Page 74 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 74
Chapter 2
show the original fill factors. This effect is related to the above mentioned accumula-
tion of negative charges at the hetero-interface and is also called the ´blue metasta-
bility´. This can be reversed by a 3 hours light soak.
Ott et al. [99] used photoluminescence (PL) and electroluminescence (EL) as well as
SCAPPS simulations, capacitance-voltage (CV) and IV measurements to study the in-
fluence of light and dark exposure at elevated temperatures. It was found that the PL
and EL intensities were governed by the net doping density and interface charges
respectively. They concluded that exposure to illumination and positive electrical bi-
ases lead to more stable solar cells. They also demonstrated that PL and EL are well
suited for the detection of the degradation mechanisms and can distinguish between
bulk and interface properties by the appropriate operation conditions during lumi-
nescence measurements, for example by the selection of the applied electrical bias.
More information on the impact of electrical biases, was provided by Ott et al. [100],
who exposed CIGS solar cells to lower positive and negative biases (+400 mV and -100
mV) under dry heat. The negative electrical bias had a negative impact on the cell be -
haviour, while the positive electrical bias initially had a positive impact on the V and
oc
fill factor, while later, these factors decreased. It was suggested that the degradation
was driven by the occurrence of a barrier at the back contact of the device. Howev-
er, this degradation phenomenon was very slow, so combined exposure to dry heat
conditions and electrical biases do probably not lead to the most critical degradation
mechanisms.
2.4.4 Superposition of degradation mechanisms in CIGS solar cells
In many references, one important mechanism regarding the degradation has been
emphasised. An overview of the separated degradation mechanisms is shown in
chapters 2.4.2 and 2.4.3. However, in most cases, multiple mechanisms seem to be
involved, which can be dominant at different times. In this chapter, an example of the
superposition of multiple mechanisms is shown.
Daume et al. studied the evolution of the degradation process of flexible CIGS solar
cells on polyimide substrates with various sodium contents [94] under damp heat ex-
posure as well as combined damp heat and illumination exposure. It was observed
that the efficiencies of two sets of CIGS solar cells with different sodium contents in
the absorber showed different trends during damp heat exposure. The solar cells with
low sodium contents showed an increase in fill factor and efficiency after 5 to 15 hours
of exposure to damp heat conditions followed by a decrease. This indicated a super-
position of an improving and a degrading mechanism involved in the degradation
process of the CIGS solar cell. On the other hand, the solar cells with high sodium con -
tents demonstrated an immediate decrease in efficiency without initial improvement.
72
show the original fill factors. This effect is related to the above mentioned accumula-
tion of negative charges at the hetero-interface and is also called the ´blue metasta-
bility´. This can be reversed by a 3 hours light soak.
Ott et al. [99] used photoluminescence (PL) and electroluminescence (EL) as well as
SCAPPS simulations, capacitance-voltage (CV) and IV measurements to study the in-
fluence of light and dark exposure at elevated temperatures. It was found that the PL
and EL intensities were governed by the net doping density and interface charges
respectively. They concluded that exposure to illumination and positive electrical bi-
ases lead to more stable solar cells. They also demonstrated that PL and EL are well
suited for the detection of the degradation mechanisms and can distinguish between
bulk and interface properties by the appropriate operation conditions during lumi-
nescence measurements, for example by the selection of the applied electrical bias.
More information on the impact of electrical biases, was provided by Ott et al. [100],
who exposed CIGS solar cells to lower positive and negative biases (+400 mV and -100
mV) under dry heat. The negative electrical bias had a negative impact on the cell be -
haviour, while the positive electrical bias initially had a positive impact on the V and
oc
fill factor, while later, these factors decreased. It was suggested that the degradation
was driven by the occurrence of a barrier at the back contact of the device. Howev-
er, this degradation phenomenon was very slow, so combined exposure to dry heat
conditions and electrical biases do probably not lead to the most critical degradation
mechanisms.
2.4.4 Superposition of degradation mechanisms in CIGS solar cells
In many references, one important mechanism regarding the degradation has been
emphasised. An overview of the separated degradation mechanisms is shown in
chapters 2.4.2 and 2.4.3. However, in most cases, multiple mechanisms seem to be
involved, which can be dominant at different times. In this chapter, an example of the
superposition of multiple mechanisms is shown.
Daume et al. studied the evolution of the degradation process of flexible CIGS solar
cells on polyimide substrates with various sodium contents [94] under damp heat ex-
posure as well as combined damp heat and illumination exposure. It was observed
that the efficiencies of two sets of CIGS solar cells with different sodium contents in
the absorber showed different trends during damp heat exposure. The solar cells with
low sodium contents showed an increase in fill factor and efficiency after 5 to 15 hours
of exposure to damp heat conditions followed by a decrease. This indicated a super-
position of an improving and a degrading mechanism involved in the degradation
process of the CIGS solar cell. On the other hand, the solar cells with high sodium con -
tents demonstrated an immediate decrease in efficiency without initial improvement.
72
