Hopping-Based Charge Transfer in Diketopyrrolopyrrole-Based Donor-Acceptor Polymers: A Theoretical Study

Diketopyrrolopyrrole (DPP) based materials have recently been considered as promising candidates for novel organic electronics. In this article, we report an investigation on intermolecular charge transfer between DPP based polymers. We use Marcus transfer theory and evaluate the required quantities, the reorganization energy, and the coupling, by density functional based tight binding (DFTB) calculations. Since the coupling is dependent on the stacking geometry we employ an energy-weighted statistical approach to derive a single quantity, which can been entered in the Marcus formula. This value contains the variation of the coupling when the stacking conformation is changed. The application of this method, as we implement it in this study, does not require a detailed analysis of the energy landscape, but samples over a large number of stacking possibilities on a regular, but very dense, grid are taken into account. These average values can been used to analyze isomeric effects such as the orientation of units, the influence of the molecular structure as functionalization, or the importance of stacking properties as parallel and antiparallel stacking. The obtained results show that enhanced charge carrier mobilities can be achieved when specific molecular configurations are considered rather than by working with a set of random orientations.