Surface Electrostatic Properties of Organic Semiconductors

Abstract

Understanding the surface/interface electrostatic properties of organic semiconductors has great implications for the fundamental transport properties of these materials and their performance in devices. Therefore, this thesis aims to correlate electrostatic properties with microstructure and mechanical strain in benchmark organic semiconductors. To this end, a number of scanning probe microscopy (SPM) techniques are employed to examine thin films and single crystals of prototypical organic semiconductors. In particular, strong variations of interfacial polarization at the organic/insulator interfaces are quantified by scanning Kelvin probe microscopy (SKPM). The roles of the dielectric type and deposition condition are identified. Moreover, striking lateral electrostatic heterogeneities are visualized in thermally deposited organic semiconductor bi-layers on various dielectrics, and are directly related to the complex microstructural motifs of the films. The mixed homoepitaxial growth modes, which give rise to the inhomogeneous microstructure, can be conveniently determined by combining two variants of lateral force microscopy (LFM), namely, friction force microscopy (FFM) and transverse shear force microscopy (TSM). Furthermore, a fundamental correlation is established between the surface electrostatic potential and mechanical strain. The effects of tensile and compressive strains in both elastic and plastic regimes are determined for the first time. Overall, organic semiconductors exhibit complex surface/interface electrostatic properties, which can be visualized by SPM and can be correlated with microstructure and mechanical properties.