Microstructure design using graphs

Thin films with tailored microstructures are an emerging class of materials with applications such as battery electrodes, organic electronics, and biosensors. Such thin film devices typically exhibit a multi-phase microstructure that is confined, and show large anisotropy. Current approaches to microstructure design focus on optimizing bulk properties, by tuning features that are statistically averaged over a representative volume. Here, we report a tool for morphogenesis posed as a graph-based optimization problem that evolves microstructures recognizing confinement and anisotropy constraints. We illustrate the approach by designing optimized morphologies for photovoltaic applications, and evolve an initial morphology into an optimized morphology exhibiting substantially improved short circuit current (68% improvement over a conventional bulk-heterojunction morphology). We show optimized morphologies across a range of thicknesses exhibiting self-similar behavior. Results suggest that thicker films (250 nm) can be used to harvest more incident energy. Our graph based morphogenesis is broadly applicable to microstructure-sensitive design of batteries, biosensors and related applications. MATERIALS DESIGN: The power of graphs Representing microstructures with graphs, and mapping their properties mathematically, can help improve material properties. Morphogenesis is the process of optimizing performance in devices; representing the candidate structure with an appropriate mathematical framework in order to probe their properties and then mapping the structure to a property. Current approaches rely on computationally heavy methods for both stages, but now a team from Iowa State University and University at Buffalo simplify the process by representing the structures with labelled, weighted, undirected graphs. On this basis, one can create a “surrogate” model through generic physics graph descriptors (e.g. path lengths, domain sizes) and weighting functions for the particular property of interest. This approach reveals new designs for improved organic solar cells, but could expanded to other devices.