Page 272 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 272
Chapter 10
cell, makes the molybdenum more stable under degrading conditions, like damp heat.
Zinc oxide front contact
The zinc oxide front contact has been degraded under conditions 1 and 3. Aluminium
doped zinc oxide (ZnO:Al) degrades via a gradual process, which starts at the air/ZnO:Al
interface and slowly proceeds in the direction of the substrate. This degradation is
caused by the slow diffusion of water and CO via the grain boundaries. In the absence
2
of either of these species, the material is very stable.
Within degraded ZnO:Al, increased concentrations of water and CO (as well as sulfur
2
and chlorine) were observed, which might indicate the formation of materials like
Zn(OH) and Zn(CO ) (OH) , or similar materials, based on chlorides or sulfides. The
5
2
6
3 2
diffusion of and the reactions with water and CO lead to an increase in the potential
2
barrier at the grain boundaries in the polycrystalline ZnO:Al. This leads to a decrease of
electron mobility and thus in conductivity, while the transmission remained majorly
constant.
Complete CIGS solar cells
The complete CIGS solar cells, consisting of soda lime glass (SLG)/molybdenum (Mo)/
Cu(In,Ga)Se (CIGS)/cadmium sulfide (CdS)/intrinsic zinc oxide (i-ZnO)/aluminium
2
doped zinc oxide (ZnO:Al) were degraded under conditions 1, 2 and 3. The solar cells
degraded due to diffusion of species from the atmosphere, like water and CO as well
2
as from the cell itself. The diffusion of water and CO mostly leads to an increased series
2
resistance of the ZnO:Al, as well as to local dissolution of the CIGS solar cells, leading to
the formation of voids and a rapid decrease of short circuit current.
It was also observed that the alkali elements and especially sodium, play an important
role in the degradation of CIGS solar cells. When alkali-rich and alkali-poor CIGS solar
cells were both exposed to damp heat and illumination, the alkali-rich samples gave
higher initial conversion efficiencies, but degraded severely within 100 hours, while the
alkali-poor samples were relatively stable. The degradation of the alkali-rich samples
led to the formation of sodium rich spots in the depletion region. This is likely caused
by to Na migration from the grain boundaries in the CIGS layer to the depletion region.
+
This migration can be due to the electric field resulting from exposure to illumination,
caused by a PID-like mechanism. The migration likely resulted in the appearance of
shunt paths, which were responsible for a very low shunt resistance and open-circuit
voltage. Furthermore, ingression of water and other species into the ZnO:Al seems to
be responsible for a slow but steady increase in series resistance for both types, while
sodium migration led to a more severe increase for the series resistance for the alkali-
rich samples.
270
cell, makes the molybdenum more stable under degrading conditions, like damp heat.
Zinc oxide front contact
The zinc oxide front contact has been degraded under conditions 1 and 3. Aluminium
doped zinc oxide (ZnO:Al) degrades via a gradual process, which starts at the air/ZnO:Al
interface and slowly proceeds in the direction of the substrate. This degradation is
caused by the slow diffusion of water and CO via the grain boundaries. In the absence
2
of either of these species, the material is very stable.
Within degraded ZnO:Al, increased concentrations of water and CO (as well as sulfur
2
and chlorine) were observed, which might indicate the formation of materials like
Zn(OH) and Zn(CO ) (OH) , or similar materials, based on chlorides or sulfides. The
5
2
6
3 2
diffusion of and the reactions with water and CO lead to an increase in the potential
2
barrier at the grain boundaries in the polycrystalline ZnO:Al. This leads to a decrease of
electron mobility and thus in conductivity, while the transmission remained majorly
constant.
Complete CIGS solar cells
The complete CIGS solar cells, consisting of soda lime glass (SLG)/molybdenum (Mo)/
Cu(In,Ga)Se (CIGS)/cadmium sulfide (CdS)/intrinsic zinc oxide (i-ZnO)/aluminium
2
doped zinc oxide (ZnO:Al) were degraded under conditions 1, 2 and 3. The solar cells
degraded due to diffusion of species from the atmosphere, like water and CO as well
2
as from the cell itself. The diffusion of water and CO mostly leads to an increased series
2
resistance of the ZnO:Al, as well as to local dissolution of the CIGS solar cells, leading to
the formation of voids and a rapid decrease of short circuit current.
It was also observed that the alkali elements and especially sodium, play an important
role in the degradation of CIGS solar cells. When alkali-rich and alkali-poor CIGS solar
cells were both exposed to damp heat and illumination, the alkali-rich samples gave
higher initial conversion efficiencies, but degraded severely within 100 hours, while the
alkali-poor samples were relatively stable. The degradation of the alkali-rich samples
led to the formation of sodium rich spots in the depletion region. This is likely caused
by to Na migration from the grain boundaries in the CIGS layer to the depletion region.
+
This migration can be due to the electric field resulting from exposure to illumination,
caused by a PID-like mechanism. The migration likely resulted in the appearance of
shunt paths, which were responsible for a very low shunt resistance and open-circuit
voltage. Furthermore, ingression of water and other species into the ZnO:Al seems to
be responsible for a slow but steady increase in series resistance for both types, while
sodium migration led to a more severe increase for the series resistance for the alkali-
rich samples.
270
