Accelerated weathering test for evaluating ...

Accelerated weathering test for evaluating “transparent encapsulants on steel substrate” and prediction of lifetime improvement of solar cell

Sanjay Ghosha, Vasil Stoickova, Luke Haponowa, Jeff Kettle*a, Ana L. Martínezb, David Gómezb, Pascal Sánchezb a School of Electronic Engineering, Bangor University, Bangor, Gwynedd LL57 2UT, UK● b ITMA Materials Technology, Photonic Area, C/ Calafates, 33417 Avilés, Spain *

Corresponding Author [email protected]

Abstract In this work, a brief review of transparent flexible encapsulant for PV modules based on steel substrates is presented followed by our recent work on developing a new easily implementable, cost effect and robust method for selecting best encapsulant for steel as the substrate is discussed. The developed method is laboratory based accelerated weathering test conducted following IEC guidelines.

Introduction In order to maintain high efficiency of the solar cell during their life time, encapsulation plays the most important part of protecting the solar cell from degradation from the external environment, particularly by blocking ingress of gas and water molecules. Traditionally, glass has been used as encapsulant for solar cell, however with the advance of flexible solar cells, for instance, solar cells fabricated onto flexible plastic and thin steel substrate, the need for efficient flexible transparent encapsulant has been growing. The desired properties of encapsulant required for PV module protection include resistance to UV-visible radiation driven discoloration reaction, embrittlement, and delamination. In addition, release of by-product from the encapsulant into the integral part of solar cell should be stopped or minimised, for example, commonly use encapsulant ethylene vinyl acetate (EVA), releases acetic acid as decomposition by-product which increases the corrosion rate of metallic contacts and lower the performance of solar cells.1,2 Therefore, new material or combination of different existing material is needed to overcome this issues to make a better encapsulates. In this work, we highlight the use of different low temperature polymer films combination as encapsulant for protecting steel as substrate. The data will enable assessing best encapsulate for protecting and

increasing lifetime of PV modules in the real outdoor environment. In the following, a brief literature review on flexible transparent enacapsulant in the context of using to protect PV module is presented. Ethylene tetrafluoroethylene: Front and back sheet Ethylene tetrafluoroethylene (ETFE) was used in sheet form for encapsulation as it is highly transparent to light, retains its transparency and strength for over 30 years, has a high level of heat retention and has been widely used as building material in recent years.3,4 ETFE usually employed as a front sheet for solar cells. ETFE coated with a hydrophobic silicon oxide (ETFESiOx) nanomaterial has been also been reported to significantly improve the barrier properties of the substrate towards water and oxygen.5,6 Ethylene-vinyl acetate: bonding

adhesion and

Ethylene-vinyl acetate is inherently flexible, tough, clear, has hot-melt adhesive water proofing properties and UV radiation resistance.7 Some of the other application of EVA includes sealants in meat and dairy packaging, insulation of cables and wires, foot wares as a shock absorber, and various outdoor applications.8 EVA has also been used as an encapsulating material for crystalline silicone in the industrial production of PV modules.9,10 It has been recently reported that the effect of temperature and relative humidity on the adhesion and debonding properties of EVA as an encapsulating layer on PV module could be used as a measure to predict their long-term reliability or to develop accelerated aging test model.11 It has been reported that using fluorescent organic dyes dissolved in the pre-existing EVA encapsulation layer results in 0.18% absolute higher efficiency of commercial screen-printed multi-crystalline silicon

photovoltaic modules and stability to UV degradation.12

improved

Polyvinyl butyral: adhesion and bonding Polyvinyl butyral (PVB) has strong binding, adhesion to many surfaces and filmforming properties, excellent optical transparency, high compatibility with other polymer and flexible properties. PVB major applications include laminated safety glass for automobile windshields, binders for ceramics and metal powders, adhesive, and printing ink for packaging.13 PVB as encapsulant material for PV module has been investigated, relatively recently, for their high stability against UV radiation and the high adhesive force to glass and other PV substrates.14,15 An advantage of PVB (Tg = 35 oC) over EVA (Tg = -16 oC) is its higher glass transition temperature which makes it suitable for PV module operating in a cold region.15 Polyethylene terephthalate: barrier layer Polyethylene terephthalate (PET) is a transparent, very lightweight, a semicrystalline resin in its natural state and can be semi-rigid to rigid depending on its processing. It is normally processed by sheets as expensive extruding equipment is otherwise necessary. PET is excellent water and moisture barrier material, widely used as plastic bottles for soft drinks and food packaging, and are 100% recyclable.16 It has been investigated as back sheet material for PV module protection with improvement in barrier properties when coated with silicon oxide (PET-SiOx).5,17,18 PET has been reported to effectively protect the polymeric multilayer from UV degradation and damage due to moisture when used as back sheet for PV modules.19,20 Planarised PET is widely used as substrate for manufacturing flexible transparent electrodes for optoelectronic applications. Polyethylene napthalate: barrier layer Polyethylene napthalate (PEN) is a polyester-based barrier layer and provides very good oxygen barrier properties and consider as emerging material for electronic industry.21,22 Other important properties of PEN include good thermooxidative resistance, high strength and modulus, chemical and hydrolytic resistance and ultraviolet light barrier resistance. Some of the important properties of the polymer films of EVA, PVB, ETFE, PET and

PET are summarised in table 1. All the polymers possessed good transparency (T) for considering as transparent encapsulate. The UV durability (X) is highest for ETFE and most commonly use as front sheet material, which provides the necessary shield against the harmful UV-radiation. Table 1 Properties of transparent polymers

Tg EVA PVB ETFE PET PEN

o

̶ 16 C 35 oC 67-81 oC 122 oC

T

X

93.9% 93.9% 90% 95% 95%

++ ++ +++ ++ +

Tg = Glass transition temperature, T = Light transmittance, X =UV durability (under 42 suns at 6000h of exposure) 23,15

Results and discussion The transparent polymer films considered for the encapsulation of steel samples in this work comprising a combination of ETFE, EVA, PVB, PET and PEN having films thickness above 100 microns. Different combination of polymer films were trialled as encapsulation materials, by considering their physical properties known in the literature as protective front sheet, back sheet, sealant, and encapsulate for PV modules. ETFE has been primarily used as outer protecting layer front sheet for the encapsulation of steel samples because it is available in sheet form and can be applied to substrates of steel using an adhesive. EVA and PVB were used as encapsulating layers, as these provide adhesive binding between the films as well as strong adhesion with the underlying steel substrates. PET and PEN were considered as barrier layer owing to their excellent gas and moisture barrier properties. As both of these materials are applied in sheet form, an adhesive (such as EVA or PVB) is needed to be used in combination with the PET/PEN. The testing protocol for evaluating the encapsulant for steel substrate is shown in figure 1. The polymer films and their different combinations were subjected to QUV accelerated weathering test to simulate the outdoor weathering condition. The test conducted according to ISO guideline (4982-3:2006). The performance was evaluated on the basis of change in percentage transmission (%T) and yellow index (YI). After QUV test, the selected films combination were used to

encapsulate two types of steel for further evaluation by thermal cycling (TC) and thermal humidity cycling test (THC) following IEC guideline (61646). Based on the extent of decomposition assessed by combination of visual inspection, microscopy, Raman spectroscopy, XRD and adhesion measurement best encapsulant type is predicted.

decomposition was observed after thermal cycling test indicating the potential of the the encapsulate to survive outdoor weathering condition for about 10 years. A more stringent test was performed by thermal-humidity test using climate chamber. Noticeable difference were observed even by visual inspection. For illustration, an encapsualted steel sample after THC of 1000 hour is shown in figure 4. The extent of corrosing and decompostion of encapsulant was analysed by combination of micrsocopy, Raman spectrsocopy, XRD, and adehesion measurement. The combined results were used to predict the best encapsulant that can be used to protect the PV module in outdoor environment for more than 15 years. The combination ETFE/PVB/PET/PVB was found to best among the different combinations investigated in this study. The best combination encapsulate has been uner investigation for protecting OPV modules for outdoor weathering test. The results will be presented elseware. THC

Figure 1 Test protocol for selecting encapsulant.

Figure 2 depicts the QUV test carried out for the combination of polymers films. The films selected for next level of study must have the transparency greater than 85% and yellow index less than 5%. Several polymer combination e. g. ETFE/PVB/PET/PVB, ETFE/EVA and ETFE/PVB was found to be passed the QUV test and were consider for lamination of steel sample. For illustration, an example of lamented steel sample (4 cm x 4cm area) is shown in figure 3.

Figure 2 Combination of polymer films exposed to QUV test.

Figure 3 Laminated steel sample.

The laminated steel samples were subjected to thermal cycling test. No visual

Figure 4 An encapsulated steel sample exposed to thermal humidity cycling (THC).

Conclusion We have presented a low cost, easily implementable and robust test method to evaluate performance of low temperature, transparent flexible encapsulate for protecting steel substrate. The data generated through the method can be used for selecting best encapsulant to protect and maintain high efficiency in their lifetime of PV module exposed to outdoor environment.

Acknowledgement We are grateful to European Union’s Research Fund for Coal and Steel (RFCS) for funding STEELPV project under grant agreement no. RFSR-CT-2014–00014.

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