EP3669168A1

EP3669168A1 - Water vapour permeation

Info

Publication number: EP3669168A1
Application number: EP18759701.8A
Authority: EP
Inventors: Hazel Assender; Ashley Te Xiang SIM; Vincent TOBIN
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2017-08-18
Filing date: 2018-08-17
Publication date: 2020-06-24
Also published as: WO2019034888A1; GB201713262D0

Abstract

A method of analysing water vapour permeation through a barrier layer.Themethod comprises: providing a sample comprising a barrier layer and a layer of moisture-sensitive material having optical properties that change on exposure to moisture, wherein the layer of moisture-sensitive material is in fluid communication with one face of the barrier layer; exposing an opposite face of the barrier layer to a humid environment to allow water vapour to permeate through the barrier layer; andoptically imaging the layer of moisture- sensitive material over time, wherein the optical images are taken in situwhile the opposite face of the barrier layer is exposed to the humid environment.

Description

WATER VAPOUR PERMEATION

[0001] This invention relates to a method of analysing water vapour permeation through a barrier layer. In particular, the invention relates to a method of method of analysing water vapour permeation through a barrier layer for flexible optoelectronic devices.

BACKGROUND

[0002] In recent years, there has been a growing interest in the development of organic optoelectronic devices (e.g. Organic Light Emitting Diodes (OLEDs) and Organic

Photovoltaics (OPVs)).

[0003] Such optoelectronic devices, however, are extremely sensitive to moisture.

Accordingly, to improve the lifetime of such devices, the devices need to be encapsulated or protected by barrier layers. Encapsulation in glass can be effective for certain applications. However, flexible barrier layers are preferred for flexible optoelectronic applications.

[0004] It is known to encapsulate flexible optoelectronic devices using barrier layers, for example, transparent polymer films. To improve the barrier properties of such polymer films, thin oxide-based or nitride-based coatings have been used to further reduce the permeation of gas species.

[0005] Various methods have been developed to determine the moisture permeability of such polymer films. For example, a calcium test may be employed. In this test, a layer of calcium is used to react with water vapour that permeates through the polymer film under test. The reaction results in the conversion of calcium metal to calcium hydroxide.

Accordingly, by measuring the extent of reaction, the permeability of the polymer film may be determined.

[0006] In one prior art method, the extent of reaction is determined by determining the change in electrical resistance of the layer of calcium. The water vapour permeation rate may be determined by the rate of change of the electrical resistance with time. In an alternative prior art method, the extent of reaction may be determined by measuring the transmission of light through the layer of calcium. When opaque calcium metal reacts with water, it forms calcium hydroxide, which is transparent. The rate of change of the intensity of light that permeates the polymer film, therefore, can provide an indication of the water vapour permeation rate of the film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a sample containing a barrier layer constructed according to a prior art method;

Figure 2 is a schematic drawing of a sample containing a barrier layer constructed according to a method of an embodiment of the present invention;

Figures 3a is a plan view of an apparatus according to one embodiment of the present invention;

Figure 3b is a cross-sectional view of the apparatus of Figure 3a;

Figure 4 shows images from a calcium test according to one embodiment of the present invention conducted on an uncoated PEN barrier;

Figure 5 is a graph showing how the water vapour transmission rate (WVTR) of a

PEN barrier varies with time;

Figure 6 shows images from a calcium test according to one embodiment of the present invention conducted on a PEN barrier coated with aluminium oxide;

Figure 7 is a graph showing how the area covered by spots varies with time; Figure 8 is a graph showing how the number of spots varies with time;

Figure 9 compares images from a calcium test according to one embodiment of the present invention at two separate time intervals; and

Figure 10 is a graph showing the WVTR of a spot on a PEN barrier coated with aluminium oxide.

DETAILED DESCRIPTION

[0008] According to one aspect of the present invention, there is provided a method of analysing water vapour permeation through a barrier layer, said method comprising:

providing a sample comprising a barrier layer and a layer of moisture-sensitive material having optical properties that change on exposure to moisture, wherein the layer of moisture-sensitive material is in fluid communication with one face of the barrier layer; exposing an opposite face of the barrier layer to a humid environment to allow water vapour to permeate through the barrier layer; and

optically imaging the layer of moisture-sensitive material over time, wherein the optical images are taken in situ while the opposite face of the barrier layer is exposed to the humid environment.

[0009] It has been found that, by optically imaging the layer of moisture-sensitive material in situ while the opposite face of the barrier layer is exposed to a humid environment, it may be possible to observe changes in the optical properties of the layer of moisture-sensitive material in situ while water vapour permeates through the barrier layer. This can allow an observer to gain information on the mechanisms by which water vapour permeates the barrier layer. This may provide an observer with diagnostic information on the inherent properties of the barrier layer and/or the manufacturing process by which the barrier layer is produced.

[0010] Optically imaging the layer of moisture-sensitive material over time as described herein may comprise capturing images of the layer of moisture-sensitive material at different times. For example, images of the layer of moisture-sensitive material may be captured periodically. In some embodiments, at least one image of the layer of moisture- sensitive material is captured at least once every two hours or less. In some

embodiments, images may be captured more frequently. For example, at least one image of the layer of moisture-sensitive material may be captured at least once every hour, at least once every 30 minutes, at least once every 15 minutes or even once every 5 minutes or less. In some embodiments, multiple images of the layer of moisture-sensitive material may be captured every minute. For example, at least one image of the layer of moisture- sensitive material may be captured at least once every few seconds or even several times a second.

[0011] Water vapour may permeate through macrodefects and/or nanodefects in a barrier layer. It has been found that water vapour permeation through macrodefects can cause optical changes to appear as localised optical features (e.g. spots or lines) in the optical images of the layer of moisture sensitive material, while water vapour permeation through nanodefects can cause optical changes in the background optical brightness of the optical images of the layer of moisture sensitive material. In the present method, the growth of these localised optical features and/or the changes in the background optical brightness can be monitored to provide diagnostic information on the barrier layer.

[0012] Thus, in another aspect, the present invention provides a method of analysing water vapour permeation through a barrier layer, said method comprising:

providing a sample comprising a barrier layer and a layer of moisture-sensitive material having optical properties that change on exposure to moisture, wherein the layer of moisture-sensitive material is in fluid communication with one face of the barrier layer; exposing an opposite face of the barrier layer to a humid environment to allow water vapour to permeate through the barrier layer

optically imaging the layer of moisture-sensitive material over time, analysing the optical images for changes in localised optical features in the optical images with time, and/or

analysing the optical images for changes in the background optical brightness of the optical images with time.

[0013] In yet another aspect, there is provided an apparatus for analysing water vapour permeation through a barrier layer, said apparatus comprising:

a humidity chamber for housing a sample comprising the barrier layer in a humid environment;

a light source for transmitting light through the sample contained in the humidity chamber;

an optical sensor for obtaining optical images of a sample contained within the humidity chamber over time; and

a processor for analysing the growth in any localised optical features in the images obtained and for analysing changes in background optical brightness of the images obtained.

[0014] The analysis steps may be carried out using imaging software, for example, Image J™.

[0015] The analysis may allow a user to distinguish between water vapour permeation rates through nanodefects and permeation rates through macrodefects. It may also be possible to determine water vapour permeation rates through nanodefects and permeation rates through macrodefects.

[0016] The optical images may be taken in situ while the opposite face of the barrier layer is exposed to the humid environment.

[0017] In one embodiment, the localised optical features may be localised spots and/or localised lines in the optical images. Spots may arise as a result of permeation through e.g. pin-hole or point macrodefects in the barrier layer. Lines may arise, for example, as a result of cracks or scratches in the barrier layer. In one embodiment, changes in the localised spots and/or lines are determined by analysing the growth in the number and/or the growth in the size of one or more of the localised spots and/or lines. As mentioned above, this analysis may be carried out using imaging software, for example, Image J™.

[0018] Changes in the background optical brightness of the layer of moisture sensitive material may comprise changes in a spatially averaged or integrated optical brightness of the moisture sensitive material. Again, changes in this background brightness may be analysed using imaging software e.g. Image J™. [0019] Analysis of optical images as contemplated herein may include determining a change in one or more properties of the optical images between images taken at different times. The one or more properties of the optical images may include at least one property indicative of water vapour permeation through nanodefects in the barrier layer and at least one property indicative of water vapour permeation through macrodefects in the barrier layer.

[0020] A property of an optical image which is indicative of water vapour permeation through nanodefects may comprise a spatially averaged or integrated brightness level of the optical image. The spatial average or integration may be made over the entire optical image or at least over a majority of an optical image of the moisture sensitive material. Analysing changes in a spatially averaged or integrated brightness level of the optical images with time may provide an indication of water vapour permeation through

nanodefects in the barrier layer with time.

[0021] A property of an optical image which is indicative of water vapour permeation through macrodefects may comprise a property associated with one or more localised features in the image, such as spots and/or lines which appear in the image. For example, a property of an optical image which is indicative of water vapour permeation through macrodefects may comprise the number of localised features which appear in the image, the size of one or more localised features which appear in the image, the proportion of the image which is occupied by localised features and/or the total and/or average brightness of all localised features which appear in the image.

[0022] The analysis of optical images as contemplated herein may, for example, comprise identifying localised features (e.g. spots and/or lines) in the optical images. For example, standard image processing techniques such as edge detection may be used to identify localised features indicative of macrodefects in the barrier layer in an optical image. The analysis may further comprise characterising one or more identified localised features in the optical image, for example, by determining at least one property of the identified localised optical features in each image. The at least one determined property of the identified localised optical features in each image may include at least one of: the total number of identified localised optical features in each image, the proportion of the area of each image which is occupied by the identified localised optical features and/or a measure of the size of the identified localised optical features.

[0023] The analysis may further comprise analysing changes in the determined at least one property of the identified localised optical features between different images taken at different times. Such an analysis may provide an indication of water vapour permeation through macrodefects in the barrier layer with time. [0024] As was explained above, optical features such as lines or spots in an image may be indicative of macrodefects in the barrier layer, whereas the background or spatially averaged optical brightness of the image may be indicative of the nanodefects in the barrier layer. According to at least some embodiments contemplated herein, optical images are analysed to obtain both an indication of water vapour permeation through macrodefects and to obtain an indication of water vapour permeation through nanodefects. This may allow the contributions of macrodefects and nanodefects to the water vapour permeation to be distinguished from one another. For example, the obtained indication of water vapour permeation through macrodefects may be compared to the obtained indication of water vapour permeation through nanodefects. Such a distinction and/or comparison may inform any changes to be made in manufacturing parameters (which might reduce the water vapour permeation of a barrier layer).

[0025] The method of the present disclosure may be used to determine water vapour permeation through any barrier layer. The barrier layer may be any barrier layer. For example, the barrier layer may be a barrier layer suitable for use as packaging materials. In one embodiment, the barrier layer may be a barrier layer for any material or device requiring encapsulation, and, in particular, protection from exposure to moisture.

Preferably, the barrier layer may be a barrier layer for a flexible optoelectronic device, for example, an organic light emitting diode (OLED) or an organic photovoltaic device (OPV).

[0026] The barrier layer may comprise a supporting substrate, for example, a flexible supporting substrate. The substrate may be a polymer substrate, for example, a polymer film. Suitable polymer substrates may be formed of polyethylene, polypropylene, polyethylene terephthalate and polyethylene napthalate. The polymer substrate may have a thickness of 5μηι to 200μηι, preferably 12μηι to 125μηι, more preferably 12μηι to 30μηι.

[0027] The barrier layer may have water vapour transmission rate (WVTR) of less than 1 x 10^"6 g/m²/ day. The polymer substrate may have an oxygen transmission rate of less than 1 x 10^"3 cm³/m²/day.

[0028] In a preferred embodiment, the barrier layer may comprise a coating deposited on the (e.g. polymer) substrate. For example, the coating may be a coating of transparent inorganic material, for instance, oxides and/or nitrides. Suitable examples include ceramics, for instance, aluminium oxide, silicon oxide, titanium oxide, titanium nitride and aluminium oxynitride. The coating may have a thickness of 5nm to 2000nm, preferably 10nm to 500nm.

[0029] In some embodiments, the barrier layer may be a multi-layer structure. The multi- layer structure may include a polymer substrate comprising a coating as described above. Additional layers may be present, for example, over the coating, under the polymer substrate or intermediate the coating and polymer substrate. The additional layers may be organic and/or carbon-rich layers.

[0030] Where the coating is present, the sample may be positioned such that it is the substrate (e.g. polymer substrate) that is exposed to the humid environment, with the coating in contact with the layer of moisture-sensitive material.

[0031] The coating may be applied by any suitable deposition technique. Examples of such techniques include chemical vapour deposition (CVD), atomic layer deposition (ALD), evaporation and sputtering. In one embodiment, the coating may be formed by sputtering.

[0032] The barrier layer may include macrodefects and/or nanodefects. Macrodefects are defects that may induced by handling, process defects or substrate contamination. In contrast, nanodefects may be inherent to the barrier layer material itself, arising, for example, from the intergranular boundaries in the structure or from the disorder of the amorphous material. Typical sizes might be greater than 100nm for macrodefects, up to mm-scale, and below 100nm for nanodefects where transport of permeation is

characterized by Knudsen flow, and below about 5nm where transport would be limited to surface diffusion. The difference between macrodefects and nanodefects can be characterized by a) the activation energy of permeation (as measured from the gradient in a plot of InWVTR vs 1/T) - for macrodefects this reflects that of the underlying substrate and for nanodefects this is some value greater than this and/or b) the localized nature of the defects observed where permeation can be imaged (e.g. in Ca test) in cases in which the macrodefect density is sufficiently low to resolve individual defects. In addition, macrodefects can typically be imaged directly by microscopy, arising from scratches or cracks in the ceramic layer, or particles or pin-holes.

[0033] Macrodefects and nanodefects may need to be limited to improve or maintain barrier performance, but the route to doing this may be different in the two cases. For example, macrodefects may be limited by limiting handling damage, arcing and/or abrasion during deposition, or substrate surface contamination or asperities, whereas to limit nanodefect populations may require appropriate surface treatment (e.g. roughness, chemistry) of the substrate, and process parameters e.g. deposition power, speed, layer thickness, vacuum environment, as well as control of barrier layer chemistry. By providing a convenient way of distinguishing and, in some embodiments, quantifying macrodefects and nanodefects in a sample, the present invention can allow manufacturing parameters to be enhanced to reduce the macrodefects and nanodefects in a sample.

[0034] The layer of moisture-sensitive material may be formed of any suitable material. In a preferred embodiment, the layer may be a layer of calcium metal. Upon contact with water vapour, calcium metal reacts to form calcium hydroxide. While the metal is opaque, calcium hydroxide is transparent. The reaction of calcium metal with water, therefore, provides a visual indication of water vapour permeating through the barrier layer.

[0035] The layer of moisture-sensitive material may be applied by any suitable technique. Examples include chemical vapour deposition (CVD), atomic layer deposition (ALD), evaporation and sputtering. In a preferred embodiment, the layer of moisture- sensitive material may be applied by evaporation. The layer of moisture-sensitive material may have a thickness of 10nm to 1000nm, preferably 50nm to 300nm, more preferably 150nm to 250nm.

[0036] To determine the water vapour permeation rate through the barrier layer, one face of the barrier layer is placed in fluid communication with a layer of moisture-sensitive material. Preferably, one face of the layer of moisture-sensitive material is placed in contact with the barrier layer. The opposite face of the layer of moisture-sensitive material may be isolated or sealed from other sources of water vapour. Accordingly, most, if not substantially all, of the water that contacts the layer of moisture-sensitive material passes through the barrier layer.

[0037] In one embodiment, the layer of moisture-sensitive material may be deposited onto the barrier layer, for example, by chemical vapour deposition (CVD), atomic layer deposition (ALD), evaporation or sputtering. Thus, the layer of moisture-sensitive material may be in contact with the barrier layer. A sheet of glass or other optically transparent material may be placed over the opposite face of the barrier layer. This may help to prevent moisture from contacting the opposite face of the barrier layer. In one

embodiment, a sealant (e.g. a transparent sealant) may be used to form a seal to isolate or seal the layer of moisture-sensitive material from moisture. Accordingly, most, if not substantially all, of the water that contacts the layer of moisture-sensitive material comes into contact with the layer of moisture-sensitive material by permeating through the barrier layer. In a preferred embodiment, a sample comprising the barrier layer and layer of moisture-sensitive material is sealed using glass and an epoxy resin in a dry or moisture- reduced or moisture-free atmosphere, for example, a glove box. In one example, an epoxy resin (e.g. a transparent epoxy resin) is used as a sealant.

[0038] In an alternative embodiment, for example, where a glove box is not available, it may be possible to deposit a protective layer over the layer of moisture-sensitive material. The protective layer may be applied prior to application of any glass or epoxy layer. The protective layer may allow epoxy and/or glass to be applied over the layer of moisture- sensitive material in e.g. a moisture-containing atmosphere without the risk of extensive degradation of the moisture-sensitive material taking place. [0039] In yet another aspect, therefore, there is provided a sample comprising: a barrier layer;

a layer of moisture-sensitive material having optical properties that change on exposure to moisture, wherein the layer of moisture-sensitive material is contact with one face of the barrier layer;

an optically transparent protective layer overlying the layer of moisture sensitive material; and

a glass layer overlying the optically transparent protective layer;

wherein the glass layer is sealed to the barrier layer by a moisture- impermeable seal.

[0040] The sample may be prepared by a method comprising:

applying a layer of moisture-sensitive material to a barrier layer in a reduced- moisture environment; applying an optically transparent protective layer over the layer of moisture sensitive material in a reduced-moisture environment; applying a glass layer over the optically transparent protective layer; and sealing the glass layer to the barrier layer to provide a moisture-impermeable seal.

[0041] The protective layer may be formed of metal. For example, an optically transparent layer of metal may be applied. Examples of suitable metals include copper silver, gold, platinum, ruthenium, rhodium, palladium, osmium, iridium, rhenium, titanium, niobium, and tantalum. For the avoidance of doubt, metal alloys (e.g. metal alloys containing at least some of the metals mentioned above) may also be employed, for example, alloys that are resistant to water.

[0042] The protective layer may be applied using the same technique used to apply the layer of moisture-sensitive material. For example, chemical vapour deposition (CVD), atomic layer deposition (ALD), evaporation or sputtering may be used. In one

embodiment, the protective layer is applied using the same apparatus as that used to apply the layer of moisture-sensitive material. For example, where the layer of moisture sensitive material is a layer of e.g. calcium metal deposited onto the barrier layer using a particular deposition technique (e.g. evaporation), a protective layer of e.g. copper may be formed over the calcium layer using the same deposition technique (e.g. evaporation). Once the layer of moisture-sensitive material and protective layer have been applied, the resulting structure may then be removed from e.g. the evaporator to the atmosphere. Glass and epoxy layers may be applied over the protective layer e.g. under normal atmospheric conditions without the risk of significant moisture degradation of the moisture- sensitive material.

[0043] Once prepared, a sample comprising the barrier layer and layer of moisture- sensitive material may be tested in, for example, a humidity chamber. Such a chamber may define an enclosure in which a humid atmosphere may be maintained. In some embodiments, the chamber can be controlled to provide an atmosphere having a predefined humidity. The chamber may include control means for keeping the humidity constant. Additionally, the chamber may include control means for controlling the temperature within the enclosure.

[0044] The chamber may be provided with a reservoir of water or other aqueous solution (e.g. salt solution). The chamber may also be provided with a heater, for example, a heating platform that heats the contents of the reservoir to evaporate water and create a humid atmosphere within the chamber.

[0045] The humidity within the chamber may be controlled at a humidity within the range of 0%RH to 100%RH, preferably 50%RH to 100%RH, more preferably 85%RH to 88%RH.

[0046] The temperature within the chamber may be controlled at a temperature within a wide range. The temperature range may be constrained in practical terms by other factors: e.g. the freezing point of water at the lower bound and the softening point of components of the barrier layer (e.g. polymer substrate) at the upper bound. In one example, the chamber may comprise temperature control means to control the

temperature within the chamber between 0 to 100 deg C, for example, 25 to 80 degrees C. In some examples, the temperature control means can control the temperature within the chamber from 25degC to 40, 50, 60 or 70 deg C.

[0047] When the sample is exposed to a humid atmosphere, for example, in the chamber, water vapour in the atmosphere may permeate the barrier layer and come into contact with the layer of moisture sensitive material. This may cause the moisture sensitive material to react, thereby changing its visual appearance. For example, where the moisture sensitive material is calcium metal, water vapour in the atmosphere may react with the calcium metal forming calcium hydroxide. Calcium metal is opaque, while calcium hydroxide is transparent. The reaction of calcium with water, therefore, can be detected as a visual change (e.g. opacity). For example, if light is transmitted through the barrier layer, this visual change can be detected as a change in the amount of light that can be transmitted through the barrier layer. Using calcium to detect the transmission of water vapour through the barrier layer may sometimes be referred to as a "calcium test". Embodiments of the present invention provide an improved calcium test that can be used to analyse macrodefects and nanodefects in a barrier layer.

[0048] As mentioned above, water vapour may permeate through macrodefects and/or nanodefects in a barrier layer. It has been found that water vapour permeation through macrodefects can cause optical changes to appear as localised features (e.g. bright spots, lines or regions) in the optical images of the layer of moisture sensitive material, while water vapour permeation through nanodefects can cause optical changes in the background optical brightness of the optical images of the layer of moisture sensitive material. In the present method, the growth of these localised spots and/or the changes in the background brightness can be monitored in situ to provide diagnostic information on the barrier layer.

[0049] The chamber may be provided with an optically transparent window through which optical images can be taken. This window allows in situ monitoring to take place using optical sensors located outside the humid environment. Alternatively, optical sensors may be positioned within the chamber to image the changes in the layer of moisture-sensitive material in situ.

[0050] Reference is now made to Figure 1. This Figure depicts a sample containing a barrier layer whose water vapour permeation characteristics require analysis. The sample 10 is of a prior art construction. In particular, the sample 10 comprises a barrier layer 12, a layer of moisture sensitive material 14 (e.g. calcium metal), a transparent sealant 16 (e.g. epoxy resin) and a layer of glass 18.

[0051] The layer of moisture-sensitive material 14 may be deposited on the barrier layer 12 by evaporation in a dry evaporation chamber (not shown). To protect the layer of moisture-sensitive material from degradation, the partially assembled sample is removed from the chamber and transferred to e.g. a glove box (not shown), which provides a dry atmosphere for further assembly of the sample.

[0052] A transparent sealant 16, for example, epoxy resin may be applied over the layer 14 of moisture sensitive material under the dry conditions of the glove box. A layer of glass 18 is then applied over the sealant 16, which seals the glass 18 to the barrier layer 12 in a moisture-impermeable seal. Accordingly, once assembled, substantially all of the moisture reaching the layer 14 of moisture-sensitive material permeates through the barrier layer 12.

[0053] Figure 2 illustrates a sample constructed according to a method of an

embodiment of the present invention. The sample 1 10 is similar to the sample 10 of Figure 1 and like parts have been labelled with like numerals. However, unlike the sample 10 of Figure 1 , the sample 1 10 may be assembled without e.g. a glove box. Specifically, an optically transparent protective layer 1 15 may be applied over the layer of moisture sensitive material 14. The layer 1 15 may be a transparent layer of metal (e.g. copper) that may be evaporated using the same dry evaporation chamber (not shown) used to evaporate the layer 14 onto the barrier layer 12. Where a metal is evaporated, a mask 1 17, for example, of Kapton™ tape may be applied over exposed areas of the barrier layer 12 to allow an adherent layer of the optically transparent protective layer 1 15 to deposit over the sample.

[0054] The layer 115 provides a temporary moisture barrier that protects the layer of moisture sensitive material 14 for sufficient time to allow the sealant 16 and glass 18 to be applied without the need for the dry atmosphere of a glove box.

[0055] Figures 3a and 3b are schematic plan and side views of an apparatus in accordance with one embodiment of the present invention. The apparatus comprises a humidity chamber 200; a light source 210; an optical camera 220 and a processor (not shown). The chamber 200 includes an optically transparent window 222 through which images may be taken by the optical sensor 220.

[0056] The chamber 200 comprises a heated stage 224 onto which a reservoir of salt solution 226 is placed. Heating the stage 224 causes the salt solution 226 to evaporate, creating a humid atmosphere.

[0057] In use, a sample, the chamber 200 may be sealed and the stage 224 heated to cause the salt solution 226 to evaporate. The level of humidity and temperature within the sealed chamber may be controlled to a desired level using a controller (not shown). Once the desired humidity and/or temperature is reached, a sample, for instance, a sample illustrated in Figure 1 or 2, may be placed within the chamber 200. Light may be transmitted through the sample using light source 210 and optical images of the sample may be taken using camera 220.

[0058] The moisture in the atmosphere permeates the barrier layer 12 of the sample and contacts the layer of moisture sensitive material 14. This causes the moisture sensitive material to degrade. Where the moisture sensitive material 14 is calcium metal, the calcium metal reacts to form calcium hydroxide. As noted above, calcium metal is opaque, while calcium hydroxide is transparent. The reaction of calcium with water, therefore, can be detected as a visual change. For example, when light is transmitted through the barrier layer 12, this visual change can be detected as a change in the amount of light that can be transmitted through the barrier layer 12. [0059] Where water vapour permeates through macrodefects and/or nanodefects in a barrier layer, this can cause optical changes to appear as localised bright spots in the optical images of the layer of moisture sensitive material, while water vapour permeation through nanodefects can cause optical changes in the background optical brightness of the optical images of the layer of moisture sensitive material. In the present method, the growth of these localised spots and/or the changes in the background brightness can be monitored in situ to provide diagnostic information on the barrier layer.

Examples

[0060] In the following examples, samples were produced in as shown in Figure 2 and analysed in an apparatus as depicted in Figures 3a and 3b.

Permeation Measurements

[0061] When a test sample is exposed to humid air, water vapour permeates through the barrier layer and reacts with the calcium beneath it to form calcium hydroxide. The opaque calcium becomes increasingly transparent as it is converted to calcium hydroxide. In this investigation, optical transmission through the calcium was directly measured by observing the changes in the shade of grey over time. The shades of grey were then processed with ImageJ to give numerical values, which were then normalised from 0% to 100% reacted. The darkest shade (opaque) represented the unreacted volume of calcium at the start of the test and the lightest shade represented the absence of all calcium metal.

[0062] The change in optical transmission over a period of time can then be converted to a WVTR as explained below:

[0063] Fluence of water is the mass of water transported across the layer per unit

[0064] Measured Fluence = Moles of water transmitted * Mrwater/Area of Ca

[0065] Ca + 2H₂0 = Ca(OH)₂ +H₂

[0066] Mole ratio of calcium:water = 1 :2

[0067] Moles of H₂0 used in the reaction Mol_Water= 2 * Mol_Ca [0068] Fluence = 2 * moles of Ca reacted * Mr_water/Area of Ca

[0069] Fluence = 2 * mass of Ca reacted * Mr_water/(Area of Ca * Mr_Ca)

[0070] Fluence = 2 * volume of Ca reacted * p_Ca Mr_water/(Area of Ca * Mr_Ca)

[0071] Fluence = 2 * thickness of Ca reacted * pc_a Mr_water/Mrc_a [0072] The thickness of Ca reacted is given by in terms of the light transmitted, Tr:

[0073] (Tr(t) - Tr(initial))*Thca/(Tr(final)-Tr(initial))

[0074] The WVTR is thus the gradient of the fluence vs time trace in the linear region between the completely opaque and completely transparent regions.

[0075] These calculations were used to calculate WVTR through a polyethylene napthalate (PEN) barrier (Teonex Q65 (bi-axially oriented polyethylene naphthalate, PEN, with thickness 125μηι) produced by DuPont Teijin) and a polypropylene barrier (PP) (Rayoart CG90 (bi-axially oriented polypropylene, PP, with thickness of 90μηι)

manufactured by Innovia Films).

Polyethylene Naphthalate

[0076] Figure 4 shows Images from a calcium test conducted on an uncoated PEN barrier sample at 24°C using an apparatus as shown in Figure 3 above. The water vapour transmission rate of the sample was calculated at various time intervals and the results plotted in Figure 5.

[0077] The experimental fluence (y-axis) is a measure of the amount of water that is passing through the barrier sample. There are three regions clearly visible in the graph, the initial transient lag time, the increase to steady state and finally the plateau. This plateau is a result of all the calcium having been reacted and hence there is no further increase in measured fluence. The spots seen in Figure 4 are most likely a result of incomplete coverage during Ca deposition or the presence of inclusions on the surface of the barrier sample

[0078] The initial transient portion of low permeation is associated with the PEN absorbing water vapour and thus only a very small quantity of water passes through the polymer. After this initial transient, the steady-state region shows a constant gradient which gives a water vapour transmission rate (WVTR) of 0.53 ± 0.05 g/m²/day.

[0079] Permeation of water vapour can be fitted to an Arrhenius relationship with respect to temperature: Equation XX

[0080] where ΔΕ is the activation energy associated with the barrier, which in this case is PEN polymer, R is the gas constant, and T is the temperature. Thus, the calcium test was conducted over a range of temperatures and the results are shown in the table below. These values were used to calculate the activation energy by plotting In(WVTR) against 1000/Temperature (not shown).

[0081] From these values, an activation energy of 60 ± 4 kJ/mol was obtained. This value is close to those found in literature using industry-standard methods. This demonstrates that activation energy can be measured for any barrier sample with the calcium test according to an embodiment of the present disclosure.

Polypropylene PP

[0082] For comparison, the calcium test was carried out on a different uncoated polymer, Polypropylene PP.

[0083] WVTR was measured over a range of temperatures :

Temperature WVTR

(°C) (g/m²/day)

25 0.31 ± 0.05

30 0.41 ± 0.07

32 0.44 ± 0.07

39 1.2 ± 0.3

[0084] The activation energy of 74 ± 7 kJ/mol was calculated. This value is comparable to values obtained using industry-standard methods.

Polyethylene Napthalate PEN and Aluminium Oxide AIOx

[0085] Once it was determined that it was possible to achieve both reliable quantitative and qualitative information from the calcium test according to one embodiment of the present disclosure, the test was used to observe the more complex situation where a thin ceramic oxide layer had been deposited on a polymer to improve its barrier properties.

[0086] In this part of the investigation, the calcium test was conducted on samples with a thin coating of aluminium oxide on a PEN substrate. This was where the potential of the calcium test to image macrodefects within a sea of nanodefects was a particular advantage.

[0087] The results of the calcium test are shown in Figure 6. These are images from the calcium test according to an embodiment of the present disclosure conducted on a PEN substrate with an aluminium oxide coating at 23°C

[0088] As can be seen from the figures, an array of transparent spots appeared on the opaque calcium as time passed, which increased both in size as well as number. The formation of these spots were associated with the presence of macrodefects in the aluminium oxide coating. Macrodefects would allow water vapour to pass through at a much higher rate, thus reacting with the calcium within a localised region. In addition, the background (the region of calcium not covered by spots) slowly transitioned from completely opaque to translucent. This could be associated with the nanodefects allowing water to permeate at a much lower rate through individual defects compared to that of macrodefects.

[0089] A first step to analysing the images was to observe the evolution of spots within the calcium. To begin with, the percentage area of spots was plotted as a function of time (Figure 7):

[0090] The graph, representing the evolution of spots over time, displays an initial transient followed by an increase in spot area. From the graph, it appears to have two different linear regions, an initial steeper portion from ~1 .2 days to 2.6 days, followed by a longer less steep portion at >2.8 days.

[0091] A possible explanation would be that the spots begin growing but start impinging on one another at the 2.6 day mark. This would therefore lead to a lower rate of area increase of the spots. To further investigate this, the number of spots was plotted as a function of time (Figure 8):

[0092] The graph shows the rapid increase in the number of spots once the lag time has passed. The rate of increase is smooth, although there is slight curvature to the graph. This reduction in gradient can be associated with the fact that there are only a limited number of macrodefects present and therefore as time passes, the probability of a macrodefect revealing itself decreases.

[0093] There is no obvious change in gradient at -2.6 day mark, which implies that the spots have yet to coalesce. In addition to this, visual observation of the pictures also further confirm that the spots do not impinge on one another at the change in gradient.

[0094] Another explanation for the higher initial gradient may be to do with the PEN having to reach equilibrium with the chamber. Initially, the PEN has a uniform water concentration across the polymer (40% RH). As it equilibrates with the conditions in the chamber (85% RH), the polymer side exposed to the humidity of the oven increases in water concentration at a higher rate than the decrease in water concentration of the polymer side in contact with AIOx. This would lead to a period where the concentration gradient across the AIOx layer is greater and thus the flux of water out of the barrier through the macrodefects is higher. A potential way to examine this further is to conduct the calcium test at differing level of humidity. A lower humidity calcium test would potentially show a less drastic change in gradient.

[0095] It was also noticed that the lag time portion is significantly longer than that of uncoated PEN. The size of a macrodefect is typically much smaller than the single pixel of the camera, so although the macrodefects can act as unhindered pathways for water to travel through, the actual area through which the water flows is smaller. This therefore means that a lower volume of water permeates through over the same period of time. This means that a longer time is required for a spot to appear and begin growing.

[0096] The percentage area covered by spots starts at a non-zero value. This is due to incomplete coverage of calcium during thermal deposition, which may be due to the deposition process itself or the presence of dust and inclusions on the surface of the barrier as seen the polymer-only samples. This therefore means that not all the spots seen in the images taken are in fact a direct result of macrodefects in the ceramic layer, however such spots to not change and grow during the course of the experiment in contrast to those spots due to macrodefects so they can be excluded by suitable image analysis. [0097] This example illustrates the benefit of using a chamber having a transparent window as illustrated in Figure 3. Apart from the fact that the conditions of the sample are not jeopardised when imaged, the time-lapse function allows for a very large number of images of identical areas to be taken over a period of time. This not only results in a great number of data points, but it is in fact possible to observe the behaviour of single spots on the sample over time.

[0098] Figure 9 shows two different types of spots that were observed. The spots that grow in size are indicative of the presence of a macrodefect, whilst the spots that remain constant were probably created at the deposition stage, the inclusion, for example, of dust particles. Prior to this, with the traditional method of conducting the calcium test, it was not possible to distinguish between these two cases. This would allow for the estimation of the dispersion and density of defects on a barrier sample to a much greater accuracy.

[0099] The percentage area increase of spots over time could then be equated to the volume of reacted calcium as discussed above. This can be used to analyse the water vapour transmission rate of the spots over time. At a steady state, the WVTR value was determined to be 8.6 ± 0.3 x 10^"4 g/m²/day, which is about two orders of magnitude lower than that of the uncoated PEN at the same temperature (4.48 ± 0.05 x 10^"1 g/m²/day). If we assume that the growth of spots are a direct result of macrodefects and that macrodefects provide no hindrance to water vapour, all barrier capabilities lie solely on the PEN within these defects. Thus we could assume that the permeation behaviour through

macrodefects to be the same as that of uncoated PEN. The lower overall WVTR obtained via spot growth does make sense as the area covered by macrodefects is much smaller. Therefore the value of 8.62 ± 0.01 x 10^"3 g/m²/day is in fact an apparent WVTR which is a result of relatively small areas of macrodefects peppered over a relatively large area of barrier coating. The WVTR of the uncoated PEN can be thought of as the absolute transmission rate through a macrodefect and thus the activation energy of the permeation through macro defects would be expected to be that of the underlying polymer.

[00100] In order to confirm this, the activation energy associated with the holes expanding had to be examined :

[00101]

T (°C) WVTR (g/m²/day)

23 0.0086 ± 0.0003

25 0.00057 ± 0.00002

32 0.012 ± 0.0002 34 0.0014 ± 0.00007

39 0.0055 ± 0.00005

46 0.022 ± 0.0009

[00102] Table WVTR values of PEN + AIOx obtained from the spot growth (as % area) over time at different temperatures. Errors calculated by compounding the accuracy of measurements, deviation from straight lines, variation in gradient, as well as taking into account a degree of experimental error. A plot of In(WVTR) against 1000/Temperature was used to calculate activation energy at 61 kJ/mol. This value is close to that of uncoated PEN (60 ± 4 kJ/mol).

Single Spots

[00103] The behaviour of individual spots was analysed to further understand the ability of the calcium test to capture the nature of a macrodefect as well as to see if it was possible to calculate the size of defects. The expansion of singular spots was analysed (Figure 10):

[00104] The range of WVTR values obtained from the expansion rate of single spots lie between 0.17 g/m²/day to 0.25 g/m²/day. It was initially speculated that the not all the water passing through the macrodefect reacted with the calcium. In other words there was the possibility that some of the water exiting the macrodefect travelled into the epoxy, rather than reacting with calcium to expand the spot. To investigate this possibility, the rate at which the opacity of a single pixel at the centre of a newly appearing spot changed was measured.

[00105] The WVTR value obtained via change in opacity was 1.71 ± 0.02x 10^"1 g/m²/day. It is therefore highly encouraging that this values lies very close to that obtained for the same spot via the spot expansion method (1.78 ± 0.01 x 10^"1 g/m²/day).

[00106] In this change in opacity method, the water vapour has yet to penetrate all the way through the calcium. Hence all water permeating through the macrodefect can be assumed to react with the calcium directly around it. The similarity in values supports the idea that in the spot expansion method, all the water permeating through the macrodefect acts to enlarge spot and none (or a negligible amount) escapes into the epoxy. This further adds to the viability of the calcium test as a method of capturing the behaviour of macrodefects, on both a qualitative and quantitative scale.

Size

[00107] The size of single defects can potentially be calculated by measuring the mass of water through the defect and comparing to the absolute WVTR values previously obtained from the uncoated polymer. To estimate the size of the macrodefect, the mass of water per unit time is divided by the absolute WVTR of the substrate polymer at the same temperature. The range of macrodefect areas obtained for PEN + AIOx at 23°C was ~ 4-7 x 10^"8m². If we assume macrodefects as circular, we obtain a macrodefect size of ~ 200- 300μηι. Physically, these sizes appear reasonable for that of macrodefects.

Background

[00108] The next step to analysing the images was to measure the rate at which the background changed in opacity (Figure 1 1):

[00109] The graph obtained contains an initial transient region followed by an increase in permeation, and finally a steady-state region. The observed change in gradient at ~2 days can be reasoned in a similar fashion as with the macrodefects. This steady state region of constant gradient was used to calculate the WVTR. Therefore a WVTR value of 5.4 ± 0.1 x 10^"3 g/m²/day was from the change in background opacity. This was measured for PEN + AIOx samples across a range of temperatures :

[00110] Table -WVTR values of PEN + AIOx obtained from the change in background opacity at different temperatures

[00111] The activation energy calculated from the values is 74 ± 7 kJ/mol. As expected this value is higher than that obtained for uncoated PEN (60 ± 4 kJ/mol). These values were not measured with a calibration of the intensity measured during the experiment, and thus the values obtained are likely to be subject to some error. In particular, in samples with more bright spots from macrodefects (such as is observed at greater temperature) will have a greater underestimate of the transmitted intensity, i.e. the apparent WVTR will be lower than the true value, and more so at higher temperature, underestimating the measured activation energy.

[00112] The overall WVTRs can then be calculated, by adding the two mechanisms above (background and spots), which would allow for an overall activation energy to be achieved.

[00113] The overall activation energy was calculated to be 63 ± 14 kJ/mol. [00114] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[00115] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[00116] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

A method of analysing water vapour permeation through a barrier layer, said method comprising:

providing a sample comprising a barrier layer and a layer of moisture-sensitive material having optical properties that change on exposure to moisture, wherein the layer of moisture-sensitive material is in fluid communication with one face of the barrier layer;

exposing an opposite face of the barrier layer to a humid environment to allow water vapour to permeate through the barrier layer; and optically imaging the layer of moisture-sensitive material over time, wherein the optical images are taken in situ while the opposite face of the barrier layer is exposed to the humid environment.

A method as claimed in claim 1 , wherein water vapour permeates through any macrodefects and/or nanodefects in the barrier layer, and wherein the method further comprises analysing the optical images to obtain an indication of water vapour permeation through macrodefects and an indication of water vapour permeation through nanodefects in the barrier layer.

A method as claimed in claim 1 or 2, wherein the method further comprises analysing the optical images for changes in localised optical features in the optical images with time, and/or analysing the optical images for changes in the background optical brightness of the optical images with time.

A method as claimed in claim 3, wherein analysing the optical images for changes in localised optical features in the optical image with time comprises: identifying localised optical features in a plurality of images captured at different times and determining at least one property of the identified localised optical features in each image; and

analysing changes in the determined at least one property of the identified localised optical features between different images taken at different times.

A method as claimed in claim 4, wherein the determined at least one property of the identified localised optical features in each image comprises at least one of: the total number of identified localised optical features in each image, the proportion of the area of each image which is occupied by the identified localised optical features and/or a measure of the size of the identified localised optical features.

exposing an opposite face of the barrier layer to a humid environment to allow water vapour to permeate through the barrier layer

A method as claimed in claim 6, wherein the optical images are taken in situ while the opposite face of the barrier layer is exposed to the humid

environment.

A method as claimed in any one of claims 6 to 7,

wherein water vapour permeates through macrodefects and/or nanodefects in the barrier layer;

wherein permeation through macrodefects causes optical changes to appear as localised optical features in the optical images; and

wherein water vapour permeation through nanodefects causes changes in the background optical brightness of the optical images.

A method as claimed in claim 8, which comprises analysing the optical images to distinguish between permeation rates through nanodefects and permeation rates through macrodefects.

A method as claimed in any one of claims 5 to 9, wherein the localised optical features are localised spots and/or localised lines in the optical images.

A method as claimed in any one of claims 5 to 10, wherein changes in the background optical brightness of the layer of moisture sensitive material comprises changes in a spatially averaged or integrated optical brightness of the moisture sensitive material.

12. A method as claimed in claim 10 or 12, wherein changes in the localised spots and/or lines are determined by analysing the growth in the number and/or the growth in the size of one or more of the localised spots and/or lines.

13. A method as claimed in any one of claims 5 to 12, wherein the analysis steps are carried out using imaging software.

14. A method as claimed in any one of the preceding claims, wherein the layer of moisture-sensitive material is a layer of calcium metal.

15. A method as claimed in any one of the preceding claims, wherein the opposite face of the barrier layer is exposed to a humid environment by placing the sample in a humidity chamber, said humidity chamber having an optically transparent window through which the optical images are taken.

16. A method as claimed in any one of the preceding claims, wherein the layer of moisture-sensitive material is in direct contact with the barrier layer.

17. A method as claimed in any one of the preceding claims, wherein the barrier layer comprises a polymer film substrate having a layer of a ceramic oxide deposited thereon.

18. A method as claimed in any one of the preceding claims, wherein the barrier layer is a barrier layer for a flexible optoelectronic device.

19. A method as claimed in any one of the preceding claims, wherein the sample comprises the barrier layer, the layer of moisture-sensitive material overlying one face of the barrier layer and a layer of glass overlying the layer of moisture-sensitive material, said layer of glass being sealed to the barrier layer by epoxy resin.

20. A method as claimed in claim 19, wherein an optically transparent protective layer is positioned intermediate the layer of glass and the layer of moisture sensitive material.

21. An apparatus for analysing water vapour permeation through a barrier layer, said apparatus comprising:

22. An apparatus as claimed in claim 21 , wherein the humidity chamber includes a controller for controlling the humidity and/or temperature within the chamber.

23. An apparatus as claimed in claim 22, wherein the optical sensor is configured to take in situ optical images of a sample contained within the humidity chamber.

24. An apparatus as claimed in claim 23, wherein the optical sensor is located outside the humidity chamber and the humidity chamber includes an optically transparent window for transmitting light transmitted through the sample to the optical sensor.

25. A sample comprising:

a barrier layer;

a glass layer overlying the optically transparent protective layer;

26. A method of preparing a sample comprising a barrier layer for water vapour permeation analysis, said method comprising applying a layer of moisture-sensitive material to a barrier layer in a reduced-moisture environment; applying an optically transparent protective layer over the layer of moisture sensitive material in a reduced-moisture environment; applying a glass layer over the optically transparent protective layer; and sealing the glass layer to the barrier layer to provide a moisture- impermeable seal.

A method as claimed in claim 26, wherein the glass layer and seal are applied under standard atmospheric conditions.

A method as claimed in claim 26 or 27, wherein the layer of moisture-sensitive material and the optically transparent protective layer are applied using the same technique.