The influence of water and atmospheric gases on the degradation of aluminum doped
zinc oxide layers for CIGS solar cells
Mirjam Theelen
1,2
, Christopher Foster
1
, Supratik Dasgupta
1
, Zeger Vroon
1
, Nicolas Barreau
3
,
Miro Zeman
2
1
TNO, Thin Film Technology - De Rondom 1, 5612 AP, Eindhoven, The Netherlands
2
Delft University of Technology, Photovoltaic Materials and Devices, Mekelweg 4, 2628 CD
Delft, The Netherlands
3
Institut des Matériaux Jean Rouxel (IMN)-UMR 6502, Université de Nantes, CNRS, 2 rue
de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France
Introduction
Aluminum doped zinc oxide (ZnO:Al) is used in among others transparent electronics and
thin film solar cells, because of its low price, high transparency and conductivity. Sputtered
ZnO:Al films are therefore used as front contacts in CIGS solar cells. It is known that contact
of ZnO:Al films water (often in vapor phase at elevated temperatures, also called “damp
heat”) leads to decreased mobility and thus conductivity of these films. However, it was
unknown whether this was mainly caused by the presence of water, or whether other
atmospheric species were also involved in the degradation process. In this study, we
investigated whether single ZnO:Al films and complete CIGS solar cells degrade in the
presence of water with and without other atmospheric species, like O
2
, N
2
and CO
2
.
Experimental
We have studied the impact of water and atmospheric gases on the degradation of ZnO:Al as
well as CIGS solar cells. In order to accelerate their physical and chemical degradation
behavior, ZnO:Al samples were exposed to liquid room temperature H
2
O purged with the
atmospheric gases air, CO
2
, O
2
and N
2
and to these gases individually. The optical, electrical
and structural characteristics were analyzed before, during and after degradation.
Results
ZnO:Al films degrade in the presence of H
2
O and CO
2
. Individually, gaseous CO
2
does not
impact the degradation, while the individual impact of H
2
O is minor: exposure to H
2
O
without CO
2
results in a small decrease in mobility, associated with slow diffusion of H
2
O
down the grain boundaries, possibly forming Zn(OH)
2
. However, in the presence of CO
2
, the
optical and electrical properties of ZnO:Al deteriorated very quickly, while a large increase of
OH and C was observed by SIMS depth profiling. We propose the formation of
Zn
5
(OH)
6
(CO
3
)
2
. We show that the impact of N
2
as well as O
2
on ZnO:Al degradation is very
small.
The CIGS solar cells exposed to H
2
O and CO
2
also degraded very rapidly: the solar cells
exposed to H
2
O/CO
2
/N
2
and H
2
O/air showed a very rapid decay in efficiency, while the other
showed a very small decrease. The losses in the H
2
O/CO
2
exposed solar cells were driven by
a steady increase in series resistance and a very rapid ‘sudden death’ decrease of the short
circuit current. Cross-section SEM showed the formation of gaps in the ZnO:Al layers,
especially at the CdS/ZnO interface.
Conclusion
Experiments showed that single ZnO:Al films and films incorporated in CIGS degraded in
the presence of water combined with CO
2
. They were stable in water combined with oxygen
and nitrogen, as well as in gaseous CO
2
. This knowledge can help the optimization of barrier
coatings for devices containing ZnO:Al films. These barriers should not only function as a
water barrier, since porosity for CO
2
should be taken into account as well.
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