Correlating Structural and Electronic Disorder in Organic Semiconductor Single Crystals

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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.