Page 229 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 229
The impact of alkali elements
they were exposed to damp heat and illumination for a very long time.
Additionally, cross-section SEM pictures were also taken, as is shown in Figure 7.12.
Although all deposition conditions, except for the introduction of the SiN barrier layer,
x
were identical, the samples clearly look different. The molybdenum layer for the alkali-
poor sample has a thickness of 0.6 μm, while its thickness is 1.0 μm for the alkali-rich
sample, which likely has a more porous structure. Another surprising difference is the
alkali-poor
alkali-rich
Figure 7.12
Cross-section SEM pictures of the alkali-poor (top) and the alkali-rich (bottom) samples before and after 165 and 778 hours
of exposure to damp heat and illumination.
CIGS grain size: while it is often stated that the alkali content influences the grain size,
an increase in grain size is mostly reported [8]. In this case, the grain size becomes
smaller, either due to the present of the alkali content, or due to the changed
molybdenum morphology.
After degradation, no changes in morphology can be detected for either types during
the first 165 hours, even though the composition and the electrical properties of the
alkali-rich samples had already clearly changed. After 365 hours (not depicted), some
small cracks seemed to occur in the alkali-rich sample. However, large impact on the
morphology is only visible for this sample after 778 hours, when cracks occurred, as
can be seen in Figure 7.12 and Figure 7.13. Additionally, Figure 7.13 shows that the
complete structure of the stack has expanded at some positions.
5 μm
Figure 7.13
Cross-section SEM picture of the alkali-rich sample after 778 hours of exposure to damp heat and illumination.
227
they were exposed to damp heat and illumination for a very long time.
Additionally, cross-section SEM pictures were also taken, as is shown in Figure 7.12.
Although all deposition conditions, except for the introduction of the SiN barrier layer,
x
were identical, the samples clearly look different. The molybdenum layer for the alkali-
poor sample has a thickness of 0.6 μm, while its thickness is 1.0 μm for the alkali-rich
sample, which likely has a more porous structure. Another surprising difference is the
alkali-poor
alkali-rich
Figure 7.12
Cross-section SEM pictures of the alkali-poor (top) and the alkali-rich (bottom) samples before and after 165 and 778 hours
of exposure to damp heat and illumination.
CIGS grain size: while it is often stated that the alkali content influences the grain size,
an increase in grain size is mostly reported [8]. In this case, the grain size becomes
smaller, either due to the present of the alkali content, or due to the changed
molybdenum morphology.
After degradation, no changes in morphology can be detected for either types during
the first 165 hours, even though the composition and the electrical properties of the
alkali-rich samples had already clearly changed. After 365 hours (not depicted), some
small cracks seemed to occur in the alkali-rich sample. However, large impact on the
morphology is only visible for this sample after 778 hours, when cracks occurred, as
can be seen in Figure 7.12 and Figure 7.13. Additionally, Figure 7.13 shows that the
complete structure of the stack has expanded at some positions.
5 μm
Figure 7.13
Cross-section SEM picture of the alkali-rich sample after 778 hours of exposure to damp heat and illumination.
227
