Environmental Mechanisms of Debonding in Photovoltaic Backsheets

The backsheets used in photovoltaic modules are exposed to aggressive field environments that may include combined temperature cycles, moisture, and mechanical loads. The effects of the field environment on backsheet debonding, which can lead to module degradation (corrosion) and loss of function, are still not well understood or quantified. Employing a newly developed quantitative mechanics technique, we report the effect of aging on backsheet debond energy, including the separate effect of temperature, mechanical stress and relative humidity on debond growth rate. The debond energy of the backsheet decreased dramatically from 1000 to 27 J/m² within the first 750 h of exposure to hot (85 C) and humid (85% RH) aging treatments. The debond growth rate increased up to 500-fold with small changes of temperature (10 C) and relative humidity (20%). To elucidate the mechanisms of environmental debonding, we developed a fracture-kinetics model, where the molecular relaxation processes at the debond front are used to predict debond growth. The model and techniques form the fundamental basis to develop accelerated aging tests and long-term reliability predictions for photovoltaic backsheets.