Page 166 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 166
Chapter 5
Before degradation After degradation
air
Mo
15 mtorr
MoSe 2
(Na)MoO x
25 mtorr ‘Foreign’
glass species
Figure 5.30
Schematic representation of the molybdenum film before and after degradation, drawn on the cross-section SEM photo of
Mo25/15. The lines represent the composition of the grain boundaries: The blue lines represent (Na)MoO x , while the red lines
represent MoSe 2 . The yellow spots indicate the presence of foreign species, like carbon, nitrogen, chlorine and sulfur. It should
be noted that the only the global position of the colours should be considered, since all materials will be present in some
concentration at all indicated positions.
5.4.2 Lift-off experiment
It has been observed that bilayer molybdenum thin films, which contain small
concentrations of MoSe as obtained by lifting off of a CIGSe layer, degrade when
2
exposed to damp heat. This exposure led to a complete loss of conductivity after
150 hours, while the reflectivity also decreased. It was observed that the degradation
behaviour for both the electrical and optical parameters was less severe for the
molybdenum thin film deposited at lower sputter pressure, as was also observed
in the ‘selenisation and pressure’ experiment. However, the degradation process
occurred slower for these bilayer Mo samples obtained by the lift-off process than for
the standard selenised monolayer molybdenum.
In order to learn more about the degradation process and its products, the samples
have been studied with SEM-EDX, XPS and SIMS. Cross-section SEM did not show the
formation of a thick MoO layer on top of the metallic molybdenum, as reported for the
x
‘selenisation and pressure’ experiment but it clearly showed the bilayer structure. SIMS
indicated the presence of a bottom layer with a high concentration of molybdenum
oxide and ‘foreign’ species, like carbon, nitrogen and chlorine. The local degradation
of this bottom layer is probably due to the porous nature of that molybdenum before
damp heat exposure, since it was deposited at a high sputter pressure. This also led
to a less degraded top layer: the degrading species diffused towards the bottom layer
and did not impact the top layer. It is proposed that the decrease of reflectivity and
conductivity is therefore a relatively slow process for the lift-off samples: the oxidation
of the bottom layer has less impact on the physical parameters than changes at the
Mo/CIGS interface.
164
Before degradation After degradation
air
Mo
15 mtorr
MoSe 2
(Na)MoO x
25 mtorr ‘Foreign’
glass species
Figure 5.30
Schematic representation of the molybdenum film before and after degradation, drawn on the cross-section SEM photo of
Mo25/15. The lines represent the composition of the grain boundaries: The blue lines represent (Na)MoO x , while the red lines
represent MoSe 2 . The yellow spots indicate the presence of foreign species, like carbon, nitrogen, chlorine and sulfur. It should
be noted that the only the global position of the colours should be considered, since all materials will be present in some
concentration at all indicated positions.
5.4.2 Lift-off experiment
It has been observed that bilayer molybdenum thin films, which contain small
concentrations of MoSe as obtained by lifting off of a CIGSe layer, degrade when
2
exposed to damp heat. This exposure led to a complete loss of conductivity after
150 hours, while the reflectivity also decreased. It was observed that the degradation
behaviour for both the electrical and optical parameters was less severe for the
molybdenum thin film deposited at lower sputter pressure, as was also observed
in the ‘selenisation and pressure’ experiment. However, the degradation process
occurred slower for these bilayer Mo samples obtained by the lift-off process than for
the standard selenised monolayer molybdenum.
In order to learn more about the degradation process and its products, the samples
have been studied with SEM-EDX, XPS and SIMS. Cross-section SEM did not show the
formation of a thick MoO layer on top of the metallic molybdenum, as reported for the
x
‘selenisation and pressure’ experiment but it clearly showed the bilayer structure. SIMS
indicated the presence of a bottom layer with a high concentration of molybdenum
oxide and ‘foreign’ species, like carbon, nitrogen and chlorine. The local degradation
of this bottom layer is probably due to the porous nature of that molybdenum before
damp heat exposure, since it was deposited at a high sputter pressure. This also led
to a less degraded top layer: the degrading species diffused towards the bottom layer
and did not impact the top layer. It is proposed that the decrease of reflectivity and
conductivity is therefore a relatively slow process for the lift-off samples: the oxidation
of the bottom layer has less impact on the physical parameters than changes at the
Mo/CIGS interface.
164
