Page 34 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 34
Chapter 2
illumination, which also leads to a reduction in the relative humidity of the
sample. Therefore, when comparing between damp heat and damp heat
illumination experiments, these deviations should be taken into account.
The temperatures used during accelerated lifetime testing (typically 85C) are much
o
higher than typical environmental temperatures. However, it should be noted that
module temperatures are often higher than the environmental temperature: McMa-
hon et al. [15] measured the temperatures at the back of modules in New Mexico as
high as 71 C.
o
It is important to note that the accelerated lifetime tests sometimes include the re-
moval of samples from the climate chamber for analysis purposes. The samples are
then returned to the chamber several hours or days later. These actions could lead
to additional degradation phenomena like the introduction of drying stains (evap-
ouration of surface water due to the lower humidity of ambient air) and stress related
effects due to large and fast temperature ('thermo-shock') and humidity changes. On
the other hand, this ‘drying effect’ can also be positive, for example due to intermedi -
ate drying of the samples, as described by Pern et al. [25].
2.3 Degradation of the individual layers
In this chapter, the changes in properties due to exposure to various test conditions
as well as the derived degradation mechanisms of the individual layers in the CIGS cell
are presented. Chapters 2.3.1 describes the vulnerability of the back contact molyb-
denum, chapters 2.3.2 and 2.3.3 describe the CIGS and buffer layer respectively, while
chapter 2.3.4 discusses the stability of the TCO front contact, with the emphasise on
the widely used material ZnO:Al. The cross-section of a typical CIGS solar cell is shown
in Figure 1.6.
2.3.1 Molybdenum degradation
Sputtered molybdenum (Mo) is used by almost all research groups and CIGS manu-
facturers as a back contact material. The properties of these as-deposited layers are
attractive for use in CIGS solar cells, since they can among others withstand high tem -
peratures and a selenium atmosphere, which is required for the further CIGS deposi-
tion [26]. Additionally, molybdenum films have relatively good conductivity and re-
flectivity, which are the main requirements for good performance in CIGS solar cells.
However, the material properties of molybdenum can change as a function of time,
leading to changes in conductivity and reflectivity. More information about the func-
32
illumination, which also leads to a reduction in the relative humidity of the
sample. Therefore, when comparing between damp heat and damp heat
illumination experiments, these deviations should be taken into account.
The temperatures used during accelerated lifetime testing (typically 85C) are much
o
higher than typical environmental temperatures. However, it should be noted that
module temperatures are often higher than the environmental temperature: McMa-
hon et al. [15] measured the temperatures at the back of modules in New Mexico as
high as 71 C.
o
It is important to note that the accelerated lifetime tests sometimes include the re-
moval of samples from the climate chamber for analysis purposes. The samples are
then returned to the chamber several hours or days later. These actions could lead
to additional degradation phenomena like the introduction of drying stains (evap-
ouration of surface water due to the lower humidity of ambient air) and stress related
effects due to large and fast temperature ('thermo-shock') and humidity changes. On
the other hand, this ‘drying effect’ can also be positive, for example due to intermedi -
ate drying of the samples, as described by Pern et al. [25].
2.3 Degradation of the individual layers
In this chapter, the changes in properties due to exposure to various test conditions
as well as the derived degradation mechanisms of the individual layers in the CIGS cell
are presented. Chapters 2.3.1 describes the vulnerability of the back contact molyb-
denum, chapters 2.3.2 and 2.3.3 describe the CIGS and buffer layer respectively, while
chapter 2.3.4 discusses the stability of the TCO front contact, with the emphasise on
the widely used material ZnO:Al. The cross-section of a typical CIGS solar cell is shown
in Figure 1.6.
2.3.1 Molybdenum degradation
Sputtered molybdenum (Mo) is used by almost all research groups and CIGS manu-
facturers as a back contact material. The properties of these as-deposited layers are
attractive for use in CIGS solar cells, since they can among others withstand high tem -
peratures and a selenium atmosphere, which is required for the further CIGS deposi-
tion [26]. Additionally, molybdenum films have relatively good conductivity and re-
flectivity, which are the main requirements for good performance in CIGS solar cells.
However, the material properties of molybdenum can change as a function of time,
leading to changes in conductivity and reflectivity. More information about the func-
32
