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[MUSIC PLAYING]

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PROFESSOR: Hello, everyone.

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In our last demo,
we demonstrated

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how the electrical
conductivity of silicon

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can be changed by over
six orders of magnitude

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by adding dopants
that can increase

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the number of free or mobile
charges in the material.

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Today, we'll show how we can use
light to break electronic bonds

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and silicon, and create
free mobile charges.

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The principles
we'll be using today

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can be applied to
everything from sun screen,

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to of course, solar cells.

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We'll use the undoped, or
intrinsic silicon sample

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from our last demo, and measure
how the conductivity changes

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when we shine light on it.

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Our set up is identical
to that of last time.

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We'll take a piece of
silicon with metal contacts.

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We'll use an ohmmeter that we
connect to our sample via metal

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wires to measure
its conductivity.

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The measured resistance
will be determined

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by the conductivity and the
size and shape of our sample.

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Finally, we represent
the connectivity

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in terms of our measured
values and relate it

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to the number of
free, mobile charges,

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and the material
properties of silicon.

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We'll first measure the
conductivity of our sample

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in the dark, and then
shine light on our sample

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and see how the
conductivity changes.

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Our ohmmeter is hooked
up to our sample,

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and we measure a resistance
of around 120,000 ohms, which

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is equivalent to a conductivity
of around 0.0002 inverse ohm

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centimeters.

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Now, let's flip on the light.

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We can see that we measure
a slightly lower resistance

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of around 40,000 ohms,
but what is light

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doing to affect the
conductivity so much.

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Let's zoom in to the atomic
level and explore why.

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We see here a 2D representation
of a pure silicon crystal where

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all the valence electrons
form rigid covalent bonds,

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are immobile, and don't allow
the flow of electricity.

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This material
structure is identical

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to our intrinsic sample
when in the dark,

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which has a very
low conductivity.

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When light hits our sample,
photons of sufficient energy

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can break these covalent bonds,
injecting the formally immobile

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electron, giving enough
energy to move around.

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The mobile electron leaves
behind a mobile hole,

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which can move
through the crystal

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by swapping positions with
neighboring covalently bonded

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electrons.

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This explains why
the light increases

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the conductivity of our sample.

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Again, our conductivity
is determined primarily

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by the number of mobile charges.

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Light creates additional
pairs of mobile electrons,

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and holes, thus increasing
n and our conductivity.

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We've demonstrated that
light is able to generate

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fee carriers in our
ultra-pure sample.

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The same effect still
happens in dope silicon,

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but the light induced
change in conductivity

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only creates a small
relative change

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that we can't measure
using our ohm meter.

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Generating these
extra mobile charges

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by breaking covalent
bonds with light

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is the source of the
electricity that we eventually

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collect in our solar cell.

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In the next video, we'll explain
how these light generated

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mobile charges will be collected
and converted into electricity.

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I'm Joe Sullivan,
thanks for watching.

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