Page 37 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 37
Stability of Cu(In,Ga)Se 2 Solar Cells
Theelen et al. [18,26] studied the chemical structure of the molybdenum oxide. The
5+
molybdenum oxide formed after 105 hours of damp heat exposure contained Mo
6+
and Mo and had an average composition of MoO . Raman spectroscopy indicat-
2.88
ed the presence of MoO and MoO bindings, suggesting the presence of suboxides,
3
2
which are oxides with a molybdenum oxide ratio in between MoO and MoO [26].
2
3
They also observed that the oxide layers can crack, caused either by the volume ex-
pansion due to the oxide formation or by the thermal and/or humidity shocks that oc -
cur when the samples are removed from the climate chamber. Since this leads to the
exposure of a new surface of metallic molybdenum to degrading species, a second
underlying layer of molybdenum oxide can be formed. Reference [26] also reported
the occurrence of needles on the oxide surface, which may consist of NaCO . More
2
3
information about these experiments can be found in chapter 5.
Pern et al. [24] reported the formation of yellow-blue materials when molybdenum
was exposed to damp heat. This might be MoO or a molybdenum hydroxide. The
3
occurrence of large structural and morphological changes like the formation of micro -
cracks, as well as the loss of reflectivity, were also observed in this case.
Wennerberg et al. [27] also observed the oxidation of molybdenum under damp heat
conditions, leading to the formation of a non-conductive milky MoO material. The
3
corrosion started at point defects randomly scattered across the surface, from where
the corrosion grew laterally with time, leading to the formation of clusters of larger
corroded areas. This led to an exponential increase in sheet resistance which had al-
ready reached a 6.5 fold increase from the initial value after 180 hours of damp heat.
It was also shown that the sputter pressure influenced the stability of molybdenum
exposed to humidity and temperature. Reference [26] showed that denser molybde-
num, deposited at lower sputter pressure, was more resistant to damp heat. These
molybdenum layers had a thinner oxide layer and thus retained higher conductivity
and reflectivity: The reflectivity (measured from 340 to 1120 nm) of the porous sample
decreased from 34% to 17% after only four hours of damp heat exposure, while the
dense sample showed a decline from 45% to 40%.
In order to discover which degradation conditions lead to the molybdenum degra-
dation, molybdenum films were exposed to temperature and humidity conditions,
which became higher in time [19]. The films showed a stable resistance up to 70 o C/70%
o
RH conditions, but became highly resistant after exposure to 85C/70% RH. Under
these conditions, the colour also changed to dark brown with small white dots. Feist
et al. [29] exposed molybdenum films for 48 hours to several degradation conditions,
o
o
like dry heat (85 C), damp heat (85 C/100% RH) and room temperature (RT) water. The
sample in the damp heat test increased in thickness from 700 to 900 nm, while the
35
Theelen et al. [18,26] studied the chemical structure of the molybdenum oxide. The
5+
molybdenum oxide formed after 105 hours of damp heat exposure contained Mo
6+
and Mo and had an average composition of MoO . Raman spectroscopy indicat-
2.88
ed the presence of MoO and MoO bindings, suggesting the presence of suboxides,
3
2
which are oxides with a molybdenum oxide ratio in between MoO and MoO [26].
2
3
They also observed that the oxide layers can crack, caused either by the volume ex-
pansion due to the oxide formation or by the thermal and/or humidity shocks that oc -
cur when the samples are removed from the climate chamber. Since this leads to the
exposure of a new surface of metallic molybdenum to degrading species, a second
underlying layer of molybdenum oxide can be formed. Reference [26] also reported
the occurrence of needles on the oxide surface, which may consist of NaCO . More
2
3
information about these experiments can be found in chapter 5.
Pern et al. [24] reported the formation of yellow-blue materials when molybdenum
was exposed to damp heat. This might be MoO or a molybdenum hydroxide. The
3
occurrence of large structural and morphological changes like the formation of micro -
cracks, as well as the loss of reflectivity, were also observed in this case.
Wennerberg et al. [27] also observed the oxidation of molybdenum under damp heat
conditions, leading to the formation of a non-conductive milky MoO material. The
3
corrosion started at point defects randomly scattered across the surface, from where
the corrosion grew laterally with time, leading to the formation of clusters of larger
corroded areas. This led to an exponential increase in sheet resistance which had al-
ready reached a 6.5 fold increase from the initial value after 180 hours of damp heat.
It was also shown that the sputter pressure influenced the stability of molybdenum
exposed to humidity and temperature. Reference [26] showed that denser molybde-
num, deposited at lower sputter pressure, was more resistant to damp heat. These
molybdenum layers had a thinner oxide layer and thus retained higher conductivity
and reflectivity: The reflectivity (measured from 340 to 1120 nm) of the porous sample
decreased from 34% to 17% after only four hours of damp heat exposure, while the
dense sample showed a decline from 45% to 40%.
In order to discover which degradation conditions lead to the molybdenum degra-
dation, molybdenum films were exposed to temperature and humidity conditions,
which became higher in time [19]. The films showed a stable resistance up to 70 o C/70%
o
RH conditions, but became highly resistant after exposure to 85C/70% RH. Under
these conditions, the colour also changed to dark brown with small white dots. Feist
et al. [29] exposed molybdenum films for 48 hours to several degradation conditions,
o
o
like dry heat (85 C), damp heat (85 C/100% RH) and room temperature (RT) water. The
sample in the damp heat test increased in thickness from 700 to 900 nm, while the
35
