Luminescence in sulfides : a rich history and a bright future

Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles

Charakterizace tenkovrstvých elektroluminiscenčních součástek

Jádrem této disertační práce bylo studovat optické a elektrické charakteristiky tenkovrstvých elektroluminiscenčních součástek řízených střídavým proudem (ACTFEL) a zejména vliv procesu stárnutí luminiforů na jejich optické a elektrické vlastnosti. Cílem této studie měl být příspěvek ke zvýšení celkové účinnosti luminoforů, vyjádřené pomocí jasu, účinnosti a stability. Vzhledem k tomu, že současnou dominantní technologií plochých obrazovek je LCD, musí se další alternativní technologie plošných displejů porovnávat s LCD. Výhodou ACTFEL displejů proti LCD je lepší rozlišení, větší teplotní rozsah činnosti, větší čtecí úhel, či možnost čtení při mnohem vyšší intenzitě pozadí. Na druhou stranu je jejich nevýhodou vyšší energetická náročnost, problém s odpovídající barevností tří základních barev a podstatně vyšší napětí nutné pro činnost displeje. K dosažení tohoto cíle jsme provedli optická, elektrická a optoelektrická měření ACTFEL struktur a ZnS:Mn luminoforů. Navíc jsme studovali vliv dotování vrstvy pomocí KCl na chování mikrostruktury a na elektroluminiscenční vlastnosti (zejména na jas a světelnou účinnost) ZnS:Mn luminoforů. Provedli jsme i některá, ne zcela obvyklá, měření ACTFEL součástek. Vypočítali jsme i rozptylový poměr nabitých barevných center a simulovali transportní charakteristiky v ACTFEL součástkách. Studovali jsme vliv stárnutí dvou typů ZnS:Mn luminoforů (s vrstvou napařenou či získanou pomocí epitaxe atomových vrstev) monitorováním závislostí svítivost-napětí (L-V), velikost vnitřního náboje - elektrické pole luminoforu (Q-Fp) a kapacitance-napětí (C-V) ve zvolených časových intervalech v průběhu stárnutí. Provedli jsme krátkodobá i dlouhodobá měření a pokusili jsme se i o vizualizaci struktury luminoforu se subvlnovým rozlišením pomocí optického rastrovacího mikroskopu pracujícího v blízkém poli (SNOM). Na praktickém případu zeleného Zn2GeO4:Mn (2% Mn) ACTFEL displeje, pracujícího při 50 Hz, jsme také studovali stabilitu svítivosti pomocí měření závislosti svítivosti na napětí (L-V) a světelné účinnosti na napětí (eta-V). Přitom byl zhodnocen význam těchto charakteristik. Nezanedbatelnou a neoddělitelnou součástí této práce je i její pedagogický aspekt. Předložený text by mohl být využit i jako učebnice pro studenty na mé univerzitě v Lybii.The objective of this thesis is to study the optical and electrical characterization of Alternating-Current Thin-Film ElectroLuminescent (ACTFEL) devices, and specifically the aging process of phosphor materials that comprise the ACTFEL display in an effort to improve the overall performance of the primary phosphor colors in terms of brightness, efficiency and stability. Since the dominant flat-panel display technology is the LCD, an alternative flat-panel display technology must gauge itself in terms of the LCD. The advantages of ACTFEL displays in comparison with LCDs are the ability to pattern much smaller pixel, performance over a wider temperature range, full-viewing angle, and readability with much greater intensity background light. The disadvantages of ACTFEL displays against LCD ones are larger power consumption, lack of adequate chromaticity of the three primary colors, and much larger driving voltages. To achieve these objectives, the optical, electrical, and opto-electric measurements of the ACTFEL structures and ZnS:Mn phosphor hosts were carried out. Moreover, the effect of KCl co-doping on the microstructure and the electroluminescent properties (mainly brightness and luminous efficiency) of ZnS:Mn phosphors has been investigated, too. A non-common electrical characterization of ACTFEL devices has also been provided. We also calculated charged center scattering rates, and simulated the electron transport process in an ACTFEL device The study of the aging characteristics of evaporated and atomic layer epitaxy ZnS:Mn phosphors has been undertaken by monitoring the luminance-voltage (L-V) internal charge-phosphor field (Q-Fp) and capacitance-voltage (C-V) electrical characteristics at in selected time intervals during aging. Short-term and long-term ACTFEL aging studies has been provided and an attempt to visualize locally the structure of phosphor with a subwavelenght resolution using Scanning near-field optical microscope (SNOM) has also been presented. The practical case of a green Zn2GeO4:Mn (2% Mn) ACTFEL device operated at 50 Hz was studied and a luminance stability by a measurement of luminance-voltage (L-V) and luminous efficiency-voltage (eta-V) characteristics has been evaluated. A non-negligible and indiscreptible task of this thesis was also its pedagogical aspect. Therefore, the presented text can be considered as a textbook suitable for our students in Libya.

Fabrication and Electro-optic Properties of MWCNT Driven Novel Electroluminescent Lamp

We present a novel, cost-effective and facile technique, wherein multi-walled carbon nano-tubes (CNTs) were used to transform a photoluminescent material to exhibit stable and efficient electroluminescence (EL) at low-voltages. As a case study, a commercially available ZnS:Cu phosphor (P-22G) was combined with a very low concentration of CNTs dispersed in ethanol and its alternating current driven electroluminescence (AC-EL) is demonstrated. The role of CNTs has been understood as a local electric field enhancer and facilitator in the hot carrier injection inside the ZnS crystal to produce EL in the hybrid material. The mechanism of EL is discussed using an internal field emission model, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.Comment: 9 Figure

Equivalent circuits and efficacy of single-layer ACPEL devices

This article has been made available through the Brunel Open Access Publishing Fund.A series resistance - capacitance equivalent circuit has been used to describe a single layer AC Powder Electroluminescence (ACPEL) lamp in operation. The two crucial components of this practical equivalent circuit are frequency and voltage dependent and have been independently determined for a single layer ACPEL device over a range of 50-800 Hz and 10-150V. The organic binder containing a ferroelectric component is mainly responsible for determining the capacitive element since it acts in series with a larger capacitative contribution mainly from the phosphor. The series resistive element will be determined by mainly the phosphor particles, and the remarkable changes in the effective series resistance and capacitance of the lamp structure are shown to be brought about by the activation of the ZnS phosphor. The effective resistance is consistent with a model where conductivity is governed by the average charge recombination time under given internal field and frequency conditions. Using the effective resistance and capacitance values of our equivalent circuit, the average rate of energy dissipation can be easily calculated as a function of applied voltage and frequency. For sinusoidal waveforms, first indications are that efficacy will be optimized at low voltages, but only weakly dependent on frequency. © The Author(s) 2014

AC Frequency Dependence of Electroluminescent ZnS Phosphor Panel Color

In this study, electroluminescent panels manufactured by Meadow River Enterprises were used to study the AC frequency dependence of their color. A project was developed to acquire their emittance color at various frequencies. Three frequencies were selected for the investigation. Data, obtained by measuring the spectrum at a constant excitation frequency, were taken for each of the three standard frequencies: 200 Hz, 1500 Hz, and 20 kHz. In general the higher the frequency, the bluer the emittance color of the panel. The lower the frequency, the more green the panel became. An explanation for this phenomenon, the frequency dependence of the emission color, is suggested using a model for the electronic structure of Cu doped ZnS. It can be concluded that the blue emission is a two step process, while the green emission is a one step process according to the model used in this project

Red Emission Of Thin Film Electroluminescent Device Based On p-GaN.

High quality GaN layers doped with Mg were grown on Si(111) substrates using high temperature AlN as buffer layer by using radio-frequency molecular beam epitaxy (RF-MBE)

Synthesis and optoelectronic properties of new ethynylated pyrazine derivatives

Several diaryleneethynylpyrazine derivatives, in which the pyrazine unit is electron-deficient, have been synthesised using Sonogashira palladium-catalysed cross-coupling reactions. Compound 32, an important intermediate in the synthesis of diaryleneethynylpyrazine derivatives was made by a modified literature procedure which improved the yield. Examination of optical absorption and photoluminescence spectra of compound 37 shows that the pyrazine unit does not change the behaviour significantly compared to analogue 42, while compound 38 shows pyridine substituents have a profound effect on the photophysics of these pyrazine systems. The redox properties of representative compound 37 were studied by cyclic voltammetry, which shows that reduction of 37 to its radical anion occurs as a reversible process at high negative potentials of ca. -1.87 V. The X-ray crystal structure of 37 is also presented. Quantum mechanical calculations of the geometry and electronic structure were performed for compound 37; the known phenylene analogue 42 was calculated at the same level for comparison. The results show that the energies of both HOMO and LUMO orbitals of 37 are decreased compared to 42. The calculated value of the HOMO-LUMO gap of 37 (3.56 eV) is close to that estimated from the red edge of the longest wavelength absorption (382 nm = 3.25 eV)

Red Y2O3:Eu-Based Electroluminescent Device Prepared by Atomic Layer Deposition for Transparent Display Applications

Y2O3:Eu is a promising red-emitting phosphor owing to its high luminance efficiency, chemical stability, and non-toxicity. Although Y2O3:Eu thin films can be prepared by various deposition methods, most of them require high processing temperatures in order to obtain a crystalline structure. In this work, we report on the fabrication of red Y2O3:Eu thin film phosphors and multilayer structure Y2O3:Eu-based electroluminescent devices by atomic layer deposition at 300 °C. The structural and optical properties of the phosphor films were investigated using X-ray diffraction and photoluminescence measurements, respectively, whereas the performance of the fabricated device was evaluated using electroluminescence measurements. X-ray diffraction measurements show a polycrystalline structure of the films whereas photoluminescence shows emission above 570 nm. Red electroluminescent devices with a luminance up to 40 cd/m2 at a driving frequency of 1 kHz and an efficiency of 0.28 Lm/W were achieved