共轭聚合物中的极化子动力学与齐纳隧穿研究
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摘要
掺杂聚乙炔高电导率的发现打破了有机聚合物都是绝缘体的传统观念,开创了导电聚合物的研究领域。导电高分子聚合物都有一个长程的π电子共轭主链,因而导电聚合物又被称为有机共轭聚合物。共轭聚合物实现了从绝缘体到半导体,再到导体、超导体的变化,是所有物质中完成形态变化跨度最大的,其特殊结构和优异的物理化学性能使它成为材料科学的研究热点。作为不可替代的新兴基础有机功能材料之一,聚合物材料在能源、光电子器件、信息、传感器、分子器件,以及电磁屏蔽、金属防腐和隐身技术上有着广泛的应用前景。到目前为止,共轭聚合物在分子设计和材料合成、掺杂方法和掺杂机理、可溶性和加工性、光、电、磁等物理性能及技术应用上的探索都已取得重要的研究进展。经过三十余年的发展,许多基于有机聚合物的光电器件已经从单纯的实验兴趣转变为新兴的实用技术。目前,人们已经研制出多种有机光电子器件,如有机发光二极管,场效应管,光伏电池等。
与传统的无机材料相比,有机分子间的相互作用较弱,它们大多具有准一维结构;此外,有机材料中存在着较强的电子—晶格相互作用,电子态和晶格态两者相互影响。即电荷的注入或光激发会诱导晶格发生畸变,反过来,晶格的变化又影响聚合物的结构。因此,聚合物中的载流子不再是电子或空穴,而是电荷的自陷元激发,如孤子、极化子、双极化子。聚合物中的这些元激发在很大程度上决定着聚合物中的电荷输运、发光等物理性质,是人们理解聚合物特性的重要方面。由于这些元激发包括电荷和晶格畸变两部分,在外场的作用下,电荷的运动必然影响到晶格畸变。因此聚合物中的载流子及其在电场下的输运性质一直是理论研究的重点。
创立于20世纪70年代的Su-Schrieffer-Heeger(SSH)模型,用半经验的紧束缚方法研究了共轭聚合物聚乙炔的电子结构和光学性质,并取得了巨大成功,此后,Bishop、Sun、Conwell、Xie等对SSH哈密顿进行了修正。目前,国际上主要有四个小组基于SSH模型,分别采用绝热近似和非绝热近似,研究了共轭聚合物中载流子动力学输运及其微观机制,这些工作的进一步开展,不但有助于对共轭聚合物中的微观物理世界的认识,更具有重要的应用价值,能够推动新型有机光电器件的研究和开发。
本论文在紧束缚的(SSH)模型基础上,对哈密顿进行了修正,利用非绝热的分子动力学方法,模拟了有序的耦合聚合物链系统中链间扩展的极化子的形成及其在外场中的输运动力学,并与一维链内定域极化子的运动作比较。同时我们还研究了温度对极化子稳定性的影响。最后讨论了强电场下聚合物中的齐纳隧穿现象。本论文具体的研究内容和主要结果如下:
1.链间扩展的极化子动力学
共轭聚合物通过电荷注入、掺杂或者光激发可以形成载流子——极化子。对于聚合物中极化子性质的研究,人们已经进行了相当多的研究,包括极化子的形成过程以及极化子在外电场下的运动。大都关注于单条聚合物链的情况,耦合链系统中极化子形成的过程尚不清楚。实验研究发现,通过自组织,有机分子由于链间耦合可形成有序的薄膜结构。光谱研究表明,在这种聚合物链有序排列的结构中极化子具有二维链间特征。因此,我们构造多条有序耦合的聚合物链来模拟自组织的薄膜,研究其中极化子形成过程。实验表明,将这种有序的有机薄膜应用于场效应管可极大地提高迁移率,因此普遍认为高的迁移率是与极化子的二维链间特征有关的。因此研究二维链间扩展的极化子的性质有重要意义。
首先,我们研究了耦合聚合物链系统中极化子的形成过程,发现在有序耦合的共轭聚合物链系统中,注入到其中的电子形成链内定域的极化子还是链间扩展的极化子取决于聚合物链间的耦合强度。不管是在较弱的链间耦合下还是在较强的耦合下,电子注入到系统中后,首先分布在多条链上,但是当链间耦合较弱时,这种分布持续一段时间后电荷就逐渐向一条链中转移,最终形成了定域在一条链内的极化子。在较强的耦合下,电荷不能完全克服链间的竞争,因此,最终形成了链间扩展的二维极化子。
计算二维链间扩展极化子的运动,包括极化子沿聚合物链的运动(外加电场平行于链的方向(x方向)施加)和极化子在聚合物链间的运动(外加电场垂直于链的方向(y方向)施加),与一维链内定域极化子的运动比较发现,在相同的电场强度下,二维的链间扩展极化子具有更大的运动速度。从这个意义上讲,以二维链间扩展极化子为载流子的有序的有机聚合物体系,其迁移率远大于以链内定域极化子为载流子的有机体系。这与实验结果是一致的。
2.温度对于极化子稳定性的影响
除了极化子在外电场下的输运,极化子的稳定性也是人们关心的一个重要问题,特别是对于有机发光二极管(OLED),它的工作原理是基于阴、阳极注入的负、正电极化子在聚合物层碰撞产生激子,因此极化子的稳定性直接关系到有机发光器件的工作效率。目前的研究发现,电场强度、电场施加模式、杂质、载流子之间的碰撞等都会对极化子的稳定性产生重要影响。另外,在一定温度下,有机聚合物材料中总是存在着晶格热涨落。而且,在某些一维材料中,当温度接近相变温度时,晶格热涨落可能比二聚化还要大几倍。因此,晶格涨落对一维材料的性质有重要影响。
本论文中,我们讨论了晶格热涨落对共轭聚合物中极化子稳定性的影响。热效应通过郎之万方程引入的热随机力来描述,温度区域选在150K-350K之间。研究发现,在这个温度区域内,极化子能级的电子占据数减少,极化子的定域性受到影响。极化子保持理想定域态的时间随温度的升高而变短。考虑到温度效应后,极化子的解离电场大大降低。
3.共轭聚合物中的齐纳隧穿现象
固体材料在强场下的电荷输运一直是备受关注的课题,20世纪30年代,Bloch和Zener对这方面的理论研究作出重要贡献。自从Zener用电子的带间隧穿(Zenertunneling)来解释固体的电击穿后,Zener隧穿现象引起了物理学家浓厚的研究兴趣。在很多无机系统中,如电流驱动的约瑟夫森结、加速光学晶格、场驱动的超晶格等,人们做了大量Zener隧穿及其相关应用的研究。但是,有机半导体中尚未有详细的相关报道。在本论文中,我们将通过研究高电场下有机半导体中电子态的演化来讨论可能的Zener效应。
以一条处于理想二聚化状态的电中性的聚合物链作为研究对象,对其施加平行于链方向的电场。发现,在足够强的电场作用下,原本无净电荷分布的链中出现了极化电荷。能带发生倾斜,满带中的电子能够穿过带隙进入导带,实现了有机半导体中的Zener隧穿。伴随着Zener隧穿的发生,带隙消失,有机半导体经历了从半导体到导体的转变(I-M相变)。同时,有机晶格原子也从二聚化的排列转变为等距离排列,原子构型的这种变化是与I-M相变相对应的。这是有机聚合物特有的软性质决定的。
The discovery of high conductivity of doped trans-polyacetylene (tPA) broke through the conventional opinion that all organic polymers were insulators, and opened a new area of conducting polymers. The p_z-orbital of neighboring carbons overlap to form conjugated n bond, so conducting polymers can be called conjugated polymers. Conjugated polymers span a wide range from insulator to superconductor. The special structures and excellent physical and chemical properties have made it a focus of material science. As a new kind of irreplaceable fundamental functional materials, polymers has promising potential application in optoelectric device, sensing probe, molecular device, electromagnetic shielding, and metal preservation. A lot of significant progress has been made for conjugated polymers in many aspects, such as molecule design, material synthesis, doping method, solubility and processability, and conducting mechanism. Now, some optoelectric devices based on conjugated polymers have been turned into practical technique from pure experimental interest, including organic light-emitting diode (OLED), field-effect transistor (FET), photovoltaic cell, etc.
Compared with the traditional organic semiconductor, organic material has its unique properties. First, most of the conjugated polymers have the quasi-one-dimensional structure due to the weak interaction between the organic molecules. Second, there are strong electron-lattice couplings in organic systems. Injected charges (electrons or holes) or photoexcitation in conjugated polymers will induce lattice deformation, and the lattice deformation in turn influences the energy band structure of the organic system. So, it is generally believed that these self-trapping excitations, including solitons (only in trans-polyacetylene), polarons and bipolarons, are charge carriers in conjugated polymers. These elementary excitations are of fundamental importance for transport and photoluminescence of conjugated polymer system. Therefore, the dynamical process of these excitations has always been the focus in both theoretical and experimental investigation, and extensive studies have been carried out on charge carrier dynamics and on the promising application of conjugated polymers in organic functional devices.
In 1970's, a tight binding model-SSH model, developed by Su, Schrieffer and Heeger, was adopted to study the electronic structure and optical properties of the simplest conjugated polymer PA and showed considerable success. Since then Bishop, Sun and Xie et al have extended the SSH Hamiltonian to research both the static and dynamic process of excitations. At present, there mainly are four groups performing researches on charge transport in conjugated polymers based on the SSH model. The further research in this field not only can broaden our understanding of the microcosmic physical world but also have a substantial impact on the applications of organic devices.
Within the tight-binding electron-lattice interacting model, by using a nonadiabatic dynamic method, we have simulated the dynamic formation process of a interchain delocalized polaron and its motion driven by the external field in a well-ordered system of coupled conjugated polymer chains. The dynamical behavior of the interchain delocalized polarons is compared with that of the intrachain localized ones. Moreover, we have also explored the effects of temperature on the polaron stability. At last, the behavior of conjugated polymer chain under high electric field is investigated. The detailed research and main results are given below:
1. Dynamics of interchain delocalized polarons in conjugated polymers
Polaron formation and motion in a single chain is now a relatively well studied subject, and a great deal of effort has been devoted to such studies. However, the dynamical behavior of polarons in a more complicated system of coupled conjugated polymer chains is not so clear. Several experiments have reported high mobilities in thin-film transistors separately, in which owing to self-assembly, organic molecules form a relatively well ordered film structure by interchain stacking. In Sirringhaus's experiment which aimed to probe the transport properties of the ordered microcrystalline domains, it revealed the 2D interchain character of the polaronic charge carriers (polarons), and suggested that radical cations on isolated 1D chains can not lead to high mobilities. Thus, it is of great interest to probe the polaron transport in well-ordered coupled conjugated polymers, such as self-assembly films.
We construct a system consisting of as many as ten well-ordered coupled conjugated polymer chains to provide enough space for the polaron to travel. First, the dynamical process of polaron formation in such a system is investigated. It is found that once the electron is added to the system, it is shared by the several chains and induces lattice distortion in the chains within a short time. With enhancing the interchain coupling, it takes longer time for the electron to overcome the interchain coupling to localize in a single chain. Beyond a certain value of the interchain coupling, the electron will evolve into a 2D interchain delocalized polaron state extending over several chains.
By applying an external electric field to the interchain delocalized polaron, we study the 2D motion of the polaron, including the motion along the polymer chains (in which case the field is parallel to the chain) and the motion perpendicular to the polymer chain (in which case the field is perpendicular to the chain). Compared with the motion of the intrachain localized one, the interchain delocalized polarons drift much faster than localized polarons under the same electric field, implying that high mobilities can be achieved by 2D charge transport. This is in agreement with earlier experiments.
2. Effect of temperature on the stability of polarons
Polaron stability is another issue to which much attention has been paid. Up to now, the external electric field was demonstrated to have significant influence on the polaron stability. Moreover, it was found that, besides the electric field strength, the application mode of the field also plays an important role in polaron stability. Other effects, such as impurities, defects, and collision between charge carriers, have also been thoroughly studied. However, all these works were based on the lattice dimerization, i.e., lattice fluctuations were not taken into account.
Dimerization has been well understood in the static limit and within the mean-field theory. As a matter of fact, however, lattice fluctuations always exist. Even in the limit of zero-temperature, lattice fluctuations are of the same order of magnitude of the lattice distortion. Furthermore, at temperature comparable to the transition temperature the thermal lattice motion can be several times larger than distortion. The lattice fluctuations should have an important effect on the one-dimensional materials. In this paper, we investigate the stability of the localized polaron in conjugated polymers in the presence of thermal lattice fluctuations.
The temperature effect is simulated by introducing random forces to the equation of the lattice motion. It is found that the localized polaron state becomes delocalized even at low temperatures. The time of polaron keeping localized depends on the magnitude of temperatures. By taking into account the thermal effect, we find that the dissociation field is weaker compared with earlier works.
3. Zener tunneling in conjugated polymers
The issue of high-field transport in solids is of great concern and has been investigated for many decades due to basic interest in the physical phenomena and many important applications. Early theoretical contributions were made by Bloch and Zener. Since Zener explained the electrical breakdown of solid dielectric in terms of interband tunneling, the phenomenon of Zener tunneling has attracted considerable interest, particularly in modern nanoscale devices. To the best of our knowledge, however, Zener tunneling in conjugated polymers (belong to the family of organic semiconductors) is rarely referred to.
In chapter IV, we consider the dynamical evolution of electronic states in a conjugated polymer chain under a high electric field, and try to explore the possible Zener tunneling effect in organic semiconductors. It is shown that under a sufficiently high field, electrons can transit from the valence band to the conduction band, which is Zener tunneling in organic semiconductors. The result also indicates a field-induced insulator-metal (I-M) transition accompanied by the vanishing of the energy gap, that is, the Peierls phase is destroyed. Meanwhile, the lattice configuration undergoes significant change from dimerization to equidistant arrangement when I-M transition occurs.
与传统的无机材料相比,有机分子间的相互作用较弱,它们大多具有准一维结构;此外,有机材料中存在着较强的电子—晶格相互作用,电子态和晶格态两者相互影响。即电荷的注入或光激发会诱导晶格发生畸变,反过来,晶格的变化又影响聚合物的结构。因此,聚合物中的载流子不再是电子或空穴,而是电荷的自陷元激发,如孤子、极化子、双极化子。聚合物中的这些元激发在很大程度上决定着聚合物中的电荷输运、发光等物理性质,是人们理解聚合物特性的重要方面。由于这些元激发包括电荷和晶格畸变两部分,在外场的作用下,电荷的运动必然影响到晶格畸变。因此聚合物中的载流子及其在电场下的输运性质一直是理论研究的重点。
创立于20世纪70年代的Su-Schrieffer-Heeger(SSH)模型,用半经验的紧束缚方法研究了共轭聚合物聚乙炔的电子结构和光学性质,并取得了巨大成功,此后,Bishop、Sun、Conwell、Xie等对SSH哈密顿进行了修正。目前,国际上主要有四个小组基于SSH模型,分别采用绝热近似和非绝热近似,研究了共轭聚合物中载流子动力学输运及其微观机制,这些工作的进一步开展,不但有助于对共轭聚合物中的微观物理世界的认识,更具有重要的应用价值,能够推动新型有机光电器件的研究和开发。
本论文在紧束缚的(SSH)模型基础上,对哈密顿进行了修正,利用非绝热的分子动力学方法,模拟了有序的耦合聚合物链系统中链间扩展的极化子的形成及其在外场中的输运动力学,并与一维链内定域极化子的运动作比较。同时我们还研究了温度对极化子稳定性的影响。最后讨论了强电场下聚合物中的齐纳隧穿现象。本论文具体的研究内容和主要结果如下:
1.链间扩展的极化子动力学
共轭聚合物通过电荷注入、掺杂或者光激发可以形成载流子——极化子。对于聚合物中极化子性质的研究,人们已经进行了相当多的研究,包括极化子的形成过程以及极化子在外电场下的运动。大都关注于单条聚合物链的情况,耦合链系统中极化子形成的过程尚不清楚。实验研究发现,通过自组织,有机分子由于链间耦合可形成有序的薄膜结构。光谱研究表明,在这种聚合物链有序排列的结构中极化子具有二维链间特征。因此,我们构造多条有序耦合的聚合物链来模拟自组织的薄膜,研究其中极化子形成过程。实验表明,将这种有序的有机薄膜应用于场效应管可极大地提高迁移率,因此普遍认为高的迁移率是与极化子的二维链间特征有关的。因此研究二维链间扩展的极化子的性质有重要意义。
首先,我们研究了耦合聚合物链系统中极化子的形成过程,发现在有序耦合的共轭聚合物链系统中,注入到其中的电子形成链内定域的极化子还是链间扩展的极化子取决于聚合物链间的耦合强度。不管是在较弱的链间耦合下还是在较强的耦合下,电子注入到系统中后,首先分布在多条链上,但是当链间耦合较弱时,这种分布持续一段时间后电荷就逐渐向一条链中转移,最终形成了定域在一条链内的极化子。在较强的耦合下,电荷不能完全克服链间的竞争,因此,最终形成了链间扩展的二维极化子。
计算二维链间扩展极化子的运动,包括极化子沿聚合物链的运动(外加电场平行于链的方向(x方向)施加)和极化子在聚合物链间的运动(外加电场垂直于链的方向(y方向)施加),与一维链内定域极化子的运动比较发现,在相同的电场强度下,二维的链间扩展极化子具有更大的运动速度。从这个意义上讲,以二维链间扩展极化子为载流子的有序的有机聚合物体系,其迁移率远大于以链内定域极化子为载流子的有机体系。这与实验结果是一致的。
2.温度对于极化子稳定性的影响
除了极化子在外电场下的输运,极化子的稳定性也是人们关心的一个重要问题,特别是对于有机发光二极管(OLED),它的工作原理是基于阴、阳极注入的负、正电极化子在聚合物层碰撞产生激子,因此极化子的稳定性直接关系到有机发光器件的工作效率。目前的研究发现,电场强度、电场施加模式、杂质、载流子之间的碰撞等都会对极化子的稳定性产生重要影响。另外,在一定温度下,有机聚合物材料中总是存在着晶格热涨落。而且,在某些一维材料中,当温度接近相变温度时,晶格热涨落可能比二聚化还要大几倍。因此,晶格涨落对一维材料的性质有重要影响。
本论文中,我们讨论了晶格热涨落对共轭聚合物中极化子稳定性的影响。热效应通过郎之万方程引入的热随机力来描述,温度区域选在150K-350K之间。研究发现,在这个温度区域内,极化子能级的电子占据数减少,极化子的定域性受到影响。极化子保持理想定域态的时间随温度的升高而变短。考虑到温度效应后,极化子的解离电场大大降低。
3.共轭聚合物中的齐纳隧穿现象
固体材料在强场下的电荷输运一直是备受关注的课题,20世纪30年代,Bloch和Zener对这方面的理论研究作出重要贡献。自从Zener用电子的带间隧穿(Zenertunneling)来解释固体的电击穿后,Zener隧穿现象引起了物理学家浓厚的研究兴趣。在很多无机系统中,如电流驱动的约瑟夫森结、加速光学晶格、场驱动的超晶格等,人们做了大量Zener隧穿及其相关应用的研究。但是,有机半导体中尚未有详细的相关报道。在本论文中,我们将通过研究高电场下有机半导体中电子态的演化来讨论可能的Zener效应。
以一条处于理想二聚化状态的电中性的聚合物链作为研究对象,对其施加平行于链方向的电场。发现,在足够强的电场作用下,原本无净电荷分布的链中出现了极化电荷。能带发生倾斜,满带中的电子能够穿过带隙进入导带,实现了有机半导体中的Zener隧穿。伴随着Zener隧穿的发生,带隙消失,有机半导体经历了从半导体到导体的转变(I-M相变)。同时,有机晶格原子也从二聚化的排列转变为等距离排列,原子构型的这种变化是与I-M相变相对应的。这是有机聚合物特有的软性质决定的。
The discovery of high conductivity of doped trans-polyacetylene (tPA) broke through the conventional opinion that all organic polymers were insulators, and opened a new area of conducting polymers. The p_z-orbital of neighboring carbons overlap to form conjugated n bond, so conducting polymers can be called conjugated polymers. Conjugated polymers span a wide range from insulator to superconductor. The special structures and excellent physical and chemical properties have made it a focus of material science. As a new kind of irreplaceable fundamental functional materials, polymers has promising potential application in optoelectric device, sensing probe, molecular device, electromagnetic shielding, and metal preservation. A lot of significant progress has been made for conjugated polymers in many aspects, such as molecule design, material synthesis, doping method, solubility and processability, and conducting mechanism. Now, some optoelectric devices based on conjugated polymers have been turned into practical technique from pure experimental interest, including organic light-emitting diode (OLED), field-effect transistor (FET), photovoltaic cell, etc.
Compared with the traditional organic semiconductor, organic material has its unique properties. First, most of the conjugated polymers have the quasi-one-dimensional structure due to the weak interaction between the organic molecules. Second, there are strong electron-lattice couplings in organic systems. Injected charges (electrons or holes) or photoexcitation in conjugated polymers will induce lattice deformation, and the lattice deformation in turn influences the energy band structure of the organic system. So, it is generally believed that these self-trapping excitations, including solitons (only in trans-polyacetylene), polarons and bipolarons, are charge carriers in conjugated polymers. These elementary excitations are of fundamental importance for transport and photoluminescence of conjugated polymer system. Therefore, the dynamical process of these excitations has always been the focus in both theoretical and experimental investigation, and extensive studies have been carried out on charge carrier dynamics and on the promising application of conjugated polymers in organic functional devices.
In 1970's, a tight binding model-SSH model, developed by Su, Schrieffer and Heeger, was adopted to study the electronic structure and optical properties of the simplest conjugated polymer PA and showed considerable success. Since then Bishop, Sun and Xie et al have extended the SSH Hamiltonian to research both the static and dynamic process of excitations. At present, there mainly are four groups performing researches on charge transport in conjugated polymers based on the SSH model. The further research in this field not only can broaden our understanding of the microcosmic physical world but also have a substantial impact on the applications of organic devices.
Within the tight-binding electron-lattice interacting model, by using a nonadiabatic dynamic method, we have simulated the dynamic formation process of a interchain delocalized polaron and its motion driven by the external field in a well-ordered system of coupled conjugated polymer chains. The dynamical behavior of the interchain delocalized polarons is compared with that of the intrachain localized ones. Moreover, we have also explored the effects of temperature on the polaron stability. At last, the behavior of conjugated polymer chain under high electric field is investigated. The detailed research and main results are given below:
1. Dynamics of interchain delocalized polarons in conjugated polymers
Polaron formation and motion in a single chain is now a relatively well studied subject, and a great deal of effort has been devoted to such studies. However, the dynamical behavior of polarons in a more complicated system of coupled conjugated polymer chains is not so clear. Several experiments have reported high mobilities in thin-film transistors separately, in which owing to self-assembly, organic molecules form a relatively well ordered film structure by interchain stacking. In Sirringhaus's experiment which aimed to probe the transport properties of the ordered microcrystalline domains, it revealed the 2D interchain character of the polaronic charge carriers (polarons), and suggested that radical cations on isolated 1D chains can not lead to high mobilities. Thus, it is of great interest to probe the polaron transport in well-ordered coupled conjugated polymers, such as self-assembly films.
We construct a system consisting of as many as ten well-ordered coupled conjugated polymer chains to provide enough space for the polaron to travel. First, the dynamical process of polaron formation in such a system is investigated. It is found that once the electron is added to the system, it is shared by the several chains and induces lattice distortion in the chains within a short time. With enhancing the interchain coupling, it takes longer time for the electron to overcome the interchain coupling to localize in a single chain. Beyond a certain value of the interchain coupling, the electron will evolve into a 2D interchain delocalized polaron state extending over several chains.
By applying an external electric field to the interchain delocalized polaron, we study the 2D motion of the polaron, including the motion along the polymer chains (in which case the field is parallel to the chain) and the motion perpendicular to the polymer chain (in which case the field is perpendicular to the chain). Compared with the motion of the intrachain localized one, the interchain delocalized polarons drift much faster than localized polarons under the same electric field, implying that high mobilities can be achieved by 2D charge transport. This is in agreement with earlier experiments.
2. Effect of temperature on the stability of polarons
Polaron stability is another issue to which much attention has been paid. Up to now, the external electric field was demonstrated to have significant influence on the polaron stability. Moreover, it was found that, besides the electric field strength, the application mode of the field also plays an important role in polaron stability. Other effects, such as impurities, defects, and collision between charge carriers, have also been thoroughly studied. However, all these works were based on the lattice dimerization, i.e., lattice fluctuations were not taken into account.
Dimerization has been well understood in the static limit and within the mean-field theory. As a matter of fact, however, lattice fluctuations always exist. Even in the limit of zero-temperature, lattice fluctuations are of the same order of magnitude of the lattice distortion. Furthermore, at temperature comparable to the transition temperature the thermal lattice motion can be several times larger than distortion. The lattice fluctuations should have an important effect on the one-dimensional materials. In this paper, we investigate the stability of the localized polaron in conjugated polymers in the presence of thermal lattice fluctuations.
The temperature effect is simulated by introducing random forces to the equation of the lattice motion. It is found that the localized polaron state becomes delocalized even at low temperatures. The time of polaron keeping localized depends on the magnitude of temperatures. By taking into account the thermal effect, we find that the dissociation field is weaker compared with earlier works.
3. Zener tunneling in conjugated polymers
The issue of high-field transport in solids is of great concern and has been investigated for many decades due to basic interest in the physical phenomena and many important applications. Early theoretical contributions were made by Bloch and Zener. Since Zener explained the electrical breakdown of solid dielectric in terms of interband tunneling, the phenomenon of Zener tunneling has attracted considerable interest, particularly in modern nanoscale devices. To the best of our knowledge, however, Zener tunneling in conjugated polymers (belong to the family of organic semiconductors) is rarely referred to.
In chapter IV, we consider the dynamical evolution of electronic states in a conjugated polymer chain under a high electric field, and try to explore the possible Zener tunneling effect in organic semiconductors. It is shown that under a sufficiently high field, electrons can transit from the valence band to the conduction band, which is Zener tunneling in organic semiconductors. The result also indicates a field-induced insulator-metal (I-M) transition accompanied by the vanishing of the energy gap, that is, the Peierls phase is destroyed. Meanwhile, the lattice configuration undergoes significant change from dimerization to equidistant arrangement when I-M transition occurs.
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