Flexible Pressure Sensors Based on the Controlled Buckling of Doped Semiconducting Polymer Nanopillars

Mechanically flexible and electrically conductive nanostructures are highly desired for flexible piezoresistive pressure sensors toward health monitoring or robotic skin applications. The popular approach for these sensors is to combine flexible but insulating polymers as a micro- or nanostructural functional medium and conductive materials covering the polymer surface, which could give rise to many practical issues, for example, durability, compatibility, and complicated processing steps. We herein report a piezoresistive pressure sensor with a functional component of nanopillars of a doped semiconducting polymer, operating at low bias voltage with a sensing mechanism based on controlled buckling. Nanopillars of poly(3-hexylthiophene-2,5-diyl) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane are patterned using anodic aluminum oxide templates. The nanopillars impart reversible current changes in response to the applied pressure over a wide pressure range (0-400 kPa). The sensor exhibits two current response regimes. Below 50 kPa, a strongly nonlinear response is observed, and above 50 kPa, a linear pressure response is demonstrated. Euler buckling theory is used to predict the deformation behavior of the nanopillars under pressure and in turn elucidate the sensing mechanism. Our results demonstrate that the contact area between the nanopillars and the top electrode increases with the application of pressure due to their elastic buckling in a two-regime fashion underlining the two electrical current response regimes of the sensor. Independent finite element modeling and scanning electron microscopy measurements corroborated this sensing mechanism. In contrast to many reported pressure sensors, the controlled elastic buckling of the nanopillars enables the detection of pressure over a wide range with good sensitivity, excellent reproducibility, and cycling stability.