Transparent barrier films such as Al 2 O 3 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 (≤3 μm lateral dimension) were less significant in determining water vapour ingress. In contrast, larger defects (≥3 μm 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.

Thin functional barrier layers of aluminum oxide (Al2O3) that are used particularly in photovoltaic (PV) modules to prevent the possibility of water vapor ingress should be applied over the entire PV surface without any defects. However, for barrier layer thicknesses within the sub-micrometer range (up to 50 nm) produced through the atomic layer deposition (ALD) method, it is common for defects to occur during the production process. To avoid defective barriers from being incorporated in the final PV unit, defects need to be detected during the barrier production process.In this paper, the implementation of in process inspection system capable of detecting surface defects such as pinholes, scratches, or particles down to a lateral size of 3 µm and a vertical resolution of 10 nm over a 500 mm barrier width is presented. The system has a built-in environmental vibration compensation capability, and can monitor ALD-coated films manufactured using roll-to-roll (R2R) techniques. Ultimately, with the aid of this in process measurement system, it should be possible to monitor the coating surface process of large-area substrates, and if necessary, carry out remedial work on the process parameters.

This paper seeks to establish a correlation between surface topographical defects and Water Vapour Transmission Rate (WVTR) measured under laboratory conditions for aluminium-oxide (Al 2 O 3) barrier film employed in flexible photovoltaic (PV) modules. Defects in the barrier layers of PV modules causing high WVTR are not well characterised and understood. A WVTR of ~10-1 g/m 2 /day is sufficient for the most packaging applications, but ≤10-6 g/m 2 /day is required for the encapsulation of long-life flexible PV modules (Carcia et al., 2010a, 2010b). In this study, Surface metrology techniques along with Scanning Electron Microscopy (SEM) were used for a quantitative characterisation of the barrier film defects. The investigation have provided clear evidence for the correlation of surface defect density and the transmission of water vapour through the barrier coating layer. The outcomes would appear to suggest that small numbers of large defects are the dominant factor in determining WVTR for these barrier layers.

Abstract. Surface topography analysis plays a very significant role in determining the functional performance for many engineering surfaces. In this paper, feature characterisation techniques, based on the 'Wolf pruning' method are implemented to characterise micro and nano-scale features which have a dominant effect on the functional lifespan of flexible Photovoltaic (PV) modules. The densities and dimensions of the potential significant features are calculated by means of the feature "characterisation toolbox". The outcome of this study has shown the potential of areal feature segmentation for detecting functionally significant defects present in Atomic Layer Deposition (ALD) barrier coatings of Al 2 O 3 on polymer films. The analysis provides the basis for the development in process metrology for Roll-to-Roll (R2R) production of barrier coatings as applied to flexible PV arrays and is a first step in the demonstration of in-process use of feature parameters.

Abstract-The Photovoltaic (PV) industry is seeking to increase efficiency and functional lifetime of PV modules manufactured on polymer substrates. High resolution and high speed surface inspection for the quality control of the manufacture of large area flexible PV modules are necessary to guarantee maximum quality, longer lifetime and enhanced product yield. Flexible PV films are the newest development in the renewable energy field and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. These modules are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll technology. However, they are at present highly susceptible to long term environmental degradation as a result of water vapor transmission through the protective encapsulation to the active layer. To reduce the WVTR the PV encapsulation includes a barrier layer of amorphous Al 2 O 3 on a planarised polymer substrate. This highly conformal barrier layer is produced by atomic layer deposition (ALD). Nevertheless water vapour transmission is still facilitated by the presence of micro and nanoscale defects in these barriers which results in decreased cell efficiency and reduced longevity. Analytical techniques including: White Light ScanningInterferometry (WLSI), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to characterise the water vapor barrier defects. Areal surface texture parameter analysis allows the efficient separation of small insignificant features from significant defects. This parametric analysis is then correlated with the water vapour transmission rate as measured on typical sets of films using standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, density and distribution, where it is postulated that small numbers of large features have more influence on the deterioration of water vapor transmission rates than large numbers of small features. This result provides the basis for developing roll-to-roll in process metrology devices for quality control.