有机电致发光器件中电子—空穴对的自旋混合过程及磁效应分析

发布时间：2018-06-14 16:10

  本文选题：有机发光二极管 + 电子-空穴对　； 参考：《西南大学》2016年博士论文

【摘要】：有机电致发光(electroluminescence,EL)器件是以有机材料为活性层,在外加电场作用下辐射发光的有机半导体器件,又称为有机发光二极管(organic light-emitting diode,OLED)。自从邓青云(C.W.Tang)博士等1987年发明了三明治型有机双层薄膜电致发光器件以来,有机电致发光材料与器件被广泛研究并取得了很大进展。运用有机材料作为电子器件有很多优势,如它们制备简单、化学可调控,可以制作柔性和透明的器件。因此,除了作OLED外,还可以用来制备有机太阳能电池(OSC),有机自旋阀(OSV),有机场效应晶体管(OFET)。近二十多年来,这些有机半导体器件迅猛发展,取得了大量进展,还催生了一门新型学科—有机电子学。它是一门既涉及化学(如有机化学、光化学和高分子化学等),又牵连物理学(界面物理、固体物理、半导体物理及有机材料与器件内电子转移、自旋输运和电子-空穴对(e-h对)的自旋混合等过程)。有机电子学中有机半导体器件的工作原理和内部微观机制的相关知识,不但可以很好地帮助我们认识有机半导体器件的性质,还为我们优化有机电子器件指明方向。但是,以碳氢为基础的有机半导体材料与传统的无机半导体材料相比,在电子结构、电子输运、载流子迁移率等方面都明显不同,我们不能完全按照无机半导体的方法来研究它们,对其内部机制认识得也并不完善。例如有机半导体器件中由正负极化子组成e-h对之间通过自旋混合相互影响和转换,从而对器件性能产生重要影响。参与的自旋相关的混合过程认识得并不全面,提出的观点有分歧甚至相互冲突,这就需要我们对e-h对自旋混合过程进行进一步的研究。分析研究e-h对自旋混合过程的方法很多,常用的方法就是测量有机器件的吸收光谱与发射光谱、瞬态光谱、发光-电压-电流(B-I-V)特征曲线等。并根据需要,通过改变器件结构、测量温度和偏置电压甚至更换功能层的方法调控混合过程从而改变它们的光谱和特征曲线等来确定内部机制的种类。但有些自旋混合过程很难用这些方法区分,如三重态激子湮灭(TTA)和反系间窜越(RISC)导致的延迟荧光的瞬态光谱特征相似,也无法用B-I-V曲线区分.幸运的是,有机半导体器件在外加磁场作用下,其发光、电流甚至光谱都会发生相应改变,且这种变化往往是磁场的函数,我们称之为有机磁场效应(OMFE)。它主要包括磁致发光效应(MEL)和磁致电导效应(MC)。不同自旋混合过程往往具有不同的MEL和MC特征曲线,这些曲线可以作为e-h对自旋混合过程的身份标签或者指纹,为我们提供高效分析有机半导体器件内部机制的非接触手段。例如,因为上面提到的TTA和RISC的MEL指纹明显不一样,可以很容易通过磁效应进行区分。目前,人们已经在OSC、OFET、OSV和OLED中都发现了磁效应,但前3种器件磁效应主要是MC。在OLED中,空穴h和电子e在电场库仑力作用下相遇形成e-h对,这些e-h对在不同分子上时为极化子对,在同一分子上是激子。自旋法则决定的单、三重态e-h对比例为1:3。荧光发光器件只有25%的单重态激子型e-h对才对发光有贡献,75%的三重态e-h对的退激辐射是自旋禁阻的。因此,三重态e-h对的寿命可以达到10~(-6)到10~2秒量级,比单重态e-h对的10~(-9)秒的量级大得多,这就使三重态e-h对有足够长的时间与其它e-h对和载流子进行自旋混合。这些自旋混合过程包括系间窜越(ISC)和单重态激子分裂(STT);还有两个T激子通过湮灭过程产生1个S激子的过程(TTA),以及T与载流子(C)或极化子(P)间的相互作用(TCA或TPI)等。它们会改变单、三重态e-h对的比例,从而对器件的发光和电流都产生重要影响。而外磁场会通过抑制这些过程反过来改变器件的发光和电流,从而产生MEL和MC。我们可以同时通过MEL和MC的特点研究这些混合作用的特点以及它们对器件性能的影响。同时,通过器件结构、温度、界面修饰和掺杂等技术调控这些自旋混合,我们还可以对MEL和MC等磁效应的调控,实现特定功能的器件,如有机磁传感。因此,通过OLED研究e-h对及其与极化子之间的自旋混合,将比其它有机半导体器件更有优势。本论文以OLED器件为研究对象,通过改变电子传输材料为活跃层的器件的电极及其蒸镀方式来修饰有机-金属界面、更换空穴传输材料为活跃层的器件的电极来调控电子注入和缺陷数目、往有机活跃层中引入Fe_3O_4杂质或者结构缺陷,对器件内的自旋混合过程进行调控。再结合不同温度下的光谱、I-B-V曲线等手段,分析不同混合过程的磁效应特征,完善自旋混合过程的指纹库。最后,利用已知的自旋混合过程的指纹特征,对器件内可能发生的自旋混合过程进行分析量化,提出了通过改变分子间距调控单、三重态e-h对相互转换来提高器件荧光效率的方法。具体内容分为如下几个章节:第1章介绍了有机自旋电子学的基本概念与研究内容相关的基本知识。特别是OLED中e-h对形成过程、种类以及与其它激子或者电荷等自旋混合过程的形式、相互作用类型与相关微观模型与机制。如超精细相互作用模型(HFI)、自旋轨道耦合机制(SOC)、“Δg”机制、三重态激子湮灭(TTA模型)、单重态激子解离(STT)、双极化子模型(Bipolaron)、三重态-电荷作用(TCA)等。第2章详细介绍了本实验小组制备OLED的方法、流程及样品的测试。重点介绍有机层和金属电极的真空蒸镀方法、以及光谱分析、磁效应分析等测量分析方法和技术。第3章选用典型的电子传输兼发光材料Alq_3作为活跃层,制备了结构为ITO/Cu Pc/NPB/Alq_3/金属电极的OLED器件。在该器件中,我们分别采用Al,Cu,Au和Li F/Al电极来改变电极功函数和原子序数。并通过改变金属电极的真空蒸镀方法(分子束沉积、热阻蒸发和电子束蒸发)来修饰有机金属界面,最终实现了对器件内e-h对自旋混合过程的调控。然后通过测量这一系列器件在20 K~300 K温度范围内的MEL和MC来分析调控结果。实验结果显示,当采用分子束沉积法去蒸镀功函数高、原子序数大电极时,器件的MEL在高场出现明显下降,且这种下降无法用已有的引起高场下降的e-h对自旋混合模型,如TTA、TCA、双极化子和Δg机制来解释。我们提出了一种新的模型来解释这种高场下降:有机金属界面修饰可以改变e-h对复合区的位置,高功函数的金属作阴极时,电子注入势垒高从而成为少子,导致复合区位置靠近阴极并与有机金属界面部分交叠。交叠区金属原子的高原子序数会产生强的SOC作用,单重态的e-h对向三重态转换,从而减弱发光。而外磁场抑制少子的迁移率,进一步提高了e-h对复合区与有机金属交界面重叠程度,发光进一步减弱,最终导致MEL在高场出现明显下降。在第4章中,为了弄清楚三重态激子型e-h对(T)与载流子(C)的相互作用(triplet-charge interaction,TCI)的磁电导的本源,我们用空穴迁移率高的红荧烯(rubrene)代替第三章中电子迁移率高的Alq_3作为活跃层进行有机-金属界面修饰,使e-h对复合区靠近金属电极。再通过改变电极来调控C,通过温度和磁场来调控rubrene器件中STT和TTA的能量共振的方向,改变STT和TTA从而调制T。最后测量并分析器件的MEL和MC。结果发现MEL不受电极修饰的影响,但Al电极rubrene器件的MC表现了随磁场增加单调减小的特征曲线。这种负的MC与其它金属电极器件的先负后正的MC特征明显不同。我们认为,Al电极器件这种负的MC不应该是T激子与过量空穴通过解离或者散射通道的TCI引起的,而是T与陷阱中的束缚电子通过TCI的去陷阱(T+C_t→S_0+C)通道淬灭直接受外磁场抑制引起的。另外,载流子注入较为平衡的Li F/Al和Ca电极器件的MC比Al电极器件的小了1个量级,且随磁场的增加先减小后增大,这并非是因为平衡注入器件内的TCI弱,而是由于器件内rubrene功能层中的陷阱容易被电子占满,TCI去陷阱淬灭通道和陷阱捕获淬灭通道对电流的影响变低。因此,载流子陷阱在TCI的磁效应中具有重要地位。在上一章节中,我们是通过有机-金属界面修饰,发现在rubrene中本身自带的缺陷作为载流子陷阱是TCA混合导致MC的本质原因。在第5章中,我们重点讨论人为在活跃层中引入陷阱,来研究陷阱参与的e-h对的自旋混合过程的MC和MEL的特征。为此,我们制备了两种类型的OLED器件。第1种是基于蒽晶体的OLED,采用分子束蒸发的方法制备蒽的多晶,引入结构缺陷,使得单、三重态e-h对和极化子都被缺陷束缚。为了简单它们分别表示为S_1_t,T1_t和P_t。再加上自由的三重态e-h对(T)和极化子(P),器件内实现了T_tTA,TPI,TP_tI和T_tPI共存。研究器件的MEL和MC,特别是通过Lorentzian经验公式拟合发现,rubrene型OLED中有陷阱束缚的e-h对MC的贡献值很小,不是因为束缚态不能产生磁电导,而是由于TP_tI和T_tPI导致的MC符号相反,线宽(饱和磁场)相近,当两者共存时相互抵消的结果。这与第四章的结论一致,即束缚态的e-h对和极化子在TCA这一自旋混合中发挥重要作用,从而大大影响器件的MC。这一发现,丰富了我们对束缚态参与的e-h对和极化子的自旋混合的认识。同时,它们的MC特征也可以作为指纹,帮助我们分析有机器件中是否有束缚态的e-h对和极化子进行自旋混合。第2种是通过向聚合物SY-PPV掺入四氧化三铁纳米颗粒的办法引入非辐射陷阱,并对器件通大电流进行老化处理,从而分析大量陷阱的磁效应。结果发现,MEL的线型特征在掺杂前后发生了巨大改变:没有掺杂时,MEL表现为随磁场单调上升再饱和的ISC特征。掺杂后,发光减弱,漏电流很大,但大电流处理前,MEL线型与纯的SY-PPV器件相似。这些器件进行大电流老化处理后,表现为随磁场直接下降的负的MEL线型。据我们所知,这种MEL线型在大电流处理前后由正变负,截然相反的情况从未报道过。经原子力显微镜(AFM)扫描发现,SY-PPV层明显出现了高温结晶导致的结构陷阱。这些结果表明,陷阱在MEL中也发挥了重要作用,它们甚至会改变激子型e-h对自旋混合的方向,导致完全相反的磁效应。在第6章内容中,我们以MEL和MC作为指纹,来分析分子间距对激子型e-h对自旋混合的方向的调控。我们以rubrene为对象,通过掺杂把它引入到具有较高三重态的磷光主体材料m CP中。通过改变掺杂浓度的办法来调控分子间距d,最终在室温下实现对e-h对自旋混合的方向的调控,即随着rubrene的分子间距d由1.8 nm增大到5.0 nm,器件内部的自旋混合过程实现了由STT向TTA转换。由于STT过程消耗S_1降低荧光效率,而TTA过程却产生额外的单重态激子S_1增强OLED荧光效率。因此,这项工作为我们提供了一个更有前景的途径去提升室温条件下OLED工作效率。另外,在这一章节中,我们基于Merrifield有关TTA引起MEL变化的理论模型,还给出了利用高场范围MEL下降量估算TTA和STT相对强弱的公式,为用MEL量化OLED内e-h对间的自旋混合过程提供一个思路。
[Abstract]:Organic electroluminescence (electroluminescence, EL) devices are organic semiconductor devices using organic materials as active layers and irradiated by external electric field, also known as organic light-emitting diodes (organic light-emitting diode, OLED). In 1987, the electroluminescence of sandwich type organic double layer films was invented by Deng Qingyun (C.W.Tang) blogger. Organic electroluminescent materials and devices have been widely studied and made great progress. The use of organic materials as electronic devices has many advantages, such as simple preparation and chemical regulation, which can be used to make flexible and transparent devices. In addition to OLED, organic solar cells (OSC) and organic spin can be used to produce organic spin. Valve (OSV), an airport effect transistor (OFET). In the past more than 20 years, these organic semiconductor devices have developed rapidly and have made great progress. It has also produced a new discipline, organic electronics. It is a kind of Chemistry (such as organic chemistry, photochemistry and polymer chemistry), and Physics (interface physics, solid physics, semi conductance). Body physics and the process of electron transfer in organic materials and devices, spin transport and electron hole pair (E-H pair) spin mixing. The working principles of organic semiconductor devices in organic electronics and the knowledge of internal micromechanisms can not only help us to understand the properties of organic semiconductor devices, but also optimize the organic matter for us. However, the organic semiconductor materials based on the hydrocarbon are obviously different from the traditional inorganic semiconductor materials in terms of electronic structure, electron transport, carrier mobility and so on. We can not study them completely according to the methods of inorganic semiconductors. For example, the internal mechanism is not perfect. For example In organic semiconductor devices, the positive and negative polarons are composed of the positive and negative polarons of the E-H pair, which influences and converts through the spin mixing, which has an important effect on the performance of the device. The spin related mixing process involved is not fully understood, and the points of view are different or even conflicting. This requires us to carry out the spin mixing process of E-H. There are many methods for the spin mixing process of E-H. The commonly used method is to measure the absorption spectra and emission spectra, transient spectra, luminescence voltage current (B-I-V) characteristic curves of organic devices and so on. According to the needs, the method of adjusting the structure of the device, measuring the temperature and bias voltage and even changing the functional layer, is used to regulate and control the mixing. However, some spin mixing processes are difficult to distinguish by these methods, such as the three heavy state exciton annihilation (TTA) and the anti series channeling (RISC), which are similar to the transient spectra of the delayed fluorescence, and can not be distinguished by the B-I-V curve. Fortunately, the organic semiconductors are semi conductors. Under the effect of applied magnetic field, the light, current and even spectrum of the body will change correspondingly, and this change is often a function of the magnetic field. We call it the organic magnetic field effect (OMFE). It mainly includes the magnetoluminescent effect (MEL) and the magnetic conductivity effect (MC). The different spin mixing process often has different MEL and MC characteristic curves, These curves can be used as the identity labels or fingerprints of E-H's spin mixing process, providing us with a non-contact means of efficient analysis of the internal mechanism of organic semiconductor devices. For example, because the above mentioned TTA and RISC MEL fingerprints are obviously different, it can be easily distinguished by magnetic efficiency. At present, people have already been in OSC, OFET, OSV. Magnetic effects are found in both OLED and OLED, but the main magnetic effects of the first 3 devices are MC. in OLED, the hole h and the electron e meet to form E-H pairs under the electric field Coulomb force. These E-H pairs at different molecules are polaron pairs and the same molecule is a exciton. The spin rule is determined by the single, the three heavy E-H pair is only 25 of the 1:3. fluorescent device. The% singlet exciton type E-H pair contributes to the luminescence, and the 75% of the three heavy state E-H pair is spin forbidden. Therefore, the lifetime of the three state E-H pair can reach 10~ (-6) to 10~2 seconds, much larger than the 10~ (-9) second of the single state E-H, so that the three heavy E-H pairs have long enough time with other E-H pairs and carriers. The process of spin mixing. These spin mixing processes include intersystem channeling (ISC) and single state exciton splitting (STT); and two T excitons producing 1 S excitons through annihilation (TTA), and the interaction between T and carrier (C) or polaron (P). They change the proportion of the single, three state E-H pairs and thus the device's hair. Both light and electric current have an important effect. The external magnetic field, by inhibiting these processes, reversely changes the light and current of the device, thus producing MEL and MC.. We can study the characteristics of these mixing and their effects on the performance of the devices simultaneously through the characteristics of MEL and MC. At the same time, the structure, temperature, interface modification and doping of the device are used. In order to regulate these spin mixing, we can also regulate the magnetic effects such as MEL and MC to achieve specific functional devices, such as organic magnetic sensing. Therefore, the OLED study of E-H and its spin mixing with the polaron will be more advantageous than other organic semiconductor devices. This paper uses OLED devices as the research object, by changing electricity. The sub transmission material is the electrode of the active layer and its evaporation method to modify the organic metal interface. The replacement of the hole transmission material is the electrode of the active layer to regulate the electron injection and the number of defects. The Fe_3O_4 impurities or structural defects are introduced into the organic active layer, and the spin mixing process in the device is regulated. The magnetic effect characteristics of different mixing processes are analyzed by means of spectral and I-B-V curves at different temperatures, and the fingerprint Library of the spin mixing process is perfected. Finally, using the known fingerprint characteristics of the spin mixing process, the possible spin mixing process in the device is analyzed and quantized, and the three heavy state e is proposed by changing the interval between the molecules. The specific content of -h is divided into the following chapters: the first chapter introduces the basic concepts related to the basic concepts of organic spintronics and the basic knowledge of the research content. In particular, the formation process of E-H in OLED, the types of spin mixing with other excitons or electric charges, and the interaction between them, are interacted with each other. Types and related microscopic models and mechanisms, such as hyperfine interaction model (HFI), spin orbit coupling mechanism (SOC), "delta G" mechanism, three heavy exciton annihilation (TTA model), single state exciton dissociation (STT), bipolar submodel (Bipolaron), three heavy state electric charge (TCA), etc. "the second chapter introduces the method of preparing OLED in this experimental group, in detail. Test of process and sample. The vacuum evaporation method of organic layer and metal electrode, spectral analysis, magnetic effect analysis and other measurement analysis methods and techniques are emphasized. In the third chapter, the typical electronic transmission and luminescent material Alq_3 is selected as the active layer, and the OLED device with the structure of ITO/Cu Pc/NPB /Alq_3/ metal electrode is prepared. We use Al, Cu, Au and Li F/Al electrodes to change the work function and the atomic number of the electrode, and modify the organic metal interface by changing the vacuum evaporation method (molecular beam deposition, thermal resistance evaporation and electronic Shu Zhengfa) by changing the vacuum evaporation method of the metal electrode. Finally, the control of the self spin mixing process of E-H in the device is realized. And then the series is measured by measuring this series. The control results were analyzed by MEL and MC in the temperature range of 20 K~300 K. The experimental results showed that when the work function of vapour plating was high and the atomic number was large, the MEL of the device decreased obviously in the high field, and this descent could not be used by the E-H with high field drop, such as TTA, TCA, double polarization. A new model is proposed to explain the high field drop: the interface modification of the organic metal interface can change the position of the E-H to the complex region. When the metal as the cathode of the high power function, the electron injection barrier is high and thus becomes a minority, leading to the overlapping of the composite location by the near cathode and with the interface part of the organic metal. The high atomic number of the metal atom in the region will produce a strong SOC effect, and the E-H of the single heavy state converts to the three heavy state, thereby reducing the luminescence. The external magnetic field inhibits the mobility of the minority, and further improves the degree of overlap between the E-H and the interface of the organic metal. The luminescence is further weakened, which eventually leads to a significant decline in the high field of MEL. In the fourth chapter In order to find out the origin of the magnetic conductance of the three state exciton type E-H pair (T) and carrier (C) interaction (triplet-charge interaction, TCI), we use the high hole mobility of the red fluorenes (rubrene) instead of the Alq_3 in the third chapter as the active layer to carry out the organic metal interface modification, so that E-H is close to the metal electricity in the complex region. By changing the electrode to regulate the C, the direction of the energy resonance of STT and TTA in the rubrene device is regulated by the temperature and magnetic field, the STT and TTA are changed and the T. is modulated and the MEL and MC. results of the device are analyzed to find that MEL is not affected by the electrode modification, but the Al electrode rubrene parts show the monotonous decrease with the increase of the magnetic field. The negative MC is obviously different from the negative positive MC characteristics of other metal electrodes. We think that the negative MC of the Al electrode should not be caused by the TCI of the T exciton and the excess hole through the dissociation or scattering channel, but the quenching of the T and the bound electrons in the trap (T+C_t to S_0+C) through the TCI. It is caused by external magnetic field suppression. In addition, the MC of the carrier injected into the more balanced Li F/Al and Ca electrode devices is 1 orders of magnitude smaller than the Al electrode device, and decreases with the increase of the magnetic field first and then increases. This is not because the TCI in the balanced injection device is weak, but because the traps in the rubrene function layer are easily occupied by the electrons in the device, TCI goes. The effect of trap quenching channel and trap capture quenching channel on the current is reduced. Therefore, the carrier trap plays an important role in the magnetic effect of TCI. In the last chapter, we found that the inherent defect of the carrier trap in rubrene is the essential reason for the TCA mixing to lead to the MC. In the fifth chapter, the carrier trap is the carrier trap. We focus on the introduction of traps in the active layer to study the characteristics of MC and MEL in the spin mixing process of E-H pairs involved in the trap. To this end, we have prepared two types of OLED devices. The first is based on the OLED of anthracene crystal, the polycrystalline anthracene is prepared by molecular beam evaporation, and the structural defects are introduced to make the single, three heavy E-H. Both the pair and the polaron are bound by the defects. In order to simply be expressed as S_1_t, T1_t and P_t. with the free three heavy E-H pair (T) and the polaron (P), T_tTA, TPI, TP_tI and T_tPI are realized in the device. The contribution value of MC is very small, not because the bound state does not produce magnetic conductance, but is the result of the opposite of the MC symbol caused by the TP_tI and T_tPI, the line width (saturated magnetic field) and the mutual counteraction when both coexist. This is in agreement with the conclusion of the fourth chapter that the E-H pair in the bound state and the polaron play an important role in the spin mixing of TCA. The discovery of MC. in large impact devices enriches our understanding of the spin mixing of E-H pairs and polarons involved in bound states. At the same time, their MC characteristics can also be used as fingerprints to help us analyze the spin mixing of E-H pairs with bound states in the machine parts and the polarons. The second is by adding four oxidation to the polymer SY-PPV. The non radiation trap was introduced into the three iron nanoparticles and the magnetic effects of the large current were analyzed. The results showed that the linear features of MEL changed dramatically before and after doping. When the doping was unadulterated, the MEL showed a ISC characteristic with the monotonous rise of the magnetic field. After doping, the luminescence was weakened and the leakage was lost. The flow is very large, but the MEL line is similar to the pure SY-PPV device before large current treatment.
【学位授予单位】：西南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TN383.1
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本文编号：2018090