The
ladder-type benzimidazobenzophenanthroline (BBL)
polymer is
one of the most important and most studied n-type conducting polymers.
It is also an organic mixed ion-electron conductor (OMIEC), which
can undergo electrochemical switching in electrolyte solutions by
accommodating opposite ions. The extensive morphological changes of
the OMIEC material during operation affect the transport properties
and, hence, the device performance. However, molecular insights into
the dynamic structural changes during the electrochemical switching
are limited, as they are difficult or impossible to access in experiments.
The computational microscope based on molecular dynamics (MD) calculations
can provide us with complete insights into the detailed dynamic morphological
changes that are currently missing, to a large extent, for the BBL
polymer. In the present study, using atomistic MD simulations, we
obtained microscopic insights into the electrochemical switching of
BBL film in two different electrolytes, namely, single-atom counterion
K+ (potassium) in water and molecular counterion DMBI+ (dimethyl-3-butyl imidazolium) in chloroform. For both cases,
the maximum crystallinity is found up to a moderate reduction level.
Beyond that, ion intercalation initiates a structural phase transition
and causes a decrease in the crystalline order of the film. At the
higher reduction levels, the single-atom K+ counterions
are stabilized within the lamellar stacked BBL chains; in contrast,
the DMBI+ counterions with higher molecular weights are
stabilized within the BBL π–π stacks, forming π–π
stacking between BBL and DMBI+. Our findings substantiate
how molecular dopants can improve the thermomechanical stability of
the material and why smaller single-atom counterions are preferred
for maintaining better crystallinity. The detailed microscopic insights
into the morphological changes during the electrochemical switching
of BBL film, which cannot be directly accessed experimentally, can
definitely help design n-type OMIEC-based devices made of BBL.