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P. 210
Chapter 6
6.4.4 Degradation mechanism based on diffusion of species
Based on the knowledge that diffusion of both small elements from glass and
small atoms or molecules from the environment occur, we propose that diffusion
occurs in two directions. The reaction down leads to an increased resistivity or even
dissolution, while the upward diffusion (green) does not seem to influence the
electronic properties. However, this upward diffusion has a minor impact on the
optical properties (Figure 6.19).
1. · Small molecules, like water, carbon dioxide as well sulphur and chlorine
species diffuse from the environment into the grain boundaries.
· Small elements, like calcium, silicon and aluminium leach from the glass and
enter the grain boundaries.
2 · The small molecules react with the grain boundaries, which results in the
increased potential barriers and thus decreased mobility. The rate of this
reaction is determined by diffusion and is different per element. The
hydroxide and CO based elements have a higher diffusion rate than sulphur
2
and chlorine species. The elemental species arrive in the grain boundaries,
where they react, leading to the formation of e.g. Zn(OH) or Zn (CO ) (OH) ,
3 2
6
5
2
the complete dissolution of the material, or to adsorption of these species.
· Calcium, silicon and aluminium reach the zinc oxide surface. When soda lime
glass is used, this likely also happens with sodium
3 · When reactions or adsorption occur, the grain boundaries can be saturated,
leading to a slower decrease in mobility.
· Spots and stains can form due to reactions of calcium, silicon and aluminium
with carbon and oxygen. When the samples are removed from the climate
chamber, drying stains are formed due to the evapouration of residual moisture
at the surface, while in solution these species do probably dissolve directly.
6.5 Conclusions
Sputtered thin ZnO:Al films on borosilicate glass were exposed to damp heat (85 o C/85%
RH) and to various combinations of atmospheric gases. This led to acceleration of
the physical and chemical degradation behaviour. Damp heat treated samples were
studied by XRD and optical measurements, which showed that the crystal structure
and transmission in the range 300-1100 nm do not change, hereby confirming that
the bulk structure stayed constant. It was also observed that the carrier concentration
stayed constant, while the Hall mobility and thus the overall resistivity decreased.
This implies the increase of potential barriers, which are likely located at the grain
boundaries.
208
6.4.4 Degradation mechanism based on diffusion of species
Based on the knowledge that diffusion of both small elements from glass and
small atoms or molecules from the environment occur, we propose that diffusion
occurs in two directions. The reaction down leads to an increased resistivity or even
dissolution, while the upward diffusion (green) does not seem to influence the
electronic properties. However, this upward diffusion has a minor impact on the
optical properties (Figure 6.19).
1. · Small molecules, like water, carbon dioxide as well sulphur and chlorine
species diffuse from the environment into the grain boundaries.
· Small elements, like calcium, silicon and aluminium leach from the glass and
enter the grain boundaries.
2 · The small molecules react with the grain boundaries, which results in the
increased potential barriers and thus decreased mobility. The rate of this
reaction is determined by diffusion and is different per element. The
hydroxide and CO based elements have a higher diffusion rate than sulphur
2
and chlorine species. The elemental species arrive in the grain boundaries,
where they react, leading to the formation of e.g. Zn(OH) or Zn (CO ) (OH) ,
3 2
6
5
2
the complete dissolution of the material, or to adsorption of these species.
· Calcium, silicon and aluminium reach the zinc oxide surface. When soda lime
glass is used, this likely also happens with sodium
3 · When reactions or adsorption occur, the grain boundaries can be saturated,
leading to a slower decrease in mobility.
· Spots and stains can form due to reactions of calcium, silicon and aluminium
with carbon and oxygen. When the samples are removed from the climate
chamber, drying stains are formed due to the evapouration of residual moisture
at the surface, while in solution these species do probably dissolve directly.
6.5 Conclusions
Sputtered thin ZnO:Al films on borosilicate glass were exposed to damp heat (85 o C/85%
RH) and to various combinations of atmospheric gases. This led to acceleration of
the physical and chemical degradation behaviour. Damp heat treated samples were
studied by XRD and optical measurements, which showed that the crystal structure
and transmission in the range 300-1100 nm do not change, hereby confirming that
the bulk structure stayed constant. It was also observed that the carrier concentration
stayed constant, while the Hall mobility and thus the overall resistivity decreased.
This implies the increase of potential barriers, which are likely located at the grain
boundaries.
208
