Ion-backbone accessibility enables unity doping efficiency in organic electrochemical transistors

Organic electrochemical transistors are commonly benchmarked using volumetric capacitance (C*); however, this metric does not distinguish between Faradaic and non-Faradaic contributions and therefore does not directly quantify the density of electronically active charge carriers that governs device-relevant doping capacity. Here we introduce effective volumetric capacitance (C_eff* = C* · η), where η denotes doping efficiency, as a metric for electronically effective volumetric doping. Using this metric, we show ionophilic side chains promote high ionic uptake but also sequester ions away from the conjugated backbone, lowering doping efficiency. By contrast, side-chain removal increases ion access to electrochemically addressable backbone sites, enabling doping efficiencies approaching unity. The resulting materials exhibited both enhanced volumetric charge density and improved charge transport, yielding transistors with transconductance among the highest reported. These results establish a practical design rule for organic mixed conductors by showing that electronically effective doping, rather than ionic uptake alone, governs transistor operation.