Page 42 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 42
Chapter 2
studied the degradation of molybdenum in CIGSSe cells by lifting off the CIGSSe layer
in order to obtain a soda lime glass/Mo sample. Doing so for a non-degraded sample,
Cu-Ga-O particles which were broken from the CIGSSe layer, as well as a Mo-Se film were
observed, while indium was not present on the molybdenum surface. When a device
o
was exposed to O purged water at 85 C, the Cu-Ga-O particles were also present, but in
2
this area a higher Ga-O concentration was found. The occurrence of these particles was
reported to likely be specific for the deposition process (e.g. incomplete selenisation)
and to likely facilitate moisture and/or oxygen transfer to the Mo/CIGSSe interface.
CIGS
Back contact: MoSe 2
Back contact:
MoO 2 /MoO 3 /MoSe 2
(potentially with Na)
Back contact: Molybdenum
SLG
1 1 Degradation due to exposure
to temperature and humidity
Figure 2.5
General reaction mechanism for the degradation of molybdenum in CIGS cells at an isolation scribe, showing the formation
of molybdenum oxides from MoSe 2 and metallic molybdenum.
A proposal for a general route for degradation of molybdenum as present in CIGS so-
lar cells is shown in Figure 2.5. This shows that the top MoSe layer and the associated
2
metallic molybdenum can also form into an molybdenum oxide layer, although this
reaction is likely slower for selenised molybdenum than for metallic molybdenum.
2.3.1.3 Summary on molybdenum degradation
When molybdenum and selenised molybdenum films are exposed to water and oxy-
gen, especially under elevated temperatures, black and blue stains can occur on the
metallic molybdenum surface. These stains contain molybdenum oxide (MoO /MoO ),
2
3
which is generally badly conducting and reflecting. This likely has a negative impact
on the solar cell. Furthermore, the molybdenum oxide layers can crack and needles
can be formed. The formation of a thick layer of non-conductive molybdenum oxide
presumably leads to a very fast decrease of the conductivity when measured from
the top. The degradation effects are the most severe for molybdenum deposited at
high pressures during sputter deposition, thereby forming more porous molybde-
num layers, which are more susceptible for ingression of among others water and
40
studied the degradation of molybdenum in CIGSSe cells by lifting off the CIGSSe layer
in order to obtain a soda lime glass/Mo sample. Doing so for a non-degraded sample,
Cu-Ga-O particles which were broken from the CIGSSe layer, as well as a Mo-Se film were
observed, while indium was not present on the molybdenum surface. When a device
o
was exposed to O purged water at 85 C, the Cu-Ga-O particles were also present, but in
2
this area a higher Ga-O concentration was found. The occurrence of these particles was
reported to likely be specific for the deposition process (e.g. incomplete selenisation)
and to likely facilitate moisture and/or oxygen transfer to the Mo/CIGSSe interface.
CIGS
Back contact: MoSe 2
Back contact:
MoO 2 /MoO 3 /MoSe 2
(potentially with Na)
Back contact: Molybdenum
SLG
1 1 Degradation due to exposure
to temperature and humidity
Figure 2.5
General reaction mechanism for the degradation of molybdenum in CIGS cells at an isolation scribe, showing the formation
of molybdenum oxides from MoSe 2 and metallic molybdenum.
A proposal for a general route for degradation of molybdenum as present in CIGS so-
lar cells is shown in Figure 2.5. This shows that the top MoSe layer and the associated
2
metallic molybdenum can also form into an molybdenum oxide layer, although this
reaction is likely slower for selenised molybdenum than for metallic molybdenum.
2.3.1.3 Summary on molybdenum degradation
When molybdenum and selenised molybdenum films are exposed to water and oxy-
gen, especially under elevated temperatures, black and blue stains can occur on the
metallic molybdenum surface. These stains contain molybdenum oxide (MoO /MoO ),
2
3
which is generally badly conducting and reflecting. This likely has a negative impact
on the solar cell. Furthermore, the molybdenum oxide layers can crack and needles
can be formed. The formation of a thick layer of non-conductive molybdenum oxide
presumably leads to a very fast decrease of the conductivity when measured from
the top. The degradation effects are the most severe for molybdenum deposited at
high pressures during sputter deposition, thereby forming more porous molybde-
num layers, which are more susceptible for ingression of among others water and
40
