Page 205 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 205
Degradation mechanisms of the aluminium doped zinc oxide front contact
contaminants in the outer 1 nm. These soluble salt ions may also penetrate into the
bulk material through cracks and defects. This might of course also occur in the grain
boundaries.
2+
The effect of the temperature and CO on Zn in thermodynamic equilibrium is
2
described in reference [22]. Under standard CO concentrations and at 85 C, zinc oxide
o
2
should be the most stable reaction product of zinc. However, at lower temperatures,
Zn (OH) (CO ) (hydrozincite) and ZnCO (smithsonite) are the most stable materials.
6
3
5
3 2
The formation of hydrozincite from zinc hydroxide follows this reaction:
-
2-
2+
Zn(OH) (s) + 4Zn (aq) + 4OH (aq) + 2CO (aq) → Zn (CO ) (OH) (s) (6.4)
5
3 2
6
3
2
-
2-
Analogue reactions can occur in the presence of Cl and SO ions, resulting in the
4
above mentioned chlorides or sulphates.
Furthermore, reference [23] showed experimentally that Zn(OH) is kinetically
2
favoured at low temperatures (under 34 C) and can thus also be formed according to:
o
Zn + 2OH → Zn(OH) 2 (6.5)
2+
-
At higher temperatures, ZnO formation was observed, which is explained by
dehydration. This might also have occurred to our samples due to long storage in
an inert atmosphere. It is expected that the compounds mentioned above might be
formed during field operation of CIGS modules.
Since these results are mainly based on zinc corrosion, this reaction matrix was
expanded further by basic thermodynamical calculations with the program Medusa
[24] on the degradation of ZnO:Al, so a rough idea could be obtained about the
expected reaction products. These results served as a guideline for further studies,
but are not per se valid for this case, which is a situation without thermodynamic
equilibrium in a crystal lattice instead of an aqueous solution. In this calculation,
the reaction products of Zn in an aqueous solution were determined under the
2+
following conditions:
· pH: 6.5
2-
-6
· CO concentration: 5x10 mol/liter
3
3+
· Al concentration: 10 mol/liter
-3
Under these circumstances, Al(OH) (aluminium hydroxide), Zn(OH) (zinc hydroxide)
3
2
and Zn (OH) (CO ) (hydrozincite) came out as the most likely reaction products. Other
5
6
3 2
203
contaminants in the outer 1 nm. These soluble salt ions may also penetrate into the
bulk material through cracks and defects. This might of course also occur in the grain
boundaries.
2+
The effect of the temperature and CO on Zn in thermodynamic equilibrium is
2
described in reference [22]. Under standard CO concentrations and at 85 C, zinc oxide
o
2
should be the most stable reaction product of zinc. However, at lower temperatures,
Zn (OH) (CO ) (hydrozincite) and ZnCO (smithsonite) are the most stable materials.
6
3
5
3 2
The formation of hydrozincite from zinc hydroxide follows this reaction:
-
2-
2+
Zn(OH) (s) + 4Zn (aq) + 4OH (aq) + 2CO (aq) → Zn (CO ) (OH) (s) (6.4)
5
3 2
6
3
2
-
2-
Analogue reactions can occur in the presence of Cl and SO ions, resulting in the
4
above mentioned chlorides or sulphates.
Furthermore, reference [23] showed experimentally that Zn(OH) is kinetically
2
favoured at low temperatures (under 34 C) and can thus also be formed according to:
o
Zn + 2OH → Zn(OH) 2 (6.5)
2+
-
At higher temperatures, ZnO formation was observed, which is explained by
dehydration. This might also have occurred to our samples due to long storage in
an inert atmosphere. It is expected that the compounds mentioned above might be
formed during field operation of CIGS modules.
Since these results are mainly based on zinc corrosion, this reaction matrix was
expanded further by basic thermodynamical calculations with the program Medusa
[24] on the degradation of ZnO:Al, so a rough idea could be obtained about the
expected reaction products. These results served as a guideline for further studies,
but are not per se valid for this case, which is a situation without thermodynamic
equilibrium in a crystal lattice instead of an aqueous solution. In this calculation,
the reaction products of Zn in an aqueous solution were determined under the
2+
following conditions:
· pH: 6.5
2-
-6
· CO concentration: 5x10 mol/liter
3
3+
· Al concentration: 10 mol/liter
-3
Under these circumstances, Al(OH) (aluminium hydroxide), Zn(OH) (zinc hydroxide)
3
2
and Zn (OH) (CO ) (hydrozincite) came out as the most likely reaction products. Other
5
6
3 2
203
