WO2004086530A1

WO2004086530A1 - Flexible electroluminescent device

Info

Publication number: WO2004086530A1
Application number: PCT/IB2004/050290
Authority: WO
Inventors: Hans Zuidema; Petrus C. P. Bouten; Giovanni Nisato; Oscar J. A. Buijk; Cornelis A. H. A. Mutsaers; Maarten Dirkzwager; Esther A. W. G. Janssen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-03-25
Filing date: 2004-03-19
Publication date: 2004-10-07
Also published as: TW200501797A

Abstract

A flexible electroluminescent device having a first (3) and second substrate (7) enclosing an electroluminescent element (5) and a brittle layer (13) which fails when stressed by flexure is made more robust by positioning the mechanical neutral line (21) associated with a flexure in or near such brittle layer. Positioning the mechanical neutral line (21) in or near the brittle layer (13) is achieved by adapting, relative to one another, the stiffness of the first and second substrate.

Description

Flexible electroluminescent device

The invention relates to a flexible electroluminescent (EL) device.

An electroluminescent (EL) device is a device which emits light when a suitable voltage is impressed on its electrodes. If the electroluminescent device comprises one or more organic compounds which facilitate charge transport and/or light emission it is generally referred to as an organic electroluminescent device. Organic electroluminescent devices are low- voltage devices which can be made to emit any color, are thin, light weight and/or of large area rendering such devices suitable for a wide range of display and lighting applications. An organic electroluminescent device may comprise organic compounds of relatively low molecular weight, also referred to in the art as small molecule electroluminescent devices, or compounds of high molecular weight, referred to as polymer electroluminescent devices. Since the electroluminescent device, in particular one of the organic type, can be made very thin and of large area such an electroluminescent device is particularly suitable for making a flexible device. A flexible organic light emitting diode is disclosed in WO 01/05205. The device according to said disclosure is encapsulated by plastic substrates which are laminated onto the surface of the diode. Although WO 01/05205 is silent with respect thereto, organic light-emitting diodes generally comprise brittle layers, that is layers which crack when subjected to relatively low levels of stress by flexure. In a flexible electroluminescent device, which essentially is a stack of thin layers, the brittle layer is the one to fail first. If, for example, the brittle layer is a conductive electrode layer of the electroluminescent device, such as an indium tin oxide (ITO) electrode layer, the formation of cracks is detrimental if not catastrophic to the conductivity of the layer and thus to the EL device as a whole. Consequently, there is a need in the art to provide flexible electroluminescent devices having one or more brittle layers which despite the presence of such one or more brittle layers is robust if stressed by flexure.

An object of the invention is, inter alia, to provide a flexible, in particular organic, electroluminescent device which is robust or at least has an improved robustness if stressed by flexure, robust meaning capable of being flexed many times during operational lifetime without device failure notwithstanding the device having one or more brittle layers.

These and other objects are achieved by means of a flexible electroluminescent device comprising an electroluminescent element, a first flexible substrate carrying the electroluminescent element, a second flexible substrate being arranged over the electroluminescent element, a brittle layer and, in a flexed state, a mechanical neutral line, the stiffness of the first and the second substrate being adapted relative to one another such that the mechanical neutral line passes through or near the brittle layer.

When a flexible structure such as a foil is stressed by flexure, strain develops in the foil. At the convex side of the flexed foil a tensile strain develops, at the concave side a compressive strain. Traversing the foil from the convex side to the concave side tensile strain becomes compressive strain and therefore there must be a location at which no strain is developed. The collective location(s) of zero strain of a cross-section taken along a direction at an angle to the direction of flexure of a flexed structure form a (curved) line which in the art is referred to as the mechanical neutral line.

By making the mechanical neutral line traverse the brittle layer of a flexible electroluminescent device in or near it, the brittle layer experiences relatively little or substantially no strain thus providing an electroluminescent device which is more robust when stressed by flexure. Those skilled in the art will appreciate that the meaning of "near" as in the mechanical neutral line being near to the brittle layer depends in general, inter alia, on the specific structure of a electroluminescent device, on the brittleness of the brittle layer and on the degree of flexibility and extent of robustness a particular application of the electroluminescent device requires and/or desires. Typically, "near" means closer than about 1 μm or better about 0.5 μm or still better about 0.1 μm to the major surface of the brittle layer which faces the mechanical neutral line.

The mechanical neutral line can be positioned anywhere in or between the first and/or second substrate by varying the stiffness of the first and second substrate relative to one another. Because the brittle layer is generally a thin layer (thickness typically less than about 500 mn) the mechanical neutral line is generally not located in or near the brittle layer unless the stiffness of the substrates is expressly adapted relative to one another to move the mechanical neutral line in or near such brittle layer. For example, the device disclosed in WO 01/05205 which has identical substrates has a mechanical neutral line which may not even be located within the electroluminescent element let alone within a brittle layer of such electroluminescent element.

The stiffness of a substrate is conveniently varied by, inter alia, varying the thickness thereof or by (partially) replacing substrate material with material having different elastic properties or by adding (layers of) material having different elastic properties to the substrate.

In the context of the invention, "flexible" means capable of being flexed to a radius of curvature of about 1 m or less, more particular 10 cm or less, even more particular 4 cm or less, and returning to original shape when stress is removed. In the context of the invention, a layer is brittle if made of a material breaking at about 1 % strain or less.

In a preferred embodiment of the invention the brittle layer is a transparent electrode layer of the electroluminescent element or more particular an indium tin oxide electrode layer. Electroluminescent elements generally require the use of a transparent electrode layer to allow the light emitted by the emissive layer to leave the element. Semiconducting inorganic oxides, such as indium tin oxides, are typically used as transparent electrode materials. However, these materials are generally brittle. Failure of the electrode layer leads to catastrophic failure of the electroluminescent element In another preferred embodiment the brittle layer is a barrier layer incorporated in the first or second substrate.

Electroluminescent elements, in particular those of the organic type, are sensitive to oxygen and/or water, the sensitivity being such that organic materials, in particular organic materials having a flexibility required for flexible electroluminescent devices, do not have the ability to block oxygen and water to the extent required. Therefore, to provide the required barrier properties the first and/or second substrate may comprise barrier layers having the required barrier properties. Such barrier layers are typically made of inorganic materials such as glass or (other) ceramic material rendering such barriers layers brittle layers. In an embodiment of the flexible electroluminescent device in accordance with the invention, the electroluminescent element has a plurality of individually addressable pixels and one or more active thin- film electronic components for addressing each one of said plurality of individually addressable pixels. The thin-film electronic component may be a thin-film diode or, preferably, a thin-film transistor. Thin-film electronic components comprise silicon-containing material such as doped elementary silicon (amorphous, polycrystalline, crystalline) and silicon-oxides. Such silicon-containing material are generally brittle and in accordance with the invention robustness of an electroluminescent device comprising such active components is improved if the mechanical neutral line is arranged in or near such components.

An embodiment of the flexible electroluminescent device in accordance with the invention comprises a further brittle layer, the brittleness of the brittle layer and the further brittle layer being substantially similar and the stiffness of the first and the second substrate adapted relative to one another such that the mechanical neutral line is located between the brittle and the further brittle layer.

The electroluminescent device may contain more than one brittle layer in particular a combination of the brittle layers identified above. Preferably, to avoid crack formation, the more than one brittle layers are arranged as close as possible to each other most preferably adjacent one another. If comprising more than one brittle layer the mechanical neutral line is best arranged somewhere between the outermost brittle layers, the exact location being determined by the brittleness of the particular brittle layers at hand.

Organic electroluminescent elements, which typically comprise an organic electroluminescent layer dispersed between a hole-injecting and an electron-injecting electrode, are particularly suitable for providing flexible EL devices. Therefore, in a preferred embodiment the invention relates to a flexible EL device in accordance with the invention comprising such organic electroluminescent element.

These and other aspects of the invention will be apparent from and elucidated with reference to the drawings and the embodiments described hereinafter.

In the drawings:

Fig. 1 shows, schematically, a cross-sectional view of an embodiment of a flexible electroluminescent device in accordance with the invention in a flexed state; Fig. 2 shows, in a cross-sectional view, a plot of lines of equal strain developing within a central part of a symmetrical flexible structure when brought into a flexed state; and

Fig. 3 shows a graph of the strain S (in dimensionless units) as a function of thickness t (in μm) developed within flexible structures stressed by flexure. Fig. 1 shows, schematically, a cross-sectional view of an embodiment of a flexible electroluminescent device 1 in accordance with the invention in a flexed state. The EL device 1 has a first flexible substrate 3 provided with an electroluminescent element 5. The EL device 1 has a second flexible substrate 7 which is attached to the first substrate 3 via an adhesive perimeter seal 9. The flexible substrate 7 covers the EL element 5. The EL element 5 comprises an EL layer 11 dispersed between a brittle ITO hole-injecting electrode layer 13 and an electron-injecting electrode layer 15. The first substrate 3 comprises a polymeric base film 17 provided with a thin barrier layer 19 to lower the permeability of the substrate 3 to water and oxygen. The second substrate 7 may also contain one or more of such barrier layers (not shown). When a suitable voltage is applied to the electrodes, the ITO layer 13 being biased positively relative to the electrode 15, the EL element 5 emits light via the transparent ITO electrode 13. With at least one of the substrates, and in the present embodiment preferably the first substrate 3, selected to be light-transmissive the emitted light is capable of leaving the EL device 1.

The dashed line 21 shown in Fig. 1 represents the mechanical neutral line of the EL device 1 associated with the flexed state shown. The stiffness of the first flexible substrate 3 and the second flexible substrate 7 is adapted, relative to one another, such that the mechanical neutral line 21 passes through the brittle ITO layer 13 and near the brittle barrier layer 19 where near would typically be about 1 μm or less. Typically, the ITO layer has a thickness in the range of about 100 to about 500 nm. The barrier layer may have a thickness typically in the range of about 5 nm to about 1 μm.

The stiffness of a substrate may be adapted may varying its shape, thickness or composition. The composition may be varied by using a material having a different elasticity or by adding or taking out layers in the composite substrate. The stiffness of a substrate is easily measured using standard methods and the elastic properties of a large variety of engineering materials are available from reference books and catalogs.

The process of adapting, relative to one another, the stiffness of the first and second substrate so as to arrange a mechanical neutral line on or near a brittle layer is straightforward and may proceed as follows: A first and second substrate each having a particular stiffness is selected and an EL device, or a series of such devices, is manufactured using the selected substrates. The EL device is then subjected to a series of flexibility tests in which the device is flexed to a predetermined radius of curvature a predetermined number of times to determine the point at which the brittle layer fails. Inspection of the failed EL device may show on which side of the brittle layer the mechanical line is located. Alternatively, further experiments wherein the stiffhess of first and/or second substrates is varied may be used to find where the mechanical neutral line is located. Having established on which side of the brittle layer the mechanical neutral line is located the stiffhess of the first and/or second substrate is adapted to move the neutral line towards the brittle layer. This process is repeated until the mechanical neutral line passes through or near the brittle layer. Instead of manufacturing and flexibility-testing real EL devices, computer simulations wherein the strain developed within a modeled version of a flexed EL device is computed may be used to great advantage in determining the stiffhess of the substrates required in the real EL device. Computer simulations wherein the strain developed within a modeled version of a flexed EL device is computed may also be used in method to establish whether, in accordance with the invention, a mechanical neutral line of a flexed flexible EL device is passes through or near a brittle layer of such device. The method comprises: measuring the stiffness of the EL device, separating the first substrate from the second substrate, measuring the stiffhess of the first substrate carrying the EL element and the second substrate, taking physical measurements of the EL devices to determine its dimensions, using the measurements data so obtained to provide a model description of the EL device suitable for input into a computer program capable of calculating the strain distribution within the model EL device when the model EL device is brought into a flexed state, computing the strain developed within the flexed EL device, and identifying the mechanical neutral line (locations where strain is zero) from a cross-section of the distribution, the cross-section being, for example, taken in a plane perpendicular to the direction of flexure. If different programs yield essentially different results use the simulation software Marc/Mentat commercially available from MSC-software (see www.marc.com).

The element may be arranged centrally between the first and second substrate but typically is arranged off-center leading to an asymmetric EL device. Such asymmetric device is obtained of the first substrate is used as the substrate onto which, during manufacture, successive layers of the EL element are applied. The first and second flexible substrate may be of a conventional construction.

If, as is the case in Fig.1 , light emission occurs via the electrode layer 13, it is convenient to make the first flexible substrate light-transmissive for the emitted light. Correspondingly, the second flexible substrate is light-transmissive if light emission occurs via electrode layer 15. To provide the necessary flexibility substrates preferably include polymeric layers or laminates of such layers. In particular elastic and/or rubbery polymers are useful. Suitable examples include polymethacrylates, polycarbonates and polyethyleneterephthalat.es. Further suitable materials are disclosed in WO 01/05205. The substrates may include barrier layers to prevent ingress of oxygen and/or moisture, suitable examples of flexible substrates comprising such barrier layers being disclosed in WO 01/05205 or US 6,268,695 or US 6,281,525.

The EL device in accordance with the invention is flexible and to achieve such flexibility the first and second substrate typically have a thickness of 1 mm or less or, more particular, 500 μm or less or, still more particular, 250 μm or less. An EL device may comprise more than one brittle layer as shown in Fig. 1 for the EL device 1. From the viewpoint of making the device more robust when stressed by flexure in accordance with the invention it is advantageous to have the one or more brittle layers near or adjacent one another so that the mechanical neutral line can be made to pass through or near all of the one or more brittle layers. If the one or more brittle layers are separated by non-brittle layers the mechanical neutral line is to be arranged somewhere between the outermost brittle layers. To first order, in the case of two brittle layers, it is preferable to position the mechanical neutral line at a distance of B1/(B1+B2) . Δd from the most brittle layer, where Bl is the brittleness of the least and B2 of the most brittle layer and Δd the distance between the least and most brittle layer. Brittleness may be defined (conventionally) as relative elongation before breakage. This first-order rule-of -thumb extends mutatis mutandis to devices having more than two brittle layers.

Except for the adaptation of the stiffness of the substrates in accordance with the invention, the EL device is of a conventional construction.

The EL device, or more specifically the EL element, in accordance with the invention may be configured as a (large area) lighting, signage, billboard signal or other display device. The EL device may be monochrome, multi-color or full-color. The (display) device may comprise a single light-emissive area or a plurality of independently addressable light-emissive areas such as a (multiplexed) segmented display. The device may also be a matrix device of the passive or active type having an array of individually addressable pixels. The EL element 5 comprises at least an EL layer dispersed between a first and second electrode layer as shown in Fig. 1. The EL element may be a unipolar device in which injection of holes or electrons only are sufficient to achieve luminescence or a bipolar device which requiring injection of holes and electrons. The bipolar device may be what is known in the art as a light-emitting chemical cell which allows the use electrodes having similar work function or a light-emitting diode wherein typically a hole- injecting electrode having a high work function ("high" meaning 4.5 eV or more) is used in combination with a low work function electrode ("low" meaning less than 4.5 eV).

The EL layer of the EL element may be fonned of inorganic or, preferably, organic, more particular polymeric, material. Suitable organic materials include compounds having conjugated systems sufficiently extended to provide luminescence in the visible range and/or having the ability accept and/or transport holes and/or electrons. Suitable organic (polymeric) EL materials are known in the art as such. The thickness of an organic layer formed of a low molecular weight compound(s), such layer being typically provided by a vacuum deposition methods is small, typically 5 to 100 nm. Polymeric EL layers, being typically provided using a wet deposition method, typically have a thickness in the range of about 30 nm to 500 nm.

Electrode layers of the EL element are used to inject charges, holes or electrons, into the EL layers. Suitable electrode materials are well known in the art. A very suitable high work function material is ITO because it is a transparent conductive material. However ITO is brittle. The invention is therefore particularly advantageous if applied to an EL device having an ITO layer. A suitable low work function material is calcium or barium or Mg:Ag. Electrode layers typically have a thickness of 50 to 200 nm.

As is known in the art, the EL element may comprise further layers such as a hole injecting/transport layer or an electron-injecting/transport layer.

EL elements having an array of individually addressable pixels will typically have a patterned top electrode layer, top referring to the electrode layer furthest away from the single substrate which carries the EL element. To provide such a patterned top electrode layer it is conventional to use a relief structure which provides shadow regions on the substrate surface for the vapor flux of material from which the electrode layer is to be formed. Typically such a relief structure is provided photo-lithographically and is 1 μm to about 5 μm in height.

EL elements having an array of individually addressable pixels wherein at least one of the organic layers such as the EL layer is ink-jet printed will typically have a relief structure outlining the boundaries of the pixels. The relief structures outline containers into which the ink-jetted fluid can be deposited to prevent fluid form spreading laterally beyond a pixels' boundary. Such relief structures are typically 1 to 5 μm high.

Referring to Fig. 1, the relatively thick perimeter adhesive seal 9 creates an interspace between the first and second substrate. The interspace may be used conveniently to PTTNT 030947 ^{rn JU}^' PCT/IB2004/050290

accommodate an oxygen and/or water getter to absorb any oxygen and/or water which is transmitted by the adhesive seal or the substrates and which would otherwise degrade the EL device. As is well known, organic electroluminescent devices are sensitive to water and/or oxygen. Alternatively, the EL device may be constructed such that the second substrate contacts the EL element 5 thus substantially eliminating the interspace.

If the EL element 5 is an active matrix device that is a device which has a plurality of individually addressable pixels and one or more active thin-film electronic components for addressing each one of said plurality of individually addressable pixels, the thin-film electronic components are typically arranged between the electrode layer 13 and the first substrate 3. The invention is particularly useful for active matrix devices as such devices allow a cathode common to all pixels to be used which creates a relatively thin electroluminescent device which can be encapsulated with a thin second substrate.

The invention will be further elucidated with examples demonstrating the principle of shifting the mechanical neutral line of a flexible structure by adapting relative to one another the stiffness of a first and a second flexible substrate.

Example 1

A model description of a structure in which a 100 nm thick indium tin oxide layer is sandwiched between a first and second substrate consisting each of a 100 μm thick fluorine polyester foil (Ferrania CR84R4 arylite) in a flexed state is provided. The Young's modulus of the fluorine polyester foil is set to 2el0 Pa. and of the ITO is set to 112.2e9 Pa.

The model description of the structure in a flexed state is fed into to Marc/Mentat simulation software version Mentat-2001 (MSC.Marc-2001 can also be used) and the strain distribution within the structure computed. Results are shown in Fig. 2. Fig. 2 shows, in a cross-sectional view, a plot of lines of equal strain developing within a central part of a symmetrical flexible structure when brought into a flexed state. Negative values indicate compressive strain, positive values tensile strain.

The flexible structure being symmetric, the mechanical neutral line is located in the center of the structure and, as the ITO is arranged in the center of the structure, the mechanical neutral line passes through the ITO layer.

Example 2

The model flexible structure is modified such that the thickness of the first substrate is reduced 10 % of the thickness in example 1, ie reduced to 10 μm. When stressed by flexure in the manner of Example 1 the strain distribution developed under such stress within the flexible structure is computed.

In Fig. 3, curve A, the resulting strain distribution within the flexible structure along a line which intersects the direction of flexure and is at right angles thereto is shown. The dot on curve A indicates the position of the ITO layer. Clearly, in this flexible structure the mechanical neutral line (where strain is zero) does not pass through the ITO layer.

Example 3

Example 2 is repeated except that the stiffness of the substrates is adapted relative one another. In particular, the Young's modulus of the material from which the substrate having the 100 μm thickness is formed is set to 2.10^s Pa.

In Fig. 3, curve B, the resulting strain distribution within the flexible structure along a line which intersects the direction of flexure and is at right angles thereto is shown. The dot on curve B indicates the position of the ITO layer. Clearly, by adapting the stiffhess of the substrates relative to one another, the mechanical neutral line (where strain is zero) is shifted so as to make the mechanical neutral line pass through the ITO layer.

Claims

CLAIMS:

1. A flexible electroluminescent device comprising an electroluminescent element, a first flexible substrate carrying the electroluminescent element, a second flexible substrate being arranged over the electroluminescent element, a brittle layer and, in a flexed state, a mechanical neutral line, the stiffness of the first and the second substrate being adapted relative to one another such that the mechanical neutral line passes through or near the brittle layer.

2. A flexible electroluminescent device as claimed in claim 1 wherein the brittle layer is a transparent electrode layer of the electroluminescent element.

3. A flexible electroluminescent device as claimed in claim 2 wherein the transparent electrode layer is an indium tin oxide layer.

4. A flexible electroluminescent device as claimed in claim 1 wherein the brittle layer is a barrier layer incorporated in the first or second substrate.

5. A flexible electroluminescent device as claimed in claim 1 wherein the electroluminescent element has a plurality of individually addressable pixels and one or more active thin-film electronic components for addressing each one of said plurality of individually addressable pixels.

6. A flexible electroluminescent device as claimed in claim 1, 2, 3, 4 or 5 wherein the electroluminescent element is an organic electroluminescent element.