HK1072661B - Electroluminescent device - Google Patents

Description

Electroluminescent device

Technical Field

The invention relates to a cathode for optical devices, in particular organic electroluminescent and optoelectronic devices.

Background

One type of opto-electronic device is a device that utilizes organic materials to emit light or as the active element of a photovoltaic cell or photodetector ("photovoltaic" device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or receiving negative charge carriers (electrons) into the organic layer and an anode for injecting or receiving positive charge carriers (holes) into the organic layer.

In organic electroluminescent devices, electrons and holes are injected into a semiconducting organic layer where they recombine to produce excitons which undergo radiative decay. In WO90/13148, the organic light-emitting material is a polymer, namely poly (p-phenylenevinylene) ("PPV"). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In US4539507, the organic light emitting material is a so-called small molecule material, such as (8-hydroxyquinoline) aluminium ("Alq 3"). In practical devices, one electrode is transparent to allow photons to escape from the device.

The structure of an organic opto-electronic device is the same as that of an organic light-emitting device, but the charges are separated, rather than complexed, as described, for example, in WO 96/16449.

Fig. 1 shows a cross-sectional structure of a typical organic light emitting device ("OLED"). The OLED is typically fabricated onto a glass or plastic substrate 1 coated with a transparent first electrode 2 (e.g. indium tin oxide "ITO"). A thin film layer of at least one electroluminescent organic material 3 covers the first electrode. Finally, the cathode 4 covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy comprising a single layer such as aluminum or multiple layers such as calcium and aluminum. For example, to enhance charge injection from the electrodes into the electroluminescent material, additional layers may be added to the device. For example, a hole injection layer such as poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be disposed between the anode 2 and the electroluminescent material 3. When a voltage is applied between these electrodes from a power supply, one electrode functions as a cathode and the other electrode functions as an anode.

The electrical properties have a large impact on the efficiency and lifetime of the device. Many materials have been proposed for the cathode electrode, with materials with low work functions generally being preferred. It has been shown that sandwiching a layer of high dipole dielectricity between the cathode and the electroluminescent layer can improve device efficiency by facilitating electron injection. For example, EP0822603 discloses a thin fluoride layer sandwiched between an EL layer and a thick conductive layer. The fluoride may be selected from the group of alkali metal fluorides or alkaline earth metal fluorides. The conductive layer may be selected from the group of elemental metals, metal alloys, and conductive materials. For the fluoride layer, a thickness in the range of 0.3 to 5.0nm has been taught. Similarly, Applied physics Letters 79(5), 2001, 563-565 discloses metal fluoride/Al cathodes. In addition, WO00/48257 describes an arrangement comprising a metal fluoride layer, a layer of calcium and a layer of aluminium.

The focus in the OLED field has been to develop full color displays using organic red, green and blue (RGB) electroluminescent materials. For this reason, a great deal of work has been reported in the development of red, green and blue emitters for small molecules and polymers. These emitters include substituted aromatic moieties. The emission color can be adjusted by appropriately selecting the aromatic moiety and/or its substituent. Sulfur-containing electroluminescent materials, such as polymers containing thiophene or benzothiadiazole repeat units, have been reported. For example, a red-green emitter comprising these cells is disclosed in WO 00/46321.

Full color OLEDs have been disclosed, for example, in Synthetic Metals 111-. The difficulty with these devices is that the overall performance (i.e., efficiency, lifetime, etc.) of the device is poor due to the incompatibility of the cathode with the at least one red, green, and blue emitter. For example, the cathode disclosed in Synthetic Metals 111-112(2000), 125-128 is LiF/Ca/Al, which is very effective for blue emitting materials, but exhibits very poor performance for green emitters, especially for red emitters. A particular problem of degradation in green and red efficiency is observed when the green and red pixels are not driven.

The present inventors have recognized that detrimental interactions exist between the cathode and the sulfur-containing material in the aforementioned devices. In addition to such interactions which are detrimental to OLEDs, similar detrimental interactions can also affect the semiconducting properties of organic materials in organic opto-electronic devices. It is therefore an object of the present invention to provide a cathode having improved compatibility with sulfur-containing organic semiconducting materials. It is a further object of the invention to provide a cathode having improved compatibility with all red, green and blue electroluminescent organic semiconducting materials.

Summary of The Invention

In a first aspect, the present invention provides an optical device comprising:

-an anode

-cathode

-an organic semiconducting material located between the anode and the cathode, and

an electron transport layer between the cathode and the organic semiconducting material

Wherein the organic semi-conductive material contains sulfur and the electron transport layer contains barium.

In a preferred embodiment of the first aspect of the invention, the optical device is an electroluminescent device, more preferably a display device. In a second preferred embodiment of the first aspect of the invention, the optical device is an optoelectronic device.

In a second aspect, the present invention provides an optical device comprising: an anode, a cathode, red, green and blue electroluminescent organic semi-conductive materials located between the anode and the cathode, and an electron transport layer located between the electroluminescent organic semi-conductive materials and the cathode, wherein the electron transport layer comprises barium.

In a preferred aspect, the electron transporting layer contains elemental barium as a main component. In this aspect, the electron transport layer can be considered to be another component of the cathode because it is a conductive material. In another preferred aspect, the electron transporting layer contains a dielectric barium compound as a main component. Preferred dielectric barium compounds include barium halides and barium oxide, most preferably barium fluoride. Preferably, the thickness of the barium-containing layer is in the range of 1-6 nm.

Without wishing to be bound by any theory, it is believed that the properties of the overlying cathode layer can affect the injection of charge into the emissive layer by the cathode when a sufficiently thin dielectric layer is used, and there is an opportunity to select materials for the cathode such that the performance of the device is enhanced by a combination of material properties. Possible mechanisms for this enhancement are believed to include: (a) avoiding detrimental interaction between the organic layer and the cathode by the dielectric layer while retaining at least some of the injection characteristics of the cathode material; and (b) forming an intermediate state through the dielectric layer (e.g., using one or more organic layers) that facilitates electron injection from the cathode. The dielectric layer should be thin enough to produce the effect, but thick enough to be deposited reproducibly and uniformly (without excessive defects). In general, possible mechanisms that lead to improved performance include surface induced dipoles, altered work function, chemically stable compounds forming charge transfer, and decomposition of the compound layer of the cathode to form a doped implant layer.

The or each organic semi-conductive material may be a small molecule, but is preferably a polymer. Examples of such materials include homopolymers and copolymers of optionally substituted poly (phenylene vinylene) and optionally substituted polyfluorenes. Copolymers are particularly preferred. The organic semiconductive material includes: for example triarylamines or heterocycles, especially sulfur-containing heterocycles. In one embodiment, the organic semiconducting material is a polymer comprising repeating units of heterocycles selected from optionally substituted 4, 7-linked benzothiadiazoles, 2, 5-linked thiophenes, and combinations thereof.

The cathode is composed of a layer of an electrically conductive material, in particular a metal such as aluminum or an alloy. Alternatively, the cathode may comprise more than one layer of conductive material, and in particular the cathode may comprise a bilayer of two metals having different work functions. In a first embodiment, the cathode comprises a layer of aluminum. Preferably, the thickness of the aluminum layer is in the range of 200 to 700 nm. In a second embodiment, the cathode comprises a bilayer of calcium and aluminum, wherein the calcium layer is in contact with the layer of dielectric material. Preferably, the thickness of the calcium layer is 5-

Preferably, a hole injection layer comprising PEDOT: PSS is disposed between the anode and the semiconducting material.

"Red electroluminescent organic semiconducting material" means an organic material capable of emitting by electroluminescence radiation having emission peaks around the wavelength of 600-750nm, preferably 600-700nm, more preferably 610-650nm, most preferably 650-660 nm.

By "green electroluminescent organic semiconducting material" is meant an organic material capable of emitting radiation by electroluminescence at a wavelength of 510-580nm, preferably 510-570 nm.

By "blue electroluminescent organic semiconducting material" is meant an organic material capable of emitting radiation by electroluminescence having a wavelength in the range 400-500nm, preferably 430-500 nm.

The "major constituent" of the electron transport layer means that it is from 50 to 100%, preferably more than 90%, most preferably substantially all of the electron transport layer.

Brief description of the drawings

The invention is described in further detail, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art electroluminescent device;

FIG. 2 shows electroluminescent devices according to these examples of the invention;

FIG. 3 shows a current density versus bias voltage curve for a red electroluminescent device;

FIG. 4 shows a current density versus bias voltage curve for a green electroluminescent device;

FIG. 5 shows a plot of luminance versus time for a red electroluminescent device;

FIG. 6 shows a graph of luminance versus time for a green electroluminescent device;

FIG. 7 shows a graph of luminance versus time for a blue electroluminescent device

Examples of the invention

A) Monochrome electroluminescent device

The following example describes the preparation of an electroluminescent device in which the electroluminescent material contains sulfur and the cathode includes a barium halide layer for comparison with a corresponding device without a barium-containing layer.

Fig. 2 shows an organic light-emitting device according to these examples. The device comprises a transparent glass or plastic substrate 5. The substrate has a transparent anode electrode 6 made of ITO. Above the anode is a hole injection layer 7 of poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS). Above the hole injection layer is an organic light-emitting material 8, above which is a dielectric electron transport layer 9 of barium fluoride, which is covered by a cathode 10 comprising a calcium layer 11 and an aluminium layer 12.

To form the device of figure 2, a transparent ITO layer is deposited on the glass plate 5 to form the anode 6. The glass sheet may be soda-lime and borosilicate glass with a thickness of, for example, 1 mm. The thickness of the ITO coating is suitably around 100 to 150 a. The substrate 5 and the anode electrode 6 may be a glass plate coated with IT0, which is commercially available and prepared in advance.

A hole injection layer 7 was deposited on IT0, the emission layer being formed from a solution containing poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT: PSS) and having a PEDOT: PSS ratio of about 1: 5. The thickness of the hole transport layer is aboutThe hole transport layer is spin-coated from a solution and then baked in a nitrogen atmosphere at about 200 c for 1 hour.

The electroluminescent material 8 is then deposited. Monochromatic red and green devices according to the invention (hereinafter red and green devices) were prepared according to this example separately, with the electroluminescent layers as follows:

and (3) red devices: mixture of F8BT, TFB and Red

Green devices: mixture of Host and F8BT

F8BT ═ poly (2, 7- (9, 9-di-n-octylfluorene) -co-3, 6-benzothiadiazole)

TFB ═ poly (2, 7- (9, 9-di-n-octylfluorene) -copoly (1, 4-phenylene- ((4-sec-butylphenyl) amino) -1, 4 phenylene)

Poly (2, 7, - (9, 9-di-n-octylfluorene) -co- (2, 5-thienylene-3, 6-benzothiadiazole-2, 5-thienylene)

Random copolymer of Host ═ 90% 2, 7- (9, 9-di-n-octylfluorene) and 10% 1, 4-phenylene- ((4-sec-butylphenyl) amino) -1, 4-phenylene

The preparation of these materials is disclosed in WO99/54385, WO00/46321 and WO00/55927, the contents of which are incorporated herein by reference.

To assess the suitability of this device structure for full color displays, similar blue devices were also prepared, in which the electroluminescent layer comprised a random copolymer of poly (2, 7- (9, 9-di-n-octylfluorene), 1, 4-phenylene- ((4-sec-butylphenyl) amino-1, 4-phenylene, and 1, 4-phenylene- ((4-n-butylphenyl) amino) -1, 4-phenylene.

The electroluminescent material of these single-color devices is spin-coated to a thickness ofLeft and right.

Then depositing by evaporation a barium-containing electron-transporting layer 9 having a thickness of about 4nm, followed by evaporation to a thickness of aboutCalcium layer 11 and a thickness of aboutThe aluminum layer 12 of (a) forms the cathode 10. Preferably, the vacuum is not interrupted between subsequent layers during evaporation, which reduces contamination of the interfaces between the layers. If a dielectric material such as barium fluoride is to be deposited, evaporation of such material is preferably carried out at least at a very low rate: preferably belowAlthough slightly higher rates may also be used. It is preferred that the material of each cathode layer is kept at an elevated temperature below its evaporation point (typically around 650 to 670 c) for about 5 to 10 minutes prior to deposition to allow for outgassing.

A power supply is connected between the anode 6 and the layer 12 of the cathode 10. The power supply is arranged to apply a voltage between the electrodes so as to charge the cathode 10 negatively with respect to the anode 6.

Finally, the device is encapsulated with epoxy.

B) Full-color electroluminescent device

A full colour display device according to the invention can be prepared in accordance with the method described above, except that the red, green and blue electroluminescent materials are included, which are advantageously deposited by an ink jet printing process rather than a spin coating process as disclosed in, for example, EP 0880303.

Device performance

The performance of each of the red, green and blue monochromatic devices comprising barium fluoride/calcium/aluminum cathodes was compared to the performance of devices comprising the prior art cathodes lithium fluoride/calcium/aluminum and calcium/aluminum. From these comparisons, the inventors have found that there is significantly no detrimental interaction between the barium-containing electron transport layer and the sulfur-containing electroluminescent layer that tends to occur with lithium-containing electron transport layers.

A comparative device was prepared in accordance with the above-described example of the single-color device, except that the electron transit layer 9 was prepared using lithium fluoride instead of the barium fluoride contained, or the electron transit layer was not deposited at all.

Fig. 3 and 4 show comparative curves of current density versus bias voltage for red and green devices, respectively. It shows the bias voltages required for the device to operate-it can be seen from the figure that the device according to the invention exhibits similar performance to the comparative device. In addition, similar comparative data for the blue device indicates that the use of these three cathodes also exhibited comparable performance.

Fig. 5 and 6 show the luminance change over time for the red and green devices, and fig. 7 shows similar data for the blue device. As shown in fig. 5, the luminance change over time for the red device with the barium fluoride/calcium/aluminum cathode was better than the luminance change over time for the red device with the lithium fluoride/calcium/aluminum cathode or the calcium/aluminum cathode. The difference between the device according to the invention with a barium fluoride/calcium/aluminium cathode and the comparative device with a lithium fluoride/calcium/aluminium cathode is very significant. In addition, fig. 7 shows that the device with a calcium/aluminum cathode exhibits a particularly poor lifetime when used with a blue light emitting material, whereas the device with a barium fluoride/calcium/aluminum cathode does not show any particular disadvantages.

Furthermore, it has been determined that green, especially red, devices (i.e., devices having a sulfur-containing organic light emitting layer) having a lithium fluoride/calcium/aluminum cathode exhibit a significant decrease in lifetime when in the off state for any length of time. In contrast, it was found that the lifetime of the red and green devices with barium fluoride/calcium/aluminum cathodes remained constant regardless of whether they were driven frequently or left off for a certain time.

Elemental barium may also be used as the electron transport layer, for example in combination with a silver layer as the cathode, which has been found to possess essentially the same advantages as the barium fluoride electron transport layer as compared to the barium-free devices described above.

Without wishing to be bound by any theory, it is believed that lithium is able to migrate into the electroluminescent material and bind with sulfur-containing species, thus quenching the electroluminescence of these species, whereas larger barium atoms or ions are less likely to undergo such migration.

While the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations, and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.

Claims (29)

1. An optical device comprising

An anode, a cathode, a anode and a cathode,

a cathode electrode, which is provided with a cathode,

an organic semi-conductive material interposed between the anode and the cathode, and

an electron transport layer between the cathode and the organic semi-conductive material,

wherein the organic semi-conductive material contains sulfur, and the electron transfer layer contains simple substance barium, barium fluoride or barium oxide.

2. The optical device according to claim 1, wherein the electron transport layer contains elemental barium as a main component.

3. The optical device according to claim 1, wherein the electron transport layer contains a dielectric barium compound as a main component.

4. An optical device according to claim 3, wherein the dielectric barium compound is barium fluoride.

5. An optical device according to any preceding claim, wherein the semi-conductive material is a polymer.

6. The optical device of claim 5, wherein at least one of the polymers comprises optionally substituted polyfluorene repeat units.

7. The optical device of claim 1, wherein the organic semi-conductive material is a red organic electroluminescent material.

8. An optical device according to claim 1, wherein the organic semi-conductive material is a green organic electroluminescent material.

9. The optical device of claim 1, comprising red, green and blue organic electroluminescent materials between the anode and the electron transport layer.

10. The optical device according to claim 1, wherein the thickness of the barium-containing layer is in the range of 1-6 nm.

11. The optical device of claim 1, wherein the cathode comprises an aluminum layer.

12. The optical device of claim 11, wherein the aluminum layer has a thickness in the range of 200 to 700 nm.

13. The optical device of claim 11, wherein the cathode comprises a calcium layer between the electron transport layer and the aluminum layer.

14. The optical device of claim 13, wherein the calcium layer has a thickness of 5-25 a

15. An optical device according to claim 1, wherein a hole injection layer comprising PEDOT: PSS is provided between the anode and the semiconducting material.

16. An optical device comprising an anode, a cathode, red, green and blue electroluminescent organic semi-conductive materials located between the anode and the cathode, and an electron transport layer located between the electroluminescent organic semi-conductive materials and the cathode, wherein the electron transport layer comprises elemental barium, barium fluoride or barium oxide.

17. The optical device according to claim 16, wherein the electron transport layer contains elemental barium as a main component.

18. The optical device according to claim 16, wherein the electron transport layer contains a dielectric barium compound as a main component.

19. The optical device of claim 18, wherein the dielectric barium compound is barium fluoride.

20. The optical device of any of claims 16-19, wherein the semiconducting material is a polymer.

21. The optical device of claim 20, wherein at least one of the polymers comprises optionally substituted polyfluorene repeat units.

22. The optical device of claim 18, wherein the semiconductive material contains sulfur-containing heterocyclic repeat units.

23. The optical device of claim 22, wherein the heterocyclic repeating unit is selected from the group consisting of an optionally substituted 4, 7-linked benzothiadiazole, a 2, 5-linked thiophene, and combinations thereof.

24. The optical device of claim 16, wherein the barium-containing layer has a thickness in the range of 1-6 nm.

25. The optical device of claim 16, wherein the cathode comprises an aluminum layer.

26. The optical device of claim 25, wherein the aluminum layer has a thickness in the range of 200 to 700 nm.

27. The optical device of claim 25, wherein the cathode comprises a calcium layer between the electron transport layer and the aluminum layer.

28. The optical device of claim 27, wherein the calcium layer has a thickness of 5-25 a

29. An optical device according to claim 16 wherein a hole injection layer comprising PEDOT: PSS is provided between the anode and the semiconducting material.