Page 75 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Stability of Cu(In,Ga)Se 2 Solar Cells
Similar solar cells were also exposed to damp heat conditions combined with illumi-
nation in the setup described in reference [23]. By the in-situ studying of the evolution
of the V and efficiency a two stage aging process was found consistently for the
oc
samples with low sodium. The IV parameters of both types of solar cells increased
during the first 50 hours, after which they showed a rapid decrease. Similar to damp
heat exposure, the improvement phase was not visible for the samples with very high
sodium contents. It was therefore concluded that the improving mechanism was neg -
ligible compared to the degradation mechanism when the sodium content is very
high [50,94].
The nature of the improving mechanism could not be revealed exactly. However, Dau -
me et al. speculated that the diffusion of copper ions within the CIGS may be relevant
[101]. Under the influence of the internal electric field, a slow tendency towards a cop -
per depletion of the CIGS surface was proposed. Since this is essentially a widening of
the ordered vacancy compound (OVC) phase, this can lead to an increase of the band
gap near the CIGS/CdS interface. It was confirmed by SCAPS simulations that this would
lead to an increase in open circuit voltage which was actually observed experimentally.
Considering the degradation mechanism, Daume et al. argued that it was mainly driv -
en by the corrosion of the back contact. Since the decrease in efficiency was mainly
caused by a decrease of the fill factor, which was caused by an increase in series resis-
tance, while V and J remained fairly constant, the pn-junction was assumed to be
oc
sc
stable compared to the planar contacts [50]. SIMS measurements of the sodium depth
distribution before and after damp heat revealed a decrease of the sodium contents
at the Mo/CIGS interface [94]. At the same time, samples with higher sodium content
exhibited a stronger degradation. While other degradation processes (such as TCO
degradation and the degradation at various interfaces) could not be ruled out, it was
thus concluded that corrosion at back contact (interface) was the main contribution to
the overall cell degradation of these solar cells on polyimide substrates.
2.4.5 Degradation of CIGS modules with encapsulation and barriers
Since the ingression of water and other atmospheric species is often a reason for CIGS
solar cell degradation, barriers are applied. The most prominent example is glass,
which is often used as front and back sheet and serves as a perfect water barrier when
combined with a good edge seal. However, for flexible modules, other flexible front-
and back sheets are required, which need to have competitive pricing combined with
a good barrier function. Additionally to the front- and back sheets, also encapsulants
like EVA and PVB play a role in the degradation of CIGS modules. In this chapter, we
describe the impact of all these package materials, like the front- and back sheets and
73
Similar solar cells were also exposed to damp heat conditions combined with illumi-
nation in the setup described in reference [23]. By the in-situ studying of the evolution
of the V and efficiency a two stage aging process was found consistently for the
oc
samples with low sodium. The IV parameters of both types of solar cells increased
during the first 50 hours, after which they showed a rapid decrease. Similar to damp
heat exposure, the improvement phase was not visible for the samples with very high
sodium contents. It was therefore concluded that the improving mechanism was neg -
ligible compared to the degradation mechanism when the sodium content is very
high [50,94].
The nature of the improving mechanism could not be revealed exactly. However, Dau -
me et al. speculated that the diffusion of copper ions within the CIGS may be relevant
[101]. Under the influence of the internal electric field, a slow tendency towards a cop -
per depletion of the CIGS surface was proposed. Since this is essentially a widening of
the ordered vacancy compound (OVC) phase, this can lead to an increase of the band
gap near the CIGS/CdS interface. It was confirmed by SCAPS simulations that this would
lead to an increase in open circuit voltage which was actually observed experimentally.
Considering the degradation mechanism, Daume et al. argued that it was mainly driv -
en by the corrosion of the back contact. Since the decrease in efficiency was mainly
caused by a decrease of the fill factor, which was caused by an increase in series resis-
tance, while V and J remained fairly constant, the pn-junction was assumed to be
oc
sc
stable compared to the planar contacts [50]. SIMS measurements of the sodium depth
distribution before and after damp heat revealed a decrease of the sodium contents
at the Mo/CIGS interface [94]. At the same time, samples with higher sodium content
exhibited a stronger degradation. While other degradation processes (such as TCO
degradation and the degradation at various interfaces) could not be ruled out, it was
thus concluded that corrosion at back contact (interface) was the main contribution to
the overall cell degradation of these solar cells on polyimide substrates.
2.4.5 Degradation of CIGS modules with encapsulation and barriers
Since the ingression of water and other atmospheric species is often a reason for CIGS
solar cell degradation, barriers are applied. The most prominent example is glass,
which is often used as front and back sheet and serves as a perfect water barrier when
combined with a good edge seal. However, for flexible modules, other flexible front-
and back sheets are required, which need to have competitive pricing combined with
a good barrier function. Additionally to the front- and back sheets, also encapsulants
like EVA and PVB play a role in the degradation of CIGS modules. In this chapter, we
describe the impact of all these package materials, like the front- and back sheets and
73
