Charge transport and spectroscopic analysis of high-mobility molecular semiconductors

Abstract

Small molecule organic semiconductors have been at the heart of charge transport studies in organic electronic materials over recent decades. Their ordered structures have played a pivotal role in the development of novel theories elucidating the charge transport phenomena and in the advancement of electronic devices with an ever-increasing efficiency. However, embracing new multi-disciplinary approaches holds the potential to uncover new material properties that have previously remained unnoticed. This thesis is organized into three experimental chapters, each covering distinct approaches to studying molecular semiconductors, and it is structured as follows. The initial chapter delves into the investigation of out-of-plane charge transport properties of the molecular semiconductor C8-DNTT-C8 using conducting atomic force microscopy (C-AFM). The chapter describes C-AFM measurements conducted on single crystal films of C8-DNTT-C8 characterized by a layered morphology and a 2D character. By studying local changes in electric current on a single molecule length scale, a novel vertical transmission line method (V-TLM) is introduced. This method enables an effective determination of the interlayer resistivity of molecular semiconductors, demonstrating a good agreement with conventional contact resistivity measurements. Herein, the described V-TLM method lays the groundwork for studying the out-of-plane conductivity as an inherent property of a material, opening pathways to address the contact resistance in organic electronic devices from a molecular engineering perspective. The second chapter of this thesis presents a study on the charge transport in the novel p-type molecular semiconductor OEG-BTBT in field-effect transistor devices. OEG-BTBT is a derivative of the high-mobility semiconductor BTBT, incorporating hydrophilic side chain substituents. The chapter outlines a meticulous optimisation of transistor device fabrication parameters and identifies the limiting factors affecting performance of this novel material. In the third chapter, spectroscopic studies of molecular semiconductor DNTT single crystals are explored. Here, for the first time, we demonstrate that under irradiation by a femtosecond laser pulse DNTT single crystals emit terahertz radiation, whose frequencies correspond to the intermolecular vibrational modes. We present a comprehensive and multi-faceted experimental study of this previously unobserved phenomenon and provide a possible explanation for the origin of terahertz generation. This work introduces novel tools and approaches for studying previously overlooked material properties of molecular semiconductors, which should contribute to a deeper understanding of these compounds and facilitate a further development of more efficient organic electronic devices.