High Efficiency White Light Research Articles

Directed emissive high efficient white transparent organic light emitting diodes with double layered capping layers

We report on white transparent organic light emitting diode (TOLED) with double layered capping layer (CL), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and ZnSe. By introducing the double layered CL, total efficiency was improved by 27% to have 90% of that of conventional bottom-emissive device. The achievement of highly improved efficiency was interpreted as the synergetic effect of constructive interference and maximized reflectance of CL. By adjusting the reflectance of the CL, it was possible to choose the emissions direction selectively and achieve spectral matching between top and bottom emissions as well as enhanced total efficiency in white TOLED.

Generation of white LED light by frequency downconversion using perylene-based dye

A high efficiency white light emitting diode (LED) was fabricated by generation of frequency downconversion from a GaN/InGaN blue LED. In place of conventional inorganic phosphors, a perylene-based dye was used for colour conversion. The resulting hybrid structure is analysed by focusing on the visual performance of the realised LEDs employing the most relevant photometric parameters of a light source. Preparation of the organic polymer is described as well. The thermal stability of the dye was investigated and a simple structure which avoids colour degradation is proposed.

High efficient white light emission of rare earth-free MnO-SnO-ZnO-P2O5 glass

White light emission of rare earth (RE)-free Mn-doped SnO–ZnO–P2O5 glass is demonstrated. Glass transition temperature of the obtained glass is below 440°C, which assures the application to sealant of solid state light emitting devices. The RE-free transparent glass shows white light emission with a high value of quantum efficiency (QE) comparable to conventional crystalline phosphor by excitation of deep UV light. The broad emission of RE-free transparent glass can be continuously tuned by both the amount of activator and the composition of the glass without decrease of the QE value.

Development of Nonpolar and Semipolar InGaN/GaN Visible Light-Emitting Diodes

AbstractThis article reviews the development of nonpolar and semipolar InGaN/GaN light-emitting diodes (LEDs), emphasizing structures on freestanding bulk GaN. A brief history of LED development on each orientation is provided, followed by a discussion of the most relevant and recent results. The context is related to several current LED issues, such as the realization of high-efficiency white solid-state lighting, potential solutions to the “green gap,” and applications for polarized emitters. The section on nonpolar LEDs highlights high-power violet and blue emitters and considers the effects of indium incorporation and substrate miscut. The section on semipolar GaN reviews the development of LEDs in the violet, blue, green, and yellow regions and highlights the potential of InGaN/GaN LEDs as an alternative technology to AlInGaP for yellow emitters. A brief review of polarization anisotropy also is included for each orientation. Finally, a two source white light system utilizing a nonpolar blue LED and a semipolar yellow LED is presented.

High efficient two color white phosphorescent organic light emitting diode

We report on high efficient white organic light emitting diodes comprising the two phosphorescent emitter materials platinum(II)[2-(4′,6′-difluorophenyl)pyridinato-N,C2′)](2,4-pentanedionato) (FPt1) and iridium(III)bis[∼4,6-difluorophenyl-pyridinato N, C2′]picolinate (FIrpic). Investigating the influence of the dopants in the emission layers on the device performance, a series of devices with different FIrpic and FPt1 concentrations have been processed. After optimizing the device with respect to efficiency and white color coordinates the influence of a thickness variation of the emission layers was investigated. The experiments revealed that the device performance is more sensitive to the parameters of the emission layer containing FPt1. Additional optical simulations showed that interference effects play a crucial role in this device setup. The optimized device resulted in an external quantum efficiency of 10%, a power efficiency of 16 lm/W, near warm white color coordinates of 0.39/.41, and a color rendering index of 81 at 1000 cd/m2.

Successful fabrication of white light emitting diodes by using extremely high external quantum efficiency blue chips

AbstractWe fabricated three types of high luminous efficiency white light emitting diodes (LEDs). The first is the white LED, which had a high luminous efficiency (ηL) of 161 lm/W with the high luminous flux (ϕv) of 9.89 lm at a forwardbias current of 20 mA. The blue LED had a high power (ϕe) of 42.2 mW and high external quantum efficiency (ηex) of 75.5%. The second is the high luminous efficiency white LED with a low voltage (Vf) of 2.80 V, which was almost equal to the theoretical limit. ηL and wall‐plug efficiency (WPE) is 169 lm/W and 50.8%, respectively, at 20 mA. They are approximately twice higher than those of a tri‐phosphor fluorescent lamp (90 lm/W and 25%). The third is the high power white LED fabricated from the high power blue LED with high ϕe of 651 mW at 350 mA. ϕv, ηL and WPE of the high power white LED are 145 lm, 134 lm/W and 39.6% at 350 mA, respectively. Moreover, at 1 A, ϕv and ηL were 361 lm and 97 lm/W, respectively. Thus ϕv is equivalent to that of a 30 W‐class incandescent lamp. And, ηL is slightly higher than that of a tri‐phosphor fluorescent lamp. Moreover, we fabricated the high power near ultra‐violet, bluish‐green and green LEDs, whose ϕe at 350 mA were 675 mA, 325 mW, and 236 mW, respectively. ϕv of the green LED was 128 lm at 350 mA. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of mass transport mechanism in InGaN epitaxy on ridge shaped selective area growth GaN by metal organic chemical vapor deposition

In this work, the evolution of the InGaN layer growth on the ridge shaped GaN was studied. A mass transport model was presented to simulate the epitaxy process of the InGaN layer. The model consisted of two consecutive components, gas-phase diffusion process and surface diffusion process. The mean lifetime of adatoms on epitaxial surface was associated with their reaction rate in this model. An InGaN layer on ridge shaped GaN, including (0002) and {112¯2} facets, was grown by metal organic chemical vapor deposition to confirm the mass transport model. Gradient indium content distribution and inhomogeneous thickness of the InGaN layer were observed. Simulation of the InGaN layer growth process was performed by finite difference method with the mass transport model. By analyzing the results from calculations and experiments, the origins of the InGaN layer characteristics were attributed to the two diffusion components in the growth process. Surface diffusion resulted in the inhomogeneous thickness and gas-phase diffusion chiefly led to the gradient indium content. In addition, the adatoms reaction rate on epitaxial surface determined their mean lifetime as speculated by the analysis. The demonstration of the growth process of InGaN layer offers valuable insight in obtaining high efficiency white light emitting diodes by selective area growth technology.

Fluorene trimers with various 9,9′-substituents: The synthesis, characteristics, condensed state structures, and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351nm, and highly efficient deep blue emission at 394nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40nm)/trimers (40nm)/BCP (10nm)/Alq3 (20nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40nm)/rubrene (0.1nm)/T4 (8nm)/Alq3(52nm)/LiF/Al is fabricated with high efficiency (6.15cdA−1), high brightness (9500cdm−2) and good white light CIE coordinates (0.32, 0.37).

25.1: Invited Paper: Achieving Efficient Solid State Lighting Using Organic Light Emitting Devices

AbstractA significant challenge facing human kind in the 21st Century is how to address the ever decreasing supply of depletable and renewable energy. One approach to this problem is to decrease our usage. for this reason, considerable attention has been focused on more efficient means for room lighting which currently consumes approximately 20% of the total energy used. Organic light emitting devices (OLEDs) provide a unique opportunity to provide this high efficiency solid state lighting at very low cost. In this talk, I will discuss several strategies for achieving very high efficiency white light emission at high brightnesses for the next generation of efficient solid state lighting sources based on small molecular weight, vapor deposited OLED structures. Key to our approach is the use of electrophosphorescence as a means for converting all electrical into optical energy. We show that the highest luminance efficiencies can be obtained by a combination of fluorescence and phosphorescence in a unique OLED structure. Furthermore, the highest brightnesses are achieved (without a significant loss in power efficiency) by stacking several fluorescent/phosphorescent elements in a single OLED structure (called a SOLED), with each emitting element in the stack separated by a transparent charge generation layer. Prospects for OLEDs as the next practical generation of interior illumination sources will be reviewed.

A high performance fluorescent white organic light-emitting device and its optimization for full-colour display

A white organic light emitting diode (OLED) was fabricated in which blue emission host 9,10-di(2-naphthyl)anthracene (ADN) doped with an orange emission dye (rubrene) was used to generate bright, high efficiency white light. A large amount of work was done to study the emission mechanism of a rubrene-doped device. Finally, to get optimized EL spectra, we introduced a green light which is emitted from tris(8-hydroxy-quinoline) aluminium (Alq3), the electron transport material, into the emission of white light.

Voltage-induced broad-spectrum reflectivity change with surface-plasmon waves

Voltage-induced broad-spectrum reflectivity change with surface-plasmon waves is reported. When white light is incident at a metal/electro-optical material interface, surface-plasmon waves can be excited under phase match conditions. This surface-plasmon resonance depends on the dielectric constants of both the metal and the electro-optical material. Photons in the surface-plasmon resonance wavelength range are absorbed by the interface. Since metals have large imaginary parts of their dielectric constants, the surface-plasmon resonances are broad and may cover all visible wavelengths. Applying voltage to the electro-optical material to change its dielectric constant can result in a change in the reflectivity at the interface. Experimental results showed a reflectivity change from almost 0% to about 40% under an applied voltage using a liquid-crystal and nickel film structure, and the results had good agreement with theoretical calculations. The theoretical calculations also predicted a 90% reflectivity recovery by exciting surface-plasmon waves at a Rh–Al/electro-optical material interface. These results demonstrate that a high efficiency white light modulator can be built using surface-plasmon excitations.

High-Efficiency White Light Emission Using a Phosphorescent Sensitizer in Organic Light-Emitting Devices

We have fabricated white organic light-emitting devices by using the phosphorescent material fac tris (2-phenylpyridine) iridium [Ir(ppy3)] as a sensitizer, as a result, the efficiency of these devices is improved dramatically. Ir(ppy)3 and the fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7- tetramethyljulolidyl-9-enyl)(DCJTB) are co-doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host, whose thickness affects both color and efficiency of the devices. Tris (8-hydroxyquinoline) aluminum (Alq) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine or BCP) are used as electron-transporting and exciton-blocking layers, respectively, and N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) as a blue light-emitting as well as hole-transporting layer. The maximum efficiencies of the devices with 15 and 20 nm co-doped BCP tuning layer are 7.5 and 8.6 cd/A, respectively, and the former presents fairly pure white emission with CIE coordinates of (0.33, 0.32) at 10 V, which is very stable at various biases (10–19 V).