Page 70 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 70
Chapter 2
The formation of particles surrounded by a circular pattern was also observed by Britt
et al. [45]. They exposed Mo/CIGS/CdS thin films deposited on stainless steel substrates
without TCO to a damp heat test. Already after fifty hours, particles at the surface of
the sample surrounded by discoloured rings were observed. They could grow up to
15 mm in diameter and the core of the defect contained high levels of carbon, sodium
and oxygen, while high levels of sodium were found through the whole sample. This
kind of defects might lead to the formation of pin-holes and shunting.
Malmström et al. [92] demonstrated that the composition of the CIGS absorber also
influenced the stability in damp heat testing. They investigated the influence of dif-
ferent gallium contents in the absorber of their Cu(In Ga )Se solar cells. When these
x
1-x
solar cells were exposed to a damp heat test for 800 hours,the lowest efficiency drop
(efficiency -27%) was for an absorber with x = 0.4. Solar cells based on CuInSe (x=0)
2
and CuGaSe (x=1) showed relative efficiency decreases of 92% and 64% relatively.
2
Daume et al. reported an influence of the sodium content of absorbers on the damp
heat stability of CIGS solar cells on flexible polyimide substrates. They reported that
solar cells with initially higher sodium contents degraded faster than those with lower
sodium contents [50]. It was found that the sodium dependent decrease in efficiency
was mostly driven by a decreasing fill factor which was mainly caused by an increasing
series resistance.
The authors showed that the degradation of CIGS solar cells is not a homogeneous
process in the lateral direction [50]. CIGS solar cells on polyimide foil without encapsu -
lation were exposed several times to damp heat tests and their electroluminescence
(EL) images were taken after each step. Darker areas evolved in some parts of the cell
after 15 hours of damp heat testing, which indicated a more severe degradation than
in other parts of the cell. The spatial inhomogeneity in the EL intensity was explained
by a laterally inhomogeneous corrosion of the back contact (interface) that limits the
current injection to the pn-junction in certain areas of the cell. While these areas ap-
pear as dark spots in EL testing, according to the reciprocity theorem [93], the exact
same areas will contribute less to the power output of the solar cell under normal op-
erating conditions. A new interpretation of the capacitance-voltage-measurements,
essentially assuming a reduced `effective contact area´, supported this argument [50].
Before damp heat exposure, SIMS measurements showed the aggregation of sodium
at two positions: in the region of the pn-junction and at the CIGS/Mo interface [94]
(chapter 2.4.3). The latter aggregation nearly disappeared after damp heat treatment.
In the context of the sodium dependent degradation it was concluded that a main
corrosion mechanism is present at the CIGS/Mo interface [50]. This spatial inhomoge-
neous corrosion leads to an increase in the contact resistance between absorber and
68
The formation of particles surrounded by a circular pattern was also observed by Britt
et al. [45]. They exposed Mo/CIGS/CdS thin films deposited on stainless steel substrates
without TCO to a damp heat test. Already after fifty hours, particles at the surface of
the sample surrounded by discoloured rings were observed. They could grow up to
15 mm in diameter and the core of the defect contained high levels of carbon, sodium
and oxygen, while high levels of sodium were found through the whole sample. This
kind of defects might lead to the formation of pin-holes and shunting.
Malmström et al. [92] demonstrated that the composition of the CIGS absorber also
influenced the stability in damp heat testing. They investigated the influence of dif-
ferent gallium contents in the absorber of their Cu(In Ga )Se solar cells. When these
x
1-x
solar cells were exposed to a damp heat test for 800 hours,the lowest efficiency drop
(efficiency -27%) was for an absorber with x = 0.4. Solar cells based on CuInSe (x=0)
2
and CuGaSe (x=1) showed relative efficiency decreases of 92% and 64% relatively.
2
Daume et al. reported an influence of the sodium content of absorbers on the damp
heat stability of CIGS solar cells on flexible polyimide substrates. They reported that
solar cells with initially higher sodium contents degraded faster than those with lower
sodium contents [50]. It was found that the sodium dependent decrease in efficiency
was mostly driven by a decreasing fill factor which was mainly caused by an increasing
series resistance.
The authors showed that the degradation of CIGS solar cells is not a homogeneous
process in the lateral direction [50]. CIGS solar cells on polyimide foil without encapsu -
lation were exposed several times to damp heat tests and their electroluminescence
(EL) images were taken after each step. Darker areas evolved in some parts of the cell
after 15 hours of damp heat testing, which indicated a more severe degradation than
in other parts of the cell. The spatial inhomogeneity in the EL intensity was explained
by a laterally inhomogeneous corrosion of the back contact (interface) that limits the
current injection to the pn-junction in certain areas of the cell. While these areas ap-
pear as dark spots in EL testing, according to the reciprocity theorem [93], the exact
same areas will contribute less to the power output of the solar cell under normal op-
erating conditions. A new interpretation of the capacitance-voltage-measurements,
essentially assuming a reduced `effective contact area´, supported this argument [50].
Before damp heat exposure, SIMS measurements showed the aggregation of sodium
at two positions: in the region of the pn-junction and at the CIGS/Mo interface [94]
(chapter 2.4.3). The latter aggregation nearly disappeared after damp heat treatment.
In the context of the sodium dependent degradation it was concluded that a main
corrosion mechanism is present at the CIGS/Mo interface [50]. This spatial inhomoge-
neous corrosion leads to an increase in the contact resistance between absorber and
68
