Tutorial: Solar Cell Operation

[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.

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