Versatile polymer-coated Ag2Se thermoelectric materials and devices for multi-scenario applications developed by direct-ink printing

Flexible thermoelectrics offer eco-friendly solutions for wearable health monitoring, smart sensors, and energy harvesting. However, challenges remain in device fabrication and performance enhancement. Herein, flexible thermoelectric films-Ag₂Se/methylcellulose, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)@Ag₂Se/methylcellulose, and polyvinylpyrrolidone@Ag₂Se/methylcellulose-are fabricated via a scalable and economical direct-ink printing method. After cold pressing and annealing, the polyvinylpyrrolidone@Ag₂Se/methylcellulose film shows a record-high power factor of 2191.5 μWm^-1K^-2 at 400 K among all the flexible organic/inorganic films prepared via direct-ink printing. The heterointerfaces, pores, boundaries, and dislocations formed in the composite films through the fabrication procedure are beneficial for enhancing the Seebeck coefficient and electrical conductivity simultaneously, as well as reducing the thermal conductivity. Three-leg flexible thermoelectric generators are fabricated using a direct-ink printing process, yielding a power density of 22.1 W/m² at a ΔT of 36.1 K. Such devices have been applied across various scenarios, including low-grade heat recovery, position identification, light-heat-electricity conversion, and respiratory monitoring. The direct-ink printing and thermoelectric performance optimization strategies have universal applicability, exhibiting potential for advancing flexible wearable electronics.