Identification of Ultrafast Relaxation Processes As a Major Reason for Inefficient Exciton Diffusion in Perylene-Based Organic Semiconductors

The exciton diffusion length (<i>L</i>_D) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials. Our model is based on simulations with a hybrid approach which combines high-level ab initio computations for the part of the system directly involved in the described processes with a force field to include environmental effects. The adequacy of the model is shown by detailed comparison with available experimental results. The model indicates that the short exciton diffusion lengths of α-perylene tetracarboxylicdianhydride (PTCDA) are due to ultrafast relaxation processes of the optical excitation via intermolecular motions leading to a state from which further exciton diffusion is hampered. As the efficiency of this mechanism depends strongly on molecular arrangement and environment, the model explains the strong dependence of <i>L</i>_D on the morphology of the materials, for example, the differences between α-PTCDA and diindenoperylene. Our findings indicate how relaxation processes can be diminished in perylene-based materials. This model can be generalized to other organic compounds