Papers by Keyword: Electroluminescence

Abstract: The heteroepitaxial growth of GeSn and Ge crystals on Si substrates are investigated for Si-based photonic applications. Light Emitting Diodes with emission wavelengths from 2,100 to 1,550 nm could be demonstrated with active intrinsic GeSn light emitting layers between Ge barriers. A clear shift of the direct band gap toward the infrared beyond 2 μm is measured. Emission intensity is increased compared to Ge Light Emitting Diodes. Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices are demonstrated. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Pérot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1,660 nm and 1,700 nm.

Abstract: We analyzed multi quantum well light emitting diodes, consisting of ten alternating GeSn/Ge-layers, were grown by molecular beam epitaxy on Si. The Ge barriers were 10 nm thick and the GeSn wells were grown with 7% Sn and thicknesses between 6 and 12 nm. Despite the high threading dislocation density of 10⁹ to 10¹⁰ cm⁻² the electroluminescence spectra measured at 300 and 80 K yield a broad and intensive luminescence band. Deconvolution revealed three major lines produced by the GeSn wells that can be interpreted in terms of quantum confinement. Biaxial compressive strain causes a splitting of light and heavy holes in the GeSn wells. We interpret the three lines to represent two direct lines, formed by transitions with the light and heavy hole band, respectively, andan indirect line.

Abstract: We report a hybrid hetero p-n junction between Zinc Oxide (ZnO) microspheres and p-GaN thin film or poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS). ZnO microspheres, which have high crystalline quality, were synthesized by ablating a ZnO sintered target with focused pulsed laser at high fluence. Recent investigation has demonstrated that ZnO microspheres had high optical property and laser actions in the WGM (whispering gallery mode) from the microshperes under pulsed laser irradiation. In this study, we fabricated a hybrid hetero p-n junction between ZnO microspheres and p-GaN or PEDOT:PSS, and both p-n junctions with p-GaN or PEDOT:PSS had a good rectifying characteristic. In the case of p-GaN, electroluminescence was observed under forward bias.

Abstract: Oxide inorganic electroluminescent device in which thin films of Pr-doped NaNbO₃ phosphor and Sb-doped SnO₂ transparent conductor are alternately stacked was prepared by sol-gel and spin-coating methods. Red electroluminescence was observed due to f-f transitions of Pr³⁺ ions by applying 5-kHz ac voltages to the device. The luminance was 1.0 cd m⁻² at 25 V ac and5.0 cd m⁻² at 34 V ac.

Abstract: Sulfide based luminescent materials have attracted a lot of attention for a wide range of photo-and electroluminescence applications. Among the sulfides, ZnS is promising host material for development of phosphors in different visible emission bands. Doping of Ag can affect the electroluminescence of the host material. Incorporation of host ZnS into the polymer matrix is one of the best method to display their special functions, which stabilize the nanoparticles. Here we report a synthesis and electroluminescence of silver doped ZnS/PVK nanocomposites thin films. Reported films were prepared by using chemical route with varying Ag doping and ZnS loading in the composite. Structural and morphological characterization were carried out through XRD and SEM techniques, which confirmed the particles in nanoregime. Though optical absorption spectra and band gap of ZnS semiconductor nanoparticles in ZnS:Ag/PVK matrix were estimated, and using EMA model, particle size was calculated which supports the results of XRD. Electroluminescence of nanocomposite samples was studied and it was found that threshold voltage depends on doping of Ag and also on loading of ZnS. Voltage brightness characteristics support the production of EL by acceleration-collision mechanism.Contents of Paper

Abstract: Currently there is a great interest in II–VI semiconductor nanoparticles, particularly organically capped soluble particles of cadmium or zinc sulphide and selenide, for their ready to use application in devices. For electroluminescence (EL) devices, it is expected to cover a broad spectrum and to tune various specific colours by preparing Cd_1-xZn_x Se instead of CdSe and ZnSe. Ternary alloys have composition dependent properties; therefore Cd_1-xZn_xSe has attracted much attention in the fields of luminescence and optoelectronic devices. It has wide optical band-gap and good stability with respect to environment. In this study, Cd_1-xZn_xSenanoparticles have been synthesized by using starch as a capping agent through a chemical synthesis route at room temperature. Samples have been prepared varying composition factor ‘x’ in ternary alloy Cd_1-xZn_xSe. Cubic structure of all has been confirmed by XRD. Crystallite size calculated from XRD was found in the range of 3-5 nm and it was observed that size reduces on increasing Zn content in ternary compound. Optical absorption spectra showed the blue shift in absorption edge with increasing Zn content. Band gap has been obtained by absorption studies and increase in band gap observed on increasing Zn content in the compound. Electroluminescence studies reveal that lower threshold voltage is required for samples with lower ‘x’ value. The Brightness increases on increasing the voltage above threshold voltage and the variation pattern is almost exponential for all samples. The voltage-current curve represents ohmic nature of the EL cell. Impedance was found to increase on increasing ‘x’ value. The increase in EL intensity is faster for higher frequency. EL spectra revealed that light emission is in violet-green region corresponding to band gap for both Cd_0.75 Zn _0.25Se and Cd_0.5 Zn _0.5Se, with a slight blue shift on increasing Zn content. A ternary system Cd_1–xZn_xSe, may be engineered better for application purpose by suitably choosing the composition parameter ‘x’.Contents of Paper

Abstract: Nanotechnology has led to a profound paradigm shift after the developments in recent years and after being recognised as one of the most important areas of impending technology. Nanomaterials are the basis of newly emerging nanotechnologies for various applications in sensors, photonics, drug delivery, proteomics, biomolecular electronics, and homeland security. Luminescent nanomaterials have attracted great interest worldwide because of their unusual structural, optical and electronic properties as well as efforts to prepare miniaturised devices. By understanding and manipulating these properties, the performance of the resulting optical structure can be tailored for desired end-use applications. Luminescence nanoparticles have tremendous potential in revolutionizing many interesting applications in today’s emerging cutting-edge optical technology such as solid state lighting. Solid-state lighting (SSL) relies on the conversion of electricity to visible white light using solid materials. SSL using any of the materials (inorganic, organic, or hybrid) has the potential for unprecedented efficiencies. The development of novel mercury-free inexpensive nanomaterials, that convert longer wavelength UV to blue light eventually into white-light and are eco-friendly with improved luminous efficacy, energy-saving, long-lifetime, and low-power consumption characteristics, is discussed. In this review, we present a general description of EL related to nanomaterials as the emitter and outlines basic research requirements that could enable solid-state lighting to achieve its potential. Continuing progress in the synthesis and purification of SSL materials are beginning to enable separation of extrinsic and intrinsic phenomena and improve device performance. This review mainly focuses on the basic mechanism, classification, synthesis and characterization of luminescent nanomaterials. The review also covers recent advances in lanthanide-based nanomaterials and photoluminescent nanofibers formed by combining electrospun polymeric nanofibers and quantum dots (QDs) for lighting applications. In spite of the remarkable scientific progress in preparation processes and applications of nanomaterials, they are still not widely used by the industry. Finally, we conclude with a look at the future challenges and prospects of the development of electroluminescence (EL) devices for lighting.Contents of Paper

Abstract: Currently, LED display is an information display technology that has strong function,high efficiency and wide scope of application.Since its development has occurred, LED display has experienced a number of technical improvements and adjustments. This paper studies the basic physical characteristics of LED displays, electroluminescent application status and ultra-high brightness of the display luminescence phenomena, and it proposes future development goals of LED display.

Abstract: Several research groups have reported that nanocrystalline II-VI semiconductors show enhanced luminescence, increased oscillator strength and shorter response time. Nanocrystalline powder samples of CdS, CdSe, ZnS and ZnSe nanocrystals and their composites with PVA and PVK have been prepared by chemical route. SEM. TEM and AFM images indicate agglomeration of particles. XRD reveal the crystal structure and size in nanometer range and absorption spectra show increased band gap due to quantum confinement.The EL studies on nanocrystalline powder samples and nanocrystal/polymer composites have shown that the light emission starts at certain threshold voltage, different for different specimens and then increases with increasing voltage. It is found that smaller nanocrystals have lower threshold voltage and higher EL brightness. It is observed that nanocomposite give much higher electroluminescence starting at lower voltage and increasing very fast with the voltage as compared to nanocrystalline powder. The emission spectra are found to depend on the material, crystalline size and doping. Electroluminescence in undoped and doped chalcogenide nanocrystals and nanocomposites is reviewed in this paper. In nanosize regime, electroluminescence (EL) is governed by the size quantization effect. Contents of Paper

Abstract: Organic light emitting diodes (OLEDs) have been the focus of intense study since the late 1980s, when the low voltage organic electroluminescence in small organic molecules such as Alq3, and large organic molecules such as polymers (PPV), was reported. Since that time, research has continued to demonstrate the potential of OLEDs as viable systems for displays and eco-friendly lighting applications. OLEDs offer full colour display, reduced manufacturing cost, larger viewing angle, more flexible, lower power consumption, better contrast, slimmer, etc. which help in replacing the other technologies such as LCD. The operation of OLEDs involves injection of charge carriers into organic semiconducting layers, recombination of charge carriers, formation of singlet and triplet excitons, and emission of light during decay of excitons. The maximum internal quantum efficiency of fluorescent OLEDs consisting of the emissive layer of fluorescent organic material is 25% because in this case only the 25% singlet excitons can emit light. The maximum internal quantum efficiency of phosphorescent OLEDs consisting of the emissive layer of fluorescent organic material mixed with phosphorescent material of heavy metal complexes such as platinum complexes, iridium complexes, etc. is nearly 100% because in this case both the 25% singlet excitons and 75% triplet excitons emit light. Recently, a new class of OLEDs based on thermally activated delayed fluorescence (TADF) has been reported, in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates of more than 10⁶ decays per second. These molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels and provides an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.The OLED technology can be used to make screens large enough for laptop, cell phones, desktop computers, televisions, etc. OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, automotive displays, and even head wearable displays. Presently, the OLEDs are opening up completely new design possibilities for lighting in the world of tomorrow whereby the offices and living rooms could be illuminated by lighting panels on the ceiling. The present paper describes the salient features of OLEDs and discusses the applications of OLEDs in displays and solid state lighting devices. Finally, the challenges in the field of OLEDs are explored. Contents of Paper