User:Tahereh Nematiaram/sandbox

Dynamic Disorder

In molecular semiconductors, molecules are kept together via weak van der Waals forces, therefore, even at room temperature, there are large thermal fluctuations. These large amplitude thermal molecular motions act as a source of a dynamic disorder which ultimately leads to a “transient localization” of the wave function on timescales shorter than the period of molecular oscillations.

The dynamic disorder is one of the main hindrances of charge carrier mobility in molecular semiconductors. Although it is desirable to have or design materials with a small dynamic disorder, this quantity depends on the electronic and vibrational structure of the materials in a very complicated way that it may seem impractical to develop an intuitive understanding of why some materials have a smaller or larger dynamic disorder.

In a recent work, utilizing a novel multi-scale QM/MM method and applying it for the dynamic disorder analysis of approximately 5,000 molecular semiconductors extracted from the Cambridge Structural Database, a list of key characteristics of structures that display a low degree of the dynamic disorder are identified. This work shows that the strength of dynamic disorder is highly correlated with the gradient of the transfer integral, a property easily computable for any molecular orientation. This work also suggests that orientations of molecular pairs with just a few atoms in contact, i.e. the head-to-tail arrangements of the molecules but with the conjugated cores of the interacting molecules not lying on the same plane, are more likely to yield low dynamic disorder. This observation is highly counterintuitive as for years the focus has been to maximize the number of pi-stacked atoms. The presence of alkyl chains, the strength of the transfer integral and the presence of heavy atoms also showed meaningful influence on the dynamic disorder. The findings of this work provide important design principles for materials exhibiting lower levels of dynamic disorder.