Electrolyte pH Drives a Crossover to Proton‐Coupled Doping in n‐Type Organic Mixed Conductors

ABSTRACT Organic mixed ionic‐electronic conductors (OMIECs) have emerged as promising materials for applications ranging from bioelectronics to neuromorphic computing to charge storage. Many of these applications require n‐type OMIECs, whose operation is strongly influenced by the electrolyte. Although factors such as counterion identity and concentration are known to impact electrochemical doping, the role of electrolyte pH, which sets the proton activity of the electrolyte, remains comparatively underexplored, especially for n‐type OMIECs. Here, we investigate how proton activity governs electrochemical doping in the n‐type polymer p(gNDI‐T2) across a pH range of 1–10 and uncover a sharp mechanistic crossover at pH ∼4. Below this threshold, doping proceeds through super‐Nernstian proton‐coupled electron transfer (PCET), while above it, conventional K + ‐compensated doping prevails. Using a suite of operando methods, we find that doping at low pH results in minimal mass transfer to the polymer and the emergence of localized polaronic states. These mechanistic differences can directly impact device operation, as in organic electrochemical transistors (OECTs), proton‐compensated species significantly decrease electronic charge‐carrier mobility. More broadly, these results establish electrolyte pH as a key design parameter for n‐type OMIECs, one that must be controlled for reliable device operation, and that may also be harnessed for applications requiring pH‐responsive behavior.