As probably most people know
theres a lot of energy that comes
out of the sun. Thousands, and
thousands, and thousands of
times more than we need. And
we know how to use the suns
energy in a number of ways. We
can try to convert it directly
into electricity and that way of
using the suns energy is called
solar photovoltaics or solar
cells for short. Solar cells are
materials that can take the
energy that comes in from the sun
in the form of photons, and
convert those photons and that
energy into a different kind of
energy which is electricity. So
its converting photons into
electrons.
You can think of it like the
electrons are being pushed up a
hill in a way. Think about water.
You pump water up a hill and
you let it roll back down. And
as it rolls back down you can
turn something, you can do work.
And if you keep it up there,
then it stores the energy for you
until you want it. Now a solar
cell doesn't store the energy,
so for that you need a battery.
But what a solar cell does is its
constantly pumping. It's
constantly pumping that water
up the hill and allowing it to
flow back down. And the important
thing is that the pump here is
the sun and the water is
electricity. It is electrons.
And so as that flows back down
from the energy that pumped it
from the suns energy, flows back
down, we can do work.
So first of all you have the
active layer semiconductor and
then that has to contact metal.
So you have, sort of, two metals
on two different sides of this
material. One of them is on the
side that the sun is shining
through so you'd like that to
either not take up a lot of
area or be transparent so that
it is not blocking the light from
the active layer. Now once
you go out from the active layer
and the metal contacts then you
have the packagings. And the way
that we do that mostly today is
with glass. So this active layer
material thats absorbing sunlight
if you look at this material from
the point of view from the
electron, what it looks like is
that there are all these energies
that I can sit at as an electron
in the material. And then there's
this big, sort of, gap where I
can't go anywhere in that gap.
And that top, before I get to
that gap, is called the valence
band. That's the highest energy
level I can be at in this material.
And then the next level up,
above this gap, is called the
conduction band. To be an
electron that can be taken out
of the material I have to make
my way into that conduction
band. How do I put it up there?
I give it a boost in energy that
makes it overcome that gap.
You can imagine that what that
means is that as a minimum the
energy from the sun that I have
to have to generate electricity
with this solar cell, is going
to be equal to that gap. That
gap is a fundamental property
of materials.
So the main limitation is cost.
On average, its around a factor
of five more expensive than say
the electricity you get from
natural gas. There's another
challenge and that's storage.
So if we wanted, say, up to ten
percent of the electricity in this
country to come from solar PV
we could do that. If we wanted
it to be more than that, then
we're going to need to figure out
a way to store that energy
efficiently, at that large of
a scale.
There's a lot of interest in
getting off of glass because if
you can get off of glass then
you can make lighter panels.
We may be able to embed solar
cells into other materials.
Like into the tiles on your roof,
for example. Even into fabric.
They have these transparent
solar cells. And because they
are transparent they're not that
efficient. You think, "Why would
I want a transparent not that
efficient solar cell?" Well, they
have a demonstration where you
put one of their solar cells on
top of an amazon kindle - you
basically can't tell that its
there - but the kindle never
needs to be plugged in again.
Even from just ambient light
from the room, thats enough to
basically keep it charged. I
think we need to get solar into
peoples hands and into people's
products so that they can see
that its actually not that
complicated and it can be very
useful and ultimately it can do
a lot of good for the world.