Page 78 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 78
Chapter 2
tion caused by the impurity level at the grain boundaries, the surface and the bulk of
the CIGS, and the shift of the Fermi level in the CIGS/CdS layer interface. The fill factor
loss was attributed to both increased resistance of the ZnO:Al and the increase of the
electronic barrier caused by a shift of the diffusion potential in the junction.
The impact of the encapsulants EVA and silicone were studied by Kempe et al. [108].
They studied damp and dry heat exposure of Shell Solar mini-modules with EVA or
the silicone GE RTV615, while glass or TEFZEL® was used as front sheet. After 457 hours
(TEFZEL®) and 935 hours (glass) of damp heat, the mini-modules demonstrated a large
increase in series resistance and a small decrease in V , which indicates that this deg-
oc
radation was driven by moisture ingress. As explained in chapter 2.3.4, this is mainly
o
caused by changes in zinc oxide conductivity. The samples were also exposed to 85 C
and 0% RH conditions (´dry heat´) for a long period (up to 8770 hours). After this time,
it was observed that exposure to damp heat conditions led to a faster increase in se-
ries resistance and decrease in fill factor for EVA than for silicone as encapsulant .
Hegedus et al. [109] reported that the use of only a single 55 nm ALD layer of Al O de -
3
2
posited directly on a gridded Cu(In,Ga)Se device provides excellent protection during
2
the standard 1000 hours damp heat exposure, while a sample only protected by a PET
foil lost 50% of its initial efficiency.
2.4.6 Summary on device degradation
Exposure of complete CIGS solar cells and mini-modules to liquid water as well as
humidity at elevated temperatures generally leads to a decrease in efficiency. This
was mostly caused by a decrease of the fill factor and the open circuit voltage, while
the short circuit current degraded less. Dry heat exposure did lead to only a small
or no reduction of the efficiency, but could influence the electrical behaviour of the
samples, e.g. visible by the occurrence of roll-overs in the IV curves. Degradation was
often accompanied by the appearance of circular structures, like spots or particles,
which can be rich in sodium and oxygen. On the other hand, long term exposure to
the atmosphere, like during storage in the dark, could have a positive impact on the
efficiency.
When humidity and elevated temperatures were combined with a bias, either by an
electrical bias or illumination, additional degradation phenomena occurred. Under
these conditions, migration of sodium (either from the glass or the CIGS layer) to the
CdS region was observed which led to a severe efficiency decrease.
Experiments with CIGS cells with varying gallium content in the absorber showed that
for a medium content of gallium, the degradation of the was the lowest, while the cell
degraded more intensely for pure CIS or CGS absorbers.
76
tion caused by the impurity level at the grain boundaries, the surface and the bulk of
the CIGS, and the shift of the Fermi level in the CIGS/CdS layer interface. The fill factor
loss was attributed to both increased resistance of the ZnO:Al and the increase of the
electronic barrier caused by a shift of the diffusion potential in the junction.
The impact of the encapsulants EVA and silicone were studied by Kempe et al. [108].
They studied damp and dry heat exposure of Shell Solar mini-modules with EVA or
the silicone GE RTV615, while glass or TEFZEL® was used as front sheet. After 457 hours
(TEFZEL®) and 935 hours (glass) of damp heat, the mini-modules demonstrated a large
increase in series resistance and a small decrease in V , which indicates that this deg-
oc
radation was driven by moisture ingress. As explained in chapter 2.3.4, this is mainly
o
caused by changes in zinc oxide conductivity. The samples were also exposed to 85 C
and 0% RH conditions (´dry heat´) for a long period (up to 8770 hours). After this time,
it was observed that exposure to damp heat conditions led to a faster increase in se-
ries resistance and decrease in fill factor for EVA than for silicone as encapsulant .
Hegedus et al. [109] reported that the use of only a single 55 nm ALD layer of Al O de -
3
2
posited directly on a gridded Cu(In,Ga)Se device provides excellent protection during
2
the standard 1000 hours damp heat exposure, while a sample only protected by a PET
foil lost 50% of its initial efficiency.
2.4.6 Summary on device degradation
Exposure of complete CIGS solar cells and mini-modules to liquid water as well as
humidity at elevated temperatures generally leads to a decrease in efficiency. This
was mostly caused by a decrease of the fill factor and the open circuit voltage, while
the short circuit current degraded less. Dry heat exposure did lead to only a small
or no reduction of the efficiency, but could influence the electrical behaviour of the
samples, e.g. visible by the occurrence of roll-overs in the IV curves. Degradation was
often accompanied by the appearance of circular structures, like spots or particles,
which can be rich in sodium and oxygen. On the other hand, long term exposure to
the atmosphere, like during storage in the dark, could have a positive impact on the
efficiency.
When humidity and elevated temperatures were combined with a bias, either by an
electrical bias or illumination, additional degradation phenomena occurred. Under
these conditions, migration of sodium (either from the glass or the CIGS layer) to the
CdS region was observed which led to a severe efficiency decrease.
Experiments with CIGS cells with varying gallium content in the absorber showed that
for a medium content of gallium, the degradation of the was the lowest, while the cell
degraded more intensely for pure CIS or CGS absorbers.
76
