WO2007098451A1 - Nanostructured electroluminescent device and display - Google Patents

Nanostructured electroluminescent device and display Download PDF

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Publication number
WO2007098451A1
WO2007098451A1 PCT/US2007/062445 US2007062445W WO2007098451A1 WO 2007098451 A1 WO2007098451 A1 WO 2007098451A1 US 2007062445 W US2007062445 W US 2007062445W WO 2007098451 A1 WO2007098451 A1 WO 2007098451A1
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Prior art keywords
electroluminescent device
layer
electron
hole
metal
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PCT/US2007/062445
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French (fr)
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Damoder Reddy
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Solexant Corporation
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Priority to JP2008555536A priority Critical patent/JP2009527876A/en
Priority to CN2007800095438A priority patent/CN101405888B/en
Priority to EP07757229A priority patent/EP1989745A1/en
Priority to AU2007216983A priority patent/AU2007216983A1/en
Priority to CA002642678A priority patent/CA2642678A1/en
Publication of WO2007098451A1 publication Critical patent/WO2007098451A1/en

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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/88—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing selenium, tellurium or unspecified chalcogen elements
    • C09K11/881—Chalcogenides
    • C09K11/883—Chalcogenides with zinc or cadmium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/0257—Doping during depositing
    • H01L21/02573—Conductivity type
    • H01L21/02576—N-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/0257—Doping during depositing
    • H01L21/02573—Conductivity type
    • H01L21/02579—P-type
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B33/00—Electroluminescent light sources
    • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
    • H05B33/14—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of the electroluminescent material, or by the simultaneous addition of the electroluminescent material in or onto the light source
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
    • H10K50/115—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers comprising active inorganic nanostructures, e.g. luminescent quantum dots
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/14—Carrier transporting layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K2102/00—Constructional details relating to the organic devices covered by this subclass
    • H10K2102/301—Details of OLEDs
    • H10K2102/331—Nanoparticles used in non-emissive layers, e.g. in packaging layer
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/17—Carrier injection layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/17—Carrier injection layers
    • H10K50/171—Electron injection layers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles

Definitions

  • the invention relates to electroluminescent devises and emissive displays containing them.
  • Emissive displays fall under three categories depending on the type of emissive device in the display: (1) Organic Light Emitting Displays (OLED), (2) Field Emission Displays (FED) and (3) Inorganic Thin Film Electroluminescent Displays (EL). Of these three categories, OLEDs have received the most attention and investment around the world. Approximately 100 companies are developing various aspects of the OLED technology. Commercial OLED products arc in the mobile phone and MP3 markets. OLED devices can be made from small molecules (pioneered by Kodak) or polymers (pioneered by Cambridge Display Technology). OLED devices can also be made from phosphorescent materials (pioneered by Universal Display Technology ⁇ . More than 90% of the commercial products use Kodak's fluorescent small molecule materials.
  • Polymer materials offer lower cost manufacturing by using solution processing techniques such as spin coating and ink-jet printing.
  • Polymeric materials are expected to offer a cost effective solution for large size (>20") OLED displays.
  • Phosphorescent materials offer higher efficiencies and reduce power consumption.
  • OLED displays suffer from several materials based and manufacturing process dependant problems. For example, OLEDs have short lifetimes, loss of color balance over time, and a high cost of manufacturing. The poor lifetime and color balance issues are due to the chemical properties of emissive device in The OLED. For example, it is difficult to improve the lifetime of blue OLEDs because the higher energy in the blue spectrum tends to destabilize the organic molecules used in the OLED.
  • Polymeric materials offer a possible route to achieve low cost high volume manufacturing by using ink-jet printing. However, polymers have even shorter lifetimes compared to small molecules. Lifetimes must increase by an order of magnitude before polymeric materials can be commercially viable.
  • next generation emissive display technology is expected to be based on newly emerging nanomateriaJs called quantum dots (QDj.
  • QDj quantum dots
  • the emission color in the QDs can be adjusted simply by changing the dimension of the dots.
  • the usefulness of quantum dots in building an emissive display has already been demonstrated in QD-OLED. See Scth Coe et al.. Nature 420, 800 (2002). Emission in these displays is from inorganic materials such as CdSe which are inherently more stable than OLED materials. Stable blue materials can be achieved simply by reducing the size of the quantum dots.
  • the electroluminescent device contains (1) first and second electrodes, at least one of which is transparent to radiation; (2) a hole conducting layer containing first nanoparticles wherein the hole conducting layer is in contact with the first electrode; (3) an electron conducting layer containing second nanoparticles where the electron conducting layer is in contact with the hole conducting layer and the second electrode; and optionally (4) a voltage source capable of providing positive and negative voltage, where the positive pole of the voltage source is connected to the first electrode and the negative pole is connected to the second electrode.
  • the electroluminescent device also includes an electron-hole combination layer between the hole and electron conducting layers.
  • the electron-hole combination layer can be a layer of metal or metal oxide. It can also be a layer of metal or metal oxide in combination with the first and/or second nanoparticles used in the hole and/or electron conducting layers.
  • the electron-hole combination layer can also be a sintered layer where the aforementioned components are treated, typically with heat, to coalesce the particles into a solid mass.
  • An electron-hole combination layer can also be made at the juncture of the hole-conducting and electron-conducting layers by simply sintering these two layers in the absence of metal or metal oxide. In general, the electron-hole combination layer is 5-10 nanometers thick.
  • the electroluminescent device can also include a hole injection layer that is between the first electrode and the hole conducting layer.
  • the hole injection layer can contain a p-type semiconductor, a metal or a metal oxide. Typical metal oxides include aluminum oxide, zinc oxide or titanium dioxide, whereas typical metals include aluminum, gold or silver.
  • the p-type semiconductor can be p-doped Si.
  • the electroluminescent device can also include an electron injection layer that is between the second electrode and the electron conducting layer.
  • This electron injection layer can be a metal, a fluoride salt or an n-type semiconductor. Examples of fluoride salt include NaF, CaF2. or BaF2.
  • the nanoparticles used in the hole conducting and electron-conducting layer are nanocrystals.
  • Exemplary nanoerystals include quantum dots, na ⁇ orods, nanobipods> nanotripods, nanomultipods, or nanowires.
  • Such nanocrystals can be made from CdSe, ZnSe, PbSe, CdTe, InP, PbS, Si or Group H-VI 1 IWV or HI-V materials.
  • a nanostrticture such as a nanotube, nanorod or nanow ⁇ re can be included in the hole conducting, electron-conducting and/or electron-hole combination layer.
  • a preferred nanostructure is a carbon nanotube. When nanostructures are used, it is preferred that the na ⁇ oparticies be covalently attached to the nanostructure.
  • Figure 1 (Prior Art) depicts quantum dots that absorb and emit at different colors because of their size differences. These quantum dots are of nanometer size. Small dots absorb in the blue end of the spectrum while the large size dots absorb in the red end of the spectrum.
  • f0016J Figure 3 depicts nanoparticks capped with a solvent such as tr-n-octyl phosphine oxide (TOPO)
  • TOPO tr-n-octyl phosphine oxide
  • FIG. 4 depicts nanoparticles functional ized with a linker.
  • Figure 5 depicts core-shell nanoparticles functional ized with a linker.
  • FIG. 1 1 depict various embodiments of the nanostructured electroluminescent device.
  • the electroluminescent device contains (1) two electrodes, at least one of which is transparent to radiation. (2) a hole conducting layer containing first nanoparticles, and (3) an electron conducting layer comprising second nanoparticles.
  • the first and second nanoparticles are different either in composition and/or size.
  • the first and second nanoparticles are chosen such that the first particles of the hole conducting layer conduct holes while the second particles of the electron conducting layer conduct electrons.
  • the nanoparticles are chosen so that their relative bandgaps produce a Group II band offset.
  • CdTe and CdSe are nanoparticles that present a Group II band offset.
  • the electroluminescent device optionally includes a voltage source capable of providing a positive and negative voltage.
  • the positive pole of the voltage source is electrically connected to the first electrode and hence to the hole conducting layer while the negative pole is connected to the second electrode and hence connected to the electron conducting layer.
  • an electron-hole combination layer is placed between the hole and electron conducting layers.
  • the electron-hole combination layer can comprise a metal, a metal oxide, or a mixture of a metal or metal oxide with the nanoparticles of the hole conducting layer or the nanoparticles of the electron conducting layer.
  • the metal or mutal oxide is in combination with the nanoparticies of the hole conducting layer as well as the nanoparticles of the electron conducting layer.
  • the type of e ⁇ ectron-hoie conducting layer present in an electroluminescent device will depend upon its method of manufacture.
  • Figure 6 shows an electroluminescent device without a power source.
  • a transparent anode such as indium dn oxide (620) is formed on the glass substrate (610).
  • a first nanoparticle layer is then deposited followed by a second nanoparticle layer.
  • a metal cathode (650) is then formed on the second nanoparticle layer.
  • the entire device can then be annealed/sintered so as to form a continuous layer between the first and second nanoparticle layers and an electron-hole combination layer.
  • the electron-hole combination layer is formed between the two layers and is made from nanoparticles from the hole and electron conducting layers. It is in this region that electrons and holes combine with each other to emit light when a positive and negative voltage is placed across the device.
  • the radiation emitted may be dependent upon the difference between the conduction band energy of the electron conducting nanoparticles and the valence band energy of the hole conducting nanoparticles. ft is to be understood that the emitted radiation need not correlate exactly to the differences in such energy levels. Rather, light having energy less than this band gap may be expected.
  • an electron-hoie combination layer is formed. If the metal or metal oxide is placed on the first nanopartiele layer and then sintered pri ⁇ r t ⁇ the addition of the second nanopartiele layer, the electron-hole combination layer comprises not only the metal or metai oxide but also nanoparticles derived from the first layer. Alternatively, the second nanopartiele layer can be deposited upon the metal or metai oxide layer and the device then sintered. In this case, the electron-hole combination layer comprises metal or metal oxide in combination with nanoparticles from the first and second layers. If the device is made by first depositing the hole conducting layer, followed by a layer metal or metal oxide and sintered, the electron-hole combination layer comprises metal or metal oxide in combination with nanoparticles from, the hole conducting layer.
  • the electroluminescent device may further contain an electron injection layer and/or a hole-injection layer.
  • the electron injection layer is positioned between the second nanopartiele layer and the cathode.
  • Electron injection layers can include n-type semiconductors, fluoride salts or metals.
  • the n-type semiconductor can be, for example, n-doped silicon while the fluoride sait can be sodium chloride, calcium chloride or barium fluoride.
  • fluorides arc used the layer can be 0.5 to 2 nanometers thick.
  • metals are used, the layer can be 5 to 20 nanometers thick.
  • the hole injection layer (730) can be a p-type semiconductor, a metal or a metai oxide.
  • the metal oxide can be, for example, aluminum oxide, zinc oxide, or titanium dioxide whereas the metal can be aluminum, gold or silver.
  • An example of a p-type semiconductor that can be used as a hole injection layer is p-doped silicon,
  • a hole blocking layer (860) is added to the embodiment previously set forth in Figure 7. Examples of the hole blocking layers include TiO 2 , ZnO and other metal oxides with a bandgap greater than 3eV.
  • an electron blocking layer can be disposed between the anode and the first nanopartiele layer or between the hole injection layer and the first nanopartiele layer.
  • electron blocking layers include those made from TiO 2 .
  • an electron injection layer can also act as a hole blocking layer.
  • two different materials can be used where one acts as an electron injection layer and the other a hole blocking layer.
  • an electron injection layer can be LiF, BaF or CaF while the hole blocking layer can be TiO?.
  • the hole injection layer can also act as an electron barrier.
  • the hole injection layer can be made from Au while the electron barrier layer can be made
  • Luminescent nanoparticle refers to luminescent materials that generate light upon the combination of holes and electrons.
  • Luminescent nanoparticles are generally nariocrysials such as quantum dots, nanorods, nanobipods, nanotripods, nanomultipods or nanowires.
  • Luminescent nanoparticles can be made from compound semiconductors which include Group H-VI, IJ-IV and HI-V materials. Some examples of luminescent nanoparticles are CdSe, ZnSe, PbSe, InP, PbS, ZnS, CdTe Si, Ge, SiGe, CdTe, CdHgTe, and Group H-VI, H-IV and III-V materials. Luminescent nanoparticles can be core type or core-shell type. In a core-shell nanoparticle, the core and shell are made from different materials. Both core and shell can be made from compound semiconductors.
  • the nanoparticles of the hole conducting layer have a bandgap such that holes are easily transferred from the anode to these nanoparticles.
  • the nanoparticles of the electron conduction layer have a bandgap such that electrons can easily transfer from cathode to these nanoparticles. Bandgaps of the materials used for the hole and electron conducting layers will be complimentary to each other to allow efficient recombination of holes and electrons in the electron-hole combination layer.
  • Quantum dots are a preferred type of nanoparticle.
  • quantum dots having the same composition but having different diameters absorb and emit radiation at different wave lengths.
  • Figure I depicts three quantum dots made of the same composition but having different diameters.
  • the small quantum dot absorbs and emits in the blue portion of the spectrum; whereas, the medium and large quantum dots absorb and emit in the green and red portions of the visible spectrum, respectively.
  • the quantum dots can be essentially the same size bui made from different materials.
  • a UV-absorbing quantum dot can be made from zinc selenide; whereas, visible and IR quantum dots can be made from cadmium selenide and lead selenide, respectively.
  • Nanoparticles having different size and/or composition are used in each of the na ⁇ oparticle layers.
  • the luminescent nanoparticle can be modified by reaction with a linker
  • X and Y can be reactive moieties such as carboxyl ⁇ e acid groups, phosphonic acid groups, sulfonic acid groups, amine containing groups etc.
  • a and b are independently 0 or 1 where at least one of a and b is L R is a carbon, nitrogen or oxygen containing group such as -CH 2 , -NH- or -O-, and n is 0-10 or 0-5.
  • One reactive moiety e.g., X
  • Y can react with another structure such as (1) the electrode, (2) the electr ⁇ n-hole combination Jayer.
  • the luminescent nanoparticles are used to decorate nanostructures which are then used in the electron and/or hole conducting layers.
  • the linkers, with or without a second reactive moiety, can also passivate the nanoparticles and increase their stability and electroluminescence. They can also improve the nanoparticle solubility or suspension in common organic solvents used to make the charge conducting layers.
  • the distance between the surface of a nanoparticle and any of the aforementioned structure can be adjusted to minimize the effect of surface states that can facilitate electron-hole combination outside of the electron-hole combination layer.
  • the distance between these surfaces is typically 10 Angstroms or less preferably 5 Angstroms or less. This distance is maintained so that electrons or holes can tunnel through this gap from the electrodes to the electron-hole combination layer,
  • the term "'nanostructure,” “'electron conducting nano- structure” or “hole conducting nanostructure” refers to nanotubes, nanorods, nanowires, etc. Electron and hole conducting ⁇ anoslructures are crystalline in nature. In general, the nanostructures are made from wide band gap semiconductor materials where the band gap is, for example, 3.2eV for T ⁇ O2- The nanostructures are chosen so that their band gap is higher than the highest band gap of the photoactive nanoparticle to be used in the solar cell (e.g. , >2.0eV). [0035] Electron conducting nanostructures can be made, for example, from titanium dioxide, zinc oxide, tin oxide, indium tin oxide (ITO) and indium zinc oxide. The nanostructures may also be made From other conducting materials, such as carbon nanotubes, especially single-wall carbon nanotubes.
  • Electron conducting nanostructures can be prepared by methods known in the art. Conducting nanostructures can also be prepared by using colloidal growth facilitated by a seed particle deposited on the substrate. Conducting nanostructures can also be prepared via vacuum deposition process such as chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), Epitaxial growth methods such -ts molecular beam epitaxy (MEB), etc.
  • CVD chemical vapor deposition
  • MOCVD metal-organic chemical vapor deposition
  • MEB molecular beam epitaxy
  • the outside diameter of the nanotube ranges from about 20 nanometers to 100 nanometers, in some cases from 20 nanometers to 50 nanometers, and in others from 50 nanometers to 100 nanometers.
  • the inside diameter of the nanotube can be from about 10 to 80 nanometers, in some cases from 20 to 80 nanometers, and in others from 60 to 80 nanometers.
  • the wall thickness of the nanotube can be 10-25 nanometers, 15-25 nanometers, or 20-25 nanometers.
  • the length of the nanotube in some cases is 100-800 nanometers, 400-800 nanometers, or 200-400 nanometers.
  • the diameters can be from about 100 nanometers to about 200 nanometers and can be as long as 50-100 microns.
  • Nanorods can have diameters from about 2-200 nanometers but often are from 5-100 or 20-50 nanometers in diameter. Their length can be 20-100 nanometers, but often are between 50-500 or 20-50 nanometers in length.
  • the electroluminescent device (without a voltage source) does not include an organic hole conducting polymer or an organic electron conducting polymer. Except when organic linkers are used, the device is essentially entirely inorganic.
  • the electroluminescent devices can be used in emissive displays.
  • Emission displays include fiat panel displays (alone or in combination with other components associated with a finished product) as well as other electronic devices.
  • Example 1 A nanostructured electroluminescent device is shown in
  • a transparent conducting layer ITO 620 is deposited on glass substrate (610) by following methods well known in the art.
  • the surface of the ITO can be exposed to plasma treatment or other processes well known in the art to adjust the work function of ITO.
  • a first charge conducting na ⁇ oparticle layer (630) is then deposited on the ITO layer.
  • Spin coating or ink-jet printing or other printing process can be used to deposit nanoparticles dispersed in a suitable solvent.
  • a continuous pin hole free nanoparticie layer can be obtained by heating the substrate to about 200 C for about 15 minutes to drive off the solvent.
  • the nanoparticles in layer 630 can be dots, , rods or wires.
  • the first nanoparticie layer in this embodiment is made from CdSe.
  • Second nanoparticie layer (640) is deposited direciiy on top of the first nanoparticie layer (630).
  • Spin coating or ink-jet printing or other printing process can be used to deposit nanoparticles dispersed in a suitable solvent.
  • a continuous pin hole free nanoparticie layer can be obtained by heating the substrate to about 200 C for about 15 minutes to drive off the solvent.
  • the nanoparticles in layer 640 can be dots, rods or wires.
  • the second nanoparticie layer (640) in this embodiment is made form CdTe.
  • the particle size of the CdSe in the first nanoparticie layer (630) and CdTe in the second nanoparticie layer (640) can be adjusted to obtain the desired emission colors. To produce blue emission 3 micron dots can be used.
  • red emission 6 micron dots can be used.
  • Other colors can be produced by adjusting the nanoparticie size by using methods well known in the art. Interface between the two nanoparticle layers can be improved by heating the substrate in a saturated CdCl 2 solution in methanol or by methods well known in the art. Such a treatment creates a suitable interface between the first nanoparticie layer and the second nanoparticie layer such that efficient electron-hole combination occurs at the interface.
  • a aluminum metal electrode (650) is then deposited on top of the second nanoparticie layer lo complete the nanostructured electroluminescent device.
  • FIG. 7 Another embodiment of a nanostructured electroluminescent device is shown in Figure 7.
  • a transparent conducting layer ITO (720) is deposited on glass substrate (710).
  • a hole injection layer (730) such as aluminum oxide is deposited on ITO layer 720 by the methods known in the art.
  • the first and second nanoparticie layers (740 and 750), are then deposited as described in Example 1.
  • An electron injecting layer (760) such as LiF is then deposited on tap of the second nanoparticie layer by methods well known in the art.
  • An Aluminum metal electrode (670) is deposited O ⁇ top of the second nanoparticle layer to complete the nanosUuctured electroluminescent device.
  • Example 3 Another embodiment of a nanostruclured electroluminescent display shown in Figure 8.
  • the ITO hole injection and first and second nanoparticle layers are formed as described in Example 2.
  • a hole blocking layer made of TiO 2 (S60) is deposited on top of the second nanoparticle layer by the methods well known in the art,
  • An electron injecting layer (870) such as LiF is then deposited by methods well known in the art and an Aluminum metal electrode (80) is deposited on top of the second nanoparticle layer to complete the nanostructured electroluminescent device.
  • Example 4 Another embodiment of a nanostructured electroluminescent display shown in Figure 9.
  • the ITO layer (920) is deposited on glass substrate (910), as described in Example 1, the first nanoparticle layer (930) is then deposited on lhe ITO layer as described in Example 1.
  • the nanoparticles (CdSe dots, rods, bipods, tripods, multipods, wires) in this example are associated with a nanostructure such as the first rianoparticle layer (930) in this embodiment, made by decorating a functionalized single wall carbon naao tube (SWCNT).
  • the second nanoparticle layer (940) is deposited directly on top of the first nanoparticle layer (930). As described in Example I , the nanoparticles in the second layer (940).
  • Example 5 Another embodiment of a nanostructured electroluminescent display shown in Figure 10.
  • the ITO, first and second nanoparticle and metal cathode layers are formed as described in Example 4.
  • an electron injecting layer (1060) such as LiF is deposited on top of the second nanoparticle layer before an aluminum metal electrode (1070) is deposited on top of the second naiioparticl ⁇ layer.
  • Example 6 Another embodiment of a nanostructured electroluminescent device shown in Fiaure 1 1. This device is made as described in
  • the thickness of the ITO layer used in the above embodiments is 1 OOnm and the thickness of the aluminum layer is 150nm.
  • the hole injection layer is about 5 Angstroms thick and the thickness of the electron injection layer is about 10 Angstroms.
  • the nanoparticle layers have a thickness in the 10-lOOnin range.
  • the above embodiments are some examples of the applying the present invention. It will be obvious to any one skilled in the art that other materials and material combinations well known in the art can be used in place of the material examples used in the above embodiments to build a nanostructure electroluminescent display according to the present invention.
  • other transparent conducting materials can be used as anode instead of ITO.
  • Other metal oxides can be used as hole injection materials instead of aluminum oxide.
  • Other metal haiides can be used as electron injecting materials instead of LiF Io build a nanostructure electroluminescent display according to the present invention.
  • Other metals such as Ag, Ca can be used instead of Aluminum as cathode Io build a nanostr ⁇ cttire electroluminescent display according to the present invention.
  • CdSe and CdTe nanoparticles are used as examples for the first and second nanoparticle layers.
  • Other luminescent nanoparticles with suitable bandgaps can be used instead of CdSe and CdTe to build a nanostructure electroluminescent display according to the present invention.

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Abstract

An electroluminescent device contains first (610) and second (650) electrodes, at least one of which is transparent to radiation; a hole conducting layer (630) containing first nanoparticles wherein the hole conducting layer (630) is in contact with said first electrode (610); an electron conducting layer (640) containing second nanoparticles where the electron conducting layer (640) is in contact with the hole conducting layer (630) and the second electrode (650); and optionally a voltage source capable of providing positive and negative voltage, where the positive pole of the voltage source is connected to the first electrode and the negative pole is connected to the second electrode. In some embodiments, the electroluminescent device also includes an electron-hole combination layer between the hole and electron conducting layers.

Description

NANOSTRUCTURED ELECTROLUMINESCENT DEVICE AND
DISPLAY
FIELD OF THE INVENTION f 0001 J The invention relates to electroluminescent devises and emissive displays containing them.
CLAIM OF PRIORITY
[0002] This application claims the benefit of U.S. Provisional Application Serial No. 60/774,794, filed February 17, 2006, under 35 U.S. C. § 119(e) and is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0003] Emissive displays fall under three categories depending on the type of emissive device in the display: (1) Organic Light Emitting Displays (OLED), (2) Field Emission Displays (FED) and (3) Inorganic Thin Film Electroluminescent Displays (EL). Of these three categories, OLEDs have received the most attention and investment around the world. Approximately 100 companies are developing various aspects of the OLED technology. Commercial OLED products arc in the mobile phone and MP3 markets. OLED devices can be made from small molecules (pioneered by Kodak) or polymers (pioneered by Cambridge Display Technology). OLED devices can also be made from phosphorescent materials (pioneered by Universal Display Technology}. More than 90% of the commercial products use Kodak's fluorescent small molecule materials. Polymer materials, on the other hand, offer lower cost manufacturing by using solution processing techniques such as spin coating and ink-jet printing. Polymeric materials are expected to offer a cost effective solution for large size (>20") OLED displays. Phosphorescent materials offer higher efficiencies and reduce power consumption. [0004] OLED displays suffer from several materials based and manufacturing process dependant problems. For example, OLEDs have short lifetimes, loss of color balance over time, and a high cost of manufacturing. The poor lifetime and color balance issues are due to the chemical properties of emissive device in The OLED. For example, it is difficult to improve the lifetime of blue OLEDs because the higher energy in the blue spectrum tends to destabilize the organic molecules used in the OLED. The cost of manufacturing small molecule full color displa>s is also very high diie ihe need to use expensive shadow masks to deposit red, green and blue materials. Kodak and others have developed white OLEDs by using color filter technology to overcome this problem. However, the use of color filters adds cost to the bill of materials and reduces the quality of display. Some of the main advantages of the OLED display are being taken away by this approach.
{0005] Polymeric materials offer a possible route to achieve low cost high volume manufacturing by using ink-jet printing. However, polymers have even shorter lifetimes compared to small molecules. Lifetimes must increase by an order of magnitude before polymeric materials can be commercially viable.
{0006] The next generation emissive display technology is expected to be based on newly emerging nanomateriaJs called quantum dots (QDj. The emission color in the QDs can be adjusted simply by changing the dimension of the dots. The usefulness of quantum dots in building an emissive display has already been demonstrated in QD-OLED. See Scth Coe et al.. Nature 420, 800 (2002). Emission in these displays is from inorganic materials such as CdSe which are inherently more stable than OLED materials. Stable blue materials can be achieved simply by reducing the size of the quantum dots.
[0007J Display devices made with QDs have quantum efficiencies which are an order of magnitude lower than OLED. QDs have been combined with OLED materials to improve efficiency. See US2004/0023010. However, this approach produces only modest improvement in efficiency while decreasing the display lifetime and complicating the manufacturing process. SUMMARY OF THE INVENTION
[0008] The electroluminescent device contains (1) first and second electrodes, at least one of which is transparent to radiation; (2) a hole conducting layer containing first nanoparticles wherein the hole conducting layer is in contact with the first electrode; (3) an electron conducting layer containing second nanoparticles where the electron conducting layer is in contact with the hole conducting layer and the second electrode; and optionally (4) a voltage source capable of providing positive and negative voltage, where the positive pole of the voltage source is connected to the first electrode and the negative pole is connected to the second electrode.
[0009] Ia some embodiments, the electroluminescent device also includes an electron-hole combination layer between the hole and electron conducting layers. The electron-hole combination layer can be a layer of metal or metal oxide. It can also be a layer of metal or metal oxide in combination with the first and/or second nanoparticles used in the hole and/or electron conducting layers. The electron-hole combination layer can also be a sintered layer where the aforementioned components are treated, typically with heat, to coalesce the particles into a solid mass. An electron-hole combination layer can also be made at the juncture of the hole-conducting and electron-conducting layers by simply sintering these two layers in the absence of metal or metal oxide. In general, the electron-hole combination layer is 5-10 nanometers thick.
(0010] The electroluminescent device can also include a hole injection layer that is between the first electrode and the hole conducting layer. The hole injection layer can contain a p-type semiconductor, a metal or a metal oxide. Typical metal oxides include aluminum oxide, zinc oxide or titanium dioxide, whereas typical metals include aluminum, gold or silver. The p-type semiconductor can be p-doped Si.
jfOOH] The electroluminescent device can also include an electron injection layer that is between the second electrode and the electron conducting layer. This electron injection layer can be a metal, a fluoride salt or an n-type semiconductor. Examples of fluoride salt include NaF, CaF2. or BaF2.
fO012] The nanoparticles used in the hole conducting and electron-conducting layer are nanocrystals. Exemplary nanoerystals include quantum dots, naπorods, nanobipods> nanotripods, nanomultipods, or nanowires. Such nanocrystals can be made from CdSe, ZnSe, PbSe, CdTe, InP, PbS, Si or Group H-VI1 IWV or HI-V materials.
[0013] In some electroluminescent devices, a nanostrticture such as a nanotube, nanorod or nanowϊre can be included in the hole conducting, electron-conducting and/or electron-hole combination layer. A preferred nanostructure is a carbon nanotube. When nanostructures are used, it is preferred that the naπoparticies be covalently attached to the nanostructure.
DESCRIPTION OF THE DRAWINGS:
[0014] Figure 1 (Prior Art) depicts quantum dots that absorb and emit at different colors because of their size differences. These quantum dots are of nanometer size. Small dots absorb in the blue end of the spectrum while the large size dots absorb in the red end of the spectrum.
[Θ015] Figure 2 {Prior Art) depicts quantum dots of the same size made from
ZnSe, CdSe and PbSe that absorb and emit in UV, visible and IR respectively
f0016J Figure 3 (Prior Art) depicts nanoparticks capped with a solvent such as tr-n-octyl phosphine oxide (TOPO)
[0017J Figure 4 depicts nanoparticles functional ized with a linker.
[0018] Figure 5 depicts core-shell nanoparticles functional ized with a linker.
[0019J Figure 6- 1 1 depict various embodiments of the nanostructured electroluminescent device.
DETAILED DESCRIPTION
[OΘ20] The electroluminescent device contains (1) two electrodes, at least one of which is transparent to radiation. (2) a hole conducting layer containing first nanoparticles, and (3) an electron conducting layer comprising second nanoparticles. The first and second nanoparticles are different either in composition and/or size. Fn addition, the first and second nanoparticles are chosen such that the first particles of the hole conducting layer conduct holes while the second particles of the electron conducting layer conduct electrons. The nanoparticles are chosen so that their relative bandgaps produce a Group II band offset. CdTe and CdSe are nanoparticles that present a Group II band offset. However, different nanoparticles can be chosen having different composition and/or size so long as the conduction and valence bands form a Type II band offset. The electroluminescent device optionally includes a voltage source capable of providing a positive and negative voltage. When present, the positive pole of the voltage source is electrically connected to the first electrode and hence to the hole conducting layer while the negative pole is connected to the second electrode and hence connected to the electron conducting layer.
[0021] In some embodiments, an electron-hole combination layer is placed between the hole and electron conducting layers. The electron-hole combination layer can comprise a metal, a metal oxide, or a mixture of a metal or metal oxide with the nanoparticles of the hole conducting layer or the nanoparticles of the electron conducting layer. In some cases, the metal or mutal oxide is in combination with the nanoparticies of the hole conducting layer as well as the nanoparticles of the electron conducting layer. The type of eϊectron-hoie conducting layer present in an electroluminescent device will depend upon its method of manufacture. Figure 6 shows an electroluminescent device without a power source. In Figure 6, a transparent anode such as indium dn oxide (620) is formed on the glass substrate (610). A first nanoparticle layer is then deposited followed by a second nanoparticle layer. A metal cathode (650) is then formed on the second nanoparticle layer. The entire device can then be annealed/sintered so as to form a continuous layer between the first and second nanoparticle layers and an electron-hole combination layer. The electron-hole combination layer is formed between the two layers and is made from nanoparticles from the hole and electron conducting layers. It is in this region that electrons and holes combine with each other to emit light when a positive and negative voltage is placed across the device. The radiation emitted may be dependent upon the difference between the conduction band energy of the electron conducting nanoparticles and the valence band energy of the hole conducting nanoparticles. ft is to be understood that the emitted radiation need not correlate exactly to the differences in such energy levels. Rather, light having energy less than this band gap may be expected.
fOO22J If a layer of metal or metal oxide is positioned between the first and second nanoparticle layers, an electron-hoie combination layer is formed. If the metal or metal oxide is placed on the first nanopartiele layer and then sintered priυr tυ the addition of the second nanopartiele layer, the electron-hole combination layer comprises not only the metal or metai oxide but also nanoparticles derived from the first layer. Alternatively, the second nanopartiele layer can be deposited upon the metal or metai oxide layer and the device then sintered. In this case, the electron-hole combination layer comprises metal or metal oxide in combination with nanoparticles from the first and second layers. If the device is made by first depositing the hole conducting layer, followed by a layer metal or metal oxide and sintered, the electron-hole combination layer comprises metal or metal oxide in combination with nanoparticles from, the hole conducting layer.
[0023] The electroluminescent device may further contain an electron injection layer and/or a hole-injection layer. Referring io Figure 7, the electron injection layer is positioned between the second nanopartiele layer and the cathode. Electron injection layers can include n-type semiconductors, fluoride salts or metals. The n-type semiconductor can be, for example, n-doped silicon while the fluoride sait can be sodium chloride, calcium chloride or barium fluoride. When fluorides arc used the layer can be 0.5 to 2 nanometers thick. When metals are used, the layer can be 5 to 20 nanometers thick.
[0024] The hole injection layer (730) can be a p-type semiconductor, a metal or a metai oxide. The metal oxide can be, for example, aluminum oxide, zinc oxide, or titanium dioxide whereas the metal can be aluminum, gold or silver. An example of a p-type semiconductor that can be used as a hole injection layer is p-doped silicon, In Figure 8, a hole blocking layer (860) is added to the embodiment previously set forth in Figure 7. Examples of the hole blocking layers include TiO2, ZnO and other metal oxides with a bandgap greater than 3eV.
{0025} In addition, an electron blocking layer can be disposed between the anode and the first nanopartiele layer or between the hole injection layer and the first nanopartiele layer. Examples of electron blocking layers include those made from TiO2.
[0026] It is to be understood that an electron injection layer can also act as a hole blocking layer. However, in some embodiments two different materials can be used where one acts as an electron injection layer and the other a hole blocking layer. For example, an electron injection layer can be LiF, BaF or CaF while the hole blocking layer can be TiO?.
[0027J Similarly, at the anode, the hole injection layer can also act as an electron barrier. However, when different materials are used for these functions, the hole injection layer can be made from Au while the electron barrier layer can be made
Figure imgf000008_0001
f0028] As used herein, the term ''nanoparticle" or "luminescent nanoparticle" refers to luminescent materials that generate light upon the combination of holes and electrons. Luminescent nanoparticles are generally nariocrysials such as quantum dots, nanorods, nanobipods, nanotripods, nanomultipods or nanowires.
[0029] Luminescent nanoparticles can be made from compound semiconductors which include Group H-VI, IJ-IV and HI-V materials. Some examples of luminescent nanoparticles are CdSe, ZnSe, PbSe, InP, PbS, ZnS, CdTe Si, Ge, SiGe, CdTe, CdHgTe, and Group H-VI, H-IV and III-V materials. Luminescent nanoparticles can be core type or core-shell type. In a core-shell nanoparticle, the core and shell are made from different materials. Both core and shell can be made from compound semiconductors.
[0030] The nanoparticles of the hole conducting layer have a bandgap such that holes are easily transferred from the anode to these nanoparticles. The nanoparticles of the electron conduction layer have a bandgap such that electrons can easily transfer from cathode to these nanoparticles. Bandgaps of the materials used for the hole and electron conducting layers will be complimentary to each other to allow efficient recombination of holes and electrons in the electron-hole combination layer.
[0031] Quantum dots are a preferred type of nanoparticle. As in known in the art, quantum dots having the same composition but having different diameters absorb and emit radiation at different wave lengths. Figure I depicts three quantum dots made of the same composition but having different diameters. The small quantum dot absorbs and emits in the blue portion of the spectrum; whereas, the medium and large quantum dots absorb and emit in the green and red portions of the visible spectrum, respectively. Alternatively, as shown in Figure 2. the quantum dots can be essentially the same size bui made from different materials. For example, a UV-absorbing quantum dot can be made from zinc selenide; whereas, visible and IR quantum dots can be made from cadmium selenide and lead selenide, respectively. Nanoparticles having different size and/or composition are used in each of the naπoparticle layers.
{0032] The luminescent nanoparticle can be modified by reaction with a linker
X4-Rn-Yb where X and Y can be reactive moieties such as carboxylϊe acid groups, phosphonic acid groups, sulfonic acid groups, amine containing groups etc., a and b are independently 0 or 1 where at least one of a and b is L R is a carbon, nitrogen or oxygen containing group such as -CH2, -NH- or -O-, and n is 0-10 or 0-5. One reactive moiety (e.g., X) can react with the nanoparticle while the other (Y) can react with another structure such as (1) the electrode, (2) the electrυn-hole combination Jayer. (3) the hole or electron injection layer, (4) the hole or electron blocking layer, or (5) other nanoparticles. In some embodiments, the luminescent nanoparticles are used to decorate nanostructures which are then used in the electron and/or hole conducting layers. The linkers, with or without a second reactive moiety, can also passivate the nanoparticles and increase their stability and electroluminescence. They can also improve the nanoparticle solubility or suspension in common organic solvents used to make the charge conducting layers.
JG033] By adjusting the components ol X4-Rn-Yb, the distance between the surface of a nanoparticle and any of the aforementioned structure can be adjusted to minimize the effect of surface states that can facilitate electron-hole combination outside of the electron-hole combination layer. The distance between these surfaces is typically 10 Angstroms or less preferably 5 Angstroms or less. This distance is maintained so that electrons or holes can tunnel through this gap from the electrodes to the electron-hole combination layer,
[0034] As used herein, the term "'nanostructure," "'electron conducting nano- structure" or "hole conducting nanostructure" refers to nanotubes, nanorods, nanowires, etc. Electron and hole conducting πanoslructures are crystalline in nature. In general, the nanostructures are made from wide band gap semiconductor materials where the band gap is, for example, 3.2eV for TΪO2- The nanostructures are chosen so that their band gap is higher than the highest band gap of the photoactive nanoparticle to be used in the solar cell (e.g. , >2.0eV). [0035] Electron conducting nanostructures can be made, for example, from titanium dioxide, zinc oxide, tin oxide, indium tin oxide (ITO) and indium zinc oxide. The nanostructures may also be made From other conducting materials, such as carbon nanotubes, especially single-wall carbon nanotubes.
[0036] Electron conducting nanostructures can be prepared by methods known in the art. Conducting nanostructures can also be prepared by using colloidal growth facilitated by a seed particle deposited on the substrate. Conducting nanostructures can also be prepared via vacuum deposition process such as chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), Epitaxial growth methods such -ts molecular beam epitaxy (MEB), etc.
{0037] ϊn the case of nanotubes, the outside diameter of the nanotube ranges from about 20 nanometers to 100 nanometers, in some cases from 20 nanometers to 50 nanometers, and in others from 50 nanometers to 100 nanometers. The inside diameter of the nanotube can be from about 10 to 80 nanometers, in some cases from 20 to 80 nanometers, and in others from 60 to 80 nanometers. The wall thickness of the nanotube can be 10-25 nanometers, 15-25 nanometers, or 20-25 nanometers. The length of the nanotube in some cases is 100-800 nanometers, 400-800 nanometers, or 200-400 nanometers.
[0038] In the case of nanowires, the diameters can be from about 100 nanometers to about 200 nanometers and can be as long as 50-100 microns. Nanorods can have diameters from about 2-200 nanometers but often are from 5-100 or 20-50 nanometers in diameter. Their length can be 20-100 nanometers, but often are between 50-500 or 20-50 nanometers in length.
(003.9] As described above, the electroluminescent device (without a voltage source) does not include an organic hole conducting polymer or an organic electron conducting polymer. Except when organic linkers are used, the device is essentially entirely inorganic.
[0040J The electroluminescent devices can be used in emissive displays.
Emission displays include fiat panel displays (alone or in combination with other components associated with a finished product) as well as other electronic devices. EXAMPLES
[0041] Example 1 : A nanostructured electroluminescent device is shown in
Figure 6, a transparent conducting layer ITO 620 is deposited on glass substrate (610) by following methods well known in the art. The surface of the ITO can be exposed to plasma treatment or other processes well known in the art to adjust the work function of ITO. A first charge conducting naπoparticle layer (630) is then deposited on the ITO layer. Spin coating or ink-jet printing or other printing process can be used to deposit nanoparticles dispersed in a suitable solvent. A continuous pin hole free nanoparticie layer can be obtained by heating the substrate to about 200 C for about 15 minutes to drive off the solvent. The nanoparticles in layer 630 can be dots,, rods or wires. The first nanoparticie layer in this embodiment is made from CdSe. Second nanoparticie layer (640) is deposited direciiy on top of the first nanoparticie layer (630). Spin coating or ink-jet printing or other printing process can be used to deposit nanoparticles dispersed in a suitable solvent. A continuous pin hole free nanoparticie layer can be obtained by heating the substrate to about 200 C for about 15 minutes to drive off the solvent. The nanoparticles in layer 640 can be dots, rods or wires. The second nanoparticie layer (640) in this embodiment is made form CdTe. The particle size of the CdSe in the first nanoparticie layer (630) and CdTe in the second nanoparticie layer (640) can be adjusted to obtain the desired emission colors. To produce blue emission 3 micron dots can be used. To produce red emission 6 micron dots can be used. Other colors can be produced by adjusting the nanoparticie size by using methods well known in the art. Interface between the two nanoparticle layers can be improved by heating the substrate in a saturated CdCl2 solution in methanol or by methods well known in the art. Such a treatment creates a suitable interface between the first nanoparticie layer and the second nanoparticie layer such that efficient electron-hole combination occurs at the interface. A aluminum metal electrode (650) is then deposited on top of the second nanoparticie layer lo complete the nanostructured electroluminescent device.
(0042] Examg!e_2 : Another embodiment of a nanostructured electroluminescent device is shown in Figure 7. A transparent conducting layer ITO (720) is deposited on glass substrate (710). As described in Example I, a hole injection layer (730) such as aluminum oxide is deposited on ITO layer 720 by the methods known in the art. The first and second nanoparticie layers (740 and 750), are then deposited as described in Example 1. An electron injecting layer (760) such as LiF is then deposited on tap of the second nanoparticie layer by methods well known in the art. An Aluminum metal electrode (670) is deposited OΏ top of the second nanoparticle layer to complete the nanosUuctured electroluminescent device.
[0043] Example 3: Another embodiment of a nanostruclured electroluminescent display shown in Figure 8. The ITO hole injection and first and second nanoparticle layers are formed as described in Example 2. A hole blocking layer made of TiO2 (S60) is deposited on top of the second nanoparticle layer by the methods well known in the art, An electron injecting layer (870) such as LiF is then deposited by methods well known in the art and an Aluminum metal electrode (80) is deposited on top of the second nanoparticle layer to complete the nanostructured electroluminescent device.
|0044] Example 4: Another embodiment of a nanostructured electroluminescent display shown in Figure 9. The ITO layer (920) is deposited on glass substrate (910), as described in Example 1, the first nanoparticle layer (930) is then deposited on lhe ITO layer as described in Example 1. The nanoparticles (CdSe dots, rods, bipods, tripods, multipods, wires) in this example are associated with a nanostructure such as the first rianoparticle layer (930) in this embodiment, made by decorating a functionalized single wall carbon naao tube (SWCNT). The second nanoparticle layer (940) is deposited directly on top of the first nanoparticle layer (930). As described in Example I , the nanoparticles in the second layer (940). functionalized CdTe dots, rods, bipods, tripods, multipods or wires arc associated with functionalized single wall carbon nanotubes (SWCNTs). An aluminum metal electrode (950) is then deposited on top of the second nanoparticle layer to complete the nanostructured electroluminescent device.
[0045] Example 5: Another embodiment of a nanostructured electroluminescent display shown in Figure 10. The ITO, first and second nanoparticle and metal cathode layers are formed as described in Example 4. However, in this embodiment, an electron injecting layer (1060) such as LiF is deposited on top of the second nanoparticle layer before an aluminum metal electrode (1070) is deposited on top of the second naiioparticlε layer.
f 0046] Example 6: Another embodiment of a nanostructured electroluminescent device shown in Fiaure 1 1. This device is made as described in
U Example 5, except a hole blocking layer (1 160) is deposited on top of the second nanoparticle layer.
j[ΘO47] The thickness of the ITO layer used in the above embodiments is 1 OOnm and the thickness of the aluminum layer is 150nm. The hole injection layer is about 5 Angstroms thick and the thickness of the electron injection layer is about 10 Angstroms. The nanoparticle layers have a thickness in the 10-lOOnin range.
[0048] The above embodiments are some examples of the applying the present invention. It will be obvious to any one skilled in the art that other materials and material combinations well known in the art can be used in place of the material examples used in the above embodiments to build a nanostructure electroluminescent display according to the present invention. For example, other transparent conducting materials can be used as anode instead of ITO. Other metal oxides can be used as hole injection materials instead of aluminum oxide. Other metal haiides can be used as electron injecting materials instead of LiF Io build a nanostructure electroluminescent display according to the present invention. Other metals such as Ag, Ca can be used instead of Aluminum as cathode Io build a nanostrυcttire electroluminescent display according to the present invention. CdSe and CdTe nanoparticles are used as examples for the first and second nanoparticle layers. Other luminescent nanoparticles with suitable bandgaps can be used instead of CdSe and CdTe to build a nanostructure electroluminescent display according to the present invention.
(O049J The above embodiments show a bottom emitting display. It will be obvious to any one skilled in the art that a top emitting display can be built according to the present invention by using appropriate cathode and anode materials well known in the art.

Claims

1. An electroluminescent device comprising first and second electrodes, at least one of which is transparent to radiation: a hole conducting layer comprising first nanoparticles wherein said hole conducting layer is in contact with said first electrode; an electron conducting layer comprising second nanoparticJcs wherein said electron conducting layer is in contact with said hole conducting layer and said second electrode; and a voltage source capable of providing positive and negative voltage, where the positive pole of said voltage source is electrically connected to said first electrode and the negative pole is connected to said second electrode.
2. The electroluminescent device of Claim 1 further comprising an electron-hole combination layer between said hole and electron conducting layers.
3. The electroluminescent device of Claim 2 wherein said electron-hole combination layer comprises a layer of metal or metal oxide.
4. The electroluminescent device of Claim 2 wherein said electron-hole combination layer comprises a metal or metal oxide in combination with said first or said second nanoparticle.
5. The electroluminescent device of Claim 2 wherein said electron-hole combination layer comprises a metal or metal oxide in combination with said first and said second nanoparticle,
6. The electroluminescent device of Claim 2 wherein said first and said second nanoparticles comprise at least one metal and wherein the metal of said electron-hole combination layer comprises at least one of the metals of said first or second nanopartides.
7. The electroluminescent device υf Claim 2 wherein said electron-hole combination layer is a sintered layer.
8. The electroluminescent device of Claim 2 wherein said electron-hole combination layer is 5-10 nanometers thick.
9. The electroluminescent device of Claim 2 further comprising a hole injection layer between and in contact with said first electrode and said hole conducting layer.
10. The electroluminescent device of Claim 9 wherein said hole injection layer comprises a p-type semiconductor, a metal or a metal oxide,
1 1. The electroluminescent device of Claim 10 wherein said metal oxide comprises aluminum oxide, zinc oxide or titanium dioxide.
12. The electroluminescent device of Claim 10 wherein said metal comprises aluminum,, gold or silver.
13. The electroluminescent device of Claim 10 wherein said p-type semiconductor is p-doped Si.
14. The electroluminescent device of Claim 2 further comprising an electron injection layer between and in contact with said second electrode and said electron conducting layer.
15. The electroluminescent device of Claim 14 wherein said electron injection layer comprises a metal, a fluoride salt or an n-type semiconductor
16. The electroluminescent device of Claim 15 wherein said fluoride salt comprises NaF5 CaF2, or BaF2.
17. The electroluminescent device of Claim 1 wherein said first and second nanopaiticles are nanocrystals.
18. The electroluminescent device of Claim 17 wherein said nanocrystals are independently selected from the group consisting of quantum dots, nanorods, nanobipods, nanotripods, nanomultipods, or nanowires.
19. The electroluminescent device of Claim 17 wherein said nanocrystals comprise quantum dots.
20. The electroluminescent device of Claim 16 wherein said nanocrystals comprise CdSe, ZnSe, PbSe, CdTe, InP, PbS, Si or Group H-VI, II-IV or IH-V materials.
21. The electroluminescent device of Claim 17 further comprising a nanostructure in said hole conducting, electron conducting or electron-hole combination layer.
22. The electroluminescent device of Claim 21 where said nanostructure comprises a nanotube, nanorod or nanowire.
23. The electroluminescent device of Claim 22 wherein said nanostructure comprises a carbon nanotube.
24. The electroluminescent device of Claim 21 wherein said nanoparticle is covalently attached to said nanostructure.
25. An electronic device comprising the electroluminescent device of claim I .
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