Page 51 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Stability of Cu(In,Ga)Se 2 Solar Cells
a damp heat test for 1000 hours. Mini-modules with both buffer technologies still
showed more than 95% of the initial power after 1000 hours damp heat, while no
differences in the degradation behaviour was found.
As will be described in chapter 2.4.2, Kushiya et al. [10] exposed CIGS modules with
a CBD Zn(O,S,OH) buffer to a combined test with damp heat and illumination. The
x
changes in fill factor as observed within this experiment might be related to this buf-
fer. The hydration of ZnO by free water molecules in the buffer might have led to
Zn(OH) formation, leading to deterioration of the properties of the pn-interface and
2
the fill factor. The reverse reaction, which is the dehydration of the Zn(OH) to ZnO,
2
might have explained increases in fill factor.
2.3.3.3 Summary on buffer degradation
Under damp heat exposure, the thin CdS buffer seems to diffuse into the CIGS and
ZnO layers. Possible reaction products are ZnSO or a similar sulphate and Cd Zn S
4
1-x
x
and ZnO S . The interaction between CdS and the doped ZnO can possibly lead to an
1-x x
increase in the sheet resistance of the latter layer. Interaction between these layers was
also found when Zn(O,S,OH) was used as buffer, which likely resulted in the hydration
x
and dehydration of ZnO into Zn(OH) and vice versa. When In S was used as alternative
2
2 3
buffer, no large differences in stability with CdS based solar cells could be found.
2.3.4 TCO degradation
The transparent conductive oxide (TCO) functions as a front contact of CIGS solar cells
or modules. It should be transparent to allow the influx of photons to the CIGS layer,
while its conductivity is required for the transport of the produced electrons. There-
fore, the main requirements for a TCO in a solar cell or module are the conductivity as
well as the transparency.
Studies on the effects of field and accelerated testing of CIGS modules have indicated
that the primary observed reason for module failure is the decrease of fill factor as a
result of an increased series resistance of the TCO [4,56]. The TCO is therefore often
stated as the key element in CIGS module degradation. This can be explained by the
vulnerability of some TCOs for water, but also by the fact that cell sizes are mostly
optimised based on initial TCO conductivity. Therefore, small decreases in the con-
ductivity can already greatly influence the series resistance and thus the efficiency of
a CIGS solar cell or module.
The main type of TCO used in CIGS modules is sputtered aluminium doped zinc oxide
(ZnO:Al), but other types, including indium tin oxide (ITO), indium zinc oxide (IZO) and
chemical vapour deposited (CVD) boron doped zinc oxide (ZnO:B) are also used.
In order to compare the different types of TCOs with each other, the degradation rate
49
a damp heat test for 1000 hours. Mini-modules with both buffer technologies still
showed more than 95% of the initial power after 1000 hours damp heat, while no
differences in the degradation behaviour was found.
As will be described in chapter 2.4.2, Kushiya et al. [10] exposed CIGS modules with
a CBD Zn(O,S,OH) buffer to a combined test with damp heat and illumination. The
x
changes in fill factor as observed within this experiment might be related to this buf-
fer. The hydration of ZnO by free water molecules in the buffer might have led to
Zn(OH) formation, leading to deterioration of the properties of the pn-interface and
2
the fill factor. The reverse reaction, which is the dehydration of the Zn(OH) to ZnO,
2
might have explained increases in fill factor.
2.3.3.3 Summary on buffer degradation
Under damp heat exposure, the thin CdS buffer seems to diffuse into the CIGS and
ZnO layers. Possible reaction products are ZnSO or a similar sulphate and Cd Zn S
4
1-x
x
and ZnO S . The interaction between CdS and the doped ZnO can possibly lead to an
1-x x
increase in the sheet resistance of the latter layer. Interaction between these layers was
also found when Zn(O,S,OH) was used as buffer, which likely resulted in the hydration
x
and dehydration of ZnO into Zn(OH) and vice versa. When In S was used as alternative
2
2 3
buffer, no large differences in stability with CdS based solar cells could be found.
2.3.4 TCO degradation
The transparent conductive oxide (TCO) functions as a front contact of CIGS solar cells
or modules. It should be transparent to allow the influx of photons to the CIGS layer,
while its conductivity is required for the transport of the produced electrons. There-
fore, the main requirements for a TCO in a solar cell or module are the conductivity as
well as the transparency.
Studies on the effects of field and accelerated testing of CIGS modules have indicated
that the primary observed reason for module failure is the decrease of fill factor as a
result of an increased series resistance of the TCO [4,56]. The TCO is therefore often
stated as the key element in CIGS module degradation. This can be explained by the
vulnerability of some TCOs for water, but also by the fact that cell sizes are mostly
optimised based on initial TCO conductivity. Therefore, small decreases in the con-
ductivity can already greatly influence the series resistance and thus the efficiency of
a CIGS solar cell or module.
The main type of TCO used in CIGS modules is sputtered aluminium doped zinc oxide
(ZnO:Al), but other types, including indium tin oxide (ITO), indium zinc oxide (IZO) and
chemical vapour deposited (CVD) boron doped zinc oxide (ZnO:B) are also used.
In order to compare the different types of TCOs with each other, the degradation rate
49
