Advancing Ag₂Se thin-film thermoelectrics via selenization-driven anisotropy control

The debate over the optimal orientation of Ag₂Se thin films and its influence on thermoelectric performance remains ongoing. Here, we report a wet-chemical selenization-based anisotropy optimization technique to control the in-plane orientation of the Ag₂Se thin film, steering it away from (002) nearly parallel planes that hinder charge carrier mobility. This approach enables us to achieve an impressive power factor of 30.8 μW cm^-1 K^-2 at 343 K. The as-fabricated Ag₂Se thin film demonstrates remarkable durability, retaining over 90% of its power factor after six months of air exposure, and outstanding flexibility, with performance variation staying within 5% after 2000 bending cycles at a 5 mm radius. These attributes are attributed to the controlled film thickness, crystallinity, and strong adhesion to the polyimide substrate. Additionally, the as-assembled slotted thermoelectric device delivers an output power of 0.58 μW and a competitive power density of 807 μW cm^-2 at a temperature difference of 20 K, alongside a high normalized power density of 1.8 μW cm^-2 K^-2, highlighting its potential for practical application. This study provides valuable insights into the design of high-performance, highly flexible thermoelectric thin films for real-world applications.