Institute of Theoretical and Simulational Chemistry, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, 150080 Harbin, People's Republic of China.
Phys Chem Chem Phys. 2011 May 28;13(20):9736-46. doi: 10.1039/c1cp00001b. Epub 2011 Apr 18.
The electronic coupling between adjacent molecules is an important parameter for the charge transport properties of organic semiconductors. In a previous paper, a semiclassical generalized nonadiabatic transition state theory was used to investigate the nonperturbative effect of the electronic coupling on the charge transport properties, but it is not applicable at low temperatures due to the presence of high-frequency modes from the intramolecular conjugated carbon-carbon stretching vibrations [G. J. Nan et al., J. Chem. Phys., 2009, 130, 024704]. In the present paper, we apply a quantum charge transfer rate formula based on the imaginary-time flux-flux correlation function without the weak electronic coupling approximation. The imaginary-time flux-flux correlation function is then expressed in terms of the vibrational-mode path average and is evaluated by the path integral approach. All parameters are computed by quantum chemical approaches, and the mobility is obtained by kinetic Monte-Carlo simulation. We evaluate the intra-layer mobility of sexithiophene crystal structures in high- and low-temperature phases for a wide range of temperatures. In the case of strong coupling, the quantum charge transfer rates were found to be significantly smaller than those calculated using the weak electronic coupling approximation, which leads to reduced mobility especially at low temperatures. As a consequence, the mobility becomes less dependent on temperature when the molecular packing leads to strong electronic coupling in some charge transport directions. The temperature-independent charge mobility in organic thin-film transistors from experimental measurements may be explained from the present model with the grain boundaries considered. In addition, we point out that the widely used Marcus equation is invalid in calculating charge carrier transfer rates in sexithiophene crystals.
相邻分子之间的电子耦合是有机半导体电荷输运性质的一个重要参数。在之前的一篇论文中，使用半经典广义非绝热过渡态理论研究了电子耦合对电荷输运性质的非微扰效应，但由于分子内共轭碳-碳伸缩振动的高频模式的存在，该理论在低温下不适用[G. J. Nan 等人，J. Chem. Phys.，2009，130，024704]。在本文中，我们应用了一种基于虚时通量-通量相关函数的量子电荷转移速率公式，而无需弱电子耦合近似。然后，虚时通量-通量相关函数用振动模式路径平均值表示，并通过路径积分方法进行评估。所有参数均通过量子化学方法计算，迁移率通过动力学蒙特卡罗模拟获得。我们评估了 sexithiophene 晶体结构在高低温相中的层内迁移率，温度范围很广。在强耦合的情况下，发现量子电荷转移速率明显小于弱电子耦合近似计算的值，这导致迁移率特别是在低温下降低。因此，当分子堆积在某些电荷输运方向导致强电子耦合时，迁移率对温度的依赖性降低。从实验测量得到的有机薄膜晶体管的温度无关电荷迁移率可以用考虑了晶粒边界的本模型来解释。此外，我们指出，广泛使用的 Marcus 方程在计算 sexithiophene 晶体中的载流子转移速率时是无效的。
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