Formation of Needle-like Poly(3-hexylthiophene) Crystals from Metastable Solutions

We investigated the formation of crystalline structures of conjugated poly(3-hexylthiophene) (P3HT) from supersaturated solutions. To this end, we induced crystallization by changing solubility through the diffusion of a poor solvent into a homogeneous P3HT solution. An adapted seeding process was employed based on an increase of the solution temperature combined with a decrease of the amount of the poor solvent in solution. By adjusting the concentration gradient of the poor solvent, we achieved spatially controlled conditions of supersaturation, including regions of slow growth. Our strategy allowed reducing the nucleation density significantly while maintaining a finite growth rate, resulting in the formation of long needle-like crystals. These crystals showed homogeneous birefringence and, with respect to thin films prepared from homogeneous solutions, bathochromically shifted absorption and emission spectra, indicating a highly ordered arrangement of P3HT.