ORIGINAL PAPER

Determination of the cross-linking degree of commercialethylene-vinyl-acetate polymer by luminescence spectroscopy

Jan Caspar Schlothauer & Rojonirina Maryline Ralaiarisoa &

Arnaud Morlier & Marc Köntges & Beate Röder

Received: 16 January 2014 /Accepted: 17 April 2014 /Published online: 30 April 2014# Springer Science+Business Media Dordrecht 2014

Abstract Ethylene-Vinyl-Acetate is the most common encap-sulation polymer in photovoltaic modules. Its degree ofcrosslinking is a critical parameter of the production process,greatly influencing the durability of the product. The lumines-cence of Ethylene-Vinyl-Acetate with UVexcitation is present-ed here as a non-destructive method to determine the degree ofcrosslinking. The luminescence intensity increases with thedegree of crosslinking. Supposing a linear correlation the meth-od allows determining the degree of crosslinking with a relativeprecision of 12 %. This optical method it is suitable and com-paratively easy to apply for an industrial in-line measurement.

Keywords Luminescence . Spectroscopy . EVA .

Photovoltaics . Cross-linking

Introduction

The use of solar energy became a factor of rising importancein the last years for the worldwide human energy budget.

Especially photovoltaics is regarded a promising way in get-ting sustainable energy. One of the most important questionsfor its worldwide extending use is the question of reliabilityand long term stability of photovoltaic (PV) modules as thisvalue is a major factor in the competitiveness of the price ofsolar power. Nowadays module manufacturers assure longtermwarranties, such as 25 to 30 years, for their solar modules[1, 2]. These warranties are ambitious and contain risks con-sidering the fact that little data exists about the long termstability of current PV modules under different climate condi-tions and that established degradation rates are about at theedge of the warranted properties [3]. To justify or extend thetodays warranties it is essential to find and standardize newmethods that can diagnose the state of a solar module duringproduction and allow a funded prediction of its service life.These refinedmethods will improve the quality of solar moduleproduction and consequently allow a better prediction of solarmodules lifetime. Finally, their use will increase the modulereliability and thus make themmore competitive on the market.

Ethylene-Vinyl-Acetate (EVA) is most commonly used asan encapsulation polymer in photovoltaic modules. It servesseveral purposes: protection and isolation of the solar cells,providing bonding and stability of the modules in general.During module lamination EVA crosslinks and forms an in-fusible network between the polymer chains. Typical EVAformulations used for solar applications contain a radicalinitiator such as peroxide. This radical initiator is activatedby temperature and generates radicals on the polymer back-bone which subsequently combine to form covalent bonds.The degree of cross-linking of the EVA achieved during thelamination process is a decisive parameter for the durabilityand long-term stability of the module. In this context thelamination process is the critical point to ensure the optimalstiffness and performance of the EVA in its abovementionedroles. An insufficient cross-linking can accordingly lead forinstance to delamination [4]. Thus, the determination of the

J. C. Schlothauer :R. M. Ralaiarisoa : B. Röder (*)Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin,Germanye-mail: roeder@physik.hu-berlin.de

J. C. Schlothauere-mail: jan.schlothauer@physik.hu-berlin.de

R. M. Ralaiarisoae-mail: ralaiarr@physik.hu-berlin.de

A. Morlier :M. KöntgesInstitute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1,31860 Emmerthal, Germany

A. Morliere-mail: morlier@isfh.de

M. Köntgese-mail: Marc.Koentges@ISFH.de

J Polym Res (2014) 21:457DOI 10.1007/s10965-014-0457-9

degree of crosslinking of EVA is relevant so as to optimize thelamination process and thereby improve the quality of theproduct and reduce the manufacturing costs. Several methodshave been reported for the determination of the degree ofcrosslinking [1, 5]. Also a phenomenological model in corre-lation to rheometric measurements has been developed, de-scribing the temperature-time-dependence of the degree ofcross-linking of EVA during the lamination process [6]. Thegel extraction (Soxhlet) still remains a standard method todetermine the curing state of material [7] but is not considereda reliable method for the determination of the degree ofcrosslinking of EVA. The Differential Scanning Calorimetry(DSC) of extracted samples from the module and the mechan-ical measurements allow determining efficiently the degree ofcrosslinking of EVA [4, 8–10]. These are however time-consuming and destructive offline methods. Non-destructivemethods for determination of the cross-linking in the modulewere developed recently. They rely on measurements of ultra-sound velocity through the module [11], dielectric propertiesmeasurement of EVA [12] or measurement of the stiffness ofEVA by indentation are promising investigation tools for theanalysis of the crosslinking process of EVA, although theyimpose limitations to the module design. A fast and non-destructive optical method newly revealed a correlation existsbetween the gel content and the correspondent measured diffusetransmission in glass-glass modules [13]. However, none of thepresent non-destructive methods is precise enough and suitablefor e.g. in-line measurements during manufacturing withoutconstraining the use of the method to specific module designs(e.g. glass-glass modules instead of the far more common glass-foil modules). As well the more relevant space between theglass and the cell is inaccessible with such methods.

Spatially resolved luminescence spectroscopy on completePV modules has recently been used by the authors as a non-destructive method to diagnose and characterize the EVA incommercial PV modules [14, 15]. During outdoor weatheringor accelerated aging the EVA and its additives decompose andsome of the degradation products emit luminescence. This lumi-nescence increases monotonously with homogenous acceleratedageing [16]. The luminescence spectra of accelerated and out-door weathered modules allow discriminating between differentaging factors [14]. Spatially resolved luminescence scans [14] orluminescence images [15] can be correlated with cell cracks.

In previous studies of EVA samples luminescence spectros-copy has been shown to be a valuable tool for the investigationof EVA properties [2]. Pern et al. investigated the effects ofcuring on EVA samples using synchronous fluorescence spec-troscopy in the range of 275 nm to 450 nm [2]. An increase inthe intensity of the spectrum with the curing time (and respec-tively the degree of cross-linking) was found there. A maxi-mum at 370 nmwas observed for the synchronous fluorescencespectrum while a 50 nm wavelength shift was used. Theseinvestigations were directed at the understanding of the role

of the curing process for the degradation of the module ratherthan a quantitative analysis of the crosslinking degree.

The purpose of the present work is to investigate theapplicability of the luminescence spectroscopy technique thatis used for the analysis of the EVA in complete PV modulesfor a quantitative analysis of the degree of EVA crosslinking.Precisely the question should be answered whether the meth-od could be used as a noninvasive tool for determining thisparameter with accuracy good enough for broad application.In a further step this will enable the determination ofcrosslinking inmodules. In difference to methods as discussedin [1] this method is non-destructive and can be appliedcomparatively easy, even in the laminating process duringmodule fabrication and to whole photovoltaic modules.

Experimental: material and methods

Samples

Commercial EVA-foils with four different degrees ofcrosslinking were prepared at the Institute for Solar EnergyResearch Hamelin (ISFH) in a photovoltaic module laminator.To prepare each sample, two 145 mm × 125 mm layers ofEVA are wrapped between two fluorinated polymer sheets toavoid adhesion to laminator parts and heated in the laminatorat a programmed set temperature of 150 °C for 11 to 19 mindepending on the targeted degree of crosslinking. To provideuncured samples with the same texture and conditioning, theuncured EVA samples are prepared by stopping the tempera-ture ramp at 90 °C and maintaining the sample at this temper-ature for 8 min. Immediately after preparation the samples arerapidly cooled over ice blocks and separately wrapped accord-ing to the degree of crosslinking in an airtight and opaquealuminized plastic packaging. The samples are kept in a darkand cooled environment until the luminescence measurement.The luminescence measurement of each EVA-foil is conduct-ed directly after opening the packaging. Once both protectivesheets have been removed, the EVA-foil is analyzed withoutany further preparation.

Determination of the degree of crosslinking

The degree of crosslinking is linearly depending on thecrosslink density, the amount of covalent bonds formed be-tween the chains per volume fraction of material. The shearmodulus G of the material is related to the cross linkingdensity νe by the linear relation:

G ¼ νe⋅R⋅T þ G t0ð Þ

Where R is the universal gas constant, T the temperatureand G(t0) the shear modulus of the uncured molten material.

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The dynamic mechanical analysis (DMA) allows measur-ing the storage modulus G’(t) of a material sample duringcrosslinking. The normalization of G’(t) by G’(t∞), the max-imal storage modulus of the totally crosslinked material al-lows calculating the degree of crosslinking χ over curing timefor a given temperature according to the relation, consideringG’(t0) can be neglected :

χ ¼ G0 tð ÞG0 t∞ð Þ ⋅100

The degree of crosslinking of an 18 mm diameter EVAsample is measured over time under a periodic deformation of0.1 % at 1 Hz with an Anton Paar Physica MCR 301 DMAprogrammed with the lamination temperature curve previous-ly measured in the laminator in different points of the samplearea between the two EVA layers of the sample described inthe sample preparation section. This allows for the calculationof the required lamination time to reach a given degree ofcrosslinking of larger samples.

The curing in the DMA of samples extracted from fivedifferent places of a 0 % laminated foil result in an absoluteuncertainty on the crosslinking grade of 1 %. Inhomogeneityof the temperature of the sample during the lamination processis determined. Curing of samples following the extremacurves lead to an uncertainty relative to the mean temperaturecurve of e.g. 7 %, 4 % and 2 % for degrees of crosslinkingwith nominal expected values of 30 %, 70 % and 95 %.

The crosslinking reaction speed is depending on the tem-perature and the opening of the laminator at the end of theplanned process duration is not instantaneous. To take thisopening time into account, the opening of the laminator isstarted 50 s before the estimated targeted cross-linking gradeis reached, causing the sample to start cooling before the endof the process time. Furthermore, there is an uncertainty on thequickness of the quenching, leading to an uncertainty on theactual curing grade of the material. Uncertainty due to late orpremature quenching is estimated to be on a time span of ±25 s. The resulting error of the degree of cross linking isdetermined following a quadratic distribution.

Thus, considering the above mentioned errors, the samplesinvestigated here have degrees of crosslinking equal to 38%±19 %, 71 %±10 % and 90 %±7 %.

Luminescence measurements

The samples are excited with a UV-Laser at 375 nm. Theluminescence emission of the sample is recorded with a fiberoptics spectrometer [Hamamatsu, CD-mini-spectrometer].Excitation and emission are separated using a dichroic mirrorand a long-pass filter to further suppress the excitation light.No spectral corrections were applied to the data. Both theexcitation and recording optics are integrated into a 3D

scanning system. The setup has been used and describedpreviously by one of the authors [14]. It has been taken careto ensure no background signals from behind the EVA foil aredetected. The excitation energy is chosen to 11.4 mW, theintegration time of the spectrometer is 100 ms. Two spatiallyresolved measurements at two different locations are per-formed for each sample. The scanning resolution is set to122 by 10 spectra across the scanned area of 2.8 cm by0.4 cm for each location.

A LabVIEW program is used to evaluate the data in combi-nation with Mathematica and Matlab routines. For a first over-view of the data the total intensity of every spectrum is calcu-lated and a false-color-map of the scanned area is generated (asin Fig. 1c). Single spectra are obtained by averaging the spectraover a certain area. Errors are reported as the maximum of thestandard deviation which is calculated for every wavelengthsimultaneously with the averaging thus the error is estimated toits upper limit. Since results indicate, that the luminescence ofthe EVA foils does not have a homogeneous distribution, thearea which will be taken into account for spectral evaluation, isselected manually for all samples (according to 4.1.). Theprincipal component analysis (PCA) is computed with thePCA-routine available in the Matlab-Toolbox (VersionR2012a). The data matrix used for the PCA consists of thespectra (800) in the range of 439 to 800 nm. Taking into accountthe resolution of the Spectrometer this yields an array of 800 by852 values. The 800 spectra are the cases of the PCA, each with852 variables. Each spectrum is then displayed as a point in thespace of the first two principal components.

Results

Observation of peculiar spots

As indicated, the luminescence maps of the EVA foils showsome inhomogeneity. Considering the total intensity map ofthe sample in Fig. 1 some pixels appear highly luminescentwhilst the major part of the area appears homogenous. Notonly the intensity but also the shape of the spectra differsstrongly from the average at these pixels. The highly lumines-cent pixels correspond very well with dust and fiber particlesfound on the sample. Even though the samples were handledin optical laboratories it seems that dust particles and e.g.fibers of clothing accumulate on the foil. A systematic remov-al is not possible, because some particles were also containedwithin the foil due to the lamination procedure. For commer-cially produced laminates it can be expected, that this effectwill be negligible. For the precision of the here presentedmethod this means that the precision of the method willincrease when an industrial process quality is available. Forthe laboratory samples investigated here the effect of theseparticles is removed by exclusion of these data points. In the

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following the exclusion procedure will be imposed on all datasets. However, for no sample it is necessary to exclude morethan 15 % of the data.

Correlation of the luminescence and the degree of crosslinking

For the investigated samples it is found that the luminescenceintensity monotonously increases with the degree ofcrosslinking. Figure 2 shows the mean spectra obtained byaveraging over the two distinct sites (see 3.3 Luminescencemeasurements) on the sample. Spectra taken from the twodifferent positions of a same sample have the same color,position 1 as a solid, position 2 as a dotted line. A slightdeviation between the intensities at the two sites can beobserved, whereby the shape of the spectra is similar. Onlyminor deviations of the spectral luminescence distribution areobserved for the samples with crosslinking degrees of 38, 71and 90 %. In contrast the spectra obtained for the 0 % samplesdiffer strongly from all others.

For a quantitative evaluation of the data the Peak intensityat 450 nm of each spectrum in Fig. 2 is plotted in Fig. 3. The

corresponding error bars are determined from the standarddeviation of the measurements on the samples with this degreeof crosslinking. Because the correlation of the luminescenceintensity and the crosslinking of EVA has not yet been de-scribed theoretically a linear correlation is used to fit the data.The Pearson’s correlation coefficient for the data shownhere is 0.996 indicating a strong positive correlation. Atthis point it seems not reasonable to fit the data with amore complex function.

The deviations in luminescence between the two investi-gated sites of each sample may indicate some large-scaleinhomogeneity in the sample or any not yet precisely con-trolled parameter. But the deviations are not beyond the errorin the measurement procedure. The evaluation of more than100 measurements on samples like the ones used here yieldthe statement, that a single measurement will have a relativeintensity error of about 10 %. Another reason for this obser-vation may be the above mentioned particles on the samplesurface. Most of those particles are visible to the naked eyesome only in the UV and some particles are not luminescent(Fig. 1). Due to the current, subjective method of determiningthe peculiarities some minor peculiarities may still have evad-ed the removal procedure.

400 500 600 700 8000

5000

10000

15000

Wavelength nm

Intensity

coun

ts

Fig. 2 Averaged spectra over the scanned area at two different positions(solid and dotted lines), color coded according to the degree ofcrosslinking: black: 0 %, red: 38 %, blue: 71 %, green: 90 %. Thedeviation between the two scanned sites may be taken as an indicationof the error of the measurement procedure

115

0a) Photo of the sample under normal light conditions. (The part of the ring-structure on the left is the support of the sample.)

b) Photo of the same sample area under UV irradiation.

c) Luminescence intensity map of the same area, obtained with the scanning set-up. The data are normalized, the intensity scale is cropped.

Fig. 1 Comparison of visible particles on a 0 %-crosslinked EVA sample (a), a UV-excited photo (b) and the scanned intensity of the same area (c)

0 20 40 60 80 1000

5000

10000

15000

Degreeof Crosslinking

Intensity

coun

ts

Fig. 3 Average of the luminescence intensity at 450 nm for the fourdegrees of crosslinking with a linear fit of the data

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The data in Fig. 3 are fitted with a linear function for thedependence of the measured intensity [I] on the degree ofcrosslinking [χrel]: I=m χrel+b. The fit yielded parametervalues for b=1,700±36 and m=133±8. The function describ-ing the degree of crosslinking [χrel] as a function of themeasured intensity [I] for this particular measurement set-upand this particular EVA thus is:

χrel ¼I − 1700

133

Using Gaussian error propagation for the function of χreland the relative intensity error of about 10 % this yields aquadratic error function as plotted in Fig. 4. The function canbe approximated linearly, estimating the error for the proce-dure to 2 % absolute and 12 % relative error to the degree ofcrosslinking.

Principal component analysis

The spectra of the samples with different degrees ofcrosslinking appear quite similar, especially for the two sam-ples with the highest degrees of crosslinking, 71 % and 90 %(Fig. 2). The principal component analysis (PCA) is used toinvestigate whether further information beyond the lumines-cence intensity is contained in the spectra which may ulti-mately enhance the precision of crosslinking degree identifi-cation. In general, PCA can be used as a supporting tool tolook for eventual underlying information in a data set [17],having a large range of applications beyond the use here [18,19]. Starting from a large set of variables, a new set of linearlyindependent variables is generated. These latter are the prin-cipal components (PCs), among which the first two compo-nents contain most of the information [17]. For the datasetwhich was evaluated here 99.8 % of the variation isexplained by the first and 0.15 % by the second prin-cipal components, so the dimension reduction to twoprinciple components is justified.

To apply the PCA all spectra of the four samples, 800spectra each, are combined in a single dataset. The spectralwavelength range for the PCA is chosen from 439 to 800 nmavoiding artifacts from the excitation light. Each spectrum isdisplayed as a point in the space of the first two principalcomponents (Fig. 5).

All PCA results in Fig. 5 are color and shape codedaccording to the degree of crosslinking. The PCA results ofFig. 5a) show a visible distinction of the different degrees ofcrosslinking. The samples are dominantly ordered with re-spect to their degree of crosslinking along the PC1 axis.Considering the good correlation of luminescence intensityto the degree of crosslinking (see 4.2), the PC1-Axis in Fig. 5may be consequently interpreted as intensity, since the PCA-routine used does not scale the variables to unit standarddeviation - thus the intensity information is retained in thedataset of the principal components. The data points of the

2000 4000 6000 8000 10000 12000 140000

2

4

6

8

10

12

0 30 50 70 100

0

30

50

70

100

Intensity counts

ErrorDeg

reeo

fCrosslinking

Degreeof Crosslinking

Deg

reeo

fCrosslinking

Fig. 4 (Left scale) Error of the degree of crosslinking (in absolute %) as afunction of the measured luminescence intensity. (Right scale) degree ofcrosslinking corresponding to the error scale (e.g. at a crosslinking densityof 70% the error is 10 %). (Top scale) degree of crosslinking correspond-ing to the intensity scale

a) PCA of the spectral data

b) PCA of spectra normalized to maximum

c) PCA of spectra normalized to total area

PC 2

PC 1

0%

30%

70%

95%

PC 2

PC 1

0%

30%

70%

95%

PC 2

PC 1

0%

30%

70%

95%

Fig. 5 EVA samples with 0 %(black), 38 % (red), 71 % (blue)and 90 % (green) crosslinking,each with 800 spectra, plotted inthe space of the two first PCs

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0 %- sample are densely gathered and clearly apart from thedata points of the other samples. These latter are more widelydistributed. Although the 71 %- and 90 %-samples are distin-guishable, the clouds of data points are partially overlapping.This indicates that the measurement of a single spectrum, at asingle location, may not yield reliable information about thedegree of crosslinking. At this point the question remains,whether this is due to a real inhomogeneity of the sample onthe scale of the measured area.

To further investigate whether information about the degreeof crosslinking can be obtained by evaluating the shape of theluminescence spectra we removed the intensity informationfrom the dataset by normalizing the spectra and conductingthe PCA as described above, but with the normalized spectra.For the PCA shown in Fig. 5 b the spectra are normalized totheir maximum at 450 nm, for Fig. 5c) to the area under thespectrum. A clear distinction of the 0 % and 38% samples canbe seen, for the two highest degrees of crosslinking only aslight difference can be observed, more pronounced in Fig. 5c.This finding indicates that adding spectral information willlikely enhance the precision of the method, especially for thelower degrees of crosslinking. Considering the systematicsobserved in the shape and position of the data points accordingto the PCA it may be assumed that it would be possible todetermine the degree of crosslinking with higher quantitativeaccuary when finding suitable spectral parameters comparedto the only use of the maximal intensity.

Conclusions

Four EVA foils with different degrees of crosslinking havebeen investigated using luminescence spectroscopy. The lumi-nescence intensity is found to increase monotonously with thedegree of crosslinking. Applying a linear correlation betweenthe intensity of the spectral peak at 450 nm and the degree ofcrosslinking for the particular EVA investigated here is deter-minable with an error of about 2 % absolute and 12 % relative.

Samples for this investigation were prepared on a labora-tory scale — factors which affect the error of the procedure,such as the dust particles, will be less for industrial-scalephotovoltaic modules, thus errors will likely be lower formeasurements on industrial samples. The ability to discrimi-nate the different degrees of crosslinking by a PCA taking intoaccount 2 principal components suggests that the preci-sion of the method can be further improved byemploying spectral information.

The measurements were conducted with an experimentalset-up which has been used for luminescence investigationson industrial photovoltaic modules. Thus an application of themethod on an industrial photovoltaic module will likely bepossible. However a calibration of the luminescence intensityto the degree of crosslinking will be necessary.

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