Visualizing Structure / Property Relationships in Organic Semiconductor Thin Films by Scanning Probe Microscopy.

摘要

Probing the electrostatic properties of organic thin films and their interfaces is critical to understanding charge transport behavior in organic semiconductor devices. By utilizing a suite of scanning probe microscopy techniques (SPM), or atomic force microscopy (AFM), related to topography, tribology, and electrochemical potentials, we can visualize important aspects of the semiconducting materials used in active thin film transistors and solar cells. Most notably scanning Kelvin Probe Force Microscopy (KFM) can record surface potential maps simultaneously with sample topography, which is ideal for probing structure-property relationships in layered systems at the nanometer scale. The surface potential is a key electrical state variable defined as the electrochemical potential energy of a fixed point charge confined to the sample surface relative to a reference state (e.g. vacuum level). The surface potential is comprised of an intrinsic material component (i.e., work function) and an extrinsic component influenced by many other externalities such as crystal structure, defects, dipoles, contaminants, doping densities, electric fields, fixed charges, and photo-activity. Surface potential maps in tandem with the microstructural landscape of an organic semiconductor thin film effectively yield an energetic map for charge carriers in these devices. The information gleaned from this technique is a powerful approach to visualizing the direct impact of microstructure on electrochemical potentials at a level of visual clarity and spatial resolution that largely remains unexploited. This thesis aims to employ this technique on a portfolio of device geometries and correlate the surface potential measurement to the fundamental physics of semiconductors and its impact on transport phenomena.