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.     

Different Er centres in Si and their use for electroluminescent devices

At low temperatures, Er in Si produces a big variety of spectra in the 1.5 μm region which can be identified by high-resolution spectroscopy as being due to either interstitial Er or different complexes of Er with oxygen, intrinsic defects and other light impurities. Although the luminescence yield can be improved by codoping with light elements (C, N, O, F, etc.) all of these centres show strong thermal quenching of the luminescence above 150–200 K. There is, however, one type of rather broad spectrum in heavily Er- and O- doped Si, which is seen up to temperatures of 400 K and above. This spectrum can be excited in Si by hot electrons generated in a reverse biased diode. The same spectrum appears also in other Si related materials like porous Si and in silica with the same temperature dependence. In these materials, excitation spectroscopy is possible and it shows also close agreement of the excitation spectra. From these findings we infer that Er is incorporated in another surrounding and we propose Si–Er–O nano-precipitates since the spectra of other candidates, like Er₂O₃, are clearly different. We review recent work on the excitation and quenching mechanisms and we discuss consequences for technology.     

Organic single-quantum-well electroluminescent device

A new kind of single-quantum-well electroluminescent (EL) device consists of a hole transport N,N-Bis(3-methyphenyl)-N,N-diphenylbenzidine(TPD) layer, and electron transport 8-(quinolinolate)-aluminum(Alq) layer and a light emitting layer of Alq doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) has been fabricated by the multisource-type high-vaccum organic molecular deposition. The dopant rubrene is as a potential well, and the undoped Alq layer is as a barrier layer. The EL spectra shows the spectral narrowing and the emission peak energy blue-shift, and the efficiency and luminance of the device have been significantly improved. The experimental phenomena is explained as the result of recombination of carriers from the quantized energy state.     

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.     

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.     

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.     

Thin-film electroluminescent devices excited by a linearly rising voltage

The average and instantaneous luminances of a thin-film electroluminescent device (TFELD) are determined as functions of the voltage rise time by solving kinetic equations for the concentration of excited emission centers in the electroluminescent layer of the device. It is shown theoretically and experimentally that the dependences of the average and peak luminances, the external and internal quantum yield, the energy yield, and the luminous efficacy as functions of the voltage rise time all have a maximum, and the position of that maximum depends on the frequency of the driving voltage. The calculated and experimental dependences make it possible to determine the main parameters of the electroluminescence process: the collisional excitation cross section for the emission centers, the concentration of emission centers, and the transition probability of the emission centers to an excited state, as well as the radiative and nonradiative recombination probabilities of these and other centers. Zh. Tekh. Fiz. 69, 58–63 (February 1999)     

有机薄膜晶体管及其在传感器方面的应用

有机薄膜晶体管因其有机材料种类的多样性、简单的制备工艺和柔性兼容性等优点,在有机半导体器件的研究领域中广受关注。介绍了有机薄膜晶体管的研究进展,并进一步分析了有机薄膜晶体管的基本结构、工作原理及电学特性。同时介绍了有机薄膜晶体管作为传感器的发展进程,重点从有机薄膜晶体管作为传感器的敏感性、选择性等方面阐述,对有机薄膜晶体管作为传感器的优势做了详细的介绍。最后分析了有机薄膜晶体管作为传感器的工作机理。     

有机薄膜晶体管及其在传感器方面的应用

有机薄膜晶体管因其有机材料种类的多样性、简单的制备工艺和柔性兼容性等优点,在有机半导体器件的研究领域中广受关注。介绍了有机薄膜晶体管的研究进展,并进一步分析了有机薄膜晶体管的基本结构、工作原理及电学特性。同时介绍了有机薄膜晶体管作为传感器的发展进程,重点从有机薄膜晶体管作为传感器的敏感性、选择性等方面阐述,对有机薄膜晶体管作为传感器的优势做了详细的介绍。最后分析了有机薄膜晶体管作为传感器的工作机理。     

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.     

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.     

有机电致发光器件中载流子的输运和复合发光

以高场作用下载流子对三角势垒的FowlerNordheim隧穿理论为基础,建立了单层有机电致发光器件中载流子输运和复合发光模型,给出了薄膜中电子空穴对的解离和复合概率及电子和空穴的密度分布.计算并讨论了外加电压和注入势垒对器件电流和复合效率的影响. 关键词： 电致发光器件 载流子输运 载流子复合     

on submicron rings and their application for coherent nanoelectronic devices

Electron-phase modulation in magnetic and electric fields will be presented in In_0.75Ga_0.25As Aharonov–Bohm (AB) rings. The zero Schottky barrier of this material made it possible to nanofabricate devices with radii down to below 200 nm without carrier depletion. We shall present a fabrication scheme based on wet and dry etching that yielded excellent reproducibility, very high contrast of the oscillations and good electrical gating. The operation of these structures is compatible with closed-cycle refrigeration and suggests that this process can yield coherent electronic circuits that do not require cryogenic liquids. The InGaAs/AlInAs heterostructure was grown by MBE on a GaAs substrate [F. Capotondi, G. Biasiol, D. Ercolani, V. Grillo, E. Carlino, F. Romanato, L. Sorba, Thin Solid Films 484 (2005) 400], and in light of the large effective g-factor and the absence of the Schottky barrier is a material system of interest for the investigation of spin-related effects [W. Desrat, F. Giazotto, V. Pellegrini, F. Beltram, F. Capotondi, G. Biasiol, L. Sorba, D.K. Maude, Phys. Rev. B 69 (2004) 245324; W. Desrat, F. Giazotto, V. Pellegrini, M. Governale, F. Beltram, F. Capotondi, G. Biasiol, L. Sorba, Phys. Rev. B 71 (2005) 153314; J. Nitta, T. Akazaki, H. Takayanagi, T. Enoki, Phys. Rev. Lett. 78 (1997) 1335] and the realization of hybrid superconductor/semiconductor devices [Th. Schäpers, A. Kaluza, K. Neurohr, J. Malindretos, G. Crecelius, A. van der Hart, H. Hardtdegen, H. Lüth, Appl. Phys. Lett. 71 (1997) 3575].     

Organic photovoltaic materials and devices

Organic photovoltaic solar cells bere an important potential of development in the search for low-cost modules for the production of domestic electricity. We review the principles and techniques needed for their development: organic semiconductors, their transport properties and photophysical characteristics, photovoltaic molecule and polymer structures, device technologies, electrical and optical behaviour of the cells, state of the art, limitations and perspectives. Despite some recent record efficiencies, research on organic solar cells is still in its infancy when stability and efficiency have to be compared with the performances of silicon cells. A nominal 10% solar efficiency is the research target for the next few years. To cite this article: J.-M. Nunzi, C. R. Physique 3 (2002) 523–542.     

Organic light emitting devices

The purpose of this paper is to provide general information about basic physical processes involved in organic electroluminescence and to present the main parameters and advantages of organic light emitting devices (OLEDs).     

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     

Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices

Received: 10 March 1998 / Accepted: 21 September 1998 / Published online: 24 February 1999