Page 263 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 263
Overview and recommendations
Overview and recommendations
Based on the results as obtained in the literature study and in the experiments as
describes in this thesis, an overview of the most dominant degradation mechanisms
in CIGS solar cells and modules is made. Additionally, the failure mechanisms are also
linked to possible changes that can be observed in the electrical parameters. Finally,
some very general suggestions, both inside and outside the scope of this thesis, are
given for the study of CIGS stability.
CIGS degradation mechanisms
The modes and mechanisms leading to degradation of Cu(In,Ga)Se (CIGS) solar cells
2
and modules have been studied in order to better predict their long term behaviour.
The changes in electrical and physical parameters associated with decreased
performance, as well as the chemical reactions that are causing these changes are
described.
The degradation of CIGS solar cells was mostly observed to lead to fill factor and open
circuit voltage decreases, while short circuit current decreases are less prominent.
The open circuit voltage was mostly impacted by changes in absorber and pn-
junction properties, while the fill factor mostly changed due to the introduction of
alternative shunt paths and due to increased series resistance. The latter was amongst
others caused by a decrease of molybdenum, TCO, grid and interconnection scribe
conductivity as well as increased contact resistance due to delamination.
For the molybdenum back contact, it was observed that a less reflective and badly
conductive molybdenum oxide layer, potentially with incorporated Na, formed on
+
top of the molybdenum due to damp heat exposure. It was also shown that ITO was
more stable as front contact than ZnO:Al, which degraded slowly but steadily due to
damp heat exposure. The degradation was probably caused by the diffusion of water
and CO via grain boundaries, and mainly led to reduced mobility and conductivity,
2
while the transparency of the ZnO:Al was mostly relatively stable. When the stability
of buffers was studied, it was observed that interdiffusion of CdS with for example
zinc oxide occurred, leading to sulphate formation. Various types of other buffers
seemed to show a similar damp heat stability as CdS.
When the CIGS absorber, and also complete CIGS solar cells were studied, it was often
observed that sodium plays a role. Sodium migration, which can just occur when the
absorber is exposed to damp heat, but also due to illumination or a bias, leads to fast
decreases in CIGS cell efficiency. This sodium can either come from the soda lime glass
substrate or from the molybdenum or CIGS layers themselves. Generally, illumination
and bias exposure led to redistribution of the electrical charges in the CIGS/CdS,
influence the diode behaviour.
261
Overview and recommendations
Based on the results as obtained in the literature study and in the experiments as
describes in this thesis, an overview of the most dominant degradation mechanisms
in CIGS solar cells and modules is made. Additionally, the failure mechanisms are also
linked to possible changes that can be observed in the electrical parameters. Finally,
some very general suggestions, both inside and outside the scope of this thesis, are
given for the study of CIGS stability.
CIGS degradation mechanisms
The modes and mechanisms leading to degradation of Cu(In,Ga)Se (CIGS) solar cells
2
and modules have been studied in order to better predict their long term behaviour.
The changes in electrical and physical parameters associated with decreased
performance, as well as the chemical reactions that are causing these changes are
described.
The degradation of CIGS solar cells was mostly observed to lead to fill factor and open
circuit voltage decreases, while short circuit current decreases are less prominent.
The open circuit voltage was mostly impacted by changes in absorber and pn-
junction properties, while the fill factor mostly changed due to the introduction of
alternative shunt paths and due to increased series resistance. The latter was amongst
others caused by a decrease of molybdenum, TCO, grid and interconnection scribe
conductivity as well as increased contact resistance due to delamination.
For the molybdenum back contact, it was observed that a less reflective and badly
conductive molybdenum oxide layer, potentially with incorporated Na, formed on
+
top of the molybdenum due to damp heat exposure. It was also shown that ITO was
more stable as front contact than ZnO:Al, which degraded slowly but steadily due to
damp heat exposure. The degradation was probably caused by the diffusion of water
and CO via grain boundaries, and mainly led to reduced mobility and conductivity,
2
while the transparency of the ZnO:Al was mostly relatively stable. When the stability
of buffers was studied, it was observed that interdiffusion of CdS with for example
zinc oxide occurred, leading to sulphate formation. Various types of other buffers
seemed to show a similar damp heat stability as CdS.
When the CIGS absorber, and also complete CIGS solar cells were studied, it was often
observed that sodium plays a role. Sodium migration, which can just occur when the
absorber is exposed to damp heat, but also due to illumination or a bias, leads to fast
decreases in CIGS cell efficiency. This sodium can either come from the soda lime glass
substrate or from the molybdenum or CIGS layers themselves. Generally, illumination
and bias exposure led to redistribution of the electrical charges in the CIGS/CdS,
influence the diode behaviour.
261
