A polymorph of the 6,13-dichloropentacene organic semiconductor: crystal structure, semiconductor measurements and band structure calculations

Abstract

Polymorphism is an important issue for the application of organic semiconductors in field-effect transistors due to the dependence of device performance on crystal packing. Recently an organic semiconductor, 6,13-dichloropentacene (DCP), was shown to give a field-effect mobility as high as 9 cm² V⁻¹ s⁻¹ when crystallized in the form of microribbons obtained by physical vapor transport (PVT), in contrast with the much lower mobilities found in crystals grown from solution. In the present manuscript we show that the crystals grown in the vapor phase are a polymorph of the structure reported for the crystals grown from solution. Both polymorphs show a π-stacking motif but differ in the interplanar distances and the displacement of the molecules along the long axes of the molecules. The DCP PVT structure compares favorably to that of rubrene, but DCP shows a much lower mobility. Band structure calculations show that the rubrene crystal exhibits greater band dispersion than DCP even though the long-axis displacement in rubrene is much greater. We conclude that the transport properties cannot be inferred simply by the magnitude of the intrastack long-axis displacement, but requires a consideration of the relative location of the HOMO/LUMO coefficients of the neighboring molecules in the stack.