HK1070182B - Light emitting component with organic layers - Google Patents

Description

Light-radiating member having an organic layer

Technical Field

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

Background

Organic light emitting diodes have been used as candidates for large area display implementations since their lower operating voltage demonstration in 1987 [ c.w.tang et al, appl.phys.lett.51(12), 913(1987) ]. It is composed of a series of thin (typically 1nm to 1 μm) layers of organic material, preferably prepared by evaporation in vacuo or by separation of a solvent, for example by centrifugation. After electrical contact through the metal layer, the organic thin coating can form a wide variety of electronic or optoelectronic components, such as diodes, light emitting diodes, photodiodes and transistors, which compete with inorganic-based coatings for the properties of the components they produce.

In the case of Organic Light Emitting Diodes (OLEDs), light emission and light emitting diode emission are carried out by means of injection of carriers (electrons on one side and holes on the other side) generated by contact between the organic layers due to an externally applied voltage, and subsequent recombination of Exzitonen (electron-hole pairs) formed in an active region and the emission of the Exzitonen.

The advantage of such organic-based components over conventional inorganic-based (semiconductor, e.g. silicon, gallium arsenide) components is that very large surface area display elements (screens ) can be manufactured. The organic feedstock is relatively less expensive (low material and energy consumption) than the inorganic feedstock. Furthermore, the materials can be coated on flexible substrates using lower process temperatures than inorganic materials, so that they can be used for new applications in a range of display and lighting technologies.

Typically, the arrangement of such components consists of one or more of the following layer sequences:

1. the thickness of the carrier, the substrate,

2. a base electrode, which is hole-injecting (positive) and transparent,

3. a hole-injecting layer for injecting holes into the substrate,

4. a hole-transporting layer (HTL),

5. a light-radiating layer (EL),

6. an Electron Transport Layer (ETL),

7. an electron-injecting layer for injecting electrons into the substrate,

8. the cap electrode, typically a metal with lower emission power, is electron-injecting (negative),

9. and the shell is used for eliminating environmental influence.

It is a common situation and often several layers (other than 2, 5 and 8) are also eliminated or multiple properties are combined in one layer.

When the top electrode is formed from an opaque metal layer, light is emitted in the layer sequence described through the transparent bottom electrode and the substrate. Materials commonly used for this purpose are indium-tin-oxide (ITO) and semiconductor oxides of the same family as hole-injecting contacts (a transparent, exfoliate semiconductor). For electron injection, non-noble metals such as aluminum (Al), magnesium (Mg), calcium (Ca) or a mixed layer of Mg and silver (Ag) or a combination of such metals with a thin layer of a salt such as lithium fluoride (LiF) can be used.

In EP 1017118 and EP 0489979, OLEDs have a doping in their hole-transport layer, by means of which a more tunable conductivity is possible and OLEDs of this type can be driven with a lower operating voltage.

For many applications, it is generally desirable that the emitted light not enter the substrate, but pass through the cap electrode. For example, for organic based light emitting diode displays, the display is made of a silicon substrate or a plastic substrate (e.g., US patent No. 5,736,754(s.q.shi et al), filed 1995 11.17.7. US patent No. 5,693,956(s.q.shi et al), filed 1996 6.29.7), or a display coated with a structural filter or absorber layer on an organic light emitting diode (e.g., US patent No. 6,137,221(d.b.roitman et al, filed 1998 7.8.7.; c.hosokawa et al, synthetic. metal., 91, 3-7 (1997); g.rajeswaran et al, SID 00 Digest, 40.1 (2000)).

For the series of organic coatings described above (the top electrode being the cathode), this emission through the top electrode can be excited by means of a very thin, commonly used metal electrode. Since a sufficiently high transmissive cap does not yet achieve high lateral conductivity, it must be coated on a transparent contact material, such as ITO or indium oxide doped with zinc (e.g., US patent No. 5,703,436(s.r. forrest et al), filed 3/6/1996; US patent No. 5,757,026(s.r. forrest et al), filed 4/15/1996; US patent No. 5,969,474(m.arai), filed 10/24/1997). Known implementations of this structure are also visible (e.g. g.parthasarathy et al, appl.phys.lett.72, 2138 (1997); g.parthasarathy et al, adv.matter.11907(1997)) for organic interlayers for improved electron injection, which can be partially doped with metal atoms such as lithium (g.parthasarathy et al, appl.phys.lett.76, 2128 (2000)). In these articles a transparent contact layer (usually ITO) is applied. ITO is not suitable for electron injection (cathode), which increases the operating voltage of the LED.

A further possibility for a transparent cathode is the inversion of the layer sequence, i.e. the transparent contact for hole injection (anode) as the top-cap electrode. One such solution is proposed in US 6046443.

The implementation of this anode inversion structure on LEDs greatly increases the difficulty in practice. Metal electrodes with acceptable transparency must be thin enough to generally not form a complete layer, especially to achieve sufficient lateral conductivity to allow a uniform current flow across the structure. If the layer sequence is made using a hole injection layer, it is required that the metal used as hole-injecting ITO (or one other metal) should be applied to the organic layer sequence (e.g., US patent No. 5,981,306(P. Burrows et al), filed 9/12 1997). Most of the processing techniques and organic layers required are difficult to handle and may be damaging. A protective layer can be formed as an improvement, but this protective layer increases the operating voltage of the component, since it increases the total thickness of the organic layer (U.S. Pat. No. 6,046,543(V.Bulovic et al), filed 12/23/1996; Z.Shen.science 276, 2009 (1997)).

Another decisive disadvantage of great difficulty is that current methods for producing light-emitting diodes require that the hole-injection layer should have an emission power as high as possible. This is necessary because the organic layer is usually undoped and therefore has an efficient implantation when there is as low an energy barrier as possible. A special preparation of the surface of the hole-injecting material is generally necessary for lower operating voltages and higher performance to be achieved (e.g. C.C.Wu et al, appl.Phys.Lett.70, 1348 (1997); G.Gu et al, appl.Phys.Lett.73, 2399 (1998)). The emission power of the ITO can be varied, for example, by ozonation and/or oxygen plasma ashing at about 4.2eV to 4.9 eV. Which has a large effect on the efficiency of hole injection and the operating voltage of a corresponding OLED. This surface preparation method is not used when applying the hole injection material to the organic layer. It will do so, typically at higher operating voltages for inverted organic light emitting diodes, to produce lower operating efficiencies (light emission versus electrical power) (e.g., v. bulovic et al, appl. phys. lett.70, 2954 (1997)).

For the reasons mentioned, no inverted structures have existed to date which have the same good optoelectronic characteristic data as a corresponding non-inverted structure. That is, such inverted OLEDs operate at higher voltages and are less efficient than OLEDs having conventional layer structures.

This inverted arrangement allows for simple integration of the OLED with standard drive electronics, such as CMOS technology with amorphous Si-NFETs. In addition, the arrangement of the cathode in the organic layer has the advantage of better protecting the influence of the environmental influence of oxygen and water. As is known, it has a negative effect on long-term stability, such as a peel-off phenomenon of the cap electrode. Since the conventional hole-injecting transparent anode material ITO already contains oxygen, its efficacy is reduced for an inverted structure. Another advantage of the inverted structure is that a very flat semiconductor substrate can be used as the substrate, while a certain residual roughness of ITO is unavoidable for conventional structures with ITO as the base contact. Which can cause problems with long-term stability due to the pin-hole effect (partial through-contact).

It has long been known for light-emitting diodes made of inorganic semiconductors that thin space charge regions can be obtained by means of highly doped boundary layers, the energy barrier present by tunneling allowing a more efficient injection of charge carriers. The doping of the organic material is described in US patent No. 5,093,698 filed on 12.2.1991. But it can cause problems in practical applications due to energy adaptation of the different layers and reduced efficacy of LEDs with doped layers. It can be improved by selecting a suitable separating layer (patent application DE 10058578.7, filed on 25/11/2000).

Disclosure of Invention

The object of the invention is to provide an inverted light-emitting diode which emits light via a cover contact and which can be operated with a reduced operating voltage and has an increased light emission efficiency. While protecting all organic layers from damage due to the mounting of the transparent top-cover electrode (all layers are considered transparent in the sense of the present invention with a transmission of > 50% in the light emission range of the OLED).

According to the invention, this object is achieved in that: the hole-transport layer is provided with a thickness which is thicker than the undoped layer in the light-radiating member in the prior art and is p-doped with an organic material. Furthermore, the transfer layer can have a larger layer thickness (typically 20-40nm) than the undoped layer without a significant increase in the operating voltage.

By adding hole injection material to the p-doped hole transport layer at the anode, a thin space charge region is obtained through which carrier injection efficiency is improved. Due to the tunnel injection, even a high energy barrier cannot block the injection through the very thin space charge region. It is advantageous that the carrier transport layer can be doped by mixing organic or inorganic substances (dopants). By virtue of the increased conductivity of the doped organic layer, it can be made sufficiently thick to achieve protection against all damage due to the transparent top electrode mounting.

It is known from x.zhou et al, appl.phys.lett.78, 410(2001) that organic light-emitting diodes with a doped transfer layer have efficient light radiation only when the doped transfer layer is combined in a suitable manner with a separating layer. In an advantageous embodiment, an inverted light-emitting diode is thus provided which also has a carrier transport layer. The spacer layer is generally located between the carrier transport layer and an optical radiation layer of a component, wherein electrical energy of the current generated by the injected carriers of the component is converted into light. The material of the spacer layer is selected according to the invention such that, on application of a voltage (in the direction of the operating voltage), due to the energy levels thereof, the majority carriers (HTL surface: holes, ETL surface: electrons) will not be strongly hindered at the boundary layer doped transport layer/spacer layer (lower energy barrier), but the minority carriers will be effectively blocked at the boundary layer light radiation layer/spacer layer (high energy barrier).

A preferred embodiment of an inverted OLED structure according to the invention comprises the following layers:

1. the thickness of the carrier, the substrate,

2. the electrode, usually a metal with low emission power, is electron-injected (cathode ═ negative)

N-doped, electron injecting and transporting layer,

4. a thin electron-surface separator made of a material with a band layer adapted to the band layer of the surrounding layers,

5. the photoradiation layer (possibly doped with a radiation pigment),

6. a barrier layer (typically thinner than layer 7) at the face of the cavity made of a material with a tape layer adapted to the tape layer of the surrounding layers,

a p-doped hole injection and transport layer,

8. an electrode, hole-injecting (anode ═ positive), preferably transparent,

9. top cover for removing environmental influence

The meaning of the invention is that, when only a spacer layer is used, only one side (hole or electron conducting side) can be doped, since the layer of the injection and transport layer and the light emission layer are already adapted to one another on one side. In addition, the carrier injection and carrier transport functions on layers 3 and 7 can be distributed to more layers, at least one of which is doped. If the doped layers are not adjacent to their respective electrodes, all layers between the doped layers and the respective electrodes should be so thin that the carriers can efficiently tunnel through. These layers can be thicker when they have a high conductivity (the orbital resistance of these layers must be lower than the adjacent doped layers). The intermediate layer according to the invention is thus meant to be part of an electrode. The typical molar doping concentration is in the range of 1: 10 to 1: 10000. If the dopant is very small compared to the matrix molecules, it is possible to use for the first time more dopant as matrix molecules in the layer (to 5: 1). The dopant may be organic or inorganic.

Drawings

The present invention will be described in detail by examples later. In the attached figures:

FIG. 1 is a schematic energy diagram of an inverted OLED in a conventional general embodiment (undoped, numbered to indicate the layer structure of the inverted OLED described above),

fig. 2 is an energy diagram of an inverted doped OLED with a spacer layer.

Detailed Description

In the embodiment shown in fig. 1, there is no space charge region on the hole injection contact. This embodiment requires a lower energy barrier for hole injection. This may not be possible with available materials. The hole injection from the anode is therefore less efficient than conventional structures, which can be modified in view of the emission power of their anode. The OLED has an elevated operating voltage.

According to the invention, the disadvantages of the prior art structures can be eliminated by an inverted OLED, the doping injection and transfer layer of which is combined with an isolation layer. A corresponding arrangement is shown in figure 2. The hole-injecting and p-doped hole-transport layer 7 is doped here in such a way that a space-charge zone is created on the boundary layer of the contact (anode 8). Provided that the doping is sufficiently high so that the space charge region can readily tunnel through. Such doping is possible and has been described in the literature on non-inverting light emitting diodes (X.Q.Zhou et al, appl.Phys.Lett.78, 410 (2001); J.Blochwitz et al, Organic Elecronics (2001) to be published).

This arrangement has the following advantages:

● significant injection of carriers from the anode in a p-doped carrier transport layer

● independence of detailed preparation of the anode 8

● it is possible to select for the anode 8 a material with a relatively high energy barrier for hole injection

As a preferred embodiment, a solvent should be specified here, with which the combination of the p-doped injection and transport layer and the isolating layer is present only on the hole side. The OLED has the following layer structure:

a substrate 1, for example glass,

-a cathode 2: 1nm LiF in combination with aluminum (LiF improves the implantation at the contacts),

electroluminescent and (in this case) conventional electron-conducting light-radiating layer 5: 65nmAlq₃Or using 30nm Alq₃And 15nm Alq₃Doped with about 1% of a radiation pigment such as quinacridone,

-a spacer layer 6 of the cavity face: 5nm of Triphenyldiamine (TPD),

p-doped hole transport layer 7: 100nm Starburst TDATA 50: 1 doped F₄-TCNQ，

Transparent electrode (anode 8): indium-tin-oxide (ITO).

The p-doped hole transport layer 7 can be prepared by an evaporation process in a vacuum mixed vapor phase. In principle, such layers can also be produced by other methods, for example by successive vaporization of substances, the interdiffusion of which is ultimately controlled as far as possible by temperature; or by separation of other mixed materials (e.g., centrifugation) under vacuum or non-vacuum. The barrier layer 6 may likewise be vaporized under vacuum, but may also be prepared by other methods of preparation, for example by centrifugation under vacuum or non-vacuum.

In another preferred embodiment, an electron transport layer 3 may be added between the cathode 2 and the light-radiating layer 5. The dopant may be vaporized by mixing two organic materials (molecular types) in an organic layer or a metal source, as described aboveSeed coating (as described in patent application DE 10058578.7 filed 11/25/2000). As an example according to the invention, the dopant mentioned here is Alq with Li₃(U.S. Pat. No. 6,013,384(J.Kido et al; J.Kido et al, appl.Phys. Lett.73, 2866, (1998)) filed on 22.1/1998). By the use of one such layer in an inverted OLED structure according to the invention, the positive effect of the lithium fluoride layer between the electron transport layer and the cathode metal layer can be achieved in a non-inverted OLED structure (where "hotter" metal atoms can be vaporized onto LiF to achieve the desired electron injection effect, as can be seen, for example, in m.g. mason, j.appl.phys.89, 2756(2001)), as can be achieved for an inverted structure. The LiF layer reduces the emission power of the cathode material (here aluminum) and the Li dopant (typical and identified in accordance with the invention of Li atoms with Alq) of the electron transport layer₃The concentration of molecules is between 5: 1 and 1: 10) so that band bending is possible at the cathode boundary surface in favor of efficient electron injection, which is similar in p-doped hole transport layers.

By using the doped layer according to the invention, it is possible to achieve a low operating voltage and a high efficiency in an inverted structure with light radiation via the top-side electrode, as in the case of the prior art structure with radiation via the substrate. As stated, it depends on efficient hole injection, which can be relatively independent of the exact emission power of the transparent anode material due to doping. Thereby, an efficient display can be manufactured based on a conventional semiconductor substrate.

Although only a few preferred applications have been described, it will be apparent to those skilled in the art that many modifications and variations of the present invention are possible and are within the meaning of the present invention. For example, other transparent contacts besides ITO can be used as anode material (e.g., H.Kim et al, appl.Phys.Lett.76, 259 (2000); H.Kim et al, appl.Phys.Lett.78, 1050 (2001)).

Further according to the invention, the transparent anode may also consist of a sufficiently thin layer of a non-transparent material, such as silver or gold, and a thick layer of a transparent conductive material.

Correlation chart

1. -a substrate

2. -cathode

3. -an electron transport layer

4. -an insulating layer

5. -optical radiation layer

6. -an insulating layer

7. -p-doped hole transport layer

8. -an anode

9. Protective layer

Claims (18)

1. Light-radiating member with organic layers, consisting of a substrate (1), at least one light-radiating layer (5) and at least one hole-transporting layer (7), the light-radiating member having a cathode (2) and an anode cap electrode (8), the cathode (2) being applied to the substrate (1) and the light radiation passing through the anode cap electrode (8), characterized in that at least one hole-transporting layer (7) is mounted directly under the anode cap electrode (8), the thickness of the hole-transporting layer (7) being in the range of 100nm to 50 μm in order to protect the layers thereunder from damage when the anode cap electrode is mounted, and in that this hole-transporting layer (7) is p-doped with an organic material.

2. Light-radiating member according to claim 1, wherein said light-radiating member is an organic light-emitting diode.

3. A light-radiating member according to claim 1 in which the hole-transporting layer has a layer thickness of up to 100 nm.

4. A light-radiating member according to one of claims 1 to 3, in which, between the hole-transporting layer (7) and the light-radiating layer (5), a spacer layer is provided which builds up a low energy barrier for holes and a high energy barrier for electrons.

5. Light-radiating member according to claim 1, in which an electron-transporting layer (3) is present between the cathode (2) and the light-radiating layer (5).

6. Optical radiation member according to claim 5, wherein the electron transport layer (3) is doped.

7. Light-radiating member according to claim 5, in which a spacer layer (4) is present between the electron-transporting layer (3) and the light-radiating layer (5) which is created for the electron low-energy barrier and for the hole high-energy barrier, and a spacer layer (6) is present between the light-radiating layer (5) and the p-doped hole-transporting layer (7) which is created for the hole low-energy barrier and for the electron high-energy barrier.

8. Light-radiating member according to claim 5, in which an improved contact layer is present between the electron-transport layer (3) and the cathode (2) and/or between the anode cap electrode (8) and the hole-transport layer (7).

9. Light-radiating member according to claim 1, in which an electron-injecting cathode (2) is present on a transparent substrate (1).

10. Optical radiating member according to claim 1, in which the optical radiating layer (5) is a hybrid layer made of several materials.

11. Light-radiating member according to claim 1, in which the p-doped hole-transporting layer (7) consists of an organic host and an acceptor-selective dopant.

12. Optical radiation member according to claim 1, wherein the anode dome electrode (8) is transparent or translucent and is provided with a protective layer (9).

13. Light radiating member according to claim 1, wherein the anode dome electrode (8) is metallic and made so thin that it is translucent.

14. Light-radiating member according to claim 13, in which a further transparent contact layer is applied over the translucent metallic anode dome electrode (8) to improve lateral conductivity.

15. Optical radiation member according to claim 5, wherein the electron transport layer (3) is n-doped by mixing an organic host and a dopant of the donor type.

16. A light-radiating member according to claim 12 in which the p-doped hole-transport layer (7) and the transparent anode cap electrode (8) are arranged in a plurality in sequence.

17. Light-radiating member according to claim 5, in which the molar concentration of the mixture in the hole-transporting layer (7) and/or in the electron-transporting layer (3) is in the range of 1: 100000 to 5: 1 in terms of the ratio of dopant molecules to host molecules.

18. A light-radiating member according to claim 7, in which the layer thicknesses of the hole-transporting layer (7), the electron-transporting layer (3), the light-radiating layer (5) and the spacer layer (4, 6) are in the range from 0.1nm to 50 μm.