Page 235 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells

The impact of alkali elements

① ZnO:Al ② ZnO:Al

i-ZnO i-ZnO
CdS Depletion CdS
Depletion region with E pn
+
E pn
region Θ Θ Θ Θ Θ Θ Θ varying Θ Θ Θ Θ Θ Θ Θ
thickness
CIGS CIGS

Mo Mo

③ ZnO:Al

i-ZnO
Depletion CdS Shunt path
region with Cation
varying Θ Θ Θ Θ Θ Θ Θ Anion
E pn
thickness Electron
CIGS Hole
Na +
Electric field
Mo Migration

Figure 7.17
Proposed degradation mechanisms for CIGS solar cells containing sodium due to exposure to damp heat and illumination
exposure.

top layers, which can explain the occurrence of Na in the ZnO:Al. These shunt paths
+
can function as a tunnel, through which additional Na still present in the CIGS layer,
+
can be transported through the depletion region of the pn junction (Figure 7.17, step
3). This migration into the top solar cell layers can also explain the faster increase in
series resistance for the alkali-rich samples.
The occurrence of shunt paths has also been confirmed by SEM and EDX measurements,
which show that the degradation spots consist of sodium, carbon and oxygen, but
have nuclei which also contain for example cadmium. Cross-section SEM also revealed
that after 778 hours, cracks occurred in the layers. We therefore propose that exposure
to damp heat and illumination as long as 778 hours even led to structural changes in
the regions of the spots. Since the cracks were not detected for solar cells that were
degraded for a shorter time, we suggest that the crack formation resulted from severe
ion migration.

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