RAMAN SPECTROSCOPIC DETERMINATION OF

RAMAN SPECTROSCOPIC DETERMINATION OF THE DEGREE OF ENCAPSULANT CROSSLINKING IN PV MODULES Ch. Hirschl1*, M. Biebl–Rydlo1, W. Mühleisen1, L. Neumaier1, G.C. Eder2, B.S. Chernev3 and M. Kraft1 1 CTR Carinthian Tech Research AG, Europastraße 4/1, 9524 Villach, Austria, E: [email protected], [email protected], [email protected], [email protected] Corresponding author: E: [email protected], T: +43 4242 56300-238; F: +43 4242 56300-400 2 OFI Österreichisches Forschungsinstitut für Chemie und Technik, Franz-Grill-Str. 5, 1030 Wien, Austria, E: [email protected] 3 Austrian Centre for Electron Microscopy and Nanoanalysis Graz and Research Institute for Electron Microscopy and Fine Structure Research, Steyrergasse 17/III, 8010 Graz, Austria. E: [email protected]

With the increasing use of PV installations and the highly competitive situation on the PV module market, the modules’ reliability and long-term performance over projected operational lifetimes of 25+ years become key factors. One component known to have a vital impact on the long-term characteristics, and hence of particular interest here, is the polymeric solar cell encapsulant. Presently, the prevalent material of choice for this is elastomeric ethylene vinyl acetate (EVA). While several analytical methods to evaluate the curing, i.e. crosslinking, state of EVA exist, an industrially applicable in-line quality control method is still lacking. The approach explored in this work uses Raman spectroscopy. In contrast to most other methods, Raman spectroscopy allows a contact-free analysis through the front glass, in-line and in real time in the manufacturing line. The Raman spectra of differently crosslinked EVA samples and modules show subtle but characteristic spectral differences in the CH-stretching vibration region (3.000 – 2.800 cm-1) that correlate unambiguously and reliably to the state of encapsulant crosslinking. Keywords: Photovoltaics, degree of crosslinking, ethylene vinyl acetate, Raman spectroscopy 1

INTRODUCTION With the enhanced competition in the photovoltaic (PV) market over the last years, increasing attention is paid to two major topics: i) cost reduction in production, combined with increased module efficiency, and ii) reliability and long-term performance. To be competitive in the market, PV module manufacturers nowadays have to warrant operational lifetimes of 25+ years with a total yield loss not exceeding 20% [1]. Although a range of off-line and in-line control and analysis methods are being offered for incoming control of PV materials, many quality parameters of relevance to the long time performance are not yet reliably controllable, or even properly defined [2]. The most important component obviously susceptible to aging, and hence likely to critically influence the longterm characteristics, is the polymeric solar cell embedding material. This encapsulation material fulfils several basic functions in the module: it connects the components, provides structural support and mechanical protection to the solar cells (preventing over-stressing and cell cracking) [3], maintains electrical insulation, limits the ingression of ambient media (humidity, oxygen etc.), and is essential to provide an optimal optical coupling between the incident solar irradiation and the solar cells. All these functions have to be maintained over the entire operational lifetime of the module. Though a number of different PV encapsulation materials have been investigated, their general characteristics are all very similar: optically transparent, electrically insulating and soft but dimensionally stable, with good adhesion properties and lasting aging resistance – all at possibly low cost. While new ideas and concepts are constantly being introduced, crosslinked elastomeric ethylene vinyl acetate (EVA) is presently the by far dominating encapsulation material for PV modules. Although the degree of crosslinking of these materials is a crucial process and quality parameter, up to today no analytical method capable of measuring this parameter in-line exists. The standard method to measure the degree

of EVA crosslinking is a Soxhlet–type extraction process [4], which determines the amount of non-linked and hence soluble/leachable polymer. While quite simple in its basic design and procedure, this method has some fundamental disadvantages: •

typical test duration is 24 h, rendering it highly unsuitable for (real-time!) quality control

•

the measurements require samples, which is easy enough for single EVA foils in method development but hard to come by from assembled modules in manufacturing control

•

absolute values may depend substantially on the exact analytical process and parameters, making comparisons problematic

•

no differentiation can be made between singly and multiply crosslinked polymer chains To overcome these shortcomings, several alternative analytical methods based on thermal or mechanical principles have been investigated [5– 14]. Still, until now none of them has proven sufficiently reliable and easy to use to replace the standard method in industrial practice. 13

2

MATERIALS AND METHODS Independent of the manufacturer, the internal structure of commercially available crystalline PV modules is very similar: a solar glass as front cover, two encapsulant layers embedding the solar cells between them, and a polymer backsheet or an additional glass layer. These components are assembled, electrically connected and then laminated using a combination of heat and pressure. For the investigation into the degree of crosslinking, two different setups have been used: single EVA foils (Vistasolar® 486, SolutiaSolar GmbH) and complete, hand-laminated single cell mini-modules comprising these foils.

2.1 Ethylene vinyl acetate EVA in general is a random copolymer of ethylene and vinyl acetate; for PV applications, the percentage of vinyl acetate is typically in the range 28 – 33 % (w/w). Thermoplastic, with a melting range of 60 – 70 °C, mildly opaque, soft and easily plastically deformable, this native EVA material would fulfil neither the mechanical nor the optical requirements requested of an embedment for solar cells. However, by crosslinking the copolymer chains in the course of module lamination, the mouldable EVA sheet is transformed into an elastomeric, highly transparent encapsulation. Crosslinking EVA is feasible only via a radical reaction, using an organic peroxide or peroxycarboxylic acid as radical initiator (“crosslinker”) [1]. This crosslinker is homolytically cleaved into two radical species in a heatinduced initial step. These radicals (R-O•, R’-O• in Fig. 1) then abstract hydrogen from the EVA chains, preferably from terminal methyl groups of the vinyl acetate sidechains. The active radical sites then react with other active sites in their vicinity, creating chemical bonds between the polymer chains and thus transforming the initially thermoplastic EVA into a “cured” three-dimensionally crosslinked elastomer (Fig. 1) [15].

Fig. 1: Basic principle of the main curing reaction of native EVA during radical crosslinking

The degree of crosslinking can thus be controlled by i) the lamination temperature, affecting the amount of crosslinker activated per time unit, ii) the lamination time and iii) the initial concentration of crosslinker. 2.2 Soxhlet Extraction Presently, the degree of EVA crosslinking is usually determined using a Soxhlet extraction method conducted according to the respective ASTM D2765 procedure [4], or some derivative thereof. To validate the results in this work, standard test method A was used: ~ 0.35 g of each EVA specimen were prepared and the exact initial weight (M1) determined on a 0.1 mg analytical precision balance. The specimen was then put into a single-use stainless steel mesh envelope and refluxed for 18 hours in a xylene isomer mixture (puriss. p.a., SigmaAldrich). After this treatment, the non-crosslinked fraction was supposed to be fully dissolved in the solvent and separated from the remaining, crosslinked and hence insoluble, elastomer matrix (“gel”). This insoluble residue was dried in a heated vacuum extractor (60 °C, 50 mbar) for 6 hours, followed by the determination of its net weight (M2). The ratio of the mass of the insoluble residue divided by the initial mass of the original test sample yields the “gel content”:

M  Gel Content [% ] =  2  ⋅100, M 2 ≤ M 1  M1 

To ensure a statistically reliable reference basis, the gel content of each type of EVA test foil (see Section 2.4) was determined on two separate Soxhlet apparatuses for at least four independent samples cut from different sections of each test specimen. 2.3 Raman Spectroscopy A recent study [14] indicated that the most promising analytical tool for non-destructive in-line determination of the degree of crosslinking could be Raman spectroscopy. The measurement of vibrational Raman spectra is a common, well-established method for the determination of the degree of crosslinking of various polymeric materials [16]. The principle exploited there is usually to detect the amount of reactive groups, like unsaturated bonds or isocyanate groups, before, during and after the crosslinking reaction by their characteristic spectral features. Done properly, this yields a direct quantitative value for the extent of crosslinking. In the case of EVA materials, however, the situation is significantly different, as EVA materials do not contain any dedicated crosslinking groups. Instead, the radical crosslinking reaction transforms terminal methyl (-CH3) groups into methylene (-CH2-) groups (see Fig. 1). The only spectroscopic detectable differences would thus be a change in the relative intensities of the characteristic CHx features. Since the extent of crosslinking in the cured state of EVA is low, these changes are expected to be weak, but might still be significant. The Raman system used to validate this fundamental assumption and explore the opportunities and limitations of the method was a Renishaw inVia Raman Microscope RE04 controlled by WiRe 3.4. Spectral post-processing was done partly in the native WiRe software package, partly using Bruker’s OPUS suite. Spectral acquisition was in point spectra mode with 7 s signal integration time, using a 5-fold microscope objective and a 532 nm / 50 mW excitation laser. 2.4 Sample Preparation The lamination process was carried out in a manual laminator following standard lamination conditions. The stacks (glass/EVA/cells/EVA/backsheet) were placed in the pre-heated laminator. The lamination chamber was then evacuated for 4 minutes to raw vacuum levels, while the module stack was shifted to the heating plate. Upon contact, the chamber was evacuated to the final fine vacuum (60 Pa), followed by applying a pressure of ~ 85 kPa to the stack via a pressure plate. Only at that stage the stack made full contact with the heating plate, thus – assumedly – initiating the EVA crosslinking and starting the clock on the lamination time. For the purpose of this study, the lamination temperature kept constant at 144 °C while the lamination time was systematically varied from 0 to 20 minutes (with 10 minutes being the industrial standard). After the end of the set time, the laminator was vented, the panel removed and the sample(s) recovered. Two configurations have been laminated: •

EVA (Vistasolar® 486, SolutiaSolar GmbH) foils between two solar glasses with release paper

•

fully wired single-cell 185 x 185 mm² mini-modules, comprising a 3.2 mm solar front glass (Petra-

glass GmbH), a first EVA sheet (Vistasolar® 486, SolutiaSolar GmbH), the solar cells (BeyondSun), cell connection ribbons (Ulbrich of Austria GmbH), a second EVA sheet, and a polyamide backsheet (Icosolar® AAA 3554, Isovoltaic AG) The investigated samples were: •

native, i.e. uncured EVA

•

EVA treated in the procedure described above until the start of the “lamination time”, subsequently denoted “S0”

•

foil samples S1, S2, S3, S4, S5, S6, S8, S10 and S20, corresponding to 1 – 20 minutes lamination time, respectively

•

mini-module samples M1, M2, M3, M4, M5, M6, M8, M10 and M20, corresponding to 1 – 20 minutes lamination time, respectively

3 RESULTS 3.1 Analysis of EVA crosslinking of foils At first glance, the Raman spectra of differently crosslinked EVA samples show hardly any difference. However, a detail spectroscopic and chemometric analysis [17] reveals subtle but significant spectral changes in the CH-stretching vibration region, i.e. 3000 – 2800 cm-1: with increasing crosslinking, the intensity of the spectral features of the methylene groups increase in intensity relative to the CH3-bands. This agrees well with the assumption that the crosslinking proceeds primarily via a radical reaction of the vinyl acetate’s terminal methyl groups, transforming them into methylene bridges. An example Raman spectrum of a 10 min-cured EVA foil (S10) is shown in Fig. 2. The spectral region found to be most relevant for the determination of the degree of crosslinking is shown in the insert.

proach1 in the mathematical correlation, the results of the Raman method can be unambiguously related to the traditional Soxhlet-derived Gel Content (Fig. 3).

Fig. 3: Degree of crosslinking of EVA foils according to the Raman approach vs. the gel content according to the Soxhlet standard method; the dash-dotted diagonal line represents the expectancy function of the calibration

With only ~1 % of the terminal CH3-groups participating in the crosslinking reaction, the spectral effect underlying the Raman approach is minor, but still sufficiently pronounced to allow building an analytical method on it. This is witnessed in both Fig. 3 and Fig. 4: First, the Soxhlet and the Raman method show an excellent correlation; only the lowest degree of crosslinking measured (S4, ~ 20 % gel content) would appear to be either overpredicted by the Raman method or under-estimated by the Soxhlet method, an observation that will have to be further investigated and eventually validated or corrected. Second, the Raman method shows a poorer analytical accuracy (higher error bars) than the standard Soxhlet method.

Fig. 2: Typical Raman spectrum of an S10 EVA sample; inset: magnification of the CH2/CH3 stretching vibration region

The Raman measurements were performed repeatedly (5 independent, non-sequential repetitions per point) on 9 predefined spots of each 10 x 10 cm² EVA foil specimen, and subjected to different spectroscopic evaluation procedures. In a first, analytical-spectroscopic approach, the ratios of the baseline-corrected band intensities (-CH2 vs. -CH3 feature intensities) were evaluated. In the second method, a chemometric evaluation model (partial least square calibration applied to vector-standardised first derivative spectra) was developed and used. Taking into account the different origin of signals of the Soxhlet standard method and the newly established Raman ap-

Fig. 4: Comparison of the Soxhlet and the Raman degrees of crosslinking of EVA foils vs. lamination time, overlaid with a proposed pseudo 1st order reaction kinetics curve fitted to the data

The inherent reliability of the agreement of the two, analytically completely unrelated, approaches also shows in 1

The Soxhlet method detects the chemical attachment of single chains to the elastomer and hence follows a – also previously observed [14] - pseudo 1st order reaction dependency on the lamination time (dash-dotted curve in Fig. 4). Contrary to this, the Raman approach measures the transformation of the chemical groups involved in the crosslinking reaction and thus – in agreement with the standard reaction model – follows 2nd order reaction kinetics.

a second observation: With the Soxhlet apparatus, the measured degree of crosslinking is zero for lamination times up to 3 minutes; only for lamination times of 4 minutes and more remains an insoluble gel in the sample container. This effect also displays in the Raman spectra: samples S0 to S3 have nearly identical Raman features, and a clear trend vs. lamination time is observable only for samples S4 and above. In a detailed analysis, the reaction kinetics curves of both methods independently predict the full onset of the crosslinking reaction to happen at 200 – 210 s lamination time, i.e. the effective crosslinking time equals lamination time minus 3.5 min. The most likely explanation for this is that the crosslinking in the EVA only fully starts after i) complete melting of the foil and ii) reaching the crosslinker activation temperature (estimated 130 – 140 °C) throughout the EVA. 3.2 Analysis of EVA crosslinking in PV modules To validate the findings of the foil experiments and evaluate the in-situ applicability of the proposed Raman method for foils encased inside an assembled PV module, the experimental procedure was repeated for a series of fully operational PV mini-modules M1 – M20, again manufactured with varied lamination times. A first concern was that the other module components – Si cells, glass etc. – might interfere spectrally, or that the intensity of the CHx bands could be diminished to a point at which the reliability of the method suffers. The comparison of typical Raman spectra of foils and modules (Fig. 5) shows that this is not the case. Though a number of additional bands appear in the module, the evaluated CHx features are not affected. Also the reduction in the band intensity is not that strong that it would cause a significant effect on the analytical figures of merit of the method.

ily compensated for by using standard spectroscopy processing

Fig. 6: Typical Raman spectra measured on the bare cell surface, a finger, a busbar and on the backsheet of an M10 test mini-module

Employing the same spectroscopic data evaluation models developed for the test foils again yields a near-perfect correlation between the degrees of crosslinking determined by the Raman method and the Soxhlet reference values2.

Fig. 7: Degree of crosslinking of EVA foils inside an assembled, fully functional PV mini-module according to the Raman approach vs. the gel content according to the Soxhlet standard method2; the dash-dotted diagonal line represents the expectancy function of the calibration

Fig. 5: Comparison of Raman spectra of an EVA foil and a laminated PV module measured through the front glass in the upper EVA encapsulant layer

A second possible source of error could be the structures of the module components – bare PV cell surface, fingers, busbars etc. Again, an experimental validation showed that the position of measurement is non-critical, as long as the measurement spot sits somewhere on the cell (Fig. 6). While some parts of the spectra change significantly with the selected measurement spot (e.g. the distinct 520 cm-1 Si band present in the on-cell spectra disappears when measuring on a busbar), the relevant CHx region containing the crosslinking information remains unaffected; differences in the baseline can be eas-

Using this calibration function yields an excellent agreement between the Raman-derived data and the corresponding identical reference values also for the in-situ measurement of the EVA curing state inside assembled modules (Fig. 7). Again, the analytical accuracy expressed in the error bars is tentatively better with the standard method, but the other figures of merit – in particular the measurement accessibility and the measurement time and effort – are clearly in favour of the new Raman method. Again the time characteristics show an obvious temporal offset between the nominal lamination times and the effective crosslinking times. To validate the assumption made, an M20 lamination process was monitored in detail using 4 different temperature sensors positioned at differ-

2

The Soxhlet reference data used here were taken from the respective S1 – S20 foil data, due to the impossibility to remove the EVA from the modules after lamination.

ent module component positions: top of the glass, beneath the glass, on the cell and on the backsheet.

6. ACKNOWLEDGMENT This work was conducted as part of the e!mission project EVAnetz. Funding by the Federal Ministries of Transport, Innovation and Technology (BMVIT) and of Economics and Labour (BMWA), managed on their behalf by the Austrian Research Promotion Agency (FFG), is gratefully acknowledged. 7. REFERENCES

Fig. 8: Comparison of the Soxhlet and the Raman degrees of crosslinking of EVA foils inside complete PV mini-modules vs. lamination time, overlaid with a proposed pseudo 1st order reaction kinetics curve fitted to the data

Their data agrees with the assumptions derived from the crosslinking measurements: at the start of the crosslinking time, the temperature on the cell is only ~ 100 °C, and it takes ~ 3.5 minutes for the laminate to be heated through. 4 CONCLUSION Standard Raman spectroscopy proved a most promising approach for realising an in-line instrument for measuring the degree of EVA crosslinking in-situ throughout PV module manufacturing. The method is fast (order of seconds), non-destructive, easy to use (once a calibration model has been established), and requires only a single optically accessible side, i.e. the front glass. On the downside, the method is inherently relative, i.e. requires a constant material composition. This, however, is a reasonable assumption for most PV manufacturing lines that use specific EVA types and qualities only; in fact, the Raman method would act as a quality control detecting deviating raw material composition. Alternatively, regular testing of the uncured material could provide the necessary reference for the data evaluation. While the standard Soxhlet method may be the more precise approach in terms of analytical figures of merit (if executed under carefully optimised and controlled conditions), the much quicker and versatile Raman analysis method gives a significant competitive edge for practical realisation. Regarding instrumentation, cost-effective compact devices suitable for process integration are commercially available. A comprehensive evaluation for each module in a factory line is possible (40 points in 1 minute). 5 OUTLOOK The envisaged next steps will be to characterise further EVA materials in the same way to validate the applicability of the method regardless of different additives and/or slightly varying percentages of vinyl acetate. In addition, work will continue on the spectral data processing and evaluation, in an effort to optimise the methods analytical accuracy to its inherent limits. Furthermore, the Raman instrumentation will be adapted and used for surveying full 60-cell modules, with a special focus in the spatial homogeneity of the degree of EVA crosslinking in the modules.

[1]

Czanderna A.W., Pern F.J., “Encapsulation of PV modules using ethylene vinyl acetate copolymer as a pottant: A critical review”, Sol. Energy Mater. Sol. Cells 43(2): 101181 (1996) http://dx.doi.org/10.1016/0927-0248(95)00150-6

[2]

Kurtz, S., Wohlgemuth, J., Sample, T., Yamamichi, M., Amano, J., Hacke, P., Kempe, M., Kondo, M., Doi, T., Otani, K., "Ensuring quality of PV modules", Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE: 842-847 (2011) http://dx.doi.org/10.1109/PVSC.2011.6186084

[3]

Dietrich, S., Sander, M., Pander, M., Ebert, M., "Interdependency of mechanical failure rate of encapsulated solar cells and module design parameters", Proc. SPIE 8472: 84720P (2012) http://dx.doi.org/10.1117/12.929289

[4]

American Society for Testing and Materials, “Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics”, ASTM D2765 – 11 (2006) http://dx.doi.org/10.1520/D2765-11

[5]

Agroui, K., Belghachi, A., Collins, G., Farenc, J., “Quality control of EVA encapsulant in photovoltaic module process and outdoor exposure”, Desalination 209(1–3): 1 – 9 (2007) http://dx.doi.org/10.1016/j.desal.2007.04.001

[6]

Xia, Z., Cunningham, D., Wohlgemuth, J., “A new method for measuring cross-link density in ethylene vinyl acetatebased encapsulant“, Photovoltaics Int. 5: 150 - 159 (2009)

[7]

Schubnell M., “Investigation of the curing reaction of EVA by DSC and DMA”, Photovoltaics Int. 7: 131 – 137 (2010)

[8]

Stark W., Jaunich M., ”Investigation of Ethylene/Vinyl Acetate Copolymer (EVA) by thermal analysis DSC and DMA“, Polym. Test. 30(2): 236 – 242 (2011) http://dx.doi.org/10.1016/j.polymertesting.2010.12.003

[9]

Hidalgo M., Medlege F., Vite M., Corfias-Zuccalli C., Voarino P., Gonzalez-Leon J., “A new DSC method for the quality control of PV modules: Simple and quick determination of the degree of crosslinking of EVA encapsulants”, Photovoltaics Int. 14: 130 – 140 (2011)

[10] Mühleisen W., Biebl-Rydlo M., Spielberger M., “Determining the degree of EVA crosslinking in assembled PV modules acoustically and in-situ”, Proc. 26th European Photovoltaic Solar Energy Conference: 3480 - 3483 (2011) http://dx.doi.org/10.4229/26thEUPVSEC2011-4AV.1.59 [11] Li, H.Y., Perret-Aebi, L.E., Théron, R., Ballif, C., Luo, Y., Lange, R.F., “Optical transmission as a fast and nondestructive tool for determination of ethylene-co-vinyl acetate curing state in photovoltaic modules”, Prog. Photovolt: Res. Appl. (2011) http://dx.doi.org/10.1002/pip.1175 [12] Oreski G., Pinter G., “Thermo-mechanical characterization of the crosslinking behavior of ethylene vinyl acetate films for solar cell encapsulation”, submitted to Polymer Testing (2013) [13] Mickiewicz R. A., Cahill E., Wu P., “Non-destructive determination of the degree of cross-linking of EVA solar module encapsulation Using DMA shear measurements”,

Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE [14] Ch. Hirschl, M. Biebl–Rydlo, M. DeBiasio, W. Mühleisen, L. Neumaier, W. Scherf, G. Oreski, G. Eder, B. Chernev, W. Schwab, M. Kraft, “Determining the degree of crosslinking of ethylene vinyl acetate photovoltaic module encapsulants—A comparative study”, Solar Energy Materials & Solar Cells 116 (2013): 203-218 http://dx.doi.org/10.1016/j.solmat.2013.04.022 [15] Dluzneski P.R., “Peroxide Vulcanization of Elastomers“, Rubber Chem. Technol. 74(3): 451-492 http://dx.doi.org/10.5254/1.3547647 [16] H. Lobo, J.V. Bonilla, “Handbook of Plastics Analysis”, CRC Press (2003) [17] B. S. Chernev, C. Hirschl and G.C. Eder, Non-destructive determination of ethylene vinyl acetate crosslinking in PV modules by Raman Spectroscopy, Applied Spectroscopy, Vol 67, 11, (2013)