Flexible substrates for organic devices

Details Flexible substrates for organic devices Flexible substrates for organic devices

1999-04-01

2003-07-15

Kelly, Cynthia A. (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Discontinuous or differential coating, impregnation or bond

C428S332000, C428S426000

Reexamination Certificate

active

06592969

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to flexible substrates for organic devices and particularly but not exclusively to organic light emitting devices (OLEDs) and to OLEDs fabricated on flexible substrates.
BACKGROUND OF THE INVENTION
Organic light-emitting devices such as described in U.S. Pat. No. 5,247,190 or in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference, have great potential for use in various display applications. According to one method, an OLED is fabricated by coating a glass or plastic substrate with a transparent first electrode (anode) such as indium tin oxide (ITO). At least one layer of a thin film of an electroluminescent organic material is then deposited prior to a final layer which is a film of a second electrode (cathode) which is typically a metal or alloy.
The electrode and organic layers used in OLEDs are typically very thin; normally of the order of a few 100 nm and typically around 100 nm and can be flexed without a great risk of damage to the device structure and functioning of the device. By using thin substrates of glass or transparent plastic, formable and/or flexible light sources and displays can be made. For this purpose substrates can be at most a few 100 &mgr;m thick.
In order to fabricate OLEDs which have good operating and shelf life it is of utmost importance to protect the active layers of the device, i.e. the electrode and organic layers, from the ingress of ambient species which can react with the active layer and deteriorate device performance, particularly oxygen and moisture. Typically, but not necessarily, an OLED emits light only from one side and this is typically through the transparent substrate and anode. The cathode is typically opaque and is made of a metal or alloy. This opaque side is relatively easy to encapsulate against the ingress of ambient reactive species as, for example, pinhole-free metal foils or metallised plastic foils can be used by, for example, lamination to the cathode.
For OLEDs fabricated on glass substrates the glass itself provides an excellent barrier against the ingress of oxygen and moisture. However, for OLEDs fabricated on transparent plastic foils it is extremely difficult to encapsulate the transparent side against the ingress of ambient reactive species. The oxygen and water permeabilities of even the most impermeable transparent plastic substrates (thin films) presently available are too high to be sufficient as a barrier for long life OLED devices. A simple estimate for this is given for example in K. Pichler, Phil. Trans. R. Soc. Lond. A (1997), Vol. 355, pp 829-842. This situation can be greatly improved by the conductive transparent coating itself, typically an inorganic conductive oxide such as indium tin oxide (ITO). Such ITO coatings on the thin plastic substrates can be very good barriers against the ingress of oxygen and water from outside into the device, as long as the ITO coatings are pinhole-free and defect free. However, these thin ITO (or other conductive oxide coatings) deposited onto thin flexible plastic substrates are prone to “cracking” if the substrates are not handled with the greatest care. The occurrence of such cracks in the ITO coating creates highly efficient diffusion channels for the ingress of ambient reactive species, just as pinholes in the coating would do. In addition to that, such cracks in the ITO coating may also result in an undesired deterioration of the surface flatness of the coating. This requirement to avoid cracking of the ITO coating puts severe constraints on the handling of the substrates and devices and hence the manufacturing process.
Alternatively, the use as an OLED substrate of thin formable and/or flexible glass with thicknesses of less than 200 &mgr;m is possible and even only 30 &mgr;m thick flexible glass, which is available commercially, is impermeable to oxygen and water and thus provides excellent barrier properties together with high transparency. Such thin glass is currently available from, for example, DESAG AG, Germany. However, such thin glass, although of a composition and specially manufactured to reduce brittleness, is still extremely difficult to handle and can very easily break if not handled with the greatest care. This puts severe limitations on the use of thin flexible glass as substrates for OLEDs due to the difficulty of manufacturing.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide an improved substrate for an electronic or optoelectronic device including at least one electrically active organic layer, which avoids or at least reduces the problems of the prior art.
According to one aspect of the present invention there is provided a transparent or substantially transparent formable and/or flexible component for use as an outer protective element in such an organic device, which component is a composite structure comprising a layer of glass of a thickness ≦200 &mgr;m and a layer of plastic. In this connection the formability of the component allows it to deviate from full planarity by bending and/or twisting so it can adapt to the shape or form of some other object. Its flexibility allows it to be bent without detrimentally affecting its barrier properties.
The invention is particularly but not exclusively concerned with an organic light-emitting device. Such a device comprises a first charge injecting electrode for injecting charge carriers of a first type and a second charge injecting electrode for injecting charge carriers of a second type. Between the first and second electrodes is arranged at least one layer of a thin film of an electroluminescent organic material. When an electric field is applied across the device, charge carriers injected into the material by the first and second electrodes recombine and decay radiatively causing light to be emitted from the electroluminescent layer. In the present description, the first electrode is referred to herein as the anode and the second electrode is referred to herein as the cathode.
Other organic devices include thin film transistors (TFTs), diodes, photodiodes, triodes, photovoltaic cells and photocouplers.
The outer protective element can constitute a substrate for the organic device and, as such, can be coated with a transparent electrode layer. That layer would normally be the anode and is preferably of indium tin oxide. In that case, the electrode coating is applied to one surface of the glass layer so that the plastic layer, adjacent the other surface of the glass layer, forms the outer layer of the protective element. As an alternative, the outer protective element constitutes an encapsulation film for a preformed organic light-emitting device.
In order to form the structure in which the outer protective element constitutes a substrate for the organic light-emitting device, the glass layer may be precoated with a transparent electrode layer prior to attachment to the plastic layer, or the transparent electrode layer may be deposited after fabrication of the composite structure. It is also possible to reverse the order of layers in the composite structure so that the plastic layer constitutes the inner layer carrying the electrode layer and the outer layer constitutes the glass layer.
The organic device with the outer protective element can be manufactured in a sequence of integrated steps which include the construction of the composite structure, deposition of the transparent electrode layer, deposition of the or each organic electrically active layer and deposition of the second electrode layer. A batch, semi-continuous or continuous process can be considered for the manufacture of the complete device. A further encapsulation layer on the second electrode layer can be provided.
Various manufacturing techniques are possible in accordance with different embodiments of the present invention.
According to one embodiment, a plastic layer carrying a coating of a first transparent electrode (e.g. ITO) is provided. Then, at least one layer of an electrically active, e.g. electroluminescent, orga

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