Piezoelectric/electroluminescent composites for low voltage input flat-panel display devices

In the present work, we report enhanced electro-mechano-optical conversion in piezoelectric/electroluminescent composites, composed of ZnS-type electroluminescence phosphor (doped with Cu, Cl, Mn) placed adjacent to a single-crystal piezoelectric transformer. The effect originates from the combined functions of electroluminescent characteristics and amplified piezoelectric effects. The composites can realize a comparatively low exciting voltage of 2 V rms to start electroluminescence in various materials. Through the naked eye strong green-light emission can be seen under an input voltage of 4.95 V rms. We can easily control the emission brightness by adjusting the driving frequency due to the resonating characteristics of the piezoelectric transformer. This provides a promising path for us to fabricate multiple function composites. PACS 77.84.-s; 77.84.Lf; 77.65.-j; 78.60.Fi; 77.65.Fs     

Thin-film waveguide devices

This paper reviews a variety of practical or potential uses of low optical loss thin-film waveguides for light modulation, deflection, and switching. Emphasis is given to those thin-film devices that most likely will become necessary replacements of the bulk for systems involving signal processing, imaging radar, and optical communication in the infrared.     

Thin-film electroluminescent structures on substrates with rough surfaces

It is shown that the use of substrates with inner rough and outer diffusely scattering surfaces in film electroluminescent structures makes it possible to substantially increase the coefficient of radiation extraction (by a factor of 1.3–2.5) and the brightness of luminescence (by a factor of 1.2–3.5) as compared to the structure on a smooth surface, which can be employed to enhance the brightness of individual colors of the luminescence of electroluminescent emitters. The results of the investigations indicate a substantial decrease in the constant of the time of brightness buildup (by a factor of 6–8 for MDSDM structures and by a factor of ∼3.8 for MDSCM structures) and the appearance of two segments of brightness decay for MDSMD structures with different constant decay times in going from the structure on a smooth substrate to the structures built on substrates with an inner rough surface. Unlike this, MDSCM structures have two segments of brightness decay on both a smooth substrate and a rough substrate. Ul’yanovsk State University, 42, L. Tolstoi St., Ul’yanovsk, Russia, 432700. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 787–793, September–October, 1998.     

Effect of driving voltage pulse shape on the luminance of thin-film electroluminescent devices

The dependence of the average luminance on the parameters of the metal-insulator-semiconductor-insulator-metal structures in thin-film electroluminescent devices (TFELDs) and on the excitation conditions is found by solving kinetic equations for the variation in the concentration of excited emission centers in the phosphor layer of TFELDs for different driving voltage pulse shapes (triangular, trapezoidal, sinusoidal, and rectangular with an exponential leading edge). It is shown that for equal amplitudes and pulse repetition rates of a sign-changing symmetric voltage, the average luminance and luminous efficacy of TFELDs increase as the rate of rise of the voltage is increased for different driving voltage pulse shapes in the following sequence: triangular, sinusoidal, trapezoidal, rectangular. The calculations are confirmed by experiment. Zh. Tekh. Fiz. 69, 64–69 (February 1999)     

Thin-film electroluminescent emitters based on calcium sulfide doped with cerium

Stavropol' State Technical University, 2 Kulakov Av., Stavropol', 355000. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 3, pp. 178–181, May–June, 1995.     

Organic electroluminescent devices and their application

Several organic electroluminescent devices have been fabricated by multi-source high vacuum deposition system. For high brightness organic electroluminescent device, the maximum brightness is over 40000 cd/m². For quantum well structures, quantum size effect has been investigated and the high light emission efficiencies of the devices have been obtained. White-light emission from organic multi-quantum well structures is proposed at first. Brightness of the white-light MQW devices reaches 4000 cd/m² at 17 V. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. The research is supported by the National “863” Project of China [No. 863-307-05-05(02)] and by the National Natural Science Foundation of China [No. 69637010].     

What is the relativistic generalization of a linearly rising potential?

Recent progress in summing graphs of non-Abelian gauge theories focuses attention on certain manifestly Lorentz-invariant classical action-at-a-distance theories, whose solution allows for the construction of field-theoretic Green functions in the WKB approximation. The sum of graphs is of QED type, except that the gluon propagator is modified to incorporate the renormalization-group-invariant charge $\text{g}\text{(k)}$. The purely phenomenological choice $\text{g}{}^2\text{～ k}{}^{\mathrm{?2}}$ is equivalent non-relativistically to a linearly rising potential and yields a fully relativistic classical action-at-a-distance theory with exactly soluble circular orbits, whose Bohr quantization yields an asymptotic approximation to the poles of the Green function. One finds asymptotically linear Regge trajectories, but other phenomenological aspects are not as promising when only a linearly rising potential is used (in common with other phenomenological studies). As a redeeming feature, there do not appear to be any obvious pathologies of the sort familiar from string theories and ad hoc generalizations of linearly rising potentials.     

Tunneling current oscillations excited by DC voltage

Investigating a polycrystalline gold layer on glass by a scanning tunneling microscope in air, tunneling current oscillations were found, which are excited by the DC voltage across the gap. The oscillation amplitude is dependent on the place on the surface of the sample and correlates with its topography. The frequency spectra of these oscillations are influenced by resonances of the mechanical system (z-piezo/sample holder/transducer/sample). A piezoelectric transducer is able to detect alternating forces originating from the tunnel junction. The resonances in the spectrum of the AC tunneling current and the mechanical resonances of the STM system seem to be related. Trapping and subsequent delayed desorption of charge carriers at localized surface states could play a role in generating the observed time-dependent forces across the gap and thereby creating tunneling current oscillations.     

GaN electroluminescent devices: Preparation and studies

It is shown that the properties of GaN:Zn mainly depend on three parameters: partial pressure of zinc, additional HCl and GaCl. Therefore, it is possible reliably to obtain layers, either N-type or semi-insulating, with P-type tendency. The cathodoluminescence spectra of these layers are composed of four bands, blue, green, yellow and red, which depend on the above parameters. Using these results it has been possible to prepare electroluminescent M-i-n devices emitting over a wide spectra range (blue to yellow) with good efficiencies. The electrical and optical characteristics of these devices are analysed. An injection model is discussed.     

Organic low-dimensional structure electroluminescent material characteristics and devices

Photoluminescence (PL) characteristics of organic doped films are studied as a function of dopant concentration, doped layer thickness. The luminescence properties of doped films are decided by Förster energy transfer rate between host and guest and fluorescence quenching effect of guest. For Alq:rubrene doped film at 5% doping level, we fabricated and characterized organic doped quantum-well structure device, and observed significant quantum size effect. For Alq:QAD doping system, at low doping level of 0.4%, the intermolecular spacing between QAD molecules is relatively large, the interaction between neighboring QAD molecules is very small due to the strong localization of the carriers in the single QAD molecules, the PL properties of Alq:QAD doped films only depends on the PL properties of single isolated QAD molecules, which is somewhat similar to the inorganic quantum dot structure. We also reported the electroluminescent performance of the Alq:QAD system.     

Luminescence and conduction charge in thin-film electroluminescent devices

The luminescence and conduction currents of a doubly-insulated thin-film electroluminescent device were studied under various excitation conditions. The conduction current waveform was calculated numerically from the luminescence waveform. It was composed of two components: a fast transient component j_fc, which rises and decays rapidly when a pulse is applied, and a dc-like component j_dc, which maintains a constant level during the duration of pulse. The ratio of these components varied considerably with applied voltage. Both the slow response of luminescence to voltage change and the change in j_fc when the excitation conditions were changed from repetitive pulses to a single pulse suggest an accumulation of charges inside the electroluminescence layer. This accumulation causes a space charge effect which explains the dominance of j_fc in the high-brightness region. The fact that luminescence intensity in this region is not related to the amplitude of individual pulses, but rather to the average amplitude of the pulse train also indicates that the space-charge-induced internal electric field is the main factor in accelerating the carriers which excite the luminescence centers. It was found that the two conduction current components showed similar characteristics in singly-insulated devices.     

基于电致发光效应的光学电压传感器

本文综述了基于电致发光效应的光学电压传感器机理、分类及其主要特性,分析总结了此类传感器的研究现状及其存在的主要问题,同时提出未来研究课题的建议。电致发光型电压传感器的主要优点在于不需要载波光源,因而可以有效避免以往光学电压器中工作光源性能不稳定所引起的传感器性能变化;此外,此类电压传感器结构简单、体积小、重量轻、成本低,可以实现较高的性能价格比。今后研究的主要问题包括合理选择电压传感材料与器件、提高传感器的温度和湿度稳定性等。电致发光型电压传感器在电力工业和航空航天等领域的科学研究与实验中将具有广泛的应用前景。     

Minimally relativistic Schrödinger equation and the linearly rising potential

In place of the usual approximation $\text{T = (m}{}^2\text{c}{}^4\text{+ p}{}^2\text{c}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{? mc}{}^2\text{= p}{}^2\text{/2m ? p}{}^4\text{/8m}{}^3\text{c}{}^2\text{+}\text{O}\text{(1/c}{}^4\text{)}$. in the “minimally relativistic” Schrödinger equation, we suggest to employ the alternative formula $\text{T = p}{}^2\text{c}{}^2\text{[(m}{}^2\text{c}{}^4\text{+ p}{}^2\text{c}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ mc}{}^2\text{]}{}^{\mathrm{?1}}\text{= p}{}^2\text{(2m + p}{}^2\text{/2mc}{}^2\text{)}{}^{\mathrm{?1}}\text{+}\text{O}\text{(1/c}{}^4\text{)}$. Its merits are illustrated on the particular linear potentials V(x) = μx + const, where a small O(c^?3) modification of the coupling (if needed) enables us to construct the ground and first M ( = O(c³)) excited states ψ(x) = e^-cx × polynomial (x) exactly.     

Erbium-doped Si nanocrystals: optical properties and electroluminescent devices

In the last decade, a strong effort has been devoted towards the achievement of efficient light emission from silicon. Among the different approaches, rare-earth doping and quantum confinement in Si nanostructures have shown great potentialities. In the present work, the synthesis and properties of low-dimensional silicon structures in SiO₂ will be analyzed. All of these structures present a strong room temperature optical emission, tunable in the visible by changing the crystal size. Moreover, Si nanocrystals (nc) embedded in SiO₂ together with Er ions show a strong coupling with the rare earth. Indeed each Si nc absorbs energy which is then preferentially transferred to the nearby Er ions. The signature of this interaction is the strong increase of the excitation cross section for an Er ion in the presence of Si nc with respect to a pure oxide host. We will show the properties of Er-doped Si nc embedded within Si/SiO₂ Fabry–Pérot microcavities. Very narrow, intense and highly directional luminescence peaks can be obtained. Moreover, the electroluminescence (EL) properties of Si nc and Er-doped Si nc in MOS devices are investigated. It is shown that an efficient carrier injection at low voltages and quite intense room temperature EL signals can be achieved, due to the sensitizing action of Si nc for the rare earth. These data will be presented and the impact on future applications discussed.     

Electroluminescence parameters of thin-film ZnS: Mn electroluminescent devices

A new technique is suggested that makes it possible to determine the probability of radiative recombination of impact-excited Mn²⁺ luminescence centers, as well as to find the time dependences of the electron multiplication factor, impact ionization length, and impact ionization coefficient from the time dependences of the instantaneous internal quantum yield. These dependences follow from the time dependences of the luminance, as well as of the current and charge passing through the phosphor layer, when the devices are excited by a low-frequency ramp voltage.     

Isophorone derivative as red dopant for organic electroluminescent devices

The donor–acceptor functionalized molecule, bis(4-(2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl)phenyl)(1-naphthyl)amine (DPN-4CN), with symmetrical structure, was investigated for its application in optoelectronic devices. Red organic light-emitting diodes (OLEDs) were fabricated by doping DPN-4CN in tris(8-hydroxyquinolino) aluminum (Alq₃) as red emitters, with a structure of ITO/NPB/Alq₃:DPN-4CN/BCP/Alq₃/LiF/Al. The device with a doping concentration of 2.5 wt% showed pure red emission with λ_max at 654 nm and CIE coordinates of (0.62, 0.36), a high brightness of 5080 cd m⁻² at a driving voltage of 12 V, a current efficiency of 2.14 cd A⁻¹ and an external quantum efficiency of 1.07% at a current density of 20 mA cm⁻². The current efficiencies and CIE coordinates of the device were almost constant over a current density from 1 to 200 mA cm⁻².