Mohamed Elrawemi

Correlation of micro and nano–scale defects with WVTR for aluminium oxide barrier coatings for flexible photovoltaic modules

This paper seeks to establish a correlation between surface topographical defects and water vapour transmission rate (WVTR) measured under laboratory conditions for aluminium–oxide (Al2O3) 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/m2/day is sufficient for the most packaging applications, but ≤10−6 g/m2/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.

Implementation of in Process Surface Metrology for R2R Flexible PV Barrier Films

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.

The Use of Feature Parameters to Asses Barrier Properties of ALD coatings for Flexible PV Substrates

This paper reports on the recent work carried out as part of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair and control systems for nano-scale thin films on large area substrates. In the present study flexible photovoltaic films have been the substrate of interest. Flexible PV films are the subject of significant development at present and the latest films have efficiencies at or beyond the level of Si based rigid PV modules. These flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes, where the whole film is approximately 3um thick prior to encapsulation. Whilst flexible films offer significant advantages in terms of mass and the possibility of building integration (BIPV) they are at present susceptible to long term environmental degradation as a result of water vapor transmission through the barrier layers to the CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) PV cells thus causing electrical shorts and efficiency drops. Environmental protection of the GIGS cell is provided by a thin (40nm) barrier coating of Al2O3. The highly conformal aluminium oxide barrier layer is produced by atomic layer deposition (ALD) where, the ultra-thin Al2O3 layer is deposited onto polymer thin films before these films encapsulate the PV cell. The surface of the starting polymer film must be of very high quality in order to avoid creating defects in the device layers. Since these defects reduce manufacturing yield, in order to prevent them, a further thin polymer coating (planarization layer) is generally applied to the polymer film prior to deposition. The presence of surface irregularities on the uncoated film can create defects within the nanometre-scale, aluminium oxide, barrier layer and these are measured and characterised. This paper begins by reporting the results of early stage measurements conducted to characterise the uncoated and coated polymer film surface topography using feature parameter analysis. The measurements are carried out using a Taylor Hobson Coherence Correlation Interferometer an optical microscope and SEM. Feature parameter analysis allows the efficient separation of small insignificant defects from large defects. The presence of both large and insignificant defects is then correlated with the water vapour transmission rate as measured on representative sets of films using at standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, where it is postulated that small numbers of large defects play a significant role in higher levels of WVTR.

Further development of surface metrology methods for predicting the functional performance of flexible photovoltaic barrier films

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 Al2O3 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.

Defect detection in thin-film photovoltaics; Towards improved efficiency and longevity

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. 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 water vapor transmission rate (WVTR) the PV encapsulation includes a barrier layer of amorphous Al2O3 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 nano-scale defects in these barriers which results in decreased cell efficiency and reduced longevity. The main aim of this research paper is to use surface metrology techniques including: White Light Scanning Interferometry (WLSI), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to characterise the water vapor barrier defects which are seemed to be responsible for the water vapor permeation. A real 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.

Metrology of Al2O3 Barrier Film for Flexible CIGS Solar Cells

Flexible Cu (In, Ga) Se2 (CIGS) solar cells are very attractive renewable energy sources because of their high conversion efficiencies, their low cost potential and their many application possibilities. However, they are at present highly susceptible to long term environmental degradation as a result of water vapor ingress through the protective encapsulation layer to the absorber (CIGS) layer. The basic methodology to prevent the water vapor permeation is to combine an oxide layer (e.g. AlOx) coating with suitable polymer substrates. Nevertheless, micro and nano-scale defects can appear at any stage of the coating process thus affecting the module efficiency and lifespan. The main aim of this research paper is to use surface metrology techniques including: White Light Scanning Interferometry (WLSI), Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM) to characterise the aluminum oxide (Al2O3) barrier film defects, which appear to be directly responsible for the water vapor permeability. This paper reports on the development of a characterisation method for defect detection based on “Wolf Pruning†method and then correlates this with measured water vapor transmission rates (WVTRs) using standard MOCON® test. The results presented in this paper provided a detailed knowledge of the nature of micro and nano-scale defects on the Al2O3 barrier films which are responsible for water vapor and oxygen ingress. This result can then be used to provide the basis for developing roll-to-roll in process metrology devices for quality control of flexible PV module manufacture.

In-situ defect detection systems for R2R flexible PV barrier films

Film processing procedures by means of Roll-to-Roll (R2R) for barrier coatings can often result in PV barrier films being manufactured with significant quantities of defects, which results in lower efficiency and a short life span. In order to improve the process yield and product efficiency, it is desirable to develop an inspection system that can detect transparent barrier film defects in the production line during film processing. Off-line detection of defects in transparent PV barrier films is difficult and time consuming. Consequently, implementing an accurate in-situ defects inspection system in the production environment is even more challenging, since the requirements on positioning, fast measurement, long term stability and robustness against environmental disturbance are demanding. This paper reports on the development and deployment of two in-situ PV barrier films defect detection systems, one based on wavelength scanning interferometry (WSI) and the other on White Light Channeled Spectral Interferometry (WLCSI), and the integration into an R2R film processing line at the Centre for Process Innovation (CPI). The paper outlines the environmental vibration strategy for both systems, and the developed auto-focusing methodology for WSI. The systems have been tested and characterised and initial results compared to laboratory-based instrumentation are presented.