Page 179 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Degradation mechanisms of the aluminium doped zinc oxide front contact
6.1 Introduction
Zinc oxide (ZnO) has been investigated extensively because of the increasing number
of possible industrial applications. Being a wide band gap semiconductor, zinc oxide
is emerging as a prospective material for gas sensors, transparent electronics and thin
film solar cells. For chalcopyrite based thin film solar cells, like Cu(In,Ga)Se , aluminium
2
doped ZnO (ZnO:Al) is used as a front contact, since it is a non-toxic, inexpensive and
abundant material. Furthermore, ZnO:Al is very attractive for CIGS solar cells, since
sputtering allows room temperature deposition, which prevents exposure of the
underlying layers to elevated temperatures.
In the Photovoltaic (PV) market, producers give out warrantees that panels will still yield
80% of the starting power after 25 years. Therefore, CIGS modules should be reliable
for at least this time, but it was observed in field testing that CIGS modules often
degrades too fast. It was observed that zinc oxide is one of the key parameters in the
panels’ degradation, indicating that ZnO:Al is not stable for 25 years in the field. From
accelerated lifetime testing [1] it has been shown that the water ingress leads to an
increase in lateral sheet resistance of the ZnO:Al, hereby increasing the series resistance
in the solar cell. This will decrease the fill factor and consequently the efficiency.
Proper encapsulation can protect the solar cells against water ingression. For rigid
modules, glass is an excellent encapsulation material, while for flexible modules often
expensive inorganic/organic multistack materials are chosen. A CIGS solar cell with a
more stable ZnO:Al front contact can contribute to lower energy costs and accelerate
the introduction of flexible CIGS modules to the market. Therefore, it is necessary to
understand and improve the degradation behaviour of ZnO:Al under water ingression.
Several studies [2-6] have already tried to simulate the field exposure, by placing the
samples under ‘damp heat’ conditions, as defined according to the International
Electrotechnical Commission (IEC) module testing procedure 61646. These studies
[2,3] have led to an increased insight in the changes in optical and electrical properties
due to degradation. Relations between these properties and the surface roughness of
the substrate [2], deposition temperatures [4], crystallinity [4] and aluminium content
[3] have been described in literature.
However, the environmental factors leading to ZnO:Al degradation are mostly still
unknown. This can be a problem, since it is expected that other atmospheric species
like O or CO might also play a role in the degradation, but this has not been studied
2
2
thoroughly for solar applications. The exact nature of the environmental molecules
that are playing a role in the relevant degradation processes in ZnO:Al are proposed in
literature, but have not been studied thoroughly. A suggestion for the main degrading
species include oxygen and water molecules [7] and water and CO [8].
2
177
6.1 Introduction
Zinc oxide (ZnO) has been investigated extensively because of the increasing number
of possible industrial applications. Being a wide band gap semiconductor, zinc oxide
is emerging as a prospective material for gas sensors, transparent electronics and thin
film solar cells. For chalcopyrite based thin film solar cells, like Cu(In,Ga)Se , aluminium
2
doped ZnO (ZnO:Al) is used as a front contact, since it is a non-toxic, inexpensive and
abundant material. Furthermore, ZnO:Al is very attractive for CIGS solar cells, since
sputtering allows room temperature deposition, which prevents exposure of the
underlying layers to elevated temperatures.
In the Photovoltaic (PV) market, producers give out warrantees that panels will still yield
80% of the starting power after 25 years. Therefore, CIGS modules should be reliable
for at least this time, but it was observed in field testing that CIGS modules often
degrades too fast. It was observed that zinc oxide is one of the key parameters in the
panels’ degradation, indicating that ZnO:Al is not stable for 25 years in the field. From
accelerated lifetime testing [1] it has been shown that the water ingress leads to an
increase in lateral sheet resistance of the ZnO:Al, hereby increasing the series resistance
in the solar cell. This will decrease the fill factor and consequently the efficiency.
Proper encapsulation can protect the solar cells against water ingression. For rigid
modules, glass is an excellent encapsulation material, while for flexible modules often
expensive inorganic/organic multistack materials are chosen. A CIGS solar cell with a
more stable ZnO:Al front contact can contribute to lower energy costs and accelerate
the introduction of flexible CIGS modules to the market. Therefore, it is necessary to
understand and improve the degradation behaviour of ZnO:Al under water ingression.
Several studies [2-6] have already tried to simulate the field exposure, by placing the
samples under ‘damp heat’ conditions, as defined according to the International
Electrotechnical Commission (IEC) module testing procedure 61646. These studies
[2,3] have led to an increased insight in the changes in optical and electrical properties
due to degradation. Relations between these properties and the surface roughness of
the substrate [2], deposition temperatures [4], crystallinity [4] and aluminium content
[3] have been described in literature.
However, the environmental factors leading to ZnO:Al degradation are mostly still
unknown. This can be a problem, since it is expected that other atmospheric species
like O or CO might also play a role in the degradation, but this has not been studied
2
2
thoroughly for solar applications. The exact nature of the environmental molecules
that are playing a role in the relevant degradation processes in ZnO:Al are proposed in
literature, but have not been studied thoroughly. A suggestion for the main degrading
species include oxygen and water molecules [7] and water and CO [8].
2
177
