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Stability of Cu(In,Ga)Se 2 Solar Cells
PV module can withstand the predicted outdoor conditions. This weak correlation be -
tween tests and reality is currently a challenge, specifically for thin film PV technologies
including CIGS [15]. These modules show different failure mechanisms from crystalline
silicon modules, on which the ALT procedure are based. As described in reference [14],
the ALT procedures have been optimised multiple times based on field experience,
while this optimisation process for thin film modules only started recently.
The degradation of (CIGS) modules can be caused by causes which only occur for CIGS
'CIGS specific', as well as 'CIGS non-specific' reasons, which can also be found in other
types of PV modules, such as connection wires or junction box corrosion. Table 2.2
shows an overview of failure modes leading to CIGS module degradation, and a glob -
al categorisation whether they are specific for CIGS or also observed for other (thin
film) modules.
When the circumstances leading to degradation specific for CIGS solar cells or mod-
ules were studied, humidity was often a key factor. ALT tests including liquid or gas-
o
eous water, like damp heat tests (exposure to 85C/85% relative humidity (RH)) very
often led to the failure of CIGS solar cells or modules. It was therefore concluded that
CIGS solar cells and modules are very sensitive to humidity ingress. Furthermore, sen-
sitivity to e.g. temperature (shocks), electrical bias and illumination has been found,
but the impacts of these loads are not necessary detrimental.
In order to protect the CIGS cells against water ingression, barrier materials are ap-
plied in modules. For rigid modules, glass is an excellent barrier material, while for
flexible modules often expensive inorganic/organic multi-stack materials are applied.
These materials have a low water vapour transmission rate (WVTR), which is a measure
of the passage of water vapour through a material. Although this low WVTR gener-
ally enhances the lifetime of CIGS modules, it also leads to higher costs and hinders
the large scale market introduction of flexible modules. Therefore, intrinsically stable
CIGS cells and modules would be very attractive. They would contribute to lower pro -
duction costs due to reduced encapsulation costs and accelerate the introduction of
flexible CIGS modules to the market. Knowledge about intrinsically more stable CIGS
material might thus also help CIGS producers that are currently producing stable, but
relatively expensive modules.
The objective of this chapter is the presentation of an overview of the research on
long term stability of CIGS solar cells and the discussion of the results obtained from
field and accelerated lifetime testing. Additionally, the failure mechanisms occurring
29
PV module can withstand the predicted outdoor conditions. This weak correlation be -
tween tests and reality is currently a challenge, specifically for thin film PV technologies
including CIGS [15]. These modules show different failure mechanisms from crystalline
silicon modules, on which the ALT procedure are based. As described in reference [14],
the ALT procedures have been optimised multiple times based on field experience,
while this optimisation process for thin film modules only started recently.
The degradation of (CIGS) modules can be caused by causes which only occur for CIGS
'CIGS specific', as well as 'CIGS non-specific' reasons, which can also be found in other
types of PV modules, such as connection wires or junction box corrosion. Table 2.2
shows an overview of failure modes leading to CIGS module degradation, and a glob -
al categorisation whether they are specific for CIGS or also observed for other (thin
film) modules.
When the circumstances leading to degradation specific for CIGS solar cells or mod-
ules were studied, humidity was often a key factor. ALT tests including liquid or gas-
o
eous water, like damp heat tests (exposure to 85C/85% relative humidity (RH)) very
often led to the failure of CIGS solar cells or modules. It was therefore concluded that
CIGS solar cells and modules are very sensitive to humidity ingress. Furthermore, sen-
sitivity to e.g. temperature (shocks), electrical bias and illumination has been found,
but the impacts of these loads are not necessary detrimental.
In order to protect the CIGS cells against water ingression, barrier materials are ap-
plied in modules. For rigid modules, glass is an excellent barrier material, while for
flexible modules often expensive inorganic/organic multi-stack materials are applied.
These materials have a low water vapour transmission rate (WVTR), which is a measure
of the passage of water vapour through a material. Although this low WVTR gener-
ally enhances the lifetime of CIGS modules, it also leads to higher costs and hinders
the large scale market introduction of flexible modules. Therefore, intrinsically stable
CIGS cells and modules would be very attractive. They would contribute to lower pro -
duction costs due to reduced encapsulation costs and accelerate the introduction of
flexible CIGS modules to the market. Knowledge about intrinsically more stable CIGS
material might thus also help CIGS producers that are currently producing stable, but
relatively expensive modules.
The objective of this chapter is the presentation of an overview of the research on
long term stability of CIGS solar cells and the discussion of the results obtained from
field and accelerated lifetime testing. Additionally, the failure mechanisms occurring
29
