Welcome to the PV online course,
in this chapter we will cover the topic how a solar cell works.
With this teaching unit I want to explain to you
what happens when you put a p-area and an n-area together.
In a first step,
we look at a p-type and an n-type semiconductor, which are physically separated.
The holes in the p-region can move around freely.
This is also the case for the electrons in the n-region,
which do have a high mobility within the semiconductor crystal lattice.
On the contrary, the core atoms of the doping materials,
namely the negatively charged boron core atoms in the p-doped region
and the positive charged phosphorous core atoms in the n-region CANNOT move at all.
They are incorporated into the silicone-crystal lattice and are, hence, stationary.
In each of the
two areas there are as many positive as negative charges.
In sum, the areas are not charged.
They are electrically quasi-neutral.
We are now bringing the two areas together.
Since the electrons are free to move,
they will diffuse into the neighbouring p-region.
In the same way, the holes now diffuse into the neighbouring n-region.
The core atoms of the dopants, however,
remain stationary.
Due to the diffusion of electrons
from the n- to the p-region
at the edge of the p-area at the border to the n-area,
an area is created (marked red here)
which is no longer electrically neutral but negatively charged.
Conversely, at the edge of the n-area at the border
to the p-area, an area is created (marked red here)
that is no longer electrically neutral but positively charged.
The negatively charged zone at the border of the p-region and the positively charged zone
at the edge of the n-region together form the space charge region.
Due to the space charges
an electric field is created.
The electric field
creates a force opposite to that of the diffusion.
The charge carriers drift
back to the region where they are majorities.
The electrons drift back to the n-region and the holes back to the p-region.
As a result,
there are hardly any free charge carrier
in the space charge region, as they are immediately
transported away from the area by the electrical-field.
As a consequence,
majority charge carriers are depleted in the space charge region,
so this region is called the depletion region
or depletion zone.
We now consider The currents of the electrons at the p/n junction.
The diffusion current density of the electrons
is determined by the electron concentration gradient,
so it flows in the opposite direction to the gradient, hence the negative sign.
The proportionality is the diffusion constant
and since we are dealing with charged particles,
it must be multiplied by the elementary charge.
The field current density of the electrons is proportional
to the electron concentration and the electrical-field.
In addition, both values must be multiplied by the mobility
and elementary charge.
In thermal equilibrium, which we consider first,
field current and diffusion current are again
exactly equal.
The same applies
to the currents of the holes at the p/n-junction.
The diffusion current density of the holes is also determined by the electron
concentration gradient, but it flows - as described in the last teching unit -
in the direction of the slope
of the electron concentration, hence there is NO negative sign.
The proportionality is again a diffusion constant
and since we are dealing with charged particles, it must be multiplied
by the elementary charge.
The field current density of the holes
is proportional to the HOLE concentration and the electrical-field.
In addition, both values must be multiplied
by the mobility and elementary charge.
In thermal equilibrium, the field and diffusion currents are exactly equal.
What did we learn in this teaching unit.
A p-n junction is formed by bringing a p
and an n semiconductor into contact.
A depletion zone is formed by diffusion of majority carriers
into the opposite region.
The immobile space charges
of the core atoms of the dopants in the depletion zone cause electrons
and holes to drift against the direction of diffusion.
In equilibrium, the drift and diffusion current
of electrons and holes exactly balance each other.
In the next 2 teaching units we will have a look on how the p-n
junction enables a diffusion voltage and thus the operation of a solar cell.
Thank you for your attention.