Page 164 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 164
Chapter 5
These results are in agreement with the results as observed in reference [7], which
reported the formation of suboxides with O/Mo ratios between MoO and MoO. In
2
3
this study, XPS measurements have shown that the O/Mo ratio is 2.88±0.01, which is
similar to the MoO -like compounds as observed here.
3
The cracked molybdenum oxide on the surface contained small amounts of nitrogen
and carbon, indicating the presence of nitrates and carbonates. The small crystalline
needle-like structures have a higher content of sodium and carbon than the underlying
layer, which might indicate the formation of Na CO . However, it is not likely that these
3
2
needle-like structures would also occur in complete CIGS solar cells.
5.4.1.2 Electrical effects
The degradation effects increased with the porosity of the molybdenum material:
porous layers resulted in the formation of the thickest oxide layer due to damp heat
exposure. This has an effect on the sheet resistance of the most porous molybdenum,
which became non-conductive after approximately thirty hours, while the most
dense layers still showed conductivity by four point probe after 105 hours exposure
o
to 85 C/85% RH. It can thus be concluded that low sputter pressures lead to the
formation of molybdenum layers that retain their conductivity for a longer time.
The presence of selenium did not have an influence of this abrupt loss of conductivity.
However, the presence of selenium seemed to lead to a smaller increase in sheet
resistance due to damp heat exposure: while selenised samples started with a
higher sheet resistance, the increase in sheet resistance as a function of damp heat
time is slower.
This is especially important for the conduction in P2 through which a thin column of
molybdenum and ZnO:Al has to transport the current. The molybdenum back contact,
which still consists of conductive molybdenum and transports the current in the
lateral direction will suffer less from the formation of a non-conductive molybdenum
oxide layer on top of a still conducting molybdenum layer.
5.4.1.3 Optical properties and visual inspection
The selenised and non-selenised samples showed large differences in degradation
behaviour, especially when the visual and optical properties are considered. This
indicates that degradation experiments on non-selenised molybdenum layers are
not representative for the degradation of the molybdenum layer in CIGS modules.
However, degradation experiments on non-selenised samples can show what is
happening to stored non-processed molybdenum sheets as well as non-selenised side
162
These results are in agreement with the results as observed in reference [7], which
reported the formation of suboxides with O/Mo ratios between MoO and MoO. In
2
3
this study, XPS measurements have shown that the O/Mo ratio is 2.88±0.01, which is
similar to the MoO -like compounds as observed here.
3
The cracked molybdenum oxide on the surface contained small amounts of nitrogen
and carbon, indicating the presence of nitrates and carbonates. The small crystalline
needle-like structures have a higher content of sodium and carbon than the underlying
layer, which might indicate the formation of Na CO . However, it is not likely that these
3
2
needle-like structures would also occur in complete CIGS solar cells.
5.4.1.2 Electrical effects
The degradation effects increased with the porosity of the molybdenum material:
porous layers resulted in the formation of the thickest oxide layer due to damp heat
exposure. This has an effect on the sheet resistance of the most porous molybdenum,
which became non-conductive after approximately thirty hours, while the most
dense layers still showed conductivity by four point probe after 105 hours exposure
o
to 85 C/85% RH. It can thus be concluded that low sputter pressures lead to the
formation of molybdenum layers that retain their conductivity for a longer time.
The presence of selenium did not have an influence of this abrupt loss of conductivity.
However, the presence of selenium seemed to lead to a smaller increase in sheet
resistance due to damp heat exposure: while selenised samples started with a
higher sheet resistance, the increase in sheet resistance as a function of damp heat
time is slower.
This is especially important for the conduction in P2 through which a thin column of
molybdenum and ZnO:Al has to transport the current. The molybdenum back contact,
which still consists of conductive molybdenum and transports the current in the
lateral direction will suffer less from the formation of a non-conductive molybdenum
oxide layer on top of a still conducting molybdenum layer.
5.4.1.3 Optical properties and visual inspection
The selenised and non-selenised samples showed large differences in degradation
behaviour, especially when the visual and optical properties are considered. This
indicates that degradation experiments on non-selenised molybdenum layers are
not representative for the degradation of the molybdenum layer in CIGS modules.
However, degradation experiments on non-selenised samples can show what is
happening to stored non-processed molybdenum sheets as well as non-selenised side
162
