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A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n‐doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV–vis–NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next‐generation conjugated polymers. [ABSTRACT FROM AUTHOR]

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p‐type (semi‐)conducting polymers as the channel material, while n‐type OECTs are yet at an early stage of development, with the best performing electron‐transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi‐walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder‐type π‐conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n‐type OECTs with record‐high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high μC* (electron mobility × volumetric capacitance) of about 1 F cm−1 V−1 s−1. This enables the development of complementary inverters with a voltage gain of >16 and a large worst‐case noise margin at a supply voltage of <0.6 V, while consuming less than 1 µW of power. [ABSTRACT FROM AUTHOR]