A very promising strategy to design highly efficient materials for organic mixed-ionic-electronic conductors, is based on the idea of using the backbone structure of “more traditional” conjugated polymers – already designed and optimised specifically for electronic transport – and to modify the chemical composition of their side-chains in order to achieve efficient ionic transport. However, it is well-known that side-chains play an essential role in determining the microstructure of polymer thin films, which in turns deeply affects the charge carrier mobilities of the material and the efficiency in devices. As a consequence, understanding the structural effects of exchanging alkyl for glycol side chains in conjugated polymers is of central importance in determining accurate structure-function relationships for this new class of materials.

In this talk I will demonstrate that by using our recently developed combination of vacuum electrospray deposition (ESD) and high-resolution scanning tunnelling microscopy (STM) [1], it is possible to image conjugated polymers used in organic electronic and bioelectronic applications at the (sub-)molecular scale, thereby revealing their conformation and assembly with unprecedented spatial resolution [2,3]

I will present results on the prototypical semicrystalline polythiophene polymer pBTTT and directly compare these with analogous results on the mixed-ionic-electronic conductors obtained by substituting the alkyl side-chains of pBTTT with ethylene glycol (EG) side-chains. Our high-resolution images identify clear differences in the tendency of assembling between the two types of polymers and show that this depends on the local polymer density. I will further demonstrate that glycol side-chain polymers are capable of interdigitating and use the observed structural characteristics to interpret X-ray diffraction measurements of 3D thin films [4].

Finally, I will show that our observations can be explained based on a simple model that accounts for the different intermolecular interactions between alkyl and EG chains and propose this as a general framework to rationalise the assembly of conjugated polymers with EG side-chains. Moreover, I will show that atomistic molecular dynamic simulations (Nelson group, Imperial College London) can reproduce very closely the experimentally observed polymer conformations and assembly patterns.