The Influence of the Bipolaron Formation Rate on the Charge Transport Properties of Chemically Doped Oligoetherized Polythiophene

In the last decades, organic semiconductors have attracted significant attention due to their biocompatibility, mechanical flexibility, solution processability, and lightweight nature. A recent achievement brought the engineering of side chains in polymer films to enhance ion intercalation, as demonstrated by the newly developed polymer P(g₃2T-T). This derivative of P3HT, featuring oligoether instead of aliphatic side chains, emerges as a promising candidate for chemical doping because of the affinity of its side chains to dopant anions as well as potential applications in bioelectronic devices. Chemical doping of the P(g₃2T-T) has been found to achieve an up to fourfold increase in macroscopic conductivity compared to P3HT. However, this enhanced conductivity is highly dependent on specific doping conditions. While general principles of doping are now well-understood, the precise effects of various doping parameters on the charge transport properties of doped thin films still need to be investigated.<br/> <br/>Our study employs <i>in-situ</i> absorbance spectroscopy as well as THz spectroscopy to investigate the effects of differing dopant and electrolyte concentrations on the conductivity of chemically doped P(g₃2T-T) films. We reveal that the early kinetics of the doping process have a significant influence on the final thin film conductivity. The results demonstrate that the bipolaron-to-polaron ratio is a rather weak indicator of favorable charge transport properties and that instead the bipolaron formation rate is a superior predictor. We presume that the bipolaron formation rate is entangled with the swelling of the polymer film, the intercalation of the dopant anions, and thus the packing of the polymer film as well as its charge transport properties. We explore two distinct doping methods—immersed doping and anion exchange doping—and use two different dopants, F₄TCNQ and Magic Blue, to rule out the impacts of specific doping techniques. Especially for the anion exchange doping method, our results offer for the first time an explanation for the dependence of the electrolyte concentration on the film conductivity. Moreover, we report an excellent restoration of long-range charge transport in chemically doped P(g₃2T-T), compared to the short range (100 nm) derived from THz spectroscopy. Overall, our results shed light on the underlying principles of chemical doping and the revealed dependencies on specific doping conditions can help facilitate the development of the next generation of organic semiconducting polymers that can be employed at the interface between biological and electronic systems.