60: Designing organic molecules and packing for highly conductive semi-conductors

Abstract: Computational models provide design insights into better functional materials by building off of fundamental structure-property relationships. Directional conductivity in a material can be achieved by favorable material packing, driven by molecular structure; finding such molecular structures that drive directional conductivity would allow for a leap forward in flexible organic semiconductors. Recently, ‘frozen-atom’ charge transport models using the Boltzmann transport equations (BTE) have calculated the directional hole conductivity of small-molecule, organic semi-conductor, crystalline materials accurately. BTE relies on the assumption that a single relaxation time parameter adequately captures the decay of this conductive hole. However, the further assumption of a local equilibrium for hole injection is problematic because it is material-dependent and environment-independent. Previous applications of BTE have focused on predicting conductivity in closely related, non-polar acene experiments that hide these local equilibrium features. Expanding applications of the BTE methodology, two polymorphs of the dipyrrolyldiketone BF2 complex (DATZAA), which are more similar to functionalized organic semiconductors with hetero-atoms and directional hydrogen bonding, are explored. The predicted conductivity direction and magnitude significantly depends on hole injection parameters, caused by different stereochemical packing. This work shows that changing the inter-molecular packing in a crystal can completely change that crystal’s hole response to an electric field. The design of new organic semiconducting materials requires accounting for details of hole injection mechanisms (electronic properties), together with details of the strength and character of inter-molecular packing (structure).