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Introduction
This is also shown in the scenarios in Figure 1.9. The blue scenario is the one that is
currently desired, with a slow but steady degradation rate, which leads to an end of
the economic lifetime after 25 years. However, in many cases, the solar modules lose
their efficiency earlier, as is for example depicted in the red line. These ‘sudden death’
scenarios, caused by infant mortalities, reduce the predictability of photovoltaic prod -
ucts, as well as the electricity output and the trust of investors, installers and potential
owners. By optimising both the stability (lower degradation rate) and the predict-
ability (no sudden death scenarios), e.g. by reduction of infant mortalities, the green
scenario could be reached. It should be noted that the total electricity yields of these
scenario can be determined by the integration of these curves.
However, for the thin film PV industry, the long term field performance of their modules
is especially hard to assess, because predicting their lifetime is a complicated process.
In order to give an assessment of the lifetime of solar modules and how fast they
lose their efficiency (their ‘degradation behaviour’), a set of Accelerated Lifetime Tests
(ALT), based on the IEC standards [13], are used. These test procedures should ideal-
ly tell whether the requirements related to performance stability and predictability
are met. However, literature [14-16] reveals that a positive outcome of these tests not
always means that the solar module can stand outdoor conditions for twenty years,
while it is likely that the opposite can also be true.
This weak correlation between tests and reality is especially true for thin film PV tech-
nologies [16], for which a set of tests named the ‘IEC 61646 standard’ [13] is used. While
crystalline silicon modules has been tested in the laboratory and used in the field
since the 1970s, for thin film modules, field experience has not yet been extensively
compared with laboratory tests. For crystalline silicon, many tests have been altered
in the last 25 years, but this learning curve has not yet been followed for thin film
modules. Furthermore, the IEC standard does not focus on fundamental understand-
ing of the degradation process within a module. However, knowledge of the failure
mechanisms, which are the processes happening inside the module leading to failure
and the possibility to link these with the observed field failure of modules is very im-
portant. This can help to link the loss of the power output of a module with its pro-
duction process, in order to obtain modules with a better stability and predictability.
Due to the limited knowledge about the field failure in thin film PV, many issues about
the stability and the predictability, like the influence of the interfaces between the
components and the role of the barrier materials, are still open.
As one of the thin film PV technologies, also for thin film CIGS, more research was re-
quired into the failure mechanisms occurring in CIGS solar cells and modules in order to
enhance their stability and predictability. Literature revealed that humidity was often a
17
This is also shown in the scenarios in Figure 1.9. The blue scenario is the one that is
currently desired, with a slow but steady degradation rate, which leads to an end of
the economic lifetime after 25 years. However, in many cases, the solar modules lose
their efficiency earlier, as is for example depicted in the red line. These ‘sudden death’
scenarios, caused by infant mortalities, reduce the predictability of photovoltaic prod -
ucts, as well as the electricity output and the trust of investors, installers and potential
owners. By optimising both the stability (lower degradation rate) and the predict-
ability (no sudden death scenarios), e.g. by reduction of infant mortalities, the green
scenario could be reached. It should be noted that the total electricity yields of these
scenario can be determined by the integration of these curves.
However, for the thin film PV industry, the long term field performance of their modules
is especially hard to assess, because predicting their lifetime is a complicated process.
In order to give an assessment of the lifetime of solar modules and how fast they
lose their efficiency (their ‘degradation behaviour’), a set of Accelerated Lifetime Tests
(ALT), based on the IEC standards [13], are used. These test procedures should ideal-
ly tell whether the requirements related to performance stability and predictability
are met. However, literature [14-16] reveals that a positive outcome of these tests not
always means that the solar module can stand outdoor conditions for twenty years,
while it is likely that the opposite can also be true.
This weak correlation between tests and reality is especially true for thin film PV tech-
nologies [16], for which a set of tests named the ‘IEC 61646 standard’ [13] is used. While
crystalline silicon modules has been tested in the laboratory and used in the field
since the 1970s, for thin film modules, field experience has not yet been extensively
compared with laboratory tests. For crystalline silicon, many tests have been altered
in the last 25 years, but this learning curve has not yet been followed for thin film
modules. Furthermore, the IEC standard does not focus on fundamental understand-
ing of the degradation process within a module. However, knowledge of the failure
mechanisms, which are the processes happening inside the module leading to failure
and the possibility to link these with the observed field failure of modules is very im-
portant. This can help to link the loss of the power output of a module with its pro-
duction process, in order to obtain modules with a better stability and predictability.
Due to the limited knowledge about the field failure in thin film PV, many issues about
the stability and the predictability, like the influence of the interfaces between the
components and the role of the barrier materials, are still open.
As one of the thin film PV technologies, also for thin film CIGS, more research was re-
quired into the failure mechanisms occurring in CIGS solar cells and modules in order to
enhance their stability and predictability. Literature revealed that humidity was often a
17
