Highlights

Recent years have seen a burgeoning interest in organic photovoltaics (OPVs) due to their low cost, lighter weight, flexible shape and versatile materials synthesis schemes.  Among the wide variety of small molecules that have been studied, the family of n-acenes, constituted of linear chains of n planar benzene rings, is certainly one of the most investigated because of their structural stability and capability of forming highly ordered polycrystalline films.  However, they usually have limited response to the solar spectrum and low charge carrier mobility.  For blue photons, current OPVs have exhibited high (>70%) quantum efficiency.  But, the efficient harvesting of red photons, which constitute a significant fraction of the solar energy, has proven a very challenging task. Dr. Huang’s first principles calculations show that by designing the interaction between metal substrates and the first layer of molecules, acenes can be packed in a face-to-face fashion instead of the conventional herringbone (face-to-edge) arrangement. Acenes in the cofacial packing would increase the p-orbital overlap and thus enhance the charge mobility by maximizing electronic coupling between adjacent molecules (as indicated by the charge density distribution in the HOMO state). Absorption spectra of cofacially packed acenes have a better overlap with the solar spectrum, which allows harvesting more of the solar energy from red photons.