Ethylene-Vinyl Acetate (EVA) Encapsulants for Photovoltaic Modules: Degradation and Discoloration Mechanisms and Formulation Modifications for Improved Photostability

Long-term photothermal stability is needed for the polymeric encapsulants made of ethylene-vinyl acetate (EVA) copolymer used in crystalline-silicon (c-Si) based photovoltaic (PV) modules. severe browning of EVA was observed in late 1980s on Carrisa Plains PV arrays installed with reflecting mirrors and resulted in large losses of the power output. The EVA browning crisis prompted NREL scientistsin 1990 to investigate the problems and search for solutions. The key degradation mechanisms and discoloration factors were investigated by using a variety of film samples, including field-degraded EVA samples and two commercial EVA formulations, regular-cure A9918 and fast cure 15295, that were laminated, cured, and subjected to accelerated environmental exposures. Chemical, thermogravimetric,absorption/transmission, colorimetric, and fluorescence analyses were employed to characterize the EVA samples. Current-voltage (I-V) measurements were used to determine the electrical performance characteristics of solar cells. The results show that photothermal degradation is greater than thermal degradation and that the EVA A9918 discolors earlier and faster than EVA 15295. The UV absorber,Cyasorb UV 531 (trademark), in commercial formulations decomposes photochemically and its decomposition rate is directly related to the temporal evolution of discoloring polyenic chromophores. In general, the loss rate of the UV absorber and the progress of EVA discoloration from light yellow to brown follow a sigmoidal pattern. Physically, UV-filtering glass superstrates that remove UV </= 350nm effectively reduce the discoloration rate of both commercial EVA formulations. Chemically, the EVA discoloration rate is affected by a number of factors, but more strongly by the formulation additives and curing conditions, in which additives and curing-generated chromophores and their concentrations show a synergistic discoloring effect. Photobleaching reactions are responsible for thenon-discolored but degraded EVA because of oxidative destruction of the curing-generated chromophores in the presence of oxygen. By using specifically selected stabilizers and curing agents in the new formulations, a large reduction in the chromophore generation by curing has been achieved and resulted in significant improvement of the photostability against discoloration.