Inorganic Electroluminescence Research Articles

CsPbBr3/Porous‐SiO2 Composite Film for Efficient Perovskite Light‐Emitting Diodes

AbstractThis study aims to enhance the performance of inorganic perovskite light‐emitting diodes (PeLEDs) by incorporating porous silica (p‐SiO2) to fabricate perovskite‐composite films for electroluminescence (EL) devices. This is because, in addition to the inorganic perovskite material, composites with inherently stable materials are needed for perovskite to ensure additional external stability. The introduced p‐SiO2 particles impede the crystal growth of perovskite during the anti‐solvent assisted crystallization process, resulting in the formation of smaller CsPbBr3 crystals in the CsPbBr3/p‐SiO2 composite film. Accordingly, compared to previous CsPbBr3 films, this composite film exhibits two folds with higher photoluminescence quantum yield (PLQY) due to improved crystalline formation. Surprisingly, the CsPbBr3/p‐SiO2 composite film additionally has good water‐resistant properties because the residual cetyltrimethylammonium bromide (CTAB) molecules are extracted from the p‐SiO2 particles and are oriented at the top surface of the CsPbBr3/p‐SiO2 composite film. The EL device fabricated with this composite film exhibits outstanding luminescence efficiency, with a current efficiency (CE) of 70.06 cd A−1 and an external quantum efficiency (EQE) of 16.97%, surpassing control samples by two folds of magnitude. Furthermore, the operational stability improves approximately sevenfold compared to the control, presenting a promising strategy for advancing the field of inorganic perovskite ELs.

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.

Reduction time effect on the dielectric characteristics of reduced-graphene-oxide--encapsulated barium titanate powder fillers

The reduction process for graphene oxide (GO) is important for this kind of graphene to be used as a conducting nanomaterial. After encapsulating barium titanate (BTO) powder particles in GO platelets, the GO-encapsulated BTO was chemically reduced with hydrazine (H2NN(CH3)2) by varying the reduction time (6, 12, 18, and 24 h). Reduced-graphene-oxide--encapsulated BTO (RGO@BTO) fillers were mixed with a polymer matrix, to boost the performance of the dielectric composites with the help of interfacial polarization. All the composite samples with RGO@BTO fillers exhibited substantially higher dielectric constants (52–72%) due to encapsulation, while suppressing dielectric losses. The best dielectric performance was obtained from 18 h (the second longest time) not from 24 h. It can be interpreted that 18 h is a balancing point between the main process of removing oxygen-functional groups and the collateral process of attaching nitrogen-functional groups from a reduction agent of hydrazine. Detailed analyses of XRD, XPS, Raman scattering, and impedance analysis, support the specialty at 18 h. Additionally, AC-driven inorganic electroluminescence devices with RGO@BTO fillers yielded to the best performance at 18 h. Therefore, during the chemical reduction of RGO, optimizing the reduction time is imperative for high dielectric performance.

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.

Optical properties of inorganic powder EL device with a multifunctional dielectric layer

In this paper, we report the optical properties of an inorganic powder electroluminescence (EL) device with a multifunctional dielectric layer. BaTiO3 doped with Yb3+ and Er3+ was used as a dielectric layer of an inorganic powder EL device. Also, BaTiO3 doped with Yb3+ and Er3+ can function as an upconversion phosphor. According to the luminance measurement, when near infrared (NIR) irradiation with 980 nm wavelengths was applied, the luminance was increased by only about 90 cd m−2. However, when both NIR irradiation and 150 root mean square voltages were applied, the luminance increased by about 400 cd m−2. Additionally, the emission spectrum and the chromaticity were changed. Therefore, BaTiO3: Yb3+, Er3+ plays two roles as a dielectric and phosphor simultaneously. The optical properties can also be manipulated by using BaTiO3: Yb3+, Er3+ as a dielectric layer of an inorganic powder EL device.

Graphene-encapsulation effect of BaTiO3 on AC electroluminescence

Alternating current (AC) driven inorganic electroluminescence (EL) was investigated with a focus on the polymer-based dielectric layer, where barium titanate (BTO) powder particles encapsulated in reduced graphene oxide (rGO) platelets. The EL devices with rGO-encapsulated BTO exhibited higher luminescence and lower current density than the non-encapsulated case with BTO only, primarily due to enhanced dielectric performance of the dielectric layer. This was originated from the encapsulation of BTO in rGO platelets through the interfacial polarization of numerous micro-capacitors within the dielectric layer. By varying the concentration of rGO (0, 0.05, 0.10, and 0.20 wt%), the optimum concentration of encapsulated rGO was found to be near 0.10 wt%, where the highest concentration case (0.20 wt%) showed the worst EL performance, possibly due to crisscross linking among rGO-encapsulated BTO. We found that the quality of the dielectric layer, if the optimum concentration of graphene was employed, was a key factor in AC EL.

Highly Stretchable Sound-in-Display Electronics Based on Strain-Insensitive Metallic Nanonetworks.

The growing importance of human–machine interfaces and the rapid expansion of the internet of things (IoT) have inspired the integration of displays with sound generation systems to afford stretchable sound‐in‐display devices and thus establish human‐to‐machine connections via auditory system visualization. Herein, the synchronized generation of sound and color is demonstrated for a stretchable sound‐in‐display device with electrodes of strain‐insensitive silver nanowires (AgNWs) and emissive layers of field‐induced inorganic electroluminescent (EL) phosphors. In this device, EL phosphors embedded in a dielectric elastomer actuator (DEA) emit light under alternating‐current bias, while audible sound waves are simultaneously generated via DEA actuation along with input sound signals. The electroluminescence and sound‐generation performances of the fabricated device are highly robust and reliable, being insensitive to stretch‐release cycling because of the presence of the AgNW stretchable electrodes. The presented principle of integrating light emission and acoustic systems in a single stretchable device can be further expanded to realize sound‐in‐display electronics for IoT and human–machine interface applications.

(Invited) Alternating Current Sensors, Memories and Displays with Ferroelectric Sound

Visualization of human senses has been of great interest for developing an emerging interactive display that can artificially stimulate synesthesia with numerous unprecedented applications. Especially, visualization of various daily sound and music in a form of flexible thin film devices can be a great challenge. In this presentation, we first present flexible artificial synesthesia devices that visualize continuous and complicated sounds [1]. The device is made of a thin composite film of a piezoelectric polymer for sound generation and inorganic electroluminescence (EL) microparticles for direct visualization of input sound signals. Field-induced EL of the microparticles in the device depends upon the source sound wave, making their EL synchronized with sound arising from the piezoelectric actuation. We also demonstrate that piezo and ferroelectric sound is capable of sensing, monitoring, and storing the information of various liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid [2]. Liquid-interactive ferroelectric sound (LIFS) is successfully developed when a liquid droplet is placed on a ferroelectric polymer layer across two in-plane electrodes underneath the ferroelectric layer under an external in-plane AC field. An AC field built up vertically between one of the electrodes and the liquid results in ferroelectric vibration of the device, leading to sound. The sound pressure level (SPL) of the device depends upon the polarity of the liquid, allowing for facile liquid sensing and identification. More importantly, as the SPL arising from LIFS of a liquid is correlated with non-volatile remnant polarisation of the ferroelectric layer, the information of a liquid is readily stored and retrieved even after the liquid is removed, resulting in a sensing memory of the liquid. By programming different LIFSs on a thin film pad, we can also identify the 2D position of a liquid droplet on the pad. Moreover, our device successfully monitored the flow of a human serum liquid passing through channels with their diameters ranging from millimetres to tens of microns and determined its velocity. Mechanically flexible and optically transparent tube-type LIFS AC devices allow for in situ analysis of the flow of a liquid in terms of SPL.

Flexible artificial synesthesia electronics with sound-synchronized electroluminescence

Visualization of human senses has been of great interest for developing an emerging interactive display that can artificially stimulate synesthesia with numerous unprecedented applications. Especially, visualization of various daily sound and music, which are much more complicated than human touch, in a form of flexible thin film devices can be a great challenge. We present flexible artificial synesthesia electronics that visualize continuous and complicated sounds. The electronic device is made of a thin composite film of a piezoelectric polymer for sound generation and inorganic electroluminescence (EL) microparticles for direct visualization of input sound signals. Field-induced EL of the microparticles in the device depends upon the source sound wave, making their EL synchronized with sound arising from the piezoelectric actuation. The flexible artificial synesthesia devices with sound-synchronized EL (FASSEL) showed extreme mechanical tolerance that can be repeatedly folded and crumpled with visible sound, allowing a variety of unexplored applications including synchronous sound-lightings and wearable, on-body sound-vision systems to facilitate emotional interaction of human being with sound in a human-friendly form.

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.

Investigation on direct-current-driven inorganic electroluminescence in rare-earth and Li co-doped ZnO pellets

The phenomena involved in direct-current-driven electroluminescence (DC EL) was investigated, which occurred in the sintered pellets of ZnO doped with rare-earth (RE) and lithium. The microstructural observation revealed that the morphology of loosely-packed small-grains was advantageous for the DC EL, where the doped RE3+ was homogeneously dispersed over the ZnO matrix as small RE2O3 grains. The EL color was dependent on the type of the doped RE3+ and green, blue, or red EL was observed with Er3+, Tm3+, or Eu3+. The EL emission was recognized on the anode side. That brought a new multilayered structure concentrating RE3+ on the anode side to enhance the EL intensity. The occurrence of the EL on the anode side in the presence of an acceptor-type dopant lithium suggested the involvement of holes in the DC EL emission.

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.

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.

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.

A New Kind of Blue Hybrid Electroluminescent Device.

Bright blue Electroluminescence come from a ITO/BBOT doped silica (6 x 10(-3) M) made by a sol-gel method/Al driven by AC with 500 Hz at different voltages and Gaussian analysis under 55 V showed that blue emission coincidenced with typical triple emission from BBOT. This kind of device take advantage of organics (BBOT) and inorganics (silica). Electroluminescence from a single-layered sandwiched device consisting of blue fluorescent dye 2,5-bis (5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) doped silica made by sol-gel method was investigated. A number of concentrations of hybrid devices were prepared and the maxium concentration was 6 x 10(-3) M. Blue electroluminescent (EL) always occurred above a threshold field 8.57 x 10(5) V/cm (30 V) at alternating voltage at 500 HZ. The luminance of the devices increased with the concentration of doped BBOT, but electroluminescence characteristics were different from a single molecule's photoluminescence properties of triple peaks. When analyzing in detail direct-current electroluminescence devices of pure BBOT, a single peak centered at 2.82 eV appeared with the driven voltage increase, which is similar to the hybrid devices. Comparing Gaussian decomposition date between two kinds of devices, the triple peak characteristic of BBOT was consistent. It is inferred that BBOT contributed EL of the hybrid devices mainly and silica may account for a very small part. Meanwhile the thermal stability of matrix silica was measured by Thermal Gravity-Mass Spectroscopy (TG-MS). There is 12 percent weight loss from room temperature to 1000 °C and silica has about 95% transmittance. So the matric silica played an important role in thermal stability and optical stability for BBOT. In addition, this kind of blue electroluminescence device can take advantages of organic materials BBOT and inorganic materials silica. This is a promising way to enrich EL devices, especially enriching inorganic EL color at a low cost.

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