Dan Warr1 Luis Perdigão1 Harry Pinfold1 Jonathan Blohm1 David Stringer2 Anastasia Leventis3 Hugo Bronstein3 Alessandro Troisi4 Giovanni Costantini1

1, University of Warwick, Coventry, , United Kingdom
2, Imperial College London, London, , United Kingdom
3, University of Cambridge, Cambridge, , United Kingdom
4, University of Liverpool, Liverpool, , United Kingdom

The structure of a conjugated polymer and its solid-state assembly are without a doubt the most important parameters determining its properties and performance in (opto)-electronic devices. A huge amount of research has been dedicated to tuning and understanding these parameters and their implications in the basic photophysics and charge transporting behaviour. The lack of reliable high-resolution analytical techniques constitutes however a major limitation, as it hampers a better understanding of both the polymerisation process and the formation of the functional thin films used in devices.
Here, by combining vacuum electrospray deposition and high-resolution scanning tunnelling microscopy (STM) we demonstrate the ability of imaging conjugated polymers with unprecedented detail, thereby unravelling structural and self-assembly characteristics that have so far been impossible to determine.
Applying this novel technique to prototypical DPP-containing polymers, we show that sub-molecular resolution STM images allow us to precisely identify the monomer units and the solubilising alkyl side-chains in individual polymer strands. Based on this, it becomes possible to determine the molecular number distribution of the polymer by simply counting the repeat units. More importantly, we demonstrate that we can identify, precisely determine the nature, locate the position, and ascertain the number of defects in the polymer backbone. This unique insight into the structure of conjugated polymers is not attainable by any other existing analytical technique and represents a fundamental contribution to the long-discussed issue of defects as a possible source of trap sites. Furthermore, the analysis of our high-resolution images, also reveals that the frequently assumed all-trans-conformation of the monomers in the polymer backbone is actually not observed, while demonstrating that the main driver for backbone conformation and hence polymer microstructure is the maximization of alkyl side-chain interdigitation.
This work has profound implications on the wider field of polymer science, representing a first, fundamental step in tackling a major and still unresolved problem, i.e. how to precisely and reliably characterise a polymeric macromolecule.