Page 57 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 57
Stability of Cu(In,Ga)Se 2 Solar Cells
The changes in electrical properties are probably caused by changes in the grain
boundaries, where a small change in composition can have a large impact on the
conductivity. Furthermore, in some cases, the composition of the grains also had
changed. At present, the chemical mechanisms leading to these composition chang-
es are not completely clarified. In this chapter, we discuss which species can be in-
volved in the degradation. Suggestions about the molecules that are formed during
ZnO:Al degradation include Al(OH) [25,64], Zn(OH) [25,64,71,76], ZnOH-complexes
2
3
and hydrates [20], ZnCO [76] and Zn(CO ) (OH) and related species [71] which are
3 2
3
5
6
mostly non-conductive materials. The presence of a hydroxide was also shown in ref-
erence [29]. These materials can function as an electrical potential barrier in the grain
boundaries as shown in Figure 2.10. They might also be present in the grains, thereby
simply decreasing the conductivity of the films.
It is clear that these chemical reactions occur due to reactions between the zinc oxide
and molecules from the atmosphere. It was suggested in reference [75] that the pen-
etration of oxygen and water molecules into the bottom part of the film can strength -
en the effect of grain boundary scattering, while other references [72,72,76] suggest-
ed a role for water and CO .
2
Theelen et al. [85] studied the role of various atmospheric species, by exposing ZnO:Al
layers at room temperature to the atmospheric gases carbon dioxide (CO), oxygen
2
(O ), ogen/argon (N/Ar) and air as well as liquid water purged with these gases, in
2
2
order to investigate their influence on the stability behaviour of these layers. It was
shown that O and CO gases in the absence of water did not cause any degradation
2
2
at all during the tested period, while water purged with O or N only led to a small
2
2
increase in resistivity, likely due to the formation of a limited amount of Zn(OH).
2
However, when CO was also present alongside with water, the concentration of OH -
2
increased greatly in the bulk and even more at the air/ZnO:Al and the ZnO:Al/glass
interfaces. It could thus be concluded that ZnO:Al was stable in water as well as CO
2
individually, but degraded quickly in these species combined. Furthermore, O expo-
2
sure does not lead to degradation of ZnO:Al. More information about these experi-
ments can be found in chapter 6.
It was proposed that migration of species during degradation of ZnO:Al on borosili-
cate glass occurs in two directions [71]. Water, CO and other atmospheric species can
2
migrate from the environment into the ZnO:Al, thereby increasing the resistivity. On
the other hand, elements from the glass, including calcium, silicon and aluminium
migrated to the ZnO:Al surface, where they react with carbon and oxygen to form
55
The changes in electrical properties are probably caused by changes in the grain
boundaries, where a small change in composition can have a large impact on the
conductivity. Furthermore, in some cases, the composition of the grains also had
changed. At present, the chemical mechanisms leading to these composition chang-
es are not completely clarified. In this chapter, we discuss which species can be in-
volved in the degradation. Suggestions about the molecules that are formed during
ZnO:Al degradation include Al(OH) [25,64], Zn(OH) [25,64,71,76], ZnOH-complexes
2
3
and hydrates [20], ZnCO [76] and Zn(CO ) (OH) and related species [71] which are
3 2
3
5
6
mostly non-conductive materials. The presence of a hydroxide was also shown in ref-
erence [29]. These materials can function as an electrical potential barrier in the grain
boundaries as shown in Figure 2.10. They might also be present in the grains, thereby
simply decreasing the conductivity of the films.
It is clear that these chemical reactions occur due to reactions between the zinc oxide
and molecules from the atmosphere. It was suggested in reference [75] that the pen-
etration of oxygen and water molecules into the bottom part of the film can strength -
en the effect of grain boundary scattering, while other references [72,72,76] suggest-
ed a role for water and CO .
2
Theelen et al. [85] studied the role of various atmospheric species, by exposing ZnO:Al
layers at room temperature to the atmospheric gases carbon dioxide (CO), oxygen
2
(O ), ogen/argon (N/Ar) and air as well as liquid water purged with these gases, in
2
2
order to investigate their influence on the stability behaviour of these layers. It was
shown that O and CO gases in the absence of water did not cause any degradation
2
2
at all during the tested period, while water purged with O or N only led to a small
2
2
increase in resistivity, likely due to the formation of a limited amount of Zn(OH).
2
However, when CO was also present alongside with water, the concentration of OH -
2
increased greatly in the bulk and even more at the air/ZnO:Al and the ZnO:Al/glass
interfaces. It could thus be concluded that ZnO:Al was stable in water as well as CO
2
individually, but degraded quickly in these species combined. Furthermore, O expo-
2
sure does not lead to degradation of ZnO:Al. More information about these experi-
ments can be found in chapter 6.
It was proposed that migration of species during degradation of ZnO:Al on borosili-
cate glass occurs in two directions [71]. Water, CO and other atmospheric species can
2
migrate from the environment into the ZnO:Al, thereby increasing the resistivity. On
the other hand, elements from the glass, including calcium, silicon and aluminium
migrated to the ZnO:Al surface, where they react with carbon and oxygen to form
55
