Light absorption of poly(3-hexylthiophene) single crystals

We report the local UV-vis absorption behavior of single crystals of conjugated poly-3-hexylthiophene (P3HT), obtained by crystallization in dilute solutions at elevated temperatures based on a self-seeding approach. Single P3HT crystals were washed in a poor solvent in order to remove surrounding molecules. Birefringence suggested high internal structural order of the conjugated polymer chains within the crystal. A noticeable red-shift of the A0-0
absorption peak by~70 nm and a record A0-0/A0-1 intensity ratio were observed, indicating that in these P3HT single crystals polymer chains were planarized, allowing for large conjugation lengths and possibly delocalized states along the whole chain. The large energy separation of the vibronic peaks with respect to the reported vibrational spectra of cast films might be related to the type of packing induced by interdigitated hexyl-side-groups of the studied form-II single crystals of P3HT which may allow for some inter-chain orbital overlap (inter-chain delocalization). Furthermore, we show that absorption properties of as deposited dispersions of P3HT single crystals can be attributed to three different contributions, related to different molecular order.