Page 207 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 207
Degradation mechanisms of the aluminium doped zinc oxide front contact
The chemical stability of zinc oxide in an aqueous solution is a function of the pH. It
is thermodynamically stable in the pH between 6 and 12, outside this pH range, zinc
2+
oxide dissolves [19]. In an acidic environment, the reaction (6.2) occurs, leading to Zn
formation.
6.4.3.4 Water and water purged with air
The effect for the HO/air and HO samples is more severe than the degradation of
2
2
the H O/N and the HO/O samples. When considering non-purged water as well as
2
2
2
2
water purged by compressed air, it can be assumed that the gases found in air will
be present in the water. A standard air mixture contains N (~78% volume), O (~21%),
2
2
Ar (~1%) and CO (0.03% volume) plus other constituents (e.g. H, Ne, He, Kr) in
2
2
concentrations under 20 ppm. Since ZnO:Al appears to be inert for N and O , while Ar
2
2
is known to be very inert, it is expected that the main reactive species in air is CO. A
2
possible reaction product, as also described above, is hydrozincite (Zn (CO ) (OH) ) as
6
5
3 2
described in reaction mechanism (6.4).
The formation of a carbonate and hydroxide containing material is also supported
by the observation of large quantities of hydrogen, hydroxide and carbon in SIMS
analysis. Furthermore, sulphur and chlorine were also present. As presented in
reference [19], various natural degradation products of zinc contain chlorides and
-
sulphates. These products are formed the presence of Cl and SO ions. The formation
2-
4
reaction are analogue to reaction (6.4), and are expected to result in the formation
of e.g. Zn(OH) Cl •H O (zinc hydroxychloride) and ZnSO (OH) •nH O (basic zinc
2
4
4
6
2
2
5
8
sulphate). The origin of the ‘foreign’ elements was not shown, but these must have
resulted either from the compressed air or the glass (vessel or substrate).
Another observation for the HO/air samples was the appearance of gaps in the
2
ZnO:Al layer near the glass/ZnO:Al interface. This can be explained by preferred
dissolution of the ZnO:Al near the interface. This indicates that ions from the glass
have played a catalytic role in the etching of the ZnO:Al, for example by a local change
in pH. As mentioned above, especially silicon and calcium can migrate through the
grain boundaries from borosilicate glass. Possible candidates for ions with a catalytic
2+
2-
role are therefore for example basic SiO or Ca . ZnO:Al dissolves both under basic
3
conditions (pH>12) and acidic conditions (pH<6), while it is stable in a pH between
6 and 12 [19], but it cannot be distinguished which reaction is more likely. However,
since CO 3 2- is probably involved, an acidic reaction is the most likely.
A significant pH change was not observed, but the volume of formed H or OH has
-
+
been very small compared to the total volume of the H O. Therefore, a local change in
2
pH is possible even when no significant pH shift in the H O has been observed.
2
205
The chemical stability of zinc oxide in an aqueous solution is a function of the pH. It
is thermodynamically stable in the pH between 6 and 12, outside this pH range, zinc
2+
oxide dissolves [19]. In an acidic environment, the reaction (6.2) occurs, leading to Zn
formation.
6.4.3.4 Water and water purged with air
The effect for the HO/air and HO samples is more severe than the degradation of
2
2
the H O/N and the HO/O samples. When considering non-purged water as well as
2
2
2
2
water purged by compressed air, it can be assumed that the gases found in air will
be present in the water. A standard air mixture contains N (~78% volume), O (~21%),
2
2
Ar (~1%) and CO (0.03% volume) plus other constituents (e.g. H, Ne, He, Kr) in
2
2
concentrations under 20 ppm. Since ZnO:Al appears to be inert for N and O , while Ar
2
2
is known to be very inert, it is expected that the main reactive species in air is CO. A
2
possible reaction product, as also described above, is hydrozincite (Zn (CO ) (OH) ) as
6
5
3 2
described in reaction mechanism (6.4).
The formation of a carbonate and hydroxide containing material is also supported
by the observation of large quantities of hydrogen, hydroxide and carbon in SIMS
analysis. Furthermore, sulphur and chlorine were also present. As presented in
reference [19], various natural degradation products of zinc contain chlorides and
-
sulphates. These products are formed the presence of Cl and SO ions. The formation
2-
4
reaction are analogue to reaction (6.4), and are expected to result in the formation
of e.g. Zn(OH) Cl •H O (zinc hydroxychloride) and ZnSO (OH) •nH O (basic zinc
2
4
4
6
2
2
5
8
sulphate). The origin of the ‘foreign’ elements was not shown, but these must have
resulted either from the compressed air or the glass (vessel or substrate).
Another observation for the HO/air samples was the appearance of gaps in the
2
ZnO:Al layer near the glass/ZnO:Al interface. This can be explained by preferred
dissolution of the ZnO:Al near the interface. This indicates that ions from the glass
have played a catalytic role in the etching of the ZnO:Al, for example by a local change
in pH. As mentioned above, especially silicon and calcium can migrate through the
grain boundaries from borosilicate glass. Possible candidates for ions with a catalytic
2+
2-
role are therefore for example basic SiO or Ca . ZnO:Al dissolves both under basic
3
conditions (pH>12) and acidic conditions (pH<6), while it is stable in a pH between
6 and 12 [19], but it cannot be distinguished which reaction is more likely. However,
since CO 3 2- is probably involved, an acidic reaction is the most likely.
A significant pH change was not observed, but the volume of formed H or OH has
-
+
been very small compared to the total volume of the H O. Therefore, a local change in
2
pH is possible even when no significant pH shift in the H O has been observed.
2
205
