Page 180 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 180
Chapter 6
Knowledge about the species playing a role in ZnO:Al degradation is very important,
because it can be used to optimise the product design and the accelerated tests
for ZnO:Al containing products. This can for example be useful in the design of
encapsulation materials in which the water and oxygen barrier properties (water
vapour transmission rate and oxygen transmission rate respectively) are optimised,
while the penetration of other atmospheric species is not defined at all.
Furthermore, information in the literature about the chemical mechanisms leading
to the change in optical and electrical properties during degradation is limited. It is
suggested that the presence of oxygen and water promotes zinc oxide degradation [7],
while Greiner, Feist and Pern [2,9,10] suggest that the formation of Al(OH) [2,10] and
3
Zn(OH) [2,9,10] plays a role in the degradation, since these materials are non-conductive.
2
The presence of a hydroxide in degraded ZnO:Al was also shown in reference [10]. An
overview about the literature on this topic can be found in chapter 2.3.4.1.
In general, literature (see chapter 2.3.4.1) gives an excellent picture on the electrical
and optical aspects, but the chemical behaviour is underexposed and does not yield
an unambiguous picture for the influence of layer composition on the film properties.
Therefore, we aimed to identify the chemical and physical failure mechanisms.
In this study, ZnO:Al samples were therefore exposed to a damp heat test as well as
to a test to learn more about the impact of atmospheric species (including HO, CO ,
2
2
N and O). The electrical, optical and compositional characteristics were analyzed
2
2
before, during and after exposure. This information is used to determine the impact
of various degradation conditions on ZnO:Al and to identify the relationship between
the chemical and physical degradation phenomena for this material. This information
is used to propose the possible chemical and physical failure mechanisms. The impact
of these mechanisms on CIGS solar cell performance are discussed.
6.2 Experimental
6.2.1 Sample preparation
6.2.1.1 Damp heat treatment
ZnO:Al films were deposited by radio frequency (RF) magnetron sputtering using a
Lesker sputtering tool. The films were sputtered on 0.7 mm thick Eagle 2000 Corning
glass substrate from a circular (7ยด) high purity ZnO ceramic target with 2% AlO from
2
3
the company Processmaterials. The target to substrate distance was fixed to 110 mm
and the flow of pure Ar gas was set to 15 standard cubic cm per minute (sccm). The
2
chamber pressure was 0.26 Pa, while the RF power was 400 W, which corresponds to
178
Knowledge about the species playing a role in ZnO:Al degradation is very important,
because it can be used to optimise the product design and the accelerated tests
for ZnO:Al containing products. This can for example be useful in the design of
encapsulation materials in which the water and oxygen barrier properties (water
vapour transmission rate and oxygen transmission rate respectively) are optimised,
while the penetration of other atmospheric species is not defined at all.
Furthermore, information in the literature about the chemical mechanisms leading
to the change in optical and electrical properties during degradation is limited. It is
suggested that the presence of oxygen and water promotes zinc oxide degradation [7],
while Greiner, Feist and Pern [2,9,10] suggest that the formation of Al(OH) [2,10] and
3
Zn(OH) [2,9,10] plays a role in the degradation, since these materials are non-conductive.
2
The presence of a hydroxide in degraded ZnO:Al was also shown in reference [10]. An
overview about the literature on this topic can be found in chapter 2.3.4.1.
In general, literature (see chapter 2.3.4.1) gives an excellent picture on the electrical
and optical aspects, but the chemical behaviour is underexposed and does not yield
an unambiguous picture for the influence of layer composition on the film properties.
Therefore, we aimed to identify the chemical and physical failure mechanisms.
In this study, ZnO:Al samples were therefore exposed to a damp heat test as well as
to a test to learn more about the impact of atmospheric species (including HO, CO ,
2
2
N and O). The electrical, optical and compositional characteristics were analyzed
2
2
before, during and after exposure. This information is used to determine the impact
of various degradation conditions on ZnO:Al and to identify the relationship between
the chemical and physical degradation phenomena for this material. This information
is used to propose the possible chemical and physical failure mechanisms. The impact
of these mechanisms on CIGS solar cell performance are discussed.
6.2 Experimental
6.2.1 Sample preparation
6.2.1.1 Damp heat treatment
ZnO:Al films were deposited by radio frequency (RF) magnetron sputtering using a
Lesker sputtering tool. The films were sputtered on 0.7 mm thick Eagle 2000 Corning
glass substrate from a circular (7ยด) high purity ZnO ceramic target with 2% AlO from
2
3
the company Processmaterials. The target to substrate distance was fixed to 110 mm
and the flow of pure Ar gas was set to 15 standard cubic cm per minute (sccm). The
2
chamber pressure was 0.26 Pa, while the RF power was 400 W, which corresponds to
178
