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Chapter 2
2.3.4.1.8 ZnO:Al annealing
Instead of annealing after degradation, which can reverse the effects of damp heat
exposure, annealing can also be applied as a part of the deposition process:
o
Hüpkes et al. [72] applied a special annealing step at 650 C in order to prepare highly
2
stable ZnO:Al films. These films, which retain their charge carrier mobility of 70 cm /Vs
after 1000 hours of damp heat treatment, probably have ‘reconstructed grain bound-
aries’. This indicates that grains grow together due to the treatment, so the grain
boundaries are ‘closed’.
ZnO:Al films prepared via a low temperature deposition and annealing process which
can be used for application in CIGS solar cells showed an increase in initial mobility
2
from 13 cm /Vs to 35 cm /Vs due to the annealing treatment. They also showed a sta-
2
ble carrier concentration and mobility after 1000 hours damp heat exposure. How-
ever, it should be noted that in the reference sample without the annealing step also
retained it mobility, but it showed a decrease in the carrier concentration.
2.3.4.2 Indium tin oxide degradation
An alternative to ZnO:Al is tin doped indium oxide (ITO). This material is especially
attractive for small area cells and is less used for modules because of its relatively high
cost. As shown in Figure 2.8, ITO is generally reported to be more stable than ZnO:Al.
Kim et al. [78] reported that the conductivity of a 1 µm thick ITO layer did not change
after 24 hours of exposure to 80C/100% RH. Guillen et al. [88] described that the
o
structural and optical properties of sputtered ITO samples did not change due to 1000
hours exposure to a damp heat test. Small changes in sheet resistance were observed:
it was found that samples with a higher partial pressure of O during the sputtering
2
process have a lower stability to a damp heat test. This is explained by the prominent
presence of (222) oriented grains, which can accommodate a large amount of inter-
stitial oxygen atoms. Due to their higher lattice distortion than for the (400) oriented
grains, they are more vulnerable for humidity and oxygen ingress, since these species
can now more easily migrate through the material via the defect paths.
Xu et al. [17] reported that a difference in degradation behaviour was observed for
different sputtering temperatures. The structure of the ITO films changes from amor-
phous to polycrystalline for an increasing deposition temperature. The films depos-
ited at higher temperature, had both a lower initial resistivity and a better stability
when exposed to a damp heat test. Since the samples deposited at low temperatures
are less crystalline, it was proposed that water and alkaline species (NaOH) can easily
penetrate through amorphous ITO films. On these samples, white spots were found –
60
2.3.4.1.8 ZnO:Al annealing
Instead of annealing after degradation, which can reverse the effects of damp heat
exposure, annealing can also be applied as a part of the deposition process:
o
Hüpkes et al. [72] applied a special annealing step at 650 C in order to prepare highly
2
stable ZnO:Al films. These films, which retain their charge carrier mobility of 70 cm /Vs
after 1000 hours of damp heat treatment, probably have ‘reconstructed grain bound-
aries’. This indicates that grains grow together due to the treatment, so the grain
boundaries are ‘closed’.
ZnO:Al films prepared via a low temperature deposition and annealing process which
can be used for application in CIGS solar cells showed an increase in initial mobility
2
from 13 cm /Vs to 35 cm /Vs due to the annealing treatment. They also showed a sta-
2
ble carrier concentration and mobility after 1000 hours damp heat exposure. How-
ever, it should be noted that in the reference sample without the annealing step also
retained it mobility, but it showed a decrease in the carrier concentration.
2.3.4.2 Indium tin oxide degradation
An alternative to ZnO:Al is tin doped indium oxide (ITO). This material is especially
attractive for small area cells and is less used for modules because of its relatively high
cost. As shown in Figure 2.8, ITO is generally reported to be more stable than ZnO:Al.
Kim et al. [78] reported that the conductivity of a 1 µm thick ITO layer did not change
after 24 hours of exposure to 80C/100% RH. Guillen et al. [88] described that the
o
structural and optical properties of sputtered ITO samples did not change due to 1000
hours exposure to a damp heat test. Small changes in sheet resistance were observed:
it was found that samples with a higher partial pressure of O during the sputtering
2
process have a lower stability to a damp heat test. This is explained by the prominent
presence of (222) oriented grains, which can accommodate a large amount of inter-
stitial oxygen atoms. Due to their higher lattice distortion than for the (400) oriented
grains, they are more vulnerable for humidity and oxygen ingress, since these species
can now more easily migrate through the material via the defect paths.
Xu et al. [17] reported that a difference in degradation behaviour was observed for
different sputtering temperatures. The structure of the ITO films changes from amor-
phous to polycrystalline for an increasing deposition temperature. The films depos-
ited at higher temperature, had both a lower initial resistivity and a better stability
when exposed to a damp heat test. Since the samples deposited at low temperatures
are less crystalline, it was proposed that water and alkaline species (NaOH) can easily
penetrate through amorphous ITO films. On these samples, white spots were found –
60
