Page 256 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 256
Chapter 8
for the CIGS solar cells. These effects mostly occurred in the HO/CO /N and H O/air
2
2
2
2
samples, so it is proposed that these gaps were formed in the presence of both HO
2
and large quantities of CO . Proposed simplified representations of the dissolution of
2
the zinc oxide layers in water in the presence and absence of large quantities of CO2
are shown in Figure 8.10.
It should be noted that gap formation in complete CIGS solar cells has been described
before. Feist et al. [4] showed the formation of small circular holes (up to 20 nm) in the
ZnO:Al layer close to the CIGSSe/ZnO:Al interface due to water exposure. In that case,
these holes were observed in encapsulated minimodules exposed for 168 hours to
o
85 C H O purged with O . These samples also showed a very rapid decrease in current,
2
2
leading to an efficiency decrease. Naturally, the high temperature contributes greatly
to the reactivity of many species present, but alongside with the limited amounts of
gaps observed in the HO/O experiment in this study, this shows that also HO with
2
2
2
O can lead to degradation under certain conditions. However, the main driving force
2
behind the dissolution of the zinc oxide appears to be H O combined with CO .
2
2
The most straightforward explanation of the dissolution of the zinc oxide can be
found in the stability of zinc oxide as a function of pH. Zinc oxide is thermodynamically
stable at a pH between 6 and 12, while it dissolves outside of this range [5]:
For acidic solutions, the following reaction can occur:
+
ZnO (s) + 2H (aq) → Zn (aq) + H O (l) [5] (8.1)
2+
2
-
In alkaline solutions, depending on among other things the OH content, this reaction
occurs:
ZnO (s) + 2OH (aq) + H O (l) → Zn(OH) 4 2- (aq)[5] (8.2)
-
2
Although the global pH of all degradation vessels was always between 6 and 12, the
local pH at the grain boundaries or in CIGS/CdS/i-ZnO region may have been different.
This might have been caused by the presence of CO. It is possible that locally in the
2
grain boundaries the following reaction occurred:
H O (l) + CO (g) H CO (aq) (8.3)
2
3
2
2
Carbonic acid (HCO ) then can ionise in water forming low concentrations of
3
2
hydronium and carbonate ions:
−
H O (l) + H CO (aq) HCO (aq) + H O (aq) (8.4)
+
2
3
3
2
3
254
for the CIGS solar cells. These effects mostly occurred in the HO/CO /N and H O/air
2
2
2
2
samples, so it is proposed that these gaps were formed in the presence of both HO
2
and large quantities of CO . Proposed simplified representations of the dissolution of
2
the zinc oxide layers in water in the presence and absence of large quantities of CO2
are shown in Figure 8.10.
It should be noted that gap formation in complete CIGS solar cells has been described
before. Feist et al. [4] showed the formation of small circular holes (up to 20 nm) in the
ZnO:Al layer close to the CIGSSe/ZnO:Al interface due to water exposure. In that case,
these holes were observed in encapsulated minimodules exposed for 168 hours to
o
85 C H O purged with O . These samples also showed a very rapid decrease in current,
2
2
leading to an efficiency decrease. Naturally, the high temperature contributes greatly
to the reactivity of many species present, but alongside with the limited amounts of
gaps observed in the HO/O experiment in this study, this shows that also HO with
2
2
2
O can lead to degradation under certain conditions. However, the main driving force
2
behind the dissolution of the zinc oxide appears to be H O combined with CO .
2
2
The most straightforward explanation of the dissolution of the zinc oxide can be
found in the stability of zinc oxide as a function of pH. Zinc oxide is thermodynamically
stable at a pH between 6 and 12, while it dissolves outside of this range [5]:
For acidic solutions, the following reaction can occur:
+
ZnO (s) + 2H (aq) → Zn (aq) + H O (l) [5] (8.1)
2+
2
-
In alkaline solutions, depending on among other things the OH content, this reaction
occurs:
ZnO (s) + 2OH (aq) + H O (l) → Zn(OH) 4 2- (aq)[5] (8.2)
-
2
Although the global pH of all degradation vessels was always between 6 and 12, the
local pH at the grain boundaries or in CIGS/CdS/i-ZnO region may have been different.
This might have been caused by the presence of CO. It is possible that locally in the
2
grain boundaries the following reaction occurred:
H O (l) + CO (g) H CO (aq) (8.3)
2
3
2
2
Carbonic acid (HCO ) then can ionise in water forming low concentrations of
3
2
hydronium and carbonate ions:
−
H O (l) + H CO (aq) HCO (aq) + H O (aq) (8.4)
+
2
3
3
2
3
254
