WO2017216557A1

WO2017216557A1 - Methods for the production of organic electronic devices

Info

Publication number: WO2017216557A1
Application number: PCT/GB2017/051738
Authority: WO
Inventors: Dawei DI; Le Yang; Richard Friend
Original assignee: Cambridge Enterprise Limited
Priority date: 2016-06-15
Filing date: 2017-06-14
Publication date: 2017-12-21
Also published as: GB201610443D0

Abstract

The present invention relates to methods of manufacturing electroluminescent devices having at least four contiguous layers that comprise the sequential deposition of at least three of functional layers from solution, wherein material and solvent selection ensures that each deposition step does not compromise the structural integrity and electronic quality of any previously deposited layer thereby allowing access to high efficiency devices.

Description

Methods for the Production of Organic Electronic Devices

Field of Invention

The present invention relates to novel methods for the production of electroluminescent devices, such as organic light-emitting diodes (OLEDs), by solution processing. The invention also relates to electroluminescent devices produced according to these methods.

Background of the Invention

Organic light-emitting diodes (OLED) are light-emitting diodes, a type of electroluminescent device, in which the emissive electroluminescent material is a film of organic material which emits light in response to an electrical current. Similarly, polymer light-emitting diodes (PLED) are electroluminescent devices in which the emissive electroluminescent material is a film of light-emitting polymer (LEP) material which emits light in response to an electrical current. The term electroluminescent device therefore encompasses OLED, PLED and various other light-emitting devices including those containing organometallic emitters. For simplicity, and without being bound by theory, the background to the invention below is presented by reference to OLED devices although it will be appreciated that the same general principles apply to the wider group of electroluminescent devices.

The emissive organic layer of an OLED is sandwiched between two electrical contact layers (the electrodes). For enhanced efficiency, in addition to this light emitting layer, OLED devices usually incorporate a layer(s) of charge injecting, transporting or blocking material between the emissive layer and the electrodes. The charge transporting/injection materials are added to the device structure to improve charge injection into the emissive layer and to prevent charges with the opposite sign from escaping the device. The charge transporting layers in general facilitate hole transport/electron blocking or electron transport/hole blocking. Consequently, by promoting charge transport into emissive layer, these charge transport materials facilitate the recombination of holes and electrons in the emissive layer to form a bound state called an exciton. The electrons in the excitons in due course relax from a high energy state to a lower energy state and in so doing emit radiation which, for an OLED device, is of a frequency most often in the visible region.

In order to obtain an OLED device with optimal emission properties and efficiency it is usually necessary to produce devices having a multilayer structure. Furthermore, the thickness and quality of each layer in an OLED need to be tailored to ensure efficient transport of holes and electrons into the emissive layer and to ensure that exciton formation and recombination occur within the emissive layer as this delivers optimal light emission. By selection of the materials from which the device layers are formed and the thickness and quality of those layers, devices with high internal quantum efficiency (IQE) and external quantum efficiency (EQE) can be generated. In addition, control of device layer thickness influences the turn-on voltage - the potential difference between the electrodes at which the OLED begins to emit light. It is highly desirable for an OLED device to have a low turn-on and drive voltages because if the device can be driven at lower energy to achieve a specified light output device efficiency and lifetime is generally improved. The method of production of OLED devices is in general dictated by the physical nature of the materials used in the device layers. Prior art techniques for the generation of OLED devices include both vacuum deposition and solution deposition methods. In the case of devices containing small molecule emitters, OLED production generally involves thermal evaporation in a vacuum commonly termed vacuum deposition (see e.g. Tang, C. W.; Vanslyke, S. A. (1987). "Organic electroluminescent diodes". Applied Physics Letters 51 (12): 913). OLEDs containing small molecule emitters are sometimes referred to as SM- OLEDs. The use of vacuum deposition can allow the formation of well-controlled, homogeneous films, and the construction of very complex multi-layer structures. This can deliver devices with high efficiency (IQE and EQE) and low turn on voltages. Notable downsides of the use of vacuum deposition for elaboration of OLED is that production costs are high and that this technique is not general considered suitable for large area devices. In addition, there usually a significant loss of material during the vacuum deposition process and a consequently increased process cost from this aspect.

For OLEDs containing a polymer emitter, sometimes referred to as polymer light- emitting diodes (PLEDs), vacuum deposition is generally not appropriate due to the physical properties of the polymer and instead a simpler, cheaper, solution-based deposition approach to device fabrication is usually adopted (see e.g. Burroughes, J. H.; Bradley, D. D. C; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burns, P. L; Holmes, A. B. (1990). "Light-emitting diodes based on conjugated polymers". Nature 347 (6293): 539-541). In more detail, the light-emitting polymer (LEP) can be dissolved in an organic solvent and deposited on a substrate in that solution without the need for a vacuum, the solvent being removed by evaporation during and/or after film formation. Solution deposition is, advantageously, cheaper than vacuum deposition and is also more suited to the production of large surface area devices. In addition, complex device structure can be accessed through, for example, printing without the need to use masks. A disadvantage of prior art solution processing approaches is that deposition of multiple layers is compromised by the fact that deposition of subsequent layers upon a layer/layers previously deposited from solution tends to degrade the previously deposited layer/layers. Solution processing for multilayer devices also often results in intermixing of layers or interfaces and these phenomena significantly impacts on (deteriorates) the luminance and electronic properties of the OLED devices produced thereby. Thus there is a need to deliver new approaches that allow the production of multilayer OLED devices, for example those with four or more layers by solution processing that result in high efficiency OLED devices.

It is an object of the present invention to provide methods for generating multilayer organic electronic devices such as OLED devices with the high IQE and EQE efficiencies associated with vacuum deposition approaches through a simple and cost-efficient solution- based approach. In particular, it is an object of the present invention to provide a method for generating OLED devices with standard materials without resorting to modification of their chemical structures to incorporate groups such as sulfonates to render them water and/or methanol soluble. In addition, it is an object of the present invention to allow material not compatible with previous OLED production techniques to be exploited as functional materials in OLED devices. The invention as described herein below also encompasses multilayer organic electronic devices such as OLED devices made by the processes of the invention.

Summary of the Invention In a first aspect of the present invention there is provided a method for the production of an electroluminescent device comprising sequential deposition of at least four contiguous layers on a substrate-side electrode wherein, in the case wherein the substrate electrode is an anode, the at least four layers comprise: i) a hole injection layer (HIL); ii) a hole transport layer (HTL) deposited from a non-polar or low-polarity aprotic solvent having a dielectric constant of less than 20; iii) an emissive layer (EML) deposited from a polar aprotic organic solvent having a dielectric constant of greater than 20; iv) an electron transport layer (ETL) deposited from a non-aqueous polar protic solvent having a dielectric constant of greater than 10; and, in the case wherein the substrate electrode is a cathode, the at least four layers comprise: i) an electron transport layer (ETL); ii) an interlayer (IL) deposited from a polar protic organic solvent with dielectric constant greater than 5; iii) an emissive layer (EML) deposited from a non-polar or low polarity aprotic solvent with dielectric constant less than 20; and iv) a hole transport layer (HTL) deposited from a polar aprotic organic solvent having a dielectric constant greater than 5; wherein in each case the method does not involve a chemically cross linking of the deposited materials and wherein the layers are deposited on the substrate-side electrode in the order i) to iv).

It can be understood that the methods of the invention use a set of at least three orthogonal solvents that do not cause any significant degradation of the functional layers (e.g. HIL, HTL, EML, ETL or IL) already deposited. The methods of the invention do not include any cross linking step in order to retain the integrity of the deposited layers. The solvents used to deposit the layers can be used to dissolve or suspend at least 2 mg/mL the material or materials from which each respective functional device layers is composed. In general, the use of a single solvent for each solvent deposition step is preferred, but a blend of two or more solvents having the specified type, Hansen Solubility Parameter indicators, and/or dielectric constant as a mixture can be used.

In contrast the solubility of the material or the combination of materials in each deposited layer in the solvent(s) used to deposit each subsequent layer is generally less than 1 mg/mL and preferably chosen to be 0.1 mg/mL or less. It should be noted that the solubility of the deposited layer can be different from the solubility of the individual materials forming that layer. This is the case, for example, when dopant guest molecules are present in a polymeric host in which case integration of the guest in the polymeric matrix renders the guest substantially insoluble in the context of the deposited layer.

In preferred embodiments, the deposition time for each solution deposited layer is up to 1000 seconds, but deposition times are generally much shorter, for example from 5 to 300 seconds and this in conjunction with the solvent and materials profiles also ensures that sequential solution deposition steps do not cause significant degradation of the any previously deposited layer of the structure. In preferred embodiments, the solvent from each solution deposition step is removed by evaporation under reduced pressure or by heating to a temperature from room temperature up to 300°C. One or more of the solution deposited layers may be annealed following deposition to deliver layers with optimal consistency and structural and electronic properties. This in turn advantageously allows access to devices with optimal internal and external quantum efficiencies as the electronic properties such as charge transport, injection and confinement can be tailored to localise exciton recombination in the emissive layer. A significant advantage of the methods of the invention allow devices to be elaborated with control of the individual layer thickness and layer thickness not attainable by solution processing methods in the prior art.

In order that the flexibility of the invention can be understood the nature of the method can be described by reference to the nature and/or composition of the layers formed by the method. As the skilled person will understand, preferred examples of each layer of the device as described herein and above may be combined with any of the preferred examples provided for the remaining layers. In addition, the properties of the layers can be expressed in terms of their functional properties such as the HOMO- or LUMO-levels or chemical nature as the key determinant is whether the orthogonality of the solvents used in the process and the compatibility of the material used to construct the layers with those solvents. Thus in implementing the methods of the invention the skilled person need only consider the solvent compatibility and the properties of the material that can perform the required functional role in the electroluminescent device.

In embodiments of the invention wherein the substrate-side electrode is an anode, the method may further comprise the step of depositing a hole blocking layer (HBL) from a polar aprotic solvents having ε > 6 in between the emissive layer and the electron transport layer. In any method involving deposition of a HBL this layer may comprise polystyrene (PS) or poly(methyl methacrylate) (PMMA) or a blend of PS and PMMA. In any method involving deposition of a HBL this layer may be deposited from acetone or butanone.

In the methods of the invention, the emissive layer deposited from solution may comprise a polymeric emitter, a blend of polymeric emitters or a polymeric host in which a guest emitter species is incorporated. Examples in which the emissive layer is polymeric host in which a guest emitter species is incorporated also known as host-guest type emissive layer. Use of host-guest type emissive layers and advantageously allow fabrication of efficient electroluminescent devices with incorporating small molecule emitters via a solution deposition approach wherein the prior art has generally required a vacuum deposition approach to access efficient electroluminescent devices. In some embodiments, the emissive guest in the host-guest type emissive layer is a light-emitting polymer. In some embodiments the emissive guest in the host-guest type emissive layer is a small molecule emitter. In embodiments wherein the emissive layer is of the host-guest type, the emissive layer may have a type 1 heterostructure. In embodiments the host in the host-guest emissive layer may be PVK or a related carbazole containing polymer. In embodiments where the emissive layer is of the host-guest type there may be a single emitter species or a plurality of emitter species. Furthermore, the emissive layer material may in addition to the emitter species contain one or more dopant that serves to modulate the charge injection, transport or balance properties of the emissive layer, these dopants have a band gap and triplet energy larger than or equal to that of the emissive species.

In embodiments wherein emissive layer is of the host-guest type, the weight percentage of the emissive guest in the EML is from 0.1 wt% to 80 wt%. In some preferred embodiments wherein emissive layer is of the host-guest type the band gap of the host material is 2 eV to 6 eV and/or the band gap of the emissive guest is from 1 eV to 3.5 eV. In embodiments described herein the EML is deposited by the method of the invention is from 5 nm to 500 nm in thickness. In any embodiment described herein the solvent from which the EML is deposited can be selected from DMF, DMSO, nitromethane, dimethyl acetamide, and dimethyl ethanolamine or a blend of these solvents satisfying the dielectric constant specified for deposition of the emissive layer. The EML may comprise any suitable polymer or small molecule (organic or organometallic) emitter species soluble solvent compounds. In some embodiments the EML deposited by the methods of the invention comprises a compound or polymer selected from PVK, PFHCz,lr(ppy)₃, lr(ppy)₂(acac), lr(piq)₃, lr(piq)₂(acac), PtOEP, PdOEP, Flrpic, 4CzlPN, Alq₃, Znq, rubrene, Eu(dmb)₃(Phen-NH₂), perylene, DPA, DPBF, TIPS-pentacene, TIPS- tetracene, TES-pentacene, FTES-ADT, anthracene, DCM, CBP, TPBi, Bphen, TAZ, PBD, Bu-PBD, BCP, TCTA, TAPC, TPD, NPD or a blend of a two or more of these materials. For example, the EML may comprise a blend of PVK and a small molecule emitter.

In embodiments of the methods of the invention wherein the substrate-side electrode is an anode the hole injection layer me be selected from materials a HOMO level or Fermi level relative to the vacuum level of from -4 eV to -7 eV and may be selected from the group comprising water-soluble/dispersible conductive polymer (e.g., PEDOT:PSS, PPy:PSS), conductive inks (e.g. NiO, W0₃), colloidal aqueous suspensions (e.g., NiO, W0₃, V2O5, Mo0₃), and other solution processable, high work-function inorganic compounds such as Cul and CuSCN, or a combination of two or more of these materials. In the embodiments of the invention the thickness of the hole injection layer is from 1 nm to 500 nm. In some preferred embodiments the HIL is PEDOT:PSS.

In embodiments of the methods of the invention wherein the substrate-side electrode is the anode the hole transport layer has a HOMO level from -4 eV to -7 eV and is optionally selected to be within 0.2 eV of the HOMO level of the HIL and the HOMO level of the emissive species in the EML. In some embodiments the HIL comprises a wide band gap, hole-transporting polyfluorene such as TFB, poly-TPD, PFO, F8BT, F8T2, PFD, PFH, PFHA, PFB, PPV or MEH-PPV. In some embodiments the HTL is be doped with one or more small molecule or polymer hole transport/electron blocking materials, such as TPD, TCTA, TAPC, NPD, CBP, PVK, PS, PMMA at a doping concentration by weight of from 0.01 wt % to 80 wt %. In preferred embodiments the solvent used to deposit the HTL has a dielectric constant of from 2 to 10. In preferred embodiments wherein the wherein the substrate-side electrode is an anode the thickness of the hole transport layer is greater than 100 nm thick and optionally from 150 to 200 nm thick. In the embodiments of the methods of the invention wherein substrate-side electrode is an anode the electron transport layer (ETL) is from 1 nm to 1000 nm. In preferred embodiments the solvent used to deposit the electrons transport layer is selected from methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. In some preferred embodiments the ETL comprises BPhen, TPBi, TAZ, PBD, Bu-PBD, n-type inorganic nanoparticle dispersion solutions of e.g. ZnO or T1O2, or admixtures of two or more of these materials.

In the methods of the invention wherein the substrate-side electrode is a cathode the nature of the layers, other than the EML, that are selected as above on the basis of functional and solubility properties differ from those described above and thus the electron transport layers and the hole transport layers differ from those described above. Accordingly, in the embodiments of the invention wherein the substrate side electrode is a cathode the electron transport layer is selected to have a Fermi level, LUMO level or conduction band position of from -2 eV to -4.5 eV and a thickness from 1 nm to 500 nm.

In preferred embodiments the interlayer of the solution deposited interlayer (IL) comprises a low work-function polymer such as PEI, PEIE or a wide band gap inorganic salt such as CS2CO3 or Ba(OH)2, or the combination of two or more of these materials. The thickness of the interlayer generated by the methods of the invention is from 0.1 nm to 20 nm. Preferred embodiments of the solvents from which the IL is deposited are 2- methoxyethanol, 1- and 2-propanol, ethane-1 ,2,-diol, propane-1 ,2-diol, the various isomeric butanediols, diethylene glycol and short chain alcohols such methanol, ethanol and the isomers of butanol.

In the methods of the invention wherein the substrate side electrode is a cathode the hole transport layer is deposited from polar aprotic solvent with ε > 5 and preferably ε > 15. A preferred solvent from which the HTL is deposited is acetone. Preferred embodiments of the hole-transporting layer in this structure comprise or consist of small molecules such as TPD or wide band gap polymers that are soluble in ketones for example polystyrene (PS) and PMMA, or the combination of two or more of these materials.

In a further aspect the invention provides an OLED device formed by a method according to the invention described herein.

Description of the Drawings

In order that the invention can be fully understood, the invention and embodiments thereof are described with reference to the following Figures, which are not intended to limit the scope of the invention. Figure 1 shows the architectures of electroluminescent devices produced by the methods described herein having 1a) the standard architecture also referred to as architecture 1 featuring 1) a hole injection layer (HIL), 2) a hole transport layer (HTL) that can also serve as an electron blocking layer (EBL), 3) an emissive layer (EML) and 4) an electron transport layer (ETL); 1 b) having a modified standard architecture also referred to as architecture 2 in which a dedicated hole blocking layer (HBL) is present between the EML and the ETL; and 1 c) the inverse architecture also referred to as architecture 3 wherein the base electrode is the cathode and upon which is located a ETL, an interlayer (IL), EML and a HTL that can also serve as a EBL.

Figure 2 shows the device characteristics of an OLED fabricated from DPA emitter having the standard architecture as described in example 1 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 3 shows the device characteristics of an OLED fabricated from Flrpic emitter having the standard architecture as described in example 2 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum. Figure 4 shows the device characteristics of a OLED fabricated from DPBF emitter having the standard architecture as described in example 3 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 5 shows the device characteristics of an OLED fabricated from perylene emitter having the standard architecture as described in example 4 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 6 shows the device characteristics of an OLED fabricated from lr(ppy>3 emitter having the standard architecture as described in example 5 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 7 shows the device characteristics of an OLED fabricated from Alq3 emitter having the standard architecture as described in example 6 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 8 shows the device characteristics of an OLED fabricated from 4CzlPN emitter having the standard architecture as described in example 7 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 9 shows the device characteristics of an OLED fabricated from rubrene emitter having the standard architecture as described in example 8 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 10 shows the device characteristics of an OLED fabricated from Eu(dbm)3(Phen-NH2) emitter having the standard architecture as described in example 9 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum.

Figure 11 shows the device characteristics of an OLED fabricated from lr(piq>3 emitter having the standard architecture as described in example 10 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset

Figure 12 shows the device characteristics of an OLED fabricated from PtOEP emitter having the standard architecture as described in example 11 below (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 13 shows the device characteristics of an OLED fabricated from TIPS- pentacene emitter having the standard architecture as described in example 12 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum, with a working device shown in inset.

Figure 14 shows the device characteristics of an OLED fabricated from PFHCz emitter having the standard architecture as described in example 13 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum.

Figure 15 shows the device characteristics of an OLED fabricated from lr(ppy)3 emitter in a PFHCz host having the standard architecture as described in example 14 a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) EL spectrum.

Figure 16 shows the device characteristics of an OLED fabricated from F8BT polymer emitter having the inverted device architecture as described in example 15 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) device EL spectrum. Figure 17 shows the device characteristics of an OLED fabricated from rubrene emitter having the inverted device architecture (architecture 3) as described in example 16 below, (a) EQE and current efficiency against current density; (b) EQE and current efficiency against luminance; (c) luminance against voltage, with the device structure shown in inset; and (d) device EL spectrum.

Detailed description of the invention

The invention relates to new methods for the generation of high efficiency multi-layer electroluminescent devices, i.e. electroluminescent devices having at least 4 functional layers, such as organic light-emitting devices (OLEDs) and polymer light-emitting diodes (PLEDs) and devices featuring a functional layer(s) in which a small molecule emitter or a polymeric emitter is blended with a polymeric host material, via solution processing. The methods of the invention include a fully solution-based approach to formation of the functional, e.g. charge transport and light emitting, layers of the electroluminescent device having a standard architecture as described herein as architecture 1 and architecture 2. In addition, the methods of the invention provide a solution-based approach to an electroluminescent device having an inverted device architecture referred to herein as architecture 3 in which three functional layers are deposited from solution onto a compatible electron transport layer formed through standard methods.

Advantageously, the sequential solution deposition approach of the methods of the invention do not involve chemical or photo-cross linking of the deposited layers but instead take advantage of a set of at least three, optionally four or five, orthogonal solvents and at least four, optionally five sets of functional materials: in the case of architectures 1 and 3 at least four orthogonally soluble material sets and in the case of architecture 2 five orthogonally soluble material sets. These orthogonal solvents are used in conjunction with sets of functional small molecule and/or polymeric materials that serve as hole injection and transport materials, electron transport and injection materials and emissive layer materials, that are soluble at the levels specified herein in each of the solvent types specified for deposition of the respective layers.

In addition, the methods of the invention described herein do not require that the chemical structures of the functional material from which the layers of the electroluminescent devices are formed are chemically modified in order to modulate their intrinsic solubility. Thus the materials used are generally standard materials and other than materials such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polypyrrole- polystyrene sulfonate (PPy:PSS) and similar materials which are suitable for use in the hole injection layers and that are deposited form highly polar solvents such as water are not sulfonate derivatives.

The electroluminescent devices prepared by the method of the invention comprise a number of functional organic layers deposited from solution on a substrate adorned with an electrode. Thus the electroluminescent devices prepared according to the method of the invention have architecture 1 , also referred to herein as the standard architecture and shown in Figure 1 a wherein the anode, located on a substrate, is a transparent conductive electrode used for hole injection or an inverted device architecture wherein the cathode, located on a substrate, is a transparent conductive electrode used for electron injection. Figure 1 b shows device architecture 2 that is related to the standard architecture of Figure 1 a and that is also referred to herein as the modified standard architecture in which an addition layer, which serves to block holes or to prevent exciton dissociation, is introduced between the emissive layer and the electron layer relative to architecture 1.

Transparent conducting electrodes typically comprise a transparent conducting metal oxide such as indium tin oxide (ITO), fluorine doped tin oxide (FTO), or a doped zinc oxide such as aluminium-doped zinc oxide on a rigid substrate such as glass or a flexible substrate such as a polymer, for example polyethylene terephthalate. Such a transparent conducting electrode at the base of the device, i.e. upon which the functional layers are deposited, would typically be used in conjunction with an opaque, optionally reflective, electrode located on top of the at least four contiguous functional organic layers of the device. In the standard architecture, the anode is a transparent conducting electrode while the cathode is typically a low work function metal such as calcium, magnesium, aluminium or silver or an inorganic buffer layer such as LiF, CS2CO3, CsF, calcium, coated with one or more metal layers selected from metals such as silver, aluminium, or magnesium. In principle however the anode can be any suitable metal or any other flexible or rigid conductive material and these materials are well known to those skilled in the art.

In a third case the device formed by a method of the invention has architecture 3, also referred to herein as the inverted architecture and shown in Figure 1 c, wherein the cathode is a transparent conducting electrode while the anode is typically a higher work function metal such as silver, gold, or aluminium, or an inorganic buffer layer such as M0O3, V2O5 or NiO coated with one or metal layers selected from metals such as silver, aluminium or gold. The term "higher" in this case refers to the cathode material having a work function that is higher than that of the low work function top anode in this inverted device architecture. Preferably the work function of the anode material is greater than 4 eV. In principal, however, the anode can be any suitable metal or any other flexible or rigid conductive material and these materials are well known to those skilled in the art.

The methods of the invention are solution processing methods wherein the electroluminescent device fabrication scheme in which all functional layers beside electrodes and optionally the first layer are deposited from solution. Furthermore, the solution processing methods of the invention does not involve cross linking of any or each functional layers deposited from solution in order to preserve their integrity and to facilitate elaboration of a structure containing at least four functional layers. Instead the solution approach of the present invention utilises a set of at least three orthogonal solvent types in the case wherein the base electrode is an anode and at least three orthogonal solvent types in the case where the base electrode is a cathode. The orthogonal solvent sets are used in conjunction with materials soluble or, as appropriate, dispersible or suspensible in these solvents. Once the materials are deposited as a layer, the deposited materials are substantially insoluble in the solvent used to deposit further layers thus allowing access to highly efficient electroluminescent devices without the need to resort to cross linking or a narrow selection of specially synthesised materials, for example polymers sulfonated to confer solubility in methanol.

A specific order of deposition is key to delivering an efficient electroluminescent device. For example, to deliver an electroluminescent device of the architecture 1 featuring four contiguous functional layers deposited on the substrate anode as shown in figure 1a the sequence of deposition that is used is i) hole injection layer (HIL) that is deposited from aqueous solution, ii) hole transport layer (HTL) that may also function as an electron blocking layer (EBL), iii) emissive layer (EML) and iv) an electron transport layer (ETL) that may also function as a hole blocking layer (HBL). Architecture 2 is a variation of the standard architecture in which an additional layer that serves as a dedicated hole blocking layer (HBL) can be inserted between the emissive layer and the electron transport layer of the standard architecture. In this case of architecture 2 the sequence of deposition for the electroluminescent device with five contiguous functional layers deposited on the substrate anode is i) hole injection layer (HIL) deposited from aqueous solution, ii) hole transport layer (HTL), iii) emissive layer (EML), iv) hole blocking layer (HBL) and v) electron transport layer (ETL). The invention also relates to a method of forming an electroluminescent device having an inverted architecture known as architecture 3 featuring four contiguous functional layers as shown in figure 1 c wherein the four contiguous layers deposited on the cathode are i) an ETL, ii) an interlayer (IL), iii) an emissive layer (EML) and iv) a HTL that can also serve as an electron blocking layer. The general solvent and material properties suitable for use in the methods of the invention are described below. Specific combinations of material and solvents are required for each of the device architectures 1 , 2 and 3 as is dictated by the need for orthogonal solvents and solubilities of the materials in each successive layer. All architectures are rendered possible by the identification of orthogonal solvent and material sets soluble therein having the properties to form efficient electroluminescent devices.

As a general consideration the solvents used in the methods of the invention have a boiling point at atmospheric pressure that allows for efficient removal of solvent after each deposition step by e.g. evaporation. In general solvents having a boiling point of less than 200°C are preferred. Evaporation of solvents from the deposited layers can be carried out at temperatures from room temperature to 300°C and pressures from atmospheric pressure to high vacuum conditions as appropriate to the solvent.

The solvent selected for each deposition step is chosen so that it does not degrade any previously deposited functional layer. A key criteria in the present invention is therefore the identification/provision of materials with a selective solubility profile in one solvent over a plurality of other solvents. In the case of formation of the first layer in each architecture the solvent is not a significant factor as long as the substrate/electrode is not degraded by the solvent and the subsequent layers are not exposed to the solvent used to deposit the first layer. Instead the key determinant for deposition of the first functional layer in each of architectures 1 , 2 and 3 is the orthogonality of solubility of the functional material of layer 1 with the solvents used to deposit the at least 3 further functional layers upon it. Correspondingly there is also more flexibility in the choice of the deposition technique for layer 1 in each case and any technique known in the art may be adopted. Although where possible deposition from solvent is generally preferred this layer, layer 1 , can thus also be deposited by an alternate technique such as atomic layer deposition (ALD) or chemical vapour deposition (CVD). More details on this are provided herein. In addition, deposition from solvent includes deposition from solution or suspension or dispersion in an appropriate solvent.

Deposition of layers from solution is carried out by techniques well known in the art that include spin-casting, drop-casting, doctor blading (including zone-casting and solution- shearing), dipcoating, spray-casting, printing and painting or a combination of two or more of these techniques. In spin-casting, a small amount of the solution/suspension containing the functional material is dropped onto the centre of a flat substrate which spins at a chosen speed, generating a uniform thin film on that substrate. Drop casting is similar to spin- casting, but the substrate is not spun during drop-casting, and a film is formed on the flat substrate as solvent evaporates statically, with or without heating. Doctor blading involves spreading of solution/suspension by a moving blade onto a stationary substrate, or stationary blade onto a moving substrate. In dip-coating, a substrate is dipped or immersed into the solution/suspension and subsequently withdrawn at a controlled speed. In spray- coating, the substrate, sometimes being heated to a specific temperature, receives a flux of vapourised solution, typically through a nozzle trajectory. Painting and printing imply a combination of one or more of the above techniques and additional patterning technique(s) well established in the art, by use of e.g. shadow masking, selective wettability, various forms of lithography, and micro-contact printing. The duration of each solution deposition step is for up to 1000 seconds before solvent removal, for example from 2 to 1000 seconds, typically from 5 to 300 seconds and most preferably from 10 to 120 seconds. Although the solvents and material for the layers are selected to not degrade the previously deposited layers it is preferable to keep the deposition time to a minimum. It is also preferred to perform the deposition process of organic materials under an inert atmosphere of a gas such as nitrogen or argon to deliver the optimal quality for the processing of, for example, oxygen or moisture sensitive materials. The deposition of layers from solution steps are carried out at atmospheric pressure and do not require application of a vacuum. Solvent removal following deposition of the layer from solution is achieved via evaporation facilitated by heating or placing the sample under reduced pressure. For example, the deposited layer can be placed in a vacuum chamber for from 10 seconds to two hours to achieve optimal device performance. Alternatively, or in addition to placing in a vacuum chamber, the deposited layer can be annealing at a temperature from room temperature to up to 300°C to deliver a film with optimal consistency and structural and electronic properties.

The solvents used in the methods of the invention are categorized by the terms polar/non-polar, and protic/aprotic as is conventional in the art. So that the range of solvents useful for application in the methods of the invention can be appreciated solvent polarities are also defined herein by reference to the dielectric constant, ε or to the corresponding Hansen Solubility Parameters (HSP) characterised by dispersion (5D), polar (δΡ), and hydrogen bonding (δΗ) values. Tables of values for these parameters can be found in "Hansen Solubility Parameters: A User's Handbook." Charles M. Hansen. CRC Press LLC. 2000.

Abbreviations and full names of the organic functional materials used herein, both small molecule and polymeric material and the various inorganic materials used for electroluminescent device formation are provided in table 1. Abbreviation Full name Abbreviation Full name

PEDOT:PSS Poly(3,4-ethylenedioxy DPBF 1 ,3-Diphenyl

thiophene) polystyrene isobenzofuran

sulfonate

PPy:PSS Polypyrrole-polystyrene TIPS-pentacene 6,13-Bis(triisopropylsilyl sulfonate ethynyl)pentacene

NiO Nickel(ll) oxide TES-pentacene 6,13-Bis((triethylsilyl) ethynyl) pentacene

M0O3 Molybdenum trioxide FTES-ADT 5,1 1-bis-

(triethylsilylethynyl) anthradithiophene

V2O5 Vanadium(V) oxide TIPS-tetracene [bis(triisopropylsilylethynyl

) tetracene

Cul Copper(l) iodide DCM 4-(dicyanomethylene)-2- methyl-6-(p-dimethyl aminostyryl)-4H- pyran

CuSCN Copper(l) thiocynate CBP 4,4-Bis(N-carbazolyl)-1 , 1 '- biphenyl

TFB Poly[(9,9-dioctylfluorenyl- TPBi 2,2',2"-(1 ,3,5-Benzinetriyl)

2,7-diyl)-co-(4,4'-(N-(4- -tris (1-phenyl-1-H- sec- butylphenyl) benzimidazole) diphenylamine)]

Poly-TPD Poly [N,N'-bis-(4- Bphen Bathophenanthroline

butylphenyl)-N,N'- bisphenylbenzidine]

Table 1 List of abbreviations used to refer to materials described herein PFO Poly(9,9-di-n- TAZ 3-(Biphenyl-4-yl)-5-(4-tert- octylfluorenyl-2,7-diyl) butylphenyl)-4-phenyl-4H- 1 ,2,4- triazole

F8BT Poly(9,9-dioctylfluorene- TPD N,N'-Bis(3-methylphenyl)- alt-benzothiadiazole) Ν,Ν' -diphenylbenzidine

PFB Poly-(9,9'-dioctylfluorene- NPD N.N'-DiO-naphthyO-N.N' co-bis-N,N'-(4- -diphenyl-(1 , 1 '-biphenyl)- butylphenyl)-bis-N,N'- 4,4'-diamine phenyl-1 ,4-phenylene

diamine

PPV Poly(p-phenylene PS Polystyrene

vinylene)

MEH-PPV Poly[2-methoxy-5-(2- PMMA Poly(methyl methacrylate) ethylhexyloxy)-1 ,4- phenylenevinylene]

PVK Poly(9-vinylcarbazole) DMF N,N-Dimethylformamide lr(ppy)₃ Tris [2-phenylpyridinato- DMSO Dimethyl Sulfoxide

C²,N] iridium(lll) lr(ppy)₂(acac) Bis[2-(2-pyridinyl-N) THF Tetrahydrofuran

phenyl-C] (2,4- pentanedionato- 02,04)iridium(lll) lr(piq)₃ Tris(1-phenyl isoquinoline) PEI Polyethylenimine

iridium(lll)

Table 1 cont'd lr(piq)2(acac) Bis[2-(1 -isoquinolinyl- PEIE Polyethylenimine

N)phenyl-C] (2,4- ethoxylated pentanedionato- 0²,0⁴)iridium(lll)

PtOEP Platinum Cs₂C0₃ Cesium carbonate

octaethylporphyrin

PdOEP Palladium Ba(OH)₂ Barium hydroxide

octaethylporphyrin

Flrpic Bis[2-(4,6-difluorophenyl) ITO Indium tin oxide

pyridinato-C²,N]

(picolinato) iridium(lll)

4CzlPN 2,4,5,6-tetrakis(carbazol- FTO Fluorine doped tin oxide

9-yl)-1 ,3- dicyanobenzene

Alq₃ Tris-(8-hydroxyquinoline) LiF Lithium fluoride

aluminium

Znq 8-Hydroxyquinoline zinc CsF Cesium fluoride

Rubrene 5,6, 11 ,12- Cr0₂ Chromium(IV) oxide

Tetraphenylnaphthacene

DPA 9,10-Diphenylanthracene WO3 Tungsten(VI) oxide

PFHCz Poly(9,9-n-dihexyl-2,7- ALD Atomic layer deposition fluorene-alt-9- phenyl-3,6- carbazole)

PBD 2-(4-Biphenylyl)-5-phenyl- AP-SALD Atmospheric-pressure

1 ,3,4- oxadiazole spatial atomic layer deposition

Table 1 cont'd Bu-PBD 2-(4-tert-Butylphenyl)-5- CVD Chemical vapour

(4-biphenylyl)-1 ,3,4- deposition

oxadiazole

BCP Bathocuproine ZnO Zinc oxide

TCTA Tris(4-carbazoyl-9- T 1O2 Titanium dioxide

ylphenyl)amine

TAPC 4,4'-Cyclohexylidene F8T2 Poly(9,9-dioctylfluorene- bis[N,N-bis(4- a/f-bithiophene),

methylphenyl) Poly[(9,9-dioctylfluorenyl- benzenamine] 2,7-diyl)-co-bithiophene],

Poly[[2,2'-bithiophene]-

5,5'-diyl(9,9-dioctyl-9H- fluorene-2,7-diyl)]

PFD Poly(9,9-di-n- PFH Poly(9,9-di-n- dodecylfluorene) hexylfluorenyl -2,7-diyl)

PFHA Poly[(9,9-dihexylfluoren- Eu(dmb)₃(Phen- Tris(dibenzoylmethane)

2,7-diyl)-co-(anthracen- NH₂) mono(5-amino-1 ,10-

9,10-diyl)] phenanthroline)europium

(III)

Table 1 cont'd

The four layer standard architecture of an electroluminescent device, architecture 1 , produced by a method according to the invention is presented in Figure 1a. This comprises four contiguous functional layers that are deposited from solution in the order 1) hole injection layer (HIL), 2) hole transport layer (HTL) that may also function as electron blocking layer (EBL), 3) emissive layer (EML) and 4) electron transport layer (ETL), that may also function as a hole blocking layer (HBL). A solvent-layer solubility matrix for architecture 1 is presented in table 2 below. 1-HI L, 2-HTL(EBL),

3-EM L,

aqueous polyfluorenes

4-ETL (H BL),

polymer,polymer/polyme

Solvent for solutions and derivatives,

r, or polymer/small

depositing each or e.g, TFB, PFB, e.g., Bphen, TAZ, TPBi, molecule blends, e.g.,

layer suspension poly-TPD, PFO etc PBD, ZnO nanoparticles,

PVK, lr(ppy)₃, Alq₃,

e.g. can also be Π0₂ nanoparticles etc

4Czl PN, rubrene etc

PEDOT:PSS doped with small

etc molecules

1, water

(aqueous S - - - solution)

2, non-polar or

slightly polar

organic, e.g.,

IS S - - toluene,

chlorobenzene,

THF etc

3, polar organic,

e.g., DMF, IS IS S - DMSO etc

4, alcohol, e.g.,

methanol,

ethanol, IS IS IS S

propanol,

butanol etc

S = soluble, IS = insoluble,- = not relevant as the solvent used in depositing underlying layers will not affect upper layers

Table 2 Solvent-layer solubility matrix for production of devices of architecture 1

The devices of architecture 1 and architecture 2 (see below) are structurally distinguished from those devices with equivalent architectures prepared using prior art solvent processing methods. In more detail, foregoing devices have a hole transport layer that is not more than 100 nm thick and in most cases, 50 nm that results from the solvent that has been previously used to deposit the emissive layer dissolving away the hole transport layer. In the prior art the absence of an orthogonal solvent/material combination also led to an intermixing of the hole transport layer and emissive layer and this in turn caused luminescence quenching and correspondingly reduced device efficiency. In developing the orthogonal solvent/material combinations described herein the inventors have devised a method that allows elaboration of electroluminescent devices that have IQEs and EQEs matching or exceeding those obtained by vacuum deposition approaches.

In more detail, to achieve well-defined (non-intermixed) device structure of architecture 1 , the following material/solvent combinations can be used. The device structure is constructed in the following order using layer-by-layer sequential deposition from solution.

The materials from which layer 1 , the hole injection layer (HIL) is formed, i.e. the hole injection materials is/are selected from the group comprising water-soluble/dispersible conductive polymer (e.g., PEDOT:PSS, PPy:PSS), conductive inks (e.g. NiO, W0₃), colloidal aqueous suspensions (e.g., NiO, WO3, V2O5, M0O3), and other solution processable, high work-function inorganic compounds such as Cul and CuSCN, or a combination of two or more of these materials. Alternative methods well known in the art can be used to deposit to high work-function metal oxides such NiO, M0O3, Cr02, or materials such as Cul and CuSCN and may be used as an alternative to solution processing for this layer. The materials have a HOMO level or Fermi level relative to the vacuum level of from -4 eV to -7 eV, preferably from -4.5 eV to -6 eV, and this is preferably between the Fermi level of cathode and the HOMO level of the hole transport layer (HTL). The HIL materials are selected from materials with a hole mobility of greater than 10^"5 cm²/Vs, and most preferably materials with a hole mobility of greater than 10^"2 cm²/Vs. The thickness of the HIL produced by the method of the invention is from 1 nm to 500 nm, and is preferably from 10 nm to 100 nm.

In cases where the hole injection layer is deposited from solution the solvent, referred to as solvent 1 , from which the hole injection layer is deposited is a polar protic solvent with a very high dielectric constant, for example water. The HIL material can from example be provided as a solution or a suspension or dispersion in water. Water has the following characteristics: a dielectric constant, ε of 80, and HSP 5D 15.5, δΡ 16.0, and δΗ 42.3. Aqueous solutions of volatile acids, for example acetic acid or other carboxylic acids, volatile bases or surfactants may also be used although the general consideration that the solvent should be removable via evaporation and preferably has a boiling point of not more than 200°C are to be taken into account. The solubility of the HIL material is less than 1 mg/mL in solvents 2, 3 and 4, i.e. the solvents from which the further HTL, EML and ETL are deposited, and preferably less than 0.1 mg/mL. As noted above, as this layer is the first layer to be deposited then the deposition time, i.e. the time allowed for film formation is not crucial as deposition of this layer does not compromise the integrity of the layer below that is in this case an anode.

In a preferred example the HIL material is PEDOT:PSS and solvent 1 is water. Other preferred examples include the material/solvent combinations of CuSCN / dipropyl sulfide and Cul / acetonitrile.

The materials from which the second layer i.e. the hole transport layer (HTL) that may also function in the device as an electron blocking layer are selected for their hole transport properties and their solubility in a polar and moderately polar organic solvents. Thus, the HOMO level of these materials is from -4 eV to -7 eV and is preferably from -5 eV to -6 eV. The HOMO level of the HTL material is ideally selected to be close (+/- 0.2 eV) to the HOMO level of the HIL and the HOMO level of the emissive species in the EML. The LUMO level of the HTL material is from -1 to -3.5 eV, preferably from -1.5 eV to -3 eV and is ideally selected to lie above the LUMO level of the EML material so that it is effective electron blocking layer at the same time. The band gap (i.e. the HOMO-LUMO gap) of the HTL is >2 eV, more preferably >2.5 eV and is preferably larger than the band gap of the emissive species in the EML. The hole mobility of the HTL material is > 10^"8 cm²/Vs, preferably >10^"5 cm²/Vs, and most preferably >10^"2 cm²/Vs. The thickness of the hole transport layer is from 1 nm to 1000 nm and is preferably from 20 nm to 300 nm.

Examples of the materials that are suitable for use as the HTL (EBL) material are wide band gap, hole-transporting polyfluorenes and their derivatives, e.g., TFB, poly-TPD, PFO, F8BT, F8T2, PFD, PFH, PFHA, PFB, PPV, MEH-PPV, or the combination of two or more of these materials. In some cases, the HTL(EBL) can be doped with one or more small molecule or polymer hole transport/electron blocking materials, such as TPD, TCTA, TAPC, NPD, CBP, PVK, PS, PMMA at a doping concentration (in weight percent) of from 0.01 wt % to 80 wt %, and preferably from 0.1 wt % to 30 wt%, to facilitate improved charge injection, transport or balance, Solvent 2, the solvent from which the HTL is deposited, is selected from the group of non-polar and slightly polar aprotic organic solvents, i.e. it is a solvent with ε < 20. Preferably, the solvent has 1 <ε <10, and in terms of HSP has 5D >16, δΡ < 8, and δΗ < 8. Examples of preferred common solvents include but are not limited to chlorobenzene, toluene, dichlorobenzene (p-, m-, o- or mixed isomers), chloroform, xylene (p-, m-, o- or mixed isomers), THF, dichloromethane, dichloroethane, trichloroethane (all isomers), trichlorobenzene (all isomers). Other possible solvents include but are not limited to pyrrole, pyridine, tetrahydropyran, tetrahydrothiophene, and 1 ,4-dioxane and halogenated and alkylated derivatives thereof. As will be apparent to the person of skill in the art there are many solvents that fall within this group of non-polar and slightly polar aprotic organic solvents. The solubility of the HTL material in solvent 2 is from 2 mg/mL to 1000 mg/mL, while its solubility in solvents 3 and 4 is less than 1 mg/mL and is preferably below 0.1 mg/mL.

The third functional layer in architecture 1 is the emissive layer (EML). The nature of the materials in the emissive layer in all three of the device architectures, namely architectures 1 , 2 and 3, described here are very similar and the materials from which the EML is derived are selected from their ability to emit light (for emitters), in the case of optional dopants to facilitate improved charge injection, transport or balance, or in the case where a blend of materials is used to serve as a host for a material that emits light, for its host properties. The emitters in the EML are polymers and small molecules that are organic or organometallic compounds. Exemplary EML materials include PVK, PFHCz, lr(ppy)3, lr(ppy)₂(acac), lr(piq)₃, lr(piq)₂(acac), PtOEP, PdOEP, Flrpic, 4CzlPN, Alq₃, Znq, rubrene, Eu(dmb)₃(Phen-NH₂), perylene, DPA, DPBF, TIPS-pentancene, TIPS-tetracene, TES- pentacene, FTES-ADT, anthracene, DCM, CBP, TPBi, Bphen, TAZ, PBD, Bu-PBD, BCP, TCTA, TAPC, TPD, NPD, that are soluble in polar organic solvents or blends of these material. Overall the EML is either a single emissive polymer, a blend of polymers or a polymer-small molecule blend. It should be noted that the EML does not consist solely of small molecule emitter species, instead when small molecule emitters are present in the EML they are present in the context of a small molecule/host polymer blend. In some cases, the small molecules or polymers within the EML function are dopant to optimize charge injection, charge transport or exciton transfer into the EML.

The methods of the invention also relate to the manufacture of devices containing an emissive layer with one or more small molecule organometallic emitter(s) of the structure L- M-X wherein M is a two-coordinate metal selected from copper, silver and gold; L is a neutral electron donor cyclic alkyl amino carbene (CAAC) ligand and X is a monodentate anionic ligand. Examples of these organometallic complexes can exhibit rotationally accessed spin- state inversion (RASI) photoemission characteristics and correspondingly exhibit excellent light emission characteristics. Examples of cyclic alkyl amino carbene (CAAC) ligands L include A/-aryl-2,2,4,4-tetralkyl pyrrolidine derivatives such as A/-(2,6-diisopropylbenzene)- 2,2-dimethyl-4,4-adamantyl pyrrolidine. Examples of the monodentate anionic ligands include halides such as Br and CI^", pseudohalides such as triflate (CF3SO2^"), alkoxides such as optionally substituted phenoxides and organic amides of the structure R^'-N-R^" wherein R^' and R^" are hydrogen and organic groups and may be the same or different. Examples of these complexes and their properties are reported in Chem. Commun., 2016, 52, 6379-6382 and UK Patent Application Nos GB1516230.8 and GB1609707.3 that are herein incorporated by reference. Exemplary complexes of this type and a generic structure of the carbene ligand wherein R^a, R^b, R^c, and R^d are alkyl groups and Ar is a substituted phenyl group are shown below.

The EML can thus be formed by a polymeric emissive species itself, or a host-guest blend in which the guest material is the emissive species and is either a polymeric emitter or a small molecule emitter. The EML layer may contain multiple emissive species to achieve white light or multiple colour generation. The EML layer may also contain dopant molecules which enhance charge injection, charge transport, charge balance or exciton transfer. As noted above, the solubility of the dopant molecules when deposited, i.e. when they are resident in a polymeric matrix is greatly reduced relative to the solubility of the dopant molecule when it is not a guest. This modulation of the solubility of guest or dopant molecules generally applies for the other layers described herein. Thus the solubility of the EML materials alone or in combination that are suitable for use in the method according to the invention is from 2 mg/mL to 1000 mg/mL, whilst their solubility in solvent 4 when deposited is < 1 mg/mL and is preferably below 0.1 mg/mL.

If a host-emissive guest structure (e.g., PVK hostemissive guest) is used, the preferred energetic alignment is a type I heterostructure. i.e., the LUMO level of the emissive guest is below the LUMO level of the host material, while the HOMO level of the emissive guest is higher the HOMO level of the host material. This ensures that the band gap of the host is larger than the band gap of the emissive guest. The weight percentage of the emissive guest in the EML is set to from 0.1 wt% to 80 wt% and is preferably from 1 % to 30%. The band gap of the host material is 2 eV to 6 eV, and is preferably from 2.5 eV to 4 eV. The band gap of the emissive guest is from 1 eV to 3.5 eV and is preferably from 1.5 eV to 3 eV. In a preferred case, the triplet energy of the host is larger than the triplet energy of the emissive guest. In addition, any additional charge transport or charge balance dopants that are present in the EML should have band gap and triplet energy larger than or equal to that of the emissive guest. If the EML is composed of a single emissive species (e.g., a light-emitting polymer), the band gap of the emissive species is 1 eV to 3.5 eV, preferably 1.5 eV to 3 eV. Any additional charge transport or charge balance dopants should have band gap and triplet energy larger than or equal to that of the emissive species.

The thickness of the EML is from 5 nm to 500 nm and is preferably from 10 to 100 nm.

Solvent 3 is an organic polar aprotic solvent, with a relatively high dielectric constant of ε > 20. Preferably, solvent 3 has a boiling point of less than 200°C at atmospheric pressure and ε > 30, and 5D > 16, δΡ > 12, and 10 < δΗ > 14. Examples of solvents that can be used as solvent 3 include but are not limited to DMF, DMSO, nitromethane, dimethyl acetamide, and dimethyl ethanolamine. DMF and DMSO are preferred examples of solvent 3.

The fourth and final functional layer deposited by the method of the invention in the electroluminescent devices of architecture 1 is the electron transport layer (ETL). In some case the electron transport layer can also function as a hole blocking layer. The materials from which the ETL layer is formed are wide band gap, electron-transporting small molecules which are soluble in solvent 4. Examples of materials that can be used to form the ETL include BPhen, TPBi, TAZ, PBD, Bu-PBD, n-type inorganic nanoparticle dispersion or solutions of e.g., ZnO or T1O2, or admixtures of two or more of these materials.

In general, the materials suitable for use as an electron transport layer have LUMO level (or conduction band energy level, CB) of from -2 eV to -4 eV, preferably from -2.5 eV to -3.5 eV, and most preferably this LUMO (CB) level is close (+/- 0.2 eV) to the LUMO level of the emissive species in the EML. In addition, the materials suitable for use as in the ETL have a HOMO-level (or valence band energy level, VB) of from -9 eV to -5 eV, preferably from -8 eV to -6.5 eV, and this HOMO level ideally lies below the HOMO (VB) level of the EML material thus advantageously allowing the ETL to also serve as an effective hole blocking layer. The band gap of the ETL material (i.e. the HOMO-LUMO or CB-VB gap) is greater than 2 eV, and preferably >2.5 eV. In optimal cases the band gap of the ETL material is larger than the band gap of the emissive species in the EML. The electron mobility in the ETL should be > 10^"8 cm²/Vs, more preferably > 10^"5 cm²/Vs, and most preferably > 10^"2 cm²/Vs.

The thickness of the electron transport layer, i.e. layer 4 of the device of architecture 1 , is from 1 nm to 1000 nm and is preferably from 20 to 300 nm. Solvent 4, the solvent from which layer 4 in architecture 1 is deposited is an organic polar protic solvent with ε from 10 to 60. Solvent 4 preferably has 15 < ε < 35, and has HSP 5D > 15, 5 < δΡ < 13, δΗ > 14, and a boiling point below 200°C. Preferred examples of solvent 4 include but are not limited to methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. Methanol and ethanol are particularly preferred. Other solvents that can be used as solvent 4 include glycol ethers (ethane-1 ,2,diol, propanediols, butanediols, diethylene glycol), alcohols (benzyl-alcohol, propenols, etc). In general, solvent 4 has good solvent properties, low molecular weight and low boiling point as this combination of properties favours efficient removal of the solvent. The solubility of the ETL materials, alone or in combination, in solvent 4 is greater than 2 mg/mL and generally from 2 mg/mL to 1000 mg/mL.

Once layer 4 is in place the top electrode, the cathode can then be applied under standard techniques. As will be appreciated by the person skilled in the art selection of the materials and solvents from which the 4 functional device layers between the electrodes of the electroluminescent device allows a large degree of flexibility in delivering practical electroluminescent devices with internal quantum efficiency and external quantum efficiencies that have previously only been accessible via vacuum deposition processes. The orthogonal nature of the solvent and material from which each successive contiguous layer is formed allows access to devices with optimal layer thickness. This in turn can be exploited to ensure that recombination of electrons and hole in the devices formed by the method of the present invention occurs in the emissive layer of the electroluminescent device and delivers the substantially optimal IQEs and EQEs observed.

The five-layer modified standard architecture of an electroluminescent device, architecture 2, produced by a method according to the invention is presented in Figure 1 b. This comprises five contiguous functional layers that are deposited from solution in the order 1) hole injection layer (HIL), 2) hole transport layer (HTL) that may also function as electron blocking layer (EBL), 3) emissive layer (EML), 4) hole blocking layer (HBL) and 5) electron transport layer (ETL). Architecture 2 differs from architecture 1 in that a separate hole blocking layer is added between the EML and the ETL. The use of this additional layer can be used to balance the charge injection and transport in the device. In addition, it is believed that use of this additional hole blocking layer can also potentially reduce exciton quenching at the EML/ETL interface. This architecture, architecture 2, can thus advantageously be used in situations where no optimal HTL/ETL combination can be identified for devices of architecture 1 that results in excessive hole injection. Use of a separate HBL above the EML therefore advantageously solves the problem of excessive hole injection and is used to ensure that recombination of electrons and hole to form excitons occurs in the emissive layer to deliver devices with optimal or near optimal efficiency (i.e. improved IQE and EQE). A solvent-layer solubility matrix for architecture 2 is presented in Table 3 below.

S = soluble, IS = insoluble, - = not relevant as the solvent used in depositing underlying layers will not affect upper layers

Table 3 Solvent-layer solubility matrix of architecture 2 devices In more detail, to achieve well-defined (non-intermixed) device structure of architecture 2, the following material/solvent combinations can be used. The device structure is constructed in the following order using layer-by-layer sequential deposition from solution.

As mentioned above, architecture 2 is a variation on architecture 1 and consequently the materials and solvents for the first three layers of architecture 2 are selected on identical criteria to the corresponding layers in architecture 1 devices as described above. Likewise layer 5 of the architecture 2 device in general corresponds to layer 4 of the architecture 1 device and the materials and solvent used for layer 5 in architecture are selected on the same criteria and from the same materials and solvent as is outlined above for layer 4 of architecture 1. For the avoidance of doubt, in the description of devices of architecture 1 the solubility of the material from which each layer is constituted is provided for the solvent from which that layer is deposited and the solubility in the solvents from which the further layers are deposited. For architecture 2, the materials from which each layer above layer 1 are constituted are soluble in the solvent at a level of at least 2 mg/mL while their solubility of the material of each layer in the solvents used to deposit each of the subsequent layers is generally less than 1 mg/mL and preferably less than 0.1 mg/mL. This orthogonality of solvent and materials provides access to devices with high quality, non-intermixed layers. In practice the short residence time of the deposited layers as well as the specification of materials allows a tolerance as to the absolute solubility but these guideline solubility parameters should generally be adhered to.

The material constituting the hole blocking layer, the hole blocking material, is thus selected from a material that is soluble in solvent 4 (see table 3) and that is insoluble or substantially insoluble in solvent 5 having the following properties. The HOMO level of the hole blocking material is from -9 eV to -5 eV and the band gap of the HBL is from 3 eV to 9 eV and preferably 3eV to 5eV. The solubility of the hole blocking layer (HBL) materials, alone or in combination, in solvent 4 is greater than 2 mg/ml and generally from 2mg/mL to 1000mg/ml. Solubility of the hole blocking layer once depositied in solvent 5 is less than 1 mg/mL and preferably less than 0.1 mg/mL. Materials that fit these criteria include but are not limited to polystyrene (PS) and poly(methyl methacrylate) (PMMA) and blends of these materials, optionally with additional dopants to further enhance the hole blocking effect.

Solvent 4 is selected from the group of polar aprotic solvents having ε > 6 and preferably having ε from 10 to 22, or HSP 5D > 15, 5 < δΡ < 13, δΗ < 10 that have a boiling point of less than 200°C at atmospheric pressure. A preferred example of solvent 4 is acetone. Other examples of solvents that can be used as solvent 4 include ethyl acetate, simple short chain ketones such as butanone, butadiene and pentanone and aniline. The fifth and final functional layer deposited by the method of the invention in the electroluminescent devices of architecture 2 is the electron transport layer (ETL). The materials from which the ETL layer is formed are wide band gap, electron-transporting small molecules or inorganic nanoparticles which are soluble/dispersable in solvent 5. Examples of materials that can be used to form the ETL include BPhen, TPBi, TAZ, PBD, Bu-PBD, n- type inorganic nanoparticle dispersion solutions of e.g., ZnO or T1O2, or admixtures of two or more of these materials.

In general, the materials suitable for use as an electron transport layer have LUMO level (or conduction band energy level, CB) of from -2 eV to -4 eV, preferably from -2.5 eV to -3.5 eV, and most preferably this LUMO (CB) level is close (+/- 0.2 eV) to the LUMO level of the emissive species in the EML. In addition, the materials suitable for use as in the ETL have a HOMO-level (or valence band energy level, VB) of from -9 to -5 eV, preferably from - 8 eV to -6.5 eV, and this HOMO level ideally lies below the HOMO level of the EML material thus advantageously allowing the ETL to also serve as an effective hole blocking layer. The band gap of the ETL material (i.e. the HOMO-LUMO gap or CB-VB gap) is greater than 2 eV, and preferably >2.5 eV. In optimal cases the band gap of the ETL material is larger than the band gap of the emissive species in the EML. The electron mobility in the ETL should be > 10^"8 cm²/Vs, more preferably >10^"5 cm²/Vs, and most preferably >10^"2 cm²/Vs.

The thickness of the electron transport layer, i.e. layer 5 of the device of architecture 2, is from 1 nm to 1000 nm and is preferably from 20 to 300 nm.

Solvent 5, the solvent from which layer 5 in architecture 2 is deposited is a polar protic organic solvent with ε >15, and preferably having ε between 15 and 35, HSP 5D > 15, 5 < δΡ < 13, δΗ > 14 and a boiling point below 200°C. Preferred examples of solvent 5 include but are not limited to methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2- butanol. Methanol and ethanol are particularly preferred. Other solvents that can be used as solvent 5 include glycol ethers (ethane-1 ,2 diol, propanediols, butanediols, diethylene glycol), alcohols (benzyl-alcohol, propenols, etc). In general, solvent 5 has good solvent properties, low molecular weight and low boiling point as this combination of properties favours efficient removal of the solvent. The solubility of the ETL materials, alone or in combination, in solvent 5 is greater than 2 mg/mL and generally from 2 mg/mL to 1000mg/mL.

Once layer 5 is in place the top electrode, the cathode can then be applied under standard techniques. As will be appreciated by the person skilled in the art selection of the materials and solvents from which the 5 functional device layers between the electrodes of the electroluminescent device allows a large degree of flexibility in delivering practical electroluminescent devices with internal quantum efficiency and external quantum efficiencies that have previously only been accessible via vacuum deposition processes. The orthogonal nature of the solvent and material from which each successive contiguous layer is formed allows access to devices with optimal layer thickness. This in turn can be exploited to ensure that recombination of electrons and hole in the devices formed by the method of the present invention occurs in the emissive layer of the electroluminescent device and delivers the substantially optimal IQEs and EQEs observed.

The third and final type of electroluminescent device architecture described herein is the inverted architecture, architecture 3, that has as the base electrode a transparent cathode. This device architecture, architecture 3, is shown in Figure 1 c. This electroluminescent device architecture comprises four contiguous functional layers that are in the order 1) electron transport layer (ETL), 2) interlayer (IL), 3) emissive layer (EML) and 4) hole transport layer (HTL), that may also function as a electron blocking layer (EBL). A solvent-layer solubility matrix for architecture 3 is presented in table 4 below. At least layers 2 to 4 in this architecture, and optionally layer 1 , are formed by deposition from solution. Thus layer 1 can be formed via standard methods including solution techniques such as deposition from a nanoparticle dispersion in for example water, from a sol gel or can be formed reactively from precursor solutions using techniques such as atomic layer deposition (ALD), atmospheric pressure spatial atomic layer deposition (AP-SALD), chemical vapour deposition (CVD). It should be noted that ALD, AP-SALD and CVD are not considered to be standard solution deposition techniques such as those used to deposit layers 2 to 4 in architectures 1 to 3 or and optionally layer 1.

As has been demonstrated in the examples, application of the method of the invention to the fabrication of an electroluminescent device allows the first successful direct solution deposition of an organic HTL on top of a conjugated polymer wherein the structural integrity of the emitter layer is preserved. This process advantageously avoids the need to resort to complicated floating/transfer protocols for incorporation of the HTL on top of a conjugated polymer containing emissive layer. Table 4 shows a solvent-layer solubility matrix for production of devices of architecture 3. 3-EM L, emissive

2-1 L, low work- polymer, a polymer-

1-ETL, low

function polymers polymer blend, or a

work-function 4-HTL (EBL), e.g.,

Solvent for depositing or inorganic salts, polymer-small

metal oxides TPD, polystyrene each layer e.g. PEI e.g., PEI, molecule blend e.g.,

e.g., ZnO, Ti0₂ (PS), PM MA etc

PEI E, Cs₂C0₃, F8BT, PFO, rubrene,

Ba(OH Alq₃, lr(ppy)₃,

4CzlPN etc

1, nanoparticle

dispersion, ink, sol- s - - - gel, ALD, etc

2, glycol ethers, e.g.,

2-methoxyethanol IS S - - etc

3, moderately polar

organic e.g., tolurene,

IS IS S - chlorobenzene, THF

etc

4, acetone etc IS IS IS S

S = soluble, IS = insoluble, - = not relevant as the solvent used in depositing underlying layers will not affect upper layers.

Table 4 Solvent-layer solubility matrix of architecture 3

In more detail, to achieve well-defined (non-intermixed) device structure of architecture 3, the following material/solvent combinations can be used. The device structure is constructed in the following order sequential layer deposition.

The materials from which the electron transport layer (ETL) is formed, the electron transport materials, and the techniques for their deposition are well known in the art. In general, layer 1 in devices of architecture 3 are low work function metal oxides, such as ZnO, T1O2, that can be presented in conductive inks, for example ZnO inks optionally doped with aluminium, or n-type conductive polymers. These materials can be deposited on the base cathode by standard techniques such as deposition from nanoparticle dispersion and sol gel processes, or they can be formed reactively from precursor solutions using techniques such as atomic layer deposition (ALD), atmospheric pressure spatial atomic layer deposition (AP-SALD), chemical vapour deposition (CVD).

The materials profile of the ETL materials are i) they have a Fermi level, LUMO level or conduction band position of from -2 eV to -4.5 eV and preferably from -3 eV to -4 eV; and ii) they have an electron mobility of greater than 10^"5 cm²/Vs, preferably > 10^"2 cm²/Vs. The thickness of the ETL is from 1 nm to 500 nm and is preferably from 10 nm to 100 nm. The solubility of these materials, alone or in combination, once deposited on the cathode in the solvents used for the formation of layer 2, 3 and 4 of architecture 3, i.e. solvents 2, 3, and 4 is less than 1 mg/mL and preferably less than 0.1 mg/mL. This lack of solubility in solvents 2, 3, and 4 is an essential key requirement for the production of efficient devices with the inverted device architecture.

Layer 2 of the electroluminescent device of architecture 3 is an interlayer (IL). This interlayer consists of a low work-function polymer such as PEI, PEIE or a wide band gap inorganic salt such as CS2CO3 or Ba(OH)2, or the combination of two or more of these materials. The interlayer (IL) is relatively thin and has a thickness of from 0.1 nm to 20 nm, and preferably 0.5 nm-10 nm. The Fermi level, LUMO level or conduction band position of the interlayer is from -2 eV to -4.5 eV and is preferably -3 eV to -4 eV. The solubility of the IL materials, alone or in combination, in solvent 2 is greater than 2 mg/mL, for example from 2 mg/mL to 1000mg/mL while its solubility in solvents 3 and 4 is less than 1 mg/mL and preferably less than 0.1 mg/mL.

Solvent 2 for the case of architecture 3 is an organic polar protic solvent with ε > 6, that preferably has ε between 10 and 20, or HSP 5D > 15, 5 < δΡ < 15, δΗ > 14, and a boiling point of less than 200°C. Preferred examples of solvents that can be used as solvent 2 include but are not limited to 2-methoxyethanol, 1- and 2-propanol. Other examples of solvent 2 include simple glycol ethers such as ethane-1 ,2,-diol, propane-1 ,2-diol, the various isomeric butanediols, diethylene glycol and short chain alcohols such methanol, ethanol and the isomers of butanol.

The third functional layer in architecture 3, layer 3, is the emissive layer (EML). The emissive material in the EML comprises polymers and small molecules that are organic or organometallic compounds such as F8BT, PFO, PFHCz, TFB, poly-TPD, PFB, PPV, MEH- PPV, PVK, lr(ppy)₃, lr(ppy)₂(acac), lr(piq)₃, lr(piq)₂(acac), PtOEP, PdOEP, Flrpic, 4CzlPN, Alq₃, Znq, rubrene, Eu(dmb)₃(Phen-NH₂), perylene, DPA, DPBF, TIPS-pentancene, TIPS- tetracene, TES-pentacene, FTES-ADT, anthracene, DCM, CBP, TPBi, Bphen, TAZ, PBD, Bu-PBD, BCP, TCTA, TAPC, TPD and NPD or one or more of these materials. Overall the EML comprises either an emissive polymer, a polymer-polymer blend in which at least one of the polymers is an emissive polymer, or a polymer-small molecule blend in which at least the small molecule is an emitter. It should be noted that the EML does not consist solely of small molecule emitter species, instead if small molecule emitters are present in the EML they are present in the context of a small molecule/ polymer blend wherein the polymer serves as the host and the small molecule is the guest. The EML can thus consist of a single polymeric emissive species, or a host-guest blend in which the guest material is a polymeric or small molecule emissive species. The EML layer may contain multiple emissive species to achieve white light or multiple colour generation. The EML layer may also contain dopant molecules which enhance charge injection, charge transport, charge balance or exciton transfer.

The approach to devices of architecture 3 as described herein has, for the first time, allowed deposition of an organic hole transport layer on top of a conjugated polymer layer without compromising the integrity of that polymer layer via simple solution processing. This could have implication in the fabrication of other devices.

The methods of the invention also relate to the manufacture of devices containing an emissive layer with a small molecule organometallic emitter of the structure L-M-X wherein M is a two-coordinate metal selected from copper, silver and gold; L is a neutral electron donor cyclic alkyl amino carbene (CAAC) ligand and X is a monodentate anionic ligand. Examples of these organometallic complexes can exhibit rotationally accessed spin-state inversion (RASI) photoemission characteristics and correspondingly exhibit excellent light emission characteristics. Examples of cyclic alkyl amino carbene (CAAC) ligand L include A/-aryl-2,2,4,4-tetralkyl pyrrolidine derivatives such as A/-(2,6-diisopropylbenzene)-2,2- dimethyl-4,4-adamantyl pyrrolidine. Examples of the monodentate anionic ligands include halides such as Br and CI^", pseudohalides such as triflate (CF3SO2^"), alkoxides such as optionally substituted phenoxides and organic amides of the structure R^'-N-R^" wherein R^' and R^" are hydrogen and organic groups and may be the same or different. Examples of these complexes and their properties are reported in Chem. Commun., 2016, 52, 6379-6382 and UK Patent Application Nos GB1516230.8 and GB1609707.3 that are herein incorporated by reference.

The solubility of the EML materials, alone or in combination, suitable for use in the method according to the invention is greater or equal to 2 mg/mL for example from 2 mg/mL to 1000 mg/mL. The solubility of the deposited layer of EML materials in solvent 4 is less than 1 mg/mL, and preferably less than 0.1 mg/mL If a host-emissive guest structure (e.g., polyfluorene host: emissive guest) is used, the preferred energetic alignment is a type I heterostructure wherein the LUMO level of the emissive guest is below the LUMO level of the host material, while the HOMO level of the emissive guest is higher the HOMO level of the host material. This ensures that the band gap of the host is larger than the band gap of the emissive guest. The weight percentage of the emissive guest in the EML is from 0.1 to 80 wt %, preferably 1 %-30 wt %. The band gap of the host material from 2 eV to 6 eV, preferably 2.5 eV to 4 eV. The band gap of the emissive guest is from 1 eV to 3.5 eV, preferably 1.5 eV to 3 eV. It is also preferred that the triplet energy of the host is larger than the triplet energy of the emissive guest. Furthermore, any dopant incorporated into the EML to modulate charge injection, charge transport or charge balance have a band gap and triplet energy that is greater than or equal to that of the guest in the emissive layer.

In the case where the EML is composed of a single emissive species (e.g., a light- emitting polymer), the band gap of the emissive species is from 1 eV to 3.5 eV and preferably from 1.5 eV to 3 eV. Any dopant incorporated into the EML to modulate charge injection, transport or balance have a band gap and triplet energy that is greater than or equal to that of the guest in the emissive layer.

The thickness of the EML is from 5 nm to 500 nm. In preferred cases the thickness of the EML in architecture 3 is from 10 to 100 nm. Solvent 3 is selected from the group of non-polar or slightly polar aprotic organic solvents, having ε <20. The solvent preferably has a boiling point of less than 200°C at atmospheric pressure. In preferred cases, the solvent has ε from 1 to 10, or, in terms of HSP, 5D > 16, δΡ < 8, and δΗ < 5. Examples of preferred common solvents include but are not limited to chlorobenzene, toluene, dichlorobenzene (p-, m-, o- or mixed isomers), trichlorobenzene (all isomers), xylene (p-, m-, o- or mixed isomers), THF, chloroform, dichloromethane, dichloroethane and trichloroethane (all isomers). Other possible solvents include but are not limited to pyrrole, pyridine, tetrahydropyran, tetrahydrothiophene, and 1 ,4-dioxane.

The fourth and final layer of the electroluminescent device of architecture 3 is the hole transport layer (HTL) that may also function as an electron blocking layer (EBL). The materials from which this layer is formed are hole-transporting small molecules such as TPD or wide band gap polymers that are soluble in ketones such as polystyrene (PS) and PMMA, or the combination of two or more of these materials. The solvent from which the HTL of architecture 3 is deposited is an organic polar aprotic solvent, where ε > 5 and preferably ε > 15, or an HSP of 5D > 15, 5 < δΡ < 13, 5 < δΗ < H and preferably 5D > 15, 5 < δΡ < 13, 5 < δΗ < 14, and preferably with a boiling point of less than 200°C. A preferred example is acetone. Other examples include simple short ketones butanone, butadione, the isomers of pentanone and aniline. The HTL materials of architecture 3 have a solubility in solvent 4 of from 2mg/ml_ to 1000mg/ml_.

The functional properties of the HTL materials in architecture 3 are as follows. The HTL materials have a HOMO level of from -4 eV to -9 eV and preferably from -5 eV to -6 eV. The HOMO level is preferably selected to be close (i.e. within 0.2 eV) to the HOMO level of the emissive species in the EML. The HTL materials have a LUMO level of from -1 to -3.5 eV, preferably from -1.5 eV to -3 eV. The LUMO level of the HTL material is ideally above that of the LUMO level of the EML material as this allows for effective electron blocking. The band gap of the HTL material, i.e. the HOMO-LUMO gap, is greater than 2 eV and preferably greater than 2.5 eV, for an optimal device performance the band gap of the HTL layer is larger than the band gap of the emissive species in the EML. Once layer 4 is in place the top electrode, an anode can then be applied under standard techniques. As will be appreciated by the person skilled in the art selection of the materials and solvents from which the 4 functional device layers between the electrodes of the electroluminescent device allows a large degree of flexibility in delivering practical electroluminescent devices with internal quantum efficiency and external quantum efficiencies that have previously only been accessible via vacuum deposition processes. The orthogonal nature of the solvent and material from which each successive contiguous layer is formed allows access to devices with optimal layer thickness. This in turn can be exploited to ensure that recombination of electrons and hole in the devices formed by the method of the present invention occurs in the emissive layer of the electroluminescent device and delivers the substantially optimal IQEs and EQEs observed.

The various options for materials for use as an emissive layer in the electroluminescent devices produced by the method of the present invention having either architecture 1 , 2 or 3 allows a great degree of flexibility with regards to device properties. The use of a pure polymer or a polymer-polymer blend can have certain advantages, for example such layers can display high luminosity at relatively low driving voltage, and can display increased stability in which one or more small molecule emitters is/are provided in a polymeric host. In addition, use of a polymer or a polymer-polymer blend as the emissive layer can minimise motility of the materials in the device and this can be advantageous for the generation of long operational lifetime devices. In addition, the present methods allow use of a small molecule emitter, or indeed a plurality of small molecule emitters, blended into a polymeric host such as a carbazole- based polymer, for example PVK in the case of architectures 1 and 2. As well as allowing EML containing small molecule emitter layers to be generated efficiently by solution processing this approach also advantageously results in dispersal of emitters in a wide band gap polymeric host matrix wherein they are physically separated at an optimum average distance, and are isolated from luminescence quenching sites, while allowing effective electron confinement. This advantageously can lead to a reduction of detrimental concentration quenching and exciton dissociation and that in turn delivers devices of improved EQE. In addition, use of small molecules as guests in a polymeric host allows access to electroluminescent devices featuring emitter species that had previously not been suitable for use in such devices. For example, the present inventors have shown that a deep blue fluorophore, 9, 10-diphenylanthracene (DPA) can be used in an electroluminescent device by using the method of the invention whereas previously problems with film formation and crystallisation had rendered use of DPA impractical. Likewise, although perylene is a well-known fluorescent emitter its complex intermolecular interactions had meant that hither to the present invention no practical electroluminescent device using this material as emitter had been produced.

So that the invention may be better understood the invention is described by reference to the examples below. These examples demonstrate the extremely broad applicability of the device architectures and fabrication methods described here, and should not be considered to limit the scope of the invention as the skilled person would appreciate that by applying the design criteria described above a large range of practical and highly efficient electroluminescent devices (comparable to or better than some of the best vacuum- deposited electroluminescent devices such as OLEDs) having both standard and inverted architectures can be generated by a low cost and flexible method.

Specific examples

Example 1. 9,10-diphenylanthracene (DPA) OLED (deep blue, fluorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:DPA/Bphen/LiF/AI Device Fabrication: ITO substrate was cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150nm HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45μηι PVDF filter, spin-coated, 30nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16.5mg/ml_ in toluene), filtered with 0.20μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PVK doped with 20 w/w% DPA (10mg/ml_ in DMF), filtered with 0.20μΓΠ PTFE filter, spin coated, 20nm. Thermal annealing in nitrogen-filled glovebox at 120°C for 10 min.

ETL: Bphen (10mg/ml_ in methanol), filtered with 0.20μηι PTFE filter, spin coated, 70nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7nm) / Al (evaporated, 120nm), under vacuum pressure of 5x10-6 mbar.

Device characterisation data is presented in Figure 2. This is the first working example of a DPA-based OLED. Although DPA is a well-known fluorescent emitter, its film formation and crystallisation have been an issue for device fabrication and have prevented the elaboration of successful deep blue OLEDs directly fabricated with DPA before development of the OLED fabrication techniques described herein.

As can be seen the method of the invention provides access to a deep blue OLED (EL emission peak at 440nm), with an EQE and current efficiency of 3.3% and 4.5 cd/A at 1.5 cd/m², 2.4% and 3.3 cd/A at 100 cd/m², and 1.25% and 1.7 cd/A at 1000 cd/m². This OLED can deliver a maximum luminance in excess of 3000 cd/m². Example 2. Flrpic OLED (sky blue, phosphorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:Flrpic/TPBi/LiF/AI

Device Fabrication: ITO substrate was cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150 nm HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45μηι PVDF filter, spin-coated, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PVK doped with 15 w/w% Firpic (10mg/ml_ in DMF), filtered with 0.20 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 90°C for 10 min.

ETL: TPBi (8mg/ml_ in methanol), filtered with 0.20μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min. Cathode: LiF (evaporated, 0.7nm) / Al (evaporated, 100 nm), under vacuum pressure of 1 *10^"6 mbar.

Device characterisation data is presented in Figure 3. This example shows a sky-blue OLED (EL emission peak at 489nm) with a EQE and current efficiency of 11.0% and 30.6 cd/A at 20.6 cd/m², 10.8% and 30.1 cd/A at 100 cd/m2, 9.4% and 26.1 cd/A at 1000 cd/m², 4.4% and 12 cd/A at 10000 cd/m². The OLED generated can deliver a luminance of 20000 cd/m².

Note that the device efficiencies achieved here are comparable to those described in the literature for vacuum-evaporated Firpic OLEDs (see for example, J. Appl. Phys. 46, L1 17- L119 (2007), reporting an EQE for Firpic OLED of 21 % at 100 cd/m²). Example 3. DPBF OLED (sky blue, fluorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:DPBF/Bphen/LiF/AI

Anode: indium tin oxide (ITO), 150nm HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin coated in air, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16.5mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 20 w/w% DPBF (10 mg/mL in DMF), filtered with 0.20 μπι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min.

ETL: Bphen (10 mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min. Cathode: LiF (evaporated, 0.7 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x 10^"6 mbar.

Device characterisation data is presented in Figure 4. This is the first working example of a DPBF-based OLED. As can be seen the method of the invention provides access to a sky- blue blue OLED (EL emission peak at 492 nm), with a EQE and current efficiency of 2.2% and 7.3 cd/A at 0.05 cd/m², 1.65% and 5.4 cd/A at 25 cd/m², 1.5% and 5.0 cd/A at 100 cd/m², and 0.9% and 2.9 cd/A at 1000 cd/m². This OLED can deliver a maximum luminance in excess of 3500 cd/m².

Example 4. Perylene OLED (green, fluorescent) Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:Perylene/Bphen/LiF/AI

Anode: indium tin oxide (ITO), 150 nm

HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin coated in air, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (15 mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 170 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 20 w/w% perylene (10mg/mL in DMF), filtered with 0.20 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min. ETL: Bphen (7.5 mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 50nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.6nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x 10^"6 mbar.

Device characterisation data is presented in Figure 5. This is the first working example of a perylene-based OLED. Although perylene is a well-known fluorescent blue emitter its complex intermolecular interactions have been an issue for device fabrication and have prevented the elaboration of successful OLEDs directly fabricated with perylene before development of the OLED fabrication techniques described herein. As can be seen the method of the invention provides access to a green OLED (main EL emission peak at 527 nm), with a EQE and current efficiency of 3.8% and 12.4 cd/A at 13.8 cd/m², 3.4% and 1 1.0 cd/A at 100 cd/m², and 2.2% and 7.2 cd/A at 1000 cd/m². This OLED can deliver a maximum luminance in excess of 8700 cd/m². The green EL emission observed arises from exciplex emission.

Example 5 lr(ppy)₃ OLED (green, phosphorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:lr(ppy)₃/Bphen/LiF/AI

Device Fabrication: ITO substrate was cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use. Anode: indium tin oxide (ITO), 150 nm

HTL: TFB (15mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 170 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PVK doped with 10 w/w% lr(ppy)3 (10mg/mL in DMF), filtered with 0.20 μπι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min.

ETL: Bphen (10mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.6nm) / Al (evaporated, 120nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 6. In this example, we demonstrate a green OLED (EL emission peak at 525 nm), with a EQE and current efficiency of 25.7% and 91 cd/A at 34.1 cd/m², 25.2% and 89 cd/A at 100 cd/m², 24% and 85 cd/A at 1000 cd/m², 16.9% and 60 cd/A at 10000 cd/m². The OLED generated in this example can deliver a luminance of 73000 cd/m².

The device efficiencies achieved using the technique described herein are higher than some of the best vacuum-evaporated lr(ppy)3 OLEDs. For comparison, see e.g. Adv. Mater. 22, 2468-2471 (2010), wherein the maximum EQE and current efficiencies reported for their best lr(ppy)3 OLED were 21.6% and 83.4 cd/A, respectively. Example 6. Alq3 OLED (green, fluorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:Alq3/Bphen/LiF/AI

Device Fabrication: ITO substrate is cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use. Anode: indium tin oxide (ITO), 150 nm

HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45μηι PVDF filter, spin coated in air, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16.5mg/ml_ in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PVK doped with 10 w/w% Alq₃ (10mg/ml_ in DMF), filtered with 0.20 μπι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min.

ETL: Bphen (10mg/ml_ in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 7. In this example a green OLED (EL emission peak at 528 nm), with a EQE and current efficiency of 3.7% and 12.8 cd/A at 0.1 cd/m², 2.2% and 7.4 cd/A at 20.9 cd/m², 2.1 % and 7.2 cd/A at 100 cd/m², 1.7% and 5.8 cd/A at 1000 cd/m² was elaborated. The OLED device of this example delivers a luminance of 1 1250 cd/m².

The device efficiencies achieved with the solution based OLED fabrication technique described herein are higher than some of the best vacuum-evaporated Alq3 OLEDs. For comparison, see e.g. Appl. Phys. Lett. 99, 153303 (201 1), wherein the maximum EQE and current efficiencies reported for the best Alq3 OLED device are reported as 1.71 % and 5.21 cd/A, respectively.

Example 7. 4CzlPN OLED (green, thermally activated delayed fluorescent (TADF))

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:4CzlPN/Bphen/LiF/AI Device Fabrication: ITO substrate was cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150 nm.

HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin coated in air, 30nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16.5mg/ml_ in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 10 w/w% 4CzlPN (10mg/ml_ in DMF), filtered with 0.20 μπι PTFE filter, spincoated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min. ETL: Bphen (10mg/ml_ in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Device characterisation data is presented in Figure 8. In this example a green OLED (EL emission peak at 528 nm), with a EQE and current efficiency of 12.8% and 44.1 cd/A at 1.85 cd/m², 9.7% and 33.2 cd/A at 100 cd/m², 5.4% and 18.5 cd/A at 1000 cd/m² was elaborated. The OLED device of this example delivers a maximum luminance of 15600 cd/m².

The device efficiencies afforded by the technique described herein are comparable with some of the best vacuum-evaporated 4CzlPN OLEDs. For comparison, see e.g. Sci. Rep. 3, 2127 (2013), wherein the maximum EQE and current efficiencies reported for the best 4CzlPN OLED described therein were 17% and 50 cd/A, respectively.

Example 8. Rubrene OLED (yellow, fluorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:rubrene/Bphen/LiF/AI

Device Fabrication: ITO substrate is cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150 nm

HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin coated in air, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min. HTL: TFB (17.5mg/ml_ in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 200 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 20 w/w% rubrene (10mg/ml_ in DMF), filtered with 0.20 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 160°C for 10 min. ETL: Bphen (10mg/ml_ in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Device characterisation data is presented in Figure 9. As can be seen therein the OLED fabrication technique described herein delivers a yellow OLED (EL emission peak at 570 and 608 nm), with a EQE and current efficiency of 6.0% and 13.9 cd/A at 0.07 cd/m², 4.0% and 12.6 cd/A at 19.4 cd/m², 3.6% and 1 1.5 cd/A at 100 cd/m², 2.3% and 7.2 cd/A at 1000 cd/m². This OLED can deliver a maximum luminance of 7900 cd/m².

The device efficiencies achieved with the present technique are higher than some of the best vacuum-evaporated rubrene OLEDs. For comparison, see e.g. Appl. Phys. Lett. 89, 183513 (2006), wherein the maximum EQE and current efficiencies reported for rubrene OLEDs are 3.4% and 8.2 cd/A, respectively.

Example 9. Eu(dbm)3(Phen-NH₂) OLED (red, lanthanide emitter)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:Eu(dbm)₃(Phen-NH₂)/TPBi or Bphen/LiF/AI

Anode: indium tin oxide (ITO), 150 nm

HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin-coated, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PVK doped with 10 w/w% Eu(dbm)₃(Phen-NH₂) (10 mg/mL in DMF), filtered with 0.20 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 90°C for 10 min.

ETL: TPBi (8mg/ml_ in methanol) or Bphen (10mg/ml_ in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7 nm) / Al (evaporated, 100 nm), under vacuum pressure of 1 *10^"6 mbar.

Device characterisation data is presented in Figure 10. This is the first working example of a red OLED (EL emission peak at 617nm, FWHM ~4nm), fabricated from Eu(dbm)₃(Phen- NH₂). The OLED has an EQE and current efficiency of 2% and 3.3 cd/A at 0.1 cd/m², 1.5% and 2.5cd/A at 1 cd/m², 1 % and 1.7 cd/A at 10 cd/m², 0.1 % and 0.15 cd/A at 100 cd/m² and can deliver a luminance of up to 420 cd/m².

Example 10. Ir(piq)₃ OLED (red, phosphorescent) Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK: lr(piq)₃/Bphen/LiF/AI

Device Fabrication:JTO substrate was cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150 nm

HTL: TFB (16.5 mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 180 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 10 w/w% lr(piq)₃ (10mg/mL in DMF), filtered with 0.45 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 120°C for 10 min. ETL: Bphen (10mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 70nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar. Device characterisation data is presented in Figure 11. As can be seen therein the OLED fabrication technique described herein delivers an orange OLED (EL emission peak at 627 nm), with a EQE and current efficiency of 14.1 % and 14.4 cd/A at 12.2 cd/m², 12.5% and 12.8 cd/A at 100 cd/m², 8.1 % and 8.3 cd/A at 1000 cd/m². The OLED of this example delivered a maximum luminance of 1 1360 cd/m².

The device efficiencies achieved with the present technique are higher than some of the best vacuum-evaporated lr(piq>3 OLEDs. For comparison, see e.g. Jpn. J. Appl. Phys. 55 03CD02 (2016), wherein the maximum EQE and current efficiencies reported for lr(piq>3 OLED were 8.8% and 5.6 cd/A, respectively Example 11. PtOEP OLED (red, phosphorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:PtOEP/Bphen/LiF/AI

Anode: indium tin oxide (ITO), 150 nm HIL: PEDOT:PSS (aqueous dispersion), filtered with 0.45 μηι PVDF filter, spin coated in air, 30 nm. Thermal annealing under nitrogen at 200°C for 20 min.

HTL: TFB (16.5 mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 200 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 5 w/w% PtOEP (10 mg/mL in DMF), filtered with 0.20μΓΠ PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min.

ETL: Bphen (10 mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 50 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.6 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar. Device characterisation data is presented in Figure 12. As can be seen therein the OLED fabrication technique described herein delivers a red OLED (EL emission peak at 653 nm), with an EQE and current efficiency of 7.6% and 3.0 cd/A at 2.6 cd/m², 6.6% and 2.6 cd/A at 10 cd/m², 1.6% and 0.65 cd/A at 100 cd/m². The OLED device of this example delivered a maximum luminance of 350 cd/m². The device efficiencies achieved with the present technique are comparable with or higher than some of the best vacuum-evaporated PtOEP OLEDs. For comparison, see e.g. Phys. Rev. B 77, 235215 (2008), wherein the maximum EQE reported for a PtOEP OLED was about 2.5%. Example 12. TIPS-pentacene OLED (red, fluorescent)

Device structure: Glass/ITO/PEDOT:PSS/TFB/PVK:TIPS-pc/Bphen/LiF/AI

HTL: TFB (17.5 mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 200nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min.

EML: PVK doped with 2 w/w% TIPS-pentacene (10 mg/mL in DMF), filtered with 0.20μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 120°C for 10 min.

ETL: Bphen (7.5 mg/mL in methanol), filtered with 0.20 μηι PTFE filter, spin coated, 50 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.6 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 13. As can be seen therein the OLED fabrication technique described herein delivers a red OLED (EL emission peak at 665 nm), with an EQE and current efficiency of 2.73% and 3.0 cd/A at 0.06 cd/m², 0.92% and 0.79 cd/A at 0.3 cd/m², 0.76% and 0.65 cd/A at 10 cd/m², 0.50% and 0.44 cd/A at 100 cd/m². The OLED device obtained in this example delivered a maximum luminance of 860 cd/m².

Using the fabrication technique described herein we have generated the first working example of a TIPS-pentacene-based OLED. TIPS-pentacene is known for being a singlet fission material (Nature Chemistry 5, 1019-1024 (2013), and is thus a potential singlet fission sensitizer in solar cells (Nano Lett. 15, 354-358 (2015)). When well-dispersed, with the lack of intermolecular coupling, singlet fission in TIPS-pentacene is suppressed and emissive properties are observed, such as has been observed in a dilute solution (Nature Chemistry 5, 1019-1024 (2013)). In our example, doping a small amount of TIPS-pentacene (w/w 2%) in a host polymer is appears to be analogous to dispersal in a dilute solution, and EL is successfully observed in these OLED devices. Example 13. PFHCz OLED (blue, fluorescent polymer)

Processing condition for the PFHCz device (unoptimised example).

EML: PFHCz (10 mg/mL in DMF), filtered with 0.20μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar. Device characterisation data is presented in Figure 14. As can be seen therein the OLED fabrication technique described herein delivers a blue OLED (EL emission peak at 453 nm), with an EQE and current efficiency of 0.26% and 0.8 cd/A at 19.1 cd/m², 0.26% and 0.80 cd/A at 10 cd/m², 0.23% and 0.72 cd/A at 100 cd/m². The OLED device obtained in this example delivered a maximum luminance of 1200 cd/m². The device efficiencies achieved with the present technique are comparable with literature PFHCz-based OLEDs. For comparison, see e.g. Macomolecules 38, 4970-4976 (2005), wherein the current efficiency reported was 0.03 cd/A at a luminance of 30 cd/m². Example 14. Ir(ppy)3 OLED (green, phosphorescent)

Processing condition for the PFHCz:lr(ppy)3 device (unoptimised).

HTL: TFB (16.5 mg/mL in toluene), filtered with 0.20 μηι PTFE filter, spin coated in nitrogen- filled glovebox, 200 nm. Thermal annealing in nitrogen-filled glovebox at 200°C for 20 min. EML: PFHCz doped with 10 w/w% lr(ppy)₃ (10 mg/mL in DMF), filtered with 0.20 μηι PTFE filter, spin coated, 20 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: LiF (evaporated, 0.7 nm) / Al (evaporated, 120 nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 15. As can be seen therein the OLED fabrication technique described herein delivers a green OLED (EL emission peak at 526 nm), with an EQE and current efficiency of 2% and 6.5 cd/A at 0.05 cd/m², 0.53% and 1.8 cd/A at 1.0 cd/m², 0.55% and 1.8 cd/A at 10 cd/m², 0.52% and 1.7 cd/A at 100 cd/m², 0.19% and 0.6 cd/A at 1000 cd/m². The OLED device obtained in this example delivered a maximum luminance of 1200 cd/m².

This fabrication technique described delivered the first working example of an OLED using PFHCz as a host material in the emissive layer, doped with lr(ppy)3 in this example. Optimisation of the device fabrication parameters are expected to deliver devices with improved efficiency.

Example 15. F8BT OLED (green-yellow, fluorescent polymer)

Device structure: Glass/ITO/ZnO/PEI/F8BT/TPD/Mo0₃/Au Device Fabrication: ITO substrate is cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use.

Anode: indium tin oxide (ITO), 150 nm

ETL: ZnO, deposited by AP-SALD at 150°C, 50-60 nm. IL: PEI (1 mg/mL in 2-methoxyethanol), filtered with 0.20 μηι PTFE filter, spincoated in nitrogen-filled glovebox, Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min.

EML: F8BT (20 mg/mL in chlorobenzene), filtered with 0.20 μηι PTFE filter, spincoated, 300 nm. Thermal annealing in nitrogen-filled glovebox at 150°C for 10 min.

HTL/EBL: TPD (5 mg/mL in acetone), filtered with 0.20 μηι PTFE filter, spincoated, 60 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min.

Cathode: M0O3 (evaporated, 10 nm) / Au (evaporated, 80nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 16. As can be seen therein the OLED fabrication technique described herein delivers a green polymer LED (EL emission peak at 567 nm), with a EQE and current efficiency of 6.8% and 22.6 cd/A at 340 cd/m², 6.5% and 21.5 cd/A at 100 cd/m², 6.6% and 21.9cd/A at 1000 cd/m², 4.9% and 16.2 cd/A at 10000 cd/m². The OLED device of this example delivered a maximum luminance of 59350 cd/m².

The device efficiencies achieved with the present technique are comparable with some of the best F8BT OLEDs. In this example, a relatively thin (300 nm) EML was used thus greatly reducing F8BT material consumption. The driving voltage required to achieve a luminance of 1000 cd/m² was only 7V. For comparison, see e.g. Adv. Mater. 22, 3194-3198 (2010), the maximum EQE and current efficiencies reported for the best F8BT OLED device described were 7.3% and 22.7 cd/A, respectively, using a much thicker (1200 nm) EML (emissive layer). To achieve a luminance of 1000 cd/m², the prior art devices were operating at a much higher voltage of 15V (cf 7V for the present example).

Example 16. Rubrene (yellow, fluorescent)

Device structure: Glass/ITO/ZnO/PEI/F8BT:rubrene/TPD/Mo0₃/Au

Device Fabrication:_ITO substrate is cleaned with acetone and isopropanol, and plasma etched with oxygen for ten minutes immediately before use. Anode: indium tin oxide (ITO), 150 nm

ETL: ZnO, deposited by AP-SALD at 150°C, 50-60 nm.

IL: PEI (1 mg/mL in 2-methoxyethanol), filtered with 0.20 μηι PTFE filter, spincoated in nitrogen-filled glovebox, Thermal annealing in nitrogen-filled glovebox at 100°C for 10 min. EML: F8BT doped with 20 w/w% rubrene (20 mg/mL in chlorobenzene), filtered with 0.20 μηι PTFE filter, spincoated, 260 nm. Thermal annealing in nitrogen-filled glovebox at 170°C for 10 min.

HTL/EBL: TPD (5 mg/mL in acetone), filtered with 0.20 μηι PTFE filter, spincoated, 60 nm. Thermal annealing in nitrogen-filled glovebox at 60°C for 10 min. Cathode: M0O3 (evaporated, 10 nm) / Au (evaporated, 80nm), under vacuum pressure of 5x10^"6 mbar.

Device characterisation data is presented in Figure 17. As can be seen therein the OLED fabrication technique described herein delivers a yellow OLED (EL emission peaks at 570nm), with an EQE and current efficiency of 6.3% and 20.7 cd/A at 1123 cd/m², 5.4% and 18.0 cd/A at 100 cd/m², 6.3% and 20.7 cd/A at 1000 cd/m², 5.4% and 18.0 cd/A at 10000 cd/m². The OLED device of the present example could reach a maximum luminance of 60000 cd/m².

The device efficiencies we achieved with the present technique are higher than some of the best vacuum-evaporated rubrene OLEDs. For comparison, see e.g. Appl. Phys. Lett. 89, 183513 (2006), wherein the maximum EQE and current efficiencies reported for the best rubrene OLED described were 3.4% and 8.2 cd/A, respectively.

Claims

1) A method for manufacturing an electroluminescent device comprising sequential deposition of at least four contiguous layers on a substrate-side electrode wherein:

A) the electrode is an anode and the at least four layers comprise: i) a hole injection layer (HIL); ii) a hole transport layer (HTL) deposited from a non-polar or low-polarity aprotic solvent having a dielectric constant of less than 20; iii) an emissive layer (EML) deposited from a polar aprotic organic solvent having a dielectric constant of greater than 20; and iv) an electron transport layer (ETL) deposited from a non-aqueous polar protic solvent having a dielectric constant of greater than 10; or

B) the electrode is a cathode and the at least four layers comprise:

1) an electron transport layer (ETL); ii) an interlayer (IL) deposited from a polar protic organic solvent with dielectric constant greater than 5; iii) an emissive layer (EML) deposited from a non-polar or low polarity aprotic solvent with dielectric constant less than 20; and iv) a hole transport layer (HTL) deposited from a polar aprotic organic solvent having a dielectric constant greater than 5; wherein the method does not involve chemical cross linking of the deposited materials and wherein the layers are deposited in the order i) to iv).

2) A method according claim 1 wherein the emissive layer comprises a light emitting polymer or a polymeric host-in combination with a) light emitting polymer or b) a small molecule emitter guest. 3) A method according to claim 1 or 2, in which the weight percentage of emitting guest in polymeric host is from 0.1 wt % to 80 wt %. 4) A method according to claim 1 , 2 or 3, wherein the host-emissive guest structured emissive layer is a type I heterostructure.

5) A method according to any preceding claim, wherein the band gap of the host material is 2 eV to 6 eV. 6) A method according to any previous claim, wherein the emissive layer is from 5 nm to 500 nm thick.

7) A method according to any preceding claim, wherein the emissive layer is deposited from DMF, DMSO, nitromethane, dimethyl acetamide, and dimethyl ethanolamine or a blend of two or more of these solvents. 8) A method according to any preceding claim, wherein the substrate-side electrode is an anode, and the method further comprises the step of depositing a hole blocking layer from a polar aprotic solvents having ε > 6 in between the emissive layer and the electron transport layer.

9) A method according to any preceding claim, wherein the substrate-side electrode is an anode and the thickness of the hole transport layer is greater than 100 nm thick, optionally from 150 to 200 nm thick.

10) A method according to any preceding claim, wherein the substrate-side electrode is an anode and the electron transport layer is deposited from a solvent selected from methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol, or a blend of two or more of these solvents.

1 1) A method according to any preceding claim wherein the substrate-side electrode is an anode, further wherein the hole transport layer comprises one or more wide band gap, hole-transporting polyfluorene such as TFB, poly-TPD, PFO, F8BT, F8T2, PFD, PFH, PFHA, PFB, PPV or MEH-PPV. 12) A method according to any of claims 1 to 7, wherein the substrate-side electrode is a cathode, further wherein the interlayer is deposited from a solvent selected from 2- methoxyethanol, 1- and 2-propanol, ethane-1 ,2,-diol, propane-1 ,2-diol, a butanediol, diethylene glycol, methanol, ethanol and butanol, or a blend of two or more of these solvents. 13) A method according to any of claims 1 to 7 or 12, wherein the substrate-side electrode is a cathode, in which the hole transport layer is deposited from acetone or butanone.

14) A method according to any of claims 1 to 7, 12 or 13, wherein the substrate-side electrode is a cathode, in which the hole transport layer is TPD or a wide band gap polymer soluble in ketones such as polystyrene (PS) and poly(methyl methacrylate), or the combination of two or more of these materials.

15) A method according to any of preceding claim, wherein the emissive layer comprises one or more compound or polymer selected from the group comprising PVK, PFHCz, lr(ppy)₃, lr(ppy)₂(acac), lr(piq)₃, lr(piq)₂(acac), PtOEP, PdOEP, Flrpic, 4CzlPN, Alq₃, Znq, rubrene, Eu(dmb)₃(Phen-NH₂), perylene, DPA, DPBF, TIPS-pentancene, TIPS-tetracene, TES-pentacene, FTES-ADT, anthracene, DCM, CBP, TPBi, Bphen, TAZ, PBD, Bu-PBD, BCP, TCTA, TAPC, TPD and NPD

16) A method according to any of preceding claim, wherein the emissive layer comprises one or more organometallic emitter of the structure L-M-X wherein M is a two-coordinate metal selected from copper, silver and gold; L is a neutral electron donor cyclic alkyl amino carbene (CAAC) ligand and X is a monodentate anionic ligand.

17) A method according to any preceding claim, further comprising the step of introducing a top electrode. 18) A method substantially as hereinbefore described with reference to the figures.

19) A device produced by a method according to any preceding claim.