Page 60 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 60
Chapter 2
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tive, low-transparent ZnO:Al samples to 85 C dry heat, 85 C/100% RH and liquid water
at room temperature. They saw a thickness increases for these conditions from an
initial 110 nm to thicknesses as large as ~130 nm, 350 nm and 1000 nm respectively. At
the same time, the ZnO:Al resistivity increased by three to four orders of magnitude
following exposure to the water bath and 85°C/100% RH environments.
Additionally, as described in chapter 3.3.4.1.1, the roughness of the layer beneath the
TCO film also greatly influences the stability.
2.3.4.1.6 Influence of doping concentration
Minami et al. [68] observed that a high concentration of the aluminium (up to 8%)
in ZnO:Al films led to a higher initial resistivity, but also to a more stable TCO when
o
exposed to mild damp heat conditions (60 C/90% RH) for 1.000 hours. The influence
of aluminium was also observed in reference [64], where aluminium contents of 1 to
10% were compared and a reduction in the internal stress of the ZnO:Al film for higher
aluminium concentration (above 3%) was observed. The best moisture resistance was
obtained for aluminium concentrations between 3 and 5 %. However, a concentration
of 2% is standard for CIGS.
Steinhauser et al. [79] observed a large difference in stability of ZnO:B for different
boron concentrations. He explained this relationship between doping concentration
and damp heat stability by the higher carrier concentrations for the boron-rich sam-
ples. This relation can be explained by the increase of charge states at grain bound-
aries during damp heat treatment. Heavily doped ZnO grains allow extra tunneling
through the potential barriers and thus more stable films. It has been shown that
a-Si:H solar cells made with ZnO:B with high boron contents are stable under damp
heat, in contrast to the cells with low boron concentrations.
Pern et al. [25] added magnesium to the ZnO:Al, leading to (ZnMg )O:Al and
0.1
0.9
(Zn 0.99 Mg )O:Al. The film with 10% magnesium was non-conductive, while the 1%
0.01
magnesium film showed the same degradation behaviour as standard ZnO:Al, so no
influence was observed. Kang et al. [16] added 1 wt-% Ga 2 O to the ZnO:Al. The electrical
3
o
changes of these films due to 60 C/90% RH were relatively small, but due to the use of
these mild conditions, comparison with ZnO:Al without gallium cannot be made.
In summary, a small increase of the aluminium concentration from values normally
used in CIGS solar cells likely has a positive influence on the damp heat stability of the
doped zinc oxide layers, while high boron doping concentrations are also desirable.
58
o
o
tive, low-transparent ZnO:Al samples to 85 C dry heat, 85 C/100% RH and liquid water
at room temperature. They saw a thickness increases for these conditions from an
initial 110 nm to thicknesses as large as ~130 nm, 350 nm and 1000 nm respectively. At
the same time, the ZnO:Al resistivity increased by three to four orders of magnitude
following exposure to the water bath and 85°C/100% RH environments.
Additionally, as described in chapter 3.3.4.1.1, the roughness of the layer beneath the
TCO film also greatly influences the stability.
2.3.4.1.6 Influence of doping concentration
Minami et al. [68] observed that a high concentration of the aluminium (up to 8%)
in ZnO:Al films led to a higher initial resistivity, but also to a more stable TCO when
o
exposed to mild damp heat conditions (60 C/90% RH) for 1.000 hours. The influence
of aluminium was also observed in reference [64], where aluminium contents of 1 to
10% were compared and a reduction in the internal stress of the ZnO:Al film for higher
aluminium concentration (above 3%) was observed. The best moisture resistance was
obtained for aluminium concentrations between 3 and 5 %. However, a concentration
of 2% is standard for CIGS.
Steinhauser et al. [79] observed a large difference in stability of ZnO:B for different
boron concentrations. He explained this relationship between doping concentration
and damp heat stability by the higher carrier concentrations for the boron-rich sam-
ples. This relation can be explained by the increase of charge states at grain bound-
aries during damp heat treatment. Heavily doped ZnO grains allow extra tunneling
through the potential barriers and thus more stable films. It has been shown that
a-Si:H solar cells made with ZnO:B with high boron contents are stable under damp
heat, in contrast to the cells with low boron concentrations.
Pern et al. [25] added magnesium to the ZnO:Al, leading to (ZnMg )O:Al and
0.1
0.9
(Zn 0.99 Mg )O:Al. The film with 10% magnesium was non-conductive, while the 1%
0.01
magnesium film showed the same degradation behaviour as standard ZnO:Al, so no
influence was observed. Kang et al. [16] added 1 wt-% Ga 2 O to the ZnO:Al. The electrical
3
o
changes of these films due to 60 C/90% RH were relatively small, but due to the use of
these mild conditions, comparison with ZnO:Al without gallium cannot be made.
In summary, a small increase of the aluminium concentration from values normally
used in CIGS solar cells likely has a positive influence on the damp heat stability of the
doped zinc oxide layers, while high boron doping concentrations are also desirable.
58
