Page 43 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Stability of Cu(In,Ga)Se 2 Solar Cells
oxygen. Selenised molybdenum also degraded in damp heat tests via the formation
of molybdenum oxide, but this process occurred slower, probably since it is easier to
oxidise metallic molybdenum than MoSe .
2
Although the oxide formation seems very detrimental for later deposited solar cells,
mild oxidation could actually result in slightly improved efficiency. When CIGS is de-
posited onto oxidised molybdenum the molybdenum oxide is likely reduced (MoO
3
MoO ) as well as transformed into MoSe .
2
2
Various references reported on the presence of sodium, which likely plays a role in
molybdenum degradation. Sodium can e.g. occur in the form of needles on the mo-
lybdenum surface, however it is also possible that it intercalates via a reduction reac-
tion into MoO , thereby forming Na MoO .
x
3
3
2.3.2 CIGS absorber degradation
The CIGS absorber layer is the core of a CIGS solar cell and also the layer with the
biggest variety of deposition techniques and material parameters. Variations include
the deposition temperature (depending on the substrate choice), one/two/multi-step
processing, gallium-grading (flat or sophisticated), sulphur/selenium ratio and alka-
li supply (quantity and source type). The variety lead to a diversity of startups and
companies that are offering CIGS based products, each with individual intellectual
property about their specific CIGS absorber [36].
There are two fundamental techniques to deposit the CIGS absorber:
1. Thermal co-evapouration of all the constituent elements, i.e. the deposition of
copper, indium, gallium and selenium onto a substrate with suitable growth
temperature (typically between 400 and 600°C)
2. Two stage processing: Deposition of copper, indium and gallium precursors onto
the cold substrate followed by annealing under a selenium atmosphere at high
temperatures (this can sometimes also be called rapid thermal processing (RTP)).
Since the chalcopyrite absorber always crystallises under selenium-supply, it should
be denoted that co-evapourated absorbers grow 'bottom-up', i.e. from the back con-
tact towards the (later) absorber surface, while two stage absorbers crystallise 'top-
down', i.e. the precursor (and thus later absorber) surface selenises first.
As noted in the introduction, the stability of non-encapsulated CIGS solar cells is main -
ly threatened by humidity [37]. Many research groups report a reduction of the open
circuit voltages and the fill factors of their CIGS solar cells after damp heat exposure
[38-41]. More information about factors majorly influencing the fill factor is given in
other chapters (the molybdenum contact (chapter 2.3.1 ), the TCO (chapter 2.3.4), the
isolation scribes (see chapter 2.5.1 ) and the grid (chapter 2.5.2)).
The changes in open circuit voltage are often related to the CIGS absorber. Lower
41
oxygen. Selenised molybdenum also degraded in damp heat tests via the formation
of molybdenum oxide, but this process occurred slower, probably since it is easier to
oxidise metallic molybdenum than MoSe .
2
Although the oxide formation seems very detrimental for later deposited solar cells,
mild oxidation could actually result in slightly improved efficiency. When CIGS is de-
posited onto oxidised molybdenum the molybdenum oxide is likely reduced (MoO
3
MoO ) as well as transformed into MoSe .
2
2
Various references reported on the presence of sodium, which likely plays a role in
molybdenum degradation. Sodium can e.g. occur in the form of needles on the mo-
lybdenum surface, however it is also possible that it intercalates via a reduction reac-
tion into MoO , thereby forming Na MoO .
x
3
3
2.3.2 CIGS absorber degradation
The CIGS absorber layer is the core of a CIGS solar cell and also the layer with the
biggest variety of deposition techniques and material parameters. Variations include
the deposition temperature (depending on the substrate choice), one/two/multi-step
processing, gallium-grading (flat or sophisticated), sulphur/selenium ratio and alka-
li supply (quantity and source type). The variety lead to a diversity of startups and
companies that are offering CIGS based products, each with individual intellectual
property about their specific CIGS absorber [36].
There are two fundamental techniques to deposit the CIGS absorber:
1. Thermal co-evapouration of all the constituent elements, i.e. the deposition of
copper, indium, gallium and selenium onto a substrate with suitable growth
temperature (typically between 400 and 600°C)
2. Two stage processing: Deposition of copper, indium and gallium precursors onto
the cold substrate followed by annealing under a selenium atmosphere at high
temperatures (this can sometimes also be called rapid thermal processing (RTP)).
Since the chalcopyrite absorber always crystallises under selenium-supply, it should
be denoted that co-evapourated absorbers grow 'bottom-up', i.e. from the back con-
tact towards the (later) absorber surface, while two stage absorbers crystallise 'top-
down', i.e. the precursor (and thus later absorber) surface selenises first.
As noted in the introduction, the stability of non-encapsulated CIGS solar cells is main -
ly threatened by humidity [37]. Many research groups report a reduction of the open
circuit voltages and the fill factors of their CIGS solar cells after damp heat exposure
[38-41]. More information about factors majorly influencing the fill factor is given in
other chapters (the molybdenum contact (chapter 2.3.1 ), the TCO (chapter 2.3.4), the
isolation scribes (see chapter 2.5.1 ) and the grid (chapter 2.5.2)).
The changes in open circuit voltage are often related to the CIGS absorber. Lower
41
