Flexible pressure sensor based on freestanding organic charge-transfer thin-film transistors

Wearable pressure sensors are important for health monitoring, soft robotics, and human-machine interfaces. Traditional sensors use single-component, flexible and solution-processable organic semiconductors. In comparison, organic donor-acceptor charge transfer (CT) systems combine the advantages of both the donor and the acceptor, with tunable CT interactions, offering multiple opportunities to enhance sensing properties. Here, we propose a solution-processed flexible organic thin-film transistor (TFT) pressure sensor based on a freestanding poly(3-hexylthiophene) (P3HT)-fullerene (C₆₀) CT film. The film is prepared by air-water interfacial self-assembly, yielding crystallised centimetre-scale, transferable thin films with retained P3HT molecular ordering and P3HT-C₆₀ CT-related interactions. When used as the active semiconductor layer in a flexible TFT, the film shows p-type transistor behavior with an ON/OFF ratio of approximately 10⁵, a maximum pressure sensitivity of 2.03 kPa^-1, and a response time of about 60 ms. The pressure response is attributed to pressure-induced modulation of molecular packing and donor-acceptor interactions within the freestanding film. These results extend the use of P3HT-C₆₀ beyond conventional photovoltaic applications and demonstrate that interfacial self-assembly can provide freestanding donor-acceptor semiconductor films for flexible, gate-modulated tactile sensing. This work may initiate the study of flexible sensors based on the CT complex.