[MUSIC PLAYING]
PROFESSOR: Hello everyone, today we're
going to learn how a Solar Cell is able to turn light
generated mobile charges into electricity.
Today's lesson will use everything
we've learned in the past videos to understand this effect.
So, make sure you understand the material
from the previous videos before watching.
First, let's go over the structure of a Solar Cell.
Here's a cell that I made.
And we can see that a metal ribbon is connected
to the top metal contacts, which form a grid.
The spaces between the grid lines
allow light to enter the cell.
If we flip over the cell we see the entire back surface
is coated with metal, which allows
easy extraction of charge from the back surface.
Additionally, we have another metal ribbon
that's connected to the backside.
Now, let's hook up our Solar Cell
to an ammeter to measure the current.
So, here we have an ammeter connected to our Solar Cell
and our light source which will simulate the sun.
And we can see that if we turn on our light source
we start to read a current flowing out of our Solar Cell.
In this case about 0.12 amps or 120 milliamps.
Now, if we turn off the light the output of the cell
drops to zero and we no longer read any current.
We know from our last demo the light
generates mobile charges and silicon.
But how do these mobile charges become a electric current
coming out of our Solar Cell?
The secret has to do with doping.
The top layer is doped with phosphorus, shown in blue.
Well the bottom layer is doped in boron, shown in red.
The different dopants interact in a way,
which we'll describe shortly, to create an electric field
in our device where the Boron-doped and
Phosphorous-doped regions meet.
It is this electric field that acts as a one way valve
in our Solar Cell for electrons and holes.
An electric field is created when
positive and negative charges are separated.
As you know, opposite charges attract and like charges repel.
We'll exploit this property by creating
a sheet of positive charges on the left and negative charges
on the right, thus producing an electric field, which
we denote with the Greek letter z.
If you were to insert a negatively charged particle,
such as an electron, into this field
it would move toward the positive charges.
Alternatively, if you put a positively charged particle
it would move toward the negative charges.
We're able to create an electric field inside our Solar Cell
by using different dopants on either side of the device.
Here we have our silicon lattice, which is un-doped.
We'll start by replacing some of the silicon atoms
with phosphorus atoms on one side.
On the opposite side we'll put in boron atoms.
To focus on our dopant atoms and the mobile
charges they introduce we'll fade out the silicon lattice.
Recall that phosphorus atoms introduce
static positive charges in mobile negative charges.
While boron atoms introduce static negative charges
in mobile positive charges.
All the mobile charges are free to move around at random.
A process known as diffusion.
Here we see a single electron moving around
on its random walk.
During this random motion if an electron and hole encounter
each other they neutralize and effectively vanish.
As this process of holes and electrons
randomly defusing and neutralizing
as the interface continues, the total number
of mobile charges in the device decreases.
This leaves a region at the interface
of immobile static charges where the net charge
is negative on one side and positive on the other.
These opposing sheets of charge create an electric field
of the interface, which at this point is very weak.
As charges continue to diffuse, they're
still able to move across this weak electric field
and neutralize.
As this happens, the sheets of net positive
and net negative static charges widen
and the electric field grows in strength.
Now that the electric field is stronger,
as other mobile charges continue to move and diffuse
around the lattice, they're now repelled
by the field and electrons to the left and holes stay
to the right.
It is this electric field that separates
light generated mobile charges and pushes them
to the extreme ends of the device.
The image we see now is our Solar Cell in the dark.
However, recall that our silicon atoms are still present.
And if light strikes our silicon atom,
a mobile hole and electron is generated.
As these mobile charges move around randomly
there's a chance that they will randomly
encounter the electric field.
The mobile electron will get repelled by the electric field.
However, the mobile hole will get swept to the other side
by the electric field.
Now, let's zoom out.
We can see that after the electric field has pushed
our light excited electron and hole to the left
and right respectively, we now have an extra negative charge
on the left and extra positive charge on the right.
If we connect a wire to short the two opposite sides
together the excess electrons are
attracted to the excess holes on the opposite side.
This attraction is what drives electricity through our wire.
As light continually shines on the Solar Cell
charges are constantly being pushed out of the device
and driving the electric current.
Now hopefully you understand the basics
of how these amazing, but rather simple, devices work.
We hope that this knowledge will provide the basic foundation
while tackling more difficult and abstract concepts while you
learn the material in this course.
I'm Joe Sullivan, thanks for watching.