Ab Initio Simulations of Interfaces between SAM-Modified Gold Electrodes and n-Type or p-Type Organic Semiconductors Based on the Benzothieno-Benzothiophene (BTBT) Architecture

We have investigated the atomistic structure and the electronic properties of selected metal-organic interfaces, commonly found in organic electronic devices, using ab initio simulations based on density functional theory. Our study focuses, in particular, on the interaction of a widely used p-type organic semiconductor (OS) like the [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) polycyclic molecule with a gold electrode, bare or functionalized with a self-assembled monolayer (SAM) of aromatic thiol used as a surface modifier. A similar investigation involved an S,S'-tetraoxide derivative of BTBT indicated as BTBTOx(2), showing promising characteristics as an n-type semiconductor, whose original synthetic protocol is also discussed. First, our simulations, including the ab initio calculation of dispersion forces that permits an accurate description of noncovalent interactions, show that minimal changes in local interactions can induce a different arrangement of the same molecules in contact with metal electrodes. In particular, we report a stronger adhesion of BTBT and BTBTOx(2) on bare gold, with the molecules lying flat on the surface, and a stronger cohesion of the same organic adlayers on gold surfaces functionalized with an SAM, with the molecules that prefer to form stacks parallel to the surface, more similar to those observed in the corresponding molecular crystals. Moreover, the functionalization of the same gold surface with self-assembled molecular modifiers having different polarities induces changes in the electronic properties of the Au/BTBT and Au/BTBTOx(2) interfaces, transforming the Schottky junctions formed when the molecules are in direct contact with the bare gold surface in more favorable Au/SAM/OS junctions characterized by a significant reduction of the injection barrier or even a nearly Ohmic contact when the SAM is applied, in both the p-type and n-type investigated cases.