Polymer encapsulants characterized by fluorescence analysis before and after degradation

Structural changes of the luminescent chromophores in various polymer encapsulants for photovoltaic modules due to processing, weathering and artificial degradation are characterized by the nondestructive, noninvasive fluorescence analysis. Undegraded, simulation-degraded, and weathering-degraded EVA films have been studied. The results show that curing the EVA films can generate new chromophores of /spl alpha/,/spl beta/-unsaturated carbonyls that can be excited by UV light up to /spl sim/400 nm. Their concentration in the EVA films made by a typical slow-curing process for EVA A9918 formulation is greater than those made by a fast-curing process for EVA 15295 formulation. Photothermal degradation of the EVA begins with an increase and lengthening of these chromophores. The UV-exposed EVA A9918 in solar cells turns brown and exhibits two types (one short and one long) of chromophores, and the thermally treated EVA shows yellow and only the short chromophores. The extent of discoloration from yellow to dark brown is a direct measure of the extent of degradation and an indication of an increase in the length of the polyconjugated carbon-carbon double bonds (polyenes). Similar results are observed for the weathering-degraded, yellow and brown PVB encapsulants in roof-top modules.<>