THE RIGHT WAY TO CHARACTERIZE ADHESION OF POLYMERS IN PV MODULES

It is widely accepted that adherence (which reports to what is measured when performing an adhesion test) of the encapsulant to the main substrates of the module plays a key role in the long term reliability of the PV module [1,2]. Consequently, adherence is commonly measured and used to assess or compare encapsulant compatibility with a given substrate. The most common procedures used in the PV field to characterize adhesion between a polymer film and a substrate are the so called peeling test, lap shear and compressive shear tests. Here we use a compressive shear setup to characterize the adherence of Poly Vinyl Butyral (PVB) and Polyethylene-covinyl acetate (EVA) to glass before and after degradation in damp-heat (DH) conditions (85°C, 85%RH). The adherence metrics that can be derived from a Compressive Shear Test (CST) are presented and discussed. We show that a single metric is not sufficient to characterize adherence and that a set of at least two indicators including the peak shear stress and the viscous dissipation should be used. Using this set it is found that the interface PVB/Glass is more affected by the degradation than the EVA/Glass interface.PV-LABIMT-NE Number: 61

Compressive-shear adhesion characterization of polyvinyl-butyral and ethylene-vinyl acetate at different curing times before and after exposure to damp-heat conditions

Photovoltaic (PV) module efficiency and reliability are two factors that have an important impact on the final cost of the PV electricity production. It is widely accepted that a good adhesion between the encapsulant and the different substrates of a PV module is needed to ensure long-term reliability. Several testing procedures exist that use a metric derived from the force at interface failure to characterize the adhesion. It has, however, not been demonstrated that those metrics relate directly to the interfacial adhesion (defined as the surface energy density needed to break interfacial bonds), and the obtained results usually relate to an apparent adhesion strength. In this work, we describe a new design for compressive-shear testing of polymer layers bonded to rigid substrates. We use it to characterize real interfacial adhesion of ethylene-vinyl acetate (EVA) and polyvinyl-butyral (PVB) to a glass substrate before and after degradation in damp-heat. Our results show that a peak-force based metric is unable to capture the evolution of adhesion through degradation, and a new metric based on the elastic strain energy of the encapsulant is proposed. Moreover, we show that PVB adhesion to glass is much more affected by damp-heat exposure where polymer saturation takes place, in comparison with the adhesion of EVA to glass. The presented characterization protocol is a powerful tool that can help in assessing the reliability of an encapsulant facing specific degradation conditions. Copyright © 2012 John Wiley & Sons, Ltd