Thermal processing of EVA encapsulants and effects of formulation additives [for solar cells]

The authors investigated the in-situ processing temperatures and effects of various formulation additives on the formation of ultraviolet (UV) excitable chromophores in the thermal lamination and curing of ethylene-vinyl acetate (EVA) encapsulants for solar cells. A programmable, microprocessor-controlled, double-bag vacuum laminator was used to study two commercial as-formulated EVA films, A9918P and 15295P, and solution-cast films of Elvax^TM (EVX) impregnated with various curing agents and antioxidants. The results show that the actual measured temperatures of EVA lagged significantly behind the programmed profiles for the heating elements and were affected by the total thermal mass loaded inside the laminator chamber. The antioxidant Naugard p^TM, used in the two commercial EVA formulations, greatly enhances the formation of UV-excitable, short chromophores upon curing, whereas other tested antioxidants show little effect. A new curing agent chosen specifically for the EVA formulation modification produces little or no effect on chromophore formation, no bubbling problems in the glass/EVX/glass laminates, and a gel content of ~80% when cured at programmed 155°C for 4 min. Also demonstrated is the greater discoloring effect with higher concentrations of curing-generated chromophores