Patent Application
20090001403
The present disclosure generally relates to light emitting displays and more particularly to free standing quantum dot light emitting display.
The market for personal portable electronic devices, for example, cell phones, personal digital assistants (PDA's), digital cameras, and music playback devices (MP3), is very competitive. Manufactures are constantly improving their product with each model in an attempt to cut costs and production requirements.
In many portable electronic devices, such as mobile communication devices, displays present information to a user. For a simple icon display on the surface of a housing, for example, light emitting diodes have provided light through a small portion of a surface of the housing to illuminate an icon to a user.
Free standing quantum dots (FSQDs) are semiconductor nanocrystallites whose radii are smaller than the bulk exciton Bohr radius and constitute a class of materials intermediate between molecular and bulk forms of matter. FSQDs are known for the unique properties that they possess as a result of both their small size and their high surface area to volume ratio. For example, FSQDs typically have larger absorption cross-sections than comparable organic dyes, higher quantum yields, better chemical and photo-chemical stability, narrower and more symmetric emission spectra, and a larger Stokes shift. Furthermore, the absorption and emission properties vary with the particle size and can be systematically tailored. It has been found that a Cadmium Selenium (CdSe) quantum dot, for example, can emit light in any monochromatic, visible color, where the particular color characteristic of that dot is dependent on the size of the quantum dot, (i.e., size tunable band gap).
FSQDs are easily incorporated (solubalized or dispersed) into or onto other materials such as polymers and polymer composites because solution processing of inorganic nanocrystals is made possible by a capping layer of organic capping groups on the surface of the FSQDs. This capping layer may be tailored to control solubility, external chemistry, and particle spacing. FSQDs are highly soluble and have little degradation over time. These properties allow FSQD polymers and polymer composites to provide very bright displays, returning almost 100% quantum yield.
Applications for FSQD polymers and polymer composites include point of purchase and point of sale posters, mobile device housings or logos, segmented displays, including infrared displays, absorbers for infrared sensors or detectors, and light emitting diodes (LEDs). The visible advantages inherent to FSQD polymers and polymer composites are attractive.
Accordingly, it is desirable to provide an improved free standing quantum dot light emitting display. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
FIG, 5 is a top view of the exemplary embodiment of
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
Free standing quantum dots (FSQDs), also known as nano-crystals, are semiconductors composed of periodic groups of II-VI, III-V, or IV-VI materials, for example, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb. Alternative FSQDs materials that may be used include but are not limited to tertiary microcrystals such as InGaP, which emits in the yellow to red wavelengths (depending on the size) and ZnSeTe, ZnCdS, ZnCdSe, and CdSeS which emits from blue to green wavelengths. Multi-core structures are also possible such as ZnSe/ZnXS/ZnS, are also possible where X represents Ag, Cu, or Mn. The inner most core is made of ZnSe, followed by the second core layer of ZnXS, completed by an external shell made of ZnS.
FSQDs range in size from 2-10 nanometers in diameter (approximately 102-107 total number of atoms). At these scales, FSQDs have size-tunable band gaps, in other words there spectral emission depends upon size. Whereas, at the bulk scale, emission depends solely on the composition of matter. Other advantages of FSQDs include high photoluminescence quantum efficiencies, good thermal and photo-stability, narrow emission line widths (atom-like spectral emission), and compatibility with solution processing. FSQDs are manufactured conventionally by using colloidal solution chemistry.
FSQDs may be synthesized with a wider band gap outer shell, comprising for example ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb. The shell surrounds the core FSQDs and results in a significant increase in the quantum yield. Capping the FSQDs with a shell reduces non-radiative recombination and results in brighter emission. The surface of FSQDs without a shell has both free electrons in addition to crystal defects. Both of these characteristics tend to reduce quantum yield by allowing for non-radiative electron energy transitions at the surface. The addition of a shell reduces the opportunities for these non-radiative transitions by giving conduction band electrons an increased probability of directly relaxing to the valence band. The shell also neutralizes the effects of many types of surface defects. The FSQDs are more thermally stable than organic phosphors since UV light will not chemically breakdown FSQDs. The exterior shell can also serve as an anchor point for chemical bonds that can be used to modify and functionalize the surface.
Due to their small size, typically on the order of 10 nanometers or smaller, the FSQDs have larger band gaps relative to a bulk material. It is noted that the smaller the FSQDs, the higher the band gap. Therefore, when impacted by a photon (emissive electron-hole pair recombination), the smaller the diameter of the FSQDs, the shorter the wavelength of light will be released. Discontinuities and crystal defects on the surface of the FSQD result in non-radiative recombination of the electron-hole pairs that lead to reduced or completely quenched emission of the FSQD. An overcoating shell (example ZnS) having, e.g., a thickness of up to 5 monolayers and higher band gap compared to the core's band gap is optionally provided around the FSQDs core to reduce the surface defects and prevent this lower emission efficiency. The band gap of the shell material should be larger than that of the FSQDs to maintain the energy level of the FSQDs. Capping ligands (molecules) on the outer surface of the shell allow the FSQDs to remain in the colloidal suspension while being grown to the desired size. The FSQDs may then be placed within the display by a printing process, for example. Additionally, a light source (preferably a ultra violet (UV) source) is disposed to selectively provide photons to strike the FSQDs, thereby causing the FSQDs to emit a photon at a frequency comprising the specific color as determined by the size tunable band gap of the FSQDs.
Referring to
In this previously known light emitting device 100, when the layer 108 of the plurality of FSQDs are impacted with light having a wavelength shorter that which would be emitted by the FSQDs, an electron in each of the FSQDs so impacted is excited to a higher level. Alternatively, a DC potential may be applied across the FSQDs to excite an electron to a higher level. When the electron falls back to its ground state, a photon is emitted having a wavelength determined by the size of the FSQD.
In accordance with the exemplary embodiments described herein, one or more layers of a plurality of FSQDs may be electrically driven (caused to emit photons) by an inductive coil. In one exemplary embodiment, the FSQDs may be driven by direct application of the electric field produced by the coil, which may be applied through a housing. This electric (electromagnetic) field results from the presence and motion of charged particles and exerts forces on them. A sub-discipline called electrodynamics describes the behavior of moving charged particles interacting with electromagnetic fields. This inductive coupling refers to the transfer of energy from one circuit component to another through a shared magnetic field. A change in current flow through one device, e.g., the coil, induces current flow in the other device, e.g., the light emitting device. The two devices may be separated as in the coil and light emitting device just mentioned or may be physically contained in a single unit, as in the primary and secondary sides of a transformer in a second exemplary embodiment. In this second exemplary embodiment, the FSQDs may be driven by the application of a current (after being converted from AC to DC) generated from a secondary coil by the application of an electric field from a primary coil. The wavelength of the emitted light comprises a color as determined by the diameter and composition of the FSQDs and may be used to illuminate, for example, icons, text, or the housing itself.
Referring to
Though various lithography processes, e.g., photolithography, electron beam lithography, and various printing processes including imprint lithography ink jet printing, may be used to fabricate the light emitting device 200, a printing process is preferred. In the printing process, the FSQD ink in liquid form is printed in desired locations on the substrate. Ink compositions typically comprise four elements: 1) functional element, 2) binder, 3) solvent, and 4) additive. Graphic arts inks and functional inks are differentiated by the nature of the functional element, i.e. the emissive quantum dot. The binder, solvent and additives, together, are commonly referred to as the carrier which is formulated for a specific printing technology e.g. tailored rheology. The function of the carrier is the same for graphic arts and printed electronics: dispersion of functional elements, viscosity and surface tension modification, etc. One skilled in the art will appreciate that an expanded color range can be obtained by using more than three quantum dot inks, with each ink having a different mean quantum dot size. A variety of printing techniques, for example, Flexo, Gravure, Screen, inkjet may be used. The Halftone method, for example, allows the full color range to be realized in actual printing.
The substrate 202 comprises any transparent material, but may comprise, for example, glass, ceramic, insulated metal, polymers, and polymer composites. The first electrode 204 comprises a transparent material, preferably indium tin oxide, and function as an anode. The second electrode 212 comprises, for example, an opaque electron source material comprising, for example, magnesium and silver, and functions as a cathode. It is recognized that the substrate 202 may comprise a rigid structure or be flexible, and although it is disposed adjacent the first electrode 204 as shown, it may alternatively be opaque when disposed adjacent the second electrode 212. The hole transport layer 206 may be organic or inorganic and comprise, e.g., N,N0-diphenyl-N,N0-bis(3-methylphenyl)-(1,1 0-biphenyl)-4,4 0-diamine (TPD). The electron injection layer 210 may be either organic or inorganic and comprise, e.g.,tris-(8-hydroxyquinoline)aluminium or 3-(4-Biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ).
The light emitting device 200, and more specifically the substrate 202 in this embodiment, is disposed against the structure 214. The structure 214 may comprise any transparent material and may be rigid or flexible. Examples of the structure 214 include a poster board, such as used in advertising, and a portion of a housing for a portable electronic device.
A coil 216 is disposed contiguous to the light emitting device 200, for example against the second electrode 212 as shown in
When the layer 108 of the plurality of FSQDs is impacted by the electric field, an electron in each of the FSQDs so impacted is excited to a higher level. When the electron falls back to its ground state, a photon having a wavelength determined by the size of the FSQD is emitted and passes through the electrode 204 and the transparent structure 214. The first electrode 202, or a layer (not shown) disposed between the first electrode 202 and the structure 214, may be shaped or patterned wherein the photons exiting the structure 214 assume a desired pattern. Alternatively, a blocking layer (not shown) of an opaque material may block the light, thereby creating the pattern. Examples of the desired pattern may include, e.g., an envelope, text, or any know icons used in electronic devices. In another version of this first embodiment, a larger portion of the structure may receive the photons from the FSQDs, effectively providing a colored surface on the housing of a portable electronic device, for example.
A coil 316 is disposed on a side of the structure 314 opposed to the second electrode 312, but near the layer of FSQDs 308. A resonant converter 318 is coupled between the coil 316 and both a power source 322 and a microprocessor 324. When instructed by the microprocessor 324, the resonant converter 318 provides an alternating current to the coil 316. This alternating current flowing through the coil 316 provides an electric field (not shown) which will exist through and on the other side of the structure 314.
When the layer 108 of the plurality of FSQDs is impacted by the electric field, an electron in each of the FSQDs so impacted is excited to a higher level. When the electron falls back to its ground state, a photon having a wavelength determined by the size of the FSQD is emitted and passes through the transparent structure 214. The first electrode 304, or a layer (not shown) disposed between the first electrode 304 and the structure 314, may be shaped or patterned wherein the photons exiting the structure 314 assume a desired pattern. Examples of the desired pattern may include, e.g., an envelope, text, or any know icons used in electronic devices.
Referring to
A primary coil 416 is disposed on a side of the structure 414 opposed to the light emitting device 400. A resonant converter 418 is coupled between the coil 416 and both a power source 422 and a microprocessor 424. A secondary coil 426 is disposed a side of the structure 414 opposed by the primary coil 416, and formed over a dielectric material 428. Electrical conductors 432, 434 are formed within the dielectric material 428 and are coupled to the respective ends of the secondary coil 426. When instructed by the microprocessor 424, the resonant converter 418 provides an alternating current to the coil 416. This alternating current flowing through the coil 416 provides an electric field (not shown) which will project through and on the other side of the structure 414. This electric field within the secondary coil 426 will generate an AC current to an AC to DC converter 436. A DC voltage is provided by electrical conductors 438, 440 to the first and second electrodes 404, 412, respectively.
When the layer 408 of the plurality of FSQDs are impacted by the electric field having an electron in each of the FSQDs so impacted is excited to a higher level. When the electron falls back to its ground state, a photon having a wavelength determined by the size of the FSQD is emitted and passes out through the anode 404. The first electrode 404, or a layer (not shown) disposed over the first electrode 404, may be shaped or patterned wherein the photons exiting the light emitting device 400 assume a desired pattern. Examples of the desired pattern may include, e.g., an envelope, text, or any know icons used in electronic devices.
Referring to
Referring to
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
