Page 83 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Stability of Cu(In,Ga)Se 2 Solar Cells
scribe where current enters the ZnO layer (Figure 2.14). This can be observed by faster
heating of this side. This effect was also observed after 8770 hours of dry heat exposure.
P1 P2 P3 P1 P2 P3
Front contact: Absorber: Back contact: Substrate
Doped ZnO CIGS molybdenum
Figure 2.14
Schematic picture of the current flow through a CIGS module containing a TCO with increased resistance. This figure is
based on reference [108].
2.5.1.5 Summary on scribe degradation
The monolithic interconnection scheme of a CIGS module plays a large role in module
degradation. The most vulnerable scribe is P2, where a Mo/ZnO:Al contact is responsi -
ble for the current transport between solar cells. Increased resistance of this scribe due
to dam heat exposure was often observed with the help of model systems. Possible
reasons are the introduction of an oxide layer at the Mo/ZnO:Al interface or decreased
resistivity of ZnO:Al films.
Furthermore, degradation of P3 was also observed, due to the transformation of mo-
lybdenum in molybdenum oxide, for example on positions that have been damaged
by the scribing process. Since the metal molybdenum has a high conductivity, its deg -
radation only impacts the module efficiency when the molybdenum layer is almost
completely oxidised, which will likely only happen after very long exposure to humid -
ity and oxygen. Finally, shunting around the P1 scribe could also influence the module
stability, but this could not be observed in a module.
Furthermore, the increased resistivity of the TCO also has an additional impact on
modules, in case the molybdenum resistivity remains largely constant. This leads to
the creation of a difference in voltage drops at different positions, resulting in local
heating of the module.
81
scribe where current enters the ZnO layer (Figure 2.14). This can be observed by faster
heating of this side. This effect was also observed after 8770 hours of dry heat exposure.
P1 P2 P3 P1 P2 P3
Front contact: Absorber: Back contact: Substrate
Doped ZnO CIGS molybdenum
Figure 2.14
Schematic picture of the current flow through a CIGS module containing a TCO with increased resistance. This figure is
based on reference [108].
2.5.1.5 Summary on scribe degradation
The monolithic interconnection scheme of a CIGS module plays a large role in module
degradation. The most vulnerable scribe is P2, where a Mo/ZnO:Al contact is responsi -
ble for the current transport between solar cells. Increased resistance of this scribe due
to dam heat exposure was often observed with the help of model systems. Possible
reasons are the introduction of an oxide layer at the Mo/ZnO:Al interface or decreased
resistivity of ZnO:Al films.
Furthermore, degradation of P3 was also observed, due to the transformation of mo-
lybdenum in molybdenum oxide, for example on positions that have been damaged
by the scribing process. Since the metal molybdenum has a high conductivity, its deg -
radation only impacts the module efficiency when the molybdenum layer is almost
completely oxidised, which will likely only happen after very long exposure to humid -
ity and oxygen. Finally, shunting around the P1 scribe could also influence the module
stability, but this could not be observed in a module.
Furthermore, the increased resistivity of the TCO also has an additional impact on
modules, in case the molybdenum resistivity remains largely constant. This leads to
the creation of a difference in voltage drops at different positions, resulting in local
heating of the module.
81
