Tip:
Highlight text to annotate it
X
Now if you submit that to, if you shine light on that,
what you have done is you will then generate an electron
whole pair, and because of the asymmetry and the concurrent
field that is produced, the charges will separate
and they will be collected as photocurrent.
The photocurrent is always represented as being
a positive current.
Remember there are all these problems with physicists
and chemists and directions that currents go, but remember that
in a solar cell the photocurrent is a positive current.
Now this is an expression for the short circuit current
density that you can get out of that cell.
Now there are two things that are important about
this particular equation.
One is this function, bs of e, which is basically what is
the light density, the spectral photon flux density,
coming into the cell.
Now that matters, it matters whether you are on the ground,
there's one solar spectrum.
There's another solar spectrum if you are up in space,
outside of the earth's atmosphere.
We call that air mass zero, meaning there is no air
between you and the sun.
Normally when we calibrate standard solar cells,
we calibrate them at air mass 1.5 because unless you live
on the equator the sun is never really right over your head,
it's off here somewhere in the sky.
In fact, in Cleveland it's 41 degrees, actually,
above the horizon.
The other part of this equation is the quantum efficiency
of the cell.
All of these are a function of energy and then integrated
over the energy spectrum.
The quantum efficiency relates to the device itself.
It represents how well does the material absorb light,
how well do we separate the charges, how well do we
actually collect them.
Now there is a lot of physics that goes into those parameters.
We won't go into the details today but I wanted to at least
do a few critical things--Is it going, yes, it will go--and
at least give you a sort of general knowledge.
If you think back some of you may have had solid state physics
where you look at solid materials.
You can take one atom and it can have an energy structure.
If you take two atoms you will find there is a splitting
of energies.
If you take three, there are more splittings.
If you take n number of atoms in a solid, you have what we refer
to as a band characteristic for a semiconductor.
Here you represent the valence band and the conduction band
for a simple device that's pictured above.
These are the minority carrier densities for the whole,
the positive if you will, part of the current
and the electrons.
Now, there are all kinds of things in this, including things
like what is the range of the interaction, what is the width
of the region as we separate the charges, and that's called
the depletion region.
The idea being you sweep the charges out of that region.
Remember that, because that's a very sensitive region
for a solar cell.
If you want to make it die, put some impurity in the depletion
region and it dies very nicely for you.
These constants, fp1 and fp2, relate to the things like
the recombination velocity of the carriers in the bulk
material and on the surface, the diffusion length
of the carriers, and a variety of other things.
The Fermi energy, by the way, for those of you who haven't
gotten so far as solid state physics, is the level at which,
at absolute zero, all of the energy bands below it are filled
and all the energy bands above it are empty.
Now we are not going to operate at absolute zero and so life
gets a good deal more complicated in an actual
solar cell.
And this is a very simplified version that you can use
to identify the basic parameters that are of interest here.
One is the open circuit voltage.
That's as if you had the light generating capability and you
disconnected the load from the circuit.
The other is the short circuit current.
So this is the open circuit voltage right here, okay,
when the current is zero, and this is the short circuit
current maximum.
That's when you have the ends of the device shorted out so you
get the maximum amount of current.
The ratio of the area, if you are operating at some power,
perhaps your max power point, that's what the MP stands for,
the ratio of that area, the blue part relative to the red part
is what is called the fill factor of a cell.
Now if it were an ideal cell it would be one.
Nothing is ideal.
The efficiency, quite simply, is the ratio of the power density
on the surface of the cell relative to the power density
of the electrical output of the cell.
So it's how well you actually convert the sunlight
into useable electric energy.
Now it does depend on how bright the light is, and you can well
imagine why that's the case, because if you have more photons
you'll generate more carriers.
What's interesting about the solar cell, though, is that it's
almost entirely an effect of the current.
You'll see very little change in the voltage as you go,
and then the middle is one sun.
By one sun what we mean is the amount of sunlight at the radius
of the earth from the sun, okay.
When you go to two suns or five suns you are concentrating
the light.
When you're going away from the sun, such as if you were to go
to distant planets, you are now diminishing the intensity
of the light, and what you can see is, of course,
that it is the current that changes predominately.
Another effect, and this is what we measure for some real cells
by the way.
One of the things you can see here is that if you look at high
intensity, high temperatures the wire bangett materials do
better, and there is some logic to that if you look into all
the mechanisms that produce the current for a cell.
Likewise, since we do take missions into the sun.
Messenger just finished a mission into the sun.
We also take, well at some radius from the sun, we also go
out to the planets where not only is the intensity low but
the temperature is low as well, and you'll see that some
materials, such as silicon here, do not necessarily do very well
under those conditions.
So this is a picture, separated out, of what temperature
actually does to a cell.
If the temperature increases you get a higher current,
that's the red, but a slightly lower voltage.
If the temperature decreases, the current decreases,
but the voltage increases.
So those are how, and in general you lose more with the voltage
than you do with the current.
This is a comparison for you of those two spectra
I talked about.
This is what it looks like in outer space at the earth's
orbit, and this is what it looks like on the ground.