silicon vs dye-sensitized solar cells

7/28/2019 Silicon vs Dye-sensitized solar cells

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Introduction

In the search for clean and renewable energy sources that are also cheap and reliable, man has

scoured the earth looking for an answer. Though, I believe we should be looking up. The sun providesapproximately 1000 watts of power per square meter to the earth on a bright sunny day. 1 Plants have

been using this energy since the dawn of time, which begs the question, How do we do that?

The answer is photovoltaic cells, more commonly known as solar cells. Over the years, man has

come up with several types of cells that produce power directly from sunlight. Silicon, being the base

element in the most common solar cells, has led the industry for years in efficiency vs cost. 1 This

means, though there are cheaper cells and there are more efficient cells, silicon based cells are the

happy medium. But, there is an up and coming competitor that dares to challenge silicon for the crown,

and that competitor is known as the dye-sensitized solar cell. Though less efficient, dye-sensitized solar

cells are drawing lots of attention from researchers in the solar industry because of their low cost and

ease of manufacturing. And due to recent research, dye-sensitized solar cells have the potential to meet,

and even beat, silicon solar cell efficiency. 2 Also, this new research has found that dye-sensitized solar

cells can be produced in more environmental friendly ways but is believed it would drive up the

costs. 2.3

With these two competitors, which one will win out? Will increases in current cost halt the dye-

sensitized solar cell? Or, will the environmental impact of silicon manufacturing be a downfall?

I believe that even with increasing cost, dye-sensitized solar cells will rise to outperform silicon solar

cells and gain tremendous backing due to their low environmental impact during manufacturing.


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How silicon solar cells work

Understanding how silicon solar cells work can be pretty difficult without a decent

understanding of physics. But, an analogy can help sum up the general idea of how it works. Consider

two plates that are separated by some height. The bottom plate has a bunch of bowl like divots filledwith water. We want to water to get up to the top plate, so, we start throwing rubber balls at the pools of

water. When a rubber ball hits the water with enough force, the water splashes up onto the top plate.

From there, the water runs over to a gutter and down a pipe where it turns a small waterwheel. After the

water turns the waterwheel, it flows back into the bottom plate and refills the divots. We can continue

to get water on the top plate to run down the pipe and turn the wheel by simply continuing to throw

rubber balls at the water. 1

Using this analogy, we can understand how solar cells

work. The water in the analogy above are electrons. The plates

are the two sides of a solar cell and are two types of silicon that

has been purposely contaminated (doped) with boron in one

and phosphorus in the other. The phosphorus-doped silicon is

the bottom plate and the boron-doped silicon is the top plate. 1

Phosphorus has five electrons in its outer shell, silicon

has four, and boron only has three. Looking at the atomic

Illustration 1: Water analogy of how a solar cell works

Illustration 2: Silicon lattice structure(4).


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structure of silicon, we see a lattice of silicon atoms (white

balls in the illustration) connected (bonded) together by their

electrons (the blue lines). These electron bonds are not easily

broken, so the electrons are not allowed to move about freely, andtherefore makes silicon a very bad conductor, or insulator. But

what if we were to replace one of the silicon atoms with a

phosphorus atom, which has five electrons to share? The

phosphorus would bond right into place no problem. But now

there is the fifth electron just sitting there all by itself, not bonded

to anything. This electron can easily be knocked free by a little bit

of energy. Because there is an extra electron on the lattice, the

lattice is no longer electrically neutral. Instead it is electrically

negative due to the extra electron having a negative charge. So

we will refer to phosphorus-doped silicon as being the n-type,

meaning negative type of doped silicon. 4

Now that we have a free electron, we run into a small

problem. The electron has nowhere to go. So, we make another

lattice of silicon, this time replacing a silicon atom with a boron atom that only has three electrons to

share. Again, the boron fits right into place. But, instead of an extra electron, there is a hole where a

free electron wants to drop into. Similarly, because there is one less electron than a normal silicon

lattice, we get a positive charge on the lattice. We will refer to boron-doped silicon as being the p-

type, meaning positive type of doped silicon. 4

If we take our two doped silicon lattices and we connect them with a piece of conducting wire,

say copper, this gives the electron a bridge to move from the phosphorus-doped (n-type) lattice to the

boron-doped (p-type) lattice. But, the electron cannot do this on its own. Though it is not bonded to

Illustration 3: Phosphorus-doped silicon (4).

Illustration 4: Boron-doped silicon(4).


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anything, we still need to give it a little push to get it going. Enter the Sun! Light is energy, and it just

so happens that it is enough energy to get the electron moving from one plate to the other. And, if we

put an electronic device between the two lattices, the electron will do work as it moves through the

device. At that point, we have successfully produced power from light!

How Dye-sensitized solar cells work

Dye-sensitized cells, on the other hand, work a bit differently. Dye-sensitized solar cells, which

will be abbreviated by DSSC, were invented by Michael Gratzel and Brian O'Regan in 1991. These

DSSC use a photoelectrochemical process to produce electricity. DSSC are made up of several

essential components. The most obvious one is a photosensitive dye that gives off electrons when the

energy from light is absorbed. When the electron is given off, there then needs to be a conductive

material for it to travel through and connect to an electronic device. 5

Once through the device, the electron heads back to a part of the DSSC that has a special

material that, when the original electron was given off by the dye, gives one of its electrons to the dye

to replace the other electron. This is done by two processes called oxidation and reduction in an

oxidation-reduction reaction, or redox reaction. A redox reaction is defined as the transfer of electrons

from one species to another. 6 During oxidation an electron is given off to the dye. This creates a

hole for the returning electron to drop into, which is the reduction, thus completing a full redox

reaction. 5,6 Now the process is free to continually repeat itself as long as there is light shining on the

dye to give off electrons. Again, this is a very simplified description of how DSSC work, but provides

one with the general idea.


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Production of Silicon solar cells

For the production of solar cells, one will require an exceptionally pure silicon wafer (about

99.9999999% pure) to begin with and this is no exaggeration. Obtaining this level of purity is actuallyeasier than it may seem. In fact, the silicon itself does a lot of the work. A clever way of creating such

purity is by using what is called the Czochralski

technique. In this technique silicon is heated until it

melts into a liquid state. Once melted, a small pure

silicon crystal (or seed crystal) is put in contact with

the surface of the liquid silicon. And, once this seed

crystal comes in contact with the liquid, silicon

molecules will begin structuring themselves around

the seed crystal and use it like a blueprint to

continually form a crystalline structure until there is

no more silicon left to be added. Then the seed

crystal and the newly formed silicon crystal is raised

from the furnace where it was melted and taken out

to cool. Pictured to the right is a single silicon crystal

that has be grown using the Czochralski technique.

This boule, as they are called, weighs about 130lbs

and is just over 6ft tall. After the silicon boule cools,

it will be cut into thin silicon wafers that can be used in a vast array of electronic devices, or in our

case, silicon solar cells. 7

Illustration 5: Single silicon crystal grown by

Czochralski technique (7)


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Production of Dye-sensitized solar cells

Dye-sensitized solar cells are made of a few

accentual components that have been described previously.In standard DSSC's a ruthenium based dye is used to cover

a photoanode that is made out of Titanium dioxide (TiO 2)

nanoparticles. This is dyed TiO 2 plate is then placed on top

of an electrolyte that is responsible for the redox-reaction

described earlier. 8 But, the production of the TiO 2 are rather

difficult and somewhat costly to produce since there is

currently no major industrial foundation for which to produce such technologically sophisticated

nanoparticles. That is, until man looked to nature for help.

Diatoms

In just the last few years, researchers at Oregon State University and Portland State University

have found that microscopic single-celled algae,

called Diatoms, could be used to greatly improve the

efficiency of DSSC. 2 These tiny little creatures make

exoskeletal shells, called frustules, out of amorphous

silica found in sand on the ocean floor.

Because the diatoms themselves are so small, they are

able to produce structures in their shells that are

smaller than what we can see with a conventional

microscope and we must resort to a Scanning Electron Microscope in order to see their incredible

structural design.

Illustration 7: A group of Diatoms (8)

Illustration 6: Standard DSSC (8)


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When we start to take a closer look at the

diatom with the use of an S.E.M. Under relatively low

magnification, we can begin to see that there is atexture pattern to the shells of the diatom (illustration

8). But as we begin to zoom-in closer and closer, we

see that the shell is covered in tiny divots (illustration

9) that are arranged in a nice uniform linear pattern

across the entirety of the shell. But, there is even more

to these divots than currently meets the eye. As we continue to look closer, we see that there are divots

inside the divots of the diatom's shell (illustration 10). These tiny shells are considered nanostructures

and are incredibly intricate and beyond the manufacturing capability of current technology!

Illustration 8: Diatom under lowmagnification with S.E.M. (8)

Illustration 9: Closer look at the structure of a diatom shell (8)

Illustration 10: A look inside the divots of adiatom's shell (8)


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Diatoms and Dye-sensitized solar cells

What do these diatoms have to do with solar cells? Well, Gregory Rorrer and his lab team at

Oregon State University have found that, if insteadof being presented with silica to create their shells,

diatoms that are fed TiO 2 will construct their shells

out it. And, these diatoms, when living together in a

large colony, will build their shells together to form a

mostly continuous layer (illustration 11). Which, if

you recall, a layer of TiO 2 is an essential element in

the production of DSSC. 2,8

So, now that we have figured out that diatoms can construct nanostructures beyond our current

manufacturing capability, how then do we use them to make DSSC? Rorrer and his team found that

when diatoms live in a colony and produce these

continuous shell, that the shell can be removed from the

diatoms without harming them, since they only need it

for protection from predators which are not present in

the lab where they are grown. 2

Yet, this is still the the most exciting part about

the diatoms' shells. Where the shells really shine (no pun

intended) is that all those nano-sized divots reflect light

inside themselves multiple times due to the concave shape of the divots (illustration 13). This cause the

light to pass through the light sensitive dye allowing it to give more of its energy to the dye with every

reflection causing the dye to give off more electrons, thus creating more electricity! 8

Illustration 11: Continuous shell of a diatomcolony (8)

Illustration 12: Light reflecting multipletimes in divots (Howard)


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As you can see in illustration 13, with a diatom

created TiO 2 layer added to a DSSC the light

is able to reflect back through the dye and generate more

electricity. Since the DSSC produces more electricity per area, its production efficiency is greater than conventional

DSSC.

Solar cell efficiency

A huge factor that needs to be looked at is the bottom line. How efficient are silicon solar cells

compared to dye-sensitized solar cells? Efficiency is usually given in percent conversion. This means

that of the 1000 watts of energy per square meter the sun provides the earth, solar cells are only able to

convert a percent of that energy into electricity that can be used. So, a solar panel that covers one

square meter and is 5% efficient will produce 50 watts of electricity.

Looking back to when the first silicon solar cell was produced in 1954 it was reported to be less

than 4% efficient. In 1983, silicon solar cells were being reported to have 16.5% efficiency. Later in

1988 researchers and developers reported 21.4% efficiency. And, as of 2009, silicon solar cells have

reached an efficiency of 25%. 9,10

Dye-sensitized solar cells today do not share the same level of efficiency with only just under

10% efficient. Rorrer and team members ran experiments on conventional DSSC vs diatom DSSC to

compare efficiency ratings under identical conditions. Their results showed that a diatom enhanced

DSSC was over twice as efficient as a conventional DSSC! This giant leap in efficiency put diatom

DSSCs at 20% efficient, just 5% behind the silicon solar cell. 2,8

Illustration 13: Diatom created TiO2 in a DSSC (8)


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Thoughts and Conclusion

Looking back at the information that has been laid before us, one would easily conclude that the

bottom line is silicon solar cells just flat outperform DSSC. But lets look at a few more factors beforewe jump to a hasty conclusion.

First off, silicon solar cells have been around for over half a century. This has given researchers

ample time to study, experiment, and understand how they work while DSSC have only been around

for less than half the time silicon cells have. Along with this half century of existence came the

monstrously huge electronics industry to back it up. In this information age, industry depends highly on

ultra-pure silicon in order to make every electronic device that we today have come to be completely

engulfed in. 7 With such a strong industrial backbone, pure silicon crystals necessary for solar cells has

become common place in today's world.

DSSC, unfortunately, cannot afford the same luxury. They are confined to small laboratories

and university research departments. Yet, even with these restrictions on facilities and funding, DSSCs

have come a long way in the last 21 years thanks to the ground breaking research performed by Dr.

Gregory Rorrer and his colleagues at Oregon State University.

I cannot argue the efficiency and strong backbone that makes silicon solar cells the current

leader in photovoltaics. But, I would not count dye-sensitized solar cells out just yet. Though they are

lacking in a couple areas, they are fast on their way to a becoming a formidable rival that could result

in cheaper, more efficient green energy for future generations to come.


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