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Origin of Meyer-Neldel-type compensation behavior in organic semiconductors at large carrier concentrations: Disorder vs. thermodynamic description

机译:大载流子浓度下有机半导体中Meyer-Neldel型补偿行为的起源:无序与热力学描述

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摘要

We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter, Singh, Sariciftci, and Bässler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors (OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We have suggested and realized a test experiment based on measurements of the compensation behavior for the temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models. We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS distribution and polaron binding energy.
机译:我们扩展了有效的介质近似理论[Fishchuk,Kadashchuk,Genoe,Ullah,Sitter,Singh,Sariciftci和Bässler,Phys。 Rev. B 81,045202(2010)]研究极化子的形成如何影响在大载流子浓度下无序有机半导体中依赖温度的电荷-载流子迁移过程中观察到的Meyer-Neldel(MN)补偿行为,如有机场效应所实现晶体管(OFET)。我们显示,只要极化子结合能比较小,可以分别使用Miller-Abrahams或Polaron Marcus速率在无序形式主义的框架中一致地描述非极化子和极化子跳跃传输中有机半导体薄膜的补偿行为。到系统中状态密度(DOS)分布的宽度。我们认为,基于热力学推理的替代模型,例如多激发熵(MEE)模型,该模型假定电荷传输以极化子和多声子过程为主,并且忽略了能量紊乱,因此固有地不适用于充分描述无序有机物中的电荷-载流子迁移半导体。我们已经建议并实现了一个基于对OFET器件中与温度相关的电导率和迁移率的补偿行为的测量结果的测试实验,以检查这些模型的适用性。我们指出,在薄膜OFET中观察到的MN行为与MEE方法所预测的真正MN规则无关,而是由于DOS部分填充的功能依赖性而产生的明显效果。在具有跳跃运输的无序系统中。实验结果充分支持了这一事实。发现表观MN能量还取决于DOS分布的形状和极化子结合能。

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