LAUSR

Abstract Understanding microscopic mixed ionic‐electronic conduction in organic electrochemical transistor (OECTs) is crucial to advance bioelectronic, neuromorphic, and sensing applications. In this work, electrochemical strain wave (ESW) microscopy is introduced as a novel approach to probe transport processes in the OECT channel at microscopic length scales. ESWs are generated in mixed conductors by charge injection at the electrodes and propagate into the device channel through combined charge transport and swelling. With AFM experiments, the local amplitude and phase of ESWs are mapped in OECTs under different operating conditions. Results demonstrate that quantitative ESW acquisitions can determine carrier concentration‐dependent mobility effects in n‐type and p‐type devices, or reveal microstructural defects in the OECT channel, altering local charge transport. The findings provide a coherent framework to interpret the structure‐functionality relation in OECTs, mapping transport properties below the diffraction limit and without limitations by screening effects or electrochemical side reactions of the probe.