Welcome to
the PV online course in photovoltaics.
In this chapter we will deal with the question "How does a solar cell work?
In this teaching unit, the properties of semiconductors are explained.
As in the last teaching unit,
we start with the two-dimensional spatial
representation of a solid.
We first consider a metal lattice.
The core atoms, consisting
of the protons, neutrons and inner electrons,
are simply positively charged.
In contrast, the electrons of the outer shell
are not bound to an atomic nucleus.
They can move freely and are also called electron gas.
This results in very good conductivity.
The specific electrical resistance of metals is less
than 10 to the power of minus 5 Ohm cm.
The opposite is true for the insulator.
Here also the electrons of the outer shell
are bound to an atomic nucleus.
So the electrons cannot move.
This causes a very bad conductivity.
The specific electrical resistance in INSULATORS
is greater than 10 to the power of 7 Ohm centimeter.
The properties of a semiconductor ate
between those of a conductor and an insulator.
The specific electrical resistance of a semiconductor is between
10 to the power of minus 5 Ohm centimeter
and 10 to the power of 7 Ohm centimeter.
In the semiconductor, the electrons are partly "bound"
to atomic nuclei
Partially, however, the electrons can move freely.
Where electrons have detached from the atomic nucleus
and leave an atomic
hull, so-called holes are formed.
which can also move.
But what is a hole now
and how does such an electrical conduction through holes work?
An analogy to a parking garage helps us here.
Imagine you want to go shopping
in downtown Cologne or another city on Saturday afternoon.
The access to the upper parking deck is blocked.
In the lower parking deck,
all parking bays are occupied.
Out of sheer desperation, you stand in the access lane.
Other drivers get the same idea.
When you return from your shopping, the entire parking deck is blocked,
no one can move.
Everything is blocked.
You are at ground zero.
This also corresponds to the situation in a semiconductor
at absolute zero temperature. At
some point,
the parking management decides to release the upper parking deck.
This corresponds to the temperature
increase in the HL.
Individual cars can now enter the upper parking deck.
These cars can then play "James Bond"
and move freely around the upper parking deck.
But also on the lower parking deck cars can start to move.
The cars can now move from gap to gap.
Of course this is more more time consuming.
For this reason, the mobility of holes in a semiconductor
is about a factor of 10 lower than the mobility of electrons.
We now also want to understand, as announced,
in addition to the spatial representation of the charge
carriers, the energetic approach.
In the metal, the valence and conduction bands overlap.
But, since the conduction band is not occupied with electrons,
the electrons from the valence band can move freely here.
It is like a fully occupied parking deck with totally free
access to the upper totally free parking deck.
Of course, the drivers will drive to the upper
park deck, in this case. In
the insulator, on the other hand, there is a large
band gap between the valence band and the conduction band.
The large band gap corresponds
to the blocking of the upper parking deck
with simultaneous overfilling of the lower parking deck.
In a semiconductor ,
the band gap is smaller
and single electrons can (depending on the temperature) pass from the valence
band into the conduction band, leaving a hole
in the valence band.
If the band gap is larger than 4
eV, the material is called an insulator, if it is smaller than 4eV,
it is called a
semiconductor.
In a semiconductor, electrons can be lifted from the valence
band into the conduction band by thermal excitation.
They leave a hole with positive charge (here
marked in dark gray) in the valence band.
But also the energy of a photon can be sufficient
to transport a hole from the valence band into the conduction band.
The photon must have at least the energy
of the band gap before it is absorbed.
We now consider semiconductors with different band gaps.
The band
gaps are given at room temperature.
First, we will look at semiconductors
with comparatively low Band gap
like Si with a band gap of 1.12eV
or Ge with a band gap of only 0.67eV.
In such semiconductors with low band gap
a significant number of electrons
are in the conduction band at room temperature.
This is different for semiconductors
with comparatively large band gap.
Here, only a few electrons are in the conduction band at room temperature.
Typical semiconductors,
which are also used in PV
and have a higher band gap, are for example GaAs,
which is used for space solar cells. Or CdTe,
which is used for low-cost solar modules in very large
solar parks.
We have now mentioned
several time the energy unit eV.
But how much energy is ONE electron volt?
Well, in
electron volt there is once the Volt
and E - the elementary charge.
The elementary charge is 1.6 times
ten to the power of minus 19 seconds.
If you put this into the equation for the energy of a bandgap,
we get 1.6 times
ten to the power of -19 Ampere second times volt.
This is 1.6 times ten to the power
-19 watt seconds.
So a 1 eV is 1.6 times 10 to the power
minus 19 JOULE
While conductors have no band gap
and insulators have a very large band gap,
the band gap of semiconductors is less than 4 eV.
In semiconductors, electrons can be lifted from the valence band
to the conduction band by thermal energy or with the help of a photon.
The conductivity of semiconductors is strongly dependent on the band gap.
The energy of 1 eV corresponds to 1.6 times
10 to the power of minus
19 joules.
We have now understood electron and hole conduction and we got a feeling
for the energy of the band gap of different semiconductors
In the next teaching unit we will look at another way
to lift electrons from the valence into the conduction band.
Thank you for your attention.