Band Filling, Electrochemical Reaction, and Re-Entrant Insulating Behavior in Electrolyte-Gated BBL Polymer Semiconductor Films

Electrochemical doping of the n-type polymer poly(benzimidazobenzophenanthroline) (BBL) in contact with ionic liquids reveals a peak in the drain current (I_D) vs gate voltage (V_G) behavior, i.e., conductivity versus electron density. The conductivity peak is related to simultaneously acquired gate current-gate voltage (I_G-V_G) charging curves that are integrated to yield total charge accumulation. The I_G-V_G traces reveal three separate redox events upon charging BBL that can be correlated with the I_D-V_G behavior. We assign the first broad I_G-V_G peak to accumulation of mobile electrons in the BBL LUMO and an increase in I_D. Two subsequent sharp I_G-V_G peaks correspond to electrochemical transformation of BBL to a polymer salt with 2:1 and 1:1 repeat unit-to-cation stoichiometries, respectively. Salt formation correlates with conductivity collapse at high V_G; the 1:1 salt phase is insulating. Ex situ grazing incidence wide-angle X-ray scattering (GIWAXS) indicates an initial 5% contraction of the lamellar spacing upon doping with subsequent retention of lamellar (para)crystalline order for repeated doping cycles. Overall, our results reveal a complex interplay between band filling and electrochemical reaction in the transport behavior of electrochemically doped BBL films and provide additional evidence of nonmonotonic conductivity versus charge behavior that appears to be general in polymer semiconductor films.