Page 172 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 172
Chapter 5
The vulnerability of molybdenum to damp heat can be a problem in CIGS PV
technology. On one hand, degradation of the back contact itself is probably not a big
risk, since this will probably not lead to the complete loss of the lateral conductivity
of molybdenum. The most likely degradation point is the molybdenum-zinc oxide
interface in P2 by the formation of an oxide layer between the molybdenum and
zinc oxide [6]. This would break down the monolithic connection, which would
have a destructive effect on CIGS modules. Another risk can be found in the volume
expansion, which can lead to delamination between the CIGS and the molybdenum
layers or the disappearance of the ohmic contact.
It is therefore recommended to use bilayer (or multilayer) molybdenum film with a
dense top layer, which can prevent the formation of a top later of molybdenum oxide.
A last remark concerns the storage of molybdenum films. In research environments,
molybdenum is often deposited and then stored, instead of direct further processing.
Exposure to humid air for several days can already lead to molybdenum oxide
formation, which can be detrimental for quality of the later processed solar cells.
It should be noted that the storage of molybdenum layers can also influence the
electrical parameters of a solar cell in a positive way, but since the impact is not
known, this can reduce experimental reproducibility and disturb the interpretation
of the obtained results.
5.5 Conclusions
All molybdenum layers already showed degradation effects which were visible by
naked eye after only several hours of damp heat exposure. The exposure resulted in
large volume expansion due to the formation of a thick non-conductive molybdenum
oxide layer consisting of various oxides and suboxides on top of the metallic
molybdenum. This MoO layer showed cracks and the appearance of needle-like
x
structures, maybe existing of Na CO .
3
2
The degradation effect was most severe for layers with the highest sputter pressure
and thus the most porous microstructure. It was less severe for bilayer molybdenum
films, which largely oxidised in the more porous bottom layer. The effect of selenisation
was observed in the visual and optical characteristics: dense selenised molybdenum
layers retained the highest reflectance. Likely, the presence of MoSe prevented rapid
2
oxidation of the molybdenum. These results show that a dense selenised molybdenum
is the most stable type.
The surface oxidation of molybdenum was further studied by XPS measurements,
which showed that the MoO material is mostly MoO with intercalated Na , leading to
+
3
x
170
The vulnerability of molybdenum to damp heat can be a problem in CIGS PV
technology. On one hand, degradation of the back contact itself is probably not a big
risk, since this will probably not lead to the complete loss of the lateral conductivity
of molybdenum. The most likely degradation point is the molybdenum-zinc oxide
interface in P2 by the formation of an oxide layer between the molybdenum and
zinc oxide [6]. This would break down the monolithic connection, which would
have a destructive effect on CIGS modules. Another risk can be found in the volume
expansion, which can lead to delamination between the CIGS and the molybdenum
layers or the disappearance of the ohmic contact.
It is therefore recommended to use bilayer (or multilayer) molybdenum film with a
dense top layer, which can prevent the formation of a top later of molybdenum oxide.
A last remark concerns the storage of molybdenum films. In research environments,
molybdenum is often deposited and then stored, instead of direct further processing.
Exposure to humid air for several days can already lead to molybdenum oxide
formation, which can be detrimental for quality of the later processed solar cells.
It should be noted that the storage of molybdenum layers can also influence the
electrical parameters of a solar cell in a positive way, but since the impact is not
known, this can reduce experimental reproducibility and disturb the interpretation
of the obtained results.
5.5 Conclusions
All molybdenum layers already showed degradation effects which were visible by
naked eye after only several hours of damp heat exposure. The exposure resulted in
large volume expansion due to the formation of a thick non-conductive molybdenum
oxide layer consisting of various oxides and suboxides on top of the metallic
molybdenum. This MoO layer showed cracks and the appearance of needle-like
x
structures, maybe existing of Na CO .
3
2
The degradation effect was most severe for layers with the highest sputter pressure
and thus the most porous microstructure. It was less severe for bilayer molybdenum
films, which largely oxidised in the more porous bottom layer. The effect of selenisation
was observed in the visual and optical characteristics: dense selenised molybdenum
layers retained the highest reflectance. Likely, the presence of MoSe prevented rapid
2
oxidation of the molybdenum. These results show that a dense selenised molybdenum
is the most stable type.
The surface oxidation of molybdenum was further studied by XPS measurements,
which showed that the MoO material is mostly MoO with intercalated Na , leading to
+
3
x
170
