Page 234 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 234
Chapter 7
Grain I Grain II
GB
Figure 7.16
Band structure of a grain boundary and two grains in a CIGS absorber with (light blue) and without (black) charge carrier
generation caused by illumination (modified after [16]).
illumination a large concentration of charge carriers is generated in the CIGS absorber
with a depth profile from a large concentration at the top of the absorber than at
the bottom. The trap states related to the defects in the grain boundaries are filled
with photogenerated carriers and consequently the band bending at the vicinity of
the grain boundaries is reduced. Therefore, the internal field at the grain boundaries
decreases. The enhanced concentration of primarily minority carriers contributes to
lowering the internal electric field. Under illumination, an upward potential gradient
+
for Na is created that facilitates the migration of ions towards the pn-junction.
Additionally, under open circuit conditions, the build-in voltage at the pn junction is
+
reduced under illumination. Consequently, the electrostatic barrier that the Na ions
have to overcome to cross the pn-junction is reduced.
These effects, induced by the illumination and open circuit conditions, favour the mi-
gration of Na through the grain boundaries (Figure 7.17, step 1). It is also proposed
+
that the elevated temperature of the samples leads to faster migration.
+
The positively charged Na ions can then arrive at the pn-junction. Since the internal
electric field of the depletion region of the pn junction does not facilitate the drift of
+
the Na ions across the depletion region, they cluster at the depletion region together
+
to form sodium rich spots. Accumulation of the Na ions in clusters counterbalances
the internal electric field of the depletion region of the pn-junction ( Figure 7.17, step 2).
The presence of sodium naturally influences the electrical behaviour of the diode.
Analogous to Shin et al. [17], who demonstrated that high concentrations of sodium
near the CIGS/CdS interface deteriorated conversion efficiencies, we propose that this
has a negative impact on the solar cell performance. The clustering of the positively
charged Na can locally reduce the electric field of the depletion region, which both
+
influences the electrical properties of the diode and favours the accumulation of
further additional Na ions (no more barrier to overcome). This can lead to the local
+
formation of a shunt path, which are then observed as the formation of spots.
+
When the shunt path is formed, the Na ions cross the pn-junction and diffuse into the
232
Grain I Grain II
GB
Figure 7.16
Band structure of a grain boundary and two grains in a CIGS absorber with (light blue) and without (black) charge carrier
generation caused by illumination (modified after [16]).
illumination a large concentration of charge carriers is generated in the CIGS absorber
with a depth profile from a large concentration at the top of the absorber than at
the bottom. The trap states related to the defects in the grain boundaries are filled
with photogenerated carriers and consequently the band bending at the vicinity of
the grain boundaries is reduced. Therefore, the internal field at the grain boundaries
decreases. The enhanced concentration of primarily minority carriers contributes to
lowering the internal electric field. Under illumination, an upward potential gradient
+
for Na is created that facilitates the migration of ions towards the pn-junction.
Additionally, under open circuit conditions, the build-in voltage at the pn junction is
+
reduced under illumination. Consequently, the electrostatic barrier that the Na ions
have to overcome to cross the pn-junction is reduced.
These effects, induced by the illumination and open circuit conditions, favour the mi-
gration of Na through the grain boundaries (Figure 7.17, step 1). It is also proposed
+
that the elevated temperature of the samples leads to faster migration.
+
The positively charged Na ions can then arrive at the pn-junction. Since the internal
electric field of the depletion region of the pn junction does not facilitate the drift of
+
the Na ions across the depletion region, they cluster at the depletion region together
+
to form sodium rich spots. Accumulation of the Na ions in clusters counterbalances
the internal electric field of the depletion region of the pn-junction ( Figure 7.17, step 2).
The presence of sodium naturally influences the electrical behaviour of the diode.
Analogous to Shin et al. [17], who demonstrated that high concentrations of sodium
near the CIGS/CdS interface deteriorated conversion efficiencies, we propose that this
has a negative impact on the solar cell performance. The clustering of the positively
charged Na can locally reduce the electric field of the depletion region, which both
+
influences the electrical properties of the diode and favours the accumulation of
further additional Na ions (no more barrier to overcome). This can lead to the local
+
formation of a shunt path, which are then observed as the formation of spots.
+
When the shunt path is formed, the Na ions cross the pn-junction and diffuse into the
232
