Characterizations of aged Glass/Ethylene Vinyl Acetate/Glass using fluorescence spectroscopy and instrumented indentation

Ethylene vinyl acetate (EVA) is widely used as an encapsulant in photovoltaic devices, which are exposed to simultaneous UV irradiation, thermal, and humidity conditions. Aging behaviors of the EVA were characterized using confocal fluorescence spectroscopy with submicron scale resolutions of cross-sections of aged EVA samples. Additionally, mechanical properties of fluorescing regions in the aged EVAs where fluorescence emission was measured were investigated using a nanoindentation technique. In this study, we characterized two different formulations of EVA in glass/EVA/glass specimens after UV irradiation at 40 °C, 60 °C and 80 °C with 50 % relative humidity for 180 days. The fluorescence intensities on the UV irradiated sides of EVA samples were higher than those for the unexposed sides over wavelengths from 400 nm to 800 nm, and intensity differences between the two sides became more pronounced when the specimens were aged at higher temperatures. Elastic moduli of UV irradiated side for samples measured by instrumented indentation decreased, compared to unaged EVA samples, while fluorescent intensity increased with increasing the aging temperature.