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Weathering degradation of EVA encapsulant and the effect of its yellowing on
solar cell efficiency
Conference Paper · November 1991
DOI: 10.1109/PVSC.1991.169275 · Source: IEEE Xplore
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WEATHERING DEGRADATION OF EVA ENCAPSULANT AND THE EFFECT
OF ITS YELLOWING ON SOLAR CELL EFFICIENCY
FJ. Pern, A.W. Czanderna, K.A. Emery, and R.G. Dhere
National Renewable Energy Laboratory
(formerly the Solar Energy Research Institute)
Golden, Colorado
ABSTRACT
The encapsulant materials provide optical coupling,
mechanical support, electrical isolation, physical isolatiod
protection, and thermal conduction for the solar cell assembly
(43). EVA copolymer (33% vinyl acetate) is extensively used
for crystalline Si-based PV module encapsulation. EVA is
formulated and processed to provide the desired mechanical
strength and stability (43). The processed EVA contains a
65% to 70% gel content (degree of cross-linking), 0.30 wt %
Cyasorb W 531 (a UV absorber identified as "Cyasorb" here),
and two antioxidants(4-6). Upon weathering degradation, the
EVA between the cover glass plate and solar cells in PV
modules may become discolored. In previous papers, we
studied the structuraleffects and relationship among the extent
of degradation,gel content, and Cyasorb concentration in EVA
(6,7). In this paper, we emphasize more the effect of EVA
yellowing on the solar cell efficiency.
After five or more years of weathering, the degradation of
ethylene-vinyl acetate (EVA) encapsulant in photovoltaic (PV)
modules resulted in a yellow to dark brown color. Degraded
EVA shows a substantial increase in the gel content and a
large to complete loss of the ultraviolet (UV) absorber,
Cyasorb UV 531. The EVA discoloration is caused by the
formation of polyconjugated (C=C), double bonds of various
lengths. Acetic acid and other volatile organic components are
also produced from the photothermal decomposition of the
EVA. The solar cell efficiency was reduced by -9% by a light
yellow brown EVA and -50% by a dark brown EVA. Weathered PV modules with dark brown EVA also show a -50%
decrease in efficiency.
EXPERIMENTAL
INTRODUCTION
EVA Materials and Analvtical Procedure
In the past two decades, significant improvements in the
efficiency and reliability of new semiconductor materials for
PV devices and modules have resulted in PV becoming an
increasingly viable energy alternative. While efficiency improvements in large-area CuInSq, CdTe, and a-Si-based thinfilm PV modules are increasingly promising, single-crystal
(and polycrystalline) Si-based PV modules have been the most
dominant products widely used. Typical efficiencies of these
PV modules range from about 11% to 14% (1). A long-term
stability of over 20-30 years for all types of PV modules is
one of the requirements to be cost-competitive.
The EVA materials we studied, our analytical procedures, and the analytical instruments we used are essentially
the same as those reported previously (6,7). The laminated
and cured EVA in PV modules that are stored in the dark for
six years remains clear. Clear and discolored (degraded) EVA
samples were cut from PV modules weathered outdoors for
more than five years. The Cyasorb concentration (wt %) and
gel content (%) are the two crucial measurements of EVA
degradation (6).
Measurement of the Effect of EVA Yellowing on Solar
Cells Efficiencv
However, some Si-based modules deployed outdoors for
five years or more develop a light yellow, to yellow-brown, to
dark brown color, depending on their locations, use temperature, and configuration. A common factor in these discolored
modules is their use of EVA films as the encapsulant. The
consequence of the EVA weathering degradation can be
significant. For example, Gay and Berman reported that EVA
browning resulted in a -30% loss in the annual energy output
from the six-megawatt PV systems at the Carrisa, California,
power plant (2). A detailed study on the module performance
of the Canisa PV systems revealed that the EVA degradation
is highly nonuniform from module to module, and that the
aveme module power output is 35.9%below that of the initial
rating (3). The mismatch between neighboring modules,
caused by nonuniform EVA degradation, contributed an
additional power loss of 11.1%.
The effect of EVA discoloration on solar cell efficiency
was evaluated as before (6), using a calibrated SpectrolabX-25
solar simulator. Quantum efficiency (spectral response) was
measured on a computerized system with periodic (440 Hz)
monochromatic light directed through one of an array of 10nm band-pass interferencefilters. The lower wavelength limit
was 290 nm. In both cases, the virgin or degraded EVA films
were pressed above the reference solar cells with or without
adding a cover glass above the film. Similar measurements
were also performed for a solar cell specimen (26 cm2) taken
from a broken PV module. The I-V performance of three
large PV modules was measured on a Spire Model 240A solar
simulator.
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CH2953-8/91/0000-0557$1.00 0 1991 IEEE
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RESULTS AND DISCUSSION
Virgin cured EVA
Exposed EVA
&
t
t
unexposed, clear + clear + light-yellow- yellow-brown
(acetic acid+)
+
EVA Degradation and Discoloration
I
Figure 1 shows the results obtained from analyzing a
large number of non-degraded and field-degraded EVA
samples. The gel content increased substantially from 65%70% in virgin EVA to 85%-88%in clear degraded EVA, to
90%-92% in light yellow EVA, to 96%-97% in yellow-brown
and dark brown EVA, while the Cyasorb concentration decreased concomitantly during the degradation process. No
Cyasorb was observed in the extensively degraded EVA.
When the Cyasorb concentration dropped below about 0.21 wt
%, the EVA films became discolored (6). As the extent of
degradation increased, the color darkened from light yellow, to
yellow, to yellow-brown, and to dark brown. The EVA
degradation across each solar cell unit on weathered PV modules was not uniform. The Cyasorb concentration was much
lower and the gel content was higher in EVA from over the
central region than from over the cell edges (6,7). No noticeable change in either the gel content or the Cyasorb concentration was observed in EVA samples taken from unweathered
modules from similar locations. The EVA mostly remained
clear around the cell edges and in the areas between neighboring solar cells. In addition to the discoloration,acetic acid and
volatile organics were present in the degraded EVA.
I
- 100
- 90
- 80
K/
,
,I
I
\
\
-I
Clear Yellow
,
\
\
- 70
Figure 1. The Cyasorb UV 531 concentration, gel content,
and EVA color at different extents of degradation, summarized from the results of analyzing a large number of undegraded and field-degraded EVA samples. The actual degradation is not known except for the virgin EVA.
Figure 2 compares the transmission spectra for virgin and
degraded (to various degrees) EVA samples. As the color on
the EVA darkens, more UV and visible light is absorbed. The
yellow-browning of degraded EVA is caused by the formation
of polyconjugated carbon-carbon double bonds (polyenes)
(6,7). The discoloredEVA luminesces strongly upon illumination by UV and visible light. In fact, the extent of EVA degradation can be detected directly using fluorescence analysis
from the shift in the emission peak position (from shorter to
longer wavelengths in the 500-650 nm region) and the increase
in peak intensity (6,7). Intense acetic acid odors and other
volatile organic components were detected when the weathered
PV modules were cut open. The presence of acetic acid is significant. In various simulated degradation experiments, acetic
acid was produced from EVA exposed to UV light at 45°C or
heated in the dark at 130°C. Acetic acid has been found to
catalyze EVA yellow-browning from 85" to 130°C. A detailed
discussion will be given elsewhere (7).
Wavelength (nm)
In summary, weathering degradation of the EVA encapsulant in PV modules results in an increased gel content, a loss
of the UV absorber, a yellow-browning of the EVA, and the
production of acetic acid and volatile organic components (6).
Figure 2. Transmission spectra measured for (1) a clear
EVA film, (2) a light yellow-brown EVA film,(3) a darker
yellow-brown EVA film, (4) a brown EVA film, and (4) a
darker brown EVA film. Samples #1, #4, and #5 were laminated in two glass plates and exposed to an RS4 sun lamp at
Effects of EVA Discoloration on Solar Cell Efficiency
90°C for 1600 h. Samples #2 and #3 were taken from weathered PV modules.
The effect of yellow-browning on the light transmission
is shown in Figure 2 for samples that are clear to dark brown
EVA. The effect of EVA yellow-browning on the electrical
performance and quantum efficiency (spectral response) was
obtained using single-crystal Si reference cells with and
without a degraded EVA sample laid over them. The results
are given in Tables 1 and 2(A), respectively. Table 1 gives
short-circuit current
the measured open-circuit voltage (V,),
density (J%), maximum power (PmJ, and calculated loss (%)
for two virgin clear and two degraded yellow-brown EVA
films. When covered with a virgin clear EVA film, the Si cell
However, the loss
lost about 6.7% in J, and 0.8% in P,.
increased to 12%-14% in J, and 14%-16% in P, with the
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are 67%-77% lower than the typical 13%-14% efficiency of
new 4"x4" mini-modules made by the same manufacturer. The
decrease is 17%-27% greater than the changes measured with
dark brown EVA films alone (see Table 2A), suggesting that
other factors such as increased series resistance may have also
contributed to the efficiency drop. This argument seems to be
supported by a f i i factor of about 0.50 as compared to a fill
factor of about 0.70 for new mini-modules. The relative spectral responses measured for the variously degraded EVA samples are compared in Figure 4.
yellow-brown EVA films. The loss in V, was relatively
small. In effect, the Si cell's 11.6% efficiency changed to
10.7% with the virgin EVA and 9.7% with the yellow-brown
EVA; this corresponds to a net loss of 9.3% of its original
efficiency when the EVA color changed from clear to yellowbrown. These effects also appear clearly in the relative quantum efficiency ratios to the bare Si reference cell, as shown in
Figure 3. The spectral responses for the two virgin cured
EVA films are virtually identical, with a cutoff below 360 nm
from the absorption of Cyasorb. For the two degraded yellowbrown EVA films that differ slightly in color, the spectral
responses begin at about 290 nm because of the absence of
Cyasorb. Because of light absorption by polyenes, the spectral
response is lower between 360 and about 900 nm than that of
the virgin EVA (Figure 2, curves 3 and 4).
Table 1. Effect of EVA Yellowing on the I-V Performance
of a Single-Crystal Si Reference Solar Cell'
As the EVA color became dark brown, the change in
measured cell efficiency increased. Table 2(A) shows the results obtained for two dark brown EVA samples with or without a 7059 glass plate pressed tightly onto the EVA over the
reference Si cell. For comparison, a solar cell specimen taken
from a broken PV module with dark brown EVA was also
similarly analyzed. With the dark brown EVA, a nearly 50%
loss in PmUwas observed; this is -35% greater than that for
the yellow-brown EVA seen in Table 1. The large difference
is simply because the dark brown EVA absorbs more UV and
visible light (Figure 2) than does the yellow-brown EVA (6,7).
The presence of a cover glass plate reduced the P,, an
additional 2%. For the solar cell specimen, the measured
efficiency for the three tape-defied areas ranged from 3.0%
to 4.4% (see Table 2B), further supporting our earlier conclusion that the degradation of EVA is non-uniform across the
cell surface (6). More importantly, the measured efficiencies
Sample
EVA
(VI
No EVA
0.474
A9918
V,
PIU,
Jffi'
Loss (%) in
Jffi
pm,
(mw)
vcc
39.87
4.06
-_-
---
--_
0.470
37.21
3.74
0.84
6.67
0.79
15295P
0.471
37.28
3.75
0.63
6.50
o.m
Yellow-1
0.466
35.00
3.48
1.69 12.21 1428
Yellow-2
0.465
34.51
3.41
1.90 13.44 16D1
* An aperture of 0.35 cm2was used over a single-crystal Si reference
solar cell for the measurements. The EVA film was placed beneath the aperture. No cover @ass was used on top of the cell or
EVA. The loss (%) would be slightly larger if a cover glass were
used. The P,, loss (%) would be about 7% less than the values
shown if they were corrected for the light scattering (see text).
Data for Yellow-1 and Yellow-2 are identified as samples #2 and
#3 in Figure 3 and Yellow-2 as sample #3 in Figure 4.
-1.oo 1
I
E
f
0.00U
250
450
650
850
1050
1250
Wavelength (nm)
250
Figure 3. Relative quantum efficiency measured for a
single-crystal Si reference solar cell covered with virgin
clear (curves #1 and #2) and degraded yellow-brown (curves
#3 and #4) EVA films. The spectral responses are ratioed to
that for the bare Si reference cell. The virgin EVA films are
EVA A9918 (curve #1) and EVA 15295P (curve #2). The
two degraded EVA films differ slightly in color.
450
850
Wavelength (nm)
650
1050
1250
Figure 4. Quantum efficiency measured for (1) a singlecrystal Si reference solar cell, (2) the cell in (1) covered
with virgin cured EVA A9918, (3) the cell in (1) covered
with yellow-brown EVA, and (4) a solar cell specimen with
dark brown EVA. Curves #1 and #4 are normalized to
100%. Curves #2 and #3 are normalized to curve #1.
559
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Table 2. Effect of Dark Brown EVA on the I-V Performance of (A) a Single-Crystal Si Reference Solar Cell' and
(B) a Solar Cell Specimen
(A) Two dark brown EVA films peeled from degraded PV modules
Loss (%) in
(mA/cm2)
Pinax
(mW)
V,
J,
P,
0.398
22.54
3.54
----
----
----
Brown-1
+ slideb
0.347
0.341
13.15
12.64
1.80
1.70
12.81 41.66 49.15
14.32 43.92 5198
Brown-2
0.345
0.343
13.21
12.76
1.79
1.72
13.32 41.39 49.44
13.82 43.39 51.41
Sample
EVA
V,
(V)
Si Cell
+ slideb
Jw
(B) A solar cell specimen with dark brown EVA'
Efficiency
P,
Fill
J,
(%)
(mA/cm2) (mW) Factor
Spot
No.
Area
(cm')
(V)
1
0.170
0.330
26.62
0.746
0.50
4.4
2
0.190
0.325
21.62
0.665
0.50
3.5
3
0.163
0.308
20.16
0.489
0.48
3.0
V,
' The results shown in the table are not adjusted for the loss of light
transmission through the EVA film due to light scattering from the
come Si-side surface. See text for details. Data for Brown-1 are
identified as sample #3 in Figure 5(a).
A glass plate was pressed tightly on top of the EVA film. An active aperture area of 0.967 cmz was used for the measurements.
The solar cell specimen (26 cm2) was taken from a broken PV
module with dark brown EVA and the superstrates and substrates
intact. The three areas were defmed by black tape. Contacts were
made by soldering wires to the front and back buslines.
One concem in our measurements of efficiency and spectral response is the accuracy offset by light scattering from the
textured surface of EVA films, and the extent of noncoupling
between the EVA and the reference Si cell. This concem is
particularly important for the dark brown EVA films (used for
measurements in Table 2A) that have a coarse surface side
after being forcefully removed from the micro-pyramidically
etched surface of the Si solar cells. The light-scattering effect
was assessed from the integrated reflectance measurements
from 250 to 1250 nm for a virgin clear EVA sample, a yellowbrown EVA sample, two dark brown EVA samples (one of
them with a coarse Si-side surface as described above and the
other with a smoother Si-side surface), and the solar cell
specimen. The results are shown in Figure 5. Figure 5(a)
shows that the integrated reflectances are nearly identical for
the virgin clear and yellow-brown EVA films (about 7%), and
double that for the dark brown EVA with a coarse Si-side
surface. Therefore, the measured solar cell performance
parameters in Tables 1 and 2(A) should be corrected by 7%
and 14%-15% of their listed values, respectively. Accordingly,
the corrected decrease in solar cell efficiency is -8.6% when
EVA changed from clear to yellow-brown and -42% when it
changed to dark brown. While the integrated reflectance
measured for the solar cell specimen is about 35% lower than
that for a dark brown EVA with a smoother Si-side surface
(Figure 5b), this small difference is not sufficient to adversely
affect the interpretation of results in Table 2.
Effect of EVA Discoloration on PV Module Performance
The effect of EVA discoloration on module I-V performance was also evaluated for three PV modules of the same
type (33 4"x4" cells). The EVA color ranged from light
yellow to dark brown. The shunt and series resistances were
determined from dark I-V measurements. Table 3 summarizes
the results. The original data on the I-V performance and
resistances are unavailable, which prevents an accurate interpretation of the data. However, the results seem to suggest
that an increase in the EVA yellow-browning is related to a
decrease in the shunt resistance and an increase in the series
resistance, which in turn results in a corresponding decrease in
the fill factor and hence the efficiency. The losses in fill factor and efficiency in the yellow-brown and dark brown EVA
modules are about 50% when compared to a typical fill-factor
value of about 0.70 and an efficiency of 210% for a new
module of the Same configuration. Our results on three modules are in good agreement with the field results obtained at
the Carrisa Plains power plant, where EVA yellow-browning
caused an average 36% loss in power output (2) and where
some modules with mirror-enhanced configuration lost nearly
70% more than average peak power (3).
0.20
t
0.00
250
(a)
I
I
450
I
I
650
I
I
850
I
I
1050
I
1250
Wavelength (nm)
Figure 5. Integrated reflectance measured for (a) a virgin
EVA A9918 film (curve #1), a yellow-brown EVA film
(curve #2), a dark brown EVA film (curve #3) with a coarse
Si-side surface, and (b) a dark brown EVA with a smoother
Si-side surface (curve #1) and a solar cell specimen
(curve #2).
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Table 3. Electrical Performance Measured using a Spire
Simulatorand ResistancesDetermined from Dark I-V Data
for Three Weathered PV Modules'
Mod. EVA
ColoP
A
V,
I,
(V)
(A)
It ye1 6.755 7.217
The authors thank R. DeBlasio for his support in this
work and S. Rummel and E. Beck for their technical contributions. This work is supported by the Department of Energy
34.0 0.70 9.34 1.35 0.109
yel-bm 6.733 6.428 16.35 0.38 4.49 0.577 0.508
C
drk bm
6.137
ACKNOWLEDGMENT
,P
Fill
Eff. %h, %,
(W) Factor (%) (kohm) (Ohm)
B
7.047
to about 50%, depending on the extent of EVA discoloration
from the degradation.
15.99 0.37
4.41 0.723
under Contract No. DE-AC02-83CH10093.
0.499
REFERENCES
' The three modules were of the same type, and each contains 33 4"x4"
1. J. P. Thomton, R. DeBlasio, and K. Zweibel, Energv Enpineerin& 87, 1990, pp. 63-79.
2. C. F. Gay and E. Berman, Chemtech, March 1990, pp.
182-186.
3. A. L. Rosenthal and C. G. Lane, Proc. PV Module Reliabilitv WorkshoD, Oct. 25-26, 1990, Lakewood, Colorado,
SERI/CP-4079, pp. 217-229.
4. C.G. Gebelein, D.J. Williams, and R.D. Deanin, ed., polvmers in Solar Energy Utilization, ACS, Washington D.C.,
1983, Ch. 22 and 23, pp. 353-385.
5. E. Cuddihy, C. Coulbert, A. Gupta and R. Liang, FlatPlate Solar Array Proiect Final ReDort. Vol. VII--Module
Encapsulation, JPL publication 86-31, DOWJPL-1012-125,
1986.
6. F.J. Pem and A.W. Czandema, Solar Cells, in press.
7. F.J. Pem and A.W. Czandema, to be published.
square solar cells.
It ye1 = light yellow, yel-bm = yellow-brown, drk bm = dark brown.
CONCLUSIONS
We have reported that weathering degradation of the
EVA encapsulant in PV modules results in an increase of
copolymer cross-linking, a large decrease in W absorber concentration, the production of acetic acid, and a yellow to dark
brown color. The discoloration results from the formation of
polyenes of various lengths. The polyenes absorb UV and visible light and luminesce in the visible region; thus, they may
partially compensate for the loss of solar cell efficiency due to
reduced light transmission. The light-absorbing nature of
polyenes reduces the net solar cell efficiency from about 9%
561
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