Page 58 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 58
Chapter 2
spots and stains of carbonates and oxides. These spots and stains likely formed after
removal from the climate chamber, due to the drying of the condensed water. Spots
were also observed by Hamasha et al. [77] where they occurred on samples exposed
to 100% humidity. They were described as drying stains.
2.3.4.1.3 Material changes during ZnO:Al degradation
The nature of the structural and compositional changes in ZnO:Al exposed to tem-
perature and moisture is hard to detect, because changes mainly happen at the grain
boundaries, thereby only influencing a small volume fraction of the thin film material.
However, two possible processes can explain the degradation at the grain boundaries
due to diffusion of atmospheric species, like water and CO [71]:
2
• Molecular adsorption (physical reaction)
• Chemical reaction of the species with the ZnO:Al
Hüpkes et al. [72] used deuterium as an isotopic marker to identify the amount of
deuteriated water (D O) taken up by the films during damp heat exposure. They con-
2
cluded that the water uptake does not saturate even after 1000 hours and that the
diffusion of the atmospheric species is the rate limiting step for the degradation rate.
This was confirmed by SIMS measurements in reference [71], which showed a gradient
in OH content even after 2876 hours of damp heat exposure.
-
Theelen et al. [71] proposed that the ingression of water leads to the formation of
Zn(OH) , which does not greatly influence the properties of the ZnO film. However, in
2
the presence of CO , materials like hydrozincite (Zn (CO ) (OH) ) can be formed, while
5
2
6
3 2
related chloride and sulphate containing species can also form. These molecules are
probably present in the grain boundaries and can then function as electrical potential
barriers (Figure 2.9). More information about this proposition can be found in chapter 6.
Kim et al. [78] described the composition of the grain boundaries before and after
degradation. It was observed that before degradation, grain boundaries had a higher
local conductance and a higher zinc to oxygen ratio than the material in the middle of
o
the grains. After 24 hours at 80 C/100% RH, the conductance and the zinc to oxygen
ratio had decreased at both positions. This suggests that the oxidation of zinc was
responsible for the loss of local conductance. In this reference, it is proposed that the
excess of zinc, which is present in conductive ZnO forms in the presence of water into
undoped, stoichiometric ZnO with a high resistance and hydrogen gas:
+
2−
Zn(excess) + H O → Zn + 2e + 2H + O (2.6)
2+
−
2
ZnO (stoichiometric, high resistance) + H ↑ [78]
2
56
spots and stains of carbonates and oxides. These spots and stains likely formed after
removal from the climate chamber, due to the drying of the condensed water. Spots
were also observed by Hamasha et al. [77] where they occurred on samples exposed
to 100% humidity. They were described as drying stains.
2.3.4.1.3 Material changes during ZnO:Al degradation
The nature of the structural and compositional changes in ZnO:Al exposed to tem-
perature and moisture is hard to detect, because changes mainly happen at the grain
boundaries, thereby only influencing a small volume fraction of the thin film material.
However, two possible processes can explain the degradation at the grain boundaries
due to diffusion of atmospheric species, like water and CO [71]:
2
• Molecular adsorption (physical reaction)
• Chemical reaction of the species with the ZnO:Al
Hüpkes et al. [72] used deuterium as an isotopic marker to identify the amount of
deuteriated water (D O) taken up by the films during damp heat exposure. They con-
2
cluded that the water uptake does not saturate even after 1000 hours and that the
diffusion of the atmospheric species is the rate limiting step for the degradation rate.
This was confirmed by SIMS measurements in reference [71], which showed a gradient
in OH content even after 2876 hours of damp heat exposure.
-
Theelen et al. [71] proposed that the ingression of water leads to the formation of
Zn(OH) , which does not greatly influence the properties of the ZnO film. However, in
2
the presence of CO , materials like hydrozincite (Zn (CO ) (OH) ) can be formed, while
5
2
6
3 2
related chloride and sulphate containing species can also form. These molecules are
probably present in the grain boundaries and can then function as electrical potential
barriers (Figure 2.9). More information about this proposition can be found in chapter 6.
Kim et al. [78] described the composition of the grain boundaries before and after
degradation. It was observed that before degradation, grain boundaries had a higher
local conductance and a higher zinc to oxygen ratio than the material in the middle of
o
the grains. After 24 hours at 80 C/100% RH, the conductance and the zinc to oxygen
ratio had decreased at both positions. This suggests that the oxidation of zinc was
responsible for the loss of local conductance. In this reference, it is proposed that the
excess of zinc, which is present in conductive ZnO forms in the presence of water into
undoped, stoichiometric ZnO with a high resistance and hydrogen gas:
+
2−
Zn(excess) + H O → Zn + 2e + 2H + O (2.6)
2+
−
2
ZnO (stoichiometric, high resistance) + H ↑ [78]
2
56
