Page 216 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 216
Chapter 7
Abstract
Cu(In,Ga)Se based thin film solar cells require incorporation of a small amount of especially
2
sodium and potassium into the absorber layer to obtain high efficiencies. However,
the influence of these elements on CIGS solar cell stability is not yet known. Therefore,
unencapsulated CIGS solar cells with high and low contents of sodium (Na) and potassium
(K) were exposed to damp heat and illumination. The solar cells with a high alkali (Na, K)
content exhibited higher initial conversion efficiencies, but degraded severely within 100
hours, while the alkali-poor samples kept relatively stable performance under damp heat
and illumination.
The degradation of the samples with a high alkali content 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 via the grain boundaries in 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 caused both by the Na
accumulation and illumination. The migration resulted in the formation of shunt paths,
which reduced the shunt resistance and open circuit voltage and can even lead to the
formation of cracks in the solar cell. Furthermore, ingression of water into the ZnO:Al is
expected to be responsible for a slow but steady increase in series resistance for both high
and low alkali solar cells. Additionally, sodium migration led to a severe increase of the
series resistance in case of alkali-rich samples.
214
Abstract
Cu(In,Ga)Se based thin film solar cells require incorporation of a small amount of especially
2
sodium and potassium into the absorber layer to obtain high efficiencies. However,
the influence of these elements on CIGS solar cell stability is not yet known. Therefore,
unencapsulated CIGS solar cells with high and low contents of sodium (Na) and potassium
(K) were exposed to damp heat and illumination. The solar cells with a high alkali (Na, K)
content exhibited higher initial conversion efficiencies, but degraded severely within 100
hours, while the alkali-poor samples kept relatively stable performance under damp heat
and illumination.
The degradation of the samples with a high alkali content 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 via the grain boundaries in 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 caused both by the Na
accumulation and illumination. The migration resulted in the formation of shunt paths,
which reduced the shunt resistance and open circuit voltage and can even lead to the
formation of cracks in the solar cell. Furthermore, ingression of water into the ZnO:Al is
expected to be responsible for a slow but steady increase in series resistance for both high
and low alkali solar cells. Additionally, sodium migration led to a severe increase of the
series resistance in case of alkali-rich samples.
214
