Page 59 - Advances in Renewable Energies and Power Technologies

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

filter if their thickness is adjusted to l a /4, where l a is the wavelength of light in
the antireflection coating material. It is given by

l
v ph
l a ¼ ¼ p ffiffiffiffi (1.47)
f ε r
where v ph is the phase velocity of light in the material, ε r is relative dielectric con-
stant, and l is the wavelength in free space.
If SiO 2 is used with ε r ¼ 4 and l ¼ 0.5 mm, then l a ¼ 0.25 mm and l a /4 ¼ the
thickness of the antireflection coating ¼ 0.0625 mm. For thick layers, thickness of
the antireflection coating <1 mm, the optical matching condition is applied to get
the suitable antireflection coating:
p ffiffiffiffiffiffiffiffiffiffiffiffiffi
ε ran ¼ ε ra ε rSi (1.48)
where ε ran is the relative dielectric constant of the antireflection coating. ε ra of air is
w1 and ε rSi ¼ 11.7. The reflectance can also be decreased by surface texturing [19]
as shown in Fig. 1.29A. By surface texturing, the reflected rays will be trapped by the
material. Only light will be reflected again after multiple incidence on the surface.
Fig. 1.29B depicts the reflectance of Si after antireflection coating and with texturing
and antireflection coating.
Remember that r of Si ¼ 0.3. It is clear from this figure that r has been decreased
appreciably with this type of surface treatment. There are new developments in anti-
reflective coating such as Al 2 O 3 [20].
• The optimum doping of the layers:
The p-Si substrate r ¼ 2.2 U cm e 10 U cm.
20
18
þ
The n -Si layer N D ¼ 10 e10 /cm 3
• The diffusion lengths:
L n in the substrate 200 mm.
þ
L p in the n layer 0.5 mm
• The metal grid resistance:
It must be much smaller than the lateral resistance of the top layer. The metal grid
covers about 10%e20% of the solar cell surface. Thickness of metal ¼ 3e5 mm
2
• The short circuit current density J sc w 30e40 mA/cm for AM1 illumination.
• The open circuit voltage V oc w 0.52e0.65 V
• FF w 0.8e0.85
2
• P out maximum, P m ¼ (15e24) mW/cm F.F. (0.8) ¼ 12e19.2 mW/cm 2
• The efficiency h ¼ P m /P in ¼ (12e19)/100 w (0.12e0.19) ¼ 12%e19%.
• The highest efficiency achieved 25.6% for special solar cell constructions:
• These cells have back surface field p -layer at the bottom of the substrate
þ
[21].
• They have very shallow n top layer to minimize the dead top layer and
þ
enhance the collection of the eeh generated by the UV radiation.
• They have double antireflection coating from SiO 2 þ TaO. The SiO 2 layer
also passivates the top layer surface and reduces its recombination velocity.
• They have surface texturing.
• Solar cells are connected in circuits, they must be wired.