Giant piezoelectricity driven by Thouless pump in conjugated polymers

Thanks to the possibility they offer to convert mechanical energy into electrical energy and vice-versa, piezoelectric materials are of great interest in various fields and find many technological applications, ranging from macro- to microscopic electromechanical devices, to energy harvesting and much more. For their high electromechanical response, the most widely used piezoelectric materials are inorganic perovskites, such as lead zirconate titanate (PZT). However, they present some limitations since they have very low mechanical flexibility, high fabrication costs and are often toxic and not bio-compatible. A promising alternative to overcome these limitations is to exploit piezoelectric properties of organic polymers.

While most organic piezoelectrics rely on the presence of intrinsic local dipoles, a highly nonlocal electronic polarization can be foreseen in conjugated polymers, characterised by delocalized and highly responsive π-electrons. These 1D systems represent a physical realization of a Thouless pump, a mechanism of adiabatic charge transport of topological nature which results, as shown in our work, in anomalously large effective charges, inversely proportional to the band gap energy. A structural (ferroelectric) phase transition further contributes to an enhancement of the piezoelectric response reminiscent of that observed in piezoelectric perovskites close to morphotropic phase boundaries. First-principles Density Functional Theory (DFT) calculations performed in two representative conjugated polymers using hybrid functionals, show that state-of-the-art organic piezoelectric are outperformed by piezoelectric conjugated polymers, mostly thanks to strongly anomalous effective charges of carbon, larger than 5e – ordinary values being of the order of 1e – and reaching the giant value of 30e for band gaps of the order of 1 eV.