We report a series of novel polymeric mixed ionic-electronic conductors based upon the incorporation of fused thieno[3,2-b]thiophene and bithiophene with isomeric sidechains. The enhanced rigidity in the polymer backbone facilitated the formation of stabilized bipolarons, while the reorganized ethylene glycol side chains not only maintained the polymer's hydrophilicity but also unexpectedly enhanced the crystallinity of the polymer film. This design strategy led to the development of 4gTT-2gT, a high-performance and stable organic mixed ionic-electronic conductor. The polymer exhibited a maximum µC* of 729 F V⁻¹ cm⁻¹ s⁻¹, one of the highest values among low-threshold voltage polythiophene derivatives, while demonstrating excellent operational stability, retaining 99% of its maximum current after 1-h device switching cycles and 94% after 9-h at lower bias. We implemented the material in OECT-based artificial synapses, which maintained functionality under a large temperature range (23–373 K). These combined properties establish 4gTT-2gT as a prime candidate for next-generation mixed conductor devices.

We present a series of newly developed donor-acceptor (D-A) polymers designed specifically for organic electrochemical transistors (OECTs) synthesized by a straightforward route. All polymers exhibited accumulation mode behavior in OECT devices, and tuning of the donor comonomer resulted in a three-order-of-magnitude increase in transconductance. The best polymer gFBT-g2T, exhibited normalized peak transconductance (g_m,norm) of 298±10.4 S cm⁻¹ with a corresponding product of charge-carrier mobility and volumetric capacitance, μC*, of 847 F V⁻¹ cm⁻¹ s⁻¹ and a μ of 5.76 cm² V⁻¹ s⁻¹, amongst the highest reported to date. Furthermore, gFBT-g2T exhibited exceptional temperature stability, maintaining the outstanding electrochemical performance even after undergoing a standard (autoclave) high pressure steam sterilization procedure. Steam treatment was also found to promote film porosity, with the formation of circular 200–400 nm voids. These results demonstrate the potential of gFBT-g2T in p-type accumulation mode OECTs, and pave the way for the use in implantable bioelectronics for medical applications.