Characterisation techniques to assess functional properties of barrier coatings for flexible PV substrates

This paper details findings from recent work undertaken as part of the EU funded NanoMend project. The aim of the project is to develop integrated process inspection, cleaning, repair and control systems for nano-scale thin films on large area substrates. The presence of surface irregularities on the uncoated film can induce "defects" within the, aluminium oxide, barrier layer, these features are in the order of magnitude of nanometres to a few microns. These "defects" are examined and quantitatively characterised. The measurements are carried out using a Taylor Hobson Coherence Correlation Interferometer an optical microscope and SEM. The efficient separation of small features from large features is achieved using data segmentation and feature parameter techniques. The presence of both large and small features is then correlated with the water vapour transmission rate as measured on representative sets of films using at standard MOCON (quantitative gas permeation) test. The paper concludes that, based on analysis of water vapour transmission rates (WVTR) and defect size that small numbers of large features have a more significant effect on the deterioration of water vapour transmission rates. It is the WVTR which indicates the potential longevity of the device as it is the effectiveness of this ALD barrier layer to water vapour which ensures the effective function of the core CIGS layers encapsulated in the device. The ability to measure and effectively characterise the features which are significant defects in the ALD barrier layers provides novel information to enable automatic detection and correction of potential restrictors to device performance in the processes.