Correlating Structural and Electronic Disorder in Organic Semiconductor Single Crystals

摘要：有机半导体是一种先进的材料，可以实现智能手机和电视中常见的明亮显示。有机发光二极管（oled）利用有机半导体薄膜将电转换为显示所需的红、绿、蓝三色光。在这个项目中，首席研究员Daniel Frisbie正在试图了解有机半导体单晶的性质，这些单晶在其他类型的器件中有应用，比如有机场效应晶体管（ofet）。对于ofet来说，电荷速度是至关重要的，但是晶体中的结构缺陷会捕获电荷，降低它们的平均速度。弗里斯比教授的目标是利用一种强大的高分辨率显微镜技术，即扫描开尔文探针显微镜（SKPM），确定有机半导体单晶中存在哪些类型的缺陷。SKPM成像电势和晶体中许多类型的缺陷具有可以检测到的“电压特征”。Frisbie将使用SKPM和其他表征技术的组合，如x射线衍射和电子显微镜来定位缺陷，确定缺陷的结构性质并表征其电学特性。这项工作将有助于更好地理解有机半导体的结构-性质关系，从而扩大现实生活中的应用。一个重要的更广泛的影响将是培养有机半导体材料科学和器件物理方面的研究生。技术摘要：本项目旨在通过阐明π共轭分子单晶中机械应变、缺陷、表面电位和电输运之间的关系，推动有机半导体材料科学的发展。该实验计划建立在先前首席研究员（PI）实验室的工作基础上，该实验室证明了通过扫描开尔文探针显微镜（SKPM）进行的表面电位成像对晶体有机半导体中的缺陷和不均匀应变非常敏感。PI寻求(1)了解应变/缺陷-表面电位关系的原因，(2)证明其与广泛类别的晶体有机半导体的相关性，以及(3)建立表面电位变化与电传输之间的关系。研究的重点是有机单晶，因为它们的低无序水平有助于识别和分析精确的结构-性质关系。采用气相输运法生长有机半导体单晶，并用SKPM、紫外光电子能谱（UPS）和x射线衍射（XRD）等方法对单晶进行了表征。PI采用了一个强大的平台，可以对运行中的SKPM、UPS和XRD仪器中的晶体施加应变。这种方法适用于0.01-0.5%的应变范围，从而可以精确定量机械应变-表面电位关系。在第二个方向上，PI揭示了到目前为止在少数有机半导体单晶中观察到的阶跃边缘电位的原因，并将这些电位与场效应（FET）输运联系起来。在第三方面，他研究了由固-固相变引起的平面缺陷对有机晶体表面电位的影响，并将结果与场效应管性能联系起来。这项工作使人们对有机半导体的结构-性质关系有了更深入的了解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical Abstract:Organic semiconductors are the advanced materials enabling bright displays commonly found in smart phones and televisions. OLEDs(organic light-emitting diodes) employ thin films of organic semiconductors to convert electricity to red, green, and blue lights necessary for displays. In this project, principal investigator Daniel Frisbie is seeking to understand the properties of single crystals of organic semiconductors that have applications in other types of devices such as organic field effect transistors (OFETs). For OFETs, charge velocity is critically important, but structural defects in crystals can trap electrical charges, lowering their average speed. Professor Frisbie aims to determine what types of defects exist in organic semiconductor single crystals using a powerful high resolution microscopy technique called scanning Kelvin probe microscopy (SKPM). SKPM images electric potential and many types of defects in crystals have a "voltage signature" that can be detected. Frisbie will use SKPM and a combination of other characterization techniques such as X-ray diffraction and electron microscopy to locate defects, to determine the structural nature of defects and to characterize their electrical properties. The work should lead to a better understanding of structure-property relationships in organic semiconductors and thus enable expanded real-life applications. An important broader impact will be the training of graduate students in organic semiconductor materials science and device physics.Technical Abstract:The goal of this project is to advance the materials science of organic semiconductors by elucidating relationships between mechanical strains, defects, surface potential, and electrical transport in single crystals of pi-conjugated molecules. The experimental plan builds on prior work in the Principal Investigator's (PI's) laboratories that demonstrated surface potential imaging by scanning Kelvin probe microscopy (SKPM) is surprisingly sensitive to defects and inhomogeneous strains in crystalline organic semiconductors. The PI seeks (1) to understand the causes of the strain/defect-surface potential relationship, (2) to demonstrate its relevance to broad classes of crystalline organic semiconductors, and (3) to establish the relationship between surface potential variations and electrical transport. The research focuses on organic single crystals as their low levels of disorder facilitate identification and analysis of precise structure-property relationships. Single crystals of benchmark organic semiconductors are grown by vapor transport and characterized with a spectrum of methods including SKPM, ultraviolet photoelectron spectroscopy (UPS), and X-ray diffraction (XRD). The PI employs a robust platform to apply strains to crystals inside operating SKPM, UPS and XRD instrumentation. Strains ranging from 0.01-0.5% are applied this way, which allows for precise quantitation of the mechanical strain-surface potential relationship. In a second direction, the PI is uncovering the causes of step edge potentials so far observed in a handful of organic semiconductor single crystals and correlates the potentials with field effect (FET) transport. In a third vein, he examines the impact of planar defects induced by solid-solid phase transitions on the surface potential of organic crystals and correlates the results with FET performance. The work is leading to a deeper understanding of structure-property relationships in organic semiconductors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.