Organic semiconductors (OSCs) have incredible prospects for next-generation, flexible electronic devices including bioelectronics, opto-electronics, energy harvesting and storage. They are flexible, biocompatible, printable, low cost, efficient and sustainable semiconductor materials with hybrid electrical and ionic conductivities, which can expand the functionality and accessibility of electronics beyond silicon.
Yet many fundamental challenges still exist in understanding the morphology, microstructure and charge transport processes of OSCs, with intimate connection to processing techniques and control over doping mechanisms. Firstly, solution processing prohibits definitive control over microstructure, which is fundamental for controlling electrical and ionic transport properties necessary for the development and advancement of new technologies. Second, OSCs generally suffer from poor electrical conductivities due to a combination of low carriers and low mobility, which limits overall efficiencies. Hence understanding different doping mechanisms and their influence on microstructure and charge transport properties is important for increased efficiencies of organic devices. To compete with their inorganic counterparts, characterization of the structural, electronic, and charge transport attributes of these organic semiconductor materials with respect to processing parameters and doping mechanism plays an important role for device engineering.
This effort considers the two questions of interest: (i) How does the microstructure change with chemical and electrochemical doping mechanisms and (ii) how do these changes in microstructure impact the electronic and charge transport properties. Addressing these questions will also allow us to connect the electronic and structural properties with fabrication processes and doping, to control the structural properties, and link their electronic properties with emphasis on charge transport.
Synchrotron based high energy X-ray scattering techniques, specifically grazing incidence wide angle X-ray scattering (GIWAXS) and small angle X-ray scattering (SAXS) are ideal techniques to measure intra- and inter-molecular spacings and phase segregation effects. In this work, GIWAXS was used to study the microstructural features ranging from sub angstrom to nanometer length scales, a key regime for transport in organic materials. GISAXS was used to study the structures greater than the nanometer length scale to understand phase segregations due to doping mechanisms. Electronic properties are measured using spectroscopy and correlated with electrochemical density of states. Transport properties are analyzed using cyclic voltammetry, mobility and conductivity measurements. Collectively, these measurements allow us to design organic semiconductor systems with controllable behaviors for next-generation technologies.