Correlating Structural and Electronic Disorder in Organic Semiconductor Single Crystals

非技术摘要：有机半导体是一种先进的材料，能够实现智能手机和电视中常见的明亮显示器。OLED(有机发光二极管)使用有机半导体薄膜将电能转换为显示器所需的红、绿、蓝光。在这个项目中，首席研究员Daniel Frisbie试图了解有机半导体单晶的性质，这些单晶可以应用于其他类型的设备，如有机场效应晶体管(OFET)。对于OFET来说，充电速度是至关重要的，但晶体中的结构缺陷会捕获电荷，降低它们的平均速度。Frisbie教授的目标是使用一种名为扫描开尔文探针显微镜(SKPM)的强大高分辨率显微镜技术来确定有机半导体单晶中存在哪些类型的缺陷。SKPM图像中的电势和晶体中的许多类型的缺陷都有一个可以检测到的“电压信号”。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，并表征其电气性能。这项工作应该有助于更好地理解有机半导体中的结构-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的研究生。技术摘要：该项目的目标是通过阐明pi-共轭分子单晶中机械应变、缺陷、表面电位和电子传输之间的关系来促进有机半导体材料科学的发展。该实验计划建立在首席调查者(PI)实验室先前工作的基础上，该实验室证明了扫描开尔文探针显微镜(SKPM)对晶体有机半导体中的缺陷和不均匀应变出人意料地敏感。PI寻求(1)了解应变/缺陷-表面电势关系的原因，(2)证明其与广泛类别的结晶有机半导体的相关性，以及(3)建立表面电势变化与电输运之间的关系。这项研究主要集中在有机单晶上，因为它们的无序程度较低，有助于识别和分析精确的结构-性质关系。用气相传输法生长了基准有机半导体单晶，并用SKPM、UPS、X射线衍射等方法对其进行了表征。PI采用了一个坚固的平台，在运行SKPM、UPS和X射线衍射仪的内部对晶体施加应变。以这种方式施加从0.01-0.5%的应变，这允许精确地量化机械应变-表面势关系。在第二个方向，PI揭示了到目前为止在少数有机半导体单晶中观察到的台阶边缘势的原因，并将这些势与场效应(FET)输运联系起来。在第三条脉络中，他研究了固-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FET性能相关联。这项工作正在加深对有机半导体中结构-性质关系的理解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.