GB2473815A

GB2473815A - Electroluminescent material and devices

Info

Publication number: GB2473815A
Application number: GB0916394A
Authority: GB
Inventors: Martin Robert Bryce; Kiran T Kamtekar; Ben Lyons; Andrew P Monkman; Helen L Vaughan; Geoff Williams
Original assignee: ZUMTOBEL GmbH; Zumtobel Lighting GmbH Austria; Thorn Lighting Ltd
Current assignee: ZUMTOBEL GmbH; Zumtobel Lighting GmbH Austria; Thorn Lighting Ltd
Priority date: 2009-09-18
Filing date: 2009-09-18
Publication date: 2011-03-30
Also published as: GB0916394D0

Abstract

A monomer for use in conjugated oligomers and polymers has a general structure based around a phenothiazine or related heterocyclic core, and is of the following formula, where X is SO or SO2 and Y comprises N, S, C, Si or P and has at least one group Z pendent therefrom, where Z comprises an aryl group. The monomer may especially be N-arylphenothiazine—S,S—dioxide and it can be copolymerised with 9,9—dioctylfluorene. The polymers manufactured from the invention may be used in a range of optoelectronic devices by convenient solution processing techniques.

Description

ELECTROLUMINESCENT MATERIAL AND DEVICES

The present invention relates to electroluminescent materials and devices.

Electroluminescence is a phenomenon whereby materials emit light in response to an electric current passed therethrough. This phenomenon is exploited, inter al/a, in the manufacture and operation of organic light emitting diodes (OLED5). OLED5 typically include an organic semiconducting material comprise of an organic small molecule and/or a polymer (e.g. an oligomer) which has a network of conjugated ti-orbitals.

In a simple OLED configuration, a thin film comprising an organic light emitting material is sandwiched between an anode and a cathode. When a bias voltage is applied across the device, holes are injected into the highest occupied molecular orbital (HOMO) of the light emitting material and electrons are injected into the lowest unoccupied molecular orbital (LUMO).

When the electrons and holes recombine, an exciton is formed which is able to decay radiatively, resulting in the emission of light. The colour of the light produced is dependent on the HOMO-LUMO bandgap of the material.

However, in order to improve stability and enhance device efficiency (photons of light emitted per injected charge) a multi-layered device is usually employed. Such devices incorporate layers of charge transporting material such as an electron transport material and/or a hole transporting material to affect the injection of charges into the emissive material resulting in enhanced light emission. A number of materials have been described for use as electron and hole transporting materials (Kulkarni et al. Chem. Mater.

2004, 16: 4556 -4573; Yan et al. AppI. Phys. Lett. 2004, 84: 3873 -3875).

White organic light emitting devices (WOLED5) have attracted increasing interest in recent years as their potential applications to full colour displays with the help of colour filters, in backlights for liquid crystal displays (LCD5) and in solid-state lighting (SSL) devices become apparent. Consequently, an efficient single-component OLED capable of white light emission is highly desirable.

A number of approaches have been described for achieving white emission from WOLED5, such as using multi-layer device structures to get white light emission at the same time from two or more active layers which each emit a different colour light (Burrows et al. AppI. Phys. Lett. 1998, 73: 435 -437; D'Andrade et al. Adv. Mater. 2002, 14: 147 -151; Lee et al. Mater. Sci. Eng., B, 2002, 95:24 -28; Huang et al. AppI. Phys. Lett. 2002, 80: 2782 - 2784), devices comprising single-layers into which different fluorescent or phosphorescent dopants are dispersed into a host polymer (where a dopant is an impurity element added in low concentrations to alter the optical/electrical properties of a semi-conductor material) (Kido et al. AppI.

Phys. Lett. 1995, 67; 2281 -2283; Mazzeo et al. , Synth Met. 2003, 139:675 -677) or pixelated structures in which the three Red, Green and Blue active layers are patterned as a matrix array using precise shadow masking (Kido et al. AppI. Phys. Lett. 1995, 67; 2281 -2283). However, the high band gap required for blue emissions has ensured that the production of efficient, long-lasting blue emitters has proved challenging.

A single layer structure WOLED is desirable since they lend themselves more readily to large scale production and low-cost manufacturing. However, the single layer fluorescent or phosphorescent doped WOLED5 that have been described suffer from a number of drawbacks. Firstly, these devices can suffer from variations in the CIE (Commission Internationale de l'Eclairage) co-ordinates of colour emission due to the applied bias, rendering them less preferable for LCD backlight and other illumination purposes.

Over the last decade polyfluorenes (PF) have emerged as effective electroluminescent materials in the class of semiconductive organic conjugated polymers. Properties of PF5 which render them suitable for electroluminescent applications include bright blue emission, high hole mobility, excellent thermal and chemical/electrochemical stability and easy tuneable properties through chemical modifications and co-polymerisations.

However, the mobility and injection off electrons are much lower than that of holes in these materials, resulting in a greatly unbalanced transport of positive and negative carriers. This poor charge balance in the electroluminescent material leads to overall low efficiency of OLED devices based on PF5. A further problem inherent to PF materials is the formation of fluorene defects as a result of thermal-, photo-, or electrical degradation, which is frequently encountered in these materials, resulting in low emission bands.

Many different approaches have been used to mitigate these problems; among them is the introduction of either electron donor or electron acceptor moieties into the homopolymers (in the backbone, as side-chains or endcaps). In OLED5, the introduction of such electron donor or electron deficient moieties represents an efficient way to modify the HOMO-LUMO levels of the polymers thus allowing improved electron/hole mobility of the material, decreased barriers of charge injection into the electroluminescent layer, and tuning of the colour emission, i.e. by reducing the LUMO energy at the interface of the electrode and the polymer it is possible to increase electron injection to balance the charges in the device and improve efficiency PF5, and indeed other conjugated polymer systems, may be synthesised by a number of methods, but are commonly made by one of the Suzuki polymerisation method or the Yamamoto polymerisation method. Each of these methods ensures a coupling of the conjugation systems of the monomer units.

Suzuki polymerisation effects the coupling of an aryl-or vinyl-boronic acid or ester with an aryl or vinyl halide to form a conjugated system. The reaction is catalysed by a palladium (0) complex such as tetrakis(triphenylphosphine)palladium(0) and is described in several review papers (such as Suzuki, A. J. Organometallic Chem. 1999, 576, 147-168).

Its use in polymerisation for organic semiconductors is described in Sakamoto 3. et al., Macrol. Rapid Commun., 2009, 30, 653-687.

Yamamoto polymerisation effects the coupling of aryl-or vinyl-magnesium halides with aryl-or vinyl-halides to form a conjugated system. The reaction is catalysed by nickel complexes and was first described by Yamamoto et al in Bull Chem. Soc. Jpn., 1978, 51, 2091. Its use in polymerisation for organic semiconductors is described in Yamamoto T. Prog. Polym. Sci., 1993, 17, 1153-1205.

Fluorene-based copolymers incorporating electron-deficient thiophene-S,S-dioxide moieties have been tested and did not demonstrate improved performance of LED over conventional PF homopolymers, showing low to moderate external EL efficiencies of iü to O.l4% (Charas et al. Chem. Commun. 2001, 1216: Charas et al., J. Mater. Chem. 2002, 12:3523; Pasini et al., J. Mater. Chem. 2003, 13:807; Destri et al., Synth. Nletals, 2003, 138, 289). Therefore, typically it is not expected that incorporation of a thiophene-S,S-dioxide moiety into a fluorene-based copolymer would be beneficial. In fact it is expected, and reported, that quenching of the emission occurs resulting in overall poor efficiency of OLED devices incorporating these materials.

In a first aspect, the invention provides an electroluminescent polymer comprising a first repeat unit comprising the structure: Where x is SO or SO2 and Y comprises N,S,C,Si or P and wherein Y has at least one group Z pendent therefrom where Z comprises an aryl group.

Without wishing to be bound by any particular theory, it is thought that the inclusion of the Y group reduces the conjugation across the structure, thereby increasing the band gap of the material and offering a deep blue electroluminescent emission spectrum.

Moreover, the inventors have surprisingly found that the inclusion of an aromatic containing group Z pendent to the structure Y, an increase in emission efficiency is observed.

When referring to para-coupling and meta-coupling the ring structures are numbered such that carbon 1 is the carbon next to the atom or group of atoms with greatest molecular weight. Position 2 is "ortho", position 3 is "meta", and position 4 is "para".

Preferably, X is SO2 and/or Y is N. Preferably, Z comprises a substituted or unsubstituted aryl group. In certain embodiments, Z is partially or completely fluorinated.

Preferably, the pendant group Z comprises a structure selected from the group: ± ± R2R3 R1 R8R9 Ar3Ar6 Ar1' Ar2 Ar4 Ar5 where R1 to R3 and R6 to R1° may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, nitro, amino, amido, halo, alkoxy, hydroxyl, thiol or thioalkyl and where Ar1 to Ar6 comprise optionally Optionally, groups R1 to R3 and R6 to R10, if present, are partially or entirely fluorinated.

In some embodiments, the pendant group Z comprises a structure selected from the group: CF3CF3 Bu CF3 Preferably, the polymer comprises a second, different, repeat unit comprising an organic semiconducting structure.

Preferably, the second repeat unit comprises the structure: R R5 Where R4 and R5 may be the same or different and may comprise H or C1 to C20 (e.g. C1 to C10) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups.

Preferably, R4 and R5 comprise C1 to C10 unsubstituted straight alkyl chains.

In certain embodiments, the polymer may comprise a third repeat unit comprising an electron transport moiety. The electron transport moiety preferably comprises a structure selected from the group: R15 R17 -OJ-(>-cL:>-o Q -Q R16 R18 ö R° where R15 to R2° may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, nitro, amino, amido, halo, alkoxy, hydroxyl, thiol or thioalkyl; where Q1 to Q4 may be the same or different and may comprise N or CH and where Z may comprise 0, NR or S. Preferably, the polymer comprises a fourth different repeat unit comprising a hole transport moiety.

Preferably, the hole transport moiety preferably comprises a structure selected from the group: c-N-/ N1" / R13 \14 / \Arbo m s m Ar7 Ar9 where R13 and R14 may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, nitro, amino, amido, halo, alkoxy, hydroxyl, thiol or thioalkyl and where Ar7 to Ar1° comprise optionally substituted aryl groups.

Preferably, the polymer comprises a block copolymer, regular or alternating copolymer or a random copolymer, including the first repeat unit and one, some or all of the second, third and fourth repeat units.

Preferably, the copolymer is a random copolymer. The term "random copolymer" denotes a copolymer consisting of macromolecules in which the probability of finding a given monomeric unit at any given site in the chain is independent of the nature of the adjacent units.

The term "regular copolymer" denotes a copolymer comprising a sequence of regularly alternating repeating units.

The term "block copolymer" denotes a copolymer where each repeat unit is arranged to be adjacent at least one identical repeat unit.

In some embodiments the polymer further comprises one or more red and/or green emitting repeat units. Preferably the red and/or green emitting repeat units comprise phosphorescent emitters, such as those selected from complexes of the form IrL3 or IrL2X, where x = R33''R34 where R31 to R34 may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, nitro, amino, amido, halo, alkoxy, hydroxyl, thiol or thioalkyl; or complexes of the form: r 1 I Ii o Alternatively or additionally, the polymer may comprise colour dopants, such as those selected from the group: R21 5R26 7R28 where R21 and R3° may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, nitro, amino, amido, halo, alkoxy, hydroxyl, thiol or thioalkyl and where X' and X2 may comprise S or C2H4.

Optionally, the copolymer or oligomer according to the first or second aspects further comprises at least one alkyne group. The at least one alkyne group can be situated between a first repeat unit and a second repeat unit or between two second repeat units.

Preferably, the optionally first repeat units comprise approximately 2 -50 mole percent of the polymer.

More preferably, the first repeat units comprise approximately 10 -35 mole percent of the polymer.

More preferably, the first repeat units comprise approximately 20 -30 mole percent of the polymer.

Preferably, at least a portion of the first repeat units functions as a blue emitting moiety.

Preferably, at least a portion of the first repeat units functions as an electron transporting moiety.

Preferably, at least a portion of the first repeat units functions as a high energy triplet host in the presence of one or more phosphorescent emitters (e.g. if the polymer comprises one or more phosphorescent emitters).

The first repeat units and the second repeat units are preferably selected such that they allow charge transfer therebetween in the excited state.

Preferably, the polymer is capable of emitting light close to white point (x,y = 0.33, 0.33).

The second repeat units may be substituted at any suitable position with any suitable functional group which results in a functionally equivalent molecule according to the present invention.

Preferably, when the second repeat units are substituted, they may be substituted at any suitable position with alkyl groups.

Optionally, when the second repeat units are substituted, they may be substituted at any suitable position by pendent moieties.

Optionally, when the second repeat units are substituted, they may be substituted at any suitable position by spiro units.

According to a third aspect of the invention there is provided an optical device comprising a polymer as described above.

Preferably, the optical device is an organic light emitting device whereby the polymer makes up at least part of an emissive layer.

Preferably, the organic light emitting device comprises first and second electrodes and the emissive layer is formed between the first and second electrodes.

Optionally, the organic light emitting device comprises at least one further layer, e.g. a hole injection layer or an electron injection layer.

Preferably, the copolymers of the invention are prepared by Suzuki polymerisation.

Optionally, the copolymers of the invention can be prepared by any suitable polymerization method, such as Yamomoto polymerization.

Embodiments of the present invention shall now be described in more detail with reference to the following drawings.

Figure 1 shows a synthesis of a monomer for forming a polymer according to the present invention; Figure 2 shows a synthesis of a polymer according to the present invention; Figure 3 shows photoluminescence data for the Polymer 1; Figure 4 shows photoluminescence data for the Polymer 2; Figure 5 shows readings for brightness against current density for a device according to Example 3; Figure 6 shows readings for electroluminescent power against electroluminescent wavelength for a device according to Example 3; Figure 7 shows current density against voltage for a device according to

Example 3;

Figure 8 shows readings for brightness against current density for a device according to Example 4; Figure 9 shows readings for electroluminescent power against electroluminescent wavelength for a device according to Example 4; Figure 10 shows current density against voltage for a device according to

Example 4.

Random 9,9-d ioctylfluorene-phenothiazine-S,S-d ioxide copolymers were synthesised by palladium-catalyzed Suzuki copolymerisation of 9,9- dioctylfluorene monomers and various derivatives of phenothiazine-S,S-dioxide, as demonstrated in the examples below.

The amount of the phenothiazine-S,S-dioxide was varied from 2 to 50 mol % to produce copolymers with different loading of phenothiazine-S,S-dioxide [PTD] units into the main chain of the conjugated polymers. The starting material, 2,7-dibromo-9,9-dioctylfluorene (1) was prepared according to literature procedures (Perepichka et al. Chem. Commun. 2005, 3397-3399).

Commercial (Aldrich) 9, 9-dioctylfluorene-2,7-bis(trimethyleneborate) (3) was used in the syntheses and Figure 3 (and Example 5) demonstrates synthesis of monomer 2,8-dibromodibenzothiophene-S, S-dioxide (6) which was used.

Preparation of N-p-butyl-phenyl phenothiazine-S,S-dioxide (Monomer 1, shown graphically in Figure 1) Stage 1: Phenothiazine (Aldrich) (25 mmol) and p-butylbromobenzene (Aldrich) (30 mmol) were dissolved in toluene (Fisher 80 ml) under a protective atmosphere of argon. Pd2dba3 (0.2 mol%) and HPtBu3BF4 (2 mol%) were added and the mixture stirred for 10 mm. Sodium tert-butoxide (Aldrich) (38 mmol) was added and the mixture was stirred at 80°C for 65 h. After cooling, the mixture was diluted with water and ethyl acetate and the aqueous layer was extracted with more ethyl acetate. The organics were combined, dried, filtered and concentrated before being passed through a silica plug and eluting with 10% dichloromethane (DCM) in petroleum ether 40-60 (both Fisher). The eluent, N-arylphenothiazine, was concentrated and crystallised from ethanol.

Stage 2: The N-arylphenothiazine (7.5 mmol) was dissolved in DCM (Fisher) (50 ml) under a protective atmosphere of argon and cooled in an ice bath. N-bromosuccinimide (NBS) (Aldrich) (15.5 mmol) was added in small portions and the solution was stirred with protection from light overnight, slowly warming to room temperature. The solution was diluted with water and the aqueous layer was extracted with DCM. The organics were washed with brine, dried, filtered and concentrated before being passed through a silica plug and eluting with petroleum ether 40-60. The eluent, dibromo-N-arylphenothiazine was concentrated and crystallised from ethanol.

Stage 3: The dibromo compound (2.2 mmol) was dissolved in acetic acid (Fisher) and hydrogen peroxide (Aldrich) (15 ml) was carefully added and the solution was refluxed for 5 h at 130°C. A further 10 ml of peroxide was added and the solution refluxed overnight. After cooling, water was added and the resultant solid was filtered and washed with water. The solid was dissolved in DCM and washed with brine, dried, filtered and concentrated to give an oil, Monomer 1, which was crystallised from ethanol.

Example 1

Synthesis of fluorene-Monomer 1 copolymer (Polymer 1, shown graphically in Figure 2).

A flask was charged with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3- propanediol) ester (400.0 mg, 99.50%, 0.713 mmol), 2,7-dibromo-9,9- dioctylfluorene (275.0 mg, 99.95%, 0.501 mmol), N-(4-butylphenyl)-3,7-dibromophenothiazine-S,S-dioxide (Monomer 1, 112.4 mg, 99.24%, 0.214 mmol) and toluene (16 m). The mixture was degassed for 15 mm before bis[(tri-ortho-tolyl)phosphine]palladiumdichloride (11 mg, 1 mol%) was added.

Degassing was continued for 15 mm before a degassed 2O% wt. solution of tetraethylammonium hydroxide in water (4 ml) was added and the mixture was vigorously stirred at 115 °C with protection from light for 18 h. Bromobenzene (0.1 ml) was added and stirring continued at 115 °C for 1 h before benzeneboronic acid (100 mg) was added and stirring continued at 115°C for 1 h. After cooling, the grey mixture was added slowly to 300 ml of methanol to precipitate the crude polymer as grey fibres. The fibres were filtered and washed with methanol, water and more methanol.

The polymer was redissolved in toluene (20 ml) and a solution of N,N-diethyldithiocarbamic acid sodium salt (15 ml) was added and the mixture was stirred at 65 °C for 16 h. The layers were separated and the aqueous layer was extracted with toluene.

The combined organic layers were washed with dilute HCI solution, sodium acetate solution and twice with water before being passed through a celite 545 plug to yield a clear yellow solution. The solution was concentrated until it became viscous and was then added dropwise to methanol (350 ml) to precipitate the polymer as white fibres which were isolated by filtration and washed with methanol and acetone, 486 mg.

Gel permeation chromatography against polystyrene showed an M of 56,833 and an M of 206,510.

Photoluminescence of Polymer 1 was measured in a fluorometer in solution in toluene and in chloroform (both having a primary emission peak at 414 nm and a secondary peak at 438 nm respectively) and as a thin film on a quartz substrate, excited at 390 nm (twin emission peaks at 425 nm and 448 nm). The results are displayed graphically in Figures 3a (solution) and 3b (film).

Preparation of N-(4-trifluoromethylphenyl)-3,7-dibromophenothiazine-S,S-dioxide (Monomer 2, shown graphically in Figure 1) Stage 1: Phenothiazine (Aldrich) (25 mmol) and p-trifluoromethylbromobenzene (Aldrich) (30 mmol) were dissolved in toluene (Fisher 80 ml) under a protective atmosphere of argon. Pd2dba3 (0.2 mol%) and HPtBu3BF4 (2 mol%) were added and the mixture stirred for 10 mm. Sodium tert-butoxide (Aldrich) (38 mmol) was added and the mixture was stirred at 80°C for 65 h. After cooling, the mixture was diluted with water and ethyl acetate and the aqueous layer was extracted with more ethyl acetate. The organics were combined, dried, filtered and concentrated before being passed through a silica plug and eluting with 10% dichioromethane (DCM) in petroleum ether 40-60 (both Fisher). The eluent, N-p-trifluorophenylphenothiazine, was concentrated and crystallised from ethanol.

Stage 2: The N-arylphenothiazine (7.5 mmol) was dissolved in DCM (Fisher) (50 ml) under a protective atmosphere of argon and cooled in an ice bath. N-bromosuccinimide (NBS) (Aldrich) (15.5 mmol) was added in small portions and the solution was stirred with protection from light overnight, slowly warming to room temperature. The solution was diluted with water and the aqueous layer was extracted with DCM. The organics were washed with brine, dried, filtered and concentrated before being passed through a silica plug and eluting with petroleum ether 40-60. The eluent, N-(4-trifluoromethylphenyl)-3,7-dibromophenothiazine-S,S-dioxide was concentrated and crystallised from ethanol.

Stage 3: The dibromo compound (2.2 mmol) was dissolved in acetic acid (Fisher) and hydrogen peroxide (Aldrich) (15 ml) was carefully added and the solution was refluxed for 5 h at 130°C. A further 10 ml of peroxide was added and the solution refluxed overnight. After cooling, water was added and the resultant solid was filtered and washed with water. The solid was dissolved in DCM and washed with brine, dried, filtered and concentrated to give an oil, Monomer 2, which was crystallised from ethanol.

Example 2

Synthesis of fluorene-Monomer 2 copolymer (Polymer 2, shown graphically in Figure 2).

A flask was charged with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3- propanediol) ester (400.0 mg, 99.50%, 0.713 mmol), 2,7-dibromo-9,9- dioctylfluorene (275.0 mg, 99.95%, 0.501 mmol), N-(4-trifluoromethylphenyl)-3,7-dibromophenothiazine-S,S-dioxide (115.3 mg, 98.96%, 0.214 mmol) and toluene (16 m). The mixture was degassed for 15 mm before bis[(tri-ortho-tolyl)phosphine]palladiumdichloride (11 mg, 1 mol%) was added.

Degassing continued for 15 mm before a degassed 2O% wt. solution of tetraethylammonium hydroxide in water (4 ml) was added and the mixture was vigorously stirred at 115°C with protection from light for 18 h. Bromobenzene (0.1 ml) was added and stirring continued at 115°C for 1 h before benzeneboronic acid (100 mg) was added and stirring continued at 115°C for 1 h. After cooling, the grey mixture was added slowly to 300 ml of methanol to precipitate the crude polymer as yellow fibres. The fibres were filtered and washed with methanol, water and more methanol.

The polymer was redissolved in toluene (20 ml) and a solution of N,N-diethyldithiocarbamic acid sodium salt (15 ml) was added and the mixture was stirred at 65°C for 16 h. The layers were separated and the aqueous layer was extracted with toluene. The combined organic layers were washed with dilute HCI solution, sodium acetate solution and twice with water before being passed through a celite 545 plug to yield a clear yellow solution. The solution was concentrated until it became viscous and was then added dropwise to methanol (350 ml) to precipitate the polymer as pale yellow fibres which were isolated by filtration and washed with methanol and acetone, 495 mg.

Gel permeation chromatography against polystyrene showed an M of 44,573 and an M of 164,899.

Photoluminescence of Polymer 2 was measured in a fluorometer in solution in toluene and in chloroform (both having a primary emission peak at 414 nm and a secondary peak at 438 nm respectively) and as a thin film on a quartz substrate, excited at 390 nm (primary emission peak at 422 nm and a secondary peak at 445). The results are displayed graphically in Figures 4a (solution) and 4b (film).

Electronic devices were fabricated using Polymer 1 and Polymer 2.

Example 3

An electroluminescent device was fabricated using a 1.lnm thick glass substrate coated with patterned indium tin oxide to give a sheet resistance of l5ohm/sq. The substrate was cleaned with detergent, deionized water, acetone and isopropanol successively in an ultrasonic bath, followed by UV ozone treatment for 10 minutes.

An approximately SOnm-thick layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT) was spincoated first and then dried for 10 minutes on a hot-plate at 150°C. Polymer 1 was dissolved in toluene and spincoated onto the PEDOT layer to give a layer approximately 65nm thick.

A 4nm thick layer of barium and then at least lOOnm of aluminium were evaporated through a mask onto the polymer layer in a vacuum of approximately 1x106 mBar.

Four devices of 4mm x 5mm were obtained on a single substrate. The devices were sealed by using a UV curable epoxy resin to fix a piece of glass over the device area.

Example 4

The method of Example 3 was repeated, replacing Polymer 1 with Polymer 2, to give four devices of 4mm x 5mm on a single substrate.

Device Testing The substrates of Examples 3, 4 and 5 were each placed in a sample holder where pins made electrical contact to the ITO anodes and barium/aluminium cathodes. The sample holder was mounted in an integrating sphere. The individual devices on each substrate could be selected via switches on the sample holder.

An Agilent 6632B power supply was used to drive the devices.

Electroluminescence was measured using an Ocean Optics USB4000 CCD connected to the sphere with a fibre optic cable. The power supply and CCD were both controlled by a personal computer.

The voltage applied to the device was increased in uniform steps and the current and electroluminescence spectrum measured at each step. These values were then used to calculate quantities such as current density, external quantum efficiency, optical power output, wall plug efficiency, luminance, luminous efficacy, brightness and CIE co-ordinates.

The results of some of these tests are shown in Figures 5 to 10.

As Figures 5 to 7 demonstrate, the device of Example 3 (containing Polymer 1) offers a powerful peak electroluminescent emission at around 425 nm (measured at 4.8V, 3.7mA/cm2, 29cd/m2. CIE co-ordinates x = 0.159, y = 0.243), thereby giving a deep blue emission, without detrimental effect on the device's on-voltage or brightness.

Figures 8 to 10, which display data for the device of Example 4 (containing Polymer 2), show that the device also offers powerful emission at 425 nm (measured at 6.8V, 5.2mA/cm2, 22cd/m2. CIE co-ordinates x = 0.158, y = 0.097), while operating well within accepted voltage levels.

No doubt many other effective alternatives will occur to the skilled person.

It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS1. An electroluminescent polymer comprising a first repeat unit comprising the structure: ccc where X is SO or SO2 and Y comprises N, 5, C, Si or P and wherein Y has at least one group Z pendent therefrom where Z comprises an aryl group.
2. An electroluminescent polymer according to Claim 1, wherein X is SO2 and/or Y is N.
3. An electroluminescent polymer according to Claim 1 or Claim 2, wherein Z comprises a substituted or unsubstituted aryl group. In certain embodiments, Z is partially or completely fluorinated.
4. An electroluminescent polymer according to any of Claims 1 to 3, wherein the pendant group Z comprises a structure selected from the group: R2 R3 R1 where R1 and R2 and R3 may be the same or different and may comprise H or C1 to C10 (e.g. C1 or C5) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups, intro, amino, amido, halo, alkoxy, hydroxyl.
5. An electroluminescent polymer according to Claim 4, wherein groups R1, R2 and R3, if present, are partially or entirely fluorinated.
6. An electroluminescent polymer according to any preceding Claim, wherein the pendant group Z comprises a structure selected from the group: CF3CF3 Bu CF3
7. An electroluminescent polymer according to any preceding Claim, wherein the polymer comprises a second, different, repeat unit comprising an organic semiconducting structure.
8. An electroluminescent polymer according to Claim 7, wherein the second repeat unit comprises the structure: R R5 Where R4 and R5 may be the same or different and may comprise H or C1 to C20 (e.g. C1 to C10) optionally substituted, straight, branched or cyclic alkyl, alkenyl or alkynyl groups.
9. An electroluminescent polymer according to Claim 8, wherein R4 and R5 comprise C1 to C10 unsubstituted straight alkyl chains.
10. An electroluminescent polymer according to any preceding Claim, wherein the polymer comprises a third repeat unit comprising an electron transport moiety.
11. An electroluminescent polymer according to Claim 10, wherein the electron transport moiety comprises the first repeat unit.
12. An electroluminescent polymer according to Claim 1, wherein the polymer comprises a fourth different repeat unit comprising a hole transport moiety.
13. An electroluminescent polymer according to Claim 1, wherein the polymer comprises a block copolymer, regular or alternating copolymer or a random copolymer.
14. An electroluminescent polymer according to any preceding Claim, wherein the polymer further comprises one or more red and/or green emitting repeat units, the red and/or green emitting repeat units optionally comprising phosphorescent emitters.
15. An electroluminescent polymer according to any preceding Claim, wherein the polymer further comprises at least one alkyne group.
16. An electroluminescent polymer according to Claim 15, wherein the at least one alkyne group can be situated between the first repeat unit and the or a second repeat unit or between two second repeat units.
17. An electroluminescent polymer according to any preceding Claim, wherein first repeat units comprise approximately 2 -50 mole percent of the polymer, e.g. 10 -35 mole percent or 20 -30 mole percent of the polymer.
18. An electroluminescent polymer according to any preceding Claim, wherein at least a portion of the first repeat units functions as a blue emitting moiety.
19. An electroluminescent polymer according to any preceding Claim, wherein at least a portion of the first repeat units functions as an electron transporting moiety.
20. An electroluminescent polymer according to any preceding Claim, wherein at least a portion of the first repeat units functions as a high energy triplet host in the presence of one or more phosphorescent emitters, e.g. if the polymer comprises one or more phosphorescent emitters.
21. An electroluminescent polymer according to any preceding Claim, wherein the first repeat units and the second repeat units are selected such that they allow charge transfer therebetween in the excited state.
22. An electroluminescent polymer according to any preceding Claim, wherein the polymer is capable of emitting light close to white point (x,y = 0.33, 0.33).
23. An optical device comprising a polymer according to any preceding Claim.
24. An optical device according to Claim 23 further comprising a phosphorescent emitter.
25. An optical device according to Claim 23 or 24, wherein the optical device is an organic light emitting device whereby the polymer makes up at least part of an emissive layer.
26. An optical device according to Claim 25, wherein the organic light emitting device comprises first and second electrodes and the emissive layer is formed between the first and second electrodes.
27. An optical device according to Claim 25 or 26, comprising at least one further layer, e.g. a hole injection layer or an electron injection layer.