Investigation of newly developed thermoplastic polyolefin encapsulant principle properties for the c-Si PV module application

Materials Chemistry and Physics 243 (2020) 122660

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Investigation of newly developed thermoplastic polyolefin encapsulant principle properties for the c-Si PV module application Baloji Adothu a, b, *, Parth Bhatt c, Sarita Zele a, Jeroen Oderkerk d, Francis Reny Costa d, Sudhanshu Mallick a, b, ** a

The National Centre for Photovoltaic Research and Education (NCPRE), Indian Institute of Technology Bombay, Mumbai, 400076, India Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, 400076, India Waaree Energies Ltd, Plot 231-236, Surat Special Economic Zone, Surat, Gujarat, 394230, India d Borealis Polyolefine GmbH, St.-Peterstraße 25, A-4021, Linz, Austria b c

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

As compared to EVA, TPO has: � A higher transmission in the UV region. � Five times slower discoloration. � Higher melting point and thermal decomposition stability. � At least 45% higher peel strength with a 30% shorter lamination time. � Robust - 29% higher energy absorption during the deformation.

A R T I C L E I N F O

A B S T R A C T

Keywords: Encapsulant Ethylene-vinyl acetate Non-crosslinking Thermoplastic polyolefin Photovoltaic module

Ethylene-vinyl acetate (EVA) is the predominating material of choice for making the encapsulant film for photovoltaic (PV) modules. The easy accessibility, low cost, high transparency, long track record, widespread know-how on processability and performance, and to some extent, ignorance of the criticality of encapsulant film on the long-term performance of PV modules have made EVA, a dominant player in the PV industry. In parallel, due to economic reasons, the majority of encapsulant development has moved to a direction of compromising the quality to meet the cost target. In recent years, the PV industry has started recognizing, polyolefin-based encapsulants as technically superior when compared to EVA. Polyolefin-based encapsulant comes in either a crosslinked or thermo-plastic version. Thermoplastic polyolefin offers several advantages related to process­ ability and performance over the crosslinked version. In the current study, we compare a newly developed thermoplastic polyolefin-based encapsulant and a state of the art EVA encapsulant from different aspects of fundamental material properties, like optical, thermal, mechanical, etc. and discuss their implication on the performance of a solar module.

* Corresponding author. Particulate Materials Laboratory, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, 400076, India. ** Corresponding author. Particulate Materials Laboratory, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, 400076, India. E-mail addresses: [email protected] (B. Adothu), [email protected] (S. Mallick). https://doi.org/10.1016/j.matchemphys.2020.122660 Received 1 August 2019; Received in revised form 8 January 2020; Accepted 10 January 2020 Available online 11 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 243 (2020) 122660

decrease in the overall performance of a PV module [1,8,10]. Besides, the reaction by-products of peroxides and unreacted peroxide released from the encapsulant during the evacuation phase can have a corrosive effect on the laminator membrane, thus reducing its service life. With so many negative effects of peroxide assisted the crosslinking process, the question arises why is this crosslinking needed at all for EVA and some polyolefin-based encapsulant. A typical un-crosslinked (uncured) EVA used for encapsulant design has a melting point in the range 60–70 � C and a high melt flow index (MFI) up to 30 g/10 min. Such a polymer matrix will be highly flowable above melting temperature and have low mechanical strength. As a result, a module made with un-crosslinked or insufficiently cross-linked EVA will show poor mechanical integrity (low adhesion strength of the encapsulant to glass and backsheet) and cannot withstand application temperature roughly above 60 � C. Therefore, with EVA encapsulant a sufficient degree of crosslinking is a prerequisite to making good modules. In this context, a non-crosslinking encapsulant film will remove all the complexities and negative effects associated with peroxide assisted crosslinking process. Their higher melting point, low MFI (due to higher molecular weight) and sufficiently high me­ chanical properties replace the need for crosslinking and make them equally functioning in PV modules like crosslinking encapsulants. One representative class of non-crosslinking encapsulant is based on non-crosslinking polyolefin, which is commonly called thermoplastic polyolefin (TPO) encapsulant. During the lamination process, TPO is simply melted, and under the pressure applied by the laminator membrane, the molten encapsulant is spread over the whole interface to create intimate sealing and surface wetting. No curing/cross-linking agent is required to ensure strong adhesion and stable laminates as necessary interfacial adhesion are established through the in-built silane functional group (either as comonomer or as a grafted functional group on polymer chain) already present in the TPO macromolecules. The lamination time, temperature, and pressure play a major role to obtain strong adhesion and good quality of modules with TPO based encapsulants. Due to the absence of a crosslinking reaction during lamination, TPO undergoes virtually no chemical change after lamination and a faster lamination cycle is real­ ized as compared to the cross-linked encapsulant. To qualify any new encapsulant for the PV module, there are several critical and fundamental properties which are to be evaluated and ful­ filled. For example, optical transmission at least 90% and adhesion strength to glass at least 80 N/cm are typical parameters often cited by encapsulant suppliers in their product datasheet. Similarly, to pass aging testing like damp heat, humidity freeze, and thermal cycling test, an encapsulant must have a sufficiently high melting point, preferably greater than 85 � C[ref- IEC 61215]. Thermal stresses are always created in the PV module due to the different thermal expansion coefficients of module components. In order to dissipate this thermal stresses through the encapsulant, it should have a low-modulus of elasticity (<50 MPa industrial recommended) [1,7–9]. Volume resistivity is another impor­ tant property of encapsulant to prevent the current leakage/potential induced degradation in the PV module. In the PV industry, most man­ ufacturers may miss the deeper insight into the material behavior of encapsulant film simply because resin and recipe providers often act as mere raw material suppliers in the whole value chain. In the current study, we try to reduce this gap by bringing expertise from module manufacturer, resin manufacturer and independent PV research institute to understand and compare the inherent properties of two different classes of encapsulants and performance of the modules made with these encapsulants keeping rest of the BOM constant. One encapsulant material included in the study belongs to the most widely used encapsulant type, namely EVA, while the other belongs to the newly developed non-crosslinking TPO encapsulant. The aim is to compare the principal properties of TPO with standard EVA as an encapsulant material and validate it as a better material for a PV module.

1. Introduction Harnessing solar energy through the photovoltaic effect to produce electricity has now become the fastest-growing sector in the renewable energy production industry. A photovoltaic module, commonly called the PV module, sits as a composite entity at the heart of the whole system deployed for electricity production from solar radiation [1]. A typical energy-producing park can contain hundreds of thousands of PV mod­ ules connected in a specific order, where the PV module itself acts as the most critical functional entity. Therefore, the first and foremost task in ensuring unperturbed performance and high-efficiency level of an en­ ergy park is to ensure that the PV module itself keeps on functioning at the desired level over the whole service life of the energy park. A PV module is typically made of multiple components (together called the ‘bill of materials’ or ‘BOM’), namely solar cells, connecting ribbons, encapsulant film, glass, backsheet film, protective frame, seal­ ants, junction box, external cable, etc. From the cost perspective, the contribution of the encapsulant is about 4.0% to the total cost of the BOM for a module [2]. However, the role encapsulant plays in the functioning and durability of the PV module is very critical. Nowadays, PV module researchers are focussing on new polymer encapsulant ma­ terial development while keeping the rest of the BOM constant [3]. Traditionally, the encapsulant films based on EVA and polyolefin elas­ tomers (POE) are to be crosslinked during the module lamination pro­ cess to function properly. These cross-linkable encapsulant films from material composition perspective are a cocktail of chemicals like mul­ tiple stabilizing additives, peroxide, UV absorbers, and Hindered Amine Light Stabilizers (HALS), free silanes, etc. [3–7]. During the PV module lamination step, all cross-linkable encapsulant films undergo significant chemical modification converting a mechani­ cally weaker thermoplastic polymer composition into a crosslinked thermoset matrix. The first and very critical step in this chemical transformation is the decomposition of peroxide molecules, which subsequently helps to create chemical bonds between polymer chains. This is known as ‘crosslinking’ or ‘curing’. An important parameter, namely ‘degree of crosslinking’ or ‘degree of curing’ is taken as a mea­ sure to assess the completion of the lamination process. The PV modules made with cross-linkable encapsulants usually need a high level (any­ thing above 70–80%) of the ‘degree of crosslinking’ to function properly over a long period of service life. Since crosslinking causes, a chemical change in EVA molecules, various encapsulant properties, like adhesion strength to glass and backsheet, optical transmittance, mechanical strength, and volume resistivity, are also influenced by the final degree of crosslinking [8]. There are multiple factors that influence the kinetics of crosslinking reaction during lamination and the final degree of crosslinking, for example, temperature, time, reactivity and concentra­ tion of the crosslinking agent, nature of encapsulant polymer resin, presence of additives, etc. [9]. Therefore, to understand the real effect of encapsulant on module lamination and module performance, it is necessary to understand the role and importance of every entity present in encapsulant composition. For example, an encapsulant based on EVA can have a wide variation in co-monomer composition, melt flow index (MFI), type, and concentration of peroxide used (slow, fast, ultra-fast) and nature of UV/antioxidant stabilizers used. The crosslinking/curing process during module lamination not only brings desired changes in the encapsulant material but also in parallel brings few undesired effects. The peroxide decomposition by-products and unreacted peroxide molecules are of no use for the PV module and need to be removed to avoid bubble formation. In the case of EVA based encapsulant, the vinyl acetate (VA) units under favorable condi­ tions can trigger the formation of volatiles (acetic acid, butane), gaseous products (CO2, CO, CH4) and conjugation in polymer chain [1,8,10]. These volatiles and gaseous products were studied by Sultan and Sorvik [11,12]. Due to the presence of carbonyl conjugation, volatiles, and VA units in EVA lead to discoloration, delamination, moisture ingression, and corrosion of metallization over the years. This leads to a significant 2

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2. Experimental materials and methods

EVA encapsulants were 9.6 & 13 min, respectively. Since TPO does not have any by-product formation during the lamination and free of crosslinking reaction, the evacuation time and the pressure hold time were shortened significantly as compared to the EVA lamination cycle.

2.1. Laminates configuration Fig. 1, (a) represents the laminate structure that has been used for the testing of encapsulant optical transmittance. Quartz glass (QG) (20 mm � 20 mm, 2 mm thickness) and standard (Borosil textured) solar glass (SG) (50 mm � 50 mm, 3.2 mm thickness) have been used for the encapsulant transmittance test. Fig. 1 (b) structure has been used for the bare film optical transmittance, thermal and mechanical properties tests. Fig. 1 (c) & (d) shown structures that have been used for the preparation of TPO and EVA based 72 cell standard PV modules (1.6 m2) for thermal stability test. Two structures have been used for the peel adhesion test. The structures like backsheet (180 mm � 500 mm)/two layers of encapsulant/glass and backsheet/encapsulant/backsheet have been used for the testing of encapsulant adhesion with the glass (180 mm � 180 mm) and with the backsheet (180 mm � 180 mm) respec­ tively shown in Fig. 1. (e) and (f). Non-ARC coated tempered textured solar glass (Gujarat Borosil Ltd., 3.2 mm thickness, 180 mm � 180 mm) and a back sheet whose structure is polyvinyl fluoride (PVF or Tedlar)/polyethylene terephthalate (PET)/ Primer (i.e., Tedlar/PET/Primer) (Coveme, dyMate KL 50/250) have been used for 180� peel test. Coveme backsheet and fast-cure type EVA (0.45 mm thickness) encapsulants have been used throughout this study. To create the initial space for gripping of peel laminates, Teflon strips have been used at the edge and removed after the lamination. For thermal properties and volume resistivity measurements, TPO and EVA bare encapsulant films were laminated between two Teflon sheets. Teflon sheets have been removed after the lamination. TPO encapsulant (0.64 mm thickness) is a Quentys BPO encapsulant supplied by Borealis Polyolefine GmbH, Austria company. Solar glass (72 cell module size), multi-crystalline solar cells, EVA, and backsheet have been provided by Waaree Energies Ltd. Surat, India.

2.3. Characterization Transmittance spectra of TPO and EVA encapsulants have been measured with an UV–Vis-Nir Spectrometer (PerkinElmer Lambda 950) in a transmittance mode from 200 to 1200 nm with a 1 nm wavelength (λ) resolution and an average integrating time of 0.1 s. Air has been used as a zero baseline and flat quartz glass (QG) & solar glass (SG) have been used as a reference during the transmittance measurement. Thermal and humidity stability of solar panels has been studied through the damp heat (DH) chamber (Make: ESPEC, Model: EWSH282-5CW) at 85% RH & 85 � C for 1000 h as per the IEC-61215 recommendation. Currentvoltage (I–V) has been measured on a Class AAA Spire Solar Simulator (model: SPI-SUN 5600 SLP BLUE) at standard AM 1.5G condition. Module/Solar String Electroluminescence (EL) tester (Model: SWT-REL 16 M, Make: RENUSCO Image Technology Co. Ltd.) has been used for visual image observation of PV modules. Yellowness Index (YI) has been measured using a portable colorimeter (make: Testronix, model: TP110), which has a D65 light source and a 10� observer angle. The thermal properties of encapsulants were investigated with dif­ ferential scanning calorimetry (DSC) (TA instruments, Q2000) and thermogravimetric analysis (TGA) (diamond model, PerkinElmer, USA). For both DSC and TGA, around 10 mg sample has been taken in a platinum pan under the dry stream of nitrogen (gas flow 20.0 ml/min) with a heating rate of 10 � C/min. Universal tensile tester from Instron (model No. 3345R3093, equipped with a 5 kN load cell) has been used for 180� peel test with a peel rate of 50 mm/min, up to 110 mm Table 1 TPO and industrial standard EVA lamination optimum conditions at 150 � C.

2.2. Laminates preparation

Lamination steps

The BPO and EVA encapsulant based standard 72 cell multicrystalline solar cell-based PV modules and laminates have been pre­ pared (lamination conditions; 150:180:20:380) individually in an automatic industrial laminator (Boost Solar, BSL22360AC-II & III laminator). The optimum lamination parameters used for the prepara­ tion of laminates and PV modules like temperature, pressure, vacuum time, pressure build-up time, and pressure hold time used for the study are given in Table 1. The same lamination temperature of 150 � C was used for both the encapsulants. Total lamination cycle times for TPO and

Evacuation Pressure build up Pressure holding

TPO lamination condition

EVA lamination condition

Time

Upper chamber

Lower chamber

Time

Upper chamber

Lower chamber

s

kPa

kPa

s

kPa

kPa

180 20

100 15

100 100

280 20

380

10

100

480

100 5 0

100 100 100

Fig. 1. Laminates structure: (a) For glass to glass encapsulation for transmittance test, (b) For bare film transmittance, thermal and mechanical properties test, (c) & (d) Are the TPO and EVA based 72 cell standard PV modules structure for thermal stability test, (e) & (f) Are for the peel adhesion strength test with respect to glass and with respect to backsheet. 3

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displacement on 10 mm width peel strip. Minimum three to five strips have been used and an average of saturation load (N) has been taken from 10 mm to 100 mm of peel extension for the final adhesion strength calculation. The mechanical testing has been done with Universal tensile tester (Instron, model No. 3345R3093, equipped with a 5 kN load cell) on 100 mm length and 15 mm width of encapsulant trips. The volume resistivity has been measured with a Hewlett Packard 16008B resistivity cell (manufacturer: Keysight technologies) together with a Keithley 6517B Electrometer/High resistance meter (Manufacturer: A Tektronix Company). Volume resistivity is measured by applying 500-volt direct current to the specimen, which is placed between two electrodes (10 cm2) for 60 s, according to ASTMD 257.

scratches or any contaminated marks, or some of its additives (UV ab­ sorbers) may be lost and stick to the Teflon sheet during the lamination process. So bare encapsulant film always shows lower τ or small UV cut-off transitions than the encapsulated laminates. This is clearly shown in Fig. 2. According to the NREL recommendation, the final UV cut off values are 289 nm, 300 nm, 346 nm, 240 nm, 350 nm for SG, SG-TPO-SG, SG-EVA-SG, TPO, and EVA bare films respectively at 10% of τ. QG-TPO-QG and QG-EVA-QG laminates have the same UV cut off value at 228 nm at 10% of τ (Table 2). The “representative solar-weighted” transmittance (τrsw,{unitless}) is determined by using the equation given in the NREL report [13]. τrsw is defined for 300 � λ � 1250 nm, which is a typical maximum operating wavelength range for the flat-panel PV module. For TPO, it has been found that τrsw and hemispheric transmittance (τ) values are a little higher than EVA encapsulant, which is given in Table 2. TPO behaves as a high UV transparent encapsulant compared to EVA, because there is a significant higher τmean value in the UV region (280–400 nm). Polymeric materials are usually loaded with different kinds of stabilizers to ensure long term durability against exposure to sunlight. The UV part of the electromagnetic radiation is highly harmful to major polymers and lower the wavelength of the UV light higher is its deleterious effect on the polymer chain. Therefore, to protect the encapsulant film against UV radiation, different kinds of UV stabilizers are used. The primary func­ tion of these UV stabilizers is either to deactivate the radicals which are formed once UV lights interact with polymer chains or directly absorb the UV light and convert it into heat energy. The stabilizers which absorb the UV light also acts as a blocker of UV light, and when an encapsulant film contains such UV absorbing stabilizer, the UV cut-off value of the film increases, thus allowing less light to reach the solar

3. Results and discussion 3.1. Optical properties Two encapsulant layers have been used in industrial PV modules production. The first one is in contact with the solar cells, and it should provide hemispherical transmittance (τ) of at least 90% from 400 nm to 1100 nm [1]. The optical (τ) of TPO and EVA encapsulants as a function of wavelength is shown in Fig. 2. SG has a UV cut-off at 289 nm, shown in Fig. 2 (a). Due to the higher UV cut-off of SG, as compared to the encapsulants, the actual UV cut-off or other UV cut-off transitions of encapsulants which are shorter than the SG’s cutoff will be blocked by SG or only shows UV cutoff which is far from SG’s UV cut-off value. Due to this reason, the SG-TPO-SG laminate UV cut-off value is shifted almost near to SG’s value even though the TPO encapsulant film has a shorter value (240 nm). Whereas SG-EVA-SG laminate shows only 346 nm (at 10% of τ) UV cut-off, which is very far from the SG value (289 nm). But other UV cut-off values of EVA at 228 nm and 290 nm (shown by QG) are blocked by SG in SG-EVA-SG laminate because they are shorter or close to the SG value. QG has an extremely short UV cut-off than the encap­ sulants and which allows all the radiations and shows all the UV cut-off transitions of encapsulants after 200 nm shown in Fig. 2 (b). Due to this shorter UV cut-off of QG, all three UV cut-off values of EVA (at 228 nm, 290 nm, and 330 nm) and one UV cut-off of TPO are clearly seen with QG in Fig. 2 (b). EVA laminate (QG-EVA-QG) shows three UV cut-off transitions from 230 nm to 330 nm with partial light transmission. This is due to the UV absorber added in the EVA formulation. In the bare EVA film, above mentioned three UV cut-off transitions appear very small. This may be due to the UV absorber loss during the bare film lamination process. TPO has shown only one UV cut off value because it does not have any UV absorber like EVA. To avoid the confusion, the National Renewable Energy Laboratory (NREL) suggested taking the UV cut-off value at 10% of τ for encapsu­ lated laminates, not for the films [13]. Because in reality, encapsulants are stacked into composite structural manner during the PV module fabrication. It is difficult to get the encapsulant bare film without any

Table 2 Optical properties of TPO and EVA encapsulants. Sample details

UV cut-off (nm) at 10% τ

τ rsw (%)

SG SG-TPO-SG SG-EVA-SG QG QG-TPOQG QG-EVAQG TPO film EVA film

289 300 346 NA 228

280–1200 nm

τ Mean (%) 280–400 nm

400–1200 nm

92 89.6 88.5 93.3 92.6

71.7 57.7 28.7 92.5 90

91.7 90 90.6 93.4 92.8

228

91

53.5

91.0

240 350

90.0 83.0

82.4 25.5

90.0 91.7

τ is the hemispheric optical transmittance; τ Mean measured from 280 to 400 nm

and 400–1200 nm. rsw is the representative solar-weighted transmittance measured from 280 to 1200 nm.

τ

Fig. 2. Transmittance spectra of (a) TPO and EVA laminated between the solar glass (SG) and (b) TPO and EVA laminated between quartz glass (QG). TPO and EVA film transmittance spectra also included for comparison, especially for UV cut-off. 4

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cell. Obviously, this explains why the EVA studied in the current study has lower UV τ. Now one way is to reduce or avoid the use of UV absorbing additives in the EVA formulation. But, since EVA molecules are very prone to degradation under UV light, especially under UV-B & UV-A radiation, hence avoiding suitable UV absorbing stabilizers in EVA will certainly lead to compromise on long –term durability of the encapsulant film [14]. The low UV cut-off EVA has been developed in recent years, which shows a 1% gain in the power output of the PV module [15,16], but its longtime durability still needs to be established. Incomplete curing, insufficient photo stabilization, and UV shielding by the UV stabilizers in EVA formulation can also be a potential cause of faster discoloration of EVA encapsulant. EVA turns to yellow/browning due to the UV ab­ sorption (UV absorber loss) over time and vinyl acetate (VA) loss in the form of acetic acid. Due to this discoloration, EVA transmittance de­ creases and finally leads to the degradation in PV module performance [1,8,10]. On the other hand, the TPO resins being free of vinyl ester (like vinyl acetate in EVA) type structural moieties does not undergo degra­ dation reaction following Norrish type degradation producing conju­ gated polyene and acetic acid [1].

Table 3 I–V parameters of TPO and EVA based PV modules. DH test (hr)

PV module

Isc (A)

Voc (V)

FF (%)

Pmp (W)

Imp (A)

Vmp (V)

0

TPO EVA TPO EVA

7.5 7.4 7.4 7.2

46.3 46.1 46.2 46.0

0.77 0.76 0.77 0.76

268 259.2 263.3 252

7.2 6.9 6.9 6.7

37.7 37.1 37.6 37.2

1000

3.2. Thermal stability of PV modules TPO shows around 57% (SG-TPO-SG) and whereas EVA shows 28% (SG-EVA-SG) of light transmission from 280 nm to 400 nm. Due to this high UV transmission behavior, TPO shows a gain of more than 1% in current and more than 3% in power output of the prepared TPO based PV module as compared to the conventionally prepared EVA based PV module, which is clearly shown in Fig. 3 (a) and (b). Thermal stability of the TPO based PV module was also studied through the damp heat test (DH) (conducted at 85% RH & 85 � C for 1000 h as per the IEC-61215 recommendation) and also compared with EVA based PV module. All the PV module I–V parameters are given in Table 3. After 1000 h of DH test, both TPO and EVA encapsulated modules show less than 1.7% power loss. But TPO PV module still shows a gain of more than 3% in power output as compared to the EVA module after 1000hr of DH test also. As per the IEC-61215 DH test, if the PV module shows less than 5% power loss, then it would be considered as a thermally stable encapsu­ lant. The thermal stability of the modules is also visually observed through the EL images. Fig. 4 (a) & (b) are the TPO based PV modules, and (c) & (d) are the EVA based PV module’s EL images taken after the lamination (taken as 0 h DH) and after the 1000 h of DH test respec­ tively. There are no significant changes observed in EL images of both the encapsulated PV modules after the 1000 h of the DH test shown in Fig. 4. From these results, it is concluded that TPO based PV modules are also resistant to the DH test. In order to check the discoloration of TPO based PV modules, YI has been measured with a colorimeter after 0 h & after 1000 h of DH test and also compared with EVA based modules. YI has been measured at seven positions, as shown in Fig. 4, and an average of these seven is taken, and

Fig. 4. EL images: (a) & (b) TPO based PV module, (c) & (d) EVA based PV module after 0 h and 1000 h of DH test.

which are given in Table 4. YI value moves from negative to positive when the PV module turns from blue color solar cells into yellow/brown color depending upon the encapsulant degradation. Negative YI value suggests that the encapsulant does not undergo much yellowing during this 1000 h of the DH test. The change in YI value is the difference be­ tween the final (1000 h) and initial (0 h) YI value. Change in YI value suggests that TPO has five times slow discoloration development than the EVA encapsulant. It is well known that in EVA, the vinyl acetate (VA) Table 4 YI of TPO and EVA based PV modules. Sr. No

TPO-PV module 0h

1 2 3 4 5 6 7 Avg of 7 ΔYI

86.4 91.3 88.0 88.4 72.6 88.9 94.3 61.0 12.7

EVA-PV module 1000 h 40.2 46.9 49.1 52.2 56.3 48.5 45.0 48.3

0h 92.5 90.9 94.9 93.2 91.9 86.6 89.0 91.3 67.9

1000 h 16.5 26.8 25.5 28.2 22.8 28.4 15.6 23.4

Fig. 3. (a) I–V curves, (b) power-voltage curves of TPO, and EVA encapsulant based PV modules thermal stabilities after 0 h and 1000 h of DH test. 5

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unit absorbs the oxygen and water vapor in the humid and hot condition and results in acetic acid or acetaldehyde or ketones formation [1]. This results in faster discoloration, and as well as it increases the encapsu­ lant’s degree of cross-linking (which makes the encapsulant in harder or brittle in nature). In TPO, there are no such VA units and crosslinking related problems, because it is a thermoplastic non-crosslinking encap­ sulant. From these results, it can be concluded that TPO can become a better candidate than the crosslinking encapsulants like EVA and POE for module encapsulant applications.

stage decomposition is due to the polyethylene main chain loss. EVA has two-stage decomposition. The first stage decomposition starts at 260 � C due to the acetic acid evolution and the second stage is due to the polyethylene main chain degradation. The process of acetic acid evo­ lution from VA unit splitting is called ‘deacetylation’ in EVA [20]. This is one of the processes of the EVA encapsulant degradation mechanism involved in the PV module discoloration [1]. When a PV module having a cell defect area around 1 � 1 mm2, a hot spot is created and its maximum temperature reaches to 294 � C [21]. At this temperature, EVA is dominated by deacetylation/acetic acid generation. TPO’s first decomposition starts from 385 � C. From these results, it would be concluded that TPO is more resistant and chemically stable than EVA.

3.3. Thermal properties Fig. 5. (a) shows the thermal behavior of TPO and EVA encapsulants before the lamination and after the lamination (BL & AL). The melt transition of TPO starts from 40 � C to ends at 108 � C while for EVA starts from 35 � C to ends at 80 � C. The peak melting point, crystallinity, crosslinking reaction, and their temperatures are given in Table 5. The low-temperature endothermic peak is related to the melting of small crystal segments and the high-temperature endothermic peak is related to the melting of large crystal segments [17]. The degree of crystallinity is calculated as per the reported literature [17,18]. Since the total crystallinity of TPO is higher than EVA, it can maintain the small frac­ tion of crystal segments up to 100 � C which is sufficient to prevent the mechanical creep. It is well reported that high melt transition encap­ sulants show less amount of creep flow in the PV modules [19]. So TPO based PV modules can withstand higher operating temperatures in the range of 85–100 � C when compared to that of EVA. There is no significant change in the DSC degree of crystallinity before and after the lamination of TPO and EVA encapsulants and this is also in good agreement with the XRD patterns shown in Fig. 5. (b) XRD pattern shows an amorphous phase broad peak at 21� of its two theta angle. It would be considered as a complex semi-crystalline phase involved in encapsulants due to the coexistence of two monomers [17]. But the XRD pattern also shows that TPO has a slightly higher degree of crystallinity due to the narrow peak observed at 21� . It is already discussed that DSC can also help to identify the chemical crosslinking reaction involved in the encapsulant film. EVA shows the crosslinking reaction before and after the lamination. Due to the cross­ linking reaction, EVA generates the acetic acids (deacetylation rate 2.8 ppm/h) and other volatiles during the PV module lamination and which leads to the degradation of encapsulant over a period of time [10–12]. DSC results show that there is no crosslinking reaction involved in TPO-BL and TPO-AL. From this result, it is concluded that TPO is a non-crosslinking encapsulant and which can reduce the cost and lami­ nation cycle time for PV module production, and also it can eliminate the volatiles generation during the lamination. Thermal stability and thermal decomposition TPO and EVA encap­ sulant have been studied by TGA. Fig. 6 shows TGA thermal stability and thermal decomposition stages of both the encapsulants. Single-stage thermal decomposition involved in TPO-BL and TPO-AL. This single-

3.4. Adhesion and mechanical properties Adhesion strength is a very important property to enhance the interface bonding for the longevity of the PV module. Fig. 7 shows the peel test profiles at glass-encapsulant and backsheet-encapsulant in­ terfaces for EVA and TPO encapsulant based laminates. The average peel force (Newton’s (N)) has been taken from 10 mm to 100 mm and divided with 1 cm peel sample width for obtaining the final adhesion strength. For TPO, the obtained average adhesion strengths are 175.9 N/cm and 113.7 N/cm at the glass (G)-encapsulant and at the backsheet (BS)encapsulant interface respectively shown in Fig. 7 (a) & (b). For EVA, the obtained adhesion strengths are 107.5 N/cm and 72.9 N/cm at the glassencapsulant and at the backsheet-encapsulant interface respectively shown in Fig. 7 (c) & (d). Peel profile shows fluctuations, which are generally observed phenomena in encapsulant peel test [22,23]. It is observed that both the encapsulant show fluctuations in the peel profile and which indicates that the peeling path is not smooth. Due to this reason, the average has been taken for the final peel adhesion strength. It is obvious that the measured adhesion strengths for both the encapsu­ lants are significantly higher than what is described as typical values in the respective datasheet of the two material provided by their supplier. For EVA as known that peel strength is a function of the degree of crosslinking or gel content of EVA after lamination and usually, peel strength increases with increasing degree of crosslinking up to a certain level [22,23]. On the other hand, the TPO, in spite of having no cross­ linking, shows very high adhesion after lamination. This only possible, where the polymer matrix of such TPO has sufficient mechanical strength and also strong chemical interaction with the glass and back­ sheet surface. An uncured or insufficiently cured EVA has no sufficient mechanical strength to withstand the peel force. Additionally, free silane added to EVA can show its maximum effect of improving peel strength only when it is grafted on the EVA polymer chain. Such silane grafting is assisted by peroxide during the lamination process in order to ensure strong and stable lamination [1,4,14,22–25]. Since in TPO, silane is already part of the polymer chain either as comonomer or grafted moiety, they do not need additional grafting process during the lamination process. Due to the absence of a crosslinking

Fig. 5. (a) DSC curves, and (b) XRD pattern of TPO and EVA encapsulants. BL & AL represents before the lamination and after the lamination. 6

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Materials Chemistry and Physics 243 (2020) 122660

Table 5 Thermal and mechanical properties of TPO and EVA encapsulants. Sample name

Crystallinity (%)

TPO-BL TPO-AL EVA-BL EVA-AL

15 13 14 10

Endothermic melting peak (� C)

Exothermic crosslinking reaction peck (� C)

Low

High

Onset

Peak

Offset

47 47 47 47

92 91 68 62

– – 126 145

– – 150 164

– – 175 178

Young’s modulus (MPa)

Energy absorption (MJ/m3)

22 24 19.4 20.8

1.06 1.11 0.82 0.86

which is clearly shown in Fig. 8 (a). But TPO shows a similar trend before and after the lamination because of its pure thermoplastic and non-crosslinking nature. The elastic modulus of both the encapsulants has been calculated from the stress/strain-curve in the elastic region. The elastic module has been calculated as per ASTMD-882-02, and its R2 value is around 0.998. It is found that Young’s modulus of TPO encap­ sulant is slightly higher than EVA due to its higher crystallinity, which is given in Table 5. In the field, PV modules are always under the me­ chanical and environmental load conditions like wind, snow, extreme temperatures, humidity, thermal cycles, etc. Ideally, the modulus value is calculated only for low strain region and also in field conditions, the encapsulant will never reach to the fracture strain in a PV module. So 50% strain has been considered for the estimation of encapsulant toughness or energy absorption. Toughness is an ability of a material to absorb the energy during the plastic deformation without failure. The area under the stress-strain curve gives a measure of the toughness of the encapsulant material. Higher this area, higher will be its ability to absorb energy and protect encapsulant film from mechanical shock. So area under the curve has been calculated via the trapezoidal method according to Equation (1):

Fig. 6. TGA curves of TPO and EVA encapsulants.

reaction, TPO can reduce the lamination time significantly to 9 min compared to that of 13–14 min for EVA and which can increase the PV module production capacity. High adhesion strength encapsulant can prevent moisture ingress in the PV module and also avoid the chance of interfacial deboning/delamination, hence enhancing the reliability of the PV module. There is a significant difference in the stress-strain curves of EVA, which is due to the cross-linking nature after the lamination [25,26] and

(1)

A ¼ A0:1% þ A0:2% þ ::… þ A50% A0:1% ¼

�y þ y � 0 0:1% ⋅ ðx0⋅1% 2

x0 Þ…… A50% ¼

�y þ y � 50% 49:9% ⋅ðx50% 2

Fig. 7. Peel test profiles: (a) TPO to glass (G), (b) TPO to back sheet (BS), (c) EVA to G, (d) EVA to BS. 7

x49:9% Þ (2)

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Materials Chemistry and Physics 243 (2020) 122660

Fig. 8. (a) Tensile stress-strain curve; (b) Energy absorption curve of TPO and EVA encapsulants.

Where A is the total area and A0.1% to A50% are the area (energy ab­ sorption) calculated under the stress-strain curve up to the 50% strain; y0.1% to y50% is the stress (MPa) and x0.1% to x50% strain values are considered from 0.1 to 50% strain. Fig. 8 (b) shows the energy absorption vs strain. The calculated total energy absorption (toughness) is given in Table 5. TPO is 29% tougher (resistance to mechanical shocks) than EVA encapsulant. In Fig. 8 (a) EVA encapsulant shows more fracture toughness (area) until the fracture strain due to its crosslinking nature. But in reality, as an encapsulant will never reach its fracture strains. With a very high ‘fracture toughness’, a crosslinked EVA can have a negative effect on the following things: 1. Repairing of the module with small defects needs more force to separate the encapsulant & other module components; 2. Mechanical recycling of modules will be also more difficult. Due to the high melt transition up to 110 � C, the TPO encapsulant still remains in a solid phase, and it can provide high mechanical strength/shock resistance, toughness, and creep resistance when a PV module reaches the maximum operating temperature of 85–100 � C [27]. It is well reported that high melt tran­ sition encapsulants show less amount of creep flow in the PV modules [19].

laminates and PV modules have been prepared at the optimum lami­ nation conditions and used for characterization as encapsulant material. TPO is a high transmittance encapsulant as compared to EVA due to its lower UV cut off, and it also has a slightly higher τrsw. Due to this, the TPO based PV module shown a gain of more than 3% in power output as compared to the EVA based PV module. The discoloration is also five times slower for TPO as compared to the EVA based PV module. DSC results show that TPO can maintain a small fraction of crystal segments up to 100 � C due to its large melting range with a slightly higher crys­ tallinity as compared to the EVA. The high melting transition tempera­ ture range is the major advantage of TPO encapsulant. DSC results show that during lamination, TPO undergoes no chemical changes, while EVA shows a crosslinking reaction, which converts a thermoplastic matrix into a thermoset. TGA evaluation shows that irreversible chemical degradation starts in EVA as early at 260 � C, while TPO is very stable, and no significant degradation starts below 400 � C. Such higher thermal stability of TPO can be advantageous when hot-spot is created in the module since the degradation of the encapsulant can be avoided. Shorter lamination cycle time (9 min) in TPO encapsulant gives stable lamina­ tion with higher peel adhesion strength as compared to the EVA (13–15 min). The strain behavior is the same for both TPO-BL and TPO-AL, but EVA shows a significant difference due to its crosslinking. From the energy absorption vs. strain curve, it is confirmed that TPO has more toughness (resistance to mechanical shocks) than EVA. The order of electrical volume resistivity remains the same for TPO before and after the lamination whereas EVA shows the difference due to its crosslinking. TPO can maintain its higher adhesion strength, crystallinity, Young’s modulus, resistance to creep, toughness, and volume resistivities even at the extreme module operating temperature of 85 � C due to its high melting transition temperature of 110 � C. These results conclude that TPO can effectively work as a better encapsulant than EVA for the long term stability of the c-Si PV modules application.

3.5. Volume resistivity In a solar PV plant, modules show a high potential difference be­ tween the solar cells and the module frame with respect to the ground. This high potential difference results in leakage current from the module frame to the solar cells. This phenomenon is called Potentially Induced Degradation (PID). This leakage current flow is due to the migration of ionic species from the front glass through the encapsulant and finally reaches the solar cell. A possible way to prevent PID is to hinder ions transportation through encapsulant material. For that, one of the important dielectric properties of the encapsulant is its volume re­ sistivity. It is found that EVA has a volume resistivity of 4.9 � 1014 and 4.4 � 1015 Ω-cm before and after the lamination respectively. The vol­ ume resistivity of EVA encapsulant increased after the lamination due to the crosslinking and proper decomposition of other additives (UV absorber, stabilizers, antioxidants). The presence of un-decomposed peroxide curing agent and other additives increase the polarity, and that leads to a decrease in the volume resistivity of EVA over the period of time [28–31]. It is found that the volume resistivity of TPO is 3.2–8.2 � 1015 Ω-cm before and after the lamination. The order of TPO volume resistivity remains the same before and after the lamination. It is ex­ pected that the TPO encapsulant can still show its high volume re­ sistivity even at an extreme module operating temperature of 85 � C due to its high melting transition (110 � C). At this extreme module tem­ perature, EVA is in liquid state and volume resistivity can be reduced.

Acknowledgments Authors would like to acknowledge Borealis Polyolefine GmbH, Austria and Waaree Energies Ltd. Surat, India for providing materials, equipment, and support for preparation and measurement of sample analysis during the course of this study. We are thankful to the Metal­ lurgical Engineering and Materials Science department, Particulate Material lab, and SAIF at IIT Bombay for the characterization facilities. This work was supported by the NCPRE funded by the Ministry of New and Renewable Energy of the Government of India through the Project No. 31/09/2015–16/PVSE-R&D dated 15th June 2016. This work also funded through the multi-institutional project No: Spons/MT/MS-1/ 2018 and dated Jun 25, 2018, by Borealis (Vienna, Austria), Waaree (Surat, India), and NCPRE, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India.

4. Conclusions Newly developed non-crosslinking TPO materials have been tested and characterized as EVA replacement encapsulant material. Firstly, 8

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Materials Chemistry and Physics 243 (2020) 122660

Appendix A. Supplementary data

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