Page 171 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 171
Degradation mechanisms of the molybdenum back contact
intercalated. Finally it should be also highlighted that the bronze electrical properties
show a large two-dimensional anisotropy. This leads to a difference in the electrical
conductivity along different axes: For K Mo O ,the resistivity is a 1000 times higher
0.9
17
6
along the b axis than along the layers [45]. The non-conductive behaviour could also,
partly, related to the orientation of the Na intercalated MoO layer.
+
3
5.4.2.3 Influence of molybdenum density
The optical and electrical measurements showed that Mo25/15 degraded faster
than Mo25/2. It was also observed that the black and blue spots, associated with
the oxidation of the molybdenum, occurred faster for Mo25/15. Therefore, it can be
assumed Mo25/2 is more stable, as can be expected based on the ‘selenisation and
pressure’ experiment.
XPS measurements also confirmed the presence of higher concentrations of oxides
for Mo25/15, therefore indicating the formation of a higher amount of molybdenum
oxide. The faster decrease for the selenium content also confirms this. However, it was
observed that the sodium concentration was the same for both samples. Therefore,
it is assumed that sodium migrates through the samples during the damp heat
exposure, independently of the oxidation of the molybdenum. Likely the sodium is
thus also present in the non-degraded MoSe regions.
2
5.4.3 Influence of molybdenum degradation on CIGS solar cell performance
It can be concluded that the slowest degradation with respect to reflectance,
conductivity and volume expansion can be obtained for molybdenum samples with
a low argon sputter pressure and a selenised surface. The former can be explained
by the dense structure with a small volume of intergrain material, which would allow
migration of the degrading species, like O and H O as well as sodium. The influence
2
2
of selenisation might be explained by the presence of MoSe in the intergrain space,
2
preventing either the diffusion of degrading species or preventing the oxidation of
the molybdenum.
Most molybdenum back contacts are obtained by the deposition of a bilayer of
molybdenum, of which the bottom layer is sputtered under high argon pressure,
while the top layer is deposited under low argon pressure. This is positive for the
stability of the molybdenum layer in CIGS solar cells, since the water and oxygen
migrate into the bottom layer, therefore degrading the top layer less and maintaining
a high conductivity for the layer. Therefore, the conditions used for the deposition
of the ‘standard’ molybdenum layers in CIGS solar cells are similar to the deposition
conditions of the slowly degrading sample in these studies.
169
intercalated. Finally it should be also highlighted that the bronze electrical properties
show a large two-dimensional anisotropy. This leads to a difference in the electrical
conductivity along different axes: For K Mo O ,the resistivity is a 1000 times higher
0.9
17
6
along the b axis than along the layers [45]. The non-conductive behaviour could also,
partly, related to the orientation of the Na intercalated MoO layer.
+
3
5.4.2.3 Influence of molybdenum density
The optical and electrical measurements showed that Mo25/15 degraded faster
than Mo25/2. It was also observed that the black and blue spots, associated with
the oxidation of the molybdenum, occurred faster for Mo25/15. Therefore, it can be
assumed Mo25/2 is more stable, as can be expected based on the ‘selenisation and
pressure’ experiment.
XPS measurements also confirmed the presence of higher concentrations of oxides
for Mo25/15, therefore indicating the formation of a higher amount of molybdenum
oxide. The faster decrease for the selenium content also confirms this. However, it was
observed that the sodium concentration was the same for both samples. Therefore,
it is assumed that sodium migrates through the samples during the damp heat
exposure, independently of the oxidation of the molybdenum. Likely the sodium is
thus also present in the non-degraded MoSe regions.
2
5.4.3 Influence of molybdenum degradation on CIGS solar cell performance
It can be concluded that the slowest degradation with respect to reflectance,
conductivity and volume expansion can be obtained for molybdenum samples with
a low argon sputter pressure and a selenised surface. The former can be explained
by the dense structure with a small volume of intergrain material, which would allow
migration of the degrading species, like O and H O as well as sodium. The influence
2
2
of selenisation might be explained by the presence of MoSe in the intergrain space,
2
preventing either the diffusion of degrading species or preventing the oxidation of
the molybdenum.
Most molybdenum back contacts are obtained by the deposition of a bilayer of
molybdenum, of which the bottom layer is sputtered under high argon pressure,
while the top layer is deposited under low argon pressure. This is positive for the
stability of the molybdenum layer in CIGS solar cells, since the water and oxygen
migrate into the bottom layer, therefore degrading the top layer less and maintaining
a high conductivity for the layer. Therefore, the conditions used for the deposition
of the ‘standard’ molybdenum layers in CIGS solar cells are similar to the deposition
conditions of the slowly degrading sample in these studies.
169
