The degree of crosslinking is dependent on i) the lamination temperature, ii) the initial concentration of crosslinker and iii) the lamination time. In industrial practice, the key process parameter is the lamination time. As module lamination is the time-limiting factor in PV module manufacturing, this creates a conflict of interest.
Therefore, the main aim of this study is to investigate the cross-linking behavior of EVA but also for optimization potentials of the EVA formulation. Currently, a degree of cross-linking higher than 70% obtained from Soxhlet extraction, is used as quality control standard in PV industry.
Altogether, Raman spectroscopy would appear the most promising approach for realising an in-line instrument for measuring the degree of EVA crosslinking in PV modules. The method is fast, non-destructive, easy to use (once a calibration model has been established), and requires only a single optically accessible side, i.e. the front glass.
This “DSC degree of crosslinking” (X) was thus determined from the reaction enthalpy Δ H(Sx) of the crosslinking reaction of the respective test sample in comparison to the reaction enthalpy Δ H(S0) of the uncured EVA reference (average of all S 0 samples) according to (3) X (S x) = Δ H (S 0) − Δ H (S x) Δ H (S 0) 2.3.2.
In this study, only two optical methods showed realistic potential for measuring the degree of crosslinking of EVA encapsulants in-line in a PV manufacturing line: UV/Vis spectroscopy and Raman spectroscopy.
For the most common encapsulant, elastomeric ethylene vinyl acetate (EVA), the degree of crosslinking is a key issue in this respect. Raman spectroscopy was researched as a fully contact-free, truly non-destructive optical method to measure the degree of EVA crosslinking of bare EVA film samples as well as in-situ inside assembled PV modules.