WO2011141715A1

WO2011141715A1 - Organic light-emitting device and method

Info

Publication number: WO2011141715A1
Application number: PCT/GB2011/000738
Authority: WO
Inventors: Thomas Kugler
Original assignee: Cambridge Display Technology Limited
Priority date: 2010-05-14
Filing date: 2011-05-13
Publication date: 2011-11-17
Also published as: GB201008092D0; GB2480323A

Abstract

An organic light-emitting device and method of making the same, in which the organic light- emitting device has an electrode for injecting charge carriers of a first type, a charge transporting layer comprising at least one organic charge transporting material having nanoparticles dispersed therein, the charge transporting material having an energy level for receiving charge carriers of the first type and the nanoparticles having an energy level for receiving energy from the charge carriers, the nanoparticle energy level value falling between the first electrode workfunction value and the charge transporting material energy level value.

Description

ORGANIC LIGHT-EMITTING DEVICE AND METHOD

Summary of the Invention

This invention relates to organic light-emitting devices and methods of making the same. Background of the Invention

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 transistors and memory array 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- coating.

With reference to Figure 1, an organic light-emitting device (OLED) comprises a substrate 1 carrying an anode 2, a cathode 4 and an organic light-emitting layer 3 between the anode and cathode comprising a light-emitting material.

Holes are injected into the device through the anode 2 and electrons are injected through the cathode 4 during operation of the device. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of the light combine to form an exciton that releases its energy as light.

In order to facilitate the transfer of holes and electrons into the light-emitting layer of an OLED additional layers may be provided between the anode and the cathode, such as a hole- transporting layer between the anode and the light-emitting layer and / or an electron- transporting layer between the cathode and the light-emitting layer.

One known class of hole transporting materials is electron-rich triphenylamines.

WO 01/66618 discloses an organic light-emitting device in which triphenylamine repeat units of a hole transporting polymer are substituted with electron-withdrawing trifluoromethyl groups to adjust the HOMO level of those repeat units.

US 6777706 discloses an optical device having a layer comprising an organic semiconductive material that includes a substantially uniform dispersion of light transmissive nanoparticles. The nanoparticles may influence an optical property of the material. US 2009/072714 discloses a light-emitting device including a layer comprising a composite of a high molecular arylamine compound and an inorganic compound. The inorganic material accepts electrons from the polymer to form a composite having an absorption peak that is not found in the spectra of either the high molecular arylamine compound or the inorganic compound.

Summary of the Invention

In a first aspect the invention provides an organic light-emitting device comprising:

a first electrode for injecting charge carriers of a first type and having a first electrode workfunction;

a second electrode for injecting charge carriers of a second type;

an organic light-emitting layer comprising an organic light-emitting material between the first electrode and the second electrode; and a charge transporting layer between the first electrode and the organic light-emitting layer comprising at least one organic charge transporting material having nanoparticles dispersed therein; wherein:

the charge transporting material has an energy level for receiving charge carriers of the first type; the nanoparticles have a first nanoparticle energy level for receiving energy from said charge carriers;

and wherein the first nanoparticle energy level value falls between the first electrode workfunction value and the charge transporting material energy level value.

Optionally, the first electrode is an anode; the second electrode is a cathode; the charge carriers of the first type are holes; the charge carriers of the second type are electrons; and the charge transporting material energy level and the first nanoparticle energy levels are both HOMO levels.

Optionally, the nanoparticles comprise at least one further nanoparticle energy level value that is different from the first nanoparticle energy level value and wherein the at least one further nanoparticle energy level value falls between the first electrode workfunction value and the charge transporting material energy value. Optionally, the nanoparticle dispersion comprises at least two chemically different nanoparticles.

Optionally, the nanoparticle dispersion comprises metal oxide nanoparticles.

Optionally, the nanoparticle dispersion comprises a mixture of metal oxide nanoparticles of more than one stoichiometry.

Optionally, the charge transporting material is a hole transporting arylamine. Optionally, the charge transporting material is a polymer.

Optionally, the first electrode comprises indium-tin oxide.

In a second aspect the invention provides a method of forming an organic light-emitting device according to the first aspect comprising the steps of depositing a composition comprising the nanoparticles and the charge transporting material to form the charge- transporting layer over the first electrode; depositing the organic light-emitting layer over the charge-transporting layer; and depositing the second electrode over the light-emitting layer.

Optionally according to the second aspect, the composition comprises the charge transporting material and the nanoparticles in a solvent and wherein the solvent is evaporated following deposition of the composition.

Optionally according to the second aspect, formation of the composition comprises the step of mixing at least two different nanoparticle types.

Optionally according to the second aspect, the at least two different nanoparticle types include nanoparticles of different average diameter.

Optionally according to the second aspect, the at least two different nanoparticle types include at least two chemically different nanoparticles.

Optionally according to the second aspect, the organic light-emitting layer is formed by depositing a composition comprising the light-emitting material and a solvent and evaporating the solvent.

In a third aspect, the invention provides an organic light-emitting device comprising:

a first electrode for charge carriers of a first type and having a first electrode workfunction; a second electrode for charge carriers of a second type; an organic light-emitting layer comprising an organic light-emitting material between the anode and the cathode; and

a charge transporting layer between the first electrode and the organic light-emitting layer comprising at least one organic charge transporting material having nanoparticles dispersed therein; wherein: the organic light-emitting material has an energy level for receiving charge carriers of the first type; the charge transporting material has an energy level for receiving charge carriers of the first type;

the nanoparticle has a first nanoparticle energy level for receiving energy from said charge and wherein the charge transporting material energy level value and the first nanoparticle energy level value fall between the first electrode workfunction value and the light-emitting material energy value.

The organic light-emitting device of the third aspect may optionally be formed using method steps as described with respect to the second aspect_} and may comprise any of the features recited in the first aspect,

Description of the Drawings

Figure 1 is a schematic illustration of a prior art OLED;

Figure 2 is a schematic illustration of an OLED according to one embodiment of the invention; and

Figure 3 is a schematic energy level diagram of the OLED of Figure 2.

Detailed Description of the Invention

Figure 2 illustrates an OLED 20 according to one embodiment of the invention having an anode 22, a hole transporting layer 24, a light emitting layer 26 comprising a light-emitting material and a cathode 28. Further layers may be provided between the anode and the cathode such as a hole injection layer between the anode 22 and the hole transporting layer 24, one or more electron transporting layers between the cathode 28 and the light emitting layer 26 and further hole transporting layers between the anode 22 and the light-emitting layer 26. With reference to Figure 3, hole transporting layer 24 comprises a hole transporting material HT carrying nanoparticles NP1 and nanoparticle NP2. The hole transporting material HT provides a matrix in which the nanoparticles are carried.

NP1 and NP2 have ionization potentials falling between the ionization potential of the anode 22 and the ionization potential (HOMO) level of the hole transporting material (and, in this example, the light-emitting material). Moreover, the ionization potentials of NP 1 , NP2 and HT are different so as to provide stepped hole transport between the anode and the hole- transporting layer 24.

In the example of Figure 3, the ionization potential of HT is nearest that of the light-emitting material and the ionization potentials of NP1 and NP2 are nearer to the ionization potential of the anode than that of HT, and as such the nanoparticles serve to improve efficiency of hole transport from the anode to the hole transporting material HT, Alternatively or additionally, it will be appreciated that the ionization potentials of the nanoparticles may likewise be selected so as to improve hole transport from the hole transporting material HT into the light- emitting material if the HOMO level of the hole transporting material HT is close to the ionization potential of the anode but separated from the HOMO level of the light-emitting material.

In this example, two different nanoparticle types are provided in order to provide two energy level steps between the anode 22 and the hole transporting layer 24, however it will be appreciated that embodiments of the invention may have one or more than one nanoparticle types in the hole transport layer.

Anode

The anode may comprise any material with a workfunction suitable for injection of holes into the OLED. Exemplary materials for use as a transparent anode in the case where light is emitted through the anode include indium tin oxide (ITO) and indium zinc oxide (1ZO). In the case where light is not emitted through the anode, for example if the cathode is transparent, then opaque conducting materials such as opaque metals may be used as the anode material.

The anode may comprise a single layer or may comprise more than one layer. For example, the anode may comprise a first anode layer and an auxiliary conductive layer between the anode and the hole transport layer, such as a layer of organic conductive material between the anode and the hole transport layer, in which case the relevant anode workfunction for the purpose of this invention is the workfunction of the layer closest to the hole-transporting layer.

Examples of doped organic hole injection materials include optionally substituted, doped polyethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in EP 0901 176 and EP 0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion ®; polyaniline as disclosed in US 5723873 and US 5798170; and optionally substituted polythiophene or poly(thienothiophene).

Nanoparticles

The nanoparticles may be selected from any nanoparticle having an energy level falling within the relevant range. The energy level can be, for example, a work function, an ionization potential, a Fermi energy level, a triplet level, a HOMO level, or a LUMO level. In embodiments in which the charge carrier to be received is a hole the energy level is preferably an ionization potential.

Suitable nanoparticles for use in a hole transporting layer include metal compounds such as transition metal oxides MOx wherein M is a transition metal, including VOx, MoOx and RuOx.

The mean average diameter of the nanoparticles may be up to about 50 nm. up to about 30 nm or up to about 10 nm.

In the case of metal compound nanoparticles, the nanoparticles may be selected according to their stoichiometry and may include sub-stoichiometric and stoichiometric metal compounds. For example, sub-stoichiometric metal oxides comprise electrons populating the conduction band. Such compounds are therefore n-doped, and the work function is smaller as compared to stoichiometric metal oxides, i.e. the Fermi energy is closer to the vacuum level.

Additionally or alternatively, nanoparticles may be selected according to their size. For example, for a given metal compound the HOMO of that metal compound nanoparticle deepens (i.e. moves further from vacuum level) as the size of the nanoparticle decreases below about 10 nm. This applies in particular to monocrystalline nanoparticles having a diameter below about 10 nm. This effect is described in more detail in, for example, Phys. Chem. Chem. Phys., 2007, 9, pp 725 - 730 "Electronic properties f Ag nanoparticle arrays. A Kelvin probe and high resolution X S study", Mathias Schnippering, Michel Carrara, Annette Foelske, Riidiger Kotz and David J. Fermin, and J. Am. Chem. Soc. 2007, 129, pp 4136-4137 "Size-Dependent Electron Injection from Excited CdSe Quantum Dots into Ti02 Nanoparticles" Istvan Robe I, Masaru Kuno, and Prashant V. Kamat

More than one type of nanoparlicle may be provided in the hole transporting layer, for example to provide more "steps" between the anode and the light-emitting material. The different nanoparlicle types may be, for example, a mixture of nanoparticles with a different average diameter or different chemical composition (e.g. nanoparticles constituted from different elements and / or nanoparticles with different stoichiometrics). In this case, the difference in workfunction between at least two of the different nanoparlicle types is optionally at least 0.2 eV.

Any ratio of hole transporting material to nanoparlicle may be used, and the ratio may be selected according to the ionization potentials of the anode, hole transporting layer and light emitting layer relative to the ionization potentials of the nanoparticles.

However, an optional nanoparlicle: hole transporting material ratio is about 1:1 - 2:1.

Hole transporting material

A wide range of organic hole transporting materials are known to the skilled person. One class of hole transporting materials include optionally substituted arylamines or

heteroarylamines, such as triarylamines.

Optionally, the hole transporting material is a hole transporting polymer. A polymer is particularly beneficial as a binder in which the nanoparticles may be carried. Hole transporting polymers may contain hole transporting repeat units in the polymer backbone or pendent from the polymer backbone. An exemplary hole transporting polymer comprises optionally substituted arylamine repeat units, in particular repeat units of formula (V):

(V)

wherein Ar¹ and Ar² in each occurrence are independently selected from optionally substituted aryl or heteroaryl groups, n is greater than or equal to 1. preferably 1 or 2, R is H or a substituent, preferably a substituent, and p and q are each independently 1, 2 or 3. R is preferably alkyl or -(Ar³), wherein Ar³ in each occurrence is independently selected from aryl or heteroaryl and r is at least 1, optionally I , 2 or 3.

Any of Ar¹, Ar² and Ar³ may independently be substituted with one or more substituents. Preferred substituents are selected from the group R³ consisting of: alkyl wherein one or more non-adjacent C atoms may be replaced with O, S, substituted

N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F or aryl or heteroaryl optionally substituted with one or more groups R⁴.

aryl or heteroaryl optionally substituted with one or more groups R⁴,

fluorine, nitro and cyano;

wherein each R⁴ is independently alkyl in which one or more non-adjacent C atoms may be replaced with O, S, substituted N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F, and each R⁵ is independently selected from the group consisting of alkyl and aryl or heteroaryl optionally substituted with one or more alkyl groups.

Any of the aryl or heteroaryl groups in the repeat unit of Formula (V) may be linked by a direct bond or a divalent linking atom or group. Preferred divalent linking atoms and groups include O, S; substituted N; and substituted C.

Where present, substituted N or substituted C of R\ R⁴ or of the divalent linking group may independently in each occurrence be NR⁶ or C ⁶2 respectively wherein R⁶ is alkyl or optionally substituted aryl or heteroaryl. Optional substituents for aryl or heteroaryl groups

R⁶ may be selected from R⁴ or R⁵.

In one preferred arrangement, R is Ar³ and each of Ar¹ , Ar² and Ar³ are independently and optionally substituted with one or more C₁.₂0 alkyl groups.

Preferred units satisfying Formula 1 include units of Formulae 1 -3:

1 2 3 wherein Ar¹ and Ar² are as defined above; and Ar³ is optionally substituted aryl or heteroaryl. Where present, optional substiluents for Ar³ may be as described above with respect to formula (V).

In another preferred arrangement, aryl or heteroaryl groups of formula (V) are phenyl, each phenyl group being optionally substituted with one or more alkyl groups.

In another preferred arrangement, Ar¹, Ar² and Ar³ are phenyl, each of which may be substituted with one or more Ci.₂₀ alkyl groups, and r = 1.

In yet another preferred arrangement, Ar', Ar² and Ar³ are phenyl, each of which may be substituted with one or more C1.20 alkyl groups, r = 1 and Ar¹ and Ar² are linked by a direct bond or by an O or S atom.

This polymer may be a homopolymer or it may be a copolymer comprising co-repeat units, for example a copolymer comprising repeat units of formula (V) and 1-99 mol % of a co- repeat unit. Exemplary co-repeat units include optionally substituted polyarylenes such as: optionally substituted polyfluorenes, in particular polymers comprising 2,7-linked fluorene repeat units; polyindenofluorenes, particularly 2,7-linked polyindenofluorenes;

poiyphenyienes, particularly poly-l,4-phenylenes. Such polymers as disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.

Particularly preferred co-repeat units comprise optionally substituted fluorenes, such as repeat units of formula IV;

(IV) wherein R¹ and R² are independently H or a substituent and wherein R¹ and R² may be linked to form a ring.

R¹ and R² are optionally selected from the group consisting of hydrogen; optionally substituted -(Ar³)_r wherein Ar³ and r are as described above; and optionally substituted alkyl wherein one or more non-adjacent C atoms of the alkyl group may be replaced with 0, S, substituted N, C=0 and -COO-.

In the case where R¹ or R² comprises alkyl, optional substituents of the alkyl group include F, CN, nitro, and aryl or heteroaryl optionally substituted with one or more groups R⁴ wherein R⁴ is as described above.

In the case where R or R comprises aryl or heteroaryl, each aryl or heteroaryl group may independently be substituted. Preferred optional substituents for the aryl or heteroaryl groups include one or more substituents R³.

Optional substituents for the fluorene unit, other than substituents R¹ and R², are preferably selected from the group consisting of alkyl wherein one or more non-adjacent C atoms may be replaced with 0, S, substituted N, C=0 and -C00-, optionally substituted aryl, optionally substituted heteroaryl, fluorine, cyano and nitro.

Where present, substituted N in repeat units of formula (IV) may independently in each occurrence be NR⁵ or NR^d.

In one preferred arrangement, at least one of R^! and R² comprises an optionally substituted Ci- o alkyl or an optionally substituted aryl group, in particular phenyl substituted with one or more C^o alkyl groups.

The hole transporting material may be substituted with a crosslinkable group. In the case of a hole transporting polymer, some of the repeat units may carry the crosslinkable group. This may be a repeat unit of formula (IV), a repeat unit of formula (V) or another repeat unit. Exemplary crosslinkable groups include groups comprising a double bond such as groups containing a vinyl or aery late moiety or groups comprising a cyclobutane moiety such as benzocyclobutane. The polymer may be crosslinked following its deposition, in particular to prevent dissolution of the hole transporting layer if an overlying layer is to be formed by solution deposition.

Hole transporting layer The thickness of the hole transporting layer must be at least as thick as the average diameter of the nanoparticles, and may be up to about 200 nni, optionally up to about 100 nm or 50 nm. If the nanoparticles used have relatively high conductivity then a thicker layer may be formed without resulting in an increase in drive voltage as compared to a thinner hole

transporting layer comprising lower conductivity nanoparticles (or, indeed, no nanoparticles).

The thickness of the hole transport layer may be controlled in order to adjust the size of a microcavity created between the anode and cathode and thereby tune the colour of light emitted by the OLED. Furthermore, if the anode has a rough surface then the hole

transporting layer may smooth out this surface roughness.

The combination of the hole-transporting material and the nanoparticles used in the hole- transporting layer may be selected so as to interact. For example, the lone pair of electrons of an arylamine hole-transporting material and a sub-stoichiometric metal o ide nanoparticle may show a Lewis acid-base interaction.

The nanoparticles may be graded through the hole-transporting layer. For example, in the arrangement illustrated in Figures 2 and 3 the concentration of nanoparticles in the hole transporting layer 24 may increase towards the anode 22. Alternatively, if the workfunction of the nanoparticles is closer to the HOMO level of the hole transporting material than the workfunction of the anode then concentration of nanoparticles may increase towards the hole transporting layer.

Gradation of the nanoparticles may be achieved using methods known to the skilled person. For example, acidic nanoparticles such as Mo0 may be used with a basic anode such as indium tin oxide. Accordingly, deposition of M0O₃ onto ITO may result in migration of acidic MoOj nanoparticles towards the ITO, in particular if the nanoparticles are deposited from a solvent formulation and if the solvent drying conditions and / or the solvent boiling point are such that at least some of the nanoparticles are able to migrate towards the anode before evaporation of the solvent reaches a point at which the nanoparticles are unable to migrate through the formulation.

Multiple hole transport layers may be provided between the anode and the light-emitting layer. For example, the embodiment of Figures 2 and 3 may be modified to include two hole transport layers wherein a first layer comprises NP 1 and a second hole transport layer comprises NP2. The provision of multiple hole transport layers is an alternative or additional means for providing gradation of nanoparticles between the anode and the light-emitting layer.

Hole transporting layer formulations

The hole transporting layer may be formed by depositing a composition of the hole transporting material and the nanoparticles in a solvent followed by evaporation of the solvent. Suitable solvents for dissolution of the hole transporting material will be known to the skilled person any may include common organic solvents such as alkylated benzenes or chlorinated alkanes (for the avoidance of doubt, "solvent" in this context- may mean a material capable of dissolving the hole transporting material, but it does not mean a material capable of dissolving the nanoparticles, but rather a solvent for carrying the nanoparticles in a dispersion).

Light emitting material

Suitable light-emitting materials for use in layer 26 include small molecule, polymeric and dendrimeric materials, and compositions thereof. Suitable light-emitting polymers for use in layer 26 include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as: polyfluorenes, particularly homopolymers or copolymers comprising repeat units of formula (IV) above; polyindenofluorenes, particularly 2,7-linked polyindenofluorenes; and polyphenylenes, particularly alkyl or alkoxy substituted poly- 1 ,4- phenylene. Examples of such polymers as disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.

Each of these repeat units is optionally substituted. Examples of substituents include solubilising groups such as Ci-₂₀ alkyl or alkoxy; electron withdrawing groups such as fluorine, nitro or cyano; and substituents for increasing glass transition temperature (Tg) of the polymer.

The light-emitting layer may comprise a host / dopant arrangement in which a host material (such as one of the light-emitting materials described above) is used in combination with a light-emitting dopant. Suitable l ight-emitting dopants include fluorescent and / or

phosphorescent (such as heavy metal complex) light-emitting dopants.

Polymer synthesis

Preferred methods for preparation of conjugated polymers, including light-emitting polymers and hole-transporting polymers, comprise a "metal insertion" wherein the metal atom of a metal complex catalyst is inserted between an aryl or heieroaryl 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. Yamamolo, "Electrically Conducting And Thermally Stable π - 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 is used. Similarly, according to the method of Suzuki polymerisation, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine.

It will therefore be appreciated that repeat units illustrated throughout this application may be derived from a monomer carrying suitable leaving groups. Likewise, an end group or side group may be 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, in particular AB, 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 groups include tosylate, mesylate and triflate.

Cathode

Cathode 28 is selected from materials that have a workfunction allowing injection of electrons into the light-emitting layer. Other factors influence the selection o the cathode such as the possibility of adverse interactions between the cathode and the light-emitting material. The cathode may consist of a single material such as a layer of aluminium.

Alternatively, it may comprise a plurality of metals, for example a bilayer of a low

workfunction material and a high workfunction material such as calcium and aluminium as disclosed in WO 98/10621 ; elemental barium as disclosed in WO 98/57381, Appl. Phys. Lett.

2002, 81(4), 634 and WO 02/84759; or a thin layer of metal compound, in particular an oxide or fluoride of an alkali or alkali earth metal, to assist electron injection, for example lithium fluoride as disclosed in WO 00/48258; barium fluoride as disclosed in Appl. Phys. Lett. 2001, 79(5), 2001 ; and barium oxide. In order to provide efficient injection of electrons into the device, the cathode preferably has a workfunction of less than 3.5 eV, more preferably less than 3.2 eV, most preferably less than 3 eV. Work functions of metals can be found in, for example, Michaelson, J. Appl. Phys. 48( 1 1), 4729, 1977.

The cathode may be opaque or transparent. Transparent cathodes are particularly

advantageous for active matrix devices because emission through a transparent anode in such devices is at least partially blocked by drive circuitry located underneath the emissive pixels. A transparent cathode will comprises a layer of an electron injecting material that is

sufficiently thin to be transparent. Typically, the lateral conductivity of this layer will be low as a result of its thinness. In this case, the layer of electron injecting material is used in combination with a thicker layer of transparent conducting material such as indium tin oxide.

It will be appreciated that a transparent cathode device need not have a transparent anode (unless, of course, a fully 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 light-emitting 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 substrate is commonly glass, however alternative substrates may be used, in particular where flexibility of the device is desirable. For example, the substrate may comprise a plastic as in US 6268695 which discloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in EP 0949850.

The device is preferably encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having suitable barrier properties such as silicon dioxide, silicon monoxide, silicon nitride or alternating stacks of polymer and dielectric as disclosed in, for example, WO 01/81649 or an airtight container as disclosed in, for example, WO 01/19142. 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 atmospheric moisture and / or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.

Solution processing

Hole transporting layer 24 and / or light-emitting layer 26 may be deposited by any process, including deposition from a solution in a solvent.

In the case where the hole transporting layer and / or light emitting layer comprises a polyarylene, such as a polyfluorene, suitable solvents for solution deposition include mono- or poly-alkylbenzenes such as toluene and xylene. Particularly preferred solution deposition techniques including printing and coating techniques, preferably spin-coating and Inkjet printing.

Spin-coating is particularly suitable for devices wherein patterning of the electroluminescent material is unnecessary - for example for lighting applications or simple monochrome segmented displays.

Inkjet printing is particularly suitable for high information content displays, in particular full colour displays. A device may be Inkjet printed by providing a patterned layer over the first electrode and defining wells for printing of one colour (in the case of a monochrome device) or multiple colours (in the case of a multicolour, in particular full colour device). The patterned layer is typically a layer of photoresist that is patterned to define wells as described in, for example, EP 0880303.

As an alternative to wells, the ink may be printed into channels defined within a patterned layer. In particular, the photoresist may be patterned to form channels which, unlike wells, extend over a plurality of pixels and which may be closed or open at the channel ends.

Other solution deposition techniques include dip-coating, roll printing and screen printing.

If light-emitting layer 26 is deposited from a solution in a solvent then hole-transporting layer 24 may be crosslinked prior to deposition of light-emitting layer 26, or the hole transporting material may be selected such that it is not soluble in the solvent used to deposit the light- emitting layer. Energy level measurements

HOMO, LUMO, electrode workfunction and nanoparticle workfunction values may be calculated by any method known to the skilled person in order to determine the nanoparticle workfunction required to provided stepped charge transport from the relevant electrode to the light-emitting material, A suitable measurement method is photoelectron spectroscopy using AC-2 apparatus available from RK Instruments Inc, as described in more detail below.

Measurements may alternatively be made using ultraviolet photoelectron spectroscopy measurements performed in a vacuum system, by irradiating the sample surface with ultraviolet light of a fixed wavelength. This measurement results in plots of the Photoelectron Intensity as a function of Kinetic Energy, which are 1:1 -projections of the Density-Of-State in the sample substrate.

In order to take measurements by photoelectron spectroscopy such as AC-2, the thickness of the polymer layer covering the surface of the nanoparticles should be within the information depth of the measurement technique. For AC-2 measurements, the information depth is around lOnm.

A further technique, which is less sensitive to the surface of the composite, is Kelvin probe microscopy which can be used to record spatially resolved maps of the ionisation potentials.

The HOMO energy level value of the hole transporting material and the nanoparticle HOMO value(s) may be the values measured for those materials in isolation, or as measured for the hole transporting material / nanoparticle composition; no doping occurs between the

nanoparticles and the hole transporting material, and consequently the HOMO levels of the two materials are substantially the same both in isolation and in the composition.

The absence of doping can be identified because there is substantially no change in

conductivity of the hole-transporting material alone and the composition of the hole transporting material and nanoparticles. Moreover, the UV-vis absorption spectrum of the composition does not comprise any significant absorption peak that is not found in the individual materials. a) Formation of a hole transporting composition

A hole transporting layer was formed from a hole transporting polymer, Polymer 1, and nanoparticles PQN933. PQN933 is sub-stoichiometric M0O₃ with oxygen deficiencies available from Intrinsic) Materials.

50% 12.5%

Polymer 1 was prepared by Suzuki polymerisation as described in WO 00/53656 of a monomer reaction mixture comprising 50 mol % of a diboronic acid ester of 9,9-bis(l,3- dihexylbenzene) with dibromo-substituted monomers corresponding to the other repeat units illustrated above in the given molar percentages,

24 mg of Polymer 1 was dissolved in 6 ml of dry chloroform and 19 mg of PQN933 was added to the solution in a glovebox.

The composition was treated for 1 hour in an ultrasonic bath in order to disaggregate the nanoparticles.

The composition was filtered through filters with successively decreasing pore sizes, specifically 2.7, 1.0, 0.45, 0.20 and 0.10 micron filters in order to remove aggregated nanoparticles that were not disaggregated by the ultrasonic bath treatment.

Drops of the filtered composition were deposited onto a glass substrate and heated to 100°C for 5 minutes.

b) Measurement of energy values

Energy level values of hole transport materials and nanoparticles were measured by photoelectron spectroscopy using the AC- 2 photoelectron spectrometer available from R Instruments Inc.

Measurements are performed in air, and result in plots of photoelectron yield vs. photon energy. The measurements are performed by probing a sample that is typically several square millimetres in area, and includes the following steps: • UV photons emitted from a deuterium lamp are monochromatized through the grating monochromator

• The monocromatized UV photons are focused on a sample surface in the air

^• The energy of UV photon is increased from 3.4eV to 6.2eV, step by step

^• When the energy of the UV photon is higher than the threshold energy of

photoemission of the sample material (i.e. the Ionisation Potential), the photoelectrons are emitted from the sample surface

^• Photoelectrons emitted from the sample are detected and counted in the air by the open counter

^• Photoemission threshold (Ionisation Potential) is determined from the energy of an intersecting point between a background line and the extended line of the square root of the photoelectric quantum yield.

The following ionization potential values were obtained:

PQN933: -5.4 eV

Polymer 1 : -5.57 eV

The ionisation potential of 1TO as measured by AC-2 is about 5 eV.

A range of other nanoparticle materials are available, for example PQN929 which is near- stoichiometric M0O₃.

ITO on a glass substrate was subjected to UV-ozone treatment and a layer of hole transporting material as described in Example 1 was spin-coated over the anode. A light- emitting layer comprising polyfluorene repeat units of formula (IV) and amine repeat units of formula (V) was deposited over the hole transporting layer by spin-coating, and a cathod of metal fluoride / aluminium was evaporated over the light-emitting layer.

Although the invention is described herein with respect to injection of holes from an anode into the HOMO of a hole transporting material by use of one or more nanoparticle types with suitable HOMO levels to provide stepped hole transport, it will be appreciated that the teachings herein may analogously be applied to an electron transporting layer between the light-emitting layer and the cathode in order to provide stepped electron transport into the LUMO of an electron transporting material by use of nanoparticles with an appropriate LUMO level.

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

1. An organic light-emitting device comprising:

a first electrode for injecting charge carriers of a first type and having a first electrode workfunction; a second electrode for injecting charge carriers of a second type:

an organic light-emitting layer comprising an organic light-emitting material between the first electrode and the second electrode; and

a charge transporting layer between the first electrode and the organic light-emitting layer comprising at least one organic charge transporting material having

nanoparticles dispersed therein; wherein:

the charge transporting material has an energy level for receiving charge carriers of the first type;

the nanoparticles have a first nanoparticle energy level for receiving energy from said charge carriers; and wherein the first nanoparticle energy level value falls between the first electrode workfunction value and the charge transporting material energy level value.

2. An organic light-emitting device according to claim 1 wherein the first electrode is an anode; the second electrode is a cathode; the charge carriers of the first type are holes; the charge carriers of the second type are electrons; and the charge transporting material energy level and the first nanoparticle energy levels are both HOMO levels.

3. An organic light-emitting device according to claim 1 or 2 wherein the nanoparticles comprise at least one further nanoparticle energy level value that is different from the first nanoparticle energy level value and wherein the at least one further nanoparticle energy level value falls between the first electrode workfunction value and the charge transporting material energy value.

4. An organic light-emitting device according to claim 3 wherein the nanoparticle

dispersion comprises at least two chemically different nanoparticles.

5. An organic light-emitting device according to any preceding claim wherein the

nanoparticle dispersion comprises metal oxide nanoparticles.

6. An organic light-emitting device according to claims 3, 4 and 5 wherein the nanoparticle dispersion comprises a mixture of metal oxide nanoparticles of more than one stoichiometry.

7. An organic light-emitting device according to any preceding claim wherein the charge transporting material is a hole transporting arylamine.

8. An organic light-emitting device according to any preceding claim wherein the charge transporting material is a polymer,

9. An organic light-emitting device according to any preceding claim wherein the first electrode comprises indium-tin oxide.

10. A method of forming an organic light-emitting device according to any preceding claim comprising the steps of depositing a composition comprising the nanoparticles and the charge transporting material to form the charge-transporting layer over the first electrode; depositing the organic light-emitting layer over the charge -transporting layer; and depositing the second electrode over the light-emitting layer.

11. A method according to claim 10 wherein the composition comprises the charge

transporting material and the nanoparticles in a solvent and wherein the solvent is evaporated following deposition of the composition,

12. A method according to claim 10 or 11 wherein formation of the composition

comprises the step of mixing at least two different nanoparticle types.

13. A method according to claim 12 wherein the at least two different nanoparticle types include nanoparticles of different average diameter.

14. A method according to claim 12 or 13 wherein the at least two different nanoparticle types include at least two chemically different nanoparticles.

15. A method according to any of claims 10-14 wherein the organic light-emitting layer is formed by depositing a composition comprising the light-emitting material and a solvent and evaporating the solvent.

16. An organic light-emitting device comprising:

a first electrode for charge carriers of a first type and having a first electrode workfunction;

a second electrode for charge carriers of a second type; an organic light-emitting layer comprising an organic light-emitting material between the anode and the cathode; and a charge transporting layer between the first electrode and the organic light-emitting layer comprising at least one organic charge transporting material having

nanoparticles dispersed therein; wherein:

the organic light-emitting material has an energy level for receiving charge carriers of the first type;

the nanoparticle has a first nanoparticle energy level;

and wherein the charge transporting material energy level value and the first nanoparticle energy level value fall between the first electrode workf unction value and the light-emitting material energy value.