有机分子器件动力学输运性质研究

The development of scanning probe techniques and the advent of self-assembly monolayers and methodologies made molecular electronics into a reality. New type molecular electronic devices, such as molecular conductance wires, conductance switching, rectifier, and field-effect transistor, have been developed successfully. In comparison with the rapid development of experimental studies, the key issue in theoretical research is to understand the fundamental mechanisms underlying the experimental phenomena, especially the electronic transport characteristics induced by the intrinsic properties of the organic materials. In this project, we will use the hierarchical equations of motion(HEOM) of the reduced-density-matrix formalism combined with the Su-Schrieffer-Heeger（SSH）model to solve the nonadiabatic dynamics of the charge and the lattice in quantum open systems self-consistently. Taking the advantage of HEOM formalism, the evolutions of the electronic injection and transport in real time domain can be obtained, then we can study the response of the system to the exciting fields and the time in which the system achieves to the steady-state. The effects of the electron-phonon coupling and the creation/annihilation of the excited states on the dynamics are focused on. Through these studies, we want to establish the quantum theory of transport in organic nanostructures and explore the new physical phenomenon and effects in organic electronic devices.

随着扫描隧道显微镜、自组装膜以及分子单层等实验技术的发展，以有机分子为功能单元的分子电子学发展迅速，分子导线、分子开关、分子整流器、分子场效应管等新型电子学器件已研制成功。实验的突破要求理论上深入理解器件中电荷的输运性质及微观机制，特别是由于有机体系独特的内禀性质导致的输运特性。本项目立足于有机材料柔性晶格和强电-声耦合特点，基于分层递阶运动方程的量子输运方法，结合Su-Schrieffer-Heeger模型，自洽求解电荷与晶格的含时演化方程，研究有机分子器件中电荷注入、输运的动力学过程，探讨系统建立稳定状态的时间尺度以及对外加激励的响应，重点关注电-声耦合作用及激发态的产生、湮灭对动力学过程的影响。一方面从理论上探索电-声耦合对输运性质的影响规律，建立有机电子学器件中的动力学输运理论；另一方面探讨有机分子器件不同于传统无机电子学器件的新效应。

有机分子器件以有机小分子或聚合物作为电荷传输层，具有许多不同于传统无机半导体器件的特有性质。实验的不断成功引发了人们对有机分子器件内部微观机制的研究热潮，特别是由于有机体系独特的内禀性质导致的输运特性。我们立足于有机材料柔性晶格和强电-声耦合特点，结合Su-Schrieffer-Heeger模型，建立并完善了基于分层递阶运动方程的量子输运方程组，研究了有机分子器件中电荷注入、输运的动力学过程，其中主要包括：（1）通过对聚乙炔分子链瞬态输运性质的研究发现，低偏压区域，电-声相互作用对电荷输运起到抑制作用，在高偏压区域，电-声相互作用对电荷输运起到促进作用。高偏压下强的电-声相互作用导致系统产生了激子态，激子态辅助输运引起了电流随电-声相互作用增强而增大的现象，体现了有机系统与传统无机半导体系统的重要区别；（2）计算了低聚噻吩体系中稳态输运性质，分析发现一定偏压附近，系统将出现负微分电阻效应，异质硫原子之间的跃迁是导致负微分电阻效应的关键。外加含时激励模式下系统伏安特性曲线呈现出回滞曲线，随电极与分子耦合增强，回滞曲线面积减小；（3）另外我们还研究了电子关联以及温度效应对动力学性质的影响，长程电子相互作用有利于链内单态激子的产生，能提高器件的发光效率。随着温度升高，极化子激子态的稳定性逐渐减弱，产率减小，光激发谱呈现蓝移现象。本项目的完成建立了瞬态量子输运理论，从量子力学角度解释了电荷输运的动力学特性以及微观机制，为探讨研制新型有机电子学器件提供了一定理论依据。