Effect of Molecular Stacking on Exciton Diffusion in Crystalline Organic Semiconductors

Exciton diffusion is at the heart of most organic optoelectronic devices' operation, and it is currently the most limiting factor to their achieving high efficiency. It is deeply related to molecular organization, as it depends on intermolecular distances and orbital overlap. However, there is no clear guideline for how to improve exciton diffusion with regard to molecular design and structure. Here, we use single-crystal charge-transfer interfaces to probe favorable exciton diffusion. Photoresponse measurements on interfaces between perylenediimides and rubrene show a higher photocurrent yield (+50%) and extended spectral coverage (+100 nm) when there is increased dimensionality of the percolation network and stronger orbital overlap. This is achieved by very short interstack distances in different directional axes, which favors exciton diffusion by a Dexter mechanism. Even if the core of the molecule shows strong deviation from planarity, the similar electrical resistance of the different systems, planar and nonplanar, shows that electronic transport is not compromised. These results highlight the impact of molecular organization in device performance and the necessity of optimizing it to take full advantage of the materials' properties.