Functional Modelling of Water Vapour Transmission through Surface Defects Using Surface Segmentation Analysis

Flexible photovoltaic films have been recently shown to have efficiencies comparable to those of solid Si based photovoltaics. Flexible PV films have significant advantages in terms of ease of manufacture by roll-to-roll (R2R) techniques and in easy building integration. A significant challenge is the protection of the flexible solar cells from water vapour ingress, which seriously reduces cell life and efficiency. Transparent barrier films are a possible solution to addressing the water vapour transmission rate (WVTR) challenge. Consequently thin barrier films such as those made from Al2O3 are the subject of increasing research interest when used for the encapsulation of flexible PV modules. The film can be produced by several thin film deposition processes such as atomic layer deposition (ALD). However, micro-scale defects in the barrier film such as pinholes and particulate debris have been shown to have serious consequences in terms of WVTR. Our previous research has empirically shown that small defects (≤3μm lateral dimension) were less significant in determining water vapour ingress. In contrast, larger defects (≥3 μm lateral dimension) have been shown to have a greater effect on the barrier functionality. The present paper illustrates the use of surface segmentation techniques to efficiently extract defect data from measured surface topography of barrier film sheets. Experimental results are presented where the defect information is correlated with the WVTR tests. A model is then presented to test the hypothesis that the major contributing defects to water vapour transmission rate (WVTR) are small numbers of large defects. The model presented in the paper shows excellent correlation with experimental results and provides a theoretical basis for the development of in process surface measurement for thin film R2R manufacture

Nano scale Characterisation of Photovoltaic Ultra Barrier Films

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

Implementation of wavelength scanning interferometry for R2R flexible PV barrier films

Roll to Roll manufacture of nano-scale thinf ilm layers faces the challenge of micro/nano-scale defects appearing in the films. Atomic Layer Deposition (ALD) coatings of aluminium oxide, Al2O3 are used as barrier layers for photovoltaic (PV) solar modules where the primary function of the barrier layer is to prevent the water vapour ingress to the PV cells. Barrier layer defects have been shown to have negative impact on the performance of the barrier layers. Poor barriers cause module degradation resulting in reduced PV efficiency and lifespan. In order to ensure the quality of manufacture of the barriers, defects should be detected during the barrier production process and the information used to optimise the production process. This paper introduces, as part of EU funded NanoMend project, a full solution for inspection of entire surface regions of Al2O3 barrier films across large area substrates. The solution principle is based on implementing an opto-mechanical in-process inspection system to measure the significant defects using a wavelength scanning interferometer (WSI) embedded within the film-rewinder stage and integrated with the substrate translation and kinematic stages. The opto-mechanical system allows full surface measurement over full substrate widths of approximately 0.5m. The system provides an auto-focus for the WSI with an accuracy and repeatability better than 6 µm at optimum optical alignment conditions. The system is combined with a porous air-bearing conveyor used to hold the film web at fixed height within the focal depth of WSI objective lens and with height variation of 3 µm is also presented as a case study to highlight the system capability

Wavelength Scanning Interferometery for large area roll to roll metrology applications in photovoltaic manufacturing environment

The wavelength scanning interferometer is currently being applied as a core metrology technology as part of the EU project NanoMend - Nanoscale Defect Detection, Cleaning and Repair for Large Area Substrates ̴500 mm width. NanoMend Project aims to develop technologies that are able to detect and correct micro and nano-scale defects in roll-to-roll produced films in order to improve product performance, yield and lifetime

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

Metrology and Characterisation of Defects in Thin-Film Barrier Layers Employed in Flexible Photovoltaic Modules

Flexible thin-film photovoltaic (PV) modules based on copper indium gallium selenide (CIGS) materials are one of the most recent developments in the renewable energy field, and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. Whilst these films offer significant advantages in terms of mass and the possibility of building-integrated photovoltaic (BIPV) applications, they are at present highly susceptible to long term environmental degradation as a result of water vapour transmission through the protective encapsulation layer to the active (absorber) layer. To maintain the PV module flexibility and to reduce or eliminate the water vapour permeability, the PV encapsulation includes a barrier layer of amorphous aluminium oxide (Al2O3) material of a few nanometres thickness deposited on a planarised polyethylene naphthalate (PEN) substrate. The highly conformal barrier layer of the Al2O3 is produced by atomic layer deposition (ALD) methods using roll-to-roll (R2R) technology. Nevertheless, water vapour permeation is still facilitated by the presence of micro and nano-scale defects generated during the deposition processes of the barrier material, which results in decreased cell efficiency and reduced unit longevity. The state of the art surface metrology technologies including: optical microscopy, white light scanning interferometry (WLSI), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were extensively deployed in this project as offline surface characterisation methods to characterise the water vapour barrier layer defects, which are postulated to be directly responsible for the water vapour ingress. Areal surface texture parameters analysis based on wolf pruning, area pruning and segmentation analysis methods as defined in ISO 25178-2; allow the efficient separation of small insignificant defects from significant defects. The presence of both large and small defects is then correlated with the barrier films functionality as measured on typical sets of Al2O3 ALD films using a standard MOCON® (quantitative gas permeation) test. The investigation results of the initial analysis finishes by drawing conclusions based on the analysis of the water vapour transmission rate (WVTR), defects size, density and distribution, where it is confirmed that small numbers of large defects have more influence on the deterioration of the barrier films functionality than large numbers of small defects. This result was then used to provide the basis for developing a roll-to-roll in process metrology device for quality control of flexible PV barrier films. Furthermore, a theoretical model approach was developed in this thesis based on the water vapour diffusion theory to determine the cut- off level between large significant defects and small insignificant defects. The results of the model would seem to reveal that, in order to build up in process, non-contact optical defect detection system for R2R barrier films, the critical spatial resolution required for defect detection need not be less than 3 μm laterally and 3Sq nm (Sq= root mean square surface roughness deviation of non-defective sample area) per field of view (FOV) vertically. Any defect that has dimensions less than this appears to have a significantly lower effect on the PV barrier properties and functionality. In this study, the surface topography analysis results and the theoretical model approach outcomes, both provide the basis for developing a R2R in process metrology device for PV barrier films defect detection. Eventually, the work in this thesis reports on the deployment of new (novel) in-line interferometric optical sensors based on wavelength scanning interferometry (WSI) designed to measure and catalogue the PV barrier films defects where they are present. The sensors have built-in environmental vibration compensation and are being deployed on a demonstrator system at a R2R production facility in the UK

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

Wavelength Scanning Interferometry for PV Production In-line Metrology

Flexible PV modules are manufactured using roll to roll (R2R) technology. These modules require a flexible barrier material to prevent water vapor ingress

A computerised data handling procedure for defect detection and analysis for large area substrates manufactured by roll-to-roll process

The development of optical on-line/in-process surface inspection and characterisation systems for flexible roll to roll (R2R) thin film barriers used for photo-voltaic (PV) modules is a core research goal for the EU funded NanoMend project. Micro and nano scale defects in the ALD (atomic layer deposition) Al2O3 barrier coating produced by R2R techniques can affect the PV module efficiency and lifespan. The presence of defects has been shown to have a clear correlation with the water-vapour-transmission-rate (WVTR). Hence, in order to improve the PV cell performance and lifespan the barrier film layer must prevent water vapour ingress. One of the main challenges for the application of in process metrology is how to assess large and multiple measurement data sets obtained from an in process optical instrument. Measuring the surface topography over large area substrates (approximately 500 mm substrate width) with a limited field-of-view (FOV) of the optical instrument will produce hundreds/thousands of measurement files. Assessing each file individually to find and analyse defects manually is time consuming and impractical. This paper reports the basis of a computerised solution to assess these files by monitoring and extracting areal surface topography parameters. Comparing parameter values to an experimentally determined threshold value, obtained from extensive lab-based measurement of Al2O3 ALD coated films, can indicate the existence of the defects within a given FOV. This process can be repeated automatically for chosen parameters and the existence of defects can be indicated for the entire set of measurement files spontaneously without interaction from the inspector. A running defect log and defect statistics associated with the captured set of data files can be generated. This paper outlines the implementation of the auto-defect logging using advanced areal parameters, and its application in a proof of concept system at the Center for Process Innovation (UK) is discussed

In-line metrology of functional surfaces with a focus on defect assessment on large area Roll to Roll substrates

This paper reports on the recent work carried out as part of the initial stages of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair for nano-scale thin films on large area substrates. Flexible photovoltaic (PV) films based on CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) have been reported to have light energy conversion efficiencies as high as 19%. CIGS based multi-layer flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes (R2R), where the whole film is approximately 3μm thick prior to final encapsulation. The resultant films are lightweight and easily adaptable to building integration. Current wide scale implementation however is hampered by long term degradation of efficiency due to water ingress to the CIGS modules causing electrical shorts and efficiency drops. The present work reports on the use of areal surface metrology to correlate defect morphology with water vapour transmission rate (WVTR) through the protective barrier coatings