Reliability of Back-Contact MWT Modules under Hot and Humid Conditions

Abstract

Aims: Back-contact metallization-wrap-through (MWT) modules made with different materials were manufactured and tested at ECN in The Netherlands. Study Design: Damp heat testing (DH, 85°C and 85% relative humidity) was performed to investigate the effect of the module materials and potential failure mechanisms on electrical performance degradation. Place and Duration of Study: ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, Netherlands and Centre de Développement des Energies Renouvelables, Division Energie Solaire Photovoltaïque, Route de l'Observatoire, BP 62, Bouzaréah, 16340, Alger, Algeria, between June 2011 and March 2015. Methodology: Fourteen back-contact MWT modules with different combinations of four conductive backsheet foils (inner isolation layers, also referred to as inner layer dielectric, ILD), two electrically conductive adhesives (CA) and two encapsulants were tested. Results: Results reveal that modules which combine conductive backsheet foil with two types of ILD and ethylene-vinyl-acetate (EVA) showed the highest degradation levels. It is suggested that effects of moisture in EVA cause a weakening of the adhesion strength at the ILD/Cu interface and then delamination between conductive adhesive and Ag-plated contacts applied to the copper interconnection foil. Removal of ILD significantly improves the stability of back-contact modules. Enhanced reliability is observed for modules which combine ILD-free foil Cu backsheet, conductive adhesive and polyolefin or EVA as encapsulant. Conclusion: Removal of ILD is the most interesting approach and is currently adopted by foil manufacturers. Importantly, the modules built with ILD-free backsheet and either polyolefin encapsulant or EVA showed respectively only 0.3% and 1.6% maximum power loss after 2000 hours of damp heat which is well below the requirement of the IEC 61215 standard.