WO2019002822A1

WO2019002822A1 - White organic light emitting device

Info

Publication number: WO2019002822A1
Application number: PCT/GB2018/051630
Authority: WO
Inventors: Philip BENZIE
Original assignee: Cambridge Display Technology Limited; Sumitomo Chemical Company Limited
Priority date: 2017-06-30
Filing date: 2018-06-14
Publication date: 2019-01-03
Also published as: GB201710542D0

Abstract

A white light-emitting organic light-emitting device (100) includes a microcavity (D) configured to use resonance effects to enhance a blue-light component of the white light generated by the white light emitting organic device. The microcavity is formed by a reflective electrode (115) and a semi-transparent reflector layer (107) of the white light-emitting organic light-emitting device (100). The white light emitting organic device includes a transparent electrode (111) and layers of differing refractive indices, where the refractive indices are selected to produce a microcavity with resonance effects that enhance the blue component of the white light generated by the white light emitting organic device (100).

Description

WHITE ORGANIC LIGHT EMITTING DEVICE

BACKGROUND

Embodiments of the present disclosure relate to white light emitting organic light-emitting devices and methods of forming the same. An organic light-emitting device comprises an anode, a cathode and an organic Hght- 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.

U.S. Patent No. 2010/0231485 discloses a white light-emitting microcavity device having colour filters and independently controllable light-emitting elements to form coloured sub- pixels. U.S. Patent Pub. No. 2005/0225237 discloses an OLED having a reflector and a semi- transparent reflector forming a microcavity structure and a colour filter. A colour shift which increasing viewing angle has been reported for microcavity devices, as disclosed in Zhang et al, IEEE Journal of Quantum Electronics, Vol. 50, No. 5, May 2014, p. 348-353.

Embodiments of the present disclosure provide a white OLED having a high CCT at wide viewing angles.

SUMMARY

Embodiments of the present disclosure provide a white light-emitting organic light- emitting device comprising an anode and a cathode. At least one light-emitting layer is disposed between the anode and the cathode. The white light-emitting organic iight- emitting device further comprises a semi-transparent reflector layer, an outcoupling layer, and first and second transparent spacer layers. The light-emitting layer or layers comprise a blue light-emitting material. One of the anode and cathode is reflective and the other of the anode and cathode is transparent. The transparent electrode is disposed between the reflective electrode and the semi-transparent reflector layer, and spaced apart from the semi-transparent reflector layer by the first spacer layer. The reflective electrode and the semi-transparent reflector layer are configured to form a microcavity for resonance of light emitted by the blue light-emitting material. The outcoupling layer is disposed over the semi-transparent reflector layer and spaced apart therefrom by the second transparent spacer layer. As used herein, the term "transparent" means a transmittance of at least 80%, optionally at least 90% for light having wavelengths in the range of 400-800 nm.

As used herein, the term "semi-transparent" means a transmittance of less than 80 %, optionally less than 75% for light having wavelengths in the range of 400-800 nm, preferably in the range of 400-490 nm. DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the drawings in which: Figure 1 A illustrates schematically a white OLED, according to some embodiments of the present disclosure, in which a semi-transparent reflector layer is provided between a transparent anode and a reflective cathode.

Figure I B illustrates schematically a white OLED, according to embodiments of the present disclosure as illustrated in Figure 1 A, in which the cathode comprises a transparent layer;

Figure 1 C illustrates schematically a white OLED, to some embodiments of the present disclosure, in which a semi-transparent reflector layer is provided between a transparent cathode and a reflective anode; Figure 2 illustrates schematically the layer structure of a modelled device according to some embodiments of the present disclosure;

Figure 3A shows emission spectra at a range of emission angles for a modelled device without a semi-transparent reflector layer;

Figure 3B shows emission spectra at a range of emission angles for a modelled device, according to some embodiments of the present disclosure, having a 20 nm semi-transparent reflector layer of silver; and

Figure 4 shows modelled variation in CCT of devices with thicknesses of a semi- transparent reflector layer of 0-20 nm, in accordance to some embodiments of the present disclosure. DETAILED DESCRIPTION

Figure 1A, which is not drawn to any scale, illustrates an OLED 100 according to an embodiment of the present disclosure. The OLED 100 comprises a transparent anode electrode 1 1 1 , a reflective cathode electrode 1 15 and a light-emitting layer 1 13 disposed between the anode and cathode. The light-emitting layer 1 13 may comprise light-emitting materials, including a blue light-emitting material, configured to emit colours that combine to produce white light. One or more further layers (not shown) may be provided between the anode and the cathode. The or each layer between the anode and cathode preferably has a refractive index n in the range of about 1.3-2.0.

The anode preferably has a refractive index n in the range of about 1 .6-2.0. The cathode comprises or consists of a reflective layer 1 15.

The transparent anode is disposed between the reflective cathode and a semi-transparent reflector layer 107. The semi-transparent reflector layer is disposed between the anode 1 1 1 and an outcoupling layer 103.

The semi-transparent reflector layer is preferably a metal. In some embodiments of the present disclosure, the metal may comprise silver, aluminium and/or the like. The semi- transparent metal layer preferably has a thickness in the range of about 10-25 nm.

In some embodiments of the present disclosure, the outcoupling layer 103 may have a structured outer surface. In some embodiments of the present disclosure, the outcoupling layer 103 may comprise an embossed or moulded outer surface, for example as illustrated in the inset of Figure 1 A, to allow outcoupling of light at the outcoupling layer-air interface, which outcoupling of light may otherwise be trapped as waveguided modes at the interface.

In some embodiments of the present disclosure, the outcoupling layer may be a plastic layer. In some embodiments of the present disclosure, the outcoupling layer may comprise a plurality of lenses. In some embodiments of the present disclosure, the outcoupling layer may be as described in "Improved Light Out-Coupling 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", N. .Patel et al., IEEE Journal on Selected Topics in Quantum Electronics, Vol 8., No. 2, 2002, the contents of which are incorporated herein by reference.

In some embodiments of the present disclosure, the outcoupling layer 103 may be a substrate on which the other layers of the device are deposited. An outer surface of the outcoupling layer may be patterned before or after the layers of the device are formed. In other embodiments, the layers of the device may be formed sequentially on a substrate (not shown), optionally a glass or plastic substrate, starting with the cathode 1 15.

In some embodiments of the present disclosure, a first transparent spacer layer 109 is provided between the transparent anode 1 1 1 and the semi-transparent reflector layer 07.

The first transparent spacer layer preferably has a refractive index greater than the refractive index of the transparent anode, optionally a refractive index at least 0.3 greater than the refractive index of the transparent anode. Preferably, the first transparent spacer layer has a refractive index at least 1.0 greater than the refractive index of the semi- transparent reflector layer.

In some embodiments of the present disclosure, a second transparent spacer layer 105 is provided between the outcoupling layer 103 and the semi-transparent reflector layer 107. Preferably, the second transparent spacer layer has a refractive index at least 1 .0 greater than the refractive index of the semi-transparent reflector layer. The distance D between a reflective surface of the semi-transparent reflector layer 107 and a reflective surface of the reflective cathode layer 1 15 is selected so as to form a microcavity for the blue light-emitting material. It will be understood that distance D may be selected according to the wavelength of light emitted by the blue light-emitting material and refractive indices of the materials between the reflective surfaces. In some embodiments of the present disclosure, D is in the range of about 200 nm to about 450 nm.

In operation of the OLED, light hv is emitted through the transparent anode, the first and second transparent spacer layers, the semi-transparent reflector layer and the outcoupling layer. The microcavity for the blue light-emitting material increases the proportion of blue light emitted from the OLED as compared to a device in which the microcavity is not present, allowing for white light with a higher CCT as compared to a white OLED in which the microcavity is not present.

In some embodiments of the present disclosure, on-axis blue emission from the OLED (i.e. emission normal to an external surface of the transparent anode) may be unchanged as compared to emission from a device in which the microcavity is not present. However, the wavelength of off-axis blue emission may be shifted to a longer wavelength than the on- axis emission.

In some embodiments of the present disclosure, the semi-transparent reflector layer and the first and second transparent spacer layers are provided between the anode 1 1 1 and the outcoupling layer 103. The present inventors have found that the outcoupling layer in combination with the semi-transparent reflector layer 107 and first and second transparent spacer layers 109, 105 may allow for a higher CCT as compared to an OLED without these layers and a smaller change, or no change, in off-axis blue emission wavelength as compared to an OLED without the outcoupling layer.

Preferably, the OLED has a CCT in the range of about 2500-5000 , preferably a CCT in the range of about 3500-5000 .

Preferably, the OLED has a colour rendering index (CRI) of at least 80, optionally at least 85. In some embodiments of the present disclosure, Duv is up to 0.1

In some embodiments of the present disclosure, the transparent anode preferably consists of a single anode layer as illustrated in Figure 1. The anode layer may comprise or consist of a transparent conducting oxide, preferably indium tin oxide or indium zinc oxide.

In some embodiments of the present disclosure, the thickness of the first transparent layer may be selected according to the required thickness for the microcavity. Accordingly, the thickness of the active layers of the OLED may be selected to optimise device properties such as lifetime and/or efficiency, rather than outcoupling of blue light.

In some embodiments of the present disclosure, the first transparent spacer layer may be a transparent inorganic or organic dielectric material, optionally a transparent polymer or an inorganic compound, such as Si0₂.

In some embodiments of the present disclosure, there may be more than one first transparent spacer layer between the anode and the semi-transparent reflector layer. There may be more than one second transparent spacer layer between the semi-transparent reflector layer and the outcoupling layer. The or each transparent layer may be an inorganic or organic transparent layer, optionally a polymer.

The cathode may consist of one reflective layer or may comprise or consist of two or more layers.

Figure I B illustrates a device as described with reference to Figure 1 A, except that the cathode comprises two layers, one of which is a reflective layer. The cathode 1 15 of Figure I B comprises a reflective layer 1 15A and a transparent layer 1 15B, wherein the reflective surface of the cathode is provided by the internal surface of reflective layer 1 15A. Preferably, transparent layer 1 15B is a metal compound, more preferably a metal fluoride, yet more preferably an alkali fluoride or alkali earth fluoride.

Figures 1 A and I B illustrate devices in which the cathode consists of or comprises a reflective layer and light is emitted through a transparent anode. In other embodiments, the anode may comprise a reflective layer and the cathode may be transparent. Figure 1 C illustrates a device according to Figure 1 A except the anode 1 1 1 is the reflective electrode and the cathode 1 15 is the transparent electrode. The microcavity is formed between the reflective anode 1 1 1 and the semi-transparent reflector layer 107. The white OLED may be formed on a substrate 101 , optionally a glass or plastic substrate.

A reflective anode may comprise or consist of a layer of reflective metal, optionally a layer of a transition metal.

The device of Figure I C may be formed by a method comprising the steps of forming the reflective anode, light-emitting layer, transparent cathode, first transparent spacer layer, semi-transparent reflector layer, second transparent spacer layer and outcoupling layer over the substrate 101. In another embodiment, a device comprising a reflective cathode and transparent anode may be formed by a method comprising the steps of forming the reflective cathode, light- emitting layer, transparent anode, semi-transparent reflector layer and outcoupling layer over the substrate 101.

The white OLED may contain only one light-emitting layer or two or more light-emitting layers. Optionally, 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.

For simplicity, Figures 1 A- 1 C illustrate OLEDs having electroactive layers of an anode, cathode and light-emitting layer. 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 Nafion ®; 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( 1 1), 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. Merely by way of example, hole-transporting polymers may comprise polymers comprising arylamine repeat units, for example as described in W099/54385 or WO2005/049546 the contents of which are incorporated herein by reference. In some embodiments of the present disclosure, a hole-injection layer is provided between the anode and the light-emitting layer or layers and a hole-transporting layer is provided between the hole-injection layer and the light-emitting layer or layers.

In some embodiments of the present disclosure, an electron-transporting layer is provided between the cathode and the light-emitting layer or layers.

The OLED may comprise only one light-emitting layer which emits light when the device is in use. The 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.

Light-emitting materials

The white OLED 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.

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 or each light-emitting layer 105 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. 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.

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

A red light-emitting material may have a photoluminescence spectrum with a peak in the range of about more than 550 up to about 700 nm, optionally in the range of about more than 560 nm or more than 580 nm up to about 630 nm or 650 nm.

A green light-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 up to about 490 nm, optionally about 400-490 nm or 450-490 nm.

The photoluminescence spectrum of a material may be measured by casting 5 wt % 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 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.

Cathode

The cathode may comprise one or more layers. In the case where the cathode is reflective, the cathode comprises or consists of a reflective layer. Suitable materials for the reflective layer are metals, preferably metals having a work function of at least 4 eV, optionally aluminium, copper, silver, gold or iron. The reflective layer preferably has a thickness of at least 50 nm, optionally a thickness of 50-500 nm or 50-200 nm.

Work functions of metals are as given in the CRC Handbook of Chemistry and Physics, 12-1 14, 87^th Edition, published by CRC Press, edited by David R. Lide. If more than one value is given for a metal then the first listed value applies. The cathode may comprise a conductive layer, optionally a metal layer, and a layer of a metal compound, optionally a metal halide.

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

White OLED formation

The semi-transparent reflector layer and the outcoupling layer may be formed by any suitable method. Optionally, the semi-transparent reflector layer and the outcoupling layer are laminated, together or separately, to the transparent electrode of the white OLED. The semi-transparent reflector layer may be applied directly to an external surface of the transparent electrode or it may be spaced apart therefrom by one or more intervening transparent spacer layers. Optionally, the semi-transparent reflector layer and / or the outcoupling layer are laminated on or over the transparent electrode in a roll-to-roll process.

Applications

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 was performed using Setfos software available from Fluxim AG for white- emitting OLEDs having the layer structure illustrated in Figure 2 in which the semi- transparent silver reflector layer was varied from 2-20 nm and in which ITO is a transparent anode electrode HIL is a hole-injection layer; HTL 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 the cathode consists of a first layer of sodium fluoride and a reflective layer of aluminium. For the purpose of comparison, a device having the structure of Figure 2 was modelled in which the silver layer was absent.

With reference to Figures 3A and 3B, devices with the semi-transparent reflector layer have a significantly stronger blue component having a peak at around 460 nm across a wide viewing angle than the comparative device in which the semi-transparent reflector layer is absent.

With reference to Figure 4, the CCT increases (i.e. moves towards a cooler white colour) with increasing thickness of the semi-transparent reflector layer up to a thickness of 20 nm. Figure 4 illustrates CCT for devices having no semi-transparent silver reflector layer (lowest CCT) and increasing CCT with increasing silver thickness up to 20 nm (highest CCT).

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.

Claims

1. A white light-emitting organic light-emitting device comprising: an anode; a cathode; a light-emitting layer disposed between the anode and the cathode; a semi-transparent reflector layer; an outcoupling layer; and

first and second transparent spacer layers, wherein: the light-emitting layer comprises a blue light-emitting material; one of the anode and cathode is reflective and the other of the anode and cathode is transparent;

the transparent electrode is disposed between the reflective electrode and the semi-transparent reflector layer and spaced apart from the semi-transparent reflector layer by the first spacer layer; the reflective electrode and the semi-transparent reflector layer form a microcavity configured to provide resonance of light emitted by the blue light-emitting material; and the outcoupling layer is disposed over the semi-transparent reflector layer and spaced apart therefrom by the second transparent spacer layer.

2. The white light-emitting organic light-emitting device according to claim 1 , wherein the cathode is reflective.

3. The white light-emitting organic light-emitting device according to claim 2, wherein the cathode comprises a reflective metal layer.

4. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the semi-transparent reflector layer comprises a metal.

5. The white light-emitting organic light-emitting device according to claim 4, wherein the metal is silver.

6. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the first transparent spacer layer has a refractive index that is larger than the refractive index of the transparent electrode.

7. The white light-emitting organic light-emitting device according to claim 6, wherein the first transparent spacer layer has a refractive index at least 0.3 larger than the refractive index of the transparent electrode.

8. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the first transparent spacer layer has a refractive index at least 1.0 larger than the refractive index of the semi-transparent reflector layer.

9. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the second transparent spacer layer has a refractive index at least 1.0 larger than the refractive index of the semi-transparent reflector layer.

10. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the transparent electrode is unpatterned.

1 1. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the reflective electrode is unpatterned.

12. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the light-emitting layer is unpatterned.

13. The white light-emitting organic light-emitting device according to any one of the preceding claims, wherein the outcoupling layer comprises a patterned outer surface.