Molecular design of photoactive acenes for organic photovoltaics

Absorption spectra of n-acenes (n from 2 to 6, for naphthalene, anthracene, tetracene, pentacene, and hexacene, respectively) have been calculated using a newly developed code based on time-dependent density-functional theory. Our calculations show that absorption spectra and charge carrier mobility of acenes not only depend on the molecular identity but also on the molecular packing. By designing the interaction between metal substrates and the first layer of acene molecules, they 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 pi-orbital overlap and thus enhance the charge mobility by maximizing electronic coupling between adjacent molecules. 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.