Simulation of Charge Distribution and Microstructure in Semicrystalline Polymeric Ionic-Electronic Conductors Using Classical Simulation at Constant Electrochemical Potential.

Understanding how charge distributions on aggregated chains change with microstructure under constant electrochemical potential is crucial for elucidating the behavior of polymeric organic mixed ionic-electronic conductors (OMEICs), yet it remains difficult to study. To address this challenge, we introduce a methodology to perform classical atomistic simulations of doped semiconductors at a constant electrochemical potential. The method allows individual polymer chains to be oxidized and reduced, taking into account their individual redox potentials and the externally tunable electrochemical potential. The implementation follows a grand-canonical molecular dynamics (GC-MD) scheme, with the local modulation of the redox potential being described by a QM/MM Hamiltonian. Applied to a semicrystalline polymer with ordered layered and lamellar structures, the method reproduces the experimentally observed minimal structural changes over the electrochemical potentials and charging levels considered. Near the redox potential, charging levels fluctuate more strongly, and variations in the interlamellar angle (defined by the normal of adjacent lamellae) are most pronounced. Moreover, analysis of the local environment reveals no detectable correlation between a chain's redox reaction and the charge distribution of neighboring chains, except at the most negative potentials, where redox events occur preferentially in more positively charged surroundings. Lastly, examination of individual chains shows minimal chain-chain charge correlation, and the single-chain conformation remains closely linked to its redox behavior. Overall, this work provides a robust framework for investigating charge distributions in dynamically doped systems and offers new conceptual routes for studying polymer structural responses under constant electrochemical potentials.