Gatsios Christos, Dreher Maximilian, Amsalem Patrick, Opitz Andreas, Jouclas Remy, Geerts Yves, Witte Gregor, Koch Norbert
Institut für Physik & Center for the Science of Materials Berlin (CSMB), Humboldt-Universität zu Berlin, Berlin 12489, Germany.
Department of Physics, Philipps-Universität Marburg, Marburg 35037, Germany.
J Phys Chem C Nanomater Interfaces. 2024 Dec 2;128(49):21228-21236. doi: 10.1021/acs.jpcc.4c06741. eCollection 2024 Dec 12.
Isomerism of molecular structures is often encountered in the field of organic semiconductors, but little is known about how it can impact electronic and charge transport properties in thin films. This study reveals the molecular orientation, electronic structure, and intermolecular interactions of two isomeric thienoacenes (DN4T and isoDN4T) in thin films, in relation to their charge transport properties. Utilizing scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARUPS), and near-edge X-ray absorption fine structure measurements (NEXAFS), we systematically analyze the behavior of these isomers from submonolayer to multilayer coverage on highly ordered pyrolytic graphite (HOPG) as substrates. We find that at submonolayer coverage both DN4T and isoDN4T molecules predominantly adopt a nearly flat-lying orientation on the surface, minimizing intermolecular interactions. The distinct emission features of the highest occupied molecular orbital (HOMO) level in ARUPS enables the determination of molecular reorganization energies. These are found to be in good agreement with theoretical predictions, suggesting superior charge transport in DN4T compared to isoDN4T. Notably, thickness-dependent photoemission measurements reveal a significant splitting (approximately 450 meV) of the HOMO level of isoDN4T, attributed to polarization-induced effects rather than wave function overlap, indicating a nuanced interplay between molecular packing and electronic properties. Our results underscore the importance of molecular packing and substrate interactions in determining the electronic structure and transport properties of organic semiconductor thin films. Substrate-induced polymorphism and the crucial role of polarization-induced effects influencing charge transport are highlighted. These insights are pivotal for future engineering of molecular and thin film structures, aiming to enhance the performance of organic semiconductor-based devices.
分子结构的异构现象在有机半导体领域经常遇到，但对于它如何影响薄膜中的电子和电荷传输特性却知之甚少。本研究揭示了两种异构噻吩并苯（DN4T和isoDN4T）在薄膜中的分子取向、电子结构和分子间相互作用，以及它们与电荷传输特性的关系。利用扫描隧道显微镜（STM）、角分辨光电子能谱（ARUPS）和近边X射线吸收精细结构测量（NEXAFS），我们系统地分析了这些异构体在以高度有序热解石墨（HOPG）为基底的情况下，从亚单层到多层覆盖时的行为。我们发现，在亚单层覆盖时，DN4T和isoDN4T分子在表面主要采取近乎平躺的取向，以最小化分子间相互作用。ARUPS中最高占据分子轨道（HOMO）能级的独特发射特征使得能够确定分子重组能。发现这些重组能与理论预测高度吻合，表明与isoDN4T相比，DN4T具有更好的电荷传输性能。值得注意的是，厚度依赖的光电子能谱测量揭示了isoDN4T的HOMO能级有显著分裂（约450 meV），这归因于极化诱导效应而非波函数重叠，表明分子堆积和电子性质之间存在微妙的相互作用。我们的结果强调了分子堆积和基底相互作用在决定有机半导体薄膜的电子结构和传输特性方面的重要性。突出了基底诱导的多态性以及极化诱导效应在影响电荷传输方面的关键作用。这些见解对于未来分子和薄膜结构的工程设计至关重要，旨在提高基于有机半导体的器件的性能。
