WO2013045869A1

WO2013045869A1 - Oled display device having some of the pixels containing two diodes with organic layers of different thickness

Info

Publication number: WO2013045869A1
Application number: PCT/GB2012/000710
Authority: WO
Inventors: Daniel Forsythe; Matthew Roberts
Original assignee: Cambridge Display Technology Limited
Priority date: 2011-09-28
Filing date: 2012-09-12
Publication date: 2013-04-04
Also published as: JP2014532260A; GB201404977D0; CN103827948A; KR20140082745A; GB2509270A; GB201116749D0; GB2495107A

Abstract

A display comprises an array of organic light emitting diode (OLED) pixels, at least some of the pixels comprising a first (3) and a further diode (4) connected in parallel to a drive current. The first diode (3) has a larger area than the further diode (4), and the further diode (4) has a thinner active organic layer than the first diode (3), such that the further diode (4) has a greater drive current density in use than the first diode (3). The further diode ages faster than the first diode because it is driven harder. As it ages, the conductivity of the further diode decreases, and so more current passes through the first diode. The light emitted by the first diode therefore decreases at a slower rate than in the absence of the further diode, improving lifetime and reducing image burn-in at a cost of a small reduction in the initial efficiency of the display.

Description

DISPLAY DEVICE

This invention generally relates to displays. More particularly the invention relates to display devices having organic electroluminescent pixels, such as for example Organic Light Emitting Diode (OLED) display devices.

BACKGROUND TO THE INVENTION

Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates.

Organic (which here includes organometallic) LEDs may be fabricated using either polymers, non-polymeric molecules, or dendrimers in a range of colours, depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of non-polymeric molecule based devices are described in US 4,539,507; and examples of dendrimer-based materials are described in WO99/21935 and WO02/067343.

A basic structure of a typical organic LED involves a glass or plastic substrate supporting a transparent anode layer comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer, an electroluminescent layer and an optional electron transport layer, and a cathode. The hole transport layer may comprise, for example, PEDOT: PSS (polystyrene-sulphorate - doped

polyethylene - dioxythiophene). The electroluminescent layer may comprise a polyfluorene polymer, PPV, polythiophene, or a luminescent guest in a non- emissive host. Many such materials are described in the book Organic Light- Emitting Materials and Devices" edited by Zhigang Li and Hong Meng, published by CRC Press (Taylor and Francis) [2007] (ISBN 10: 1 -57444-574-X) . The cathode layer typically comprises a low work function metal such as calcium and may include an additional layer immediately adjacent electroluminescent layer, such as a layer of sodium or lithium fluoride or a conductive polymer, for improved electron energy level matching. Contact wires to the anode and the cathode respectively provide a connection to a power source. The same basic structure may also be employed for small molecule devices. In this structure, light can be emitted through the transparent anode and substrate and devices with this structure are referred to as "bottom emitters". Devices which emit through the cathode may also be constructed, for example, by keeping the thickness of the cathode layer to less than around 50- 100mm so that the cathode is substantially transparent.

Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multi-coloured display may be constructed using groups of red, green and blue emitting pixels. Pixels emitting different colours need not have equal areas.

In finished display devices, the pixels will be connected to driver circuitry to drive the pixels in response to video input signals, or the like. Such circuitry and OLED devices in general are well known in the art. Further information on conventional OLED materials and devices can be found in a variety of textbooks, such as for example Organic Light-Emitting Materials and Devices" edited by Zhigang Li and Hong Meng, published by CRC Press (Taylor and Francis) [2007] (ISBN 10: 1 - 57444-574-X).

In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a CRT picture, to give the impression of a steady image. A problem with organic light emitting materials is that the lifetime is short

compared with other technologies. For example, the time taken for the light from a blue pixel to decrease by 50% can be of the order of 20,000 hours. Also, the eye is very sensitive to brightness differences, so if a fixed image is present on a display for a length of time sufficient to reduce the brightness by about 3% in bright areas it results in image burn-in or image sticking, where a persistent ghost image is permanently apparent on the display regardless of the image being displayed subsequently.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a display comprising an array of organic light emitting diode (OLED) pixels each having an active organic layer, at least some of the pixels comprising a first diode and a further diode being connected in parallel to one another and being connected to a source of drive current in use, the first diode having a larger area than the further diode, and the further diode having a thinner active organic layer than that of the first diode, such that the further diode is provided with a greater drive current density in use than the first diode.

In one embodiment, the display comprises an array of organic light emitting diode (OLED) pixels, at least some of the pixels comprising a first diode and a further diode being connected in parallel to a source of drive current in use, the first diode having a larger area than the further diode, and the further diode having a thinner active organic layer than the first diode, such that the further diode is provided with a greater drive current density in use than the first diode. In use, the further diode ages faster than the first diode because it is driven harder. As it ages the conductivity of the further diode decreases so that more current passes through the first diode. Thus the light emitted by the first diode decreases at a slower rate than in the absence of the further pixel, thus improving lifetime and reducing the problem of image burn-in in exchange for a small reduction in the initial efficiency of the display.

Other preferred arrangements are described in the dependent claims.

In particular, the further diodes can be adapted to be non-emissive, or may further comprise an opaque body arranged to prevent any light being emitted by the further diode from being emitted by the display.

The array of pixels may comprise a plurality of groups of pixels having different colors, and wherein the at least some pixels comprising said first diode and said further diode all belong to a group of pixels having the same color (for example, blue).

Preferably, each further diode has an area of less than 20% of the area of each first diode, very preferably less than or equal to 10% of the area of each first diode.

The thickness of the active area of the further diodes is advantageously at least 10% thinner than the thickness of the first diodes. Preferably at least 20% thinner than the thickness of the first diodes.

The active organic layer of the first diode and further diode may each comprise a plurality of layers each layer comprising a different material or different materials.

The active organic layer or layers can comprise a polymer materia), a non- polymeric material, or a mixture of both types of material.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a plan view of (a) a standard prior art pixel and (b) a pixel according to the present invention,

Figure 2 shows a cross-section of a pixel according to the present invention,

Figure 3 shows how the first and further diode are connected in parallel to one another,

Figure 4 shows how the luminance of a prior art pixel and a pixel according to the present invention changes with time.

Figure 5 shows the decay in luminance of the first diode together with the reduction in current through the smaller further diode.

DETAILED DESCRIPTION The present invention provides a display in which the active pixels have an in-built mechanism for changing the current density in the pixel during driving. This relies on using a small area "sacrificial" diode running in parallel with the active pixel. The small area diode can be made using the same materials as those of the main pixel. In this case the smalt area diode will emit light when driven. To prevent this light from being emitted by the display, a light blocking element comprising an opaque layer (such as a thick metal electrode) can be provided located in the path of light from the small area diode. Alternatively, it is possible to use materials in the small area pixel which result in minimal intrinsic light emission. It is important that light from this small area diode is not emitted by the display, because the fact that it is driven harder and ages quicker than the main pixel means that image sticking or burn-in might be visible to the eye from the small diode. As the active pixel is driven the small area non emissive pixel is running at a higher current density. This causes the small area pixel to degrade faster than the emissive pixel. As it degrades the conductivity reduces, so more current is switched to the active pixel. This increase in current in the active pixel compensates for the drop in PL efficiency due to driving. The result is the initial decay of luminescence is significantly improved. In other words, the light emitted by the first diode decreases at a slower rate than in the absence of the further pixel, improving lifetime and reducing image burn-in at a cost of a small reduction in the initial efficiency of the display.

Figure 1 shows a plan view of (a) a standard prior art pixel 1 and (b) a pixel according to the present invention (2). Pixel 2 includes a first diode 3 and a further diode 4 which comprises, in essence, a parallel pixel that consists of the same LEP device stack, but has a thinner LEP and a smaller area than pixel 2. This ensures the current density through this pixel is higher than the active pixel, but the total current drawn is kept to a minimum, ensuring high panel efficiency.

The pixel 2 according to the present invention is shown in cross-section in Figure 2. This figure shows a substrate (6), typically glass, on which a patterned layer of ITO (8) is provided to act as an anode. In this embodiment, an opaque metal layer (10) is provided between the further diode 4 and the viewing surface of the display (12) so that light from further diode 4 is not emitted by the display in use. A bank layer (14) is provided on top of the ITO, and is patterned to provide a perimeter wall defining and independently surrounding the first diode and the further diode. The bank wall thus forms two wells, one for the first diode (3) and one for the further diode (4). Active organic materials can then be deposited into the wells by ink jet printing.

The active materials comprising the OLED device stack (16) may for example include a hole transport layer deposited by ink-jet printing, which is baked or optionally cross-linked prior to deposition of a light emitting organic layer through a subsequent ink-jet printing step. The -sacrificial" further diode (4) is preferably arranged to have a thinner OLED device stack (18), for example by depositing a thinner light emitting organic layer.

The deposition of a thinner layer can be achieved with a printing recipe, the big (first) diode will require a certain number of drops to achieve the target thickness, in the small (further) diode the number of drops can be reduced to achieve a thinner light emitting layer.

An alternative approach is to run two print passes for the light emitting layer, the first one to fill in the first diode, the second to fill in the smaller further diode.

A specific example of a device made according to the present invention will now be described in detail.

A device was fabricated on a glass substrate. having a transparent ITO anode. The diode and further diode were defined by banks made of an insulating material. A hole injection layer 35 nm in thickness was deposited by inkjet printing onto the anode within the banks, followed by baking and the deposition of an intertayer material 22 nm in thickness. The structure was again baked to remove solvent.

The diode area was 10 square millimeters, and the further diode area was 1 square millimeter. Next, a light emitting polymer layer 90 nm thick was deposited into bank defining the diode, and a layer of the same materia! 55 nm thick was deposited into the further diode. These layers were again baked to remove excess solvent. A cathode comprising separate sequential layers of Naf and aluminium was then evaporated onto the top of the devices through a shadow mask.

The hole injection material used was Poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy)-2,5-diyl), sulfonated solution (commercially available from Sigma-Aldrich).

The interlayer material comprised a copolymer having the following repeat units :

(repeat unit (b) described in WO2005/049546), and

The solvent used for this copolymer was ortho-xylene.

The light emitting polymer layer comprised a copolymer having the following repeat units: (d) (9, 9) di-ociy! fluorene, (e) (9, 9,) di-hexyl fluorene, and (f) phenoxazine. The solvent used for this copolymer was also ortho-xylene.

These copolymers were synthesized by the well known Suzuki process. The decay in tuminence of the device described above and a normal pixel without the further diode is shown in Figure 4. The X axis is the measured luminance divided by the initial luminance at a drive current of 8mA/cm². The y axis is time in hours. It will be seen that for the pixel of the present invention the time taken to reach 80% of the initial luminance is more than doubled from7 hours to 16 hours.

Figure 5 shows a plot of the initial decay of the luminance of the first (large area) diode together with the reduction in current passing through the smaller further diode as it ages. In this figure the top graph shows the luminance of the first diode according to the axis on the left hand side, which shows measured luminance in arbitrary units. The lower graph shows the current passing through the further diode according to the axis on the right hand side, which is read in milliamps.

Of course, any materials capable of being used to form the layers of an OLED device can be used instead of the specific materials mentioned above in the main and further diode structure of the present invention. Specifically, alternative polymers such as PVK and PPV can be used for the light emitting layer, which can as an alternative also comprise a phosphorescent guest compound in a host polymer. PEOOT and triaryl amine copolymers can be used as the hole transport or interlayer material.

As a further alternative, non-polymeric organic materials can be used as the active iayers. Such materials include Alq₃, PBD, NPD, triaryl amines, etc.. Such materials are sometimes called "small molecule" (sm) materials, and may comprise dimer, trimer or tetramer structures, or dendrimers. Descriptions of suitable polymer and non-polymeric materials can be found in a variety of textbooks, such as for example "Organic Light- Emitting Materials and Devices" edited by Zhigang Li and Hong Meng, published by CRC Press (Taylor and Francis) [3007] (ISBN 10: 1 -57444-574-X), especially chapters 2 and 3.

The diode and further diode may comprise 2 or more active layers, for example electron transport layers, hole transport layers, exciton blocking layers, hole and electron injecting layers, in addition to light emitting layers. In general, small molecule (non-polymeric) OLEDs tend to have many active layers in the device. Although the devices in the present embodiment were fabricated using ink-jet printing, other solution processing techniques such as nozzle printing, spin coating, slot die coating, and gravure or flexograpftic printing can be used as an alternative. The devices may also comprise layers deposited using vapour deposition techniques such as for example evaporation or sputtering.

It will be appreciated that although only a few particular embodiments of the invention have been described in detail, various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention as defined in the following claims.

Claims

1 . A display comprising an array of organic light emitting diode (OLED) pixels each having an active organic layer, at least some of the pixels comprising a first diode and a further diode being connected in parallel to one another and being connected to a source of drive current in use, the first diode having a larger area than the further diode, and the further diode having a thinner active organic layer than that of the first diode, such that the further diode is provided with a greater drive current density in use than the first diode.

2. A display as claimed in claim 1 in which the further diodes are adapted to be non-emissive.

3. A display as claimed in claim 1 , further comprising an opaque body

arranged to prevent any light being emitted by the further diode from being emitted by the display.

4. A display as claimed in any preceding claim in which the array of pixels comprise a plurality of groups of pixels having different colors, and wherein the at least some pixels comprising said first diode and said further diode all belong to a group of pixels having the same color.

5. A display as claimed in claim 4 in which the same color is blue.

6. A display as claimed in any preceding claim in which each further diode has an area of less than 20% of the area of each first diode.

7. A display as claimed in any preceding claim in which each further diode has an area of less than 10% of the area of each first diode.

8. A display as claimed in any preceding claim in which the thickness of the active area of the further diodes is at least 10% thinner than the thickness of the first diodes.

9. A display as claimed in any preceding claim in which the thickness of the active area of the further diodes is at least 20% thinner than the thickness of the first diodes.

10. A display as claimed in any preceding claim in which the active organic layer of the first diode and further diode each comprise a plurality of layers each layer comprising a different material or different materials.

1 1.A display as claimed in any preceding claim in which the active organic layer or layers comprise a light-emissive polymer material.

12. A display as claimed in any one of claims 1 to 10 in which the active organic layer or layers comprise a light-emissive non-polymeric material.