Thin-film Electroluminescent Research Articles

Effect of TPD films as electron blocking layer on EL spectrum

In an organic thin film electroluminescence (OTFEL) device with an ITO/PPV/Alq:DCM1/A1 structure, a TPD layer was inserted between the PPV and Alq:DCM1 layers. A device without the TPD layer or with a 5-nm thick TPD layer operating at higher DC voltage (≥8 V) exhibited EL emission from both the PPV (513 nm) and Alq:DCM1 (591 nm) layers, however, a device with a 5-nm thick TPD layer operating at lower voltage (<7 V) or with a 10-nm thick TPD layer had an EL emission peak at 591 nm, implying that the TPD layer can effectively block electron transportation. Hence we can spatially control the carrier recombination zone and obtain different EL emissions by adjusting the thickness of the carrier blocking layer and the driven voltage.

Ultraviolet-blue electroluminescence from Gd3Ga5O12:Ag

UV-blue light was obtained from a thin-film electroluminescence device using Gd3Ga5O12:Ag as a light-emitting layer, which was deposited by using electron-beam evaporation. The crystal composition and structure of Gd3Ga5O12:Ag were studied by x-ray powder diffraction. The Gd3Ga5O12:Ag has a photoluminescence emission which peaked at around 397 and 467 nm, which were attributed to the oxide vacancies and Ag+, respectively. The brightness of 32 cd/m2 was obtained when an alternating voltage of 130 V at 1 kHz was applied.

Visible emission from electroluminescent devices using an amorphous AlN:Er3+ thin-film phosphor

Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. Thin films of Er-doped AlN, ∼200 nm thick, were grown on indium–tin–oxide/aluminum–titanium–oxide/glass substrates using rf magnetron sputtering in a nitrogen atmosphere. The turn-on voltage was found to be around 70–80 and 100 V for ACTFEL devices without and with a top insulator layer. Sharp emission lines in the visible region were observed which correspond to known transitions of the Er3+ ion. Temperature-dependent cathodoluminescence studies corroborate the EL results, and show that optimum device performance is attained near 300 K.

Characteristics of a SrS:Ce Thin-Film Electroluminescent Device Prepared by Post-Annealing in H2S

Strontium sulfide doped with cerium (SrS:Ce) thin-film electroluminescent (TFEL) devices annealed in the presence of H2S are investigated. X-ray diffraction patterns of SrS phosphor layers show a preferential (220) orientation, and cerium accumulated in the lower portion of the phosphor in samples annealed in H2S. The relative concentrations of S and Sr (S/Sr ratios) of the phosphors before annealing are thought to be responsible for the EL characteristics. Annealing in H2S had a significant effect on the crystallinity of phosphors with low S/Sr ratios before annealing.

Multicolor-emitting thin-film electroluminescent devices using Ga2O3 phosphors co-doped with Mn and Cr

High-luminance multicolor-emitting thin-film electroluminescent (TFEL) devices have been developed using a new Mn- and Cr-co-doped Ga2O3 phosphor with variations of both Mn and Cr from 0 to a content of 20 at. %. Ga2O3:Mn,Cr TFEL devices driven at 1 kHz exhibited a change from green to red in emission color, maintaining luminances above 100 cd/m2, when the co-doped Cr content was varied from 0 to 20 at. % under a constant Mn content of 0.3 at. % doping. In addition, the emission color of a Ga2O3:Mn,Cr TFEL device co-doped with a Mn content of 0.3 at. % and a Cr content of 3 at. % changed from green to red with an increase of the applied voltage.

Hot-wall deposition using alternative source supply for SrS based electroluminescent phosphor thin films

SrS based blue electroluminescent phosphor thin films, such as SrS : Ce and SrS : Cu, have been prepared by hot-wall deposition using alternating source supply. The SrS films deposited at 600°C are formed by columnar polycrystalline grains of 0.5–1 μm, mainly oriented along the [1 0 0] axis. From the average deposition rate, it is deduced that 1 Sr–S layer is deposited for one cycle of Sr and S vapor supply. The SrS : Cu,Cl device shows pure blue EL emission. The luminance and efficiency measured at 1 kHz are 100 cd/m 2 and 0.10 lm/W, respectively. The SrS : Ce,Cl device shows a greenish–blue EL emission; however, the luminance and efficiency are much lower than those for the devices prepared by other deposition methods.

Luminescent properties of Zn xMg 1− xS : Mn thin film electroluminescent devices

Zn x Mg 1− x S : Mn thin film electroluminescent (TFEL) devices have been developed in a wide range of compositions, x between 0.16 and 1.0, for use as yellow/green-light-emitting displays. Electroluminescence shows the blue shift alike photoluminescence with the increase in Mg composition. The Zn 0.98Mg 0.02S : Mn TFEL device driven at 1 kHz has a high luminance of 1056 cd/m 2 and an efficiency of 1.2 lm/W. The CIE color coordinates varied from ( x, y)=(0.54, 0.45) at x=0.98 to (0.47, 0.52) at x=0.16 with the decrease in Zn composition x in consistence with the peak shift from 585 to 565 nm.

Alternating-current thin-film electroluminescent device modeling via SPICE Fowler-Nordheim diode

A novel SPICE model is proposed for simulating the electrical characteristics of alternating-current thin-film electroluminescent (ACTFEL) devices. The model consists of two capacitors, representing the top and bottom insulators of the ACTFEL device, connected to a Fowler-Nordheim diode shunted by a resistor, which account for conduction and leakage, respectively, in the ACTFEL phosphor layer. A software interface is developed so that the user may conveniently input the two adjustable model parameters (trap barrier height and phosphor shunt resistance) as well as five parameters corresponding to the physical structure of the ACTFEL device (electron effective mass, device area, insulator capacitance, phosphor thickness, and phosphor dielectric constant). Excellent agreement is obtained between measured and simulated electrical characteristics for evaporated ZnS:Mn ACTFEL devices.

Gallium oxide as host material for multicolor emitting phosphors

Ga 2O 3 as the host material for multicolor-emitting phosphors, in particular, for thin-film electroluminescent (TFEL) devices has been described. High-luminance multicolor emissions were realized by TFEL devices with a Mn-, Cr-, Co-, Sn- or rare-earth-activated Ga 2O 3 phosphor.

Unusual disparity in electroluminescence and photoluminescence spectra of vacuum-evaporated films of 1,1-bis ((di-4-tolylamino) phenyl) cyclohexane

The electroluminescence (EL) and photoluminescence (PL) spectra of thin films of 1,1-bis((di-4-tolyloamino)phenyl) cyclohexane (TAPC) appear to be completely different. Whereas the broad PL spectra reveal major maxima at ≅370 and 450 nm, a strong regular band at 580 nm is characteristic of the EL spectra. It is shown that in contrast to PL, which is composed of molecular exciton (monomer) and excimer emission, EL can be explained by the direct cross recombination transition between electrons and holes trapped on tritolylamine (TTA) subunits of different TAPC molecules. Such a pair of trapped carriers, formed selectively under recombination of statistically independent (here injected at opposite electrodes) electrons and holes, is considered as a particular excited state (“electromer”) responsible also for the EL emission of some other compounds containing TTA or triphenylamine moieties, as for example an amino-substituted triphenylbenzene.

Synthesis and characterization of some ZnS-based thin film phosphors for electroluminescent device applications

ZnS:Mn, ZnGaS:Mn and ZnS:CuCl 2 phosphor thin films for alternating-current thin-film electroluminescent (ACTFEL) devices were prepared by thermal evaporation from two and three sources, respectively. Films are polycrystalline, strongly oriented and stoichiometric or nearly stoichiometric with high optical transmission in the visible part of the spectrum. The band gap of ZnS:Mn, ZnGaS:Mn and ZnS:CuCl 2 films was found to be 3.63, 3.86 and 3.56 eV, respectively. The sheet resistance of the ZnS:Mn and ZnGaS:Mn films was greater than 100 MΩ. The resistivity of ZnS:CuCl 2 films was between 1.5–80 Ω cm. Photoluminescent (PL) and electroluminescent (EL) characteristics were also studied. The results indicate that after the optimization the investigated phosphor thin films will be suitable for ACTFEL device applications.

Properties of barium titanate (BaTiO3) thin films grown on silicon by rf magnetron sputtering

Abstract Thin films of BaTiO3 were deposited on p-Si substrates by rf magnetron sputtering in order to investigate their suitability for use in ac thin film electroluminescent (ACTFEL) devices and dynamic RAM (DRAM) applications. Post-growth annealing at 700°C and the subseauent deposition of Al contacts resulted in the creation of Al/BaTiO3/p-Si metal-insulator-semiconductor devices. The electronic and structural properties of the films were examined by admittance spectroscopy, current-voltage and transient current measurements, and X-ray diffraction (XRD) characterization. Analysis of the XRD spectra showed the polycrystalline nature of the films but also the presence of an amorphous phase. The electrical measurements revealed a high dielectric constant, around 60, a charge storage capacity exceeding 3 μC cm−2 and a total charge trapped inside the oxide of around 50 nC cm−2 while the density of traps at the BaTiO3/p-Si interface was found to be as high as 1 × 1012 cm−2 eV−1. These results indicate that ...

The effects of multiple KrF laser irradiations on the electroluminescence and photoluminescence of rf-sputtered ZnS:Mn-based electroluminescent thin film devices

Multiple pulse KrF laser annealing and ablation effects upon the luminescent properties of ZnS:Mn-based electroluminescent structures are investigated. The beneficial increase of the number of active luminescent centres is assessed via sub-bandgap photoluminescence analysis and an optimum is determined due to the competition of the annealing and ablation processes. The maximal photoluminescence signals obtained via laser treatment are 67% and 19% greater than the signals obtained from samples thermally annealed for 1 h at 500°C and 700°C, respectively. Under certain irradiation conditions, the electroluminescence analysis of laser-annealed samples also shows enhanced values compared with samples treated thermally at 500°C. However, these improvements do not reflect those expected from the photoluminescence analysis. We suggest that multiple pulse irradiation leads to detrimental modifications at the electronic interface for electroluminescence applications.

Energy transfer and white emitting organic thin film electroluminescence

Energy transfer between in N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1′-biphenyl-4-4′-diamine (NPB) and rubrene was investigated. The device ITO/CuPc/NPB:rubrene/blocking layer /Alq/MgAg, in which copper phthalocyanine (CuPc) is used as buffer layer, NPB as the hole transporting layer (HTL), trimer of N-arylbenzimidazoles (TPBi) [or 2–(4–biphenylyl)–5–(4–tertbutylphenyl)–1,2,3–oxadiazole(PBD) or 1,2,4–triazolederivative(TAZ)] as the blocking layer, Tris(8-quinolinolato)aluminum complex (Alq) as electron transporting layers (ETL), can not give white emitting light. White emitting light can be realized in a new device with the structure ITO/CuPc/NPB/blocking layer:rubrene/Alq/MgAg, in which rubrene is doped in blocking layer instead of in NPB. The emission spectrum of this device covers a wide range of visible region and can be adjusted by the concentration of rubrene. The white emitting devices with CIE coordinates x=0.31, y=0.32, maximum luminance 8635 cd/m 2 and luminous efficiency 1.39 Lm/w, were obtained with the blocking layer TPBi doped with 1.5% rubrene.

Examination of EL and PL properties of MCHM-PPV and MEH-PPV: a study towards introduction of a new series of thin film EL devices

The effect of the side group, employed during synthesis, on the PL (photoluminescence), EL (electroluminescence) profiles and the light intensity of PPV derivatives has been investigated using MEH (methoxy-ethyl-hexyloxy) and MCHM (methoxy-cyclo-hexyl-methyloxy) as side groups. The measurements show that even the most subtle change in molecular structure of PPV has a marked improvement on the EL, PL profiles and the light emission intensity. It has been found that the PL and EL intensities of the MCHM-PPV are higher than those of MEH-PPV. MCHM-PPV also demonstrated higher light emission at lower potentials and currents.

Ablation study on pulsed KrF laser annealed electroluminescent ZnS:Mn/Y 2O 3 multilayers deposited on Si

The ZnS:Mn active layer of a thin film electroluminescent (TFEL) device has been annealed under 1.034 MPa (10.34 bar, 150 psi) of argon pressure using a 20-ns pulsed KrF excimer laser. We investigate the effects of multiple shots at various power densities upon the ablation rates of the ZnS:Mn layer. The results are compared to a thermal simulation of the laser-matter interaction using single pulse irradiation, and it is inferred that the cubic to hexagonal transition and melting of ZnS:Mn decrease the ablation rate.

Mobility of sodium in SrS based thin film electroluminescent structures grown by atomic layer epitaxy

Mobility of sodium in SrS-based electroluminescent thin film stacks grown by atomic layer epitaxy was studied in the temperature range 600–850 °C using ion implantation for doping and nuclear resonance broadening for profiling of sodium. The crystallinity and orientation of SrS and SrS:Ce films were studied using wide angle x-ray scattering (WAXS). It was found that sodium diffuses rapidly to the interfaces between SrS and upper and lower Al2O3 insulators. These insulators act effectively as diffusion barriers for sodium up to a temperature of about 700 °C. In the case of SrS:Ce films, even after annealing at high temperatures for long times, the Na concentration in the bulk of the film remained in the range 0.4–0.6 at. % which is about two to three times the concentration of Ce. WAXS measurements revealed the preferred [111] orientation of SrS and SrS:Ce films with an average size of crystallites ⩾120 nm. It turned out that annealing for a few hours at about 700–750 °C was needed to recover the implantation damage and restore the crystallinity of SrS to its original level.

Electroluminescence thermal quenching in SrS:Cu thin-film electroluminescent devices

Electroluminesence (EL) thermal quenching refers a reduction in luminance, concomitant with a reduction in transferred charge, when an alternating-current thin-film electroluminescent (ACTFEL) device is operated at an elevated temperature. EL thermal quenching is found to be significant in SrS:Cu ACTFEL devices operated above ∼60-80 °C. Maximum transferred charge-maximum applied voltage (Qmax-Vmax) and transferred charge capacitance (i.e., dQmax/dVmax vs Vmax) measurements as a function of temperature in conjunction with ACTFEL device simulation are employed in order to establish that EL thermal quenching arises from a thermally activated annihilation of positive space charge and a corresponding increase in the threshold voltage.

PREPARATION AND LUMINESCENT CHARACTERISTICS OF AN OXIDE PHOSPHOR Gd3G a5O12:Ag

A composite oxide material Gd3Ga5O12:Ag was pre pared by solid state reaction. The composition and the crystallinity of phosphor were determined by X-ray diffraction. The thin film electroluminescence (EL) de vices based on Gd3Ga5O12:Ag were prepared by el ectron beam evaporation. We have obtained a good UV-blue EL emission with a peak at 397nm and a shoulder at 467nm. The EL peaks located at 397 and 467nm are fou nd to originate from the oxide vacancies and 4d95s0 to 4d10 of Ag+, respectively.

Electroluminescent Devices with Nanostructured ZnS:Mn Emission Layer Operated at 20 V0-p

ABSTRACTWe have developed a new type of a thin-film electroluminescence (TFEL) device with nano- structured (NS)-ZnS:Mn utilizing its enhanced luminescent efficiency due to the quantum confinement (QC) effects. As NS-ZnS:Mn, ZnS:Mn/Si3N4 multilayers with thicknesses of 1.9–3.5 nm for ZnS were prepared by a rf-magnetron sputtering method. From the results of grazing incidence X-ray reflectometry and X-ray diffractmetry, formation of ZnS:Mn nanocrystals in the ZnS layers are confirmed. With a decrease in the ZnS:Mn layer thickness, the photoluminescence (PL) efficiency associated with the Mn2+ transitions is increased, and the PL excitation spectrum is shifted toward higher energies, indicating appearance of the QC effects. As the results of the application of NS-ZnS:Mn to the emission layer of the TFEL device, we have successfully observed reddish-orange emission above the threshold voltage of 12 V0-p, and the maximum luminance is 3.0 cd/m2 operated with the 1-kHz sinusoidal voltage of 20 V0-p.