5 Crystalline Silicon Solar Cells: Homojunction Cells
125
Fig. 5.14 A typical PERC cell. The drawing is not to scale. See the text for a complete description
of the processes and interactions shown in the figure
The Al 2 O 3 layer is highly hygroscopic and binds water vapour very easily. The
layer is, thus, sensitive to moisture. This means that one needs a protective layer, a
SiN y layer, which is usually implanted in the industrial environment using the same
PECVD method. In addition to the protective effect, an optical adaptation of the back
side of the solar cell is thereby achieved (see Chap. 4, Light Trapping), provided
the layer thickness is correctly chosen. Low-energy photons with large wavelengths,
which have penetrated through the entire solar cell can be reflected back into the solar
cell very effectively through such an optimized layer stack, consisting of Al 2 O 3 and
SiN y : the stack functions as an optical mirror. In this way, the photons get another
chance to create an electron-hole pair in the bulk silicon. Compared to the Al-BSF
standard cell with an average back-reflection of 65%, the dielectric layers of the
PERC technology can increase the average back-reflection to 89%.
In contrast to the standard Al-BSF solar cell, the silicon nitride passivation layer
(SiN y ) hinders the actual contacting of the cell with the metallization paste, because
SiN y is an insulator and the generated charge carriers therefore cannot contribute to
the current flow. To counteract this problem, we must enable a local contact between
the metal paste and the underlying silicon wafer. This is done by locally opening the
SiN y layer as well as the Al 2 O 3 layer with a laser, so that the backside aluminium
paste can be connected to the silicon and forms a contact. If these local contacts,
that is to say the laser openings, are located at a certain distance from each other,
the corresponding charge carriers can reach them. The distance between the contact
points has an influence on the series resistance. The design and optimization of the
openings, their size and their distance from each other has been the subject of many
studies. Figure 5.13 shows the cross-section of a PERC cell and a “top view” of the
125
Fig. 5.14 A typical PERC cell. The drawing is not to scale. See the text for a complete description
of the processes and interactions shown in the figure
The Al 2 O 3 layer is highly hygroscopic and binds water vapour very easily. The
layer is, thus, sensitive to moisture. This means that one needs a protective layer, a
SiN y layer, which is usually implanted in the industrial environment using the same
PECVD method. In addition to the protective effect, an optical adaptation of the back
side of the solar cell is thereby achieved (see Chap. 4, Light Trapping), provided
the layer thickness is correctly chosen. Low-energy photons with large wavelengths,
which have penetrated through the entire solar cell can be reflected back into the solar
cell very effectively through such an optimized layer stack, consisting of Al 2 O 3 and
SiN y : the stack functions as an optical mirror. In this way, the photons get another
chance to create an electron-hole pair in the bulk silicon. Compared to the Al-BSF
standard cell with an average back-reflection of 65%, the dielectric layers of the
PERC technology can increase the average back-reflection to 89%.
In contrast to the standard Al-BSF solar cell, the silicon nitride passivation layer
(SiN y ) hinders the actual contacting of the cell with the metallization paste, because
SiN y is an insulator and the generated charge carriers therefore cannot contribute to
the current flow. To counteract this problem, we must enable a local contact between
the metal paste and the underlying silicon wafer. This is done by locally opening the
SiN y layer as well as the Al 2 O 3 layer with a laser, so that the backside aluminium
paste can be connected to the silicon and forms a contact. If these local contacts,
that is to say the laser openings, are located at a certain distance from each other,
the corresponding charge carriers can reach them. The distance between the contact
points has an influence on the series resistance. The design and optimization of the
openings, their size and their distance from each other has been the subject of many
studies. Figure 5.13 shows the cross-section of a PERC cell and a “top view” of the