Powder Electroluminescent Devices Research Articles

Thermal Diffusion Films with In-Plane Anisotropy by Aligning Carbon Fibers in a Cellulose Nanofiber Matrix.

For highly efficient heat dissipation of thin electronic devices, development of film materials that exhibit high thermal conductivity in the in-plane direction is desired. In particular, it is important to develop thermally conductive films with large in-plane anisotropy to prevent thermal interference between heat sources in close proximity and to cool in other directions by diffusion. In this study, we developed flexible composite films composed of a uniaxially aligned carbon-fiber filler within a cellulose nanofiber matrix through liquid-phase three-dimensional patterning. The film exhibited a high in-plane thermal conductivity anisotropy of 433%, with combined properties of a thermal conductivity of 7.8 W/mK in the aligned direction and a thermal conductivity of 1.8 W/mK in the in-plane orthogonal direction. This remarkable thermal conductivity and in-plane anisotropy showed the ability to significantly cool powder electroluminescent devices formed on the composite film and also to cool two heat sources in close proximity without thermal interference. In addition, the carbon-fiber filler could be extracted from the composite films by heat treatment at 450 °C and reused as a thermally conductive material.

Latest Research Developments in Powder Electroluminescent Devices

Latest Research Developments in Powder Electroluminescent Devices

Enhancement of Luminance in Powder Electroluminescent Devices by Substrates of Smooth and Transparent Cellulose Nanofiber Films.

Powder electroluminescent (EL) devices with an electric field type excitation are surface light sources that are expected to have a wide range of practical applications, owing to their high environmental resistance; however, their low luminance has hindered their use. A clarification of the relationship between the properties of the film substrates and the electroluminescence is important to drastically improve light extraction efficiency. In this study, powder EL devices with different substrates of various levels of surface roughness and different optical transmittances were fabricated to quantitatively evaluate the relationships between the substrate properties and the device characteristics. A decrease in the surface roughness of the substrate caused a clear increase in both the current density and the luminance. The luminance was found to have a direct relationship with the optical transmittance of the substrates. The powder EL device, which was based on a cellulose nanofiber film and was the smoothest and most transparent substrate investigated, showed the highest luminance (641 cd/cm2) when 300 V was applied at 1 kHz.

Development of Electrochromic Device with Poly(3,4-ethylenedioxythiophene) on Invisible Silver-Grid Transparent Electrode

Electrochromic (EC) reaction is defined as a reversible color change which induced by the electrochemical redox reactions. The EC device is promising as an optical modulation device such as a display and a window. However, a flexibilities of the EC device is unsatisfied because conventional EC device consists of transparent conductive oxide on the glass substrate. To overcome this, we have developed a gravure offset printed invisible Ag-grid transparent electrodes by using Ag-nanoparticles ink,[1] and demonstrated that a powder electroluminescent device by this transparent electrode showed the high bending stability.[2], [3] Therefore, it is also expected the realization of EC device showing high bending durability.In this study, the novel EC device composed of poly(3,4-ethyenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has been fabricated on a gravure offset printed fine Ag-grid transparent electrode and evaluated optical and electrochemical properties. Scanning the potential from 0 V (vs. Ag/Ag+) to the negative direction, the cathodic current due to the reductive reaction of PEDOT:PSS was observed, and the absorbance correspond to the coloration of PEDOT:PSS was increased. Then, when scanning the potential to the positive direction, the anodic current due to the oxidative reaction of PEDOT:PSS was observed, and the absorbance was decreased. This changing in the absorbance were observed several tens of times. These evaluation results demonstrate that PEDOT:PSS develops a reversible color change with the EC reaction on the fine silver-grid transparent electrode. Reference [1] M. Ohsawa, N. Hashimoto, Mater. Res. Express, 5, 085030 (2018).[2] N. Takeda, S. Tsuneyasu, K. Yanagawa, N. Hashimoto, M. Ohsawa, T. Satoh, Electron. Lett., in press.[3] M. Ohsawa, N. Hashimoto, N. Takeda, S. Tsuneyasu, T. Satoh, Flex. Print. Electron. (under review). Figure 1

Novel powder electroluminescent device enabling control of emission color by thermal response

AbstractWe demonstrated the emission of multicolored light from a powder electroluminescent device using a thermochromic material through temperature control. At 20°C, the emission of a sapphire blue light was observed. Upon increasing the temperature, the color of the thermochromic materials changed, leading to sky‐blue emission.

CuO Nanowire-Enhanced Alternating Current-Driven Powder Electroluminescent Device with High Performance

We report here the first use of a vertically aligned copper oxide (CuO) nanowire array in an alternating current-driven zinc sulfide (ZnS:Cu)-based powder electroluminescent (ACPEL) device. As a re...

Enhanced electroluminescence in fully printable powder electroluminescent device with flexible invisible silver‐grid transparent electrode

Powder electroluminescent devices (PELDs) have a high potential for commercial application because of flat light emission devices and simple printing process fabrication with low-cost production, without semiconductor processes using vacuum and plasma. In conventional PELDs, indium tin oxide (ITO) has been used as a transparent electrode. However, ITO is unfavourable in flexible electronic devices due to its intrinsic brittleness. The authors evaluated the current and luminance characteristics of PELDs prepared on gravure offset-printed invisible silver-grid (Ag-grid) electrodes that were laminated with and without poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) (PEDOT:PSS) transparent electrodes. The PELD on the Ag-grid laminated with PEDOT:PSS electrode shows the maximum luminance of ∼250 cd/m2 at 200 μm spacing, which is five-fold greater than the luminance of PELD without PEDOT:PSS electrode lamination.

In brief

PAGE 632 Chinese researchers propose a symbol cyclic reconstruction scheme for universal-filtered multi-carrier systems (UFMC). The overlap-add technique is used to lower the computational complexity of the system transmitter, with ISI cancellation used at the receiver to remove interference between neighbouring symbols. Simulation results show the superiority of the scheme over state-of-the-art systems. PAGE 616 Researchers from Iran present a monitoring technique for tracking the state-of-health of batteries, suitable for use in battery management systems. The method can detect early degradation of batteries by monitoring the switching and battery transitions in a battery management system. Experiments on a typical charge balancing circuit verify the performance of the technique. Block diagram of SCR-UFMC with multipath fading channels. PAGE 589 Indian researchers propose a novel estimation technique for a noise-influenced electronic circuit, unifying the Carleman linearisation with the Fokker-Planck equation for Itô's Stochastic Differential Equations. Numerical simulation results are presented for a benchmark tunnel diode oscillator circuit. The method may prove useful for estimating state trajectories under the influence of randomness when observations are not possible. Laboratory test bench. PAGE 612 A collaboration of researchers from Japan present an evaluative study on the current and luminance characteristics of powder electroluminescent devices prepared on gravure offset-printed invisible Ag-grid electrodes. Results show that the luminance is strongly dependent on the transparency and light-extraction efficiency of the Ag-grid electrodes. Flow of the proposed estimation technique.. PAGE 609 Researchers from China propose a wideband differential filtering power divider, using a three-lined coupled structure with multiple lumped elements. Compared with other differential filtering power dividers, the proposed design shows the advantages of a wide bandwidth, compact size and low insertion loss. A prototype validates the simulated results. Optical microscopy image of the emitted light from PELDs on the Ag-grid electrodes. Photograph of the proposed wideband differential filtering power divider.

Flexible powder electroluminescent device on transparent electrode of printed invisible silver-grid laminated with conductive polymer

Invisible Ag-grid transparent electrodes at a line width of the Ag-grid of 5 μm, spacing between the grid lines of 150 μm and a line thickness of 0.6 μm have been prepared on a 50 μm thick flexible polyethylene naphthalate (PEN) film substrate by a conventional gravure offset printing using our newly developed Ag nanoparticle ink. Furthermore, the Ag-grid electrode was laminated with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer. The Ag-grid laminated with PEDOT:PSS (Ag-grid/PEDOT) transparent electrode was applied to a commercial alternating-current powder electroluminescent device (ACPELD) as an ITO replacement. The as-prepared Ag-grid/PEDOT-based ACPELD shows uniform luminescence with a luminance of 292 cd m−2 at 150 V0-p and 2 kHz. Repeated bending durability of the fabricated ACPELD was evaluated by a cyclic bending test at a bending radius of 1.5 mm up to 20 000 bending cycles. Luminance of the Ag-grid/PEDOT-based ACPELD is stable throughout the bending cycles compared to that of an ITO-based ACPELD. Luminance of the 20 000 cycle-bended Ag-grid/PEDOT-based ACPELD maintains 97% of that of the as-prepared. These results demonstrate that the Ag-grid/PEDOT on PEN-based ACPELDs develop stable luminance characteristics throughout the bending cycles.

Bending reliability of transparent electrode of printed invisible silver-grid/PEDOT:PSS on flexible epoxy film substrate for powder electroluminescent device

Invisible Ag-grid transparent electrode at a grid line width of 5 μm, spacing between the grid lines of 200 μm and a grid line thickness of 0.6 μm have been prepared on a 50 μm-thick flexible epoxy film substrate by a conventional gravure offset printing using our developed Ag nanoparticle ink. Furthermore, the Ag-grid were laminated with a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer. Bending durabilities of the Ag-grid laminated with PEDOT:PSS (Ag-grid/PEDOT:PSS) transparent electrode were investigated via bending tests. The Ag-grid/PEDOT:PSS transparent electrode on the epoxy film substrate shows no noticeable resistance change throughout 20,000 outer bending cycles at a bending radius of 1.0 mm. The Ag-grid/PEDOT:PSS transparent electrode was applied to a commercial alternating-current powder electroluminescent device (ACPELD). Repeated bending durability of the fabricated ACPELD was investigated via a cyclic bending test. Luminance of the fabricated ACPELD is stable throughout the 30,000 outer bending cycles at a bending radius of 1.0 mm. The transparent electrode-based ACPELD develops also excellent bending durability.

EL emission enhancement in powder electroluminescent device via insertion of receptive layer

A method to enhance the electroluminescence (EL) of powder electroluminescent devices via the insertion of a receptive layer between the letter substrate and the transparent electrode is proposed. This approach affords uniformity of distance between the substrate and electrode, which ensures a more uniform electric field on application of a voltage to the device. Both the current and luminance are observed to increase, and the device capacitance and capacitive reactance improve on introduction of the receptive layer, which enhances the EL properties. The emission-enhancement mechanism is examined via measurements of the frequency dependence of the current, luminance, and loss tangent tan δ. Regardless of the receptive layer, the current decreases in the frequency regime over which tan δ increases, which indicates that the EL properties are affected by electrical energy losses due to dielectric loss. The frequency-dependence behaviours of tan δ and temperature are found to be different because of heat production due to both dielectric loss and radiationless deactivation, which means that dielectric loss must be reduced to improve device efficiency. Their approach can form an integral aspect of the evaluation of powder electroluminescent devices.

Powder electroluminescent device based on paper substrate for designing multi-functional digital signage

Powder electroluminescent device based on paper substrate for designing multi-functional digital signage

Effect of Introducing Receptive Layer to Paper Substrate in Powder Electroluminescent Device

Effect of Introducing Receptive Layer to Paper Substrate in Powder Electroluminescent Device

Alternating Current‐Driven Oxide Powder Electroluminescent Device Employing Vertically Aligned ZnO Nanowire Array

A novel type of AC‐driven oxide powder electroluminescent (EL) device is demonstrated. The device is based on stable heterogeneous junctions between Mn2+‐activated zinc germanate (Zn2GeO4:Mn) oxide phosphor and a vertically aligned ZnO nanowire array. The ZnO nanowire array is synthesized on an ITO‐coated glass substrate via a hydrothermal growth method. A stable phosphor–nanowire junction is formed by simple drop‐coating of a Zn2GeO4:Mn oxide phosphor suspension on the nanowire layer followed by annealing at 400 °C for 3 h. Bright green light (≈20 cd m−2) with a wavelength of 535 nm from the as‐fabricated EL device is observed, and the as‐fabricated device shows excellent stability in air, maintaining a brightness of ≈10 cd m−2 for 15 days with less than 2.5% intensity loss. The electroluminescence emission mechanism is discussed in detail. This new and facile strategy to build an oxide powder EL device can enable cost‐effective, highly stable, and energy‐efficient devices for lighting or display applications.

Photoluminescence and Electrical Properties of ZnO Nanopowders Prepared by a Solution Combustion Method for Optoelectronic Device Application.

Nano-sized powders of ZnO phosphor were prepared by a solution combustion method (SCM). The ZnO powder prepared using Zn(OH)₂ and glycine as the oxidant and fuel, respectively, (fuel/oxidant = 0.8), show good powder characteristics such as an average grain size of 30 nm and specific surface area of 120 m²/g. Single-phase ZnO powders were obtained regardless of the fuel/ oxidant ratio. The photoluminescence spectra of the obtained ZnO powders show a single sharp peak near 390 nm corresponding to a deep blue color. This was confirmed by cathodoluminescence measurement and CIE color coordinate values. The PL spectra, powder characteristics and electrical properties show very good consistency. Furthermore, the electron carrier concentration of the ZnO nanopowders prepared by SCM is more than 3 orders of magnitude higher than that of the purest ZnO single crystal, which is commercially available. These powders could be potentially applied as blue phosphors of displays such as A.C. powder electroluminescent devices and plasma display panels.

Efficient Solid-State Photoluminescence of Graphene Quantum Dots Embedded in Boron Oxynitride for AC-Electroluminescent Device.

Emerging graphene quantum dots (GQDs) have received much attention for use as next-generation light-emitting diodes. However, in the solid-state, π-interaction-induced aggregation-caused photoluminescence (PL) quenching (ACQ) in GQDs makes it challenging to realize high-performance devices. Herein, GQDs incorporated with boron oxynitride (GQD@BNO) are prepared from a mixture of GQDs, boric acid, and urea in water via one-step microwave heating. Due to the effective dispersion in the BNO matrix, ACQ is significantly suppressed, resulting in high PL quantum yields (PL-QYs) of up to 36.4%, eightfold higher than that of pristine GQD in water. The PL-QY enhancement results from an increase in the spontaneous emission rate of GQDs due to the surrounding BNO matrix, which provides a high-refractive-index material and fluorescence energy transfer from the larger-gap BNO donor to the smaller-gap GQD acceptor. A high solid-state PL-QY makes the GQD@BNO an ideal active material for use in AC powder electroluminescent (ACPEL) devices, with the luminance of the first working GQD-based ACPEL device exceeding 283 cd m-2 . This successful demonstration shows promise for the use of GQDs in the field of low-cost, ecofriendly electroluminescent devices.

A CuO Nanowire‐Based Alternating Current Oxide Powder Electroluminescent Device with High Stability

AbstractAn AC‐driven powder electroluminescent (EL) device has been achieved by constructing a CuO nanowire–Zn2GeO4:Mn phosphor heterogeneous junction. The CuO nanowires enhance the local electric field, resulting in electroluminescence of an oxide‐based phosphor in EL devices owing to field injection at the nanowire tips. The CuO nanowire array was synthesized by an in situ thermal oxidation method at 400 °C in air and employed as an electric field enhancement layer in the EL device. The heterogeneous structures were created through drop coating of a phosphor suspension on the CuO nanowire array. The initial EL device tests show good luminescent performance with very promising brightness maintenance for over 360 h, with a loss of luminescent intensity of under 1 % at over 10 cd m−2 luminance. The fabrication method offers the prospect of simple, low‐cost, large‐scale EL devices with the potential to solve the limited operational lifetime of sulfide‐based AC powder EL devices.

A CuO Nanowire-Based Alternating Current Oxide Powder Electroluminescent Device with High Stability.

An AC-driven powder electroluminescent (EL) device has been achieved by constructing a CuO nanowire-Zn2 GeO4 :Mn phosphor heterogeneous junction. The CuO nanowires enhance the local electric field, resulting in electroluminescence of an oxide-based phosphor in EL devices owing to field injection at the nanowire tips. The CuO nanowire array was synthesized by an in situ thermal oxidation method at 400 °C in air and employed as an electric field enhancement layer in the EL device. The heterogeneous structures were created through drop coating of a phosphor suspension on the CuO nanowire array. The initial EL device tests show good luminescent performance with very promising brightness maintenance for over 360 h, with a loss of luminescent intensity of under 1 % at over 10 cd m-2 luminance. The fabrication method offers the prospect of simple, low-cost, large-scale EL devices with the potential to solve the limited operational lifetime of sulfide-based AC powder EL devices.

Printing Smart Designs of Light Emitting Devices with Maintained Textile Properties

To maintain typical textile properties, smart designs of light emitting devices are printed directly onto textile substrates. A first approach shows improved designs for alternating current powder electroluminescence (ACPEL) devices. A configuration with the following build-up, starting from the textile substrate, was applied using the screen printing technique: silver (10 µm)/barium titanate (10 µm)/zinc-oxide (10 µm) and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (10 µm). Textile properties such as flexibility, drapability and air permeability are preserved by implementing a pixel-like design of the printed layers. Another route is the application of organic light emitting devices (OLEDs) fabricated out of following layers, also starting from the textile substrate: polyurethane or acrylate (10–20 µm) as smoothing layer/silver (200 nm)/poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (35 nm)/super yellow (80 nm)/calcium/aluminum (12/17 nm). Their very thin nm-range layer thickness, preserving the flexibility and drapability of the substrate, and their low working voltage, makes these devices the possible future in light-emitting wearables.

Optical and electrical properties of Zn2SiO4:Mn2+ powder electroluminescent device

Green-emission Zn2SiO4:Mn2+ powder phosphor was applied for powder electroluminescence device through a screen printing method. The EL device consisted of silver nanowires as top electrode, 6μm-thick Zn2SiO4:Mn2+ phosphor layer, 45µm -thick BaTiO3 dielectric layer, and metal bottom electrode. The EL device showed the strong 525nm green emission spectrum. Its luminance-voltage dependence showed the exponential increase, and its luminance-frequency dependence showed the linear increase and then saturation behavior. It is notable that its temperature dependence showed the constant behavior at lower temperature, and then the drastic rising pattern up to Curie temperature of the dielectric layer (~ 120°C), and then the thermal quenching trend. The maximum luminance was 0.96cd/m2 where the power consumption was 250W/m2 at 420Vp and 400Hz, and thus the luminous efficiency was 0.012lm/W.