Si3N4做电子加速层的聚［2-甲氧基-5-(2-乙基-己氧基)-1，4-苯撑乙烯撑］固态阴极射线发光

用Si₃N₄作为电子加速层制备了固态阴极射线发光器件,其中发光层为聚［2-甲氧基-5-(2-乙基-己氧基)-1,4-苯撑乙烯撑］(MEH-PPV).在交流电压的驱动下,实现了MEH-PPV的固态阴极射线发光.与SiO₂做电子加速层的器件进行了对比研究,两种器件在交流电场的驱动下都得到了波峰位于417nm的短波长发光峰,它来自有机物中电子从最低未占分子轨道到最高占据分子轨道的直接复合发光,这进一步证明了固态阴极射线理论的正确性.在交流高场下比 关键词： 固态阴极射线发光 3N₄')" href="#">Si₃N₄ 电子加速层 电致发光     

驱动电压频率对固态阴极射线发光影响的研究

固态阴极射线发光(SSCL)是发光学中一种新的激发方式,引发出一些发光学中的重要问题,但是固态阴极射线发光的性质还不是十分清楚,需要进一步研究。文章用SiO₂作为电子加速层, 有机材料MEH-PPV为发光层, 在正弦交流电压驱动下实现了固态阴极射线发光,得到410和580 nm两个发光峰。通过研究这两个发光峰的性质,证实它们分别符合能带理论和分子理论。改变驱动电压的频率时,长波峰的发光强度随频率的增加而增加,而短波峰的发光强度随频率的增加而减小,这是由于这两个发光峰对应的上能级寿命不同引起的。     

有机场致发光中能带模型与分子理论的讨论

在有机场致发光中,能带模型及分子理论从20世纪就存在尖锐的矛盾。在分层优化方案中,经SiO₂加速后的电子能量可以到达10eV,这足以激发发光材料发光,将分层优化方案应用到有机场致发光材料中。发现了固态阴极射线发光(SSCL),经过对它的交叉证明、普适性的验证,肯定了固态阴极射线的发光确实是在发光二极管,无机及有机场致发光之外的一种完全新型的电场诱导的发光。SSCL的特征是在它的光谱中出现短波发光峰,实验证明长波发光峰的减弱是由于电场离化效应。研究了这种效应出现的电压阈值并和SSCL的短波峰出现的电压相比,发现短波峰的出现是在激子的电场离化之后,从而找出了电子处于局域态与扩展态的分水岭,解释了在有机场致发光中能带模型和分子理论并不矛盾,只是适用的条件不同。激子的离化是随电场强度而渐进的变化,因此会有一个两种过程并存的范围。     

提高电子注入的固态阴极射线发光的研究

设计了SiO₂与电极间加入ZnS的复合加速层结构，进一步说明靠电子注入的增强来扩大初电子源.同时，在电子发生倍增前后的电压下，对比了单层和夹层结构的不同加速层的发光性能.得到在低压直流下单侧复合加速层器件的发光优于单层SiO₂加速层器件而劣于单层ZnS器件；在高压交流下复合加速层表现出明显的优势，在复合加速层中，ZnS不仅对电子有加速作用，而且对增强电子注入贡献很大，同时ZnS和SiO₂/ZnS界面还提供了初电子源.另外，与SiO _{关键词： 电致发光 复合加速层 过热电子 固态阴极射线}     

Stochastic and Relaxation Processes in Argon by Measurements of Dynamic Breakdown Voltages

Statistically based measurements of breakdown voltages U _b and breakdown delay times td and their variations in transient regimes of establishment and relaxation of discharges are a convenient method to study stochastic processes of electrical breakdown of gases, as well as relaxation kinetics in afterglow. In this paper the measurements and statistical analysis of the dynamic breakdown voltages U _b for linearly rising (ramp) pulses in argon at 1.33 mbar and the rates of voltage rise k up to 800 V s ^–1 are presented. It was found that electrical breakdowns by linearly rising (ramp) pulses is an inhomogeneous Poisson process caused by primary and secondary ionization coefficients α , γ and electron yield Y variations on the voltage (time). The experimental breakdown voltage distributions were fitted by theoretical distributions by applying approximate analytical and numerical models. The afterglow kinetics in argon was studied based on the dependence of the initial electron yield on the relaxation time Y ₀ (τ ) derived from fitting of distributions. The space charge decay was explained by the surface recombination of nitrogen atoms present as impurities. The afterglow kinetics and the surface recombination coefficients on the gas tube and cathode were determined from a gas‐phase model. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

有机磷光材料的固态阴极射线发光研究

采用有机磷光材料三-(2-苯基吡啶)-铱(Ir(PPY)₃)与无机材料SiO₂复合制成夹层结构器件,用交流电压驱动获得了Ir(PPY)₃主峰位于517nm的发光和主峰位于435nm的蓝色发光.通过分析器件的光谱特性,发现这两个发光峰都是源于SiO₂中加速电子直接碰撞激发有机层引起的固态阴极射线发光.继实现多种有机聚合物材料和有机小分子材料八羟基喹啉铝(Alq₃)的固体阴极射线发光之后,又证实了有机 关键词： 夹层结构器件 有机磷光材料 固态阴极射线发光     

AC modulation of the recombination electroluminescence in anthracene single crystal

The recombination electroluminescence (REL) in an anthracene single crystal has been modulated by ac voltage at different dc biasing voltages. The frequency and ac voltage dependences observed are interpreted in terms of the presented model of sinusoidal current wave. The dc-voltage-induced changes in the modulation are explained by the voltage variation of the triplet exciton lifetime and wavelength of the current wave based on concomitant evolution of the triplet-to-singlet exciton concentration ratio responsible for the contribution of delayed and prompt components of the REL.     

三波段白光有机电致发光器件及其色稳定性的控制

制备并研究了基于3种小分子荧光材料的白光有机电致发光器件。发光层采用红色发光材料DCJTB掺杂绿光材料Alq3构成混合发光层,与OXD-7构成的蓝色发光层形成异质结的结构,并以NPB为空穴注入层。通过改变结构参数详细研究了发射光谱及其色坐标的电场依赖性。通过分析载流子注入/传输特性,控制激子的复合区域等措施得到了色坐标稳定性好、光谱丰富的高性能白光电致发光器件。经过优化的器件结构可以覆盖更大的可见光区域,当电压从9 V增加到13 V时,色坐标仅从CIE(x,y)=(0.364,0.314)偏移到CIE(x,y)=(0.332,0.291),具有较好的色稳定性。     

Application of Electron-Beam Ionized Discharges to Switches?a Comparison of Experiment with Theory

A theoretical investigation of high-pressure discharges ionized by an external electron beam (e-beam) was conducted. Only when secondary emission from the cathode and electron-impact ionization of metastable states were included in the analysis did calculated current-voltage (I-V) characteristics for argon and methane discharges compare well with experimental data. The I-V characteristics obtained reveal a sharp rise in the current at a certain threshold voltage. This threshold voltage and the entire I-V characteristic are shifted to lower voltages when metastable ionization is significant. Below the threshold voltage and at low external ionization source strengths, a region of negative differential conductivity is obtained. In the high-current region, the I-V slope is controlled by the secondary emission coefficient. The additional cathode sheath ionization from secondary emission and ionization from metastable states significantly reduces the discharge voltage. This important effect can be used to reduce e-beam switch losses and increase lifetime through judicious gas mixture selection and proper cathode conditioning.     

Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes

Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at E_c-0.51 eV and E_c-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

“Thermal barrier” effect in energy relaxation of polaritons in HgI2

The paper presents the results of measurements of polariton radiation spectra, polariton band excitation spectra and spectra of resonance Raman scattering HgI₂. These are registered at different temperatures and units of energetic derangement between the exciting radiation quantum energy and that of transverse exciton. The threshold value of the temperature (T=12 K) starting from which there is attained the equilibrium between the polariton and phonon subsystems is found. The scattering mechanism that guides the formation of “thermal barrier” favouring the polaritons thermalization, is revealed.     

The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N,N′-bis-(1-naphthyl)-N,N′-1-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq₃) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq₃/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq₃ increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq₃ benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).     

激子复合区厚度对有机磁效应的影响

在常规型有机发光二极管的基础上, 通过改变发光层tri-(8-hydroxyquinoline) aluminum (III) (Alq₃)厚度, 研究了激子复合区厚度对有机发光二极管磁效应的影响.测量了器件在不同温度及偏压下电致发光及注入电流在外加磁场作用下的变化, 着重研究了低温下的有机磁电导效应和有机磁电致发光效应.实验发现, 低温(50 K)高磁场 (500 mT)下, 器件表现出随Alq₃厚度的减薄, 磁电导值由正到负再到正的非单调变化.利用磁场调控的超精细相互作用、 磁场抑制的三重态激子-电荷反应以及激子在界面的淬灭效应, 对有机磁电导在低温下表现出的现象进行了定性的解释.实验结果表明, 通过改变激子复合区的厚度, 可以实现对激子浓度的有效调节, 进而实现对有机磁电导和磁电致发光效应的调节. 该研究进一步丰富了有机磁效应的实验现象, 同时提供了一种调控有机磁效应的手段. 关键词： 激子复合区 激子浓度 有机磁电导 有机磁电致发光     

Picosecond time-resolved X-ray absorption spectroscopy of ultrafast aluminum plasmas

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.     

Low-temperature X-ray and photoluminescence of lead halide crystals

The emission spectra of PbF₂, PbCl₂ and PbBr₂ monocrystals are measured under optical and X-ray excitation at liquid helium temperature. Certain emission bands are attributed to the radiative transitions in the self-trapped cation exciton (excited Pb²⁺ion). The increase of trapped exciton via electron-hole recombination is discussed.     

Analysis of the photovoltaic efficiency of a molecular solar cell based on a two-level system

A molecular solar cell is modelled as a two-level system connected to electrodes by chains of electron-transporting and hole-transporting orbitals. Light absorption and emission are simulated using the generalised Planck equation and intermolecular charge transfer using non-adiabatic Marcus theory. Quantum efficiency–voltage characteristics, open-circuit voltage and monochromatic power-conversion efficiency are calculated as a function of the following parameters: charge-separation rate, interfacial recombination rate, charge mobility, light intensity and built-in bias. We find that slow charge separation, fast recombination and low mobility all contribute to a decrease in efficiency compared to the ideal (detailed balance) limit. When charge-separation and interfacial recombination rates are related through the intermolecular coupling, maximum efficiency is achieved at some optimum, but not the maximum, charge-separation rate. Two regimes are distinguished for the open-circuit voltage: when interfacial recombination is important, V_oc varies approximately linearly with the donor–acceptor energy gap; but when recombination is insignificant, V_oc is determined by the optical gap. Including exciton binding energy in the driving force for charge separation reduces V_oc. In systems with significant recombination, V_oc first increases and then saturates with increasing light intensity. Low mobility and interfacial recombination are the main avoidable sources of loss when realistic parameters are used, but the effects of low mobility can be partly compensated by applying a built-in bias between the electrodes. PACS 72.40.+w; 73.40.Lq; 72.80.Le     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

Observation of a donor-acceptor pair recombination in the edge emission of ZnSe crystal by electro-luminescence

An injection type electro-luminescence in ZnSe crystal has been studied by using ZnSe-SnO₂ hetero-junction at 20°K. In the emission peak observed around 2·70 eV, a clear energy shift toward the higher energy side with increasing injection current density has been found at low temperatures, which could be associated with the D-A pair (donor-acceptor pair) recombination process. The energy sum of the donor and acceptor activation is estimated to be larger than 137 meV. In the higher temperature region, this emission line turns out due to the ‘free-to-bound’ recombination, and the related acceptor ionization energy is considered to be ～120 meV. By taking into consideration the energies of bound exciton emission, the exciton localization energies and the related donor and acceptor ionization energies are evaluated.     

Electromodulation spectroscopy of excitons: High field regime

The effects of electric field on excitons have been investigated experimentally by means of electroabsorption (EA). The behavior of excitons with ionization fields F_I of the same order of magnitude as or smaller than typical applied fields is reported. EA spectra associated with excitons in TlBr, PbI₂, BiI₃, trigonal Se, and α-monoclinic Se will illustrate this case. In this situation the EA signal arises principally from the effect on the exciton ground state, and no finer structure could be detected. The exciton ground state undergoes a shift to higher energies with the applied field. Characteristic dependences with field of the sizes, energy position of the peaks, and zero crossing points of the EA signals have been found. Estimates of exciton parameters can be readily obtained from the EA spectra. Unusual features of the exciton EA spectra PbI₂ are discussed. Qualitatively, at least, theoretical predictions are obeyed.