Solid-state Lighting Technology Research Articles

Novel Na(+) doped Alq3 hybrid materials for organic light-emitting diode (OLED) devices and flat panel displays.

Pure and Na(+) -doped Alq3 complexes were synthesized by a simple precipitation method at room temperature, maintaining a stoichiometric ratio. These complexes were characterized by X-ray diffraction, Fourier transform infrared (FTIR), UV/Vis absorption and photoluminescence (PL) spectra. The X-ray diffractogram exhibits well-resolved peaks, revealing the crystalline nature of the synthesized complexes, FTIR confirms the molecular structure and the completion of quinoline ring formation in the metal complex. UV/Vis absorption and PL spectra of sodium-doped Alq3 complexes exhibit high emission intensity in comparison with Alq3 phosphor, proving that when doped in Alq3 , Na(+) enhances PL emission intensity. The excitation spectra of the synthesized complexes lie in the range 242-457 nm when weak shoulders are also considered. Because the sharp excitation peak falls in the blue region of visible radiation, the complexes can be employed for blue chip excitation. The emission wavelength of all the synthesized complexes lies in the bluish green/green region ranging between 485 and 531 nm. The intensity of the emission wavelength was found to be elevated when Na(+) is doped into Alq3 . Because both the excitation and emission wavelengths fall in the visible region of electromagnetic radiation, these phosphors can also be employed to improve the power conversion efficiency of photovoltaic cells by using the solar spectral conversion principle. Thus, the synthesized phosphors can be used as bluish green/green light-emitting phosphors for organic light-emitting diodes, flat panel displays, solid-state lighting technology - a step towards the desire to reduce energy consumption and generate pollution free light.

Discovery of New Nitridosilicate Phosphors for Solid State Lighting by the Single-Particle-Diagnosis Approach

Discovery of novel luminescent materials is of fundamental importance in the advancement of solid state lighting and flat panel display technologies. In this work, we report a single-particle-diagnosis method for the discovery of new phosphors by just characterizing a luminescent crystalline particle as small as 10 μm in diameter. We explored single-particle fluorescence imaging and spectroscopy techniques to evaluate the photoluminescence of a phosphor particle distinguished from a complex powder mixture and applied a high-resolution single-crystal X-ray diffractometer to determine its crystal structure. The approach enabled us to discover two new phosphors in the Ba3N2–Si3N4–AlN ternary system: Ba5Si11Al7N25:Eu2+ and BaSi4Al3N9:Eu2+. Ba5Si11Al7N25:Eu2+ crystallizes in the space group of Pnnm (no. 58) with a = 9.5923(2), b = 21.3991(5), c = 5.8889 (2) Å and Z = 2, while BaSi4Al3N9:Eu2+ in the space group of P21/C (no.14) with a = 5.8465(4), b = 26.7255(18), c = 5.8386(4) Å, β = 118.897° and Z = 4. The single-particle photoluminescence of Ba5Si11Al7N25:Eu2+ shows yellow emission (λem = 568 nm, fwhm = 98 nm) and a quantum efficiency of 36% under the 405 nm excitation. BaSi4Al3N9:Eu2+ shows blue emission (λem = 500 nm, fwhm = 67 nm) upon the 365 nm excitation.

Engineering design and assessment of a demand-sensitive LED streetlighting system

A solid-state lighting technology was demonstrated at a US Navy Research Center in Maryland – where light-emitting diode (LED) luminaires were substituted for existing high pressure sodium (HPS) streetlighting units, and an intelligent lighting control system was deployed. This paper summarizes the prudent engineering design and implementation of the demand-sensitive feature of the LED streetlighting system at Naval Surface Warfare Center (NSWC) Carderock Division that provides an annual average electricity and CO2 savings of 74%. In addition, technical system performance and life-cycle assessment of the HPS and the new LED systems are discussed. Best practices and field experience that can help with other similar smart lighting implementation elsewhere are also summarized.

A flicker-free CMOS LED driver control circuit for visible light communication enabling concurrent data transmission and dimming control

Recent breakthroughs in solid-state lighting technology have opened the door to a variety of applications using light-emitting diodes (LED's) for not only illumination, but also optical wireless co...

(Invited) Thin Film Technologies for Micro/Nano Systems; A Review

Thin film technology is employed since ages in the field of art, household appliances, optics, metallurgy etc. But the invention of solid state electronic devices, more specifically integrated circuits, really boosted thin film technology to a level of atomic precision in a high volume manufacturing environment. Today the mass production of nano electronic devices principally relies on the deposition and patterning on thin films. Consequently the performance of these devices and systems is determined by the relevant properties of patterned thin films.For conventional applications in the microelectronic domain, the function of a thin film is either an insulator or a conductor; the electrical properties of such films are of overriding importance in the application. And because silicon is the preferred substrate material for current and future CMOS technologies, the thin film deposition technologies must be silicon compatible. However, the continuous scaling of CMOS brings conventional thin films beyond physical limitations. Therefore novel materials like high K gate dielectrics and low K intermetallic dielectrics where introduced in the CMOS process flow. Moreover in combination with the electrical parameters also other domains like mechanical stress needs to be controlled to enhance the charge carrier mobility, often complicating the formation of the film stack and subsequent processing. In addition the increasing aspect ratio of downscaled and novel device structures like fin-fet also put constraints on the deposition techniques. Consequently, over time a broad portfolio of thin film deposition technologies and thin films properties has been developed.But the application of thin films is not limited to IC technology only, also the fabrication microelectronic mechanical systems (MEMS), biomedical devices, solid state lighting technology, and novel approaches to micro and nano system integration, all rely (partly) on thin film technologies. This class of non-CMOS technologies will be further referred to as micro/nano systems technology. The device scaling in micro/nano system technology follows different rules compared with CMOS and spans multi domains and multi scales, even more than in CMOS.The broad portfolio of thin film technologies originally developed for CMOS now employed in micro/nano system technology today churns out a large number of different process sequences to yield a broad spectrum of device structures process flows. Novel discoveries in the domain of nano science, biology, the technology push in solid state lighting and the further industrialisation of MEMS and NEMS will broaden this technology portfolio even more. And contrary to a CMOS transistor, the structure and manufacturing process of micro/nano devices is strongly dictated by its application. Hence a micro/nano system does not have a generic structure, and in some cases very specific thin films and corresponding deposition technologies are required.CMOS scaling as we know may end in the not too distant future, while the exploration of micro and specifically nano systems rise from the ranks. Novel 3D devices, heterogeneous integration schemes and micro/nano system will emerge to provide increasing functionality. The expanding application domain of newly developed micro/nano systems will result in new thin film deposition technologies, tools and new requirements. In addition, the diversity in device structures, the multi scale and multi domain applications are a challenging task for the numerical simulation of such systems.In this paper, the major applications of thin film technologies, applied in micro/nano system technology today will be reviewed, the diversity will be addressed and novel structures will be discussed with an outlook into the future.

(Invited) Point Defect Characterization of Group-III Nitrides by Using Monoenergetic Positron Beams

The nitride semiconductor material system has played a dominant role in the current success of the solid-state lighting (SSL) industry. Because the recent progress of the SSL technology has been mainly driven by light-emitting diodes (LEDs) that use GaN and Ga-rich InGaN, many studies have been carried out on those alloys. The potential of InGaN as an SSL material comes from the ability to control band gaps from 0.65 eV to 3.43 eV, which correspond to entire wavelengths from infrared to ultraviolet regions. Because those band gap energies can cover the solar spectrum, InGaN is also recognized as having a high potential as a photovoltaic device material. The external quantum efficiency (EQE) of an InGaN-based LED, however, is known to drop rapidly with increasing InN composition. The EQE decrease is attributed to (i) piezoelectric fields in multiplayer structures which cause a separation between electron and hole wavefunctions, and (ii) the degradation of crystal quality. For InGaN-based tandem solar cells, the degradation of conversion efficiency occurs when InN composition increases. During growth of InGaN layers, the large difference in the lattice constants of InN and GaN cause unusually high strain to be introduced into the layer, and this causes dislocations, stacking faults, misorientations, phase separations, etc. Native point-defects are also known to act as non-radiative recombination centers in group III nitrides. Thus, further knowledge regarding such defects is needed to improve the optical and electrical properties of InGaN-based LEDs and solar cells. Positron annihilation is a powerful technique for evaluating vacancy-type defects in semiconductors [1], and the defects in group-III nitrides have been investigated using this method [2-6]. In the present study, we have used monoenergetic positron beams to probe native vacancies and deformation-induced defects in GaN and InGaN grown by MOCVD, MBE, and HVPE. For InGaN, it was found that vacancy-type defects were introduced with increasing InN composition, and the major defect species was identified as complexes between a cation vacancy and a nitrogen vacancy. The presence of the defects correlated with lattice relaxation of the InGaN layer and the increase in photon emissions from donor-acceptor-pair recombination. The concentration of the defects, however, was found to be suppressed by Mg doping, suggesting that Mg is an excellent suppressor of cation vacancies in InGaN. The introduction mechanism of vacancy-type defects and their interactions with impurities will be discussed. 1. R. Krause-Rehberg and H. S. Leipner, Positron Annihilation in Semiconductors, Solid-State Sciences (Springer-Verlag, Berlin, 1999) vol. 127.2. A. Uedono, S. Ishibashi, T. Ohdaira, and R. Suzuki, J. Crystal Growth 311, 3075 (2009).3. A. Uedono, S. Ishibashi, N. Oshima, and R. Suzuki, Jpn. J. Appl. Phys. 52, 08JJ02 (2013).4. A. Uedono, S. Ishibashi, K. Tenjinbayashi, T. Tsutsui, K. Nakahara, D. Takamizu, and S. F. Chichibu, J. Appl. Phys. 111, 014508 (2012).5. A. Uedono, T. Tsutsui, T. Watanabe, S. Kimura, Y. Zhang, M. Lozac’h, L. W. Sang, S. Ishibashi, and M. Sumiya, J. Appl. Phys. 113, 123502 (2013).6. A. Uedono, I. Yonenaga, T. Watanabe, S. Kimura, N. Oshima, R. Suzuki, S. Ishibashi, and Y. Ohno, J. Appl. Phys. 114, 084506(1-6) (2013).

Solid state lighting review – Potential and challenges in Europe

According to IEA estimates, about 19% of the electricity used in the world is for lighting loads with a slightly smaller fraction used in the European Union (14%). Lighting was the first service offered by electric utilities and still continues to be one of the largest electrical end-uses. Most current lighting technologies can be vastly improved, and therefore lighting loads present a huge potential for electricity savings. Solid State Lighting (SSL) is amongst the most energy-efficient and environmentally friendly lighting technology. SSL has already reached a high efficiency level (over 276 lm/W) at ever-decreasing costs. Additionally the lifetime of LED lamps is several times longer than discharge lamps. This paper presents an overview of the state of the art SSL technology trends. SSL technology is evolving fast, which can bring many advantages to the lighting marketplace. However, there are still some market barriers that are hindering the high cost-effective potential of energy-efficient lighting from being achieved. This paper presents several strategies and recommendations in order to overcome existing barriers and promote a faster penetration of SSL. The estimated savings potential through the application of SSL lighting systems in the European Union (EU) is around 209 TWh, which translates into 77 million tonnes of CO 2 . The economic benefits translate into the equivalent annual electrical output of about 26 large power plants (1000 MW electric). Similar impacts, in terms of percentage savings, can be expected in other parts of the World.

A Review of Solid State White Light Emitting Diode and Its Potentials for Replacing Conventional Lighting Technologies in Developing Countries

Lighting is an indispensable energy end use of man. A significant number people in developing countries live without electric lighting and depend on oil-based lamps. Globally, lighting consumes substantial amount of energy and is a major contributor to green house emissions. Conventional electric lighting systems such as incandescent lamps are highly inefficient and waste energy while the fluorescent/CFL lamps contain toxic chemicals like mercury. Incandescent lamps with short lifespan as well as the fluorescent/CFL lamps pose environmental waste disposal problems (e.g.). The use of renewable energy resource such as solar and wind power systems for example as sources of power for the most efficient and long life lighting source such as high-brightness light emitting diodes (LEDs) (also known as solid state lighting) would reduce global energy consumption for lighting by half, with corresponding reduction in green house emissions. In developing countries solar powered solid state lighting (SPSSL) would ensure access to electric lighting by the disperse population who may not be connected to the national grid in the near feature. In this article we elaborately review the upcoming solid state lighting technology. The physics and principles of operation of LEDs are also reviewed. The impact of this new lighting technology on developing countries in the areas of commerce, education, health and environmental impacts in comparison to conventional lighting technologies is the main thrust of this review.

Photoluminescence, Non-Reduction and Thermal Stability Characteristics of Red Phosphors Ca3La3(1-x)Eu3x(BO3)5 Prepared in Reducing Atmosphere

A series of Ca3La3(1-x)Eu3x(BO3)5 phosphors were prepared via a solid-state reaction under reducing atmosphere. Rietveld refinements were performed by adopting the powder X-ray diffraction data, which indicates the occupations of Eu3+ on both La3+ and Ca2+ sites with a preferred location on the La3+ site over the Ca2+ site. This kind of phosphors could be excited by UV and blue LED in solid-state lighting technology and the CIE chromaticity coordinates for phosphors CLBO: 0.06Eu3+ (λex = 254 nm) and CLBO: 0.12Eu3+ (λex = 393 nm) are (0.6712, 0.3328) and (0.6685, 0.3287), respectively, which are compliant with the national television system committee (NTSC) standard for red chromaticity. The energy transfer rate between the Eu3+-Eu3+ pairs is weak that proved by the results of the decay process and the efficiency of the Eu3+ 5D0→7F2 emission. The mechanism responsible for the non-reduction of Eu3+ was analyzed, which was determined by two factors: 1) no element of the host matrix can be oxidized and 2), cationic vacancies which can be regarded as the electron donor cannot exist. The temperature dependence study shows thermal quenching behavior is attributed to crossover from the 5D0 excited state of the Eu3+ to the charge transfer state band and these phosphors have good thermal stability.

Comparative Evaluation of Light-Emitting Diode Cap Lamps With an Emphasis on Visual Performance in Mesopic Lighting Conditions

Conducted at the National Institute for Occupational Safety and Health's (NIOSH) Office of Mine Safety and Health Research, the experiment described in this paper is part of ongoing mine illumination research designed to explore the benefits of solid-state lighting technologies when applied to the underground mining industry. This experiment involves the comparative evaluation of cap lamps with similar spectral power distributions, focusing on the electrical and battery discharge characteristics, with a secondary objective being the benefits gained through alternative light beam distributions. NIOSH researchers conducted the investigation by comparing three commercially available light-emitting diode cap lamps and an NIOSH prototype cap lamp at varying power settings. Visual performance for the detection of hazards was quantified by recording times of detection for finding rotating targets in the peripheral field of view and objects representing trip and fall hazards on the ground. The NIOSH prototype cap lamp resulted in improvements ranging from 15% to 43% for peripheral motion detection time and 5%-23% for slip, trip, and fall object detection time, respectively, as compared with the referent incandescent cap lamp.

Solid-State Lighting: A System Review

T his article presents the background on the development of solid-state lighting technology, which is gaining popularity as a light source application. This review focuses on the main characteristics of solid-state lighting devices as well as their supply requirements and the effect of temperature on light-emitting diode (LED) performance. LED drivers are designed to achieve the best operation conditions without degrading the longer lifetime that this technology achieves in comparison to other popular light sources. Offline LED drivers include active power factor correction while current control with low ripple is required to supply the LED units or string arrangements. Methods to achieve balanced current sharing on paralleled LED strings and some of the latest contributions on LED drivers are also explained.

Enhanced light-extraction from hierarchical surfaces consisting of p-GaN microdomes and SiO2 nanorods for GaN-based light-emitting diodes

We report an efficient light-extraction scheme employing the hierarchical structure, p-GaN microdomes/SiO2 nanorods (NRs), on GaN light-emitting diodes (LEDs). Compared with the flat LED, the LEDs with hierarchical surfaces exhibits a light-output improvement of 36.8%. The considerable enhancement in light-extraction efficiency is attributed to the multiple tilted surfaces of microdomes and the graded refractive indexes provided by the SiO2 NRs, reducing total internal reflection and Fresnel reflection. The enhanced optical performances are supported by the finite-difference time-domain analysis. Advances in light extraction scheme employing hierarchical structures demonstrated here pave the way to solid-state lighting technology.

Guest Editorial Recent Advances in Solid State Lighting

This Special Issue on Solid State Lighting, comprising the April/May/June 2013 issues of the IEEE/OSA Journal of Display Technology, focuses on the material physics and innovations, device physics and simulations, new device concepts and engineering, and system implementation of high efficiency solid state lighting technologies. The 45 papers of this Special Issue on Solid State Lighting covers both the innovations in inorganic and organic LEDs technologies, as well as the new concepts required for system implementation of the technology. We are very fortunate to have several excellent manuscripts in the key topics areas related to the progress for achieving high internal quantum efficiency and novel approaches for improving light extraction in III-Nitride LEDs, new concept for achieving phosphor-converted white LEDs, and key advances in the important field of organic LEDs. The progress in both organic and inorganic LEDs is very important, and the advances are complementary of one another attributed to the different key applications enabled by these two technologies. This Special Issue on Solid State Lighting covers three separate categories: 1) innovations for addressing the internal quantum efficiency and efficiency-droop in III-Nitride LEDs; 2) novel approaches for addressing light extraction efficiency in III-Nitride LEDs, and enabling circuits and systems for LEDs; and 3) novel approaches for white LEDs, and key advances and review on the material and device concepts in organic LEDs.

Red is the new black: how the colour of urban skyglow varies with cloud cover

The development of street lamps based on solid-state lighting technology is likely to introduce a major change in the colour of urban skyglow (one form of light pollution). We demonstrate the need for long-term monitoring of this trend by reviewing the influences it is likely to have on disparate fields. We describe a prototype detector which is able to monitor these changes, and could be produced at a cost low enough to allow extremely widespread use. Using the detector, we observed the differences in skyglow radiance in red, green and blue channels. We find that clouds increase the radiance of red light by a factor of 17.6, which is much larger than that for blue (7.1). We also find that the gradual decrease in sky radiance observed on clear nights in Berlin appears to be most pronounced at longer wavelengths.

Interference Rejection Using Filter-Based Sensor Array in VLC Systems

With the recent development of solid-state lighting (SSL) technologies, visible light communication (VLC) systems using light-emitting diodes (LEDs) has been a promising technology to complement wireless communication. While LEDs offers advantageous properties such as high brightness, lower power consumption, long lifetime, and short transient time for high transmission rates, few researches have been exploited on the receiver side. Conventionally, photoelectric-diodes are implemented to convert optical signals into electronic signals. Since conventional photoelectric-diodes cannot distinguish inputs of different spectra, using conventional photoelectric-diodes have the disadvantage that the system is vulnerable to interference, and that it is hard to achieve wavelength-division-multiplexing (WDM) for light sources of different wavelengths. In this work, a spectrum sensor array is proposed to be implemented on the receiver side to achieve interference rejection. Due to recent advances in semiconductor technologies, spectrum sensors with different spectral transmission properties can be integrated into a chip-scale sensor-array. By proper design of the weightings for individual spectrum sensor, the effective output signal-to-interference ratio (SIR) can be maximized. Following the concept of coherent multi-antenna communication systems, signal fusion algorithms are presented. Our simulation is conducted based on the specification of a prototype filter-array spectrum sensor from nanoLambda. Simulation results demonstrate that robust interference rejection is possible using the low-cost spectrum sensor array.

Novel red-emitting perovskite-type phosphor CaLa1−xMgM′O6: xEu3+ (M′ = Nb, Ta) for white LED application

Single phase of CaLa1−xMgM′O6: xEu3+ (M′=Nb, Ta; 0<x⩽1) red phosphors was prepared by solid-state reaction method, and their structural and optical properties were also investigated in detail. The excitation spectra of CaLa1−xMgM′O6: xEu3+ revealed two excitation bands at 396 and 466nm, which correspond to the sharp 7F0→5L6 and 7F0→5D2 transitions of Eu3+ and matches well with the two popular emissions from near-UV and blue light-emitting diode (LED) in solid-state lighting technology. The intensity of the red emission for Ca(La0.7Eu0.3)MgTaO6 and Ca(La0.6Eu0.4)MgNbO6 are respectively three and four times higher than that of (Y0.95Eu0.05)2O3 under near-UV light irradiation, while under blue light irradiation are 2 and 2.5 times respectively. The quantum efficiencies of the entitled phosphors excited under 396 and 466nm are also investigated and compared with commercial phosphors Y2O3:Eu3+, Sr2Si5N8:Eu2+ and Y3A5G12:Ce3+. The chromaticity coordinates of Ca(La0.7Eu0.3)MgTaO6 and Ca(La0.6Eu0.4)MgNbO6 (x=0.662, y=0.338) are close to the standard of National Television Standard Committee (NTSC) values (x=0.670, y=0.330).

EPD of Phosphors for Display and Solid State Lighting Technologies

Electrophoretic deposition (EPD) has been used for phosphor screening for a variety of emissive information displays and more recently, for solid state lighting. EPD is well suited to deposit the fine (nanometer to micrometer diameter) phosphor particles needed for high resolution displays. The fundamentals of the EPD process in an isopropanol (IPA) bath have been characterized by the dissociation behavior of nitrate salts in IPA, measurement of the effects of pH and nitrate salt concentration on the zeta potential of the particles, studying of the processing conditions and modeling of the deposition rates. The electrochemical precipitation reactions form an adhesive agent for the particles and the adhesion strength can be enhanced by various methods to meet the requirements of these technologies.

Fabrication of red organic light emitting diodes (OLEDs) using EuxY(1-x)(TTA)3Phen organic complexes for solid-state lighting

The mechanism of energy transfer leading to electroluminescence (EL) of a lanthanide complex, EuxY(1-x)(TTA)3Phen (TTA= thenoyltrifluoro-acetone, phen=1,10-phenanthroline), doped into TPBi(1,3,5-tris(N-Phenyl-benzimidizol-2-yl) benzene host at 15 wt% of host is investigated. With the device structure of anode/hole transport layer/EuxY(1-x)(TTA)3Phen (15%): TPBi/electron transport layer/cathode, maximum luminescence of 185.6 cd/m2 and 44.72 cd/m2 was obtained from device I made of Eu0.4Y0.6(TTA)3Phen and device II made of Eu0.5Y0.5(TTA)3Phen, respectively at 18 volts. Saturated red Eu 3+ emission based on 5 D0→ 7 F2 transition is centered at a wavelength of 612 nm with a full width at half maximum of 5 nm. From the analysis of I-V, J-V-L characteristics and electroluminescent (EL) spectra, we conclude that direct trapping of holes and electrons and subsequent formation of the excitation occur on the dopant, leading to high quantum efficiencies at low current densities. These results show that fabricated OLED devices can successfully emit saturated red light and can be used in applications such as opto-electronic OLED devices, displays and solid-state lighting technology.

Combustion synthesis of multicomponent ceramic phosphors for solid state lighting

About 50% of the world’s need for lighting is provided by artificial lighting. For over the last decade, the world has witnessed rapid shift from conventional Hg-based lighting to LED-based solid state lighting (SSL). SSL technology extensively uses YAG:Ce3+ phosphor for production of white light emitting devices (LEDs). Part of the blue light from the (In,Ga)N LED chip is absorbed by a thin layer of Ce3+-doped YAG and is converted into yellow light. The combination of blue and yellow gives a bright white light source with an overall energy efficiency exceeding that of a compact fluorescent lamp. Several soft chemical routes have been explored for synthesis of YAG but were discarded due to their complex nature, high cost for the industrialization, phase impure materials, etc. In this paper we describe rapid one-step modified combustion synthesis of YAG:Ce3+ and related phosphors carried out at 500°C using a mixed fuel. Photoluminescence spectra of YAG:Ce3+ and LED prepared thereof are comparable with those of commercial phosphors.

Sol–Gel Synthesis of Red-Emitting Phosphor Ca–Sr–Mo–W–O–(Eu3+,La3+) Powders and Luminescence Properties

The object of this study is to synthesize (Ca0.6,Sr0.16)(Mo0.4,W0.6):Eu3+0.08,La3+0.08 by sol–gel technology. The results show that the dominant emission peak of phosphor is 5D0 → 7F2 (616 nm). The International Commission on Illumination (CIE) chromaticity coordinates of the red-emitting phosphor are (0.67, 0.33), which is the NTSC system standard for red chromaticity. Because there are two regions in the excitation spectra of the phosphor, one is assigned from the charge transfer state (CTS) band at about 200–350 nm, and the other is from the intra-4f transitions from 350 to 600 nm. Hence, the phosphors could be strongly excited by near-UV and blue LEDs in solid state lighting technology. X-ray diffraction patterns indicated that red-phosphor powders present a tetragonal structure, I41/a[88](Z = 4) space group.