Flexoelectricity in soft conjugated polymer films – Flex-SCO

We studied polyelectrolyte conjugated polymer films based on PEDOT)/Poly(styrene sulfonate) (PEDOT :P SS) mostly synthesized by Moltech. Thus, various key parameters could be evaluated, including the degree of oxidation of PEDOT, the PEDOT/PSS ratio and different pre- and post-deposition treatments of the films. Then the synthesis of several monomers corresponding to structural modifications of EDOT was carried out in order to modify the interactions between the conjugated polymer and PSSH and study the resulting effects on flexoelectric properties. In particular, the introduction of alcohol functions, to multiply intermolecular interactions by hydrogen bonds, on the ethylene dioxy bridge and also on a propylenedioxy bridge (PRODOT) has been carried out. Finally, an extension of the system combined with the use of bisEDOT or terthiophene units of the EDOT-Thiophene-EDOT type with the central thiophene functionalized with alcohols or polyether chains were synthesized. For all these modified EDOT systems, their polymerization by chemical oxidation in the presence of PSSH was carried out in order to obtain PEDOT :PSS films with controlled ratios between the doped conjugate system and the PSSH polymer. This systematic study made it possible to specify that the structural modifications of EDOT must be carried out in such a way that the oxidation potential of the modified monomer must always be at most equal to that of EDOT in order to obtain PEDOT :P SS solutions leading to films with a flexoelectric effect. Finally, EDOT or PRODOT monomers carrying polyether chains (PEGs) or an imidazole function (to obtain ionic liquid-like systems) have been synthesized with the aim of obtaining PEDOT-like conjugated polymers by direct polyarylation reactions. Only a polypolymer (EDOT-PEG) was able to obtain flexible films with a flexoelectric effect.

For all these polymers, dielectric and electrical measurements were systematically carried out, as well as the measurement of the flexoelectric coefficient by the so-called bending method in a cantilever beam configuration on films deposited on stainless steel substrate and self-supported.

From the different PEDOT :PSS film structures, it has been shown that the key to achieving high flexoelectricity lies in a strategic engineering of the composition, structure and doping of the material in order to position its conductivity ideally close to 10⁻⁵ S/m. Our results demonstrate that the adjustment of the PEDOT:PSS ratio plays a critical role in optimizing flexoelectric performance. Formulations with intermediate ratios, such as 1:20, achieve the right balance between electronic and ionic conductions, which promotes controlled charge mobility and improves flexoelectric response, achieving a flexoelectric coefficient of 12 μC/m. The introduction of plasticizers, such as xylitol, is proving to be a powerful tool for fine-tuning conductivity and has resulted in a flexoelectric rating of 13 μC/m. It has also been shown that flexoelectricity optimization can be achieved by electrochemical doping, which directly modulates the level of PEDOT doping. By partially doping PEDOT:PSS 1:4, its electronic conductivity can be reduced to the optimal range, doubling the flexoelectric coefficient compared to its fully doped counterpart. On the other hand, by grafting alcohol functions, either on an EDOT unit or on terthiophenes of the EDOT-ThiopheneCH2OH-EDOT type, flexible polymers also presenting an optimization of electronic and ionic conductivity, made it possible to achieve flexoelectric coefficients of 14 μC/m. For the non-doped poly-EDOT-PEG conjugated polymer, without PSSH, a flexoelectric coefficient of 7 μC/m is achieved, which is 70 times higher than what was obtained with poly(3-hexylthiophene). Ultimately, this work highlights that the path to high-performance flexoelectric polymer materials lies in the ability to fine-tune their electrical properties. The very high flexoelectric coefficients of the systems studied have made it possible to develop flexible and sensitive sensors with very large curvature and deformation (up to 62 m-1). In order to improve the Figure of Merit for mechanoelectric energy conversion, multi-layer blends and composites based on conjugated polymer and elastomer or biopolymer (starch) have been prepared in order to reduce the effective permittivity of conjugated polymers, while maintaining a high flexoelectric response.

The Flex-SCO project opens up research perspectives on the tribovoltaic effect induced by friction/slippage between a metal and a semiconductor. The tribovoltaic effect is based on the friction between the two materials of different Fermi energy levels constituting the pair generating a Schottky contact. The frictional energy excites the charge carriers (electron-hole pairs) out of equilibrium at the interface and induces a current in the external circuit and a unipolar voltage (DC), without the rectification step of the output signal, which greatly simplifies the structure of the sensor/generator and decreases the energy cost. The families of conjugated systems studied in the project will make it possible to initiate a study on the development and tribovoltaic characterization of composite films based on semiconducting polymers. It is planned to favor the use of green solvents and synthetics with low environmental impact and in compliance with REACH and RoHs regulations.

The project deals with experimental and theroretical study of ambient mechanical energy harvesting with flexoelectric-like polymer films. The considered transduction mechanism is the flexoelectric effect which corresponds to an electrical polarization induced by a strain gradient. This effect is expected to be exalted in the case of flexible compounds like soft polymers, which are able to withstand very large curvature. However, this presently understudied coupling remains weak in comparison with piezoelectric effect in organic films. The objective of Flex-SCO is to develop all organic films, with flexoelectric coefficients up to 1000 times larger than those actually measured, allowing to propose an alternative to piezoelectric materials. The methodology consists in using semi-conducting conjugated molecules/polymers exhibiting high polarizability. Hence, the objective is to enhance both the interfacial and ionic polarizations in polymer films to induce dipolar motion and free charges transport (i.e. large polarization changes) under strain gradient. The combination of the two functionalities- flexibility and enhanced flexoelectricity- would enable integration of these films in task-oriented devices for ambient low frequency (<100 Hz) micro-energy harvesting in the µW-mW range and from different sources (wind flow, pressure,..). Thus, flexoelectric polymer-based energy harvesters could contribute to power distributed and isolated low consumption sensors for the Internet of Things (IoT) applications.
The work program consists of 4 work packages (WPs).
The first WP deals with the project coordination and dissemination.
The second WP aims to bring out the structures which will allow to reach the highest flexoelectric coefficients, while maintaining good flexibility of the films and easy processing by conventional methods such as electrodeposition, spin coating or doctor blade. Mainly, based on the partners’ preliminary results and works recently published about flexoelectric effect in polyelectrolyte materials (PEM), three different axes of work around the conjugated materials have been identified: (1) structural modifications of conjugated systems; (2) conjugated polyelectrolyte systems and (3) dielectric polymers with conjugated side chains.
The third WP aims at gaining detailed information on structure and properties of the flexoelectric films in both pristine and blend forms, and the development of devices based on this knowledge. Structural, dielectric and flexoelectric properties of polymer thin films will be studied in purpose to selecting the compositions with the largest flexoelectric coefficients but also the ability for mechanical energy harvesting. A comprehensive study of the relationships between morphological and electrical, dielectric parameters and the flexoelectric response in polymer compositions will be realized as a function of the film thickness (range : 0.5 – 100 µm) in order to assess the size scaling effect. The influence of electrode configuration will also be studied. Based on this knowledge, energy harvesters will be developed and optimized in WP4.
WP4 is devoted to the realization and evaluation of flexoelectric polymer-based vibrational energy harvesters. Different types of harvesting devices will be studied depending on the mechanical excitation (forced vibrations, air flow, acoustic pressure,…). Design and assembly of specific test benches (like wind tunnel experiments) will be carried out to verify the electro-mechanical models and identify the optimal energy harvester configuration.