5 Crystalline Silicon Solar Cells: Homojunction Cells
131
they finally leave the solar cell and contribute to the production of electricity. In the
passivated emitter rear contact (PERC) cell, the back surface has been improved with
respect to the Al-BSF cell, by building up an Al 2 O 3 layer, covered by a protective
layer of SiN. In addition to an improved passivation, an additional back surface
field is also created here. In the Al-BSF cell the entire back side is coated with
aluminium and constitutes an Ohmic contact with high recombination rates. In the
PERC cell, contacting the back side is only done via small openings. This improves
the passivation of the back side; the surface recombination decreases. Such contacts
are also applied to the PERT cell. The difference between a PERC cell and a PERT
cell is that the back side of the PERT cell is further improved. This will be described
now.
To further improve the back side, one can dope the latter even more to further
reduce recombination of electrons at that location. This leads us to the PERT cell
(Passivated Emitter and Rear Totally Diffused). In the PERT cell a dopant diffusion
is also carried out at the back side, in the same way as on the front side, but with the
opposite “polarity” of the dopant: with boron on the back side, instead of phosphorous
as used on the front side. Now the back side of the PERT cell is very strongly doped
with boron, so that a p
++ layer is created. Figure 5.19 shows the cross-sections of
the three types of homojunction cells: the basic homojunction Al-BSF, PERC and
PERT cells. For the sake of simplicity, all three cells are drawn here with p-type
wafer material so that it becomes easier to compare the different layers. The PERT
Fig. 5.19 Cross-sections of the three main types of homojunction cells: a Al-BSF (Al-dopedBSF) cell; b PERC cell; c PERT cell. The drawings are not to scale. The charge carrier transport
corresponds to that in Fig. 5.11. For the sake of simplicity, all three cells are drawn here with p-type
wafer material
131
they finally leave the solar cell and contribute to the production of electricity. In the
passivated emitter rear contact (PERC) cell, the back surface has been improved with
respect to the Al-BSF cell, by building up an Al 2 O 3 layer, covered by a protective
layer of SiN. In addition to an improved passivation, an additional back surface
field is also created here. In the Al-BSF cell the entire back side is coated with
aluminium and constitutes an Ohmic contact with high recombination rates. In the
PERC cell, contacting the back side is only done via small openings. This improves
the passivation of the back side; the surface recombination decreases. Such contacts
are also applied to the PERT cell. The difference between a PERC cell and a PERT
cell is that the back side of the PERT cell is further improved. This will be described
now.
To further improve the back side, one can dope the latter even more to further
reduce recombination of electrons at that location. This leads us to the PERT cell
(Passivated Emitter and Rear Totally Diffused). In the PERT cell a dopant diffusion
is also carried out at the back side, in the same way as on the front side, but with the
opposite “polarity” of the dopant: with boron on the back side, instead of phosphorous
as used on the front side. Now the back side of the PERT cell is very strongly doped
with boron, so that a p
++ layer is created. Figure 5.19 shows the cross-sections of
the three types of homojunction cells: the basic homojunction Al-BSF, PERC and
PERT cells. For the sake of simplicity, all three cells are drawn here with p-type
wafer material so that it becomes easier to compare the different layers. The PERT
Fig. 5.19 Cross-sections of the three main types of homojunction cells: a Al-BSF (Al-dopedBSF) cell; b PERC cell; c PERT cell. The drawings are not to scale. The charge carrier transport
corresponds to that in Fig. 5.11. For the sake of simplicity, all three cells are drawn here with p-type
wafer material