Inorganic-organic interfaces present a versatile class of systems, providing the opportunity to achieve intriguing functionalities, e.g. as molecular switches, thermoelectrics, cargo-lifters, memories, or transistors, amongst others. The key to optimizing their functionalities lies in a systematic, fundamental understanding of the geometric and electronic structure of the interface. In many experimental setups or technologically relevant devices, organic materials are deposited as disordered or amorphous material.
Most phenomena occurring at the interface between inorganic electrodes and well-ordered organic materials are reasonably well understood. At the same time, the crucial impact of defects and disorder for e.g., the conduction in organic bulk materials has been recognized. Yet a systematic, assessment from first-principles of the impact of defects for transport properties at the interface is has not yet gained appropriate attention. The reason for this can be, in part, traced back to the high computational effort of jointly describing the defect and the surrounding crystalline layer, as well as to the vast configurational space that gives rise to a potential energy surface (PES) with a huge number of local minima. In the present project, we intent to close this gap and obtain an in-depth understanding of the role of defects and disorder in organic monolayers on various observables, including interface dipole, density of states, and interfacial level alignment.
In close collaboration with experimental partners, we will develop an efficient strategy based on Basin Hopping methods to sample the PES and obtain a set of low-energy structures that are occupied at finite temperature. Coarse-graining the PES by describing the layer as a combination of adsorption structures for isolated molecules that are arranged on a regular meshed imposed on the inorganic substrate allows to track the number of minima and keeps exploring the configurational space tractable. The Basin Hopping will be coupled with an atomistic description via density functional theory to predict the electronic structure at defect-containing interfaces and compare it to ideal, well-ordered structures. For selected model systems, such as the adsorption of the small organic electron acceptor Tetracyanoethene on Cu and ZnO substrates, this will allow answering important question such as: “What is the equilibrium concentration of defect at various temperatures?”, “What are common defect motifs?”, or “How do defects affect the energy-levels of the surrounding material?”.