Understanding polymorph transformations in core-chlorinated naphthalene diimides and their impact on thin-film transistor performance

Though charge transport is sensitive to subtle changes in the packing motifs of molecular semiconductors, research addressing how intermolecular packing influences electrical properties has largely been carried out on single-crystals, as opposed to the more technologically relevant thin-film transistors (TFTs). Here, independent and reversible access to the monoclinic and triclinic crystal structures of a core-chlorinated naphthalene tetracarboxylic diimide (NTCDI-1) is demonstrated in polycrystalline thin films via post-deposition annealing. Time-resolved measurements of these transitions via UV-visible spectroscopy and grazing-incidence X-ray diffraction indicate that the polymorphic transformations follow second-order Avrami kinetics, suggestive of 2D growth after initial nucleation. Thin-film transistors comprising triclinic NTCDI-1 consistently outperform those comprising its monoclinic counterpart. This behavior contrasts that of single-crystal transistors in which devices comprising monoclinic crystals are consistently superior to devices with triclinic crystals. This difference is attributed to more uniform in-plane charge transport in polycrystalline thin films having the triclinic compared to the monoclinic polymorph. As the mobility of TFTs is a reflection of ensemble-average charge transport across crystalline grains having different molecular orientations, this study suggests that among different polymorphs of a particular molecular semiconductor, those with smaller in-plane anisotropy are more beneficial for efficient lateral charge transport in polycrystalline devices. Post-deposition annealing enables reversible thin-film structural rearrangement of a core-chlorinated naphthalene tetracarboxylic diimide between two distinct polymorphs. In situ spectroscopy and grazing-incidence X-ray diffraction reveal the transformations to follow second-order Avrami kinetics. Thin-film transistor performance indicates in-plane anisotropy to dominate charge transport, and implicates 2D π-stacking to be more efficient for lateral charge transport than 1D π-stacking in polycrystalline thin films.