam Villanueva, Andrew Zimmerman

Advised by Zac Gray

AUGUST 1, 2016 GROUP 4

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Group Members Page

James Laskey, Reading Area Community College

Jessica Sirney, Westmoreland County Community College

Elliot Taylor, California University of Pennsylvania

Adam Villanueva, Andrew Zimmerman, Millersville University of Pennsylvania

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Table of Contents

Group Members Page 1

Table of Contents 2

Grading Rubric 3

Feasibility Report 4

Index of Figures and Graphs 7

Group Statement 9

Objective 9

Renewable Vs Non-renewable Resources 10

Si PV Technologies 12

PECVD Altering structure of amorphous Si 25

TCOs, Electrode material, and how to pick them 30

Solar Cell Efficiency 33

NOlar Cells 34

Theoretical Solar Cell Plan 39

Alternate Solar Cell Technologies 56

Glossary 62

References 64

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Group 4: Silicon Solar Cell Grading Checklist: Zac

CHECK MARK FOR ZAC

LIST PAGE(S) IN REPORT

DELIVERABLE

TILE PAGE WITH DATE, DICLAMER Member page with pictures, first and last name.

This grading checklist Achieved feasibility report in italics, with current changes in bold (strawman)

Table of contents Index of pictures and graphs linked to references

Proper citation in document Objective stated in the introduction section

As a minimum 12 references; these must be referenced in the document and follow the standardized format. At least 8 of these references must not be from the internet.

25 words defined in glossary

Group statement on what makes an effective group project. A few well written sentences would be adequate.

Conclusion that states limit of success

Feasibility report and block diagram done at mid point Technical points

Discussion of renewable versus non-renewable energy resources

Discuss Si PV technologies: a-Si:H, nc-Si:H, poly-Si, and mono-Si. Compare physics, performance, cost, and applications.

Discuss and compare the device fabrication of a-Si:H, nc-Si:H, poly-Si, and mono-Si solar cells.

Table contrasting a-Si:H, nc-Si:H, poly-Si, & mono-Si PV cells.

Provide a detailed fabrication scheme for a thin film Si solar cell based on either a-Si:H, nc-Si:H or both (tandem). Your device must be unique and innovative. Use at least 5 characterization steps along the way to assess progress.

Discuss electrode materials used in solar cells: top contact, back contacts. Discuss TCO’s and how they could be deposited. Discuss what criteria go into selecting an electrode material.

Apply as much as possible learned in the lab to your fabrication scheme. Use real data obtained from various labs when fabricating your device (e.g. PECVD, ALD, RIE, Sputtering, Lithography, etc.)

Discuss the process-structure-properties-performance relationships of PECVD grown nc-Si:H and a-Si:H. How does changing the processing parameters (e.g. P, p, T, SiH4 vs. SiF4, etc.) change the microstructure of the nc-Si:H and a-Si:H?

Include at least 10 AFM images of surface morphologies of various solar cell materials (ALD AZO, PECVD SiNx, Sputtered metal, glass, etc.). Also include FESEM/optical if relevant. Ideally these will be from your own lab work from throughout the semester.

Discuss how solar cell efficiency is assessed.

Compare and contrast Si solar cell technology to other thin film solar cell technologies: provide similarities and differences.

Handout PowerPoint lecture on the presentation day. It should be 6 slides to a page and copies on both sides.

Report to be bound as shown in class.

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Achieved Feasibility Report (Strawman)

Renewable vs. nonrenewable energy sources: We plan to discuss the various types of

renewable and nonrenewable energy sources and their advantages and

disadvantages. This part was pretty easy to write about as an abundance of research

has been done on all the types of renewable and nonrenewable resources.

Si PV Technology: We will discuss the structure of various types of silicon used for PV

technology, their properties, efficiencies when used in solar cells, and various methods of

production. This section was relatively easy to write for, as many sources gave good

descriptions of the structure of the types of silicon, and how the structures influenced

the device performance.

Device Fabrication, different Si comparison: We will discuss a few production methods

for producing the four previously discussed types of silicon, namely PECVD methods

where applicable. This section did not have as much PECVD as we had originally

thought, as some types of silicon required higher temperatures to produce and could

not be done with other methods. That being said, this section was completed

successfully and gave examples of types of production for each type of silicon.

Electrode materials, top contacts/bottom contacts: We will discuss what TCOs are and

talk about a few of the more popular electrode materials and TCOs, including ITO and

AZO. We will also talk about different top and bottom contacts that will be used in our

procedure. This section was fairly easy to write about. We ended up following our

original idea of describing various TCOs and comparing them. We also talked about

the top and bottom contact materials used in our procedure.

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Process-Structure-Properties-Performance relationships: We plan to describe how

PECVD parameters will influence these qualities. The difficult part will be constructing

the table and gathering the resources for changing each parameter and its effect on the

solar cell. We examined the effect of changing the parameters of each processing

variable. The variables indicated whether the material would gain more or less of a

crystalline structure. The difficult part was that none of the resources had examined

changing all of the process parameters, so the variables changed with the source.

Solar Cell Efficiency: We are going to use a graph to show how different values of a solar

cell are found and how they are used in calculating a solar cell’s efficiency. We will

describe the graph and, also go into detail about what each value is and how it fits

together. We ended up pulling off our original plan of how to write this section. We

did end up having to make our own graph in order to showcase every value necessary.

A brief discussion of the most efficient and average efficiency of solar cells was also

included.

Compare and Contrast Si solar cells to other solar cell technologies: We plan to research

two different types of solar cells that do not rely on silicon as a semiconductor: cadmium

telluride cells, and copper indium-gallium diselenide/sulfur cells, covering their basic

structure, fabrication, bandgaps, efficiencies, problems, and in general comparing them

with Si based technologies. This section was very easy to complete; both cells are

widely being researched and there is a wealth of knowledge and literature on the

subjects. One could in fact write a whole paper on the variations on these types of

cells. This objective was easily met.

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Solar Cell Device Fabrication Scheme: We plan to come up with one of three types of

solar cells. One is a solar cell that would be cheap and easy enough to make in 3rd world

countries without much technology. One is a solar cell that mimics a plant’s

photosynthesis. The last is a solar cell that is more efficient because the top contact is

removed and only glass is between the Si and the sun. The top contact removal is the

most promising and probably the one that we will research as a group. None of those

ideas worked. Instead of removing the top contact, we discovered a way to make a

keyhole defect incorporated into the solar cell as an asset instead of a defect. By using

ALD, we could coat the keyhole defect, thus providing more conductive pathways.

Even if the keyholes are not attached to the top or bottom contact, they make the

whole solar cell overall more conductive, and in theory, increase efficiency.

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Index of Pictures and Graphs

Figure 1: Structure of amorphous Si

Figure 2: Multi junction of amorphous Si cell

Figure 3: Hydrogen flow rate VS Film composition chart

Figure 4: Multi junction microcrystalline cell

Figure 5: Hydrogen flow rate VS Deposition rate chart

Figure 6: PERL

Figure 7: Decline of monocrystalline Si thickness over time

Figure 8: TEM images of different Si structures

Figure 9: Roman spectra of thin film growth

Figure 10: Comparison of TCOs

Figure 11: Graph for calculating the efficiency of a solar cell

Figure 12: Sputter Al, then grow p-type and intrinsic Si

Figure 13: Design 1 of Ag wires

Figure 14: Design 2 of Ag wires

Figure 15: Robotic arm holding a shadow mask for Ag wire deposition

Figure 16: Formation of conductive n-type layer with tungsten

Figure 17: Keyhole defect

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Objective

The objective of this project is for us to create a new and innovative solar cell while

being informative about the research being done today. We will describe how solar cells work,

and the methods by which to grow different types of silicon for them. We will discuss

renewable energy. We will provide tables and charts to support our data along with references

to peer reviewed journal articles. As we describe our own solar cell, we will include pictures

that we made of many of the steps that we took. We will include process parameters and

explanations about why we chose to take certain steps. We will also include a small section

about the research we did and why some of our ideas would not work. Alternative materials for

solar cells will also be explained.

Group Statement:

To be an effective group, communication is a very important factor. If we all know what

the others are doing, we can successfully delegate and complete all the tasks assigned in the

project. A shared objective and timeline is also necessary to keep everybody in the group on

task and punctual. We had several group meetings and many conversations about the project,

it was a daily theme and we were always adding and removing ideas. Another part of making a

productive and successful group is to assign people to their strengths, which I believe was done

expertly for this project. The most essential aspect of a successful group is a shared drive for

everyone to complete the project and to do their best work.

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Renewable vs. Non-renewable Resources

Since 1975, the Earths global temperature has been increasing by about one degree every

decade, with scientists expecting it to hit close to a nine degree increase by the end of this

century if our habits do not change. This may not seem like much, but Carlowicz, an author and

scientist at NASA, informs us that when all it took in the past was an approximate one to two

degree drop to send us into the Little Ice Age, nine degrees is surely something to cause

concern (1). Even though things look bad as of now, there is hope. Alternative fuel sources have

been sprouting up everywhere, whether they be solar, or wind. Of course, there are a lot of

non-renewable resources out there, such as coal, crude oil, and nuclear power. Each of these

forms of fuel come with advantages and disadvantages. Coal is extremely cheap for electricity

production, making it viable for third world countries, but also very dirty and a contributor to

global warming. There is also crude oil, which is great because of how easy it is to handle. It is

also much easier to extract than coal and is relatively cheap for the masses. Conversely, oil is

very bad due to the fact that it can be difficult to initially obtain, the threat of oil spills is also a

problem, and lastly, for the same reason as coal, oil proves to be bad for the environment and

contributes to global warming. Especially problematic, it is currently estimated that there is

only enough crude oil left to last, approximately, another 60 years. This presents a dire need to

search out other resources. It is clear that these non-renewable resources are not set to last,

nor should they as they have adverse effects on the environment. This is why renewable

resources are to be sought after and need to have a greater attempt made towards them to

make them a more viable option. There are many renewable resources out there that are

worth mentioning, two of the most popular ones being solar and wind. Much like the non-

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renewable resources, they each come with their own set of advantages and disadvantages.

First, wind energy is produced by using blades to collect the kinetic energy produced by the

wind and turning it into energy to power our homes and businesses. One downfall to wind

energy, though it pales in comparison to the downfall of crude oil, is that it poses a threat to

nearby avian life. In addition to wind there is solar energy which comes in a few different forms

itself, such as thermal, and electric. With thermal solar energy, “electricity [is] produced from

sunlight through direct heating of fluids to generate steam for large scale centralized electrical

generation” (2). With electric solar energy, electricity is produced from sunlight through

photovoltaics. Electric solar energy is what will be focused on in this paper with respect to the

use of solar cells.

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Si PV Technologies

Amorphous Silicon

Physics:

Amorphous silicon consists of silicon atoms bound in a disorganized fashion when

compared to single crystal silicon. Most of the atoms in this crystal are bonded to three other

silicon atoms, and the ones that are not will have a hydrogen bonded to them. For this reason,

amorphous silicon is very easy to deposit on many substrates and requires much lower

temperatures, under 300oC, when compared to single crystal silicon.

In amorphous silicon (a-Si:H) exists an indirect band gap of 1.7-1.8 eV higher than that of

crystalline silicon, allowing these types of solar cells to absorb light ranging from 688 nm to

730nm roughly [1]. This property is also responsible for the large absorption coefficient greater

than 10^5 cm^-1 for photons greater than this band gap. With that said however, a-Si:H

exhibits a short minority-carrier lifetime, especially when doped and must make use of an

electric field. Most of the later discussed types of silicon will need to make use of the p-i-n

photodiode, so it will be assumed that this is true for most of the types of silicon [1]. This

electric field is established using a p-i-n photodiode which consists of an intrinsic layer of

amorphous silicon sandwiched between a p-type doped layer, and an n-type doped layer.

It should also be noted that the band gap causes the cell to exhibit a higher open-circuit

voltage allowing the solar cell to operate at a higher conversion efficiency. a-Si:H cells also have

their current limited by a smaller portion of the solar spectrum.

Performance/Applications:

The basic structure of a-Si:H is shown in Figure 1 to the left. a-Si:H cells usually operate

around 7% efficiency when produced in a single-junction manner for most commercially

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produced cells, but those constructed in labs will be around 12% and degrade over a span of

months to around 5-6% [1]. This degradation is due to the Staebler-Wronski effect. This

happens because when amorphous silicon is exposed to light, the electron hole pairs inside the

crystals will recombine. Large amounts of energy are released with this happens and can cause

hydrogen atoms bound to silicon to diffuse throughout the crystal, causing dangling bonds to

form [17].

The performance of these cells can usually be improved through various methods. One

such method is done by growing the amorphous layers in a manner that causes the

microstructure to be close to the nano-crystalline silicon region. This can cause the efficiency to

be stable around 10.1% [1].

Another method is using a

multilayer stack of several solar

cells. Such a cell is shown in

Figure 2 to the right. Since a

Figure :

Figure : (A)

Figure 2: Multi-junction amorphous silicon cell [1]

Figure 1: Image depicting the structure of amorphous silicon when the light induced degradation effect takes a toll on a solar cell [17]

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single cell will only convert a small range of wavelengths, these other cells will be tuned to

absorb other wavelengths of light. The absorbance spectrum and a cross section of one of these

cells is shown to the right, with the absorber layers having bandgaps from 1.1 eV to 1.7 eV using

amorphous silicon germanium cells. This can also reduce the degradation of performance for a -

Si:H cells by having regions within the solar cell that absorb higher energy light, and allow lower

energy light to enter a region where it would be less detrimental to the structure.

Alternatively, the cell could be annealed at 200oC to diffuse the hydrogen atoms back to

their original position. This is a quick fix, and the hydrogen will eventually migrate again.

Constantly annealing the solar cell over time could cause damage, so the aforementioned

methods are preferred [17].

Production Methods of a-Si:H:

Numerous production methods of amorphous silicon for solar cells exist, such as plasma

enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition

(HWCVD).

HWCVD essentially works by thermally dissociating silane gas on a tungsten filament at

temperatures over 1500 C. This method has advantages of higher deposition rates, a better

uniformity, and no dust/ion damage like PECVD methods [1]. The reason why HWCVD is not

used much is the thermal radiation from the hot wires causes the temperature control of the

substrate to be very difficult. For this reason, HWCVD is not as practical as PECVD.

PECVD deposition of a-Si:H works by using either SiH4, or SiF4 as well as hydrogen and

argon gasses. When depositing on a glass substrate it must be heated to 150oC, and the RF

electrode heated to 200oC at a pressure of 3.8 Torr. The flow rate for argon is held constant at

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88 sccm, and the flow rate for hydrogen would be held somewhere between 0-3.5 sccm with

the RF power held at 40 W [6].

More specifically for the gas ratios, there seems to be a spectrum as far as how much

hydrogen is present in the crystal, and whether or not the deposited layer is amorphous or

microcrystalline. Using a SiF4 flow

rate of 10 sccm, with an RF power

of 40 Watts, one can vary the flow

rate of hydrogen gas to observe this

transition [6]. At flow rates of

hydrogen less than 4.5 sccm, the

layer deposited constitutes

at least 90% of its volume as

amorphous silicon [6]. Figure 3

shown to the right illustrates this

transition.

For SiF4 flow, it does not appear that the concentration of SiF4 effects whether or not

the deposited layer is amorphous our microcrystalline, and that this is dependent upon the flow

of H2 only.

Cost:

The cost of amorphous silicon solar cells is hard to determine, as it is highly dependent

on the quality and the desired efficiency. But according to Solar Energy For Us, the cost per

watt for an amorphous silicon solar cell is roughly $0.45-0.53/watt. This low price helps offset

Figure : (A)

Figure 3: Hydrogen flow rate versus the film composition fraction, note the transition point at 4.5 sccm. This is where the deposited film transitions from amorphous to microcrystalline silicon [6].

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the low efficiency and lifespan of a-Si cells when compared to other more efficient designs.

These costs are only expected to fall as more and more companies start up and design more

efficient methods in which to produce these solar cells.

Microcrystalline/Nanocrystalline Amorphous Silicon

Physics:

Microcrystalline silicon is very similar to amorphous silicon in terms of structure. The

difference with microcrystalline silicon is that the structure is composed of a mixture of

crystalline and amorphous silicon. The ratio of crystalline and amorphous silicon depends highly

on the gas ratios used during manufacturing, which will be discussed later. This structure gives

µc-Si a similar band structure to crystalline Silicon, around 1.1 eV, allowing it to absorb red and

infrared light. Much like amorphous silicon, these types of cells will usually make use of a p-i-n

structure [10]. An advantage to using this type of silicon is that p-type and n-type doped µc-Si

have a much higher conductivity when compared to p-type and n-type doped amorphous

silicon layers.

Because this structure is not entirely amorphous, nor is it entirely crystalline, it is not

affected by light-induced degradations that the amorphous cells were prone to. This al lows for

these types of cells to have a much more stabilized efficiency.

It should also be noted that a major disadvantage for choosing µc-Si is its lower

absorption coefficient when compared to amorphous silicon. This requires the use of more

material, and much thicker layers [10].

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Performance/Applications:

As stated before in the paper, a major disadvantage for amorphous silicon is the

degradation of efficiency. Since µc-Si cells have a mixture of amorphous and crystalline silicon,

they do not experience this effect as much, thus allowing them to operate at their initial

efficiency for much longer.

For single-junction cells, microcrystalline cells can be produced with efficiencies greater

than 10%, with the highest confirmed efficiency of 10.8%. While this efficiency is stable, it also

comes at the cost of having to use a greater thickness film of µc-Si. In a practical application,

µc-Si will usually be used in a multilayer stack solar cell, combining amorphous, and sometimes

amorphous silicon doped with germanium. This would then allow the cell to have a broader

spectrum of absorption of 1.1 eV due to the µc-Si and 1.7 to 1.8 eV due to the a-Si:H regions of

the cell. a-SiGe would be used in between these layers, and can be used to cut the production

cost to 0.38 cents/Wp [1]. However, if the a-SiGe layer is replaced by a microcrystalline layer,

this can further decrease the cost of production due to the cheaper silane gas versus GeH4.

Figure 4: Multi-junction cell using microcrystalline silicon and an intermediate reflector [1]

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Such a cell is depicted in Figure 4 below, with its absorbance spectrum shown to the right. The

intermediate reflector is needed due to the low absorption of the top cell and aids in increasing

the absorbed light. These types of multi-junction cells have theoretical efficiencies estimated at

17% [1].

A multi-junction cell can be made using this type of silicon and can use the configuration

of p-type µc-Si, over intrinsic µc-Si, on top of n-type µc-Si, over ZnO, and the bottom most layer

being gold. This was done on both a SnO2 coated substrate, and a ZnO coated substrate with

efficiencies of 8.9% and 9.4%. The VOC for such a cell was 0.526V, with the fill factor at 0.71 [1].

Production Methods:

The majority of µc-Si:H films are deposited with PECVD techniques, at around 200oC and

a very high hydrogen concentration [3]. As stated before, there is a transition point where the

deposited silicon goes from microcrystalline to amorphous silicon. Usually microcrystalline is

flowed into the chamber at rates greater than 4.5 sccm. As shown in Figure 5 to the left, the

deposition rate for

amorphous/microcrystalline silicon increases

until the crossover point of 4.5 sccm of H2,

and falls slowly [3]. So one can assume that

the deposition rate for µc-Si:H will be

relatively unaffected by the concentration of

hydrogen.

Figure : (A)

Figure : (A)

Figure 5: Deposition rate versus hydrogen flow rate. Note that

the deposition rate only slowly declines after the crossover point of 4.5 sccm [3].

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In terms of practicality, this type of silicon is usually deposited right at the transition

point for PECVD when the deposited silicon goes from amorphous to micro/nanocrystalline.

This can give a solar cell with an optimal stabilized efficiency and absorption coefficient.

Depositing at this transition point is also optimal because you can not only deposit the most,

but you also will not waste as much of the hydrogen gas [3].

Similar to the deposition of amorphous silicon with PECVD, the same precursor, SiF4, will

be used as well as argon gas for bombardment. The main difference here as stated above is

that µc-Si:H has an increased concentration of hydrogen gas, and slightly increased

temperature compared to a-Si:H [3].

Cost:

The cost for microcrystalline solar cells is a little more complex due to the fact that most

cells that use this type of silicon, are a multi-junction cell. Thus the price is usually higher for

these types of cells. According to Solar Energy For Us, the price for these multi -junction cells is

usually around $15-23/watt, namely due to the complexity and various materials required. This

high price for many is offset by the longer lifespan, and higher efficiency when compared to

amorphous silicon solar cells.

Polycrystalline Silicon

Physics:

Polycrystalline silicon is another material of interest when it comes to silicon based solar

cell technologies. The structure of Poly-Si can come in two forms, randomly oriented small

crystals, or columnar oriented crystals. These individual crystals are usually referred to as

grains, and can vary in size. The individual crystals are usually monocrystalline in structure and

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the crystal structure of this is face-centered diamond-cubic. This gives Poly-Si a bandgap of 1.1

eV, similar to the other two types of silicon previously discussed.

Performance/Applications:

An interesting property about Poly-Si cells is that the performance is not due to the high

crystallinity of the grains, but of the grain boundaries. Thus, a Poly-Si cell with many small

grains rather than a few large grains can obtain a higher efficiency [4].

Because the size of the crystalline grains is directly related to the solar cell performance

one can fine tune the grain size to obtain a desired efficiency. For example, the grain sizes are

usually larger than the film thickness and can produce a single stack p-i-n type solar cell with

efficiencies around 4.4%. They can produce an open circuit voltage of 0.36 V, and a fill factor of

0.61 when produced with hot wire CVD, and can go as high as 15% when using direct thermal

CVD for production [2]. This is for a cell thickness of 1.2 µm. Thus, with a crystal size smaller

than 100 nm, poly-Si can create a higher VOC by using the p-i-n structure. It should also be

noted that the orientation of these grains with respect to one another does not have a large

impact on the performance, and relies mainly on the grain size. This is most likely due to the

hydrogen passivation at the grain boundaries [14].

Owing to the monocrystalline nature of these cells, they are less prone to the Stabler-

Wronski effect like the amorphous silicon cells were. This allows these types of cells to work at

a given efficiency much longer than amorphous cells, similar to the micro/nanocrystalline cells.

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Due to the nature of poly-Si, these cells once again will use a p-i-n configuration for a

solar cell. A single stack poly-Si cell produced by Yamamoto in 2000 had an efficiency of 10.7%

[7]. These cells had a VOC of 0.539 V, and the fill factor was not listed. This cell was able to show

that poly-Si is better in a single-junction cell, due to greater control of the back reflectors for

the films.

A generalization can be made for poly-Si cells as far as efficiency. Conergy, an Australian

solar cell company, produces many poly-Si cells with efficiencies around 15% [11].

Production:

Production of intrinsic, p-type, and n-type poly-Si can be done using a PECVD process

similar to those discussed for other types of silicon. The main difference here is the elevated

temperature to obtain the polycrystalline structure.

One PECVD method is to deposit an amorphous silicon film, and then anneal the film at

a high temperature, usually around 500 – 800oC, to convert the film to poly-silicon. An

important aspect to note about this process is that the annealing is usually done gradually [4]. If

the anneal was done rapidly, the hydrogen would accumulate at the interface with the

substrate, in this case glass, and could burst destroying the film.

The best cells produced using this method would give a VOC of around 0.420 V, and a low

fill factor of 0.55 [4].

Cost:

The cost of poly-Si cells can obviously vary greatly depending on the type of cell. But

according to PVInsights, a website that displays the market price for various types of solar cells,

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Taiwan produced poly-Si cells are priced at $0.26/watt [13]. This is up to date as of August 20th,

2016. These price continues to fall due to the gradual advances in this field.

Monocrystalline Silicon

Physics:

Monocrystalline silicon (c-Si) is the purest among these four materials. It consists of a

single crystal of silicon, arranged in a diamond cubic lattice. Like some of the other types of

silicon, the band gap here is 1.12 eV meaning that these cells are best for wavelengths around

1100 nm, in the infrared region of light [15].

The high purity allows single crystal silicon to produce some of the most efficient silicon

based solar cells. They also eliminate the need for a p-i-n structure, as most single crystal silicon

solar cells will act like a p-n junction to generate charge carriers.

It should also be noted that due to the lack of hydrogen present in the crystal,

monocrystalline silicon does not undergo the same performance degradation that some of the

other types of silicon do. Usually the loss of efficiency is around 0.5% per year of use.

While the above is true for intrinsic silicon, p-type crystalline silicon is more susceptible

to light-induced degradation caused by recombination of reactive boron-oxygen complexes

[15]. There are methods of reducing this, such as using a boron-doped magnetic-field CZ wafer

[15].

Performance/Applications:

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The best efficiencies reported as of 2010 for c-Si

can run up to 25%, with a VOC = 0.706 V and the fill factor

= 0.828 [15]. A c-Si cell with these properties is shown in

Figure 6 to the right. This type of cell, unlike the others,

does not utilize an intrinsic layer.

The cell discussed above is a passivated emitter

rear localized cell, where the rear contact has a

passivation layer an was produced in a lab. Realistically, commercial c-Si cells can reach

efficiencies of 16%, only limited by economic factors forcing high output production of low cost

cells [15].

Production:

Since these types of cells usually use single crystal silicon, it can be assumed that the

production of a c-Si cell will usually start with a silicon wafer produced using the

Czochralski process [15]. This is a very high temperature process as the purity required for

monocrystalline silicon is 99.9%. It can also be assumed that the production of the p-n junction

would be as simple as doping the top layer of a wafer to be p or n type, depending on the type

of wafer that the cell originated as.

Cost:

One of the largest factors holding back monocrystalline silicon PV technology is the

much higher cost associated with it, namely due to the higher price of monocrystalline silicon

wafers. Because of this, the price for a 156 mm c-Si solar cell is around $1.375/watt according

to PVInsight [13].

Figure

Figure 6:Passivated emitter rear localized cell (PERL). Note

the lack of an intrinsic layer of silicon contrary to other discussed solar cells [15]

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One of the most efficient manners of reducing the cost is to produce even thinner

silicon wafers. Sharp Coporation has done work to do this, and since 1997 the cell thickness has

decreased from about 375 µm to

around 190 µm in 2005 [15]. This

decline in price is shown in

Figure 7 to the left.

Table contrasting Si based cells

Table 1: Table comparing the four major types of silicon PV materials. These values can shift depending on the production method used, the ones listed are those obtained in labs. All these values were obtained using the research used to complete this portion.

Type of Silicon Lab

Efficiency

(%)

Common

Efficiency

(%)

Band

Gap

(eV)

VOC

(Volts)

Fill

Factor

Cost

($/watt)

Monocrystalline

Silicon

25.6 16-18 1.12 0.740 0.827 1.375

Polycrystalline Silicon 20.8 15-17 1.1 0.662 0.803 0.26

Micro/Nanocrystallin

e Silicon

11.4 8.9 1.1 0.535 0.698 15-23

Amorphous Silicon 10.2 6-10.2 1.1-

1.7

0.896 0.731 0.38

Figure Figure 7: Decline of moncrystalline wafer thickness overtime. This has had a direct effect on the price of monocrystalline silicon based solar cells [15]

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PECVD Altering Structure of Amorphous Silicon

Solar cells typically use two different silicon allotropes: nanocrystalline silicon (nc-Si:H)

or amorphous silicon (a-Si:H). Silicon thin films are commonly used in solar cells because the

films are easy to make and can be formed at moderately low temperatures. The main

difference between these two types of silicon is their structure. The structure of the material

influences the properties and performance of the materials in solar cell applications. Plasma

enhanced chemical vapor deposition (PECVD) can be used to create each type of sil icon by

varying the recipe parameters. By varying the parameters to form different types of sil icon,

solar cells can function properly and utilize the advantages of the desired allotrope.

As previously stated, nc-Si:H has a different structure than a-Si:H. Each allotrope of

hydrogenated silicon is porous due to the silicon-hydrogen bonds throughout the material.

However, nc-Si:H differs from a-Si:H because it is a type of amorphous silicon that forms nano-

sized crystallites to form large grains in the amorphous silicon. The two structures can be seen

below in Figure 8.

26

Figure 8 (a) X-TEM image of nc-Si:H structure. (b) X-TEN high resolution image of a-Si:H [A].

Clearly, the nano crystalline has a more crystalline structure at 2 nm and the amorphous

structure shows less crystal uniformity.

Although the crystal structure is not entirely different, the properties of the materials

change quite drastically across the two. First, the nano-crystalline structures of silicon are

commonly desired in solar cell fabrications because they provide better carrier mobility than a -

Si:H [B]. Since nc-Si:H has a more organized structure, the carrier mobility is increased. The

carrier mobility in solar cells is a critical factor in determining the overall efficiency of the cell.

Another advantage of nano crystalline silicon as opposed to amorphous silicon is its stability

under illumination [C]. After some time, amorphous silicon based solar cells lose efficiency due

to a phenomena known as the Stabler-Wronski effect. Using nano-crystalline silicon is an

effective way to reduce this phenomena. The downside to using nc-Si:H is the weak absorption

spectrum compared to amorphous silicon. The nano-crystalline silicon has a strong IR-red light

absorption but lacks in the rest of the UV and visible light spectrum. To fix this issue, the silicon

layers are made thicker in solar cells [C]. Since the two allotropes are amorphous, the materials

are also prone to impurities in the structure [D]. In some cases, solar cells are annealed to

minimize the amount of impurities on the material.

PECVD is the most common method of thin film silicon growth for solar cells. By

changing the recipe parameters in PECVD, the tool can grow either a-Si:H or nc-Si:H on a

substrate. The power, pressure, temperature, gas type, and gas ratio each have an effect on

which type of amorphous silicon will form. Table X.1 below shows how the changing of process

parameters will affect the growth of material.

27

Raised Parameter a-Si:H nc-Si:H

Power X

Pressure X

Temperature Depends on precursor gases

[H2]/[SiH4] X

He dilution with SiF4 X

Table 2- Effect of raising PECVD parameters on silicon allotropes.

The first altered parameter is the effect of increasing the power in a PECVD process. By

increasing the power, more electrons are free in the plasma. The free electrons are able to

break apart the silane molecules and allow the a nano-crystalline silicon film to form. In a 2016

study done by Elarbi et al, increasing the PECVD power created a more nano-crystalline

structure with the largest grain sizes at the highest power density (500mW/cm2) [E].

In another study conducted by Gope et al, PECVD of a-Si:H were run slightly above the

typical pressures (2-8 Torr). The grown films were thicker than typical thin films, thus the

pressure needed to be higher than usual. The process used a steady flow of precursor gases

(hydrogen and silane) but altered the amount of the inert gas, argon, to change the pressure.

To analyze when the silicon formed in a crystalline structure at the nanoscale, a laser Raman

spectra of the films were analyzed. A peak at roughly 520 cm-1 indicates the film is nc-Si:H

(Figure X.2).

28

Figure 9- (A) Raman spectra of thin films grown in PECVD system. (B) Crystalline volume fraction as a function of

pressure [F].

As shown in Figure 9, the nc-Si:H was grown between 2-4 Torr. As the pressure increased, the

PECVD system grew more amorphous material. Lower chamber pressure allows nano-

crystalline silicon to grow as opposed to amorphous silicon because the increased mean free

path allow the microstructure of the material to be more organized and structured when

grown.

When examining the effect of temperature on nc-Si:H growth, the precursor gas type

alters the desired temperature. In many cases, nc-Si:H will be grown using halogenated

precursors such as SiF4 to maintain Si-Si bonds in the film [G]. Therefore, SiF4 gas will be used as

opposed to silane in growing films. When using a halogenated precursor, a stronger etch

selectivity is present at lower temperatures. When using silane and hydrogen as precursor

gases, the etch selectivity is stronger at higher temperatures. Therefore, increasing the

temperature will allow nc-Si:H to grow [H].

Bruno et al also examined the addition of He to the SiF4 based plasma to analyze the

role of He as a diluent and how it may affect the Si structure. The results of this experiment

29

shows that increased amounts of He in the plasma improved the materials ’ crystallinity [H]. By

doing so, the He in the plasma also slows the etching rate. The increase of He gas in a plasma is

another way that changing PECVD parameters may influence the structure of silicon.

The understanding of a-Si:H and nc-Si:H was important in this unique solar cell design.

The effects of process parameters in PECVD aided selection of fabrication steps in the project.

The use of this information will aid in growing amorphous silicon allotropes with PEECVD

fabrication steps of growing the p-i-n junction in the solar cell.

30

Discussion of TCOs, Electrode Materials, and How to Pick Them

A lot of consideration goes into picking out materials for solar cells. Among those

materials are the electrode materials and TCO’s. These are quite possibly one of the most

crucial components on a solar cell as they are the transparent and electrically conductive

materials that provide the solar cell with its power. TCO’s are doped meta l oxides that are used

in opto-electrical devices such as photovoltaics. There are many desirable qualities that one

should observe when picking out which TCO would work for a desired project. Ideally TCOs

should be fully transparent in a wide range of wavelengths and they should also have metal-like

conduction properties. [3] TCOs are important as they are a happy medium for use in solar cells

when compared to either glass or metals as a top contact material. This is because glass has a

very high resistivity value, about 1016 Ω cm, meaning it has a very low conductive value. Even

though this is true, in return, it also has a very high transmittance value, approximately 96%,

allowing more light to pass through. Conversely, the use of metals as a top contact material

provide a very low resistivity value, about 10-6 Ω cm, giving it a high conductive property, while

also allowing absolutely no light through. This is where the use of TCOs come into play. TCOs

are a good mid-ground as most of them provide a relatively good transmittance value around

80%, while also having decent conductive properties. The properties of these values also

change depending on what kind of substrate the TCOs are deposited on. For the process stated

later on, glass was used as the substrate. There are a few particularly popular TCOs to consider

when making solar cells. Of them there is ITO, and aluminum doped zinc oxide (ZnO:Al, or

commonly abbreviated as AZO). ITO is a transparent and colorless thin film consisting of

approximately 90% In2O3 and 10% SnO2. It is a more popular material than AZO. In fact, it is

31

actually the most commonly used TCO in general. This is “because of its two key properties, its

electrical conductivity and optical transparency” [4]. There are a few major drawbacks to ITO

though, and those are: its price, its growing scarceness, and most importantly, its brittleness.

AZO is the other popular TCO and is the one that will be discussed in the solar cell design. AZO

consists of about 2% aluminum. It can be produced with most deposition techniques, but in this

paper the use of ALD will be discussed to achieve an extremely conformal coating.

Two other things that must be considered for solar cells are what the top and bottom

contact materials will be made of. In this paper the use of AZO as the top contact material will

be discussed, while aluminum will be discussed as the bottom contact material. The reasons

AZO was used as the top contact material and as the TCO in general, is because of a few

different reasons, one of which simply being because the parameters were already known. In

addition to that it was found that AZO can handle higher temperatures better than other TCOs,

such as ITO, and thus would be safer to use during some of the processing steps for the

deposition of other materials. AZO is also much cheaper than ITO and other TCOs [4]. Lastly,

“patterning of films by etching is easier [with AZO] than with ITO films” [4]. In fig 10, found

below, is a simplistic approach at showing the differences between major TCOs.

Fig 10. Comparison of different TCO qualities. [5]

32

In addition to the top contact, the bottom contact material also needs to be discussed.

The bottom contact material is simply a metallic electrode that is used to “collect the minority

carriers and produce an output current proportional to the intensity of the incident light” [6].

Aluminum was chosen for this purpose simply because it is very cheap and incredibly reflective.

This is good as it will undoubtedly be able to reflect the light at a relatively similar intensity. A

coating of 60nm of aluminum was found to be the best as it provides the most reflectivity [7].

As shown, the determination of what TCOs and what electrode materials are being used for the

top and bottom contacts are extremely important. It needs to be certain that all of the process

parameters will work well with each material and also that each material will work well with

each other. That is why AZO was chosen as the top contact material for this process and

aluminum was chosen as the bottom contact material.

33

Solar Cell Efficiency

Essentially, what solar cell efficiency boils down to is how much power is gotten out of it

from how much power goes into it. In a perfect world there would be 100% efficiency and a

solar cell would be able to produce as much energy as it gets. But to this day, records indicate

that the best overall cell efficiency achieved is around 46%. This was achieved in December of

2014 by using multi-junction concentrator solar cells, which are solar cells that consist of

different semiconductor materials to create multiple p-n junctions. Now, while even this 46%

efficient solar cell may not

sound great it is actually a

significant improvement on

modern day solar panels which

still only achieve about 15%

efficiency. As stated earlier, in

order to determine a solar cell’s

efficiency one must divide the

solar cell’s power output by it’s power

input. This formula of 𝜂 =𝑃𝑜𝑢𝑡

𝑃𝑖𝑛, where η

is the symbol for efficiency, expands out

to 𝜂 =𝑉𝑜𝑐∗𝐼𝑠𝑐∗𝐹𝐹

𝑃𝑖𝑛. In this equation Voc is defined as the open-circuit voltage, Isc is the

short-circuit current, and FF is the fill factor. Figure 11 to the right displays a graph of how these

values are found and how they are all connected. The fill factor is calculated by dividing the

value found for A by the value found for B.

Figure 11. Graph showing the different values used

for calculating the efficiency of a solar cell where the

x value is voltage and the y value is current.

34

NOlar cells

Within 1 week of the start of this project, three main ideas were discussed. The first was

to create a solar cell that would be very cheap and easy enough to make in a 3rd world country.

This idea, however, was not unique or challenging. The second idea was to make a solar cell

that would mimic a plant’s photosynthesis. While a few companies are researching this, it was

not feasible as the topic of this project because it was too challenging. The third idea purposed

in this project was to eliminate the need for a top contact that blocks any of the light coming

into the solar cell. If this could be achieved, then the top could just be covered in glass and 96%

of the sunlight could penetrate through the p and n junction, increasing efficiency. This idea

was unique and seemed attainable.

Figure 12 depicts the first plan which

included doing a flat column of

aluminum via sputtering with a

shadow mask, followed by the normal

sequence of p-type, then intrinsic

silicon via PECVD. After those were

laid, a wire would have to be placed

between the intrinsic and n-type

silicon. Placing the wire became the

Figure 12: Sputter Al, then grow p-type and intrinsic silicon issue with this idea. Many thoughts

were considered. After the intrinsic silicon was set, nanowires could be grown in a pattern on

top of the device. However, growing nanowires in a certain direction in specific placement

35

posed a challenge. Also, after the nanowires were grown, the solar cell would be exposed to air,

creating an oxide on the intrinsic silicon, so the cell would have to be cleaned using HF, and

then the n-type silicon could be grown. The addition of an HF clean added danger and extra

steps to the process that could be done without exposure. The key was to make this solar cell in

as little steps as possible to eliminate mistakes and lower cost while still achieving the goal of

higher efficiency.

Another way to deposit the wire would be to grow the column of aluminum, then p-

type, then intrinsic silicon, and then to perform liftoff to place silver in the shape of a wire

between the intrinsic and n-type silicon. Figures 13 and 14 show the wire designs created for

the solar cell. This was promising for a week or so, and then when the details needed to be

worked out, there were more problems. First, to perform liftoff, the solar cell would have to be

removed from the PECVD and exposed to the air. Again, having to use HF in solar cell

production increases price too much and adds too many steps. Also, the wire would block a lot

of photons from connecting with the p-type silicon underneath it. It was purposed that a

robotic arm could be utilized inside a PECVD cluster tool to sputter the silver wire through a

shadow mask. Figure 15 depicts this plan. This would not work, however, because a shadow

mask can only be used to make features in the macro scale.

36

Figure 13: The first design of

the silver wire implanted in

the solar cell

Figure 14: The second wire

design of the silver wire

implanted in the solar cell

Figure 15: A robotic arm

holding a shadow mask

to sputter Ag into the

shape of a wire.

At this point, there were two new ideas: first, if there was a way to make a channel

inside of the solar cell without removing it from PECVD, the metal could possibly be

37

electroplated into the channel. Second, since tungsten comes in a gas form, could it be

introduced with the n-type silicon making the top layer of the solar cell itself making it

conductive? Figure 16 shows this process. The second idea was quickly dismissed because to

grow tungsten, a tungsten seed crystal is needed. The tungsten seed crystal could be implanted

in a cluster tool without exposing the solar cell to the air, but the tungsten would only g row

where the seed crystals were, and this would lead to nanocolumns that block light rather than a

conductive n-type silicon layer. Also, a byproduct of n-type silicon and tungsten would be HF,

which could etch the glass.

Figure 16: PECVD of n-type

silicon and tungsten to make

a conductive top layer,

eliminating the need for a

top contact.

The first idea sat idle until a breakthrough occurred; a defect when using PECVD at high

pressure creates holes between columns rather than growing conformally. Instead of growing a

single flat line of aluminum and having a flat solar cell, aluminum columns could be deposited,

then the p-type and intrinsic silicon could be grown conformally while the n-type silicon is

grown non-conformally. This would create a keyhole within the solar cell that could be filled

with a metal. This is the idea that was selected to be the focus of the project and is shown in

Figure 17.

38

Figure 17: A view of the keyholes to be

filled with a conducting material.

Filling the voids with a metal was a difficult task to purpose. First, electroplating was

discussed, but there would be no way to make the metal travel through the voids all

throughout the solar cell. Second, planting tungsten seeds was discussed, but again, the

planting would require extra steps that should be eliminated to make a more profitable solar

cell. Also, tungsten is not very conductive compared to other metals. Finally, ALD was

suggested. ALD would coat inside the voids while also coating the top. This idea is the most

promising, even though a top contact is still created. The focus of the project shifted from

eliminating a top contact, to utilizing a catastrophic machine defect.

In conclusion, a lot of time was spent trying to figure out what kind of solar cell to make

for this project. The focus shifted a few times before finally settling on incorporating a defect

into the solar cell to improve it. All the research that was done to determine the final solar cell

and the parameters that would be needed is invaluable. One must try, and fail, before one can

succeed. One group, that is. All of the pictures and ideas discussed were made by Group 4.

39

Theoretical Solar Cell Plan

Introduction

Though many ideas were introduced, one idea stood out and could be feasible. It was

time consuming and difficult to come up with a solar cell in and of itself, and coming up with

the process parameters was even more challenging. Some of these steps have been performed

by themselves and were not done to make a solar cell. Those pictures are included in a section

of their own. This process as a whole to make a solar cell is theorized to work based on the

parameters. Sadly, the solar cell could not be brought to fruition in time for this project

presentation, however, someday it could happen. The following is an outline of the steps and

parameters needed to theoretically build the keyhole defect solar cell.

Step 1: Substrate clean and aluminum deposition

A glass substrate will be squirt cleaned on both sides with acetone, followed by a rinse

in IPA, and a rinse in DI water to clean the wafer of any and all impurities (see Figure 1). These

rinses will be followed by a dry with a N2 gun. After the N2 gun, a hot plate bake for 10 minutes

(or longer if deemed necessary) at 200˚ C will be used to remove any water that was not

removed with the N2 gun.

With the substrate clean, a shadow mask will be taped with kapton tape to the

substrate. The shadow mask will cover most of the wafer, except for two rectangles which will

be the bottom contacts for the two solar cells. The substrate will be sputtered in an RF

40

sputtering tool with 60 nm of Al (see Figure 2).

The power will be tuned to 100 W, with the idea that a lower power will give the film a greater

density, at the cost of more time. The operating temperature will be 300˚ C which will also aid

in increasing the film density. The base pressure will be around 10-5 Torr, with operating

pressure kept around 5 mT. The throttle valve and incoming Ar gas will maintain the higher

pressure of 5 mT. A starting estimate of the pressure of gas to be flowed will be 5 mT of Ar into

the chamber, which will change based on the the throttle valve parameters. After deposition,

the substrate will be removed from the chamber. This step is estimated to take approximately 1

hour.

The substrate clean parameters are standard, common knowledge parameters that

nano-technicians use frequently in the lab. The parameters of the sputtering are also relatively

standard, with a low power and high temperature so the film is more dense and thus more

conductive. This film is the back contact, which needs to be conductive. As such, Al was chosen,

due to it’s good conductivity. Al is also reflective and will reflect back any light waves that were

not adsorbed by the silicon absorbing layer on the first pass. The thickness of an Al film that is

the most reflective is 60 nm, and that is based on a table found in an article published by

41

Midwest Tungsten Service [A]. The Al will also bond well will the p-type Si that will be deposited

on top of it several steps later, as certain metals will bond better with n-type or p-type Si.

Step 2: Electron beam lithography

ZEP520A electron beam photoresist will be diluted with 2 parts anisole to 1 part ZEP and

spun at 2000 RPM on the substrate to obtain a 1500 Å thickness of the photoresist (see Figure

3) [B]. This 2:1 ratio dilutes the ZEP so a desired thickness can be reached in a shorter amount

of time. This will be followed by a pre-exposure bake of 170˚ C on a hot plate [B]. The wafer

will then be moved to an electron beam exposure tool to be exposed at 35 µC/cm2 and 20 keV

(see Figure 4) [B]. The exposure pattern will create dots in a rectangular grid only on the central

square

42

portions of the deposited Al (see Figure 5). The pattern will not take up the whole Al rectangles

deposited in step 1. These two unpatterned portions will later be exposed, allowing for a wire

to be attached so the cell can be connected to a circuit. The diameter of the dots will be 30 nm.

The spacing between each dot on the rectangular grid will be 500 nm from the

center of the exposed dots. The ZEP photoresist will be developed in n-amyl acetate for 3

minutes followed by a rinse with IPA. A post bake will not be used, as the next step is

electroplating. This step will take 2-4 hours, due to electron beam lithography taking a

significant amount of time.

The predominant reason for the parameters chosen in this step were based upon a

technical report on ZEP520A [B] and information provided by Zac Gray. This is relatively

standard process, so going with what is known to work was the most prudent decision. As the

43

resolution of typical optical lithography does not allow for submicron features, electron

lithography was used.

Step 3: Electroplating aluminum into ZEP holes

The next step is to electroplate 118 nm of aluminum into the patterned holes (see

Figure 6). An alligator clip will be pushed through the ZEP photoresist to the unpatterned

portion of Al to allow for an electrical connection to

form an anode. That clip will then be hooked to a DC

power supply so that the aluminum on the substrate

will be negatively biased. Another clip will be

attached to a plate of Al, and will be hooked to the

other side of the power supply so that the Al will be

positively biased. Then the two electrodes will be

submerged in a solution of diethyl ether, 2 M

anhydrous AlCl3, and .5 M LiOH, chosen according to

a novel hydride bath for electroplating Al developed

by Couch and Brenner [V]. Since this solution is extremely dangerous and pyrophoric, the

electroplating process must be conducted in an N2 purged glovebox. The exact amount of

current applied by the power supply will need to be experimentally determined, as the Couch

and Brenner experiment operated on the order of millimeters, not nanometers as this

application requires [V]. Thus, the time the sample is left in the bath will also differ from the

cited experiment. The ZEP photoresist will then be removed by a gentle O2 ICP plasma. The ICP

ashing recipe is as follows: a base pressure of 30 mT, 45 sccm O2 with 200 W power on the coil

44

and 45 W power on the chuck. This step could vary in time depending on the parameters. The

total time for this step should be one hour or less.

Step 4: PECVD to deposit p-type silicon in PECVD cluster tool

Note that while depositing doped silicon is not conventional for PECVD processes and

that the process is relatively dangerous due to the reactions occurring, it can be done if care is

taken [C]. In this case, this PECVD chamber (one of several in a PECVD cluster tool) is outfitted

with a mechanical pump, a turbo pump, and a cryogenic pump for this application. All are

corrosive type models and the volatile byproducts must pass through a high quality scrubber

before being released into the atmosphere. The base pressure needs to be on the order of 10 -8

or 10-9 Torr for this process. The goal of this step is to deposit 20 nm of p-type doped µC:Si

conformally on the substrate, covering all surfaces relatively equally (see Figure 8). The recipe

is as follows: 0.007 sccm B2H6, 0.7 sccm SiH4, 100 sccm H2, and 200 sccm N2 as a carrier gas [C].

The chamber will be pre-conditioned with these gases. The RF power used will not be a

standard 13.56 MHz supply; this power supply will be a 110 MHz supply instead [C]. The power

will be kept low, but high enough so that a plasma ignites; a good estimate is 100 W or lower.

The temperature will be kept at 250˚ C. This step should take approximately 45 minutes to 1

45

hour.

Generally speaking, PECVD is not traditionally done to deposit doped Si due to the

number of dangerous gases present in the chamber at one time and the potential contaminants

that can be ignited. However, it can be done and has been proven to provide thin films

according to Hollingsworth and Bhat [C]. The variation in parameters, specifically the ultra high

vacuum, was done in this step for safety purposes. The ratio of gases, the different RF power

supply, power, and temperature were also chosen according to the paper by Hollingsworth and

Bhat on depositing doped silicon in PECVD [C].

The reason why PECVD was chosen over ion implantation and the furnace method for

placing dopants was two different reasons. First, the furnace method would melt the Al metal ;

Al has a melting point of around 660˚ C, and furnace methods require upwards of 900˚ C for the

temperature. Second, ion implantation could destroy the delicate Al structures made in step 3.

Thus, the two standard methods were both eliminated and an uncommon way needed to be

discovered and devised. The PECVD was the next idea, and also seemed to be the best idea.

A p-type layer is essential to a solar cell and the cell will not function without it.

However, since, µC:Si is being used as the primary adsorbing layer, this p-type doped Si layer

does not need to be thick. It only needs to be thick enough to create a depletion region. Thus,

20 nm will suffice.

46

Step 5: PECVD to deposit intrinsic silicon in PECVD cluster tool

The sample will be transitioned via the robotic arm from the first PECVD specially

outfitted chamber to a standard PECVD chamber in the same cluster tool, while still under

vacuum. This chamber will be preconditioned to deposit typical µC:Si:H with a standard (13.56

MHz) RF power supply (see Figure 9). The recipe includes 60 sccm of SiF4, 28 sccm H2 (as a

diluent), and 500 sccm of N2 (as a carrier), all flown into the chamber. The pressure will be kept

at 5 Torr, and the temperature will be held at 300˚ C. The power will be held between 200 and

300 W. 470 nm of µC:Si:H will be deposited relatively conformally under these conditions. This

part of the process should take under 30 minutes.

The parameters described are typical for any PECVD deposition. Pressures for PECVD are

a few Torr, temperatures are generally between 100 and 300˚ C, and powers range from 100 to

400 W. The only difference here is that a usual Si deposition uses SiH4 gas. Here, for µC:Si:H, H2

gas is required, and instead of SiH4, SiF4 is used instead. PECVD is the best way deposit Si; PVD

and atomic layer deposition (ALD) can not deposit Si, and sputtering Si is less than ideal.

The intrinsic silicon for this tool will be the primary absorbing layer. As it is sandwiched

between an n-type Si layer and a p-type Si layer, this layer is able to generate electron-hole

pairs when sunlight is incident upon the material. The thickness of 470 nm was chosen because

47

this is the lowest wavelength of visible light. A thinner layer would have less efficiency due to

the thickness being outside of the visible light spectrum. Making the layer thicker, however,

would mean that our Al columns would need to be more widely spaced and much more

susceptible to damage. Combined with the thicknesses of the doped films above and below it,

the combined thickness should be adequate to handle visible light.

Step 6: PECVD to deposit n-type silicon in a PECVD cluster tool.

The sample will be transitioned from the second, standard PECVD chamber via robotic

arm while still in vacuum to a third, standard PECVD chamber within the same cluster tool that

has been conditioned with the recipe for n-type Si. Fifteen nm of n-type Si will be deposited

non-conformally so as to form keyhole defects. Typically, keyhole defects are undesirable, but

in this cell, the idea is to have keyhole defects that will be filled by a different process later on.

Similar proportions of gas to step 4 will be used, only replacing B2H6 with PH3 (see Figure 10).

Thus, 0.007 sccm of PH3, 0.7 sccm of SiH4, 100 sccm of H2 (as a diluent), and 200 sccm of N2 (as

a carrier) will all be flowed into the chamber. However, the pressure will be kept at 5 Torr

unlike the p-type Si doped layer in an attempt to make the keyhole defects. A lower

temperature of 100˚ C will be used as a way of attempting to make the keyhole defects, as well

as a lower power of 100 W.

48

The process should take 30-45 minutes. A theory may be that the 470 nm deposition of µC:Si

(step 5) will start to form a keyhole with an open top in the latter part of the process, and step

6 will finish this keyhole completely. After the gases have been evacuated from the chamber

and the deposition is stopped, the sample will be removed from the PECVD cluster tool.

This time, the recipe taken from Hollingsworth and Bhat was modified in an attempt to

make the keyhole defects. Similar gas ratios were used; however, the pressure, temperature,

and power were all changed in an attempt to make keyhole defects. Note the pressure change

from step 4 to this step; the base pressure of a typical PECVD is a few mT. This massive pressure

increase is what makes this step more dangerous than step 6. These parameters may need to

be tweaked in order to optimize safety during operation and to form the keyhole defects.

The deposition of n-type Si here creates the ability to generate electron-hole pairs in the

presence of sunlight through the creation of a depletion region. However, because this is the

top and the µC:Si:H is the primary absorber, this n-type layer cannot be too thick; doped

enough to create the junction, but thin enough to let most of the light through to the absorbing

layer.

Step 7: ALD aluminum doped zinc oxide

In this step, 100 nm of AZO will be deposited as a top contact. The concept is that the

AZO will not only function as the top contact, but will also fill in at least partially the keyhole

defects made in step 6 (and possibly started in step 5) if there is a path to the keyhole defects

on the side into the interior of the device. The design of this device should allow for the AZO to

grow inside all of the keyhole defects. The idea is that a TCO inside the keyholes will allow for

better conductivity, but also allow for unabsorbed light to pass through and still be absorbed by

49

the lower part of the intrinsic silicon layer. One supercycle of the AZO will consist of 10 pulses

of diethylzinc (DEZ), 1 pulse of trimethyl aluminum, and another 10 pulses of DEZ. In between

each pulse of metal there will be a pulse of water vapor. Twenty sccm of nitrogen gas will flow

through the chamber for the entire deposition to remove volatile byproducts (see Figure 11).

For 100 nm, a total of 37 super cycles will be completed. If the wait time in between each pulse

is 10 seconds, then one super cycle will take about 7 minutes. For 37 super cycles the

deposition should take about 4 hours and 19 minutes if the temperature of the hot plate in the

ALD tool is held at 200˚ C. However, due to temperature stabilization and conditioning of the

ALD chamber, the whole process will take approximately 5 hours.

A top contact is necessary for the solar cell circuit to be completed. One could deposit

thin wires on top as the top contact, but this reduces the amount of sunlight that can hit the

solar cell. Thus, a TCO like AZO was the only other choice. As an added benefit, the AZO layer

will hopefully fill the keyhole defects, aiding the conductivity of the Si layers by letting the

electrons move more quickly to the top contact, but also allowing more sunlight to reach the

absorbing layer.

50

The recipe used in this section was learned in the MSC lab at Penn State using the ALD

machine. Since the parameters were already known, it was the best choice for this solar cell as

time did not permit trial and error with other TCOs.

Step 8: Lithography and wet etching the AZO

To dictate the positions of the top contacts, standard positive contact photolithography

will be performed with Shipley 1827 photoresist. HMDS will be spin cast onto the sample,

followed by an HMDS bake of 60 seconds at 105˚ C. S1827 will then be spin cast onto the

sample. The dispense will be a dynamic dispense for good uniformity, and the final spin speed

will be a standard 4000 RPM (see Figure 12). The wafer will then be soft baked for 90 seconds

at 105˚ C.

The photomask used will be in the shape described in Figure 13, with the exposed part being

the area surrounding the L shape. The wafer will then be exposed for 6.3 seconds in an i -line

exposure tool (365 nm wavelength) such as the Karl Suss MA6/B6.

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The purpose of this step is to make the exposed photoresist on the substrate weak so it can be

removed. Once it has been developed in a TMAH developer (e.g. CD-26) for 55 seconds, only

the portion of the AZO that will become the top contact will be covered in photoresist (see

Figure 14). This layer of photoresist will protect that section of AZO from being etched in the

next step (see Figure 15). The wafer will then be rinsed with DI water.

52

After the exposed photoresist has been removed, the AZO will be wet etched by a 5%

diluted solution of HCl in DI water (see Figure 16). In this step, it is crucial that only the top of

the wafer will be etched so that no etchant can work it’s way into the side keyhole defects to

etch the AZO theoretically deposited in the defects. Though surface tension may prevent the

wet etchant from entering such small defects, the precaution will still be taken. As such, the

etching will be done by dipping a q-tip into the solution and wiping the top with the HCl. This

process will be repeated several times, as is deemed necessary. As there is approximately 100

nm of AZO present, it will take multiple passes for the acid to etch through this material. There

is no need to worry about over etching, as the Si underneath will be etched very little, if at all,

53

by HCl. HCl gives a strong vapor that is also capable of etching inside of the keyholes. To avoid

etching the desired areas, the substrate will be etched one section at a time, taking care around

the solar cell area, and quickly rinsed after each pass. When rinsed the substrate will be held in

such a way that the HCl will not enter the region of the solar

cell.

After the etching is complete, the areas around the photoresist will be tested with a digital

multimeter in many areas to be sure the AZO has been removed. The sample will then be rinsed

with DI water once more and dried with an N2 Gun.

After the HCl has been removed, the substrate will be rinsed with acetone to remove

the photoresist (see Figure 17). After the acetone has removed all of the photoresist, the

substrate will be rinsed with IPA, then DI water, and then dried with a nitrogen gun.

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The sample is not sonicated because a simple squirt with acetone will be sufficient to

remove all of the photoresist. The photoresist was not exposed to a plasma, so its removal

should be quite easy. Sonication combines the chemical removal of photoresist with the

physical bombardment of the photoresist, which could damage the pillars that were created in

earlier steps or even induce defects in the Si. The wet etch and photoresist clean parameters

are based on lab experience and advice from the advisor, Zac Gray.

Step 9: Dry etching the silicon

The Si that is exposed from step 8 will now be etched in a SF6/O2 dry etch in an RIE tool.

The AZO that remains on the substrate acts like a mask, as it does not etch in SF6 (see Figure

18). The ratio of SF6 to O2 should be 18:1 [D]. The base pressure for the RIE tool is 10 mT.

Exactly 36 mT of SF6 gas will be flowed into the chamber alongside 2 mT of O2 gas. A power of

300 W will be used to ensure that the etch is strongly anisotropic and does not etch under the

AZO block left behind from step 8. The full process will take approximately 45 minutes.

Finished Product

55

Once all of these steps have been followed, the completed solar cell should look like

Figure 19. If put to use, this solar cell has the potential to increase efficiency by optimizing

carrier collection and light absorption in the conductivity bands in the cell.

The shape of the solar cells was chosen (obviously) because of the group number that

created it. With this shape, two (or possibly more) solar cells could fit onto one glass substrate,

which is ideal for research and learning purposes. The photomasks would have alignment marks

to ensure that the 4 shape is created perfectly. All of the images in this section were created by

Group 4.

56

Alternate Solar Cell Technologies: Silicon Alternatives

Though silicon is generally used for the manufacturing of solar cells, there are other

potential photovoltaic systems that may satisfy humanity’s energy needs. But why invest in

silicon alternatives to solar cells if silicon is so naturally abundant? There are two main reasons:

cost and efficiency. For example, silicon wafers account for nearly 50% of the cost of solar cell

manufacturing [A]. Using a glass substrate (e.g. microscope slide) as opposed to a silicon wafer

would greatly decrease the cost of solar cells. Another manufacturing idea is to make the solar

cells in roller-based manufacturing systems, which would greatly reduce the cost of

manufacturing [B]. This can be done because many alternate solar cell technologies are

generally thin films that range from 1-2 µm in thickness [B]. The other main reason is efficiency;

alternate solar cells may be more efficient than silicon based solar cells. A measure of the

efficiency and cost together is in the measurement of dollars per watt peak ($/Wp) [C]. The US

Department of Energy says that the cost per watt peak of a solar cells needs to be $0.33/Wp for

solar cells to be efficient [C]. Si based solar cells have yet to even break $1/Wp. Two alternate

solar cell types stand out: cadmium tellurium (CdTe) and copper indium-gallium

diselenide/sulfide (CIGS or CIS if gallium is not included). In figure I, a graph of their efficiencies

is shown [A]. CdTes efficiency is higher than most Si based technologies other than a

microsilicon-amorphous Si hybrid. CIGS (labelled CIS in the figure), has efficiencies that are

higher than Si or CdTe based solar cells.

Cadmium Telluride Based Solar Cells

CdTe is a IIB-VIA compound semiconductor that was discovered by Frerichs in 1947 [D].

It has a direct band gap of 1.5 eV, which is perfect for photovoltaic conversion efficiency [C]. It

57

has also broken

$1/Wp, and has been

reported to have a

cost per watt peak

efficiency as low as

$0.85/Wp. As shown

in figure 1, it is just as

or more efficient than

Si based solar cells.

CdTe cells work

in a similar fashion to

Si cells. CdTe solar

cells are a diode that

generates an

electron-hole pair in

response to being

bombarded with

sunlight. Instead of

silicon based

p-n junction,

however, they use a

CdS-CdTe p-n junction, with CdS being the

Figure I: A graph of the efficiencies of different solar cells. The x axis is the model name/number, and are labelled according to the type of solar cell [A]

Figure II: Diagram of a CdTe based solar cell [A]

58

n, and CdTe being p [C]. See figure II for a diagram of the solar cell [A]. The primary absorber is

the CdTe, with the CdS being there specifically to create the p-n junction.

The CdTe cell is made by starting with a glass substrate and depositing a transparent

conductive oxide (TCO) [C]. In figure II, the TCO used is tin oxide [A]. Then, the CdS “window”

layer is deposited through solution methods [C] [E]. Then, CdTe is deposited through either RF

sputtering, close spaced sublimation, chemical bath deposition, electrodeposition, or screen

printing [C]. This naturally creates an alloy layer that has Cd, S, and Te in between the CdTe/CdS

layers. Then a back metal contact is deposited via either sputtering or physical vapor deposition

[C]. Note that this cells starts with the top contact and ends with the bottom contact.

There are drawbacks to CdTe solar cells. This is because cadmium is toxic and tough to

dispose of properly [A]. This also brings up environmental concerns as, if cadmium is disposed

of improperly, it can cause damage to the environment. The argument against this is that

cadmium is a natural byproduct of zinc mining, and that companies may as well put it to good

use rather than disposing of it as companies would other byproducts. In conclusion, CdTe cells

may be a potential way forward. If their efficiency improves and new technologies for their

disposal are invented, they may provide a real solution to sustainable solar energy.

Copper Indium/Gallium Diselenide/sulfide Based Solar Cells

Copper indium gallium diselenide/sulfide (CIGS) solar cells are another thin film solar

cell type. They have a direct bandage of 1.0-1.7 eV, which can be tuned depending on the ratio

of indium to gallium [E]. They have gotten up to 19.5% efficiency for small cells [A]. As shown in

figure I, CIGS cells seem to be a cut above standard silicon based photovoltaics [A].

59

As with both CdTe and Si solar cells, CIGS are a diode with a p-n junction. See figure III

for a diagram of a CIGS solar cell. As with CdTe, the CdS window layer functions as the n type,

and the CIGS functions as the p type [A]. However, there are other materials currently under

research as a replacement for the CdTe layer, including In(OH)3, In2S3, SnO2, Sn(S,O)2, ZnSe,

Zn(Se,OH), In(OH,S), ZrO2, ZnS, ZnO, Zn(O,S,OH), Zn(OH)2, and ZnInSe [F]. Also note that the

adsorption layer may be composed of copper and selenide/sulfide with only indium (CIS), only

gallium (CIG), an alloy of both (CIGS), or in alternating layers of copper indium selenide/sulfide-

copper gallium selenide/sulfide (also

CIGS) [E]. However, only sulfur or

selenide is used for the last element in a CIGS material [E].

CIGS cells, unlike CdTe cells, start with the bottom contact first. Molybdenum (Mo) is

deposited onto a glass substrate through electron beam physical vapor deposition [E]. Then,

the CIGS material is deposited. There are several ways of doing this: selenization of metal

layers, alternating sputtering and evaporating stacked layers (e.g. CIS-CIG-CIS-CIG etc), or co-

evaporation, which has two different evaporators in the same chamber working at the same

time [E]. Then, the window layer is deposited. If the window layer is CdS, it is deposited via

solution methods; if not, other methods are used [C]. The final layer is the top TCO; in figure II,

it is ZnO [A]. This could be doped with aluminum to make aluminum doped zinc oxide (AZO) or

Figure III: Diagram of a CIGS solar cell.

60

a grid of aluminum could

be deposited, then the

ZnO/AZO [B]. This grid does

have a tradeoff in that

some incoming sunlight is

adsorbed by the aluminum

grid, but the conductivity

the aluminum grid gives to

the overall cell can also

improve efficiency [B].

CIGS still have some problems. The first is their use of the CdS layer as a window layer,

which is the same drawback as the CdTe cells [A]. However, because CIGS is a different

material, the other materials listed as potential window layers (In(OH)3, In2S3, SnO2, Sn(S,O)2,

ZnSe, Zn(Se,OH), In(OH,S), ZrO2, ZnS, ZnO, Zn(O,S,OH), Zn(OH)2, and ZnInSe) could prove to be

much more environmentally friendly [F]. This removes CIGS from harming the environment,

which makes them much more attractive than CdTe cells. Another drawback is that CIGS

degrade over time if not properly sealed off from moisture [C]. This degradation could lead to

lowered efficiency, which is the reason why CIGS are attractive in the first place. They are also

hard to mass produce and commercialize due to the amount of indium that the earth has [A].

Indium is a rare element, and there may not be enough indium that exists on earth to totally

satisfy earth’s energy needs through CIGS solar cells.

61

In conclusion, cost and efficiency are the main reason any new product is produced. Si

may be abundant, but there are other sources of materials for photovoltaics emerging that

could possibly improve today’s solar cell. For now, Si seems to be the best option in bulk.

62

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