Page 163 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 163
Degradation mechanisms of the molybdenum back contact
5.4 Discussion
5.4.1 Selenisation and pressure experiment
5.4.1.1 Molybdenum oxide formation
o
The degradation of molybdenum is visible after less than two hours of 85C/85%
RH exposure. After only two hours under these conditions, there were already
large stains of molybdenum oxide on the non-selenised molybdenum samples.
These areas became larger and thicker, until a thick cracked layer of molybdenum
oxides was present on the surface. The cracking allowed further progression of the
degradation due to the exposure of new metallic molybdenum located deeper in
the bulk of the layer. Furthermore small needle-like structures were observed on
the degraded material.
Raman spectroscopy showed MoO, MoO and Mo O bonds were present on both
3
2
23
8
selenised and non-selenised samples, while the signal of suboxide MoO was
11
4
predominantly observed on the selenised samples. The presence of MoOspecies
x
was confirmed by XRD. Preliminary identification of the formed species by XRD and
Raman measurements showed the presence of MoO , as well as the suboxides Mo O
3
26
9
(MoO 2.89 ) Mo O (MoO 2.78 ), Mo O (MoO 2.88 ) and Mo O (MoO 2.75 ). These are all MoO -
47
17
23
3
4
11
8
like compounds with different amounts of oxygen vacancies.
Porous non -selenised Dense non -selenised Porous selenised Dense selenised
① Damp heat treatment ① Damp heat treatment ① Damp heat treatment ① Damp heat treatment
O 2 / H 2 O O 2 / H 2 O O 2 / H 2 O O 2 / H 2 O
Molybdenum Molybdenum Molybdenum Molybdenum
② Oxide formation ② Oxide formation ② Oxide formation ② Oxide formation
Molybdenum oxide (MoO x ) Molybdenum oxide (MoO x )
Molybdenum oxide (MoO x ) )
Molybdenum oxide (MoO x
Molybdenum Molybdenum Molybdenum Molybdenum
③ Removal from
o
④ The newly exposed Mo 85 C/85% RH ④ The newly exposed Mo
can be oxidised again to can be oxidised again to
yield multilayer stacks of yield multilayer stacks of
Molybdenum MoO x
MoO x
Grain boundary containing MoSe 2 Cracking due to evaporation of
Grain boundary containing a mixture of water and difference in
MoO 3 and Na 2 MoO 4 ●2H 2 O thermal expansion
Figure 5.29
Proposed degradation route for different thin film molybdenum layers on soda lime glass. The differences between porous
and dense as well as selenised and non-selenised molybdenum films is shown.
161
5.4 Discussion
5.4.1 Selenisation and pressure experiment
5.4.1.1 Molybdenum oxide formation
o
The degradation of molybdenum is visible after less than two hours of 85C/85%
RH exposure. After only two hours under these conditions, there were already
large stains of molybdenum oxide on the non-selenised molybdenum samples.
These areas became larger and thicker, until a thick cracked layer of molybdenum
oxides was present on the surface. The cracking allowed further progression of the
degradation due to the exposure of new metallic molybdenum located deeper in
the bulk of the layer. Furthermore small needle-like structures were observed on
the degraded material.
Raman spectroscopy showed MoO, MoO and Mo O bonds were present on both
3
2
23
8
selenised and non-selenised samples, while the signal of suboxide MoO was
11
4
predominantly observed on the selenised samples. The presence of MoOspecies
x
was confirmed by XRD. Preliminary identification of the formed species by XRD and
Raman measurements showed the presence of MoO , as well as the suboxides Mo O
3
26
9
(MoO 2.89 ) Mo O (MoO 2.78 ), Mo O (MoO 2.88 ) and Mo O (MoO 2.75 ). These are all MoO -
47
17
23
3
4
11
8
like compounds with different amounts of oxygen vacancies.
Porous non -selenised Dense non -selenised Porous selenised Dense selenised
① Damp heat treatment ① Damp heat treatment ① Damp heat treatment ① Damp heat treatment
O 2 / H 2 O O 2 / H 2 O O 2 / H 2 O O 2 / H 2 O
Molybdenum Molybdenum Molybdenum Molybdenum
② Oxide formation ② Oxide formation ② Oxide formation ② Oxide formation
Molybdenum oxide (MoO x ) Molybdenum oxide (MoO x )
Molybdenum oxide (MoO x ) )
Molybdenum oxide (MoO x
Molybdenum Molybdenum Molybdenum Molybdenum
③ Removal from
o
④ The newly exposed Mo 85 C/85% RH ④ The newly exposed Mo
can be oxidised again to can be oxidised again to
yield multilayer stacks of yield multilayer stacks of
Molybdenum MoO x
MoO x
Grain boundary containing MoSe 2 Cracking due to evaporation of
Grain boundary containing a mixture of water and difference in
MoO 3 and Na 2 MoO 4 ●2H 2 O thermal expansion
Figure 5.29
Proposed degradation route for different thin film molybdenum layers on soda lime glass. The differences between porous
and dense as well as selenised and non-selenised molybdenum films is shown.
161
