Welcome everyone to the PV online course.
So far we have looked at the behavior of a pn junction
in thermal equilibrium in the dark.
In this lesson,
we will look at the pn junction without thermal equilibrium.
We will take a closer look at what happens when we apply
a voltage to the pn junction or when we illuminate it.
First, however,
we consider a p/n junction in the dark,
which we short-circuit.
If we now look at the diffusion and drift current densities of the holes
and of the electrons, we can see that all currents are exactly equal.
The charge carrier currents are in equilibrium.
So, in the dark, there is no net current flow
in the short-circuited pn junction.
We recall that the drift current moves the minorities in both regions,
that is, electrons in the p-region and holes in the n-region.
It counteracts the diffusion current.
The diffusion current, on the other hand, moves the majorities,
that means, the electrons in the n-region and the holes in the p-region.
We now apply a voltage source with approximately
0.6V to the p/n junction so that the negative pole
is connected to the n-region and the positive pole to the p-region.
At this polarity we call the bias voltage forward voltage.
The forward voltage creates an electric field which counteracts
the electric field created by the p/n junction.
This reduces the electric field of the p/n junction
and the width of the depletion region is narrowed.
Let us first consider
what this means for the drift current densities.
As the field E decreases,
the drift current densities of the minorities also decrease.
For the diffusion current densities the effect is opposite:
Since the width of the depletion region is narrowed, dx is decreased
and thus dn/dx is increased at the same time.
That means the diffusion current densities of the majorities increase.
In sum, holes are transported into the n-region,
where they are attracted by the negative pole and electrons
into the p-region, where they are attracted by the positive pole.
In sum, the forward voltage results
in a current flow in the forward direction.
As a reminder, the band diagram
of a pn junction in thermal equilibrium is shown again.
The minority charge carriers are transported
to the opposite regions by the drift current.
At the same time,
diffusion causes energetic
majority charge carriers to overcome the electric field and diffuse
into the region where they are minority charge carriers.
Now let us apply a forward voltage to the pn junction.
As we have seen, the electric field is reduced.
At the same time, the band bending is reduced.
The electric field causes the
electrons to drift from the p- to the n-region.
Since the electrons are the minorities in the p-region,
the drift current is very small.
At the same time,
energetic electrons diffuse from the n-region to the p-region.
Since the band bending is now lower,
more electrons have enough energy to climb
the potential barrier caused by the electric field.
The same is true for the holes, of course.
The difference between the two
energy levels equals elementary charge times
diffusion voltage minus applied voltage.
Since there is now an external voltage
applied to the semiconductor, it is no longer in equilibrium.
Therefore, the Fermi level is no longer constant across the semiconductor.
Far away from the space charge zone,
the Fermi level corresponds to the Fermi level at equilibrium.
We now consider the p-n junction
at a reverse bias of about 0.6V (in the dark).
The n-region is now connected to the positive pole and the p-region to the negative pole.
The electric field of this voltage
amplifies the electric field of the p/n junction.
In sum, the electric field increases
and so does the depletion region.
Due to the larger electric-field,
the drift current densities of the minorities increase.
The larger width of the depletion region decreases dn/dx
and thus the diffusion current densities of the majorities.
Overall, the drift currents dominate.
However, since the drift currents
are proportional to the minority charge carrier concentration
and these are very small, only a very small current
flow occurs overall.
This extremely small current flow is also called reverse saturation current.
Let us have a look at the band
diagram of the pn junction under reverse bias.
In contrast to forward bias,
the band bending increases here.
The minority charge carriers are transported
to the opposite region by the drift current.
However, there are very few
minority charge carriers, so the drift current is very small.
For the diffusion current, the picture is as follows:
since the potential barrier is now very steep,
there are hardly any
electrons that have enough energy
to be transported to the p-region by diffusion.
Although the drift current is very small,
it dominates and the reverse saturation current flows.
Again, the difference between the two energy levels equals
elementary charge times diffusion voltage minus applied voltage.
But since the voltage now has a negative sign, the overall voltage increases.
As under forward bias - under reverse bias, the Fermi level is not constant
across the semiconductor anymore.
If we illuminate the p/n-junction with light,
additional electron-hole pairs are generated.
The majorities (for example electrons in the n-region)
do not matter in each case.
However, the concentration of the minorities
increases by several orders of magnitude.
Remember the example with the princes and princesses.
Since the minority charge carrier densities
increase by several orders of magnitude, the drift current densities
of the minorities increase in the same way.
The electrons are transported
into n-region, the holes into p-region.
A significant current flow occurs.
This current flows through the circuit
even though no external voltage has been applied.
This is the short circuit current .
The current caused by
solar radiation is called photocurrent.
At full solar radiation of 1000W/m2, it is about 10 billion times larger
than the dark current in a Si solar cell.
Thus, the short-circuit current is approximately equal to the photocurrent.
It flows in the same direction as the reverse current in reverse direction.
Let us now open the circuit,
Additional electron/hole pairs are created by illumination,
the minorities are transported by the drift currents
and a voltage will built up at the open terminal
caused by the additional charge carrier pairs separated at the depletion region.
This voltage is called open circuit voltage.
It has the same polarity as a pn junction
operated in forward direction.
Let's look at the current flow in the band diagram.
The photoelectric effect significantly
increases the number of minority charge carriers.
The electrons are transported to the neighboring n-region
by the electron slide.
Since there is no change
in the band bending compared to thermal equilibrium,
electrons continue to diffuse in the reverse direction,
but they are few compared to the photogenerated charge carriers.
In sum, a significant current flows in the reverse direction.
Let's summarize: In thermal equilibrium,
drift & diffusion current densities balance each other out.
If we put the pn-junction under forward bias, the diffusion current densities
of the majorities dominate and overall
a current in forward direction occurs.
Under reverse bias on the other hand, the drift current densities
of the minorities dominate.
Since there are only a few minority carriers, the overall current is very small.
The resulting current is called reverse saturation current.
Finally, if we illuminate the pn-junction,
many electron-hole-pairs are created
and the minority densities
increase significantly.
The electron-hole pairs are separated
by the depletion region and transported to the electrodes.
A current much higher than in the dark
is flowing - the photocurrent.
Thank you for your attention.