有机半导体中载流子的自旋极化性质研究

发布时间：2018-05-18 19:10

本文选题：有机半导体 + 有机磁电阻　；参考：《山东大学》2016年博士论文

【摘要】：与无机半导体和全碳材料(如碳纳米管、石墨烯等)相比,有机半导体材料有诸多不同。大部分有机半导体材料在形貌上高度无序,化学纯度不高,元素之间通过共价键结合,分子之间通过范德瓦耳斯力结合。过去三十多年来,有机半导体作为重要的电磁光功能材料已取得长足发展。与无机器件相比,有机半导体器件具有价格低、易制备、柔性等优点,因而在应用方面有十分广阔的前景。有机发光二极管,有机太阳能电池,有机场效应管,有机激光器和有机传感器等均已研制出来。随着以有机发光二极管为代表的有机电子学器件在实际应用中获得巨大成功,有机自旋电子学也已进入到大规模研究活动中。有机自旋电子学的研究催生了一系列对磁场敏感的概念器件的发明,如磁场传感器、自旋阀、自旋有机发光二极管等。以器件为导向的研究进展迅速,但有机自旋电子学不仅包含自旋相关的技术,也包含对器件中电子自旋物理过程的理解。目前文献中已经提出大量的物理模型和假设。有机半导体中的载流子为极化子和双极化子,而且输运多为分子间跃迁的模式,这导致有机半导体载流子迁移率很低。材料的高无序性导致载流子密度在同一种材料的不同样品中也可能是不同的。有机自旋电子学不仅包含信息载流子的自旋交换,还包含对磁场敏感的载流子输运,这些现象都与自旋弛豫有关。在有机自旋阀器件中,自旋极化电流从具有不同矫顽力的铁磁电极注入到有机活性层中。由于两电极的磁化方向可以独立改变,该器件可以制成载流子自旋过滤器。反过来,由于器件对外磁场敏感,又可用作磁场探测器。自旋阀器件要求载流子迁移率高,自旋弛豫时间长。有机半导体中自旋-轨道耦合较弱,这似乎对自旋极化的保持有利。但有机半导体迁移率低、超精细场强、高无序、载流子自旋高定域性又都不利于自旋极化的保持。这些特点一方面会使有机自旋电子学在基本原理以及技术上显得丰富多样,另一方面也给自旋现象的实验研究带来了困难。由于存在电极杂散场,传统的磁光克尔效应显微技术不再适用。迄今为止,最有可能在微观上追踪自旋极化的实验有两个,一是μ子自旋转动谱法,根据实验测量,有机半导体中自旋扩散长度只有纳米量级。另一个实验是双光子光电子发射谱实验,这个实验给铁磁／有机层界面自旋注入提供了令人信服的证据,但却不能探测有机半导体材料中的自旋极化输运。发现电荷输运受磁场影响的现象,进一步使问题变得复杂。有机磁电阻效应(OMAR)指的是有机发光二极管器件在弱磁场(几十个mT)下,其电阻可以有高达1-10%的变化率。近十年来,在不同有机材料,特别是溶液和真空蒸发制备的聚合物薄膜有机发光二极管器件中,有机磁电阻效应均有报道。磁电阻的大小、正负、偏压依赖关系均有研究。几个mT量级的OMAR和磁电致发光也有报道。还有研究发现OMAR可以在磁场变化过程中改变符号。一个有意思的观点是,OMAR可能揭示了有磁场感官能力的鸟利用地磁场判断方向的物理机制,而且有可能涉及宏观室温量子相干现象。目前关于OMAR效应,主要的理论模型包括：电子-空穴对机制(或者叫载流子对机制),双极化子机制和激子-极化子碰撞机制。在电子-空穴对模型中,自旋弛豫过程控制了单态和三重态电子-空穴对的相互转化,而单、三重态电子-空穴对的浓度共同决定了器件的电导率。外加静磁场可以改变超精细场对自旋弛豫的影响,从而出现磁电阻。通常认为,这一模型只能描述或正、或负的OMAR,但不能同时描述两者。激子极化子碰撞模型认为,三态激子与极化子碰会撞导致极化子迁移率下降,而外磁场可以改变三重态激子的浓度,进而改变碰撞次数,产生磁电阻效应。这两个模型都需要器件中同时存在正负载流子,但OMAR还可以在单极器件中出现。Bobbert等人提出,通过引入双极化子,可以很好地解释单极器件中的OMAR。模型认为外磁场可以调控极化子和双极化子比率,由于极化子和双极化子的迁移率不同,导致加磁场前后电导率不同。有实验在单极器件中发现了高达2000%的OMAR,也可以很好地用双极化子模型来解释。对于器件中磁相互作用的来源,有研究考虑了超精细相互作用和自旋-轨道相互作用,还有研究认为外磁场对载流子的洛仑兹力改变了载流子在格点间的跃迁积分,进而改变电流,产生磁电阻现象。目前,有机磁电阻研究最大的挑战在于从实验上直接检验这些理论的正确性。目前有实验组利用光谱技术开展相关的研究。OMAR效应的发现促进了人们对有机半导体材料中载流子自旋特性的深入思考。2010年,Tarafder等人通过第一性原理计算发现,Alq3分子中注入电子会出现净磁矩。计算发现,Alq3分子在电子注入前后,三个A1-N键和三个A1-O键会发生变化,同时费米面附近会发生自旋劈裂。他们研究了诸如电荷量从0到1.5 e时分子自旋极化的情况,发现分子的自旋极化随注入电荷量成正比。因为Alq3分子中包含金属元素,其中的物理过程更为复杂。Hou等人计算了纯有机聚噻吩分子中电荷注入引起的自发自旋极化情况。根据他们的计算,注入电荷量少的时候,系统不会出现自旋极化,只有注入电荷量的达到一定值后,系统才会出现自旋极化。另外分子的聚合度也会对自旋极化的大小有影响。有机半导体中注入载流子的自发自旋极化以及载流子迁移率受磁场调控等现象的发现,都预示着有机半导体中载流子有丰富的电荷和自旋特性。而室温有机多铁现象的发现更加丰富了人们对于有机半导体材料的认识。多铁材料指的是能同时显示出两个及以上铁性序参量的材料,如铁电性、铁磁性和铁弹性。近十年来,多铁材料以其丰富的物理性质,以及在自旋电子学、光电、热电和传感器等领域的潜在应用,迅速成为科学研究的热点。而室温有机多铁现象的发现和研究,为低成本、大面积地制备和开发实用型器件提供了新的思路。2009年,Giovannetti等人采用第一性原理计算结合模型方法,首次预言了在TTF-CA电荷转移盐材料中会出现多铁现象。在这之后,几种具有磁电耦合性质的复合物器件在实验上被制备出来。2012年,Ren等人报道了在nw-P3HT/C60复合物中观测到光激发铁磁性现象。他们发现,在P3HT纳米线单晶中掺入C60后,光照会使器件呈现明显的铁磁性。实验进一步发现,外电场、应力等外界刺激也会对材料磁化强度有影响。通常在无机多铁材料中,磁电序出现的居里温度较低。但激发铁磁性不受这一限制,从而为室温多铁开辟了新的领域。综上所述,有机半导体材料中的载流子有丰富的自旋特性,实验上也发现了很多新现象,但很多具体问题背后的物理机制还需要仔细研究,以便更好地理解有机半导体中载流子的自旋特性。本论文从机半导体中载流子的自旋特性入手,采用经典的跃迁理论和一维Su-Schrieffer-Heeger (SSH)模型,分别研究了有机磁电阻现象,有机半导体中电荷注入引起的自发自旋极化和有机电荷转移复合物器件中的光激发铁磁性现象。以下是具体的研究内容和基本结论。1.OLED器件中的磁电阻效应。关于实验上发现的有机磁场效应,目前人们已经提出几个理论。我们这里采用经典的Marcus跃迁模型,提出磁场引起载流子能级的赛曼劈裂,并改变电子在分子间跃迁率的模型,给出有机磁电阻效应的一个经典的解释。此外我们还考虑了超精细相互作用,考虑了外磁场对载流子的洛伦兹力,计算发现,在低温下,赛曼劈裂引起的载流子跃迁率的改变可能是有机磁电阻效应的一个来源。但是在高温下,有机非磁性器件中外磁场引起电流的变化,更多是磁场对载流子的洛仑兹力引起的。2.有机半导体中电荷注入引起的自发自旋极化。我们在Hou等人研究基础上,采用扩展的一维SSH模型,考虑电子-电子相互作用,自旋反转相互作用以及自旋-轨道耦合,理论研究了聚噻吩中电荷注入引起的自发自旋极化现象。模型方法的优势在于可以方便地计算更长的分子链,更多的电荷注入和更方便的参数分析。计算发现,随着电-声耦合常数的增大,系统电子态会出现从扩展到局域的突变,伴随着电子态局域的出现,自旋极化随之产生,因此有机材料强的电-声相互作用是出现自发自旋极化的前提。相同电荷注入量下,不同聚合度的聚噻吩,其自旋磁矩也会有所不同,我们认为这都与电荷在分子中的局域度有关。对于小分子,分子尺寸很小,注入到分子中的电荷很容易形成局域态,因此即使注入很少电子,系统仍然可以出现自旋极化。而在高分子中,注入电荷量很少时,注入的电荷先是形成扩展态,注入电荷量增加到一定程度时局域态才会出现,并与之相伴产生自旋极化。对于自旋-轨道耦合,因为有机材料中自旋-轨道耦合强度很小,计算发现自旋-轨道耦合对自旋极化几乎没有影响。3.有机电荷转移复合物中的激发铁磁性。围绕Ren等人在电荷转移复合物中发现的光激发铁磁性,我们采用扩展的一维紧束缚模型,计算了有机电荷转移复合物中,激子和电荷转移态的自旋极化情况和变化规律。一般认为,带间光激发产物是单态激子,三态激子因为跃迁禁阻,其数目可以忽略。而考虑自旋相关的相互作用之后,自旋不再是好的量子数,所有的激子都会变成单态和三态激子的叠加态。计算发现,对于电荷转移态,电子给体和受体上的自旋极化是不一样的,因此,不论如何激发,电荷转移态总会对外显示净磁矩。关于电荷转移态之间的耦合,我们在计算中考虑了不同的耦合方式,计算发现,对于不同的自旋排列,系统的能量有高低,但是总能对外显示净磁矩。
[Abstract]:Compared with inorganic semiconductors and all carbon materials (such as carbon nanotubes, graphene, etc.), there are many different organic semiconductors. Most organic semiconductors are highly disordered and with low chemical purity. The elements are combined by covalent bonds between the elements and the molecules are combined by Fan De Valis. Over the past thirty years, organic semiconductors have been made. Compared with inorganic devices, organic semiconductor devices have many advantages, such as low price, easy preparation and flexibility. Therefore, organic light-emitting diodes, organic solar cells, organic field effective tubes, organic lasers and organic sensors have been developed. As organic electronic devices represented by organic light emitting diodes have gained great success in practical applications, organic spintronics have also entered into large-scale research activities. The study of organic spintronics has led to a series of concept devices sensitive to magnetic fields, such as magnetic field sensors, spin valves, spin organics. Light emitting diodes and so on. The progress of device oriented research is rapid, but organic spintronics includes not only spin related technology, but also the understanding of the physical process of electron spin in the device. A large number of physical models and hypotheses have been proposed in the current literature. The mode of intermolecular transition, which leads to the low mobility of the organic semiconductor carrier, may lead to the carrier density in different samples of the same material. Organic spintronics includes not only the spin exchange of the information carrier, but also the carrier transport of the magnetic field, which are sensitive to the magnetic field. In organic spin valve devices, the spin polarization current is injected into the organic active layer from the ferromagnetic electrodes with different coercivity. Because the magnetization direction of the two electrode can be changed independently, the device can be made into a carrier spin filter. In turn, it is sensitive to the external magnetic field and can be used as a magnetic field probe. The spin valve device requires high carrier mobility and long spin relaxation time. The spin orbit coupling in organic semiconductors is weak, which seems to be beneficial to spin polarization. However, the mobility of organic semiconductors is low, hyperfine field strength, high disorder, and high domain of carrier spin is not conducive to the maintenance of spin polarization. It will make organic spintronics rich in basic principles and techniques. On the other hand, the experimental study of spin phenomena is difficult. Due to the existence of the heterostray field, the traditional magneto-optical Kerr effect microscopy is no longer applicable. So far, there are two experiments that are most likely to trace spin polarization on microviews, one is mu. According to the experimental measurements, the spin diffusion length of the organic semiconductor is only nanometers. The other experiment is a two-photon photoelectron emission spectrum experiment. This experiment provides convincing evidence for the spin injection of the ferromagnetic / organic layer interface, but it can not detect the spin polarization transport in the organic semiconductors. The phenomenon that the current charge transport is affected by the magnetic field further complicates the problem. The organic magnetoresistance effect (OMAR) refers to the resistance of organic light-emitting diodes in the weak magnetic field (dozens of mT) to a change rate of up to 1-10%. In the last ten years, the polymer films prepared in different organic materials, especially in solution and vacuum evaporation, have been prepared. The effects of organic magnetoresistance are reported in the light emitting diode (LED) devices. The size of magnetic resistance, positive and negative, and bias dependence have been studied. Several mT orders of OMAR and magnetoelectric luminescence are also reported. And the study found that OMAR can change symbols in the process of magnetic field change. A deliberate view that OMAR may reveal the sense of the magnetic field The capable birds use the geomagnetic field to determine the physical mechanism of the direction, and may involve the quantum coherence phenomena at the macroscopic room temperature. At present, the main theoretical models of the OMAR effect include: the electron hole pair mechanism (or the carrier pair mechanism), the dipole mechanism and the exciton dipole collision mechanism. In the electron hole pair model, the spin is spins. The relaxation process controls the mutual conversion of the single state and the three state electron hole pair, while the concentration of the single and three heavy electron hole pairs determines the electrical conductivity of the device together. The external magnetostatic magnetic field can change the effect of the hyperfine field on the spin relaxation, thus producing the magnetoresistance. The shock model of exciton polaron shows that the collision between the three state exciton and the polaron leads to the decrease of the mobility of the polaron, and the external magnetic field can change the concentration of the three heavy exciton, and then change the number of collisions and produce the magnetoresistance effect. The two models all need both positive and negative carriers in the device, but OMAR can also be in the monopole. In the device,.Bobbert et al. Put forward that by introducing the dipole, the OMAR. model in the unipolar device can be well explained that the external magnetic field can regulate the ratio of the polaron and the dipolaron. Due to the different mobility of the polaron and the dipole, the conductivity is different before and after the addition of the magnetic field. The experiment is found to be up to 20 in the monopole device. The OMAR of 00% can also be well explained by the dipolaron model. For the source of magnetic interaction in the device, the study considers the hyperfine interaction and spin orbit interaction, and the study considers that the Lorentz force of the external magnetic field changes the transition integral between the carriers at the lattice, and then changes the current and produces the magnetoelectricity. At present, the greatest challenge for the research of the organic magnetoresistance is to test the correctness of these theories directly. At present, the experimental group is using the spectral technology to carry out the related research on the.OMAR effect and promote the people to think deeply about the spin characteristics of the carrier in organic semiconductors for.2010 years. Tarafder et al. The calculation of the principle of sexual principle shows that the net magnetic moment of the injection of electrons in the Alq3 molecule is found. It is found that the three A1-N and three A1-O keys will change before and after the electron injection, and the spin splitting will occur near the surface of the Fermi surface. They have studied the spin polarization such as the charge amount from 0 to 1.5 e, and found the spin polarization of the molecule. As the injection charge is proportional to the amount of charge, because the Alq3 molecule contains metal elements, the physical process is more complicated by.Hou et al. The spontaneous spin polarization caused by charge injection in the pure organic polythiophene is calculated. According to their calculations, the system will not have spin polarization when the charge amount is less, only the amount of injection charge is reached. After a certain value, the spin polarization will occur in the system. The degree of polymerization of the molecules also affects the spin polarization. The discovery of the spontaneous spin polarization of the carriers in the organic semiconductors and the discovery of the carrier mobility by the magnetic field all indicate the rich charge and spin properties of the carriers in the organic semiconductors. The discovery of the phenomenon of organic iron at room temperature has enriched the understanding of organic semiconductors. Multi iron materials refer to materials that can show two and more iron order parameters at the same time, such as ferroelectricity, ferromagnetism and ferroelasticity. In the last ten years, the material has its rich physical properties, as well as spintronics, photoelectric, thermoelectric, and thermoelectric properties. The potential applications of sensors and other fields have rapidly become the focus of scientific research. The discovery and study of the phenomenon of organic iron at room temperature provides a new way of thinking for the low cost, large area preparation and development of practical devices. Giovannetti et al. Used the first principle calculation method to predict the charge transfer in TTF-CA for the first time. After this, several composite devices with magnetic and electrical coupling properties have been prepared for.2012 years after this. Ren et al. Reported the observation of the ferromagnetism in the nw-P3HT/C60 complex. They found that after the incorporation of C60 in the P3HT nanowire single crystal, the light illuminated the device with an obvious ferromagnetic field. The experiment further found that external electric field, stress and other external stimuli also affect the magnetization of the material. Generally, in the inorganic multi iron material, the Curie temperature in the magnetoelectric order is lower. However, the excitation of ferromagnetism is not limited, which opens up a new field for the room temperature multi iron. The carrier of organic semiconductors is abundant. Many new phenomena have been found in the spin properties of the rich, but the physical mechanisms behind many specific problems need to be carefully studied in order to better understand the spin characteristics of the carriers in the organic semiconductors. This paper starts with the spin properties of the carrier in the machine semiconductor, and uses the classical transition theory and the one-dimensional Su-Schrieffer-Heeger. (SSH) model, the phenomenon of organic magnetoresistance, the spontaneous spin polarization induced by charge injection in organic semiconductors and the light excitation ferromagnetism in the organic charge transfer complex devices are studied. The following is the specific research content and the basic conclusion of the magnetoresistance effect in the.1.OLED device. Several theories have been put forward by the former people. Here we use the classical Marcus transition model to suggest that the magnetic field causes the Zeeman splitting of the carrier energy level, and changes the model of the electron transition rate between the molecules and gives a classic explanation of the effect of the organic magnetoresistance. In addition, we consider the hyperfine interaction and consider the external magnetic field to load. It is found that the change of the carrier transition rate caused by Zeeman splitting at low temperature may be a source of the organic magnetoresistance effect at low temperature. But at high temperature, the magnetic field caused by the magnetic field in the organic nonmagnetic device causes the change of the current, and more is the charge of the.2. organic semiconductor caused by the magnetic field to the Lorentz force of the carrier. On the basis of Hou et al. Based on the extended one-dimensional SSH model, we consider the electron electron interaction, spin reversal interaction and spin orbit coupling, and study the spontaneous spin polarization phenomena caused by the charge injection in polythiophene. The advantage of the model method is that it can be easily calculated. The longer molecular chain, more charge injection and more convenient parameter analysis. It is found that with the increase of the electrical acoustic coupling constant, the electronic state will appear from the expansion to the local, with the appearance of the electronic state, the spin polarization is produced, so the spontaneous spin polarization of the organic material is strong. Under the same charge injection amount of polythiophene with different degree of polymerization, the spin magnetic moment of the polythiophene will vary. We think that this is all related to the local degree of the charge in the molecule. For small molecules, the size of the molecule is very small, the charge into the molecule is very easy to form a local state, so the system can still appear even if there is little electron injection. When the injection charge is very small, the injected charge is first formed, and the local state will appear when the charge amount is increased to a certain extent, and the spin polarization is associated with it. For spin orbit coupling, the spin orbit coupling is found to be found in the organic material. The spin polarization has little effect on the magnetization of the excited ferromagnetism in the.3. organic charge transfer complex. We have calculated the spin polarization and the variation of the exciton and charge transfer states in the organic charge transfer complex by using the extended one-dimensional tight binding model around the light excited ferromagnetism found in the charge transfer complex of Ren et al. It is generally believed that the interband light excitation product is a single state exciton, and the number of the three state excitons can be ignored because of the transition. Considering the spin dependent interaction, the spin is no longer a good quantum number, and all the excitons will become the superposition state of the single state and the three state exciton. The calculation is found that the charge transfer state, the electron donor and the receptor are found. The spin polarization is different. Therefore, no matter how excited, the charge transfer state always shows the net magnetic moment.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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