WO2004084259A2

WO2004084259A2 - Organic light-emitting diode with improved light disengaging properties

Info

Publication number: WO2004084259A2
Application number: PCT/EP2004/002128
Authority: WO
Inventors: Arvid Hunze; Lothar Rau; Wolfgang Rogler; Marcus Scheffel; Sven VÖGELE
Original assignee: Osram Opto Semiconductors
Priority date: 2003-03-19
Filing date: 2004-03-03
Publication date: 2004-09-30
Also published as: DE10312219A1; WO2004084259A3

Abstract

The invention relates to an organic light-emitting diode (OLED) with an improved extraction efficiency, i.e. the probability with which a generated photon (7) can be disengaged from the diode and thus be identified. This is achieved by the incorporation of a light disengaging structural layer located between the substrate and the anode.

Description

description

Organic light emitting diode with improved light decoupling

The invention relates to an organic light-emitting diode (OLED) in which the extraction efficiency, that is to say the probability with which a photon generated can be coupled out of the diode and thus contribute to the brightness, is improved.

OLEDs with improved efficiency are known, for example, from the publication by G. Gu, et. al .: High-external - quantum-efficiency organic light-emitting devices in "Optics Letters", vol. 22, no. 6, p. 396, from 1997. The method described therein for increasing efficiency is based on so-called mesa structures. These include structures made from truncated pyramids or truncated cones, which serve as reflectors for laterally emitted emissions. The illuminated area is located on the flat upper side of the stump. Disadvantages of this method are: The structures used are introduced directly into the substrate. When using glass substrates, this can only be achieved with great effort or the use of highly toxic substances (hydrofluoric acid). The mesa structures enclose one pixel each. In order to create structures with a suitable aspect ratio, the structure depth must be at least a few 10 μm. Such structures cannot be produced photolithographically or only with great effort. The active illuminated area covers only a small part of the available area.

In addition, US 5,834,893 discloses mesa structures on inverted OLEDs. The structures described here for improving the coupling-out efficiency are based on an inverted OLED structure, ie the transparent anode is deposited as the last layer. The OLEDs are in Substrate depressions that serve as reflectors, analogous to the structure described above.

This technique has the following disadvantages:

- For passive matrix displays, the inverse structure of an OLED currently represents a great challenge. There is still no technically usable process available, as the common anode material ITO usually. in the case of deposition or structuring leads to damage to the underlying organic layers.

Corrugated (wave-shaped) organic layers (J.M. Lupton, et. Al.: Bragg scattering from periodically microstructured light emitting diodes, Appl. Phys. Lett. 77

(21), p. 3340, 2000) are described as a way to improve extraction efficiency.

In this experiment, a polymer LED was applied to a one-dimensional periodic structure with a period of 388 nm and depths of 10 - 100 nm. The structure acts as a Bragg reflector and in turn leads to a scattering of optical modes in the emitter material. Disadvantages with this procedure are: - The periodicity of the structure leads to a strong angular dispersion. For display elements, however, a color impression that is independent of the viewing angle is required.

- A thin gold layer (15 nm) was used as the anode, which already shows strong absorption despite the small layer thickness. A transfer of the corrugation to the ITO, which is otherwise used as a standard anode, seems difficult to achieve due to the larger ITO layer thicknesses and high process temperatures.

The object of the invention is to provide an OLED with a conventional, that is to say non-inverted structure, which shows a significantly increased extraction efficiency. The subject matter of the invention is an OLED, a substrate, on which a first positive and transparent electrode (anode), subsequently comprising at least one emitting layer of predominantly organic material and in turn subsequently a negative reflecting electrode (cathode), between the lower first electrode and the substrate and / or at least one light decoupling structure layer is integrated into the substrate.

The extraction efficiency is increased by introducing at least one light coupling structure layer, between the substrate and the first electrode and / or integrated into the substrate. OLEDs can also be combined with color filter structures to generate certain colors or full colors. These are generally introduced between the substrate and the first positive electrode (ITO anode) and, for example, covered with an organic planarization material before the ITO electrode is applied. The light decoupling structure layer can be combined particularly well with such a color filter structure and / or integrated directly into a color filter.

The structure resolution of the light decoupling structure layer, that is to say the distance between two structure features and / or the size of a structure feature, is less than / equal to one pixel, preferably less than one pixel and particularly preferably in the range from 1-50 μm.

The individual structures of the light decoupling structure layer can be worked into the substrate or structured on it (eg photolithographically). For this purpose, for example, one or more structured layer (s) of a photoresist is applied to a substrate, on which the transparent electrode, the organic layer (s) and the second electrode are then applied. The entire structure is then repeated if necessary, i.e. if there are no planning layer is introduced, the structuring of the photoresist layer.

According to an advantageous embodiment, the structures of the light decoupling structure layer are not periodic.

According to one embodiment, the outcoupling efficiency can be optimized by a further layer between the substrate and the structure. The refractive index of this layer should be smaller than that of the two adjacent layers. This leads to a greater probability of total reflection at the interface between the light decoupling structure layer and the low index layer. Light reflected in this way can in turn be reflected on the oblique cathode surface and thus coupled out.

The light decoupling structure layer can also be planarized. Materials suitable for planarization are known from microelectronics and can either be predominantly organic or inorganic material. They can be applied by means of processes known from microelectronics, such as spin coating, knife coating and / or vapor deposition. Large-area structured application by means of a suitable printing process is also possible. In order to achieve an optical contrast, the refractive index of the planarization differs from that of the light decoupling structure layer. Alternatively, an additional layer with a higher or lower refractive index or a semi-transparent metal layer can be inserted between the two. Planarization with a high refractive index is advantageous in order to achieve the most efficient possible coupling of the light from the OLED material into the planarization layer.

The decoupling structures (or simply "structures") of the

Light decoupling structure layer can be connected to one another or executed individually. The structures can be be periodic or non-periodic. The structures can be, for example, conical, drop-shaped, pyramidal or polygonal.

Glass, plastic, plastic / ceramic and plastic / glass composites come into consideration as substrate materials. Particularly suitable substrates are plastic films, in particular flexible films, since the structures according to the invention can be generated particularly easily by simple molding processes such as embossing in film production.

Five figures are described below for the exemplary embodiment of the invention.

FIG. 1 shows an embodiment in which the substrate 1, for example a glass substrate, has a light coupling structure layer 2, which was produced by means of a suitable and known, for example photolithographic process and is provided with periodic or non-periodic (statistically distributed) structures. On top of that, an OLED is built up, which reproduces the structures in all layers. The light coupling structure layer is followed by the first electrode, the transparent anode 3, for example made of ITO (indium tin oxide). The anode is followed at least by an organic active layer 4 on which the cathode 5, for example the last deposited and / or metallic one, then comes to rest, which in turn has a structured, non-planar shape.

The layer structure is for example the following: 80nm PEDOT

(Hole transport material), βOnm polyfluorene derivative (emitter material), 5nm calcium, 200nm aluminum. As can be seen in FIG. 6, there is a significantly increased efficiency (+ 41% at 10 mA / cm ² ) (line with the filled boxes "structured device") in comparison to the reference diodes processed on the same substrate (shown by the line with the empty box "reference"). Furthermore, consider that the emission occurs mainly at the edges of the structures, see FIG. 7, which shows a microscopic picture of structured OLED during operation. The structured areas can be recognized by the brightness (generally increased luminance) and in particular the even brighter edges are striking.

The arrows 7 in FIG. 1 show an exemplary beam path of an emitted photon.

The non-planarity of the upper electrode or cathode 5 can serve to reflect photons emitted laterally from the OLED. These are deflected in the forward direction and at least partially decoupled. Overall, reflector-like structures of the light decoupling structure layer 2 lead to the decoupling of otherwise wave-guided photons.

Coupling structures that consist of a material with a high refractive index are advantageous. It should be larger than that of the functional organic materials arranged above it, that is to say preferably greater than 1.6 and particularly preferably greater than 1.8 and particularly preferably greater than 2.0, in order to make the coupling of light from the active organic layers as efficient as possible.

Organic materials with a high refractive index are usually characterized by high polarizability. Materials with a refractive index greater than 1.6 which are advantageously used here as a light decoupling structure layer are therefore particularly highly aromatic polymer systems such as, for example, aromatic polyepoxides, polyimides and / or aromatic heterocyclic polymers or any mixtures thereof. Substitution with heavy atoms, such as halogens, generally leads to high refractive indices. FIG. 2 shows a structure similar to that of FIG. 1, with the difference that a low index layer 6 is arranged between the substrate 1 and the light decoupling structure layer 2, by means of which the decoupling efficiency between substrate and structure is further optimized. The refractive index of this layer 6 is preferably smaller than that of the two adjacent layers substrate 1 on the one hand and light coupling structure layer 2 on the other. This leads to a greater probability of total reflection at the interface between the decoupling structure 2 and the low-index layer 6. In this way, light reflected in turn can be reflected on the oblique cathode surface 5 and thus coupled out.

FIG. 3 shows an exemplary embodiment in which the light decoupling structure layer 2 is embedded in a further layer 8. This planarization can take place with various materials, which are preferably known from microelectronics and are organic and / or inorganic in origin. Examples of suitable planarization materials are poly-epoxides, novolac resins, phenolic resins, polyimides or polyacrylates. The planarization is preferably carried out with a high index layer, that is to say particularly advantageously with a material of the substance classes described above, the refractive index of which is greater than or equal to 1.6.

FIG. 3 shows a light decoupling structure layer 2 which essentially comprises individual truncated cones 2. Such a light decoupling structure layer 2 can be produced by means of a suitable photolithographic process, so that only free-standing islands of the originally applied layer 2 are retained after the development.

FIG. 4 essentially shows the structure known from FIG. 3 with the low index layer 6 known from FIG. 2. The coupling-out efficiency is optimized by a low-index layer 6 between substrate and structure and / or high-index layer or planarization layer 8 analogously to FIG. 2. Finally, FIG. 5 shows how the processing sequence of low-index layer 6 and light decoupling structure layer 2 can be interchanged. The low index layer 6 lies between the substrate 1 and the high index layer 8 but above the light coupling structure layer 2.

The exemplary embodiments can, for example, also be combined with one another in such a way that the decoupling structures are only partially planarized.

Finally, FIGS. 6 and 7 show how the efficiency of these OLEDs is increased and that the emission mainly occurs at the edges of the structures.

Quantum mechanical calculations show that from a conventional OLED component approx. 50% to 80% of the photons are lost through emission suppression and total reflection without additional measures for light extraction (MH Lu and JC Sturm, Optimization of external coupling and light emission in organic light-emitting devides: modeling and experiment, J. Appl Phys., 91 (2), p. 595, 2002). This illustrates the particular advantage of the invention, which results from the improved coupling-out of light from the component, that a large part of the photons otherwise lost by waveguiding can be used to increase the efficiency of the OLED. The decoupling can be increased by at least a factor of 1.4 with the invention. The resulting increased component efficiency means lower operating voltage and thus a longer service life.

The invention relates to an organic light-emitting diode (OLED) in which the extraction efficiency, that is to say the probability with which a photon generated can be coupled out of the diode and thus detected, is improved. this will achieved by introducing a light extraction structure layer between the substrate and the anode.

Claims

claims

1. Organic light-emitting diode, a substrate, thereon a first positive and transparent electrode (anode), then at least one active layer of predominantly organic material and then again a negative reflecting electrode (cathode), wherein between the lower first electrode and the substrate and / or at least one light coupling structure layer is arranged integrated in the substrate.

2. Organic light emitting diode according to claim 1, wherein the light coupling structure layer is combined with a color filter and / or is integrated in a color filter.

3. Organic light-emitting diode in which the light decoupling structure layer is at least partially planarized.

4. Organic light-emitting diode according to one of the preceding claims, in which between the substrate and the first

Electrode is at least partially arranged a low index layer.

5. Organic light-emitting diode according to claim 3, in which the low-index layer is arranged at least in part between the light-decoupling structure layer and the first electrode.

6. Organic light-emitting diode according to claim 3, in which the low-index layer is arranged at least in part between the substrate and the light decoupling structure layer.

7. Organic light emitting diode according to one of the preceding claims, which still has a high index layer between the

Substrate and the first electrode u summarizes. Organic light emitting diode according to one of the preceding claims, in which the light decoupling structure layer has a refractive index of greater than 1.6.