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February, 2018

Conjugated polyelectrolytes (CPEs) have shown remarkable promise as active and charge transport layers in organic solar cells. Inkjet or screen-printing provide an easy route to low-cost fabrication of these devices. However, the morphology of the thin films made in this way is strongly influenced by the conformation of the CPE in the ink, which has a knock-on effect on intrinsically-linked optoelectronic properties, and the performance of the end-device. It is therefore crucial to develop robust processing methods that enable control of the organisation of individual and clustered polymer chains in the printing solution.

To tackle this challenge, we have developed a family of nanostructured “designer” inks in which the conformation of polythiophene CPEs is tailored through careful selection of the solvent or by co-assembly with everyday soaps, or surfactants. Surfactants self-assemble into a variety of well-defined nanoaggregate structures, whose shape varies with concentration and/or temperature. In this study, we used a combination of small angle neutron scattering, atomic force microscopy and optical spectroscopy to demonstrate that this ability can be translated to CPE-surfactant aggregates, resulting in nanostructured inks with tailored organisation which could then be deposited as the analogous thin film. Our results demonstrate the versatility of self-assembly to access a range of nanoscale assemblies, which could be crucial for the design of the next generation of organic solar cells.

Figure: Conjugated polythiophenes are transformed into a range of nanoscale morphologies using simple and versatile self-assembly strategies. 

J. E. Houston, M. Chevrier, M.-S. Appavou, S. M. King, S. Clément and R. C. Evans, "A Self-Assembly Toolbox for Thiophene-Based Conjugated Polyelectrolytes: Surfactants, Solvent and Copolymerisation", Nanoscale, 9 (2017) 17481-17493.

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