Extensive efforts have been taken to correlate the performance of organic optoelectronic devices with changes in molecular conformation, differences in long range order and various morphologies formed by conjugated polymers, e.g., regio-regular poly(3-alkylthiophene)s (P3AT). Gaining full understanding of structure/processing/performance interrelation-ships requires optimal control of all structural parameters. In this context, large single crystals, characterized by a unique order of all molecules across all length-scales, may provide suitable model systems allowing to unveil how optoelectronic properties in semiconducting polymers depend on to structural features ranging from the molecular to the macroscopic level.

Here, we demonstrate that large single crystals can be grown not only from short regio-regular oligomers but also from long poly(3-hexylthiophene) chains by employing a self-seeding approach and precisely adjusting self seeding temperature, crystallization temperature and time. The procedure allows controlling the number density, size and internal structure of these crystals. Materials obtained via different synthetic protocols with different degrees of region-regularity and different molecular weights all form single crystals of the monoclinic form II with inter-digitated hexyl side-groups when appropriately crystallized. We show how differences in long range order impact on optoelectronic properties.