Page 48 - Advances in Renewable Energies and Power Technologies

4. The Solar Cells 21

FIGURE 1.17
Illustration of the photovoltaic effect.

The remaining part of the holes flows to the load in the outer circuit. The same hap-
pens with electrons where they accumulate partly at the n-side edge of the field re-
gion neutralizing part of it while the rest of the electrons flow to the outer load
circuit.
The eeh pairs generated outside the field region diffuse toward the field region.
Only the eeh pairs generated within a diffusion length from the edges of the field
region succeed to reach the field region. The eeh pairs generated outside these diffu-
sion regions are lost by recombination. This means that W p must be made L n and
W n must be made L p such that all the semiconductor volume becomes active.
Metalesemiconductor interfaces must form Ohmic contacts [13] which are char-
acterized by high recombination velocity or tunnel MS contacts. So, the eeh pairs
generated near these interfaces survive only for very short times as they recombine
there. So, the regions outside the diffusion regions, near the contacts, are dead layers.
Once the diffusing eeh pairs in the neutral regions reach the field region, they act
similar to those pairs generated in the field region. They will increase the accumu-
lated charges at the edges of the field region increasing the cell voltage and driving
more current in the outer load circuit. From the previous discussion, it is now clear
that a solar cell has a voltage across it and drives current in the load connected to its
terminal. It acts as a battery. Hence, the name PV generator.

4.2 THE IeV CHARACTERISTICS OF THE SOLAR CELL

As a source of electricity, the main performance of the solar cell is determined by its
IeV characteristics. The illuminated solar cell characteristics can be considered as a