光伏背板老化研究

PV BACKSHEET MATERIALS UNDER ACCELERATED AGING CONDITIONS:A PERFORMANCE STUDYSteiner A. J., Krumlacher W., Muckenhuber H., Kraxner M.ISOVOLTAIC AG, Department for Research & DevelopmentIsovoltaicstraße 1, A-8403 Lebring, AustriaPhone: +43 5 9191 9978; Fax: +43 5 9191 79978E-Mail: andreas.steiner@ABSTRACT: Over the last few years, a variety of different PV backsheet materials from various suppliers have emerged in the PV market. Whilst some of these new products rely on well-known and established materials and material combinations, others break new ground by introducing highly engineered new materials and concepts for the use in photovoltaic applications. The aim of the work presented is to compare the performance of different commercially available PV backsheet materials and material combinations regarding their stabilities under various prolonged accelerated aging conditions (e.g.: UV-exposure, damp-heat testing) according to the internationally valid standards (IEC 61730, IEC 61215, UL1703) but exceeding the requirements of those standards by a factor of 2 to 3 with respect to testing times and irradiation energy inputs. Key advantages as well as possible disadvantages of the tested materials are discussed critically, focusing on safety and performance aspects of the backsheet setups under evaluation.Keywords: Qualification and Testing, Reliability, Safety, Durability, PV Backsheet Materials1 INTRODUCTIONDuring its entire lifetime, a photovoltaic module isexposed to various different environmental and electricalstresses. These harmful effects may drastically influencethe module’s electrical performance and lifetime. Beingone of the most outer surfaces of a PV module, thebacksheet has to withstand those environmentalstresses as it has to protect the module’s activecomponents from being degraded or harmed in order toguarantee a long and efficient module lifetime andperformance. Furthermore, the backsheet represents themost vital e lectrical insulation layer on the module’sbackside, therefore being highly relevant by means ofsafety for all glass/backsheet module technologies.High UV exposure, mechanical stresses caused byabrasion or forced deformation, thermal stresses relatedto high temperatures and significant temperature changesas well as hydrolytic material degradation in course oftime just represent some of the most importantenvironmental effects a backsheet has to deal with.Regarding the electrical safety of a PV module, thebacksheet has to guarantee its insulation capacity over themodule’s lifetime in the field.The presented work aims to assay nine differentcommercially available PV backsheet materials andmaterial combinations (see Table I) in terms of vitalsafety, stability and performance criteria in course ofprolonged accelerated aging testing procedures.Table I: Structural Setup of investigated backsheetsMat. No.Structural Setup(outer layer / core layer / inner layer)1 PA / PA /PA2 PA / PET / PA3 PET / PET / Tie Layer4 PVDF / PET / Tie Layer5 PFAVE / PET / Tie Layer6 THV / PET / Tie Layer7 ECTFE / PET / Tie Layer8 PVDF / PET / PVDF9 PVF / PET / PVF 2 DESCRIPTION OF TESTING PROCEDURESThe nine PV backsheet materials under consideration (see Table I) have been tested according to various accelerated aging procedures based on internationally valid standards but exceeding the requirements of those standards by a factor of 2 to 3 with respect to testing times and irradiation energy inputs. The applied testing conditions are briefly mentioned in the following.2.1 ReflectivityReflectivity of the materials under consideration has been measured on the entire backsheet structure in accordance with DIN 5036. Irradiation was done on the inner layer (EVA-side) for a wavelength range of 400 –1000nm and the mean values of spectral reflection in this respective range has been calculated for comparison of the analyzed structures.2.2 UV testingUV testing of the different materials has been done according to ISO 4892-3, using a fluorescent UV lamp with an irradiation maximum of 1.10 W/m² @ 340nm. 1524h of testing according to a 8 hours irradiation / 4 hours bedew-cycle at 60°C yield in a total irradiation energy of 5.03 kWh/m² in the UV B range (280 – 320nm; being in accordance with the requirements of IEC 61215) and 57.8 kWh/m² in the combined UV A + UV B range (nearly 4 times higher than the required 15 kWh/m² according to IEC 61215). The testing has been done on the complete backsheet structures. The respective outer surfaces have been irradiated and changes in yellowing indices (ΔYI) have been calculated according to ASTM 313-05.2.3 Damp heat testingDamp heat testing has been performed on the test specimen at 85°C/85%rh in accordance to IEC 61215 for a total testing time of 2000 hours (twice IEC-standard). The tested backsheet samples have been regularly investigated during this testing time. Optical parameters have been evaluated according to ASTM 313-05 (change in yellowing index, ΔYI). Mechanical parameters (tensilestrength, elongation at break, loss in tensile strength) have been measured according to ISO 527-2 for 10 testing specimen and mean values have been calculated out of these individual testing results.Inner layer adhesion of the materials under consideration (weakest point of adhesion) and EVA adhesion values have been monitored over 2000 hours of damp heat testing foll owing ISOVOLTAIC’s in-house testing procedure (IPV 70; based on 180° peel-testing experiments employing EVA Sanvic Fast Cure).3 RESULTS AND DISCUSSION3.1 Reflectance valuesHigh reflectance values in the range of 400 – 1000nm may positively influence the overall electrical performance of a PV module [1]. The highest values of reflectivity in the wavelength range under consideration have been found for backsheets 1and 8, nearly approaching 90% of mean reflectivity. Whilst all other evaluated materials prove to show reflectivity values between 80 and 90%, backsheet 9(PVF/PET/PVF) shows a rather low reflectivity of only 71% in average (see Table II). These considerably low values can be contributed to the low inherent reflectance of the PVF inner layer of the analyzed backsheet.Table II: Reflectivity of evaluated backsheetsMat. No.Reflectivity [%] (mean value; 400 – 1000nm)1 89 %2 82 %3 79 %4 82 %5 81 %6 81 %7 85 %8 89 %9 71 %3.2 Optical performance in UV and damp heat testingNone of the tested commercially available PV backsheet structures exhibited significant yellowing during 2000 hours of damp heat testing. The measured values for the change in yellowing index (ΔYI) are all found to be below 5 after accelerated aging testing.During UV testing only backsheet 3(PET/PET/Tie layer) showed significant yellowing with a change in yellowing index by more than 10 (see Table III).Table III: Optical performance of analyzed backsheets Mat. No. UV testing DHT testing1 0.9 3.72 0.8 2.03 10.5 3.04 1.6 4.35 0.7 3.46 0.8 0.47 0.8 1.38 0.8 0.49 2.8 2.2This effect might be contributed to insufficient UV-protection of the PET outer layer or of the adjacent adhesive systems, respectively. Similar findings for backsheets using PET as outer surface have been reported earlier by different authors [1, 2].3.3 Mechanical performance during damp heat testingIn course of 2000 hours of damp heat testing, all evaluated PV backsheets showed a decrease in mechanical properties. These changes are considered to be closely related to chemical degradation of the backsheets’ core layers by thermal and hydrolytic attack. The core layers of the materials clearly dominate the backsheets’ overall mechanical performance as they represent the layers of the highest thickness in a PV backsheet. Therefore, electrical safety (insulation) and mechanical stability (tensile strength, elongation at break) is clearly related to the mechanical properties and stabilities of the core layer materials.The evaluated properties for the nine different backsheet setups under consideration are given in Table IV, respectively.Table IV: Mechanical properties of evaluated backsheets Mat.No.tensilestrength(MD)[N/cm]0hDHTtensilestrength(MD)[N/cm]2000hDHTtensilestrength(MD)[% loss]2000hDHTelong.atbreak(MD)[%]0hDHTelong.atbreak(MD)[%]2000hDHT1 125 120 4 220 702 580 480 17 230 1603 320 200 38 190 754 315 255 19 145 805 260 190 27 170 1006 155 105 32 135 557 240 175 27 185 1008 360 break 100 100 break9 490 350 29 200 85All PET-based material setups show a significant drop in their tensile strength after 2000 hours of damp heat testing as a result of thermal and hydrolytic stress on the core layer. Backsheet 8(PVDF/PET/PVDF) even proved to show cracks in the material’s c ore layer after the testing period (see Figure 1). Therefore, testing of the remaining tensile strength and elongation at break was not possible anymore.Figure 1:Cracks in the PET core layer of PVDF/PET/PDVF backsheet 8after 2000 hours DHT.This severe degradation may be contributed to a poor quality of the PET core layer material itself, to insufficient protection of the core layer by the used PVDF outer layers or to a combination of both effects.Backsheet 1(PA/PA/PA) which uses a polyamide based core layer clearly behaves different in terms of tensile strength testing. Whilst initial tensile strength values are found to be very low compared to PET-based backsheets, virtually no decrease in tensile strength is found after 2000 hours of damp heat testing. Therefore, hydrolytic and thermal degradation of the polyamide core layer seems to be lower than degradation of PET based core layers in this respect.In course of damp heat testing also elongation at break is decreasing with all tested backsheets. Apart from the evaluated PVDF/PET/PVDF setup, all tested specimen still showed more than 50% elongation at break after 2000 hours of accelerated aging.3.4 Adhesion during damp heat testingPolyamide based backsheet structures 1and 2 showed the highest and most stable adhesion to the tested EVA after lamination and 2000 hours damp heat testing. All other tested backsheet materials either proved to show considerably low EVA-adhesion even prior to aging or suffered from severe loss in adhesion during damp heat testing (see Table V).Table V: EVA-adhesion of analyzed backsheetsMat. No.EVA adhesionafter Lamination[N/cm]EVA adhesionafter 2000h DHT[N/cm]1 105 802 115 1003 35 324 40 305 70 206 105 207 60 408 50 109 115 40Material combinations using thermoplastic tie layers (backsheets 3 –7) proved to show the lowest adhesion values after 2000 hours of damp heat testing. THV/PET/Tie layer setup 6even exhibited severe delamination on the edges of the tested glass/EVA/EVA/backsheet dummy module setup (see Figure 2).Furthermore, some tie-layer based materials also showed significant loss in their inner layer adhesion after 2000h of damp heat testing, the weakest point of adhesion being the PET/Tie Layer interface (see Table VI).The low EVA-adhesion values of backsheet 8after accelerated aging testing may also be contributed to the poor mechanical properties of the backsheet. After 2000 hours of damp heat testing, breakage of the whole backsheet setup was observed during adhesion testing at approx. 10N/cm. Figure 2: Delamination of THV/PET/Tie layer backsheet 6 after 2000 hours DHT (dummy-module setup).Table VI:Layer adhesion of analyzed backsheets (weakest adhesion of layers)Mat. No.Layer adhesionwithout aging[N/cm]Layer adhesionafter 2000h DHT[N/cm]1 not separable not separable2 8.0 4.03 not separable not separable4 5.0 <1.05 4.5 2.06 not separable not separable7 not separable <1.08 5.0 <1.09 not separable 2.54 CONCLUSIONSIn course of accelerated aging testing, all tested PV backsheet materials have shown signs of degradation due to high temperature, UV-irradiation and hydrolysis effects, respectively. Whilst UV-stability seems to be closely related to the performance characteristics of the outer layers and the adjacent adhesive systems, mechanical properties are largely dominated by the physical properties of the used core layers. High attention has to be paid to the quality of core layer materials and core layer protection by the corresponding outer layers. Thermoplastic tie-layers have shown considerably high initial EVA adhesion for the materials under investigation but generally suffered from a drastic decrease in adhesion values after 2000 hours of damp heat testing.The authors of this paper are well aware of the fact that some of the properties described and evaluated above might be batch and manufacturer-related. Therefore, the authors want to emphasize that the study published in this paper represents the outcome of a testing program with statistical evaluation which has been carried out on commercially available samples of the respective materials. General statements regarding the suitability of various materials or material combinations for their use in PV-module applications based on this study thereforestill remain questionable.5 REFERENCES[1] W. Gambogi, S. Kurian, B. Hamzavytehrany, A.Bradley, J. Trout, O. Fu, Y. Chao, The Role ofBacksheet in Photovoltaic Module Performance and Durability, Proceedings EUPVSEC, Hamburg,Germany (2011).[2] W. Gambogi, Comparative Performance ofBacksheets for Photovoltaic Modules, Proceedings EUPVSEC, Valencia, Spain (2010).。