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Degradation mechanisms of the molybdenum back contact
molybdenum layers a separate Mo layer. Therefore, the molybdenum film had the
exact composition of molybdenum in a CIGS solar cell (Figure 5.1) .
This lift-off process was also described in reference [12], which also reports on the
composition of the non-degraded interface of similar samples, which have also been
deposited with different molybdenum sputter pressures. Extensive information about
similar non-degraded samples can be found there.
5.2.2 Sample degradation
The samples were submitted to a damp heat test, according to certification standard IEC
o
61646 [13], which specifies 1000 hours exposure to damp heat (85 C and 85% RH). The
damp heat tests were performed in a temperature and humidity controlled chamber
(Espec Humidity Cabinet LHU-212).
Due to the rapid degradation reported in literature [6], initial degradation periods of one
and two hours were chosen for both degradation studies. Later the exposure time to
o
85 C/85% RH was increased to 24 or 32 hours. This allowed excellent study of the initial
degradation mechanisms, but also induced many rapid warm-cold transitions, thereby
introducing an additional weak form of the ‘thermal cycling’ load [13] to the samples.
o
These temperature transitions combined with 85 C/85% RH will be more representative
for molybdenum that is exposed in the field than continuous exposure to damp heat.
The samples were characterised by various analysis techniques (see section 2.3) before,
during and after the degradation periods. When the samples were not in the climate
chamber or being measured, they were stored in an inert atmosphere or wrapped in a
container that was evacuated to low pressure.
During the ‘selenisation and pressure’ experiment, the samples were placed in the
climate chamber at 85 C/85% RH for varying intervals accumulating to 105 hours. The
o
degradation periods were 2/2/1/1/1/2/4/4/8/8/8/16/24/24 hours.
During the ‘lift-off’ experiment, the samples were placed in the climate chamber at
85 C/85% RH for varying intervals accumulating to 105 hours. The degradation periods
o
were 1/1/1/1/2/2/4/4/8/8/8/16/32/30/32 hours.
5.2.3 Characterisation of film properties
A Jandel Engineering Cylindrical Probe four-point was used to determine the sheet
resistance of the films. The optical properties were determined by a Shimadzu UV-
3600 UV-VIS-NIR, which allowed analysis of the reflectance of the molybdenum
samples. A Leica Wild M400 microscope and a digital camera were used to determine
131
molybdenum layers a separate Mo layer. Therefore, the molybdenum film had the
exact composition of molybdenum in a CIGS solar cell (Figure 5.1) .
This lift-off process was also described in reference [12], which also reports on the
composition of the non-degraded interface of similar samples, which have also been
deposited with different molybdenum sputter pressures. Extensive information about
similar non-degraded samples can be found there.
5.2.2 Sample degradation
The samples were submitted to a damp heat test, according to certification standard IEC
o
61646 [13], which specifies 1000 hours exposure to damp heat (85 C and 85% RH). The
damp heat tests were performed in a temperature and humidity controlled chamber
(Espec Humidity Cabinet LHU-212).
Due to the rapid degradation reported in literature [6], initial degradation periods of one
and two hours were chosen for both degradation studies. Later the exposure time to
o
85 C/85% RH was increased to 24 or 32 hours. This allowed excellent study of the initial
degradation mechanisms, but also induced many rapid warm-cold transitions, thereby
introducing an additional weak form of the ‘thermal cycling’ load [13] to the samples.
o
These temperature transitions combined with 85 C/85% RH will be more representative
for molybdenum that is exposed in the field than continuous exposure to damp heat.
The samples were characterised by various analysis techniques (see section 2.3) before,
during and after the degradation periods. When the samples were not in the climate
chamber or being measured, they were stored in an inert atmosphere or wrapped in a
container that was evacuated to low pressure.
During the ‘selenisation and pressure’ experiment, the samples were placed in the
climate chamber at 85 C/85% RH for varying intervals accumulating to 105 hours. The
o
degradation periods were 2/2/1/1/1/2/4/4/8/8/8/16/24/24 hours.
During the ‘lift-off’ experiment, the samples were placed in the climate chamber at
85 C/85% RH for varying intervals accumulating to 105 hours. The degradation periods
o
were 1/1/1/1/2/2/4/4/8/8/8/16/32/30/32 hours.
5.2.3 Characterisation of film properties
A Jandel Engineering Cylindrical Probe four-point was used to determine the sheet
resistance of the films. The optical properties were determined by a Shimadzu UV-
3600 UV-VIS-NIR, which allowed analysis of the reflectance of the molybdenum
samples. A Leica Wild M400 microscope and a digital camera were used to determine
131
