Page 76 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 76
Chapter 2
barrier and encapsulation materials on the stability of CIGS solar cells and modules.
Extensive information about the stability of encapsulants and its impact on CIGS solar
cells was reported by Coyle et al. [13,102,103]. The authors described the development
of a model for the diffusion of moisture into the module package and the subsequent
degradation of the CIGS solar cells [13]. This model is based on both experimental
work and calculations. The degradation rate was proportional to the degree of satura -
tion of the encapsulant. It was observed that moisture ingress and thus degradation
rate scale with both the climate and the characteristic package diffusion half-time.
The latter determines the rate at which the module approaches its environment’s av-
erage humidity. Diffusion half-times of the package should be approximately equal to
the target years of the module, which translates into moisture barriers with a WVTR of
-4
2
10 g/m /day at 25 C for a module with EVA encapsulant.
o
It was also concluded that the ´acceleration factor´ between accelerated ‘damp heat’
o
testing (85 C/85% RH) and real exposure in ‘Miami’ is actually non-linear and ranges
from 10 to 700 times, depending on the package and the kinetics of the cell degrada-
tion. It should be noted that the standard comparison between 1000 hours exposure
to 85 C/85% RH and 20 years of exposure the climate in Miami translated into an ac-
o
celeration factor of 175 times.
Coyle et al. noted that the degradation rate and the water permeability of the pack-
age both depend exponentially on temperature, while the degradation rate is strong -
ly increasing, in a non-linear way, with humidity. This can explain why both moisture
ingress and degradation occur mostly at higher temperatures and humidity.
Coyle et al. [102] describe that the degradation rate is proportional to the relative sat-
uration of the encapsulant and not to the absolute water concentration. Furthermore,
for encapsulants with a small WVTR, the water solubility is important: thick encapsu-
lants with a high water solubility will extend cell life, whereas thin encapsulants with
low water solubility shorten the lifetime.
Various references [104-109] have described experimental data of studies of encapsu-
lation and barrier materials on CIGS solar cells and (mini)modules.
Westin et al. [104] showed that modules without encapsulation were stable under dry
heat testing, but degraded mostly in fill factor and V when exposed to a damp heat
oc
test. It was observed that the severity of the V loss was similar for 85 C/85% RH and
o
oc
o
85 C/65% RH exposure, while the fill factor loss was slightly worse for the 85C/85%
o
RH conditions. When EVA/glass encapsulation was applied, it was observed that only
the fill factor and the series resistance deteriorated, while the V remained constant.
oc
Westin et al. described that encapsulation can impact the degradation in three ways:
74
barrier and encapsulation materials on the stability of CIGS solar cells and modules.
Extensive information about the stability of encapsulants and its impact on CIGS solar
cells was reported by Coyle et al. [13,102,103]. The authors described the development
of a model for the diffusion of moisture into the module package and the subsequent
degradation of the CIGS solar cells [13]. This model is based on both experimental
work and calculations. The degradation rate was proportional to the degree of satura -
tion of the encapsulant. It was observed that moisture ingress and thus degradation
rate scale with both the climate and the characteristic package diffusion half-time.
The latter determines the rate at which the module approaches its environment’s av-
erage humidity. Diffusion half-times of the package should be approximately equal to
the target years of the module, which translates into moisture barriers with a WVTR of
-4
2
10 g/m /day at 25 C for a module with EVA encapsulant.
o
It was also concluded that the ´acceleration factor´ between accelerated ‘damp heat’
o
testing (85 C/85% RH) and real exposure in ‘Miami’ is actually non-linear and ranges
from 10 to 700 times, depending on the package and the kinetics of the cell degrada-
tion. It should be noted that the standard comparison between 1000 hours exposure
to 85 C/85% RH and 20 years of exposure the climate in Miami translated into an ac-
o
celeration factor of 175 times.
Coyle et al. noted that the degradation rate and the water permeability of the pack-
age both depend exponentially on temperature, while the degradation rate is strong -
ly increasing, in a non-linear way, with humidity. This can explain why both moisture
ingress and degradation occur mostly at higher temperatures and humidity.
Coyle et al. [102] describe that the degradation rate is proportional to the relative sat-
uration of the encapsulant and not to the absolute water concentration. Furthermore,
for encapsulants with a small WVTR, the water solubility is important: thick encapsu-
lants with a high water solubility will extend cell life, whereas thin encapsulants with
low water solubility shorten the lifetime.
Various references [104-109] have described experimental data of studies of encapsu-
lation and barrier materials on CIGS solar cells and (mini)modules.
Westin et al. [104] showed that modules without encapsulation were stable under dry
heat testing, but degraded mostly in fill factor and V when exposed to a damp heat
oc
test. It was observed that the severity of the V loss was similar for 85 C/85% RH and
o
oc
o
85 C/65% RH exposure, while the fill factor loss was slightly worse for the 85C/85%
o
RH conditions. When EVA/glass encapsulation was applied, it was observed that only
the fill factor and the series resistance deteriorated, while the V remained constant.
oc
Westin et al. described that encapsulation can impact the degradation in three ways:
74
