Page 61 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 61
Stability of Cu(In,Ga)Se 2 Solar Cells
2.3.4.1.7 Reversibility
As described in chapter 2.3.4.1.3, it is likely that moisture and temperature exposure
lead to changes in the grain boundaries. These changes can either be physical ad-
sorption and/or chemical reactions. The adsorption of species like water is a reversible
process under elevated temperatures, since the water incorporated during damp heat
treatment is only weakly bound and can therefore be easily removed [76].
Chemical reactions, on the other hand, can also be reversible. It is likely that the for-
o
mation of Zn(OH) is reversible at temperatures above 114C, which is its decompo-
2
sition temperature. At this temperature, this material can decompose into ZnO and
water. The same effect can likely occur for hydrozincite (chapter 2.3.4.1.3), which is
o
known to decompose at temperatures around 220-250 C [86]. It should also be noted
o
that complete CIGS cells are likely not stable at temperatures above 200C, so heat
treatment at high temperature cannot easily be used to reverse the ZnO:Al degrada-
tion on CIGS solar cell.
This reversibility effect was shown by various researchers for ZnO:Al on glass or sil-
icon substrates. Tohsophon et al. [70] showed that the resistivity increase could be
reversed by annealing in vacuum at temperatures of at least 150C. At this tempera-
0
ture, the effusion of water was reported. As a result of the annealing, the resistivi-
ty decreased and the carrier concentration increased to its initial values. At a higher
temperature (500 C), effusion of CO and H was also involved, indicating tat least two
o
2
2
different reversibility phenomena, like molecular adsorption and chemical reactions
occurred during ZnO:Al degradation.
Minami et al. [67,68] reported that resistivity increase of 50 – 300 nm ZnO:Al films in-
duced by damp heat exposure could be completely restored to the initial resistivity by
annealing in a reducing atmosphere (Ar with H ) at approximately 300 C.
0
2
These results showing reversible behaviour could not be imitated in an N atmosphere
2
by Ntinas et al. [87]. ZnO:Al films were deposited on both soda lime glass and boro-
silicate glass and degraded for 1032 hours in a damp heat test. This led to an increase
in resistivity. Afterwards, they were exposed for two to eight hours to temperatures
o
between 100 and 250C under a N atmosphere. This did not lead to a decrease of
2
the resistivity. It must be noted that heating to 250C led to a small improvement of
o
the sheet resistance of the film deposited on sodium rich substrate, but it was un-
clear whether this was significant. It might be possible that a low pressure or reducing
environment is required for annealing, allowing easy out-diffusion of environmental
species.
59
2.3.4.1.7 Reversibility
As described in chapter 2.3.4.1.3, it is likely that moisture and temperature exposure
lead to changes in the grain boundaries. These changes can either be physical ad-
sorption and/or chemical reactions. The adsorption of species like water is a reversible
process under elevated temperatures, since the water incorporated during damp heat
treatment is only weakly bound and can therefore be easily removed [76].
Chemical reactions, on the other hand, can also be reversible. It is likely that the for-
o
mation of Zn(OH) is reversible at temperatures above 114C, which is its decompo-
2
sition temperature. At this temperature, this material can decompose into ZnO and
water. The same effect can likely occur for hydrozincite (chapter 2.3.4.1.3), which is
o
known to decompose at temperatures around 220-250 C [86]. It should also be noted
o
that complete CIGS cells are likely not stable at temperatures above 200C, so heat
treatment at high temperature cannot easily be used to reverse the ZnO:Al degrada-
tion on CIGS solar cell.
This reversibility effect was shown by various researchers for ZnO:Al on glass or sil-
icon substrates. Tohsophon et al. [70] showed that the resistivity increase could be
reversed by annealing in vacuum at temperatures of at least 150C. At this tempera-
0
ture, the effusion of water was reported. As a result of the annealing, the resistivi-
ty decreased and the carrier concentration increased to its initial values. At a higher
temperature (500 C), effusion of CO and H was also involved, indicating tat least two
o
2
2
different reversibility phenomena, like molecular adsorption and chemical reactions
occurred during ZnO:Al degradation.
Minami et al. [67,68] reported that resistivity increase of 50 – 300 nm ZnO:Al films in-
duced by damp heat exposure could be completely restored to the initial resistivity by
annealing in a reducing atmosphere (Ar with H ) at approximately 300 C.
0
2
These results showing reversible behaviour could not be imitated in an N atmosphere
2
by Ntinas et al. [87]. ZnO:Al films were deposited on both soda lime glass and boro-
silicate glass and degraded for 1032 hours in a damp heat test. This led to an increase
in resistivity. Afterwards, they were exposed for two to eight hours to temperatures
o
between 100 and 250C under a N atmosphere. This did not lead to a decrease of
2
the resistivity. It must be noted that heating to 250C led to a small improvement of
o
the sheet resistance of the film deposited on sodium rich substrate, but it was un-
clear whether this was significant. It might be possible that a low pressure or reducing
environment is required for annealing, allowing easy out-diffusion of environmental
species.
59
