Analytical evaluation of the ageing induced changes of the adhesive within Photovoltaic backsheets

Abstract

AIT, Austrian Institute of Technology GmbH, Energy Department, Austria. OFI Austrian Research Institute for Chemistry and Technology; Franz Grill Strase 5, Objekt 213, 1030 Wien, Austria. T: +43-1-7981601-250 F: +43-1-7981601-303 yuliya.voronko@ofi.at; gabriele.eder@ofi.at http://www.ofi.at ABSTRACT: The ageing stability of multilayer laminates - which are commercially used as PV backsheets - was examined in artificial weathering experiments. The impact of environmental factors like temperature, humidity and/or corrosive gases (ammonia) on the materials as well as on the ply adhesion within the multilayer laminate was investigated. The cross-sections of single backsheets and of backsheets integrated in PV-modules were investigated spectroscopically in the original and aged state in order to describe the impact of the environmental factors and lamination process on the adhesive layers. The spectroscopic information obtained on the evenness, thickness and chemical structure of the adhesive layer by Raman-imaging and infrared (IR)-imaging was in good accordance. Storage under damp heat conditions and humid ammonia atmosphere resulted in changes (chemical and physical degradation) of the adhesive. The magnitude of the effects, however, was strongly depending of the backsheet components and the influencing factors. Adhesive degradation was not always correlated with delamination behaviour. Keywords: Photovoltaic materials, Characterization, Environmental effect, Spectroscopy 1 Introduction Performance, quality and reliability of PV modules are key factors for the favourable development of emerging photovoltaic markets worldwide. During their lifetime in the field, PV-modules have to withstand environmental influences which may vary drastically in dependence on the surrounding climate [1,2]. The role of the backsheet within the multi material composite of a PV-module is to provide protection for the photovoltaic active layer and polymeric encapsulation against environmental, mechanical and chemical influences and to ensure electrical insulation. Thus, the weathering stability of the polymeric materials used in the multilayer laminates of a backsheet is crucial for the reliability of PV modules over their life cycle [3,4,5,6]. The backsheets usually comprise of a core layer of e.g. polyester (PET) or polyamide (PA) which provides the mechanical stability and electrical isolation. This core is laminated on both sides with a thin adhesive film to highly protective fluoropolymer outer layers [7]. The adhesiveness of the layers within the backsheet is of high importance for the functionality of the backsheet over a demanded lifetime of up to 25 years [8]. Thus, it is of great interest to investigate the changes in the adhesive layer induced by the impact of environmental factors like radiation, temperature, humidity and corrosive gases on the material and ply adhesion within the multilayer laminate. In addition, the effect of the lamination process on the adhesive layers has to be evaluated, as high temperatures (e,g,145°C) and varying pressure conditions are applied to the multilayer backsheet during the PV-production process. As analytical tools to visualize and characterize the original and aged states, these thin adhesive layers within (i) the single backsheets and (ii) backsheets incorporated in PV-modules, spectroscopic imaging techniques such as infrared (IR) imaging and Raman imaging seem to be the most promising methods. These techniques combine the spatial information (microscopy) with the chemical information (vibrational spectroscopy) and thus, might yield a complete description on the variances in the compactness, thickness and chemical structure of the adhesive upon ageing. 2 Experimental 2.1 Materials Two types of backsheets, “B1” and “B2”, were chosen for this study which comprise a PET core layer (providing the mechanical stability and electrical isolation of the multilayer material) and weathering resistant outer layers of a fluoropolymer: (i) polyvinyl fluoride (PVF, Tedlar) = B1 and (ii) polyvinylidene fluoride (PVDF) = B2. For the microscopic and spectroscopic investigations, polished cross-sections of the samples, embedded in an epoxy resin, were prepared. The structure of the backsheets was investigated at first by visual microscopy (see. Fig.1). The composition of both laminates, can be described as a symmetrical structure with a core layer from approximately 250 µm