Broadband organic LED structure

Abstract

An OLED structure comprises a plurality of device layers sandwiched between two metal electrode layers. The emissive layer comprises an emissive material having an emission spectrum with a maximum emission at an emission peak wavelength in the range of 370 to 750 nm. The plurality of sandwiched device layers has thicknesses and refractive indices constituting an optical path length between the first and the second metal electrode layers. The metal electrode layers and the plurality of sandwiched device layers form an optical microcavity having a cavity resonance with a local reflectivity minimum at a reflection valley wavelength defined by the optical path length between the metal electrodes, the optical path length being configured to result in the reflection valley wavelength lying in the range of 370 to 750 nm. The emission peak wavelength and the reflection valley wavelength are separated by at least 100 nm.

Description

[0001] BROADBAND ORGANIC LED STRUCTURE

[0002] BACKGROUND

[0003] Organic Light Emitting Diodes OLEDs are increasingly used as the light emitting structure of not only displays about al so in lighting devices for general illumination purposes .

[0004] The feasibility of OLEDs in lighting devices for general illumination purposes may depend on their capability to produce broadband or white light .

[0005] As one emissive layer material only emits light at a limited wavelength range , specific arrangements need to be done to achieve white light emission . Conventionally, white OLEDs , WOLEDs , have been implemented by incorporating different emissive materials of different emission spectra into the OLED structure . Different material s , which may differ from each other , for example , by doping thereof , may form superposed layers or be spatially distributed within one single layer .

[0006] The feasibility of OLEDs for general illumination applications may also be affected by the materials used therein . The use of toxic materials is desired to be avoided and may even be banned . On the other hand, the cost pressure may require using only low-cost materials , the supply of which is steady and the price development predictable .

[0007] Some conventional WOLEDs also suffer from a deficient long-term stability performance . SUMMARY

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description . This summary is not intended to identify key features or essential features of the claimed subj ect matter, nor is it intended to be used to limit the scope of the claimed subj ect matter .

[0009] According to a first aspect , an organic light-emitting diode OLED structure may be implemented . The OLED structure may be configured to emit broadband light . The broadband light may be white light suitable for generic lighting or illuminating purposes . In addition to illumination, the OLED structure emitting broadband light may be used for any other application where broadband light is needed . For example , the OLED structure may be used for front lighting of various display devices , for example , those of e-readers and other handheld devices .

[0010] The OLED structure comprises a plurality of device layers , including an emissive layer, sandwiched between a first and a second metal electrode layers .

[0011] The emissive layer comprises an emissive material having an emission spectrum with a maximum emission at an emission peak wavelength Xpin the range of 370 to 750 nm, for example , 390 to 660 nm, for example , 430 to 520 nm, for example , in the range of 450 to 500 , for example , in the range of 460 to 470 nm .

[0012] Each of the plurality of the sandwiched device layers has a thickness and a refractive index . The thicknesses and refractive indices of the sandwiched device layers constitute an optical path length L between the first and the second metal electrode layers .

[0013] The first and the second metal electrode layers and the plurality of sandwiched device layers form an optical microcavity having a cavity resonance with a local reflectivity minimum at a reflection valley wavelength defined by the optical path length, the optical path length being configured to result in the reflection valley wavelength lying in the range of 370 to 750 nm, for example , 390 to 660 nm, for example , 570 to 670 nm, for example, in the range of 600 to 640 nm, for example , in the range of 615 to 625 nm .

[0014] The emission peak wavelength and the reflection valley wavelength are separated from each other by at least 100 nm, for example , at least 150 nm, for example , about 150 to 160 nm or about 180 to 200 nm, for example , about 190 nm .

[0015] According to a second aspect , a method for manufacturing an OLED structure in accordance with the first aspect may be implemented . The method comprises forming a first metal electrode layer, forming the plurality of device layers , and forming a second metal electrode layer .

[0016] Further embodiments of the above aspects may be implemented within the scope of the claims .

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present disclosure will be better understood from the following detailed description read in view of the accompanying drawings , wherein : FIG . 1 shows a cross-section of a part of an OLED structure ;

[0019] FIGs . 2 and 3 show the optical performance of OLED structures ; and

[0020] FIG . 4 shows a flow chart of a method for manufacturing an OLED structure .

[0021] Unless specifically stated to the contrary, any drawing of the aforementioned drawings may be schematic and drawn not to scale such that any element in said drawing may be drawn with inaccurate proportions with respect to other elements in said drawing in order to emphasize certain structural aspects of the embodiment of said drawing .

[0022] Moreover, corresponding elements in the embodiments of any two drawings of the aforementioned drawings may be di sproportionate to each other in said two drawings in order to emphasi ze certain structural aspects of the embodiments of said two drawings .

[0023] DETAILED DESCRIPTION

[0024] The organic light-emitting diode OLED 100 partially illustrated in FIG . 1 may form, or serve as , a lightemitting part of a lighting device . Then, the illustration of FIG . 1 may be considered representing also a part of such lighting device 150 .

[0025] The lighting device 150 may be a general lighting device for illumination purposes . Such lighting device may be implemented, for example , as a replaceable light bulb for luminaires . As discusses in more detail below, the OLED may be a broadband OLED, for example , a white OLED, WOLED, emitting substantially white light .

[0026] "White" light may refer to light comprising a broadband spectrum making the light fal l within the white domain defined in accordance with any color standard used in the field of illumination . "White" light may refer to the color temperature of the light emitted out of the OLED structure lying, for example , in the range of 2700 to 6500 K .

[0027] The OLED 100 of FIG . 1 has a layered OLED structure 110 formed on and lying on a substrate 101 .

[0028] Being partially illustrated means that a complete , operable OLED may comprise also any appropriate further layers , parts , or elements not shown in FIG . 1 .

[0029] The substrate may be formed, for example , of any appropriate glass or plastic material . One possible glass material is quartz .

[0030] The OLED structure 110 serves as the actual light emitting part of the OLED 100 . In the case of the OLED forming a part of or serving as a light emitting part of a lighting device 150 , the OLED structure then serves as a light-emitting element of the lighting device .

[0031] The OLED structure 110 comprises a first and a second metal electrode layers 111 , 112 . A "metal" electrode layer refers the electrode layer comprising, possibly being completely formed of , a metal . In the example of FIG . 1 , the metal may be aluminum . In other embodiments , other metals such as silver of gold may be used in at least one of the two electrodes .

[0032] The first metal electrode layer lies on the substrate 101 .

[0033] A "layer" may refer to a substantially planar platelike structure extending laterally in two dimensions . A layer may have a thickness in a third dimension perpendicular to those two dimensions . The thickness in the third dimension may be substantially smaller than the extension of the layer in the two other dimensions .

[0034] In a layer structure having a plurality of layers stacked on each other, all the layers may have substantially the same lateral dimensions , i . e . substantially the same si ze . Alternatively, one or more layers may be larger or smaller, i . e . have different lateral or hori zontal dimensions , than one or more of the other layers of the layer structure . For example , in the OLED structure 100 of FIG . 1 , the second metal electrode layer 112 may be smaller than the first metal electrode layer 111 , so that it covers only a part of the area thereof . In FIG . 1 illustrating a laterally limited part of the OLED structure 110 only, this option is not shown .

[0035] A layer formed or lying "on" another layer of structure may refer to the layer being in direct contact with the other layer or structure . Alternatively, there may one or more intermediate layer ( s ) or structure ( s ) therebetween . A plurality of device layers lies on top of each other, sandwiched between the first and the second metal electrode layers 111 , 112 .

[0036] The plurality of device layers comprises an emissive layer 113 . The emissive layer serves as the actual light-generating part of the plurality of device layers . It comprises an electroluminescent material which, upon supply of electrical current through it , emits light . Such material may be considered an "emissive material" .

[0037] In the example of FIG . 1 , the OLED structure is a single emissive layer OLED structure . "Single emissive layer" refers to the plurality of device layers comprising not more than one single integral emissive layer . One single integral layer refers to a single layer formed of substantially the same material without any material interfaces between two different materials or compositions dividing the layer into different sub-layers .

[0038] The plurality of device layers further comprises a hole inj ection layer HIL 114 on the substrate 101 ( separated therefrom by the first metal electrode layer 111 ) , and a hole transport layer HTL 115 on the HIL . The emissive layer 113 lies on the HTL . Further, there is an electron transport layer ETL 116 on the emiss ive layer, and an electron inj ection layer EIL 117 on the ETL .

[0039] In other embodiments , a plurality of device layers may be implemented differently . For example, one or more of the hole inj ection layer, hole transport layer, electron transport layer, and electron inj ection layer may be missing . On the other hand, for example, two layers may be combined such that one single layer provides the functions of the two layers .

[0040] "Organic" light emitting device OLED refers to the materials of the plurality of device layers between the metal electrodes being organic materials .

[0041] The metal electrode layers 111 , 112 serve as electrical contacts of the OLED structure , between which current can be supplied through the OLED device layers to effect light emission in the emissive layer 113 .

[0042] In the embodiment of FIG . 1 , the first metal electrode 111 may be configured to serve as an anode of the OLED structure 110 . The second metal electrode may be configured to serve as a cathode of the OLED structure 110 .

[0043] Various metals may be used to form the first and the second metal electrode layers . For example , one or both of them may comprise , or be substantially completely formed of , aluminum . Other suitable metals to form at least one of the electrode layers are silver and gold .

[0044] The metal electrode layers 111 , 112 are optically at least partially reflective . They thus reflect at least part of the light energy at visible wavelengths incident thereon . Then, light emitted or generated in the emis sive layer and propagating in the device layers becomes at least partially reflected at the interfaces between the metal electrode layers and the device layers .

[0045] Due to said reflectivity, the first and the second metal electrode layers and the plurality of sandwiched device layers form an optical microcavity 120 . To enable light transmission out of the microcavity 120 , one of the first and the second metal electrode layers may be formed so as to have a thickness sufficiently low to make the layer in question partially transparent . "Partially transparent" refers to that in addition to reflecting part of the light incident thereon, such layer al so allows part of the incident light energy to be transmitted through it . Suitable thickness may be , for example , at least about 1 nm but less than or equal to 60 nm, for example les s than or equal to 50 nm, for example, less than or equal to 40 nm, for example, less than or equal to 30 nm, for example , less than or equal to 20 nm . In the case of aluminum, for example , the thickness of about 10 to 20 nm, for example , about 15 nm may provide appropriate partial transparency .

[0046] In the example of FIG . 1 , the second metal electrode layer 112 has a thickness sufficiently low to allow partial transmission of light through it . Thereby, light generated in the emissive layer may escape the OLED structure through the partially transparent second metal electrode layer 112 .

[0047] In the example of FIG . 1 , the first metal electrode 111 lying on the substrate 101 and may be considered located in the bottom part of the OLED structure . Then, the second metal electrode 112 may be considered located in the top part of the OLED structure 110 . Light emiss ion through the top second electrode layer then results the OLED structure being a top-emitting OLED structure .

[0048] In other embodiments , bottom-emitting OLED structures may be implemented . Then, an OLED structure may lie upside down in comparison to the example of FIG. 1, i.e. with a partially transparent electrode layer lying at the bottom part of the OLED structure, on or adjacent a substrate . In the case of a bottom-emitting OLED structure, the substrate may be formed of a transparent glass material, such as quartz, or a transparent plastic material.

[0049] The emissive layer 113 of the OLED structure comprises an emissive material selected so as to have an emission spectrum with a maximum emission at an emission peak wavelength lying in the range of 370 to 750 nm. It may lie, for example, in the range of 390 to 660 nm. In some embodiments, it may lie substantially in the blue part of the visible light spectrum, for example, in the range of 430 to 520 nm, for example, in the range of 450 to 500, for example, in the range of 460 to 470 nm.

[0050] An example of the device layers' configuration resulting an emission peak wavelength at about 466 nm is disclosed below with reference to FIG. 2. "Maximum emission" may refer to the highest intensity of the light generated in or emitted by the emissive layer .

[0051] The emissive material may comprise, being possibly substantially completely formed of, a Thermally Activated Delayed Fluorescence TADF material. A TADF material may have a substantively broad emission spectrum, which may be advantageous especially in the case of using the OLED structure for a light emitting part of an OLED for general lighting purposes. A TADF material may have, for example, an emission spectrum with a full width half maximum FWHM of at least 50 nm, for example, at least 75 nm.

[0052] Another way to consider broadness of an emission spectrum is to define a width of a wavelength range with a minimum intensity level. It may be advantageous that the emissive material produces an emission spectrum with an intensity of at least 2 %, for example, at least 5 %, for example, at least 10 % of the maximum emission throughout a wavelength range having a width of at least lOOnm, for example, at least 150 nm, for example, at least 200 nm, for example, at least 250 nm. That wavelength range may lie between 370 and 750 nm, for example, between 390 to 660 nm.

[0053] One suitable choice for the TADF material is DMAC-DPS as an emissive material.

[0054] DMAC-DPS, or 10 , 10 '- ( 4 , 4 ' -Sulf onylbis ( 4 , 1 -phenylene) ) bis ( 9, 9-dimethyl-9, 10-dihydroacridine) , is a blue emissive material in Thermally Activated Delayed Fluorescence (TADF) OLED structures. It has a FWHM of about 80 nm.

[0055] Other possible alternatives of the TDAF emissive layer material comprise, for example, DMAC-TRZ or 10- (4- (4, 6- Diphenyl-1, 3, 5-triazin-2-yl) phenyl) -9, 9- dimethyl- 9, 10- dihydroacridine, SpiroAC-TRZ or 10 - ( 4 - ( 4 , 6-diphenyl- 1, 3, 5-triazin-2-yl) phenyl) -10H- spiro [acridine -9, 9 ' - fluorene, and 2PXZ-TAZ or 10, 10 ' - (4, 4 ' - (4-Phenyl-4H- l,2,4-triazole-3,5-diyl)bis (4, 1 -phenylene) ) bis (10H- phenoxazine) as an emissive material. As known by the s kil led person, an optical microcavity refers to an optical resonance structure which may have one or more cavity resonances . An optical resonance structure may serve for filtering light by passing only some wavelengths through or out of it . A cavity resonance refers to a wavelength where the l ight transmis sion through or out of the microcavity is at its maximum . On the other hand, then the reflectance of the microcavity is at its minimum .

[0056] A cavity resonance is related to the concepts of cavity mode and resonance mode . A cavity mode or a resonance mode refers to the wavelength of the cavity resonance . The optical microcavity 120 of the OLED structure 100 has been configured so as to have at least one cavity resonance in the visible part of the light spectrum, at a wavelength higher than the emission peak wavelength .

[0057] The wavelength at which the local reflectivity minimum lies may be called a reflection valley wavelength .

[0058] A "valley" refers here to a local minimum point of the reflectivity in function of wavelength, i . e . the reflectivity spectrum . Thus , the reflectivity increases when the wavelength is either increased or decreased from the reflection valley wavelength . A valley may also be called a through . A valley and a through are concepts opposite to the "peak" , i . e . a local maximum value of a set of data points .

[0059] A "local" reflectivity minimum refers to that there may be other reflection valley wavelengths also , i . e . other wavelengths of locally lowered reflectivity, in the re- flectivity spectrum of the microcavity . A local reflectivity minimum is therefore not necessarily the absolutely lowest reflectivity .

[0060] The optical operation of the microcavity, such as the resonance wavelengths , are determined by the optical properties of the various material layers forming the microcavity . In the OLED structure of FIG . 1 , one factor affecting the optical operation is the optical path length L between the first and the second metal electrode layers .

[0061] Optical path length of a layer refers to the physical thickness and the refractive index of the material of the layer . The optical path length of a single layer may be defined as the product of the thickness and the refractive index . The optical path length L of the entire stack of device layers between the first and the second metal electrode layers 111 , 112 may be defined as the sum of the optical path lengths of each of the plurality of device layers .

[0062] The total optical path length L between the first and the second metal electrode layers may be configured, by selecting the materials and thicknesses of the device layers , so that at least one cavity resonance and thus the reflection valley wavelength lie in the range of 370 to 750 nm . It may lie , for example, in the range of 390 to 660 or 670 nm . In some embodiments , it may lie substantially in or close to the red part of the visible light spectrum, for example , in the range of 570 to 670 nm, for example , in the range of 600 to 640 nm, for example , in the range of 615 to 625 nm . An example of the device layers' configuration resulting a reflection valley wavelength at about 622 nm is disclosed below with reference to FIG. 2. In another example shown in FIG. 3, there is a reflection valley wavelength at about 655 nm.

[0063] As specified above, the emission peak wavelength and the reflection valley wavelength may each lie in the range of 370 to 750 nm. However, they are advantageously separated from each other by at least 100 nm. The separation may be, for example, at least 150 nm. In some embodiments, the separation may be in the range of about 150 to 160 nm, for example, about 155 nm. In yet other embodiments, the separation may be, for example, in the range of about 180 to 200 nm, for example, about 190 nm.

[0064] An example is disclosed below with reference to FIG. 2, with the separation being about 156 nm. In the example of FIG. 3, the separation is about 189 nm.

[0065] Said separation means that the emission peak wavelength and the reflection valley wavelength may correspond to different colors of the visible spectrum. For example, the emission peak wavelength may lie in or close to the blue part of the visible spectrum, and the reflection valley wavelength may lie in or close to the red part of the spectrum. In other embodiments, the situation may be the opposite. In yet other embodiments, the emission peak wavelength and the reflection valley wavelength may correspond to any two different colors or wavelength regions in the range of 370 to 750 nm, as long as they are sufficiently separated. Separation between the emission peak wavelength and the reflection valley wavelength may advantageously enable tailoring of the output emission spectrum of the OLED structure, as discussed further below.

[0066] The total physical thickness D of the plurality of device layers between the first and second metal electrode layers may lie, for example, in the range of 40 to 300 nm, for example, 80 to 250 nm, for example, 100 to 140 nm. The accurate thickness may be selected on the basis of the desired location of the local reflectivity minimum, i.e. the desired reflection valley wavelength, and the refractive indices of the device layers.

[0067] On the other hand, the optical path length L formed by the plurality of device layers may lie in a range defined by the range of physical thickness D multiplied by the thickness-weighted average of the refractive indices of the plurality of device layers between the first and second metal electrode layers. The thickness-weighted average of the refractive indices may lie, for example, in the range of 1.2 to 1.8. It may be, for example, about 1.5. For example, for the thickness-weighted average of the refractive indices of 1.5, the optical path length may lie in the range of 60 to 450 nm, for example, 120 to 375 nm, for example, 150 to 210 nm.

[0068] Figures 2 and 3 illustrate the optical performance of two OLED structures. The OLED structure of FIG. 2 is in accordance with that of FIG. 1. The OLED structure of FIG. 3 differs from that of FIG. 2 by the emissive layer thickness as defined below. The photoluminescence spectra of the TADF emitter comprising DMAC-DPS was obtained by exciting a thin-film of DMAC-DPS with a thickness of 100 nm formed on a quartz substrate by 250 fs pulses at 375 nm and 200 kHz repetition rate .

[0069] The reflectivities of the microcavity OLED structures were measured, from the side of the top partially transparent metal electrode , using an ellipsometer ( J . A . Woollam VASE ) .

[0070] The electroluminescence spectra of the OLED structures were obtained using a spectrometer (OceanOptics USB2000+ ) .

[0071] The measurements were carried out in nitrogen N2 atmosphere .

[0072] The OLED structures of Figures 2 and 3 have , called "WOLED1" and "WOLED2" , respectively, a plurality of device layers sandwiched between a first and a second metal electrode layers formed of aluminum . The OLED structures are top emitting with the second metal electrode layer serving as a cathode of the OLED structure having a thickness of about 15 nm . The plurality of device layers comprise a hole inj ection layer HIL formed of molybdenum trioxide M0O3 , a hole transport layer HTL comprising (C30H20N2 ) : 1 , 3-Di ( 9H-carbazol- 9-yl ) benzene mCP, the emissive layer EML formed of DMAC-DPS , an electron transport layer ETL formed of (O36H28O3P2 ) : Bis [ 2 - (diphenylphosphino ) phenyl ] ether oxide DPEPO, and an electron inj ection layer EIL formed of lithium fluoride LiF . The layer thicknesses of the plurality of the device layers may be varied such that the desired properties of the cavity resonance and the separation between the emission peak wavelength and the reflection valley wavelength are achieved. For example, the EML may have a thickness in the range of 15 to 140 nm, for example, in the range of 15 to 120 nm, for example, in the range of 15 to 95 nm.

[0073] The device layers of the OLED structure of FIG. 2, thus WOLED2, have the following thicknesses: HIL 5 nm, HTL 40 nm, EML 65 nm, ETL 50 nm, and EIL 1 nm. In other embodiments, the thickness may vary and lie, for example, in the following ranges: HIL 3 to 7 nm, HTL 35 to 45 nm, EML 50 to 90 nm, ETL 40 to 60 nm, and EIL 0.5 to 10 nm. The thickness of each device layer may depend on the material (s) thereof.

[0074] It has been found that certain advantages in the form of high efficiency and good electrical properties of

[0075] WOLED structures are achievable with the EML thicknesses, for example, in the range of 50 to 90 nm, for example, in the range of 60 to 80 nm, for example, in the range 65 to 75 nm. In other embodiments, an EML may have a thickness, for example, in the range of 25 to 35 nm, for example, about 30 nm.

[0076] As shown in FIG. 2, the emissive layer has its maximum emission at an emission peak wavelength of about 466 nm. The emission is rather broadband with a FWHM of the intensity of about 85 nm. On the other hand, there is an intensity of at least 2 % of the maximum intensity throughout a wavelength range having a width of about 220 nm . Said wavelength range lies between 350 nm and 650 nm and extends from about 395 nm to about 615 nm . Further, there is an intensity of at least 5 % of the maximum intensity throughout a wavelength range having a width of about 190 nm . Said wavelength range lies between 350 nm and 600 nm and extends from about 400 nm to about 590 nm . On the other hand, there is an intensity of at least 10 % of the maximum intensity throughout a wavelength range having a width of about 160 nm . Said wavelength range lies between 350 nm and 600 nm and extends from about 410 nm to about 570 nm .

[0077] The reflectivity of the microcavity, i . e . the reflection of light incident on the second metal electrode 112 from the outside of the microcavity, has two local minima in the measured wavelength range . One of them lies at a reflection valley wavelength of 622 nm .

[0078] The optical microcavity serves for filtering the light initially emitted in the emissive layer . The separation between the emission peak wavelength and the reflection valley wavelength results in light at the wavelengths of the highest initial intensity being suppressed or attenuated . On the other hand, the microcavity efficiently passes or couples out of the structure light at the wavelengths of the lower initial intensity .

[0079] In result of the operation of the optical microcavity, the output emission spectrum of the white OLED WOLED1 structure has its maximum at 605 nm, and there is remarkable emission throughout an emission wavelength band with a FWHM of about 150 nm ( from about 470 or 480 nm to about 620 or 630 nm) . Thus , in comparison to the initial emission spectrum of the emissive layer, the optical microcavity advantageously tailors the spectrum of the emitted light making the light emitted out of the WOLED1 structure broadband . That broadband light can be used as white light for general illumination purposes .

[0080] In other embodiments , varying the materials and thicknesses of as well of the emissive layer and the other layers of the plurality of device layers , the spectrum of light emitted out of an OLED structure may be tailored in a versatile manner .

[0081] The White OLED (WOLED2 ) structure of FIG . 3 measurement results distinguishes from that of FIG . 2 (WOLED2 ) in that the EML layer has a thickness of 75 nm .

[0082] As shown in FIG . 3 , similarly to WOLED1 , the reflectivity of the microcavity of the WOLED2 has two local minima in the measured wavelength range . One of them lies at a reflection valley wavelength of 655 nm .

[0083] The output emission spectrum of the WOLED2 structure has its maximum at 516 nm, and there is remarkable emission throughout an emission wavelength band with a FWHM of about 220 nm ( from about 470 to about 660 nm) .

[0084] As FIG . 3 shows , change of the EML thickness changes the emission spectrum, shifting the maximum intensity thereof towards the shorter wavelengths . The color temperature of the emission thus changes . This illustrates the adj ustability of the optical properties of the WOLED structures based on adj usting the layer thickness (es ) and thus the optical path length L of the microcavity . As the name W0LED2 indicates , despite the change of the emiss ion spectrum in compari son to that of WOLED1 , the light emitted by WOLED2 may also fal l within the white domain . OLED structures as discussed in the above with reference to FIGs . 1 to 3 may be manufactured using methods in accordance with those discussed below with reference to FIG . 3 . On the other hand, the OLED structures manufactured by such method may be in accordance with any of those discussed above with reference to FIGs . 1 to 3 .

[0085] The method 400 of FIG . 4 starts by forming a first metal electrode layer in step 410 .

[0086] Thereafter, a plural ity of device layers are formed on the first metal electrode layer in step 420 . That step is carried out so as to comprise forming an emissive layer as one of the device layers with an emission peak wavelength in the range of 430 to 520 nm, for example , in the range of 450 to 500 nm . Further, the device layers are formed in such a way that a total optical path length through all the device layers results , the device layers forming an optical path of an optical resonance cavity, in a reflection valley wavelength in the range of 570 to 670 nm, for example , in the range of 600 to 640 nm .

[0087] In other embodiments , the materials and the thicknesses of the device layers may be selected to produce emiss ive peak wavelength and the reflection valley wavelength lying anywhere in the range of 370 to 750 nm, separated from each other by at least 100 nm, as specified above .

[0088] Once the device layers are ready, a second metal electrode is formed on top of the device layers in step 430 . The second metal electrode layer may be formed sufficiently thin to allow light to escape through it from the microcavity formed by the metal electrode layers and the device layers there between .

[0089] In the manufacturing method, processes and equipment as such known in the art may be utili zed . For example , Physical Vapor Deposition PVD or Chemical Vapor Deposition CVD or Spin-Coating or Inj ect Printing may be used to form one or more of the various layers of the OLED structure .

[0090] For example , an OLED structure in accordance with that discussed above with reference to FIG . 2 or 3 can be manufactured using the following Physical Vacuum Deposition method with the deposition pressure of approximately 2 >< 10“7mbar . The device can be fabricated on a glas s substrate with the dimensions of 15 mm x 15 mm x 1 mm . The manufacturing method may comprise the following steps : a . First , metal anode (Aluminium) and hole-in ecting layers (M0O3 ) are deposited through a patterned shadow mask on the substrate . b . Subsequently, the hole transport layer (mCP) , emissive layer ( DMAC-DPS ) , electron transport layer ( DPEPO) , and electron inj ection layer (LiF) are deposited through a patterned shadow mask, c . The metal cathode layer (Aluminium) is deposited through a patterned shadow mask, resulting the cathode covering only partially the area of the underlying layers . The rate for deposition may be, for example, 1 A / s for Aluminium, 0.2 A / s for M0O3, 0.2 A / s for LiF, and 1 A / s for mCP, DMAC-DPS and DPEPO.

[0091] It is apparent to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims.

[0092] It will be understood that any benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages.

[0093] The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts. It will further be understood that reference to 'an' item refers to one or more of those items.

Claims

CLAIMS1. An organic light-emitting diode OLED structure (110) comprising a plurality of device layers, including an emissive layer (113) , sandwiched between a first and a second metal electrode layers (111, 112) , wherein the emissive layer (113) comprises an emissive material having an emission spectrum with a maximum emission at an emission peak wavelength Xpin the range of 370 to 750 nm, for example, 390 to 660 nm, for example, 430 to 520 nm, for example, 450 to 500, for example, 460 to 470 nm ; the plurality of sandwiched device layers has thicknesses and refractive indices constituting an optical path length L between the first and the second metal electrode layers (111, 112) ; the first and the second metal electrode layers (111, 112) and the plurality of sandwiched device layers form an optical microcavity (120) having a cavity resonance with a local reflectivity minimum at a reflection valley wavelength Xi defined by the optical path length L, the optical path length being configured to result in the reflection valley wavelength Xi lying in the range of 370 to 750 nm, for example, 390 to 660 nm, for example, 570 to 670 nm, for example, 600 to 640 nm, for example, 615 to 625 nm; the emission peak wavelength Xpand the reflection valley wavelength Xi being separated from each other by at least 100 nm, for example, at least 150 nm, for example, about 150 to 160 nm or about 180 to 200 nm, for example, about 190 nm.

2. An organic light-emitting diode OLED structure (110) as defined in claim 1, wherein the emission spectrum has an intensity of at least 2 %, for example, at least 5 %, for example, at least 10 % of the maximum emission throughout a wavelength range having a width of at least 100 to 150 nm, for example, at least 200 nm, for example, at least 250 nm between 370 and 750 nm.

3. An organic light-emitting diode OLED structure (110) as defined in claim 1 or 2, wherein the emissive material comprises a Thermally Activated Delayed Fluorescence TADF material.

4. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 3, wherein the TADF material comprises at least one of DMAC-DPS, DMAC-TRZ, SpiroAC-TRZ, and 2PXZ-TAZ as an emissive material .

5. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 4, wherein the OLED structure is a single emissive layer OLED structure .

6. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 5, wherein the thicknesses D of the plurality of sandwiched device layers has a total thickness in the range of 40 nm to 300 nm, for example, 80 to 250 nm, for example, 100 to 140 nm.

7. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 7, wherein the optical path length L lies in the range of 60 nm to 450 nm, for example, 120 to 375 nm, for example, 150 to 210 nm.

8. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 8, wherein at least one of the first and the second metal electrode layers (111, 112) comprises aluminum, silver, or gold.

9. An organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 8, wherein at least one of the first and the second metal electrode layers (111, 112) has a thickness of less than or equal to 60 nm, less than or equal to 50 nm, for example, less than or equal to 40 nm, for example, less than or equal to 30 nm, for example, less than or equal to 20 nm, to allow transmission of the emitted light out of the optical microcavity (120) through it.

10. An organic light-emitting diode OLED structure (110) as defined in claim 9, wherein the at least one of the first and the second metal electrode layers (111, 112) having a thickness to allow transmission of the emitted light out of the optical microcavity through it comprises aluminum and has a thickness in the range of 1 to 60 nm, for example, 1 to 50 nm, for example, 1 to 40 nm, for example, 5 to 30 nm, for example, 10 to 20 nm, for example, about 15 nm.

11. An organic light-emitting diode OLED structure (110) as defined in claim 9 or 10, wherein the one of the first and the second metal electrode layers (111, 112) having a thickness to allow transmission of the emitted light out of the optical microcavity through it serves as a cathode of the OLED structure.

12. An organic light-emitting diode OLED structure (120) as defined in any of claims 1 to 11, wherein the plurality of sandwiched device layers comprises, superposed in the following order on the anode:a hole injection layer (114) comprising molybdenum trioxide M0O3, a hole transport layer (115) comprising mCP, the emissive layer (113) comprising DMAC-DPS, an electron transport layer (116) comprising DPEPO, and an electron injection layer (117) comprising lithium fluoride LiF.

13. An organic light-emitting diode OLED structure (110) as defined in claim 12, wherein the emissive layer (113) has a thickness in the range of 15 to 140 nm, for example, in the range of 15 to 120 nm, for example, in the range of 15 to 95 nm.

14. An organic light-emitting diode OLED structure (110) as defined in claim 12 or 13, wherein the metal electrode (111) serving as the anode has a thickness of higher than or equal to 40 nm, for example, higher than or equal to 60 nm; the hole injection layer (114) has a thickness in the range of 3 to 7 nm, for example, about 5 nm; the hole transport layer (115) has a thickness in the range of 35 to 45 nm, for example, about 40 nm; the emissive layer (113) has a thickness in the range of 25 to 35 nm, for example, about 30 nm; the electron transport layer (116) has a thickness in the range of 40 to 60 nm, for example, about 50 nm, and the electron injection layer (117) has a thickness in the range of 0.5 to 10 nm, for example, 0.5 to 5 nm, for example, about 1 nm.

15. An organic light-emitting diode OLED structure (110) as defined in any of claims 12 or 13, wherein the metal electrode (111) serving as the anode has a thickness of higher than or equal to 40 nm, for example, higher than or equal to 60 nm; the hole injection layer (114) has a thickness in the range of 3to 7 nm, for example, about 5 nm; the hole transport layer (115) has a thickness in the range of 35 to 45 nm, for example, about 40 nm; the emissive layer (113) has a thickness in the range of 50 to 90 nm, for example, about 60 to 80 nm, for example, in the range of 65 to 75 nm; the electron transport layer (116) has a thickness in the range of 40 to 60 nm, for example, about 50 nm, and the electron injection layer (117) has a thickness in the range of 0.5 to 10 nm, for example, 0.5 to 5 nm, for example, about 1 nm.

16. A lighting device (150) comprising an organic light-emitting diode OLED structure (110) as defined in any of claims 1 to 15, configured to serve as a light-emitting element of the lighting device.

17. A method (400) for manufacturing an organic light-emitting diode OLED structure having a plurality of device layers, including an emissive layer, sandwiched between a first and a second metal electrode layers, the method comprising forming a first metal electrode layer (410) , forming the plurality of device layers (420) , and forming a second metal electrode layer (420) , wherein the emissive layer is manufactured so as to comprise an emissive material having an emission spectrum with a maximum emission at an emission peak wavelength in the range of 370 to 750 nm, for example, 390 to 660 nm, for example, 430 to 520 nm, for example, in the range of 450 to 500, for example, in the range of 460 to 470 nm ; the plurality of sandwiched device layers are manufactured so as to have thicknesses and refractive indices constituting an optical path length between thefirst and the second metal electrode layers , the f irst and the second metal electrode layers and the plurality of sandwiched device layers forming an optical microcavity having a cavity resonance with a local reflec- tivity minimum at a reflection valley wavelength defined by the optical path length; and the plurality of device layers are manufactured so as to provide the optical path length to result in the reflection valley wavelength lying in the range of 370 to 750 nm, for example , 390 to 660 nm, for example ,570 to 670 nm, for example , in the range of 600 to 640 nm, for example , 615 to 625 nm; the emission peak wavelength Xpand the reflection valley wavelength Xi being separated from each other by at least 100 nm, for example , at least 150 nm, for example , about 150 to 160 nm .

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