Thermomechanical pressing and immediate quenching: enhanced piezoelectricity and transparency in piezoelectric nanofibers

Highly transparent piezoelectric films hold great promise for smart wearable devices and energy harvesting applications. However, their fabrication is often hampered by complex processes. The development of new strategies for the preparation of transparent piezoelectric films is urgently needed. This study presents an efficient method to convert electrospun nanofibers into dense piezoelectric films by thermomechanical pressing coupled with immediate quenching. By applying this treatment at 140 °C and 600 MPa for only 3 min, the resulting dense films have nearly twice the piezoelectricity (d₃₃ of 14.09 pC N^-1) compared to the nanofibers. In addition, this poling-free process increases film transparency, significantly reduces haze, and greatly improves mechanical robustness and stiffness. These improvements are attributed to the flash crystallization effect induced by hot pressing and immediate cooling, which elongates the crystal chains, increases the lamellar dimensions, and expands the oriented amorphous fraction regions. As a result, a well-organized semi-crystalline structure is formed over the expanded regions. Our method is simple and allows precise control over the crystal structure, which promises to produce transparent piezoelectric films ideal for force sensing and energy harvesting applications.