Recently, as interest in soft bioelectronic interfaces has grown, active research is being conducted on organic mixed ionic-electronic conductors which are key channel materials for organic electrochemical transistors. Herein, we present the polymer structure-property relationship through the rational design of the side-chains of conjugated polymers that exhibit crucial electrical/electrochemical characteristics as organic mixed ionic-electronic conductors, and how this relationship as well as thin film processing can contribute to the development of high-performance organic electrochemical transistors.1 In parallel, a new electrochemical device modality (i.e., organic electrochemical diodes) is introduced and its application to neuromorphic electronics is described.