BACKGROUND

The invention relates to organic light-emitting devices and methods of forming the same.

An organic light-emitting device has an anode, a cathode and an organic light-emitting layer containing at least one light-emitting material between the anode and cathode.

In operation, holes are injected into the device through the anode and electrons are injected through the cathode. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of the light-emitting material or materials combine to form an exciton that releases its energy as light, for example a combination of emission from red, green and blue light-emitting materials.

White light-emitting OLEDs (hereinafter "white OLEDs") are known in which white emission is achieved by combining the emission of different organic light-emitting materials.

The colour correlated temperature (CCT) of a white OLED may be altered by altering one or more of the light-emitting materials of the white OLED. However, this may adversely affect the performance of the device, for example altering the hole / electron balance of the white OLED. Furthermore, achieving a "cool white" high CCT having a relatively strong blue emissive component is particularly problematic due to the high excited state energy level of blue light-emitting materials.

It is an object of the invention to provide an OLED producing cool white light over a wide viewing angle.

It is a further object of the invention to provide an efficient infrared emitting OLED.

SUMMARY

In a first aspect the invention provides a white light-emitting organic light-emitting device comprising a transparent substrate comprising a first substrate layer; a transparent first electrode comprising a first electrode layer; a second electrode; at least one light-emitting layer between the first and second electrodes; and a transparent refracting layer between the substrate and the transparent first electrode, wherein the transparent refracting layer comprises a first surface in contact with the first substrate layer and an opposing second surface in contact with the transparent first electrode layer, and wherein the refractive index of the refracting layer is at least 0.2 greater than the refractive index of the first electrode layer and at least 0.5 greater than the refractive index of the first substrate layer.

Embodiments of the present disclosure use refractive index contrast between the transparent anode layer, the transparent refracting layer and the transparent substrate layer to enhance a blue component of the white light emitted by the light emitting layer.

In a second aspect the invention provides an infrared emitting organic light-emitting device comprising a transparent substrate comprising a first substrate layer; a transparent first electrode comprising a first electrode layer; a second electrode; at least one light-emitting layer between the first and second electrodes; and a transparent refracting layer between the substrate and the transparent first electrode, wherein the transparent refracting layer comprises a first surface in contact with the first substrate layer and an opposing second surface in contact with the transparent first electrode layer, and wherein the refractive index of the refracting layer is at least 0.2 greater than the refractive index of the first electrode layer and at least 0.5 greater than the refractive index of the first substrate layer.

Embodiments of the present disclosure use refractive index contrast between the transparent anode layer, the transparent refracting layer and the transparent substrate layer to enhance a blue component and/or a near infrared component of the white light emitted by the light emitting layer. The term "transparent" means that the layer is transparent to the white light and/or the infrared radiation. The refractive index contrast between the transparent anode layer, the transparent refracting layer and the transparent substrate layer provides that the transparent refracting layer acts as a kind of Fabry-Perot interferometer adjusting the spectral properties of the white light emitted into the transparent refracting layer. In embodiments of the present disclosure, the interferometer effect adjusts the spectral properties of the white light emitted into the transparent refracting layer to enhance blue and/or near infrared components of the white light.

The substrate, transparent first electrode and transparent refracting layer of the device of the second aspect may be as described anywhere herein with reference to the device of the first aspect.

DRAWINGS

The invention will now be described in more detail with reference to the drawings in which:

Figure 1 A illustrates schematically an OLED according to an embodiment of the invention in which light is emitted through a transparent anode;

Figure IB illustrates schematically an OLED according to an embodiment of the invention in which light is emitted through a transparent cathode;

Figure 2 is a modelled emission spectrum of power per unit area vs. wavelength for a device according to an embodiment of the invention and a comparative device; and

Figure 3 is an emission spectrum for a device according to an embodiment of the invention and a comparative device.

DESCRIPTION

Figure I A, which is not drawn to any scale, illustrates an OLED 100 according to an embodiment of the invention. The OLED 100 comprises a transparent anode 10S, a cathode 109, a light-emitting layer 107 between the anode and cathode, a transparent substrate 101 and a transparent refracting layer 103 between the substrate 101 and the anode 105. The transparent substrate, transparent refracting layer and transparent electrodes as described herein may each have a transmittance of at least 80%, optionally at least 90% for light having wavelengths in the range of 400-800 nm.

One or more further layers (not shown) may be provided between the anode and the cathode. In some embodiments, the OLED is a white light-emitting OLED.

In some embodiments, the OLED is an infrared emitting OLED.

A first surface of the refracting layer 103 is adjacent to a layer of the substrate 101 and a second surface of the refracting layer is adjacent to a layer of the anode 10S.

The refracting layer 103 has a refractive index at least 0.5 higher man the substrate layer it is adjacent to, and optionally up to about 1.2 or 1.0 higher than the anode layer.

The refracting layer 103 has a refractive index at least 0.2, optionally at least 0.3, higher than the anode layer it is adjacent to, and optionally up to about 0.8 higher than the anode layer.

Preferably, the refracting layer has a thickness in the range of 50-200 nm.

Preferably, the refracting layer comprises or consists of a material having a refractive index of at least 2.0, optionally at least 2.2. The material may have a refractive index of up to 2.S. Exemplary materials include, without limitation, ZnO, ZnS, ZnSe, ZnTe and T1O2.

Preferably, the refracting layer is formed by evaporation

The substrate layer adjacent to the refracting layer preferably has a refractive index of less man 1.8, optionally less than 1.6. The substrate layer is preferably a glass or plastic layer. Preferably, the transparent substrate 101 consists of a single substrate layer.

The anode layer adjacent to the refracting layer preferably has a refractive index of less than 2.0, optionally less than 1.9 or 1.8. The anode layer is preferably a transparent conducting oxide, preferably indium tin oxide or indium zinc oxide. Preferably, the anode 105 consists of a single substrate layer.

Refractive indices as provided herein are as measured with an ellipsometer available from J A Woollam Co., Inc.

Light emitted from the device may escape the device through the substrate 101, the substrate having an outer surface having an interface with air. Preferably, an outcoupling layer (not shown) is provided over an external surface of the substrate (that is, a surface opposing the surface of the substrate in contact with the refracting layer 103). The outcoupling layer may have a structured outer surface, optionally an embossed or molded outer surface. The outcoupling layer may comprise a plurality of lenses. Outcoupling layers may be as described in "Improved light outcoupling in organic light emitting diodes employing ordered microlens arrays", S. Moller and S. R. Forrest, Journal of Applied Physics 91, 3324 (2002) or "High efficiency organic light-emitting diodes", NJLPatel et al., IEEE Journal on Selected Topics in Quantum Electronics, vol 8., no.2, 2002, the contents of which are incorporated herein by reference.

The outcoupling layer may allow outcoupling of light which may, in the absence of the outcoupling layer, be trapped as waveguided modes within the substrate.

Preferably, a white OLED as described herein has a CCT in the range of about 2500- 5000K, preferably a CCT in the range of about 3500-5000K.

Preferably, a white OLED as described herein has a colour rendering index (CRI) of at least 80, optionally at least 85.

Preferably, Duv is up to 0.1.

In the device of Figure 1 A, the refracting layer is provided between a transparent substrate and a transparent anode.

In other embodiments, the refracting layer may be provided between a transparent substrate and a transparent cathode, as illustrated in Figure IB wherein the refracting layer 103 has a refractive index at least 0.S higher than the substrate layer it is adjacent to and a refractive index at least 0.5 higher than the cathode layer it is adjacent to.

For simplicity, Figures 1 A and IB illustrate OLEDs having electroactive layers of an anode 107, cathode 111 and light-emitting layer 109. It will be appreciated that one or more further layers may be provided between the anode and the cathode including, without limitation, one or more of a hole-injection layer, a hole transporting layer, one or more further light-emitting layers, an electron transporting layer, an electron injection layer, a hole-blocking layer and an electron-blocking layer.

Preferably, a hole-injection layer is provided between the anode and the light-emitting layer or layers.

Examples of hole-injection materials include optionally substituted, doped poly(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS), polyacrylic acid or a fluorinated sulfonic acid, for example Nation ®; polyaniline; and optionally substituted polythiophene or poly(thienothiophene); and conductive inorganic materials including transition metal oxides such as VOx MoOx and RuOx as disclosed in Journal of Physics D: Applied Physics (1996), 29(11), 2750- 2753.

Preferably, a hole-transporting layer is provided between the anode and the light-emitting layer or layers.

The hole-transporting layer may comprise polymeric or non-polymeric hole-transporting materials. Exemplary hole-transporting polymers are polymers comprising arylamine repeat units, for example as described in WO 99/54385 or WO 2005/049546 the contents of which are incorporated herein by reference.

Preferably, a hole-injection layer is provided between the anode and the light-emitting layer or layers and a hole^ransporting layer is provided between the hole-injection layer and the light-emitting layer or layers.

Preferably, an electron-transporting layer is provided between the cathode and the light- emitting layer or layers.

The or each light-emitting layer of the OLED may contain at least one light-emitting material that emits phosphorescent light when the device is in operation, and / or at least one light-emitting material that emits fluorescent light when the device is in operation. Preferably, the or each light-emitting material is a phosphorescent material. Light-emitting materials as described herein may be polymeric or non-polymeric light- emitting materials. Exemplary light-emitting polymers are conjugated polymers, for example polyphenylenes and polyfluorenes examples of which are described in Bernius, M. T., Inbasekaran, M ., O'Brien, J. and Wu, W., Progress with Light-Emitting Polymers. Adv. Mater., 12 1737-1750, 2000, the contents of which are incorporated herein by reference. Light-emitting layer 107 may comprise a host material and a fluorescent or phosphorescent light-emitting dopant. Exemplary phosphorescent dopants are row 2 or row 3 transition metal complexes, for example complexes of ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum or gold.

A white OLED as described herein may comprise only one light-emitting layer which emits white light when the device is in use. The white OLED may comprise two or three light- emitting layers between the anode and the cathode which emit light when the device is in use wherein each light-emitting layer contains at least one light-emitting material and wherein the light-emitting materials together emit white light when the white OLED is in use.

A white OLED as described herein may contain two or more light-emitting materials including a blue light-emitting material which together produce white light when the device is in use.

Preferably, the white OLED comprises red, green and blue light-emitting materials which combine to produce white light when the white OLED is in use.

Optionally, a white OLED as described herein comprises two light-emitting layers wherein one of the light-emitting layers contains two light-emitting materials selected from red, green and blue light-emitting materials and the other light-emitting layer contains the remaining one of the red, green and blue light-emitting materials.

The or each light-emitting layer of the white OLED may consist of one or more light- emitting materials or may comprise one or more further materials. Optionally, at least one light-emitting layer comprises a host doped with one or more light-emitting materials. The light-emitting materials may be separate compounds or may be covalently linked. A white light-emitting polymer may comprise light-emitting compounds in the main chain, side chain and / or end groups of the polymer, the light-emitting compounds together producing white light.

The OLED as described herein may emit infrared light. Preferably, emission from the infrared OLED consists essentially of infrared light. An infrared OLED preferably contains only one light-emitting layer which emits light when the device is in use. Infrared emitting materials for use in OLEDs include fluorescent and phosphorescent light-emitting materials, for example as disclosed in Chuk-Lam Ho, Hua Li and Wai-Yeung Wong, "Red to near-infrared organometailic phosphorescent dyes for OLED applications", J. Organomet Chem.751 (2014), 261-285, the contents of which are incorporated herein by reference.

An infrared light-emitting material may have a photoluminescence spectrum with a peak of about more than 650 up to about 1000 nm, preferably in the range of about 700-850 nm.

A red light-emitting material may have a photoluminescence spectrum with a peak in the range of about more than 550 up to about 650 nm, optionally in the range of about more than 560 nm or more man 580 nm up to about 630 nm.

A green Eight-emitting material may have a photoluminescence spectrum with a peak in the range of about more than 490 nm up to about 560 nm, optionally from about 500 nm, 510 nm or 520 nm up to about 560 nm.

A blue light-emitting material may have a photoluminescence spectrum with a peak in the range of 400 nm up to about 490 nm, optionally about 430-490 nm.

The photoluminescence spectrum of a light-emitting material as described herein may be measured by casting 5 weight % of the material in a PMMA film onto a quartz substrate and measuring in a nitrogen environment using apparatus C9920-02 supplied by Hamamatsu.

The cathode comprises one or more layers including at least one conducting layer. Preferably, the conducting layer has a thickness of at least 50 nm, optionally a thickness of 50-500 nm or 50-200 nm. The conducting layer is preferably a metal layer, preferably a metal layer of a metal having a work junction of at least 4 eV, optionally aluminium, copper, silver, gold or iron.

Work functions of metals are as given in the CRC Handbook of Chemistry and Physics, 12-114, 87 ^th Edition, published by CRC Press, edited by David R. Lide. If more man one value is given for a metal then the first listed value applies.

The cathode may comprise the conductive layer and a layer of a metal compound, preferably a metal halide, more preferably a metal fluoride, yet more preferably an alkali fluoride or alkali earth fluoride. The metal compound layer may have a thickness of 0.5- 10 nm, preferably 0.5-5 nm.

In the case where the anode is transparent, the cathode is preferably opaque and is more preferably reflective.

A transparent cathode may comprise a layer of a transparent conducting oxide, optionally indium tin oxide or indium zinc oxide.

The white-emitting OLED described herein may be used as a display, for signage or for area lighting. Optionally, the light-emitting layer is not patterned. Optionally, one or both of the anode and cathode are not patterned. A white-emitting OLED in which the anode, cathode and light-emitting layer are unpatterned may be used for area lighting. The white- emitting OLED preferably does not comprise colour filters for filtering light emitted from the transparent electrode.

Examples Modelling Example

A white OLED containing the layer structure glass substrate / ZnS / ITO anode was modelled. For comparison, the same device without the layer of ZnS was modelled.

Modelling was performed using Setfos software available from Fluxim AG for a device having the following structure:

Glass / Zns (100 nm) / OLED wherein the OLED is a white light-emitting OLED having a 45 nm thick ITO anode layer adjacent to the ZnS layer.

Figure 2 is a graph of the modelled emission into the glass substrate integrated over multiple angles, showing an increase at both about 470 nm and 540 nm.

Device Example 1

A white-emitting OLED having the following structure was formed:

Glass / Zns (100 nm) / ITO (45 nm) / HIL / LEL (R) / LEL (G, B) / HBL / ETL / Cathode wherein HIL is a hole-injection layer; LEL (R) is a red light-emitting hole-transporting layer; LEL is a green and blue light-emitting layer; HBL is a hole-blocking layer; ETL is an electron-transporting layer, and Cathode is a cathode of a first layer of sodium fluoride and a second layer of aluminium.

To form the device, ZnS was evaporated onto glass and ITO was sputtered onto the ZnS layer. A hole injection layer was formed to a thickness of about 35 nm by spin-coating a formulation of a hole-injection material available from Nissan Chemical Industries. A red light-emitting layer was formed to a thickness of about 22 nm by spin-coating a crosslinkable red-emitting hole-transporting polymer, and crosslinking the polymer by heating at 180°C. The green and blue light-emitting layer was formed to a thickness of about 80 nm by spin-coating Host 1 (74 wt %), a green phosphorescent emitter (1 wt %) and a blue phosphorescent emitter (24 wt %) wherein the green phosphorescent emitter is a tris(phenylpyridine)iridium emitter wherein each phenylpyridine ligand is substituted with an alkylated 3,5-diphenylbenzene dendron and the blue phosphorescent emitter was an iridium complex with phenylimidazole ligands. A hole-blocking layer of Hole Blocking Compound 1 was evaporated onto the light-emitting layer to a thickness of 20 nm. An electron-transporting layer was formed by spin-coating a polymer comprising Electron- Transporting Unit 1 to a thickness of 10 nm. A cathode was formed on the electron- transporting layer of a first layer of sodium fluoride of about 3 nm thickness and a layer of aluminium of about 100 nm thickness. Host 1 has formula:

The red-emitting hole transporting polymer was formed by Suzuki polymerisation as described in WO 00/53656 to give a polymer comprising crosslinkable phenylene repeat units; amine repeat units and a 3 mol % of a red phosphorescent group of formula:

Electron-Transporting Unit 1 has formula:

Hole Blocking Compound 1 has formula:

Comparative Device 1

A white-emitting OLED was formed as described in Device Example 1, except that the ZnS layer was omitted.

With reference to Figure 3, on-axis blue light having a peak at about 470 nm is stronger for Device Example 1 than for Comparative Device 1. The infrared component at about 600- 700 nm is also stronger for Device Example 1.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.