Combined diffraction and modelling approaches: a key for an atomistic understanding of the structure and properties of polythiophenes?

Poly(3-alkythiophenes) (P3AT) are one of the most promising classes of electroactive polymers and represent a key component in a number of different organic electronic and photovoltaic devices which are being presently developed. Although extensive structural and morphological studies have been accumulating, only very few detailed atomistic models of the structural organization of these polymers have been developed. It is accepted that the structural organization of P3ATs commonly present in devices is the form I family, which is tightly stacked, yet generally disordered in ways that are difficult to detail. The study of the polymorphism of poly(3-butylthiophene) on the other hand has evidenced that the enthalpically favoured but relatively rare form II polymorph is highly ordered, interdigitated and shows relatively loose stacking. This form melts surprisingly more than 100°C below the disordered form I. A similar situation is found also in the more widely applied poly(3-hexylthiophene) for which the stable form II is so rare that it has been reported only as an impurity so far. Detailed modelling with an ad-hoc developed forcefield is able to mirror in reasonable detail much of the described polymorphic behaviour opening viable perspectives to the study of disordered states of P3ATs.