122
S. Leu and D. Sontag
on the quality of the silicon (lifetime). Lifetime and Doping must be balanced. Conclusion: The higher the doping, the better one can contact the silicon. Conversely,
the higher the doping, the lower is the lifetime of the minority carriers. Here there is
a conflict between doping and lifetime.
5.4.3 Surface Recombination Velocity on Front and Back
Sides
However, some of the most active recombination centres are at the surfaces of the
silicon wafer with the partially unsaturated bonds of the surface atoms. Indeed, at
these surfaces, the crystal structure is abruptly interrupted in its periodicity. Defects
occur which provide energy levels within the bandgap, thereby making it easier for
the carriers to recombine. This also changes the profile of the bands and it creates band
distortions. As discussed in Chap. 4, the surface recombination velocity S is a measure
of how fast a charge carrier recombines on the surface. For open, unpassivated silicon
surfaces, it is in the range of 10
5 –10
6 cm s
−1 and drastically limits the open-circuit
voltage V OC of the solar cell. This can be visualized in such a way that the minority
carriers (in p-type wafers the electrons) flow with the speed S to the surface and
recombine there, emptying, thus, the space charge zone. For an increase in V OC and
an associated increase in the efficiency of the solar cell, it is therefore imperative to
minimize the surface recombination velocity by passivation of the surface, e.g. by
deactivation of the recombination centres.
27 We can now discuss how that can be
done at the front and back surfaces.
Surface Recombination on the Front Side
As the standard solar cell has demonstrated, excellent surface passivation has
been achieved thanks to silicon nitride. The recombination velocity is about 200–
600 cm s
−1 for standard solar cells (Al-BSF-cells), so that an improvement over
unpassivated silicon surfaces by several orders of magnitude was obtained.
Surface Recombination on the Back Side
The surface recombination velocity of the solar cell on the back side is 500–
1000 cm s
−1 and is about three times higher than that on the front side. In standard solar cells (Al-BSF cells), the passivation of the back side usually takes place
via the metallization with an aluminium layer. Aluminium has the advantage that
it is, like boron, trivalent and results in p-doping of silicon. If aluminium diffuses
into a p-doped (boron-doped) silicon crystal, the doping concentration at this point
increases, so that even more holes are created on the back side and the Fermi level
27 A measure for assessing the quality of the passivation is the open circuit voltage V OC . The
better the passivation, the less electrons recombine and the higher open circuit voltage V OC . On
average, V oc is 650 mV for standard solar cells. The recombination velocity depends strongly on
the surface doping concentration.
S. Leu and D. Sontag
on the quality of the silicon (lifetime). Lifetime and Doping must be balanced. Conclusion: The higher the doping, the better one can contact the silicon. Conversely,
the higher the doping, the lower is the lifetime of the minority carriers. Here there is
a conflict between doping and lifetime.
5.4.3 Surface Recombination Velocity on Front and Back
Sides
However, some of the most active recombination centres are at the surfaces of the
silicon wafer with the partially unsaturated bonds of the surface atoms. Indeed, at
these surfaces, the crystal structure is abruptly interrupted in its periodicity. Defects
occur which provide energy levels within the bandgap, thereby making it easier for
the carriers to recombine. This also changes the profile of the bands and it creates band
distortions. As discussed in Chap. 4, the surface recombination velocity S is a measure
of how fast a charge carrier recombines on the surface. For open, unpassivated silicon
surfaces, it is in the range of 10
5 –10
6 cm s
−1 and drastically limits the open-circuit
voltage V OC of the solar cell. This can be visualized in such a way that the minority
carriers (in p-type wafers the electrons) flow with the speed S to the surface and
recombine there, emptying, thus, the space charge zone. For an increase in V OC and
an associated increase in the efficiency of the solar cell, it is therefore imperative to
minimize the surface recombination velocity by passivation of the surface, e.g. by
deactivation of the recombination centres.
27 We can now discuss how that can be
done at the front and back surfaces.
Surface Recombination on the Front Side
As the standard solar cell has demonstrated, excellent surface passivation has
been achieved thanks to silicon nitride. The recombination velocity is about 200–
600 cm s
−1 for standard solar cells (Al-BSF-cells), so that an improvement over
unpassivated silicon surfaces by several orders of magnitude was obtained.
Surface Recombination on the Back Side
The surface recombination velocity of the solar cell on the back side is 500–
1000 cm s
−1 and is about three times higher than that on the front side. In standard solar cells (Al-BSF cells), the passivation of the back side usually takes place
via the metallization with an aluminium layer. Aluminium has the advantage that
it is, like boron, trivalent and results in p-doping of silicon. If aluminium diffuses
into a p-doped (boron-doped) silicon crystal, the doping concentration at this point
increases, so that even more holes are created on the back side and the Fermi level
27 A measure for assessing the quality of the passivation is the open circuit voltage V OC . The
better the passivation, the less electrons recombine and the higher open circuit voltage V OC . On
average, V oc is 650 mV for standard solar cells. The recombination velocity depends strongly on
the surface doping concentration.