5 Crystalline Silicon Solar Cells: Homojunction Cells
115
12.6% silicon in aluminium. In this process, aluminium atoms diffuse into the silicon
crystal and displace the silicon atoms from the lattice sites. As a result, in addition
to the boron doping of silicon, doping by the trivalent aluminium atoms takes place.
This results in a small potential jump similar to the that of the pn-junction (see also
Fig. 5.10 right): The potential jump ensures that electrons generated by sunlight are
directed from the back to the front. The probability that electrons recombine at the
back is, thus, significantly reduced. Thanks to the Aluminium Back Surface Field,
an improvement in the effective surface recombination velocity is achieved. It is
about 200–600 cm s
−1 and is, thus, several orders of magnitude lower than in an
aluminium-coated cell without the firing process.
5.2.5 Metallization
(a) Silver Pastes
On the front there is an antireflective layer consisting of SiN x (silicon nitride) and on
the back there is an aluminium layer. The aluminium layer conducts very well, so that
we can easily take off the current on the back. On the front side, this is not possible
because silicon nitride is an insulator. For this reason, metal layers are printed on the
front side by employing a Screen Printing process. The pastes used for the Screen
Printing process contain silver flakes and glass frits. At about 800 °C, glass frits are
fired through the silicon nitride and make an Ohmic contact with the silicon. In this
way, depending on cell size and layout, about 70–90 thin fingers (see also Chap. 9)
in the range of 50 μm, and about 5 busbars are connected to the front side of the
solar cell. During the construction of a solar module, one solders copper ribbons
on the busbars, and the current can be taken away through these copper ribbons.
To make the back side solderable, busbars containing silver are also printed on the
backside of the cell before the rest of the backside is covered by an aluminium layer;
aluminium itself is not easily solderable. Thus, the free charge carriers can leave the
solar cell via the metal contacts on both sides and contribute to the flow of current
in an external circuit.
(b) Cell connections in the solar module (see also Chap. 9)
With the p-type wafer material, the front side delivers negative and the back side
positive charge carriers. For the construction of a whole module, the copper ribbons
are used to connect the front of the first cell to the back of the second cell, etc. This
corresponds to a series connection of the solar cells and leads to an addition of the
individual voltages, whilst the current remains constant. Typically, 1–12 cells are
processed into a so-called “string”. Six strings are then connected in series to finally
make up a whole module.
115
12.6% silicon in aluminium. In this process, aluminium atoms diffuse into the silicon
crystal and displace the silicon atoms from the lattice sites. As a result, in addition
to the boron doping of silicon, doping by the trivalent aluminium atoms takes place.
This results in a small potential jump similar to the that of the pn-junction (see also
Fig. 5.10 right): The potential jump ensures that electrons generated by sunlight are
directed from the back to the front. The probability that electrons recombine at the
back is, thus, significantly reduced. Thanks to the Aluminium Back Surface Field,
an improvement in the effective surface recombination velocity is achieved. It is
about 200–600 cm s
−1 and is, thus, several orders of magnitude lower than in an
aluminium-coated cell without the firing process.
5.2.5 Metallization
(a) Silver Pastes
On the front there is an antireflective layer consisting of SiN x (silicon nitride) and on
the back there is an aluminium layer. The aluminium layer conducts very well, so that
we can easily take off the current on the back. On the front side, this is not possible
because silicon nitride is an insulator. For this reason, metal layers are printed on the
front side by employing a Screen Printing process. The pastes used for the Screen
Printing process contain silver flakes and glass frits. At about 800 °C, glass frits are
fired through the silicon nitride and make an Ohmic contact with the silicon. In this
way, depending on cell size and layout, about 70–90 thin fingers (see also Chap. 9)
in the range of 50 μm, and about 5 busbars are connected to the front side of the
solar cell. During the construction of a solar module, one solders copper ribbons
on the busbars, and the current can be taken away through these copper ribbons.
To make the back side solderable, busbars containing silver are also printed on the
backside of the cell before the rest of the backside is covered by an aluminium layer;
aluminium itself is not easily solderable. Thus, the free charge carriers can leave the
solar cell via the metal contacts on both sides and contribute to the flow of current
in an external circuit.
(b) Cell connections in the solar module (see also Chap. 9)
With the p-type wafer material, the front side delivers negative and the back side
positive charge carriers. For the construction of a whole module, the copper ribbons
are used to connect the front of the first cell to the back of the second cell, etc. This
corresponds to a series connection of the solar cells and leads to an addition of the
individual voltages, whilst the current remains constant. Typically, 1–12 cells are
processed into a so-called “string”. Six strings are then connected in series to finally
make up a whole module.