The role of counter ion structure on the spatial distribution and molecular configuration of charge carriers in solid state electrochemically doped conjugated polymers

In a chemically doped or electrochemically doped conjugated polymer, electronic charge carriers are inherently associated with a countercharge in the form of a molecular or atomic counterion. It is expected that the structure of the counter ion will affect its distribution in the polymer film along with that of associated electronic charge carriers. Furthermore, the effect of counter ion structure on the charge storage configuration (i.e. polaron vs. bipolaron) is unknown. Here, we quantify the charge configuration and the distribution of charge carriers between aggregated and amorphous domains as a function of the counter ion structure in semicrystalline conjugated polymer films. We employ an organic electrochemical transistor with a cation tethered polymeric ionic liquid (PIL) as the gate dielectric, where the size and structure of the mobile anionic counterion in the PIL can be varied to modulate the counterion that is associated to charge carriers in the semiconductor. Operando electron paramagnetic resonance and in situ device measurements reveal the polaron to bipolaron ratio as a function of the gate bias for a range of counter ions. This is paired with in situ optical spectroscopy to determine the distribution of charge carriers between amorphous and aggregated domains.