4 Solar Cells: Optical and Recombination Losses
87
wafers, as well as metal-catalytic processes.
12 Figure 4.11b shows the surface of
a multicrystalline cell cut with diamond wire and texturized by plasma. Thanks to
these newer methods, diamond-wire-cut, multicrystalline wafers can be processed
into solar cells, whose cell efficiency is higher than those of slurry-cut wafers, whilst
at the same time the manufacturing costs are reduced.
4.1.7 Passivation of the Back Surface and Mirror Formation
at the Back
On the back we can apply two improvements:
• Passivation and light reflection with a metal paste (aluminium paste)
• Texturing the back side.
At the back of the solar cell, a reflector is used. Thus, the light that travels through
the cell is reflected there and the optical path is doubled. The light, thus, receives a
second chance to be absorbed in the silicon crystal. The back side of a solar cell has to
be passivated in all cases, i.e. the recombination centres formed by the surface have to
be deactivated; otherwise there is a substantial reduction in current and efficiency (see
Chap. 5). The metallization pastes (aluminium pastes) one uses to form electrical
contacts enhances the reflection at the back. In this case, a clever arrangement of
dielectric layer stacks can achieve practically total reflection and, thus, an “ideal”
optical mirror can be implemented (see Chap. 5).
If the back surface itself is textured or roughened, one should ideally obtain a
Lambertian reflector. With this type of reflector it is possible to increase the optical
path by a factor of
4 × n silicon
2
= 4 × 3.92
2
≈ 50
(4.12)
If the cell thickness is 200 μm, then the Lambertian reflector will result in a 10%
increase of the generated electric power. If the cells are very thin, then the increase
in electric power is much higher: As an example, if the cell thickness is 10 μm, then
the power yield can be almost doubled, thanks to a Lambertian back reflector. Of
course, there are no materials, whose surface fully complies with Lambert’s Law. An
example for partial implementation of the Lambertian effects is given by texturing
the back side of solar cells (as e.g. in bifacial HJT cells).
Now, if we have absorbed as much light as possible and created electron-hole pairs,
the goal is for the electrons and holes to reach the corresponding metal contacts on
the surface of the solar cell, enter from there into the external power circuit and
contribute to the generation of electricity for the benefit of an external consumer. For
the electrons (and holes) to reach their goal at all, they have to “live” long enough.
12 Silver ions are used here which adhere to the silicon surface, so that the etching liquid has points
of attack to structure the surface.
87
wafers, as well as metal-catalytic processes.
12 Figure 4.11b shows the surface of
a multicrystalline cell cut with diamond wire and texturized by plasma. Thanks to
these newer methods, diamond-wire-cut, multicrystalline wafers can be processed
into solar cells, whose cell efficiency is higher than those of slurry-cut wafers, whilst
at the same time the manufacturing costs are reduced.
4.1.7 Passivation of the Back Surface and Mirror Formation
at the Back
On the back we can apply two improvements:
• Passivation and light reflection with a metal paste (aluminium paste)
• Texturing the back side.
At the back of the solar cell, a reflector is used. Thus, the light that travels through
the cell is reflected there and the optical path is doubled. The light, thus, receives a
second chance to be absorbed in the silicon crystal. The back side of a solar cell has to
be passivated in all cases, i.e. the recombination centres formed by the surface have to
be deactivated; otherwise there is a substantial reduction in current and efficiency (see
Chap. 5). The metallization pastes (aluminium pastes) one uses to form electrical
contacts enhances the reflection at the back. In this case, a clever arrangement of
dielectric layer stacks can achieve practically total reflection and, thus, an “ideal”
optical mirror can be implemented (see Chap. 5).
If the back surface itself is textured or roughened, one should ideally obtain a
Lambertian reflector. With this type of reflector it is possible to increase the optical
path by a factor of
4 × n silicon
2
= 4 × 3.92
2
≈ 50
(4.12)
If the cell thickness is 200 μm, then the Lambertian reflector will result in a 10%
increase of the generated electric power. If the cells are very thin, then the increase
in electric power is much higher: As an example, if the cell thickness is 10 μm, then
the power yield can be almost doubled, thanks to a Lambertian back reflector. Of
course, there are no materials, whose surface fully complies with Lambert’s Law. An
example for partial implementation of the Lambertian effects is given by texturing
the back side of solar cells (as e.g. in bifacial HJT cells).
Now, if we have absorbed as much light as possible and created electron-hole pairs,
the goal is for the electrons and holes to reach the corresponding metal contacts on
the surface of the solar cell, enter from there into the external power circuit and
contribute to the generation of electricity for the benefit of an external consumer. For
the electrons (and holes) to reach their goal at all, they have to “live” long enough.
12 Silver ions are used here which adhere to the silicon surface, so that the etching liquid has points
of attack to structure the surface.