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Chapter 10
Summary
Large scale commercial introduction of CIGS photovoltaics (PV) requires modules
with low costs, high efficiencies and long and predictable lifetimes. Unfortunately,
knowledge about the lifetime of CIGS PV is limited, which is reflected in the results
of field studies: degradation rates varying from 0% to about 4% per year have been
observed. Since warrantees are given out that the modules will still yield 80% of their
initial power after 20 years of field exposure, degradation rates are often too high as
well as too unpredictable. In order to decreasing these degradation rates, knowledge
about the degradation behaviour is very important.
Knowledge about this degradation would also enable lower production costs:
Since both field and accelerated laboratory tests have already shown that elevated
humidity and temperature have a negative impact on CIGS PV, modules are laminated
with barrier materials to keep the moisture out. For rigid modules, glass is an excellent
barrier choice, but for flexible modules, expensive organic-inorganic multilayer
coatings are often used. An intrinsically more stable CIGS solar cell would therefore
also limit the barrier costs and facilitate the large scale market introduction of flexible
CIGS PV.
Therefore, I have focused in this work on the identification of de degradation
mechanisms in thin film CIGS solar cells. Since these cells consist of at least five
individual layers, that all influence each other and the degradation behaviour,
various layers have also been studied individually. The knowledge obtained on these
individual layers was then used to interpret the degradation behaviour of complete
CIGS solar cells.
Within this thesis, I propose the degradation mechanisms for the molybdenum
back contact and the zinc oxide front contact as well as for complete Cu(In,Ga)Se
2
solar cells. The study of these mechanisms has been executed by the exposure of
samplesdeposited at different conditions to various accelerated lifetime testing. This
was combined with extensive analysis of the samples before, during and after lifetime
tests. These tests should give an indication about the long term field exposure of CIGS
modules, but it should be mentioned that the validity of the comparison of laboratory
testing on solar cells and field exposure on modules it still under discussion.
The following tests have been used in this study.
o
1. Standard ‘damp heat’ – exposure of samples to 85 C and 85% relative humidity
(RH). A thousand hours exposure to these standard conditions should be
comparable to 20 years of field exposure in Miami (acceleration factor 175x).
It should be noted that e.g. literature reported that the acceleration factors
of these tests range from 10x to 700x, so this extrapolation should be treated
with care.
268
Summary
Large scale commercial introduction of CIGS photovoltaics (PV) requires modules
with low costs, high efficiencies and long and predictable lifetimes. Unfortunately,
knowledge about the lifetime of CIGS PV is limited, which is reflected in the results
of field studies: degradation rates varying from 0% to about 4% per year have been
observed. Since warrantees are given out that the modules will still yield 80% of their
initial power after 20 years of field exposure, degradation rates are often too high as
well as too unpredictable. In order to decreasing these degradation rates, knowledge
about the degradation behaviour is very important.
Knowledge about this degradation would also enable lower production costs:
Since both field and accelerated laboratory tests have already shown that elevated
humidity and temperature have a negative impact on CIGS PV, modules are laminated
with barrier materials to keep the moisture out. For rigid modules, glass is an excellent
barrier choice, but for flexible modules, expensive organic-inorganic multilayer
coatings are often used. An intrinsically more stable CIGS solar cell would therefore
also limit the barrier costs and facilitate the large scale market introduction of flexible
CIGS PV.
Therefore, I have focused in this work on the identification of de degradation
mechanisms in thin film CIGS solar cells. Since these cells consist of at least five
individual layers, that all influence each other and the degradation behaviour,
various layers have also been studied individually. The knowledge obtained on these
individual layers was then used to interpret the degradation behaviour of complete
CIGS solar cells.
Within this thesis, I propose the degradation mechanisms for the molybdenum
back contact and the zinc oxide front contact as well as for complete Cu(In,Ga)Se
2
solar cells. The study of these mechanisms has been executed by the exposure of
samplesdeposited at different conditions to various accelerated lifetime testing. This
was combined with extensive analysis of the samples before, during and after lifetime
tests. These tests should give an indication about the long term field exposure of CIGS
modules, but it should be mentioned that the validity of the comparison of laboratory
testing on solar cells and field exposure on modules it still under discussion.
The following tests have been used in this study.
o
1. Standard ‘damp heat’ – exposure of samples to 85 C and 85% relative humidity
(RH). A thousand hours exposure to these standard conditions should be
comparable to 20 years of field exposure in Miami (acceleration factor 175x).
It should be noted that e.g. literature reported that the acceleration factors
of these tests range from 10x to 700x, so this extrapolation should be treated
with care.
268
