By electrochemically introducing phosphonate ester groups into conductive polymer films, researchers at Science Tokyo have addressed a fundamental trade-off between electronic charge transport and ion transport, overcoming a key performance limitation in organic electrochemical transistors (OECTs). The method enables precise tuning of polymer properties and can be applied to semicrystalline materials without redesigning monomers, supporting the development of improved biosensors and flexible electronic devices.
Organic electrochemical transistors (OECTs) have garnered increasing attention for wearable electronics and biosensors due to their low-voltage operation. They consist of an electrolyte, a channel made from an organic semiconductor, and gate, source, and drain electrodes. By controlling the gate voltage, researchers can regulate the movement of ions between the electrolyte and the conductive polymer channel. However, OECTs have faced a key limitation: it is difficult to efficiently transport both electronic charges and ions simultaneously, which restricts device performance.
Now, a research team from Institute of Science Tokyo (Science Tokyo), Japan, has developed a new strategy to overcome this problem by electrochemically introducing phosphonate ester groups into conductive polymer films. Their approach, published online in the journal Angewandte Chemie International Edition on April 18, 2026, enables precise control over the balance between electronic charge transport and ionic conductivity, leading to improved transistor performance.
The study was led by Professor Shinsuke Inagi from the Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Science Tokyo, together with Assistant Professor Kosuke Sato and Dr. Kohei Taniguchi, who was a graduate student at the time of the study, from the same institute.
"Achieving the coexistence of smooth charge transport and affinity to hydrophilic ions is difficult; thus, the molecular design of the active layers in OECTs is still under development," says Inagi.
The trade-off between charge transport and ion transport largely arises from the balance between the hydrophilic (water-attracting) and hydrophobic (water-repelling) characteristics of the polymer film. Polymers with hydrophilic side chains can improve ion transport, but their synthesis often requires multiple steps. Moreover, they can absorb water from the air and trap charges, reducing electronic charge transport. In contrast, hydrophobic polymers typically show poor ion transport because hydrophilic ions have difficulty penetrating them.
The proposed method balances these properties by introducing hydrophilic phosphonate ester groups into existing hydrophobic conductive polymers without redesigning the polymer backbone itself.
The researchers electrochemically oxidized two semicrystalline conductive polymers, PBTTT (poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) and DPP-DTT (diketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene]), and reacted them with trialkyl phosphite to attach phosphonate ester groups directly onto the polymer backbone. Because the rigid crystalline structure of the polymers limited phosphite penetration, the researchers incorporated an ion-conducting material called Nafion into the polymer films. This created pathways that allowed phosphite molecules to penetrate more effectively into the tightly packed polymer structure.
By carefully controlling the amount of electric charge passed during the electrochemical reaction, the researchers were able to precisely tune the degree of functionalization or the amount of phosphonate ester incorporated into the films. When the modified polymers were used as active layers in OECT devices, the researchers observed significant improvements in transistor performance. OECT performance is commonly evaluated using the product µC*, where µ represents how easily electronic charges move through the material and C* reflects how effectively ions can enter and interact with the polymer film.
The researchers found that moderate levels of phosphonate ester incorporation, corresponding to a degree of functionalization (DOF) of approximately 0.06–0.16, produced the best results. For PBTTT, a DOF of 0.12 increased µC* to 90 mS cm⁻¹, while for DPP-DTT, the µC* value nearly doubled at a DOF of 0.06. However, excessive phosphonate incorporation reduced performance because it began to interfere with efficient electronic charge transport.
Importantly, the study overcomes a major limitation of previous phosphonylation methods, which mainly worked with amorphous polymers. By extending the approach to rigid semicrystalline polymers, the researchers broadened the range of materials available for high-performance OECT development.
"Our results demonstrate that post-functionalization of existing semicrystalline polymers is a viable strategy for balancing ionic and electronic transport without requiring entirely new monomer design and synthesis," says Inagi.
