Metrology and characterisation of defects on barrier layers for thin film flexible photovoltaics

This paper reports on the recent work carried out as part of the EU funded NanoMend project. Part of the project seeks to develop integrated process inspection, cleaning, repair and associated control systems for the manufacture of large area, nano-scale thin films of CIGS (Copper Indium Gallium Selenide CuIn_xGa _(1-x) Se₂) flexible PVs. Transparent barrier films such as aluminium-oxide (Al₂O₃) can be used as a barrier to oxygen and/or water vapor permeation and such layers are of increasing interest for the encapsulation of flexible PV modules. However, the existence of micro and nano-scale defects in the barrier surface topography has been shown to have the potential to facilitate water vapor ingress, thereby reducing cell efficiency and causing internal electrical shorts in the CIGS cells. Therefore, improvement of the quality of these barrier films is critical to reduce the susceptibility of these photovoltaic systems to environmental degradation. This paper reports on the development of a characterisation method for defect detection and then correlates this with measured water vapor transmission rates (WVTRs). The results show small numbers of large defects play the dominant role in determining the WVTR. A unified classification system for defects is proposed where the classification breaks down the defects types into four main functional groupings.