Page 193 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Degradation mechanisms of the aluminium doped zinc oxide front contact
through the grain boundaries.
This important role of grain boundaries in the degradation process was also concluded
by various references [6, 7, 10]. Reference [16] also concludes this, based on experimental
work where the decrease of electrical properties of optical measurements (intra grain) and
electrical measurements (inter grain) were compared. Calculations based on these experi -
ments allowing the barrier width and height, are explained further in reference [16].
As reported in references [2,13] and chapter 2.3.4.3, stronger degradation of ZnO:Al
electrical properties will occur when the films are grown on rough substrates, due to the
formation of ‘extended grain boundaries’. These are local perturbations of the ZnO:Al
morphology, which also allow fast diffusion of environmental species into the material.
Generally stated, it is proposed that the extended grain boundaries are the most likely
to be prone to degradation, followed by the grain boundaries. If both these elements
are not present or are stable, then the grain will probably become the next problem.
A high number of grain boundaries thus leads to a low probability for the electrons
to travel through the material. This makes zinc oxide with larger grains more stable.
The probability of electrons travelling through the grain boundaries naturally
decreases as the number of grain increases, so large grains reduce the grain boundary
o
scattering [18]. Since the 200 C grains are larger than RT grains, it is expected that an
increase in grain boundary scattering is larger for the RT sample. Therefore, the larger
mobility decrease can be explained by a difference in grain size between the RT and
200 C sample, which is shown in Table 6.2 .
o
Additionally, the difference in grain orientation could also have played a role. It is
possible that the grain boundary surface of the small grains that do not have the (002)
and (004) orientations are more sensitive for degrading species.
6.3.1.5 Composition
The composition as a function of the depth was measured by SIMS through the layer
before and after degradation. The depth profiles taken in the negative mode of the
RT sample are shown as graphs (Figure 6.11) and as cross-sections maps (Figure 6.12).
These values show the difference in concentration of various important ions within
the layers as well as due to degradation. It should be noted that the concentrations
depend on the ionisation probability of the atoms and molecules, which varies per
species and per background material. Therefore, no absolute concentrations are
obtained in this study.
It is immediately clear that the damp heat test introduces hydroxides, chlorine, sulphur
and carbon concentrations in the film. The hydroxyl profile decays by about an order
191
through the grain boundaries.
This important role of grain boundaries in the degradation process was also concluded
by various references [6, 7, 10]. Reference [16] also concludes this, based on experimental
work where the decrease of electrical properties of optical measurements (intra grain) and
electrical measurements (inter grain) were compared. Calculations based on these experi -
ments allowing the barrier width and height, are explained further in reference [16].
As reported in references [2,13] and chapter 2.3.4.3, stronger degradation of ZnO:Al
electrical properties will occur when the films are grown on rough substrates, due to the
formation of ‘extended grain boundaries’. These are local perturbations of the ZnO:Al
morphology, which also allow fast diffusion of environmental species into the material.
Generally stated, it is proposed that the extended grain boundaries are the most likely
to be prone to degradation, followed by the grain boundaries. If both these elements
are not present or are stable, then the grain will probably become the next problem.
A high number of grain boundaries thus leads to a low probability for the electrons
to travel through the material. This makes zinc oxide with larger grains more stable.
The probability of electrons travelling through the grain boundaries naturally
decreases as the number of grain increases, so large grains reduce the grain boundary
o
scattering [18]. Since the 200 C grains are larger than RT grains, it is expected that an
increase in grain boundary scattering is larger for the RT sample. Therefore, the larger
mobility decrease can be explained by a difference in grain size between the RT and
200 C sample, which is shown in Table 6.2 .
o
Additionally, the difference in grain orientation could also have played a role. It is
possible that the grain boundary surface of the small grains that do not have the (002)
and (004) orientations are more sensitive for degrading species.
6.3.1.5 Composition
The composition as a function of the depth was measured by SIMS through the layer
before and after degradation. The depth profiles taken in the negative mode of the
RT sample are shown as graphs (Figure 6.11) and as cross-sections maps (Figure 6.12).
These values show the difference in concentration of various important ions within
the layers as well as due to degradation. It should be noted that the concentrations
depend on the ionisation probability of the atoms and molecules, which varies per
species and per background material. Therefore, no absolute concentrations are
obtained in this study.
It is immediately clear that the damp heat test introduces hydroxides, chlorine, sulphur
and carbon concentrations in the film. The hydroxyl profile decays by about an order
191
