Page 30 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 30
Chapter 2
These degradation rates are determined assuming a linear decrease of output. This
linear determination of the degradation rate is the simplest approximation and in
many cases it does not represent the real progression of the degradation accurately.
However, without any further assumptions the linear approximation allows to com-
pare the degradation data from different sources which are represented differently in
each publication.
Table 2.1 shows that the tested modules vary from very stable (no degradation after
seven years) to very vulnerable to outdoor exposure. It should be noted that deg-
radation rates do not need to be linear as a function of time, since large differences
between years have been observed. These mixed stability results show that a general
statement about the lifetime of CIGS modules cannot be made at this moment. The
main reason for this limited knowledge about CIGS module lifetime can be found in:
1. Limited field experience with CIGS modules, which generally have not been in
the field for a long time.
2. Results from field experiments can vary greatly. Degradation depends on many
parameters such as module production techniques, module type, production
year, orientation of the panel and climate of the installation location, as well
as installation parameters like system voltage. So far, not enough field testing
data is available to apply reliable statistics.
3. Lessons learned for crystalline silicon modules (for both field and accelerated
testing) cannot always be applied to CIGS modules due to deviations in module
design and cell composition and build-up.
More information on the field performance of many types of PV modules, including
CIGS can be found in an excellent review paper [12]. In general, Table 2.1 and refer-
ences [12,13] show that some CIGS modules show excellent outdoor stability, while
other modules degrade very quickly. When the change of the individual PV electrical
parameters of the degraded modules was studied, it was observed that the efficien-
cy decrease was mostly caused by deterioration of the fill factor (FF), while a small
change in open circuit voltage (V ) and minimal change in short circuit current (I)
sc
oc
were also observed [4,12]. Unfortunately, most references only report the changes in
electrical parameters of their modules and not in their material properties.
In order to better predict the lifetime and reliability of CIGS modules, they are exposed
to Accelerated Lifetime Testing (ALT) according to International Electrotechnical Com -
mission (IEC) module testing procedure 61646. These tests should ideally show whether
the requirements related to performance stability are met. However, literature [12,14]
reveals that a positive outcome of the IEC tests does not necessarily indicate that the
28
These degradation rates are determined assuming a linear decrease of output. This
linear determination of the degradation rate is the simplest approximation and in
many cases it does not represent the real progression of the degradation accurately.
However, without any further assumptions the linear approximation allows to com-
pare the degradation data from different sources which are represented differently in
each publication.
Table 2.1 shows that the tested modules vary from very stable (no degradation after
seven years) to very vulnerable to outdoor exposure. It should be noted that deg-
radation rates do not need to be linear as a function of time, since large differences
between years have been observed. These mixed stability results show that a general
statement about the lifetime of CIGS modules cannot be made at this moment. The
main reason for this limited knowledge about CIGS module lifetime can be found in:
1. Limited field experience with CIGS modules, which generally have not been in
the field for a long time.
2. Results from field experiments can vary greatly. Degradation depends on many
parameters such as module production techniques, module type, production
year, orientation of the panel and climate of the installation location, as well
as installation parameters like system voltage. So far, not enough field testing
data is available to apply reliable statistics.
3. Lessons learned for crystalline silicon modules (for both field and accelerated
testing) cannot always be applied to CIGS modules due to deviations in module
design and cell composition and build-up.
More information on the field performance of many types of PV modules, including
CIGS can be found in an excellent review paper [12]. In general, Table 2.1 and refer-
ences [12,13] show that some CIGS modules show excellent outdoor stability, while
other modules degrade very quickly. When the change of the individual PV electrical
parameters of the degraded modules was studied, it was observed that the efficien-
cy decrease was mostly caused by deterioration of the fill factor (FF), while a small
change in open circuit voltage (V ) and minimal change in short circuit current (I)
sc
oc
were also observed [4,12]. Unfortunately, most references only report the changes in
electrical parameters of their modules and not in their material properties.
In order to better predict the lifetime and reliability of CIGS modules, they are exposed
to Accelerated Lifetime Testing (ALT) according to International Electrotechnical Com -
mission (IEC) module testing procedure 61646. These tests should ideally show whether
the requirements related to performance stability are met. However, literature [12,14]
reveals that a positive outcome of the IEC tests does not necessarily indicate that the
28
