Welcome to the very first chapter in the module “grid connected PV-systems”.
In this chapter we will look at the basics and learn how solar cells work.
Since most solar cells are made of silicon,
we will use silicon as an example.
In this first lesson, we will first look at different atomic models.
First of all, let's take a look at the silicon atom
as it is represented in Bohr's atomic model.
In this model, there are discrete shells,
each of which has a certain number of electrons orbiting the nucleus.
The silicon atom has three shells.
There are 2 electrons in the innermost shell.
This shell ist fully occupied.
The second shell is also fully occupied with 8 electrons.
However,
there are only 4 electrons in the third shell –
if the shell were fully occupied, there would still be 4 more electrons.
A further development of Bohr's atomic model is the orbital model.
Unlike in Bohr's atomic model, the electrons here
do not move in circles around the atomic nucleus.
Instead, only a certain probability is given for each electron
with which it would be found at a certain location.
This probability is described by the orbitals.
The simplest orbital has the shape of a sphere
- it is the so-called s orbital.
For us, the dumbbell-shaped p-orbitals are also important.
They can have three different orientations.
The first shell of Bohr's atom model corresponds to an s orbital
in the orbital model.
We call it the 1s orbital.
The second shell consists of a 2s orbital and three p orbitals.
The third shell is constructed in exactly the same way as the second,
but for the sake of clarity we no longer
show it in the three-dimensional model.
For us, however, the shape of the orbitals
is only of secondary importance.
What actually interests us is the energy of the electrons.
We can assign specific energies
to the electrons of the different orbitals.
Similar to Bohr's atomic model,
only discrete energies are possible.
An electron bound to the nucleus can therefore only occupy certain energy levels.
The electrons in the s orbital of the first shell have the lowest energy.
Next comes the energy level of the s orbital of the 2nd shell
and slightly higher the energy level of the p orbitals of the 2nd shell.
It is the same with the orbitals of the 3rd shell.
However, there cannot be
any number of electrons in an orbital.
According to the so-called Pauli principle,
the electrons in an atom must differ
in at least one quantum state.
The only
quantum state in which the electrons can differ within an orbital
is the spin – in other words the sense of rotation.
It can take on exactly two values, denoted spin-up and spin-down.
Therefore, only two electrons can reside in each orbital.
Let's look at the energies of the electrons in a silicon atom.
The silicon atom has a total of 14 electrons.
The electrons always assume the energetically most favorable state in the atom.
This means that the shells in this representation
are filled from bottom to top.
As we have already seen in Bohr's atomic model,
the p orbitals of the 3rd shell are not fully occupied.
The electrons in this not fully occupied energy level
are called valence electrons.
All atoms strive to either fill up their outermost shell
or lose enough valence electrons to expose the next lower shell.
This state is called the noble gas configuration.
In order to reach this configuration,
atoms form a chemical bond with other atoms.
Let's look at this using silicon as an example.
Silicon has 4 valence electrons.
In order to reach the noble gas configuration, it lacks 4 electrons.
Therefore, it forms a bond - for example with other silicon atoms.
In a silicon crystal, each silicon atom is surrounded by
4 neighboring silicon atoms.
The silicon atom now shares one of its 4 valence electrons
with each of these neighboring atoms.
The other silicon atoms also share their valence electrons.
This type of bond - where two valence electrones are shared -
is called a covalent bond.
However, the single atom thinks that it owns both electrons
and thus has reached the noble gas configuration.
Such a bond, in which the noble gas configuration is reached
,is very stable,
the electrons are very strongly bound here.
Let's look at the 3-dimensional representation of a silicon crystal.
You can see that the neighboring atoms
do not arrange themselves in a flat plane around the silicon atom.
Instead, together they form a tetrahedron
A three-dimensional silicon crystal
is composed of many of these tetrahedra.
Now let's look at how the electron energies behave in a silicon crystal.
Here you can see again the energy levels
in a single silicon atom.
Because of the Pauli principle, there can only be two electrons in each orbital.
We now consider
the energy states of two silicon atoms
at some distance from each other,
both have the same orbitals with identical occupation.
However, if we bring them very close together,
their orbitals overlap.
The electrons of the two-silicon molecule
can no longer occupy
the energetically most favorable position because they are already occupied
and, based on the Pauli principle, must not be occupied twice.