Degradation of adhesion strength within mini-modules during damp-heat exposure

The degradation of adhesion strength between back-sheet and encapsulant due to moisture ingress was investigated for commercial crystalline silicon photovoltaic (PV) mini-modules. The damp-heat test originated from qualification test was carried out at five different temperature and humidity conditions (95oC/85% RH, 85oC/85% RH, 65oC/85% RH, 85oC/65% RH and 85oC/45% RH) to assess the impact of stress levels on test outcomes. The adhesion strength was measured by 90o peel tests, carried out at specified degradation intervals. Several visual defects were observed, including delamination, moisture ingress and bubble formation. The adhesion strength showed a stretched exponential decay with time and significant influences of test conditions was demonstrated. A humidity dose model was proposed by assuming micro-climates seen by the modules, i.e. surface relative humidity of the back-sheet as the driving factor for an Arrhenius based model using temperature as accelerating factor. The correlation between adhesion degradation and humidity dose was investigated and an exponential model was developed to represent this correlation with extracted activation energy (Ea) of 63kJ/mol. This supplies a potential model for the estimation of adhesion strength decay triggered out by humidity in dependence of the humidity conditions.