Among the transduction mechanisms of interest for sensing and/or actuation applications at nano/micro scale, the piezoelectric effect has been widely exploited owing to the solid state nature of piezoelectrics, the large ability of specific classes of materials for the mechanical-to-electrical energy conversion and easy integration. However, every piezoelectric (also generally ferroelectric) presents well-known intrinsic drawbacks such as required poling step and related aging. In contrast, uniquely flexoelectric materials do not suffer from these disadvantages because flexoelectricity, a universal effect in all dielectric solids defined as the electrical polarization induced by a strain gradient, does not imply preliminary electric field-induced macroscopic polarization. Besides, strain gradient may be easily obtained by bending plate or cantilever-shaped structure and in this case it is nothing but the local curvature of the flexible system. Thus, as strain gradient (curvature) inversely scales with both elastic stiffness and thickness, this study will focus on the evaluation of the potentialities of flexoelectric effect in soft polymer films for electromechanical applications, with an emphasis on the thickness influence. In this way, analytical results combined to experimentally obtained effective flexoelectric coefficients for some typical polymer classes may provide guidelines for the development of soft and low frequency flexoelectric mechanical transducers.

We report the use of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a transducing material in the fabrication of mechanical-to-electrical conversion devices based on the flexoelectric-like response of this polymer. Devices are made in a cantilever-based three-layer stainless-steel/PEDOT:PSS/top metal electrode configuration to assess the effective transverse flexoelectric coefficient μ12′. We investigated the influence of the nature of the top electrode in the flexoelectric response comparing samples with gold and aluminum top electrodes and demonstrated the huge impact of adding a small fraction of a second dopant such as xylitol to the PEDOT:PSS polymer blend and the benefits of a post-treatment of the polymer film with ethanol and methanol on the flexoelectric coefficient. The combination of xylitol addition and the rinsing of the polymer films with ethanol and methanol, along with the use of gold as a top electrode, led to a significant improvement of μ12′ to ca. 24 μC m−1, which is in the range of those reported for high permittivity oxide materials. These findings support the use of conjugated polymers as an alternative to inorganic materials in flexoelectric-based applications, where large flexibility is required.

In the last decade, the polymeric poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) semiconductor has been widely studied for their electrical and electrochemical properties and used in various applications such as flexible and transparent electrodes, optoelectronics and thermoelectric devices. Here, we report the intrinsic electromechanical coupling in trilayered stainless steel/PEDOT:PSS/Aluminum composites. The PEDOT:PSS thin film integrated in the multi‐layer serves as the transduction material itself and yields a remarkable flexoelectric‐like response when the cantilever shaped‐ device assembly is subjected to beam bending. Two orders of magnitude larger flexoelectric coefficients (up to μ′12 = 6 μC/m) were obtained for semiconducting PEDOT:PSS thin films, in comparison with those measured for insulating polymers. Besides, it is observed that flexoelectric effect may be enhanced when the conductivity of the trilayer device increases to a certain extent, especially after a simple dilution treatment that induces partial removal of excess hydrophilic PSS. These first results may open a new avenue for simply preparing flexible and highly sensitive curvature sensors with single electroactive semiconducting polymer layer between metal electrodes.