13.2.1
Figure 13.4 shows the electronic band dispersion diagram of gallium arsenide. As
mentioned before, GaAs is a direct bandgap material, i.e., the highest energy level in the
valence band is vertically aligned with the lowest energy level in the conduction band.
Hence, only transfer of energy is required to excite an electron from the valence to the
conduction band, but no transfer of momentum is required.
Figure 13.4: The electronic band dispersion diagram of gallium arsenide.
The bandgap of GaAs is 1.424 eV [24]. Here we look again at the absorption
coefficient versus the wavelength. Consequently, as you can see in Figure 12.4, the
absorption coefficient of GaAs is significantly larger than that of silicon. The same is true
for InP, another III-V material that also is shown in Fig. 12.4. Because of the high
absorption coefficient, the same amount of light can be absorbed in a film more than one
order of magnitude as thin when compared to silicon. Another advantage of direct III-V
semiconductor materials is their sharp bandgap. Above E g , the absorption coefficient
increases quickly.
Let us now take a look at the utilization of the bandgap energy. Since GaAs is a direct
bandgap material, radiative recombination processes become important. On the other
hand, Shockley–Read–Hall recombination can be kept at a low level because III-V films
can be deposited by epitaxy processes that result in high purity films.
Multi-junction cells
III-V PV devices can reach very high efficiencies because they are often based on the
multijunction concept, which means that more than one bandgap is used. As discussed in
Chapter 10, the maximum theoretical efficiency of single-junction cells is described by the
Shockley-Queisser limit. A large fraction of the energy of the energetic photons are lost as
heat, while photons with energies below the bandgap are lost as they are not absorbed.
Figure 13.4 shows the electronic band dispersion diagram of gallium arsenide. As
mentioned before, GaAs is a direct bandgap material, i.e., the highest energy level in the
valence band is vertically aligned with the lowest energy level in the conduction band.
Hence, only transfer of energy is required to excite an electron from the valence to the
conduction band, but no transfer of momentum is required.
Figure 13.4: The electronic band dispersion diagram of gallium arsenide.
The bandgap of GaAs is 1.424 eV [24]. Here we look again at the absorption
coefficient versus the wavelength. Consequently, as you can see in Figure 12.4, the
absorption coefficient of GaAs is significantly larger than that of silicon. The same is true
for InP, another III-V material that also is shown in Fig. 12.4. Because of the high
absorption coefficient, the same amount of light can be absorbed in a film more than one
order of magnitude as thin when compared to silicon. Another advantage of direct III-V
semiconductor materials is their sharp bandgap. Above E g , the absorption coefficient
increases quickly.
Let us now take a look at the utilization of the bandgap energy. Since GaAs is a direct
bandgap material, radiative recombination processes become important. On the other
hand, Shockley–Read–Hall recombination can be kept at a low level because III-V films
can be deposited by epitaxy processes that result in high purity films.
Multi-junction cells
III-V PV devices can reach very high efficiencies because they are often based on the
multijunction concept, which means that more than one bandgap is used. As discussed in
Chapter 10, the maximum theoretical efficiency of single-junction cells is described by the
Shockley-Queisser limit. A large fraction of the energy of the energetic photons are lost as
heat, while photons with energies below the bandgap are lost as they are not absorbed.