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[MUSIC PLAYING]

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PROFESSOR: Hello
everyone, today we're

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going to learn how a Solar
Cell is able to turn light

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generated mobile charges
into electricity.

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Today's lesson
will use everything

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we've learned in the past videos
to understand this effect.

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So, make sure you
understand the material

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from the previous
videos before watching.

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First, let's go over the
structure of a Solar Cell.

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Here's a cell that I made.

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And we can see that a
metal ribbon is connected

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to the top metal contacts,
which form a grid.

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The spaces between
the grid lines

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allow light to enter the cell.

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If we flip over the cell we
see the entire back surface

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is coated with
metal, which allows

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easy extraction of charge
from the back surface.

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Additionally, we have
another metal ribbon

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that's connected
to the backside.

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Now, let's hook
up our Solar Cell

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to an ammeter to
measure the current.

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So, here we have an ammeter
connected to our Solar Cell

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and our light source which
will simulate the sun.

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And we can see that if we
turn on our light source

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we start to read a current
flowing out of our Solar Cell.

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In this case about 0.12
amps or 120 milliamps.

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Now, if we turn off the
light the output of the cell

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drops to zero and we no
longer read any current.

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We know from our
last demo the light

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generates mobile
charges and silicon.

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But how do these mobile charges
become a electric current

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coming out of our Solar Cell?

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The secret has to
do with doping.

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The top layer is doped with
phosphorus, shown in blue.

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Well the bottom layer is
doped in boron, shown in red.

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The different dopants
interact in a way,

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which we'll describe shortly,
to create an electric field

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in our device where
the Boron-doped and

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Phosphorous-doped regions meet.

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It is this electric field
that acts as a one way valve

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in our Solar Cell for
electrons and holes.

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An electric field
is created when

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positive and negative
charges are separated.

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As you know, opposite charges
attract and like charges repel.

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We'll exploit this
property by creating

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a sheet of positive charges on
the left and negative charges

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on the right, thus producing
an electric field, which

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we denote with the
Greek letter z.

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If you were to insert a
negatively charged particle,

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such as an electron,
into this field

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it would move toward
the positive charges.

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Alternatively, if you put a
positively charged particle

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it would move toward
the negative charges.

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We're able to create an electric
field inside our Solar Cell

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by using different dopants
on either side of the device.

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Here we have our silicon
lattice, which is un-doped.

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We'll start by replacing
some of the silicon atoms

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with phosphorus
atoms on one side.

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On the opposite side
we'll put in boron atoms.

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To focus on our dopant
atoms and the mobile

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charges they introduce we'll
fade out the silicon lattice.

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Recall that phosphorus
atoms introduce

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static positive charges in
mobile negative charges.

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While boron atoms introduce
static negative charges

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in mobile positive charges.

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All the mobile charges are
free to move around at random.

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A process known as diffusion.

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Here we see a single
electron moving around

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on its random walk.

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During this random motion if
an electron and hole encounter

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each other they neutralize
and effectively vanish.

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As this process of
holes and electrons

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randomly defusing
and neutralizing

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as the interface
continues, the total number

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of mobile charges in
the device decreases.

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This leaves a region
at the interface

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of immobile static charges
where the net charge

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is negative on one side
and positive on the other.

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These opposing sheets of
charge create an electric field

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of the interface, which at
this point is very weak.

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As charges continue
to diffuse, they're

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still able to move across
this weak electric field

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and neutralize.

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As this happens, the
sheets of net positive

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and net negative
static charges widen

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and the electric field
grows in strength.

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Now that the electric
field is stronger,

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as other mobile charges
continue to move and diffuse

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around the lattice,
they're now repelled

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by the field and electrons
to the left and holes stay

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to the right.

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It is this electric
field that separates

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light generated mobile
charges and pushes them

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to the extreme
ends of the device.

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The image we see now is
our Solar Cell in the dark.

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However, recall that our
silicon atoms are still present.

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And if light strikes
our silicon atom,

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a mobile hole and
electron is generated.

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As these mobile charges
move around randomly

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there's a chance that
they will randomly

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encounter the electric field.

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The mobile electron will get
repelled by the electric field.

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However, the mobile hole will
get swept to the other side

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by the electric field.

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Now, let's zoom out.

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We can see that after the
electric field has pushed

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our light excited electron
and hole to the left

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and right respectively, we now
have an extra negative charge

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on the left and extra
positive charge on the right.

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If we connect a wire to
short the two opposite sides

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together the excess
electrons are

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attracted to the excess
holes on the opposite side.

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This attraction is what drives
electricity through our wire.

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As light continually
shines on the Solar Cell

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charges are constantly being
pushed out of the device

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and driving the
electric current.

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Now hopefully you
understand the basics

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of how these amazing, but
rather simple, devices work.

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We hope that this knowledge will
provide the basic foundation

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while tackling more difficult
and abstract concepts while you

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learn the material
in this course.

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I'm Joe Sullivan,
thanks for watching.