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     

有机发光器件的磁电导效应

制备了基于三(8-羟基喹啉)铝(tris-(8-hydroxyquinoline) aluminum (III), Alq₃) 的有机发光二极管, 并在不同偏压下测量了器件的室温磁电导效应.在小偏压下, 发光器件展示出明显的负磁电导效应.偏压增加后, 磁电导由负值变为正值, 出现了正负转变的现象. N, N'-二苯基-N, N'-(1-萘基)-1, 1'-联苯-4, 4'-二胺(N, N′-Di(naphthalen-1-yl)-N, N′ diphenyl-benzidine, NPB) 与铜酞菁 (Copper phthalocyanine, CuPc) 单极器件磁电导的测量结果表明, 发光器件在小偏压下的负磁电导效应来源于器件中的CuPc层. 双极电流的磁电导效应可用电子-空穴对模型进行解释, 而单极电流的磁电导效应可归因于器件中的极化子-双极化子转变. 在注入电流的变化过程中, 发光器件的正负磁电导转变是两种机理共同作用的结果.     

磁场作用下的有机电致发光

分别制备了荧光器件ITO/NPB/Alq₃/LiF/Al和磷光器件ITO/NPB /CBP: Ir（ppy）₃/BCP/Alq₃/LiF/Al, 测试了50 mT磁场作用下器件的I-V和L-V特性. 与零磁场相比荧光器件和磷光器件的效率分别增加了44%和7%. 从有机电致发光的机制出发对该现象进行了解释, 认为外加磁场可以使三重态激子发生塞曼分裂, 进而在荧光器件中引起了三重态激子T     

磁场对有机电致发光的影响

制备了常规有机发光二极管ITO/NPD/Alq₃/LiF/Al，测量了该器件发光的磁场效应.发现尽管在1T范围内的磁场对发光层Alq₃的光致发光没有影响，但磁场作用下器件的电致发光(MEL)却呈现出明显的磁效应，且MEL与器件的偏压有很强的依赖关系：在小偏压时，随着磁场的增加MEL是单调递增，且在大小约为40kA/m的磁场下达到饱和，之后即使磁场增大到约1T 的情况下也没有变化；但当偏压变大时，MEL则呈现先增加，在40kA/m处达到峰值后却又减弱，而且偏压越大该MEL的减弱则越明显.对所观察到的实验结果进行了定形解释，即三线态激子相互淬灭产生单线态激子和三线态激子与器件中的非平衡载流子相互作用是此效应的物理机理. 关键词： 有机发光 磁场效应 激子淬灭     

电场作用下高分子中自陷束缚激子的极化

用同时计入电场、对称破缺项t_e和电子-晶格相互作用的紧束缚模型研究了电场作用下高分子中的自陷束缚激子.发现电场使高分子中自陷束缚激子内电荷发生转移，出现极化.极化程度随场强增加，也与t_e有关.并发现t_e≤0.1 eV时，双激子态表现出反向极化特性，这一特性可根据极化的量子力学理论得到理解：对高分子的自陷束缚激子，其禁带中央附近存在两个靠得很近的定域电子态即上定域态和下定域态，用微扰论说明了上定域态是反向极化而下定域态为正向极化；双激子态即上     

基于Alq3的有机发光二极管的磁电导效应

制备了结构为 ITO/CuPc/NPB/Alq₃/LiF/Al 的有机发光二极管,并在300,260,220和180 K 四个温度测量了器件在恒压偏置下注入电流的磁场效应(磁电导效应).在注入电流从双极电流过渡到单极电流的过程中,随电流减小,器件的磁电导呈现先上升后下降的变化趋势.当温度降低,磁电导的值下降.但在任何测量条件下,器件的磁电导始终为正,没有出现如文献报道的磁电导从正到负的变化.实验结果表明,有机发光二极管中正负磁电导现象的产生,并非仅取决于注入电流是单极电流还是双极电流,它还与有机材料、器件结构等密切相关.利用受磁场调控的“电子-空穴对”机理与“双极化子”模型,分别解释了器件双极电流和单极电流的正磁电导效应. 关键词： 有机发光二极管 磁电导 双极化子     

蓝色荧光有机发光二极管中的激子-电荷相互作用

有机发光二极管(OLED)作为新一代显示技术已广泛商用,但蓝光有机发光二极管在效率和稳定性等方面仍存在不足,虽用磷光染料能明显地提高效率,其制作成本之高却限制了产业化发展.因此,本文对蓝色荧光有机发光二极管中的效率滚降现象进行了深入探究.首先,从稳态和瞬态两个角度研究了电子电流和空穴电流对单极器件光致发光行为的影响,表明空穴对激子的淬灭效果更显著.实验证明激子-电荷相互作用是荧光OLED中效率滚降的主要机制,且激子主要是被束缚电荷淬灭而非移动电荷.另一方面制备了不同掺杂浓度的有机发光二极管器件以探究掺杂浓度对激子-极化子相互作用的影响,得到了综合性能较好的蓝色有机荧光器件,分析表明调控发光层电荷俘获可以平衡界面堆积电荷和发光层束缚电荷对激子的淬灭.本文完善了激子-极化子淬灭的内在机制,给减缓蓝色荧光有机发光二极管的效率滚降提供了有益参考.     

4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺掺杂MoOx作为空穴传输层对有机太阳电池性能的影响

通过采用4,4′,4″-三(N-3-甲基苯基-N-苯基氨基)三苯胺 (m-MTDATA)掺入MoO_x作为器件的空穴传输层来提高酞菁铜(CuPc)/C₆₀小分子 有机太阳电池的效率. 采用真空蒸镀的方法制备了一系列器件, 其中结构为铟锡氧化物 (ITO)/m-MTDATA:MoO_x(3:1)(30 nm)/CuPc(20 nm)/C₆₀(40 nm)/4,7-二苯 基-1,10-菲罗啉 (Bphen)(8 nm)/LiF(0.8 nm)/Al(100 nm)的器件, 在AM1.5 (100 mW/cm²)模拟太阳光的照射条件下, 开路电压V_oc=0.40 V, 短路电流J_sc=6.59 mA/cm², 填充因子为0.55, 光电转换效率达1.46%, 比没有空穴传输层的器件ITO/CuPc(20 nm)/C₆₀(40 nm)/Bphen(8 nm)/LiF(0.8 nm)/Al(100 nm) 光电转换效率提高了38%. 研究表明, 加入m-MTDATA:MoO_x(3:1)(30 nm)空穴传输层减小了有机层和ITO电极之间的接触电阻, 从而减小了整个器件的串联电阻, 提高了器件的光电转换效率.     

ZnO纳米棒／MEH-PPV异质结的复合电致隧穿发光

利用低温水热法生长的ZnO纳米棒（ZnO-NRs）,和p型有机半导体材料聚[2-甲氧基-5-（2-乙基己氧基）-1,4-苯撑乙烯撑]（MEH-PPV）复合制备了结构为“ITO/ZnO晶种/ZnO-NRs/MEH-PPV/Al”的发光器件。测试结果发现,该器件具有非常好的二极管整流特性。对ZnO-NRs/M EH-PPV异质结施加超过17 V的反向偏压时,可同时获得两种半导体材料的发光,且ZnO近紫外光（380 nm ）发射强度远大于 M EH-PPV的红橙光强度,发光功率随着反向偏压的增加迅速增强,然而施加正向偏压时未探测到发光。该器件的发光机理不同于其他文献报道的正偏压发光,而属于反偏压发光器件,其发光机理归因于有机无机复合异质结的界面特殊性和ZnO-NRs的纳米尺寸效应,反偏压下器件实现的是载流子隧穿发光,而正偏压时载流子以表面态的无辐射复合及漏电流方式消耗。     

有机薄膜电致发光器件在反向偏压下的电致发光

合成了性能优良的聚合物--聚对苯乙烯,并采用甩胶方法制备了单层薄膜电致发光器件ITO/PPV/Al.在所制备的器件上观察到反向偏压下的电致发光现象,记录了在不同反向偏压下的电致发光光谱,测量了在反向偏压下的电流和发光强度,初步讨论了有机发光器件在反向偏压下的发光机理.     

Magnetooptics and dynamics of a magnetic polaron in semimagnetic CdSe/ZnMnSe quantum dots

The magnetooptics and picosecond dynamics of the radiative recombination of excitons in self-assembled semimagnetic CdSe/ZnMnSe quantum dots is studied at low temperatures. The behavior of individual quantum dots in a magnetic field and with an increase in temperature is indicative of a strong exchange interaction of excitons and magnetic Mn ions giving rise to a quasi-zero-dimensional exciton magnetic polaron. When the exciton energy exceeds the intracenter transition energy in Mn, the energy is rapidly transferred from excitons to Mn ions (faster than 20 ps). In the opposite case, a substantial red shift (～15 meV) of the emission line maximum is observed during the exciton lifetime ～500 ps, presenting the dynamics of the formation of a magnetic polaron with a characteristic time of ～110 ps.     

Magnetic Field-Effect on Photoluminescence of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Nanoparticles in a Poly[vinyl butyral] Matrix

Abstract

In this work the magnetic field effects (MFE) on the photoluminescence of MEH-PPV film and nanoparticles were obtained using the MFE technique with continuous-wave photoexcitation. The analysis of the MFE dependence for the MEH-PPV film gave a HWHM (half-width at half maximum) ～ 65?mT and a total MFE value about 102.8% at 200?mT whereas the MFE dependence of the MEH-PPV nanoparticles showed significant broadening, with their HWHM ～ 280?mT and a total MFE value of 99% at 550?mT. By a simple theoretical model based on rate equations, the mechanism of the negative magnetic field effect was explained. The observed negative effect was the result of the dominant singlet exciton formation rate in the MEH-PPV nanoparticles. For explaining this negative effect the enhanced triplet-triplet annihilation due to confinement of triplet excitons in the nanoparticles and singlet molecular oxygen-triplet exciton annihilation were proposed.     

室温下磁场对基于Alq3的有机电致发光器件的影响

制备了ITO/NPB/LiF/Alq₃/LiF/Al的器件,测量了该组器件效率和亮度的磁效应.结果表明,在50 mT磁场中,当LiF缓冲层厚度为0.8 nm时,器件的效率最大增加了12.4%,磁致亮度最大变化率17%.同时,制备的磷光器件ITO/NPB/LiF/CBP:6 wt% Ir(ppy)₃/BCP/Alq₃/ LiF/Al,在50mT磁场作用下,当LiF缓冲层的厚度为0.8 nm时,器件的效率最大增加12.1%.在Alq_{3 关键词： 有机发光 磁场 效率 磁致亮度}     

磁场对基于Co铁磁薄膜的有机发光器件效率和电流的影响

制备结构为ITO/Co/NPB/Alq₃/LiF/Al的有机发光器件,测量了室温下磁场对器件发光效率和电流的影响.发现磁场强度小于80 mT时,器件发光效率随磁场强度的增加而增大,最大为18.8%,随磁场强度的继续增加发光效率的增强趋于饱和.效率的增加是Co的自旋极化的注入和磁场效应共同作用的结果,其中自旋极化注入起主要作用.在磁场强度小于60 mT时电流随磁场增强而增加,最大为6.9%,随磁场强度的进一步增加电流的增加有所减弱.产生这种现象的原因可归结为磁场相关的单线态极化子对的解 关键词： 有机电致发光 自旋极化 磁场效应     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

NON-EQUILIBRIUM MICROWAVE RESPONSE OF Yba2Cu3O7-δ GRANULAR THIN FILMS UNDER MAGNETIC FIELDS

Microwave responses of YBa₂Cu₃O_7-δ(YBCO) granular film have been studied at the microwave frequency of 30.5 GHz. In the absence of a magnetic field the dependence of a normal microwave response on the bias current is observed at a temperature close to T_c. When a magnetic field ranged from 5.0 mT to 33.0 mT is applied, the responses broaden and shift toward a lower temperature. In the superconducting state, the responses were found to be highly dependent on the magnetic field. For the current equal to 5.0mA and a magnetic field above 17.0mT the response increases and did not vanish even at a very low temperature, the fact is believed to be correlated to the anisotropic character of the structure.     

Magneto-optical properties and exciton dynamics in diluted magnetic semiconductor nanostructures

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd_0.97Mn_0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn_0.69Cd_0.23Mn_0.08Se and quantum wires (QWRs) of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn_0.97Mn_0.03Te and double QWs of Cd_0.95Mn_0.05Te–CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd_0.97Mn_0.03Se QDs shows the confined exciton energy due to the dot size of 4–6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn_0.69Cd_0.23Mn_0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn_0.97Mn_0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd_0.95Mn_0.05Te–CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW.     

Studies of trapped strontium ions in low magnetic fields

The effect of weak magnetic fields ( 1 mT) on small clouds and single ions of strontium has been studied. The fluorescence rate and hence the cooling efficiency is sharply reduced for fields of less than around 0.2 mT. The magnetic field also splits the various levels involved through the Zeeman effect. In particular, the² S _1/2–² D _5/2 transition at 674 nm, which is the basis for a potential optical frequency standard, is split into a maximum of 10 components, depending upon the experimental geometry. This is studied for various configurations of the magnetic field and 674 nm laser polarization. The implication for frequency-standards work is that the standard will need to be operated in a small but highly stable field.