Page 63 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 63
Stability of Cu(In,Ga)Se 2 Solar Cells
it was proposed they were formed due to a reaction with alkaline species.
Spots were also observed by Mei-Zhen et al. [89], who studied the degradation of
o
amorphous ITO deposited at RT and 75 C and observed the formation of two types of
spots. These circular patterns are likely formed by a reaction of the amorphous indium
with water, which leads to decomposition and the formation of crystalline In O , while
3
2
tin granules are formed at the same time. These effects can be caused by the hetero-
geneous distribution of tin and indium in the samples, which allows galvanic corro-
sion. This is an electrochemical process in which one metal corrodes preferentially to
another when both metals are in electrical contact, in the presence of an electrolyte
such as water.
2.3.4.3 Degradation of TCOs in CIGS solar cells
The impact of the TCO on the stability of a complete CIGS solar cell or module can be
found in four ways:
1. The impact of increasing resistivity and transmission due to the diffusion of
species. The effect of the resistivity increase can mainly be seen in a series
resistance increase and thus a fill factor decrease of a solar cell or module, while
transmission changes may impact the current of the solar cell or module.
2. Changes in TCO properties due to chemical reactions between TCO and, e.g.,
the encapsulation. The effects on a solar cell can be similar to the first point.
3. The changes in carrier concentration in both the TCO and CIGS layer, can lead
to changes in the Fermi level, which will induce a change in V .
oc
4. The possibility for atmospheric species to diffuse through the TCO, thereby
allowing the underlying CIGS/buffer layers to react with these atmospheric
species, leading to a change in the absorber or buffer properties. This can e.g.
result in changes of V .
oc
In this chapter, we will mainly focus on the impact of increasing resistivity (point 1),
while the other points are mostly discussed in chapter 2.4.
Lee et al. [56] investigated the impact of damp heat exposure on CIGS modules and
concluded that fill factor and efficiency losses of the modules were linked to the ap-
pearance of discoloured areas. The failure analysis of the ZnO:Al film indicated that
a change in resistivity and surface morphology occurred in these areas. This was at-
tributed to the formation of Zn(OH) due to water penetration. Non-encapsulated
2
solar cells were also exposed to a damp heat test: zinc hydroxide was found in small
amounts on their ZnO:Al surface, while encapsulated modules contained the hydrox-
ide inside the ZnO:Al as well as on the its surface. Furthermore, carboxylic acid was
61
it was proposed they were formed due to a reaction with alkaline species.
Spots were also observed by Mei-Zhen et al. [89], who studied the degradation of
o
amorphous ITO deposited at RT and 75 C and observed the formation of two types of
spots. These circular patterns are likely formed by a reaction of the amorphous indium
with water, which leads to decomposition and the formation of crystalline In O , while
3
2
tin granules are formed at the same time. These effects can be caused by the hetero-
geneous distribution of tin and indium in the samples, which allows galvanic corro-
sion. This is an electrochemical process in which one metal corrodes preferentially to
another when both metals are in electrical contact, in the presence of an electrolyte
such as water.
2.3.4.3 Degradation of TCOs in CIGS solar cells
The impact of the TCO on the stability of a complete CIGS solar cell or module can be
found in four ways:
1. The impact of increasing resistivity and transmission due to the diffusion of
species. The effect of the resistivity increase can mainly be seen in a series
resistance increase and thus a fill factor decrease of a solar cell or module, while
transmission changes may impact the current of the solar cell or module.
2. Changes in TCO properties due to chemical reactions between TCO and, e.g.,
the encapsulation. The effects on a solar cell can be similar to the first point.
3. The changes in carrier concentration in both the TCO and CIGS layer, can lead
to changes in the Fermi level, which will induce a change in V .
oc
4. The possibility for atmospheric species to diffuse through the TCO, thereby
allowing the underlying CIGS/buffer layers to react with these atmospheric
species, leading to a change in the absorber or buffer properties. This can e.g.
result in changes of V .
oc
In this chapter, we will mainly focus on the impact of increasing resistivity (point 1),
while the other points are mostly discussed in chapter 2.4.
Lee et al. [56] investigated the impact of damp heat exposure on CIGS modules and
concluded that fill factor and efficiency losses of the modules were linked to the ap-
pearance of discoloured areas. The failure analysis of the ZnO:Al film indicated that
a change in resistivity and surface morphology occurred in these areas. This was at-
tributed to the formation of Zn(OH) due to water penetration. Non-encapsulated
2
solar cells were also exposed to a damp heat test: zinc hydroxide was found in small
amounts on their ZnO:Al surface, while encapsulated modules contained the hydrox-
ide inside the ZnO:Al as well as on the its surface. Furthermore, carboxylic acid was
61
