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Chapter 2
oxide (ZnO:Al), which is used by most companies and research centres as TCO on CIGS,
while other zinc oxide compounds, like CVD ZnO:B are also included. The latter is for
example used in industrial production by Solar Frontier. Relations between the elec-
trical and optical properties and deposition temperatures [68,71], crystallinity [68,71],
sample thickness [25,67,68,75] and doping content [64,79] were found.
Literature [29,77] reported that ZnO:Al is generally stable in dry heat tests while damp
heat exposure led to a decrease in conductivity. This is mostly caused by a decrease in
electron mobility, while changes in carrier concentration were observed in some cases
but can be minor to negligible for other ZnO:Al films. Changes in the optical properties
were also observed, but in general, these minor changes will have a relatively small im -
pact on module performance, when compared to the impact of the changes in electri -
cal properties. Pern et al. [25,59] for example observed the disappearance of fringes in
the transmission spectra, which did not necessary lead to a lower transmission. Greiner
et al. [62] and Theelen et al. [71] have also shown the minor impact of damp heat treat -
ment on optical properties, like transmission and the Drude frequency.
Various references [71,76,79] suggested that ZnO:Al degradation is mostly driven by an
enhanced potential barrier at the grain boundaries, while in-grain degradation proba -
bly has a smaller impact. The difference between these processes is shown in Figure 2.8.
Figure 2.8:
Possible electron scattering mechanisms in zinc oxide samples based on references [71,82].
Several fitting models were reported to distinguish between degradation based on
grain boundary and grain degradation. These are based on the comparison of the
decrease of electrical properties obtained by optical ('intra grain') and electrical Hall
measurements ('inter grain').
52
oxide (ZnO:Al), which is used by most companies and research centres as TCO on CIGS,
while other zinc oxide compounds, like CVD ZnO:B are also included. The latter is for
example used in industrial production by Solar Frontier. Relations between the elec-
trical and optical properties and deposition temperatures [68,71], crystallinity [68,71],
sample thickness [25,67,68,75] and doping content [64,79] were found.
Literature [29,77] reported that ZnO:Al is generally stable in dry heat tests while damp
heat exposure led to a decrease in conductivity. This is mostly caused by a decrease in
electron mobility, while changes in carrier concentration were observed in some cases
but can be minor to negligible for other ZnO:Al films. Changes in the optical properties
were also observed, but in general, these minor changes will have a relatively small im -
pact on module performance, when compared to the impact of the changes in electri -
cal properties. Pern et al. [25,59] for example observed the disappearance of fringes in
the transmission spectra, which did not necessary lead to a lower transmission. Greiner
et al. [62] and Theelen et al. [71] have also shown the minor impact of damp heat treat -
ment on optical properties, like transmission and the Drude frequency.
Various references [71,76,79] suggested that ZnO:Al degradation is mostly driven by an
enhanced potential barrier at the grain boundaries, while in-grain degradation proba -
bly has a smaller impact. The difference between these processes is shown in Figure 2.8.
Figure 2.8:
Possible electron scattering mechanisms in zinc oxide samples based on references [71,82].
Several fitting models were reported to distinguish between degradation based on
grain boundary and grain degradation. These are based on the comparison of the
decrease of electrical properties obtained by optical ('intra grain') and electrical Hall
measurements ('inter grain').
52
