1.
Samantha Fisher
414 M
Fall 2015
(1)
The Bright Future of Photovoltaic Cells

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Explosions and large bubble letters are what first catches the viewer’s eye as
they scroll down their news feed on Facebook and other similar sites. The “Solar
Roadways” video is gaining recognition and views exponentially as it is shared to pages
and emails, an unusual topic to receive such a large following on social media, but is it
really that unusual? For years we as a people have recognized our dependency on
fossil fuels and the ever pressing knowledge that they’re running out, so it comes as no
surprise that we have supplied many efforts and much funding to the advancement of
efficiency in the storage and energy generation processes as well as alternate sources
of energy. ​For years we as a people have recognized our dependency on fossil fuels, and the ever
pressing knowledge that they’re running out. As a result, we have supplied much funding and focused
research on the advancement of the storage and energy generation processes.​ Some energy
generation alternatives being explored include wind power plants, solar energy, and
nuclear power, however are facing many difficulties in integration into mainstream. High
operation and repair costs are at the top of our list for these obstacles.​ ​Therefore in
order to improve these technologies’ we have focused a large amount of our time and
resources on their construction and efficiency.
One renewable energy source in recent years has pressed ahead of the pack
and received significant backing by the masses, solar energy, because of its efficiency,
unlimited power supply, and lack of harmful emissions. These reasons make it
environmentally friendly in our modern environment and health conscious society.
They’re also supported by the scientific community because of their material
components that are consistently being researched and are creating many new
opportunities. ​ ​This combined interest in solar energy allows for fast development and
research into this newer undeveloped field.
Solar energy is most commonly applied in the form of solar cells that are
traditionally composed of an anti­reflective coating, emitter, base, and rear contact all of
which is connected to an external circuit. The cell itself is a circuit, with the emitter as
the negatively charged plate and the rear contact as the positive plate (4).

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Figure 1 (4)
The electron energy transferred between the two plates occurs from electron
hole­pairs, which in lay terms means having the electron move between the valence
band energy and the conduction band energy levels. The electrons gain the energy to
move states from the light, or photons, they’re exposed to. This energy is then sent
through the circuit and collected on the connected external circuit. (4)
The cell can be made out of a number of different materials and​ ​multiple designs.
For example, the semiconducting material can be one of the many forms of silicon
wafers, which are the most commonly used, cadmium telluride, and copper indium
gallium diselenide (2). Because solar cell construction has so many options , a plaguing
issue of this alternative energy’s market penetration is rooted in the inability to decide
on the best cost and large­scale production efficient method​.​ It is because this
technology is on the brink of becoming integrated into everyday society that this
literature review focuses on the history of the technology, the many materials and

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design options, as well as future applications so that the general public may be informed
and knowledgeable.
Previously discussed was how, generally, a solar cell operates and collects
energy from the photons emitted from the sun, and from that explanation it is easy to
deduce the importance of choosing the correct materials for each part. In this particular
examination the focus will be on the emitter and base materials. The emitter and base
parts are very similar in their comparisons to semiconductors and their ability to carry a
current. Semiconductors, as their name suggests, are materials with an electrical
conductivity that lies between that of an insulator and conductor.
Figure 2
(5)
It also has a full upper level of electron energy levels and narrow band gap, which is the
distance between the upper and lower level electron energy levels. The narrower the
band gap of of a semiconductor the easier it is for electrons to move from the upper to
the lower electron energy levels, and visa versa (6). The easier the electron movement
is, the easier it is for excited electrons to break the covalent bonds in the material which
allows for more electron movement, or electrical current conduction (7,8). With this
knowledge one can see that the band gap size can play a significant role in how
efficient the solar cells operate. Alongside the importance of the band gap size is the
efficiency of the semiconductors, especially in the cases of large solar panels, and

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therefore it is important to keep efficiency in mind when debating construction materials.
A comparison of these band gap sizes and relative efficiency is included in Figure 2.
Figure 3
Material silicon wafers
(crystalline)
Gallium Arsenide
(crystalline)
Copper Indium Gallium
Diselenide (Cell)
Cadmium Telluride
Band Gap 1.12 eV (9) 1.42 eV (10) 1.68 eV (11) 1.5 eV (12)
Efficiency
(13)
25.6 ± 0.5% 18.4 ± 0.5% 20.5 ± 0.6% 19.6 ± 0.4%
Another major aspect that may contribute significantly to the efficiency, cost, &
choice of material is the physical design of the systems. Discussed above was the most
common form of photovoltaic cells, composed of two semiconductor layers creating
electron hole pairs to create an electrical current. This type of semiconductor is called a
crystalline silicon cell and, as it sounds, is composed of two silicon parts that are
oppositely charged. Usually the top piece is negatively doped, meaning it is given
excess electrons to create a negative charge, while the other side is positively doped,
has an excess of holes resulting in an overall positive charge. This set up is used to
create an electric field within the cell that moves the electrons once they get excited by
the photon light energy (4).
Figure 4 (3)

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Next, dye sensitive cells. These cells operate using a semiconductor like before,
however only one semiconductor is used. Here, the semiconductor material is coated
with a light sensitive dye that separates it from an electrolyte. The process begins with
the light photons exciting the electrons in the semiconductor, then those electrons move
through the material and through the circuit. They are then reintroduced into the
electrolyte that surrounds the semiconductor. The electron is then transferred through
the electrolyte and reunited with the dye where the process begins again (14). An
illustration of the process can be seen below in Figure 5 (15).
Figure 4 (15)
Alternatively there are the thin film cells. These cells differ from the crystalline
silicon cells in their size and materials. In these cells, the semiconductor is composed of
thinly layered materials, such as those discussed above, which allow sheets of this type
of cell to be flexible. This also gives the added benefit of being more cost efficient
because the thin layers use a smaller amount of each material than traditional cells.
These thin film cells can utilize the methods of crystalline silicon cells with varying
materials or use the dye sensitive cells discussed above (16). An example of a thin film
photovoltaic cell can be seen below.

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Figure 6 (16)
There are also photovoltaic cells known as multijunction cells. One of the main
limitations of the traditional crystalline silicon and thin film photovoltaic cells is that their
absorbance only uses photons of energy equal to or greater than that of the band gap.
Here we can apply the equation of
where E represents the energy, h is Planck's constant, c is the speed of light, and
lambda is the wavelength of the photons being collected. With this equation you can
see that only a specific set of wavelengths will equate to the energy of the band gap, the
smaller the wavelength the larger the amount of energy. To allow more photons to be
collected the wavelength spectrum needs to be expanded, which can be accomplished
by including more materials with varying band gap sizes. In a multijunction cell multiple
semiconductor materials are stacked on top of each other in descending band gap order
(17). This approach allows for a much broader spectrum of wavelengths to be utilized.
Below are diagram examples to visually explain this process.

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Figures 7 & 8 (17)
Now that there is a foundational understanding of photovoltaic cell
semiconductors and cell design, what situations and scenarios depict which set to use​?
In some cases the design is limited to certain types of materials as was briefly described
earlier, in other cases it's the environment and cost that decides what shape the cell will
take.
In regards to environment, the temperature of the operating cell can dramatically
affect its efficiency. As is generally known, the warmer the environment, the more
atoms/electrons move or vibrate within a substance. The increased mobility of these
atoms and electrons allow for a greater electric current to occur within the device.
Hence, certain cell designs have optimal temperature environments that allow them to
operate at their greatest efficiency potential based on the electron mobility of the design.
A comparison of the devices discussed at can be found in Figure 8.
The major factor preventing the integration of this technology into the majority
consumer market is the cost of photovoltaic cells. Most individuals won’t invest in an
expensive conversion or set up of a new system when they have an alternative system
in place, the question becomes though, will installing at a high price this renewable
energy source out value the savings that can be currently accrued through not
converting? The answer for that question can be determined in a general sense by

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comparing the costs of the cells and their operations in comparison to the cost of
current methods of energy. If the price comparison of these two shows that long term
renewable energies such as photovoltaics is more cost efficient then it is advantageous
for businesses to convert over for monetary value, public relations (as environmental
preservation is a current focus in today’s society) as well as for environmental reasons.
Below is a graph of known and projected cost of kilowatt per hour trends of
photovoltaics compared to nuclear power
Figure 9 (18) Figure 10 (19)
.
The reason for the price decline of photovoltaics is due to the increased
availability and operation costs. As with any product, the more common it becomes the
cheaper it becomes, a general rule of supply and demand. Also, as the supply of
photovoltaic systems increases, so will the research in developing alternative, cutting
edge systems driven by the competitive market. Also, it is recognized that the initial

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photovoltaic power plant cost is rather costly, however the lack of fuel costs and the low
operating costs allow these plants to begin to pay for themselves as seen in the below
comparison of capital investment and operating costs.
Figure 11 (20)
The photovoltaic projections bring with them the question of how will photovoltaic
technology transform going forward. Currently there are many different designs and
alterations being researched. To start, there is focus on making the photovoltaic cells
more efficient
by expanding the spectrums they can accept. One possible solution is down
conversion which splits photons with energy greater than that of the band gap into
energies that perfectly align with the band gap. The desire for this arises because when
some photons with energies greater than the band gap match up with and electron hole
pair, the energy difference is lost as heat. Another possible solution in up conversion
which does the exact opposite. It combines photons with less than the band gap energy
until they align. Lastly there is photoluminescence, which fixes the issues arising in
inefficient collection of photons near the edges of the wavelength spectrum by shifting
these energies farther into the spectrum (21). Another method of improving the
photovoltaic technology is making the photon absorption process more efficient.
Currently a photon follows a path of multiple reflections within the semiconductor before

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being entirely absorbed. This path can be simplified and shortened by etching the
surfaces on a nanoscale (22). Lastly, the matter of cost can be addressed to affect the
current barrier that photovoltaics has to the general market. One possible solution to
curbing this cost is vacuum processing which produces pure uniform materials with the
added ability of producing these in complex multilayer organizations which may be
applied to the multijunction devices discussed earlier. In addition there is the process of
wet processing which utilizes the capabilities of microscale printing of these materials
into the desired system and comes with the added benefit of being relatively cost
efficient in it’s production (23).
Although the process of converting light to usable energy is similar across all
materials and designs, it is clear that the prospective environment determines and costs
dictate the type of photovoltaic cell used. The advancement of this technology is
progressing forward at increasing rates as the technology improves in its methods of
collecting the maximum amount of energy from a given light source and as it develops a
competitive market for itself. As society evolves and focuses on renewable energy sources
to satisfy our energy needs​,​ it is clear the vast effect that this technology will have can be
measured by the current research focus on improving this technology. The question
becomes then, will crystalline silicon photovoltaics remain our main source of solar cell or
will it be replaced by one of the more recently developed designs? I think that the answer to
this question lies in the increasing efficiency of the cells. All the cells have been modified to
accomplish more energy conversion per surface area, however it’s been noted that the
multijunction cells have pushed ahead of the pack since their development in efficiency
standards (24). This is an addition to the cost of production of the multijunction vs the
crystalline silicon difference backs the favorable aspects of the shift. The cost of the
materials included in multijunction production usually include gallium and indium. The cost
of gallium is significantly lower than the price of silicon, and indium is slightly more costly,
yet these two materials are used in significantly lesser amounts than the whole of silicon
which depending on design could result in a cheaper build (25). The increased efficiency
and possibility of cheaper production lends the idea that in coming years, as the use of

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photovoltaic cells increases, the popularity of crystalline silicon will be replaced by the
multijunction cells. Regardless the future of photovoltaics is going to pervade the energy
world and has a bright future, in both prosperity and literal light absorption.
Figure 12 (24) Figure 13 (25)

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