Influence of Structural Disorder on Charge Transport and Stability in Organic Field-Effect Transistors with Molecular Semiconductors

Abstract

Organic semiconductors (OSCs) are distinguished by the soft van der Waals interactions among π-conjugated moieties, garnering substantial interest for potential use in large-area and flexible electronics due to their inherent lightweight, versatile processing capabilities, and compatibility with stretchable substrates. In the realm of organic field-effect transistors (OFETs) using molecular semiconductors (MSCs), remarkable advancements have been made in enhancing charge carrier mobility through the synthesis of novel semiconducting materials, a refined understanding of the impact of molecular structure and packing on charge transport properties, as well as the optimization of device components to eliminate extrinsic factors that limit transport. Despite these achievements, the full potential of OSCs is often constrained by an incomplete comprehension of structure-property relationships and challenges posed by device traps and stability issues. The soft intermolecular interactions and typically large molecular units of molecular semiconductors create a unique charge transport regime where the prevalent static disorder and complex structural dynamics couple with charge carrier motion, fundamentally affecting the delocalization of electronic wavefunctions and the (opto-)electronic properties of devices on a macroscopic scale. Our systematic study, underpinned by experimental techniques and theoretical frameworks, delves into the charge transport mechanisms and device stability of organic thin-film transistors (OTFTs) fabricated from a p-type, asymmetric molecular solid, 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-C10). The research findings indicate that the inhomogeneity of MSC thin films profoundly influences the temperature-dependent carrier mobility, transitioning from band-like transport signatures to scenarios that involve both delocalized and localized charge carriers as temperatures vary. These shifts across different transport regimes are further inferred by spectral density analyses of charge trap states near the band edge, derived from electrical measurements of OTFTs. Furthermore, the thesis thoroughly addresses the problems of charge carrier trapping and the operational stability of thin-film transistors through detailed device characterization and microscopic investigations. We pinpoint intrinsic origins of electronic trap states associated with net molecular dipoles and propose sophisticated device engineering strategies to mitigate the stress-induced degradation, thereby enhancing the performance and reliability of OFETs. Additionally, our research underscores the importance of morphology control and process optimization in overcoming the challenges related to the solution-based self-assembly of MSC molecules. The systematic exploration of thermal effects on Ph-BTBT-C10 thin films and transistors provides critical insights into structural transformations at elevated temperatures approaching the phase transition, which could be essential for developing thermally durable electronics suitable for applications in space exploration, smart textiles, automotive technologies, and other fields. Ultimately, this research aims to enhance our understanding of charge transport mechanisms and device degradation in organic thin-film transistors. Additionally, it seeks to pave the way for innovative solutions in developing next-generation organic electronics, which are essential for sustainable technological advancements across diverse sectors.