GB2545432A

GB2545432A - Organic electronic device fabrication method

Info

Publication number: GB2545432A
Application number: GB1522069.2A
Authority: GB
Inventors: Anderson Graham
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2015-12-15
Filing date: 2015-12-15
Publication date: 2017-06-21
Also published as: GB201522069D0

Abstract

A method of producing an Organic Electronic Device comprising a substrate, an anode, a cathode and a light emitting layer (LEL) is described in which the thickness of the LEL is adjusted so that each pixel of a given color displays substantially the same total resistance value. The adjustment of the LEL thickness is preferably carried out by printing corresponding amounts of ink in order to obtain the targeted thickness at each pixel of a given color resulting in the targeted total resistance value.

Description

ORGANIC ELECTRONIC DEVICE FABRICATION METHOD FIELD OF THE INVENTION

This invention relates to techniques for fabricating Organic Electronic Devices, in particular OLED (organic light emitting diode) devices such as polymer OLED displays.

BACKGROUND OF THE INVENTION

Organic Electronic devices, such as OLEDs, have gained importance in a variety of applications. Organic electronic devices provide many potential advantages including inexpensive, low temperature, large scale fabrication on a variety of substrates including glass and plastic. Organic light emitting diode displays provide additional advantages as compared with other display technologies - in particular they are bright, colourful, fast- switching and provide a wide viewing angle. OLED devices (which here includes organometallic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multicoloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, W095/06400, W099/48160 and US4,539,570, as well as to Organic Light Emitting Materials and Devices" edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10:1-57444-574X, which describes a number of materials and devices, both small molecule and polymer. (Here “small molecule” refers to non-polymeric small molecules - some so- called small molecules such as dendrimers end may be relatively large, but nonetheless have the characterizing feature that they do not comprise multiple repeat units assembled by polymerization).

One important requirement for such devices is the ability to enable a homogenous luminance display, to enable a desired color tuning and to improve the lifetime of a device. Measures applied in order to ensure a desired homogenous display concern in particular the device driving means, such as driving circuits for light emitting devices disclosed in US 2007/0008254 A1 and US 2007/0222716 A1. However, such means often do not prove sufficient and/or are relatively expensive, thereby running contrary to the ever increasing demand for inexpensive devices. In addition these systems typically require the use of rather expensive high conductivity materials for anode/cathode lines and/or a current based driving system which might again increase complexity of the overall system. US 2008/0203903 A1 discloses an OLED device with a patterned Light Emitting (Polymer) Layer (LEL) for the purpose of color tuning. By providing independently addressable domains of different thickness within the LEL different colors may be displayed while using only one type of material for the LEL. US 2007/0018153 A1 finally discloses an OLED device having a relatively thick Light Emitting Polymer Layer in order to improve efficiency and lifetime. However, even with these measures available to allow improvements in relation with efficiency, lifetime and color tuning the problem remains that the existing technologies require the use of high conductivity electrodes, the use of driving circuits in order to properly apply a required bias for color display/emission, as well as the problem that the desired homogenous luminance display cannot be ensured.

In view of these problems associated with the prior art the present inventors aimed at providing a method for producing Organic Electronic Devices, such as OLED devices which overcomes the problems associated with the prior art. The inventors in particular aimed at the provision of production methods which allow the use of solution based processes, as such processes may conveniently make use of existing inkjet printing technologies, thereby facilitating production processes. In addition the inventors aimed at the provision of a possibility to ensure homogenous luminance display.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides the method as defined in claims 1 to 4 as well as the Organic Electronic Devices according to claims 8 to 10. Further preferred embodiments are now described.

The present invention is based on the surprising finding that it is possible, in a very simple but efficient manner, to control and adjust luminance display by adjusting the layer thickness of a LEL (including also Light Emitting Polymer Layers (LEPL)), namely by adjusting the thickness of the LEL at each pixel position relative to its position within the array of pixels, as explained in detail below. Therefore the present invention will be described below in relation to a polymer based OLED device, however, it should be understood that the general methodology provided by the present invention is also applicable to other types of devices, as will also be outlined below. However, for the sake of illustration the detailed description will focus on OLED devices, and the skilled person will understand, based on the disclosure and his common knowledge, how these principles may also be employed for other type of devices.

DETAILED DESCRIPTION OF THE INVENTION

As indicated in Figure 1a a cross section through a typical OLED device 10 shows the various functional layers required, such as a substrate 12 bearing a transparent conductive oxide layer 14, typically ITO (indium Tin Oxide), which may be patterned, typically around 40 nm in thickness. Over this is deposited a hole injection layer (HIL) 16 typically comprising a conducting polymer such as PSS:PEDOT (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This helps match the hole energy levels of the ITO anode and light emitting polymer (and can also assist in planarising the ITO), and is typically around 30 nm in thickness though potentially up to around 150 nm. A similar layer is generally present in an organic photovoltaic device to facilitate the extraction of holes. Commercial hole injection materials are available, inter alia, from Plextronics Inc. The hole injection layer is, in this example, followed by an intermediate polymer layer, interlayer (IL) 18 - also known as a hole transport layer (HTL). This is made of a hole transport material which allows efficient transport of holes; it typically has a thickness in the range 20 nm to 60 nm and is deposited over the hole injection layer and, generally, is cross-linked. One example material from which the interlayer may be fabricated is a copolymer of polyfluorene-triarylamine or similar (examples of other suitable materials are described by Bradley et al. in Adv. Mater, vol 11, p241 - 246 (1999) and in Chapter 2 of Li and Meng - see below). Over this is deposited one or more layers of light emitting polymer (LEP) 20 to form an LEP layer or stack. One example of a light emitting polymer is PPV (Poly(p- phenylenevinylene)). A cathode 22 is deposited over the LEP stack, for example comprising a layer of sodium fluoride (NaF) followed by a layer of aluminium. Optionally an additional electron transport layer may be deposited between the LEP stack 20 and cathode 22.

The device illustrated in Figure 1 a is a bottom-emitting device, that is light generated in the LEP stack is coupled out of the device through the substrate, via the transparent ITO anode layer. It is also possible to fabricate top-emitting devices using a thin 5 cathode layer, for example less than around 100 nm in thickness. Although the structure of Figure 1 shows an LEP stack the same basic structure may also be employed for small molecule (and dendrimer) devices, i.e. devices wherein the LEL comprises small molecules and/or dendrimers and not polymeric light emitting materials. The various layers of such a devices may be produced in different manner, however, using polymer based or small molecule/dendrimer based LEL allows the use of printing processes for the fabrication of this layer, such as inkjet printing, or other printing methods. An example of a suitable inkjet printing method applicable within the context of the present invention is the method described in GB 2509497 A, incorporated herein by reference.

The skilled person will appreciate that there are many variants to produce an organic electronic device fabrication as exemplified above. For example, the ITO layer may be omitted and instead the hole injection layer 16 used as the anode layer. Additionally or alternatively the electrical conductivity of the hole injection layer 16 may be supported by an underlying metallic grid (which may optionally be transparent by using fine grid lines and/or thin metal). Such an approach may be employed, for example, in an OLED lighting tile with a large area of coverage and connections at the edge. Optionally a flexible substrate such as PET (polyethylene terephthalate) or polycarbonate may be employed. A similar basic structure may be employed for other molecular organic diodes, for example for an organic photovoltaic device.

Figure 1 b shows a view from above of a portion of an example three-colour active matrix pixellated OLED display 200 after deposition of one of the active colour layers. The figure shows an array of banks 112 and wells 114 definina pixels of the display. In a colour display different coloured (sub)pixels may comprise respectively green, red and blue light emitting polymer layers.

The materials to fabricate an OLED or other organic electronic device may be deposited by inkjet printing. Figures 2a and 2b, are taken from EP 1,219,980 and show inkjet printing apparatus for depositing red, green and blue colour filters of an electroluminescent display, and illustrate the general principle. Figures 2a and 2b show "sideways" printing; Figure 2c shows an example of an alternative print head orientation.

Thus Figure 2a shows an inkjet printer 200 comprising a base 209 supporting first and second linear positioners 206,208 for moving a substrate 212 and inkjet print head 222 relative to one another along two orthogonal axis Y and X. Positioner 206 comprises a pair of rails 254 mounting a slider 256 provided with a turntable 251 supporting a table or bed 249 on which the substrate 212 is supported. The substrate 212 is aligned on table or bed 249 by means of stops 250 against which two edges of the substrate abut. Turntable 251 allows for some limited rotation of the table and substrate 249,212 relative to the print head 222 for alignment purposes (although the ability for full rotation may be provided if desired).

Positioner 208 comprises a pair of rails 252 mounting a slider 253 which carries rotary positioners 244,246,247 which allow a print head unit 226 carrying the print head to be rotated independently about three orthogonal axes. The print head has ~90 degrees of rotation available and small amounts of screw-controlled movement are available in both other axes (for example, to allow the nozzle plate to be aligned parallel with the substrate). A further linear positioner 248 is also mounted on slider 253 to allow the print head unit and print head to be translated in the Z-direction, that is towards and away from substrate 212. Inkjet printer system 200 is controlled by a computer terminal 202 via an umbilical 204. Terminal 202 may comprise a general purpose computer with interface hardware for interfacing to the above-described linear and rotary positioners, running operating system, user interface and other inkjet printer drive and control software, in a conventional manner. Thus terminal 202 typically includes a data input device such as a network interface of floppy disk drive for receiving data defining a pattern to be printed, and printer control software to control the printer hardware to print a pattern in accordance with stored or input data. Other conventional functions such as test functions, head cleaning functions and the like are generally also provided by software running on terminal 202.

Figure 2b shows an example print head 222 in more detail. The print head has a plurality of nozzles 227, typically orifices in a nozzle plate for ejecting droplets of fluid from the print head onto the substrate. A fluid supply for printing {not shown in Figure 2b) may either be provided by a reservoir within the print head or print head unit or fluid may be supplied from an external source. In the illustrated example the print head 222 has a single row 228 of nozzles 227, but in other examples of print heads more than one row of nozzles may be provided with nozzles offset in one or two dimensions. The diameter of the orifices of nozzles 227 is tvDicallv between 10Dm and 100Dm. and drop sizes are similar. The space or pitch between adjacent nozzle orifices is typically between 50pm and 1000pm.

Generally the volume distribution of drops is non-uniform, increasing or falling off at nozzles at the edge of the print head (that is, near an end of a row of nozzles), and further non-uniformity arises from variations in drive efficiency between elements within the print head. However when depositing materials for molecular electronic devices such as OLEDs, there is a need for both high resolution, generally than better than that required for the best high resolution graphics, and accurate control of the volume of material deposited (for example to control brightness/drive current/lifetime), which implies accurate control of ink drop volume.

One known strategy for more accurately controlling the volume of material deposited is to cover a pixel or fill a well using a plurality of sequentially deposited drops rather than a single drop. However this has disadvantages and W02004/049466, incorporated herein by reference, therefore describes alternative techniques to achieve a uniform drop volume to within 1%. In addition, GB 2509497 A mentioned above describes a further method enabling a controlled printing of functional layers of an OLED device, which is applicable in the context of the present invention. In order for example to adjust LEL thickness it is for example possible to employ in inkjet printing processes drive waveforms comprising pulses of different respective durations, a longer duration for a greater target thickness of deposited material. In particular where a piezoelectric drive head is employed, the pulse duration corresponds in some sense to the duration for which the "gate” of the nozzle is open, and a voltage step within the pulse (either up or down) corresponds to the degree of "kick” given to the piezoelectric material. It has been found preferable to use the duration of a pulse for coarse control of the volume/thickness of deposited material and then to vary one or more voltage steps for fine adjustment. Thus it is not necessarily the case that the waveforms for a greater target thickness of deposited material will employ a greater voltage step (see Figure 6). Examples of printers which may be emloyed for OLED fabrication are the Litrex 1408, Litrex 142 and Litrex 140P printers, with a Dimatix SX3 inkjet print head (from Fujifilm Dimatix, Inc.). Typically the deposited materials which can be deposited using such printers in order to produce for example OLED devices, have a syrupy consistency, such as a viscosity of 8-10 centipoise; and an ejected droplet may have a volume in the order of order 8pL.

As outlined above, the present invention is based on the surprising finding that changing in a controlled manner the thickness of an LEL (including LEPL) enables an adjustment of luminance thereby providing homogenous displays. OLED displays are made up of an array of OLED pixels sandwiched between an anode and a cathode. Typically each pixel is addressable individually and luminance is achieved by applying a drive voltage over the anode and cathode. Typically such a drive scheme is of a Dulsatina tvDe so that each Dixel will be at either 0V or the drivina voltaae Vh. The relative time SDent in each state will determine the perceived brightness of the pixel. As outlined above, it is a desire in the art to obtain a brightness as homogenous as possible in order to generate a display which is perceived as being of high quality by the user. However, as already outlined above, numerous attempts in the art to ensure such a homogenous display have not been fully satisfactory.

The present invention however, enables the further improvement of the display by considering the importance of the pixel position relative to the anode/cathode structure, i.e. the position of the pixel within a given matrix. Namely, the present inventors have determined that the total resistance value of a pixel will depend from the thickness of the LEL at the given pixel position (for one given color) as well as from the respective anode and cathode resistances, which in turn depend from the distance of the anode and cathode, respectively, at the pixel position, from the anode and cathode source respectively. Pixels of the same color within a given matrix/array are therefore not identical with respect to their total resistance value. The inventors furthermore have been able to prove that luminance at a given pixel position depends from the total resistance value, i.e. that differences in luminance are caused by differing total resistance values. Accordingly the present invention provides a means for addressing this problem, by adjusting the thickness of a LEL at a given pixel position in order to ensure that over the entire array of pixels the total resistance value remains constant and in a range so that the performance of the display is considered by an user as being homogenous. Such layer thicknesses can easily be adjusted using printing techniques, for example by adjusting the number of droplets of ink deposited at a given pixel position, by using multi-pass printing processes or processes wherein a desired number of droplets is ejected at a given pixel position. Other printing methods likewise allow for a selective and adjustable amount of ink deposited, i.e. different LEL thickness, such as screen printing, gravure printing as well as slot dye printing.

In addition to the advantage of providing improved visual performance the present invention also provides additional advantages relating to the anode and cathode materials suitable as well as in relation to the way a display is driven. As briefly indicated in the introductory part describing the prior art systems, often the way of driving/targeting a pixel within a display is current based, i.e. the respective hardware has to enable a current driven mode. However, it is often desired but difficult to achieve to switch from a current driven mode to a voltage driven mode. Disadvantages associated with current driven system are for example slow response times, high energy consumption, compromised efficiency as well as requiring rather complex hardware systems to allow a current driven modus. Surprisingly it has been found that systems prepared in accordance with the present invention, due to the good resistivity matching allow for a voltage driven mode, thereby avoiding the problems associated with the current driven mode. By matching the thickness and thereby the total resistance value it is possible to drive a respective display be means of a voltage based driving system, thereby overcoming the drawbacks associated with the current driven mode. In addition it is a further surprising effect of the present invention that, due to the thickness/resistivity matching for the LEL it becomes possible to employ a anode and cathode patterns with a higher resistivity, for example it is possible to employ a broader range of materials for the anode and cathode materials. Typically high conductivity patterns and/or materials (i.e. low resistivity pattems/materials) are required in order to produce the anode and cathode patterns (such as lines) in a display which has satisfactory visual performance properties, such as display uniformity. Such materials (or patterns) however are rather expensive components for displays. However, due to the improved electrical properties of the LEL achieved by the present invention based on the resistivity matching, it has become possible to also employ materials for the anode and/or cathode patterns (or patterns as such) which display a much lower conductivity (i.e. higher resistivity) without sacrificing display performance. The present invention thereby enables the use of cheaper components/materials for anode and cathode patterns. This additional advantage, based on a thickness/resistivity matching of the LEL is surprising it concerns other layers/components of a display device. Overall, the advantages provided by the present invention far exceed the expectations, as initially only improved display performance (uniformity) was desired.

According to the present invention there is therefore provided a method of fabricating an organic electric device, the method comprising: depositing a LEL, preferable by inkjet printing, wherein the thickness of each pixel of the LEL of the same color is adjusted so that the total resistance value, i.e. the sum of the resistance of the pixel, and the respective resistances of anode and cathode at the given pixel position, is equal for all pixels of a given color within the array of pixels of a given device. The result of this method is a uniform display in particular within an OLED display, including polymer based OLED displays as the method ensures that the current through each pixel is identical, resulting in uniform and homogenous luminance. Considering the total resistance value in a method of producing an OLED display enables for example the preparation of high quality displays of larger size, as well as the use of low conductivity materials for cathode and anode, respectively, as the thickness adjustment of the LEL thickness in order to create the desired identical total resistance value over the whole array of pixels allows for the required control of the luminance. This in turn further facilitates the production method for this type of display, as cheaper materials as well as solution based deposition techniques may be employed, not only for the LEL as well as other functional layers, but also for the electrodes, thereby replacing vapor deposition methods requiring high purity metals.

Overall, the present invention therefore provides an improved methodology for the production of Organic Electronic Devices, in particular OLED devices, such as displays, Accordingly the present invention also provides an OLED display, wherein not all pixels of a given color do show an substantially identical LEL thickness, and wherein the total resistance value for each pixel of the same color within an array of pixels within the display is substantially the same, independent from the position of the pixel in the array/display. Substantially the same or substantially identical in the context of the present invention defines a variation of 5% or less, preferable 3% or less.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figures 1 a and 1 b show, respectively, a first example of a cross section through an OLED structure, and a view from above of a portion of a three colour pixelated OLED display;

Figures 2a to 2c show, respectively, an example of an inkjet printer and an example of an inkjet printer head and an example of an alternative print head orientation;

Figures 3a to 3d show, respectively, examples of ring-bank structures for a colour OLED display and droplet-based deposition of dissolved molecular electronic material into wells formed by the structures, and a side view of a well of a passive matrix OLED display in cross-section and perspective view;

Figure 4 shows, schematically, short and long pulse widths and corresponding droplet volumes produced by print head nozzles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 3a illustrates the passage of a sweeping head 222 over a number of pixels of a portion of a colour OLED display. The figure shows, diagrammatically, deposited droplets within a “polo"-type ring-bank 112. In the figure the red (R) green (G) and blue (B) sub-pixels each have a separate well with anode metal 14 at the base. Merely by way of example, in a small flat panel display a pixel may have a width of 50 pm and a length of 150 pm -250 pm with, say, 10 pm or 20 pm wide banks; in larger displays more suitable for applications such as a colour television a pixel width may be approximately 200 pm. In embodiments of the invention the ink volume deposited is adjusted in order to arrive at a desired target total resistance value

Figure 3b illustrates an arrangement in which the ring-banks 112 define longitudinal channels each holding material for a plurality of colour sub-pixels, the sub-pixels themselves being defined by anode metal 14. In embodiments the anode islands may be separated by an underlying passivation layer such as silicon oxide or nitride or SOG (spin-on-glass). In embodiments of Figures 3a and 3b no part of the ring-bank of one pixel is shared with another pixel.

Figure 3c shows a cross-section through a portion of a display such as a passive matrix OLED display in which a laver of insulatina material 116 is Drovided over Dortions of the anode metal in order to insulate this from later-deposited cathode material. This is seen more clearly in Figure 3d where it insulates cathode metal 118 (to provide electrodes at right angles to the anode metal electrodes). The insulator may comprise oxide, nitride or SOG or a resist material.

Figure 3c also illustrates that when a droplet is first deposited its volume is much larger than that of the well, though it dries to leave a thin layer of the previously dissolved material across the bottom of the well. For example, the depth of a well may be ~1 pm, the thickness of the dry deposited material may be ~0.1 pm, and the initial height of the droplet may be ~10pm. Accordingly, in order to achieve the required thickness adjustments of the LEL either multiple passes of inkjet printing, allowing the evaporation of the solvent in order to allow for subsequent droplet depositions, or the provision of wells large enough to hold the entire deposited volume of ink may be required. In the context of the provision of high quality displays it may furthermore be advantageous to consider not only LEL thickness but also the total thickness of LEL and HIL. As a relatively thin HIL is preferred for optimal Blue performance, whereas a particularly thick layer is preferred for Red there are a number of thicknesses at which the optical output is peak, and it is further preferable that the overall LEL + HIL thickness matches one of these. For example, in one structure R, G and B coloured (sub)pixels have the following layer thicknesses (*: The LEL thickness shown below is the average thickness, the individual pixels will have a differing thickness in accordance with the present invention, as illustrated above):

Typically the other layers in the device have substantially the same thickness for R, G and B coloured (sub)pixels. The present invention accordingly provides a simple yet efficient way of producing highly satisfactory organic electronic devices, in particular OELD devices, which do show an improved homogenous luminance. The average skilled person will understand that the method of the present invention may be used not only for preparing LEL with individual thicknesses of the respective pixels, but that the method may also be employed for preparing other functional layers of an OLED where thickness adjustment is desired, for example for HILs. In addition the average skilled person will also understand that the method of the present invention is not limited to printing processes, in particular injet printing (being however one of the preferred methods applicable in the context of the present invention), as the adjustment of LEL thickness may for example also be achieved using other deposition methods, such as evaporation methods (evaporation of small molecule light emitting materials).

Claims

1. Method of producing an Organic Electronic Device comprising a substrate, an anode, a cathode and a light emitting layer (LEL), characterized in that the thickness of the LEL is adjusted so that each pixel of a given color displays substantially the same total resistance value.

2. Method in accordance with claim 1, wherein the adjustment of the LEL thickness is carried out by printing corresponding amounts of ink in order to obtain the targeted thickness at each pixel of a given color resulting in the targeted total resistance value.

3. Method in accordance with claim 2, wherein printing is inkjet printing.

4. Method in accordance with any of the preceding claims, wherein the substantially the same total resistance value of the pixels of a given color varies by 3% or less.

5. An Organic Electronic Device comprising a substrate, an anode, a cathode and a light emitting layer (LEL), wherein at least the pixels for one given color are characterized in that the thickness of the respective LEL is adjusted so that each pixel of the given color displays substantially the same total resistance value.

6. Device according to claim 5, wherein the device is an OLED display and the pixels of a given color differ in their LEL thickness by 5% or more (minimum thickness to maximum thickness).

7. Device according to claim 5 and/or 6, wherein the pixels for more than one given color are characterized in that the thickness of the respective LEL is adjusted so that each pixel of the respective given color displays substantially the same total resistance value.