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PROFESSOR: Hello, everyone.
Today we'll be taking a look at how light interacts
with the surface of a solar cell.
Right now I'm standing next to a solar module made up
of individual silicon solar cells.
If you look closely, these cells actually appear black.
And they appear black for a very important reason.
Solar engineers work very hard to make their solar cells as
efficient as possible.
Reflected light is lost energy, so good engineers
will want to minimize the total amount of reflected light.
To make solar cells absorb as much light as possible,
and appear black, solar engineers do two things.
First, they grow this very thin film
of a dielectric layer on the surface.
This layer is aptly called an anti-reflection coating.
Second, they texture the wafer.
And today we'll demonstrate how texturing is performed,
and quantify its enhancement for reducing light reflection.
Silicon wafers don't start out black.
In fact, they appear gray.
Polished wafers even look mirror-like,
and reflect quite a bit of light.
Here we see a polished wafer, which reflects around 1/3
of the light off its surface.
And to create the rough surface that reflects less light,
solar engineers immerse their silicon wafers
into a hot, wet chemical bath, which
helps create tiny surface features.
In this example, we use a solution
of potassium hydroxide, or KOH, which
is heated to around 80 degrees Celsius.
This violent reaction is actually
etching into the silicon, and carving out
little tiny pyramids on the surface.
And the result is a wafer that loses its shiny appearance,
and appears to have a dull finish.
The textured wafer is left with a surface that
is covered with microscopic pyramids, whose base
is around a micron, or about 1/50
of the width of a human hair.
It turns out that this wafer only reflects about 1/3
as much light as it previously did.
The reason this KOH which bath work so well at texturing
the silicon surface is due to the fact
that these silicon wafers are large crystals, which
in this case means that the atoms are formed in an ordered,
repeating pattern.
I have a model of the silicon crystal structure right here.
On our model, I've highlighted the surface in red.
Note that each silicon atom below the surface
is actually bonded to four other silicon atoms,
with four covalent bonds.
Note that the surface atoms are only
bonded to two other silicon atoms,
and it has two bonds that are unbonded.
The alkaline etch is able to remove silicon atoms more
rapidly, when they have fewer bonds holding to the lattice.
Hence, a solution quickly removes atoms on the surface.
Now if I were acting as the KOH solution,
I would remove all the atoms that
only have two covalent bonds.
Let's remove a few atoms.
So this one only has two covalent bonds.
It gets removed.
These three atoms on the surface only
have two covalent bonds holding them,
so I'll remove them as well.
Let's go ahead.
Now we can see that after we have removed a few atoms,
we have created some atoms below our original surface that only
have two covalent bonds holding to the lattice.
These atoms will also get removed by the KOH.
Now if we continue this process, it
would look something like this.
So what we're going to do is measure the reflectivity
of both a flat and a textured wafer.
First, we'll measure the flat wafer,
and use a laser pointer as a light source
to simulate sunlight coming from very far away point.
We'll shine light down on the surface,
and measure the amount of light that
gets reflected or bounced back away
from the surface, which we'll label as R.
So I'm standing next to our first experiment, which I just
outlined in the previous sketch.
We'll be using this laser pointer,
shining it onto this silicon wafer, and into our photodiode.
For those of you who don't know what a photodiode is,
it is a tool that can measure the amount of light
hitting its surface.
The current that's read off of this ammeter
will be proportional to the amount of light
hitting our photodiode.
To help visualize the beam path, we're going to use some steam.
Now if we turn on our laser pointer, hits our photodiode,
and we can get a good reading.
And right now we see that it's reading around 0.9 milliamps.
But how does this compare to a textured wafer?
Let's find out.
Now it's hard to measure the reflectivity
of a textured wafer with a laser pointer,
because it bounces the light off at several different angles,
and we can't measure the entire beam with a photodiode alone.
However, we can simulate what this would be like.
Let's go to our sketch board to show
how we can approximate this measurement.
To measure the reflectivity of a textured surface,
we'll create a 10,000 to one scale model.
We'll approximate our textured surface
using two pieces of silicon.
Again, we use a laser pointer as our light source,
and shine it on one side of the pyramid.
It'll bounce off that next surface, and off
the adjacent side, and then we'll
measure the amount of light reflected.
Let's go to our experimental setup.
So let's clarify our set up.
The two angled lines in our drawing, the adjacent sides,
on our atomic models, and the two angled, non-textured wafers
in our set up, are all representing the pyramid
structure of a textured wafer, just
like the one visible in our scanning electron microscope
image.
Let's use some steam to visualize the beam path.
Now that the steam has settled, we
can get an accurate reading of the reflectivity
of our modeled silicon surface.
And I turn on our laser pointer.
So now we can see that our photodiode
is reading around 0.33 milliamps.
This corresponds to around a 9% reflectivity,
which is quite a huge reduction from what
we had before, by about a factor of 3.
So in summary, today we learned how
silicon is etched using a KOH solution,
and how the resultant pyramids increase
the efficiency of our solar cells,
by reducing the amount of reflective losses
by a factor of 3.
If you found this interesting, please
watch our other solar demos to learn more about
how these exciting devices work.
I'm Joe Sullivan from MIT, and thanks for watching.
I'm out of here.
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