Transport in Doped Polythiophenes and Polythienothiophenes with Alkyl or Glycol Sidechains

Organic semiconductors have a wide range of near future applications, such as: bioelectronics, thermoelectronics and optoelectronics^1,2 where their low production cost and chemical tunability make them very attractive candidates for the aforementioned fields. Current challenges include the low conductivity which can be addressed via doping. In particular for conjugated polymers doping increases the conductivity by several orders of magnitude.^2-4 Recently, sidechain engineering has been shown to be a useful tool for the modification and improvement of conjugated polymers.⁵ By exchanging the commonly used alkyl sidechains with glycol sidechains the dopant-polymer miscibility and the ionic uptake is improved.⁶ Herein we investigate how these improvements affect chemical and anion exchange doping of thiophene and thienothiophene type polymers in respect to their conductivity. To characterize these changes, we use steady state absorbance spectroscopy, four-point probe, time domain THz spectroscopy, in situ absorbance measurements and chronoamperometry. For P(g₃BTTT), we find extraordinarily high conductivities of over 2 000 S/cm combined with a high charge carrier mobility. We show that such high conductivities are achieved in the presence of a high bipolaron density, and this is observed for all the polymers with thienothiophene backbones we studied. In contrast, for thiophene based backbones there is an optimal bipolaron to polaron ratio (0.8) where the maximum conductivity is achieved. In addition, for the high conductive polymers studied we show that the DC conductivity is similar in amplitude to the THz conductivity, which means that the factors that limit the transport over few nanometers are also the limiting factors over longer distances. We demonstrate the benefit of glycol side chains for the conductivity in thiophene and thienothiophene type polymers especially when combined with anion exchange doping.