We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to ∼500 μm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.
Keywords: P3HT; directed crystallization; organic electronics; pentacene; thermal gradient.