Energy-efficient Solid-state Lighting Research Articles

Doping-Dependent Optical Response of a Hybrid Transparent Conductive Oxide/Plasmonic Medium.

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate.

Transparent conductive oxide-based architectures for the electrical modulation of the optical response: A spectroscopic ellipsometry study

Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.

White Light Emission from Thin-Film Samples of ZnO Nanocrystals, Eu3+ and Tb3+ Ions Embedded in an SiO2 Matrix.

In this work, a method was developed to determine the concentration of Eu3+ and Tb3+ ions in a thin-film sample of SiO2, co-doped with ZnO-nanocrystals (ZnO-nc), to produce a sample of any desired colour in the International Commission on Illumination (CIE) colour space. Using this method, a white light emitting sample was fabricated. The thin-film sample combines red, green and blue emissions from the Eu3+ ions, Tb3+ ions and ZnO-nc, respectively, to create white light or light of any desired colour. The emissions at 614 nm and 545 nm from Eu3+ and Tb3+ ions, respectively, is due to the energy transfer from the excited ZnO-nc to the rare-earth (RE) ions. In this way, only a single excitation wavelength is needed to excite the ZnO-nc, Eu3+ and Tb3+ ions in the sample to produce emission of a desired colour from the sample. We developed an empirical 4th-degree polynomial equation to determine the concentrations of Eu3+ and Tb3+ ions to produce light of any desired colour in the CIE colour space. Based on the above empirical equation, the concentration of Eu3+ and Tb3+ ions for a white light emitting sample was found to be 0.012 and 0.024 molar fractions, respectively. The white light emission from the sample was confirmed by fabricating the sample using the low-cost sol–gel process. The stimulated emission spectra and the experimental emission spectra of the white light sample fit very well. The results presented in this work are important to develop energy efficient solid state lighting devices.

Highly-Efficient Solution-Processed Organic Light Emitting Diodes with Blend V2O5-PEDOT:PSS Hole-Injection/Hole-Transport Layer

ABSTRACTOrganic light-emitting diodes (OLEDs) have attracted huge concern because of their intrinsic characteristics and ability to reach the pinnacle in the field of high-quality flat-panel displays and energy-efficient solid-state lighting. High-efficiency is always a key crux for OLED devices being energy-saving and longer life-span. OLEDs have encountered enormous difficulties in meeting the requirements for large-sized devices due to a major limitation in vacuum thermal evaporation technology. In multilayered OLED devices, the characteristics of the charge injection/transport layer is a crucial factor for the operating-voltage, power-efficiency and stability of the device. Transition metal oxides have shown great potential owing to their wide range of possible energy level alignments, balanced charge injection, and improvement of carrier mobilities. In this study, we report a solution-processed blend V2O5-PEDOT:PSS hole-injection/hole-transport layer (HIL/HTL) for efficient orange phosphorescent OLEDs. The electroluminescent characteristics of blend V2O5-PEDOT:PSS based devices were studied with the structure ITO/V2O5-PEDOT:PSS/CBP:Ir(2-phq)3/TPBi/LiF/Al. The V2O5-PEDOT:PSS based OLEDs displayed relatively higher device performance and low roll-off than that of the counter PEDOT:PSS device in terms of a maximum luminance of 17,670 cd m-2, power efficiency of 19.4 lm W-1, external quantum efficiency of 8.7%, and more importantly, low turn-on voltage. These results demonstrate an alternative approach based on metal oxide/organic blend HIL/HTL as a substitute of PEDOT:PSS for high-efficiency solution process OLEDs.

Design and fabrication of enhanced lateral growth for dislocation reduction in GaN using nanodashes

The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing {1−101} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing {11−22} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109cm−2 to 3.0×107cm−2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

ChemInform Abstract: Shining Light on Indium Phosphide Quantum Dots: Understanding the Interplay Among Precursor Conversion, Nucleation, and Growth

AbstractReview: 89 refs.

Shining Light on Indium Phosphide Quantum Dots: Understanding the Interplay among Precursor Conversion, Nucleation, and Growth

InP quantum dots have emerged as an exciting class of phosphors for displays and energy-efficient solid state lighting. Unfortunately, the synthesis of these materials has lagged behind that of related II-VI and IV-VI materials. It is becoming increasingly apparent that this may be due, in many cases, to the inability to control quantum dot nucleation and crystallization using precursor conversion kinetics. In this perspective, recent work on understanding the nucleation and growth of InP from the perspective of nonclassical nucleation models is discussed. In particular, the recent discovery that kinetically persistent magic-size nanoclusters build up during the high temperature synthesis of this material will be highlighted. Isolation and complete structural characterization of one such InP nanocluster has offered unprecedented insight into the structure and surface chemistry of InP and its deviation from how we think about quantum dot structure and composition based on models built for II-VI materials. ...

Hybrid Tamm plasmon-polariton/microcavity modes for white top-emitting organic light-emitting devices

White top-emitting organic light-emitting devices (WTOLEDs), emitting white light through a transparent top metallic electrode, have emerged as promising candidates as energy-efficient solid-state lighting sources and full-color flat-panel displays. The microcavity effect due to usage of metallic electrodes results in emission enhancement solely at a particular color, and therefore sets an obstacle for WTOLEDs, where at least two colors with balanced intensity should be emitted. Current efforts solving the problem basically rely on the relaxation of the microcavity effect, resulting in sacrificed light outcoupling efficiency in the original resonance region. Here, we demonstrate that by integrating a photonic crystal structure upon the top metallic electrode, an additional emission enhancement peak other than the one determined by the microcavity resonance could be provided by the Tamm plasmon-polariton mode. Mode hybridization induced dual hybrid modes with comparable light outcoupling efficiency can then be excited, from which two colors with balanced intensity could be emitted. Both experimental and theoretical results demonstrate that the proposed mode hybridization strategy may pave the way for the realization of WTOELDs towards high white color quality, improved viewing characteristics, and electroluminescence efficiency.

Novel approaches for energy efficient solid state lighting by RGB organic light emitting diodes – A review

Abstract This paper emphasizes on the novel approaches for energy efficient and eco-friendly solid state lighting. Limitations and global haphazards of currently used lighting systems such as Incandescent lamps, compact fluorescent lamps can be overcome by replacing the present lighting system by green technology called solid state lighting, which is possible only with organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs). This paper also explains important characterization techniques used to evaluate the performance, efficiency, life time; colour rendering index (CRI), Internationale de l’Eclairage (CIE) coordinates and correlated color temperature (CCT) of OLEDs. Review of literature on red, blue, green (RBG) light emitting materials and OLED devices is illustrated since the very first synthesized complex and a range of device architectures are presented and appraised. Measures to increase the efficiency and the life time of OLEDs and handling the degradation issues of the organic materials for OLEDs are also discussed. With these measures if we succeed in improving the efficiency, performance and life time, the present lighting system can be replaced by eco-friendly, energy efficient green technology called Solid state lighting, which would play a significant role in reducing global energy consumption.

Residential energy-efficient lighting adoption survey

Artificial lighting has transformed how humans relate to the world, by improving productivity and making spaces habitable. The adoption of energy-efficient solid-state lighting, light emitting diodes (LED), has been suggested as a way of reducing energy used for lighting. Such predictions rest on the assumptions of constant light density and time of use in the future, assumptions not supported by past trends. In order to better understand how consumers choose to adopt energy-efficient artificial lighting for their homes; a survey was administered in four major urban areas: Chicago, Houston, New York, and San Francisco. Major insights from the survey indicate that if lighting becomes less expensive through adopting energy-efficient light sources, there is the potential for consumers to use considerably more, and the point at which the price for LED lighting yields broad market penetration is about one half of current prices. Regional factors such as lighting subsidies, taxation policies, laws, and educational information are also explored.

Improved electrode performance in gallium nitride LEDs

Light-emitting diodes based on III-nitride semiconductors have made their way into applications such as backlighting for displays, and solid-state white lighting. High-power LEDs with efficacies larger than conventional fluorescent lamps are now commercially available, bringing the world closer toward the realization of energy-efficient solid-state lighting.1, 2 However, despite significant breakthroughs in light extraction and internal quantum efficiencies, the formation of low resistance, transparent ohmic contact to the highly resistive p-GaN (gallium nitride) layer continues to be a challenge.3 Modern commercial LEDs employ either translucent nickel-gold or indium tin oxide (ITO) as a transparent conductive electrode. Nevertheless, the low transparency of a nickel-gold stack—as well as increasing concerns over the soaring prices and future demand for indium—have initiated a search for an alternative material.4 In addition, next-generation LEDs require transparent conductive electrodes to be flexible (unlike ITO, which is brittle), cheap, and compatible with large-scale manufacturing methods. Graphene, the most recently discovered form of carbon, offers exceptional characteristics such as high transparency, mechanical flexibility, and superior thermal and electrical conductivities.5, 6 Despite its comparatively high sheet resistance (Rs/, recent work has shown that graphene can be applied to the p-GaN layer in GaN-based LEDs as a transparent conductive electrode in place of ITO.7 However, because of the large difference in work function (ˆ), integrating pristine graphene directly with p-GaN forms a Schottky barrier height (SBH, q®B ) at the interface. A large SBH is undesirable because it can create low contact resistance, which leads to strong current crowding and a high operating voltage. Figure 1.Ultraviolet photoelectron spectra of pristine and doped—with 10 and 20mM solutions of gold (III) chloride (AuCl3)—multilayer graphene (MLG) films. The inset bar diagram shows the variation in work function versus AuCl3 concentration. a.u.: Arbitrary units.

The Mixed Blessing of Phosphor-Based White LEDs

The Mixed Blessing of Phosphor-Based White LEDs

Metal-nitride-oxide-semiconductor light-emitting devices for general lighting

The potential for application of silicon nitride-based light sources to general lighting is reported. The mechanism of current injection and transport in silicon nitride layers and silicon oxide tunnel layers is determined by electro-optical characterization of both bi- and tri-layers. It is shown that red luminescence is due to bipolar injection by direct tunneling, whereas Poole-Frenkel ionization is responsible for blue-green emission. The emission appears warm white to the eye, and the technology has potential for large-area lighting devices. A photometric study, including color rendering, color quality and luminous efficacy of radiation, measured under various AC excitation conditions, is given for a spectrum deemed promising for lighting. A correlated color temperature of 4800K was obtained using a 35% duty cycle of the AC excitation signal. Under these conditions, values for general color rendering index of 93 and luminous efficacy of radiation of 112 lm/W are demonstrated. This proof of concept demonstrates that mature silicon technology, which is extendable to low-cost, large-area lamps, can be used for general lighting purposes. Once the external quantum efficiency is improved to exceed 10%, this technique could be competitive with other energy-efficient solid-state lighting options.

Solid-state semiconductors are better alternatives to arc-lamps for efficient and uniform illumination in minimal access surgery

Current arc-lamp illumination systems have a number of technical and ergonomic limitations. White light-emitting diodes (LEDs) are energy-efficient solid-state lighting devices which are small, durable and inexpensive. Their use as an alternative to arc-lamp light sources in minimal access surgery has not been explored. This study aims to develop an LED-based endo-illuminator and to determine its lighting characteristics for use in minimal access surgery. We developed an LED endo-illuminator using a white LED mounted at the tip of a steel rod. Offline image analysis was carried out to compare the illuminated field using the LED endo-illuminator or an arc-lamp based endoscope in terms of uniformity, shadow sharpness and overall image intensity. Direct radiometric power measurements in light intensity and stability were obtained. Visual perception of fine details at the peripheral endoscopic field was assessed by 13 subjects using the different illumination systems. Illumination from the LED endo-illuminator was more uniform compared to illumination from an arc-lamp source, especially at the closer distance of 4 cm (0.0006 versus 0.0028 arbitrary units--lower value indicates more uniform illumination). The shadows were also sharper (edge widths of 16 versus 44 pixels for the first edge and 15 versus 61 pixels for the second edge). The overall mean image intensity was higher (127 versus 100 arbitrary units) when using the autoshutter mode despite the lower direct radiometric power, about one tenth of the arc-lamp endoscopic system. The illumination was also more stable with less flickering (0.02% versus 5% of total power in non-DC components). Higher median scores on visual perception was also obtained (237 versus 157, p < 0.001). The LED endo-illuminator provides more uniform illumination with sharper shadows, less flickering and better illumination for visual perception than the arc-lamp-based system currently used.

Carbon nanotubes: a multi-functional material for organic optoelectronics

In recent years carbon nanotubes have emerged as a versatile material suitable for hybrid macro-electronic devices employing organic semiconductors, with the potential to address many of the barriers to the widespread exploitation of organic semiconductors as the basis for low cost, large area, flexible optoelectronics. This paper focuses on the applications of carbon nanotubes in organic light-emitting diodes and photovoltaics. These related devices are currently being pursued with much anticipation as energy efficient solid state lighting and a scalable means of harvesting electrical energy from sunlight. In this paper particular emphasis is placed on the utility of multi-wall carbon nanotubes which have received much less attention than single wall carbon nanotubes for these applications despite offering a number of key advantages.