Relating Structure and Electrostatic Potentials in Organic Semiconductor Thin Films

技术摘要：在NSF材料研究部固态和材料化学计划的更新提案中，首席研究员（pi）将继续他们的实验努力，以揭示有机半导体中基本的微观结构-性能关系。特别是，目的是探索薄膜结构与表面静电势之间的联系。表面电位影响界面处的电荷输运，因此与有机电子器件的性能直接相关。例如，在薄膜晶体管（TFT）中，有机半导体/绝缘体界面处的表面电位决定了电荷浓度，表面电位的梯度决定了载流子流动的方向。表面电位反映了许多与表面（或界面）相关的因素，包括晶体结构、电子能级、缺陷、偶极子、固定电荷、污染物和照明条件。pi将使用高分辨率电磁力显微镜（EFM）和开尔文探针力显微镜（KFM）来测量和绘制有机半导体薄膜中的表面电位，并将表面电位域和梯度与结构特征相关联。这项工作旨在解决一些问题，包括：有机半导体薄膜中的外延生长模式和晶粒形态如何影响表面电位？在有机半导体中，被捕获的电荷在哪里？这些被捕获的电荷区是否与表面电位峰或谷相关？这项工作的一个主要成果将显著提高对有机半导体薄膜和界面中的静电复杂性以及这种复杂性与微观结构的相关性的理解。薄膜和界面是OTFTs和有机太阳能电池操作的核心，将被调查，因此结果的相关性将立即对有机电子研究界显而易见。非技术概要：有机半导体是一类重要的薄膜电子材料，具有许多吸引人的特性，包括高效发光，液相可加工性和与塑料衬底的兼容性；这些优势正在推动从灵活、坚固的电子阅读器和智能卡到低成本太阳能电池的新应用。特别是有机发光二极管（oled），已经达到了适合显示技术的性能，并正在进行商业化，有机半导体在生物传感器和印刷电子领域也有令人兴奋的前景。重要的是，成熟的有机半导体技术的发展取决于对结构-加工-性能关系的透彻理解。本提案的总体目标是通过揭示模型有机半导体系统中基本的微结构-性能相关性来推进有机半导体的材料科学。这项工作将由明尼苏达大学的两名教师研究人员与博士生合作进行。因此，一个主要的更广泛的影响将是研究生水平的培训学生在材料科学和工程。此外，每年夏天，奖学金获得者将为一名明尼阿波利斯地区的高中生和一名少数族裔本科生提供暑期研究经验。少数族裔本科生将从德克萨斯大学泛美分校（UTPA）的理工科二年级和三年级学生中挑选，这是一所以拉美裔为主的服务机构。对于学生的选择，pi将由一名曾在明尼苏达大学进行过夏季研究的UTPA教员协助。我们的目标是激发这些年轻的潜在科学家对材料研究的机会，同时为他们提供真正的动手技术培训。pi还将继续为K-12学生提供扫描探针技术的演示，就像他们在之前的合同中所做的那样。

TECHNICAL SUMMARY: In this renewal proposal to the Solid State and Materials Chemistry Program, in the NSF Division of Materials Research, the Principal Investigators (PIs) will continue their experimental efforts to uncover fundamental microstructure-property relationships in organic semiconductors. In particular, the goal is to probe the connection between film structure and surface electrostatic potential. Surface potentials impact charge transport at interfaces and thus are directly relevant to the performance of organic electronic devices. For example, in a thin film transistor (TFT) the surface potential at the organic semiconductor/insulator interface determines the charge concentration, and the gradient in the surface potential determines the direction of carrier flow. Surface potentials reflect many factors associated with surfaces (or interfaces) including crystal structures, electronic energy levels, defects, dipoles, fixed charges, contaminants, and illumination conditions. The PIs will use high resolution Electric Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KFM) to measure and map surface potentials in organic semiconductor films, and to correlate surface potential domains and gradients with structural features. The work is designed to address a number of questions including: How do epitaxial growth modes and grain morphologies in organic semiconductor films impact surface potentials? Where do trapped charges reside in organic semiconductors and can the trapping zones be correlated with surface potential peaks or valleys? A principal outcome of this work will be significantly improved understanding of electrostatic complexity in organic semiconductor films and interfaces and correlation of this complexity with microstructure. Films and interfaces that are central to the operation of OTFTs and organic solar cells will be investigated, so that the relevance of the results will be immediately apparent to the organic electronics research community. NON-TECHNICAL SUMMARY: Organic semiconductors are an important class of thin film electronic materials that have many attractive properties including efficient luminescence, liquid phase processability, and compatibility with plastic substrates; these advantages are driving new applications ranging from flexible, rugged e-readers and smart cards to low cost solar cells. Organic light emitting diodes (OLEDs), in particular, have attained performance suitable for display technologies and are undergoing commercialization, and there are also exciting prospects for organic semiconductors in biosensors and printed electronics. Importantly, the development of a mature organic semiconductor technology hinges on thorough understanding of structure-processing-performance relationships. The overarching goal of this proposal is to advance the materials science of organic semiconductors by uncovering fundamental microstructure-property correlations in model organic semiconductor systems. The work will be carried out by two University of Minnesota faculty researchers in collaboration with PhD students. Thus, a principal broader impact will be graduate level training of students in materials science and engineering. In addition, the PIs will provide summer research experiences for one Minneapolis area high school student and one minority undergraduate each summer over the course of the award. The minority undergraduate will be selected from a pool of science and engineering sophomores and juniors at the University of Texas Pan American (UTPA), a largely Hispanic serving institution. For student selection, the PIs will be assisted by a UTPA faculty member who has performed summer research previously at Minnesota. The goal will be to excite these young potential scientists about the opportunities in materials research, while providing them real hands-on technical training. The PIs will also continue to provide demonstrations of scanning probe techniques to K-12 students, as they have done under their previous award.