In a theoretical study combining molecular dynamics simulations, quantum-chemical calculations, and Marcus theory-based charge migration simulations, we investigate the electronic structure and the charge transport of three members of a promising class of organic near-infrared absorber materials: aza-BODIPYs, which are partially already successfully used as the donor material in organic solar cells.[1] The local character of the frontier molecular orbitals and their coupling to the intramolecular dynamics significantly influence the size of the total charge carrier mobility and determine whether a material has a higher electron or hole conductance. These effects depend also on the molecular packing and correspondingly on steric effects causing differences in the ratio between the electron and the hole mobility of highly ordered materials. Interestingly, the fluctuations of the transfer integrals can influence both the total value of the charge carrier mobility as well as its anisotropy. This demonstrates that both local and non-local energetic disorder effects have to be considered to formulate design rules for the class of aza-BODIPY derivatives to help to further improve the efficiency of organic solar cells.

[1] T. Mueller et al. Solar Energy Materials & Solar Cells 99, 176-181 (2012).