Page 195 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 195
Degradation mechanisms of the aluminium doped zinc oxide front contact
Looking at the bottom 300 nm of the layer, the hydroxide and hydrogen concentrations
are lower than in the top layer, indicating that the diffusion and thus the degradation
can still develop further. In addition, there is a large amount of chlorine in the films after
degradation, which decreases an order of magnitude for 100-200 nm. In the region near
the surface, the concentration is probably comparable to the hydroxide concentration,
assuming that the ionisation probabilities of hydroxide and chlorine in ZnO are similar.
Degradation thus induces relatively high concentration of chlorine in the top layers of
the samples. Similarly, there is a slightly increased sulphur signal in the top 200 nm after
degradation, which is visible in the cross-section mappings as a very thin surface layer.
2-
The sulphur is probably present as sulphite or sulphate (SO 3 2- or SO ). It should be noted
4
that these SIMS measurements do not provide absolute concentration values. Since
chlorine and sulphur were barely detected by EDX, the total amount of these species
in ZnO:Al is probably very low. The treatment thus only induces these species near the
surface of degraded ZnO:Al.
The carbon signal is only significantly raised by damp heat testing in the top 100
nm of the film. The carbon concentration is difficult to estimate, because the carbon
ionisation probability is usually much lower than that of hydroxide and chlorine,
but the concentrations are likely low in the bulk. The chemical state of the carbon
cannot be determined easily from a depth profile in an oxide film, but it might be (bi-)
carbonate.
The oxygen concentration did not change due to degradation and is completely
stable as a function of depth, so it is unlikely that adsorption of O is the main reason
2
for ZnO deterioration.
Looking at the spots on top of the samples, higher concentrations of sulphur, chloride,
carbon and hydroxide were found. On the other hand, the ZnO, ZnOH and AlO
2
concentrations were very low, indicating the absence of ZnO:Al. TOF-SIMS measurements
were also executed in the positive mode, in order to map the positive ions in the atomic
+
+
+
+
+
+
layers near the surface. The most striking cations were Zn , Al , Na , K , H , Si and Ca ,
+
which confirms the presence of silicon and calcium in the white spots.
Since the complete thickness of the ZnO:Al layer is used for lateral transport of the
electrons, it can be assumed that the impact of the surface elements on the electrical
properties is small. Therefore, chlorine and especially sulphur, carbon, calcium and
silicon, which are mainly present at the surface, will not be the main reasons for
decreased conductivity. Since ZnOH and OH are present in the complete layer, these
will have the largest influence.
193
Looking at the bottom 300 nm of the layer, the hydroxide and hydrogen concentrations
are lower than in the top layer, indicating that the diffusion and thus the degradation
can still develop further. In addition, there is a large amount of chlorine in the films after
degradation, which decreases an order of magnitude for 100-200 nm. In the region near
the surface, the concentration is probably comparable to the hydroxide concentration,
assuming that the ionisation probabilities of hydroxide and chlorine in ZnO are similar.
Degradation thus induces relatively high concentration of chlorine in the top layers of
the samples. Similarly, there is a slightly increased sulphur signal in the top 200 nm after
degradation, which is visible in the cross-section mappings as a very thin surface layer.
2-
The sulphur is probably present as sulphite or sulphate (SO 3 2- or SO ). It should be noted
4
that these SIMS measurements do not provide absolute concentration values. Since
chlorine and sulphur were barely detected by EDX, the total amount of these species
in ZnO:Al is probably very low. The treatment thus only induces these species near the
surface of degraded ZnO:Al.
The carbon signal is only significantly raised by damp heat testing in the top 100
nm of the film. The carbon concentration is difficult to estimate, because the carbon
ionisation probability is usually much lower than that of hydroxide and chlorine,
but the concentrations are likely low in the bulk. The chemical state of the carbon
cannot be determined easily from a depth profile in an oxide film, but it might be (bi-)
carbonate.
The oxygen concentration did not change due to degradation and is completely
stable as a function of depth, so it is unlikely that adsorption of O is the main reason
2
for ZnO deterioration.
Looking at the spots on top of the samples, higher concentrations of sulphur, chloride,
carbon and hydroxide were found. On the other hand, the ZnO, ZnOH and AlO
2
concentrations were very low, indicating the absence of ZnO:Al. TOF-SIMS measurements
were also executed in the positive mode, in order to map the positive ions in the atomic
+
+
+
+
+
+
layers near the surface. The most striking cations were Zn , Al , Na , K , H , Si and Ca ,
+
which confirms the presence of silicon and calcium in the white spots.
Since the complete thickness of the ZnO:Al layer is used for lateral transport of the
electrons, it can be assumed that the impact of the surface elements on the electrical
properties is small. Therefore, chlorine and especially sulphur, carbon, calcium and
silicon, which are mainly present at the surface, will not be the main reasons for
decreased conductivity. Since ZnOH and OH are present in the complete layer, these
will have the largest influence.
193
