Molecular p-type dopants, widely used to electrically dope semiconducting polymers, are challenging to design because of the requirements for their redox potentials. Redox-active dopants with radical cations present a strategy to achieve effective doping and to increase their synthetic versatility. A p-type dopant from a phenothiazine radical-cation bearing polymeric ionic liquid (PIL) with an associated bis(trifluoromethanesulfonyl)azanide ((CF3SO2)2N−, TFSI−) counterion was investigated. The structure of the polymeric dopant limits mass diffusion into solid polymer films allowing charge transfer to occur only at interfaces. Electron transfer between the phenothiazine radical cation and model semiconducting polymers was found to be similar in both the small molecule and pendant attached polymer. The kinetics of charge transfer upon forming bilayers with the PIL and poly(3-hexylthiophene) (P3HT) was investigated and the limiting factor was found to be mass diffusion of the charge balancing TFSI− counterion in the solid state. The electrical conductivity and microstructural changes in P3HT film were consistent with the doping levels expected based on the radical cation fraction in the polymeric dopant. This redox-active PIL allows facile charge generation and broadens the choice of p-type dopants for semiconducting polymers.