Page 41 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 41
Stability of Cu(In,Ga)Se 2 Solar Cells
efficiency of CIGS solar cells is considered. However, molybdenum degradation seems
to impact CIGS modules at the scribes that separate the individual solar cells within a
monolithically integrated module. In these regions, the molybdenum is less protected
against humidity ingression [27] and more vulnerable due to scribing damage [28].
The chemical composition of the molybdenum surface might also be different due to
the scribing process. More information about the specific impact of scribe degrada-
tion can be found in chapter 2.5.1 .
In order to learn more about the long term stability of molybdenum within CIGS solar
cells, the information on the stability of bare molybdenum films, described in chapter
2.3.1.1 , must be considered. The chemical environment of molybdenum in a CIGS solar
cell is different than the environment of metallic molybdenum without further treat-
ment. One difference can be found in the migration of sodium into the molybdenum
layer due to high temperatures used during CIGS absorber deposition [35].
Reference [26] showed that the presence of MoSe can greatly influence the degrada-
2
tion of this molybdenum. This work showed that the decrease of conductivity due to
degradation was not large and more or less similar for selenised and non-selenised
molybdenum. However, dense selenised molybdenum retained most of its reflec-
tance after 105 hours at 85C/85% RH, while non-selenised molybdenum became
o
non-reflecting. It is proposed therefore that the presence of MoSe prevents the for-
2
mation of molybdenum oxide.
Theelen et al. [35] also reported on the degradation of selenised bilayer molybdenum
on soda lime glass. Bilayers consisting of a porous bottom layer with good adhesion
and a dense top layer with good conductivity are mostly used in CIGS solar cells. Expo -
sure to damp heat led to the formation of MoO on top of the samples, but also within
x
the more porous bottom layer. Furthermore, XPS studies showed the presence of a
potentially conductive material, which was proposed to be a ‘molybdenum bronze’.
This material was likely formed by the so-called ‘intercalation’ of Na into a matrix of
+
MoO , which according to the following redox reaction:
3
+
Na MoO xNa + MoO + xe - (2.1)
3
x
3
It was thus proposed that the formed molybdenum oxide layer contained NaMoO
3
x
+
with different Na contents and different grades of conductivity. This intercalation pro -
cess can also explain the high mobility of Na via the grain boundaries in molybdenum.
+
More information about this intercalation can be found in chapter 5.
It should be noted that the deposition method of CIGS on the molybdenum film can
also impact the stability of molybdenum in the presence of moisture: Feist et al. [21]
39
efficiency of CIGS solar cells is considered. However, molybdenum degradation seems
to impact CIGS modules at the scribes that separate the individual solar cells within a
monolithically integrated module. In these regions, the molybdenum is less protected
against humidity ingression [27] and more vulnerable due to scribing damage [28].
The chemical composition of the molybdenum surface might also be different due to
the scribing process. More information about the specific impact of scribe degrada-
tion can be found in chapter 2.5.1 .
In order to learn more about the long term stability of molybdenum within CIGS solar
cells, the information on the stability of bare molybdenum films, described in chapter
2.3.1.1 , must be considered. The chemical environment of molybdenum in a CIGS solar
cell is different than the environment of metallic molybdenum without further treat-
ment. One difference can be found in the migration of sodium into the molybdenum
layer due to high temperatures used during CIGS absorber deposition [35].
Reference [26] showed that the presence of MoSe can greatly influence the degrada-
2
tion of this molybdenum. This work showed that the decrease of conductivity due to
degradation was not large and more or less similar for selenised and non-selenised
molybdenum. However, dense selenised molybdenum retained most of its reflec-
tance after 105 hours at 85C/85% RH, while non-selenised molybdenum became
o
non-reflecting. It is proposed therefore that the presence of MoSe prevents the for-
2
mation of molybdenum oxide.
Theelen et al. [35] also reported on the degradation of selenised bilayer molybdenum
on soda lime glass. Bilayers consisting of a porous bottom layer with good adhesion
and a dense top layer with good conductivity are mostly used in CIGS solar cells. Expo -
sure to damp heat led to the formation of MoO on top of the samples, but also within
x
the more porous bottom layer. Furthermore, XPS studies showed the presence of a
potentially conductive material, which was proposed to be a ‘molybdenum bronze’.
This material was likely formed by the so-called ‘intercalation’ of Na into a matrix of
+
MoO , which according to the following redox reaction:
3
+
Na MoO xNa + MoO + xe - (2.1)
3
x
3
It was thus proposed that the formed molybdenum oxide layer contained NaMoO
3
x
+
with different Na contents and different grades of conductivity. This intercalation pro -
cess can also explain the high mobility of Na via the grain boundaries in molybdenum.
+
More information about this intercalation can be found in chapter 5.
It should be noted that the deposition method of CIGS on the molybdenum film can
also impact the stability of molybdenum in the presence of moisture: Feist et al. [21]
39
