Hence, the total current is given by
Note that the last step implies that the current density at V = V oc is zero. The current
density can only be zero if
which implies that the quasi-Fermi levels are horizontal in the entire band diagram of the
solar cell.
Figure 8.9 (b) shows the band diagram of the short circuited p-n junction. In this
situation, the photogenerated current will also flow through the external circuit. In the
short circuit condition the electrostatic-potential barrier is not changed, but from a strong
variation of the quasi-Fermi levels inside the depletion region one can determine that the
current is flowing inside the semiconductor.
When a load is connected between the electrodes of the illuminated p-n junction, only
a fraction of the photogenerated current will flow through the external circuit. The
electrochemical potential difference between the n-type and p-type regions will be
lowered by a voltage drop over the load. This in turn lowers the electrostatic potential over
the depletion region which results in an increase of the recombination current. In the
superposition approximation, the net current flowing through the load is determined as the
sum of the photo- and thermal-generation currents and the recombination current. The
voltage drop at the load can be simulated by applying a forward-bias voltage to the
junction. Therefore Eq. (8.23), which describes the behaviour of the junction under
applied voltage, is included to describe the net current of the illuminated p-n junction,
Both the dark and illuminated J-V characteristics of the p-n junction are represented
in Figure 8.10. Note that in the figure the superposition principle is reflected. The
illuminated J-V characteristic of the p-n junction is the same as the dark J-V characteristic,
but it is shifted down by the photogenerated current density J ph . The detailed derivation of
the photogenerated current density of the p-n junction is carried out in Appendix B.2.
Under a uniform generation rate, G, its value is
Note that the last step implies that the current density at V = V oc is zero. The current
density can only be zero if
which implies that the quasi-Fermi levels are horizontal in the entire band diagram of the
solar cell.
Figure 8.9 (b) shows the band diagram of the short circuited p-n junction. In this
situation, the photogenerated current will also flow through the external circuit. In the
short circuit condition the electrostatic-potential barrier is not changed, but from a strong
variation of the quasi-Fermi levels inside the depletion region one can determine that the
current is flowing inside the semiconductor.
When a load is connected between the electrodes of the illuminated p-n junction, only
a fraction of the photogenerated current will flow through the external circuit. The
electrochemical potential difference between the n-type and p-type regions will be
lowered by a voltage drop over the load. This in turn lowers the electrostatic potential over
the depletion region which results in an increase of the recombination current. In the
superposition approximation, the net current flowing through the load is determined as the
sum of the photo- and thermal-generation currents and the recombination current. The
voltage drop at the load can be simulated by applying a forward-bias voltage to the
junction. Therefore Eq. (8.23), which describes the behaviour of the junction under
applied voltage, is included to describe the net current of the illuminated p-n junction,
Both the dark and illuminated J-V characteristics of the p-n junction are represented
in Figure 8.10. Note that in the figure the superposition principle is reflected. The
illuminated J-V characteristic of the p-n junction is the same as the dark J-V characteristic,
but it is shifted down by the photogenerated current density J ph . The detailed derivation of
the photogenerated current density of the p-n junction is carried out in Appendix B.2.
Under a uniform generation rate, G, its value is