CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry of PCT Patent Application No. PCT/EP2015/059419, filed Apr. 29, 2015 and entitled “Electroluminescent Device,” which claims priority to Great Britain Patent Application No. 1407606.1, filed Apr. 30, 2014. The foregoing applications are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention relates to a solid state light-emitting device, and in particular to a perovskite-based light-emitting diode and related fabrication methods.

BACKGROUND TO THE INVENTION

In its most basic form a light emitting diode (LED) comprises a light emitting layer which is positioned between an anode and a cathode. A hole injection layer may be incorporated between the anode and the light emitting layer (also known as the active or emissive layer). It functions to decrease the energy difference between the work function of the anode and the highest occupied molecular orbital (HOMO) of the light emitting layer, thereby increasing the number of holes introduced into the light emitting layer. Broadly speaking, in operation holes are injected through the anode, and if present the hole injection layer, into the active layer, and electrons are injected into the active layer through the cathode. The holes and electrons combine in the light emitting layer to form an exciton which then undergoes radiative decay to provide light. Equivalently, an electron injection layer between cathode and light-emitting layer can play the same role in controlling the injection of electrons into the light-emitting layer. A further role for these injection layers is to confine carriers within the device, so that under forward electric bias, electrons injected from the cathode into the light-emitting layer are significantly prevented from leaving this layer via the hole-injecting layer, and equivalently, holes injected into the light-emitting layer from the anode are significantly prevented from leaving this layer via the electron-injecting layer.

Some devices also incorporate a thin polymer interlayer between the hole injection layer and the light emitting layer. This plays an important role in improving the device efficiency and the lifetime of LEDs. For example, with an interlayer, blue LEP OLEDs with an external quantum efficiency of greater than 5% can be achieved, which is 35% higher than without the interlayer. It is believed that this may be due to the prevention of exciton quenching at the hole injection layer/light emitting layer interface.

Solid state light-emitting devices based on direct bandgap semiconductors have, over the past two decades, been utilized as energy efficient sources of lighting. However, the fabrication of these devices typically relies on expensive high temperature and high vacuum processes, such as molecular beam epitaxy or thermal sublimation, rendering them uneconomical for use in large area displays. Recent reports on earth-abundant organometal halide based materials with perovskite stoichiometries and crystal structures for high efficiency photovoltaics have demonstrated this class of materials to be excellent semiconductors for optoelectronic devices. Background information can be found in: M. M. Lee et al,Science2012, 338, 643; H.-S. Kim et al,Sci. Rep.2012, 2; J. Burschka et al,Nature2013, 499, 316; M. Liu et al,Nature2013, 501, 395; S. D. Stranks et al,Science2013, 342, 341; G. Xing et al,Science2013, 342, 344; J. Heo et al,Nat Photon2013, 7, 486.

Example advantages of the perovskite class of materials are that they can be very easily solution processed or vacuum deposited under moderate process conditions, require no high temperature heating steps, and they possess an optical bandgap which is tunable in the visible to infrared regions, making them very attractive materials for use in low-cost and large area optoelectronic applications. See for example, C. C. Stoumpos et al,Inorganic Chemistry2013, 52, 9019 and J. H. Noh et al,Nano Letters2013, 13, 1764. Organometal halide based perovskites also possess strong photoluminescent properties, hence making them suitable candidates for use in light-emitting devices. However, electroluminescence could only be achieved at liquid nitrogen temperatures, rendering those devices impractical for applications.

The present applicant has identified a need to develop light-emitting devices in which electroluminescence can be achieved at room temperature.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention provides perovskite-based light-emitting devices (PeLEDs), in which electroluminescence is achieved at room temperature. The applicant has identified a number of device structures or architectures formed using solution-processed perovskites, and which may be used to achieve high brightness near-infrared, green and red electroluminescence. Three example device architectures are described to illustrate the ability to use perovskite at room temperature for light emitting diodes.

Thus, according to a first aspect of the invention, there is provided a solid state light-emitting device comprising: a first electrode coupled to a first charge injecting layer; a second electrode coupled to a second charge injecting layer; an emissive layer comprising a perovskite material, wherein the emissive layer is provided between the first and second charge injecting layers; and wherein the bandgaps of the first and second charge injecting layers are larger than the bandgap of the emissive perovskite layer.

Here, each electrode may be coupled to a charge injecting layer, and the combination of the electrode and charge injecting layer may result in injection of charge into the emissive perovskite layer. The charge injecting layers may actively inject charge, or they may act as a charge transporting layer.

According to a second aspect of the invention, there is provided a solid state light-emitting device comprising: a first electrode coupled to a charge injecting layer; a second electrode; an emissive layer comprising a perovskite material, wherein the emissive layer is provided between the charge injecting layer and the second electrode; and wherein a bandgap of the charge injecting layer is larger than a bandgap of the perovskite layer.

Here, the second electrode may be in direct contact with the emissive perovskite layer, and thus, the second electrode may be considered a second charge injecting layer.

According to a third aspect of the invention, there is provided a solid state light-emitting device comprising: a first electrode coupled to a first charge injecting layer; a second electrode coupled to a second charge injecting layer; an emissive layer comprising a perovskite material, wherein the emissive layer is provided between the first and second charge injecting layers; and wherein the first charge injecting layer is a hole injecting polyelectrolyte, and the second charge injecting layer has a bandgap that is larger than the bandgap of the emissive perovskite layer.

Here, each electrode may be coupled to a charge injecting layer, and the combination of the electrode and charge injecting layer may result in injection of charge into the emissive perovskite layer. Unlike the first aspect of the invention, one of the charge injecting layers is replaced by a hole injecting polyelectrolyte.

The following features apply to all aspects of the invention.

The emissive perovskite layer may be comprised of: a pure perovskite material, pure halide perovskite, pure organometal halide perovskite such as CH3NH3PbI3-xClx, or CH3NH3PbBr3, and/or a mixed perovskite material.

The first electrode may be formed of a transparent conductive material. Thus, the PeLED devices may use a standard device architecture (where electrons are injected from the transparent conductive electrode), or an inverted device architecture (where the transparent conductive electrode is used for hole injection), following the convention adopted in perovskite and dye-sensitised photovoltaics.

In order to confine the injected charges (holes and electrons) within the active (emissive) perovskite layer, the PeLED is formed using a thin perovskite film by forming one or both of the charge injecting layers from a material that has a large bandgap relative to the bandgap of the perovskite layer. Without wishing to be bound by theory, it is believed that the small exciton binding energy in the perovskites causes slow electron-hole capture and associated radiative recombination. An important aspect of the invention is the selection of at least one charge injecting layer having a bandgap that is greater than that of the perovskite layer, and with LUMO and HOMO levels correctly selected, so as to achieve spatial confinement of the electrons and holes in the active perovskite layer, resulting in improved radiative recombination and light emission.

Preferably the perovskite layer is a thin film having a thickness of less than 100 nm; more preferably less than 60 nm, e.g. less than 20 nm. Again without wishing to be bound by theory, it is believed that the confinement of injected charges within the thin “well” enhances electron-hole capture and improves radiative recombination.

At least one of the charge injecting layers may have an optical bandgap of between 1.5 eV and 5 eV.

One or both of the charge injecting layers may be formed of a semiconducting material, and/or of an organic semiconducting material.

One of the charge injecting layers may be a hole injecting organic semiconducting material and may be, for example, selected from the group consisting of PANI (polyaniline), polypyrrole, optionally substituted, doped poly(ethylene dioxythiophene) (PEDOT) and PEDOT:PSS (which is of use as the first charge injecting layer in the third aspect of the present invention).

One of the charge injecting layers may be an electron injecting organic semiconducting material and may be, for example, selected from the group consisting of poly(fluorene)s such as F8, TFB, F8BT and F8-TFB AB copolymer (95:5 F8:TFB).

Alternatively, one of the charge injecting layers may be an electron injecting inorganic semiconducting material and is selected from the group consisting of titanium dioxide (TiO2), zinc oxide (ZnO), magnesium zinc oxide (MgZnO) and aluminium-doped ZnO (AZO).

Alternatively, one of the charge injecting layers may be a hole injecting organic semiconducting material and is selected from the group consisting of polyfluorenes (preferably F8, TFB, PFB or F8-TFB), spiro-OMeTAD, polycarbazoles (preferably poly(9-vinylcarbazole)) and 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl.

One or both of the first and second electrodes may be formed of a transparent conductive material. In embodiments, the first electrode is an anode and the transparent conductive material may be selected from: indium tin oxide (ITO), fluorine doped tin oxide (FTO), indium zinc oxide, graphene, carbon nanotubes, and a metal with a thickness of less than 20 nm.

In one preferred embodiment of the first and second aspects of the present invention, a thin insulating layer may be formed between either or both of the electrodes, or charge injection layers if present and the light-emitting layer. In one embodiment, the insulating layer is preferably formed of an oxide or nitride. More preferably the insulating layer is selected from the group consisting of aluminium oxide, silicon dioxide, silicon nitride, zinc oxide modified with aluminium oxide, nickel oxide or magnesium oxide. The insulating layer may be deposited by any suitable means, e.g. atomic layer deposition, ALD.

In another preferred embodiment, the insulating layer may be formed from an insulating polymer, particularly from the group consisting of poly(ethyleneimine) (PEI), polyethylenimine-ethoxylated (PEI E), polystyrene (PS) or poly(methylmethacrylate) (PMMA).

For example, the transparent conductive electrode material may be coated with a thin layer (preferably <30 nm thick) of an insulating layer selected from the group consisting of aluminium oxide, silicon dioxide, zinc oxide, magnesium oxide, nickel oxide and silicon nitride. The thin scaffold or interlayer may improve the luminescence efficiency of the device, by preventing quenching at the interface between the charge injection layer and the emissive perovskite layer, or by preventing electrons or holes from escaping out of the perovskite layer.

In a further aspect of the invention, there is provided a solid state light-emitting device comprising: a first electrode coupled to a first charge injecting layer; a second electrode coupled to a second charge injecting layer; an emissive layer comprising a perovskite material, wherein the emissive layer is provided between the first and second charge injecting layers; and wherein there is a thin insulating layer formed between either or both of the charge injecting layers and the light-emitting layer.

A further thin layer (e.g. <30 nm thick) of a material selected from molybdenum trioxide and tungsten trioxide may be deposited between: the transparent conductive electrode and the perovskite layer, between a charge injecting layer and a conductive electrode, between the transparent conductive electrode and a charge injecting layer, between the perovskite layer and a charge injecting layer, or between the perovskite layer and a conductive electrode. The further thin layer may increase the charge injection efficiency between layers of the light-emitting device.

As mentioned above, the emissive perovskite layer may comprise an organometal halide perovskite material or a metal halide perovskite material.

The organometal halide perovskite may have an AMX3structure, where A is a monovalent organic cation or a monovalent metal cation, M is a divalent cation and X is a halide anion.

The divalent cation M may be a divalent metal cation, such as, but not limited to, tin (Sn2+) or lead (Pb2+).

The monovalent organic cation may be a primary, secondary or tertiary ammonium cation [HNR1R2R3], wherein each of R1, R2and R3may be the same or different and is selected from hydrogen, an unsubstituted or substituted C1-C20alkyl group and an unsubstituted or substituted C5-C18aryl group. Examples of suitable substituents for the alkyl groups are alkoxy groups having from 1 to 20 carbons atoms, hydroxyl groups, mono and dialkylamino groups wherein each alkyl group may be the same or different and has from 1 to 20 carbon atoms, cyano groups, nitro groups, thiol groups, sulphinyl groups, sulphonyl groups and aryl groups having from 5 to 18 carbon atoms. Examples of suitable substituents for the alkyl groups are alkyl groups having from 1 to 20 carbon atoms, alkenyl and alkynyl groups each having from 2 to 20 carbon atoms, alkoxy groups having from 1 to 20 carbons atoms, haloalkyl groups having from 1 to 20 carbon atoms, hydroxyl groups, mono and dialkylamino groups wherein each alkyl group may be the same or different and has from 1 to 20 carbon atoms, cyano groups, nitro groups, thiol groups, sulphinyl groups and sulphonyl groups.

In embodiments, the monovalent organic cation may be of the form [R1R2N—CH═NR3R4]+:


wherein each of R1, R2, R3and R4may be the same or different and is selected from hydrogen, an unsubstituted or substituted C1-C20alkyl group and an unsubstituted or substituted C5-C18aryl group.

In embodiments, the monovalent organic cation may be of the form (R1R2N)(R3R4N)C═N+R5R6:


wherein each of R1R2, R3, R4, R5and R6may be the same or different and is selected from hydrogen, an unsubstituted or substituted C1-C20alkyl group and an unsubstituted or substituted C5-C18aryl group.

In embodiments, the monovalent metal cation may be an alkali metal cation.

In embodiments, the monovalent metal cation is caesium (Cs+) or rubidium (Rb+).

The halide anion X may be chosen from chloride, bromide, iodide, and fluoride, and a chalcogenide anion may be chosen from sulphide, selenide, arsenide and telluride.

The halide anion X may be selected from chloride, bromide, iodide, and fluoride and, in the above-mentioned AMX3structure, each halide may be the same or different.

The organometal halide perovskite material or metal halide perovskite material may have an A1-iBiMX3structure, wherein: A and B are each a monovalent organic cation or monovalent metal cation as described above, where A and B are different; M is a divalent metal cation as described above; X is a halide anion as described above; and i is between 0 and 1.

The organometal halide perovskite material or metal halide perovskite material may have an AMX3-kYkstructure, wherein: A is a monovalent organic cation or a monovalent metal cation as described above; M is a divalent metal cation as described above; X and Y are each a halide anion as described above, where X and Y are different; and k is between 0 and 3.

The organometal halide perovskite material or metal halide perovskite material may have an AM1-jNjX3structure, wherein: A is a monovalent organic cation or a monovalent metal cation as described above; M and N are each a divalent metal cation as described above; X is a halide anion as described above; and j is between 0 and 1.

The organometal halide perovskite material or metal halide perovskite material may have an A1-iBiM1-jNjX3-kYkstructure, wherein: A and B are each a monovalent organic cation or a monovalent metal cation as described above, where A and B are different; M and N are each a divalent metal cation as described above; X and Y are each a halide anion as described above, where X and Y are different; and where i is between 0 and 1, j is between 0 and 1, and k is between 0 and 3.

According to a related aspect of the invention, there is provided a method of manufacturing a solid state light-emitting device, the method comprising: providing a first electrode on a substrate; depositing a first charge injecting layer over the first electrode; depositing an emissive perovskite layer over the first charge injecting layer; depositing a second charge injecting layer over the emissive perovskite layer; and depositing a second electrode over the second charge injecting layer; and wherein the bandgaps of the first and second charge injecting layers are larger than the bandgap of the emissive perovskite layer.

The substrate may be formed of a transparent material, preferably glass. The first electrode may be an anode and may be formed by the deposition of a transparent conductive material, preferably indium tin oxide (ITO) or fluorine doped tin oxide (FTO) on the transparent substrate.

The active perovskite layer may be composed of a single, homogenous phase perovskite material, which may help to achieve efficient light emission. The emissive layer may have a thickness of less than 100 nm.

In one preferred embodiment of the method, a thin insulating layer may be deposited between either or both of the charge injecting layers and the light-emitting layer. Preferably, the insulating layer is formed of an oxide or nitride. More preferably the insulating layer is selected from the group consisting of aluminium oxide, silicon dioxide, silicon nitride, nickel oxide or magnesium oxide. The insulating layer may be deposited by any suitable means, e.g. atomic layer deposition, ALD. In another preferred embodiment of the method, the insulating layer may be formed from an insulating polymer, in particular from the group consisting of poly(ethyleneimine) (PEI), polyethylenimine-ethoxylated (PEIE), polystyrene (PS) or poly(methylmethacrylate) (PMMA).

In one preferred embodiment of the method, a thin layer of <30 nm of a material selected molybdenum trioxide and tungsten trioxide is deposited between:the transparent conductive electrode and the perovskite layer,between a charge injecting layer and a conductive electrode,between the transparent conductive electrode and a charge injecting layer,between the perovskite layer and a charge injecting layer, orbetween the perovskite layer and a conductive electrode.

The deposition of the various layers forming the device may be performed using one or more of the following deposition techniques: vacuum thermal evaporation, spin coating, direct-write printing, inkjet printing, lithographic patterning, and solution deposition.

In a further aspect of the invention, there is provided an electronic device comprising an LED display, wherein the display comprises light-emitting devices as described above. The electronic device may be a consumer electronic device, a mobile device such as smartphone or tablet PC, or a display screen/panel.

