Page 192 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 192
Chapter 6
showed variation as a function of exposure time, but did not show a clear trend. The
initial and final values of the carrier concentrations only show a very small decrease,
which is within the error margin.
From these data, it can be concluded that the change in carrier concentration cannot
be a major source for resistivity increase, which indicates that the bulk of the zinc
oxide grains has a constant composition. This does not completely agree with the
small changes in carrier concentrations as reported in literature [4,15], but it should be
noted that these films were thinner (100-240 nm) than our films.
The Hall mobility decreased the first 1000 hours, while afterwards, the decline slowly
continues. This decrease cannot be explained by the small white dots as described
above, since these mainly occur at the top of the ZnO:Al layer, which will not block
lateral electron movement. Furthermore, the size of the spots is a factor 1000 larger
than the grain size, so any influence on the electrical properties would also affect the
carrier concentration. Therefore the reason for the mobility decrease should be found
within the zinc oxide layer itself. Figure 6.10 shows the dominant electron scattering
mechanisms in zinc oxide limiting the mobility: potential barriers at the grain boundaries
and ionised impurity scattering in the grain. In the case of the Hall effect measurements,
electrons travel macroscopic lengths and cross several grain boundaries, so the grain
boundary density can be a limiting factor on the value of Hall mobility [16].
Electron
carrier transport
Grain boundary
Electron scattering
Impurity
within grain
Figure 6.10
Possible scattering mechanisms in zinc oxide samples based on reference [13].
Based on the stable carrier concentration and the decreasing mobility, it can be
expected that the formation of detrimental compounds in zinc oxide occurs at the
grain boundaries and not in the bulk material.
This important role for the grain boundaries in the electrical properties of zinc oxide is
also confirmed in reference [17]. It was reported that neon cannot effuse in or out of the
bulk material of sputtered zinc oxides for temperatures as high as 1000C. However,
o
transport through the grain boundaries was possible. Therefore, the transport of
molecules like large molecules like oxygen, carbon dioxide and water is only possible
190
showed variation as a function of exposure time, but did not show a clear trend. The
initial and final values of the carrier concentrations only show a very small decrease,
which is within the error margin.
From these data, it can be concluded that the change in carrier concentration cannot
be a major source for resistivity increase, which indicates that the bulk of the zinc
oxide grains has a constant composition. This does not completely agree with the
small changes in carrier concentrations as reported in literature [4,15], but it should be
noted that these films were thinner (100-240 nm) than our films.
The Hall mobility decreased the first 1000 hours, while afterwards, the decline slowly
continues. This decrease cannot be explained by the small white dots as described
above, since these mainly occur at the top of the ZnO:Al layer, which will not block
lateral electron movement. Furthermore, the size of the spots is a factor 1000 larger
than the grain size, so any influence on the electrical properties would also affect the
carrier concentration. Therefore the reason for the mobility decrease should be found
within the zinc oxide layer itself. Figure 6.10 shows the dominant electron scattering
mechanisms in zinc oxide limiting the mobility: potential barriers at the grain boundaries
and ionised impurity scattering in the grain. In the case of the Hall effect measurements,
electrons travel macroscopic lengths and cross several grain boundaries, so the grain
boundary density can be a limiting factor on the value of Hall mobility [16].
Electron
carrier transport
Grain boundary
Electron scattering
Impurity
within grain
Figure 6.10
Possible scattering mechanisms in zinc oxide samples based on reference [13].
Based on the stable carrier concentration and the decreasing mobility, it can be
expected that the formation of detrimental compounds in zinc oxide occurs at the
grain boundaries and not in the bulk material.
This important role for the grain boundaries in the electrical properties of zinc oxide is
also confirmed in reference [17]. It was reported that neon cannot effuse in or out of the
bulk material of sputtered zinc oxides for temperatures as high as 1000C. However,
o
transport through the grain boundaries was possible. Therefore, the transport of
molecules like large molecules like oxygen, carbon dioxide and water is only possible
190
