WO2008134056A1

WO2008134056A1 - Photon energy coversion structure

Info

Publication number: WO2008134056A1
Application number: PCT/US2008/005465
Authority: WO
Inventors: David G. Deak; Joseph Lam
Original assignee: Deak-Lam Inc.
Priority date: 2007-04-26
Filing date: 2008-04-28
Publication date: 2008-11-06

Abstract

A photon energy conversion device uses at least one ultraviolet light emitting diode (UV- LED) with a wavelength shifting medium such as phosphor or quantum dots. The device can be used as a light source, and shaped like incandescent light bulbs, fluorescent tubes, circles or compact fluorescent bulbs.

Description

PHOTON ENERGY CONVERSION STRUCTURE

BACKGROUND OF THE INVENTION

The present invention relates to structures using ultraviolet light emitting diodes (UV- LEDs), one example being a highly efficient solid state lighting source based on UV- LED and phosphor combinations. Such a structure can provide an UV-LED energy efficient light source for exact replacement of conventional incandescent light bulbs and fluorescent lighting systems. Another aspect of the invention is a solar panel which produces electrical energy in response to incoming photons. The invention also provides methods for improving the phosphor coating conversion efficiency in UV-LEDs, where the fundamental quenching mechanisms for phosphor coatings can be determined and quantified.

Incandescent light bulbs have been using for providing light, but their efficiency is only about 10%. Fluorescent light bulbs are not esthetically pleasing and contain toxic mercury vapor. One light source that has replaced both incandescent and fluorescent lighting is LED lighting. At first LEDs were made in colors, the primary on being red. Of course, colored LEDs are not generally acceptable as a light source to replace white light incandescent and fluorescent lighting. Attempts have been made to produce LEDs to emit white light.

There are currently two methods commonly used for LED-based white light generation: (1) individual red-green-blue (RGB) LED combinations that mix to generate white light, and (2) In_xGa_{1 -x}N-based blue and near-UV (NUV; 380 to 410 nm) LED systems incorporating fluorescent phosphors that down-convert some of the emission to generate a mix of light. The RGB approach requires at least three LEDs, and each device must be adjusted by individual supply circuits to balance the emission intensity of each color for proper white light generation. Several problems currently exist with white-light devices composed of blue LEDs and Ce³⁺-doped yttrium aluminum garnet (Ce:YAG) yellow phosphors that mix blue and yellow light to produce what appears to be white light. These include the halo effect of blue/yellow color separation, strong temperature and current dependence of chromaticity, and poor color rendering caused by the lack of green and red components.

A lighting source requires high-quality light radiation because when we look at objects, we see the reflected light. The spectrum of the illumination source affects the appearance of objects in a phenomenon we call color rendering. If the illumination source does not include a spectrum close to that of incandescent bulbs or the sun, then the color of objects will be different than what we are accustomed to and there will be reluctance to use a light source which has a different color rendering that people are accustomed to.

There is a need for a light source which is highly efficient in producing light energy and which also produces acceptable color rendering. When considering efficiency, one should consider the spatial region over which the light energy is being produced and whether that meets the user's needs, and also over what frequency range or spectrum the light energy is being produced and whether that meets the user's needs, especially from a color rendering standpoint.

SUMMARY OF THE INVENTION

An objective of the invention is to contribute to the reduction of greenhouse gases by reducing the amount of energy spent on artificial illumination. The present invention provides an improvement over prior art lights with improved light output per energy consumed, color temperatures that simulate the color range of incandescent light, Halogen lights, and the general classification of all fluorescent lights that includes the novel " low wattage" fluorescent bulb replacements for the incandescent light bulb. The present invention provides a replacement for incandescent and Halogen bulbs as well as certain types of neon lighting components. Light sources according to the present invention reduce the power, efficiency, complexity, cost, and compromise to the greenhouse effect by using ultraviolet light emitting diodes as opposed to fluorescent light, incandescent light, and Halogen light. An object of the invention is to provide an LED light source which provides high luminous efficiency with high color rendering. According to the invention, this can be accomplished by matching an appropriate multicolor phosphor and encapsulation material to the near ultraviolet (NUV) region, to obtain white LEDs with both high color rendering and high luminous efficacy. The high efficiency of white-light LEDs means that the active potential exists for enormous energy savings.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a side elevational view of a light source according to the invention;

Figure 2 is a top plan view in cross-section of the light source of Figure 1, showing the

UV-LED array;

Figure 3 is a plan view of a staggered wavelength UV-LED array;

Figure 4 shows a single linear UV-LED array; Figure 5 shows multi-arrayed UV-LEDs;

Figure 6 shows UV-LEDs mounted in a disk array;

Figure 7 shows UV-LEDs arranged in a tubular array;

Figure 8 shows an octagonal UV-LED array, in plan and perspective side view;

Figure 9 shows a UV-LED light in the size and shape of a fluorescent tube; Figure 10 shows UV-LEDs in a horizontal circular array;

Figure 11 shows UV-LEDs in a series circular array;

Figure 12 shows an elevated view in cross section of a different embodiment of a light source from that of Figure 1 ;

Figure 13 shows a perspective view of the embodiment of Figure 12; Figure 14 shows portions of the embodiment of Figure 12; Figure 15 shows the underside of the portion of Figure 14; Figure 16 shows portions of the embodiment of Figures 12-15; Figure 17 shows the embodiment of Figures 12-16; Figure 18 shows the embodiment of Figures 12-17, without the dome; Figure 19 shows another embodiment with multiple LEDs arranged on a hexagonal shaped center post; and

Figure 20 shows the same embodiment of Figure 19, in elevated view in cross section, but with the reflector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One or more preferred embodiments of the invention will be described, but the embodiments are merely exemplary ways of implementing the invention and the invention is not limited to these exemplary embodiments.

As shown in Figure 1 one can incorporate currently manufactured, low cost, ultra-violet light emitting diodes of various UV wavelengths and situate the UV-LED and the voltage down converter(s) into a conventional light bulb design. This light bulb, which can either be made of glass or polycarbonate(non-breakable) will be coated with a thin film(uniform coating) of a high efficiency phosphor that can have a range of emitted colors for various applications and the shape of the light bulb can be conventional or any novel shape. Another advantage of this invention is that the bulb or other chosen novel shape does not need to be evacuated of ambient air, or contain any mercury vapour or Nobel gases such as Neon, Xenon, Argon, or Krypton.

As shown in Figure 1 the UV-LED light can be mounted in the screw-in base, and the bulb or whatever shape is desired and chosen is coated with the high efficiency phosphor and the bulb shape acts as the final enclosure for the UV-LED and the voltage down converter. The voltage down converter converts the 110 VAC (US, and Pan American countries) or 240 VAC (UK, EU, Middle East, etc) to a level of 3.9 VDC @ 30 ma. Using the voltage down converter, which has an efficiency of about 90%, using high photon energy output phosphor the light output from this arrangement can save a huge amount of electrical energy versus light emission. The US version of the down converter converts 110 VAC to 3.6 VDC @ 20 ma; so the power according to Ohm's Law is P = E x I (power equals voltage time current) or 3.6 volts times .020 amperes equals 0.072 watts. If the conversion circuitry is 90% efficient, then the actual power consumed is 0.0792 watts, still less than 1 watt to give the light output of a 100 watt incandescent light bulb. In the case for the UK or EU countries, the efficiency remains the same. There are also reflecting mirrors that assist in reflecting and re-directing the emitted UV light so as to propagate all throughout the enclosed volume coated with the phosphor compound layer.

The light can have one UV-LED or a plurality of UV-LEDs forming an array as shown in Figure 2. A series array would have a higher voltage and lower current while a parallel array of UV-LEDs would have a lower voltage and higher current. By Ohm's Law is realized that for an equal number of UV-LEDs in either a series or parallel array, the power consumed by each array is the same. With a parallel array, each diode would have its own current limiting element, such as a resistor or inductor.

The invention provides high-brightness blue and UV devices based on Ill-nitrides for the purpose of white-light LED sources. To develop efficient high-brightness white LED light sources, research has focused on fundamental studies of emission mechanisms in ZnS- and GaN-based wide-band gap compound semiconductors; improvement of epitaxial growth methods of multiple quantum wells (MQWs) and of external quantum efficiency of NUV LEDs; production of large substrates for homoepitaxial growth; development of multicolor, UV-excited phosphors that generate white light; and realization of illumination sources and fixtures using white LEDs.

Rare earth ions which are doped into solid host materials can give rise to sharp emissions in the visible spectral range. For example, Eu³⁺:Y₂O₃ is one of the most efficient red phosphors. Other rare earth ions have their emissions at different wavelengths. Yttrium aluminium borate (YAB) and Yttrium aluminium scandium borate are suitable hosts for rare-earth ions. In the quest for a material which shows rare earth emission in the visible range such that this emission stimulates a white colour perception we have produced dozens of doped YAB crystals containing different combinations and amounts of rare earth ions.

Among these many samples those systems which contained Tm³⁺ and D_y ³⁺ simultaneously in the proper ratio were the only ones showing the desired effect. The phosphor is represented by the general formula (Yl-x-yTmxDyy) A13-zScz(BO3) (where 0 < (x+y) _ 1, 0 _ _z _ 3).

This phosphor, when irradiated by ultraviolet light having a wavelength of 350 nm or less, can produce white light having composed of the wavelengths 451, 455, 470, 474, 481, 485, 564, 567, 571, 574, 579 and several peaks in the range of ±5 nm around these wavelengths, having a colour temperature of approximately 4600 - 10000 degrees K.

The white emission results from luminescence of the rare earth ions alone and does not require the presence of mercury vapour emission bands. Further, the "white light LED" emits light beyond the warm white wavelength of 3,500 to 4,000 degrees Kelvin. The white light LED, through secondary quantum photon emission stimulated by ultraviolet LED light emission, gives a colour temperature that can be anywhere between 3,500 to 4,000 degrees K with a high yield of light intensity.

Unlike conventional super bright LEDs that use "high light output" efficient phosphors, which are coated in the well that holds in place and completely surrounds, the LED semiconductor material used here coats the inner Gaussian surface of the light enclosure with this phosphor, thus allowing for a uniform thin film of this phosphor embedded within a polycarbonate plastic injected moulded embodiment. The potential applications for this phosphor material can include coatings for UV emitting gas discharge lamps, UV LEDs, plasma panels, or any other UV light emitting device. The white light generated by this phosphor is produced by luminescence only and can be generated by different kind of UV excitations. The substance is insoluble in water, acid or base and is heat resistant up to 1100°C.

Uses include flat panel displays, specialty lighting, biological sensors, quantum dot lasers, and novel floating gate memory structures.

An LED consists of several layers of semiconducting material. When a LED is operated with DC voltage light is generated in the active layer. The generated light is radiated directly or by reflections. An LED emits light in a certain color, which color depends on the material used. Two systems of material (AlInGaP and InGaN) produce LEDs with a high luminance in all colors from blue to red and also in white (luminescence conversion). Therefore different voltages operate the diode in conducting direction.

Typical super bright 5 mm UV-LEDs have color ranges from ultraviolet, blue to red, and infra-red. The method used to produce a complete visible spectrum color range of LEDs is to coat the well that holds and completely surrounds the small piece of semiconductor material, ranging in size from .1 mm to 1 mm. The cathode base and anode connection is electrically connected to the outside world by two stiff silver coated copper leads, the anode lead being the longer lead than the cathode. An ultraviolet LED having no phosphor coating along its well will emit UV light upon excitation. If phosphor is uniformly coated along the inner Gaussian surface of a typical incandescent style and size polycarbonate hollow bulb component, then the UV light will propagate outward from the LED well and will excite the phosphor atoms (coated along the inner Gaussian surface) to emit light, throughout the outer Gaussian surface, of a longer wavelength in accord with its quantum chemical characteristics. The phosphor acts as a wavelength shifting material or medium to shift the energy from the UV range to a longer wavelength which has a white color. Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit light in a very narrow range of wavelengths in the visible spectrum, resulting in nearly monochromatic light. This is why LEDs are so efficient for colored light applications such as traffic lights and exit signs. However, general light source, usually need white light. LED technology has the potential to produce high-quality white light with unprecedented energy efficiency.

White light can be achieved with LEDs in two main ways: 1) phosphor conversion, in which a blue or ultraviolet (UV) chip is coated with phosphor(s) to emit white light; and 2) RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light. The phosphor conversion approach is most commonly based on a blue LED. When combined with a yellow phosphor (usually cerium-doped yttrium aluminum garnet or YAG:Ce), the light will appear white to the human eye. A more recently developed approach uses an LED emitting in the near-UV region of the spectrum to excite multi-chromatic phosphors to generate white light. The RGB approach produces white light by mixing the three primary colors red, green, and blue. Color quality of the resulting light can be enhanced by the addition of amber to "fill in" the yellow region of the spectrum.

Correlated color temperature (CCT) describes the relative color appearance of a white light source, indicating whether it appears more yellow/gold or more blue, in terms of the range of available shades of white. CCT is given in degrees Kelvin (the unit of absolute temperature) and refers to the appearance of a theoretical black body (visualize a chunk of metal) heated to high temperatures. As the black body gets hotter, it turns red, orange, yellow, white, and finally blue. The CCT of a light source is the temperature (in K) at which the heated theoretical black body matches the color of the light source in question.

Incongruously, light sources with a higher CCT are said to be "cool" in appearance, while those with lower CCT are characterized as "warm."

Color Rendering Index (CRI) indicates how well a light source renders colors, on a scale of 0 - 100, compared to a reference light source. The test procedure established by the International Commission on Illumination (CIE) involves measuring the extent to which a series of eight standardized color samples differ in appearance when illuminated under a given light source, relative to the reference source. The average "shift" in those eight color samples is reported as Ra or CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an "R9" score, which indicates how well the light source renders a saturated deep red color.

Three ultraviolet emitting diodes may lie directly next to each other and each having a different wavelength. Each diode has a narrow bandwidth. If three ultraviolet light emitting diodes having different but close wavelengths and proximity, both Q and bandwidth can be increased as compared to any single UV-LED. For two UV-LEDs of Q = 20, and UV-LED of Q = 10, then the overall bandwidth and total resultant amplitude of all three increases as well. Another feature of this approach is that diode (a) and diode (b) operate at a higher intensity, which means its excitation current is great than diode (m) that has the lower Q due to this methodology of staggering the wavelength and the power output of the individual diodes in comparison to each other. This minimizes the current load, and gives more UV intensity over a wider bandwidth; some power is conserved. As this blend of photons of various energy levels and angular velocity strikes the ambient target phosphor atoms, they are excited and emit secondary emission at a lower angular velocity. If the phosphor coating is of an optimum thickness, the secondary emitted photons will pass through a polycarbonate hollow light enclosure and radiate through the phosphor layer to the enclosure's outer surface and in essence; provide an energy efficient and useful alternative light source to directly replace the incandescent light bulb, fluorescent tube type lights along with any and all variations of a theme.

The coefficient Aa, is the wavelength of the one diode rated at a smaller value of wavelength, which is to say that its frequency (angular velocity) is higher. This also means, from a quantum viewpoint, it emits photons with more photon energy. Planck's constant, h, ( a coefficient of Plank's Law) was proposed in reference to the problem of black-body radiation. The underlying assumption to Planck's law of black body radiation was that the electromagnetic radiation emitted by a black body could be modeled as a set of harmonic. oscillators with quantized energy of the form

E is the quantized energy of the photons of radiation having frequency (Hz) of V (nu)

or angular frequency (rad/s) of W(omega). The coefficient Aj,, is the wavelength of the

one diode rated at a larger value of wavelength than A₈, which is to say that its frequency (angular velocity) is lower. From a quantum viewpoint, it emits photons with less photon energy in accordance with Plank's Law. The term A_m represents the middle wavelength (1 /frequency), which is not used in the equation for solving the differential bandwidth

Δ_o but rather used, since the resultant staggered bandwidth is flat over the majority of the resultant region, as a midway value representing a third Q ( = 10) value for peak intensity level of photon emission of diode (m). The equation for finding the resultant value of Q and photon spectrum at the half power points (-3dB) is:

Δo = λ_b - λ_a the differential bandwidth at the -3dB (half power) points.

Using this exampled value, which is an actual value used in the present invention; A₃ = 395 nanometers in wavelength.

Further, this exampled value, which is an actual value used in the present invention; A_b = 450 nanometers in wavelength. The middle exampled value, which is an actual value

used in the present invention; Am = 422 nanometers in wavelength. For practical purposes, the actual off the shelf available wavelength may not be produced but the nearest 3^rd UV-LED value of wavelength is to be considered.

The staggered Gaussian distribution of photon emission can be realized in Figure 3. The circular array of ultraviolet light emitting diodes in will have an overall photon emission pattern and spectral effect upon the thin filmed coating of high energy and high efficiency phosphor. The current fluorescent tube lights of various lengths can be replaced directly with a UV-LED equivalent of similar size and shape. The advantages are not only realized in watt-hour savings, but unlike present fluorescent lights which use toxic mercury (which is vaporized during operation) and the phosphor is coated on a glass tube, UV-LED lights according to the invention do not need any mercury, nor do they need to be constructed out of breakable glass. Rather polycarbonate can be used which can withstand tremendous punishment without any damage or breakage.

The UV-LED light can have a plurality of arrayed ultraviolet light emitting diodes as shown in Figures 4, 5 and 6. A single linear array of UV-LEDs, as shown in Figure 4, can be utilized as the internal source of ultraviolet energy within a polycarbonate tube, covered with a uniform thin film coating of high efficiency phosphor throughout its inner Gaussian surface. Having an enclosure along with electrodes that are the same as conventional fluorescent light tubes of various tubular shapes, the light can be retrofitted as a direct replacement for the current fluorescent lights. In addition it requires no mercury/mercury vapor for its operation, and needs no hazardous glass tubes. The phosphor may be thermally encapsulated within the polycarbonate plastic during the process of an injection molding process.

For applications that require larger light output, such as for commercial lighting sign systems, hospital operating room applications or any replacement for track type lighting systems (either incandescent or Halogen), and light tiles that can be installed on walls, ceilings, floors, etc.; multi-arrayed UV-LED banks, as shown in Figure 5 can be used as direct replacement or "original idea" applications.

A linear array, linear multi-array, or a disk array, as shown in the image of Figure 6, can be used for specialty lighting systems that are categorized as non-conventional or architectural design configurations.

A lighting system can utilize ultraviolet light emitting diodes in a tubular array as shown in the computer rendering of Figure 7. In Figure 7 a circular cascaded linear array of ultraviolet light emitting diodes are connected in circular series and linear paralleled configuration (as illustrated). The grey regions are electrically conductive tinned copper strips (no-lead, RoHS compliant solder).

Figure 8 is a cross section of the octagonal UV-LED array, which is connected in series with a current limiting resistor (not shown). By design, the series of eight UV-LEDs are connected in parallel and consequently the complete array is connected to a voltage down converter that produces a DC voltage from an AC mains source. The combination of UV wavelength can be varied and stagger tuned for colour control from warm to cool "white light."

This UV-LED array of Figure 7 and Figure 8 will be placed in a polycarbonate hollow cylinder that has embedded within its composition; a high efficiency phosphor compound that will emit a broad band of "white light" when excited by ultraviolet light from the diodes. Through secondary quantum photon emission, this produces a color range of white light from 3,500 to 4,100 nanometers in wavelength, producing a range of color temperature from warm to cool light, depending on application. The substrate upon which the UV-LEDs are mounted in Figures 7 and 8 can be a flat substrate like a flexible circuit board material, having crease lines. The substrate can then be rolled up to form the shape shown in these or other Figures.

A typical "commercial direct replacement" for the fluorescent tube light is shown by a computer rendering in Figure 9, which shows another example of a light which can be produced in any size or shape as any current fluorescent tube type configuration. A vertical array as shown in the computer rendering of Figure 10 is also a possible alternative configuration for vertical insertion into the hollow bulb structure. There is another configuration of using a single circular series array of UV-LEDs as shown in Figure 11. These figures generally show a central LED structure with a plurality of LEDs mounted on a post, a reflector below the LED structure, and the LED structure mounted in a base like that for a screw-in incandescent bulb. The base has a power supply for changing the 110 VAC input power to a lower level of DC power for driving the LEDS. The device has an outer shell which may be polycarbonate, either clear or semi-opaque. The semi-opaqueness may be due to wavelength shifting material on or in the shell material, from other coated or embedded material, or from having a textured surface during formation of the shell or from an etching or like process to produce a frosted appearance. An inner shell may be provided which can have any of the characteristics or features described above for the outer shell. The wavelength shifting material may be a coating on the inside or outside surface of one or both of the inner and outer shell, embedded into the shell material, or in any other way known to those skilled in the art.

One may use "quantum based spheres," which can be used as "quantum secondary emission" light sources can be used. One may encapsulate semiconductor quantum spheres (nanoparticles approximately one billionth of a meter in size) and engineering their surfaces so they efficiently emit visible light when excited by near-ultraviolet (UV) light-emitting diodes (LEDs). The quantum dots strongly absorb light in the near UV range and re-emit visible light that has its color determined by both their size and surface chemistry.

LEDs for solid-state lighting typically emit in the near UV to the blue part of the spectrum, around 380-450 nanometers. Conventional phosphors used in fluorescent lighting are not ideal for solid state lighting. One may encapsulate the phosphor, using fine powders or a quantum nano-sphere approach.

The nanometer-size quantum spheres are synthesized in solvent containing soap-like molecules called surfactants as stabilizers. The small size of the quantum dots, being much smaller than the wavelength of visible light, eliminates all light scattering and the associated optical losses. Optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50 percent.

Nanophosphors based upon quantum spheres have two significant advantages over the use of conventional bulk phosphor powders. First, while the optical properties of conventional bulk phosphor powders are determined solely by the phosphor's chemical composition, in quantum spheres the optical properties such as light absorbance are determined by the size of the sphere. Changing the size produces dramatic changes in color. The small sphere size also means that, typically, over 70 percent of the atoms are at surface sites so that chemical changes at these sites allow tuning of the light-emitting properties of the spheres, permitting the emission of multiple colors from a single size sphere. This provides two additional ways to tune the optical properties in addition to chemical composition of the quantum sphere material itself. For the quantum spheres to be used for lighting, they need to be encapsulated, usually in epoxy or silicone. One should not alter the surface chemistry of the quantum spheres in transition from solvent to encapsulant. When altering the environment of the spheres from a solvent to an encapsulant, the quantum spheres could potentially "clump up" or agglomerate, causing the spheres to lose their light-emitting properties. By attaching the quantum spheres to the "backbone" of the encapsulating polymer they are close, but not touching. This allows for an increase in efficiency from 10-20 percent to 60 percent.

Quantum dot phosphors can be made from materials such as; nontoxic nanosize silicon or germanium semiconductors with light-emitting ions like manganese on the quantum sphere surface. Silicon, which is abundant, cheap, and non-toxic, is an ideal an ideal material to be considered. The quantum spheres can be fabricated easily at very low production cost.

Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and luminescence spectra (or fluorescence) of the same electronic transition. When a molecule or atom absorbs light, it enters an excited electronic state. The Stokes shift occurs because the molecule loses a small amount of the absorbed energy before re-releasing the rest of the energy as luminescence or fluorescence (the so-called Stokes fluorescence), depending on the time between the absorption and the reemission. This energy is often lost as thermal energy.

Stokes fluorescence is the reemission of longer wavelength (lower frequency) photons (energy) by a molecule that has absorbed photons of shorter wavelengths (higher frequency). Both absorption and radiation (emission) of energy are unique characteristics of a particular molecule (structure) during the fluorescence process. Light is absorbed by molecules in about 10^~15 seconds which causes electrons to become excited to a higher electronic state. The electrons remain in the excited state for about 10^'8 seconds then, assuming all of the excess energy is not lost by collisions with other molecules, the electron returns to the ground state. Energy is emitted during the electrons' return to their ground state. Emitted light always has a longer wavelength than the absorbed light due to limited energy loss by the molecule prior to emission.

A quantum sphere or rod is a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), due to the presence of an interface between different semiconductor materials (e.g. in the case of self-assembled quantum dots), due to the presence of the semiconductor surface (e.g. in the case of a semiconductor nanocrystal), or due to a combination of these. A quantum dot or rod has a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a small finite number (of the order of 1-100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.

Small quantum dots as well as quantum rods, such as colloidal semiconductor nanocrystals, can be as small as 2 to 10 nanometers or 20-100 for rods, corresponding to

10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the quantum dot volume. Self-assembled quantum dots are typically between 10 and 50 nanometers in size. Quantum dots defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100 nanometers. At 10 nanometers in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb.

Quantum dots can be contrasted to quantum wires, which confine the motion of electrons or holes in two spatial directions and allow free propagation in the third, and quantum wells, which confine the motion of electrons or holes in one direction and allow free propagation in two directions. Quantum dots containing electrons have a discrete energy spectrum and bind a small number of electrons. In contrast to atoms, the confinement potential in quantum dots does not necessarily show spherical symmetry. In addition, the confined electrons do not move in free space, but in the semiconductor host crystal. The quantum dot host material, in particular its band structure, does therefore play an important role for all quantum dot properties. Typical energy scales, for example, are of the order of ten electron volts in atoms, but only 1 millielectron volt in quantum dots. Quantum dots with a nearly spherical symmetry, or flat quantum dots with nearly cylindrical symmetry can show shell filling according to the equivalent of Hund's rules for atoms. Such dots are sometimes called "artificial atoms". In contrast to atoms, the energy spectrum of a quantum dot can be engineered by controlling the geometrical size, shape, and the strength of the confinement potential. Also in contrast to atoms it is relatively easy to connect quantum dots by tunnel barriers to conducting leads, which allows the application of the techniques of tunneling spectroscopy for their investigation.

Like in atoms, the energy levels of small quantum dots can be probed by optical spectroscopy techniques. In quantum dots that confine electrons and holes, the inter-band absorption edge is blue shifted due to the confinement compared to the bulk material of the host semiconductor material. As a consequence, quantum dots of the same material, but with different sizes, can emit light of different colors.

Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. In electronic applications they have been proven to operate like a single-electron transistor and show the Coulomb blockade effect. Quantum dots have also been suggested as implementations of qubits for quantum information processing.

One of the optical features of small excitonic quantum dots immediately noticeable to the unaided eye is coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, more significant in terms of coloration is the size. The case is the larger the dot, the redder (the more towards the red end of the spectrum) the fluorescence. The smaller the dot, the bluer (the more towards the blue end) it is. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the bandgap energy that determines the energy (and hence color) of the fluoresced light is inversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energy levels which are more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e. those closer to the red end of the spectrum. Recent articles in nanotechnology and other journals have begun to suggest that the shape of the quantum dot may well also be a factor in the colorization, but as yet not enough information has become available.

The ability to tune the size of quantum dots is advantageous for many applications. For instance, larger quantum dots, have spectra shifted towards the red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely the smaller particles allow one to take advantage of quantum properties.

It should be understood that the wavelength shifting medium can be any single or any combination of two or more of the materials disclosed herein, such as phosphor and quantum dots. The medium may also include flakes as reflectors. A thin layer of gold would provide a warm glow or color whether the light was powered on or not. The flakes could be any desired size, including down to nanometer size.

A photon energy conversion device may be in the form of a first electrode layer being generally transmissive to photon energy, a second electrode layer, and a layer of photon energy conversion material in the form of quantum dots disposed between the first layer and second layer. The electrode materials may include metals, such as copper, gold and/or aluminum.

While several embodiments have been disclosed, the invention is not limited to these embodiments and is defined only by way of the following claims.

Claims

I Claim:

1. A light source comprising: a photon emission source in the wavelength range of ultraviolet to near ultraviolet (less than about 350 run); and a wavelength shifting medium which receives emitted photons and emits lower frequency energy in the wavelength range of about 400 - 700 nm, and in a substantially uniform, unfocussed, omni directional radiation pattern.

2. The light source according to claim 1, wherein said wavelength shifting medium comprises quantum material embedded within an oxygen free encapsulation.

3. The light source according to claim 2, wherein the quantum material comprises at least one of quantum dots and quantum rods.

4. The light source of claim 1, where the wavelength shifting medium comprises high luminosity phosphor material.

5. The light source of claim 4, wherein the phosphor material is embedded within an oxygen free encapsulation.

6. The light source of claim 1, wherein the photon emission source and wavelength shifting medium are mounted on a flexible substrate.

7. The light source of claim 1, wherein the wavelength shifting medium comprises quantum material and phosphor material.

8. The light source of claim 3, wherein the quantum material is formed in at least one uniform layer.

9. The light source of claim 8, wherein the quantum material is formed in a plurality of layers.

10. The light source of claim 1, wherein the wavelength shifting material is spaced from the photon emission source.

11. The light source of claim 1, wherein the photon emission source comprises at least one light emitting diode (LED).

12. The light source of claim 11, wherein the photon emission source comprises a plurality of light emitting diodes (LEDs)

13. The light source of claim 12, wherein at least some of the LEDs are connected in series.

14. The light source of claim 12, wherein at least some of the LEDs are connected in parallel.

15. The light source of claim 1, wherein the light source is in the shape of a standard incandescent bulb adapted to be used in place of a standard in candescent bulb.

16. The light source of claim 15, wherein the light source is in comprises a bulb shell of the type used for incandescent light bulbs, wherein the photon emission source is disposed near the based inside the shell and herein the wavelength shifting medium is disposed as part of the bulb shell.

17. The light source of claim 15, wherein, the bulb has a base consisting of one of a screw shape or a bayonet shape.

18. The light source of claim 1, wherein the wavelength shifting medium comprises a coating on the bulb shell.

19. The light source of claim 1, wherein the wavelength shifting medium is integral with the bulb shell.

20. The light source of claim 1, wherein the light source is in the shape of a fluorescent light bulb having a shape selected from the group consisting of a tube, a circle and a compact florescent light (CFL) spiral.

21. The light source of claim 1, wherein the wavelength shifting medium emits light which appears to be white light to a human.

22. The light source of claim 1, wherein the wavelength shifting medium emits light which has a selected wavelength corresponding to a single color.

23. The light source of claim 1, further including a reflector.

24. The light source of claim 20, wherein the light source is in the shape of a tube, and wherein the photon emission source is a plurality of LEDs arranged in circular fashion throughout the length of the tube, and wherein the wavelength shifting material comprises a polycarbonate tube with embedded phosphor compound.

25. The light source of claim 16 wherein the light source further comprises a mirror reflector encircling a circuit board at the base of the bulb for carrying a plurality of LEDs, a mirror reflector at the base of the bulb encircling the circuit board, and a substantially uniform phosphor coating on substantially the entire surface of the bulb.

26. The light source of claim 1, wherein the wavelength shifting material comprises rare earth ions doped into solid host materials to provide light emissions in the wavelength range of about 400-700 run.

27. The light source of claim 26, wherein the wavelength shifting material comprises Tm ³⁺ and Dy ³⁸.

28. The light source of claim 1, wherein photon emission source comprises at least one LED formed from one of AlGaP and InGaN.

29. The light source of claim 1, wherein the wavelength shifting material comprises nanophosphors.

30. A photon energy conversion device in the form of a first electrode layer being generally transmissive to photon energy, a second electrode layer, and a layer of photon energy conversion material in the form of quantum dots disposed between the first layer and second layer.