Page 203 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 203
Degradation mechanisms of the aluminium doped zinc oxide front contact
Fermi level [19]. Therefore films with higher carrier concentration, in which the carriers
have a higher probability for thermionic emission, are typically found to have more
stable electrical properties in a harsh environment.
6.4.1.2 Reaction mechanisms in the grain boundaries
It was observed in the 'atmospheric species' experiment that H O in combination with
2
CO lead to ZnO:Al degradation, while nitrogen and oxygen only have a minor impact
2
on the stability of this layer. Since chlorine and sulphur were also encountered in the
degraded zinc oxide, we also propose a role for these materials.
The diffusion of species into the grain boundaries can influence the resistivity in two
ways, which are not easy to distinguish:
· Molecular adsorption (physical reactions)
· Chemical reactions
A combination of those two is the most likely scenario. This can also be expected
based on the reversibility experiments in reference [6], where a change in resistivity
o
was reported when annealing at 150 C as well as effusion of water, CO and H at higher
2
2
temperatures (around 500 C), indicating two different phenomena were involved.
o
6.4.1.3 Molecular Adsorption
The adsorption of molecules would occur in the grain boundaries or at the surface and
is considered to result in the formation of electron traps. Zinc oxide is known to have
great absorptivity for H, CO and CO after being cleaned of absorbed HO and CO
2
2
2
2
by vacuum heating [20]. We propose HO can be adsorbed at the grain boundaries,
2
-
-
based on the fact that OH and H were encountered.
6.4.2 Degradation due to chemical reactions
The other possibility for degradation is chemical degradation and begins with presence
of unbound zinc, oxygen or aluminium species. These species can either be present
due to dangling bonds or defects or occur due to the dissolution of ZnO:Al in the thin
water film present at the surface and possible in the grain boundaries. Looking at the
dissolution of zinc oxide, both chemical and electrochemical dissolution can result
in the formation of a Zn cation. Electrochemical reactions depend on the electrode
2+
potential which regulates the absorption of protons to the surface. This is not expected
for these experiments, but might occur in solar cells, which are electrically active.
Zinc oxide is an amphoteric material and the chemical stability of zinc oxide in aqueous
solution is a function of the pH. It is thermodynamically stable in the pH between 6
and 12, however in solutions with a high or low pH zinc oxide, zinc oxide can react [20]:
201
Fermi level [19]. Therefore films with higher carrier concentration, in which the carriers
have a higher probability for thermionic emission, are typically found to have more
stable electrical properties in a harsh environment.
6.4.1.2 Reaction mechanisms in the grain boundaries
It was observed in the 'atmospheric species' experiment that H O in combination with
2
CO lead to ZnO:Al degradation, while nitrogen and oxygen only have a minor impact
2
on the stability of this layer. Since chlorine and sulphur were also encountered in the
degraded zinc oxide, we also propose a role for these materials.
The diffusion of species into the grain boundaries can influence the resistivity in two
ways, which are not easy to distinguish:
· Molecular adsorption (physical reactions)
· Chemical reactions
A combination of those two is the most likely scenario. This can also be expected
based on the reversibility experiments in reference [6], where a change in resistivity
o
was reported when annealing at 150 C as well as effusion of water, CO and H at higher
2
2
temperatures (around 500 C), indicating two different phenomena were involved.
o
6.4.1.3 Molecular Adsorption
The adsorption of molecules would occur in the grain boundaries or at the surface and
is considered to result in the formation of electron traps. Zinc oxide is known to have
great absorptivity for H, CO and CO after being cleaned of absorbed HO and CO
2
2
2
2
by vacuum heating [20]. We propose HO can be adsorbed at the grain boundaries,
2
-
-
based on the fact that OH and H were encountered.
6.4.2 Degradation due to chemical reactions
The other possibility for degradation is chemical degradation and begins with presence
of unbound zinc, oxygen or aluminium species. These species can either be present
due to dangling bonds or defects or occur due to the dissolution of ZnO:Al in the thin
water film present at the surface and possible in the grain boundaries. Looking at the
dissolution of zinc oxide, both chemical and electrochemical dissolution can result
in the formation of a Zn cation. Electrochemical reactions depend on the electrode
2+
potential which regulates the absorption of protons to the surface. This is not expected
for these experiments, but might occur in solar cells, which are electrically active.
Zinc oxide is an amphoteric material and the chemical stability of zinc oxide in aqueous
solution is a function of the pH. It is thermodynamically stable in the pH between 6
and 12, however in solutions with a high or low pH zinc oxide, zinc oxide can react [20]:
201
