Mastering heterostructured colloidal nanocrystal properties for light-emitting diodes and solar cells

Solution-grown colloidal nanocrystal (NC) materials represent ideal candidates for optoelectronic devices, due to the flexibility with which they can be synthesized, the ease with which they can be processed for devicefabrication purposes and, foremost, for their excellent and size-dependent tunable optical properties, such as high photoluminescence (PL) quantum yield, color purity, and broad absorption spectra up to the near infrared. The advent of surfactant-assisted synthesis of thermodynamically stable colloidal solutions of NCs has led to peerless results in terms of uniform size distribution, composition, rational shape-design and the possibility of building heterostructured NCs (HNCs) comprising two or more different materials joined together. By tailoring the composition, shape and size of each component, HNCs with gradually higher levels of complexity have been conceived and realized, which are endowed with outstanding characteristics and optoelectronic properties. In this review, we discuss recent advances in the design of HNCs for efficient light-emitting diodes (LEDs) and photovoltaic (PV) solar cell devices. In particular, we will focus on the materials required to obtain superior optoelectronic quality and efficient devices, as well as their preparation and processing potential and limitations

High bandwidth freestanding semipolar (11–22) InGaN/GaN light-emitting diodes

Freestanding semipolar (11–22) indium gallium nitride (InGaN) multiplequantum-well light-emitting diodes (LEDs) emitting at 445 nm have been realized by the use of laser lift-off (LLO) of the LEDs from a 50- m-thick GaN layer grown on a patterned (10–12) r -plane sapphire substrate (PSS). The GaN grooves originating from the growth on PSS were removed by chemical mechanical polishing. The 300 m × 300 m LEDs showed a turn-on voltage of 3.6 V and an output power through the smooth substrate of 0.87 mW at 20 mA. The electroluminescence spectrum of LEDs before and after LLO showed a stronger emission intensity along the [11–23]InGaN/GaN direction. The polarization anisotropy is independent of the GaN grooves, with a measured value of 0.14. The bandwidth of the LEDs is in excess of 150 MHz at 20 mA, and back-to-back transmission of 300 Mbps is demonstrated, making these devices suitable for visible light communication (VLC) applications.

Thermal and structural properties of commercial dental resins light-cured with blue emitting diodes (LEDs)

We have investigated the thermal and structural properties of different commercial dental resins: Filtek(TM) Z-350, Grandio(A (R)), Tetric Ceram(A (R)), and TPH Spectrum(A (R)). The purpose of the present study was to evaluate quantitatively the photo-polymerization behavior and the effect of filler contents on the kinetic cures of the dental resins by using Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FT-IR) techniques. We have successfully obtained the low and high glass transition T (g) values of the dental composite resins from DSC curves. It was also observed a good agreement between the both T (g) values, activation energies from thermal degradation, and the degree of conversion obtained for all samples. The results have shown that Tetric Ceram(A (R)) dental resin presented the higher T (g) values, activation energy of 215 +/- A 6 KJ mol(-1), and the higher degree of conversion (63%) when compared to the other resins studied herein.

Thermal and structural properties of commercial dental resins light-cured with blue emitting diodes (LEDs)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes

In this paper we present the effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes (OLEDs). A number of OLED devices have been fabricated with combinations of hole injecting and hole blocking layers of varying thicknesses. Even though hole blocking and hole injection layers have opposite functions, yet there is a particular combination of their thicknesses when they function in conjunction and luminous efficiency and power efficiency are maximized. The optimum thickness of CuPc (Copper(II) phthalocyanine) layer, used as hole injection layer and BCP (2,9 dimethyl-4,7-diphenyl-1,10-phenanthroline) used as hole blocking layer were found to be 18 nm and 10 nm respectively. It is with this delicate adjustment of thicknesses, charge balancing is achieved and luminous efficiency and power efficiency were optimized. The maximum luminous efficiency of 3.82 cd/A at a current density of 24.45 mA/cm(2) and maximum power efficiency of 2.61 lm/W at a current density of 5.3 mA/cm(2) were achieved. We obtained luminance of 5993 cd/m(2) when current density was 140 mA/cm(2). The EL spectra was obtained for the LEDs and found that it has a peaking at 524 nm of wavelength. (C) 2012 Elsevier B.V. All rights reserved.

Electroluminescence analysis and simulation of the effects of injection and temperature on carrier distribution in InGaN-based light-emitting diodes with color-coded quantum wells

This paper reports on an extensive analysis of the electroluminescence characteristics of InGaN-based LEDs with color-coded structure, i.e., with a triple quantum well structure in which each quantum well has a different indium content. The analysis is based on combined electroluminescence measurements and two-dimensional simulations, carried out at different current and temperature levels. Results indicate that (i) the efficiency of each of the quantum wells strongly depends on device operating conditions (current and temperature); (ii) at low current and temperature levels, only the quantum well closer to the p-side has a significant emission; (iii) emission from the other quantum wells is favored at high current levels. The role of carrier injection, hole mobility, carrier density and non-radiative recombination in determining the relative intensity of the quantum wells is discussed in the text. © 2013 The Japan Society of Applied Physics.

Precision transfer printing of ultra-thin AlInGaN micron-size light-emitting diodes

We report the fabrication of a mechanically-flexible 16×16 array of thin-film, micron-size LEDs emitting at 480 nm. Devices were transfer-printed onto a mechanically-flexible ITO backplane using a modified, high-precision (placement accuracy ±25 nm) assembly system. © 2013 IEEE.

Fabrication and optical characterization of GaN-based nanopillar light emitting diodes

InGaN/GaN-multiple-quantum-well-based light emitting diode ( LED) nanopillar arrays with a diameter of approximately 200nm and a height of 700nm are fabricated by inductively coupled plasma etching using Ni self-assembled nanodots as etching mask. In comparison to the as-grown LED sample an enhancement by a factor of four of photoluminescence ( PL) intensity is achieved after the fabrication of nanopillars, and a blue shift and a decrease of full width at half maximum of the PL peak are observed. The method of additional wet etching with different chemical solutions is used to remove the etch-induced damage. The result shows that the dilute HCl ( HCl:H2O=1:1) treatment is the most effective. The PL intensity of nanopillar LEDs after such a treatment is about 3.5 times stronger than that before treatment.

Sterilization system for air purifier by combining ultraviolet light emitting diodes with TiO2

BACKGROUND: Ultraviolet light emitting diodes (UV LEDs) were used as a light source in TiO2 photocatalysis because of their many advantages, such as, long life, safety, low pollution, etc. In this experiment, a light source panel was successfully fabricated with UV LEDs, the light intensities of which were relatively uniform.

Improvement of GaN-based light emitting diodes performance grown on sapphire substrates patterned by wet etching - art. no. 684107

An effective approach to enhance the light output power of InGaN/GaN light emitting diodes (LED) was proposed using pyramidal patterned sapphire substrates (PSS). The sapphire substrates were patterned by a selective chemical wet etching technique. GaN-based LEDs were fabricated on patterned sapphire substrates through metal organic chemical deposition (MOCVD). The LEDs fabricated on patterned sapphire substrates exhibit excellent device performance compared to the conventional LEDs fabricated on planar sapphire substrates in the case of the same growth and device fabricating conditions. The light output power of the LEDs fabricated on patterned sapphire substrates was about 37% higher than that of LEDs on planar sapphire substrates at an injection current of 20 mA. The significant enhancement is attributable to the improvement of the quality of GaN-based epilayers and improvement of the light extraction efficiency by patterned sapphire substrates.

Novel photonic crystal structure GaN-based light-emitting diodes - art. no. 68410J

For enhancing the output efficiency of GaN light-emitting diode(LED), we calculated the band structure of photonic crystal(PhC), and designed and fabricated several novel GaN LEDs with photonic crystal on Indium-Tin-Oxide(ITO), which as p-type transparent contact of GaN LED. In this fabricating process, we developed conventional techniques in order that these methods can be easily applied to industrial volume-production. And we have done some preliminary experiments and obtained some results.

Plasma induced damage in GaN-based light emitting diodes - art. no. 68410X

The effects of plasma induced damage in different conditions of ICP and PECVD processes on LEDs were presented. For ICP mesa etch, in an effort to confirm the effects of dry etch damage on the optical properties of p-type GaN, a photoluminescence (PL) measurement was investigated with different rf chuck power. It was founded the PL intensity of the peak decreased with increasing DC bias and the intensity of sample etched at a higher DC bias of -400V is less by two orders of magnitude than that of the as-grown sample. Meanwhile, In the IN curve for the etched samples with different DC biases, the reverse leakage current of higher DC bias sample was obviously degraded than the lower one. In addition, plasma induced damage was also inevitable during the deposition of etch masks and surface passivation films by PECVD. The PL intensity of samples deposited with different powers sharply decreased when the power was excessive. The PL spectra of samples deposited under the fixed condition with the different processing time were measured, indicating the intensity of sample deposited with a lower power did not obviously vary after a long time deposition. A two-layer film was made in order to improve the compactness of sparse dielectric film deposited with a lower power.

Highly efficient single-emitting-layer white organic light-emitting diodes with reduced efficiency roll-off

By codoping blue and orange phosphorescent dyes into a single host material, a highly efficient white organic light-emitting diode (WOLED) with Commission Internationale de L'Eclairage coordinates of (0.38, 0.43) at 12 V is demonstrated. Remarkably, this WOLED achieves reduced current efficiency roll-off, which slightly decreases from its maximum value of 37.3-31.0 cd/A at 1000 cd/m(2). The device operational mechanism is subsequently investigated in order to unveil the origin of the high performance.

Achieving highly efficient white organic light-emitting diodes with reduced efficiency roll-off

A highly efficient and colour-stable three-wavelength white organic light-emitting diode with the structure of indium tin oxide (ITO)/MoO3/N,N'-diphenyl-N,N'-bis (1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/4,4'-N,N'-dicarbazole-biphenyl (CBP): bis(2,4-diphenylquinolyl-N,C-2') iridium( acetylacetonate) (PPQ)(2)Ir(acac)/NPB/p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph):2-methyl-9,10-di(2-naphthyl) anthracene (MADN)/tris (8-hydroxyquinoline) aluminum (AlQ): 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T)/AlQ/LiF/Al is fabricated and characterized. A current efficiency of 12.3 cdA(-1) at an illumination-relevant brightness of 1000 cd m(-2) is obtained, which rolls off slightly to 10.3 cdA(-1) at a rather high brightness of 10 000 cd m(-2). We attribute this great reduction in the efficiency roll-off to the wise management of singlet and triplet excitons between emissive layers as well as the superior charge injection and diffusion balance in the device.

Investigations on polyaniline based systems for technologically important applications and some novel polymers as prospective candidates for fabricating polymer light emitting diodes

Light emitting polymers (LEP) have drawn considerable attention because of their numerous potential applications in the field of optoelectronic devices. Till date, a large number of organic molecules and polymers have been designed and devices fabricated based on these materials. Optoelectronic devices like polymer light emitting diodes (PLED) have attracted wide-spread research attention owing to their superior properties like flexibility, lower operational power, colour tunability and possibility of obtaining large area coatings. PLEDs can be utilized for the fabrication of flat panel displays and as replacements for incandescent lamps. The internal efficiency of the LEDs mainly depends on the electroluminescent efficiency of the emissive polymer such as quantum efficiency, luminance-voltage profile of LED and the balanced injection of electrons and holes. Poly (p-phenylenevinylene) (PPV) and regio-regular polythiophenes are interesting electro-active polymers which exhibit good electrical conductivity, electroluminescent activity and high film-forming properties. A combination of Red, Green and Blue emitting polymers is necessary for the generation of white light which can replace the high energy consuming incandescent lamps. Most of these polymers show very low solubility, stability and poor mechanical properties. Many of these light emitting polymers are based on conjugated extended chains of alternating phenyl and vinyl units. The intra-chain or inter-chain interactions within these polymer chains can change the emitted colour. Therefore an effective way of synthesizing polymers with reduced π-stacking, high solubility, high thermal stability and high light-emitting efficiency is still a challenge for chemists. New copolymers have to be effectively designed so as to solve these issues. Hence, in the present work, the suitability of a few novel copolymers with very high thermal stability, excellent solubility, intense light emission (blue, cyan and green) and high glass transition temperatures have been investigated to be used as emissive layers for polymer light emitting diodes.