US20080087899A1

US20080087899A1 - Optically-Regulated Optical Emission Using Colloidal Quantum Dot Nanocrystals

Info

Publication number: US20080087899A1
Application number: US11/952,066
Authority: US
Inventors: Edward Sargent; Lioudmila Bakoueva; Sergei Musikhin
Original assignee: Individual
Current assignee: InVisage Technologies Inc
Priority date: 2004-04-19
Filing date: 2007-12-06
Publication date: 2008-04-17
Also published as: WO2005101530A1; US7326908B2; US20050236556A1

Abstract

The present invention relates to the emission of light which occurs in proportion with an electrical signal, an optical signal, or the combination of both. The emission of light may occur due to the passage of current through a light-emitting polymer, or due to energy transfer of excitons from this polymer to light-emitting quantum dots. Optical sensitivity is achieved through the inclusion of another species of quantum dots whose absorption is generally at longer wavelengths relative to the light-emitting material. Light incident upon the device results in an enhanced current flow in the presence of an applied bias, and thus enhanced excitation of the light-emitting moity is achieved in proportion with the optical power absorbed by the light-absorbing moity. Two device architectures are presented, one based on a mutilayer structure in which the functions of light absorption and light emission are separated, and the other in which these functions are integrated within a single active region.

Description

CROSS REFERENCE TO RELATED U.S PATENT APPLICATIONS

This patent application relates to U.S. provisional patent application Ser. No. 60/563,012 filed on Apr. 19, 2004 entitled MULTI-COLOR OPTICAL AND INFRARED EMISSION USING COLLOIDAL QUANTUM NANOCRYSTALS.

FIELD OF THE INVENTION

The present invention relates to visible-, or infrared-emission using colloidal quantum nanoparticles to achieve a spectrally-tailored multi-color emission. More particularly, the present invention provides several light-emitting device structures using colloidal quantum dots including a structure using multiple separated layers each containing a pre-selected diameter of colloidal particles and a structure which uses layers containing mixtures of nanocrystals of different diameters which emit at different wavelengths depending on the diameter. Devices using electrical or a combination of electrical and optical stimulation of luminescence are disclosed. The spectrum of luminescence of a given device is controllable through the combination of optical and electrical stimuli.

BACKGROUND OF THE INVENTION

Efficient light emission at a chosen wavelength, or combination of wavelengths, or a designed spectrum of wavelengths, is of great interest in many applications. In displays, it is desirable to be able to choose three distinct colours in the visible range. For lighting applications, tailoring of a single spectral profile is desired to achieve bright white light generation. For communications, it is desirable to generate and modulate light in the infrared range 1270 nm-1610 nm, as specified by the coarse wavelength-division multiplexing standard. In night vision, it is important to convert invisible (infrared light) signals into proportional, spatially-resolved signals which are visible. In all such applications, efficiency, brightness, and spectral control form key requirements.
Quantum dots, or colloidal quantum nanocrystals, are particles of semiconductor on the length scale 1 to 20 nm broadly, but preferably 2-10 nm, and can provide size-tunable luminescence spectra which the present inventors and others have shown to be customizable across the infrared and visible spectral ranges. When incorporated into semiconducting polymers, it is possible to inject charge into these matrix materials and ensure that either separate electrons and holes, or electron-hole pairs known as excitons, are transferred from the semiconducting polymer to the nanocrystals. This energy transfer process is a necessary step for subsequent net emission of light from the quantum dots. It is of critical importance in applications involving electroluminescence to ascertain and control the efficiency of energy transfer from the polymer to the nanocrystals.
Solution-processible devices based on colloidal quantum dots embedded in a semiconducting polymer matrix represent a promising basis for monolithic integration of optoelectronic functions on a variety of substrates including silicon, glass, III-V semiconductors, and flexible plastics. Reports of electroluminescence in the visible and infrared [Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites (ed. H. S. Nalva), American Scientific Publishers, 2003], as well as photovoltaic [W. U. Huynh, X. Peng, A. P. Alivisatos, Adv. Mater. 11, 923 (1999)] and optical modulation [S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420, 19 (2002)] phenomena in the visible, point to the possibility of combining a variety of useful optical and optoelectronic functions on a single platform.
Much work, including that on size-selective precipitation of nanocrystals to achieve the greatest possible monodispersity, has focused on narrowing emission, absorption, and modulation linewidths. Once such control over spectral properties has been achieved, it then becomes attractive to combine a number of different families of quantum dots in order to engineer a broader spectral shape: applications of such broadband or spectrally-engineered devices include multi-color light emitters for color displays; white light emitters for illumination; and, in the infrared, multi-wavelength emitters for coarse wavelength-division multiplexing and code-division multiple access [A. Stok, E. H. Sargent, IEEE Network 14, 42 (2000)}, useful in for example telecom integrated circuits (ICs).

BRIEF DESCRIPTION OF THE DRAWINGS

The layered structure and the light-emitting device produced according to the present invention will now be described, by way of example only, reference being made to the accompanying drawings, in which:
FIG. 1 shows an absorption spectra of two groups of nanocrystals (1, 2) in solution before (dashed lines) and after (solid lines) ligand exchange. Curve 3 provides the absorption spectrum of the light-emitting structure;
FIG. 2 is a schematic view of a first embodiment of a display device structure incorporating quantum dots constructed in accordance with the present invention;
FIG. 3 is a schematic view of a second embodiment of a display device structure incorporating quantum dots;
FIG. 4 shows the photoluminescence spectra of the double-layer structure for excitation through the substrate (1) and from the upper side (2), its electroluminescence spectrum (3), and photoluminescence spectrum of the mixture layer (4); and
FIG. 5 shows a third embodiment of a display device structure incorporating quantum dots.

SUMMARY OF THE INVENTION

The present invention provides methods and devices useful for the emission of light which occurs in proportion with an electrical signal, an optical signal, or a combination of both. The emission of light may occur due to the passage of current through a light-emitting polymer, or due to energy transfer of excitons from this polymer to light-emitting quantum dots or quantum nanoparticles. Optical sensitivity is achieved through the inclusion of another species of quantum nanoparticles whose absorption is generally at longer wavelengths relative to the light-emitting material. Light incident upon the device results in an enhanced current flow in the presence of an applied bias, and thus enhanced excitation of the light-emitting moity is achieved in proportion with the optical power absorbed by the light-absorbing moity. Two device architectures are presented, one based on a mutilayer structure in which the functions of light absorption and light emission are separated, and the other in which these functions are integrated within a single active region.
Thus, an embodiment of the present invention provides a device and method for converting an infrared (IR) image, projected onto the devices into a visible image. The method of conversion is indirect in that the process involves converting the IR light into an electrical current; and then turns this electrical current into visible light emission, thereby producing visible light in proportion to the infrared light incident on the device.
The present invention provides designs and methods of fabrication of efficient polymer-nanocrystal light-emitting devices. The invention describes the series of layers to be deposited to form an efficient light-emitting device based on quantum dots or nanocrystals embedded in a semiconducting polymeric host. The density and spatial distribution of nanocrystals in the active layer of the device must be suitably chosen. The disclosure describes the means of determining areal concentration of the nanocrystals required for capture of the majority of excitons into the nanocrystals prior to their recombination at opposite contacts. The areal concentration of nanocrystals N should be such that Na²>>1 where a is the nanocrystal radius; this requirement in turn describes the thickness of the emitting layer required if the packing density of the nanocrystals is fundamentally, or through design, limited to a known upper bound. The disclosure describes the placement of this electroluminescent layer relative to the injecting cathode and anode.
The present invention provides a design and method of fabrication of controlled-spectrum light-emitting devices based on semiconducting polymers and a mixture of different quantum nanocrystals. This invention concerns the formation of devices which produce a chosen spectrum of light, either in the visible or in the infrared spectral range, or both. The spectrum may be chosen to be white for lighting applications; or may alternatively represent a particular code for applications in detection and sensing in resource management and biomedical assays.
The present invention describes: 1) the size and spacing of the spectral layers required to achieve a particular luminescence spectrum from the nanocrystals; 2) the choice of nanocrystal properties and passivating ligand properties which simultaneously ensure efficient injection into the nanocrystals while keeping to a minimum the extent of inter-quantum-nanocrystal energy transfer; 3) the choice of multilayer device structure, and the manner in which to realize this device structure through combinations of water-soluble and organic-solvent-based nanocrystals and polymers.
In one aspect of the invention there is provided a device for detecting light in a pre-selected wavelength range and converting the detected light into light of at least one pre-selected wavelength and emitting the light at said at least one pre-selected wavelength, comprising:
a substrate and a first electrically conducting electrode layer on the substrate;
a first layer of first nanocrystals located on the first electrically conducting electrode layer which absorb light in said pre-selected wavelength range;
at least second layer of at least second nanocrystals which emit light at said at least one pre-selected wavelength located on the first layer of first nanocrystals; and
a second electrically conducting electrode layer on the at least second layer of the second nanocrystals, wherein at least one of said substrate and first electrically conducting electrode layer and said second electrically conducting electrode layer is substantially transparent to the light in the pre-selected wavelength range and light at the at least one pre-selected wavelength, and wherein when the light in said pre-selected wavelength range is incident on said first layer of first nanocrystals a photocurrent is responsively produced when a voltage is applied between the first and second electrically conducting electrode layers, and wherein said photocurrent acts to pump the at least second layer of the at least second nanocrystals which responsively emit light at the at least one preselected wavelength.
In another aspect of the present invention there is provided device for detecting light in a pre-selected wavelength range and converting the detected light into light of at least one pre-selected wavelength and emitting the light at said at least one pre-selected wavelength, comprising:
a substrate and a first electrically conducting electrode layer on the substrate;
a polymer matrix located on the first electrically conducting electrode layer, the polymer matrix containing a mixture of nanocrystals, the mixture including at least first nanocrystals which absorb light in a pre-selected wavelength range, and a light emitting member located in the polymer matrix which emits light at said at least one pre-selected wavelength; and
a second electrically conducting electrode layer located on the polymer matrix, wherein at least one of said substrate and first electrically conducting electrode layer and said second electrically conducting electrode layer is substantially transparent to light in the pre-selected wavelength range and light at said at least one pre-selected wavelength, wherein when light in the pre-selected wavelength range is incident on the polymer matrix and absorbed by the first nanocrystals, a photocurrent is responsively produced when a voltage is applied between the first and second electrically conducting electrode layers, and wherein said photocurrent acts to pump the second nanocrystals which responsively emit light at said at least one pre-selected wavelength.
The present invention also provides a method for detecting light in a pre-selected wavelength range and converting the detected light into light of at least one pre-selected wavelength and emitting the light at said at least one pre-selected wavelength, comprising the steps of:
applying a pre-selected voltage across a laminate structure which includes first and second electrode layers, and a polymer matrix located between said first and second electrode layers, said polymer matrix containing first nanocrystals which absorb light in a pre-selected wavelength range, and a light emitting member located in the polymer matrix which emits light at said at least one pre-selected wavelength; and
wherein at least one or both of said first and second electrode layers is substantially transparent to the light in the pre-selected wavelength range and the light at said at least one pre-selected wavelength, and wherein when the light in the pre-selected wavelength range is incident on the polymer matrix and absorbed by the first nanocrystals, a photocurrent is responsively produced when said pre-selected voltage is applied between the first and second electrode layers, and wherein said photocurrent acts to pump the light emitting member located in the polymer matrix which responsively emits light at said at least one pre selected wavelength.
The present invention also provides a method for detecting light in a pre-selected wavelength range and converting the detected light into light of at least one pre-selected wavelength and emitting the light at said at least one pre-selected wavelength, comprising the steps of:
applying a pre-selected voltage across a structure which includes first and second electrically conducting electrode layers, and located between the first and second electrode layers, a first layer of first nanocrystals on said first electrode layer which absorb light in said pre-selected wavelength range, and at least a second layer of at least second nanocrystals on the layer of first nanocrystals which emit light at said at least one pre-selected wavelength, and said second electrically conducting electrode layer being located on the at least a second layer of second nanocrystals;
wherein at least one or both of said first and second electrode layers is substantially transparent to light in the pre-selected wavelength range and light at said at least one pre-selected wavelength; and
wherein when light in said pre-selected wavelength range is incident on said first layer of first nanocrystals a photocurrent is responsively produced when to a voltage is applied between said first and second electrode layers, and wherein said photocurrent acts to pump the at least a second layer of at least second nanocrystals which responsively emit light at said at least one pre-selected wavelength.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

As used herein, the phrase “colloidal quantum dots” (or “colloidal quantum nanocrystals”, or “nanocrystals”) means particles of inorganic semiconductor, broadly in a range from about 1 to about 20 nm, but preferably in the range from about 2 to about 10 nm in diameter, which, through the use of appropriate capping ligands (organic molecules), may be dispersed in a solvent such as toluene, water, hexane, etc. It will be appreciated that the upper size limit on the colloidal quantum nanocrystals useful in the present invention is reached when the larger nanocrystals have too small quantum size effect to provide effective spectral tuning.
In certain applications it is important that the nanocrystals show a sudden onset of absorbance. For this to be achieved it is important that the nanocrystals be substantially the same shape and size, thus exhibiting monodispersity. For example, the nanocrystals may have a distribution of dimensions such that no more than about 10% of the nanocrystals have a dimension greater than about 10% of the average dimension of the nanocrystals.
In other applications, monodispersity among the nanocrystals is not important.
As used herein, the term “conducting” means sufficiently inclined to flow electrical current as to provide sufficient injection of charge carriers without incurring an excessive voltage drop.
As used herein, the term “semiconducting” means a material having a bandgap between about 0.4 and about 4.0 eV.
As used herein, the term “insulating” means a material which does not pass current strongly under the application of a voltage bias.
As used herein, the term “ligand” means a molecule on the surface of a nanoparticle which achieves passivation and renders nanoparticles soluble in certain solvents.
As used herein, the term “passivating” in regards to the quantum dots or nanoparticles, means reducing the density of defect states on the surface of a nanoparticle, thereby increasing the lifetime of excited states (excitons) within.
As used herein, the term “unsatisfied bonds” refers to atoms at the surface of the nanocrystal which, by virtue of being at the surface, do not have all of their chemical bonds satisfied.
As used herein, the term “electroluminescent” or “electroluminescence” means the production of light driven by the injection of current.
As used herein, the term “photoluminescent” or “photoluminescence” means the production of light driven by the absorption of light.
As used herein, the phrase “ligand exchange” means the replacement of one class of organic ligands with a new class. Replacement may occur in part or to in whole.
As used herein, the phrase “energy transfer” means the conveyance of energy from an initial state to a final state.
As used herein, the phrase “areal concentration” means the number of nanoparticles in an area divided by that area.
As used herein, the term “exciton” means a pair consisting of an electron and a hole which are bound to one another by virtue of their opposite charge.
The inventors disclose herein for the first time the fabrication and investigation of electroluminescent devices which combine two families of colloial quantum dots to achieve spectrally-tailored two-color emission. This is exemplified using devices produced which employ PbS quantum dot nanocrystals in the 1.1 to 1.6 μm spectral range.
Colloidal PbS nanocrystals were synthesized using an organometallic route requiring a single, short nucleation followed by slower growth of existing nuclei. The method included using the hot injection technique with rapid addition of reagents into the reaction vessel that contains the hot coordinating solvent as disclosed in Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution Advanced Materials, Volume 15, Issue 21, Date: November, 2003, Pages: 1844-1849; M. A. Hines, G. D. Scholes.
In the present invention the inventors based the synthesis after the method disclosed in C. B. Murray, D. J. Norris, M. G. Bawendi, J. Amer. Chem. Soc. 115, 8706 (1993) to provide monodisperse colloidal PbS nanocrystals over a wide range of possible sizes. For the studies disclosed herein, two groups of nanocrystal with different sizes were chosen. Group I labels the larger nanocrystals with a smaller effective bandgap; and group II the smaller nanocrystals with a larger effective bandgap.
As disclosed in L. Bakueva, S. Musikhin, M. A. Hines, T.-W. F. Chang, M. Tzolov, G. D. Scholes, E. H. Sargent, Appl. Phys. Lett. 82, 2895 (2003) a post-synthetic ligand exchange was performed to replace the initial oleate ligands used for passivating the nanocrystal surface with pre-selected ligands.
The ligands attached to the surface of the colloidal nanocrystals serve the combined purposes of: 1) passivating electronic states (unsatisfied bonds) associated with atoms at the surface of the nanocrystals; 2) allowing the nanocrystals to be dispersed in the solvent; 3) providing a spatial separation between the nanocrystals, given approximately by the length of the ligands, thereby minimizing or regulating the rate of energy transfer among nearby nanocrystals.
Ligands used by previous researchers include TOPO (tri-octyl phosophine oxide); amine chains such as octadecylamine, dodecylamine, and octylamin; and pyridine. An example of a useful ligand is octadecylamine (C18). In contrast, Bakueva et al. reported the use of octylamine (C8).
Light-emitting structures were fabricated in accordance with the present invention on glass substrates with a transparent ITO anode electrode covered with layer of poly(p-phenylenevinylene) (PPV). Light emitting devices were fabricated which employed two distinct nanocomposite layer structures. The first, as shown generally at 10 in FIG. 2, includes an ITO anode electrode coated substrate 12 which forms a transparent anode electrode substrate with the substrate 12 comprised of typically glass or any other optically transparent material (polymeric, glass etc.), with the electrically conductive transparent ITO layer on the substrate. The transparent ITO anode electrode is coated with layer of poly(p-phenylenevinylene) (PPV) 14. A layer 16 of narrow-gap group I nanocrystals covers the PPV layer 14, an intervening hydrophilic polymer layer 18 covers layer 16 and a layer 20 of wider-gap group II nanocrystals was then formed which was about twice as thick as the first layer 16 covers layer 18 with the thin PPV layer 18 being present to prevent mixing between the two groups of nanocrystals I and II. A metal bilayer cathode 22 includes a magnesium (Mg) layer 24 on layer 20 and a gold (Au) layer 26 on top of the Mg layer 24. It will be understood the thickness of layer 20 is does not need to be twice as thick as layer 16.
In operation, light incident penetrates through ITO coated substrate 12 onto the first IR-absorbing layer 16, containing the group I nanocrystals, which responsively produces a photocurrent when this device is biased by a sufficient voltage V as shown. The flow of this photocurrent acts to pump the visible light emitting layer 20 of the electroluminescent device containing the larger bandgap group II nanocrystals, which produces and emits visible light which is proportional to the IR light incident on the IR-absorbing layer 16.
It will be understood that while in certain applications it is important that the substrate and ITO electrode combination 12 be transparent, as is the case in the device 10 in FIG. 2, other embodiments falling within the scope of the present invention may comprise both top illumination and top-readout, so that only the final top electrode 22 needs to be transparent. Thus FIG. 2 shows a preferred embodiment of the multilayer device in which the IR-absorbing layer 16 is located on the side of the device from which light is incident, and thus as shown in FIG. 2 is located adjacent to the PPV layer 14 which is located on the ITO coated substrate 12.
Thus, device 10 of FIG. 2 comprises at least two active layers. One layer 16 serves the purpose of absorption/detection and the other layer 20 serves the purpose of light emission. It will be understood that the nanoparticles in layer 20 could be selected to emit not in the visible per se but in the IR as well. In this case the particles would be selected to emit at a different wavelength falling outside the range of wavelengths being detected.
While the above example used two active layers (16 and 20) of nanocrystals which absorb and emit light at different wavelengths, it will be understood that this is exemplary only, and that devices could be made with more than two layers of nanocrystals which emit light at different wavelengths. For example, instead of one visible light emitting layer 20, there could be two or more such layers each having nanoparticles which emit a different color. Alternatively, layer 20 could include different types of nanoparticles each emitting light at a different wavelength.
The density and spatial distribution of nanocrystals in each of the active layers of the device must be suitably chosen. The areal concentration of the nanocrystals required for capture of the majority of excitons into the nanocrystals prior to their recombination at opposite contacts is important. The areal concentration N should be such that Na²>>1 where a is the nanocrystal radius; this requirement in turn describes the thickness of the emitting layer required if the packing density of the nanocrystals is fundamentally, or through design, limited to a known upper bound.
Thus, for the nanoparticles in the first layer, the areal concentration N₁of the first colloidal quantum nanocrystals is such that N₁a²>>1 where a is a radius of the first nanocrystals, and in the second layer, the areal concentration N₂of the second colloidal quantum nanocrystals is such that N₂b²>>1 where b is a radius of the second nanocrystals.
The PPV layer 14 may be replaced with any other conducting, semiconducting or insulating polymer layer as long as it functions to regulate the transport of electrons and holes into the active layers of the device. In the case of an insulating layer, a thin layer which is tunnel-transparent, to different degrees, for electrons and holes, allows control over the relative rate of injection of each carrier type into the layers on either side of it. A semiconducting layer, through its choice of band structure (homo/lumo offsets relative to the active polymers), similarly allows control over electron and hole injection rates. Similarly, a conducting polymer layer, through its choice of band-structure (homo/lumo offsets relative to the active polymers) allows control over electron and hole injection rates.
The conducting, semiconducting or insulating polymer layer 18 may be comprised of 1) semiconducting polymers such as MEH-PPV (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene)) and associated poly-phenylene-vinylene derivatives, polyfluorine (PFO) and associated polyfluorine derivatives, and poly-thiophenes such as poly(3-octyl-thiophene) (P3HT), and poly(9,9-dioctylfluorene-co-bitiophene) (F8T2); 2) dielectric materials such as insulating polymers e.g. PMMA (poly-methyl-methacrylate) and inorganic or mixed organic-inorganic insulating dielectrics such as SiO₂, SiO, and SiNxOy; and 3) conducting polymer layers which may be used include PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) aqueous dispersion) and doped MEH-PPV or PPV (poly-phenylene-vinylene).
FIG. 2 shows a preferred structure which includes conducting, semiconducting or insulating layers for regulating charge flow however it will be understood the device would work without these layers 16 and 18.
The second structure, shown generally at 40 in FIG. 3 includes an ITO anode electrode-coated substrate 12 which forms a transparent anode electrode substrate 12 with the substrate again comprised of typically glass or other optically transparent polymer material, with the electrically conductive transparent ITO layer on the substrate. The transparent ITO anode electrode is coated with layer of poly(p-phenylenevinylene) (PPV) 46. Device 40 employs a uniform mixture of the two families 42 (group I) and 44 (group II) of nanocrystals inside a PPV matrix 46, resulting in total thickness of 1 μm. While a PPV matrix was used in this embodiment, it will be understood that other materials could be used. The polymer matrix serves the function of allowing the electronic transport of at least one of electrons and holes. A predominantly hole-conducting polymer would allow the transport of holes, with electrons being transported via nanoparticle-to-nanoparticle hopping. Alternatively, a balanced semiconductor with comparable electron and hole conductivities would allow balanced injection of electrons and holes through the same polymer matrix. The areal coverage by the light-emitting nanocrystal needs to exceed the threshold defined earlier with respect to the device 10 of FIG. 2. Metal bilayer cathode 22 includes magnesium (Mg) layer 24 on layer 46 and a gold (Au) layer 26 on top of the Mg layer 24. The electrical connections are the same as for device 10 in FIG. 2.
Device 40 may include PPV layer 14 or may be replaced with any other conducting, semiconducting or insulating polymer layer as long as it functions to regulate the transport of electrons and holes into the polymer matrix.
In operation, IR (or visible) light is incident on layer 46 through ITO/substrate 12 and PPV layer 14 and is absorbed by the larger group IIR-absorbing quantum dots 42, which responsively produces a photocurrent. This photocurrent flows and pumps the visible-emitting group II quantum dots 44 (or just the polymer itself in an embodiment that uses the polymer to generate the light and non nanocrystals) and these visible light-emitting nanoparticles responsively emit light.
Thus, device 40 of FIG. 3 comprises at least one active layer. The active layer 46 of interest in this device combines the function of detection and light emission. Again as with device 10 in FIG. 2, the nanoparticles which serve to emit light may be chosen to emit in the IR, and is not restricted to emission in the visible, the wavelength be outside the range of IR wavelengths being detected. Similarly, the nanoparticles which are used in the layer for absorption and detection could be tailored to absorb in the visible as well as the nanoparticles in the second emitting layer emitting in the visible, but preferably the absorption/detection nanoparticles would absorb in a longer wavelength portion of the visible spectrum than the particles in the emitting layer.
The polymer matrix may be made of semiconducting polymers such as MEH-PPV and associated poly-phenylene-vinylene derivatives, PFO and associated polyfluorine derivatives, and poly-thiophenes such as P3HT. The polymer matrix is selected to provide for photoconduction when combined with the light-detecting nanocrystals; and for electrical excitation of the light-emitting nanocrystals.
The device configurations of FIGS. 2 and 3 provide devices for converting an infrared (IR) image, projected onto the device, into a visible image. The method of conversion is indirect in that the process involves converting the IR light into an electrical current; and then turns this electrical current into visible light emission, thereby producing visible light in proportion to the infrared light incident on the device.
Therefore, both types of device comprise light-detecting and light-emitting nanoparticle or quantum dot components. Monodispersity is required in the light-emitting group II quantum nanoparticles of the active layer which emits the visible light. However monodispersity is not necessarily required of the nanoparticles which implement the light-detecting function, namely the IR absorbing group I nanoparticles. The light-emitting function can be implemented by 1) a polymer alone or 2) a blend of polymer and nanocrystals. If the light-emitting part of the device is required to produce a narrow spectrum of emission, then monodispersity becomes important.
In another embodiment of the invention in which the light-emitting function can be implemented by a polymer alone, the function of infrared optical detection is again implemented using infrared-absorbing nanocrystals. The process of emission now occurs from the polymer itself. The polymer is chosen from the same set enumerated above: MEH-PPV, P3OT, etc. since these are light-emitting polymers which can be further tailored to produce light in the red, green, or blue spectral regions. Thus this device strategy is as mentioned above, except that it does not involve the use of visible-emitting quantum dots, but the polymer matrix itself takes over this role.
In other embodiments of the device, additional layers may be included, for example, a semiconductor (e.g. PPV, the s/c polymer), a conductor (e.g. a metal, potentially an ultrathin layer), and/or a dielectric/insulator (e.g. SiO₂, LiF, etc.) may be inserted anywhere in the device, for example between the layer 16 and the PPV layer 14 of device 10. The layers may be thin enough to enable them to be optically transparent, or nearly optically transparent in cases where they are located in between the substrate and the first layer or in between the two main layers 16 and 20 in the multilayer device, or they may be placed on the very top of the device on layer 26.
As shown in FIG. 5 the devices may be configured to receive light that may originate not from a specific light source but could be incident light from a viewing field which one desires to image, which is intended to strike the sample, and this light illuminates the first and at least second layers of first and second colloidal quantum nanocrystals. The light incident may be accepted without deliberate spectral filtering, or its spectral content may be selected using an optical filtering layer. Its wavelength is chosen to promote absorption, and thus photoconduction, in one, but typically not both, active layers of the device structure.
Both structures exemplified by the devices of FIGS. 2 and 3 exhibited absorption spectra with two maxima corresponding to the two types of nanocrystals (FIG. 1). The structures were investigated using optical methods and then completed via the deposition of a metallic cathode similar to that used in L. Bakueva, S. Musikhin, M. A. Hines, T.-W. F. Chang, M. Tzolov, G. D. Scholes, E. H. Sargent, “Size-tunable infrared (1000-1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” Applied Physics Letters, vol. 82, no. 17, pp. 2895-2897, 2003, comprising a Mg film obtained by vacuum evaporation and protected from the ambient atmosphere with a thin capping Ag film. In both device structures, the active region is separated from the anode by a polymer layer, but is in direct contact with the cathode, as required (see Bakueva, S. Musikhin, E. H. Sargent, H. E. Ruda, and A, Shik, “Luminescence and Photovoltaic Effects in Polymer-Based Nanocomposites,” Handbook of Organic-inorganic Hybrid Materials and Nanocomposites, vol. 2, chapter. 5, pp. 181-215, 2003) by the less efficient transport of electrons in PPV.
Photoluminescence spectra of the structures were obtained using continuous wave (c.w.) excitation by a semiconductor laser at 831 nm. At this wavelength the polymer matrix is transparent and the exciting light absorbed only in the nanocrystals. The resulting spectra for a typical sample with two separate layers of different nanocrystals are shown in FIG. 4. These have two distinct peaks at λ=1240 nm and λ=1475 nm. Comparison with the absorption spectra shows a noticeable Stokes shift≅106 meV, the same for both peaks. Relative intensities of the peaks depend on the geometry of excitation: illumination through the substrate increases absorption and hence luminescence in the narrow-gap nanocrystal layer adjacent to the substrate, while illumination from the opposite side excites preferentially the wide-gap layer adjacent to the illuminated surface. If we define the absorbance of the narrow- (group I) and wide-gap (group II) layers to be, respectively, α and β, then for illumination from the substrate the fractions of absorbed light for these two layers are α and β(1-α) so that the ratio of luminescence peak intensity should be α/β(1−α). For illumination from the opposite side this ratio is α(1−β)/β. Comparing these formulae with relative peak intensities for curves 1 and 2 in FIG. 4, we find that α≈30% and β≈60%, in good agreement with the thickness ratio of these two layers mentioned above.
In contrast to the two-layer structure 10 of FIG. 2, the device 40 containing a mixture of different nanocrystals of FIG. 3 demonstrated one wide luminescence band covering the entire spectral region of interest, from about 1.1 to about 1.6 μm. Since the absorption spectrum in the same samples does have a distinctive double-peak structure (FIG. 1), the widening of the spectrum is attributed to re-absorption, re-emission, as well as interdot energy transfer amongst nanocrystals within and between the two groups. In the structures with well-separated layers, the latter process is expected to be negligible.
While the above disclosed device 40 used two types of nanocrystals 42 and 44 which emit at different wavelengths, it will be understood that this is exemplary only, and that devices could be made with more than two types of nanocrystals. For example, instead of one visible light emitting type of nanocrystal 44, there could be two or more visible light emitting nanocrystals, each emitting a different wavelength.
Prior to electroluminescence experiments, the samples were subjected to electrophysical measurements with the complex impedance measured in large intervals of applied bias and signal frequencies. The current-voltage characteristic in all samples was almost symmetric and slightly superlinear. Noticeable electroluminescence began at V=3 V. All data for spectral dependence and internal efficiency of electroluminescence provided herein were obtained at biases of V=3.5 V and current densities of 10 mA/cm².
Curve 3 in FIG. 4 shows the spectrum of electroluminescence in a two-layer structure. The position of the long-wavelength peak is almost the same as in the photoluminescence spectrum while the short-wavelength peak has a noticeable red shift compared to the photoluminescence spectrum. In contrast to photoluminescence, electroluminescence can be measured only through the transparent substrate. The shift of short-wavelength peak might be attributed to partial re-absorption by the layer of group I nanocrystals.
Absolute values of internal electroluminescence efficiency were found to vary from sample to sample; the largest measured value was 3.1% [7]. When the polarity of the applied bias was reversed, electroluminescence was still observed, consistent with the essentially symmetric character of the current-voltage characteristic.
In the present work, the approach to ligand exchange was found to play an important role in the realization of increased-efficiency electroluminescent devices. The nanocrystals capped by oleate ligands as a result of the synthetic procedure exhibited a good photoluminescence efficiency as high as 23% in solution. When used to make devices, these nanocrystals, before being capped having their long ligands replaced by shorter ones, exhibited no measurable electroluminescence. It was necessary to use nanocrystals on which ligand exchange had been carried out, thus which were capped with octadecylamine, to achieve 3.1% measured electroluminescence internal efficiency. Some of the present inventors have previously disclosed the necessity of carrying out ligand exchange to achieve an observable electroluminescence signal, see L. Bakueva, S. Musikhin, M. A. Hines, T.-W. F. Chang, M. Tzolov, G. D. Scholes, E. H. Sargent, “Size-tunable infrared (1000-1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” Applied Physics Letters, vol. 82, no. 17, pp. 2895-2897, 2003. It was disclosed in this reference that the ligand exchange alters both the end function group passivating the nanocrystal surface and also the length of the ligand which presents a potential obstacle to energy transfer from electrodes and polymer matrix into nanocrystals. The present work suggests that the longer octodecylamine ligand used in the present work, compared to the octylamine ligand used in the reference just cited to Bakueva et al. (Size-tunable infrared (1000-1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer), provides more effective passivation while not seriously impeding energy transfer.
Examples of the ligand attached to a surface of the colloidal quantum nanocrystals include, but not limited to, oleate, hexylamine, octylamine, dodecylamine, octadecylamine, tri-octyl phosphine oxide, and pyridine.
In a third embodiment the display device structure incorporating quantum nanoparticles may include a light source for illuminating the colloidal quantum nanocrystals, the light source emitting light at one or more pre-selected wavelengths for absorption by the different colloidal quantum nanocrystals. The use of a tunable light source permits variation in the selective excitation of a defined subset of the multi-component nanocrystal mixture or multi-layer structure. This in turn results in different degrees of energy transfer (if any) among excited vs. unexcited dots, and correspondingly a different spectrum of emission from the variously excited quantum nanoparticles in proportion with each subpopulation's level of excitation, its areal density, and its internal emission efficiency.
Copending U.S. patent application Ser. No. 60/641,766 filed Jan. 7, 2005, entitled “Quantum Dot-Polymer Nanocomposite Photodetectors and Photovoltaics” discloses a means of detecting infrared light using quantum dots combined with a polymer. This relates to the present work in that this patent application describes in greater detail the composition of layers required to make efficient infrared detectors. The active region of these detectors (the layer made up of infrared-absorbing nanocrystals combined with a polymer) can be used as the light-detecting portion of the device described herein.
In summary, the inventors have fabricated and disclosed herein nanocomposite structures containing PbS nanocrystals of two different sizes which allow tailoring of the emission spectrum of luminescent devices. Depending on device structure selected, in particular the use of two separated layers vs. a mixture of nanocrystals, the structures demonstrated light emission either in two infrared frequency peaks corresponding to the spectral region of about 1.1 to about 1.6 μm or in a wide band spanning this same spectral region. For two-color structures, it was shown that it is possible to vary the relative intensity of the peaks over a wide range through the choice of excitation conditions. The replacement of oleate with octodecylamine ligands allowed us to increase the internal efficiency of electroluminescence to 3.1%.
While the results disclosed herein employed colloidal PbS nanocrystals of two different sizes to give two different colors upon excitation, it will be understood that the present invention is in no way limited to devices using either just PbS or two different sizes of nanocrystals to give two emission wavelengths. For example, three or more differently sized colloidal quantum dots of PbS could be used to give three or more different colors or emission wavelengths. The same issues and degrees of freedom would apply: the exchange of energy amongst the three spectral components, and the freedom to make distinct choices of ligands to passivate, stabilize, and separate the three different nanocrystals. Similarly, many inorganic semiconductors in addition to PbS can be used to form quantum dots: examples in the infrared include III-V semiconductors such as InAs, and other IV-VI semiconductors such as PbSe; and in the visible include CdS, CdSe. The colloidal quantum nanocrystals may comprise a core/shell structure including a core made of a pre-selected type of material and a shell material surrounding the core made of a different pre-selected type of material. A non-limiting example of such a core/shell structure is the core-shell system ZnS(CdSe).
As used herein, the terms “comprises”, “comprising”, “including” and “includes” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms “comprises”, “comprising”, “including” and “includes” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.
The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

Claims

1. A device for detecting light in a pre-selected wavelength range and converting the detected light into light of at least one pre-selected wavelength and emitting the light at said at least one pre-selected wavelength, comprising:

a substrate and a first electrically conducting electrode layer on the substrate;

a first layer of first nanocrystals located on the first electrically conducting electrode layer which absorb light in said pre-selected wavelength range;

at least second layer of at least second nanocrystals which emit light at said at least one pre-selected wavelength located on the first layer of first nanocrystals; and

a second electrically conducting electrode layer on the at least second layer of the second nanocrystals,

wherein at least one of said substrate and first electrically conducting electrode layer and said second electrically conducting electrode layer is substantially transparent to the light in the pre-selected wavelength range and light at the at least one pre-selected wavelength, and

wherein when the light in said pre-selected wavelength range is incident on said first layer of first nanocrystals a photocurrent is responsively produced when a voltage is applied between the first and second electrically conducting electrode layers, and

wherein said photocurrent acts to pump the at least second layer of the at least second nanocrystals which responsively emit light at the at least one pre-selected wavelength.

2-71. (canceled)