GB2523986A

GB2523986A - Method and device

Info

Publication number: GB2523986A
Application number: GB1322585.9A
Authority: GB
Inventors: Simon James Goddard
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2013-12-19
Filing date: 2013-12-19
Publication date: 2015-09-16
Also published as: GB201322585D0

Abstract

A method of forming an organic light-emitting electrochemical cell 100 comprising forming a light-emitting layer formulation 103 over an anode 101 and a cathode 105, wherein the light-emitting layer formulation 103 comprises a semiconducting polymer, an electrolyte, such as polyethylene oxide and a salt, and preferably wherein the semiconducting polymer has a polystyrene-equivalent weight-average molecular weight of less than 300,000 as measured by gel permeation chromatography.

Description

Method and Device

Background

Electronic devices comprising active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes, organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic trailsistors and memory anay devices. Devices comprising organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coatillg.

An organic light-emitting electrochemical cell (LEC) may have a substrate carrying an anode, a cathode and an organic light-emitting layer between the mode and cathode comprising a light-emitting material, a salt prnviding mobile ions and an electrolyte, for example a polymer electrolyte ("polyelectrolyte"). LECs are disclosed in, for example, During operation of the device, holes are injected into the device through the anode and electrons are injected through the cathode. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of the light-emitting material combine in the light-emitting layer to form an exciton that releases its energy as light. The cations and anions of the salt may respectively p-and n-dope the light-emitting material, which may provide for a low drive voltage.

Suitable light-emitting materials include small molecule, polymeric and dendrimeric materials. Suitable light-emitting polymers for use in the light-emitting layer include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as polyfluorenes.

US 5900327 discloses a LEC comprising the polymer BDOH-PF:

BDQH-PF

The ethylene oxide side groups of BDOH-PF are said to improve compatibility with the ion-conducting polymer poly(ethylene oxide) and increase solubility of the polymer in common organic solvents.

The light-emitting layer of a LEC may be formed by depositing an ink containing the materials of the light-emitting layer and a solvent followed by evaporation of the solvent.

WO 2011/032010 discloses luminescent ink formulations containing a plurality of salts providing at least two cations or two anions.

WO 2003/053707 discloses screen-printable light-emitting polymer based inks containing a non-electrolLiminescent polymer with a molecular weight between about 300,000 and 20,000,000 to provide a viscosity of above about 50 centipoises. Use of polyethylene oxide (PEO) is described as an acceptable non-electroluminescent polymer.

it is an object of the invention to provide LECs having uniform light-emitting films.

it is a further object of the invention to provide LECs having smooth light-emitting films.

it is a further object of the invention to provide an improved process for printing of LECs.

Summary of the Invention

in a first aspect the invention provides a method of forming an organic light-emitting electrocheniical cell, the method comprising the steps of forming a light-emitting layer by roll printing a formulation over one of an anode and a cathode, and providing the other of an anode and a cathode over the light-emitting layer, wherein the formulation comprises a semiconducting polymer, an electrolyte and a salt and wherein the semiconducting polymer has a polystyrene-equivalent weight-average molecular weight measured by gel permeation chromatography of less than 300,000.

In a second aspect the invention provides an organic light-emitting electrocheniical cell obtainable by a method according to the first aspect.

Description of the Drawings

The invention will now he described in more detail with reference to the drawings in which: Figure 1 illustrates an orgallic LEC according to an embodiment of the invention; Figure 2A illustrates a partial LEC structure of an LEC according to an embodiment of the invention wherein the anode is patterned to form a plurality of individual pixel anodes and a light-eithtting film is formed over each pixel anode; Figure 2B illustrates a partial LEC structure of an LEC according to an embodiment of the invention wherein the anode is patterned to form a plurality of individual pixel anodes and the light-emitting layer is formed from a plurality of light-emitting films wherein each light-emittillg film extends over a plurality of pixel allodes; Figure 3A is a schematic illustration of the gravure printillg process: Figure 3B is a schematic illustration of a gravure cylinder; Figure 4 is a graph of roughness vs. solid content for films formed from a formulation containing a serniconducting polymer having a Mw of 147,000 and a formulation containing a serniconducting polymer having a Mw of 400,000; Figure 5 is a graph of thickness vs. solid content and thiclaiess vs pnllt speed for films formed from a formulatioll containing a semiconducfing polymer having a Mw of 147,000 and a formulation containing a senlicondLicting polymer having a Mw of 400,000; Figure 6A is a photograph of a pixel of a comparative device; Figure ÔB is a photograph of a pixel of a device according to an embodiment of the invention: Figure 7 is a plot of maximum luminance at turn on for a device according to an embodiment of the invention and a comparative device; Figure 8 is a plot of luminance vs. time for a device according to an embodiment of the invention and a comparative device; Figure 9 is a plot of voltage vs. time for a device according to an embodiment of the invention and a comparative device; and Figure 10 is a plot of maximum cd/A efficiencies for a device according to an embodiment of the invention and a comparative device.

Detailed Description of the Invention

Figure 1 illustrates an organic LEC 100 according to an embodiment of the invention.

The LEC 100 has an anode 101, for example ITO, a metal or a conductive oTganic material such as a polythiophene, for injection of positive charge carriers, a cathode 105 for injection of negative charge carriers and a light-emitting layer 103 between the anode and the cathode. Further layers maybe provided between the anode and the cathode, for example a hole-injection layer maybe provided between the anode i 0 I and the light-emitting layer 103. The cell is supported on a substrate 107. If light is emitted through the anode then the substrate i 07 is a transparent material, for example glass or a transparent plastic. If light is emitted through the cathode 105 then the substrate 107 may he an opaque or transparent material.

The light-emitting ayer contains at least one semiconducting polymer, at least one electrolyte and at least one sail. The light-emitting layer may contain one, two or more of each of the light-emitting material, electrolyte and salt.

The light-emitting layer preferably has a thickness in the range of about 200 -2000 nm, optionally 500-1200 nm.

In operation, light maybe emitted directly from the semiconducting polymer in which case the semiconducting polymer is a light-emitting polymer, or the semiconducting polymer maybe a host material used in combination with a light-emitting dopant provided in the light-emitting layer. The light-emitting dopant maybe a fluorescent dopant that accepts singlet excitons from the host polymer to produce fiLlorescence by radiative decay of singlet excitons, or a phosphoresceilt dopant that accepts triplet excitons from the host polymer, and optionally singlet excitons, from the light-emittifig material and emits phosphorescent light by radiative decay of triplet excitons.

If a light-emitting dopailt is present then all light may be emitted by the dopant, or both the semwollducting polymer aild the light-emitting dopant may emit light. More than one light-emitting dopant may be present. Light-emission from multiple light-emittillg materials (including emission, if any, from the polymer md any dopants) may combine to produce white light.

The light-emitting layer may have a thickness in the range of about tOO nm -2 microns, preferahly 100 nm -1 micron; preferably 100 nm -750 nm, preferably 100-500 nm The light-emitting layer 103 illustrated in Figure 1 is a film that extends across the whole of the surface area of the anode 101 and cathode 105, however in other embodiments the light-emitting layer 103 of organic LEC 100 may comprise two or more separate light-emitting films. The dimensions of the light-emitting film or films of a light-emitting layer may be selected according to the required device architecture, for example as described with reference to Figures 2A and 2B. Optionally, the or each light-emitting film has a length and I or width of up to about 10 crn up to about 5 cm, up to about 2 cm, up to about 1 cm, or up to about 5 mm. The light-emitting film or films may have a length and width of at least 0.5 mill.

Light is emitted from the area or areas in which the anode and the cathode overlap. One or both of the anode and cathode may be patterned to form a desired emission area.

The anode or cathode may be patterned such that a single, static image is produced when the LEC is switched on. in other embodiments, patterned anode areas or cathode areas maybe individually addressable to produce a moving image, for example an alphanumeric display.

The LEC may emit white light. The LEC maybe a monochrome (for example red, green blue or yellow) display, or may be a full-colour display.

Figure 2A schematically illustrates a plan view of a partial LEC structure of a LEC according to an embodiment of the invention in which the anode 101 is patterned to form a plurality of individually addressable anode areas 209. A light-emitting film 211 is formed over each anode area 209 to provide a light-emitting layer 103 comprising a plurality of separate light-emitting films. The cathode (not shown) may be a continuous layer extending across the area of all of the anodes areas 209 and light-emitting films 211. In another embodiment (not shown), a LEC may have a plurality of individually addressable cathode areas and a continuous anode area extending across the area of all of the cathode areas.

Figure 2B schematically illustrates a plan view of a partial LEC structure of a LEC according to another embodiment of the invention, with a similar structure to the device of Figure 2A except that each light-emitting film extends across a plurality of patterned anodes l0I.Conductive tracks (not shown) are provided on the substrate to connect the LEC to a power source and, if the LEC is a display, to drive circuitry.

In a further embodiment (not shown) the anode 101 and cathode 105 may be in the form of intersecting (e.g. perpendicular) stripes, with emission from the intersection of anode and cathode stripes. In this embodiment the light-emitting layer may extend over the whole of the anode and / or cathode area, or maybe provided in the form of a plurality of films wherein each film extends across an anode or cathode stripe area.

The light-emitting layer is formed by depositing a formulation comprising the components of the light-emitting layer and at least one solvent, and evaporating the at least one solvent.

One method of depositing the formulation is roll printing. "Roll printing" as used herein means a printing process in which a formulation is transferred directly or indirectly from a printing roller onto a printing surface. In indirect printing, the formulation maybe transferred from a printing roller to a second roller and from the second roller to the printing surface.

Exemplary roll printing processes include: -gravure printing in which a printing roller (the gravure cylinder) has recessed areas on the surface thereof wherein formulation is absorbed in the recessed areas and is transferred directly or illdirectly from the gravure cylinder to the printing surface; and -flexographic pnlltrng in which the printing roller carries a relief structure on a surface thereof wherein the relief structure defines the pattern to he printed.

Roll printing processes as described herein may be batch or continuous processes.

Figure 3A is a schematic illustration of a gravure printing process. The formulation 301 is provided in a tray 303. A gravure cylinder 305 absorbs the formulation in the tray and transfers the absorbed formulation to a printing surface 307, for example an anode carried on a glass or plastic substrate 309. Excess formulatioll may be removed from the surface of the gravure cylinder 305 by a doctor blade 311 before the absorbed formulation reaches the printing surface 307.

The printing surface may be carried on a glass or plastic substrate. Preferably, the substrate is a flexible material, for example a plastic, and the substrate is a web that is fed from a roll. Exemplary plastic substrates are PET and PEN substrates.

An impression roller 31 3 may he provided on the side of the substrate 309 opposite the printing surface 307 to press the printing surface 307 into contact with the gravure cylinder 305.

Figure 3A ill ustrates a continuous printing process. In another embodiment, the printing process may he a hatch process.

The light-emitting layers for a plurality of LECs maybe printed onto the printing surface.

Following light-emitting layer formation, the substrate may he broken into substrate areas for individual LECs before or after any one of cathode deposition and encapsulation.

Figure 3A illustrates a direct gravure printing process. In another embodiment, the printing process is an offset gravure printing process.

Figure 3B is a schematic illustration of the surface of a gravure cylinder 305 suitable for use in gravure printing. The cylinder surface comprises a plurality of printing areas 315 formed as recesses on the surface of the cylinder. The cylinder surface may be a metal having recesses formed by any suitable method, for example etching or engraving. The printing areas 315 are of a size, shape and distribution corresponding to the pattern to be printed on the printing surface. The internal area of the printing areas may be patterned into a plurality of cells, for example a grid or honeycomb structure into which the formulation is absorbed.

Figure 3C schematically illustrates a profile of a printing area containing cells 317. The cells 317 may define a square-based pyramid shape.

If the final LEC contains a single light-emitting film then this layer may be formed from the formulation deposited from a single printing area. If the final LEC contains a plurality of light-emitting films, for example as illustrated in Figure 2B, then each film may be formed by a different printing area.

If the printing surface carries patterned electrodes then the cells of the gravure cylinder are aligned with the patterned cells such that the formulation is printed over the patterned electrodes.

The light-emitting layer of the LEC maybe printed in a single pass of the gravure printer over each electrode, or two or more passes may be used if needed to achieve a desired thickness of the light-emitting layer. Preferably, a single pass is used to minimise TACT time and to avoid having to align the gravure cylinder 305 and printing surface 307.

S

The semiconducting polymer has a polystyrene-equivalent weight-average molecular weight (Mw) measured by gel permeation chromatography of less than 300,000, optionally no more than 250,000, optionally no more than 200,000, optionally no more than i 00,000 or no more than 50,000-Preferably, the semiconducting polymer has a Mw of at least I 0,000.

Use of a low molecular weight semiconducting polymer enables formation of low viscosity formulations. The present inventors have found that low viscosity formulations may improve uniformity of films of the light-emitting layer formed by gravure printing.

Without wishing to be bound by any theory, it is believed that light-emitting film uniformity may affect performance of LECs. The lower viscosity may also increase the amount of the formulation that is transferred from the cells of the gravure cylinder onto the printing surface.

A preferred viscosity range of the ink for roll printing is in the range of 10-50 cP, optionally iO-30 cP.

Viscosities as described herein are as measured at a shear rate of 1000 / s at 20°C using a cone and plate rheometer.

Use of a low molecular weight semiconducting polymer allows for a higher solids content at a given viscosity, which may allow all of the solids content required to form a light-emitting layer to be deposited in a single pass. "Solids content" as used herein means the proportion by weight of the formulation of the materials that form the light-emitting layer, including the semiconducting polymer(s), the electrolyte(s), the salt(s) and any other components of the light-emitting layer. Preferably, the solids content of the formulation is 1-10 w/w %, preferably at least 3 or 4 wt %.

Following deposition, solvent may be allowed to evaporate from the formulation at ambient pressure and temperature or may be heated and / or placed under vacuum.

Electrolyte The electrolyte may be a polymeric or non-polymeric material. Preferably, the electrolyte is a polymer Exemplary polymer electrolytes include: polyalkylene oxides, for example polyethylene oxide (PEO) and polypropylene oxides copolymers of alkylene oxide, for example polyethylene-block(ethylene glycol) polymer and poly(ethylene glycol)-block-poly(propylelle glycol)-block poly(ethylene glycol) polymer; esters of polyalkyleneglycols such as polycarbonates; polyolefins; and polysiloxanes.

A polyalkylene oxide polymer electrolyte may carry hydroxyl end-capping groups.

The polymer electrolyte may have a viscosity average molecular weight (Mv) of 10,000 - 1,000,000 Da, optionally 10,000 -500,000 Da. Optionally, the polyelectrolyte has a Mv of less than 300,000, less than 200,000 or less than 100,000.

An exemplary class of non-polymeric electrolyte is crown ethers.

Optionally, only one electrolyte is present in the light-emitting layer.

The electrolyte may make up at least I weight %, 2 weight %, 5 weight %, optionally at least tO weight % of the composition of the components of the light-emitting layer, and are optionally provided in an amount of up to 20 weight % or up to 30 weight %.

The weight percentages of components of the light-emitting layer provided anywhere herein are the weight percentages following evaporation of the solvent(s). Salts

Salk wi iii relaLi vely sinai I anions or calioris nay be more inohi ie Wart sails wi U Ijul kier ions.

Preferred cadons of the salt include alkali, alkali earth and ammonium cations.

Ammonium cations include NH4 cations and mono-, di-tri and tetraalkylarnrnonium cations.

Preferred anions of the salt include halogen-containing anions, in particular fluorine-containing anions, for example hexafi Lloropllospllate and tetrafi uorohorate.

The light-emitting composition may include only one salt or more than one salt. The ionic salt or salts may make up 0.1 -25 % by weight, optionally 1-15 % by weight, of the components of the light-emitting layer.

Light-emitting layer The semiconducting polymer of the light-emitting layer may emit light directly, or may he used as a host in combination with a fluorescent or phosphorescent light-emitting dopant. The light-emitting dopant, if present, may be a small molecule or polymeric material.

Suitable senñconducting polymers include homopolymers or copolymers comprising two or more different repeat units.

A semiconducting polymer may be a conjugated polymer having a backbone containing repeat units that are conjugated to adjacent repeat units, or may contain a substantially non-conjugated backbone with conjugated groups pendant from the non-conjugated backbone.

An exemplary polymer with a non-conjugated backbone is poly(vinylcarhazole).

Substantially all repeat units in the backbone of a conjugated polymer may be conjugated to adjacent repeat units. Conjugation along the backbone of a conjugated polymer may he broken by non-conjugating repeat units.

Exemplary polymers with at least partially conjugated backbones include polymers containing arylene, heteroarylene, arylenevinylene or heteroarylenevinylene repeat units in the polymer backbone, wherein said arylene, heteroarylene, arylenevinylene or heteroarylenevinylene repeat units maybe substituted or unsubstituted, for example substituted with one or more hydrocarhyl groups, for example one or more C140 hydrocarhyl groups, wherein one or more non-adjacent carbon atoms in a carbon chain of the hydrocarhyl groups maybe replaced with 0. Exemplary Ciohydrocarhyl groups include Ci_20 alkyl groups and phenyl substituted with one or more Ci_19 alkyl groups.

If used in the same layer as, or in a layer adjacent to, a light-emitting material with a high singlet or triplet energy level then the extent of conjugation along the backbone of the polymer maybe limited by selection of repeat units. Exemplary repeat units that may limit the extent of conjugatioll include: (i) repeat units that are twisted out of the plane of adjacent repeat units, limiting the extent of p-orbital overlap betweell adjacent repeat units; (ii) conjugation-breaking repeat units that do not provide a conjugation path between repeat units adjacent to the conjugation breaking repeat units: and (iii) repeat units that are linked to adjacent repeat units through positions that limit the extent of conjugation between repeat units adjacent to the repeat unit.

One preferred class of arylene repeat units is phenylene repeat units, such as phenylene repeat units of formula (TIT): (ill) wherein pin each occurrence is independently 0, 1, 2, 3 or 4, optionally 1 or 2; n is 1, 2 or 3; and R1 independently in each occurrence is a substituent.

Where present, each R' may independently be selected from the group consisting of: -alkyl, optionally C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with optionally substituted aryl or heteroaryl, 0, S, substituted N, C=O or -COO-, and one or more H atoms may be replaced with F; -aryl and heteroaryl groups that ma)' he unsubstituted or substituted with one or more suhstituents, preferably phenyl substituted with one or more C120 alkyl groups; and -a linear or branched chain of aryl or heteroaryl groups, each of which groups may independently he substituted, for example a group of formula -(Ar3)r wherein each Ar3 is independently an aryl or heteroaryl group and r is at least 2, preferably a branched or linear chain of phenyl groups each of which may be unsubstituted or substituted with one or more C1-20 alkyl groups.

Substituted N, where present, may be -NR2-wherein R2 is C120 alkyl; unsubstituted phenyl; or phenyl substituted with one or more C120 alkyl groups.

One or more substituents R' may be polar substituents. Polar substituents R' may improve compatibility of the semiconducting polymer with polymer electrolytes such as polyethylene oxide.

Polar substituents R1 include substituents having the following formula (X): * (S p2)v((O KCR92)m)p)CH (X) wherein * represents a point of attachment of the substituent to the repeat unit; 5p2 is a spacer group; b isO or 1; c is at least 1, optionally 1, 2 or 3; m independently in each occurrence is at least 1, optionally 1, 2 or 3; p is at least I, optionally 1, 2 or 3; and R9 in each occurrence is independently H or a substituent, preferably H or C15 alkyl.

5p2 is preferably a Ciiohydrocarhyl group, preferably unsubstituted phenyl or phenyl substituted with one or more C110 alkyl groups.

Polar substituents R1 may contain one or more polar oligo-ether groups, for example substituents containing one or more polar groups -(OCH2CH2)-R8 wherein w is at least I, optionally 1-5, and R8 is H or a suhstituent, optionally H, C110 alkyl or C110 alkoxy.

Preferably, each R' is independently selected from C140 hydrocarhyl wherein one or more non-aromatic C atoms in a chain of the hydrocarhyl group maybe replaced with 0, and is more preferably selected from C120 alkyl wherein one or more non-adjacent C atoms may he replaced with 0; unsuhstituted phenyl; and phenyl substituted with one or more C140 alkyl groups wherein one or more non-adjacent C atoms of the alkyl group or groups may be replaced with 0.

A further class of arylene repeat units are optionally substituted fluorene repeat units, such as repeat units of formula (IV): R3 R3 (IV) wherein R3 in each occurrence is the same or different and is H or a substituent, and wherein the two groups R3 maybe linked to form a ring.

Each R3 is preferably a substituent, and each R3 may independently he selected from the group consisting of: -alkyl, optionally C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with optional'y substituted ary or heteroaryL 0, S, substituted N, C=O or -COO-, and one or more H atoms maybe replaced with F; -aryl or heteroaryl that may be unsubstituted or substituted with one or more -a linear or branched chain of aryl or heteroaryl groups, each of which groups may independently he substituted, for example a group of formula -(Ar3)r as described above with reference to formula (ill).

In tile case where R3 comprises an aryl or heteroaryl group, or a linear or branched chain of aryl or heteroaryl groups, the or each aryl or heteroary] group may he substituted with one or more sLibstitlients R4 selected from the group consisting of: alkyl, for example C120 alkyl, wherein one or more non-adjacent C atoms may be replaced with 0, S, substituted N, C=O and -COO-and one or more H atoms of the alkyl group maybe replaced with F; NR52, OR5, SR5, and fluorine, nitro and cyano; wherein each R5 is independenfly selected from the group consisting of alkyl, preferably C120 alkyl; and aryl or heteroaryl, preferably phenyl, optionally substituted with one or more C129 alkyl groups.

The aromatic carbon atoms of the fluorene repeat unit may be unsubstituted, or may be substituted with one or more substituents. Exemplary substituents are alkyl, for example Ci2o ailcyl, wherein one or more non-adjacent C atoms may be replaced with 0, S, NH or substituted N, C=O and -COO-, optionally substituted aryi, optionally substituted heteroaryl, alkoxy, alkylthio, fluorine, cyano aild arylalkyl. Particularly preferred substituents include C120 alkyl aild substituted or unsubstituted aryl, for example phenyl.

Optional substituents for the aryl include one or more Ci20 alkyl groups.

Substituted N, where preseilt, may be -NR2-wherein R2 is C120 alkyl; unsubstituted phenyl; or phenyl substituted with one or more C120 alkyl groups.

One or more substituents R3 may be polar substituents. Polar substituents R3 may improve compatibility of the serniconducting polymer with polymer electrolytes such as polyethylene oxide. Polar substituents R3 may contain one or more polar oligo-ether groups, for example substitujents containing one or more polar groups -(OCH2CH2)-R8 as described above with reference to formula (III) Preferably, each R3 is independently selected from C140 hydrocarbyl wherein one or more non-aromatic C atoms in a chain of the hydrocarhy] group maybe replaced with 0, and is more preferably selected from: C120 alkyl wherein one or more non-adjacent C atoms maybe replaced with 0: unusuhstitiited pheriyl; and phenyl substituted with one or more C120 alkyl groups wherein one or more non-adjacent C atoms of the alkyl group or groups maybe replaced with 0.

The repeat unit of formula (IV) may be a 2,7-linked repeat unit of formula (IVa): R3 R3 (Wa) Optionally, the repeat unit of formula (IVa) is not substituted in a position adjacent to the 2-or 7-positions.

The extent of conjugation of repeat units of formulae (IV) may be limited by (a) linking the repeat unit through the 3-and I or 6-positions to limit the extent of conjugation across the repeat irnit, arid / or (h) substituting the repeat irnit with one or inure further substituents R' in or more positions adjacent to the linking positions in order to create a twist with the adjacent repeat unit or units, for example a 2,7-linked fluorene carrying a C120 alkyl substituent in one or both of the 3-and 6-positions.

The semiconducting polymer may contain repeat units carrying polar substituefits, for example substituents of formula &(SP2)h((0(CR92)m)p)cH or (OCH2CH2)wRR as described with reference to formula (X), and repeat units carrying non-polar substituents, for example C140 hydrocarbyl sijbstituents. For example, a light-emitting polymer may contain repeat units of formula (IV) having polar suhstituents such as substituents of formula < (SP2)h((O(CR92)m)p)cH or _(OCH2CH2)RR and repeat units of formula (IV) having non-polar substituents such as Ci4ohydrocarbyl.

The polymer may contain amine repeat units in particular amines of formula (IX): ((Ar8)cft(Ar9)d \R13 / (TX) wherein Ar8 and Ar9 in each occurrence are independently selected from substituted or ullsubstituted aryl or heteroaryl, g is greater than or equal to I, preferably 1 or 2, R13 is H or a substituent, preferably a substituent, and c and d are each independently 1, 2 or 3.

R'3, which may he the same or different in each occurrence when g > I, is preferably selected from the group consisting of alkyl, fur example C120 alkyl, Ar0, or a branched or linear chain of Ar'0 groups, wherein in each occurrence is independently optionally sLibstituted aryl or heteroaryl. Exemplary spacer groups are Ci20 alkyl, phenyl and phenyl-Ci2o alkyl.

Any of Ar8, Ar9 and, if present, Ar'° bound directly to a N atom in the repeat unit of Formula (IX) may he linked by a direct bond or a divalent linking atom or group to another of Ar8, Ar9 and Ar10 bound directly to the same N atom. Preferred divalent linking atoms and groups include 0, S: substituted N; and substituted C. Any of Ar8, Ar9 and, if present, Ar'° may be substituted with one or more substituents.

Exemplary substituents are substituents R'4, wherein each R'4 may indepeildently be selected from the group consisting of substituted or unsubstituted alkyl, optionally C120 alkyl, wherein oe or more non-adjaceilt C atoms may be replaced with optionally substituted aryl or heteroaryl, 0, S, substituted N, C=O or -COO-and one or more H atoms may be replaced with F. Substituted N or substituted C, where present, may he N or C substituted with a hydrocarbyl group (in the case of substituted N) or two hydrocarbyl groups (in the case of substituted C), for example a C140 alkyl, unsubstituted phenyl or phenyl substituted with oe or more C110 alkyl groups.

Preferred repeat units of formula (IX) have formulae 1-3: ( rs /Ar) ( Aç7Ar9) ( Aç7Ar) NArN I Ar1° Ar1° Ar1° Ar1° 1 2 3 In one preferred arrangement, R'3 is Ar0 and each of Ar8, Ar9 and Ar'0 are independently unsubstituted or substituted with one or more C,2o alkyl groups.

Ar8, Ar9 and Ar10 are preferably phenyl, each of which may independenfly be substituted with one or more substituents as described above.

In another preferred arrangement, Ar8 and Ar9 are pheny], each of which may he substituted with one or more C120 alky] groups, and R13 is 3,5-diphenylhenzene wherein each phenyl maybe substituted with one or more C120 alkyl groups.

In allother preferred arrangement, c, d and g are each 1 mid Ar8 and Ar9 are phenyl linked hy an oxygen atom to form a phenoxazine ring Amine repeat units may be provided in a molar amount in the range of about 0.5 mol % up to about 50 rn51 %, optionally up to 40 mol %.

The light-emitting ayer may contain a host material and a light-emitting dopant.

Exemplary host rnaterias include materials that are capable of emitting light in the absence of a light-emitting doparit, for example a light-emitting poyiner as described above.

The light-emitting polymer may comprise conj ugation-breaking repeat units that break any conjugation path between repeat units adjacent to the conjugation-breaking repeat unit. An exemplary conjugation-breaking repeat unit has formula (I): -(Ar2-Sp1 2) (I) wherein Ar2 in each occurrence independently represents a substituted or unsubstituted aryl or heteroaryl group; Sp1 represents a spacer group that does not provide any conjugation path between the two groups Ar2.

Ar2 is preferably phenyl that may be unsubstituted or substituted with one or more substituents, preferably one or more C120 alkyl groups. I. I

Sp may contain a single non-conjugating atom only between the two groups Ar, or Sp may contain non-conjugating chain of at least 2 atoms separating the two groups Ar2.

A non-conjugating atom may he, for example, -0-, -5-, -CR72-or -SiR72-wherein R7 in each occurrence is H or a substituent, optionally C120 alkyl.

A spacer chain Sp' may contain two or more atoms separating the two groups Ar2, for example a C120 alkyl chain wherein one or more non-adjacent C atoms of the chain may be replaced with 0 or S. Preferably, the spacer chain Sp1 contains at least one sp3-hybridised carbon atom separating the two groups A?.

Preferred groups Sp' are selected from C120 alkyl wherein one or more non-adjacent C atoms may be replaced with 0. An ether spacer or oligo-ether spacer chain, for example a chain of formula --(CH,CH20)-, wherein v is I or more, optionally 1-10, may improve miscibility of the semiconducting polymer with electrolytes such as poly(ethylene oxide).

Examples of cyclic non-conjugating spacers are optionally substituted cyclohexane or adamantane repeat units that may have the structures illustrated below: Exemplary substituents for cyclic conjugation repeat units include C1.40 alkyl.

Conjugation breaking repeat units may make up 0.5-3 0 mol % of repeat units of a polymer, preferably 1-20 mol % of repeat units.

Conjugated semicondLicting polymers as described herein maybe formed by a process comprising a "metal insertion" wherein the metal atom of a metal complex catalyst is inserted between an aryl or heteroaryl group and a leaving group of a monomer.

Exemplary metal insertion methods are Suzuki polymerisation as described in, for example, WO 00/53656 and Yamamoto polymerisation as described in, for example, T. Yamamoto, "Electrically Conducting And Thermally Stable it -Conjugated Poly(arylene)s Prepared by Organometallic Processes', Progress in Polymer Science 1993, 17, 1153-1205. In the case of Yamamoto polymerisation, a nickel complex catalyst is used; in the case of Suzuki polymerisation. a palladium complex catalyst is used.

For example, in the synthesis of a linear polymer by Yamamoto polymerisation, a monomer having two reactive halogen groups maybe used. Similarly, according to the method of Suzuki polymerisation, at least one reactive group is a boron derivative group such as a horonic acid or horonic ester and the other reactive group is a halogen.

Preferred halogens are chlorine, bromine and iodine, most preferably brotnine.

It will therefore be appreciated that repeat units illustrated throughout this application maybe derived from a monomer carrying suitable leaving groups. Likewise, an end group or side group maybe bound to the polymer by reaction of a suitable leaving group.

Suzuki polymerisation may be used to prepare regioregular, block and random copolymers. In particular, homopolymers or random copolymers may be prepared when one reactive group is a halogen and the other reactive group is a boron derivative group.

Alternatively, block or regioregular copolymers may be prepared when both reactive groups of a first monomer are boron and both reactive groups of a second monomer are halogen.

As alternatives to halides, other leaving groups capable of participating in metal insertion include sulfonic acids and sulfonic acid esters such as tosylate, mesylate and triflate.

The molecular weight of a conjugated polymer may be controlled by adding a compound substituted with only one reactive group to the polymerisation mixture either before or during polymerisation, for example only one halogen or only one boronic acid or ester group in the case of Suzuki polymerisation, in order to limit polymer growth by end-capping. In the case where two different reactive groups are used, for example Suzuki polymerisation, the molecular weight maybe controlled by providing the different reactive groups in a molar ratio other than 1:1 -For example, in the case of Suzuki polymerisation an excess of monomers having halogen leaving groups or an excess of monomers having horonic acid or ester leaving groups maybe used to redLice the molecular weight of the polymer as compared to the case where the molar ratio is 1:1 -The light-emitting material or materials of the composition may make up at least 50 or at least 60 weight % of the composition, and may form up to 80 or 90 weight % of the composition. In the case of a host / dopant system, the weight of the light-emitting materials includes the weight of the host material.

A formulation of one or more salts, an electrolyte, a semiconducting polymer and (if present) one or more dopants may contain 40-97, optionally 50-95 weight % of the semiconducting polymer.

Suitable dopants include fluorescent dopants and phosphorescent dopants. fluorescent dopants suitably have a lowest excited state singlet energy level that is no higher than, and optionally lower than, that of the host material such that singlet excitons may be transferred from the light-emitting material to the dopailt. Phosphorescent dopants suitably have an lowest excited state triplet energy level that is no higher than, and optionally lower than, that of the host material such that triplet excitons may be transferred from the light-emitting material to the dopailt.

Phosphorescent light-emitting materials Exemplary phosphorescent light-emitting materials include metal complexes comprising substituted or unsubstituted complexes of formula (11): ML'qL2rL3s (IT) wherein M is a metal; each of L', L2 and L3 is a coordinating group; q is an integer: rand s are each independently 0 or an integer; and the sum of (a. q) + (h. r) + (as) is equal to the number of coordination sites available on M, wherein a is the number of coordination sites on L', h is the number of coordination sites on L2 and c is the number of coordination sites on L -Heavy elements M induce strong spin-orbit coupling to allow rapid intersystem crossing and emission from triplet or higher states. Sutable heavy metals M include d-block metals, in particular those in rows 2 and 3 i.e. elements 39 to 48 and 72 to 80, in particular ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum and gold.

Iridium is particularly preferred.

Exemplary ligands L1, L2 and L3 include carbon or nitrogen donors such as porphyrin or bidentate ligands of formula (III): r6 (Ill) wherein Ar5 and Ar6 may be the same or different and are independently selected from substituted or unsubstituted aryl or heteroaryl; X' and Y' may be the same or different and are independently selected from carbon or nitrogen and Ar5 and Ar6 may be fused together. Ligands wherein X1 is carbon and Y' is nitrogen are preferred, in particular ligands in which Ar5 is a single ring or fused heteroaromatic of N and C atoms only, for example pyridyl or isoquinoline, and Ar6 is a single ring or fused aromatic, for example phenyl or naphthyl.

Examples of bidentate ligands are illustrated below: () c"e CO Other ligands suitable for use with d-hlock elements include diketonates, in particular acetylacetonate (acac); triarylphosphines and pyridine, each of which may he substituted.

Each of A? and Ar6 may carry one or more substituents. Two or more of these substituents may be linked to form a ring, for example an aromatic ring.

Exemplary substituents of ligands of formula (III) include groups as described above with reference to Formula (IV), preferably C149 hydrocarbyl. Particularly preferred substituents include fluorine or trifluoromethyl which may be used to blue-shift the emission of the complex, for example as disclosed in WO 02/45466, WO 02/44189, US 2002-117662 and US 2002-182441; alkyl or alkoxy groups, for example C120 alkyl or alkoxy, which may be as disclosed in JP 2002-324679; carbazole which may be used to assist hole transport to the complex when used as an emissive material, for example as disclosed in WO 02/8 1448; bromine, chlorine or iodine which can serve to functionalise the ligand for attachment of further groups, for example as disclosed in WO 02/68435 and EP 1245659; and dendrons which maybe used to obtain or enhance solution processahility of the metal complex, for example as disclosed in WO 02/66552.

A light-emitting dendrimer comprises a light-emitting core, such as a metal complex of formula (II), bound to one or more dendrons, wherein each dendron comprises a branching point and two or more dendritic branches. Preferably, the dendron is at least partially conjugated, and at least one of the branching points and dendritic branches comprises an aryl or heteroaryl group, for example a phenyl group. In one arrangement, the branching point group and the branching groups are all phenyl, and each phenyl may independently be substituted with one or more substituents, for example alkyl or ailcoxy.

A dendron may have optionally substituted formula (IV) / BP\ (TV) wherein BP represents a branching point for attachment to a core and 01 represents first generation branching groups.

The dendron may be a first, second, third or higher generation dendron. Ci maybe substituted with two or more second generation branching groups 02, and so on, as in optionally substituted formula (IVa): p 5G3 k2 H / B\NH (IVa) wherein u isO or 1; v isO if ii isO or may be 0 or 1 if u is 1: BP represents a branching point for attachment to a core and Ci, 02 and 03 represent first, second and third generation dendron branching groups. bone preferred embodiment, each of BP and 01, 02 C is phenyL and each phenyl BP, 01, 02 G-is a 3,5-linked phenyl.

A preferred dendron is a substituted or unsubstituted dendron of formula (lYb): (lYb) wherein * represents an attachment point of the dendron to a core.

BP and / or any group G may be substituted with one or more substituents, for example one or more C120 alkyl or alkoxy groups.

Phosphorescent light-emitting materials of a light-emitting composition may be present in an amount of about 0.05 mol % up to about 20 mol %, optionally about 0.1-10 mol % relative to their host material. A light-emitting composition may contain one or more phosphorescent light-emitting materials.

A phosphorescent material he physically mixed with the light-emitting material as host or may be chemically bound to the light-emitting material. In the case of a polymeric light-emitting host, the phosphorescent material may he provided in a side-chain, main chain or end-group of the polymer. Where a phosphorescent material is provided in a polymer side-chain, the phosphorescent material may he directly hound to the backbone of the polymer or spaced apart therefrom hy a spacer group, for example a C120 alkyl spacer group in which one or more non-adjacent C atoms may he replaced hy 0 or S or -C(=O)O-.

White light emission in the case of a white light-emitting LEC or composition, the light emitted may have Cifi x coordinate equivalent to that emitted by a black body at a temperature in the range of 2500-9000K and a Cifi y coordinate within 0.05 or 0.025 of the CIE y co-ordinate of said light emitted by a black body, optionally a CIE x coordinate equivalent to that emitted by a black body at a temperature in the range of 2700-4500K.

Formulations An ink formulation suitable for forming a light-emitting layer maybe formulated by mixing the components of the composition with one or more suitable solvents.

Optionally, more than one solvent is used wherein the semiconducting polymer is soluble in at least one of the solvents and wherein the electrolyte is soluble in at least one of the other solvents.

Solvents suitable for dissolving semiconducting polymers, particularly polymers comprising alkyl substituents, include henzenes substituted with one or more C110 alkyl or C119 alkoxy groups, for example toluerie, xylenes and methylanisoles.

Solvents suitable for dissolving polymer electrolytes, for example PEO, include C110 alkyl henzoates and henzenes substituted with polar ops.,for example electron-withdrawing groups, such as groups with a positive Hammett constant. Suitable poiar groups include cifiorine, cyano and C110 alkoxy substituents. Exempthry solvents include chlorobenzene aild methythenzoate.

Solvents for the semicoilducting polymer mid for the electrolyte preferably have a boilillg point in the range of 100-300°C, optionally 150 -250°C.

The formu ation may be a solution ill which all componeilts of the composition are dissolved in the solvent or solvents, or it maybe a dispersion wherein one or more components of the composition are suspended in the formulation. Preferably, the formulation is a solution.

Optionally, the electr&yte, the light-emitting material and salt together form 0.2-10 weight % of the formulation, optionally 0.5 -3 weight % of the formulation.

The formu'ation may contain further components such as surfactants and I or compatihilisers. Suitable compatibilisers include polymers comprising di alkylsiloxane repeat units,for example a dimethylsiloxane -ethylene oxide copolymer.

Hole injection layers A coilductive hole injection layer, which may be formed from a conductive orgaxñc or inorganic material, may be provided between the anode and the light-emitting layer of an LEC to improve hole injection from the anode into the light-emitting layer. Examples of doped organic hole injection materials include optionally substituted, doped poly(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in EP 0901176 and EP 0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nation ®; polyaniline as disclosed in US 5723873 and US 5798170; and optionally substituted polythiophene or poly(tliienothiophene). Examples of conductive inorganic materials include transition metal oxides such as VOx MoOx and RuOx as disclosed in Journal of Physics D: Applied Physics(1996), 29(1 i), 2750-2753.

Cathode The cathode may consist of a single material such as a layer of aluminium or silver.

Alternatively, it may comprise a plurality of layers, for example a bilayer of metals such as calcium and alumillium as disclosed in WO 98/10621, or elemental barium, either alone or with one or more cathode layers, for example a bilayer of barium and aluminium as disclosed in WO 98/5738 1, Appl. Phys. Lett. 2002, 81(4), 634 and WO 02/84759. The cathode may contain a thin layer (e.g. of about 0.5-5 nm) of metal compoulld, in particular an oxide or fluoride of an alkali or alkali earth metal between the light-emitting layer and one or more conductive layers (e.g. one or more metal layers) to assist electron injection, for example lithium fluoride as disclosed in WO 00/48258; barium fluoride as disclosed in AppI. Phys. Lett. 2001, 79(5), 2001; and barium oxide.

The cathode may be in direct contact with the light-emitting layer.

The cathode maybe an air-stable conductive material, for example a metal, optionally aluminium or silver. The cathode may be deposited by evaporation or sputtering, or by deposition of a paste of the metal. A paste of the metal may be deposited by a printing method, for example screen printing.

The cathode may be opaque or transparent. It will be appreciated that a transparent cathode device need not have a transparent anode (unless, of course, a frilly transparent device is desired), and so the transparent anode used for bottom-emitting devices may be replaced or supplemented with a layer of reflective material such as a layer of aluminium.

Examples of transparent cathode devices are disclosed in, for example, GB 2348316.

Encapsulation Organic optoelectronic devices tend to be sensitive to moisture and oxygen.

Accordingly, the substrate preferably has good barrier properties for prevention of ingress of moisture and oxygen into the device.

The device may he encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having sLlitahle harrier properties such as silicon dioxide, silicon monoxide, silicon nitride or alternating stacks of polymer and dielectric or an airtight container. In the case of a transparent cathode device, a transparent encapsulating layer such as silicon monoxide or silicon dioxide may be deposited to micron levels of thickness, although in one preferred embodiment the thickness of such a layer is in the range of 20-300 nm. A getter material for absorption of any residual moisture or any atmospheric moisture and / or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant. A getter, where present, may be screen printed.

Examples

Example 1

The effect of molecular weight of a setniconducting polymer on gravure-printed film roughness and film thickness was investigated by printing a formulation containing a semiconducting polymer having a Mw of 147,000 Da and a semiconducting polymer having a Mw of 400,000.

The formulation contained the following components in a I:1 volume ratio of 4-methylanisole: methylbenzoate: White-Emitting Polymer 1 76. lwt% PEO l3wt% DBE-821 6.4wt% SALT THAPF6 2.7 wt% SALT THPBF4 1.8wt% wherein: THAPF6 is tetrahexylamnionium hexafluorophosphate; THPBF4 is triltexyltetradecylphophonium tetrafluoroborate; DBE-821 is dimethylsiloxane-ethylene oxide block copolymer available from Gelest, Inc. and Lised as a compatihiliser; and White-Emitting Polymer 1 is a semiconducting polymer of either 147,000 Da or 400,000 Da Mw. White-Emitting Polymer 1 is a conjugated polymer having a phosphorescent dopaiit bound thereto as an end-group. The polymer is formed by Suzuki polymerisation as described in WO 00/53656 of the following monomers: mo % 19.95 mol % B Br LjJ 30mo% 30m&% Br Br mol

I

0.05 inol % With reference to Figure 4 roughness Ra, measured by a Veeco Nanoscope -V AFM system used in tapping mode, is greater for the higher molecular weight light-emitting polymer. Without wishing to be bound by any theory, this greater roughness is attributed to a greater driving force for phase separation between the high molecular weight polymer and polyelectrolyte during drying of the film as compared to the low molecular weight polymer and polyelectrolyte.

With reference to Figure 5, the lower molecular weight light-emitting polymer produces a thicker film for a given solids content of the formulation, and produces a thicker film at a given gravure printing speed.

Device Example I

An ink formulation was prepared as described in Example I, wherein White Emitting Polymer I had a Mw of 100,000 Da.

The formulation was gravure printed using a printer having cell dimensions of 26 lines/cm, 40° angle using 120° stylus.

The formulation was gravure printed onto a flexible substrate carrying patterned ITO.

The substrate is aligned with the gravure cylinder such that cells release the formulation onto the ITO. The solvent was dried on a hotplate at 120°Cto form a light-emitting layer, and a silver cathode was formed over the light-emitting layer by evaporation.

Comparative Device 1 For the purpose of companson a device was prepared as described above except that White-Emitting Polymer 1 was replaced with Comparative White Polymer having the same composition as White Emitting Polymer I bitt having a Mw of 400,000.

Figure 6A is a photograph of a pixel of Comparative Device 1. The cell pattern of the cell used to print this pixel is visible. In contrast, the cell pattern is not visible in the pixel of Device Example I shown in Figure ÔB.

Without wishing to be bound by any theory, it is believed that the lower viscosity of the formulation used to print the light-emitting layer of Device Example 1 allows the prillted formulation to flow more freely than the formulation used to print Comparative Device 1, allowing formation of a uniform pixel film during solvent evaporation.

With reference to Figure 7, luminance of Device Example I at turn-on is approximately cdt m2 compared to less than 80 cd / in2 for Comparative Device 1.

With reference to Figure 8, the brightness of Device Example 1 decays much more slowly over time than that of Comparative Device I. With reference to Figure 9, device failure occurs at a much lower voltage for Comparative Device 1 than for Device Example 1.

With reference to Figure 10, cd/A peak efficiency of Device Example 1 is higher than that of Comparative Device 1.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.

Claims

Claims A method of forming an organic light-emitting electrochemical cell, the method comprising the steps of forming a light-emitting layer by roll printing a formulation over one of an anode and a cathode, and providing the other of an anode aild a cathode over the light-emitting layer, wherein the formulation comprises a semiconducting polymer, an electrolyte and a salt and wherein the serniconthwting polymer has a polystyrene-equivalent weight-average molecular weight measured by gel permeatioll chromatography of less than 300000.
2. A method according to claim 1 wherein the weight average molecular weight of the semiconducting polymer is no more than 200,000.
3. A method according to claim 1 wherein the weight-average molecular weight of the semiconducting polymer is no more than 100,000.
4. A method according to any preceding claim wherein the light-emitting layer is formed by gravure printing.
5. A method according to any preceding claim wherein the formulation has a viscosity at measured at a shear rate of 1000! s at 20°C in the range 10-50 cP.
6. A method according to any preceding claim wherein the formulation has a solids content in the range of 1-10 weight %.
7. A method according to any preceding claim wherein the electrolyte is a polyelectrolyte.
8. A method according to claim 7 wherein the polyelectrolyte is polyethylene oxide.
9. A method according to claim 6 or 7 wherein the polyelectrolyte has a viscosity average molecular weight of less than 200 Da
10. A method according to any preceding claim wherein the semiconducting polymer emits light when the organic light-emitting electrochemical cell is in use.I I -A method according to any preceding claim wherein the composition further comprises a light-emitting clopant ailci the semiconducting polymer is a host polymer.12. A method according to claim 11 wherein the light-emitting dopailt is covalently hoLind to the host polymer.13-A method according to claim II or 12 wherein the light-emitting dopant is a phosphorescent light-emitting doparit.14. A method according to any preceding claim wherein the organic light-emitting eectrochemical cell emits white light.15-A method according to any of claims 1-13 wherein the organic light-emitting electrochemical cell is a monochrome or full colour display.16-A method according to any preceding claim wherein the polyelectrolyte is polyethylene oxide.17. A method according to any preceding claim wherein the device is prillted in a single pass.I 8. A method according to any preceding claim wherein the anode or cathode that the formulation is printed over is supported on a flexible substrate.19. All organic light-emitting electrocheniical cell obtaillable by a method according to any preceding claim.