Page 206 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 206
Chapter 6
2-
calculations indicate that the CO concentration has a large impact on the [Zn(OH) ]/
3
2
[Zn (OH) (CO ) ] ratio. The concentration of CO is very important in the consideration
3 2
2
6
5
whether the less stable hydroxide is converted in the more stable hydrozincite.
6.4.3 Atmospheric species treatment
6.4.3.1 Gases
The impact of exposure of ZnO:Al to different kinds of gases (pure CO, O and
2
2
argon) was minimal. Both the electrical and the optical properties did not change
significantly. In general, it can be stated that ZnO:Al seems to be stable for middle
long term exposure (for hunderds of hours) to these conditions.
6.4.3.2 Water purged with nitrogen or oxygen
When the H O/N and H O/O samples are considered, a minor increase in resistivity during
2
2
2
2
the first 100 hours was observed. The electrical changes could majorly be attributed to
changes in mobility, while the carrier concentration did not change significantly.
Reference [15] describes the dominant electron scattering mechanisms in zinc oxide
limiting the electron mobility for ZnO:Al: potential barriers at the grain boundaries
and ionised impurity scattering in the grain. For the H O/N and H O/O samples, based
2
2
2
2
on the stable carrier concentrations and the decreasing mobility, it can be expected
that the formation of detrimental compounds in zinc oxide occurs mainly at the grain
boundaries and not in the bulk material.
This is supported by the observation in reference [16], which shows that neon
cannot effuse through the bulk of the grains, while it can migrate through the grain
boundaries. Therefore, the transport of molecules like oxygen, nitrogen and water
is likely also only possible through the grain boundaries. Since the effects for HO/
2
N and HO/O on both the electrical and the optical parameters are minor and
2
2
2
nitrogen is known for its inertia, it is not expected that oxygen plays a large role in
the degradation of ZnO:Al. Therefore, it is expected that the main component leading
to ZnO:Al degradation under these conditions is HO. We therefore propose that a
2
reaction between Zn and OH occurs, leading to the formation of Zn(OH) according
-
+
2
to reaction (6.5). Due to the relatively stable carrier concentration, it is likely that
also in this case, degradation of the grain boundaries is the driving force behind the
observed mobility decrease.
6.4.3.3 Water purged with CO 2
The sample in H O/CO dissolved directly, which is caused by the decreased pH (4.7),
2
2
that occurred due to the dissolution of CO in water.
2
H O (l) + CO (g) → 2H (aq) + CO (aq) (6.6)
+
2-
2
3
2
204
2-
calculations indicate that the CO concentration has a large impact on the [Zn(OH) ]/
3
2
[Zn (OH) (CO ) ] ratio. The concentration of CO is very important in the consideration
3 2
2
6
5
whether the less stable hydroxide is converted in the more stable hydrozincite.
6.4.3 Atmospheric species treatment
6.4.3.1 Gases
The impact of exposure of ZnO:Al to different kinds of gases (pure CO, O and
2
2
argon) was minimal. Both the electrical and the optical properties did not change
significantly. In general, it can be stated that ZnO:Al seems to be stable for middle
long term exposure (for hunderds of hours) to these conditions.
6.4.3.2 Water purged with nitrogen or oxygen
When the H O/N and H O/O samples are considered, a minor increase in resistivity during
2
2
2
2
the first 100 hours was observed. The electrical changes could majorly be attributed to
changes in mobility, while the carrier concentration did not change significantly.
Reference [15] describes the dominant electron scattering mechanisms in zinc oxide
limiting the electron mobility for ZnO:Al: potential barriers at the grain boundaries
and ionised impurity scattering in the grain. For the H O/N and H O/O samples, based
2
2
2
2
on the stable carrier concentrations and the decreasing mobility, it can be expected
that the formation of detrimental compounds in zinc oxide occurs mainly at the grain
boundaries and not in the bulk material.
This is supported by the observation in reference [16], which shows that neon
cannot effuse through the bulk of the grains, while it can migrate through the grain
boundaries. Therefore, the transport of molecules like oxygen, nitrogen and water
is likely also only possible through the grain boundaries. Since the effects for HO/
2
N and HO/O on both the electrical and the optical parameters are minor and
2
2
2
nitrogen is known for its inertia, it is not expected that oxygen plays a large role in
the degradation of ZnO:Al. Therefore, it is expected that the main component leading
to ZnO:Al degradation under these conditions is HO. We therefore propose that a
2
reaction between Zn and OH occurs, leading to the formation of Zn(OH) according
-
+
2
to reaction (6.5). Due to the relatively stable carrier concentration, it is likely that
also in this case, degradation of the grain boundaries is the driving force behind the
observed mobility decrease.
6.4.3.3 Water purged with CO 2
The sample in H O/CO dissolved directly, which is caused by the decreased pH (4.7),
2
2
that occurred due to the dissolution of CO in water.
2
H O (l) + CO (g) → 2H (aq) + CO (aq) (6.6)
+
2-
2
3
2
204
