5 Crystalline Silicon Solar Cells: Homojunction Cells
121
5.4 Motivation for the Development of the PERC Cell
(Passivated Emitter Rear Cell)
In the following, some important physical relationships are explained, in order to
improve the Al-BSF cell. This shows where exactly we can improve the Al-BSF
cell. And this leads us to the concept of the PERC cell (Passivated Emitter Rear
Cell).
5.4.1 Lifetime and Diffusion Length
Lifetime and diffusion length are described in Chap. 3. Here we consider the absolute
magnitude of lifetime and diffusion length in a solar cell. In very good quality Cz
mono-crystalline silicon wafers, the lifetime can be several milliseconds (ms). For
example, assuming a lifetime of 1 ms, the diffusion length L of electrons in p-type
silicon is approximately 1900 μm, with a boron doping of about 10
16 cm
−3 . This is
the statistical distance that electrons can migrate on an average before recombining.
In comparison to the thickness of the solar cell of typically 160 to 180 μm, the
average transmission distance of the electron is thus far enough to reach the frontside solar cell surface and reach the n-region via the space charge zone. If it does not
reach the front of the solar cell within its lifetime, the electron recombines with a
hole, loses its energy mainly in the form of heat and falls back into the valence band.
The lifetime is, thus, a measure of the quality of the wafer material at a given doping
concentration. At a first glance it seems that lifetime is not the limiting factor.
5.4.2 Doping Versus Recombination
The more the silicon is doped, the higher the recombination rate will be, because there
will be more impurities. Particularly in p-type silicon, electrically active complexes
of boron and oxygen can be formed, which act as strong recombination centres. On
the other hand, if the material is not so strongly doped, the dark current (J 0 , see
Chap. 3) increases. This can be imagined in such a way that with low doping less
excess carriers are generated and the few recombine at the metal contacts. However,
if the lifetime of the silicon is increased, high cell efficiencies can also be achieved
with lowly doped solar cells. This requires better silicon material. If the pn-junction
is lowly doped the sheet resistivity (see Chap. 4) is increased and can reach more
than 150 /square. Today it is assumed that the optimal sheet resistance is approx.
100 /square with regards to the most commonly used metallization paste materials.
The contact resistance between the phosphorus-doped n-region and the contacting
pastes on the front side also depend on the doping concentration in the silicon and
121
5.4 Motivation for the Development of the PERC Cell
(Passivated Emitter Rear Cell)
In the following, some important physical relationships are explained, in order to
improve the Al-BSF cell. This shows where exactly we can improve the Al-BSF
cell. And this leads us to the concept of the PERC cell (Passivated Emitter Rear
Cell).
5.4.1 Lifetime and Diffusion Length
Lifetime and diffusion length are described in Chap. 3. Here we consider the absolute
magnitude of lifetime and diffusion length in a solar cell. In very good quality Cz
mono-crystalline silicon wafers, the lifetime can be several milliseconds (ms). For
example, assuming a lifetime of 1 ms, the diffusion length L of electrons in p-type
silicon is approximately 1900 μm, with a boron doping of about 10
16 cm
−3 . This is
the statistical distance that electrons can migrate on an average before recombining.
In comparison to the thickness of the solar cell of typically 160 to 180 μm, the
average transmission distance of the electron is thus far enough to reach the frontside solar cell surface and reach the n-region via the space charge zone. If it does not
reach the front of the solar cell within its lifetime, the electron recombines with a
hole, loses its energy mainly in the form of heat and falls back into the valence band.
The lifetime is, thus, a measure of the quality of the wafer material at a given doping
concentration. At a first glance it seems that lifetime is not the limiting factor.
5.4.2 Doping Versus Recombination
The more the silicon is doped, the higher the recombination rate will be, because there
will be more impurities. Particularly in p-type silicon, electrically active complexes
of boron and oxygen can be formed, which act as strong recombination centres. On
the other hand, if the material is not so strongly doped, the dark current (J 0 , see
Chap. 3) increases. This can be imagined in such a way that with low doping less
excess carriers are generated and the few recombine at the metal contacts. However,
if the lifetime of the silicon is increased, high cell efficiencies can also be achieved
with lowly doped solar cells. This requires better silicon material. If the pn-junction
is lowly doped the sheet resistivity (see Chap. 4) is increased and can reach more
than 150 /square. Today it is assumed that the optimal sheet resistance is approx.
100 /square with regards to the most commonly used metallization paste materials.
The contact resistance between the phosphorus-doped n-region and the contacting
pastes on the front side also depend on the doping concentration in the silicon and