Grant: Alliance Grants

Electrical dopants used for tuning the electronic properties of organic semiconductors (OSCs) are diverse, ranging from molecules to polymers and acids. This results in complex doping mechanisms that are only partially understood. Recently, fascinating doping efficiencies surpassing those of prominent oxidative p-dopants were reported for the Lewis acid tris(pentafluorophenyl)borane (BCF). As its electronic structure precludes oxidation of common OSCs, a protonation model was proposed suggesting the formation of a BCF-water complex that acts as Brønsted acid instead. This model, however, entails several inconsistencies that cannot be explained properly with the given scenario. In addition, only a very limited number of different Lewis acids have been used as dopants for only a few OSCs, which largely precludes drawing general conclusions. With this project, we aim to identify and systematically modify parameters affecting the doping by Lewis acids to understand fundamental processes and leverage their full potential as highly efficient p-dopants. Informed by initial density-functional theory calculations, we will develop a library of Lewis acids with systematically different acidity, size, planarity, and Lewis acidic heteroatoms to be related to their doping efficiency. The Lewis acids will be used to dope prototypical poly- and oligomeric OSCs of different basicity, thereby probing the role of the OSC conjugation length from small molecules to the polymer limit. This will enable us, to (i) understand the impacts of steric bulk on the Lewis acids, whether they form classical Lewis acid-base adducts or frustrated Lewis pairs with the OSCs, and correlate this with the capacity of hole generation, (ii) work under perfectly anhydrous conditions (ultrahigh vacuum) using vacuum-stable Lewis acids instead of BCF to isolate the role of water, and to (iii) identify the most promising acids for their subsequent use in thermoelectric model devices. This project combines the expertise of materials and device physicists as well as synthetic main group chemists that allows us to comprehensively tackle the project in truly interdisciplinary fashion for the highest possible impact.