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S. Leu and D. Sontag
(b) Charge carrier diffusion
First, both areas (n- and p-type) are electrically neutral. Due to the concentration
gradient of electrons and holes, however, charge carrier compensation takes place:
the electrons of the phosphorus-doped layer migrate into the boron-doped layer,
because there are fewer electrons in that region, and conversely, the holes migrate
into the phosphorus-doped layer. The first effect is thus a concentration-related
diffusion.
16
(c) Drift
Now, holes are positively charged and electrons are negatively charged, and charge
carriers have moved because of diffusion: Thus, an electric field is created. This is
the second effect. The electric field counteracts the concentration balance and equilibrium is established between the force of concentration compensation (diffusion)
and the force of the electric field (drift).
(d) Space charge zone
At the transition point between the p- and n-type regions, the pn-junction is formed.
Due to the displacement of charge carriers, a so-called “space charge zone” is formed
(see Chap. 3). Now if charge carriers reach this electrically active zone, charge
separation takes place. In our case, the charge carriers are predominantly generated
in the p-type silicon bulk,
17 so that the electrons drift through the electric field into
the n-type region, while the holes are repelled and thrown back into the bulk.
5.2.3 Light Trapping by ARC
Now we cover the front side of the wafer with an antireflection coating (ARC), so
that the reflection of the light is minimized and as many photons as possible are
absorbed in the silicon. The texturization on the front side combined with a good
antireflection coating (see Chap. 4) allows one to improve the absorption of photons.
The antireflection layer is usually realized with silicon nitride SiN x .
On the other hand, a good reflectivity on the back side improves the absorption
of low energy (long wavelength) photons, which are weakly absorbed by silicon and
which pass through the entire solar cell and are reflected on the back side, back into
the solar cell. This gives them a second chance to generate an electron-hole pair—in
another passage, some more electron-hole pairs will be generated: the process can
be repeated through multiple reflections. The better the light trapping is, the higher
the electric current I sc , which is generated, will be (see also Sect. 5.2.4d).
16 This process can be clearly compared to a glass of water in which salt is sprinkled. Diffusion
ensures that the concentration is equalized.
17 The bulk is very much thicker than the pn-junction. Therefore, most of the charge carriers are
generated in the bulk and not in the thin n-type layer.
S. Leu and D. Sontag
(b) Charge carrier diffusion
First, both areas (n- and p-type) are electrically neutral. Due to the concentration
gradient of electrons and holes, however, charge carrier compensation takes place:
the electrons of the phosphorus-doped layer migrate into the boron-doped layer,
because there are fewer electrons in that region, and conversely, the holes migrate
into the phosphorus-doped layer. The first effect is thus a concentration-related
diffusion.
16
(c) Drift
Now, holes are positively charged and electrons are negatively charged, and charge
carriers have moved because of diffusion: Thus, an electric field is created. This is
the second effect. The electric field counteracts the concentration balance and equilibrium is established between the force of concentration compensation (diffusion)
and the force of the electric field (drift).
(d) Space charge zone
At the transition point between the p- and n-type regions, the pn-junction is formed.
Due to the displacement of charge carriers, a so-called “space charge zone” is formed
(see Chap. 3). Now if charge carriers reach this electrically active zone, charge
separation takes place. In our case, the charge carriers are predominantly generated
in the p-type silicon bulk,
17 so that the electrons drift through the electric field into
the n-type region, while the holes are repelled and thrown back into the bulk.
5.2.3 Light Trapping by ARC
Now we cover the front side of the wafer with an antireflection coating (ARC), so
that the reflection of the light is minimized and as many photons as possible are
absorbed in the silicon. The texturization on the front side combined with a good
antireflection coating (see Chap. 4) allows one to improve the absorption of photons.
The antireflection layer is usually realized with silicon nitride SiN x .
On the other hand, a good reflectivity on the back side improves the absorption
of low energy (long wavelength) photons, which are weakly absorbed by silicon and
which pass through the entire solar cell and are reflected on the back side, back into
the solar cell. This gives them a second chance to generate an electron-hole pair—in
another passage, some more electron-hole pairs will be generated: the process can
be repeated through multiple reflections. The better the light trapping is, the higher
the electric current I sc , which is generated, will be (see also Sect. 5.2.4d).
16 This process can be clearly compared to a glass of water in which salt is sprinkled. Diffusion
ensures that the concentration is equalized.
17 The bulk is very much thicker than the pn-junction. Therefore, most of the charge carriers are
generated in the bulk and not in the thin n-type layer.