Temperature-Assisted Molecular Reordering in n-Type Organic Semiconductor Films: The Impact on Nanoscale Morphology, Photophysical Properties, and Electron Transport

Charge carrier transport in bulk organic semiconductors, especially in solution-grown films, is invariably dictated by the semiconductor layer morphology and molecular level organization of the charge hopping centers. Therefore, even tiny positional displacements of molecules will lead to significant changes in the bulk carrier transport. Various pre- and postfilm deposition techniques have been developed to achieve the ideal layer morphology and molecular arrangement on desired substrates. Herein, to demonstrate the findings on temperature-assisted morphological restructuring initiated by molecular scale reordering of n-type molecular semiconductors, a series of positional isomers of end-substituted naphthalene diimide (NDI) derivatives were subjected to study. For one derivative, namely, m-picolylnaphthelenediimide (m-PyNDI), a dramatic improvement in field-effect electron mobility and overall device performance was observed upon postdeposition thermal treatment. To elucidate the mechanism behind this drastic change, a bottom-up approach of tracing the assembly pathways from molecular scale aggregates in solution to bulk films was carried out and systematically summarized. Spectroscopic signatures of the aggregates were carefully studied by following the exciton coupling model, and the evolution of the morphological transformation was monitored using microscopic techniques.