Rolling-enabled high-performance free-standing Ag2Se/PVDF composite films for flexible thermoelectric devices

Abstract

The development of wearable electronics demands flexible thermoelectric (TE) materials that simultaneously possess high energy-conversion efficiency and robust mechanical compliance. Conventional strategies often rely on passive substrates or suffer from a trade-off between electrical and mechanical properties. In this work, we report a hot-rolling process to fabricate high-performance, free-standing Ag₂Se/PVDF composite films, which synergistically optimizes both microstructure and functionality. By systematically tuning the Ag₂Se content (80–100 wt%), we demonstrate that hot rolling not only aligns Ag₂Se nanowires into a highly oriented, continuous lamellar conductive network along the rolling direction, but also induces the α → β crystal phase transition in PVDF, forming an interpenetrating network structure with Ag₂Se as the rigid conductive skeleton and PVDF as the flexible toughening phase, thus significantly enhancing interfacial bonding and carrier transport. The optimized composite film with 90 wt% Ag₂Se (R-90) exhibits an exceptional room-temperature power factor of 1129.5 µW m⁻¹ K⁻² and a planar TE device constructed using R-90 legs achieves a high power density of 7.0 W m⁻² under a temperature difference of 35.6 K, at the highest level of free-standing inorganic–organic composite flexible thin films and devices. Moreover, the R-90 film exhibits outstanding mechanical flexibility, retaining 96.3% of its electrical conductivity after 3000 bending cycles (3 mm radius), along with a tensile strain of 1.32% at room temperature (3.3 times that of pure Ag₂Se). This study highlights rolling as a potent microstructural-control strategy that concurrently enhances TE and mechanical properties in organic–inorganic composites, paving a promising route toward substrate-free, high-performance flexible TE devices for wearable energy harvesting.