5 Crystalline Silicon Solar Cells: Homojunction Cells
129
advantages and is in places 30% higher than the energy yield of a standard cell. In
addition, it can be observed that the reflection for the PERC cell increases from about
1000 nm onwards. The reason for this is the improved optical mirror formed by the
passivation layer on the back side of the cell: this improved optical mirror allows
a second passage of unused photons through the solar cell, thereby giving them a
second chance to generate an electron-hole pair.
In addition to the improved optical mirror on the back and the associated higher
short-circuit current density J SC , higher open-circuit voltages V OC are found in PERC
solar cells. V OC is a function of the saturation current density J 0 and, thus, depends
on the quality of the surface passivation—which is excellent in PERC cells. The fill
factor FF increases with the increase in V oc , but, on the other hand, FF also depends
on the series resistance R s . The contacts on the PERC back side are limited to very
small laser openings. Thus, the charge carriers have to make a longer path to reach
the contact. The series resistance R s is thereby increased. As a consequence, the fill
factor FF tends to be lower for PERC solar cells, but this loss is minimised in welldesigned PERC cells and is compensated by the increase in V oc and the increase in
short-circuit density J sc .
A further development is the bifacial design of the back side. This is illustrated
in Fig. 5.18. In PERC bifacial solar cells the back side has to be transparent. This is
achieved by using aluminium tracks, joined together in one direction, instead of a full
aluminium layer. Indeed, the aluminium layer is interrupted in the bifacial design.
Between the aluminium tracks the back SiN y is lying
As a result, stray light that hits the back of the solar cell due to the albedo
32 effect
(see Chaps. 2 and 10) can be captured. However, less light coming in on the front
side is captured by the solar cell, as a part of the this light exits from the back of the
solar cell, without having been used; this occurs, because the aluminium mirror on
the back is now partially transparent. Furthermore, the resistance losses also increase
due to the lower conductance of the narrow Al tracks.
On the whole, the positive effects of the rear side of the PERC predominate and
lead compared to the standard solar cell to a significant increase in efficiency—of
more than 1% abs . Today’s PERC cells achieve efficiencies of 22–23%. Aspirations
are aimed at further improving the passivation, for example via a passivation of
the contact area as promoted by the TOPCon
33 cell concept. The TOPCon (Tunnel
Oxide Polysilicon Contact) cell concept is described in Sect. 5.5.2. Cell concepts
with passivation on both sides, like the concept of the HJT cell will be discussed in
the next chapter (Chap. 7).
32 Albedo is here the ratio between the light striking the back of the cell and the light striking the
front. (See also Chaps. 2 and 10)
33 The TOPCon cell concept was first presented by Fraunhofer (ISE) in 2013.
129
advantages and is in places 30% higher than the energy yield of a standard cell. In
addition, it can be observed that the reflection for the PERC cell increases from about
1000 nm onwards. The reason for this is the improved optical mirror formed by the
passivation layer on the back side of the cell: this improved optical mirror allows
a second passage of unused photons through the solar cell, thereby giving them a
second chance to generate an electron-hole pair.
In addition to the improved optical mirror on the back and the associated higher
short-circuit current density J SC , higher open-circuit voltages V OC are found in PERC
solar cells. V OC is a function of the saturation current density J 0 and, thus, depends
on the quality of the surface passivation—which is excellent in PERC cells. The fill
factor FF increases with the increase in V oc , but, on the other hand, FF also depends
on the series resistance R s . The contacts on the PERC back side are limited to very
small laser openings. Thus, the charge carriers have to make a longer path to reach
the contact. The series resistance R s is thereby increased. As a consequence, the fill
factor FF tends to be lower for PERC solar cells, but this loss is minimised in welldesigned PERC cells and is compensated by the increase in V oc and the increase in
short-circuit density J sc .
A further development is the bifacial design of the back side. This is illustrated
in Fig. 5.18. In PERC bifacial solar cells the back side has to be transparent. This is
achieved by using aluminium tracks, joined together in one direction, instead of a full
aluminium layer. Indeed, the aluminium layer is interrupted in the bifacial design.
Between the aluminium tracks the back SiN y is lying
As a result, stray light that hits the back of the solar cell due to the albedo
32 effect
(see Chaps. 2 and 10) can be captured. However, less light coming in on the front
side is captured by the solar cell, as a part of the this light exits from the back of the
solar cell, without having been used; this occurs, because the aluminium mirror on
the back is now partially transparent. Furthermore, the resistance losses also increase
due to the lower conductance of the narrow Al tracks.
On the whole, the positive effects of the rear side of the PERC predominate and
lead compared to the standard solar cell to a significant increase in efficiency—of
more than 1% abs . Today’s PERC cells achieve efficiencies of 22–23%. Aspirations
are aimed at further improving the passivation, for example via a passivation of
the contact area as promoted by the TOPCon
33 cell concept. The TOPCon (Tunnel
Oxide Polysilicon Contact) cell concept is described in Sect. 5.5.2. Cell concepts
with passivation on both sides, like the concept of the HJT cell will be discussed in
the next chapter (Chap. 7).
32 Albedo is here the ratio between the light striking the back of the cell and the light striking the
front. (See also Chaps. 2 and 10)
33 The TOPCon cell concept was first presented by Fraunhofer (ISE) in 2013.