Bistetracene Thin Film Polymorphic Control to Unravel the Effect of Molecular Packing on Charge Transport

Polymorphism, the ability for a given material to adopt multiple crystalline packing states, is a powerful approach for investigating how changes in molecular packing influence charge transport within organic semiconductors. In this study, a new "thin film" polymorph of the high-performance, p-type small molecule N-octyldiisopropylsilyl acetylene bistetracene (BT) is isolated and characterized. Structural changes in the BT films are monitored using static and in situ grazing-incidence X-ray diffraction. The diffraction data, combined with simulation and crystallographic refinement calculations, show the molecular packing of the "thin film" polymorph transforms from a slipped 1D pi-stacking motif to a highly oriented and crystalline film upon solvent vapor annealing with a 2D brick-layer pi-stacking arrangement, similar to the so-called "bulk" structure observed in single crystals. Charge transport is characterized as a function of vapor annealing, grain orientation, and temperature. Demonstrating that mobility increases by three orders of magnitude upon solvent vapor annealing and displays a differing temperature-dependent mobility behavior.