电—声相互作用对聚合物链中载流子瞬态输运性质的影响

发布时间：2018-11-11 19:31

【摘要】：与量子耗散密切相关的开放量子系统中的电输运问题一直是科研人员关注的焦点。在开放量子系统中,若中间传输层采用有机材料,电-声相互作用对于体系中载流子输运性质有着重要的影响。为了得到有机聚合物器件的实时输运性质,并且充分考虑有机材料中强的电-声耦合,本文采用基于格林函数的hierarchical equations of motion(HEOM)与非绝热分子动力学相结合的方法,详细讨论了共轭聚合物系统中元激发动力学以及载流子瞬态输运过程。对于中间体系电子部分哈密顿我们采用紧束缚的Su-Schrieffer-Heeger(SSH)模型来描述,源电极和漏电极则采用无相互作用的费米子库来描述。通过基于格林函数的HEOM方法可以得到描述体系电子部分的密度矩阵和一阶伴随密度矩阵的微分方程组;对晶格部分采用的是经典处理,其满足牛顿运动方程。利用Runge-Kutta方法对整个系统演化进行数值模拟。我们首先运用HEOM方法研究了开放量子系统中不同强度电-声耦合下体系载流子的输运性质。计算结果表明:偏压较小时,电-声相互作用阻碍体系中载流子输运,但随着偏压的增加,电-声耦合反而促进载流子输运。这是由于偏压的增加使得电极中的载流子能够注入到体系中,电-声耦合作用下,诱导晶格发生畸变,导致激子态的形成,激子态的出现促进了载流子输运。并且电-声相互作用越大,晶格弛豫能力越强,能级越向带隙中心偏移,使得偏压窗内部的态越多,电流越大。另外我们还研究了中间体系尺寸效应的影响,发现,随着中间体系长度的增加,能级分布变密,在相同的偏压下,相对于小尺寸体系更容易形成新的激子态,新激子态的出现会促进电流的再次增加。另外,当体系与电极的耦合减弱时,电-声相互作用越强,VI-曲线中出现的电流平台越明显。这是因为体系与电极耦合减小时,会使得体系中能级展宽越小,并且电-声耦合越大,晶格弛豫能力越强,所以出现的阶梯状平台越明显。我们还模拟了Sweep偏压下,系统的动力学输运行为。计算发现:当偏压以递增的形式加入,增加到最大值后再递减,体系伏-安特性曲线中,电流会出现回滞现象。这是由于有机聚合物中存在较强的电-声相互作用,运动的电子和空穴会诱导晶格发生畸变,畸变的晶格会产生一个局域的势场,电子和空穴会束缚到这个势场中形成激子态。由于这种“自陷”效应的存在,当偏压逐渐递增时,体系会形成激子态;当偏压增加到最大值再逐渐递减时,体系的激子态会湮灭。并且激子态形成的偏压值和激子态湮灭的偏压值不同,进而导致电流会出现回滞效应。我们进一步研究发现:当中间体系尺寸增加时,在一定的偏压范围内,电流的回滞效应会多次出现。增强电-声耦合强度,使得体系带隙变大,会导致电流回滞效应越明显;增强体系与电极的耦合,使得体系能级展宽变大,电流回滞效应越微弱。
[Abstract]:The problem of electrical transport in open quantum systems, which is closely related to quantum dissipation, has always been the focus of attention of researchers. In an open quantum system, if organic materials are used in the intermediate transport layer, the electro-acoustic interaction plays an important role in the carrier transport properties in the system. In order to obtain the real time transport properties of organic polymer devices and to fully consider the strong electro acoustic coupling in organic materials, the method of combining hierarchical equations of motion (HEOM) based on Green's function with non adiabatic molecular dynamics is used in this paper. The excitation kinetics and carrier transient transport in conjugated polymer systems are discussed in detail. For the electronic part of the intermediate system Hamiltonian is described by a tight-binding Su-Schrieffer-Heeger (SSH) model and the source electrode and leakage electrode are described by a fermionic library without interaction. By using the HEOM method based on Green's function, the differential equations describing the electronic part and the first order adjoint density matrix of the system can be obtained, and the lattice part is treated by classical method, which satisfies the Newtonian equation of motion. The Runge-Kutta method is used to simulate the whole system evolution. We first use HEOM method to study the transport properties of carriers in open quantum systems with different intensities of electro-acoustic coupling. The results show that the electro acoustic interaction hinders the carrier transport when the bias voltage is small, but with the increase of the bias voltage, the electro acoustic coupling accelerates the carrier transport. This is due to the increase of the bias voltage, the carriers in the electrode can be injected into the system, and the lattice distortion is induced by the electro-acoustic coupling, which leads to the formation of the exciton state, and the emergence of the exciton state promotes the carrier transport. The larger the electric-acoustic interaction is, the stronger the lattice relaxation ability is, and the more the energy level shifts to the center of the band gap, the more the states in the bias window are and the larger the current is. In addition, we also study the effect of the size effect of the intermediate system. It is found that with the increase of the length of the intermediate system, the energy level distribution becomes denser, and at the same bias voltage, it is easier to form a new exciton state than the small size system. The appearance of the new exciton state will promote the increase of the current again. In addition, when the coupling between the system and the electrode is weakened, the stronger the electro-acoustic interaction is, the more obvious the current platform appears in the VI- curve. This is because when the coupling between the system and the electrode decreases, the energy level in the system becomes smaller, and the larger the electro-acoustic coupling is, the stronger the lattice relaxation ability is, and the more obvious the ladder platform is. We also simulate the dynamic transport behavior of the system under Sweep bias. It is found that when the bias voltage is added in the form of increasing voltage and then decreasing when the bias voltage is increased to the maximum value, the hysteresis of the current will occur in the volt-ampere characteristic curve of the system. This is due to the existence of strong electro-acoustic interaction in organic polymers, in which moving electrons and holes will induce lattice distortion, and distorted lattices will produce a localized potential field in which electrons and holes will be bound to form exciton states. Due to the existence of this "self-trapping" effect, the exciton state of the system will be formed when the bias voltage increases gradually, and the exciton state of the system will annihilate when the bias voltage increases to the maximum value and then decreases gradually. Moreover, the polarization voltage of exciton state is different from that of exciton state annihilation, which leads to hysteresis effect of current. It is found that the hysteresis effect of the current will occur many times in a certain range of bias voltage when the size of the intermediate system increases. The stronger the electro-acoustic coupling intensity, the larger the band gap, the more obvious the hysteresis effect is, and the stronger the coupling between the system and the electrode is, the wider the energy level is, and the weaker the current hysteresis effect is.
【学位授予单位】：河北师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O469

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