Electrostatic Design of Materials

The common strategies for designing new materials for organic and molecular electronics rely on well-established schemes, like changing the degree of -delocalization, attaching electron donating or withdrawing moieties or exploiting the properties of heterocyclic building-blocks. In this project we will explore the potential of a radically different approach: We will use collective electrostatic effects arising from the superposition of the fields generated by ordered 2D assemblies of polar building blocks to manipulate and control the physical properties of materials. For exploring the capacity of this strategy, we will apply an extensive pool of quantum-mechanical modelling techniques to a wide range of materials. By locally shifting electronic states through electrostatic effects and by, concomitantly, localizing orbitals in well-defined regions in space, we will devise strategies for the design of complex objects like monolayer quantum-cascades, type-2 monolayer quantum-wells, electrostatically designed 3D materials formed by energetically shifted -stacks, and quasi 1-D electron and hole wires induced in inorganic semiconductor layers through the assembly of polar organic adsorbates. In these structures, we expect to be able to control ballistic transport properties, the energetics of the encountered charge-transfer excitons and the confinement of excess charge carrier densities. The primary goal of this project is assessing and benchmarking the potential of the envisioned novel materials-design scheme. Consequently, we will apply it to a wide range of material-types within the broader topic of organic, molecular, and hybrid electronics. Additionally, we will study a wide variety of materials properties. These will range from the simulation of the electronic structure and orbital localization, via the modelling of doping and the simulation of ballistic transport to the description of excitation processes. Thus, this project will also provide methodological insight regarding the simulation of the (opto)electronic properties of complex organic and hybrid materials. A clear asset in this context is the broad expertise the PI has built over the past nearly two decades studying organic semiconductors by a wide variety of modelling approaches as well as by a number of experimental spectroscopic techniques complemented by the development of unconventional but efficient concepts for (opto)electronic devices. This experience will be complemented by the expertise of a number of international collaborators.

Isolated dipoles produce electric fields, which decay rather quickly with the distance from the dipoles. This changes fundamentally, when the dipoles are assembled into two-dimensional sheets. These sheets cause (infinitely) extended steps in the electrostatic energy, which shifts the electronic states of a material below the dipole sheets relative to those in a material above the sheet. This is referred to as a collective (or cooperative) electrostatic effect and is omnipresent at interfaces, for example between metallic electrodes and organic semiconductors. Such interfaces, for example, occur in the AMOLED displays of modern cellphones or in prototypes of organic solar cells. The goals of the finished project were to better understand these effects and, most importantly, to devise strategies for using collective electrostatic effects to design materials with unprecedented properties. To achieve that, we have performed highly complex computer simulations, considering the quantum mechanical nature of the considered materials and employing Austria's most powerful supercomputer (the Vienna Scientific Cluster). By means of these simulations we have, for example shown, how the relative alignment of the electronic states in stacks of two-dimensional semiconductor materials as well as graphene can be controlled by incorporating dipolar molecules between the 2D sheets. In this way one can tune between structures, which favor the separation of charge carriers (relevant for applications like photodetectors or solar cells) or which promote the recombination of charges (as desired for light-emitting devices). We have also proposed similar concepts for covalent-organic and metal-organic frameworks, which represent one of the most interesting classes of materials, which can be generated by self-assembly processes. Here, combining metal-oxide nodes and organic molecules as linkers highly porous structures are formed, whose topology is reminiscent of the steel skeletons of skyscrapers. Considering the vast variety of building blocks that can be used in the self-assembly process forming these materials, it is not surprising that already more than 80.000 such structures have been reported. With our approach, one can again shift the electronic states in neighboring, nominally identical building blocks, generating, for example, materials with band diagrams similar to p-i-n diodes. Such device structures are omnipresent in conventional inorganic solar cells and photodetectors, but there they require the combination of differently doped semiconductor regions. This doping can be avoided by the electrostatic design concept proposed in our work. Besides that, the resulting polar networks would also be highly relevant for applications dealing with first order non-linear optical effects. Finally, it should be noted that the project has been embedded into a network of international collaborations and the obtained results have been presented on numerous scientific conferences and published in several scientific papers.