132
S. Leu and D. Sontag
cell has a better passivation than the PERC cell due to the additional p
++ layer, as
shown in Fig. 5.19 with the boron layer. Thanks to these improvements, the PERT
cell has a slightly higher efficiency than a PERC cell: about 0.5% abs higher.
In practice, hardly any p-type PERT cells are produced—for the production of
PERT cells one chooses monocrystalline n-type wafer material. The reason is: A solar
cell with a very high degree of passivation (like the PERT cell) should only be used
together with silicon material of a very high quality, such as n-type monocrystalline
wafers. In the PERT cell aluminium is only used for contacting the cell and not for
forming the BSF, which is formed by the second diffusion (Fig. 5.19c). PERT cells
have the following economic disadvantages compared to PERC cells:
1. Formation of the back surface field (BSF): In a p-type PERC cell, the aluminium generates the local BSF with a built-in field created by the negative
charges in the aluminium oxide layer. In a type PERT cell a second diffusion step
is needed for generating the BSF.
2. Co-Firing
34 : In a p-type PERC cell, there is co-firing of the phosphorous diffusion and metallisation. No additional protection layer is needed. In a n-type
PERT cell, there is co-firing of the boron diffusion and metallisation. An additional protection layer (SiO x ) is needed, to protect the boron-diffused p
++ layer
from interference with the Al 2 O 3 layer (Fig. 5.20).
In the case of the n-type rear-emitter PERT, there is no BSF (there is one rearemitter and one FSF (Front Surface Field). For the fabrication of a PERT cell the
following additional production steps have to be introduced: (a) boron diffusion, (b)
deposition of a protective layer
35 to protect the boron diffusion and (c) the retroactive
etching of boron glass. Otherwise, the process steps are the same:
Although the PERT technology constitutes an evolutionary process step with
respect to the PERC technology, with only a few additional steps, PERT modules
have only slightly higher efficiencies than PERC modules, so that the additional
economic outlay is, in our opinion, not justified (Table 5.3).
34 During the Co-Firing process, two steps take place simultaneously: (1), the metallization paste
applied by the screen printing step is fired through the SiN and (2) the dopands are driven into the
bulk to form the pn-junction.
35 After the boron diffusion, a protective layer is applied so that subsequent phosphorus diffusion
and chemical texturing do not “destroy” the effects of the boron diffusion.
S. Leu and D. Sontag
cell has a better passivation than the PERC cell due to the additional p
++ layer, as
shown in Fig. 5.19 with the boron layer. Thanks to these improvements, the PERT
cell has a slightly higher efficiency than a PERC cell: about 0.5% abs higher.
In practice, hardly any p-type PERT cells are produced—for the production of
PERT cells one chooses monocrystalline n-type wafer material. The reason is: A solar
cell with a very high degree of passivation (like the PERT cell) should only be used
together with silicon material of a very high quality, such as n-type monocrystalline
wafers. In the PERT cell aluminium is only used for contacting the cell and not for
forming the BSF, which is formed by the second diffusion (Fig. 5.19c). PERT cells
have the following economic disadvantages compared to PERC cells:
1. Formation of the back surface field (BSF): In a p-type PERC cell, the aluminium generates the local BSF with a built-in field created by the negative
charges in the aluminium oxide layer. In a type PERT cell a second diffusion step
is needed for generating the BSF.
2. Co-Firing
34 : In a p-type PERC cell, there is co-firing of the phosphorous diffusion and metallisation. No additional protection layer is needed. In a n-type
PERT cell, there is co-firing of the boron diffusion and metallisation. An additional protection layer (SiO x ) is needed, to protect the boron-diffused p
++ layer
from interference with the Al 2 O 3 layer (Fig. 5.20).
In the case of the n-type rear-emitter PERT, there is no BSF (there is one rearemitter and one FSF (Front Surface Field). For the fabrication of a PERT cell the
following additional production steps have to be introduced: (a) boron diffusion, (b)
deposition of a protective layer
35 to protect the boron diffusion and (c) the retroactive
etching of boron glass. Otherwise, the process steps are the same:
Although the PERT technology constitutes an evolutionary process step with
respect to the PERC technology, with only a few additional steps, PERT modules
have only slightly higher efficiencies than PERC modules, so that the additional
economic outlay is, in our opinion, not justified (Table 5.3).
34 During the Co-Firing process, two steps take place simultaneously: (1), the metallization paste
applied by the screen printing step is fired through the SiN and (2) the dopands are driven into the
bulk to form the pn-junction.
35 After the boron diffusion, a protective layer is applied so that subsequent phosphorus diffusion
and chemical texturing do not “destroy” the effects of the boron diffusion.