Molecular interfaces are ubiquitous and perform critical functions in organic electronic and photovoltaic devices. In typical organic solar cells sunlight is harvested by organic species and charge separation is driven by the potential energy difference at an interface between donor and acceptor species. In any organic device, charge transport to external circuits depends on interfaces between active molecular layers and electrode materials. The properties and functionality of these critical interfaces cannot be deduced directly from those of their isolated constituents. Rather, they emerge from quantum mechanical interactions at the atomistic scale. Predicting the properties of molecular interfaces thus requires a fully quantum mechanical first principles approach.
The configuration space of molecular interfaces is infinitely vast, owing to the endless possibilities of combining layers of one or more molecular species with different substrates. Layers of the same molecular species may adopt different structures on different substrates and therefore exhibit different electronic and optical properties. Furthermore, epitaxial templating may enable stabilizing meta-stable crystal structures with desirable properties in thin film form. Efficient algorithms may significantly accelerate the discovery and design of molecular interfaces with enhanced properties. We are developing a first-principles framework for structure and property prediction of molecular interfaces.
Ogre: A Python package for molecular crystal surface generation with applications to surface energy and crystal shape prediction
We have developed a new code, Ogre, for generating molecular crystal surface slabs, and the accompanying OgreSWAMP utilities for streamlining the calculation of surface energies and Wulff shapes. Ogre takes as input a bulk crystal structure and generates surface slab models with user-specified Miller indices, number of layers, and vacuum space. When cleaving molecular crystal surfaces, Ogre uses a graph representation to identify molecules and ensure the molecules at the surface remain intact. Ogre identifies all the non-equivalent surfaces based on the crystal’s space group symmetry and outputs slab structures with all possible terminations for each unique surface. These can be used to calculate the surface energies and predict the crystal shape. The Wulff shape of aspirin, produced by Ogre, is in good agreement with experimental data from Journal of Pharmaceutical Sciences 96, 2134 (2007) and Journal of Pharmaceutical Sciences 97, 1361 (2008).