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Chapter 2
2.3.2.3 Chemical changes in the absorber
In this chapter, we present data about chemical changes in CIGS absorbers that are
not embedded in a solar cell. In chapter 2.4, more extensive information about chang -
es in full solar cells is given.
Pern et al. [44] exposed CIGS absorbers to damp heat tests (15 minutes to 5 hours) and
for 8-12 months to the laboratory atmosphere. Both the 5 hours damp heat and the
air exposure led to the formation of spots (Figure 2.6), which were also reported by
references [21] and [45]. The spotty areas showed low gallium, copper and selenium
concentrations, while the concentration of sodium was relatively high.
It was expected that hydrolysis or oxidation led to these degradation processes. These
spots did not have an impact on the optical properties between 250 and 900 nm. Scan -
ning Kelvin Probe (SKP) and Atomic Force Microscopy (AFM) measurements were used
to observe the correlation grain topography and the work function distribution. Before
degradation, the work function depended closely on the grains' surface facets and its
values on the grain boundaries might have depended on specific grain boundary struc -
tures. After one and five hours of damp heat exposure, SKP AFM measurements showed
that small spots occurred on the surface. It was also observed that the work function
had lost its correlation with the grain geometric feature due to damp heat exposure.
After five hours, the initially well-defined work-function images became blurred, which
could be a results of the electronic property breakdown of the CIGS grains.
Solar cells were made from the absorbers exposed to ambient conditions and to 15
and 30 minutes damp heat. The cells exposed to ambient conditions showed a se-
vere decrease of all solar cell parameters, while the damp heat degradation showed a
much smaller decrease of only V and FF.
oc
The degradation of bare CIGS absorbers under damp heat or under ambient condi-
tions agreed with the large decrease in carrier lifetime observed by Metzger et al.
[46], who studied air exposed CIGS absorbers by Time Resolved Photo Luminescence
(TRPL). This degradation process can be prevented by CdS deposition, which leads to
absorbers that are stable in the air for months.
Suggestions about the possible degradation products on the absorbers were giv-
en by Würz et al. [47] and Heske et al. [48]. Würz et al [47] reported the formation of
Cu(OH) on CuGaSe solar cells that were stored under ambient conditions for months.
2
2
Heske et al. [48] reported the formation of sulphate, which resulted from the damp
heat induced oxidation of sulphur in Cu(In,Ga)(Se,S) solar cells.
2
44
2.3.2.3 Chemical changes in the absorber
In this chapter, we present data about chemical changes in CIGS absorbers that are
not embedded in a solar cell. In chapter 2.4, more extensive information about chang -
es in full solar cells is given.
Pern et al. [44] exposed CIGS absorbers to damp heat tests (15 minutes to 5 hours) and
for 8-12 months to the laboratory atmosphere. Both the 5 hours damp heat and the
air exposure led to the formation of spots (Figure 2.6), which were also reported by
references [21] and [45]. The spotty areas showed low gallium, copper and selenium
concentrations, while the concentration of sodium was relatively high.
It was expected that hydrolysis or oxidation led to these degradation processes. These
spots did not have an impact on the optical properties between 250 and 900 nm. Scan -
ning Kelvin Probe (SKP) and Atomic Force Microscopy (AFM) measurements were used
to observe the correlation grain topography and the work function distribution. Before
degradation, the work function depended closely on the grains' surface facets and its
values on the grain boundaries might have depended on specific grain boundary struc -
tures. After one and five hours of damp heat exposure, SKP AFM measurements showed
that small spots occurred on the surface. It was also observed that the work function
had lost its correlation with the grain geometric feature due to damp heat exposure.
After five hours, the initially well-defined work-function images became blurred, which
could be a results of the electronic property breakdown of the CIGS grains.
Solar cells were made from the absorbers exposed to ambient conditions and to 15
and 30 minutes damp heat. The cells exposed to ambient conditions showed a se-
vere decrease of all solar cell parameters, while the damp heat degradation showed a
much smaller decrease of only V and FF.
oc
The degradation of bare CIGS absorbers under damp heat or under ambient condi-
tions agreed with the large decrease in carrier lifetime observed by Metzger et al.
[46], who studied air exposed CIGS absorbers by Time Resolved Photo Luminescence
(TRPL). This degradation process can be prevented by CdS deposition, which leads to
absorbers that are stable in the air for months.
Suggestions about the possible degradation products on the absorbers were giv-
en by Würz et al. [47] and Heske et al. [48]. Würz et al [47] reported the formation of
Cu(OH) on CuGaSe solar cells that were stored under ambient conditions for months.
2
2
Heske et al. [48] reported the formation of sulphate, which resulted from the damp
heat induced oxidation of sulphur in Cu(In,Ga)(Se,S) solar cells.
2
44
