(19)
(11)EP 3 005 436 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
29.07.2020 Bulletin 2020/31

(21)Application number: 14738884.7

(22)Date of filing:  28.05.2014
(51)International Patent Classification (IPC): 
H01L 51/56(2006.01)
G02B 5/18(2006.01)
H01L 51/52(2006.01)
H01L 51/50(2006.01)
(86)International application number:
PCT/GB2014/051619
(87)International publication number:
WO 2014/191734 (04.12.2014 Gazette  2014/49)

(54)

METHOD OF FORMING AN ORGANIC LIGHT EMITTING DIODE STRUCTURE

VERFAHREN ZUR HERSTELLUNG EINER ORGANISCHEN LED-STRUKTUR

PROCÉDÉ DE FORMATION D'UNE STRUCTURE DE DIODE ÉLECTROLUMINESCENTE ORGANIQUE


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 29.05.2013 GB 201309602

(43)Date of publication of application:
13.04.2016 Bulletin 2016/15

(73)Proprietor: Lomox Limited
Abercynon, Mid Glamorgan CF45 4SN (GB)

(72)Inventor:
  • COPNER, Nigel Joseph
    Ebbw Vale Gwent NP23 5AB (GB)

(74)Representative: Boult Wade Tennant LLP 
Salisbury Square House 8 Salisbury Square
London EC4Y 8AP
London EC4Y 8AP (GB)


(56)References cited: : 
EP-A1- 2 501 207
JP-A- 2001 250 692
WO-A1-2012/035083
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description


    [0001] This invention relates to electroluminescent devices and in particular to a method of fabricating electroluminescent devices based on organic light emitting diodes (OLEDs).

    [0002] Known organic light-emitting diodes (OLEDs) generally comprise an emissive electroluminescent organic semiconductor material layer formed between a cathode and an anode, whereby the electroluminescent organic semiconductor material emits light when a voltage is applied across the anode and cathode electrodes.

    [0003] In order for visible light to be emitted from the OLED, at least one of the electrodes must be transparent to radiation in the 350nm-800nm wavelength range.

    [0004] OLEDs are considered to be suitable candidates for the next generation of displays e.g. flat panel displays because of their advantages over conventional technologies used in traditional displays e.g. liquid Crystal displays (LCDs) and plasma display panels (PDPs). OLEDs are also being increasingly used in lighting applications, replacing the more common incandescent bulbs.

    [0005] Such advantages over LCDs and PDPs include, lower fabrication costs, light-weight, flexible plastic substrates, wider viewing angles & improved brightness, increased power efficiency and faster response time. However, whilst efficient in comparison to LCDs and PDPs, the efficiency of the OLED is limited, due to incomplete light extraction from the active, light-emitting layer due to losses in the OLED.

    [0006] Figure 1 shows in section, a conventional OLED 1 of the prior art, whereby an organic electroluminescent material layer 2 is located between two electrodes; a transparent anode 4 and a reflecting cathode 6.

    [0007] The specific type of electroluminescent layer to be used may vary depending on the application of the OLED. For example, such material for the electroluminescent material may include organometallic chelates, for example Alq3, fluorescent and phosphorescent dyes and conjugated dendrimers. Alternatively, organic polymer molecules may be used, whereby typical polymers include derivatives of poly(p-phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer may determine the colour of emitted light or the stability and solubility of the polymer for performance and ease of processing. Furthermore, a polymer such as poly(n-vinylcarbazole) may be used as a host material to which an organometallic complex is added as a dopant. Iridium complexes such as Ir(mppy)3 or complexes based on other heavy metals such as platinum may also be used.

    [0008] The anode 4 is fabricated from a transparent material (e.g. indium-tin-oxide (ITO)), whilst the cathode 6 may be fabricated from a reflective metal (e.g. magnesium-silver or lithium-aluminium alloy). A protective substrate (not shown) is then deposited over the anode 4. The substrate may be flexible. Alternatively, the anode, cathode and electroluminescent material may be deposited on the substrate.

    [0009] In operation, when a voltage 10 is applied across the electrodes 4 and 6, holes from the anode 4 and electrons from the cathode 6 are injected into the organic layer 2. These holes and electrons migrate through the organic layer 2 until they meet and recombine to form an exciton. Relaxation from the excited to ground states then occurs, causing emission of light 12 through the transparent anode 4.

    [0010] Furthermore, it is known to use different layers and materials to increase the efficiency of OLEDs, whereby hole and electron injection and/or blocking and/or transport layers are used to optimise the electric properties of the OLED. For example, hole injection (HIL) e.g. Cu/Pc and/or hole transport layers (HTL) e.g. aNPD, Triarylamines, and/or electron transport (ETL) e.g. Alq3 and/or Hole Blocking layers (HBL) e.g. BCP may be used to improve electrical efficiency as required. When a voltage 10 is applied between the anode and the cathode in the OLED having the above-described structure, holes generated in the anode move to the emission layer through the HIL and the HTL, and electrons generated in the cathode move to the emission layer through the HBL and the ETL. The holes and electrons moved to the emission layer are recombined in the emitting layer to emit the light. The light generated in the emission layer is emitted to the outside through the anode. Whilst the HIL, HTL, ETL, and HBL address the electrical efficiency of OLEDs, one of the key challenges in the design of OLEDs is to optimize their light extraction efficiency.

    [0011] For conventional OLEDs as depicted in Figure 1, light extraction inefficiencies exist because light generated within a high-index organic material has difficulty propagating into the surrounding lower-index Anode/Glass substrate owing to total internal refection (TIR) at the glass/air interface, coupling to dielectric waveguide modes of the organic layers, in-plane emission, and dissipation into the metal contacts of the OLED. Approximately, 30% of the generated photons remain trapped in the glass substrate and 50% in the organic layers. Therefore, output coupling efficiency of known OLEDs is approximately only 20%.

    [0012] Various approaches for improving the optical out coupling efficiency of OLEDs have been put forward. For example, the planar substrate/air interface may be modified in order to reduce repeated TIR e.g. by using a micro lens array, or a large half sphere lens on the substrate surface.

    [0013] Other methods of improving out-coupling efficiency attempt to extract the light trapped in the organic/ITO layers for example by using a low refractive index porous aerogel, micro-cavity effects, or an embedded low-index grid photonic crystal pattern on the glass substrate. However, using these methods, the improvement in relation to OLED efficiency is limited. Moreover, some of these methods have disadvantages associated with them such as a reduction in electrical efficiency, a decrease in lifetime, a viewing angle dependent colour, complicated fabrication processes and high costs. For instance, eliminating TIR using antireflective coatings does not work as each of the layers will refract the light and ultimately the final interface with air will still meet the TIR condition. In addition, establishing gratings at pitches less than half the UV exposure wavelength is very difficult.

    [0014] Document JP 2001 250692 A discloses an organic electroluminescent device containing a diffraction grating. The manufacturing method involves applying a source material on a substrate and exposing the source material by interference fringes which are generated by laser irradiation.

    [0015] It is an object of the present invention to address the above-identified problems.

    [0016] In an aspect there is provided a method of fabricating a nano-structured grating pattern in a photo-sensitive organic electroluminescent layer of an OLED according to claim 1.

    [0017] In an example not forming part of the invention there is provided an organic light emitting diode (OLED) comprising: an organic electroluminescent layer formed between a first electrode and a second electrode, characterised in that organic electroluminescent layer comprises a nano-structured grating pattern provided therein, wherein the grating is configured to modify the refractive index of the electroluminescent layer.

    [0018] Preferably, the organic electroluminescent layer comprises a photo-crosslinkable liquid having dichroic molecules therein.

    [0019] Preferably, the photo-crosslinkable liquid is sensitive to UV-laser radiation, wherein the refractive index of the electroluminescent layer is dependent on an intensity of the UV-laser radiation.

    [0020] Preferably, the grating pattern is configured to provide a periodic refractive index change. Preferably, wherein the grating pattern has a rectangular profile.

    [0021] Preferably, the organic electroluminescent layer comprises a Fresnel lens structure profile. Preferably, the OLED further comprises one of an HIL, HTL, ETL and/or HBL.

    [0022] In an embodiment of the invention as claimed there is provided a method of fabricating a nano-structured grating pattern in a photo-sensitive organic electroluminescent layer of an OLED as recited in claim 1.

    [0023] Preferably, the first and/or second light beams have a wavelength between 10nm and 400nm.

    [0024] Preferably, the first and second light beams are provided by a Mach-Zehnder interferometer. In an example not forming part of the invention there is provided an OLED comprising a nano-structured grating pattern in a photo-sensitive organic electroluminescent layer.

    Figure 1 shows in section, a conventional OLED of the prior art;

    Figure 2 shows in section, a grating-assisted OLED;

    Figure 3 shows in side-view, a system for forming a grating in an organic layer of the OLED of Figure 2;

    Figure 4a shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the grating-assisted OLED of Figure 2 in comparison to the OLED of Figure 1;

    Figure 4b shows a graphical representation of the calculated emission properties (calculated output average electric filed (E2) versus emission angle (degrees)) of the OLED of Figure 1 in comparison to the grating-assisted OLED of Figure 2;

    Figure 4c shows a graphical representation of the electric field distributions for the grating-assisted OLED of Figure 2 at a wavelength of 410nm;

    Figure 5a shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the grating-assisted OLED of Figure 2 having a refractive index of 2.0 in comparison to the OLED of Figure 1;

    Figure 5b shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the grating-assisted OLED of Figure 2 having a refractive index of 2.1 in comparison to the OLED of Figure 1;

    Figure 6a shows in section, a grating-assisted OLED according to a second embodiment of the present invention;

    Figure 6b is a representation of a Finite-difference time-domain (FDTD)-simulated Fresnel lens structure in the OLED of Figure 6a; and

    Figure 7 shows a graphical representation of the calculated emission properties (calculated output average electric field (E2) versus emission angle (degrees)) of the OLED of Figure 1 in comparison to the grating-assisted OLED of Figure 6a.

    Figure 2 shows in section, an OLED 20 according to an example of the present disclosure, whereby an organic electroluminescent material layer 22 having a depth of 110nm is located between two electrodes; an ITO anode 24 of 120nm depth, and a reflecting cathode 26 of 100nm depth, whilst a layer of protective glass 27 is provided atop the anode 24. As detailed above HIL, HTL, ETL and HBL could also be used in the OLED 20 to improve electrical efficiency as required.



    [0025] The organic layer 22 is formed of a photo-crosslinkable liquid crystal with rod-shaped dichroic molecules. An optical grating 28 having a defined period P, is subsequently defined in the organic layer 22, whereby the grating 28 provides the organic layer 22 with a periodic refractive index change. The grating 28 is formed in the organic layer 22 using a Mach-Zehnder interferometer as described below in Figure 3.

    [0026] Initially, the anode 24 is deposited on a glass backplane 27. The organic layer 22 is then deposited on the anode 24 e.g using an inkjet printing process, spin coating, evaporation etc. The cathode 26 is then deposited using a suitable technique. Various layers e.g. HIL. HTL, HBL and/or ETL can also be deposited as required.

    [0027] When a voltage (not shown) is applied across the electrodes 24, 26, electromagnetic radiation in the form of an electromagnetic light wave is generated in the organic layer 22. The grating assisted organic layer 22 provides enhanced light extraction in comparison to a conventional OLED, as shown in detail in Figures 4 and 4b below.

    [0028] Figure 3 shows a suitable method for forming a grating in the organic layer 22. Once the organic layer is deposited on the anode 24, a Mach-Zehnder interferometer is used to combine two split beams 41 & 42 such that the beams converge on the surface of the organic layer 22.

    [0029] Illuminating the UV-sensitive organic layer 22 with an intensity-dependent-interference fringe using a Mach-Zehnder interferometer incorporating e.g. a UV laser source, provides the grating 28 in the organic layer 22, whereby the photosensitive organic material subjected to the UV light cross-links. The cross-linked material provides the grating 28 in the organic layer. An anode (not shown in Figure 3) may be deposited on the surface of the OLED 20.

    [0030] Alternatively, the grating may be formed in a UV photo-sensitive resist provided atop the organic layer e.g. evaporation, spin coating. Depending on whether the resist is positive or negative, the resist and organic material may then be etched away to provide an air/material grating 28 in the organic layer 22.

    [0031] The period (p) of the grating 28 is defined by:

    Where: λ = laser wavelength (nm); and

    θ = angle between the two incident light beams.



    [0032] For the OLED 20 above, p = 300nm, and the refractive index is 1.9.

    [0033] Figure 4a shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the conventional OLED 1 as represented by trace 31, in comparison to the grating assisted OLED 20 as represented by trace 32. From figure 4a it is evident from a comparison of the traces 31 and 32 that light enhancement of an OLED 20 comprising the grating 28 is increased in comparison to the conventional OLED 1 at all wavelengths between 400nm and 500nm.

    [0034] Figure 4b shows a graphical representation of the calculated emission properties (calculated output average electric filed (E2) versus emission angle (degrees)) of the conventional OLED 1 as represented by trace 33, in comparison to the grating-assisted OLED 20 as represented by trace 34. It is evident from a comparison of traces 33 and 34 in Figure 4b that both larger radiation and broader angle responses are obtained for the OLED 20 comprising the grating 28 for the majority of angles.

    [0035] Figure 4c shows a graphical representation of the electric field distributions for the grating-assisted OLED 20 at a wavelength of 410nm, whereby the organic layer regions are indicated by the dashed lines 29.

    [0036] Figure 5a shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the OLED 1, as represented by the trace 35 in comparison to the grating-assisted OLED 20 having a refractive index of 2.0 as represented by the trace 36; whilst Figure 5b shows a graphical representation of the calculated emission properties (output average electric field (E2) versus wavelength (nm)) of the OLED 1 as represented by the trace 37 in comparison to the grating-assisted OLED 20 having a refractive index of 2.1 as represented by the trace 38.

    [0037] Furthermore, the refractive index of the grating 28 is tuneable by using different intensities of laser light for the interferometer. Varying the refractive index effects a variation in the peak wavelength of a grating assisted OLED 20. Such an effect is demonstrated in Figures 5a and 5b, whereby the refractive index of the gratings are 2.0 and 2.1 respectively. It will be seen from Figure 5a that the peak wavelength of the grating-assisted OLED 20 having a refractive index of 2.0 is ∼430nm, as represented by trace 36.

    [0038] Furthermore, it will be seen from Figure 5b that the peak wavelength of the grating-assisted OLED 20 is 430nm having a refractive index of 2.1 is ∼440nm, as represented by trace 38. The OLEDs 20 represented by Figures 5a and 5b comprise identical structural parameters, whereby the depths of the cathode layer, organic layer and anode layer are 100nm, 110nm and 120nm respectively, whilst the grating period P is 300nm.

    [0039] Varying the depth of the grating 28 from 100nm to 120nm also results in a change in the output intensity.

    [0040] Figure 6a shows in section, a Fresnel lens structure grating-assisted OLED 40 according to an example of the present disclosure, whilst Figure 6b is a representation of a Finite-difference time-domain (FDTD)-simulated Fresnel lens structure in the OLED 40, whereby an organic electroluminescent material layer 42 having a depth of 110nm is located between two electrodes; an ITO anode 44 of 120nm depth, and a reflecting cathode 46 of 100nm depth, whilst a layer of protective glass 47 is provided atop the anode 44. As detailed above HIL, HTL, ETL and HBL could also be used in the OLED 40 to improve electrical efficiency as required.

    [0041] As above, the organic layer 42 is formed of a formed of a photo-crosslinkable liquid crystal with rod-shaped dichroic molecules. An optical grating 48 is subsequently defined in the organic layer 42, whereby the grating 48 provides the organic layer 42 with a periodic refractive index change. The grating 28 is formed in the organic layer 22 using a Mach-Zehnder interferometer as described in relation to Figure 3, but unlike the rectangle grating 28 of the first embodiment, the grating 48 is formed as a Fresnel lens structure.

    [0042] Using such Fresnel-lens structure in the OLED 40, means that when light propagates in the organic layer 42, the light will be focussed at the far field.

    [0043] As demonstrated at Figure 7, which shows a graphical representation of the calculated emission properties (calculated output average electric field (E2) versus emission angle (degrees)) of the OLED 1 as represented by trace 51, in comparison to the Fresnel lens grating-assisted OLED 40 as represented by trace 52, the grating 48 provides increased light extraction at broader angles in comparison to the conventional OLED 1.

    [0044] Although, rectangle and Fresnel-lens shaped profiles are disclosed in OLEDs 20 and 40 above respectively, it is also possible to use other grating profiles such as chirped, apodised, or any that will become apparent to the skilled person after reading this specification.

    [0045] As demonstrated above, using an intensity-dependent-interference fringe technique to create a nanostructure grating in an organic layer of an OLED, provides a method of varying the refractive index of the organic layer, thereby inter alia increasing the out-coupling efficiency of the OLED in a low cost manner in comparison to techniques currently used to provide nanostructured gratings in materials. Furthermore, by varying the intensity of the light used to fabricate the gratings, it is possible to tune the refractive index of the organic layer.

    [0046] The invention is defined by the appended claims.


    Claims

    1. A method of fabricating a nano-structured grating pattern (28) in a photo-sensitive organic electroluminescent layer of an organic light emitting diode (OLED), the method comprising the steps of:

    providing the photo-sensitive organic electroluminescent layer (22) formed of a photo-crosslinkable liquid crystal having rod-shaped dichroic molecules therein;

    defining a grating pattern in the organic electroluminescent layer by illuminating the organic electroluminescent layer with first and second light beams having an angle θ therebetween, wherein the first and second light beams (41, 42) are configured to create an interference fringe in the organic electroluminescent layer, and wherein the photo-sensitive organic electroluminescent layer is sensitive to radiation having a wavelength between 10 nm and 400 nm;

    characterised in that the refractive index of the defined grating pattern is tuned by varying the intensity of the light to which the photo-crosslinkable liquid crystal is exposed.


     
    2. A method as claimed claim 1, wherein the first and/or second light beams have a wavelength between 10 nm and 400 nm.
     
    3. A method as claimed claim 1, wherein the first and second light beams are provided by a Mach-Zehnder interferometer.
     
    4. The method of claim 1, wherein the illuminated regions of the organic electroluminescent layer are configured to form a rectangular refractive index profile.
     
    5. The method of claim 1, wherein the illuminated regions of the organic electroluminescent layer are configured to form a chirped grating profile.
     
    6. The method of claim 1, wherein the illuminated regions of the organic electroluminescent layer are configured to form an apodised grating profile.
     
    7. The method of claim 1, wherein the peak wavelength of the OLED device is selected by tuning the refractive index of the defined grating pattern.
     


    Ansprüche

    1. Verfahren eines Herstellens eines Nano-strukturierten Gittermusters (28) in einer Foto-sensitiven organischen Elektro-lumineszierenden Schicht einer organischen Licht-emittierenden Diode (OLED), wobei das Verfahren die Schritte umfasst:

    Bereitstellen der Foto-sensitiven organischen Elektro-lumineszierenden Schicht (22), welche aus einem Foto-quervernetzbaren Flüssig-Kristall gebildet ist, welcher stangenförmige dichroitische Moleküle darin aufweist;

    Definieren eines Gittermusters in der organischen Elektro-lumineszierenden Schicht durch Beleuchten der organischen Elektro-lumineszierenden Schicht mit ersten und zweiten Lichtstrahlen, welche einen Winkel θ dazwischen aufweisen, wobei die ersten und zweiten Lichtstrahlen (41, 42) dazu eingerichtet sind, einen Interferenz-Randbereich in der organischen Elektro-lumineszierenden Schicht zu erzeugen, und wobei die Foto-sensitive organische Elektrolumineszierende Schicht auf Strahlung sensitiv ist, welche eine Wellenlänge zwischen 10 nm und 400 nm aufweist;

    dadurch gekennzeichnet, dass der Brechungsindex des definierten Gittermusters durch Variieren der Intensität des Lichts eingestellt wird, welchem der Foto-quervernetzbare Flüssig-Kristall ausgesetzt ist.


     
    2. Verfahren nach Anspruch 1, wobei die ersten und/oder zweiten Lichtstrahlen eine Wellenlänge zwischen 10 nm und 400 nm aufweisen.
     
    3. Verfahren nach Anspruch 1, wobei die ersten und zweiten Lichtstrahlen durch ein Mach-Zehnder-Interferometer bereitgestellt werden.
     
    4. Verfahren nach Anspruch 1, wobei die beleuchteten Bereiche der organischen Elektro-lumineszierenden Schicht dazu eingerichtet sind, ein rechteckiges Brechungsindex-Profil auszubilden.
     
    5. Verfahren nach Anspruch 1, wobei die beleuchteten Bereiche der organischen Elektro-lumineszierenden Schicht dazu eingerichtet sind, ein chirped Gitterprofil auszubilden.
     
    6. Verfahren nach Anspruch 1, wobei die beleuchteten Bereiche der organischen Elektro-lumineszierenden Schicht dazu eingerichtet sind, ein apodisiertes Gitterprofil auszubilden.
     
    7. Verfahren nach Anspruch 1, wobei die Spitzen-Wellenlänge der OLED-Vorrichtung durch Einstellen des Brechungsindex des definierten Gittermusters ausgewählt wird.
     


    Revendications

    1. Procédé de fabrication d'un motif de réseau nanostructuré (28) dans une couche électroluminescente organique photosensible d'une diode électroluminescente organique (OLED), le procédé comprenant les étapes consistant à :

    fournir la couche électroluminescente organique photosensible (22) formée d'un cristal liquide photoréticulable contenant des molécules dichroïques en forme de tige ;

    définir un motif de réseau dans la couche électroluminescente organique en éclairant la couche électroluminescente organique avec des premier et second faisceaux lumineux ayant un angle θ entre ceux-ci, dans lequel les premier et second faisceaux lumineux (41, 42) sont configurés pour créer une frange d'interférence dans la couche électroluminescente organique, et dans lequel la couche électroluminescente organique photosensible est sensible à un rayonnement ayant une longueur d'onde comprise entre 10 nm et 400 nm ;

    caractérisé en ce que

    l'indice de réfraction du motif de réseau défini est accordé en faisant varier l'intensité de la lumière à laquelle le cristal liquide photoréticulable est exposé.


     
    2. Procédé selon la revendication 1, dans lequel les premier et/ou second faisceaux lumineux ont une longueur d'onde comprise entre 10 nm et 400 nm.
     
    3. Procédé selon la revendication 1, dans lequel les premier et second faisceaux lumineux sont fournis par un interféromètre de Mach-Zehnder.
     
    4. Procédé selon la revendication 1, dans lequel les régions éclairées de la couche électroluminescente organique sont configurées pour former un profil d'indice de réfraction rectangulaire.
     
    5. Procédé selon la revendication 1, dans lequel les régions éclairées de la couche électroluminescente organique sont configurées pour former un profil de réseau distordu.
     
    6. Procédé selon la revendication 1, dans lequel les régions éclairées de la couche électroluminescente organique sont configurées pour former un profil de réseau apodisé.
     
    7. Procédé selon la revendication 1, dans lequel la longueur d'onde de pic du dispositif OLED est sélectionnée en accordant l'indice de réfraction du motif de réseau défini.
     




    Drawing

















    Cited references

    REFERENCES CITED IN THE DESCRIPTION



    This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

    Patent documents cited in the description