WO2006012841A1 - Light-emitting component - Google Patents

Abstract

The invention relates to a light emitting component comprising a layered construction that contains an arrangement of organic layers (2), in particular to an organic light-emitting diode. The component is equipped with a diffractive assembly composed of one or more holographic layers (1), which transfers light from internal non-emitting modes to external emitting modes.

Description

 Light-emitting component

The invention relates to a light-emitting component with an arrangement of organic layers, in particular organic light-emitting diode.

State of the art

Organic Light Emitting Diodes (OLEDs) have recently been extensively studied, as they offer interesting applications in flat displays and for illumination purposes Organic light-emitting diodes usually consist of a glass substrate onto which a transparent conductor (usually indium tin oxide, Subsequently, one or more organic layers are applied with a thickness of about 100 nm, completed by a mostly metallic cathode.When a suitable voltage is applied, the organic layer emits light.On-light-emitting diodes are of interest The high switching speed as well as the comparatively high efficiency of the light emission are affordably available over large areas, and commercially successful displays based on organic light-emitting diodes are already available for these reasons.

Another future application of organic light-emitting diodes relates to illumination. In this case, homogeneous high luminances on comparatively large areas are required. In order to be able to compete with existing concepts such as fluorescent lamps, white emitters with the highest possible efficiency (> 501 m / W) should be aimed at.

However, a major disadvantage of organic light-emitting diodes realized so far is that the predominant part of the resulting light is not decoupled from the layer structure, but remains in the component and is ultimately lost. According to the previously known literature (Lu et al., J. Appl. Phys. 91, 595 (2002)), only about 20% of the resulting light is coupled out in a conventional planar OLED structure. Approximately 30% of the resulting light is totally reflected on exit from the glass substrate and then passes through the substrate ("substrate modes"), a further 50% run as waveguide mode in the anode-organic layer system, mostly indium tin oxide organic ("ITO Organic Modes "). The exact distribution of the stated proportions depends on the exact structure of the components and is not completely known to this day. Nevertheless, it can be assumed that the said Distribution approximately describes the physical reality and therefore about 80% of the light generated in the component is not coupled out in the forward direction to the outside, but remains in internal modes and thus lost for the application.

In principle, three possibilities are known for increasing the light output from organic light emitting diodes: i) by applying periodic structures or roughening the substrate, the coupling out of substrate modes can be achieved; (ii) the coupling is improved by introducing an intermediate layer with a very low refractive index and (iii) by introducing a periodic structure into the layer structure of the OLED structure, the waveguide modes can be outcoupled to the outside.

The literature already discloses a large number of works for improving the decoupling. Thus, some work describes the simple roughening of the substrate; Further work is concerned with the application of lenses or periodic arrays of lenses to the substrate (Moller et al., J. Appl. Phys. 91, 3324 (2002)). The improvement in the outcoupling by introducing a layer with a low refractive index has also been demonstrated (Tsutsui et al., Adv. Mat. 13, 1149 (2001)). A number of documents also refer to the introduction of a periodic structure (US 6,630,684; US 6,476,550; US 2001/0033135 A1; US 2004/0027062 A1).

However, all these methods have several disadvantages. Thus, the periodic structures can accomplish the substrate or the roughening only the decoupling of the modes from the substrate. Since these absorb only about 30% of the intensity, this gives only a modest improvement from the outset compared to a structure without decoupling methods. Furthermore, these structures lead to a local blurring in displays with very small picture elements (pixels) and thus a use of this improved coupling in displays is no longer possible. Furthermore, such structures produce non-planar surfaces, which is undesirable in many cases.

Using low refractive index layers also has night parts. Such layers are usually realized in the form of porous materials, which can lead to instabilities. Furthermore, oxygen and water can easily diffuse through such layers, which is disadvantageous because of the high sensitivity of the OLED to these substances. The introduction of periodic structures into the active layers of OLED has the best prospects of increasing the efficiency for the above-mentioned reasons: in a conventional OLED emitting towards the substrate, about 50% of the resulting light is transferred into the organic / substrate Modes coupled. However, the approaches presented so far, such as the periodic structuring of the substrate, have sensitive disadvantages since, owing to their small layer thickness, OLEDs have to be built up on very smooth substrates. Therefore it is necessary to planarize the periodic structure very well afterwards.

The invention

The object of the invention is to provide a light-emitting component in which the possibilities for varying the optical properties are expanded and can be implemented in a simplified manner.

This object is achieved by a light-emitting component according to independent claim 1. An¬. Advantageous embodiments of the invention are the subject of dependent subclaims. ^ι

Holographic layers have a number of advantages: On the one hand, regular periodic optical structures can be produced in holographic layers with relatively simple means. Second, these structures can be made without significant roughening of the layers. Thirdly, in holographic structures, it is comparatively easily possible to realize complex arrangements of refractive index variations by superimposing a plurality of light waves when writing the holograms, by means of which the decoupling can be improved.

The simplest variant is, in principle, a one-dimensional lateral grating, which, however, has only limited efficiency, since essentially only modes in the direction perpendicular to the strips of the grating are influenced. The next most complex step is a two-dimensional grating, which can be generated, for example, by superposing respectively second lasers perpendicular to one another. In this case, modes are influenced in both lateral directions. In further embodiments, almost any two-dimensional periodic arrangement can be realized; in thicker holographic layers can also be three-dimensional Structures are realized. The advantage of a three-dimensional imaging structure is that a diffraction effect thereby becomes selective in wavelength. This can be used very advantageously in organic light-emitting diodes, for example, in order to optimize the color coordinates of the OLEDs for display applications or for special lighting applications. As will be described below, it is thus also possible to realize emitters which emit different colors in different directions.

An essential advantage of holographic imaging in comparison to simple grid structures lies in the fact that diffractive structures are formed such that a selected optical input signal, for example a substrate mode for a given angle, is defined in an output signal, for example an external mode in the form of a plane wave in the vertical direction, is converted, without the unavoidable diffraction for simple grating occurs in several orders in a different direction. Finally, the holographic approach can be understood as a reconstruction of Huygens elementary waves, so that, in principle, internal modes of the OLED are transferred into the external half-space in almost any desired manner. For example, it is possible to create a spherical wave that leads to a focus at one point, or to create a holographic light object in space.

There is further provided an arrangement for generating periodic optical gratings in which object beams are coupled into angular ranges of total internal reflection in holographic layers and superimposed with reference beams outside the angular range of total internal reflection.

The described advantages of using one or more holographic layers arise when using any organic light-emitting diodes.

Embodiments of the invention

The invention is explained below with reference to embodiments with reference to a drawing. Hereby show:

1 shows a schematic representation of a light-emitting component with an arrangement of organic layers and a holographic layer in a first embodiment; FIG. 2 shows a schematic representation of a light-emitting component with an arrangement of organic layers and a holographic layer in a second embodiment; FIG.

Fig. 3 is a schematic representation of a light-emitting device with an arrangement of organic layers and a holographic layer in a third

embodiment;

Fig. 4 is a schematic diagram of a structure of an arrangement for exposing a holographic layer;

5 shows a graphic representation of the emission behavior of an organic light-emitting diode with and without a holographic layer; and

Fig. 6 is a schematic representation of an arrangement in which three object beams (Oi, O ₂ , O ₃ ) and a reference beam (R) are provided.

One embodiment of the proposed arrangement comprises an organic light-emitting diode emitting through the substrate, in which a holographic film is applied to the side of the substrate which is remote from the light-emitting diode, for example by a grid being imprinted. The optical modes running back and forth in the substrate are bent at this grating and are thereby diffracted at an initial or subsequent reflection into an angular range, which can finally be coupled out as an external mode.

1 shows a schematic representation of a structure of a light-emitting component with a holographic layer. In Fig. 1, there are shown: a holographic layer 1, a substrate 2, a base electrode 3 (holes injecting, positive pole) made transparent, an array of organic layers 4, which in the embodiment, a hole injecting layer, holes transporting layer (HTL), a light-emitting layer (EL), an electron-transporting layer (ETL) and an electron-injecting layer, and a cover electrode 5, which may be formed from a metal having a low work function ( Injecting electrons; negative pole). Furthermore, an encapsulation (not shown) may be provided for the exclusion of environmental influences. Layers can be omitted, except for the base electrode 3, the light-emitting layer (EL) and the cover electrode 5. It can also be provided that several layers are combined into one layer. The intended use of one or more holographic layers can be done in conjunction with organic light emitting diodes of any kind, in particular with an OLED having one or more doped transport layers, as are known as such. It is also possible to use one or more holographic layers for organic light-emitting diodes which emit away from the substrate or in both directions. Here, the holographic layer 1 can be arranged between the substrate 2 and the substrate-near contact (Basis¬ electrode 3), as shown in Fig. 2.

Alternatively, however, the holographic layer can also be applied to the semitransparent electrode facing away from the substrate, which is shown in FIG. FIG. 3 shows a carrier / substrate 20, a substrate-near contact layer 21, an organic layer system 22, a contact layer 23 remote from the substrate, and a holographic layer 24.

In the following, an example of the control of the emission of a light emitting diode will be explained. For this purpose, a hologram was exposed, which superimposed on a perpendicular to the substrate entering Objekt¬ beam with a second reference beam, which was coupled via a prism at an angle that would already be subject to the tal¬ reflection in reverse beam direction in the planar substrate. Fig. 4 shows the structure of an arrangement for exposing the holographic layer.

The light emission of a light-emitting diode can also be controlled with the aid of holograms, which are obtained by superposition of several waves, which is shown in FIG. 6 with four waves (R, Oi, O ₂ , O ₃ ). Also, holograms of a rotational body, which arise when the truncated pyramid is replaced in Fig. 6 by a truncated cone, that is, with a non-planar object wave instead of three object beams O ₁ to O ₃ is used, the normal of the object wave describes a cone, contribute to this To convert light from internal modes into external modes.

Fig. 5 shows the emission distribution of an organic light emitting diode without (dotted line) and with (solid line) holographic grating. The grid was attached with index liquid on the substrate side. Without a grating, the light-emitting diode shows a slightly increasing intensity as a function of the angle, thus only approximately following the Lambert distribution usually assumed for light-emitting diodes, which would correspond to a constant intensity. With the holographic grating, the LED has a strong vertical direction Exaggeration of the intensity: Obviously, the hologram forms intensity from the substrate modes in the direction of the object beam. This shows that the imaging effect is given for substrate modes.

From these results it follows that in principle all substrate modes can be coupled out by means of suitable exposure of the hologram. These modes are distributed in a conventional planar planar OLED to the range of angles from about 36 to about 62 degrees to normal (Lu et al., J. Appl. Phys. 91, 595 (2002)). If these modes are to be coupled out vertically, the object beam can be held vertically during the exposure of the hologram. The reference beam can then be increased in steps in the specified angular range in order to achieve a superposition of holographic images for all substrate modes. If a wider forward feed-out is to be achieved, then a superposition of multiple holographic images can be created, in which the object and reference beams are varied in steps. As can be seen from FIG. 5, in the exemplary embodiment selected here, a half-width of approximately 10 degrees is emitted. This means that a similar step size could be chosen for this case.

With this arrangement, however, it is also possible to realize organic light-emitting diodes that emit targeted in a particular direction. This is realized simply by selecting the object beam in this direction and by selecting the direction of the reference beam as many modes as possible in this direction. In addition, for example, by means of a high refractive index substrate, one can suppress the external modes of the OLED so that the contrast of such an image is increased.

Furthermore, it is possible to influence the color coordinates of an organic light-emitting diode with the aid of a suitable choice of the holographic layer. For example, if a blue emitter is used which has too strong green parts and thereby produces a whitish-blue color impression, then the holographic Structure are written with a wavelength that leads to an additional extraction of the deep blue portion. As a result, this component is preferably deflected on the resulting three-dimensional structure and produces a more suitable color coordinate in the forward direction. Similarly, it may be provided to form an organic light-emitting diode which emits different Spek¬ tren in different directions, for example for special lighting purposes. In addition to the previously mentioned holograms, which are partly transparent and are usually used in a transmissive manner, reflective holograms can also be used. These holograms can be produced in a particularly simple manner, for example by means of embossing. For the purposes of the present invention, by way of example, the reflectiveogram can be arranged under the semitransparent substrate contact in the case of an OLED emitting away from the substrate; alternatively or additionally, the hologram may also be mounted on the opposite side of the substrate.

As mentioned, with the described arrangement, the modes coupled into the substrate can be converted into external modes. Rays emitted even further from the normal can only be coupled into the ITO organic modes. If these ITO organics modes are to be decoupled, then other considerations must be made. While the substrate modes are distributed quasi-continuously over all angles, the ITO organics modes are limited to a few angles. For thin OLED structures it is expected that only the fundamental mode is allowed, for thicker structures the second TE and TM modes should each exist (T. Fuhrmann et al., Organic Electronics 4, 219 (2003)).

To decouple such a mode, the Bragg diffraction higher order known from the wave theory can be used (Ebeling, "Integrated Optoelectronics", Springer Verlag Berlin Heidelberg New York, 2nd edition, 1992, p. 67) grating K integrated into the structure for the diffraction in the order j of the following equation:

where θ _{m is} the angle of the m-th mode, k is its wave vector and nf is the refractive index of the film. Especially for j = 2, it is possible to couple into a mode which is emitted perpendicular to the substrate to the outside. However, a periodic structure of this wavevector must be arranged in the organic light-emitting diode in such a way that the substrate mode in this structure still has sufficient intensity. This should be an affix between ITO and Substrate should be the case, since here the evanescent portion of the film wave is still significant (T. Fuhrmann et al., Organic Electronics 4, 219 (2003)).

The use of holographic films in organic light-emitting diodes offers further interesting possibilities for special displays. It is thus possible to create a stereoscopic impression by overlaying two holographic images. If these two images are generated by an OLED pixelated as a display, it can be achieved by exposure of the holographic images with different wavelengths that the light of different segments of the display emitting different colors is diffracted by one of the two respective holograms. This provides a very simple option for a holographic 3D display.

Finally, reference should be made to the possibility of using a transfer hologram, that is, a hologram of a hologram. In this way it can be achieved that the diffractive plane is realized virtually and can be placed in an application of the substrate in the active layer of the OLED.

The features of the invention disclosed in the foregoing description, in the claims and in the drawing may be of importance both individually and in any combination for the realization of the invention in its various embodiments.

Claims

claims 1. Light-emitting component with a layer structure which comprises an arrangement of organic layers (4, 22), in particular organic light-emitting diode, g e k e n n, characterized by a diffractive arrangement with one or more holographic elements Layers (1, 24), which convert light from internal, non-emitting modes into external, emitting modes. 2. Component according to claim 1, characterized in that a holographic layer (1) on one of the arrangement of organic layers (4) facing side of a substrate (2) is arranged. 3. The component according to claim 1 or 2, characterized in that another holographic layer between the substrate (2) and a base contact (3) of the arrangement of organic layers (4) is arranged. 4. The component according to one of the preceding claims, characterized in that a holographic layer (24) on one of the arrangement of organic Schich¬ th (22) facing side of the substrate (20) is arranged. 5. The component according to one of the preceding claims, characterized in that a holographic layer is disposed on a side remote from the substrate side of the arrangement of organic layers. 6. Device according to one of the preceding claims, characterized in that the one or all holographic layers (1; 24) were exposed with a periodic grating formed by means of a superposition of plane waves, wherein a beam direction of a reference wave (R) is selected in that the beam direction of the reference wave extends at an angle that corresponds to an angular range of guided modes in the layer structure, and a beam direction of one or more object waves (Oi, O ₂ , O ₃ ) is selected such that the beam direction of the one or all object waves (Oi, O ₂ , O ₃ ) corresponds to one or more external modes of the layer structure. 7. Component according to claim 6, characterized in that by means of selecting an exposure direction of the one or all object waves (Oi, O ₂ , O ₃ ), the decoupling of the one or all holographic layers (1; 24) is optimized in one direction. 8. Component according to claim 6 or 7, characterized in that Periodizitä- ten of the periodic grating are selected so that a spontaneous emission spectrum of the arrangement of organic layers (4; 22) auskoppel¬ bar only partially in the forward direction, so that color coordinates of a decoupled Mode of color coordinates deviate from spontaneous emission. 9. Component according to one of claims 6 to 8, characterized in that the periodicities of the periodic grating are selected so that different Lichtwellen¬ lengths can be emitted in different directions. 10. The component according to one of claims 6 to 8, characterized in that a plurality of gratings in two or more spatial directions in a plane of the substrate (2, 20) are superimposed. 11. The component according to one of claims 1 to 5, characterized in that in the one or all holographic layers (1; 24) an object is imaged, so that in a light emission of in the arrangement of organic layers (4; 22) er¬ zeugtem light the object is displayed. 12. The component according to one of the preceding claims, characterized by a second order backscattering Bragg grating for coupling out in the Anord¬ tion of organic layers (4; 22) and / or in contact layers ridden (s) Wellen¬ lei termode (s). 13. The component according to claim 12, characterized in that the second order backscattering Bragg grating is formed in the one or all holographic layers (1, 24). 14. The component according to claim 12 or 13, characterized by several rück¬ scattering Bragg gratings second order, so that in the arrangement of organic Layers (4; 22) and / or waveguide mode (s) guided in the contact layers can be scattered with different grating vectors in different directions of the plane of the substrate (2; 20). 15. The component according to claim 14, characterized in that the plurality of second-order backscattering Bragg gratings have different lattice constants, such that the layers (4, 22) and / or in the contact layers arranged in the arrangement of organic layers Waveguide mode (s) with different emission wavelengths can each be scattered in external modes. 16. Component according to one of the preceding claims, characterized in that doped organic transport layers in the arrangement of organic Schich¬ th (4; 22) have a layer thickness, so that in the arrangement of organic Schich¬ th (4; 22) and / or higher waveguide modes can be guided in the contact layers. 17. The component according to claim 16, characterized in that the one or all holographic layers (1; 24) are designed to decouple all modes guided in the layer structure by means of periodic structures. 18. The component according to claims 16 or 17, characterized in that the do¬ tierten organic transport layers are formed by co-evaporation in a vacuum. 19. The component according to claims 16 or 17, characterized in that the doped organic transport layers are formed by application of a solution. A device according to any one of claims 16 to 19, characterized in that the doped organic transport layers comprise a doped hole transport layer of a hole transport material doped with an acceptor dopant. 21. The component according to one of claims 16 to 21, characterized in that the doped organic transport layers a doped electron transport layer of an electron transport material doped with a donor dopant. 22. Component according to one of the preceding claims, characterized in that the one or all holographic layers (1; 24) are formed by two Ob¬ jektwellen are exposed, so that a stereoscopic hologram is formed. 23. Component according to one of the preceding claims, characterized by a subdivision of the arrangement of organic layers (4; 22) at different wavelengths light-emitting pixels (pixels), wherein emission wavelengths exposure wavelengths of holograms in one or all holographi¬ rule Layers (1, 24) correspond. 24. Component according to one of the preceding claims, characterized in that the one or all holographic layers (1, 24) have a transfer hologram auf¬. 25. The component according to one of the preceding claims, characterized in that the one or all holographic layers (1; 24) of silver halide are formed in a carrier layer. 26. The component according to one of claims 1 to 24, characterized in that the one or all holographic layers (1; 24) are formed from dichromate gelatin. 27. The component according to one of claims 1 to 24, characterized in that the one or all holographic layers (1; 24) are formed from a photopolymer. 28. Component according to one of claims 1 to 24, characterized in that the one or all holographic layers (1; 24) formed by means of embossing Layer are.