respect to p-type wafers. First, the n-type wafers do not suffer from light induced
degradation. In p-type wafers both boron and oxygen are present, which under light
exposure start to make complexes that act like defects. This light-induced degradation
causes a reduction of the power output of 2–3% after the first few weeks of installation.
No such effect is present in n-type wafers. The second advantage is that n-type silicon is
not that sensitive to impurities such as iron. As a result, less effort has to be made to
fabricate high quality n-type silicon, and thus high quality n-type silicon can be processed
cheaper than p-type silicon. On the other hand, p-doped wafers have the advantage that the
boron doping is more homogeneously distributed across the wafer as this is possible for ntype wafers. This means that within one n-type wafer the electronic properties can vary,
which lowers the yield of solar cell production based on n-type monocrystalline wafers.
Although IBC cells are made from n-type wafers, they lack one large p-n junction.
Instead, IBC cells have many localized junctions. The holes are separated at a junction
between the p
+ silicon and the n-type silicon, whereas the electrons are collected using n
+
-
type silicon. The semiconductor-metal interfaces are kept as small as possible in order to
reduce the undesired recombination at this defect-rich interface. Another advantage is that
the cross-section of the metal fingers can be made much larger, because they are at the
back and therefore do not cause any shading losses. Thus, resistive losses at the metallic
contacts can be reduced. Since both electric contacts are on the back side, it contains two
metal grids, as illustrated in Figure 12.16 (b). The passivation layer should made from a
low refractive index material such that it operates like a backside mirror. It will reflect the
light above 900 nm, which is not absorbed during the first pass back into the absorber
layer. Thus, this layer enhances the absorption path length.
At the front side of the IBC cell, losses of light-excited charge carriers due to surface
recombination are suppressed by a front surface field similar to the back surface field
discussed earlier. This field is created with a highly doped n
+ region at the front of the
surface. Thus, an n
+
-n junction is created that acts like an n-p junction. It will act as a
barrier that prevents the light-excited minority holes in the n-region from diffusing
towards the front surface. The front surface field behaves like a passivation for the defects
at the front interface and allows higher levels for the hole minority density in the n-doped
bulk.
Reflective losses at the front side are reduced in a similar way as for PERL solar
cells: deposition of double-layered anti-reflection coatings and texturing of the front
surface.
The IBC concept is commercialized by the US company SunPower Corp., who have
achieved high solar cell efficiencies of 24.2%.
degradation. In p-type wafers both boron and oxygen are present, which under light
exposure start to make complexes that act like defects. This light-induced degradation
causes a reduction of the power output of 2–3% after the first few weeks of installation.
No such effect is present in n-type wafers. The second advantage is that n-type silicon is
not that sensitive to impurities such as iron. As a result, less effort has to be made to
fabricate high quality n-type silicon, and thus high quality n-type silicon can be processed
cheaper than p-type silicon. On the other hand, p-doped wafers have the advantage that the
boron doping is more homogeneously distributed across the wafer as this is possible for ntype wafers. This means that within one n-type wafer the electronic properties can vary,
which lowers the yield of solar cell production based on n-type monocrystalline wafers.
Although IBC cells are made from n-type wafers, they lack one large p-n junction.
Instead, IBC cells have many localized junctions. The holes are separated at a junction
between the p
+ silicon and the n-type silicon, whereas the electrons are collected using n
+
-
type silicon. The semiconductor-metal interfaces are kept as small as possible in order to
reduce the undesired recombination at this defect-rich interface. Another advantage is that
the cross-section of the metal fingers can be made much larger, because they are at the
back and therefore do not cause any shading losses. Thus, resistive losses at the metallic
contacts can be reduced. Since both electric contacts are on the back side, it contains two
metal grids, as illustrated in Figure 12.16 (b). The passivation layer should made from a
low refractive index material such that it operates like a backside mirror. It will reflect the
light above 900 nm, which is not absorbed during the first pass back into the absorber
layer. Thus, this layer enhances the absorption path length.
At the front side of the IBC cell, losses of light-excited charge carriers due to surface
recombination are suppressed by a front surface field similar to the back surface field
discussed earlier. This field is created with a highly doped n
+ region at the front of the
surface. Thus, an n
+
-n junction is created that acts like an n-p junction. It will act as a
barrier that prevents the light-excited minority holes in the n-region from diffusing
towards the front surface. The front surface field behaves like a passivation for the defects
at the front interface and allows higher levels for the hole minority density in the n-doped
bulk.
Reflective losses at the front side are reduced in a similar way as for PERL solar
cells: deposition of double-layered anti-reflection coatings and texturing of the front
surface.
The IBC concept is commercialized by the US company SunPower Corp., who have
achieved high solar cell efficiencies of 24.2%.