Organic electrochemical transistors (OECTs) have attracted great attention as they hold significant promise for a variety of applications ranging from printable logic circuits for electronic textiles to drivers for sensors and flat panel display pixels, as well as to artificial synapse for neuromorphic computing. [1] Because of the low working bias, high sensitivity, and stability in aqueous environments, as well as biological and mechanical compatibility with live tissues, OECTs have also recently emerged as a technological solution to a variety of diagnostic and therapeutic applications. [2] A considerable amount of work has focused, for example, on approaches exploiting the principle of OECTs for the development of biomedical tools for chemical and biological sensing, [3] electrophysiological recording, [4] monitoring of cell viability, and barrier tissue integrity, [5] to name just a few. In an OECT, the electroactive polymer constituting the channel is in direct contact with an electrolyte and with the source and drain metal electrodes ( Figure 1A). Because of the soft and permeable nature of the electroactive polymers, ions are able to penetrate into the bulk of the transistor channel. [6] The operation of an OECT relies then on a reversible ion exchange and charge compensation process, which leads to a bulk doping of the organic conducting channel and to a modulation of the electronic conductivity between the source and drain contacts. Hence, OECTs transduce a modulation in the gate voltage (V G ) to a modulation in the drain current (I D ) running through the entire bulk of the channel. The figure-of-merit that quantifies the efficiency of this transduction is the transconductance, defined as g m = ∂I D /∂V G .The current state-of-the-art active material for OECTs is the mixed ion-electron conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The volumetric doping and dedoping of PEDOT:PSS result in a modulation of the drain-source current of several orders of magnitude with a consequent high transconductance. [7] As PEDOT:PSS is doped in its pristine state, and thus highly conducting, the OECT operates in depletion mode. In addition, several polythiophene-based polymers have been reported as efficient electroactive channel materials for enhancement mode OECTs. [8] To date, however, essentially all reported OECTs have relied on hole transport (p-type), while the development of electron Organic electrochemical transistors (OECTs) have been the subject of intense research in recent years. To date, however, most of the reported OECTs rely entirely on p-type (hole transport) operation, while electron transporting (n-type) OECTs are rare. The combination of efficient and stable p-type and n-type OECTs would allow for the development of complementary circuits, dramatically advancing the sophistication of OECT-based technologies. Poor stability in air and aqueous electrolyte media, low electron mobility, and/or a lack of electrochemical reversibility, of available high-...

Ladder-type “torsion-free” conducting polymers (e.g., polybenzimidazobenzophenanthroline (BBL)) can outperform “structurally distorted” donor–acceptor polymers (e.g., P(NDI2OD-T2)), in terms of conductivity and thermoelectric power factor. The polaron delocalization length is larger in BBL than in P(NDI2OD-T2), resulting in a higher measured polaron mobility. Structure–function relationships are drawn, setting material-design guidelines for the next generation of conducting thermoelectric polymers

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N-doping of conjugated polymers either requires a high dopant fraction or yields a low electrical conductivity because of their poor compatibility with molecular dopants. We explore n-doping of the polar naphthalenediimide–bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole–dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high electrical conductivity of more than 10–1 S cm–1 for a dopant concentration of only 10 mol % when measured in an inert atmosphere. We find that the doped polymer is able to maintain its electrical conductivity for about 20 min when exposed to air and recovers rapidly when returned to a nitrogen atmosphere. Overall, solution coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelectric power factor of up to 0.4 μW K–2 m–1 compared to other NDI-based polymers.

The synthesis of a novel naphthalenediimide (NDI)-bithiazole (Tz2)-based polymer [P(NDI2OD-Tz2)] is reported, and structural, thin-film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI-bithiophene (T2) polymer [P(NDI2OD-T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD-Tz2) exhibits a more planar and rigid backbone, enhancing π-π chain stacking and intermolecular interactions. In addition, the electron-deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor-acceptor character. When n-doped with amines, P(NDI2OD-Tz2) achieves electrical conductivity (≈0.1 S cm ) and a power factor (1.5 µW m K ) far greater than those of P(NDI2OD-T2) (0.003 S cm and 0.012 µW m K , respectively). These results demonstrate that planarized NDI-based polymers with reduced donor-acceptor character can achieve substantial electrical conductivity and thermoelectric response.