WO2008059264A1

WO2008059264A1 - Droplet deposition method

Info

Publication number: WO2008059264A1
Application number: PCT/GB2007/004378
Authority: WO
Inventors: Laura Webb; Mark Bale
Original assignee: Cambridge Display Technology Limited
Priority date: 2006-11-15
Filing date: 2007-11-15
Publication date: 2008-05-22
Also published as: GB0622789D0

Abstract

The invention provides a method of reducing the non-uniformity of an organic electronic device deposited on a substrate by an inkjet print-head, the substrate forming part of a display, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the non-uniformity of the device being caused by drying effects of the ink on the substrate, the method comprising: calculating an improved nozzle driving signal; and driving the inkjet print-head with the improved nozzle driving signal, wherein the improved nozzle driving signal defines an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink that reduces the non-uniformity of the device on the substrate.

Description

DROPLET DEPOSITION METHOD

This invention generally relates to the deposition of material for electronic devices, particularly molecular electronic devices such as organic light emitting diodes, by an ink jet-type process. The invention is particularly concerned with droplet deposition methods.

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 LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), 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 so-called small molecule based devices are described in US 4,539,507.

A basic structure 100 of a typical organic LED is shown in Figure Ia. The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 40 to 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are widely available. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.

A substantially transparent hole injection layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p- phenylenevinylene)) and the hole injection layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOTrPSS (polystyrene-sulphonate- doped polyethylene-dioxythiophene) from H.C. Starck of Germany. In a typical polymer-based device the hole injection layer 106 may comprise around 200 nm of PEDOT. The light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case, banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.

Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved or enhanced through the use of cathode separators (not shown in Figure 1).

The same basic structure may also be employed for small molecule devices.

Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress. Alternatively, the displays can be encapsulated prior to scribing and separating.

To illuminate the OLED, power is applied between the anode and cathode by, for example, battery 118 illustrated in Figure 1. In the example shown in Figure 1 light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective. Such devices are referred to as "bottom emitters". Devices which emit through the cathode ("top emitters") may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent and/or using a transparent cathode material such as ITO.

Referring now to Figure Ib, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of Figure Ia are indicated by like reference numerals. As shown, the hole injection layer 106 and the electroluminescent layer 108 are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).

The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal or glass can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.

It is known to deposit material for organic light emitting diodes (OLEDs) using ink jet printing techniques. This is described in, for exampleY. Yang, "Review of Recent Progress on Polymer Electroluminescent Devices," SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, Jan., 1998; EP O 880 303; and "InIc- Jet Printing of Polymer Light-Emitting Devices", Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A.H. Mutsaers, Peter van de Weijer, Ivo GJ. Camps, Ton J.M. van den Biggelaar, Jan-Eric J.M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). InkJet techniques can be used to deposit materials for both small molecule and polymer LEDs, although these applications present their own particular problems, which are different to the problems encountered in conventional ink jet printing of images on paper or plastic, as will be explained more fully below. The term "ink" in the following disclosure is taken to mean a dissolved molecular electronic material, which can include semi conductor material, Light Emitting Polymers (LEP) or small molecules.

Use of an ink jet printer to deposit red, green and blue colour filters for an electroluminescent display is described in EP 1,219,98OA. A similar technique is described in US 2002/0105688. Figures 2a and 2b, which are taken from EP 1,219,980, show ink jet printing apparatus which maybe employed for this type of application. Figure 2a shows an ink jet printer 200 comprising a base 209 supporting first and second linear positioners 206, 208 for moving a substrate 212 and ink jet 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 the table and substrate 249, 212 to be rotated relative to the print head 222.

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. 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 ink jet 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 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 typically between 10μ,m and lOOjUm, and drop sizes are similar. The space or pitch between adjacent nozzle orifices is typically between 50μm and lOOμm.

Figure 3 a shows a conventional printing strategy in which print head 222 prints successive swathes 302, 304 in the Y-direction, stepping in the X-direction between each swathe. The technique illustrated in Figure 3b may be employed to produce a finer dot pitch. The print head is positioned at an angle Φ to the X-direction to reduce the dot pitch by a factor of cos Φ. Figure 3c shows two examples 306 and 308 of the distribution of drop volume ejected from nozzles 227 across the width of print head 222. Generally the size or 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 arise from small variations in nozzle heights. Figure 3c shows variations in drop volume, but in general similar variations are also observed in drop velocity. This problem is sometimes addressed in conventional ink jet printing by overlapping the swathes 302, 304.

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 graphics applications it is drop placement that is significant and volume variations of 5 to 10% are acceptable. However when constructing molecular electronic devices it is deposited "ink" volume which is important since this will determine the eventual film thickness which, for an OLED, impacts upon brightness and hence drive current and device lifetime. Thus it is desirable to achieve a volume variation of better than 2%, preferably better than 1%, across an entire OLED display.

To deposit a molecular electronic material a volatile solvent such as toluene or xylene is employed with 1-2% dissolved solvent material. This results in a relatively thin film in comparison with the initial "ink" volume. The drying time is dependent upon the solvent mix and the atmosphere above the substrate, but typically varies between a few seconds and some minutes. It is strongly preferable all the drops comprising material which are eventually to make up a pixel are deposited before drying begins. Solvents which may be used include alkylated benzenes, in particular toluene or xylene. Other solvents for inkjet printing are described in WO 00/59267, WO 01/16251 and WO 02/18513.

The pattern of material to be deposited is made up of pixels formed by depositing the electroluminescent material into a well (as described, for example, in EP 0 880 303) on a substrate. The wells are usually formed by photolithography of a photoresist as described in EP 0 862 156 to which reference may be made. In the case of OLEDs and other molecular electronic devices such as polymer FETs (Field Effect Transistors) these pixels and wells generally have regular shapes and a regular pattern, but in other cases the pixels can have irregular shapes. The substrate typically comprises a substantially non-absorbent material such as, for OLED displays, glass, clear plastics such as polyethylene or PET or other materials such as polyvinylidene fluoride or polyimide. In an OLED display the pixels are typically around 50μm wide and 40- 50μm long in a colour display or approximately three times this length in a monochrome display. The pixel spacing is typically 10-20μm. By contrast the print head is typically around lcm wide and a few centimetres long.

InkJet printing processes may also be used in the creation of thin film transistors (TFT). An example structure of such a TFT is shown in Figure 4. The TFT structure comprises a substrate 400 on which is deposited a gate electrode 402 followed by a dielectric layer 404 (for example, BCB (Benzocyclobutene); also inorganic materials such as SjO_x or SiN_x) and source and drain electrodes 406, 408. A layer of organic thin film transistor material 410, generally an organic semiconductor such as a polythiophene derivative is then deposited over the source and drain and dielectric layer.

InkJet printing processes are useable in at least the deposition of the organic semiconductor and dielectric materials.

However, despite advances in ink formulations, swathe joins are still a major issue for OLED devices printed and manufactured using ink jet printing processes. The swathe joins are not limited to the electroluminescent layer: swathe joins may also be present in other layers printed using an ink jet printing process. Swathe joins in displays cause the devices to exhibit 'stripes' of varying emission levels. Clearly, swathe joins are undesirable.

The source of swathe joins is not clear, but may be related to a combination of ink drying effects (possibly leading to physical film profile changes) and ink containments that may influence electrical properties of pixels. There is some evidence to suggest that these effects my also have influence on the volume of material contained within a pixel.

Figure 5 shows volume data measured across a section of a 14" active matrix display using a Zygo New View 5000 series white light interferometer . As can be seen, systematic volume variation is apparent and coincides with the swathes used to print the Light Emitting Polymer. Systematic variation is taken to mean a regular variation. Crucially, the variations in volume are not thought to be related to the inkjet head since similar variations were evident in other measurements. These artefacts may be unrelated to systematic hardware variations; instead, these artefacts may be related to physical processes such as systematic (i.e. regular) ink drying effects. Various techniques exist to overcome systematic hardware variations that cause unwanted variations in the volume of ink being deposited by an ink jet print head. One 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, and this strategy is described in EP 1,219,980, in which the print head makes multiple passes in the Y-direction (referring to Figure 3 a), depositing one drop onto a pixel on each pass. However this has the disadvantage that there is a relatively long period between successive drops landing on a single pixel, which can result in the undesirable artefacts. Furthermore because a zig-zag scanning strategy is adopted for the X-direction the intervals between successive drops landing is non-uniform, depending upon the position of a pixel in the X-direction. With the technique described in EP 1,219,980 a slow drying solvent must be employed to prevent drying between successive swathes, but a greater flexibility in solvent choice is preferable and for some applications relatively quick drying solvents such as toluene and xylene, for example with drop drying times of the order of one second, are useful. The technique of EP 1,219,980 is directed towards averaging out drop landing errors (thus reducing "banding") as much as averaging out drop volume variations.

Another technique for drop volume control is to calibrate one, or preferably a plurality of nozzles of a print head by measuring the volume of an ej ected drop whilst in flight for a range of print head drive signals. Data collected in this way may then be used to determine or adjust a print head drive signal in order to obtain a desired drop volume. Such a calibration procedure may be performed as part of a commissioning process for ink jet or droplet-based deposition apparatus, or a calibration procedure may be performed by the apparatus at switch on. However, a problem with such a calibration procedure is the difficulty in obtaining an accurate determination of the volume of an ejected droplet of dissolved material. Often, the calculations are based on a spherical drop being ejected from a nozzle. However, the shape of the ejected ink droplet is influenced by factors such as the driving signal.

Various techniques also exist in the field of ink jet printing to compensate for swathe or banding effects. US 6,830,306 describes a method by which banding in a graphic image, caused by slowly varying nozzle-to-nozzle variation, is reduced by introducing a "line correction factor" based on measurement of an optical density parameter for each nozzle on the print head. US 5,677,716, and references therein, describe the use of interlacing and print masking to reduce banding in graphic media that could also be applicable to display printing.

However, multiple passes and interlacing when using ink such as Light Emitting Polymers can cause unwanted drying effects.

It will be recognized from the foregoing discussion that improved methods of driving an ink jet print head, such that the non-uniformity in deposited ink volume caused by systematic ink drying effects is minimised, are desirable. It is also desirable to reduce the emissions non-uniformity in displays printed using ink jet type processes.

According to a first aspect of the present invention there is provided a method of reducing the non-uniformity of an organic electronic device deposited on a substrate by an inkjet print-head, the substrate forming part of a display, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the non-uniformity of the device being caused by drying effects of the ink on the substrate, the method comprising: calculating an improved nozzle driving signal; and driving the inkjet print-head with the improved nozzle driving signal, wherein the improved nozzle driving signal defines an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink that reduces the non-uniformity of the device on the substrate.

By intentionally introducing variation in the volume of ink deposited by an ink jet print head, systematic ink drying effects causing a variation in the volume of ink on the substrate may be mitigated. Advantageously, the improved nozzle driving signals are calculated, which enables any substrate to be printed in any formation whilst achieving a reduction in the non-uniformity of the volume of deposited ink There is no need to predict optimum nozzle driving signals beforehand.

In another aspect of the present invention there is provided a method of generating an improved image map for printing ink onto a substrate using an inkjet print-head, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the method comprising: retrieving adjustment data, the adjustment data defining at least an ink volume adjustment factor for each of the plurality of nozzles; calculating a plurality of improved nozzle driving signals using the adjustment data and an image map, the image map defining at least a volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink; and generating an improved image map using the calculated improved nozzle driving signals, wherein the improved image map defines at least an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink that reduces a non-uniformity in the volume of ink deposited on the substrate, the non-uniformity in the volume being caused by drying effects of the ink on the substrate.

The present invention also provides a method of printing a display on a substrate using an inkjet print-head, the display comprising a plurality of pixels on the substrate, each pixel being formed by a volume of ink deposited on the substrate by the inkjet print- head, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink onto the substrate in response to a nozzle driving signal, the method comprising: receiving an image map, the image map defining at least a volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of pixels; generating an improved image map according to the method of claim 6 or 7; driving the inkjet print-head using the improved image map in order to print a desired pattern of pixels, wherein the improved image map defines at least an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of pixels that reduces a non-uniformity in the volume of ink on the substrate resulting from drying effects of the ink.

The present invention also provides a method of reducing the visible artefacts of swathe joins in a process for manufacture of a molecular electronic device using an ink jet printing process, the method comprising: determining a waveform for driving a set of nozzles of an ink jet print head for use in said process to at least partially compensate for said swathe joins; and manufacturing said device using said determined waveform. The above described methods and apparatus may be implemented using processor control code such as conventional program code or code for setting up or controlling an ASIC (application specific integrated circuit) or FPGA (field programmable gate array). This processor control code may be provided on a carrier medium such as a hard or floppy disk, CD- or DVD-rom, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. As the skilled person will appreciate such code may be distributed between a plurality of coupled components in communication with one another, for example across a network.

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

Figures Ia and Ib show, respectively, cross sections through organic light emitting diode and a passive matrix OLED display;

Figures 2a and 2b show, respectively, an ink jet printer and an ink jet printer head;

Figures 3 a to 3 c show, respectively, conventional swathe printing, skewed printing for reduced dot pitch, and typical ink jet drop volume variations across a print head;

Figure 4 shows the construction of a thin film transistor (TFT);

Figure 5 shows volume data measured across a section of a 14" active matrix display;

Figure 6 shows substrates printed with and without a reduced nozzle-to-nozzle variation in the volume of ink deposited on the substrate;

Figure 7 shows a method of generating an improved image map for printing ink onto a substrate using an ink jet print head.

Broadly speaking we describe techniques to reduce the non-uniformity of the volume of ink deposited on a substrate, and therefore reducing the emission uniformity of a display, by intentionally introduced variation in the volume of ink deposited by an ink jet print head. Swathe effects, caused by fast-drying inks or pixel overspill effects on displays printed using an ink jet type process, may be mitigated by adjusting the drop volume.

Generally, the signals used to drive an ink jet print head are provided as an image map, which defines a pattern of ink to deposit on a substrate. The signals comprising the image map include, amongst others, nozzle driving signals to drive the nozzles and positioning data to position the print head over predefined areas of substrate. The nozzle driving signals comprise a voltage signal for each of the nozzles required to print at a particular location. The magnitude and duration of the signal define the velocity and volume of ink ejected from a particular nozzle in the print head.

We have disclosed a method of ink volume control that allows for precise correction of nozzle-to-nozzle volume variation to improve the. appearance of P-OLED devices in our co-pending application entitled "Droplet Volume Control". The relative variation of the ink being ejected by the ink jet print head has been reduced to within ±1% and given a demonstrable improvement in display emission uniformity in passive matrix devices. Figure 6 shows a display printed using (1350-04) the improved method and a display printed using a standard printing scheme (1350-01). As can been seen, emission non- uniformity is still apparent in the visible stripes hi the display printed using the improved method. It is believed that the emissions non-uniformity is caused, at least partly, by systematic ink drying effects.

A new method of reducing the non-uniformity of the volume of ink deposited on a substrate by an ink jet print head is therefore suggested. The new method intentionally introduces variations in the volume of ink deposited by an ink jet print head to compensate for systematic ink drying effects. Effectively, a known profile of deposited volume of ink across an ink jet print head is used to determine a complementary nozzle driving signal in order to compensate for a tilt in the deposited volume of ink.

Figure 7 shows a method of generating an improved image map for printing ink onto a substrate using an ink jet print head. Firstly, adjustment data is retrieved (702). The adjustment data defines at least an ink volume adjustment factor for each of the plurality of nozzles in the ink jet print head. The ink volume adjustment factor will depend on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles. Previously, this has been calculated as -5μm³ /Volt, although this value may change between different ink jet print heads. The adjustment data may be stored in a form of memory device, including, but not limited to, ROM, RAM and other memory storage devices.

Following retrieval of the adjustment data, improved nozzle driving signals are calculated (704) using the adjustment data and an image map, which defines the volume of ink to be deposited on the substrate by the ink jet print head in order to print a desired pattern of ink. The improved image map is then generated (706) using the calculated improved nozzle driving signals. The improved image map therefore defines improved nozzle driving signals that provide an improved volume of ink to be deposited on the substrate by the ink jet print head.

Driving an ink jet print head using the improved image map enables a display to be printed on a substrate. The display may exhibit a reduce non-uniformity in the volume of ink on the substrate resulting from systematic drying effects of the ink. A reduction in the non-uniformity of the volume of deposited ink would also improve the uniformity of the emission characteristics of the display.

Taking the display of Figure 5 as an example, an improved image map would be calculated to compensate for the tilting in the deposited volume of ink across the print head. The improved nozzle driving signals would be calculated to reduce the tilting by reducing the volume of ink deposited by nozzles associated with increased levels in the volume of ink on the substrate increasing the volume of ink deposited by nozzles associated with decreased levels in the volume of ink on the substrate. In this example, the tilt is generally linear although other non-linear tilts may occur. It is therefore preferable to measure the non-uniformity of the volume of ink deposited on the substrate in order to characterise the non-uniformity such that an improved nozzle driving signal and improved image map can be generated. The actual shape of the non- uniformity across the ink jet print head will be influenced by the composition of the ink. The above method is likely to have the greatest use with the LEP layer, where film thickness and uniformity has the most influence over display quality. However, the method may also be used in printing ink in other layers, too.

No doubt many other effective alternatives will occur to the skilled person and it will be understood the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

CLAIMS:

1. A method of reducing the non-uniformity of an organic electronic device deposited on a substrate by an inkjet print-head, the substrate forming part of a display, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the non-uniformity of the device being caused by drying effects of the ink on the substrate, the method comprising: calculating an improved nozzle driving signal; and driving the inkjet print-head with the improved nozzle driving signal, wherein the improved nozzle driving signal defines an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink that reduces the non-uniformity of the device on the substrate.

2. The method according to claim 1, wherein calculating an improved nozzle driving signal involves calculating the improved nozzle driving signals using adjustment data and a nozzle driving signal, the adjustment data defining at least an ink volume adjustment factor, and a nozzle driving signal defining at least a volume of ink to be deposited on the substrate by the nozzle for a desired pattern of ink on the substrate.

3. The method according to claim 2, wherein the volume adjustment factor depends on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles.

4. The method according to claim 2 or 3, wherein the nozzle driving signal is extracted from an image map, the image map defining at least a volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink.

5. The method according to any preceding claim, wherein the improved nozzle driving signal forms part of an improved image map, the improved image map defining at least an improved volume of ink to be deposited on the substrate by the inkjet print- head in order to print a desired pattern of ink.

6. A method of generating an improved image map for printing ink onto a substrate using an inkjet print-head, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink on the substrate in response to a nozzle driving signal, the method comprising: retrieving adjustment data, the adjustment data defining at least an ink volume adjustment factor for each of the plurality of nozzles; calculating a plurality of improved nozzle driving signals using the adjustment data and an image map, the image map defining at least a volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink; and generating an improved image map using the calculated improved nozzle driving signals, wherein the improved image map defines at least an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of ink that reduces a non-uniformity in the volume of ink deposited on the substrate, the non-uniformity in the volume being caused by drying effects of the ink on the substrate.

7. The method according to claim 6, wherein the ink volume adjustment factor depends on a relationship between a change in the nozzle driving signal and a change in the volume of ink deposited on the substrate by the nozzles.

8. The method according to any preceding claim, wherein the ink deposited on the substrate forms a plurality of pixels.

9. A method of printing a display on a substrate using an inkjet print-head, the display comprising a plurality of pixels on the substrate, each pixel being formed by a volume of ink deposited on the substrate by the inkjet print-head, the inkjet print-head comprising a plurality of nozzles, each nozzle depositing a volume of ink onto the substrate in response to a nozzle driving signal, the method comprising: receiving an image map, the image map defining at least a volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of pixels; generating an improved image map according to the method of claim 6 or 7; driving the inkjet print-head using the improved image map in order to print a desired pattern of pixels, wherein the improved image map defines at least an improved volume of ink to be deposited on the substrate by the inkjet print-head in order to print a desired pattern of pixels that reduces a non-uniformity in the volume of ink on the substrate resulting from drying effects of the ink.

10. The method according to any preceding claim, wherein the ink comprises a dissolved molecular electronic material.

11. The method according to claim 10, wherein the dissolved molecular electronic material comprises semiconductor material or small molecules.

12. The method according to claim 10, wherein the dissolved molecular electronic material comprises a light emitting polymer.

13. The method of claim 12, wherein the reduced non-uniformity in volume reduces the emission non-uniformity of the Light Emitting Polymer resulting from drying effects of the Light Emitting Polymer on the substrate.

14. The method according to any preceding claim, wherein the ink drying effects are systematic ink drying effects.

15. A display printed by an inkjet print-head using the method according to any preceding claim.

16. A method of reducing the visible artefacts of swathe j oins in a process for manufacture of a molecular electronic device using an inkjet printing process, the method comprising: determining a waveform for driving a set of nozzles of an ink jet print head for use in said process to at least partially compensate for said swathe joins; and manufacturing said device using said determined waveform.

17. The method according to claim 16, wherein said waveform defines an approximately linear profile of deposited volume per nozzle across said set of nozzles, and wherein said linear profile is tilted such that a greater volume of material is deposited by nozzles at one edge of a said swathe than at the other edge of said swathe.

18. The method according to claim 16 or 17, wherein the ink comprises a dissolved molecular electronic material.

19. The method according to claim 15, wherein the dissolved molecular electronic material comprises semiconductor material, small molecules or a light emitting polymer.