Electroluminescent Device Structure Research Articles

Electricity−free electroluminescence excited by droplet impact driven triboelectric field on Solid−Liquid interface

In this work, a novel solid-liquid interfacing triboelectric field based electroluminescence (SL-TEL) was proposed. When a droplet impacting on the SL-TEL device surface, specific textured green SL-TEL emit on the two cross-overlapped areas. As a coupling effect of solid-liquid interfacing triboelectrification and electroluminescence, the SL-TEL is essentially a type of electroluminescence excited by changing triboelectric field of droplet impact or wave scour. It relies on converting mechanical energy of liquid to visual light energy utilizing an intermediate triboelectric field without any electric power consumption. The SL-TEL device simply consist of three thin flexible layers, which is greatly simplifying the electroluminescent device structure and showing strong advantages of simple structure such as easy to integration and excellent stability. Experiments show that sequential liquid motions of droplet impinging on solid surface excite a corresponding dynamic series of electroluminescence patterns. The features of the electroluminescence also reveal the characteristics of liquid and the solid surface, such as contour edge of liquid spreading, capillary convergence of liquid at the apex (advancing and receding), and so on, which is capable of both sensing liquid leakage and displaying the dynamic information of droplets impacting motions.

Centimeter‐Scale and Visible Wavelength Monolayer Light‐Emitting Devices

AbstractMonolayer 2D transition metal dichalcogenides (TMDCs) have shown great promise for optoelectronic applications due to their direct bandgaps and unique physical properties. In particular, they can possess photoluminescence quantum yields (PL QY) approaching unity at the ultimate thickness limit, making their application in light‐emitting devices highly promising. Here, large‐area WS2 grown via chemical vapor deposition is synthesized and characterized for visible (red) light‐emitting devices. Detail optical characterization of the synthesized films is performed, which show peak PL QY as high as 12%. Electrically pumped emission from the synthetic WS2 is achieved utilizing a transient‐mode electroluminescence device structure, which consists of a single metal–semiconductor contact and alternating gate fields to achieve bipolar emission. Utilizing this aforementioned structure, a centimeter‐scale (≈0.5 cm2) visible (640 nm) display is demonstrated, fabricated using TMDCs to showcase the potential of this material system for display applications.

Maleimide–based donor-π-acceptor-π-donor derivative for efficient organic light-emitting diodes

Herein, we report a highly fluorescent material 3,4-bis(4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)-1-hexyl-1H-pyrrole-2,5-dione (Cbz-MI) based on donor-π-acceptor-π-donor (D-π-A-π-D) backbone. We explored maleimide as an acceptor, phenyl as π-conjugated spacer and carbazole as donor. Photophysical properties revealed high photoluminescence quantum yield of 0.84 and 0.72 in solution and doped matrix, respectively. An encouraging external quantum efficiency of 2.5% with emission peak at 550 nm is achieved utilizing Cbz-MI as emitting layer in doped electroluminescent device structure.

Switchable two-color electroluminescence based on a Si metal-oxide-semiconductor structure doped with Eu

A Si metal-oxide-semiconductor electroluminescent device structure is reported which emits two colors, while being doped with a single rare-earth element. Thermally grown SiO2 oxide layers were implanted with Eu and subseqently annealed. Depending on the electrical excitation current, the luminescence is red or blue, which can be ascribed to electronic transitions in tri- and divalent europium (Eu3+ and Eu2+), respectively.

The light-emitting device consisting of organic white-light components

A material compound, 9,9-bis{4′-[2″-(carbazolyl)-vinyl]-phenyl}fluorene (F-CZV), was specially synthesized and used to fabricate the efficient white organic light-emitting devices (WOLEDs). The absorption peaks appear at 340 nm and 346 nm in dilute dichloromethane solution and film, respectively. The photoluminescence peaks appear at 350 nm and 400 nm in the solution and film, respectively. Photoluminescent quantum yield in solution is ca 0.82 by using quinine sulfate as the standard. In an optimized electroluminescent device structure of ITO/NPB (40 nm)/F-CZV (30 nm)/BPhen (40 nm)/Mg:Ag, the saturated white-light emission was observed at Commission International De L’Eclairage (CIE) coordinates of (0.30, 0.33) at 10 V. The El spectrum of the device is close to independent of the applied driving voltage. It’s maximum brightness and current efficiency is 700 cd/m 2 and 0.41 cd/A, respectively.

White organic light-emitting diodes based on bis(2-methyl-8-quinolinolato) (para-phenylphenolato)aluminium(III) doped with tiny red dopant

A bright white electroluminescence (EL) emission can be achieved using a minimum doping technique, that is, the concentration of the dopant DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljuloli-dyl-9-enyl)- 4H-pyran) was controlled to be 0.5 wt% of the host BAlq (bis(2-methyl-8-quinolinolato)(para-phenylphenolato)aluminium(III)). The white EL device structure is ITO/copper phthalocyanine (CuPc) (5 nm)/N, N′-bis(naphthyl)-N, N′-diphenyl-1,1′- biphenyl-4,4′-diamine(NPB)(50 nm)/BAlq : DCJTB(0.5 wt%, 30 nm)/tris(8-hydroxyquinolinato) aluminium(III)(Alq3)(20 nm)/LiF(0.5 nm)/Al(100 nm). The white emission is composed of a red band from DCJTB and a blue emission from BAlq. A maximum luminance of 21 000 cd m−2 at the direct current driving voltage of 15 V, with a power efficiency of 1.10 lm W−1 at 11 V is achieved. The Commission Internationale de L'Eclairage coordinates of the white emission are well within the white zone which moves from (0.32, 0.34) to (0.30, 0.32) when the applied voltage is varied from 8 to 11 V.

White organic electroluminescence from an exciplex based on the novel substituted aluminium quinolate complex

A saturated white organic light-emitting device was obtained from an exciplex mechanism by using a novel blue-green fluorescent tris(5-mesitylsulfone-8-hydroxyquinolato)aluminium(III), Al(MSq)3. Its absorption and photoluminescence appear at 3.25 eV and 2.56 eV. In photoluminescence, an exciplex, generated from an Al(MSq)3/TPD blend, was found at 2.10 eV. The lifetime of the Al(MSq)3/TPD blend is 22.3 ns, which is longer than that of either Al(MSq)3 or TPD. Due to the exciplex formation, the orange light generated can be combined with the blue-green light generated from Al(MSq)3 to achieve a white light emission. In an optimized electroluminescence device structure of [glass/ITO (200 nm)/PEDOT:PSS (75 nm)/blend (72 nm)/BCP (8 nm)/LiF (1 nm)/Al (200 nm)], the saturated white light emission was observed at Commision Internationale De L'Eclairage (CIE) coordinates of (0.35, 0.34). The EL spectrum of the device is also independent of the applied voltage. Its maximum brightness and efficiency are 26 cd m−2 at 15.2 V/67 mA cm−2, and 597 µW A−1 at 12.5 V/28 mA cm−2, respectively.

Electroluminescent property of tetrakis(phenyl)porphyrin carbonyl ruthenium (II)

Luminescent tetrakis(phenyl)porphyrin carbonyl ruthenium (II) (RuTPPCO) complex was employed as a doped emitting material to fabricate red organic electroluminescent (EL) devices. The EL device structure was [ITO/copper phthalocyanine (Cu-Pc) (15.0 nm)/N,N′-di(α-naphthyl)-N, N′-diphenyl-(1,1′-biphenyl)-4, 4′-diamine (NPB) (60.0 nm)/tris(8-hydroxyquinolinato)aluminum (Alq3): RuTPPCO (50.0 nm)/LiF (1.0 nm)/Al (200.0 nm)]. The code-posited films of Alq3:RuTPPCO were utilized as the emitting layer to construct EL devices. Experimental results showed that energy transfer from Alq3 to RuTPPCO occurred in the codeposited films. The EL property of the codeposited Alq3:RuTPPCO films with different RuTPPCO concentrations was described. For the EL device with the RuTPPCO concentration of 15% by weight, deep red electroluminescence at 656 nm with the maximum EL efficiency of 0.32 cd/A was achieved.

Visible electroluminescence from amorphous hydrogenated silicon carbide prepared by using organic carbon source

A series of amorphous silicon carbide (a-SiC:H) films were prepared by using an organic carbon source, xylene, in a plasma chemical vapor deposition system. Visible electroluminescence (EL) was observed from the device structures of Al/a-Si:H/a-SiC:H/ITO/glass at room temperature. The EL peak was changed from 2.0 to 2.3 eV with increasing carbon content in a-SiC:H films and the EL devices have open voltages at approximately 2.5 V. The EL behavior was investigated and the influence of the EL device structure was also discussed in the paper.

Sputter deposited GaN doped erbium thin films: Photoluminescence and 1550 nm infrared electroluminescence

Erbium doped gallium nitride (GaN) thin films were deposited on Si substrates by reactive rf magnetron cosputtering of a commercial GaN target, together with a metallic erbium target. Nitrogen was employed as the reactive sputtering gas. The gallium nitride doped erbium (GaN:Er) films thus obtained, exhibited the characteristic 525, 540, 660, and 1550 nm photoluminescence emission associated with the Er+3 ion 4f–4f intraband transitions. In addition, 1550 nm IR electroluminescence (EL) emission was observed from the sputter deposited GaN:Er phosphor films. The EL device was an inverted half-stack ac thin-film EL device structure. The 1550 nm EL emission is consistent with impact excitation and subsequent I13/24→4I15/2 radiative relaxation of Er+3 ions. Impact excitation requires conduction electrons with sufficient energy from electrical field acceleration, to excite the transition.

Photoluminescent and electroluminescent properties of Mn-doped ZnS nanocrystals

ZnS:Mn nanocrystals with sizes between 3 and 4 nm were synthesized via a competitive reaction chemistry method, where the surface capping organic species (p-thiocresol) is used as an inhibitor of the crystal growth. The x-ray diffraction and photoluminescent (PL) properties of ZnS:Mn bulk and nanocrystals were compared. A direct current electroluminescent (EL) device having a hybrid organic/inorganic multilayer structure, indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT-PSS)/PVK/ZnS:Mn NC/Al, was tested. In this multilayer EL device structure, the PEDOT-PSS leads to enhanced hole injection, while the poly(N-vinylcarbazole) (PVK) serves as a passivation layer between the PEDOT-PSS and nanocrystal layers. Electron–hole recombination was not confined to the ZnS:Mn nanocrystal layer, but also occurred in the PVK layer. The result was emission from both the blue-emitting PVK and yellow-emitting ZnS:Mn nanocrystals. The EL emission spectrum was dependent upon the voltage, showing an increasing ratio of PVK emission to nanocrystal emission with increased voltage. The dependence of PL and EL emissions on Mn concentration (0.40 to 2.14 mol %) is also reported.

Contrast-enhancement in black dielectric electroluminescent devices

A high contrast electroluminescent (EL) device structure is presented. The diffuse luminous reflectivity from the metal/dielectric/phosphor/indium-tin-oxide/glass EL device structure is /spl sim/3%. A Eu-doped GaN phosphor is used to demonstrate the contrast-enhanced operation. Low reflectivity is achieved by inserting a light-absorbing black thick-film BaTiO/sub 3/ layer between the phosphor and the rear metal electrode. In addition to providing contrast enhancement, the opaque thick dielectric film exhibits capacitance and high voltage reliability (40 nF/cm/sup 2/, dielectric constant /spl epsi//sub d/ /spl sim/ 500-1000, breakdown field E/sub d,br/ /spl sim/ 0.1-0.4 MV/cm) similar to that of the highest performance transparent thin-film dielectrics. An EL device luminance of only 20 cd/m/sup 2/ is sufficient for a display contrast ratio of /spl sim/10:1 under 140 lux indoor ambient lighting (illumination). Under sunlight illumination of 100000 lux, a display contrast ratio of >3:1 is expected with application of additional contrast enhancement techniques.

Color Passive-matrix Organic Electroluminescent Displays.

We have developed 2.2-inch diagonal (132 x RGB x 162 dots) Color Passive-matrix Organic Electroluminescent (OEL) Displays through the use of patterned lateral RGB emitters, and have started the pilot production. The devices have achieved practical level in their performance. The organic electroluminescent layers and the metal cathodes have been fabricated using metal shadow (slits) masks. Further examination of the panel fabrication process and the organic materials has been made, and the results have been applied to the display devices. The organic electroluminescent device structure of the display panel is a four-layer stack consisting of a hole injection layer, a hole transport layer, an emitting layer, and an electron transport layer. The display pixel consists of three-color elements, which are green, red, and blue. These three emitting materials, the hole injection layer and the electron transport material have been newly developed. The display panel has been encapsulated by a stainless steel cap and driven by the dual-scan method. That display is thin (2.5mm, the driver circuit included) and light (10g, the driver circuit included), and has a wide range of viewing angles. That is proven to be practical for commercial purposes.

Bright red light‐emitting organic electroluminescent devices based on a novel thiophene‐containing europium complex as an emitting layer

A novel europium complex, Eu(DTP)3(dipphen) (DTP = 1,3-di(2-thienyl)propane-1,3-dione, dipphen = 4,7-diphenyl-1,10-phenanthroline), has been synthesized and applied to organic electroluminescent (EL) devices as an emitting and electron-transporting material. The EL device structure can be described as indium tin oxide (ITO)/hole-transporting layer (triphenyldiamine derivative (TPD))/emitting layer (Eu(DTP)3(dipphen))/electron transport layer (Alq3)/AlLi (99∶1). The emitting layers were formed by a codeposition technique. The deposited EL devices were observed to emit a bright red light originating from Eu(DTP)3(dipphen) with a maximum luminance of 450 cd m–2 at 15 V and 200 mA cm–2.

Characterization of Thin-Film Electroluminescent Devices with Multiple Ta2O5 Interlayers Incorporated into SrS:Pr,Ce Phosphor

A thin-film electroluminescent device (ELD) has been fabricated by incorporating multiple Ta2O5 interlayers into the SrS:Pr,Ce phosphor layer using rf-magnetron sputtering. X-ray measurement results confirm the enhanced crystallinity of the phosphor layer and can be explained by the extra thermal-cycle annealing due to the different deposition temperatures for Ta2O5 (100° C) and SrS (500° C). The inclusion of thin barrier films is expected to redistribute the internal field and introduce additional interface states in the ELD structure. Based on the results of Sawyer-Tower circuit measurements, it is found that the transferred charge density in the active layer can increase with the number of intermediate layers. This contributes to an increase in brightness and the reduction of the threshold voltage. Moreover, a stacked structure for SrS:Pr and SrS:Ce phosphor layers with window effects is proposed to alleviate the absorption problem in the SrS:Pr,Ce multi-barrier ELD structure. Details of the mechanism and the improvement in the chromaticity will also be described.

High electric field electroluminescence in hydrogenated amorphous silicon carbide alloys

High electric field effects in hydrogenated amorphous silicon carbide alloys have been investigated employing a double insulating ac-driven electroluminescent device structure. The carbon content was systematically changed by deposition conditions. Two distinguished high electric fields effects, namely, hot-electron induced electroluminescence tailing into above-gap spectral domain and avalanche multiplication indicated from saturation of the electric field were found at the electric field strength of about 1.4-2.3 MV/cm in amorphous silicon carbide as the effects in hydrogenated amorphous silicon. From an analysis of the decay of the emission spectrum in terms of the lucky-drift model, the electron mean free path can be estimated; the mean free path tends to be reduced with an increase of the optical energy gap, which implies it would be reduced with an increase of structural disorder due to carbon alloying as in lower electric field.

Electroluminescence and Avalanche Multiplication at Electric Field Strength Exceeding 1 MV/cm in Hydrogenated Amorphous SiC Alloy

High electric field effects in hydrogenated amorphous silicon carbide alloy have been investigated employing a double insulating ac-driven electroluminescent device structure. The carbon content was systematically varied by changing the deposition conditions. Hot-electron-induced electroluminescence and avalanche multiplication occurred at an electric field of 1.4–2.3 MV/cm in amorphous silicon carbide as in hydrogenated amorphous silicon. From an analysis of the decay of the emission spectrum in terms of the lucky-drift model, the electron mean free path tends to be reduced with an increase in structural disorder due to carbon alloying.

Hot-electron induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon

High electric field effects above 1 MV/cm in a-Si:H, have been investigated employing an ac-driven double insulating electroluminescent device structure. An emission tailing into energies above the optical energy gap has been observed, giving direct evidence for existence of hot-electrons in a-Si:H generated by an electric field. An analysis of the emission spectrum due to the lucky-drift model indicates the mean free path of 1.0 nm for hot-electrons. Electroabsorption measurements reveal that the internal electric field in a-Si:H saturates at ∼ 1.5 MV/cm, which implies multiplication of charges transferred across the a-Si:H layer at this field strength. Avalanche multiplication would be the most consistent mechanism with the results of the hot-electrons induced light emission.

Visible electroluminescence from porous silicon/hydrogenated amorphous silicon pn-heterojunction devices

We report the fabrication and characterization of a p-type porous silicon/ n-type hydrogenated amorphous silicon (a-Si:H) pn-heterojunction electroluminescent device structure. The devices exhibit electroluminescence in forward bias, demonstrating minority carrier injection from n-type a-Si:H into p-type porous silicon.

Hot-electron-induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon

High-electric-field phenomena above 1 MV/cm in a-Si:H have been investigated using an ac-driven double-insulating electroluminescent device structure. An emission tailing into energies above the optical energy gap has been observed, giving direct evidence for the existence of hot electrons in a-Si:H generated by an electric field. An analysis of the emission spectrum indicates an average energy of 0.13 eV and a mean free path of 1.0 nm for hot electrons. Electroabsorption reveals that the internal electric field in a-Si:H saturates at about 1.2–1.5 MV/cm, which implies multiplication of charges transferred across the a-Si:H layer at this field strength. Experimental data suggest avalanche multiplication due to field-induced hot electrons.