Charge transport in amorphous and smectic mesophases of dicyanovinyl-substituted oligothiophenes

By analyzing electrostatic and polarization effects in amorphous dicyanovinyl-substituted oligothiophenes, we conclude that local molecular dipole moments result in a large, spatially correlated, energetic disorder. This disorder increases with the number of thiophene units in the oligomer and leads to an unexpected reduction of charge carrier mobility in a more ordered (smectic) mesophase, observed for the longest of the studied oligomers (hexamers). This reduction in mobilities contradicts the common belief that more ordered phases of organic semiconductors have a better charge carrier mobility. In this particular case, the amorphousness leads to a better-connected charge percolating network, helping to bypass deep energetic traps. By comparing mobilities of amorphous and crystalline mesophases we conclude that vacuum deposited thin organic films have well ordered polycrystalline morphologies.