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Stability of Cu(In,Ga)Se 2 Solar Cells
This chapter will first discuss the long term stability tests on the individual layers in
CIGS cells, including the molybdenum (Mo) back contact (chapter 2.3.1 ), the CIGS and
buffer layers (chapters 2.3.2 and 2.3.3) and the transparent conductive oxide (TCO)
front contact (chapter 2.3.4). Then, the degradation behaviour of complete CIGS solar
cells and the physical degradation mechanisms are presented (chapter 2.4). In the final
chapter, the impact of module formation techniques on the stability of CIGS modules
is described (chapter 2.5).
In this chapter, the terms ‘CIGS solar cells’ and ‘CIGS modules’ are used to generally
refer to solar cells and modules based on Cu(In,Ga)Se , but also to related materials
2
like CuInSe and Cu(In,Ga)(Se,S) . However, when the word ‘CIGS’ is used to describe
2
2
only the absorber material, this refers to Cu(In,Ga)Se , while it is separately mentioned
2
when solar cells based on other materials, like CuInSe , are described.
2
2.2 Degradation test conditions
In the chapters below, the results of various types of accelerated lifetime tests are
described. The most important tests are introduced here:
1. 'Standard' damp heat exposure at 85C/85% RH - according to IEC standard
o
61646, 1000 hours exposure to these conditions should mimic 25 years field
exposure in Miami. However, the similarities between field and accelerated
testing are still under debate [12,13]. In this thesis – as in most publications
– the term 'damp heat' refers to these conditions unless other conditions are
explicitly stated.
2. Mild damp heat, e.g. 60 C/90% RH or 60C/60% RH [e.g. 16-19]. It should be
o
o
o
noted that the relative water concentration is higher for 60C/90% RH than
o
for 85 C/85% RH, but the absolute water concentration is lower, due to lower
saturation concentrations at lower temperatures (e.g. 90% RH at 60 C is 109 g/
o
3
o
3
m , while 85% RH at 85 C is 241 g/m [20]).
o
3. Dry heat is mostly 85C or 90 C combined with low relative humidity grade
o
(e.g. 10% RH) [21]. Articles about pure heat treatments at temperatures above
100 C are only included in this review chapter if these results can be assumed
o
relevant for long term stability.
4. Combined damp heat and illumination testing [10,22], which allows the in-
situ monitoring of the degradation behaviour [23]. Furthermore, damp heat
can be combined with UV illumination as well [24]. It should be noted that
the temperature and humidity conditions as reported in combined damp
heat illumination experiments are generally the chamber conditions. The
sample conditions can be different due to additional heating caused by the
31
This chapter will first discuss the long term stability tests on the individual layers in
CIGS cells, including the molybdenum (Mo) back contact (chapter 2.3.1 ), the CIGS and
buffer layers (chapters 2.3.2 and 2.3.3) and the transparent conductive oxide (TCO)
front contact (chapter 2.3.4). Then, the degradation behaviour of complete CIGS solar
cells and the physical degradation mechanisms are presented (chapter 2.4). In the final
chapter, the impact of module formation techniques on the stability of CIGS modules
is described (chapter 2.5).
In this chapter, the terms ‘CIGS solar cells’ and ‘CIGS modules’ are used to generally
refer to solar cells and modules based on Cu(In,Ga)Se , but also to related materials
2
like CuInSe and Cu(In,Ga)(Se,S) . However, when the word ‘CIGS’ is used to describe
2
2
only the absorber material, this refers to Cu(In,Ga)Se , while it is separately mentioned
2
when solar cells based on other materials, like CuInSe , are described.
2
2.2 Degradation test conditions
In the chapters below, the results of various types of accelerated lifetime tests are
described. The most important tests are introduced here:
1. 'Standard' damp heat exposure at 85C/85% RH - according to IEC standard
o
61646, 1000 hours exposure to these conditions should mimic 25 years field
exposure in Miami. However, the similarities between field and accelerated
testing are still under debate [12,13]. In this thesis – as in most publications
– the term 'damp heat' refers to these conditions unless other conditions are
explicitly stated.
2. Mild damp heat, e.g. 60 C/90% RH or 60C/60% RH [e.g. 16-19]. It should be
o
o
o
noted that the relative water concentration is higher for 60C/90% RH than
o
for 85 C/85% RH, but the absolute water concentration is lower, due to lower
saturation concentrations at lower temperatures (e.g. 90% RH at 60 C is 109 g/
o
3
o
3
m , while 85% RH at 85 C is 241 g/m [20]).
o
3. Dry heat is mostly 85C or 90 C combined with low relative humidity grade
o
(e.g. 10% RH) [21]. Articles about pure heat treatments at temperatures above
100 C are only included in this review chapter if these results can be assumed
o
relevant for long term stability.
4. Combined damp heat and illumination testing [10,22], which allows the in-
situ monitoring of the degradation behaviour [23]. Furthermore, damp heat
can be combined with UV illumination as well [24]. It should be noted that
the temperature and humidity conditions as reported in combined damp
heat illumination experiments are generally the chamber conditions. The
sample conditions can be different due to additional heating caused by the
31
