Page 128 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 128
Chapter 5
Abstract
Sputtered polycrystalline molybdenum films on glass with or without a MoSe top layer
2
was exposed to damp heat. All molybdenum films 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 structures,
x
maybe existing of Na CO .
2
3
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, thereaction of molybdenum and selenium, which led to MoSe 2
formation, prevented rapid oxidation of the film. These results show that a dense selenised
molybdenum is the most stable type.
The oxidised surface of molybdenum was further studied by XPS measurements, which
+
showed that the MoO material was mostly MoO with intercalated Na , which led to the
3
x
formation of Na MoO (molybdenum bronze) following this redox reaction :
3
x
+
-
MoO + xNa + xe → Na MoO 3
x
3
+
Intercalation of Na can explain both the high mobility of sodium in the grain boundaries
and the important role this element play in the degradation process. It is thus proposed
that both the grain boundaries as well as the degraded molybdenum contains MoO
3
+
+
which is intercalated with Na . Finally the intercalation of Na in MoO is a key point in the
3
understanding of the evolution of cell performance under damp heat exposure in presence
of Na. While the MoO is an insulator, Na MoO molybdenum bronze exhibits occupied states
3
3
x
+
near the Fermi level, making this material conductive. Therefore, intercalation of Na in the
MoO can greatly influence the band alignment in CIGS cells, while the formation of non-
3
conductive MoO can completely destroy the solar cell.
3
126
Abstract
Sputtered polycrystalline molybdenum films on glass with or without a MoSe top layer
2
was exposed to damp heat. All molybdenum films 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 structures,
x
maybe existing of Na CO .
2
3
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, thereaction of molybdenum and selenium, which led to MoSe 2
formation, prevented rapid oxidation of the film. These results show that a dense selenised
molybdenum is the most stable type.
The oxidised surface of molybdenum was further studied by XPS measurements, which
+
showed that the MoO material was mostly MoO with intercalated Na , which led to the
3
x
formation of Na MoO (molybdenum bronze) following this redox reaction :
3
x
+
-
MoO + xNa + xe → Na MoO 3
x
3
+
Intercalation of Na can explain both the high mobility of sodium in the grain boundaries
and the important role this element play in the degradation process. It is thus proposed
that both the grain boundaries as well as the degraded molybdenum contains MoO
3
+
+
which is intercalated with Na . Finally the intercalation of Na in MoO is a key point in the
3
understanding of the evolution of cell performance under damp heat exposure in presence
of Na. While the MoO is an insulator, Na MoO molybdenum bronze exhibits occupied states
3
3
x
+
near the Fermi level, making this material conductive. Therefore, intercalation of Na in the
MoO can greatly influence the band alignment in CIGS cells, while the formation of non-
3
conductive MoO can completely destroy the solar cell.
3
126
