Inorganic Electroluminescent Devices Research Articles

Film characteristics of ZnS:Mn phosphor prepared by metal precursor sulfurization and its application to inorganic electroluminescent devices

A ZnS:Mn phosphor film for an inorganic electroluminescent (EL) device fabricated by metal precursor sulfurization was evaluated. Metallic ZnMn was thermally evaporated at room temperature and then sulfurized by sulfur gas flowing in a tube furnace at various temperatures from 400 to 800 °C. X-ray diffraction, energy dispersive x-ray spectroscopy, and photoluminescence observations indicated that a ZnS:Mn phosphor layer was formed after sulfurization. A thick-dielectric EL device was manufactured using the obtained ZnS:Mn phosphor. The EL luminance of 216 cd/m2 at 1.0 kHz was measured when the phosphor layer was sulfurized at 500 °C. A charge density versus voltage (Q-V) curve was also analyzed to evaluate the EL phosphor characteristics.

UVC Emission Devices Based on Alternating Current Inorganic Electroluminescence Using Y2SiO5:Pr3+ Phosphors

Ultraviolet C (UVC: 200-280 nm) lighting is urgently required for a number of sterilization applications such as water and air disinfection, food processing, and medical sterilization. Compared to other UVC emission sources, AC-driven inorganic phosphor electroluminescent (EL) devices are appealing due to their wide viewing angle, fast response time, low power consumption, high contrast, simple manufacture, and cost effectiveness. Although the luminance of inorganic phosphor EL devices is typically low, it is sufficient for most low-cost lighting and large-area display applications. In addition, inorganic phosphor EL devices have the advantage of emitting light without generating much heat. In this study, UVC emission was demonstrated through EL devices using a UVC phosphor. Pr3+-doped Y2SiO5 (Y2SiO5:Pr3+) is used for UVC light, Y2SiO5:Pr3+ phosphor shows a broad UVC emission at 280 nm from the f-d transition of Pr3+ ions as well as a sharp red emission at 610 nm from the f-f transition of Pr3+ ions. The phosphors were synthesized in three types to fabricate EL devices. The Y2SiO5:Pr3+ powder, film, and nano-powder formed the phosphor layers of EL devices by screen printing, spin coating, and electrophoretic deposition, respectively. In order to emit UVC light, patterned gold was used for transparent electrodes of the devices instead of conventional indium tin oxide (ITO), which absorbs the UV region. The EL devices were demonstrated in two types: powder EL and electrolyte EL. The fabricated EL devices showed a wide range of UVC emission from 260 nm to 330 nm. Finally, luminescent and electrical analysis of each device was performed to analyze the characteristics.

Effect of inductor types and shapes on the characteristics of an inorganic EL driven by wireless power transfer

A wireless power transfer (WPT) circuit was developed for an inorganic electro-luminescent (EL) device. When transmission and receiver circuits are both in resonant mode, low operating voltage of 30 V was enough to achieve the EL luminance as high as 180 cd m−2. As for the transmission and the receiver coils, different types and shapes are studied to clarify those effects on the EL luminance and transmission efficiency. Solenoid coils generally presented higher luminance than other types. Polygonal solenoid coils resulted in even higher luminance and transmission efficiency than circular solenoids. A spiral or a spider coil, in which air core area is much smaller than the solenoid coil, was found to be less sensitive to coil displacement. The difference in EL performances with the variation of coils suggests the possibility of further improvement the WPT system.

Spectral-temporal luminescence properties of Colloidal CdSe/ZnS Quantum Dots in relevant polymer matrices for integration in low turn-on voltage AC-driven LEDs.

This work employs spectral and spectral-temporal Photoluminescence (PL) spectroscopy techniques to study the radiative mechanisms in colloidal CdSe/ZnS Quantum Dot (QD) thin films without and with 1% PMMA polymer matrix embedding (QDPMMA). The observed bimodal transient-spectral PL distributions reveal bandgap transitions and radiative recombinations after interdot electron transfer. The PMMA polymer embedding protects the QDs during the plasma-sputtering of inorganic layers electroluminescent (EL) devices, with minimal impact on the charge transfer properties. Further, a novel TiO2-based, all-electron bandgap, AC-driven QLED architecture is fabricated, yielding a surprisingly low turn-on voltage, with PL-identical and narrow-band EL emission. The symmetric TiO2 bilayer architecture is a promising test platform for alternative optical active materials.

Electroluminescence in perovskite oxide nanocrystals

Chemically stable and easily printable inorganic surface-emitting devices are essential for portable, disposable, outdoor-safe, and wearable information display devices. However, inorganic electroluminescent devices with both chemical stability and printability have never been realized. We present the first report of an oxide-nanocrystalline powder-type self-emitting device without both capping and dispersion treatments. The Ca0.6Sr0.4TiO3:Pr nanocrystal was prepared by a hydrothermal synthesis method under supercritical water conditions. The red light-emitting electroluminescence intensity increased with an increase in the driving voltage. The starting electric field for electroluminescence generation is sufficiently low (6.9 × 104 V cm−1). The proposed electroluminescent device is achieved by minimizing the voids into the inside of the light-emitting layer and the interface between the light-emitting and insulating layers by using nanometer-sized oxide-phosphor-crystals. The oxide-nanocrystal electroluminescent device will be developed into future powder-type surface-emitting devices that may contribute in inventing numerous applications, such as outdoor information displays and wearable information terminals.

Chlorobenzene-treated carbon nanotubes in a dielectric layer and their impact on inorganic electroluminescence

We investigated alternating current driven inorganic electroluminescence (EL) devices, in which chlorobenzene (CB) treated carbon nanotubes (CNTs) were incorporated within the dielectric layer. The devices having CB-treated CNTs exhibited better performance than the reference device without CNTs. Especially, 0.01 wt% of CB-treated CNTs yielded the best results—an enhancement of up to 50% in the luminance, along with a 30% reduction in the current density compared with the reference one. The improvement in EL performance was attributed to the enhanced dielectric constant of the dielectric layer by inclusion of CB-treated CNTs. This is considered to arise from the combined effect of the micro-capacitors from numerous CNTs in the dielectric layer and the interfacial polarization from the CCl bonds originating from the CB treatment. In addition, it is essential to control the concentration of CNTs low enough to prevent leakage current paths by percolation, here, 0.01 wt% sufficed this condition. Thus, incorporating CB-treated CNTs into the dielectric layer improved the characteristics of the EL devices, which would be considered as a candidate for future display applications.

Emission of dispersed‐type inorganic EL devices with frequency‐variable high‐voltage oscillation circuit

AbstractDispersed‐type inorganic electroluminescent (EL) devices composed of a transparent electrode, a phosphor, a dielectric, and a back electrode were prepared under various conditions using a zinc sulfide (ZnS)‐based phosphor. Additionally, a voltage/frequency variable circuit was designed. A compact high‐voltage/frequency variable circuit including three modules for boosting, frequency conversion, and voltage conversion was designed. A 140 Vpp voltage and a frequency in the range of 270 Hz to 2.4 kHz can be controlled by this circuit. The emission has begun to be observed at a voltage about 60 Vpp and a frequency of 400 Hz, at a voltage about 40 Vpp and a frequency of 1.4 kHz, 2.4 kHz, respectively. The emission intensity increased with an increase in frequency; emission with a wavelength of 450 nm was strongly influenced by the frequency. The luminescence and the electrical properties were affected by the preparation conditions including device structures, dispersion of ZnS:Cu, and Cl particles because of the current path generated by defects in the EL cell.

High-efficiency all-inorganic full-colour quantum dot light-emitting diodes

All-inorganic quantum dot light-emitting diodes (QLEDs) with excellent device stability have attracted significant attention for solid state lighting and flat panel display applications. However, the performance for the present all-inorganic QLEDs is far inferior to that of the well-developed QLEDs with organic charge transport layers. Our all-inorganic full-colour QLEDs show the maximum brightness and efficiency values of 21,600 cd/m2 and 6.52%, respectively, which are record-breaking among the existing all-inorganic QLEDs. The outstanding performance is achieved by an efficient design of device architecture with solution-processed charge transport layers (CTLs). Meanwhile, the ultrathin double-sided insulating layers are inserted between the quantum dot emissive layer and their adjacent oxide electron transport layers to better balance charge injection in the device and reduce the quenching effects for inorganic CTLs on QD emission. This study is the first account for high-performance, all-inorganic QLEDs insightfully offering detailed investigations into the performance promotion for inorganic electroluminescent devices.

Freely Deformable Liquid Metal Grids as Stretchable and Transparent Electrodes

A new structure of stretchable and transparent metal-grid electrodes is described, where a liquid metal alloy, eutectic gallium–indium (EGaIn) is employed as a stretchable conducting material. By a “roll-painting and lift-off” technique based on photolithography, stretchable EGaIn-grids with the linewidth of 20 $\mu \text{m}$ and the line pitch of 400–1000 $\mu \text{m}$ are produced, providing the transmittance of 75%–88% with the sheet resistance less than 2.3 $\Omega $ /square. Powered by the freely deformable nature of the liquid phase conductor, the EGaIn-grid provides stable conductivity under highly stretched condition up to 40%, as well as high reliability against cyclic deformation. By combining the EGaIn-grid with a homogeneous and transparent poly(3,4-thylenedioxythiophene):poly(styrensulfonate) (PEDOT:PSS) layer in a hybrid manner, we can construct highly stretchable and uniformly conducting transparent electrodes. The hybrid PEDOT:PSS/EGaIn-grid electrodes can be utilized as electrodes for a stretchable inorganic electroluminescence device with a ZnS phosphor-elastomer composite as the emission layer, which maintains uniform and stable emission under highly and repeatedly stretched conditions.

Photomechanical modification of ZnS microcrystal to enhance electroluminescence by ultrashort-pulse laser processing

A ZnS microcrystal was treated with an ultrashort-pulse laser and applied to an inorganic electroluminescence (EL) phosphor. We found that the emission intensity of the EL phosphor was increased by laser-induced photomechanical modification. The pulse duration dependence of the emission enhancement and structural analysis by scanning electron microscopy indicated that the structural modification was induced inside the ZnS microcrystal, although a mechanical grinding would induce the structural modification mainly on the crystal surface. The results suggested a new way of enhancing the emission of inorganic EL devices.

A novel electroluminescent device based on a reduced graphene oxide wrapped phosphor (ZnS:Cu,Al) and hexagonal-boron nitride for high-performance luminescence.

Reduced graphene oxide (rGO) has recently emerged as a very promising family of exotic carbon material with augmented performance in electronic and optoelectronic devices. Herein, we report an efficient and novel inorganic electroluminescent device geometry, where a new phosphor composite, reduced graphene oxide wrapped ZnS:Cu,Al, acts as an active emitting layer and an exfoliated hexagonal boron nitride (h-BN) as a dielectric layer. The roles of rGO in the active layer as a conductive support and local electric field enhancing agent are attributed to its wrinkles being unraveled compared with other carbon exotic nano-forms such as carbon nanotubes, graphite, charcoal and activated carbon, which significantly improves the brightness of the device (∼50 cd m-2 for 0.50 wt% rGO/ZnS:Cu,Al at 10 kHz and 110 V with an external quantum efficiency of ∼6.3% ± 0.1% and current efficiency of ∼0.81 ± 0.09 cd A-1). This new and facile strategy to construct the luminescent devices could be a paradigm shift towards cost effective, highly stable in air (for several days) and energy efficient next generation display devices.

Efficiency Enhancement of Flexible Inorganic Powder Electroluminescent Devices Using the BaTiO3 -MWNT Composite Dielectric Layer

Alternating current inorganic powder electroluminescent (IPEL) devices possess many advantages including wide viewing angle, wide operating temperature range, and easily manufactured by using a screen printing process. This study developed an IPEL devices with low power consumption and high brightness on a transparent polyethylene terephthalate substrate by introducing a composite dielectric layer, which was formed by incorporating single- or multiwall carbon nanotubes (SWNTs or MWNTs) into the BaTiO3-based dielectric layer. The luminous efficiency of the developed devices, called the IPEL: carbon nanotubes (CNT) devices, increased with appropriate incorporation ratios of CNTs, regardless of SWNTs or MWNTs. The optimal device performance was achieved with a 0.5 wt $\%$ mixing ratio of MWNTs, in which the power consumption of the device decreased 33.7 $\%$ and the luminous efficiency increased 50.9 $\%$ at the brightness of 100 cd/m2. The result of bending test with a curvature radius of 1 cm for 104 times showed that the IPEL:MWNT device had a superior luminous reliability and its variation of the efficiency was less than 6 $\%$ . The developed IPEL:MWNT device has potential as a candidate in future flexible displays.

High Electroluminescence of ZnS:Cu Phosphor with a CNT-Incorporated Dielectric Layer

High brightness was obtained for alternating current-driven inorganic ZnS:Cu powder electroluminescence (EL) devices by incorporating carbon nanotubes in a barium titanate (BaTiO3) based dielectric layer. The inorganic EL devices were consisted of four layers: a top electrode, a dielectric layer, an emitting layer, and an ITO layer which was pre-coated on the glass substrate. As an emitting material, ZnS:Cu power of particle size 9 µm (green emission) was mixed with the cyanoethyl pullulan polymer (CEP), as an organic binder, and the dimethyl formamide (DMF) organic solvent. The emitting solution was spin-coated on the ITO glass at the fixed speed of 1200 rpm. The BaTiO3 power were mixed with a CNT-CEP binder, which was also spin-coated on the emitting layer. In order to optimize the EL performance, the concentration of CNTs were varied by controlling the mixing ratio of CNTs—0, 0.0018, 0.0039, 0.0071, 0.011 and 0.014 wt%. Finally, a 500-nm-thick aluminum top electrode was thermally evaporated onto the dielectric layer. Up to 50% higher brightness was obtained for the CNT concentration of 0.011 wt%. The high brightness might be caused by the high dielectric constant of the CNT-incorporated BaTiO3 dielectric layer, thanks to the micro-capacitor effect. [Z. M. Dang et al., Small 12, 1688 (2016).] Roughly speaking, the higher concentration of the CNT yielded to the higher brightness, until a certain concentration, here, between 0.011 and 0.014 wt%. It implies that too high concentration of CNTs might lead to the formation of the unwanted percolation network of a leakage current within the dielectric layer. Finally, the role of the chlorine was investigated by inclusion of chlorobenzene into the CNT-containing dielectric layer. [J.-Y. Kim et al., Adv. Mater. 25, 2308 (2013).] Based on the above results, incorporation of CNTs within the dielectric layer enhanced the brightness (and the efficiency, possibly) of the ZnS:Cu-based EL device, which would help to extend the potential applications of EL devices.

Optical properties of inorganic electroluminescent devices with nanostripe electrodes

In this paper, we report on the luminescence (emission) characteristics of a laminated dispersion-type inorganic electroluminescent (EL) device with a nanostripe electrode made of thin Al film, instead of a conventional indium–tin oxide (ITO) transparent electrode, on the emission side of the device. The transmittance of the Al nanostripe electrode, with 60-nm line-and-space widths, was 45%. We compared an inorganic EL device positioned between two thin films of Al and the inorganic EL device with the Al nanostripe electrode using electric field simulations and actual experiments. We were able to apply the same electric field intensity to the phosphor layer in the conventional structure and to the new structure. Therefore, with an Al nanostripe electrode on one side of the EL device, it is possible to fabricate an ITO-free display.

Graphene을 첨가하여 스크린 프린팅으로 제작한 ZnS:Cu,Al 무기 전계발광 소자의 광특성

Graphene을 첨가하여 스크린 프린팅으로 제작한 ZnS:Cu,Al 무기 전계발광 소자의 광특성

Optical effect on TiO2 additive in ZnS based inorganic electroluminescence device fabricated by screen printing

Optical effect on TiO2 additive in ZnS based inorganic electroluminescence device fabricated by screen printing

Effect of halohydrocarbon solvents on the dielectric performance of a polymer film and its application to inorganic electroluminescent devices

Polymer films with halohydrocarbon solvents as additive solvents were fabricated to investigate the solvent effect on dielectric performance. Bromobenzene, chlorobenzene (CB), chloroform, and fluorobenzene (FB) were used as halohydrocarbon solvents. The cyanoethyl pullulan (CRS) polymer film with CB had a dielectric constant of 21.5 with a dielectric loss of 0.017 at 100 Hz, which exhibited a 37% increase in dielectric constant compared to the neat CRS polymer film. Increment of the dielectric constant is attributed to the high polarizability of carbon–chlorine bonds originated from CB absorbed in CRS. For the case of the CRS polymer film with FB, the dielectric constant did not increase due to the low polarizability of carbon–fluorine bonds. In order to verify the dielectric performance of the CRS polymer film with CB (CRS/CB), AC-driven inorganic powder electroluminescent (EL) device was fabricated using the green emission ZnS:Cu,Cl phosphor particles. An EL device with CRS/CB polymer film as a dielectric layer exhibited a 37% and 55% increase in luminance and efficiency, respectively compared to that with neat CRS polymer. This increase in EL performance was attributed to the increase of charge carrier density at the interface between the emitting and dielectric layers, resulting in the increased possibility of the charge carrier tunneling into phosphor particles.

Electroluminescence of a Multilayer in which Thin Films of NaNbO<sub>3</sub>:Pr Phosphor and SnO<sub>2</sub>:Sb Transparent Conductor Are Alternately Stacked

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.

Evaluation of distributed-type inorganic electroluminescence devices using comb-shaped electrodes

We deposited comb-shaped metal electrodes with narrow gaps on a glass substrate and succeeded in achieving a high electric field intensity. Using conventional structures, this result could only be achieved by applying transparent electrodes, and in this study, we used a comb-shaped metal (Au) electrode and a comb-shaped indium tin oxide (ITO) electrode. When we measured the luminescence from the opposite side of the comb-shaped electrodes, at 15 V/µm, the luminance in the ITO electrode sample was identical (92–95 cd/m2) to that measured from the side of the comb-shaped electrodes. Conversely, a 111-cd/m2 brightness (a 1.9-fold increase) was observed from the opposite side of the Au electrode. Electrode transmittance was unnecessary when the luminescence was observed from the opposite side of the comb-shaped electrodes, and by using metals such as Au, it was possible to produce a display that did not require transparent electrodes. Finally, we simulated the electric field distribution for the inorganic electroluminescence (EL) using comb-shaped electrodes by semiconductor device simulator Atlas.

스크린프린팅법으로 인쇄한 단일층 구조의 무기전계 발광소자의 특성에 관한 연구

스크린프린팅법으로 인쇄한 단일층 구조의 무기전계 발광소자의 특성에 관한 연구