GB2439594A

GB2439594A - A method for forming a predetermined pattern of an organic semiconductor

Info

Publication number: GB2439594A
Application number: GB0611226A
Authority: GB
Inventors: Yajun Xia; Christopher Newsome; Richard Henry Friend
Original assignee: Cambridge University Technical Services Ltd CUTS; Seiko Epson Corp
Current assignee: Cambridge University Technical Services Ltd CUTS; Seiko Epson Corp
Priority date: 2006-06-07
Filing date: 2006-06-07
Publication date: 2008-01-02
Also published as: GB0611226D0

Abstract

An organic insulator layer 12 is patterned using inkjet deposited solvent droplets 11. The solvent dissolves the organic insulator material and then re-deposits the organic insulator material in a "coffee stain" pattern to define a crater in the insulator layer. The crater patterns may be used to form bank structures for an OLED display. Subsequently, droplets of organic semiconductor material either dissolved in a further solvent or dispersed in a dispersant, where the further solvent or dispersant does not affect the insulator layer, may be deposited by inkjet printing into the craters.

Description

A METHOD FOR FORMING A PREDETERMINED PATTERN OF

SEMICONDUCTOR

The present invention relates to a method for forming a predetermined pattern of semiconductor on a substrate. More particularly, the present invention relates to such a method in which the pattern of semiconductor has a sufficiently high resolution that it may be used in the production of a flat-panel display.

The method uses the technique of ink-jet printing without the need for a lithographic patterning step.

Flat-panel light emitting diode (LED) displays have become commonplace due to their relative ease of fabrication and high luminescent efficiency. In such displays, the emitting materials are based on inorganic semiconductors. Examples of these materials include aluminium gallium arsenide (red emitter) , aluminium gallium phosphide (green emitter) and indium gallium nitride (blue emitter) In recent years, LED displays based on organic semiconductors have been developed. The devices operate in much the same way as their inorganic counterparts (i.e. light emission in forward bias mode) . The attraction of organic materials in this area of technology is that they permit simpier and more flexible device fabrication methods to be adopted compared to those which are necessary when working with inorganic semiconductors. In particular, conjugated polymers show particular promise in this respect, as they can be easily synthesised to exhibit high solubilities in solvents.

Such a property opens numerous possibilities for device fabrication methods based on solution processes.

OLED displays are seen as an advantageous technology compared to liquid crystal displays (LCD5) . This is because the requirement for polarised light (thus decreased luminance) and associated viewing angle loss in LCDs is circumvented by the intrinsic emissive nature of OLEDs. A further advantage of OLED technology compared to LCD technology is that OLED5 may be formed on a flexible substrate so that there is potential to fabricate OLED displays that are capable of being wound and unwound from cylindrical form.

The OLEDs of an OLED-based flat-panel display have to be formed in a high density pattern in order that the display is capable of displaying bright high resolution images. Each OLED includes a discrete region of semiconductor and all of these regions making up the OLEDs forming the display must be fabricated as a dense pre-determined pattern.

So far, appropriate patterning of the semiconductor regions in OLED-based flat-panel displays has been carried out by inkjet printing the semiconductor into pre-fabricated structures produced by the use of conventional photolithographic techniques.

Photolithography is a highly mature technology capable of fabricating sub-micron resolution patterns and is widely used in the mass production of microelectronic components. Photolithography usually requires a step in which a photomask has to be accurately aligned to previously fabricated fine structures. Typically, such alignment has to be achieved within a tolerance of about 0.1 tm otherwise the resulting electronic devices will not function because their various components will not be correctly aligned with one another. This need to achieve such a precise alignment is technically demanding as a consequence of which alignment steps represent a significant cost in the context of the manufacturing process as a whole.

Using the technique of photolithography in the manufacture of flat panel displays has a number of additional disadvantages. Firstly, the alignment between the different photolithographic processes is difficult to achieve over large areas because large substrates are prone to flexing. Secondly, as multiple photolithographic steps are involved, it is difficult to prevent contamination at critical interfaces. This problem is heightened in the production of OLEDs which require the fabrication of multi-layer structures. This is because most organic polymers which are semiconducting are soluble in, or sensitive to, aqueous solutions and organic solvents that are used to process conventional photoresists. Finally, photolithography requires the use of several sub-processes such as photoresist application, baking, exposure to UV light, development, subsequent etching and further deposition steps. The use of more than one photolithographic process multiplies the number of these sub-processes considerably. This is both time consuming and inefficient from the manufacturers' point of view and significantly increases the manufacturing cost of flat panel displays.

Depositing solutions by ink-jet printing to create fine structures has many advantages over conventional photolithography. The ink-jet process can directly pattern a substrate with a variety of materials such as solution-processable semiconductors without the need for an etching step. A combination of photolithography and ink-jet printing is disclosed in EP-A-0989778 which teaches forming bank structures using photolithography and then using these bank structures to confine subsequently deposited ink-jet printed droplets containing dissolved semiconductor. Whilst this technique is capable of forming a high resolution predetermined pattern of semiconductor, it nevertheless still suffers from the principal disadvantages of photolithography previously discussed.

Kawase et al in Adv. Mater, 2001, 13, No. 21, pages 1601-1605 describe forming via holes through a polymer film by ink-jet printing droplets of a solvent in a predetermined pattern which is capable of locally dissolving the polymer. It was found that when the solvent droplet containing the dissolved polymer dries, the insulating material is preferentially re-deposited at the contact line (that is a generally circular line which corresponds to the circumference of the droplet where it lies in contact with the polymer film) of the solvent droplet. This results in the film thickness decreasing at a location corresponding to the centre of the droplet together with the formation of a generally circular ridge located above a position corresponding to the droplet's contact line with the film. Thus a crater and associated well are formed. When solvent droplets are repeatedly deposited at the same location, the well becomes deeper resulting in the formation of a via hole through the polymer film.

The mechanism responsible for forming a via hole by means of repeated deposition of solvent droplets at the same location is fundamentally different from that of a conventional wet etching step. In the latter, etchant and rinse liquid respectively etch and then wash away the target material. Thus unwanted material dissolves in the etchant and is removed by the rinse liquid. In contrast, material is not removed from the substrate by depositing the solvent droplets. Rather the material is transferred locally from the centre region of the film corresponding to where the droplet lands to the droplet's edge. This etching mechanism can be understood in terms of a micro-fluid-flow in a sessile drop. This micro-fluid-flow has been used to explain the formation of a ring-shaped "coffee stain" from a droplet of solution deposited on a solid surface. The physics of this are described in detail by Deegan et al in Nature, Vol 389, October 1997, pages 827-829. By using this technique, via holes can be formed by depositing solvent droplets by means of ink-jet printing. The droplets dissolve and re-distribute the polymer with the underlying substrate acting as an etch-stopping layer.

In the specification which follows, the terms

"insoluble" and "soluble" in connection with a solid and a solvent are respectively defined to mean that no more than 0.5 grams (preferably no more than 0.1 grams) of the solid can be dissolved in 1 litre of the solvent at room temperature (20 C); and greater than S grams of the solid can be dissolved in 1 litre of the solvent at room temperature (20 C) A first aim of the present invention is to make use of the phenomenon of the coffee stain effect to form bank structures in a predetermined pattern over a large area substrate followed by deposition of a solution of semiconductor by ink-jet printing droplets within each of the bank structures so that the semiconductor adopts a predetermined pattern by virtue of it being confined within the banks.

A second aim of the present invention is to use such a technique in the fabrication of an OLED array.

According to a first aspect, the present invention provides a method for forming a predetermined pattern of semiconductor on a substrate comprising the steps of: (i) forming a film of an organic insulator on the substrate, (ii) depositing droplets (A) of a first solvent in which the organic insulator is soluble by ink-jet printing, the droplets being deposited on the film at locations corresponding to the predetermined pattern to form a crater from the organic insulator at each of the locations, and (iii) depositing droplets (B) comprising a semiconductor dissolved in a second solvent or dispersed in a dispersant in the craters by ink-jet printing using the same deposition pattern as used in step (ii) , the organic insulator being insoluble in the second solvent or the dispersant, the semiconductor being confined by the well of each crater.

Such a patterning method provided by the first aspect of the present invention permits the formation of a relatively high resolution pattern of semiconductor using the relatively simple and cheap technique of ink-jet printing. This method in turn enables the fabrication of complex electronic structures and devices such as large area OLED arrays at reduced cost.

It will be understood that the craters are formed from the organic insulator essentially by the latter's redistribution by means of the coffee stain effect. The craters which are formed in this way each includes a well surrounded by a roughly circular bank or wall into which the semiconductor-containing droplets can be deposited as a solution in a second solvent or as a dispersion in a dispersant also by ink-jet printing using the same deposition pattern as was used to form the craters. It is important that the organic insulator is insoluble in the second solvent or the dispersant otherwise the walls of the craters would be disrupted on contact with it and the patterning effect imposed on the ink-jet printed semiconductor by the crater walls would be lost.

Preferably the film formed in step (i) has a thickness of 40-2000 nm. If the film is too thin, then it will contain insufficient material to form craters which have walls capable of confining the later ink-jet deposited semiconductor-containing droplets (B) . On the other hand, if the film is too thick, then this could prevent interaction between the semiconductor and any previously patterned microelectronic structures which pre-exist as part of the substrate. In this context, it should be understood that the term "substrate' used throughout this specification refers not only to simple substrates such as glass, a silicon wafer or a plastics material such as polyethylenenaphthalate (PEN) or polyethyleneterephthalate (PET), but also extends to encompass any material on which the film of the organic insulator is formed including surfaces already coated and/or patterned either internally or externally with areas of conductor or semiconductor which are to act as components of microelectronic devices such as for instance might be included in a flat-panel display.

Preferably, both the droplets (A) and the droplets (B) have an average diameter of less than 100 pm.

Evidently there is a minimum size droplet which can be ejected from an ink-jet recording head. This is currently about 2 p1. As the droplet size increases, the resolution of the pattern decreases.

Preferably, a sufficient number of droplets (A) of the first solvent are deposited at each location in step (ii) so that either the substrate is exposed at the bottom of the well of each crater or the thickness of the organic insulator film is reduced to the extent that electrical transport is easily possible through it. In this preferred embodiment, all or substantially all of the organic insulator underneath the central part of the droplets (A) is carried to the droplets' sides due to the coffee stain effect where it forms the wall of the crater. In this embodiment it is important to select a solvent as the first solvent which does not degrade or interfere with the underlying substrate. This embodiment is particularly advantageous because the semiconductor will then come into direct contact, or at least electrical contact, with the underlying substrate where there may be previously patterned conductive and/or semiconductive areas with which the semiconductor may interact in use. For instance, the upper surface of the substrate may include a pattern of anodes in correspondence with the predetermined pattern of the semiconductor. Alternatively, the upper surface of the substrate may be formed from a layer of a conducting hole injection material. As a further alternative, the substrate may comprise an uppermost layer of a semiconducting hole-injecting material supported in turn by a layer of a conducting hole-injecting material.

Preferably, the craters have an average diameter of 40-200 urn. The minimum average diameter is controlled by the need to deposit the droplets (B) within the craters.

If the diameter of the craters is too small, it may not be possible to deposit the droplets (B) with sufficient accuracy to land within the crater well. On the other hand, if the craters have a too large diameter, then this would in turn reduce the resolution of the resulting predetermined pattern of semiconductor.

Preferably the average of the distance from the centre of a crater to the centre of each adjacent crater is 100-500 JLm. The craters are preferably patterned with a relatively high density on the substrate such that the resulting patterned semiconductor may be used as an intermediate for forming an OLED array.

Preferably, the substrate is heated during the deposition step (ii) to promote the evaporation of the first solvent. Whilst complete drying of the solvent is not essential, it is preferred that at least the majority of the solvent is evaporated in order that the walls of the craters become rigid and so have sufficient structural integrity to contain the semiconductor-containing droplets.

According to a second aspect, the present invention provides a method for forming an OLED array comprising: (1) forming a predetermined pattern of a semiconductor in accordance with the above first aspect of the present invention in which the substrate incorporates a pre-patterned array of anodes and driving circuitry in correspondence with the predetermined pattern of craters; and (ii) depositing a cathode layer over the pattern of craters with patterned semiconductor regions on the substrate.

It is preferred in the second aspect that the surface area of the OLED array is at least 3 cm x 3 cm.

More preferably, the OLED array can be a flat panel display, such as for instance a television, whose screen has a diagonal length of up to 100 cm or even more.

The present invention will now be described in greater detail making reference to the accompanying drawings in which like numerals are used throughout, and in which: Figure 1 schematically illustrates in cross-section the formation of a crater structure by ink-jet printing a solvent droplet onto a film of an organic insulator, Figure 2 is a graph which illustrates the cross-sectional profile of a crater structure formed by the method illustrated in Figure 1, Figure 3 is a schematic plan view of a substrate which has been cratered in a predetermined fashion corresponding to the product of step (iii) according to the first aspect of the present invention, and Figure 4 schematically illustrates in cross-section a view through the line X -X of Figure 3 and after deposition of a top layer.

In order to appreciate the ingenuity of the present invention, it is helpful to have an understanding of the "coffee stain" effect. In this regard, attention is drawn to Figure 1, part (a) of which illustrates a droplet of solvent 10 descending onto a film of an organic insulator 12 which is soluble in the solvent.

The organic insulator film 12 is in turn supported by a substrate 14 the nature of which is unimportant in this explanation.

After the impact of the solvent droplet onto the film, the droplet forms a sessile drop 11 which dissolves the organic insulator at their mutual interface as illustrated in Figure 1, part (b) . The drying rate of the droplet varies across its diameter. The air at the perimeter of the droplet contains a relatively low concentration of solvent in contrast to the air above the centre of the droplet where the concentration is much higher. This concentration gradient results in the solvent vapour pressure varying across the diameter of the drop and hence to the evaporation rate of the solvent rate being greatest at the droplet's edge when viewed from above. This differential evaporation rate results in solvent being transported from the centre of the solvent droplet to the outside edge due to the enhanced rate of evaporation there.

Consequently, the organic insulator which has dissolved in the solvent thickens at the edge of the sessile drop due to the enhanced rate of evaporation of the solvent there, resulting in the cross-sectional profile illustrated in Figure 1, part (c) . This cross-sectional profile illustrates that a roughly circular crater generally illustrated by 15 is formed which includes a central well and a generally circular wall 16 surrounding it. Such a crater structure is the consequence of the so-called coffee stain effect. A more detailed explanation of this effect is provided in the Adv. Mater. and Nature papers previously acknowledged.

Figure 2 illustrates the height profile of a crater formed when three droplets of ethanol having an average diameter of about 30 l.lm are successively deposited by ink-jet printing at the same location of a 250 nm thick film formed from poly(4-vinylphenol) (PVP) . The thickness profile illustrates that after the droplets have dried, most of the polymer from the central region where the droplets landed is redistributed around the perimeter of a central well surrounded by a generally circular wall about 500 nm high leaving little polymer in the central region. The resulting 3D structure is referred to herein as being a crater.

If the layer of the organic insulator coated on the substrate is relatively thick compared to the droplet diameter then a single droplet of the solvent can result in a thin wetting layer being present in the central well region of the crater. In this case, not all of the insulator is transferred to the crater edge. However, repeated deposition of solvent droplets onto the same position eventually results in the removal of all of the insulator from the central region and its re-deposition as the crater walls.

Having thus discussed the coffee stain effect, the utility of this in the context of the present invention will now be described with reference to Figures 3 and 4.

Figure 3 is a schematic plan view of a pattern of discrete areas of deposited organic semiconductor 30 confined within craters (unseen in Figure 3) formed from organic insulator 32. This is more clearly illustrated in Figure 4 which is a schematic cross-sectional view through the line X -X of Figure 3 after a top layer 48 has been deposited over the pattern of craters with patterned semiconductor regions.

The structures illustrated in Figures 3 and 4 are formed in accordance with the present invention as follows. Firstly a substrate generally indicated by 40 is coated with a film of an organic insulator. As previously mentioned, the substrate 40 might be a simple unpatterned substrate formed for instance from glass or plastics such as polyethyleneterephthalate or polyethylenenaphthalate. Alternatively, it may be a somewhat complex laminate already including pre-patterned elements intended to act as active components of a final microelectronic device. For instance, the substrate 40 may be formed as a laminate comprising firstly a transparent indium tin oxide layer 42 capable of acting as an anode. This may be spin-coated with an injection layer 44 having a thickness of 10 to 100 nm which may suitably be formed from poly(ethylene dioxythiophene) doped with either poly(styrene sulphonic acid) (PEDOT:PSS) or polyanaline. A semiconducting polymer layer 46 of thickness 10 to 100 nm formed for instance from polyarylamine is then spin-coated on the injection layer.

A film of an organic insulator is then formed on the substrate 40. The organic insulator is preferably an organic polymer such as for instance poly(4-vinylphenol) PVP, polyvinyl alcohol (PVA), polystyrene, poly(methyl- pentene) , polyisoprene or a polyester such as polymethyl-methacrylate. The film is typically formed by spin coating a suitable solution or dispersion of the organic insulator. The film preferably has a thickness of 40- 2000 nm, preferably 50-1000 nm, most preferably 100-300 nm.

Droplets (A) of a (first) solvent in which the organic insulator is soluble are then deposited on the film by ink-jet printing at locations corresponding to a predetermined pattern. One or more droplets are deposited at each location. This deposition forms craters 35 by redistributing the organic insulator at each of the locations by virtue of the coffee stain effect previously described. The droplets (A) preferably have an average diameter of less than 100 pm, more preferably an average diameter of 10-80 jim, and most preferably an average diameter of 20-60 jim. Each crater includes an approximately circular wall 36. The height of this wall from the bottom of the crater's well is about 100-1,000 nm, preferably 250-750 nm, more preferably 350-650 nm. The craters formed in this way typically have an average diameter, corresponding to the diameter across opposite points of the wall top, of 40- jim, more preferably 50150 jim and most preferably 60-jim.

The choice of solvent forming the droplets (A) naturally depends upon the choice of organic insulator.

The organic insulator must be soluble in the solvent.

The solvent may for instance be an organic solvent such as an alcohol (e.g. 2-propanol or ethanol) , an aldehyde, a ketone or an ether. The precise chemistry of the solvent is not particularly limited as long as it is capable of inducing the coffee stain effect when deposited as droplets on the organic insulator film.

If the substrate 40 already includes certain pre-patterned structures within it, then preferably sufficient droplets are deposited at each location so that the surface of the substrate 40 is exposed at the bottom of each crater. In this case, the predetermined pattern of the craters should correspond and align with the pre-patterned structures included in the substrate 40. This can for instance be achieved by the use of alignment markers on the substrate, a technique well known to those skilled in the art.

If the patterned organic semiconductor formed in accordance with the present invention is used as an intermediate in the production of an OLED array, then the craters are patterned as densely as possible as this results in improved image quality, particularly brightness and contrast. Typically, the average of the distance from the centre of a crater to the centre of each adjacent crater is 100-500 tm, more preferably 130- 400 trn, most preferably 150-300 tm.

The substrate may be heated during the deposition of the droplets (A) to for instance 30-90 C to promote evaporation of the first solvent.

Then using the same deposition pattern as was used to deposit the droplets (A), droplets (B) comprising a semiconductor dissolved in a second solvent or dispersed in a dispersant are then deposited in the craters again by ink-jet printing. It is essential that the organic insulator is insoluble in the second solvent or dispersant otherwise the droplets (B) will disrupt the crater structures whereby the patterning will be lost.

Instead, because the organic insulator is insoluble in the second solvent or dispersant, the droplets (B) are confined within the wells of the craters resulting in a pattern of discrete areas 30 of the organic semiconductor corresponding to the pattern of the craters. For instance, if the organic insulator is formed from PVP, then the first solvent used to form the craters may be an alcohol such as ethanol and the second solvent used to form the droplets (B) should be non-polar. Such solvents include for instance an unsubstituted or alkyl substituted thiophene, a cyclic hydrocarbon such as cyclohexane or a compound of formula: R2 wherein R', R2, R3 and R4 independently represent hydrogen or C.6 alkyl. Preferably the organic solvent (A) is benzene, toluene or o-, m-or p-xylene.

The droplets (B) preferably have an average diameter of less than 100 Itm, more preferably an average diameter of 10-80 tm, and most preferably an average diameter of 20-60 tm.

Appropriate alignment of the droplets (B) with the wells of the craters formed from the organic insulator can be assured by using the same printer and deposition pattern to deposit both the droplets (A) and the droplets (B) . In fact the substrate need not be removed at all from the printer during the process. If the substrate is removed however, then it is necessary that deposition of the droplets (B) within the pre-formed craters 35 is assured by for instance using alignment markers on the substrate.

The semiconductor which is deposited by ink-jet printing may be either an organic semiconductor or an inorganic semiconductor. Preferably the semiconductor is an organic semiconductor such as a polyfluorene, a poly(phenylene vinylene), a polyarylamine, a polythiophene, (e.g. 3-hexyithiophene), a polyphenylene (e.g. a poly(paraphenylene vinylene)) or a copolymer of any combination thereof. Each of these materials is soluble in non-polar organic solvents such as benzene optionally substituted by up to four alkyl groups, unsubstituted or alkyl substituted thiophenes or cyclic hydrocarbons such as cyclohexane. Examples of such solvents include benzene, o-, m-and p-xylene, toluene, trimethyl benzene, tetramethyl benzene and chloro benzene. Examples of inorganic semiconductors include nanoparticles consististing of Il-VI materials, such as Cadmium Selenide (CdSe) and Cadmium Telluride (CdTe) These may be dissolved or dispersed in solvents/dispersants such as chloroform. If the semiconductor is provided as a dispersion, then the dispersion should not contain particles whose longest dimension is more than 10 jim, preferably not more than 1 jim. This is because larger particles can block the nozzle of the ink-jet head which is self-evidently disadvantageous.

After deposition, the second solvent or dispersant is evaporated away to complete the deposition of the semiconductor 30 within the well of each crater as illustrated in Figure 4. This can be carried out at room temperature or more preferably at 30-100 C by, for instance, heating in an oven, on a hotplate or by vacuum drying. Higher temperatures may be used, but the temperature used in such a drying step is limited by the thermal sensitivity of any preceding layer or structure of the device.

If the patterned semiconductor 30 is used as a component of an OLED, then, in a final step, a cathode layer 48 is deposited over the cratered structure. A typical cathode comprises a film of calcium having a thickness of 20-50 nm deposited by evaporation followed by a film of aluminium having a thickness of about 200 nm which protects the calcium from reacting with the atmosphere.

It will be understood by those skilled in the art that the structure, apart of which is illustrated in plan view in Figure 3, can be used to form an OLED array provided that appropriate driving thin film transistors are pre-patterned in the substrate 40 beneath each of the craters where the organic semiconductor 30 is deposited.

In this case, light from each LED is emitted through the transparent indium tin oxide layer 42, the calcium/aluminium cathode acting as a mirror. The layer 44 acts an injection layer facilitating hole transfer into the semiconducting polymer layer 46 and the discrete areas of the light emissive semiconductor 30.

As alternatives to the above described sequence of fabrication steps, the organic insulator film may be coated directly on a layer of a conducting hole injection material. Such injection materials are typically insoluble in alcohols. The craters may then be formed without disturbing the uniformity of the injection layer.

Alternatively, the organic insulator film may be coated on a substrate which comprises an uppermost layer of a semiconducting hole-injecting material supported in turn by a layer of a conducting hole-injecting material.

It is also possible to deposit the organic insulator film directly on the indium tin oxide layer 42, form the predetermined pattern of craters therein, and then ink-jet print the injection material and semiconductor(s) successively into the wells of the craters.

A group of three adjacent areas of semiconductor illustrated as 33 in Figure 3 may ultimately form a pixel of a flat-panel display. The side of the imaginary triangle illustrated as 33 would typically be about 200 j.xrn in length. In this case, different semiconductors will be deposited in the three craters so that a first emits red light, a second emits green light and the third emits blue light. This can be achieved if for instance one of the craters has deposited therein the red emitter poly(2-methoxy-5-(3-,7-dimethyl-octyloxy)-l,4-phenylene vinylene) , a second has deposited therein the green emitter poly(p-phenylene vinylene), and the third has deposited therein the blue emitter poly(2-(6-cyano-6-methyl-heptyloxy)-l,4-phenylene). This group of three craters would then form a pixel in the final display panel capable of emitting light of practically any colour by suitable activation of the LEDs corresponding to the three different semiconductors.

It will be understood by those skilled in the art that the illustration of the group 33 of semiconductor areas forming a pixel is one of many possibilities. Not every pixel needs to have the same number of red, green and blue sub-pixels. For instance, one pixel could contain four red LED5, two green LEDs and one blue LED.

The actual number of each type of LED to be used depends upon the relative brightness of each LED. As is also well known, the colour in each pixel can be varied by altering the driving current to each LED. Typically, 8-bit values are assigned to each LED of a pixel to give a specified colour for that pixel in the form of [0, 0, 0] (black) up to [255, 255, 255] (white) . Varying each of the values between 0 and 255 alters the colour, tone and brightness of the pixel.

As an alternative to the fabrication of LEDs so far discussed, the present invention may also be applied to the patterning of cells for a polymeric photovoltaic device which converts incident light into electric current. Such devices incorporate patterns of semiconductors much the same as those previously described.

An example for forming a predetermined pattern of semiconductor on a substrate will now be provided.

Example

A substrate incorporating a pre-pattern of anodes was cleaned by sonication in acetone for 15 minutes. It was then subjected to sonication in isopropanol for a further period of 15 minutes followed by blow-drying using filtered nitrogen gas. The resulting dried substrate was then exposed to a 250 W oxygen plasma stream for 5 minutes to increase the hydrophilicity of its surface.

An aqueous dispersion of PEDOT:PSS (0.5 wt. % solid content) was then spin-coated on the substrate surface to form a PEDOT:PSS layer having a thickness of 50 nm. The resulting layer was then dried at 80 C for 10 minutes. A solution of a polyarylamine semiconductor dissolved in a toluene solution was then spin-coated to provide a film having a thickness of 50 nm. The solution had a concentration of 10 mg of polyarylamine per ml of toluene. The resulting semiconductor film was then dried at 60 C for 10 minutes.

A 300 nm thick film of PVP was then formed by spin-coating a solution of 60 mg/mi of PVP in isopropanol.

The molecular weight of PVP was around 20,000. The resulting PVP film was then dried at 60 C for 10 minutes.

A pattern of craters was then formed in the PVP film by ink-jet printing ethanol droplets from a nozzle with a diameter of 30 JLm, the pattern corresponding to the pre-pattern of anodes in the substrate. The resulting craters had an average diameter of 115 tm as measured across their rims. During the deposition of the ethanol droplets, the substrate was heated to 40 C which helped to prevent spreading of the ethanol droplets across the PVP surface.

Using the same deposition pattern, droplets of a xylene solution containing poly(2-methoxy-5-(3-,7-dimethyl-octyloxy) -1,4-phenylene vinylene) at a concentration of 15 mg/mi were then deposited in the craters by ink-jet printing followed by heating at 80 C for 10 minutes to remove the xylene solvent. This resulted in discrete areas of the semiconductor being confined by and within the well of each crater corresponding to the predetermined pattern.

The resulting structure was then loaded into an evaporator which was used to evaporate a 30 nm thick film of calcium followed by a 200 nm thick film of aluminium at a base pressure of 0.1 Pa. These layers act as a cathode.

The resulting structure could be used as a red-emissive flat panel display.

The method provided by the present invention may be used in the commercial manufacture of flat panel displays. The key advantage of this method is that the process and associated costs of photolithography are replaced by a direct patterning method with minimal material consumption. The process is also compatible with large scale devices for which conventional photolithography is unsatisfactory due to the need to ensure precise alignment with previously fabricated device architectures.

Claims

Claims: 1. A method for forming a predetermined pattern of

semiconductor on a substrate comprising the steps of: (i) forming a film of an organic insulator on the substrate, (ii) depositing droplets (A) of a first solvent in which the organic insulator is soluble by ink-jet printing, the droplets being deposited on the film at locations corresponding to the predetermined pattern to form a crater from the organic insulator at each of the locations, and (iii) depositing droplets (B) comprising a semiconductor dissolved in a second solvent or dispersed in a dispersant in the craters by ink-jet printing using the same deposition pattern as used in step (ii) , the organic insulator being insoluble in the second solvent or the dispersant, the semiconductor being confined by the well of each crater.

2. A method according to Claim 1, wherein the film formed in step (I) has a thickness of 40-2000 nm.

3. A method according to Claim 1 or Claim 2, wherein both the droplets (A) and the droplets (B) have an average diameter of less than 100 m.

4. A method according to any preceding Claim, comprising depositing sufficient droplets (A) of the first solvent at each location in step (ii) to expose the substrate at the bottom of each crater.

5. A method according to any preceding Claim, wherein the craters have an average diameter of 40-200 pm.

6. A method according to any preceding Claim, wherein the average of the distance from the centre of a crater to the centre of each adjacent crater is 100-500 pm.

7. A method according to any preceding Claim, further comprising the step of heating the substrate during or subsequent to step (ii) to 30-90 C to promote evaporation of the first solvent.

8. A method according to any preceding Claim, wherein the upper surface of the substrate includes a pattern of anodes in correspondence with the predetermined pattern of the semiconductor.

9. A method according to any of Claims 1-7, wherein the upper surface of the substrate is formed from a layer of a conducting hole injection material.

10. A method according to any of Claims 1-8, wherein the substrate comprises an uppermost layer of a semiconducting hole-injecting material supported in turn by a layer of a conducting hole-injecting material.

11. A method for forming an OLED array comprising: (1) forming a predetermined pattern of a semiconductor according to any preceding claim, in which the substrate incorporates a pre-patterned array of anodes and driving circuitry in correspondence with the predetermined pattern of craters; and (ii) depositing a cathode layer over the pattern of craters with patterned semiconductor regions.

12. A method according to Claim 11, wherein the surface area of the OLED array is at least 3 cm x 3 cm.