Structuring Semi-Conducting (Ultra)thin Films

• Applicants: Dr. Ahmed Mourran; Professor Dr. Ulrich Ziener
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2008 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 69512070

We seek to establish the fundamental principles that determine the structure and function of interfaces in ultrathin films of oligothienoacene-based materials. A central goal of our work is to understand how molecular incommensurability and discontinuities that arise at an interface, whether these are structural, compositional, or electronic, are accommodated and what are the implications of these accommodations (relaxations) on the behavior and properties of the interfacial layer. By exploring the new molecular design and the interplay of interfacial structure, composition, and transport properties, we seek to develop fundamental insights that will enable new materials with higher functionality for organic electronics demonstrated exemplary for sensing with anticipated ultimate sensitivity. Our approach closely integrates the following points. 1. Synthesis of multifunctional long beta-unsubstituted oligothienoacenes with coplanar branched endgroups partially equipped with additional functional sensing groups (see below). The fine-tuning of the endgroups ensures sufficient solubility but should improve order due to coplanarity designed to relieve steric constraints and to strengthen pi-pi coupling. The high solubility of intermediates in the course of the synthetic approach guarantees high-purity compounds in sufficient quantity in the 100 mg scale.2. To devise methods to control nucleation and growth of physisorbed monolayers as the solution undergoes a phase transformation. Issues of structure, phase transitions, and growth of monolayers require monitoring the drying process with sufficient temporal and spatial resolution. Total internal reflection fluorescence microscopy (TIRF) will be elaborated to simultaneously track the substrate/solvent interface and the three-phase contact line during the drying process. Detailed ex-situ observations using scanning force and electron microscopy on carefully controlled sub-monolayers and multi-layers samples will be combined with x-ray scattering techniques encompassing WAXS, GIWAXS, and X-ray reflectivity. We will pay particular attention to creation or annihilation of interfacial defects such as grain boundaries and terraces since this will provide a route to extend our understanding of surface diffusion and permeation in a layered crystal. While in the past we have worked primarily on oligothiophenes, we will increasingly focus on the new materials, thienoacenes, with high bulk charge carrier mobilities. In particular, we will concentrate on monolayers because of the important, yet poorly understood, influence of interfaces on the electronic transport processes that underpin the sensing principle with an organic field effect transistor.

• DFG Programme: Research Grants