Page 85 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 85
Stability of Cu(In,Ga)Se 2 Solar Cells
comparable efficiencies. Due to high mechanical stress, some of the nickel-only grids
exhibited cracking and peeled off, resulting in damaged or failed devices. However,
four-point resistance measurements on simple test structures consisting of a 0.1 µm
thin nickel layer demonstrated no change in resistance after 50 to 100 hours of damp
heat exposure, thus motivating further optimisation of a potential grid purely out of
nickel.
A different grid application technology is screen-printing. Typically, polymer matri-
ces with silver particles are printed and subsequently annealed. Britt et al. reported
that silver containing grid fingers exhibited visual signs of corrosion after 50 hours of
damp heat exposure [45]. Widespread pinhole-like defects were found on the surface
of damp heat treated stainless steel/Mo/CIGS/CdS samples. These defects consisted
of sodium, oxygen and carbon in their core. It was suggested that shunting paths
would be produced if a silver grid was printed on top of these pinholes. It remained
unclear and subject to further investigation whether such a pinhole defect could form
and would be harmful to the solar cell performance if a TCO and a gridwere applied
before damp heat exposure.
Similar to TCOs, grids can also degrade due to the interaction with barrier or encapsu -
lation materials. DeGroot et al. [114] and Elowe et al. [115] exposed SLG/Mo/CIGS/CdS/
ITO/NiAg grid cells with various moisture barriers to a damp heat test. It was observed
that Si N moisture barriers had poor adhesion to the silver grid, while SiN kept its
3
4
3
4
adherence to the ITO during damp heat exposure. A test structure consisting of SLG/
Al/ITO/NiAg and moisture barriers was used to study the degradation mechanisms.
It was found that the SiN film could oxidise into an oxynitride or oxide. It was also
4
3
measured that sodium, which likely had migrated from the glass often accumulated
together with oxygen in the neighbourhood of the grid fingers. This indicated a rela-
tionship between oxidation and sodium migration, possible due to the facilitation of
the oxidation by the presence of sodium. A possible explanation was given a chem-
ical reaction between SiO and sodium species (e.g. NaOH or NaO) from the glass.
x
2
Na O was reported to dissolve SiO to generate a liquid sodium silicate (Na O•x(SiO )).
2
2
2
2
Therefore, the combination of possible oxidation of the nitride and the presence of
sodium is detrimental for the stability of the barrier. It was predicted that mitigation
of the sodium migration to the top layer and an improved interface integrity could
overcome the observed failure mechanism. This was experimentally confirmed by
introducing a 10 nm thin TaN layer between ITO and SiN thus mitigating Na diffusion.
x
x
Therefore oxidation of the SiN film was prevented, leading to significantly improve of
x
the stability of the samples. When this was applied to CIGS solar cells, 90% of the initial
performance could be retained after 1900 hours under damp heat exposure.
83
comparable efficiencies. Due to high mechanical stress, some of the nickel-only grids
exhibited cracking and peeled off, resulting in damaged or failed devices. However,
four-point resistance measurements on simple test structures consisting of a 0.1 µm
thin nickel layer demonstrated no change in resistance after 50 to 100 hours of damp
heat exposure, thus motivating further optimisation of a potential grid purely out of
nickel.
A different grid application technology is screen-printing. Typically, polymer matri-
ces with silver particles are printed and subsequently annealed. Britt et al. reported
that silver containing grid fingers exhibited visual signs of corrosion after 50 hours of
damp heat exposure [45]. Widespread pinhole-like defects were found on the surface
of damp heat treated stainless steel/Mo/CIGS/CdS samples. These defects consisted
of sodium, oxygen and carbon in their core. It was suggested that shunting paths
would be produced if a silver grid was printed on top of these pinholes. It remained
unclear and subject to further investigation whether such a pinhole defect could form
and would be harmful to the solar cell performance if a TCO and a gridwere applied
before damp heat exposure.
Similar to TCOs, grids can also degrade due to the interaction with barrier or encapsu -
lation materials. DeGroot et al. [114] and Elowe et al. [115] exposed SLG/Mo/CIGS/CdS/
ITO/NiAg grid cells with various moisture barriers to a damp heat test. It was observed
that Si N moisture barriers had poor adhesion to the silver grid, while SiN kept its
3
4
3
4
adherence to the ITO during damp heat exposure. A test structure consisting of SLG/
Al/ITO/NiAg and moisture barriers was used to study the degradation mechanisms.
It was found that the SiN film could oxidise into an oxynitride or oxide. It was also
4
3
measured that sodium, which likely had migrated from the glass often accumulated
together with oxygen in the neighbourhood of the grid fingers. This indicated a rela-
tionship between oxidation and sodium migration, possible due to the facilitation of
the oxidation by the presence of sodium. A possible explanation was given a chem-
ical reaction between SiO and sodium species (e.g. NaOH or NaO) from the glass.
x
2
Na O was reported to dissolve SiO to generate a liquid sodium silicate (Na O•x(SiO )).
2
2
2
2
Therefore, the combination of possible oxidation of the nitride and the presence of
sodium is detrimental for the stability of the barrier. It was predicted that mitigation
of the sodium migration to the top layer and an improved interface integrity could
overcome the observed failure mechanism. This was experimentally confirmed by
introducing a 10 nm thin TaN layer between ITO and SiN thus mitigating Na diffusion.
x
x
Therefore oxidation of the SiN film was prevented, leading to significantly improve of
x
the stability of the samples. When this was applied to CIGS solar cells, 90% of the initial
performance could be retained after 1900 hours under damp heat exposure.
83
