Page 72 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 72
Chapter 2
heat exposure. It was proposed that the fill factor degradation was caused by the for-
mation of Zn(OH) in the Zn(O,S,OH) buffer, which can dehydrate into ZnO under light
x
2
soaking. It was additionally proposed that electrons could compensate the increased
number of shallow defects, which occurs in the module when it is in open-circuit condi -
tions. In general, it was concluded that the degradation of PV modules was remarkably
reduced if the module is tested in open-circuit conditions under weak illumination [10].
Thompson et al. [22] also exposed CIGS cells to combined damp heat and illumina-
tion. The results of this study are described in chapter 2.3.4.3.
Theelen et al. [23] studied the impact of sodium and potassium on CIGS solar cell
stability by in-situ measurements in a hybrid damp heat-illumination setup. CIGS
solar cells with low, medium and high sodium and potassium (alkali) concentrations
showed large differences in initial V and efficiency values, but also in degradation
oc
rates. The initial values were the highest for the samples with the highest alkali con-
tents. The solar cells with the high alkali content degraded very quickly, especially
in R , V and fill factor and demonstrated the appearance of many small spots. The
oc
sh
medium alkali sample also demonstrated rapidly loss of efficiency and the occurrence
of spots. The low alkali samples were more or less stable and barely any spots could
be found. Therefore, it was proposed that the rapid loss of efficiency was dominantly
driven by the presence of a high alkali content. The effects of sodium and potassium
could not yet be distinguished.
A possible explanation for this phenomenon was introduced by Theelen et al. [96].
It was proposed that in the presence of the illumination and water, especially sodi-
um migrates from the CIGS/Mo interface and the CIGS bulk to the pn-junction. In
this region, the positive sodium ions cluster together, forming sodium rich spots. On
these positions, the local electric field is reduced, leading to the formation of shunting
paths. This leads to the decrease of the V and the fill factor, and the appearance of
oc
sodium in the ZnO:Al layers.
Fjällström et al. [97] studied the impact of sodium oxide (NaO) and potassium ox-
2
ide (K O) in the glass on the presence of Potential Induced Degradation (PID) in CIGS
2
solar cells. Experiments were conducted on CIGS solar cells deposited on glass sub-
strates with different sodium and potassium contents. All samples, regardless of the
substrate composition, contained CIGS absorbers with additionally evapourated NaF,
except for one soda lime glass sample. These solar cells were exposed to the following
conditions for up to 50 hours:
• Heat (85 C) and voltage (50V) (´PID sample´),
o
• Heat (85 C) (´heat sample´)
o
• Air at room temperature (´reference´)
70
heat exposure. It was proposed that the fill factor degradation was caused by the for-
mation of Zn(OH) in the Zn(O,S,OH) buffer, which can dehydrate into ZnO under light
x
2
soaking. It was additionally proposed that electrons could compensate the increased
number of shallow defects, which occurs in the module when it is in open-circuit condi -
tions. In general, it was concluded that the degradation of PV modules was remarkably
reduced if the module is tested in open-circuit conditions under weak illumination [10].
Thompson et al. [22] also exposed CIGS cells to combined damp heat and illumina-
tion. The results of this study are described in chapter 2.3.4.3.
Theelen et al. [23] studied the impact of sodium and potassium on CIGS solar cell
stability by in-situ measurements in a hybrid damp heat-illumination setup. CIGS
solar cells with low, medium and high sodium and potassium (alkali) concentrations
showed large differences in initial V and efficiency values, but also in degradation
oc
rates. The initial values were the highest for the samples with the highest alkali con-
tents. The solar cells with the high alkali content degraded very quickly, especially
in R , V and fill factor and demonstrated the appearance of many small spots. The
oc
sh
medium alkali sample also demonstrated rapidly loss of efficiency and the occurrence
of spots. The low alkali samples were more or less stable and barely any spots could
be found. Therefore, it was proposed that the rapid loss of efficiency was dominantly
driven by the presence of a high alkali content. The effects of sodium and potassium
could not yet be distinguished.
A possible explanation for this phenomenon was introduced by Theelen et al. [96].
It was proposed that in the presence of the illumination and water, especially sodi-
um migrates from the CIGS/Mo interface and the CIGS bulk to the pn-junction. In
this region, the positive sodium ions cluster together, forming sodium rich spots. On
these positions, the local electric field is reduced, leading to the formation of shunting
paths. This leads to the decrease of the V and the fill factor, and the appearance of
oc
sodium in the ZnO:Al layers.
Fjällström et al. [97] studied the impact of sodium oxide (NaO) and potassium ox-
2
ide (K O) in the glass on the presence of Potential Induced Degradation (PID) in CIGS
2
solar cells. Experiments were conducted on CIGS solar cells deposited on glass sub-
strates with different sodium and potassium contents. All samples, regardless of the
substrate composition, contained CIGS absorbers with additionally evapourated NaF,
except for one soda lime glass sample. These solar cells were exposed to the following
conditions for up to 50 hours:
• Heat (85 C) and voltage (50V) (´PID sample´),
o
• Heat (85 C) (´heat sample´)
o
• Air at room temperature (´reference´)
70
