Thin-film Electroluminescent Research Articles

AC thin-film electroluminescence: A historical overview with a look ahead

50 years ago, television screens used bulky cathode ray tubes (CRTs) with a size limited to about 70 cm in diagonal (28 in.), while flat panel displays belonged to the realm of science fiction. Although the performance of CRTs in terms of energy consumption, size, and color rendering slowly improved, there were numerous applications where flat panel displays would present a tremendous advantage, notably for confined spaces and in portable equipment. Starting from the 1970s, AC thin-film electroluminescence (ACTFEL) was investigated as a high performance alternative to the—also emerging—liquid crystal displays (LCDs) and plasma display panels (PDPs). For a number of years, ACTFEL developed from the phase of fundamental research to commercial products (for monochrome displays) and full color prototype displays. However, due to the fast development and strong cost reduction of LCDs, ACTFEL did not remain competitive and most of the research activities were ceased. The current overview sketches a brief history of the ACTFEL technology, its merits and limitations, and the reasons why such displays still have their use in some niche applications.

ZnS:Mn electroluminescent display based on nanostructured indium tin oxide films

Thin-film electroluminescent (TFEL) displays have found applications in various industries and areas due to their unique advantages, where high brightness, a wide temperature range, and reliability are crucial. However, despite their benefits, they also face certain limitations and challenges. TFEL displays typically demand relatively high operating voltages. Total internal reflection in the multilayered structure can significantly impede light extraction from the display. In this study, we propose an optimization of the TFEL display structure to enhance light extraction efficiency and reduce operating voltage. Nanostructured indium tin oxide films, obtained by electron beam evaporation on a heated substrate, were utilized to amplify the electric field in a phosphor layer and to create a non-regular light-scattering relief that effectively breaks internal reflection conditions within the display structure. As a result, the display brightness at operating voltage increased by more than 1.5 times.

Fabrication and characterization of green ZnS:Tb AC- electroluminescent devices by ultrasonic spray pyrolysis

MISIM (metal-insulator-semiconductor-insulator-metal) structures of alternating current thin film electroluminescent (ACTFEL) devices were deposited on ATO/glass substrates by the ultrasonic spray pyrolysis technique. Zirconium oxide and terbium doped zinc sulfide were used as dielectric and semiconductor phosphor layers, respectively. A focused ion beam equipment was used to prepare a lamella of the MISIM structure, for cross-sectional scanning electron microscopy analysis in the STEM mode. Chemical analysis by energy-dispersive X-ray spectroscopy (EDS) elemental maps was also made for the identification of the distribution of the elements through the layers composing the MISIM structure. Characteristic Tb3+ emission bands due the 5D4 ->7FJ transitions were obtained by AC voltage application at 10 kHz with the main peak centered at 542 nm. An increase in the electroluminescent intensity and a shift of the CIE coordinates toward a purer green color with increasing the applied voltage was observed. The threshold voltage of these green electroluminescent devices was 46 Vrms, and a luminance up to 41 cd/m2 was obtained for the maximum applied voltage of 76.9 Vrms.

Strategies for fourfold enhancement of narrowband green AC-driven electroluminescence from zinc gallate co-doped with Ce3+ and Mn2+ ions by field-enhanced energy transfer

The alternating current (AC)-driven metal-oxide-semiconductor (MOS) structured electroluminescent (EL) devices currently received great attention and are continuously developed due to low-voltage system requirements compared to conventional thin-film EL devices. They shared great advantages of facile fabrication and low power consumption. However, they are not ready for commercial use due to inadequate optical performance. To resolve this issue, this paper presents three enhancement strategies of the narrowband green emission of MOS-EL device on a silicon substrate, which results from the 4T1 → 6A1 transition of the dopants from ZnGa2O4:Mn2+, Ce3+ emitting layer by effective energy transfer from the blue emission of Ce3+ to the green emission of Mn2+ under high electric field. The concentrations of Mn2+ and Ce3+ co-dopants (1), and the annealing temperatures (2) are optimized, and the electrical-optical properties with varying voltage and frequency (3) are investigated. As a result, our MOS EL device generates a large surface-emission EL light with a fourfold-enhanced EL performance. In summary, this research offers a new strategy to resolve the insufficient performance of MOS-EL devices with promising optical properties.

Carbazole-Fluorene Copolymers with Various Substituents at the Carbazole Nitrogen: Structure-Properties Relationship.

Carbazole derivatives, carbazole-containing polymers and iridium complexes are of interest due to many possible applications in photonics, electronics and biology, particularly as active or hole-transporting layers in organic as well as perovskite devices due to their interesting properties. Here, a series of carbazole-fluorene conjugated copolymers with various substituents at the N-carbazole position (2-methoxycarbonylethyl, 2-carboxyethyl, 2-ethylhexyl, and nonan-2,4-dionatoiridium(III)bis(2-phenylpyridine-N,C2')-9-yl) was prepared by Suzuki coupling. Their photophysical, electrochemical and electroluminescence (EL) properties were studied. Effects of molecular weight and substituents attached to carbazole unit on their properties are reported. The carbazole-fluorene copolymers in dilute solutions exhibited intense photoluminescence (PL) emission in the blue spectral region with high PL quantum yields (78-87%) except for the copolymer with the iridium complex (23%). Similar PL spectra were observed in dilute solutions. More pronounced differences were found in thin film PL and EL properties due to excimer/aggregate formation. Light-emitting devices (LEDs) made of copolymers with 2-ethylhexyl as N-carbazole substituent exhibited efficient EL emission with the best performance and the lowest EL onset voltages (3-4 V), while the LEDs made of copolymers with other substituents were not as efficient, but showed anomalous behavior and memory effects in current-voltage characteristics promising also for bio-inspired electronics.

White electroluminescence from Ce3+-doped bi-phase calcium silicate in metal-oxide-semiconductor structure

Thin-film electroluminescent (TFEL) device has been widely introduced with various emitting materials owing to their flexible application in optoelectronics. Particularly, the development of the white emission TFEL has increased the interest in fabrication approaches and structures. Herein, a white-emitting Ce3+-doped bi-phase calcium silicate film as emitting oxide has been synthesized by a facile all-solution process on a silicon wafer, so-called, metal-oxide-semiconductor-EL. Under alternating current source, it emits a white color from blue and yellow spectra due to the f-d transitions of Ce3+ ions in two different crystal phases (CaSiO3 and Ca2SiO4, respectively), which is close to the CIE standard illuminant D65. Beyond a threshold voltage of 10 V, it shows a gradual increment in EL intensity with varying voltage and frequency.

Photophysical and Electrical Properties of Highly Luminescent 2/6-Triazolyl-Substituted Push-Pull Purines.

New push–pull N(9)-alkylated 6-piperidino-2-triazolylpurine and 2-piperidino-6-triazolylpurine derivatives are synthesized, and their optical and optoelectronic properties are comprehensively characterized with experimental and computational methods. The compounds possess intense violet or blue fluorescence with fluorescence quantum yields of up to 91% in solution and 40% in host-free films. Depending on their structural composition, the compounds have ionization energy in the range of 5.25–6.04 eV, electron affinity of 2.18–3.15 eV, and triplet energy of 2.52–2.95 eV. Due to the presence of hole-transporting purine and electron-transporting triazole fragments, compounds exhibit bipolar charge-transportation ability. Despite the favorable emissive properties of the studied push–pull purines, their electroluminescence in thin films is quenched owing to large current densities that are present even at a moderate driving voltage. This marks application directions related to a predominantly charge-transportation functionality as the most suitable for this compound class.

Chiral Oligothiophenes with Remarkable Circularly Polarized Luminescence and Electroluminescence in Thin Films.

This work reports the first observation of circularly polarized electroluminescence (CPEL) in thin films of self-organized oligothiophenes. Four new 1,4-phenylene and 9H-carbazole-based oligothiophenes were ad hoc designed to ensure efficient spontaneous formation of chiral supramolecular order. They were easily synthesized and their chiroptical properties in thin films were measured. Circularly polarized luminescence (CPL) spectra revealed glum in the order of 10-2 on a wide wavelengths range, originating from their self-organized chiral supramolecular organization. These molecules have reasonable properties as organic semiconductors and for this reason they can constitute the active layer of circularly-polarized organic light-emitting diodes (CP-OLEDs). Thus, we could investigate directly their electroluminescence (EL) and CPEL, without resorting to blends, but rather in a simple multilayer device with basic architecture. This is the first example of a CP-OLED with active layer made only of a small organic compound.

Study of photoluminescent and photometric properties of ZnS:Mn2+ quantum dots for white light emission

Abstract In this paper, ZnS, Mn2+-doped ZnS, and ZnS:Mn2+/ZnS core-shell quantum dots (QDs) are successfully prepared by the co-precipitation route at 80 °C in basic aqueous solutions. X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), UV–Visible absorption (UV–Vis), photoluminescence (PL), photoluminescence excitation (PLE) spectroscopy and photometric studies were employed to characterize the samples. The QDs have the zinc blende structure, and their CIE (x, y) chromaticity coordinates can be tuned by the Mn2+ impurity and the TGA-Zn2+ concentrations. The overlap between these QDs spectral power and the human eye sensitivity spectra is significantly enhanced with the efficient energy transfer process. The scotopic/photopic (S/P) ratios, the luminous efficacy of optical radiation (LER), and the color correlated temperature (CCT) increase from 0.93 to 1.69, 389.84–706.53 l m/Wotp, and 2782–14,037 K, respectively. As a result, the ZnS:Mn2+ QDs are suitable for the development of tune color components in white thin-film electroluminescent (TFEL) devices as well as light-emitting diodes (LEDs).

Electroluminescence of thin film p-i-n diodes based on a-SiC:H with integrated Ge nanoparticles

Hydrogenated amorphous substoichiometric silicon carbon alloys (a-SiC:H) with and without embedded Ge nanoparticles (NPs) have been prepared by plasma enhanced chemical vapour deposition combined with in-situ Ge evaporation and annealing on semi-transparent boron doped nano-crystalline diamond coated Ti grids. The presence of Ge NPs embedded in the amorphous phase has been confirmed by transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses. Current-voltage (I–V) characteristics and near infrared electroluminescence (EL) spectra were measured to compare performance of diodes. The relatively strong EL appears in diodes with integrated Ge NPs near the direct band-gap transition of Ge at about 0.82 eV with an intensity strongly correlating with current density. However, it has also been found that Ge NPs integrated into a-SiC:H significantly deteriorates diode I–V characteristic.

Review Article: Atomic layer deposition of optoelectronic materials

Optoelectronic materials can source, detect, and control light wavelengths ranging from gamma and x rays to ultraviolet, visible, and infrared regions. Optoelectronic devices are usually systems that transduce electricity to optical signal or vice versa. Optoelectronic devices include many modern necessities such as lamps, displays, lasers, solar cells, and various photodetectors. Some important research topics in the field of optoelectronics materials are development of new materials, new technologies for fabricating materials, and design of device structures. Atomic layer deposition (ALD) is a technology that was developed in the early 1970s for manufacturing high-quality luminescent and dielectric films to be used in AC-driven thin film electroluminescent (TFEL) displays. Monochromic yellow-black displays based on a ZnS:Mn luminescent layer have been manufactured industrially using ALD since the mid-1980s. Multicolor displays (green-yellow-red) were successfully realized by filtering the broad emission band of ZnS:Mn or adding another luminescent material, e.g., green-emitting ZnS:Tb or SrS:Ce. However, applicable full-color AC TFEL devices could not be developed because of the lack of an efficient deep blue-emitting phosphor. Currently, the most promising application area in TFEL displays is transparent displays, which are commonly used in various vehicles. In the mid-1980s, epitaxial III-V semiconductors were studied using ALD. It was shown that manufacturing real epitaxial [atomic layer epitaxy (ALE)] films is possible for different III (Al, Ga, In) and V (N, P, As) materials. The advantages of ALE processing compared to more traditional metalorganic chemical vapor deposition or molecular beam epitaxy methods have remained low, however, and ALE is not used on a large scale. Research continues to be carried out using ALE, especially with nitride films. Thin film solar cells have continuously received attention in ALD research. ALD films may be used as both an absorber (CdTe, SnS) and a passivation [In2S3, Zn(O,S)] material. However, in the solar cell field, the real industrial-level use is in passivation of silicon cells. Thin ALD Al2O3 film effectively passivates all types of silicon cells and improves their efficiency. Transition metal dichalcogenides are emerging 2D materials that have potential uses as channel materials in field-effect transistors, as well as phototransistors and other optoelectronic devices. The problem with achieving large-scale use of these 2D materials is the lack of a scalable, low-temperature process for fabricating high-quality, large-area films. ALD is proposed as a solution for these limitations. This review covers all of these ALD applications in detail.

Advances in Alternating Current Electroluminescent Devices

AbstractAlternating current (AC)‐driven electroluminescent (EL) devices have recently attracted attention as potential alternatives to direct current (DC)‐driven organic light‐emitting diodes (OLEDs), as they have the great advantage of easy integration into the AC power system of 110/220 V at 50/60 Hz without complicated back‐end electronics. However, the high driving voltage and low power efficiency inherent to AC‐driven EL devices limit their widespread application. While researchers have made some remarkable progress in this field, the underlying causes during the development process remain to be explored. The strategies for improving the performance of AC‐driven EL devices with different configurations, such as the conventional sandwiched structure and multilayer‐based light‐emitting devices, are summarized in this review. For example, it is crucial to enhance the effective electric field around the emitters for AC‐driven thin film electroluminescent (AC‐TFEL) devices, while the unbalanced generation/injection of charge carriers is the main limiting factor for the performance of AC‐driven light‐emitting devices (AC‐LEDs). The recent advances in AC‐driven EL devices, with some new configurations or new‐type emitting materials, are presented by category. The challenges and opportunities for the further development of AC‐driven EL devices are also discussed.

Thin-film perovskite electroluminescence with BaTiO3 films as insulating layers

ABSTRACTWe have prepared perovskite thin-film electroluminescence with multilayered BaTiO3/((Ca0.6Sr0.4)0.997Pr0.002)TiO3/BaTiO3, obtaining low-electric-field driving. Complete epitaxial growth of all layers and very flat interfaces with monoatomic steps have been obtained. With increasing driving voltage, the intensity of the electroluminescence dramatically increases. The sharp electroluminescence peak at around 610 nm at 11 V becomes much stronger with increasing ac voltage. As a result, more than 95% of the source voltage could be effectively applied to phosphor layers. To effectively apply a source voltage to a phosphor layer, insulating layers with large dielectric constants are recommended.

Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device

The SrS: Ce3+ thin film active phosphor layer of different thickness based alternating current thin-film electroluminescent (ACTFEL) devices are fabricated by electron beam evaporation deposition (EBED) method. The morphology and chemical composition of the deposited films are investigated by field emission scanning electron microscopy (FESEM). The optical transmittance in visible range for the optimum film and optical band gap of SrS: Ce3+ film is described on the basis of Ce3+ doping concentration.

Surface morphology and dielectric behavior of perovskite SrTiO3 thin film in heterostructure electroluminescence devices

Epitaxial SrTiO3 thin film was grown on 1%-Nb-doped SrTiO3 (001) substrate by pulsed laser deposition (PLD). Post-annealing at 1050 °C drastically changes the particle-type surface morphology of the as-grown film to a well-defined step-terrace structure. An atomically smooth surface was obtained with a step height of about 0.4 nm, which corresponds to one-unit-cell of SrTiO3 (a = 0.3905 nm). The dielectric constants εr at 300 and 2 K were found to be 120 and 520 at 100 kHz, respectively. The dielectric losses at 300 and 2 K were 2.0 × 10−5 and 1.2 × 10−5 at 100 kHz, respectively. Temperature dependence of the dielectric properties showed suppressed quantum paraelectricity. Post-annealed films with an atomically smooth surface used as an insulating layer contribute to the high performance of thin-film electroluminescent (TFEL) heterostructure devices. Preparation of ideal interfaces of different materials may aid in the development of heterostructure electronic devices.

Roll-to-roll thin film coating on fluoropolymer webs – Status, challenges and applications

Thin film deposition on fluoropolymer webs – both in vacuum and at atmospheric pressure – faces several critical challenges such as poor mechanical and thermo-mechanical properties – especially low dimensional stability and low elastic modulus, high and textured surface roughness and low adhesion of thin films.This paper discusses critical process parameters in roll-to-roll processing for both vacuum processes and wet coating processes with respect to the unique properties of fluoropolymer webs. Reactively sputtered oxide layers as well as wet coated ORMOCER® layers are deposited on ETFE, PVDF and ECTFE webs to form single- and multi-layer permeation barrier systems. The influence of relevant process parameters on the layer adhesion and permeation barrier performance is reviewed. Permeation barrier coatings have been selected as main application for this study, because they are very sensitive to substrate surface irregularities and mechanical damage due to low adhesion, high strain or roll-to-roll processing issues and therefore are a good measure for the quality of the coating on the substrate.Both the wet coated ORMOCER® layers and reactively sputtered zinc-tin oxide (ZTO) layers show surprisingly good adhesion on both ETFE and PVDF surfaces. Both ZTO and – with adhesion promotion treatment – also aluminum oxide (Al2O3) layers have the potential for low water transmission rates (WVTR) below 5·10−2g/(m2d) at 38°C/90% r.h. on these substrates. Further reduction of the WVTR is demonstrated with a combination of wet coated ORMOCER® layers and sputtered ZTO layers yielding a WVTR of 1·10−3g/(m2d) at 38°C/90% r.h. These values were achieved using adapted process parameters and layer stack designs such as reduced web tension and lower layer thicknesses. The results shown in this paper demonstrate the potential of coated fluoropolymer webs for functionalization of membrane roofs and façades with flexible thin film solar cells; thermal insulation and solar control functionality; flexible thin film electroluminescent and OLED lighting panels as well as electrochromic devices.

Cathodoluminescence in Rare Earth Doped Perovskite Films for RGB Colors

We have studied PL and EL properties of chemically stable perovskite phosphor films. Pr-doped Ca0.6Sr0.4TiO3 perovskite films were grown epitaxially on SrTiO3 (001) substrates and their intense red luminescence were observed. The PL intensity was markedly increased by post-annealing treatments, and the maximum PL intensity was obtained by the combination of relatively low-temperature film growth at 600 °C and sequential rapid post-annealing at 1000 °C. 1 We have discovered perovskite thin film electroluminescence, opening a new optical application of regarding perovskite materials. Complete epitaxial growth of all of the layers and very flat interfaces with monoatomic steps have been obtained. With increasing driving voltage, the intensity of electroluminescence dramatically increases. The sharp electroluminescence peak at around 610 nm at 12 V becomes much stronger with increasing ac voltage. High-quality red color is produced and the working voltage for whole-surface electroluminescence is as low as 10 V. 2 The electron beam irradiation was a method of obtaining electrical and optically emitting. In this report, we show preparation of cathodoluminescence in perovskite films for RGB colors. The films were grown on double-side polished SrTiO3 (001) substrates by pulsed laser deposition. The single-phase polycrystalline targets were prepared by a conventional solid-state reaction method. Compositions of green, blue and red were [(Ca0 . 97Mg0 . 03)0 . 98Tb0 . 02]SnO3, Sr2(Sn0.95Ti0.05)O4 and Pr0.002(Ca0.6Sr0.4)0.997TiO3, respectively. During film growth, the SrTiO3 substrates were heated to 600 °C for film and the oxygen partial pressure was controlled at 50 Pa. An ArF excimer laser (λ = 193 nm) was used that had a repetition rate of 8 Hz and a fluence of around 1.2 J cm−2 pulse−1 at the target surface. In order to take out luminescence excited by the electron beam from the side opposite to with a film adhesion side, the both-side polished substrate which has about 70% (at 500nm) of transparent was used. The field emitter with a gate electrode was used for the source of electrons. The transmittance of film with both-side polished substrate after 1000°C post-annealing treatments was 67 % (at 500nm), indicating that the transmittance of the epitaxially grown alkaline-earth stannates film is very high. When the phosphor film of [(Ca0 . 97Mg0 . 03)0 . 98Tb0 . 02]SnO3was irradiated with the electron beam, the cathode luminescence peaks were observed at around 490, 543, 582, 620 nm. These results clearly show that CL has been successfully obtained in the perovskite films. By using thin film phosphor for FED, high resolution could be realized and it is efficient and also possibility of a long-life. REFERENCES (1) H. Takashima, et al, Applied Physics Letters. 89 261915 (2006). (2) H. Takashima, et al, Advanced Materials . 21 3699 (2009)

High-Resolution Organic Light-Emitting Diodes Patterned via Contact Printing.

In this study, we report a contact printing technique that uses polyurethane-acrylate (PUA) polymers as the printing stamps to pattern electroluminescent layers of organic light emitting diodes (OLEDs). We demonstrate that electroluminescent thin films can be printed with high uniformity and resolution. We also show that the performance of the printed devices can be improved via postprinting thermal annealing, and that the external quantum efficiency of the printed devices is comparable with the efficiency of the vacuum-deposited OLEDs. Our results suggest that the PUA-based contact printing can be used as an alternative to the traditional shadow mask deposition, permitting manufacturing of OLED displays with the resolution up to the diffraction limit of visible-light emission.

The automated system of technological preparation of production of thin-film electroluminescent indicator devices—TFEL DDS

The actual task in the field of instrument making--automation of technological preparation of production of perspective thin-film electroluminescent (TFEL) indicators is considered. The main problems in the field is high labor-intensive characteristic of calculations, tardiness of process of calculations, and also the large volume of the processed data connected by that today the large number of methods and means of a thin-film sputtering are developed. For the purpose of the solution of the problems the automated system of technological preparation of production of thin-film electroluminescent indicator devices--TFEL DDS--was developed. The main features of this system are the automated design of a configuration and a choice of materials of layers of an electroluminescent indicator element taking into account results of the analysis of indicators of TFEL structure, and also drawing up a package of design-technology documentation on the designed TFEL indicator.

Interface-Free Area-Scalable Self-Powered Electroluminescent System Driven by Triboelectric Generator.

Self-powered system that is interface-free is greatly desired for area-scalable application. Here we report a self-powered electroluminescent system that consists of a triboelectric generator (TEG) and a thin-film electroluminescent (TFEL) lamp. The TEG provides high-voltage alternating electric output, which fits in well with the needs of the TFEL lamp. Induced charges pumped onto the lamp by the TEG generate an electric field that is sufficient to excite luminescence without an electrical interface circuit. Through rational serial connection of multiple TFEL lamps, effective and area-scalable luminescence is realized. It is demonstrated that multiple types of TEGs are applicable to the self-powered system, indicating that the system can make use of diverse mechanical sources and thus has potentially broad applications in illumination, display, entertainment, indication, surveillance and many others.