Page 48 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 48
Chapter 2
of Na Se compounds. During degradation, the oxidation of the absorbers leads to
X
2
the oxidation of both CIGS and NaSe . If that oxidation is incomplete, an increased
2
X
production of elemental selenium from CIGS and NaSe following the above de-
2
X
scribed reactions occurs, which severely affects the device performance, mainly due
to the decrease of the shunt resistance.
The impact of sodium content on absorber stability was also reported by Sakurai et al.
[51], who varied the molybdenum sputter pressure. Since this impacted the sodium
migration, the resulting CIGS absorbers had different sodium contents. The stability
of these layers was studied: at high sputter pressures, large holes in the molybde-
num layer perpendicular to the substrate were observed. CIGS layers deposited on
these layers showed low efficiencies and poor reproducibility. Dark spots occurred on
the absorbers under air exposure and increased in size and number with time. These
spots were physical distortions of the CIGS layers and contained high concentrations
of sodium and oxygen. These elements were found up to 1 µm depth in the CIGS lay-
er. It is proposed that this resulted from NaOH damage, formed by a reaction from a
sodium product with water vapour. When the sputter pressure was decreased and the
molybdenum became denser, the spots were not formed.
2.3.2.5 Summary on CIGS absorber degradation
The CIGS absorber has a variety of defects with complex interactions that determine
their benign electronic behaviour. Due to the synergetic interaction of a large de-
fect pool and the possibility of ionic migration (mainly copper), CIGS is a radiation
hard and impurity tolerant material. CIGS grains themselves are thus intrinsically sta-
ble, irrespective of their complicated structure. Changes of the CIGS absorber due to
extrinsic influences such as damp heat are therefore generally with respect to grain
boundaries within the polycrystalline absorber film and the interfaces to other layers
(back contact and buffer layer).
Due to experimental limitations, CIGS absorbers are mostly studied within the thin
film layer stack of CIGS thin film solar cells, so generally limited experimental informa -
tion is available about the stability of the absorbers themselves.
However, it is known that the exposure of bare CIGS layers to ambient conditions
leads rapidly to a significant reduction of the carrier lifetime. This degradation pro-
cess can be prevented by CdS deposition on top of the CIGS which yields stability for
several months.
Under the exposure to damp heat, spots were formed on the surface of bare CIGS lay -
ers. These spots exhibited low gallium, copper and selenium concentrations, but high
sodium contents. It was assumed that hydrolysis or oxidation led to these degradation
processes. The formation of Cu(OH) on CuGaSe solar cells that were stored under
2
2
46
of Na Se compounds. During degradation, the oxidation of the absorbers leads to
X
2
the oxidation of both CIGS and NaSe . If that oxidation is incomplete, an increased
2
X
production of elemental selenium from CIGS and NaSe following the above de-
2
X
scribed reactions occurs, which severely affects the device performance, mainly due
to the decrease of the shunt resistance.
The impact of sodium content on absorber stability was also reported by Sakurai et al.
[51], who varied the molybdenum sputter pressure. Since this impacted the sodium
migration, the resulting CIGS absorbers had different sodium contents. The stability
of these layers was studied: at high sputter pressures, large holes in the molybde-
num layer perpendicular to the substrate were observed. CIGS layers deposited on
these layers showed low efficiencies and poor reproducibility. Dark spots occurred on
the absorbers under air exposure and increased in size and number with time. These
spots were physical distortions of the CIGS layers and contained high concentrations
of sodium and oxygen. These elements were found up to 1 µm depth in the CIGS lay-
er. It is proposed that this resulted from NaOH damage, formed by a reaction from a
sodium product with water vapour. When the sputter pressure was decreased and the
molybdenum became denser, the spots were not formed.
2.3.2.5 Summary on CIGS absorber degradation
The CIGS absorber has a variety of defects with complex interactions that determine
their benign electronic behaviour. Due to the synergetic interaction of a large de-
fect pool and the possibility of ionic migration (mainly copper), CIGS is a radiation
hard and impurity tolerant material. CIGS grains themselves are thus intrinsically sta-
ble, irrespective of their complicated structure. Changes of the CIGS absorber due to
extrinsic influences such as damp heat are therefore generally with respect to grain
boundaries within the polycrystalline absorber film and the interfaces to other layers
(back contact and buffer layer).
Due to experimental limitations, CIGS absorbers are mostly studied within the thin
film layer stack of CIGS thin film solar cells, so generally limited experimental informa -
tion is available about the stability of the absorbers themselves.
However, it is known that the exposure of bare CIGS layers to ambient conditions
leads rapidly to a significant reduction of the carrier lifetime. This degradation pro-
cess can be prevented by CdS deposition on top of the CIGS which yields stability for
several months.
Under the exposure to damp heat, spots were formed on the surface of bare CIGS lay -
ers. These spots exhibited low gallium, copper and selenium concentrations, but high
sodium contents. It was assumed that hydrolysis or oxidation led to these degradation
processes. The formation of Cu(OH) on CuGaSe solar cells that were stored under
2
2
46
