Electroluminescence quenching mechanism in Rubrene doped host-guest system

In this paper, the electroluminescence quenching mechanism in a 5,6,11,12-tetraphenylnaphthacene (Rubrene) doped host--guest system is studied by utilizing a specially designed organic light-emitting diode with an emission layer consisting of a few periodic host/guest structures. Tris-(8-hydroxyquinoline) aluminium (Alq₃) and Rubrene are used as the host and the guest, respectively. The thickness variation of the guest layer in each period enables the study of the effect of molecule aggregation, and the thickness variation of the host layer suggests a long range quenching mechanism of dipole--dipole interaction. The long range quenching mechanism is a F?rster process, and the F?rster radius of Rubrene is between 3 and 10~nm.     

Single-step triplet-triplet annihilation: an intrinsic limit for the high brightness efficiency of phosphorescent organic light emitting diodes

We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (F?rster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4'-N,N'-dicarbazole-biphenyl.     

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.     

Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film

Solid-state self-quenching processes of highly efficient Ir(III) phosphorescent emitters are investigated by the measurement of thin film photoluminescence quantum efficiency and transient lifetime as a function of doping concentration in a host matrix. The radiative decay rate constant is found to be independent from the average distance between dopant molecules (R), and the concentration-quenching rate constant is shown to be dependent on R(-6). The quenching dependence on R strongly suggests that luminescent concentration quenching in a phosphorescent Ir(III) complex:host film is controlled by dipole-dipole deactivating interactions as described by the F?rster energy transfer model.     

基于mCP与UGH2为母体的双发光层高效率蓝色磷光OLED

以9,9'-(1,3-苯基)二-9H-咔唑(m CP)和1,4-二(三苯甲硅烷基)苯(UGH2)为母体,将常用的蓝光染料二(3,5-二氟-2-(2-吡啶)苯基-(2-吡啶甲酸根))合铱(Ⅲ)(FIrpic)掺入这两种母体材料中,制得具有双发光层结构的蓝色磷光有机电致发光器件,并对整个物理机制进行了阐述。该器件较基于m CP或UGH2为母体的单发光层器件有着更高的器件效率。器件的最大电流效率、功率效率、外量子效率分别为21.13 cd/A、14.97 lm/W、10.56%。器件亮度从100 cd/m2到3 000 cd/m2时,效率滚降为34.2%。     

基于连续性掺杂的高效全荧光白色有机电致发光器件的研究

利用荧光材料PT-01, PT-86, PT-05作为黄色荧光客体, 蓝色荧光客体以及荧光母体制备了一种基于连续性掺杂结构的全荧光白光有机电致发光器件. 其发光层为主体/客体薄层/主体/客体薄层···交替蒸镀的重复单元. 通过优化发光层中主体的厚度并检测发光层中单线态激子的分布, 将黄、蓝两种客体染料生长在发光层中适当的位置, 得到了高效且光谱稳定的全荧光白光器件. 其最大电流效率为11.2 cd/A, 亮度在159–20590 cd/m²范围内色坐标仅有(±0.004,±0.005)的改变. 基于这种连续性掺杂结构制备的器件, 其性能不但可以达到传统主-客体共掺结构所制备的器件的性能,而且具有较高的可重复性, 更适合产业化大批量生产. 关键词： 白色有机电致发光器件 连续性掺杂 能量转移 可重复性     

Variation of efficiency droop with quantum well thickness in In GaN/GaN green light-emitting diode

In GaN/GaN multiple quantum well(MQW) green light-emitting diodes(LEDs) with varying In GaN quantum well layer thickness are fabricated and characterized. The investigation of luminescence efficiency versus injection current reveals that several physical mechanisms may jointly influence the efficiency droop, resulting in a non-monotonic variation of droop behavior with increasing quantum well(QW) thickness. When the QW is very thin, the increase of In GaN well layer thickness makes the efficiency droop more serious due to the enhancement of polarization effect. When the QW thickness increases further, however, the droop is alleviated significantly, which is mainly ascribed to the enhanced nonradiative recombination process and the weak delocalization effect.     

聚合物级联发光器件

基于溶液加工方法制备了聚乙撑二氧噻吩-聚(苯乙烯磺酸盐)(PEDOT∶PSS)/氧化锌(ZnO)/乙氧基化聚乙烯亚胺(PEIE)电荷产生层的聚合物级联发光器件, 发现PEDOT∶PSS层电导和厚度对器件的电流-电压特性影响较小, 不同PEDOT∶PSS对器件发光效率的影响主要来自于其对发光层激子不同的猝灭作用, PEDOT∶PSS厚度为60 nm的级联器件比PEDOT∶PSS 厚度为30 nm的级联器件的发光效率稍高, 原因是PEDOT∶PSS较厚时, 其表面形貌更均匀。级联器件的发光效率和驱动电压分别与发光子单元的发光效率和驱动电压之和相近, 说明在较低的电压下电荷产生层就能够有效产生电荷并注入到发光子单元中,级联器件的发光光谱中包含两个发光子单元的发光光谱,说明两个发光子单元在级联器件中都能正常工作。通过对电荷产生层的电容-电压(C-V)特性的测试, 确认了在电荷产生层中存在电荷的积累过程。证明了PEDOT∶PSS/ZnO/PEIE为有效的电荷产生层。首次报道了包含三个SY-PPV发光单元的级联器件, 三个发光子单元发光效率之和与级联器件的发光效率相当, 其最大发光效率和最大外量子效率分别为21.7 cd·A-1和6.95%。在器件亮度为5 000 cd·m-2时, 器件的发光效率和外量子效率分别为20.5 cd·A-1和6.6%。说明并没有由于发光子单元数目增加而影响级联器件的发光效率。并且其发光光谱和发光子单元的发光光谱相接近。通过 进一步降低CGL中空穴注入层对级联器件的影响有望提高级联器件的发光效率。     

Temperature dependence of the photoluminescence of CdTe/ZnTe quantum-dot superlattices

The temperature dependence of the luminescence of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) with ZnTe spacers of various thicknesses was studied. Luminescence quenching observed to occur with increasing temperature is shown to depend substantially on the thickness of the ZnTe spacer. Particular attention is focused on the temperature dependence of the luminescence of a structure with the smallest ZnTe layer thickness, containing clusters of regularly arranged quantum dots. The luminescence line of tunneling-coupled quantum dots appearing in this structure exhibits an unusual temperature dependence, more specifically, an anomalously large shift of the peak position and fast luminescence quenching with increasing temperature.     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (d _CdSe = 2.8 nm) and cationic porphyrins (H₂TMPyrP⁴⁺ and ZnTMPyrP⁴⁺) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS??Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the F?rster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H₂TMPyrP⁴⁺ and its metal complex ZnTMPyrP⁴⁺. It has been revealed that, as the QDs ? Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H₂TMPyrP⁴⁺ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP⁴⁺. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S^?(CH₂) _n COO^? and its dependence on the number of CH₂ groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ? S^?(CH₂)nCOO^? hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)??ZnTMPyrP⁴⁺, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP⁴⁺ molecule (QD* ? ZnTMPyrP⁴⁺).     

掺杂型红色有机电致发光显示器件

全色显示是有机电致发光显示(OLED)器件发展的目标,而高性能红色发光器件一直是制约全彩色OLED器件实用化的瓶颈,也是目前有机电致发光显示研究的热点。制作了掺杂DCJTB和不同浓度的rubrene两种荧光染料的红色有机电致发光显示器件,以NPB和Alq₃分别作为空穴传输层和电子传输层,发现器件性能与只掺杂DCJTB的器件相比有明显提高,发光效率提高到2～3倍。通过Frster理论和能带理论分析了器件的能量转移机理,研究发现Frster能量转移不是掺杂器件能量转移的主要形式,载流子俘获机制才是器件效率提高的主要原因;rubrene的引入使得能量能够更有效地从Alq₃转移到DCJTB,从而显著地提高了器件的发光效率和性能。     

量子阱结构对有机电致发光器件效率的影响

实验中共制备了五种有机量子阱结构电致发光器件,分别对这五种量子阱结构器件的电致发光特性进行了研究,分析了量子阱结构的周期数和势垒层的厚度对器件电学性能的影响.实验结果表明适当周期数的量子阱结构器件的亮度和电流效率比传统的三层结构器件的要大,主要原因是量子阱结构对电子和空穴的限制作用,这种限制作用提高了电子和空穴在发光层中形成激子和复合的概率,从而提高了发光的亮度和效率.当改变阱结构器件中势阱层的厚度时,也会对器件的亮度和效率产生影响,采用适当的势阱层厚度能够提高器件的亮度和效率. 关键词： 量子阱结构 电致发光 电流效率 光谱     

Single-molecule measurements of gold-quenched quantum dots

We report the study of the quenching of quantum dots (CdSe) by gold nanoparticles at the single-molecule level. Double-stranded DNA is used as a rigid spacer to tune the distance between the two nanoparticles. The width of the fluorescent intensity distribution, monitored at different interparticle distances, reflects both the nanoparticle heterogeneity and the fluorescence intermittency of the quantum dot. The fluorescence distribution emitted by single CdSe nanocrystals can easily be distinguished from the fluorescence of partially quenched CdSe. Our results show that the distance-dependence quenching is compatible with a F?rster-type process.     

以弱p型为有源区的新型p-n结构GaN紫外探测器

提出了以弱p型（p^－-GaN）为有源区的p-n结构GaN紫外探测器.由于弱p型层的载流子浓度较低,很容易增加耗尽区的宽度,从而可以增加器件的量子效率.通过模拟计算,研究了金属与p^－-GaN层的肖特基接触势垒高度、p^－-GaN层厚度等参数对器件性能的影响.研究结果表明,降低金属与p^－-GaN层的接触势垒高度、适当减小p^－-GaN层厚度能够实现有源层方向单一的内建电场,从而提高器件的量子效率.要制备出 关键词： 弱p型GaN 紫外探测器 量子效率     

Red top-emitting organic light-emitting device using 6, 13-di-(3, 5-diphenyl)phenylpentacene doped emitting system

Fluorescent red organic light-emitting devices (OLEDs) with narrow emission and negligible current-induced quenching by using a pentacene derivative of 6,13-di-(3,5-diphenyl)phenylpentacene (PDT) doped emitting system were demonstrated. With the incorporation of C₆₀ outcoupling layer, the PDT-based top-emitting OLED showed superior electroluminescence performances with Commission Internationale d’Eclairage color coordinates of (0.64, 0.36) and luminous efficiency of ～3 cd/A. To clarify the actual energy transfer process in PDT-doped emitting system, the Förster’s radius was calculated. The results indicated that the energy transfer process is predominantly from the host of tris(8-hydroquinoline) aluminum to guest of PDT via the assistant dopant of 5,6,11,12-tetraphenylnaphthacene.     

利用级联式能量传递的有机电致发光器件

为了提高掺杂型有机电致发光器件(OLED)中主体发光材料与客体荧光染料间能量传递的效率,2-对联苯-8-羟基喹啉锌(Zn ₂)作为NPB : DCJTB掺杂体系的能量助传递剂,制备了结构如:ITO/NPB/NPB : DCJTB/Zn ₂/BCP/Al的有机电致发光器件。助传递剂Zn ₂的加入,能够两次利用Frster能量转移,实现NPB向DCJTB级联式的能量传递过程,提高低浓度时掺杂染料DCJTB红光发射的纯度;此外,还探讨了三者间能量传递的有效距离,即当助传递剂与掺杂体系的距离在小于10 nm的范围内,其参与能量传递的效率随着距离的增加而逐渐下降。     

Porphyrin films — 5: Strong fluorescence

Films of tetra(perfluorophenyl)porphyrin (TFPP) show comparable strong host fluorescence whether deposited on room temperature or heated (135°C) mica substrates. Films of tetraphenylporphyrin (TPP) show only 9% as much fluorescence when deposited on unheated mica but 60% as much fluorescence when deposited on heated mica; moreover, the latter films show enhanced fluorescence from impurity guests. Octaethylporphyrin (OEP) behaves like tetraphenylporphyrin. It is observed that crystallite size is larger for films deposited on heated as compared to unheated mica. Larger crystallites could explain increased host and guest fluorescence in films of TPP and OEP prepared on heated substrates if grain boundaries serve as sites both for quenching and for trapping host excitons. The strong host fluorescence of films of TFPP can be explained either (a) by very slow exciton diffusion rates or (b) very few host quenching sites and very few guest trapping sites.     

Organic LED device based on PtOEP phosphor without doping in host material

Electroluminescence (EL) and photoluminescence (PL) have been studied on multi-layer organic light-emitting diode (OLED) devices based on phosphorescent platinum octaethyl porphine (PtOEP) molecule. A multi-layer OLED (called Pt5) which has 100% PtOEP without doping in host as the emitting layer is investigated and compared its EL and PL characteristics with those of the other OLEDs (Pt2 and Pt3) with emitting layer of PtOEP doped in 4,4′-N,N′-dicarbazole-biphenyl (CBP) host material. It is observed that Pt5 shows a lower EL efficiency than Pt2 and Pt3. Three broad EL bands are observed at 500, 527 and 570 nm in the multi-layer device in addition to red sharp EL band due to PtOEP in Pt5, while only the red PtOEP EL is observed in Pt2 and Pt3. The 500, 527 and 570 nm EL peaks arise from absorption of the broad 525 nm Alq₃ emission band by PtOEP layer. The emission from the Alq₃ electron-transport layer is caused by the carrier leakage from the hole-blocking BAlq layer. The intensity of red EL due to PtOEP is much weaker in Pt5 than in Pt2. Taking into account the result of PL, it is suggested that highly efficient energy transfer from CBP host to PtOEP guest occurs in Pt2 and Pt3, giving rise to higher PtOEP luminance, while concentration quenching occurs in PtOEP layer in Pt5.     

背照射和正照射p-i-n结构GaN紫外探测器的i-GaN和p-GaN厚度设计

研究了i-GaN和p-GaN厚度对背照射和正照射p-i-n结构GaN紫外探测器响应光谱的影响。模拟计算发现:对于背照射结构, 适当地减小i-GaN厚度有利于提高探测器的响应, 降低i-GaN层的本底载流子浓度也有利于提高探测器的响应;p-GaN的欧姆接触特性好坏对探测器的响应影响不大, 适当地增加p-GaN厚度可以改善探测器性能。而正照射结构则不同, i-GaN厚度对探测器的响应度影响不大, 但欧姆接触特性差将严重降低探测器的响应, 适当地减小p-GaN厚度可以大幅度改善探测器的响应特性。能带结构和入射光吸收的差别导致了正照射和背照射探测器结构中i层和p层厚度的选择和设计不同。