So, sunlight to electricity - the photovoltaic
effect does it without moving parts, without
a chemical effect, without the need for heat.
Now, there will be heat generated because
it's unfortunately the bi-product. It's the
waste that happens in a conversion process,
but it doesn't require heat, it doesn't require
chemistry, it doesn't require moving parts.
And, that's why PV panels have this long life
because there's less to actually degrade compared
with usual batteries or photo chemical effects,
or photo electrical chemical effects. There's
no chemical degradation. So here we are..
I've summarised it by saying these are the
3 steps - you could say 4 but it's 3 steps
- and I've taken this cross section which
comes from the Murdoch University Australia
website because it's a nice picture
and it saves me drawing a new one. You'll
see lots of similar pictures. So, here's a
typical photovoltaic panel with a slice of
material which we'll come to in a minute,
an electrical contact on the top which is
gridded to let the light through. And on the
back surface, a complete opaque contact which
isn't shown well on this picture. Here's a
cross-section of what it looks like with some
of the explanation of what's going on. On
the right hand side you'll see that the effect
is basically that light is absorbed to produce
electrical charge. That's the basic photovoltaic
effect. If we did nothing else then that would
happen anyway. So, I'll pass round a piece
of silicon so you can see what the stuff looks
like. It's - I mean don't lick it - it's not
toxic but that's a lump of silicon. You'll
see it looks vaguely metallic but it isn't
a metal, it's a semi conductor - it's not
a full conductor. So what you're doing is
you're taking this material and while that
piece is being handed around it's absorbing
light from the room and it's generating electrical
charges. And its resistance will change but
that's not the photovoltaic effect, that's
just photo conductivity. Lots of materials
do that. The secret to having a photovoltaic
effect is you separate those charges by a
built-in electrical field. If you don't separate
the charges, they rapidly meet each other,
recombine and you're back to where you started.
So, sunlight generates a pair of positive
and negative charges. You can't generate just
one charge without having the other. They're
always generated together. And if you have
a positive charge and a negative charge, and
they're close together then they'll meet,
recombine and the energy will come out again.
In LEDs it comes out as light but in silicon
it comes out as heat. The nature of the material
is it's difficult to get it to emit light.
So, we produce charges, now we then have this
separation and it's the separation that produces
a voltage that we can measure across one of
the panels. The charges themselves are the
current that is then flowing. So, when we
talk about electrics and current, its a flow
of charge. Okay. Audience: "Is this doped?".
It probably is because it's - most silicon
is doped with boron or phosphorus anyway because
most manufacturers want it partially doped,
yes. But it will be a very small part and
the doped material looks the same as the un-doped
so it has that metallic appearance. So, we
generate pairs of charges which don't go anywhere,
but as soon as you apply the field that's
built into them then the charges will flow
and you've got a current and the voltage.
And we need both to do anything useful. So,
we need the contacts on the cell to pass the
charges into the load and that produces power.
And current times voltage is power.
