Page 71 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
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Stability of Cu(In,Ga)Se 2 Solar Cells
back contact, thus increasing the overall series resistance of the solar cell as well as the
inhomogeneous appearance in electroluminescence imaging.
Radue et al. [11] looked at the changes caused by outdoor exposure of CIGS modules.
It was observed that the modules with large cell mismatch showed most severe effi-
ciency decrease, influencing the I and fill factor. Generally, the degradation mostly
sc
resulted in the increase in series resistance and the large decrease of shunt resistance.
An observed change in the V was probably caused by the latter effect.
oc
The effect of simple indoor storage of CIGS modules was described by Yanagisawa
et al. [95]. After six months in darkness, the efficiency of the sixteen modules had
increased by 7% on average (from 2.5% to 12%), largely due to an increase in short
circuit current. However, large differences between the modules were observed, es-
pecially due to changes in short circuit current and fill factors. The latter showed both
increase and decrease. Furthermore, it was also observed that modules with a low
initial series resistance were stable during indoor storage, while sample with a high
initial series resistance showed a rapid decrease in efficiency.
2.4.3 Degradation under illumination or electrical bias
Since PV modules in the field are exposed to light, various degradation tests have
used illumination for the testing of CIGS cells or modules. In various cases, the illu-
mination was also combined with elevated temperatures and humidity, to simulate
outdoor exposure [10,22]. An additional advantage of the presence of illumination
is the possibility to do in-situ measurements of the degradation, which was done by
e.g. Theelen et al. [23,96]. Since combined damp heat-illumination testing requires a
specially designed setup, another reference [97] used an electrical bias to simulate the
current flow in an operating solar cell. This also allows the possibility to speed up or
slow down certain degradation processes by the change of the direction and magni-
tude of the electrical bias. In this chapter, we describe the impact of both illumination
and electrical bias on CIGS cells and modules.
Kushiya et al. [10] reported already in 2006 on the impact of illumination. This refer-
ence reported that modules exposed to 1000 hours of dark damp heat conditions
(-25% reduction of the efficiency) recovered when exposed to illumination (+20% in-
crease of efficiency). Therefore, the impact of light exposure during degradation was
studied by the placement of CIGS modules in a damp heat chamber with 150 W/m
2
illumination in open-circuit and short-circuit conditions.
It was observed that modules in open circuit degraded less than modules in short-cir -
cuit conditions, which can mostly be observed if the fill factor is studied. The modules
under open-circuit conditions also recovered after two hours light soaking at 1000 W/
2
o
m and 25 C. The degradation in short-circuit condition was similar to standard damp
69
back contact, thus increasing the overall series resistance of the solar cell as well as the
inhomogeneous appearance in electroluminescence imaging.
Radue et al. [11] looked at the changes caused by outdoor exposure of CIGS modules.
It was observed that the modules with large cell mismatch showed most severe effi-
ciency decrease, influencing the I and fill factor. Generally, the degradation mostly
sc
resulted in the increase in series resistance and the large decrease of shunt resistance.
An observed change in the V was probably caused by the latter effect.
oc
The effect of simple indoor storage of CIGS modules was described by Yanagisawa
et al. [95]. After six months in darkness, the efficiency of the sixteen modules had
increased by 7% on average (from 2.5% to 12%), largely due to an increase in short
circuit current. However, large differences between the modules were observed, es-
pecially due to changes in short circuit current and fill factors. The latter showed both
increase and decrease. Furthermore, it was also observed that modules with a low
initial series resistance were stable during indoor storage, while sample with a high
initial series resistance showed a rapid decrease in efficiency.
2.4.3 Degradation under illumination or electrical bias
Since PV modules in the field are exposed to light, various degradation tests have
used illumination for the testing of CIGS cells or modules. In various cases, the illu-
mination was also combined with elevated temperatures and humidity, to simulate
outdoor exposure [10,22]. An additional advantage of the presence of illumination
is the possibility to do in-situ measurements of the degradation, which was done by
e.g. Theelen et al. [23,96]. Since combined damp heat-illumination testing requires a
specially designed setup, another reference [97] used an electrical bias to simulate the
current flow in an operating solar cell. This also allows the possibility to speed up or
slow down certain degradation processes by the change of the direction and magni-
tude of the electrical bias. In this chapter, we describe the impact of both illumination
and electrical bias on CIGS cells and modules.
Kushiya et al. [10] reported already in 2006 on the impact of illumination. This refer-
ence reported that modules exposed to 1000 hours of dark damp heat conditions
(-25% reduction of the efficiency) recovered when exposed to illumination (+20% in-
crease of efficiency). Therefore, the impact of light exposure during degradation was
studied by the placement of CIGS modules in a damp heat chamber with 150 W/m
2
illumination in open-circuit and short-circuit conditions.
It was observed that modules in open circuit degraded less than modules in short-cir -
cuit conditions, which can mostly be observed if the fill factor is studied. The modules
under open-circuit conditions also recovered after two hours light soaking at 1000 W/
2
o
m and 25 C. The degradation in short-circuit condition was similar to standard damp
69
