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Characterisation of EVA encapsulant material by thermally stimulated current technique
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 80 (2003) 33–45
Characterisation of EVA encapsulant material by thermally stimulated current technique K. Agrouia,*, G. Collinsb a
Silicon Technology Development Unit UDTS, 2, Bd. Dr. Frantz Fanon B.P. 399, Alger-Gare, 16000 Algiers, Algeria b Molecular Material Science, 11 Ohio, St. Maplewood, NJ 07040, USA Received 20 March 2002; received in revised form 1 March 2003
Abstract The purpose of this investigation is to better define the thermal behaviour of EVA-based encapsulant during photovoltaic module encapsulation process and also in field exposure in desert climate using the thermally stimulated current (TSC) technique. TSC experiments were conducted on EVA in the temperature range from 150 C to 70 C, the measurements were carried out on uncured and cured specimens of EVA and on EVA samples especially prepared using the laminator equipment. When performing the measurements with the TSC instrument it was noted that the EVA exhibits two peaks assigned to dipole relaxation processes. The peak maximum current and the area under the TSC current peak were used for the determination of the glass transition temperature, activation energy and relaxation frequency. For original EVA, we found that glass transition temperature at constant polarisation voltage and under different polarisation temperatures remain unchanged and is located around 38 C. Also, the activation energy has been determined using initial rise method to be about 0.32 eV. At gel content of 70%, the cured EVA shows a reduced integrated area under the depolarisation peak, especially for the high temperature. The combined change in TSC peak parameters of EVA encapsulant is correlated with the degree of curing. r 2003 Elsevier B.V. All rights reserved. Keywords: EVA; Encapsulation process; Thermal characterisation; TSC
*Corresponding author. E-mail address: [email protected] (K. Agroui). 0927-0248/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-0248(03)00112-0
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1. Introduction The polymer encapsulant used in PV modules serves to provide the functions of structural support, electrical isolation, physical isolation/protection and thermal conduction for the solar cell circuit [1]. Normally the encapsulant materials are made of a high insulating polymer to maintain good circuit isolation. Long-term stability of the encapsulant is important for PV modules deployed in the field, where a long service lifetime and reliable power output are needed [2]. Elvax 150 is manufactured and trademarked by Dupont. It is an ethylene vinyl acetate copolymer (33% vinyl acetate). Original EVA refers to stabilised and uncured Elvax 150. The stabilised film is simply referred to as original EVA and is formulated with a curing agent, a UV absorber, a photoantioxidant and a thermoantioxidant [3]. The trade names of these additives and their weight percents added in the formulation are Lupersole 101 (1.5%), Cyasorb UV 531 (0.3%), Tinuvin 770 (0.1%), and Naugard P (0.2%), respectively. The EVA film containing the above additives is one of the polymer materials developed for the encapsulation of crystalline Si-based PV modules. The EVA-based photovoltaic encapsulant is used as an interlayer in the industrial laminating process. The process involves the use of vacuum, heat and pressure to encapsulate a sandwich of materials consisting of a glass superstrate, an EVA film, an assembly of solar cells, another EVA layer and a sheet of Tedlar [4]. Several thermal techniques already exist to analyse, determine performance and check polymer product quality both in the laboratory and in the production site [5]. Thermally stimulated current (TSC) is a more sensitive alternative to other thermal analysis techniques for detecting transitions that depend on changes in mobility of molecular scale structural units and allows the detection of weak glass transition and sub-glass relaxations that sometimes cannot be detected by other thermal analysis techniques such as DSC. The TSC measurements provide information on the molecular scale mobility in the internal structure of a solid material and is based on the motions of dipoles in response to temperature and a static electric field. TSC is well suited to study the degree of curing and the determination of the glass transition temperature. In this paper we have investigated the characterisation of EVA encapsulant material during module manufacturing using the TSC technique. The effect of time and temperature during encapsulation process on the glass transition behaviour of the copolymer is examined and the characteristic parameters are estimated.
2. TSC principle The principle of TSC is to orient polar molecules or pendant polar groups of macromolecules by applying a high-voltage field at an elevated temperature. When an electrical field, E0 ; is applied at the polarisation temperature, Tp ; during a time, tp ; longer than the relaxation time, tðTp Þ; these dipoles are polarised. By cooling the material to temperature, T0 ; where T0 {Tp ; the dipole orientation is quenched into
ARTICLE IN PRESS K. Agroui, G. Collins / Solar Energy Materials & Solar Cells 80 (2003) 33–45
35
Fig. 1. Principle of TSC.
the structure because the relaxation time at this low temperature, t(T0 ), is long. Hence the polarisation remains in the sample after the electrical field is cut off. As the dielectric is heated, the relaxation time as a function of temperature, tðTÞ; decreases and motion of the dipoles is induced. The dielectric is connected to an electrometer, and the motion of the relaxing dipoles is responsible for the observed depolarisation current, IðTÞ: The dipolar motion as manifested as depolarisation current will appear at temperatures where relaxation processes take place and will depend on the morphological structure of the material [6]. Fig. 1 illustrates the principle of TSC.
3. Theoretical background of TSC The current density released during thermally stimulated current measurement can be expressed in the general form [7] Z W B T W JðTÞ ¼ A exp exp exp dT 0 ; ð1Þ kT q T0 kT where A and B are the constant of the process, W the activation energy of the process, T and T 0 the temperature, T0 the initial temperature, k Boltzman’s constant and q the heating rate. The relaxation process during a glass transition is typically the superposition of several relaxation processes. For the determination of the characteristic parameters of the process, i.e. the activation energy and the relaxation time factor, the initial rise method is mostly used. This method is based on the fact that the integral term in Eq. (1) is low for T{Tmax ; as a result, only the lower-temperature processes contribute and the first exponential factor in Eq (1) dominates at the initial low-temperature portion of the TSC curve. In this situation the current density during a thermal-stimulated
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measurement can be expressed by the Arrhenius equation W : JðTÞ ¼ A exp kT
ð2Þ
The activation energy can be determined by the slope of the plot lnðJðTÞ) vs. 1=T: The relaxation frequency aðTmax Þ at the peak temperature Tmax is defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qW Tmax ¼ : ð3Þ kaðTmax Þ The Debye formalism that is represented in Expressions (1)–(3) strictly only applies for a single relaxation process. That is to say, the equations can only be applied for an isolated relaxation. The global polarisation experiments that were performed produce a depolarisation current that represents the entire distribution of relaxations. In our study we applied Eq. (2) to the initial low-temperature portion of the relaxation process in order to get a first approximation of the magnitude of the activation energy. We used that activation energy value to estimate the relaxation frequency at Tmax using Eq. (3).
4. Experimental conditions 4.1. Samples preparation Samples were prepared with commercial EVA standards based on 9918P grade. The process is executed using the SPI-LAMINATOR 240 programmed to perform the complete cycle. For EVA-based encapsulant one cycle that has been proven uses two steps: 1. Lamination: 100 C for 5 min to permit evacuation and adhesion. 2. Cure: 160 C for 25 min. During the first 8–10 min the encapsulant temperature is raised to 160 C, where it is maintained to the end of cycle. Samples prepared with commercial EVA standards based on 9918P were inserted between two pieces of Teflon and placed on a piece of glass of the same size as used for the actual module in order to produce the correct thermal conditions during the cure. Samples submitted to the lamination step and to both lamination and cure steps of the heating cycle are referred to as laminated and cured EVA, respectively. 4.2. Gel content measurements First, all samples are subjected to a gel content measurement which represents the amount of crosslinking occurring during the cure. Gel tests are useful in determining the proper cure cycle. The Work done at Springborn Laboratory recommends a
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Table 1 Gel content measurements of EVA during thermal processing Sample
Gel content (%)
Original EVA Laminated EVA Cured EVA
0 0 70
40 20
T f = Tp + 10°C
Tp = 0˚C
Temperature (°C)
0 -20 -40 -60 -80 -100 -120 -140 -160 0
5
10
15
20
25
30
Time (minutes)
Fig. 2. Thermal program for the TSC experiment.
minimum acceptable gel content of 65%. The results of our analysis are given in Table 1. 4.3. Experimental set-up The instrument used for the study is TSC/RMA Model 9000 from Thermold, LP at Stamford, Connecticut (USA). The global TSC experiments were carried out following the procedure described in Section 2 and using the parameters schematised in Fig. 2. A voltage of 200 VDC is applied across the sample in order to align the dipoles in the internal structure at temperature Tp : The electric field is about 0.44 106 V/m. The alignment is frozen in by rapidly lowering the temperature to 130 C. During the heating process from T0 ; the depolarisation current was read by a sensitive electrometer which is able to read a current as small as 1017 A. The field is turned off, and the sample is heated using a linear temperature ramp (7 C/min). When the temperature is reached the dipoles will lose the imposed orientation by relaxing to the equilibrium state. The disorientation process produces a depolarisation current that is detected by the TSC instrument.1 Basically all experiments were 1
TSC-RMA: a fully automated instrument for thermally stimulated current and relaxation map analysis, from Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA.
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conducted by cutting a small specimen, about 8 8 mm2, from the large sample piece. The thickness of the specimen is 0.45 mm.
5. Results and discussion When performing the measurements it was noted that the original EVA exhibits two peaks representing two relaxation processes. In order to assure that the peaks we observed were dipole relaxation processes, we ran original EVA under various conditions. Fig. 3 shows the TSC for this material using polarisation temperatures 0 C, 25 C and 60 C. The repeatability of the low-temperature peak at these different polarisation temperatures is an indicator that this peak represents the glass transition temperature, Tg : Figs. 4–6 provide the TSC results for each sample polarised at 60 C. For each material there appears to be a low-temperature relaxation process and a hightemperature process. Original EVA (Fig. 4) has a low-temperature peak at about 37 C, and a high-temperature peak at about 30 C. The high-temperature peak appears to have a shoulder above 50 C. Laminated EVA (Fig. 5) has a lowtemperature peak at about 35 C, and high-temperature at about 26 C. Cured EVA (Fig. 6) has a somewhat broad low-temperature peak at about 38 C, and a hightemperature peak at about 26 C. The significance of the peaks that are observed in these experiments relates to the modes of structural relaxation that occur as the temperature is increased. The observation of two relaxations suggests that the EVA material is heterogeneous with relaxations occuring in two distinct domains. When a material undergoes a structural relaxation at the glass transition, the specimen between the electrodes will sometimes physically move because of stress relaxation. This physical movement can be detected as noise-like features. For some 4x10-12
Current (A)
3x10-12
Tp = 60° C Tp = 25° C Tp = 0° C
2x10-12
1x10-12
0 -160 -140 -120 -100 -80 - 60 -40 -20
0
20 40 60 80 100
Temperature (°C)
Fig. 3. TSC spectra for original EVA at various polarisation temperatures.
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-12
4x10
-12
Current (A)
3x10
-12
2x10
-12
1x10
0
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 4. TSC spectra for original EVA at 60 C polarisation temperature.
-12
9x10
-12
8x10
-12
7x10
-12
Current (A)
6x10
-12
5x10
-12
4x10
-12
3x10
-12
2x10
-12
1x10
0 -12
-1x10
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 5. TSC spectra for laminated EVA at 60 C polarisation temperature.
specimens the noise-like features that appear at higher temperatures have been eliminated using a sliding average method. The investigation of the relaxations in the low-temperature region was of particular interest. In order to examine this region more carefully, each sample was run using a polarisation temperature of 0 C. Figs. 7–9 show those results for each sample material. For this series of experiments, original EVA (Fig. 7) has a peak at about 36 C; laminated EVA (Fig. 8) has a peak at about 36 C; and cured EVA (Fig. 9) has a peak at about 37 C [8]. The results at two polarisation temperatures are summarised in Tables 2 and 3. In Figs. 10–12 we illustrate the plot of lnðJÞ as a function of 1=T of original, laminated
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5x10
-12
4x10
-12
Current (A)
3x10
-12
2x10
-12
1x10
0 -160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 6. TSC spectra for cured EVA at 60 C polarisation temperature.
-13
3x10
-13
Current (A)
2x10
-13
1x10
0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 7. TSC spectra for original EVA at 0 C polarisation temperature.
and cured EVA, respectively. Table 4 summarises the results of the characteristic parameters. The activation energy, relaxation frequency at Tmax and integrated current of cured EVA decreased due to crosslinking effect [9,10]. The changes indicated in Table 4 are not large enough to be significant. Also, using the data from Figs. 4 and 6 and applying a factor of 1.6 to the uncured data in order to normalise the results, the comparison of uncured and cured results are indicated in Fig. 13. The observation of two peaks suggests that the EVA is heterogeneous. There are two phases that are detected: a low-temperature phase and a high-temperature phase. Also, we observe a decrease in current intensity for the cured EVA particularly for the high-glass-transition temperature. There is a large
ARTICLE IN PRESS K. Agroui, G. Collins / Solar Energy Materials & Solar Cells 80 (2003) 33–45
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-13
4x10
-13
Current (A)
3x10
-13
2x10
-13
1x10
0
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 8. TSC spectra for laminated EVA at 0 C polarisation temperature.
-13
3x10
-13
Current (A)
2x10
-13
1x10
0 -160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 9. TSC spectra for cured EVA at 0 C polarisation temperature.
Table 2 Peak positions at 60 C polarisation temperature Sample
Low temperature ( C)
High temperature ( C)
Original EVA Laminated EVA Cured EVA
37 35 38
30 (50) 26 26
reduction in the depolarisation current that is especially apparent for the hightemperature peak. However, it is clear that in the high-temperature transition, there is a distinct difference between the cured and the uncured EVA. This strongly
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Table 3 Peak positions at 0 C polarization temperature Sample
Low temperature ( C)
Original EVA Laminated EVA Cured EVA
36 36 37
-18 -19
Original EVA sample W = 0.319 eV
ln(J) [A/m2]
-20 -21 -22 -23 -24 4.0
4.5
5.0
5.5
-1 1000/T [˚K ]
Fig. 10. Plot of lnðJÞ vs. 1=T for original EVA. -18 Laminated EVA sample W = 0.3148 eV
-19
ln(J) [A/m2]
-20
-21
-22
-23
-24 4.0
4.5
5.0
5.5
-1
1000/T [°K ]
Fig. 11. Plot of lnðJÞ vs. 1=T for laminated EVA.
suggests that the EVA is cured selectively in the high-temperature phase, and this is consistent with the large reduction in depolarisation current for the hightemperature Tg in the TSC data.
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-18 Cured EVA sample W = 0.281 eV
-19
ln(J) [A/m2]
-20 -21 -22 -23 -24
4.0
4.5
5.0
5.5
-1
1000/T [°K ] Fig. 12. Plot of lnðJÞ vs. 1=T for cured EVA.
Table 4 Main results based on TSC measurements at 0 C polarisation temperature Sample
Tmax ( C)
Peak current ( 1013) (A)
W (eV)
aðTmax Þ ( 103) (Hz)
Integrated current ( 1012) (A C)
Original EVA Laminated EVA Cured EVA
36 36 37
2.57 3.17 2.80
0.319 0.315 0.281
7.68 7.58 6.80
8.62 7.51 8.39
-12
6x10
-12
5x10
Original EVA (x1.6) Cured EVA
Current (A)
-12
4x10
3x10-12 -12
2x10
-12
1x10
0
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80
Temperature (°C)
Fig. 13. Field normalised TSC results for original and cured EVA at 0 C polarisation temperature.
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3.0x10
-12
Before exposition After exposition
2.5x10
-12
Current (A)
2.0x10
-12
1.5x10
-12
1.0x10
-13
5.0x10
0.0 -100
-80
-60
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 14. TSC spectra for cured EVA before and after outdoor exposure at 60 C polarisation temperature.
Fig. 14 shows the TSC spectra of cured EVA at polarisation temperature of 60 C before and after exposure at the PV plant of Adrar (south-west of Algeria) under desert environmental conditions characterised by high irradiance and temperature levels for 8 years of continuous exposure. This figure suggests that prolonged exposure selectively effects the poly(vinyl acetate)-rich phase, with much less impact on the polyethylene-rich phase. We found that the exposed cured EVA encapsulant showed considerable decrease in current intensity for the high-temperature peak. The difference of the magnitude of peak current suggests increased crosslinking occurring selectively in the high-temperature phase as a result of exposure [11].
6. Conclusion TSC is a very sensitive probe of polymer relaxation processes. The polymer literature indicates that polyethylene can have a Tg in the region of 30 C, and poly(vinyl acetate) can have a Tg in the range of 25–39 C. TSC experiments on EVA samples show a low-temperature peak at 37 C, and a high-temperature peak at 30 C for original EVA. For cured EVA we reported a low-temperature peak at 38 C and a high-temperature peak at 26 C. These are close enough to the literature values for the pure components to suggest that the low-temperature peak is the Tg for a polyethylene-rich phase, and the high-temperature peak is the Tg for a poly(vinyl acetate)-rich phase. All the sample materials appeared to have reproducible TSC peaks in the low- and high-temperature regions. The EVA samples all showed very similar behaviour showing a single low-temperature and a single high-temperature peak. We found that by using TSC, reproducible experiments could be carried out when polarisation temperature was 0 C and 60 C. The repeatability of the low-temperature peak at these different polarisation temperatures is an indicator that this peak results from dipole relaxation. Similarly,
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the evolution of the high-temperature peak is also suggestive that it originates in dipole relaxation behaviour and is not an artefact. It can be difficult to probe the level of cure in EVA and its laminate products. The TSC technique allows the level of cure of EVA and its laminated components to be determined in a facile manner. These results demonstrate the utility of TSC for this purpose. We show that during PV module encapsulation process, the TSC peak of cured EVA is characterised by a decrease in its current intensity and energy activation induced by the effect of crosslinking.
References [1] K.J. Lewis, Encapsulant material requirements for photovoltaic modules, in: C.G. Geblein, D.J. Williams, R.D. Deanin (Eds.), Polymers in Solar energy Utilisation, ACS, Washington, DC, 1983, pp. 367–385 (Chapter 23). [2] F.J. Pern, A.W.C. Zanderma, Characterisation of ethylene vinyl acetate (EVA) encapsulant effects on thermal processing and weatherability degradation on its discoloration, Sol. Energy Mater. Sol. Cells 25 (1992) 3–23. [3] K. Agroui, Y. Mesbahi, F. Moussa, Study of electrical properties of EVA during PV module encapsulation process, Proceedings of the Second World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria, 6–10 July 1998. [4] K. Agroui, N. Benrekaa, Characterisation of EVA for photovoltaic application, Proceedings of the 11th International Conference on Microelectronic (ICM’99), Kuwait City, 22–24 November 1999, IEEE Catalogue Number 99EX388, pp. 271–274. [5] J.R. Saffell, A. Matthiesen, R. McIntyre, J.P. Ibar, Comparing thermal stimulated current (TSC) with other thermal analytical methods to characterize the amorphous phase of polymers a, Thermochim. Acta 192 (1991) 243–264. [6] K. Agroui, Y. Segui, J. Farenc, N. Benrekaa, Characterisation of EVA encapsulant by glass transition temperature method for photovoltaic application, International Conference on Energy (IEC 2000), University of El-Ain, UAE, 7–9 May 2000. [7] E.R. Neagu, et al., Analysis of the thermally stimulated discharge current around glass–rubber transition temperature in PET, J. Appl. Phys. 82(5) (1997) 2488–2496. [8] G. Collins, EVA Test Report, 17 January, Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA, 2001. [9] L. Zong-Fu, R.E. Callahan, C. Hanson, J.M. Howard III, In situ cure characterisation of coatings by thermally stimulated current (TSC): spectroscopy and relaxation map analysis (RMA), TSC Application note, 1995. Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA. [10] J.J. lieari, L.A. Hughes, Functions of Coating in Handbook of Polymer Coatings for Electronics Chemistry, Technology and Applications, Noyes Publications, 1990, pp. 103–149 (Chapter 4). [11] R.H. Liang, S. Chung, A. Clayton, S.D.I. Stefeno, K. Oda, S.D. Hong, A. Gupta, Photochemical degradation of ethylene acetate copolymer, Polymer Science and Technology, Plenum, 1983, 20, Polymer Alloys 3: Blends, Blocks, Grafts, Interpenetrating Networks, pp. 267–278.
Solar Energy Materials & Solar Cells 80 (2003) 33–45
Characterisation of EVA encapsulant material by thermally stimulated current technique K. Agrouia,*, G. Collinsb a
Silicon Technology Development Unit UDTS, 2, Bd. Dr. Frantz Fanon B.P. 399, Alger-Gare, 16000 Algiers, Algeria b Molecular Material Science, 11 Ohio, St. Maplewood, NJ 07040, USA Received 20 March 2002; received in revised form 1 March 2003
Abstract The purpose of this investigation is to better define the thermal behaviour of EVA-based encapsulant during photovoltaic module encapsulation process and also in field exposure in desert climate using the thermally stimulated current (TSC) technique. TSC experiments were conducted on EVA in the temperature range from 150 C to 70 C, the measurements were carried out on uncured and cured specimens of EVA and on EVA samples especially prepared using the laminator equipment. When performing the measurements with the TSC instrument it was noted that the EVA exhibits two peaks assigned to dipole relaxation processes. The peak maximum current and the area under the TSC current peak were used for the determination of the glass transition temperature, activation energy and relaxation frequency. For original EVA, we found that glass transition temperature at constant polarisation voltage and under different polarisation temperatures remain unchanged and is located around 38 C. Also, the activation energy has been determined using initial rise method to be about 0.32 eV. At gel content of 70%, the cured EVA shows a reduced integrated area under the depolarisation peak, especially for the high temperature. The combined change in TSC peak parameters of EVA encapsulant is correlated with the degree of curing. r 2003 Elsevier B.V. All rights reserved. Keywords: EVA; Encapsulation process; Thermal characterisation; TSC
*Corresponding author. E-mail address: [email protected] (K. Agroui). 0927-0248/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-0248(03)00112-0
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1. Introduction The polymer encapsulant used in PV modules serves to provide the functions of structural support, electrical isolation, physical isolation/protection and thermal conduction for the solar cell circuit [1]. Normally the encapsulant materials are made of a high insulating polymer to maintain good circuit isolation. Long-term stability of the encapsulant is important for PV modules deployed in the field, where a long service lifetime and reliable power output are needed [2]. Elvax 150 is manufactured and trademarked by Dupont. It is an ethylene vinyl acetate copolymer (33% vinyl acetate). Original EVA refers to stabilised and uncured Elvax 150. The stabilised film is simply referred to as original EVA and is formulated with a curing agent, a UV absorber, a photoantioxidant and a thermoantioxidant [3]. The trade names of these additives and their weight percents added in the formulation are Lupersole 101 (1.5%), Cyasorb UV 531 (0.3%), Tinuvin 770 (0.1%), and Naugard P (0.2%), respectively. The EVA film containing the above additives is one of the polymer materials developed for the encapsulation of crystalline Si-based PV modules. The EVA-based photovoltaic encapsulant is used as an interlayer in the industrial laminating process. The process involves the use of vacuum, heat and pressure to encapsulate a sandwich of materials consisting of a glass superstrate, an EVA film, an assembly of solar cells, another EVA layer and a sheet of Tedlar [4]. Several thermal techniques already exist to analyse, determine performance and check polymer product quality both in the laboratory and in the production site [5]. Thermally stimulated current (TSC) is a more sensitive alternative to other thermal analysis techniques for detecting transitions that depend on changes in mobility of molecular scale structural units and allows the detection of weak glass transition and sub-glass relaxations that sometimes cannot be detected by other thermal analysis techniques such as DSC. The TSC measurements provide information on the molecular scale mobility in the internal structure of a solid material and is based on the motions of dipoles in response to temperature and a static electric field. TSC is well suited to study the degree of curing and the determination of the glass transition temperature. In this paper we have investigated the characterisation of EVA encapsulant material during module manufacturing using the TSC technique. The effect of time and temperature during encapsulation process on the glass transition behaviour of the copolymer is examined and the characteristic parameters are estimated.
2. TSC principle The principle of TSC is to orient polar molecules or pendant polar groups of macromolecules by applying a high-voltage field at an elevated temperature. When an electrical field, E0 ; is applied at the polarisation temperature, Tp ; during a time, tp ; longer than the relaxation time, tðTp Þ; these dipoles are polarised. By cooling the material to temperature, T0 ; where T0 {Tp ; the dipole orientation is quenched into
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Fig. 1. Principle of TSC.
the structure because the relaxation time at this low temperature, t(T0 ), is long. Hence the polarisation remains in the sample after the electrical field is cut off. As the dielectric is heated, the relaxation time as a function of temperature, tðTÞ; decreases and motion of the dipoles is induced. The dielectric is connected to an electrometer, and the motion of the relaxing dipoles is responsible for the observed depolarisation current, IðTÞ: The dipolar motion as manifested as depolarisation current will appear at temperatures where relaxation processes take place and will depend on the morphological structure of the material [6]. Fig. 1 illustrates the principle of TSC.
3. Theoretical background of TSC The current density released during thermally stimulated current measurement can be expressed in the general form [7] Z W B T W JðTÞ ¼ A exp exp exp dT 0 ; ð1Þ kT q T0 kT where A and B are the constant of the process, W the activation energy of the process, T and T 0 the temperature, T0 the initial temperature, k Boltzman’s constant and q the heating rate. The relaxation process during a glass transition is typically the superposition of several relaxation processes. For the determination of the characteristic parameters of the process, i.e. the activation energy and the relaxation time factor, the initial rise method is mostly used. This method is based on the fact that the integral term in Eq. (1) is low for T{Tmax ; as a result, only the lower-temperature processes contribute and the first exponential factor in Eq (1) dominates at the initial low-temperature portion of the TSC curve. In this situation the current density during a thermal-stimulated
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measurement can be expressed by the Arrhenius equation W : JðTÞ ¼ A exp kT
ð2Þ
The activation energy can be determined by the slope of the plot lnðJðTÞ) vs. 1=T: The relaxation frequency aðTmax Þ at the peak temperature Tmax is defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qW Tmax ¼ : ð3Þ kaðTmax Þ The Debye formalism that is represented in Expressions (1)–(3) strictly only applies for a single relaxation process. That is to say, the equations can only be applied for an isolated relaxation. The global polarisation experiments that were performed produce a depolarisation current that represents the entire distribution of relaxations. In our study we applied Eq. (2) to the initial low-temperature portion of the relaxation process in order to get a first approximation of the magnitude of the activation energy. We used that activation energy value to estimate the relaxation frequency at Tmax using Eq. (3).
4. Experimental conditions 4.1. Samples preparation Samples were prepared with commercial EVA standards based on 9918P grade. The process is executed using the SPI-LAMINATOR 240 programmed to perform the complete cycle. For EVA-based encapsulant one cycle that has been proven uses two steps: 1. Lamination: 100 C for 5 min to permit evacuation and adhesion. 2. Cure: 160 C for 25 min. During the first 8–10 min the encapsulant temperature is raised to 160 C, where it is maintained to the end of cycle. Samples prepared with commercial EVA standards based on 9918P were inserted between two pieces of Teflon and placed on a piece of glass of the same size as used for the actual module in order to produce the correct thermal conditions during the cure. Samples submitted to the lamination step and to both lamination and cure steps of the heating cycle are referred to as laminated and cured EVA, respectively. 4.2. Gel content measurements First, all samples are subjected to a gel content measurement which represents the amount of crosslinking occurring during the cure. Gel tests are useful in determining the proper cure cycle. The Work done at Springborn Laboratory recommends a
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Table 1 Gel content measurements of EVA during thermal processing Sample
Gel content (%)
Original EVA Laminated EVA Cured EVA
0 0 70
40 20
T f = Tp + 10°C
Tp = 0˚C
Temperature (°C)
0 -20 -40 -60 -80 -100 -120 -140 -160 0
5
10
15
20
25
30
Time (minutes)
Fig. 2. Thermal program for the TSC experiment.
minimum acceptable gel content of 65%. The results of our analysis are given in Table 1. 4.3. Experimental set-up The instrument used for the study is TSC/RMA Model 9000 from Thermold, LP at Stamford, Connecticut (USA). The global TSC experiments were carried out following the procedure described in Section 2 and using the parameters schematised in Fig. 2. A voltage of 200 VDC is applied across the sample in order to align the dipoles in the internal structure at temperature Tp : The electric field is about 0.44 106 V/m. The alignment is frozen in by rapidly lowering the temperature to 130 C. During the heating process from T0 ; the depolarisation current was read by a sensitive electrometer which is able to read a current as small as 1017 A. The field is turned off, and the sample is heated using a linear temperature ramp (7 C/min). When the temperature is reached the dipoles will lose the imposed orientation by relaxing to the equilibrium state. The disorientation process produces a depolarisation current that is detected by the TSC instrument.1 Basically all experiments were 1
TSC-RMA: a fully automated instrument for thermally stimulated current and relaxation map analysis, from Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA.
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conducted by cutting a small specimen, about 8 8 mm2, from the large sample piece. The thickness of the specimen is 0.45 mm.
5. Results and discussion When performing the measurements it was noted that the original EVA exhibits two peaks representing two relaxation processes. In order to assure that the peaks we observed were dipole relaxation processes, we ran original EVA under various conditions. Fig. 3 shows the TSC for this material using polarisation temperatures 0 C, 25 C and 60 C. The repeatability of the low-temperature peak at these different polarisation temperatures is an indicator that this peak represents the glass transition temperature, Tg : Figs. 4–6 provide the TSC results for each sample polarised at 60 C. For each material there appears to be a low-temperature relaxation process and a hightemperature process. Original EVA (Fig. 4) has a low-temperature peak at about 37 C, and a high-temperature peak at about 30 C. The high-temperature peak appears to have a shoulder above 50 C. Laminated EVA (Fig. 5) has a lowtemperature peak at about 35 C, and high-temperature at about 26 C. Cured EVA (Fig. 6) has a somewhat broad low-temperature peak at about 38 C, and a hightemperature peak at about 26 C. The significance of the peaks that are observed in these experiments relates to the modes of structural relaxation that occur as the temperature is increased. The observation of two relaxations suggests that the EVA material is heterogeneous with relaxations occuring in two distinct domains. When a material undergoes a structural relaxation at the glass transition, the specimen between the electrodes will sometimes physically move because of stress relaxation. This physical movement can be detected as noise-like features. For some 4x10-12
Current (A)
3x10-12
Tp = 60° C Tp = 25° C Tp = 0° C
2x10-12
1x10-12
0 -160 -140 -120 -100 -80 - 60 -40 -20
0
20 40 60 80 100
Temperature (°C)
Fig. 3. TSC spectra for original EVA at various polarisation temperatures.
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-12
4x10
-12
Current (A)
3x10
-12
2x10
-12
1x10
0
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 4. TSC spectra for original EVA at 60 C polarisation temperature.
-12
9x10
-12
8x10
-12
7x10
-12
Current (A)
6x10
-12
5x10
-12
4x10
-12
3x10
-12
2x10
-12
1x10
0 -12
-1x10
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 5. TSC spectra for laminated EVA at 60 C polarisation temperature.
specimens the noise-like features that appear at higher temperatures have been eliminated using a sliding average method. The investigation of the relaxations in the low-temperature region was of particular interest. In order to examine this region more carefully, each sample was run using a polarisation temperature of 0 C. Figs. 7–9 show those results for each sample material. For this series of experiments, original EVA (Fig. 7) has a peak at about 36 C; laminated EVA (Fig. 8) has a peak at about 36 C; and cured EVA (Fig. 9) has a peak at about 37 C [8]. The results at two polarisation temperatures are summarised in Tables 2 and 3. In Figs. 10–12 we illustrate the plot of lnðJÞ as a function of 1=T of original, laminated
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5x10
-12
4x10
-12
Current (A)
3x10
-12
2x10
-12
1x10
0 -160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80 100
Temperature (°C)
Fig. 6. TSC spectra for cured EVA at 60 C polarisation temperature.
-13
3x10
-13
Current (A)
2x10
-13
1x10
0 -140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 7. TSC spectra for original EVA at 0 C polarisation temperature.
and cured EVA, respectively. Table 4 summarises the results of the characteristic parameters. The activation energy, relaxation frequency at Tmax and integrated current of cured EVA decreased due to crosslinking effect [9,10]. The changes indicated in Table 4 are not large enough to be significant. Also, using the data from Figs. 4 and 6 and applying a factor of 1.6 to the uncured data in order to normalise the results, the comparison of uncured and cured results are indicated in Fig. 13. The observation of two peaks suggests that the EVA is heterogeneous. There are two phases that are detected: a low-temperature phase and a high-temperature phase. Also, we observe a decrease in current intensity for the cured EVA particularly for the high-glass-transition temperature. There is a large
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-13
4x10
-13
Current (A)
3x10
-13
2x10
-13
1x10
0
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 8. TSC spectra for laminated EVA at 0 C polarisation temperature.
-13
3x10
-13
Current (A)
2x10
-13
1x10
0 -160
-140
-120
-100
-80
-60
-40
-20
0
20
40
Temperature (°C)
Fig. 9. TSC spectra for cured EVA at 0 C polarisation temperature.
Table 2 Peak positions at 60 C polarisation temperature Sample
Low temperature ( C)
High temperature ( C)
Original EVA Laminated EVA Cured EVA
37 35 38
30 (50) 26 26
reduction in the depolarisation current that is especially apparent for the hightemperature peak. However, it is clear that in the high-temperature transition, there is a distinct difference between the cured and the uncured EVA. This strongly
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Table 3 Peak positions at 0 C polarization temperature Sample
Low temperature ( C)
Original EVA Laminated EVA Cured EVA
36 36 37
-18 -19
Original EVA sample W = 0.319 eV
ln(J) [A/m2]
-20 -21 -22 -23 -24 4.0
4.5
5.0
5.5
-1 1000/T [˚K ]
Fig. 10. Plot of lnðJÞ vs. 1=T for original EVA. -18 Laminated EVA sample W = 0.3148 eV
-19
ln(J) [A/m2]
-20
-21
-22
-23
-24 4.0
4.5
5.0
5.5
-1
1000/T [°K ]
Fig. 11. Plot of lnðJÞ vs. 1=T for laminated EVA.
suggests that the EVA is cured selectively in the high-temperature phase, and this is consistent with the large reduction in depolarisation current for the hightemperature Tg in the TSC data.
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-18 Cured EVA sample W = 0.281 eV
-19
ln(J) [A/m2]
-20 -21 -22 -23 -24
4.0
4.5
5.0
5.5
-1
1000/T [°K ] Fig. 12. Plot of lnðJÞ vs. 1=T for cured EVA.
Table 4 Main results based on TSC measurements at 0 C polarisation temperature Sample
Tmax ( C)
Peak current ( 1013) (A)
W (eV)
aðTmax Þ ( 103) (Hz)
Integrated current ( 1012) (A C)
Original EVA Laminated EVA Cured EVA
36 36 37
2.57 3.17 2.80
0.319 0.315 0.281
7.68 7.58 6.80
8.62 7.51 8.39
-12
6x10
-12
5x10
Original EVA (x1.6) Cured EVA
Current (A)
-12
4x10
3x10-12 -12
2x10
-12
1x10
0
-160 -140 -120 -100 -80 -60 -40 -20
0
20
40
60
80
Temperature (°C)
Fig. 13. Field normalised TSC results for original and cured EVA at 0 C polarisation temperature.
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3.0x10
-12
Before exposition After exposition
2.5x10
-12
Current (A)
2.0x10
-12
1.5x10
-12
1.0x10
-13
5.0x10
0.0 -100
-80
-60
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 14. TSC spectra for cured EVA before and after outdoor exposure at 60 C polarisation temperature.
Fig. 14 shows the TSC spectra of cured EVA at polarisation temperature of 60 C before and after exposure at the PV plant of Adrar (south-west of Algeria) under desert environmental conditions characterised by high irradiance and temperature levels for 8 years of continuous exposure. This figure suggests that prolonged exposure selectively effects the poly(vinyl acetate)-rich phase, with much less impact on the polyethylene-rich phase. We found that the exposed cured EVA encapsulant showed considerable decrease in current intensity for the high-temperature peak. The difference of the magnitude of peak current suggests increased crosslinking occurring selectively in the high-temperature phase as a result of exposure [11].
6. Conclusion TSC is a very sensitive probe of polymer relaxation processes. The polymer literature indicates that polyethylene can have a Tg in the region of 30 C, and poly(vinyl acetate) can have a Tg in the range of 25–39 C. TSC experiments on EVA samples show a low-temperature peak at 37 C, and a high-temperature peak at 30 C for original EVA. For cured EVA we reported a low-temperature peak at 38 C and a high-temperature peak at 26 C. These are close enough to the literature values for the pure components to suggest that the low-temperature peak is the Tg for a polyethylene-rich phase, and the high-temperature peak is the Tg for a poly(vinyl acetate)-rich phase. All the sample materials appeared to have reproducible TSC peaks in the low- and high-temperature regions. The EVA samples all showed very similar behaviour showing a single low-temperature and a single high-temperature peak. We found that by using TSC, reproducible experiments could be carried out when polarisation temperature was 0 C and 60 C. The repeatability of the low-temperature peak at these different polarisation temperatures is an indicator that this peak results from dipole relaxation. Similarly,
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the evolution of the high-temperature peak is also suggestive that it originates in dipole relaxation behaviour and is not an artefact. It can be difficult to probe the level of cure in EVA and its laminate products. The TSC technique allows the level of cure of EVA and its laminated components to be determined in a facile manner. These results demonstrate the utility of TSC for this purpose. We show that during PV module encapsulation process, the TSC peak of cured EVA is characterised by a decrease in its current intensity and energy activation induced by the effect of crosslinking.
References [1] K.J. Lewis, Encapsulant material requirements for photovoltaic modules, in: C.G. Geblein, D.J. Williams, R.D. Deanin (Eds.), Polymers in Solar energy Utilisation, ACS, Washington, DC, 1983, pp. 367–385 (Chapter 23). [2] F.J. Pern, A.W.C. Zanderma, Characterisation of ethylene vinyl acetate (EVA) encapsulant effects on thermal processing and weatherability degradation on its discoloration, Sol. Energy Mater. Sol. Cells 25 (1992) 3–23. [3] K. Agroui, Y. Mesbahi, F. Moussa, Study of electrical properties of EVA during PV module encapsulation process, Proceedings of the Second World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria, 6–10 July 1998. [4] K. Agroui, N. Benrekaa, Characterisation of EVA for photovoltaic application, Proceedings of the 11th International Conference on Microelectronic (ICM’99), Kuwait City, 22–24 November 1999, IEEE Catalogue Number 99EX388, pp. 271–274. [5] J.R. Saffell, A. Matthiesen, R. McIntyre, J.P. Ibar, Comparing thermal stimulated current (TSC) with other thermal analytical methods to characterize the amorphous phase of polymers a, Thermochim. Acta 192 (1991) 243–264. [6] K. Agroui, Y. Segui, J. Farenc, N. Benrekaa, Characterisation of EVA encapsulant by glass transition temperature method for photovoltaic application, International Conference on Energy (IEC 2000), University of El-Ain, UAE, 7–9 May 2000. [7] E.R. Neagu, et al., Analysis of the thermally stimulated discharge current around glass–rubber transition temperature in PET, J. Appl. Phys. 82(5) (1997) 2488–2496. [8] G. Collins, EVA Test Report, 17 January, Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA, 2001. [9] L. Zong-Fu, R.E. Callahan, C. Hanson, J.M. Howard III, In situ cure characterisation of coatings by thermally stimulated current (TSC): spectroscopy and relaxation map analysis (RMA), TSC Application note, 1995. Thermold Partners, LP, Instrumentation Division at Stamford, Connecticut, USA. [10] J.J. lieari, L.A. Hughes, Functions of Coating in Handbook of Polymer Coatings for Electronics Chemistry, Technology and Applications, Noyes Publications, 1990, pp. 103–149 (Chapter 4). [11] R.H. Liang, S. Chung, A. Clayton, S.D.I. Stefeno, K. Oda, S.D. Hong, A. Gupta, Photochemical degradation of ethylene acetate copolymer, Polymer Science and Technology, Plenum, 1983, 20, Polymer Alloys 3: Blends, Blocks, Grafts, Interpenetrating Networks, pp. 267–278.