Page 237 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 237
The impact of alkali elements
indicate that diffusion of sodium through the whole CIGS absorber layer can easily
occur within 10 hours.
o
In order to predict how likely sodium diffusion is in the field, the values at 20C can
2
be considered. Extrapolation indicates a coefficient of 1.6x10 cm /s, which leads to
-12
expected diffusion lengths of 3 and 11 micrometres after 10 and 100 hours respectively.
Therefore, it can be expected that this also happens for CIGS PV in the field. It should
o
however be noted that extrapolation of an emperical function from 157 C to 20 C can
o
introduce a large error in the result of this calculation.
However, regardless of the limitations and assumptions in these calculations, it is very
likely that sodium migration over several micrometres via the grain boundaries can
occur both at elevated and room temperatures within several hours.
7.5 Conclusions
CIGS solar cells with a high alkali (sodium, potassium) content exhibited higher initial
conversion efficiencies, but degraded rapidly under exposure to both damp heat and
illumination. On the other hand, CIGS solar cells with a low alkali content did barely
degrade under combined damp heat and illumination.
The degradation of the alkali-rich samples especially impacted the shunt resistance
and open circuit voltage and resulted in the formation of sodium rich spots on the
top ZnO:Al surface of the samples. This is likely caused by light-induced Na migration
+
through the grain boundaries of the absorber to the depletion region, accumulation
+
of Na at the depletion region and subsequent transport through the depletion
region due to the lowering of the internal electric field. The migration resulted in
the formation of shunt paths, which reduced the shunt resistance and consequently
the open circuit voltage. Calculations showed that sodium can migrate the required
distance to cause these effects.
Additionally, ingression of water into the ZnO:Al is expected to be responsible for a
slow but steady increase in series resistance for both samples. The alkali-rich solar
cells show an additional increase in series resistance, probably due to the ingression
of sodium in the ZnO:Al.
7.6 Acknowledgements
I would like to thank Denis Mangin (Université de Lorraine) for the SIMS measurements
and Arjan Hovestad (TNO), Guy Brammertz (imec) and Felix Daume (Solarion) for the
fruitful discussions.
235
indicate that diffusion of sodium through the whole CIGS absorber layer can easily
occur within 10 hours.
o
In order to predict how likely sodium diffusion is in the field, the values at 20C can
2
be considered. Extrapolation indicates a coefficient of 1.6x10 cm /s, which leads to
-12
expected diffusion lengths of 3 and 11 micrometres after 10 and 100 hours respectively.
Therefore, it can be expected that this also happens for CIGS PV in the field. It should
o
however be noted that extrapolation of an emperical function from 157 C to 20 C can
o
introduce a large error in the result of this calculation.
However, regardless of the limitations and assumptions in these calculations, it is very
likely that sodium migration over several micrometres via the grain boundaries can
occur both at elevated and room temperatures within several hours.
7.5 Conclusions
CIGS solar cells with a high alkali (sodium, potassium) content exhibited higher initial
conversion efficiencies, but degraded rapidly under exposure to both damp heat and
illumination. On the other hand, CIGS solar cells with a low alkali content did barely
degrade under combined damp heat and illumination.
The degradation of the alkali-rich samples especially impacted the shunt resistance
and open circuit voltage and resulted in the formation of sodium rich spots on the
top ZnO:Al surface of the samples. This is likely caused by light-induced Na migration
+
through the grain boundaries of the absorber to the depletion region, accumulation
+
of Na at the depletion region and subsequent transport through the depletion
region due to the lowering of the internal electric field. The migration resulted in
the formation of shunt paths, which reduced the shunt resistance and consequently
the open circuit voltage. Calculations showed that sodium can migrate the required
distance to cause these effects.
Additionally, ingression of water into the ZnO:Al is expected to be responsible for a
slow but steady increase in series resistance for both samples. The alkali-rich solar
cells show an additional increase in series resistance, probably due to the ingression
of sodium in the ZnO:Al.
7.6 Acknowledgements
I would like to thank Denis Mangin (Université de Lorraine) for the SIMS measurements
and Arjan Hovestad (TNO), Guy Brammertz (imec) and Felix Daume (Solarion) for the
fruitful discussions.
235
