采用瞬态电致发光研究磷光掺杂体系发光瞬时过冲的发射机理

掺杂型有机电致发光器件中载流子累积、载流子复合等物理过程的深入了解对提高器件效率和稳定性有重要作用。通过瞬态电致发光测量可以研究掺杂型有机电致发光器件内部载流子累积。对结构为: ITO/NPB(30 nm)/host: Ir(ppy)₃/BCP(10 nm)/Alq₃(20 nm)/LiF(0.7 nm)/Al(100 nm)的器件分别研究主体材料以及客体掺杂浓度变化对有机掺杂型器件瞬态发光行为的影响。实验发现,当单脉冲驱动电压关闭后,只有TAZ: Ir(ppy)₃掺杂器件出现发光瞬时过冲现象,即发光强度衰减到一定时间时突然增强;且随着客体掺杂浓度的增加,瞬时过冲强度逐渐增强。通过分析TAZ: Ir(ppy)₃掺杂器件的瞬时过冲强度对主体材料与掺杂浓度的依赖关系,进一步发现,瞬时过冲效应强度主要受限于发光层内部积累的电子载流子;TAZ: Ir(ppy)₃发光层内电子容易被客体材料分子俘获并积累,电场突变时陷阱电子容易跳跃到主体材料上并与主体材料上积累的空穴形成激子,激子能量传递到客体材料上并复合发光继而出现发光强度的瞬时过冲现象。研究发光瞬时过冲行为可探究器件发光层内的载流子和激子的动态行为,有利于指导器件的设计,从而减少积累电荷的影响,提高器件的性能。     

Untersuchung des „Auroral Afterglow“ und seiner Präparationsstadien

The intensity decay of negative bands of N ₂ ⁺ and of first positive bands of N₂ in the auroral afterglow has been measured. An explanation, seeming possible for the decay curves of the first ones is proposed. By this the intensity decay in the beginning of the afterglow at high ionic densities is determined by the recombination of N ₂ ⁺ with electrons. Later, the ambipolar diffusion of the charge carriers to the wall causes the time dependance. A value of the ambipolar diffusion coefficient in nitrogen, derived from the measurements is given by the relationD _a ·p ≈ 124cm² sec^?1 Torr. The temperature of the charge carriers is estimated to be 313? K. The intensity decay of the pos. group in the afterglow seems mainly to be due to the diffusion of metastable particles to the wall.     

LiF修饰阴极的双层有机电致发光器件的延时效应

基于载流子的注入、传输和复合过程,建立了双层有机发光器件的电致发光延时理论模型;讨论了电致发光延时随电压、注入势垒、内界面势垒、阳极区厚度及LiF缓冲层(BL)厚度的变化关系。结果表明:(1)低电压下,EL延时由复合过程主导,而高电压下,输运过程起着更重要的作用;(2)当δe/δh2时,M/O界面属于欧姆接触,电流是空间电荷限制的,注入势垒的变化对复合时间trec影响较大,当δe/δh2时,M/O界面成为接触限制,注入势垒的变化对trec几乎没有影响;(3)当内界面势垒超过0.3eV,H′h对trec的影响明显变弱,复合延迟时间基本上由电压和其它因素控制;(4)当电压较小时,随Lh/L的增大,trec增大;当电压超过某一值后,trec几乎不随Lh/L的变化而变化;(5)对于LiF/Ag阴极,在不同的偏压下,LiF的厚度在3.1nm左右时的复合时间最短,对应的EL延迟时间也最短,这与实验中从电致发光效率的角度得出的LiF最佳厚度一致。     

An Electroluminescence Delay Time Model of Bilayer Organic Light-Emitting Diodes

Based on the mechanism of injection, transport and recombination of the charge carriers, we develop a model to calculate the delay time of electroluminescence （EL） from bilayer organic light emitting diodes. The effect of injection, transport and recombination processes on the EL delay time is discussed, and the relationship between the internal interface barrier and the recombination time is revealed. ＂]~he results show that the EL delay time is dominated by the recombination process at lower applied voltage and by the transport process at higher applied voltage. When the internal interface barrier varies from 0.15 eV to 0.3 eV, the recombination delay time increases rapidly~ while the internal interface barrier exceeds about 0.3eV~ the dependence of the recombination delay time on applied voltage is almost undiversified, which may serve as a guideline for designing of a high-speed EL response device.     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Luminescence induced by elastic deformation of ZnS:Mn nanoparticles

When the thin film of ZnS:Mn nanoparticles deposited on a glass substrate is elastically deformed by applying a load, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. The rise and decay characteristics of the ML produced during release of the load are also similar to those produced during the application of load. Similar rise, occurrence of peak and then decrease in ML intensity are also found, when the film is deformed impulsively by dropping a steel ball of small mass from a low height; however, in this case, the time durations for the occurrence of ML and decay time of ML are very short. In the cases of loading and impulsive deformation ,after t_m, initially the ML intensity decreases at a fast rate and then at a slow rate, in which the decay time of fast decrease is equal to the time-constant for rise of pressure and the decay time for slow decrease is equal to the relaxation time of the surface charges. In the case of loading, the peak intensity I_m and the total intensity I_T of ML increase quadratically with the magnitude of applied pressure; however, in the case of impulsive deformation, both the I_m and I_T increase linearly with the height through which the ball is dropped on to the sample. In the case of deformation of the samples at a fixed strain rate, I_m should increase linearly with the applied pressure. The elastico ML in ZnS:Mn nanoparticles can be understood on the basis of the piezoelectrically-induced electron detrapping model, in which the local piezoelectric field near the Mn²⁺ centres reduces the trap-depth, and therefore, the detrapping of filled electron traps takes place, and subsequently the energy released non-radiatively during the electron-hole recombination excites the Mn²⁺ centres and de-excitation gives rise to the ML. The equal number of photons emitted during the application of pressure, release of pressure, and during the successive applications of pressure, indicates that the detrapped electron-traps get filled during the relaxation of the surface charges induced by the application and release of pressure because the charge carriers move to reduce the surface charges. On the basis of the piezoelectrically-induced electron detrapping model, expressions are derived for different characteristics of the ML of ZnS:Mn nanoparticles and a good agreement is found between the theoretical and experimental results. The expressions explored for the dependence of ML intensity on several parameters may be useful in tailoring the suitable nanomaterials capable of exhibiting ML during their elastic deformation. The values of the relaxation time of surface charges, time-constant for the rise of pressure, and the threshold pressure can be determined from the measurement of the time-dependence of ML. It seems that the trapping and detrapping of charge carriers in materials can be studied using ML.     

驱动电压和主体材料对染料掺杂有机电致发光器件的影响

采用对比使用掺杂系统的有机电致发光器件(OLEDs)在不同偏置电压下电致发光(EL)光谱的方法,观察了主体材料8-hydroxyquinoline aluminum(Alq3)中掺杂红色荧光染料4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran(DCJTB)发射光谱的变化,研究了偏置电压对掺杂系统中载流子复合区域的影响。另外,通过对比采用不同主体材料的掺杂系统的电致发光(EL)和光致发光(PL)光谱,讨论了主体材料Alq3和N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′-diamine(NPB)不同的掺杂效果。结果表明,随着偏置电压的增大,使用掺杂系统的器件中载流子复合区域逐渐向空穴传输层(HTL)移动,作为主体材料应具有高的能量传递效率,且应对染料具有低的浓度猝灭效应。     

Electroluminescence kinetics ofp-n-transitions with leakages

The kinetics of the decay in the electroluminescence intensity after gating a rectangular pulse of current in the p-n-transition with inhomogeneities, in particular, accumulations of dislocations is calculated. The basic assumptions are: on gating the rectangular pulse of current, a portion of the electrons injected into the thin p-region is extracted into the n-region, whereas the same number of electrons are injected into the inhomogeneity where they recombine without emission; the intensity of nonradiative recombination on the inhomogeneity is limited by drift of majority charge carriers (by leakages). It is shown that nonradiative recombination on inhomogeneities leads to nonexponential kinetics of electroluminescence, whose instantaneous relaxation time decreases with time. Odessa State Naval Academy, 8 Didrikhson St., Odessa, 270029, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 450–453, May–June, 1998     

1.53 μm electroluminescence from ErF3-doped organic light-emitting diodes

Near-infrared (NIR) organic light-emitting devices (OLEDs) are demonstrated by employing erbium fluoride (ErF₃)-doped tris-(8-hydroxyquinoline) aluminum (Alq₃) as the emitting layer. The device structure is ITO/N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB)/Alq₃: ErF₃/2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole) (TPBI)/Alq₃/Al. Room-temperature electroluminescence around 1530 nm is observed due to the ⁴I_13/2–⁴I_15/2 transition of Er³⁺. Full width at half maximum (FWHM) of the electroluminescent (EL) spectrum is ～50 nm. NIR EL intensity from the ErF₃-based device is ～4 times higher than that of Er(DBM)₃Phen-based device at the same current. Alq₃–ErF₃ composite films are investigated by the measurements of X-ray diffraction (XRD), absorption, photoluminescence (PL) and PL decay time. Electron-only devices are also fabricated. The results indicate that energy transfer mechanism and charge trapping mechanism coexist in the NIR EL process.     

ZnS:Ho3+和ZnS:HoF3交流电致发光薄膜发光特性的研究

利用高真空条件,分舟蒸发制备ZnS:Ho3+和ZnS:HoF3绿色交流电致发光薄膜屏。讨论了ZnS:Ho3+及ZnS:HoF3薄膜的发光特性和光谱差异。确定其激发机理为热电子直接碰撞激发,以及660nm的发射在低浓度下来自于5F3→5I7的跃迁。并认为在两种材料中,由于热电子在外电场中的加速过程中受到不同散射中心的散射,改变了热电子的统计分布,从而直接引起两种材料发射光谱的明显差异。     

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).     

NPB厚度对异质结OLED载流子复合区域的调控

通过调控p型半导体N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl)benzidine(NPB)层的厚度,制备了结构为ITO/NPB/aluminum(Ⅲ)bis(2-methyl-8-quinolinato)-4-phenylphenolate(BAlq)/NPB(0~18nm)/tri-(8-hydroxyquinoline)-aluminum(Alq₃)/Mg:Ag的多层有机电致发光器件.分析结果表明,在该类异质结器件中,NPB不仅可以作为空穴传输材料,在适当的厚度范围内,它还可以起到调控载流子复合区域的作用;当NPB厚度在0~18nm之间变化时,随着其厚度增加器件发光颜色可由蓝色变为绿色.通过器件发光光谱的表征可以得知,器件的载流子复合区域相应地由BAlq层转移至Alq₃层.     

Bright electroluminescent devices with tunable spectra obtained by strictly controlling the doping concentration of electron injection sensitizer

In this paper, we report an efficient strategy to design bright blue and blue-green electroluminescent (EL) devices by slightly doping tris(8-hydroxyquinoline) aluminum (Alq₃) into N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as the light-emitting layer (EML). Bright EL devices with tunable spectra were obtained by strictly controlling the doping concentration of Alq₃. With increasing current density, EL efficiencies of these devices increase first and then decrease gradually after reaching the maximum. Analyzing the current density-voltage (J−V) characteristics of hole-only and electron-only devices, we found the presence of Alq₃ molecules in EML not only facilitates the injection of electrons from hole block layer (HBL) into EML but also stays the transport of holes in EML, thus causing significant enhancement of EL efficiency and brightness due to improved carriers balance and broadening of recombination zone. More interestingly, the doping concentration of Alq₃ strongly influences the injection and transport processes of electrons, thus determining the distribution of holes and electrons on NPB and Alq₃ molecules.     

Charge tunneling and cross recombination at organic heterojunction under electric fields

Electroluminescence (EL) of bilayer organic light-emitting diodes based on N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) and 2-(4'-biphenyl)-5-(4”-tert-butylphenyl)-1,3,4-oxadiazole (PBD) were reported. The EL spectra of bilayer device ITO/TPD/PBD/Al consist of monomolecular emission from TPD, exciplex emission and charge carriers cross recombination at the TPD/PBD interface. By varying the thickness of each organic layer while keeping the thickness of the whole device constant, three kinds of bilayer devices were fabricated and their EL and photoluminescence spectra were compared with each other, and our experimental data show that charge tunneling and cross recombination coexist at the TPD/PBD interface, and these two processes compete with each other under high electric fields.     

Pulse-induced mechanoluminescence of ultraviolet-irradiated SrAl2O4:Eu,Dy phosphors

The SrAl₂O₄:Eu,Dy phosphors prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H₂ exhibit very intense mechanoluminescence (ML) which can be seen in daylight with naked eye. When the phosphors are deformed by the impact of a low-power electric hammer, initially the ML intensity increases with time, attains a maximum value and then decreases with time. After the threshold pressure, the peak of ML intensity I_m and the total ML intensity I_T increase with the increasing value of the impact pressure. For the ML excited by the pressure pulse of short duration, two decay times of ML are observed; however, for the ML excited by the pressure pulse of long duration, only one decay time is observed. The ML intensity decreases with successive applications of pressure on SrAl₂O₄:Eu,Dy phosphors. For the low applied pressure in the range below the limit of elasticity recovery of ML intensity takes place when the sample is exposed to ultraviolet (UV) light. This fact indicates that the vacant traps produced during the application of pressure pulses get filled during the exposure of the sample to UV light. The ML in the elastic region of SrAl₂O₄:Eu,Dy phosphors can be understood on the basis of the piezoelectrically induced detrapping model. The non-irradiated SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors exhibit ML during the fracture of the compact mass of phosphors whose ML intensity is less when compared to that of the UV-irradiated compact masses. The ML induced by pressure pulses may be useful for determining the magnitude and rise time of unknown pressure pulses and to determine the lifetime of charge carriers in shallow traps.     

利用瞬态电致发光方法研究有机电致发光器件内部电荷行为

利用自主搭建的瞬态电致发光测量系统,连续施加两个电压相同的矩形脉冲作为器件驱动电压并且两个矩形脉冲之间存在一定的时间间隔,通过测量器件的瞬态EL和瞬态电流,从而分析研究器件内部电荷存储行为和发光过程。之前的研究发现了m-MTDATA∶3TPYMB混合发光层是激基复合物的发光,并且发现了其较长延迟发光是因为空穴传输层和电子传输层内储存的电荷再复合造成的。制备了以m-MTDATA∶3TPYMB(1∶1)混合层作为发光层、m-MTDATA作为空穴传输层、3TPYMB作为电子传输层的一组器件,通过对器件瞬态EL的分析,发现在第二个脉冲驱动下器件的EL强度稳定值比第一驱动驱动下的EL强度稳定值大,且第二脉冲的EL强度稳定值与第一脉冲EL强度稳定值的比值随通过器件的电流增大而减小,实验还发现第二脉冲撤销时的延迟发光衰减速度要比第一脉冲撤销时的快,这是由于第二脉冲撤销时发光层内极化子(电荷)对激子的猝灭(TPQ)比较严重。     

Transient behaviour of the mechanoluminescence induced by impulsive deformation of fluorescent and phosphorescent crystals

When a crystal is fractured impulsively by the impact of a moving piston, then initially the mechanoluminescence (ML) intensity increases quadratically with time, attains a peak value and later on it decreases with time. Considering that the solid state ML and gas discharge ML are excited due to the charging and subsequent production of electric field near the tip of moving cracks, expressions are derived for the transient ML intensity I, time t_m and intensity I_m corresponding to the peak of ML intensity versus time curve, respectively, the total ML intensity I_T, and for fast and slow decays of the ML intensity. It is shown that the decay time for the fast decrease of the ML intensity after t_m, is related to the decay time of the strain rate of crystals, and the decay time of slow decay of ML, only observed in phosphorescent crystals, is equal to the decay time of phosphorescence. The value of t_m decreases with the increasing impact velocity, I_m increases with the increasing impact velocity, and I_T initially increases and then it tends to attain a saturation value for higher values of the impact velocity. The values of t_m, I_m and I_T increase linearly with the thickness, area of cross-section and volume of the crystals, respectively. So far as the rise, attainment of ML peak, and fast decay of ML are concerned, there is no any significant difference in the time-evolution of solid state ML, gas discharge ML, and the ML emission consisting of both the solid state ML and gas discharge ML. From the time-dependence of ML, the values of the time-constant for decrease of the surface area created by the movement of a single crack, the time-constant for the decrease of strain rate of crystals, and the decay time of phosphorescence of crystals can be determined. A good agreement is found between the theoretical and experimental results. The importance of fracto ML induced by impulsive deformation of crystals is discussed.     

Degradation of organic light-emitting diodes based on different-ligand complexes of terbium (III) salicylate and 2-phenoxybenzoate

Degradation (D) of organic light-emitting diodes (OLEDs) based on new different-ligand complexes of terbium salicylate (Tb(Sal)₃(TPPO)₂) and 2-phenoxybenzoate (Tb(pobz)₃(TPPO)₂) with triphenyl phosphinoxide (TPPO) was studied. Initial reversible D is associated with charge carrier trap filling. A temperature increase, applied stress relief, and short-term irradiation with UV light lead to trapped charge carrier release and electroluminescence (EL) intensity recovery. During strong heating, irreversible D is observed after the recovery; this D requires in addition an electric field (EF). Intense UV light causes D without EF. The use of ac bias voltage eliminates reversible D, and sealing retards irreversible long-term D.     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

基于ZnSe的有机-无机异质结电致发光器件

以电子束蒸发的方法制备硒化锌(ZnSe)薄膜，研究了基于ZnSe的有机-无机异质结电致发光器件.在双层器件ITO/ZnSe(50nm)/Alq₃(12nm)/Al中看到了峰值位于578nm的ZnSe电致发光，却很难得到单层器件ITO/ZnSe(50—120nm)/Al的电致发光；在此基础上进一步引入有机空穴传输层(HTL)，通过改变器件的结构，讨论了ZnSe对有机-无机异质结器件ITO/HTL/ZnSe/Alq₃/Al电致发光特性的影响.其电致发光光谱的研究结果证实了ZnSe在器件中的作用：ZnSe既起传输电子的作用，也起到传输空穴的作用，还作为发光层.并对ZnSe的发光机理进行了讨论. 关键词： 硒化锌 有机-无机异质结 电致发光 空穴传输层