  We study the organic electrochemical transistors (OECTs) performance of the
ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to
better understand how an apparently hydrophobic side-chain-free polymer is able
to operate as an OECT with favorable redox kinetics in an aqueous environment.
We examine two BBLs of different molecular masses from different sources. Both
BBLs show significant film swelling during the initial reduction step. By
combining electrochemical quartz crystal microbalance (eQCM) gravimetry,
in-operando atomic force microscopy (AFM), and both ex-situ and in-operando
grazing incidence wide-angle x-ray scattering (GIWAXS), we provide a detailed
structural picture of the electrochemical charge injection process in BBL in
the absence of any hydrophilic side-chains. Compared with ex-situ measurements,
in-operando GIWAXS shows both more swelling upon electrochemical doping than
has previously been recognized, and less contraction upon dedoping. The data
show that BBL films undergo an irreversible hydration driven by the initial
electrochemical doping cycle with significant water retention and lamellar
expansion that persists across subsequent oxidation/reduction cycles. This
swelling creates a hydrophilic environment that facilitates the subsequent fast
hydrated ion transport in the absence of the hydrophilic side-chains used in
many other polymer systems. Due to its rigid ladder backbone and absence of
hydrophilic side-chains, the primary BBL water uptake does not significantly
degrade the crystalline order, and the original dehydrated, unswelled state can
be recovered after drying. The combination of doping induced hydrophilicity and
robust crystalline order leads to efficient ionic transport and good stability.
