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Stability of Cu(In,Ga)Se 2 Solar Cells
Therefore, electrical measurements are typically performed on complete devices. This is
also necessary to obtain the very basic information about the conversion efficiency. The
main challenge is to separate the influences of all individual layers (from substrate to
TCO) in the interpretation of these electrical measurements. A common approach is to
investigate a series of solar cells that are manufactured the in same way, except for some
intentional variations in the absorber layer (e.g. alkali doping, Ga grading, thickness). By
comparing the degradation results from different samples, conclusions of the influence
of the absorber and its composition on the stability of the solar cells are possible.
2.3.2.2 Intrinsic CIGS stability
Intrinsic instabilities of Cu(In,Ga)Se , like the mobility of constituent elements or inter-
2
face reactions between the layers of the final device, are potentially detrimental for the
long-term stability of a CIGS solar cell. However, Guillemoles et al. [37,42,43] argue that
these fundamental instabilities are actually beneficial to the device: possible interface
reactions (back contact/absorber, absorber/buffer, buffer/window) are thermodynam -
ically or kinetically limited. In contrast to a-Si:H solar cells, defects are self-annealed at
room-temperature and are actually beneficial to the cell performance, since they effec -
tively increase the doping level in the CIGS absorber.
Regarding the defects in CIGS absorbers, the two most important features considering
the stability are the presence of a large defect pool and the possibility of ionic migra-
tion, mostly of copper. Their synergetic action makes CIGS absorbers radiation hard and
impurity tolerant materials. In that sense, the stability of CIGS absorbers is not static but
dynamic or, as Guillemoles et al. [42] put it, ´CIGS absorbers are strong because they are
flexible´. In that sense, they are called a ´smart material´, which is by their definition ´ca -
pable of sensing changes in its environment and responding to them´.
Figure 2.6
Optical image of spot formation on ambient-degraded CIGS film at a magnification of 16x obtained from reference [44] with
artificial colours.
43
Therefore, electrical measurements are typically performed on complete devices. This is
also necessary to obtain the very basic information about the conversion efficiency. The
main challenge is to separate the influences of all individual layers (from substrate to
TCO) in the interpretation of these electrical measurements. A common approach is to
investigate a series of solar cells that are manufactured the in same way, except for some
intentional variations in the absorber layer (e.g. alkali doping, Ga grading, thickness). By
comparing the degradation results from different samples, conclusions of the influence
of the absorber and its composition on the stability of the solar cells are possible.
2.3.2.2 Intrinsic CIGS stability
Intrinsic instabilities of Cu(In,Ga)Se , like the mobility of constituent elements or inter-
2
face reactions between the layers of the final device, are potentially detrimental for the
long-term stability of a CIGS solar cell. However, Guillemoles et al. [37,42,43] argue that
these fundamental instabilities are actually beneficial to the device: possible interface
reactions (back contact/absorber, absorber/buffer, buffer/window) are thermodynam -
ically or kinetically limited. In contrast to a-Si:H solar cells, defects are self-annealed at
room-temperature and are actually beneficial to the cell performance, since they effec -
tively increase the doping level in the CIGS absorber.
Regarding the defects in CIGS absorbers, the two most important features considering
the stability are the presence of a large defect pool and the possibility of ionic migra-
tion, mostly of copper. Their synergetic action makes CIGS absorbers radiation hard and
impurity tolerant materials. In that sense, the stability of CIGS absorbers is not static but
dynamic or, as Guillemoles et al. [42] put it, ´CIGS absorbers are strong because they are
flexible´. In that sense, they are called a ´smart material´, which is by their definition ´ca -
pable of sensing changes in its environment and responding to them´.
Figure 2.6
Optical image of spot formation on ambient-degraded CIGS film at a magnification of 16x obtained from reference [44] with
artificial colours.
43
