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这份更新提案中，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结构-性质关系。特别是，我们的目标是探索薄膜结构和表面静电势之间的联系。表面电势影响界面上的电荷传输，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/绝缘体界面的表面电势决定电荷浓度，而表面电势的梯度决定载流子流动的方向。表面电势反映了许多与表面(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将使用高分辨率电场力显微镜(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系列问题，包括：有机半导体薄膜中的外延生长模式和颗粒形态如何影响表面电位？有机半导体中的陷阱电荷位于何处，陷阱带能否与表面电势峰或谷相关联？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静电复杂性的理解，以及这种复杂性与微结构的关联。将对OTFTs和有机太阳能电池运行的核心薄膜和界面进行调查，以便有机电子研究界立即清楚地看到结果的相关性。非技术综述：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、可液相加工以及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，并正在商业化，有机半导体在生物传感器和印刷电子设备中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。这项建议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-性质关联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职研究人员与博士生合作进行。因此，主要的更广泛的影响将是对材料科学和工程方面的学生进行研究生水平的培训。此外，PIS将在每年夏天的颁奖过程中为明尼阿波利斯地区的一名高中生和一名少数民族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大学(UTPA)的理工科二年级和三年级学生中挑选出来。UTPA是一所以西班牙裔为主的服务机构。对于学生的选择，PIS将由一名曾在明尼苏达州进行暑期研究的UTPA教员协助。目标将是让这些年轻的潜在科学家对材料研究的机会感到兴奋，同时为他们提供真正的动手技术培训。PIS还将继续向K-12学生提供扫描探头技术演示，就像他们在上一次获奖时所做的那样。

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.