Method of making electroluminescent devices having varying...

Details Method of making electroluminescent devices having varying... Method of making electroluminescent devices having varying...

2000-09-06

2003-05-06

Ramsey, Kenneth J. (Department: 2879)

Electric lamp or space discharge component or device manufacturi

Process

With assembly or disassembly

C445S023000, C427S066000, C313S504000, C313S506000

Reexamination Certificate

active

06558219

ABSTRACT:

This invention relates to electroluminescent devices, especially those that employ an organic material for light emission.
BACKGROUND OF THE INVENTION
Electroluminescent devices that employ an organic material for light emission are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes recombine in the organic layer generating photons. In PCT/WO90/13148 the organic light emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light emissive material is of the class known as small molecule materials, such as tris-(8-hydroxyquinolino)aluminium (“Alq3”). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
As a preliminary point, it should be noted that the values stated here for energy levels, workfunctions etc. are generally illustrative rather than absolute. The workfunction of ITO can vary widely. Numbers quoted in the literature suggest a range between 4 and 5.2 eV. The 4.8 eV value used here serves as an illustrative rather than an absolute value. The applicant has carried out Kelvin probe measurements which suggest that 4.8 eV is a reasonable value. However, it is well known that the actual value can depend on ITO deposition process and history. For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (“HOMO”) and “lowest unoccupied molecular orbital” (“LUMO”) levels. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in the field that such energies are affected by a number of factors, such as the local environment near an interface, so the use of such values is indicative rather than quantitative.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time.
FIG. 1
shows a cross section of a typical device.
FIG. 2
shows the energy levels across the device. The anode
1
is a layer of transparent indium-tin oxide (“ITO”) with a workfunction of 4.8 eV. The cathode
2
is a Ca:Al layer (a calcium layer capped with aluminium) with a workfunction (for the calcium at the interface with the light emissive layer) of 2.9 eV. Between the electrodes is a light emissive layer
3
of poly (2,7-(9,9-di-n-octylfluorene) (“F8”) doped with 5% poly-(2.7-(9,9di-n-octylfluorene)-3,6-benzothiadiazole) (“F8BT”), having a LUMO energy level
4
at around 2.8 eV and a HOMO energy level
5
at around 5.8 eV. (From now on the term “5BTF8” will be used to refer to this doped emissive layer blend). The emitter dopant LUMO and HOMO levels are around 3.4 and 5.8 eV respectively.
FIG. 3
adopts a convention that the HOMO and LUMO energy levels for the dopant (F8BT) of the blend are shown by means of a rectangle inserted in the zone that corresponds to the major component (F8) of the blend. In this convention the width and lateral position of the rectangle has no particular meaning, and where a blend comprise three or more materials then two or more inserted rectangles are used. Holes and electrons that are injected into the device recombine radiatively in the 5BTF8 layer. An important feature of the device is the hole transport layer
6
of poly(styrenesulphonic acid) doped poly(ethylenedioxythiophene) (“PEDOT:PSS”). This provides an intermediate ionisation potential a little above 4.8 eV, which helps the holes injected from the ITO to reach the HOMO level in the F8. However, there is still a large barrier (approximately 1.0 eV) between the hole transport layer and the light emissive layer. The presence of high barriers is undesirable, for example because it may increase the drive voltage, build up high internal fields or cause accumulation of holes. One view is also that accumulation of charge at an interface is undesirable because it can promote chemical reactions between the polymer and contaminants, leading to conjugation reduction or deep localised states that may then be charged by the accumulation layer. The charge “trapping” is believed to result in higher bias being required to pass the same current through the device, leading to a relatively rapid voltage increase with time, as the device is used.
It is well-known to use oxygen plasma treatment to clean substrates, and especially to remove organic material. It is also well known that such plasma treatment of ITO can be used to modify the ITO's work function and potentially reduce the hole injection barrier. (See, for example, WO 97/48115).
Processes have been described for the formation of ultra-thin films with to layer-by-layer control. For example, W. B. Stockton and M. F. Rubner, “Molecular-level processing of conjugated polymers. 4. Layer-by-layer manipulation of polyaniline via hydrogen-bonding interactions,” Macromolecules, Vol. 30, pp. 2717-2725, 1997 describes polymer self-assembly via hydrogen bonding interactions; Y. Shimazaki, M. Mitsuishi, S. Ito and M. Yamamoto, “Preparation of the layer-by-layer deposited ultrathin film based on the charge-transfer interaction,” Langmuir, Vol. 13, pp. 1385-1387, 1997 describes polymer self-assembly via charge-transfer interactions; A. C. Fou and M. F. Rubner, “Molecular-level processing of conjugated polymers. 2. Layer-by-layer manipulation of in-situ polymerized p-type doped conducting polymers,” Macromolecules, Vol. 28, pp. 7115-7120, 1995 describes a process for ultra-thin film formation by active in-situ polymerization; M. Ando, Y. Watanabe, T. Iyoda, K Honda and T Shimidzu, “Synthesis of conducting polymer Langmuir-Blodgett multilayers,” Thin Solid Films, Vol. 179, pp. 225-231, 1989 describes a Langmuir-Blodgett deposition process; and K. Kaneto, K. Yoshino and Y. Inuishi, “Electrical and optical properties of polythiophene prepared by electrochemical polymerization,” Solid State Communications, Vol. 46, pp. 389-391, 1983 describe electrochemical polymerization on conducting substrates. None of these documents describes gradation of charge transport layers for light emissive devices.
Further details of the manufacture of self-assembled polymer interlayers are given in our co-pending PCT patent application number PCT/GB98/02671, the entire contents of which are incorporated herein by reference. It will be apparent that the techniques described in this application can be combined in various ways with those in that prior application.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a method of forming an electroluminescent device, comprising: forming a first charge carrier injecting layer for injecting charge carriers of a first polarity; forming an organic charge carrier transport layer over the first charge carrier injecting layer, the transport layer having an electrical and/or optical property which varies across the thickness of the transport layer; forming an organic light emissive layer over the transport layer; and forming a second charge carrier injecting layer over the light emissive layer for injecting charge carriers of a second polarity.
The step of forming a transport layer suitably comprises steps of first depositing the transport layer and then processing the transport layer to create the variation in the electronic and/or optical property

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