Page 265 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 265
Overview and recommendations
Changes in fill factor
o Series resistance (R )
s
Decrease of Mo conductivity (including scribe P3)
Decrease of TCO conductivity (including scribe P2)
Decrease of grid conductivity
Increase of contact resistances, e.g. due to delamination
o Shunt or parallel resistance (R shunt )
Introduction of alternative shunt paths (including scribe P1)
o Diode ideality factor
Changes in the quality of the pn-junction
Changes in inhomogeneity of the material
Recommendations
The most straightforward way to stop CIGS module degradation is the use of a good
water barrier. This can limit the increase in resistivity of the ZnO:Al and possibly the
molybdenum backcontact, thereby preventing the series resistance increase that is
often observed. Additionally, many phenomena related to migration of species into
the absorber layer, thereby affecting the pn-junction and thus the shunt resistance
and the V might be solved. However, alongside with the considerable costs of these
oc
barriers, this cannot solve all problems, as degradation was also observed for glass-
encapsulated modules exposed to field testing.
In order to prevent degradation issues, identification of the degradation modes
and mechanisms occurring during field testing are crucial. This information should
be compared with accelerated lifetime test results, in order to be able to accelerate
and study the correct phenomena during these tests. Additionally, lifetime studies
should focus more on better understanding of the degradation, instead of simple
go/no go judgments based on IEC tests. Possible laboratory routes to obtain better
understanding include, but are certainly not limited to:
· In-situ analysis of cell characteristics, like electrical or optical parameters,
during the degradation.
· The use of combined loads for CIGS solar cell degradation, for example
combinations of external bias (positive or negative) or different levels of
illumination combined with damp heat.
· The execution of single load experiments, like dry heat and room temperature
illumination, in order to separate the impact of various loads.
· The use of spatially resolved microscopy techniques (EL, PL, LIT), combined
with material analysis in order to identify the local degradation mechanism
due to exposure.
263
Changes in fill factor
o Series resistance (R )
s
Decrease of Mo conductivity (including scribe P3)
Decrease of TCO conductivity (including scribe P2)
Decrease of grid conductivity
Increase of contact resistances, e.g. due to delamination
o Shunt or parallel resistance (R shunt )
Introduction of alternative shunt paths (including scribe P1)
o Diode ideality factor
Changes in the quality of the pn-junction
Changes in inhomogeneity of the material
Recommendations
The most straightforward way to stop CIGS module degradation is the use of a good
water barrier. This can limit the increase in resistivity of the ZnO:Al and possibly the
molybdenum backcontact, thereby preventing the series resistance increase that is
often observed. Additionally, many phenomena related to migration of species into
the absorber layer, thereby affecting the pn-junction and thus the shunt resistance
and the V might be solved. However, alongside with the considerable costs of these
oc
barriers, this cannot solve all problems, as degradation was also observed for glass-
encapsulated modules exposed to field testing.
In order to prevent degradation issues, identification of the degradation modes
and mechanisms occurring during field testing are crucial. This information should
be compared with accelerated lifetime test results, in order to be able to accelerate
and study the correct phenomena during these tests. Additionally, lifetime studies
should focus more on better understanding of the degradation, instead of simple
go/no go judgments based on IEC tests. Possible laboratory routes to obtain better
understanding include, but are certainly not limited to:
· In-situ analysis of cell characteristics, like electrical or optical parameters,
during the degradation.
· The use of combined loads for CIGS solar cell degradation, for example
combinations of external bias (positive or negative) or different levels of
illumination combined with damp heat.
· The execution of single load experiments, like dry heat and room temperature
illumination, in order to separate the impact of various loads.
· The use of spatially resolved microscopy techniques (EL, PL, LIT), combined
with material analysis in order to identify the local degradation mechanism
due to exposure.
263
