Polarons in DPP Polymers - How Glycol Side Chains and Elongated Conjugated Backbone Influence the Formation and Transport

Abstract

Understanding polaron formation in conjugated polymers is critical for advancing solid-state organic electronics. Here, we investigate diketopyrrolopyrrole (DPP)-based polymers with tailored side chains to elucidate the impact of glycolation on charge transport and polaron formation. We demonstrate that glycol side chains enhance p-type character and charge carrier density, while backbone elongation improves planarity and mobility. Electrochemical doping using a semicrystalline solid-state ionic liquid (SSIL) can increase conductivity by four orders of magnitude. In situ field-dependent Raman spectroscopy probes polaron formation, showing increased pi-electron redistribution in glycolated DPP. Polaron formation of the DPPT-T conjugated backbone shows a more localised polaron with structural changes to the thiophene donor unit. Backbone elongation results in greater polaron delocalisation with lower reorganisation energy. Finally, ion-gel gated organic synaptic transistors (IGOSTs) demonstrate significant performance gains for glycolated polymers with gDPPT-T and gDPPT-TVT exhibiting strong excitatory post-synaptic currents. The more facile polaron formation pathway for gDPPT-TVT offers a significant advantage in the dynamics of ion migration and retention. This work provides molecular-level insight into the incorporation of glycol side chains to high-performance conjugated polymers for solid-state applications.