Stoichiometry‐Regulated Ag 2 Se Flexible Thin Films With High Power Factor and Mechanical Stability

ABSTRACT Ag 2 Se‐based thermoelectric thin films are attractive for near‐room‐temperature energy harvesting, yet suffer from severe non‐stoichiometry (Ag/Se > 2:1) due to Se vacancies, which induce excessive carrier concentration and degrade thermoelectric performance. Here we overcome this limitation through stoichiometry engineering by using a combined liquid‐phase etching, physical exfoliation, and Se vapour annealing strategy. Such an approach suppresses vacancy formation while preserving high crystallinity, enabling optimized carrier concentration and reduced thermal transport. The resulting films exhibit a high power factor of ∼32.2 µW cm −1 K −2 at 398 K and a competitive thermoelectric figure of merit ( ZT ) of ∼1.1 at 300 K. A five‐leg flexible device delivers a normalized power density of ∼4.2 µW cm −2 K −2 and maintains > 97% performance after 1000 bending cycles (4 mm radius). These results highlight stoichiometry control as an effective strategy for enhancing the performance and durability of flexible thermoelectric devices.