Page 57 - Advances in Renewable Energies and Power Technologies

30 CHAPTER 1 Solar Cells and Arrays: Principles, Analysis, and Design

CIGS cells are available as commercial products [15]. GaAs cells are used for
space applications [16]. This is because the conversion efficiency decreases as
the area of the solar cell increases as the defects increase with area.
• Solar cell material:
The efficiency of the solar cell depends on its material as discussed previously.
Fig. 1.26 shows the ideal solar cell efficiency as a function of the energy gap of
the materials [17]. The efficiency of GaAs cells is slightly higher than that of Si.
Most commercial cells are made of Si [15] because the Si technology is the most
advanced among all materials.
• The thickness of the Si-solar cells:
As the thickness of the solar cell increases, the cell output power increases for the
same illumination intensity. Fig. 1.27 shows the effect of the solar cell thickness
on the output power. From this curve, a thickness of about 0.3 mm is sufficient
to collect the incident solar radiation. An amorphous layer thickness of 1 mmis
also sufficient. Therefore, the a-Si cells are thin film cells.
• The spectral response of the solar cells:
Fig. 1.28 shows the spectral response of solar cells made of different materials. It
agrees with the analysis on the short circuit current as a function of the wave-
length. It can be noted that silicon solar cell response for ultraviolet radiation is
low. This is because of the presence of the dead layer at the top of the cell.
• Antireflection coating [18].
To decrease the reflectance from the surface of the Si-cells, its surface is coated
with an antireflection film made of SiO x and/or TiO 2 . These layers have inter-
mediate refractive index between Si and air. They can act also as an interference

FIGURE 1.26
Theoretical solar cell efficiency as a function of the energy gap.