Modelling water vapour permeability through atomic layer deposition coated photovoltaic barrier defects

Transparent barrier films such as Al2O3 used for prevention of oxygen and/or water vapour permeation are the subject of increasing research interest when used for the encapsulation of flexible photovoltaic modules. However, the existence of micro-scale defects in the barrier surface topography has been shown to have the potential to facilitate water vapour ingress, thereby reducing cell efficiency and causing internal electrical shorts. Previous work has shown that small defects (≤3μm lateral dimension) were less significant in determining water vapour ingress. In contrast, larger defects (≥3μm lateral dimension) seem to be more detrimental to the barrier functionality. Experimental results based on surface topography segmentation analysis and a model presented in this paper will be used to test the hypothesis that the major contributing defects to water vapour transmission rate are small numbers of large defects. The model highlighted in this study has the potential to be used for gaining a better understanding of photovoltaic module efficiency and performance. •A model of water vapour permeation through barrier defects is presented.•The effect of the defects on the water vapour permeability is investigated.•Defect density correlates with water vapour permeability.•Large defects may dominate the permeation properties of the barrier film.