Solid-state lighting (SSL) is a pivotal emerging technology that promises to fundamentally alter and improve lighting systems of the future. Successful development and commercialization of SSL technology will require coordinated efforts that leverage the strengths and capabilities of industry, research and academic organizations, national laboratories, and government. This paper discusses the U.S. Department of Energy's role as a catalyst in accelerating SSL technology advances. Through DOE's SSL R&D program, the collaborative efforts of our nation's best and brightest lighting experts are moving this promising technology from the laboratory to the marketplace.

“The light for the 21st century” METI national (Akari) project, which is based on the high-efficient white light-emitting diode (LED) lighting technologies using near ultraviolet (UV) LED and phosphor system, has started from 1998 and its first phase programme finished in 2004. The near UV white LED system linked with semiconductor technologies on GaN LED and compound phosphors for the general lighting applications has for the first time been proposed in the world. In particular, we have demonstrated high-efficient near UV LED having external quantum efficiencies more than 43% around an emission wavelength of 400 nm. Basic illumination properties, and applications of the high luminous efficacy (>40 lm/W) and the high general color rendering index (R_a>90) white LED sources are described. The near UV white LED technologies in conjunction with phosphor blends can offer superior color uniformity, high R_a and the excellent light quality, and the second phase programme has been performed in the MEXT national project “White LEDs for medical applications” from 2004 to 2009 for 5 years.

Since the introduction of the Americans With Disabilities Act in 1990, the number of visual fire alarm signals installed in the United States has grown exponentially. Virtually all of these fire alarm visual signals consist of the Xenon gas flashtube type. This technology offers high intensity along with moderate cost in a relatively small package. Typical intensities offered range from 15cd (candela) up to 185cd. With the recent advances in solid state LEDs (Light Emitting Diodes) the possibility exists to develop visual fire signals using this technology. When used in lower intensity visual appliances, LEDs offer comparable light output with much smaller optical footprints (albeit at somewhat higher estimated costs). This paper will examine the optical performance of a prototype LED visual fire signaling appliance as compared to a more conventional device. It will also evaluate a series of tests, which were run in an office environment to compare the response time of workers for both the conventional Xenon fire signal appliance as well as the prototype LED device. Measurements for each test subject were taken over a two to three day period. Parameters measured included time of day, size of office and general ambient lighting. Results of these experiments indicate that the general response times of the test subjects were similar for the two types of fire signals. The paper concludes with a discussion of the potential for LED-type fire signaling devices as well as some of the potential technology obstacles still to be overcome.

Deep ultraviolet light emitting diodes (LEDs) with emission wavelengths shorter than 300 nm have been grown by metalorganic vapor phase epitaxy. A bottom emitting LED design is used which requires a high-Al content Al_xGa_1-xN (x = 0.5 - 0.8 ) buffer layer which has sufficient conductivity and is transparent to the quantum well emission wavelength. LEDs were flip chip mounted to a silicon submount which provides for good thermal performance as well as improved light extraction. For large area 1 mm x 1 mm LEDs emitting at 297 nm, an output power as high as 2.25 mW under direct current operation has been demonstrated at 500 mA with a forward voltage of 12.5 volts. For shorter wavelength LEDs emitting at 276 nm, an output power as high as 1.3 mW has been demonstrated under direct current operation at 300 mA with a forward voltage of 9.2 volts. Recent improvements in heterostructure design have resulted in quantum well emission at 276 nm with a peak intensity that is 330 times stronger than the largest sub-bandgap peak. LEDs with emission wavelengths as short as 237 nm have also been demonstrated.

Blue and near-ultraviolet (UV) InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) were grown on GaN and sapphire substrates using metalorganic chemical vapor deposition. The homoepitaxial LEDs exhibited greatly improved microstructural and electrical properties compared to the devices grown on sapphire. As a result of defect reduction, the reverse-bias leakage current was reduced by more than six orders of magnitude. At forward bias, thermally activated current rather than carrier tunneling was dominant in the LEDs on GaN. The improvement of optical characteristics was found to be a strong function of In content in the active region. At low and intermediate injection levels, the internal quantum efficiency of the UV LED on GaN was much higher compared to that on sapphire, whereas the performance of the blue LEDs was found to be comparable. At high injection currents, both the blue and UV LEDs on GaN greatly outperformed their counterparts on sapphire. The homoepitaxial LEDs with a vertical geometry had a much smaller series resistance and were capable of operating at 600 A/cm² in cw mode due to uniform current spreading and efficient heat dissipation.

Mg doped Al-rich AlGaN epilayers with Al content as high as 0.7 is needed for obtaining deep UV LEDs with wavelengths shorter than 300 nm. This is one of the most crucial layers in deep UV LEDs and plays an important role for electron blocking and affects the hole injection into the active layer. Not only is this layer critical for the efficiency of deep UV LEDs, it could also introduce long wavelength emission components in UV LEDs. However, it is difficult to obtain high quality Mg doped Al-rich AlGaN epilayers and the resistivity of the grown films is usually extremely high. We report here on the growth, optical and electrical properties of Mg doped Al_0.7Ga_0.3N epilayers. Mg doped Al_0.7Ga_0.3N epilayers of high crystalline and optical qualities have been achieved after optimizing MOCVD growth conditions. Moreover, we have obtained a resistivity around 12,000 Ω cm (near the theoretical limit with Mg doping) at room temperature and confirmed p-type conduction at elevated temperatures for optimized Mg-doped Al_0.7Ga_0.3N epilayers. The growth conditions of the optimized epilayer have been incorporated into deep UV LEDs with wavelength shorter than 300 nm. A significant enhancement in power output with a reduction in forward voltage, V_f, was obtained by employing this optimized Mg doped Al_0.7Ga_0.3N epilayer as an electron blocking layer. The long wavelength emission components in deep UV LEDs were also significantly suppressed. The fundamental limit for achieving p-type Al-rich AlGaN alloys is also discussed.

We report on high output power from the quaternary AlGaInN multiple quantum well (MQW) ultraviolet light emitting diodes (UV LEDs) in the 340 nm and 280 nm wavelength range. The output power up to 1.5 mW from a 100 μm diameter device with bare-chip configuration was measured under room temperature cw operation. The internal quantum efficiency was estimated to be between 7 and 10%. In addition, the output power and external quantum efficiency for fully packaged 1x1mm² large area device were as high as 54.6 mW and 1.45%, respectively, at the injection current of 200 A/cm² under pulsed operation. The devices were incorporated into prototype system for fluorescence based bio-sensing. We also report the performance of 285 nm UV LEDs.

Various new light-emitting diodes (LEDs) including white LEDs are being actively developed for solid-state lighting and many other applications, and there are great needs for accurate measurement of various optical quantities of LEDs. Traditional lamp standards do not suffice for specific measurement needs for LEDs. The National Institute of Standards and Technology (NIST) has recently established calibration services for photometric quantities (luminous intensity and luminous flux) of LEDs, but the measurement needs are expanding. This paper covers the current capabilities and services NIST provides for calibration of LEDs and discusses the future needs for optical metrology of LEDs. Work is just completed at NIST to provide official color calibrations of LEDs (chromaticity coordinates, peak wavelength, correlated color temperature, etc.). Another urgent need addressed is radiometric calibration of LEDs, particularly the total radiant flux (watt) of ultraviolet (UV) LEDs used to excite phosphors for white LEDs. Also, as spectroradiometers coupled with an integrating sphere are increasingly used total spectral radiant flux standards from NIST are in urgent demand. Presented is the scope of NIST plans to realize these new radiometric calibration capabilities for LEDs in the near future.

Current white LED color has a wide range of CCT and varying distance to Planckian Loci, which cause different tint in white color. This color variation keeps a number of illumination and indication applications away from LEDs. This article introduces a passive method, adding correction filters, to correct the color and achieve visual equivalence of the LED white light. Efficiency of using correction filters is discussed.

White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multi-chip and phosphor-type white LED models were analyzed by simulation on their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting. Color rendering characteristics were evaluated based on the CIE Color Rendering Index (CRI), using not only R_a but also the special color rendering indices R_i as well as the CIELAB color difference ΔE*_ab for the 14 color samples defined in CIE 13.3. Several models of 3-chip and 4-chip white LEDs as well as phosphor-type LEDs are optimized for various parameters, and some guidance is given for designing these white LEDs. The simulation analysis also demonstrated several problems with the current CIE Color Rendering Index (CRI), and the need for improvements is discussed.

The emission characteristics of different LEDs are measured using an optical Fourier transform instrument originally designed for flat panel display characterization. In this approach all the light coming from the device is collected by a dedicated optics and imaged on a CCD sensor. The angular aperture of the instrument extends up to ±88° and all the azimuths for 0 to 360° are collected at the same time. The angular resolution is better than 0.5°. The luminous intensity and the color are measured rapidly and accurately. A large measurement spot size (~2mm) ensures to get all the emission of the device at the same time. It avoids any error and alignment requirements are reduced. In addition to get the luminous intensity distribution very accurately, we have noticed in some cases a dependence of the spectral emission with the angles. This dependence depends on the working conditions of the devices and in particular of the current injected in the LED. Polarization analysis of light emitted by different laser diodes is also presented.

In this paper, based on the Monte-Carlo ray tracing simulation we present the study of the light extraction efficiency of GaN-based LEDs as a function of the position of the light source over the active layer with several parameters, including chip dimensions, absorption coefficients and package. Besides, the light extraction efficient characteristic of a ThinGaN LED is studied.

Two life tests were conducted to compare the effects of drive current and ambient temperature on the degradation rate of 5 mm and high-flux white LEDs. Tests of 5 mm white LED arrays showed that junction temperature increases produced by drive current had a greater effect on the rate of light output degradation than junction temperature increases from ambient heat. A preliminary test of high-flux white LEDs showed the opposite effect, with junction temperature increases from ambient heat leading to a faster depreciation. However, a second life test is necessary to verify this finding. The dissimilarity in temperature effect among 5 mm and high-flux LEDs is likely caused by packaging differences between the two device types.

The common approaches to solid state white lighting include phosphor conversion of blue or UV sources and combining red, green, and blue LEDs. However these methods only begin to address the reproduction of the complex, time-varying spectra humans call light. This paper will suggest a massive primary approach to solid state lighting and communication. LEDs allow an economic means to divide up the visible spectrum into many individually controllable slices, which can then be used in harmony to reproduce the desired illumination. It will also be suggested that the evolution and development of the radio frequency spectrum and high fidelity audio can provide guidance to exploiting the visible spectrum.

We propose and demonstrate the use of Engineered Diffusers for control and distribution of LED light for general lighting applications. These diffusers are based on refractive microstructures and enable the efficient use of energy by controlling light propagation and directing it to specific regions of space. The microstructures are generally microlens-based arrays with each microlens elements individually designed to meet the desired scatter properties. In addition to light control, Engineered Diffusers can be used for RGB mixing to produce white light with variable color temperature, depending on the RGB content of the source. A single Engineered Diffuser component can be used for efficient color mixing and illumination control. We also discuss the fabrication of Engineered Diffusers by means of a single-point laserwriting method with capability to manufacture the deep refractive structures needed for LED beam shaping.

Solid state lighting devices have made their way into a number of niche markets and continue to make inroads into other markets as their price / performance ratios improve. One of these markets is aviation lighting. Although this paper will focus on the use of LEDs for aircraft position lights, much of the discussion is applicable to other installations on the interior and exterior of the aircraft. The color, light distribution and intensity levels for a position light are all closely regulated through Code of Federal Regulation (CFR; formerly Federal Aviation Regulation (FAR)) documents. These lighting requirements, along with harsh thermal and environmental requirements, drive the design. In this paper, we will look at these requirements and discuss what is required in order to use LEDs for this type of application. We will explore the optical, thermal and electrical issues associated with the use of LEDs for position lights and examine the specific case study of the Astreon forward position lights. Finally, we will discuss some of the challenges that we see with solid state lighting in current and future aircraft applications.

A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and being developed at NIST. The source is designed to have a capability of producing any spectral distributions mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as photometric and colorimetric standard. The use of the tunable source mimicking spectra of display colors, for example, rather than a traditional incandescent standard lamp for calibration of colorimeters, can reduce the spectral mismatch errors of the colorimeter measuring displays significantly. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of colorimeters. The results indicate that the errors can be reduced by an order of magnitude compared with those when the colorimeters are calibrated against Illuminant A. Stray light errors of a spectroradiometer can also be effectively reduced by using the tunable source producing a blackbody spectrum at higher temperature (e.g., 9000 K). The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.

We report on high-power solid-state lighting facility for cultivation of greenhouse vegetables and on the results of the study of control of photosynthetic activity and growth morphology of radish and lettuce imposed by variation of the spectral composition of illumination. Experimental lighting modules (useful area of 0.22 m²) were designed based on 4 types of high-power light-emitting diodes (LEDs) with emission peaked in red at the wavelengths of 660 nm and 640 nm (predominantly absorbed by chlorophyll a and b for photosynthesis, respectively), in blue at 455 nm (phototropic function), and in far-red at 735 nm (important for photomorphology). Morphological characteristics, chlorophyll and phytohormone concentrations in radish and lettuce grown in phytotron chambers under lighting with different spectral composition of the LED-based illuminator and under illumination by high pressure sodium lamps with an equivalent photosynthetic photon flux density were compared. A well-balanced solid-state lighting was found to enhance production of green mass and to ensure healthy morphogenesis of plants compared to those grown using conventional lighting. We observed that the plant morphology and concentrations of morphologically active phytohormones is strongly affected by the spectral composition of light in the red region. Commercial application of the LED-based illumination for large-scale plant cultivation is discussed. This technology is favorable from the point of view of energy consumption, controllable growth, and food safety but is hindered by high cost of the LEDs. Large scale manufacturing of high-power red AlInGaP-based LEDs emitting at 650 nm and a further decrease of the photon price for the LEDs emitting in the vicinity of the absorption peak of chlorophylls have to be achieved to promote horticulture applications.

An AlGaN Light-emitting diode (LED) emitting with a peak wavelength at 291 nm and a radiant power of 0.5 mW @ 100 mA was fabricated on a sapphire substrate. A compact gated fluorescence detection system was built using this LED as the excitation light source. We demonstrate that it provides sufficient power using Terbium enhanced fluorescence to detect subnanomolar concentrations of dipicolinic acid (DPA, 2, 6-pyridinedicarboxylic acid), a substance uniquely present in bacterial spores such as that from B. anthracis, providing a basis for convenient early warning detectors. We also describe initial results from a novel approach for biological aerosol detection using long lived fluorescence from a Europium tagged dye that binds to proteins.

As chip size and power levels continue to increase, thermal management, thermal stresses and cost have become key LED packaging issues. Until recently, low-coefficient-of-thermal-expansion (CTE) materials, which are needed to minimize thermal stresses, had thermal conductivities that are no better than those of aluminum alloys, about 200 W/m-K. Copper, which has a higher thermal conductivity (400 W/m-K), also has a high CTE, which can cause severe thermal stresses. We now have over a dozen low-CTE materials with thermal conductivities ranging between 400 and 1700 W/m-K, and almost a score with thermal conductivities at least 50% greater than that of aluminum. Some of these materials are low cost. Others have the potential to be low cost in high volume production. Emphasizing low cost, this paper reviews traditional packaging materials and the six categories of advanced materials: polymer matrix-, metal matrix-, ceramic matrix-, and carbon matrix composites; monolithic carbonaceous materials; and metal-metal composites/alloys. Topics include properties, status, applications, cost and likely future directions of new advanced materials, including carbon nanotubes and inexpensive graphite nanoplatelets.

In this research, experiments and optical simulations have been carried out to study the effect of bevelled sidewalls and geometric shapes on the light extraction efficiency of GaN LED chips. Besides the conventional rectangular chips, hexagonal LED chips were experimentally processed for the fist time on a novel island-like GaN substrate. The bevelled sidewalls could be naturally formed on the chips during the growth of GaN islands by HVPE technology. The results of simulations and experiments are consistent with each other, and show that the output power of LED will be improved doubly when the sidewalls were beveled on the chip. The light output from hexagonal LED chips is also proved better than that from conventional rectangular chips.

The efficiency and reliability of the solid-state lighting devices strongly depend on successful thermal management. Light emitting diodes, LEDs, are a strong candidate for the next generation, general illumination applications. LEDs are making great strides in terms of lumen performance and reliability, however the barrier to widespread use in general illumination still remains the cost or $/Lumen. LED packaging designers are pushing the LED performance to its limits. This is resulting in increased drive currents, and thus the need for lower thermal resistance packaging designs. As the power density continues to rise, the integrity of the package electrical and thermal interconnect becomes extremely important. Experimental results with high brightness LED packages show that chip attachment defects can cause significant thermal gradients across the LED chips leading to premature failures. A numerical study was also carried out with parametric models to understand the chip active layer temperature profile variation due to the bump defects. Finite element techniques were utilized to evaluate the effects of localized hot spots at the chip active layer. The importance of “zero defects” in one of the more popular interconnect schemes; the “epi down” soldered flip chip configuration is investigated and demonstrated.

In this paper we will present our recent work aimed at developing deep ultraviolet light-emitting diodes with emission from 250-280 nm. These devices were fabricated using AlGaN multiple quantum wells that were deposited on basal plane sapphire substrates using low-pressure MOCVD. Innovative MEMOCVD grown AlN buffer layers and AlN/AlGaN superalttices were also employed in the device structures to manage strain and allow the deposition of thick AlGaN layers, which was necessary to reduce the lateral spread resistance. Devices with square, multifinger and micro-LED geometries were fabricated and flip-chip mounted on AlN carriers for improved thermal management and light extraction. We have now succeeded in obtaining devices at 275 and 280 nm with cw powers in excess of 1.5 mW and pulsed powers well over 20 mW. Recently we have also succeeded in obtaining nearly milliwatt powers using an innovative micro-LED design at 250 nm. Now, for the first time, we also present dc operation of micro-pixel design deep UV LED.

Colour mixing of red, green and blue (RGB) LEDs is demonstrated for a 6 cm long PMMA cylindrical rod with a transparent refractive index matched micro particle (TRIMM) diffuser sheet at the output end. Ray tracing simulations have been performed, and the output light distributions, transmittances and losses modelled and compared with experiment. Photographed and modelled colour mixing results are presented for rods with and without TRIMM sheet mixers. The TRIMM particles homogenize the light output of plain PMMA rods to form white light, with negligible backscattering. A simple method for measuring the concentration of the particles in the diffuser sheet is described, and computer modeling and analysis of TRIMM particle systems is discussed.

We report on the successful nano-fabrication and characterization of III-nitride blue and ultraviolet (UV) photonic crystal light emitting diodes (PC-LEDs) using electron beam lithography and inductively coupled plasma dry etching. Triangular arrays of holes with different diameters/periodicities were etched on the LEDs. Optical measurements on the photonic crystals (PCs) performed using near-field scanning optical microscopy (NSOM) showed a 60° periodic variation with the angle between the propagation direction of emission light and the PCs lattice. Under optical pumping, an unprecedented enhancement factor of 20 in emission light intensity of wavelength 475 nm was achieved at room temperature with emission light parallel to the Γ-K direction of the PCs lattice. Guided by the optical pumping results, new design geometry of LEDs with PCs has been employed to optimize the light extraction. Enhancement in optical power of current injected blue and UV PC-LEDs over conventional LEDs is discussed. It was observed that the optical enhancement factor depends strongly on the PC lattice constant and hole size. The achievement of nitride photonic crystal emitters with enhanced light extraction efficiency is expected to benefit many new applications of III-nitrides including solid-state lighting for general illumination and photonic integrated circuits operating in the visible and UV spectral regions.

In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.

Currently, the highest color rendering index (CRI) value obtained in commercially available LED devices is around 90. This falls short of the CRI values typical for incandescent lamps (defined at 100). Similarly, the commercially available LEDs for higher color temperature have CRI values of 65-85, well below the theoretical maximum of 100. New phosphor blends are proposed for use with LED chips emitting in the 350-450 nm range. The application of such blends can afford CRI values greater than 95, over the entire range of color temperatures of interest for general illumination (2500K - 8000K). In some cases, the CRI values approach the theoretical maximum of 100. LED based lamps with a steady state performance of 23 LPW and 25 lumens per chip at 3000K, with a general CRI (Ra) of 97 and a mean CRI (R1-R14) of 96 are demonstrated.

Nanocrystalline yttrium aluminum garnet doped with Cerium (YAG:Ce³⁺), was synthesized by means of a modified sol-gel method that consists of a mixture of salts in an aqueous media. Structure and morphology were characterized by X-ray diffraction and Transmission Electron Microscopy. Single crystalline phase were obtained and the crystallite size range from 26 nm to 96 nm depending on the annealing temperature (from 800 to 1150 °C, respectively). The photoluminescence dependence on the crystallite size and ion concentration was performed. The experimental results show that the best ion concentration where the highest luminescence was obtained correspond to 0.1 mol% and that increases as the crystallite size increases. The feasibility of the modified sol-gel method for the preparation of nanocrystalline YAG is discussed.

In commercial high brightness phosphor coated (PC) LED packages the phosphor is put down on the die in the center of the hemispherical encapsulation, representing a quasi-point source that provides convenient optical control in lighting fixture design. However, specific applications may benefit from other package geometries and beam shapes regarding efficiency, color uniformity and thermal management. In order to examine optical arrangements the solid model of an InGaN LED die and the optical system including the phosphor were simulated using Monte-Carlo forward ray tracing technique. Photoluminescence was implemented as two separate processes: short wavelength LED emission and phosphor absorbtion was traced first, followed by re-emission of the down-converted radiation by the phosphor layer, optical properties of existing phosphors were used. Output parameters of the two ray traces were combined and evaluated for the geometries examined.

Modern light-emitting diode (LED) technology holds great promise for remote or stand-alone photovoltaic (PV) lighting applications. Acceptable intensities can be obtained for a fraction of the energy consumed by incandescent or fluorescent lighting, resulting in smaller and less costly PV/battery systems. Applying PV technology to solid-state applications seems straightforward at first glance. Yet, all too often, PV-powered products fall short of expectations. There can be many reasons for failure. As often as not, we find that failure results from misunderstanding or ignoring well-established principles of PV system design, or by assuming maintenance is unnecessary because of PV's apparent simplicity. Most of these fatal errors have simple and easily applied solutions. The most common fatal errors are discussed, and approaches are recommended that can help ensure a successfully operating system. The methodology described below is applicable to all sizes of PV power systems, ranging from one needed for a single LED to one capable of supplying many kilowatts.

Although PV (photovoltaic)-powered lighting systems have existed for many years, they are not widely used, especially in lighting for buildings, due to their high initial cost and low conversion efficiency. One of the technical challenges facing PV-powered lighting systems has been how to use the DC power generated by the PV module to energize common light sources that are designed to operate efficiently under AC power. Usually, the efficacy of DC light sources is very poor compared to AC light sources. Rapid developments in LED (light-emitting diode) lighting systems have made this technology a potential candidate for PV-powered lighting systems. This study analyzed the efficiency of each component of PV-powered lighting systems to identify optimum system configurations for different applications.

The direct gap of the In_1-xGa_xN alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells. A number of measurements of the intrinsic properties of InN and In-rich In_1-xGa_xN alloys (0 < x < 0.63) are presented and discussed here. To evaluate the suitability of In_1-xGa_xN as a material for space applications, extensive radiation damage testing with electron, proton, and alpha particle radiation has been performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In_1-xGa_xN retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. Results are evaluated in terms of the positions of the valence and conduction band edges with respect to the average energy level of broken-bond defects (Fermi level stabilization energy E_FS). Measurements of the surface electron concentration as a function of x are also discussed in terms of the relative position of E_FS. The main outstanding challenges in the photovoltaic applications of In_1-xGa_xN alloys, which include developing methods to achieve p-type doping and improving the structural quality of heteroepitaxial films, are also discussed.

As we contemplate a revolution in the lighting industry, it is yet unclear in what form tomorrow's solid-state lighting will emerge. Similarly, photovoltaic (PV) power supplied on a utility scale may take a different form from today's flat-plate silicon modules. The success of the PV industry-now a multibillion dollar a year industry and growing at more than 25% per year-has largely come from integrating solar cells into other products. In many cases, this integration required the formation of new business entities. The solid-state lighting industry faces hurdles that are similar to those faced by the PV industry. Therefore, based on the experiences of the PV industry and others, we predict that the growing pains of the solid-state lighting industry will include: (1) identifying entry markets, (2) integrating light-emitting diodes into attractive products, (3) attaining high reliability for these products, and (4) increasing production of these products, thus lowering costs and opening up new markets. These activities must be implemented, keeping in mind that most consumers do not care about buying “solid-state lighting” and “solar cells.” Rather, they want to buy attractive lighting and inexpensive electricity.

GaInP₂ lattice-matched to GaAs or Ge plays an important role in state-of-the-art III-V multijunction solar cells. The fundamental band gap of constant-composition GaInP₂ can be varied by as much as 100meV in metal-organic chemical vapor deposition (MOCVD) grown material by adjusting growth parameters that affect the degree of Cu-Pt ordering. These changes in the band gap of GaInP₂ due to ordering can be exploited in the design of III-V solar cell devices. In order to accurately model the performance of these devices, accurate values of the optical constants of all layers are required. Previous literature reports of the optical properties of GaInP₂ have primarily focused on highly disordered material or higher energy transitions in ordered material. While it has been noted that ordered GaInP₂ material results in anisotropic optical properties, the bulk optical properties of GaInP₂ as a function of ordering have not been sufficiently recorded in the literature for good optical modeling of III-V solar cell devices. In this paper, we present the dielectric functions for a range of ordered/disordered GaInP₂ measured over the range 0.7-5.0 eV using spectroscopic ellipsometry. Data analysis of generalized ellipsometry data utilizing anisotropic multilayer models allows us to report accurate dielectric functions for both the ordinary and extraordinary optical axes.

We propose a new electrode design for InGaN/Sapphire LED chips. In the new design, the thin p-ohmic metal layer on the top surface of the chip is partitioned into standardized multiple patches. Each patch is then connected to the p-electrode pad by a metal-film type of series resistor whose value is tailored to its own patch such that the current density distribution in the active region under the patch is almost the same, eliminating the severe current crowding phenomenon observed in the conventional design. As a consequence, both the maximum output power achievable from a unit InGaN/sapphire LED chip and the device reliability would be significantly improved.

Lighting based on sources of light composed of colored light-emitting diodes (LEDs) offers versatile control of color and a possibility of trade-off between efficiency and color rendering. However, psychophysical issues related to such polychromatic solid-state sources have to be addressed. In this work, studies of the perception of standard colors under illumination with a quadrichromatic red-amber-green-blue (RAGB) solid-state source were carried out. An RAGB lamp containing primary LEDs with the emission peaks at 638 nm, 594 nm, 523 nm, and 441 nm and optimized for the highest value of the general color rendering index (86 points) was investigated and compared to a tungsten lamp. 40 standard Munsell samples of value 6, chroma /6, and hue incremented by 2.5 were used in the investigation. Changes in the saturation and hue of the Munsell samples illuminated by the RAGB lamp versus tungsten lamp (both with the correlated temperature of 2600 K) were obtained by colorimetric calculation comparisons and by psychophysical experiments on subjective matching of the samples. Subjective differences in hue and subjective color discrimination differences under the tungsten and RAGB lamps were found in the wavelength range of 440-500 nm and 560-580 nm. We attribute these differences to non-optimal peak wavelengths of the primary LEDs and to the narrow-band components of the RAGB spectrum.

We report the fabrication and the characterization of white organic light-emitting diodes that exhibit high efficiency and very stable color coordinates over the wide range of bias voltages. The blue-emitting layer of 1,4-bis(2,2-diphenyl vinyl)benzene (DPVBi) is sandwiched between the red-emitting layers in which red fluorescent dyes of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-8-yl)vinyl]-4H-pyran) (DCM2) are doped into the hole-transporting layer of 4,4’bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (α-NPD) and the electron-transporting layer of tris(8-hydroxyquinoline) aluminum (Alq₃). The device structure is ITO/PEDOT:PSS/α-NPD(50 nm)/α-NPD:DCM2 (5 nm, 0.2 %)/DPVBi(10 nm)/Alq₃:DCM2(5 nm, 0.2 %)/Alq₃(40 nm)/LiF(0.5 nm)/Al. The partial energy transfer from the blue layer to the nearby red layers results in white light emission with the stable color coordinates of (0.36, 0.37). The device shows the luminous efficiency of about 3.6 lm/W at 100 cd/m² and the maximum luminance of 40,650 cd/m² at the bias of 12 V.

Non-line-of-sight (NLOS) ultraviolet (UV) communication appears to be a viable alternative to RF communication for many short-range applications. It exploits both atmospheric scattering and absorption to achieve modest data rates under non line-of-sight (ground-to-ground) and obstructed line-of-sight (foliage penetration) conditions. In this paper, we introduce NLOS optical communication and discuss the advantages of UV over radio (RF) for covert, short-range communication. We then discuss both line-of-sight (LOS) and NLOS measurements performed outdoors in full daylight, and use these measurements to refine a propagation model developed to characterize link performance under various range and background conditions.

We demonstrate efficient (η_p=11±1 lm/W at 1000 cd/m²), bright electrophosphorescent white organic light emitting devices (WOLEDs) employing three dopants in a 9-nm-thick inert host matrix. The emissive layer consists of 2 wt.% iridium (III) bis(2-phenyl quinolyl-N,C^2') acetylacetonate (PQIr), 0.5 wt.% fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) and 20 wt.% bis(4’,6’-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) co-doped into a wide energy gap p-bis(triphenylsilyly)benzene (UGH2) host. Devices were characterized in terms relevant to both display and general lighting applications, and have a peak total power efficiency of 42±4 lm/W at low intensities, falling to 10±1 lm/W at a drive current of 20 mA/cm² (corresponding to 1.4 lm/cm² for an isotropic illumination source). The Commission Internationale de l’Eclairage coordinates shift from (0.43,45) at 0.1 mA/cm² to (0.38,0.45) at 10 mA/cm², and a color rendering index >75 is obtained. Three factors contribute to the high efficiency: thin layers leading to low voltage operation, a high quantum efficiency blue dopant, and efficient confinement of charge and excitons within the emissive region. The highest occupied and lowest unoccupied energy levels of component layers will be discussed to elucidate charge and exciton confinement within the emissive layer. Additionally, we will explain energy transfer between dopants based on photoluminescent transient analysis of triple-doped thin films.

The spectral properties of commercially available LED-based and halogen MR16s were investigated. The measurements taken include TLF (Total Luminous Flux), CCT (Correlated Color Temperature), CRI (Color Rendering Index), angular variation of CCT, and luminous efficacy. The halogen MR16s were used as a baseline for comparison with LED-based MR16s. It is shown at this time that LED-based MR16s are not suitable as a direct replacement for existing alternatives due to high initial cost, low power efficiency, poor CRIs, and undesirable CCTs.