Thin-film Electroluminescent Devices Research Articles

Near-infrared thin-film electroluminescent device from ZnGa2O4:Cr3+ on silicon wafer

This study systematically investigated the Cr3+-doped ZnGa2O4 electroluminescent (EL) device, demonstrating the emission of near-infrared (NIR) light. The ZnGa2O4:Cr3+ film was prepared by a facile and cost-effective method within all-air atmosphere processes. It consists of a naturally grown amorphous SiOx and an optically active ZnGa2O4:Cr3+ layer. Under a sinusoidal voltage waveform, the device emits an NIR spectrum without the need for additional background gas or other implantation methods. The proposed mechanism suggests that light generation occurs through hot electrons released from the amorphous SiOx layer, injected into the active layer, leading to impact-ionization. The optical characteristics of the EL device are presented, and its response to applied polarities is thoroughly investigated. Finally, given the cost-effectiveness of our EL device preparation, this proposed research is of considerable importance for advancing the development of EL devices in silicon photonics.

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.

Low voltage thick-film electroluminescent device with evaporated ZnS:Mn and spin coated BaTiO3 dielectric layer

In a thick-dielectric electroluminescent (TDEL) device, which is a variation of thin-film electroluminescent devices, one of the insulation layers is replaced by a thick-film dielectric layer. High brightness and reliability are the features of TDEL owing to a higher withstanding voltage of the thick-film dielectric layer. This paper presents a new type of TDEL with a bottom emission structure. The thick-film dielectric layer was manufactured by spin coating using an ink material, which is a mixture of BaTiO3 powder and a binder resin. The thicknesses of the phosphor and dielectric layers were minimized to reduce the operating voltage. A threshold voltage as low as 50 V and a maximum luminance of 1000 cd/m2 were obtained at 60 V.

The effect of surfactants and precursors on the structure and properties of ZnS:Cu nanocrystalline particles

Doped ZnS materials exhibit excellent optical and electrical properties, making them a superior material for alternating current (AC) thin film electroluminescent devices, solar panels, light-emitting diodes, laser diodes and other luminescent device designs and fabrications. This study involves the hydrothermal synthesis of nanocrystalline Cu-doped ZnS materials without a surfactant and using two surfactants – hexadecyltrimethylammonium bromide (HTAB) and sodium dodecyl sulfate (SDS). The structure, morphology and elemental content of the synthesized nanopowders were characterized by X-ray powder diffraction, X-ray absorption spectroscopy (XAS), scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) techniques, while the optical properties were studied by photoluminescence (PL) spectroscopy. The sizes of the nanocrystallites in all the synthesized samples were below 20 nm. XAS results suggest that Cu ions substitute Zn ions in cubic zinc blende ZnS structure and are located in a locally distorted environment. The addition of a surfactant allows control of the morphology of the particles and their photoluminescent properties.

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.

Super-stretchable polymer-AgPdCu superlattice electrodes for high-performance wearable electronics

Highly-stretchable and semi-transparent polytetrafluoroethylene (PTFE)/AgPdCu (APC)-PTFE-based superlattice electrodes were realized by alternately sputtering very thin APC-PTFE hybrid layers on a polyurethane (PU) substrate at room temperature. The electrical and mechanical properties of the superlattice electrodes were optimized by controlling the PTFE thickness and the number of layers of the stacked superlattice. The optimal PTFE/APC-PTFE/PTFE/APC-PTFE/PTFE superlattice electrode showed a sheet resistance of 8.5 Ohm/square and maintained a high conductivity, even at a strain of 100%. The outstanding mechanical stretchability of the superlattice electrode is attributed to the 3-dimensional conductive APC network structure in the PTFE polymer matrix . In addition, the external strain applied to the superlattice electrode led to a change in the optical transmittance that could be used for optical switching in stretchable electronics. The superlattice electrodes can be utilized to fabricate stretchable electroluminescent devices, stretchable thin film heaters, and wearable temperature sensors . This successful demonstration of stretchable devices that include highly stretchable superlattice electrodes demonstrated the feasibility of this manufacturing strategy for use in next-generation stretchable and wearable electronics . • Fabrication of highly conductive and stretchable metal-polymer 3D nanocomposites by continuous co-sputtering of polymer and metal target. • The number of stacked layers and thickness of the polymer and polymer-metal hybrid films were controlled to form an alternating superlattice structure. • The optimized electrode showed a sheet resistance of 8.5 Ohm/square, an average transmittance of 34.0% for visible light, and stretchability that reversibly maintained a high conductivity at up to 100% strain. • The optimized electrode was applied as stretchable thin-film heater, electroluminescent device, and wearable temperature sensor.

Field-induced light emission from a close-packed Mn-doped ZnS quantum-dot layer in an alternate-current thin-film electroluminescent configuration

By exploiting the configuration of an alternate-current thin-film electroluminescent device, we observe the emission of light from a close-packed spin-cast layer of manganese-doped zinc sulfide nanoparticles (NPs), sandwiched between two dielectric layers, induced by the application of an AC voltage. By means of a microwave-assisted polyol synthesis, we are able to obtain high-crystalline surfactant-free NPs, which can be closely packed in the emitting layer, allowing an emission mechanism due to local charge creation and transport, instead of the typical impact-excitation process. This synthesis route allows us to get rid of a bulk semiconductor layer, implying that far less expensive routes for the active layer deposition such as spin coating, inkjet printing, or spray coating can be used. Our work allows us to get a deeper insight into the role played by the NP structure on the charge-transfer mechanism behind light emission and paves the way to a new generation of non-toxic quantum-dot-based displays.

Highly flexible Ag nanowire network covered by a graphene oxide nanosheet for high-performance flexible electronics and anti-bacterial applications

ABSTRACT We investigated a flexible and transparent conductive electrode (FTCE) based on Ag nanowires (AgNWs) and a graphene oxide (GO) nanosheet and fabricated through a simple and cost-effective spray coating method. The AgNWs/GO hybrid FTCE was optimized by adjusting the nozzle-to-substrate distance, spray speed, compressor pressure, and volume of the GO solution. The optimal AgNWs/GO hybrid FTCE has a high transmittance of 88% at a wavelength of 550 nm and a low sheet resistance of 20 Ohm/square. We demonstrate the presence of the GO nanosheet on the AgNWs through Raman spectroscopy. Using scanning electron microscopy and atomic force microscopy, we confirmed that the nanosheet acted as a conducting bridge between AgNWs and improved the surface morphology and roughness of the electrode. Effective coverage by the GO sheet improved the conductivity of the AgNWs electrode Effective coverage of the GO sheet improved conductivity of the AgNWs electrode with minimum degradation of optical and mechanical properties. Flexible thin film heater (TFH) and electroluminescent (EL) devices fabricated on AgNWs/GO hybrid FTCEs showed better performance than devices on bare AgNWs electrodes due to lower sheet resistance and uniform conductivity. In addition, an AgNWs/GO electrode layer on a facial mask acts as a self-heating and antibacterial coating. A facial mask with an AgNWs/GO electrode showed a bacteriostatic reduction rate of 99.7 against Staphylococcus aureus and Klebsiella pneumonia.

Role of Pr3+ ions as hole-trapping centers for the electroluminescence of (Ca0.6Sr0.4)0.998Pr0.002Ti0.9Al0.1O3−δ thin films

Thin-film electroluminescent devices composed of an (In0.975Sn0.025)2O3 anode, a phosphor, and a Sn0.95Sb0.05O2 cathode were prepared by a sol–gel and firing method. The phosphor thin films were composed of four layers, (Ca0.6Sr0.4)0.998Pr0.002Ti0.9Al0.1O3–δ or Ca0.6Sr0.4Ti0.9Al0.1O3–δ. The relation between the space distribution of Pr3+ ions and the electric/electroluminescence properties were investigated. The electroluminescence occurred predominantely at (Ca0.6Sr0.4)0.998Pr0.002Ti0.9Al0.1O3–δ layer closest to the anode. We suggested the electroluminescence mechanism that a hole injected from the anode is trapped at a Pr3+ ion, an electron injected from the cathode recombines with the trapped hole, and the Pr3+ ion is excited to emit a photon.

Thin film electroluminescent device based on magnetron sputtered Tb doped ZnGa2O4 layers

Photoluminescent (PL) layers and electroluminescent (EL) systems prepared by different methods have been systematically studied for the fabrication of flat panel displays, monitoring screens, and lighting systems. In this work we report about a new procedure of preparing Tb doped ZnGa2O4 green luminescent thin films at low temperature that consists of the simultaneous reactive magnetron sputtering (R-MS) deposition of a Zn–Ga mixed oxide acting as a matrix and the plasma decomposition (PD) of evaporated terbium acetylacetonate. The resulting films were transparent and presented a high PL efficiency making them good candidates for EL applications. Layers of this phosphor film with thickness in the order of hundreds nanometers were sandwiched between two dielectric layers of Y2O3 and AlSiNxOy that were also prepared by R-MS. The response of the resulting EL device was characterized as a function of the applied voltage and the type of AC excitation signal. The high luminance and long-term stability of these thin film electroluminescent devices (TFELDs) proves the reliability and efficiency of this kind of transparent R-MS multilayer system (with a total thickness in order of 650 nm) for display and lighting applications.

Time dependence of current density, luminance, and efficiency under dc voltages for the thin-film electroluminescent device containing praseodymium and aluminum co-doped perovskite titanate phosphor

Time dependence of current density, luminance, and efficiency under dc voltages were measured for the thin-film electroluminescent device containing (Ca0.6Sr0.4)0.998Pr0.002Ti0.9Al0.1O3−δ phosphor in order to understand the electroluminescence mechanism. The current density was not zero and the luminance was close to zero immediately after voltages were applied to the device. Then, the current density and the luminance increased, reached a maximum, and decreased. The luminance and the efficiency did not recovered at room temperature but recovered at 400 °C. We proposed the band structures of the device depending on time due to electromigration of oxygen vacancies.

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.

Ultraviolet emission from YAlO3:Gd3+ thin film electroluminescent devices fabricated on perovskite-type oxide substrates

Inorganic ultraviolet-electroluminescent (UV-EL) devices with a Gd3+-doped YAlO3 thin film emitting layer were fabricated by radio frequency sputtering. Three kinds of perovskite-type oxide substrates, namely a SrTiO3 single crystal (STO-SC), BaTiO3 single crystal (BTO-SC), and BaTiO3 polycrystalline disk (BTO-PC), were used for the devices. The influence of the substrates on the UV-EL properties of YAlO3:Gd3+ was investigated. It was found that EL threshold voltage, emitting locations, and EL intensity have changed depending on the substrates. The UV-EL attributed to the 6PJ-8S7/2 transitions in Gd3+ was observed in the BTO-PC substrate device with applied voltage of approximately over 200 V and in the STO-SC and BTO-SC substrate devices with that of approximately over 500 V. The STO-SC and BTO-SC devices showed EL only near the edges of a back electrode, whereas the BTO-PC device showed surface EL over the whole electrode. It was considered the local EL at the electrode edges in the SC substrate devices was caused by a concentrated electric field in the vicinity of the electrodes. The BTO-PC device showed not only the lowest EL threshold voltage but also the most intense EL in the three devices. Because the BTO-PC substrate has a high dielectric constant and appropriate surface roughness, which results in a high and uniform electric field over the back electrode, the YAlO3:Gd3+ emitting layer showed intense surface UV-EL with lower applied voltage.

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.

Investigation the effect of annealing temperature on the optical properties of CdSe thin films

Introduction: CdSe is an important II–VI semiconducting material dueto its typical optical properties such as small direct band gap (1.7 eV) anda high refractive index and, thus, a major concern is focused on the investigation of optical properties of CdSe thin films which is important topromote the performances of the devices of solid -state such as SC (solar cells), thin film transistors, LED (light-emitting diodes), EBPL (electron–beam pumped lasers) and electroluminescent devices. In the presentwork, CdSe thin films were deposited by thermal evaporation method andthe results have been analysed and presented. Materials and Methods:CdSe thin films has been deposited on glass microscopic slides as substrates of (75×25×1 mm) under room temperature using PVD technique.CdSe blended powders gets evaporated and condensed on the substrate.The film thickness (t = 100 5 nm) which is measured using Michelsoninterferometry method. Transmission spectrum, from 200-1100 nm, arescanned using two beams UV–VIS Spectrophotometer (6850 UV/Vis.Spectrophotometer-JENWAY). The deposited films then were annealedat temperature range of (1500C to 3500C) under vacuum to have a stable phase of the material and prevent surface oxidization. Results andDiscussion: A transmittance spectrum of CdSe thin film is scanned overwavelength range 200 to 1100 nm using a (6850 UV/Vis. Spectrophotometer-JENWAY) at room temperature. The transmittance percentagebetween the as-deposited film and the annealed films change varies from(17.0%) to (47.0%). It is clearly seen that there is a shift toward higher energy (Blue Shift) in the transmittance spectrum. As annealing temperatureincreased the transmittance edge is shifted to the longer wavelength (i.e.,after annealing the CdSe films shows red shifts in their optical spectra).The band gap was found within the range 1.966-1.7536 eV for CdSe thinfilm. As annealing temperature increases, the Eg continuously decreases.Conclusions: CdSe thin films have been deposited using Physical VaporDeposition (PVD) Technique. It is found that the transmission for asdeposited films is (17%) and increases to (47%) as annealing temperature increases. Beside this the energy gap for as- deposited CdSe film is(1.966eV) and decreased from (1.909 eV) to (1.7536eV) as the annealingtemperature increases. There is a strong red shift in optical spectrum ofthe annealed CdSe films. There is a gradual shift of the annealed filmsthin film spectra as compared of bulk CdSe films

Effects of doping by aluminum or lanthanum on the electrical and electroluminescence properties of Ca0.6Sr0.4TiO3:Pr thin films

Thin-film electroluminescent devices containing Ca0.6Sr0.4TiO3:Pr, Al-doped Ca0.6Sr0.4TiO3:Pr, or La-doped Ca0.6Sr0.4TiO3:Pr phosphors were prepared by a conventional sol–gel and firing method. Doping by Al improved the performance of the device markedly, in that its current-starting voltage and emission-starting voltage decreased, its luminance response became more rapid, and its relative quantum efficiency increased. Almost all the effects of doping with La were opposite to those of doping with Al. On stacking thin films of Al-doped Ca0.6Sr0.4TiO3:Pr and La-doped Ca0.6Sr0.4TiO3:Pr, the current-starting voltage and the emission-starting voltages decreased further and the relative quantum efficiency increased further in comparison with the Al-doped film. The effects of doping by Al and La can be understood by assuming that Ca0.6Sr0.4TiO3:Pr separates into n-type and p-type semiconducting regions as a result of the electromigration of oxygen vacancies, and that the oxygen-vacancy density increases or decreases on doping by Al and La, respectively.

Recent developments of truly stretchable thin film electronic and optoelectronic devices.

Truly stretchable electronics, wherein all components themselves permit elastic deformation as the whole devices are stretched, exhibit unique advantages over other strategies, such as simple fabrication process, high integrity of entire components and intimate integration with curvilinear surfaces. In contrast to the stretchable devices using stretchable interconnectors to integrate with rigid active devices, truly stretchable devices are realized with or without intentionally employing structural engineering (e.g. buckling), and the whole device can be bent, twisted, or stretched to meet the demands for practical applications, which are beyond the capability of conventional flexible devices that can only bend or twist. Recently, great achievements have been made toward truly stretchable electronics. Here, the contribution of this review is an effort to provide a panoramic view of the latest progress concerning truly stretchable electronic devices, of which we give special emphasis to three kinds of thin film electronic and optoelectronic devices: (1) thin film transistors, (2) electroluminescent devices (including organic light-emitting diodes, light-emitting electrochemical cells and perovskite light-emitting diodes), and (3) photovoltaics (including organic photovoltaics and perovskite solar cells). We systematically discuss the device design and fabrication strategies, the origin of device stretchability and the relationship between the electrical and mechanical behaviors of the devices. We hope that this review provides a clear outlook of these attractive stretchable devices for a broad range of scientists and attracts more researchers to devote their time to this interesting research field in both industry and academia, thus encouraging more intelligent lifestyles for human beings in the coming future.

Rb添加によるSrS:Ce薄膜EL素子の青色純度とEL動作の安定性の改善(材料・デバイス連合研究会)

Rb添加によるSrS:Ce薄膜EL素子の青色純度とEL動作の安定性の改善(材料・デバイス連合研究会)

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.