44
A. Shah
Notes
1. These values apply to the first step only. The second step will result in further
losses. The plot for the overall efficiency (first and second step) will be given
later, in Sect. 3.5.1 (Fig. 3.19).
2. Furthermore, these values correspond to the case of a solar cell with a single
junction, as we have only considered one single bandgap. When two (or more)
semiconductor materials with different gaps are superposed, the limit for the
spectral conversion efficiency η S can indeed be increased.
3.3 Separation of Electrons and Holes: The Solar Cell
as Diode
We have just seen, in Sect. 3.2, that when light is absorbed within a semiconductor,
a pair of electrical carriers is generated, i.e. an electron-hole pair is created for each
absorbed photon. Now, in a second step, in order to generate electricity, electrons
and holes have to be separated. If this does not take place, electrons and holes simply
recombine again—thereby heating up the semiconductor.
Drift transport: (Fig. 3.8) In a solar cell, there is only one way of separating electrons
and holes—by the action of an electric field E. Indeed, under the influence of an
electric field E, electrons, whose charge is negative, will travel in a direction opposite
to that of the electric field, whereas holes, whose charge is positive, will travel in the
same direction as the electric field. This phenomenon is called “drift” transport and
is shown schematically in Fig. 3.8.
We now need a semiconductor structure, which has an internal electric field.
The simplest such structure is the diode. Therefore (almost) all solar cells are just
semiconductor diodes.
In the case of most solar cells (crystalline silicon solar cells, CIGS, CdTe, GaAs
and other similar solar cells), we use a p-n diode, i.e. a diode with a p-doped region
immediately adjacent to an n-doped region. Here, the internal electric field is at the
border of the p-doped and n-doped regions, in what is called the “depletion region”.
In such a p-n type solar cell, holes and electrons are photo-generated in the bulk
of the p- and n-regions; they then travel by diffusion to the depletion region, where
they are separated by the action of the internal electric field. (Note that diffusion is
Fig. 3.8 Drift transport of electrons and holes under the influence of an electric field E
A. Shah
Notes
1. These values apply to the first step only. The second step will result in further
losses. The plot for the overall efficiency (first and second step) will be given
later, in Sect. 3.5.1 (Fig. 3.19).
2. Furthermore, these values correspond to the case of a solar cell with a single
junction, as we have only considered one single bandgap. When two (or more)
semiconductor materials with different gaps are superposed, the limit for the
spectral conversion efficiency η S can indeed be increased.
3.3 Separation of Electrons and Holes: The Solar Cell
as Diode
We have just seen, in Sect. 3.2, that when light is absorbed within a semiconductor,
a pair of electrical carriers is generated, i.e. an electron-hole pair is created for each
absorbed photon. Now, in a second step, in order to generate electricity, electrons
and holes have to be separated. If this does not take place, electrons and holes simply
recombine again—thereby heating up the semiconductor.
Drift transport: (Fig. 3.8) In a solar cell, there is only one way of separating electrons
and holes—by the action of an electric field E. Indeed, under the influence of an
electric field E, electrons, whose charge is negative, will travel in a direction opposite
to that of the electric field, whereas holes, whose charge is positive, will travel in the
same direction as the electric field. This phenomenon is called “drift” transport and
is shown schematically in Fig. 3.8.
We now need a semiconductor structure, which has an internal electric field.
The simplest such structure is the diode. Therefore (almost) all solar cells are just
semiconductor diodes.
In the case of most solar cells (crystalline silicon solar cells, CIGS, CdTe, GaAs
and other similar solar cells), we use a p-n diode, i.e. a diode with a p-doped region
immediately adjacent to an n-doped region. Here, the internal electric field is at the
border of the p-doped and n-doped regions, in what is called the “depletion region”.
In such a p-n type solar cell, holes and electrons are photo-generated in the bulk
of the p- and n-regions; they then travel by diffusion to the depletion region, where
they are separated by the action of the internal electric field. (Note that diffusion is
Fig. 3.8 Drift transport of electrons and holes under the influence of an electric field E