Correlation of spatially resolved photoluminescence and viscoelastic mechanical properties of encapsulating EVA in differently aged PV modules

The photoluminescence of ethylene vinyl acetate (EVA) in aged photovoltaic modules shows specific spatial patterns on the scale of the cells in the module that depend on the aging conditions. It is the aim of this work to investigate the correlation of these photoluminescence patterns to the viscoelastic mechanical properties of the encapsulating EVA. For this, the degradation under various conditions of two specimen sets of polycrystalline silicon photovoltaic mini-modules, comprising EVA as encapsulating polymer, is investigated using spatially resolved, ultraviolet-excited photoluminescence. Samples of the encapsulation EVA are systematically extracted from the modules for spatially resolved dynamic mechanical characterization. A correlation between the spatial distribution of the photoluminescence intensity and the values of storage modulus and loss factor of the EVA is found. An increase in the values for storage modulus and a decrease of the loss factor of the EVA from the edge of the cell towards the center are observed for all samples. We conclude that diffusion-limited oxidation must be considered for aging studies of the encapsulation EVA and that photoluminescence detection can reveal the scale on which such diffusion processes occur. Copyright (C) 2016 John Wiley & Sons, Ltd.