velocity at the emitter front surface will lead to significant charge carrier losses and
consequently lower short circuit current densities. In high quality monocrystalline silicon
wafers, for example, no defect-rich boundaries are present in the bulk. Thus, the lifetime
of charge carriers is limited by the recombination processes at the wafer surface.
In order to reduce the surface recombination two approaches are used. First, the
defect concentration on the surface is reduced by depositing a thin layer of a different
material on top of the surface. This material partially restores the bonding environment of
the silicon atoms. In addition, the material must be an insulator; it must force the electrons
to remain in and move through the emitter layer. Typical materials used for these chemical
passivation layers are silicon oxides (SiO x ) and silicon nitrides (Si x N y ).
1
A silicon oxide
layer can be formed by heating up the silicon surface in an oxygen-rich atmosphere,
leading to the oxidation of the surface silicon atoms. Si 3 N 4 can be deposited using plasmaenhanced chemical vapour deposition (PE-CVD) that we will discuss in more detail in
Chapters 13 and 14.
A second approach for reducing the surface recombination velocity is to reduce the
minority charge carrier density near the surface. As the surface recombination velocity is
limited by the minority charge carrier’s density, it is beneficial to have the minority charge
carrier density at the surface as low as possible. By increasing the doping of the emitter
layer, the density of the minority charge carriers can be reduced, which results in lower
surface recombination velocities. However, this is in competition with the diffusion length
of the minority charge carriers. The blue part of the solar spectrum leads to the generation
of many charge carriers very close to the surface, i.e. in the emitter layer. For utilizing
these light-excited minority charge carriers, the diffusion length of the holes has to be
large enough to reach the depletion zone at the p-n junction. However, increasing the
doping levels leads to a decreasing diffusion length of the minority holes in the emitter.
Therefore, too high doping levels or too thick emitter layers would result in a poor blue
response or – in other words – low external quantum efficiency (EQE, see Chapter 9)
values in the blue part of the spectrum. Such an emitter layer could be called a ‘dead
layer’ as the light-excited minority charge carriers cannot be collected.
Next, we take a closer look at the metal-emitter interface. Because electrons must be
easily conducted from the emitter to the metal, insulating passivation layers such as SiO x
or Si x N y cannot be used. Therefore, the metal-semiconductor interface has more defects
and hence an undesirably high interface recombination velocity. Additionally, a
metalsemiconductor junction induces a barrier for the majority charge carriers, as we have
seen in Section 8.3. Hence, this barrier will give rise to a higher contact resistance. Again
high doping levels can reduce the recombination velocity at the metal-semiconductor
interface and also reduce the contact resistance. In order to minimize the recombination at
consequently lower short circuit current densities. In high quality monocrystalline silicon
wafers, for example, no defect-rich boundaries are present in the bulk. Thus, the lifetime
of charge carriers is limited by the recombination processes at the wafer surface.
In order to reduce the surface recombination two approaches are used. First, the
defect concentration on the surface is reduced by depositing a thin layer of a different
material on top of the surface. This material partially restores the bonding environment of
the silicon atoms. In addition, the material must be an insulator; it must force the electrons
to remain in and move through the emitter layer. Typical materials used for these chemical
passivation layers are silicon oxides (SiO x ) and silicon nitrides (Si x N y ).
1
A silicon oxide
layer can be formed by heating up the silicon surface in an oxygen-rich atmosphere,
leading to the oxidation of the surface silicon atoms. Si 3 N 4 can be deposited using plasmaenhanced chemical vapour deposition (PE-CVD) that we will discuss in more detail in
Chapters 13 and 14.
A second approach for reducing the surface recombination velocity is to reduce the
minority charge carrier density near the surface. As the surface recombination velocity is
limited by the minority charge carrier’s density, it is beneficial to have the minority charge
carrier density at the surface as low as possible. By increasing the doping of the emitter
layer, the density of the minority charge carriers can be reduced, which results in lower
surface recombination velocities. However, this is in competition with the diffusion length
of the minority charge carriers. The blue part of the solar spectrum leads to the generation
of many charge carriers very close to the surface, i.e. in the emitter layer. For utilizing
these light-excited minority charge carriers, the diffusion length of the holes has to be
large enough to reach the depletion zone at the p-n junction. However, increasing the
doping levels leads to a decreasing diffusion length of the minority holes in the emitter.
Therefore, too high doping levels or too thick emitter layers would result in a poor blue
response or – in other words – low external quantum efficiency (EQE, see Chapter 9)
values in the blue part of the spectrum. Such an emitter layer could be called a ‘dead
layer’ as the light-excited minority charge carriers cannot be collected.
Next, we take a closer look at the metal-emitter interface. Because electrons must be
easily conducted from the emitter to the metal, insulating passivation layers such as SiO x
or Si x N y cannot be used. Therefore, the metal-semiconductor interface has more defects
and hence an undesirably high interface recombination velocity. Additionally, a
metalsemiconductor junction induces a barrier for the majority charge carriers, as we have
seen in Section 8.3. Hence, this barrier will give rise to a higher contact resistance. Again
high doping levels can reduce the recombination velocity at the metal-semiconductor
interface and also reduce the contact resistance. In order to minimize the recombination at